Part 1: A Potential Role for Novelty/Attention and Noradrenergic/Cholinergic Signaling in Gamma Frequency Stimulation
Abstract:
Gamma oscillations within the sensory cortices have been shown to play a key role in feature binding, multisensory information encoding, selective attention, and increased information transmission to the hippocampus. The process of the cortex promoting arousal, attention, and determining novelty is thought to rely on a complex interplay of dopaminergic and noradrenergic signaling for contextual novelty and cholinergic and noradrenergic signaling encoding absolute novelty. Gamma entrainment using sensory stimulation (GENUS) has been found to have significant impacts on 1) glial cells and neuroinflammation 2) increasing blood and glymphatic flow 3) plasticity, neurogenesis, and preventing neurodegeneration, and 4) hippocampal dependent memory in preclinical models of Alzheimer’s disease (AD) and neurodegeneration. Related phenotypes have been observed in a variety of disease models including chemobrain, multiple sclerosis, and stroke. Additionally, similar results have been observed using vagus nerve and transcranial stimulation in the gamma range. We propose a mechanism in which the positive impact of GENUS and gamma range stimulation is largely mediated by activation of midbrain nuclei, promotion of noradrenergic and cholinergic signaling, influencing of glial and synaptic dynamics, and promotion of circuit health. We would hypothesize that these phenotypes are based on neuromodulatory communication and diffusion rather than direct neuron-glia interactions within the cortex or corticocortical communication. We discuss the ways in which this circuit has been shown to be functionally impacted by the pathophysiology of AD and its relationship with cognitive reserve and decline. Finally, we discuss improvements in stimulation protocol to more directly promote this signaling pathway.
Visual Abstract:
Novelty/Selective attention circuit
LC – Locus coeruleus
BF – Basal forebrain
AC – Auditory cortex
VC – Visual cortex
PFC – Prefrontal cortex
HPC – Hippocampus
NA/NE – Noradrenaline/Norepinephrine
(A)Ch – (Acetyl)Choline
Proposed influence of 40 Hz sensory stimulation on Novelty/Selective attention circuit
Gamma Oscillations in Sensory Processing and Encoding
Gamma oscillations in the cortex, characterized by rapid excitation and decreased inhibition, have been shown to play a fundamental role in network information flow and communication during attention, cognitive processing, and memory encoding and retrieval (Guan, 2022, Tiesinga et al., 2004). Selective attention, the process of filtering salient stimuli from irrelevant stimuli, is thought to involve gamma rhythmic coordination of cells responding to salient stimuli (Fries, 2015).
Sensory stimulation and behavioral responses dynamically modulate the functional connectivity between pyramidal cells and inhibitory interneurons, including parvalbumin expressing, somatostatin expressing, and vasoactive intestinal peptide interneurons (Dipoppa et al., 2018; Antonoudiou et al., 2020). Synaptic connections between inhibitory interneurons and their associated excitatory neurons mediate the short-term plasticity of synaptic inhibition and the consequent endogenous gamma oscillations. The prevailing understanding is that the inhibitory interneuron network characterized by these unique discharges and synaptic activities is essential for the generation of gamma rhythm (Buzsáki, 2012; Cardin, 2018).
In addition to GABAergic transmission, a neuronal network with asynchronous firing can produce gamma oscillations in response to acetylcholine (Ach) pulses via muscarinic Ach receptors (mAChRs), which enables the dynamic network adaptation during attentional tasks (Lu et al., 2020). Cue detection in behavioral attention tasks is dependent on cholinergic-driven gamma oscillations in the frontal cortex, in which both mAChRs and nicotinic Ach receptors (nAChRs) make distinct contributions (Howe et al., 2017). Recent experimental results have shown that the detection of cues in behavioral attention tasks relies on transient increases of acetylcholine (ACh) release in frontal cortex and cholinergically driven oscillatory activity in the gamma frequency band (Howe, 2017). Norepinephrine (NE or noradrenaline) input also modulates rhythmic neuronal activity during selective attention (Dahl, 2021).
Cortical acetylcholine and norepinephrine input are largely mediated by the basal forebrain (BF) and locus coeruleus (LC) respectively. In the adult mammalian brain, cholinergic projection neurons of the basal forebrain span the entire extent of the cortex (Ananth, 2023). Cholinergic neurons participate in essential functions including respiration, sleep, attention, mood, and memory and act via two major classes of receptors: G protein-coupled muscarinic ACh receptors (mAChRs) and ionotropic nicotinic ACh receptors (nAChRs) (Ananth, 2023). The LC is a cluster of relatively large neurons containing NE that is located bilaterally in the brainstem just under the cerebellum and lateral to the fourth ventricle (Poe, 2020). The distribution of LC-NE axonal fibers is nonuniform across the ratand primate neocortex and coordinated to target functionally related circuits, such as those coordinating aversion and/or anxiety in the anterior cingulate and amygdala (Poe, 2020). However, additional evidence for projections from a subgroup of LC neurons to the caudate–putamen, along with increased NE turnover in striatum and the functional connectivity of striatal‒motor networks is revising previous conceptions that LC-NE projections do not influence striatal‒motor function (Zerbi, 2019). The projection of the LC to other motor structures such as the cerebellum has been under-categorized.
Overall, it has been shown that interaction between the midbrain nuclei and anterior hippocampus encode salience and novelty of a given stimulus. In response to novelty, noradrenergic and cholinergic input to the hippocampus (HPC) increases gamma oscillations and promotes plasticity and encoding while also increasing information transmission through the perforant path. Cholinergic and dopaminergic input to the prefrontal cortex from the basal forebrain (BF) and ventral tegmental area respectively produce gamma oscillations, which in turn promote attention, interaction, and sustained gamma oscillations in the relevant sensory cortices (Kafkas, 2018).
In the setting of multisensory input and cross-talk, it has been shown that corticocortical synchronized gamma oscillations are induced in order to modulate information flow during a visual-tactile stimuli task and that circuit modulation using transcranial alternating current stimulation at 40 Hz can impact these relationships (Misselhorn et al., 2019).
Fast gamma rhythms in the hippocampus are coupled with fast gamma inputs from the medial entorhinal cortex responsible for processing current sensory information (Colgin, 2009). It has been proposed that fast gamma promotes the transmission of current sensory information to the hippocampus during memory encoding (Colgin, 2013). Fast gamma power dominates within the hippocampus during novel object location tasks (Zheng, 2016). Glutamatergic and cholinergic agonists are also able to produce hippocampal oscillations in the 30-50 Hz range (Buhl, 1998; Fisahn, 1998; Betterton, 2017).
Outside of promotion and involvement in memory encoding in response to salient and/or novel sensory stimuli, slow gamma power in the hippocampus paired to sharp-wave ripples (SWRs) during rest and slow wave sleep may support a functional role in memory retrieval (Mably, 2018).
Summary: Gamma oscillations within the sensory cortex can be a result of spontaneous endogenous production based on asynchronous inhibitory activity and, in the setting of midbrain activation, sustained as a result of cholinergic input from the basal forebrain driving gamma oscillations in the prefrontal cortex promoting sustained attention. Prefrontal gamma oscillations are also related to dopaminergic input for interaction, motor planning, and executive function. Simultaneously, gamma oscillations in the hippocampus result from noradrenergic, cholinergic, serotonergic, and BDNF input to promote memory encoding of novel stimulus or experience.
Stimulus -> spontaneous gamma oscillations -> determination of novelty/saliency -> cholinergic/noradrenergic activation -> sustained gamma oscillations in the sensory, prefrontal, and hippocampal cortex, as well as increased transmission along perforant path.
This response has been well-characterized.
Figure 1: Role of LC and BF in novelty/attention encoding and gamma oscillations (Left – modified from Sara, 2012; Right – Hagena, 2016).
The focus in Part 1 is on the function of gamma oscillations within sensory memory encoding in response to cholinergic and noradrenergic input. Other functions of gamma may be discussed in later sections.
Gamma entrainment using sensory stimulation (GENUS)
GENUS, which uses light and sound presented at 40 Hz, 1 hour per day for an extended period, has shown results in ameliorating amyloid and tau pathology, reducing neuroinflammation and neurodegeneration, and reducing cognitive deficits and decline in various mouse models of AD and human studies (Blanco-Duque, 2024). Impact on circadian genes and sleep structure have been observed in mouse models and subjects with mild AD respectively (Yao, 2020; Chan, 2022). Significant changes in glial cell populations, immune profiles, and cerebral spinal fluid (CSF) proteomics across multiple cohorts have been reported (He,2021). Many of these benefits are thought to be associated with increased glymphatic clearance during and following 40 Hz stimulation as a result of VIP+ interneuron activity and adenosine signaling (Murdoch, 2023; Sun, 2024). Additionally, GENUS has also been found to show benefits in models of stroke and ischemia (Niu, 2023), reducing demyelination and promoting remyelination in multiple sclerosis (Rodrigues-Amorim, 2024), myelination and cognitive function in chemobrain (Kim, 2024), and neurogenesis, cognition, and mitochondrial dynamics in a mouse model of Down syndrome (Jackson, 2023).
40 Hz stimulation using transcranial magnetic stimulation of the angular gyrus found reduced volume loss in a variety of regions in patients with probable AD (Liu, 2022). 40 Hz transcranial alternating current stimulation (tACS) of the temporal lobe found increased perfusion and cognition in patients with mild to moderate AD (Sprugnoli, 2021). Transauricular vagus nerve stimulation (taVNS) at 40 Hz found reduced pathology, modulated microglia, and improved spatial memory in a mouse model of AD (Yu, 2023).
Figure 2: From Blanco-Duque, 2024.
Overall, across these disease models and stimulation protocols, impact can generally be summarized similar to Blanco-Duque as 1) changes in glial cells and neuroinflammation 2) increased blood and glymphatic flow 3) plasticity and neurogenesis within the hippocampus, and as a result, 4) improved hippocampal-dependent memory
We hypothesize that inducing gamma oscillations in multiple sensory cortices mimics activation of this novelty/salience circuit; inducing similar activation patterns and corresponding noradrenergic and cholinergic input. Doing this chronically mimics day-to-day strong sustained novelty/learning-inducing experiences involving the sensory cortices of interest.
In other words, driving sensory cortical firing at 40 Hz mimics the endogenous gamma activity relationship between LC-NE activation, acetylcholine input and the inhibitory/excitatory neuron relationship encoding novel or salient sensory stimuli processing. We hypothesize that this would feedback into the circuit to promote sustained attention and novelty encoding through noradrenergic and cholinergic activation. We would then hypothesize that the therapeutic benefits of GENUS are largely due to increased neuromodulatory tone.
Below, we will outline how each of the above categories may be a result of activation of this salience and novelty circuit producing an increase in cholinergic and noradrenergic output to the sensory, prefrontal, and hippocampal cortex.
Glial cells and neuroinflammation
Several studies indicate that microglia and astrocytes express adrenergic receptors, whose activation influences the release of pro-inflammatory mediators, controlling the extent of glial reactivity (Sugama, 2019). Acetylcholine receptors are also expressed by glial cells (Gamage, 2020). Microglial cells express the nicotinic receptor α7 and its activation attenuates the pro-inflammatory response of microglial cultures, suggesting that acetylcholine may control brain inflammation, in analogy to what is demonstrated in peripheral tissues (Minghetti, 2007). This highlights the role that noradrenergic and cholinergic input have on both overall central nervous immune system reactivity and immediate inflammatory response respectively.
The cholinergic anti-inflammatory pathway (CaiP) has been well-documented and studied (Pavlov, 2006; Han, 2017; Martelli, 2014). Noradrenaline inhibits microglial activation and suppresses pro-inflammatory mediator production (e.g., tumor necrosis factor-α, interleukin-1β & inducible nitric oxide synthase activity), thus limiting the cytotoxicity of midbrain dopaminergic neurons in response to an inflammatory stimulus (O’Neill, 2018). The anti-inflammatory effect of vagal stimulation depends on the presence of noradrenaline-containing nerve terminals in the spleen as a result of LC activation, as well as a variety of neuroplasticity and anti-inflammatory VNS outputs being directly related to CaiP (Mota, 2022, Berger 2024).
Neuroinflammation and oxidative stress driven by microglia can disrupt normal gamma oscillations. Minocycline, a microglia inhibitor, alleviates neuroinflammation and synaptic loss, which consequently restores gamma oscillations in prefrontal circuit and improves cognitive deficits (Chini, 2020; Ji, 2020).
Figure 3: Impact of noradrenaline and acetylcholine signaling on peripheral and central immune systems (From Minghetti, 2007)
Astrocytes are responsive to a range of neuromodulators. NE evokes robust calcium transients in astrocytes across brain regions through activation of α1-adrenoreceptors. Astrocytes ensheathe neurons at synapses and are known to modulate synaptic activity. Hence, astrocytes are in a key position to relay, or amplify, the effects of NE on neurons, most notably by modulating inhibitory transmission (Wahis, 2021). Cortical NE-astrocyte interactions have been shown to be key to learning behavior (Drummond, 2024 – Preprint).
Previous data suggest a role of cholinergic precursors independent from acetylcholine on maturation and differentiation of astroglial cells in vitro, rather than on their growth, proliferation and development in culture (Bramanti, 2008)
Using cholinergic agonists in hippocampal slices, Lee et al. investigated gamma oscillations and found that the transient increase of calcium concentration in astrocytes precedes the onset of oscillations and that the release of astrocyte vesicles is necessary for the maintenance (but not the initiation) of cholinergic-induced gamma oscillations, as well as normal cognition and memory in animals (Lee et al., 2014). Astrocyte specific S100 calcium binding protein B (S100B) in mPFC enhances theta-gamma PAC in vivo and improves cognitive flexibility, indicating that astrocytes might participate in the complicated signaling that constitutes the neural circuits of advanced cognitive function (Brockett et al., 2018). Furthermore, PV+ interneurons in mPFC recruit astrocytes to support the generation of gamma oscillations and to rectify decision-making behavior, as astrocyte regulation of signal transmission is critical to the maintenance of gamma oscillations (Guan, 2022, Makovkin, 2022).
Overall, both microglia and astrocytes have been shown to extensively interact with cholinergic and noradrenergic inputs and gamma oscillations and that these inputs not only impact immediate microglial state, but also glial proliferation and propensity towards a chronic inflammatory state. Neurotransmitter-glial interactions have been shown to have a profound impact on circuit function and oscillatory dynamics through interactions across the neurovascular unit and impact on inhibitory transmission during learning behavior, discussed more below. Given the hypothesized increase in noradrenergic and cholinergic input following GENUS, it is reasonable to expect significant changes in inflammatory tone and microglial surveillance, as well as astrocyte maturation and differentiation. Phenotypes that have been consistently observed across disease models after chronic exposure to GENUS (Iaccorino, 2016; Martorell, 2019; Adaikkan, 2020).
Blood flow and glymphatics to relevant circuits
The intracerebral cholinergic fibers projecting from the basal forebrain are able to act as autonomic nerve fibers for the regulation of cortical and hippocampal blood flow (Sato, 2004). Activation of these cholinergic vasodilator fibers results in the release of acetylcholine (ACh) within the cortex, activation of both nicotinic and muscarinic ACh receptors, and vasodilatation without coupling to glucose metabolic rates. This cholinergic vasodilator system has been shown to decline with age in rats mainly due to age-related declines of nicotinic ACh receptor activity (Sato, 2002).
Choline and its downstream products such as choline-containing phospholipid particles have been shown to play a critical role in multiple components of the neurovascular unit including neurons, astrocytes, microglia, and endothelial cells themselves (Roy, 2022).
At the local neuronal level, NE suppresses most activity, but amplifies the strongest activity as a result of the differential effects on adrenoreceptor subtypes. The amplification of strong activity occurs via “NE hotspots,” where positive feedback loops between local NE and glutamate release increase the strength of activated representations. To sustain higher levels of activity, hotspots also recruit additional metabolic resources. At the circuit level, the increased glutamate and NE produced at hotspots recruit nearby astrocytes that supply energy to active neurons. On a broader scale, NE facilitates the redistribution of blood flow towards hotspots and away from areas of lower activity (Mather, 2015)
Work from Maiken Nedergaard recently showed that in the setting of traumatic brain injury, noradrenergic input uniquely influences central glymphatic flow (Hussain, 2023).
“Analysis showed that norepinephrine also is a key regulator of glymphatic activity, and that norepinephrine might be responsible for suppression of glymphatic activity during wakefulness. Local application of a cocktail of norepinephrine receptor antagonists in awake mice resulted in an increase in CSF tracer influx almost comparable to that observed during sleep or anesthesia. In contrast, norepinephrine application, mimicking the wakeful state, significantly decreased the interstitial volume fraction. An increase in interstitial space volume in the sleep state reduces tissue resistance towards convective flow thus permitting CSF-ISF exchange. Thus the burst release of norepinephrine during arousal increases the cellular volume fraction resulting in a decrease in the interstitial space. In turn, the resistance toward convective exchange of CSF and ISF increases and this results in a suppression of glymphatic fluxes during wakefulness. Norepinephrine also acts directly on choroid plexus epithelial cells and inhibits CSF production. Conversely, removal of norepinephrine signaling, mimicking the sleep state, enhances CSF production. The concerted effect of norepinephrine thus acts via different mechanisms on both fluid availability and convective fluxes to suppress glymphatic function and norepinephrine can therefore be considered both a key regulator of the switch between the sleep and wakeful state and solute clearance from the brain” (Jessen, 2015)
This might explain how GENUS increases glymphatic clearance during wakefulness without overt influence on arousal state: a reduction in base noradrenergic and cholinergic tone to central glymphatic regulators as LC shifts NA production towards sensory cortices and neural mechanisms of learning.
We would strongly hypothesize that the mechanism of noradrenergic and cholinergic control over central glymphatic flow is a combinatorial influence on astrocytes, GABAergic interneurons, and the interaction between the two neuromodulators influencing the neurogliavascular unit. Additionally, the increased blood flow, vasomotion, and glymphatic clearance observed following GENUS are likely to be mediated by neuromodulator input. Changes in adenosine signaling following GENUS observed by Sun, et al. 2024 are also unsurprising given the interactions between noradrenaline, arousal, glymphatic flow and sleep state.
Plasticity, neurogenesis, and hippocampal dependent encoding of novel/salient sensory stimuli
Stimulation of noradrenergic receptors in the hippocampus alters neuronal excitability and synaptic plasticity during learning and memory encoding. Consistent with this notion, NE enhances plasticity, information transfer, and encoding during stimuli exposure for a variety of hippocampus-dependent tasks. The effects of NE receptor activation on cellular plasticity may account for NE-dependent modulation of memory in the hippocampus (Gelinas, 2007).
Depending on which subtype of adrenoceptor is activated, NE differently affects intrinsic membrane properties of postsynaptic neurons and synaptic plasticity. For example, α-adrenoceptor activation is mainly related to the potentiation of synaptic responses during learning and memory processes. α2-adrenoceptor activation may contribute to a higher-order information processing such as executive function, but currently the direction of synaptic plasticity modification by α2-adrenoceptors has not been clearly determined. The activation of α1-adrenoceptors appears to mainly induce synaptic depression in the brain (Marzo, 2009).
“At the cellular level, locus coeruleus burst release of norepinephrine transiently inhibits feedforward interneurons and either excites or inhibits subpopulations of feedback interneurons. Consistent with reduced feedforward inhibition, granule cell firing is transiently increased. Norepinephrine selectively promotes the processing of medial perforant path spatial input. This effect is mediated both through short- and long-term potentiation of cell excitability and through delayed potentiation of synaptic input. A critical level of norepinephrine release is required for long-term effects to norepinephrine alone. Norepinephrine also promotes frequency-induced potentiation of granule cell output at the mossy fiber to CA3 connection. Local increases in norepinephrine accompany glutamate release and release of other neurotransmitters providing a mechanism for norepinephrine enhancement effects independent of locus coeruleus firing. Stimuli, such as novelty and reward and punishment, which activate locus coeruleus neurons, enhance responses to medial perforant path input and engage late phase frequency-induced long-term potentiation through β-adrenoceptor activation. Behavioral studies are consistent with the mechanistic evidence for a norepinephrine role in promoting learning and memory and assisting retrieval” (Harley, 2007).
Loss of NE-rich afferents from the LC ascending to the hippocampal formation has been reported to dramatically affect distinct aspects of cognitive function, in addition to reducing the proliferation of neural progenitors in the dentate gyrus. Transplantation of LC-derived neuroblasts significantly ameliorated working memory performance and reinstated a normal density of proliferating progenitors after lesion of original LC. Indicating that LC-derived NE inputs may act as positive regulators of hippocampus-dependent spatial working memory via the maintenance of normal progenitor proliferation in the dentate gyrus (Gulino, 2023).
Crucially, norepinephrine has been found to have opposite effects on two fundamental neurogenic niches of the adult brain with norepinephrine being a negative regulator of adult periventricular neurogenesis and a positive regulator of NPC proliferation within the hippocampus (Weselek, 2020).
Combined destruction of the cortical noradrenergic and cholinergic innervations reduces the physiological response to monocular deprivation although lesions of either system alone are ineffective. We also find that 6-OHDA can interfere directly with the action of acetylcholine on cortical neurons. This suggest that intracortical 6-OHDA disrupts plasticity by interfering with both cholinergic and noradrenergic transmission and raise the possibility that ACh and NE facilitate synaptic modifications in the striate cortex by a common molecular mechanism (Mark Bear, *1986*).
Noradrenaline can also robustly activate dopamine D1 receptors in the mouse hippocampus. Noradrenaline-activated D1 receptor signaling was highly sensitive to the neuronal activity and experience of mice. Chronic stress and voluntary exercise synergistically augmented noradrenaline–D1 receptor signaling. This augmented noradrenaline–D1 receptor signaling promoted the induction of hippocampal neuronal plasticity by an antidepressant drug acting on the noradrenergic system (Kobayashi, 2022)
Loss of LC neurons and noradrenergic innervation has been observed across neurodegenerative diseases including AD and Parkinson’s disease (PD) where it has been found to profoundly correlate with neuropathology and cognitive/behavioral deficits (McMillan, 2011). The relationship between LC-health, noradrenaline, and the development of neurogenerative disease is an ongoing topic within the field (Leanza, 2018; Gutierrez, 2022; Weinshenker, 2018).
Overall, the above sections highlight the potential role of cholinergic and noradrenergic inputs to the phenotypes observed following GENUS, as well as offers a potential circuit explanation for the relationship between these neuromodulators and gamma oscillation dynamics across the sensory, prefrontal, and hippocampal cortex. The specific mechanism highlighted by Murdoch, et al. is proposed to be a result of increased noradrenergic and cholinergic signaling to relevant cortices influencing astrocytes, inhibitory neuron transmission, and other cells in the neurovascular unit. Many current GENUS outcomes have focused on specific inhibitory interneuron subtypes (PV+, VIP+) and their key role in therapeutic benefit while also observing changes in glial cell populations and phenotypes. We would suggest that these changes in GABA transmission are mediated both through direct noradrenergic and cholinergic input to these neurons and indirect communication between neuromodulators altering astrocyte behavior and their impact on GABAergic neuronal firing.
Figure 4: Altered from Blanco-Duque, 2024
Vagus nerve and attention/novelty circuit
The vagus nerve could be viewed as a stimulus reporter from ‘other’ senses. Proprioception, satiety, peripheral inflammation, and other inputs all make up important contextual information that the midbrain nuclei and anterior hippocampus need to determine novelty and salience when deciding whether to make behavioral changes in response to an experience or input.
Gamma stimulation of vagus nerve branches may induce similar saliency/novelty activation resulting in cholinergic and noradrenergic output to relevant regions, including those outside the central nervous system. This technique likely has strong utility in terms of ‘resetting’ or promoting plasticity along hard-coded autonomic response pathways.
Outside of the gamma range, we would postulate that 10 Hz stimulation using tACS/TMS/vagus nerve stimulation would reflect cholinergic, alpha, and attention-related activity, but likely has less influence on the LC-NE system.
Other topics
Ketamine interacts with LC receptors, influences plasticity and inflammation, and has been shown to be able to complement and interact with external neuromodulation techniques (Debowska, 2023).
GLP1 receptors in the LC play a significant role in satiety and food-seeking behavior (Fortin, 2023).
Is it likely that an attention/novelty circuit is impacted in AD
Attention is reduced in AD (Malhotra, 2018). Ability to encode new information and ability to determine novelty of stimulus all seem to be impaired in AD (Bastin, 2019). LC is one of the earliest sites of phosphorylated tau tangle accumulation in AD (Harley, 2021). Both cholinergic and noradrenergic agonists have shown some impact on Alzheimer’s pathology and cognition (Chen, 2022; David, 2022). There is significant loss in LC volume and MRI contrast in both AD and PD (Betts et al., 2019a, Jacobs et al., 2021a, Madelung et al., 2022, Theofilas et al., 2017, Zarow et al., 2003). (Krohn, 2023)
The functional relationship between primary mechanisms of Alzheimer’s, amyloid accumulation, and cognitive decline and this circuit are discussed in Part 3. Part 1 discusses the circuit mechanism of HOW gamma oscillations and GENUS impact glial cells, network activity and plasticity, blood flow, and glymphatic flow through noradrenergic and cholinergic influence on the neurovascular unit within sensory cortices and hippocampus, but does not cover WHY these two things might be so heavily interrelated with the core function of Alzheimer’s and other neuromodulation-associated disorders.
Ways to improve stimulation protocols
With the proposed circuit above, we would suggest that gamma-frequency neuromodulation is mirroring salient and/or novel activation (based on frequency) of relevant inputs to the midbrain arousal system causing a noradrenergic/cholinergic response to the region to promote plasticity and response to the novel stimuli. This response likely decreases with age, neurodegeneration, and LC depletion.
Stimulation in the gamma frequency induces a similar ‘novel response’ in the region that recruits LC response, but delivery could be improved by interacting with the circuit not just in terms of frequency, but also in terms of function – intermittent input or burst delivery should result in a stronger response. Specifically, the midbrain nuclei and anterior hippocampus initiate early gamma power changes within 30–180 ms from stimulus onset in a network. Fixation patterns and the duration of the first fixation, discriminates between weakly familiar and weakly novel stimuli within 320 ms of stimulus onset (Kafkas, 2015). This point, between weakly familiar and weakly novel responses, is the point on the continuum where the intersection between novelty and familiarity occurs; where the behavioral responses, while accurate, are less confident (Kafkas, 2018). Crossing this critical threshold repeatedly in separate events may improve response.
Critically, it has been shown that LC influence on the brain is uniquely reliant on activation pattern (Vazey, 2018). Phasic, but not tonic activation of LC efferents are involved in salience and attention signals across the brain and that the two different firing patterns uniquely modulate the integration of sensory information in the cortex (Devilbiss, 2011). Burst stimulation protocols have shown increased therapeutic effect and reduced treatment duration in Parkinson’s and Major Depressive Disorder respectively (Horn, 2020; Chung, 2014).
Given this information, we would propose that burst presentation of 40 Hz multi-sensory stimulation for 300-500 ms and an inter-stimulation interval (~600-3000 ms) would produce greater activation of novelty/saliency circuit within the midbrain nuclei compared to constant 40 Hz stimulation which induces a single sustained experience of salience or novelty. Some benefits may be observed by modulating gamma-theta oscillation interactions as well. Significant increases in noradrenergic output using burst stimulation may allow for reduced stimulation duration required for therapeutic efficacy.
Additionally, delivery of stimulation to the prefrontal cortex and sensory cortex simultaneously would increase efficacy of this specific circuit arm. That is, multi-sensory 40 Hz stimulation, delivered simultaneously with transcranial stimulation or during learning behavior of a particular cognitive task, may increase LC-NE activation and changes in phenotypes of interest.
Finally, given this circuit function, we would suggest that rather than maximizing 40 Hz response within the sensory cortex, the interaction between sensory and prefrontal gamma oscillations may be more relevant to therapeutic benefits. Studies should focus on stimulation protocols that produce a maximum response from the LC (fMRI, heart rate variability, or pupillometry) and/or noradrenergic and cholinergic input to the circuit of interest to maximize impact on plasticity, inflammation, neurodegeneration, and cognition in the disease model of interest.
Part 2: The Role of the Locus Coeruleus, Neuromodulatory Tone, and Gamma Oscillations in Consciousness and Neuropsychiatric Disease
Abstract
The initial review discussed how interfacing with gamma oscillations and a circuit for acute recording and encoding of novel sensory stimuli may have a profound effect on Alzheimer’s disease (AD) neuropathophysiology, network dysfunction, and cognitive decline. While counterintuitive when only thinking about attention and encoding, Alzheimer’s research has historically indicated a prominent role for noradrenergic and cholinergic signaling in disease and symptom-modifying pharmaceutical interventions. Part 3 will return to a proposed mechanism of AD.
In this article, we propose a larger role for the midbrain nuclei and resulting gamma oscillations in managing consciousness, awareness, and encoding in the short- and long-term through modulation of neurogliavascular interactions and inhibitory transmission to influence attention, salience detection, and novelty encoding. The locus coeruleus (LC) and midbrain nuclei receive more than just sensory information. We have already discussed how the vagus provides other environmental and internal stimuli such as satiety, peripheral inflammation, proprioception, and cardiovascular state, but additional connections between the LC and amygdala play a pivotal role in fight, flight, and freeze response to threatening stimuli, along with hormonal and vagal output pathways. Activation of midbrain nuclei and downstream hypothalamic nuclei can induce significant shifts in body state through changes in autonomic and inflammatory tone in response to emotion or stress. The default mode network (DMN), regulating internal thought and memory, is largely maintained by the precuneus and cingulate cortex; both of which show strong connectivity with the LC. Additionally, the LC and norepinephrine (LC-NE) system also acts as the switch between the DMN and central executive and salience networks responsible for external attention. LC activity plays a key role in the micro and macrostructure of sleep function and behavior. The role of gatekeeping new information being encoded while also managing basic needs and responding to stress or potential harmful stimuli indicates a prominent role for the neuromodulator system not just in attention, but also in consciousness, sleep, managing allostatic load, and evolutionary drive.
Overall, we would propose that the hierarchy of midbrain nuclei and neuromodulators reflects the harmonic nature of information carried in network cortical oscillations. We discuss the possibility of attention, engagement, and encoding, observed through 10 Hz, 20 Hz, and 40 Hz activity, reflecting activation of different (sets of) midbrain nuclei.
Given this proposed hierarchy, we would expect to see alterations in signaling from the midbrain nuclei to play a prominent role in neuropsychiatric disorders. In fact, the significance of this circuit can be observed in altered gamma oscillations and neuromodulatory tone across a variety of such disorders.
Post-Traumatic Stress Disorder (PTSD) – as an override of the cingulate and amygdalar input to the LC in response to specific stimuli leading to downregulation of novelty seeking and memory encoding due to activation of fight-flight-freeze state through autonomic tone. Altered gamma oscillations in the precuneus and cingulate cortex and reduced hippocampal gamma have been observed.
Attention-deficit (Hyperactivity) Disorder (AD(H)D) – as an activation of salience, but an inability to execute prefrontal planning and therefore failing to achieve proper LC-NE activation for learning behavior. The result is reduced dopaminergic input (or dopaminergic/cholinergic ratio) to and gamma oscillations in prefrontal cortex. Common ADHD treatments are thought to work through influence on noradrenergic and dopaminergic release or reuptake.
Schizophrenia – as a loss of LC ability to appropriately shift between phasic and tonic activity; resulting in a disruption of coordinated network gamma oscillations. Previous studies associated positive symptoms of schizophrenia such as hallucinations and delusions with hyperactivity of the LC-NE system, while negative symptoms such as asociality were related to hypoactivity.
Autism – as a reduced ability to provide saliency to or encode human/social context of stimuli during learning experiences. Proposed as altered connectivity between temporal lobe regions responsible for social/human-human recognition and the LC during memory encoding due to either genetic or environmental factors.
Major Depression Disorder (MDD)– Long-term allostatic inadequacy results in chronic down regulation of noradrenaline input to the hippocampus, reduced activation of downstream attention networks, decreased gamma oscillations, and reduced ability to respond to and encode emotional and salient stimuli. Feedback circuit results in reduced novelty-seeking, motivation, and prefrontal response to salient stimuli.
Overall, a large amount of research has shown that midbrain nuclei and neuromodulatory tone, and therefore gamma oscillations, attention, and memory encoding, are heavily impacted in major neuropsychiatric disorders. This indicates a role for neuromodulators not just in responding to cortical input, but in organization of brain state, consciousness, and memory encoding. However, rather than passive responders to altered neuronal firing within the circuit, we propose midbrain nuclei dysfunction as the primary mechanism of the above disorders and resultant changes in oscillatory dynamics and that the LC-NE system may be a therapeutic target to induce plasticity, reset network circuits, and influence these disorders.
Model of allostatic load from McEwen, 2011
A Proposed Role for the Locus Coeruleus as the Primary Orchestrator of Consciousness and Allostatic Load Management Through Influence on Neuromodulatory Tone
As of Breton-Provencher, 2021, the two predominant theories of the function of the locus coeruleus (LC) and norepinephrine (NE) system in consciousness were the adaptive gain theory (Aston-Jones and Cohen, 2005) and the network reset theory (Bouret and Sara, 2005; Yu and Dayan, 2005; Dayan and Yu, 2006). The adaptive gain theory proposes that phasic activity in the LC during optimal behavioral performance facilitates task-specific decision processes (Usher et al., 1999; Aston-Jones and Cohen, 2005), while tonic activity prevails during periods of poor performance, where a general increase in LC-NE activity increases the gain of a network indiscriminately, making targeted circuits more responsive to any stimulus (Usher et al., 1999; Aston-Jones and Cohen, 2005). Thus, through adaptive gain, LC-NE activity optimizes the tradeoff between exploitation and exploration behaviors. The network reset theory suggests that contexts requiring a change in behavior transiently activate LC-NE neurons (Bouret and Sara, 2005; Yu and Dayan, 2005; Dayan and Yu, 2006). These activating contexts lead LC-NE neurons to induce widespread cortical arousal and causing a reset in network activity to enable an updating of priors (Yu and Dayan, 2005; Dayan and Yu, 2006). While portions of both theories may be accurate, neither account for (or intend to within the cited theories) the prominent role the LC plays in management of sleep state or the full characterization of its inputs.
Figure 1: Influence of tonic and phasic LC activity. From Atzori, 2016.
“The LC is tightly linked to sleep architecture: Decreases in LC activity (Foote et al., 1980) and noradrenergic levels (Kalen et al., 1989) are linked to the transition from wakefulness to sleep where the LC is virtually silent during REM sleep (Foote et al., 1980). Similarly, increases in LC activity precede unprovoked rousing from sleep (Aston-Jones and Bloom, 1981). Increasing noradrenergic levels pharmacologically shortens REM sleep and prolongs wakefulness across species (De Sarro et al., 1987, Spiegel and DeVos, 1980). Interestingly, LC activity during NREM sleep has also been linked to memory consolidation and sleep spindle activity (Kjaerby et al., 2022, Osorio-Forero et al., 2021). Several afferents to and efferents from sleep promoting regions enable this tight involvement in sleep (Lew et al., 2021, Samuels and Szabadi, 2008, Saper and Fuller, 2017). Despite its small size, the LC projects to and receives input from widespread brain regions (Liebe et al., 2022, Szabadi, 2013) and is thus involved in numerous functions related to cognition such as memory formation (Amaral and Foss, 1975, Gibbs et al., 2010, Hansen, 2017, Kety, 1972, Zornetzer and Gold, 1976), attention, sensory processing (Bouret and Sara, 2002, Lecas, 2004), novelty (Vankov et al., 1995, Yamasaki and Takeuchi, 2017) and emotional memory (Hämmerer et al., 2018). It is involved in autonomic functions such as blood pressure (Sved and Felsten, 1987), immune function (Lehnert et al., 1998, Rassnick et al., 1994) and the sleep-wake cycle (for a review see Osorio-Forero et al., 2022). Furthermore, it is involved in the fight or flight response by modulating heart rate, blood pressure, salivation and pupil dilation (Ross and Van Bockstaele, 2021, Samuels and Szabadi, 2008).” (Krohn, 2023)
The LC plays a crucial role in modulating the dopaminergic system. Notably, noradrenergic terminals in cortical regions and the hippocampus have been found to co-release dopamine (DA), indicating a functional overlap (Devoto et al., 2020, Devoto et al., 2008, Devoto et al., 2005, Kempadoo et al., 2016, Pozzi et al., 1994, Smith and Greene, 2012, Takeuchi et al., 2016). Moreover, the LC projects to the ventral tegmental area (VTA), a key hub of the dopaminergic pathway, and modulates dopamine release in the nucleus accumbens (nACC) and prefrontal cortex (PFC) (Sara, 2009). Additionally, the LC has direct projections to the PFC, which, in turn, sends inhibitory relay signals to the VTA. Reciprocal connections exist between the VTA, PFC, and LC, forming a complex network that allows for bidirectional modulation (Sara, 2009). Research suggests that parvalbumin-positive cells within the basal forebrain (BF) are required for transient alertness, but that coordinated LC-NE recruitment of BF cholinergic neurons were required for sustained attention and interaction (Maness, 2022).
We would propose the following:
The midbrain nuclei and basal forebrain act as a set of neuronal frequency modulators, interacting with and influencing interneuron-neuron firing through modulation of the neurogliavascular unit, operating on different frequency harmonics. That is, initial deviation from local neuronal oscillations would be represented by spontaneous asynchronous firing producing gamma oscillations in response to unfamiliar or unexpected stimuli. In terms of external stimuli, initial attention and response occurs through cholinergic release to relevant sensory neurons producing 10 Hz oscillations and subconscious system attention, followed by cholinergic input to prefrontal cortex for conscious attention. The second step would require dopaminergic input to the prefrontal and motor cortex to initiate executive function and motor planning respectively; often associated with 20 Hz oscillations. Temporary encoding within the entorhinal/hippocampal cortex may depend on serotonergic input, but, most importantly, the long-term extraction of relevant memory engrams and learning are represented by LC activation inducing 40 Hz or gamma oscillations through direct (NE) and indirect (ACh, dopamine) mechanisms. LC-NE activation would therefore be at the top of the hierarchy and required for sustained attention, interaction, and learning. The synchrony, network organization, and harmonicity of the system are represented by relevant midbrain nuclei projections, interactions with each other, transport, and reuptake. The hierarchical nature of this evolutionary process (attention, planning, action, encoding, and learning as sequential filters) are reflected in the function of large system oscillations and production of harmonics. Additionally, the proposed maximum of learning-relevant cortical neuronal oscillations (40 Hz/gamma) may be a natural reflection of previously observed interaction time between interneuron-neuron units (Milicevic, 2024). Finally, the maintenance and daily reset of this system is performed by noradrenergic LC input, glymphatic flow, amyloid clearance, and transfer of memory engram from temporary (hippocampal) to long-term (cingulate cortex) neurons during sleep. Dreaming would be a conscious representation of acquired information combined with previous memory engrams in novel patterns or with new network properties.
Role of LC-NE and Gamma Oscillations in Neuropsychiatric Disorders
LC plays a role not only in arousal, attention, and encoding response to novel environmental experience, but also manages the cognitive, behavioral, and body response to new internal experiences, emotional stimuli, and unfulfilled basic needs. Additionally, the LC plays a prominent role in managing wakefulness and the process of sleep, influence on hormonal signaling, and pain, autonomic and inflammatory tone. Overall, we would suggest that the LC-NE in vertebrates plays a primary role in management of consciousness, evolutionary drive, and development and learning. We would hypothesize that changes in LC-NE signaling and functioning would thus play a primary role in major neuropsychiatric disorders and that this impact could be observed through altered gamma oscillations, changes in neuromodulatory tone, and impairments in attention, encoding, and recall.
Post-Traumatic Stress Disorder (PTSD)
In the setting of acute or chronic traumatic exposure, particularly during development, some individuals may develop PTSD or complex PTSD, respectively. Symptoms of re-experience, avoidance of associated stimuli, negative alterations in cognition and mood, and alterations in arousal and response characterize the disorder (DCM-5, 2013). Additionally, resultant depression and anxiety disorders are significantly associated with altered oxidative stress and inflammation profiles compared to those without PTSD; highlighting the significant interaction between conscious experience, neuropsychiatric symptoms, inflammation, and oxidative stress (Kim, 2020).
Evidence suggests that many of the primary symptoms of PTSD, including hyperarousal and sleep dysregulation, can be viewed as a disorder of the underlying fear response of an organism. The neural circuitry underlying fear and threat-related behavior and learning in mammals has largely focused on amygdala–hippocampus–medial prefrontal cortex circuitry (Ressler, 2022). Additionally, the anterior cingulate plays a significant role in long-term storage of emotional content and its recall (Rolls, 2019). Another extremely significant component in this fear response and resultant alterations in autonomic tone is the LC-NE system, which plays a role in long-term impairment in neuroplasticity (Giustino, 2019; Borodovitsyna, 2018).
Several studies have highlighted the important role of stress hormones and norepinephrine during emotional memory encoding (Hruska, 2014). Functional changes in LC activity and output have been observed in many studies of PTSD and to significantly correlate with behavioral phenotypes (McCall, 2024; Naegali, 2018).
Figure 2: Inputs and outputs of amygdala nuclei. (From Zhang, 2021).
In terms of changes in gamma oscillations across the cortex, PTSD patients’ response to trauma scripts showed increases in high-gamma band power in visual areas (Reuveni, 2022). Increased frontal gamma power has also been observed in PTSD groups compared to anxiety disorder (Moon, 2018).
Figure 3: Caliskan, 2018 provides a full review of PTSD and alterations in hippocampal, prefrontal, and amygdalar gamma oscillations during fear retrieval and extinction.
In our model, with the midbrain nuclei as the central component, we would hypothesize that PTSD represents inappropriate overactivation of initial fear response to similar/remembered stimuli and an inability to achieve the level of plasticity required to alter these associations. That is, the propensity of the LC to respond to and promote anterior cingulate/amygdalar activation and communication of fear/alert with the prefrontal cortex overrides the ability of the LC to promote plasticity and altered association encoding within the hippocampus (fear > novelty). As a result, we observe an increase in gamma oscillations in the prefrontal cortex, activation of peripheral arousal system, amygdalar-LC activation in response to fear-associated stimuli, and reduced hippocampal noradrenergic tone indicating a decrease in encoding. Additionally, areas of long-term emotional memory storage, such as anterior cingulate cortex and basolateral amygdala, are likely to show altered gamma-activity as fear memories are recalled and activated even without environmental input.
In terms of neuromodulation, we have seen large improvements in PTSD biological and cognitive phenotypes using transcranial alternating current stimulation (tACS) and transcranial magnetic stimulation (TMS) targeting the posterior cingulate cortex, hippocampus, and amygdala in the gamma frequency range (Koek. 2019). We would suggest that posterior cingulate cortex activation allows for episodic memory engram reactivation without the emotional component. Ketamine therapy, previously suggested to operate mainly on LC receptors, has also shown a significant role in modulating plasticity of amygdalar connections (Scheidegger, 2016). We would suggest that accompanying neuromodulation techniques with traumatic memory recall and gamma frequency sensory stimulation may promote plasticity and circuit reset through promotion of LC-NE system activation.
Attention-Deficit (Hyperactivity) Disorder (AD(H)D)
Attention-deficit (hyperactivity) disorder (ADHD) is a developmental disorder that negatively affects several life domains. The main behavioral symptoms of the disorder, as described in the Diagnostic and Statistical Manual of Mental Disorders (DSM), are manifestations of inattention, hyperactivity, and impulsivity, which start in early childhood and persist (especially the inattentive symptoms) into adulthood in about two-thirds of diagnosed cases (Mohamed, 2021). Impairments in complex cognitive functions such as executive functioning and memory have also been reported in ADHD (Banschewski, 2018; Fuermaier, 2013) .
ADHD in its current framework experiences both high variability in cognitive domains impacted and in clear diagnosis and patient populations (Pettersson, 2018). However, when viewed as primarily an issue in activation of midbrain nuclei and neuromodulatory tone, some of this variability may be explained. We would propose that the natural trigger for salience ((acetyl)choline to prefrontal cortex (PFC)) is present or hyperactive, but that the lack of dopaminergic neuron activation or transport to the PFC results in a failure of executive function activation, stimuli interaction and discovery, and therefore fails to result in LC-NE input to the hippocampus to encode relevancy of salient stimuli. This could be due to genetic or developmental factors in certain subgroups of ADHD, but similar cognitive issues could arise in any issue impacting the ratio of cholinergic to dopaminergic inputs to the prefrontal cortex or upstream issues in LC-NE activation.
A frequency-based cluster analysis yielded higher high-gamma power for ADHD over posterior sensors and lower high-gamma power for ADHD over frontal-central sensors (Dor-Ziderman, 2021). These results were shown to be stable over three measurements, unaffected by methylphenidate treatment, and linked to cognitive accuracy and state anxiety. Furthermore, the differential high-gamma activity evidenced substantial ADHD diagnostic efficacy, comparable to the cognitive and emotional factors. A dopaminergic theory of ADHD has long been hypothesized (Levi, 1991). Mouse models of ADHD have indicated a reduction in nicotine-stimulated norepinephrine release in the hippocampus (Sterley, 2014).
Through neuromodulatory influences over fronto-striato-cerebellar circuits, dopamine and noradrenaline play important roles in high-level executive functions often reported to be impaired in ADHD. Medications used in the treatment of ADHD (including methylphenidate, dextroamphetamine and atomoxetine) act to increase brain catecholamine levels (Campo, 2011). A trial using atomoxetine, a norepinephrine reuptake inhibitor approved for ADHD, in individuals with mild cognitive impairment and biological markers for AD showed reduced phosphorylated tau, reduced inflammation, and increased brain-derived neurotrophic factor (Levey, 2022).
From this framework, we would propose that genetic predisposition or environmental stimuli altered neuromodulatory tone during development that persists into adulthood in certain subgroups of ADHD. This change in midbrain nuclei interactions and dopaminergic input to the prefrontal cortex can be observed by changes in hippocampal norepinephrine release and PFC dopaminergic input, altered hippocampal and prefrontal gamma oscillations and issues in attention and encoding. Additionally, we could also hypothesize that increased exposure to rapid salient stimuli without interaction or encoding, especially during development, could potentially alter neuromodulatory tone or the cholinergic/dopaminergic ratio in a persistent manner. Gamma stimulation of dopaminergic input sites in prefrontal cortex or direct LC manipulation may have cognitive benefits.
Schizophrenia
“Individuals with schizophrenia show diverse symptoms and deficits in multiple domains of perception and cognition. These include cognitive disturbances, such as attention deficits and delusional ideation; disturbances of self-awareness and agency; alterations in emotional expression; disturbed motor behavior; and sensations in the absence of external stimulation, or hallucinations. One model that has garnered much attention in the past decade is the “disconnection hypothesis,” which proposes that schizophrenia disrupts signaling among brain regions, systems or cellular circuits.” (Bowie, 2006).
Previous studies have associated positive symptoms of schizophrenia (hallucinations, delusions, changes in speech) with hyperactivity of the NE system, while negative symptoms (anhedonia, social withdrawal, and reduced speech output) were related to hypoactivity of the NE system (Yamamoto, 2004). Indicating a loss of differentiation in LC activation between phasic and tonic activity (Pellegrino, 2023).
“Deficits in ASSR have been reported in patients with schizophrenia. The spectral power on frontal EEG channels evoked by 40 Hz click trains was lower, whereas the phase delay, defined as the time gap between a click and the EEG peak response while listening, was higher in patients with schizophrenia than in control subjects.
Patients with schizophrenia showed a deficit in gamma activity in the frontal area during mental arithmetic tasks. Gamma activity during the retrieval period of working memory was reduced in patients with schizophrenia compared with healthy subjects. Healthy subjects had increased gamma activity in the prefrontal cortex in response to increased demands of executive control in a working memory task, a finding not observed in patients with schizophrenia. Similarly, healthy subjects, but not patients with schizophrenia, showed increased gamma activity with increased working memory load. Patients with first onset schizophrenia were recently found to show reduced gamma power in the frontal area during the delay period of a preparatory cognitive task compared with control subjects, regardless of medication status.
Interestingly, symptoms of schizophrenia have been reported to correlate with increased synchronization of gamma oscillation, although mean gamma synchronicity was lower in patients with schizophrenia than in control subjects.” (Shin, 2011)
Findings suggest that changes in LC-NE activity – measured by task-evoked pupil dilation – when task demand increases are associated with schizophrenia symptoms (reduction in response). (Pellegrino, 2023)
“We propose that schizophrenia phenomenology, in particular cognitive symptoms, may be explained by an abnormal interaction between genetic susceptibility and stress-initiated LC-NE dysfunction. This in turn, leads to imbalance between LC activity modes, dysfunctional regulation of brain network integration and neural gain, and deficits in cognitive functions” (Maki-Marttunen, 2020)
Figure 4: From Maki-Marttunen, 2020
Overall, similar to Maki-Marttunen, et al., we would suggest that schizophrenia may be a genetic predisposition for LC dysfunction, exacerbated by trauma, stress, or environmental input, which impairs the circuit ability to shift appropriately between tonic and phasic activity; therefore, reducing the ability of the cortex to determine what are internal and external stimuli and/or differentiate between sensation and memory recall. The potential impact of gamma frequency stimulation on cognitive deficits is unclear, but appropriate deep-brain stimulation of the LC may provide cognitive benefits.
Autism (ASD)
“Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by social communication impairments and restricted, repetitive behaviors. Pharmacological intervention can be an important component of treatment for ASD symptoms. However, most currently available agents target psychiatric symptoms associated with ASD such as hyperactivity and repetitive behaviors.
Agents that decrease activity of the noradrenergic system have been used for anxiolytic and behavioral purposes in ASD. Early reports indicated benefits in language and social behaviors in a consecutive case series of individuals with ASD treated with β-adrenergic antagonists. Early reports also indicated benefits with other agents that act on the noradrenergic system. α2 adrenergic agonists, drugs which act to presynaptically inhibit norepinephrine release, improve hyperactivity, impulsivity, hyperarousal and social relationships in double-blinded placebo-controlled crossover trials in individuals with ASD. A number of researchers have demonstrated findings suggestive of increased noradrenergic activity in ASD, including increased plasma epinephrine and norepineprine, and altered urinary excretion of various catecholaminergic metabolites. However, there may be a number of alternative explanations for these findings. Subsequent studies demonstrating no abnormalities in basal noradrenergic functioning have led to the suggestion that increased reactivity to clinical procedures such as blood drawing and urine collection in ASD may have led to the earlier atypical findings.” (Beversdorf, 2020).
Resting state gamma oscillations have found conflicting results, but meta-analysis indicates an increase in gamma power in individuals with ASD (Neo, 2023).
Gabor patches are stimuli that are thought to have characteristics that match the receptive field properties of neurons in the primary visual cortex (Fredericksen, 1997), with increases in gamma-band power observed in response to increasing grating size of the Gabor patches in non-human primate research (Geiselmann, 2008). Two EEG studies have reported reduced gamma responses in posterior cortical regions in autism in response to increases in the contextual modulation of the stimuli (Milne, 2009; Snijders, 2013).
Others have found that a deficit in ascending cerebellar Purkinje fibers, which provide significant regulatory input to the LC, is often observed in individuals with autism (Hampson, 2015). It has also been observed that febrile episodes exhibit a dramatic impact on autism behavior and communication, which may be a result of functional normalization of LC-NE systems (Mehler, 2009).
The interaction between LC-NE activation, gamma oscillations, and ASD is not as clear as in other disorders discussed. While the underlying mechanism seems to be related and similar in terms of LC-NE and gamma oscillation involvement, the exact impairment remains unclear. We would hypothesize that ASD is not an issue with the LC itself, but may relate to an inability or dysfunction in extracting salient cues from complex human-related/social stimuli and giving them proper valence during memory encoding due to alterations in function or connectivity with cortical regions relevant to these interactions. We might expect to see reduced gamma oscillations in hippocampus in response to social stimuli; indicating a lack of associated salience or importance of these stimuli to LC encoding. An inability to produce proper LC response to social stimuli means a reduction in overall gamma oscillation power in sensory/relevant cortices compared to control as the individuals have reduced motivation to interact and engage and therefore a reduction in behavioral adaptation in response to social/human-human stimuli.
Major Depressive Disorder (MDD)
Major depressive disorder (MDD) is characterized by physical changes such as tiredness, weight loss, and appetite loss (Cui, 2024). Anhedonia or emotional blunting is a classic feature of MDD and is often accompanied by lack of drive, sleep issues, and cognitive challenges. The clinical symptoms of MDD include a depressed mood, loss of interest, changes in weight or appetite, and increased likelihood of committing suicide (Rice, 2019).
Additionally, MDD has been found to have high comorbidity with cardiovascular disease and reduced heart rate variability associated with functional nodes of the frontal-vagal network (Iseger, 2020).
Overall, across a variety of major neuropsychiatric disorders, a multitude of streams of research have converged around alteration in noradrenergic signaling (and downstream neuromodulators – cholinergic basal forebrain, dopaminergic neurons in ventral tegmental area, serotonergic neurons in raphe nucleus) having a significant impact on conscious experience and ability to interpret, encode, and respond appropriately to external stimuli which can also be observed with alterations in gamma oscillations during stimuli or task presentation.
Figure 5: From Avery, 2017
Many of these disorders represent acute response interpretation and the altered neuromodulatory signaling altering biological and conscious response. We would hypothesize that depression is a short-term downregulation of serotonergic tone and that MDD involves a long-term reduction in general noradrenergic input due to inability to meet allostatic needs. Mechanistically, MDD would be a reduced likelihood for LC-NE activation for learning behavior. This downregulation of the LC-HPC-PFC pathway could occur for a variety of reasons including chemical, environmental, genetic, or behavioral, but that the result is decreased ability to achieve salient, emotional or executive response to stimuli, reduced novelty seeking, and decreased likelihood to encode new information. Consistent with our hypotheses is previous research indicating altered gamma oscillations, across the prefrontal, hippocampus, and cingulate cortex in major depression, which has prompted its proposed use as a biomarker (Fitzgerald, 2018).
Acute instances of depression may be treated with serotine reuptake inhibitors, but major depressive disorder (MDD) is more likely to require LC-specific or noradrenergic transport intervention.
Conclusion
Overall, the focus of this review is to illustrate the midbrain nuclei and neuromodulatory tone, particularly the LC-NE, not as passive responders to decisions made in the cortex, but as primary conductors of consciousness, awareness, attention, and memory encoding through interactions with the neurogliavascular unit and inhibitory transmission as observed by network gamma activity. The LC is not a long-range signaling pathway used by the cortex to enact something, but the seat of arousal, the maintenance of sense-of-self throughout sleep-wake transitions, and provides immense influence on motivation, evolutionary drive, and basic function of memory encoding for relevant stimuli both directly through NE input or downstream activation of other relevant nuclei and circuits. For this reason, gamma oscillation modulation techniques; invasive or otherwise, when considering the neuromodulatory dysfunction of the disorder and its relevant cortical regions, may be useful for symptom alleviation and disorder modification.
Part 3: A Potential Role for the Locus Coeruleus in Managing Brain and Body Aging and Its Failure in Alzheimer’s Disease
Abstract
So far, we have reviewed a large set of literature highlighting the functional role of the locus coeruleus and norepinephrine (LC-NE) system in attention, salience, and memory encoding through interaction with the neurogliavascular unit, inhibitory transmission, and gamma oscillations, as well as a possible role across several major neuropsychiatric disorders due to its function in managing consciousness and arousal. Part 3 will attempt to propose an evolutionary theory on the functional hierarchy of the LC-NE system and its downstream signaling pathways, as well as its potential impact on biological aging and Alzheimer’s disease (AD).
The evolutionary goal of any organism and its nervous system is to find new and improved ways to sense and interact with its environment for pro-survival information or resources without expending extraneous energy. Higher cognition correlates with increased ability to determine what information is relevant for a given stimuli and ability to store and apply that information to novel or unrelated situations. While the ways in which we are able to acquire information and the ways our neocortex can store and recall memory have grown increasingly complex, the ultimate evolutionary decision comes down to “what is worth attention, what is worth spending energy on interaction, and what information is most important to encode?”. We have outlined how the LC-NE system is primarily responsible for making and enacting these decisions through its influence on cortical networks, plasticity, and autonomic tone. It is important that these decision-making processes come down to (relatively) simple neuronal function because it is a calculation that is carried out in every living vertebrate. In the absence of higher cognition, the ability to alter this process to new information is not especially prominent or useful. Higher cognition animals are more likely to alter the basic decision-making process involved in response to new information.
The next consideration we would make is that plasticity, encoding, consciousness, and the decision-making process made for the above questions all change drastically throughout, and influence, development and aging in vertebrates. Additionally, the environmental fulfillment we have, the amount of novel engagement and socialization, and exposure to bacterial, viral, physical, and emotional stress all play a significant role in how we develop and age. The exact mechanism of these influences has not always been clear, but it is thought that these various unrelated stimuli may have an impact on organism-wide plasticity, myelination, inflammation, resting autonomic tone, cortical oscillations, development, and cognition.
We would propose that the aging process in vertebrates is an orchestrated process influenced by LC-NE dynamics. That is, there is generally a set ‘lifespan’ plan, that for most organisms involves a period of high neuronal and cellular plasticity during development (particularly long for humans), a shift into adulthood and reproductive stage, followed by steady downregulation of higher cognitive function until expiration. In a few rare cases such as humans, there is a post-reproductive or post-menopausal stage. This process is mostly stored in genetic code; however, to allow for deviations, fluctuations, and environmental input to this process, it must be systemically influenced by a circuit that is able to modulate inflammatory tone, plasticity, and behavior to evolutionarily relevant stimuli and perceived allostatic load in both the short and long-term. The role of this circuit, level of activity, and responsiveness of this system to new information would correlate with increased LC connectivity, higher order cognition, and increased lifespan ‘complexity’. We see this in the difference in LC-NE structure in mice versus rats (Gasparini, 2020).
Finally, given the importance of this system, we would propose that dementia is primarily a result of NE insufficiency or LC-NE system dysfunction. AD would then be described as depletion or failure of the LC itself, supported with research indicating it as the initial site of tau accumulation. Many factors that we consider impactful in terms of organism functional age are found to be correlated with incidence of AD – socialization, physical, emotional, and inflammatory stress, novelty, cardiovascular health all influence LC-NE health because the goal of the system is to monitor these allostatic signals over time. Negative evolutionary/organism stressors are quantified, encoded, and provided as input to the LC to attempt to adjust accordingly. The more resources spent without resolution and the less exercise it receives in terms of positive mood, social interactions, and novelty, the more likely it is to fail somewhere in the circuit during an extended aging process. The rest of this article will attempt to describe the process of aging and development of dementia in the context of LC-NE function.
From Evans, 2022
What is Aging?
“At the biological level, aging results from the impact of the accumulation of a wide variety of molecular and cellular damage over time. This leads to a gradual decrease in physical and mental capacity, a growing risk of disease and ultimately death. These changes are neither linear nor consistent, and they are only loosely associated with a person’s age in years. The diversity seen in older age is not random. Beyond biological changes, aging is often associated with other life transitions such as retirement, relocation to more appropriate housing, and the death of friends and partners” – World Health Organization
“Aging is a gradual and irreversible pathophysiological process. It presents with declines in tissue and cell functions and significant increases in the risks of various aging-related diseases, including neurodegenerative diseases, cardiovascular diseases, metabolic diseases, musculoskeletal diseases, and immune system diseases. Although the development of modern medicine has promoted human health and greatly extended life expectancy, with the aging of society, a variety of chronic diseases have gradually become the most important causes of disability and death in elderly individuals. Current research on aging focuses on elucidating how various endogenous and exogenous stresses (such as genomic instability, telomere dysfunction, epigenetic alterations, loss of proteostasis, compromise of autophagy, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing) participate in the regulation of aging. Furthermore, thorough research on the pathogenesis of aging to identify interventions that promote health and longevity (such as caloric restriction, microbiota transplantation, and nutritional intervention) and clinical treatment methods for aging-related diseases (depletion of senescent cells, stem cell therapy, antioxidative and anti-inflammatory treatments, and hormone replacement therapy) could decrease the incidence and development of aging-related diseases and in turn promote healthy aging and longevity.” (Guo,2022).
With regards to the central nervous system:
“Pathophysiologically, brain aging is associated with neuron cell shrinking, dendritic degeneration, demyelination, small vessel disease, metabolic slowing, microglial activation, and the formation of white matter lesions.” (Blinkouskaya, 2021)
“Various aspects of physiological deterioration are controlled by the hypothalamus, a critical brain region that connects the neuroendocrine system to physiological functions. In addition, functional alterations in a set of neurons (including SST, VIP, GnRH) contribute to age-related physiological decline in energy metabolism, hormone regulation, circadian rhythm, and reproduction. The underlying cellular mechanism for the hypothalamus-mediated aging progression comprises dysregulation of nutrient sensing, altered intercellular communication, stem cell exhaustion, loss of proteostasis, and epigenetic alterations.” (Kim, 2018)
Complications with Current Structures of Aging
Treats complex organisms as an assortment of independent local/semi-local cellular interactions; all governing that specific cell populations’ functional age. However, complex interactions between basic biological systems and their cellular environment are known to play a significant role. The cardiopulmonary system determines delivery of oxygen and nutrients. The digestive system determines nutrient content ingested, absorbed, and delivered. These systems are determined by neuronal control of behavior and mood in vertebrates with higher cognition. Also fails to account for the drastic changes we see across different systems in response to significant life changes, stress, and trauma that are reflected in biological age and organism function.
Additionally, given the complexity of tissue and environment, the diversity and heterogeneity of outcomes would be much wider without unifying or normalizing systemic forces. Lack of coordination would lead to different experiences and ‘functional age’ across the body that would be easily observed just on a left/right symmetry level. The organism, as a whole, requires a central regulator that normalizes the impact of environmental stimuli and experiences on different portions of the body into a central response which allows for influence on the progression or trajectory of biological age (outside of disease contexts).
We see this coordination over time as multiple systems transition from high turnover rates, pluripotency, and plasticity to more maintenance states as the development period closes. However, a large amount of research has indicated that the when and how the closing of this development period occurs is heavily influenced by experience and salient stimuli (Sisk, 2004).
If instead, we look at age from an evolutionary perspective, the goal of an organism is to develop, procreate, promote offspring survival, and eventually pass on to prevent resource restrictions. Every evolutionary drive is aimed at completing this common pathway. We would propose that as organisms developed complex nervous systems to manage increasingly complex tissue interactions and stimuli that a primary role of this central controller would be to manage or at least influence the increasingly advanced mechanisms that govern transition from development and high neuronal plasticity to procreation, and then death.
Justification for a neural basis of age
Aging is complex and involves multi-system organization and coordination. The idea that the complicated process that is lifespan is governed primarily by local biochemical reactions and genetic expression without guiding input from the central nervous system does not seem to be consistent with reported data. The interconnected system that influences cardiac, metabolic, inflammatory, and pluripotency tone of a local tissue environment is coordinated by neural circuits and impact cellular function, metabolism, stress, and genetic and epigenetic expression. While local stress and metabolism certainly play a role in the functional outcome of age, the overall coordination of guiding an organism through development, procreation, and death is likely to be heavily influenced by neuronal systems and planning.
The Potential Role of LC-NE in Influencing/Managing Biological Age
Important factors we have already outlined as relevant to aging include inflammatory tone, pluripotency and plasticity, metabolism, sleep structure, and cognitive ability. Importantly, the LC-NE system would need to be able to influence these factors in the CNS and peripherally. In previous sections, we have discussed how the LC-NE system is able to modulate system behavior both acutely and by setting chronic tone.
Although the main rostro-caudal direction of LC is consistent across age groups, its spatial features varied with increasing age, emotional memory, and emotion regulation (Vereb, 2023). Furthermore, participants with higher-than-normal Hospital Anxiety and Depression Scale (HADS) ratings exhibited alterations in LC gradient manifested as greater asymmetry between bilateral LC.
In addition to its influence on closing the development period (Sisk, 2004), in rats, the LC develops unique sex differences between males and females in puberty (Pinos, 2001). The greater dendritic extension and complexity seen in females predicts a higher probability of communication with diverse afferents that terminate in the peri-LC. This may be a structural basis for heightened arousal in females, an effect which may, in part, account for the sex bias in incidence of stress-related psychiatric disorders (Bangassar, 2011). We would also hypothesize that the sexual dimorphism in structural and functional connectivity of the LC-NE may contribute to the differences in AD incidence rate between sexes (Pike, 2017).
In terms of diet and metabolic fulfillment, NE transporter availability has been found to be reduced in the thalamus and insular cortex in obese individuals; indicating a prominent role of the LC-NE system in maladaptive eating disorders and long-term impact of diet (Li, 2014; Bresch, 2017). Mutations in noradrenaline transporter gene SLC6A2 have been correlated with anxious arousal, PTSD, and metabolic syndrome (Lemche, 2016). Generally, these results indicate a role for the LC-NE in cumulative, generative measurements of allostatic fulfillment in diet.
Due to its exposed location next to the 4th ventricle, the LC is more vulnerable to inflammatory molecules and toxins (for a review see Evans et al., 2022; Matchett et al., 2021). Moreover, LC axons are partially myelinated and less cost-efficient, resulting in higher energy consumption, which in turn leads to higher levels of reactive oxygen species (ROS) (Lushchak et al., 2021).
Reduced leukocyte telomere length, a marker of immunological aging, has repeatedly been shown to be associated with stress response and central and peripheral (nor)epinephrine (Eitan, 2015). Chronic psychosocial stress through neuroendocrine mediators, specifically LC-NE activation, may drive aging through influence on cellular metabolic activity, DNA damage, telomere length, cellular senescence, and inflammatory response patterns (Polsky, 2022).
During normal aging, many of the cognitive functions that exhibit decline are supported by the LC-NE, including verbal intelligence (Clewett et al., 2016), response inhibition (Liu et al., 2020, Tomassini et al., 2022), memory (Calarco et al., 2022, Dahl et al., 2019, Langley et al., 2022, Liu et al., 2020), emotional memory (Hämmerer et al., 2018, Sterpenich et al., 2006), attention and processing speed (Calarco et al., 2022; Harada et al., 2013; Krohn, 2023).
One potential hypothesis is that the LC is simply the cognitive influence of the brain on aging, however, this would ignore the vast amount of information delivered to the LC of vertebrates in terms of metabolism, peripheral inflammation and infection, oxidative stress, and a dozen other metrics reported by the vagus nerve.
Instead, we propose that the LC-NE system plays a primary role in the management of biological age and the closing of the developmental period, the impact of cognitive experience on age and autonomic function, and the impact of environmental and body experience on cognition through its influence on plasticity, pluripotency, inflammation, neurogliavascular unit function, and consciousness in response to input it receives from higher cortical circuitry and the vagus nerve. Evolutionarily, this would place the LC-NE system at the center of learning, lifetime stress or allostatic load, and biological age in vertebrates.
The Role of LC-NE in Dementia and Alzheimer’s
The locus coeruleus is the initial site of Alzheimer’s disease neuropathology, with hyperphosphorylated tau appearing in early adulthood followed by neurodegeneration in dementia (Harley, 2021). Locus coeruleus dysfunction contributes to Alzheimer’s pathobiology in experimental models, which can be rescued by increasing norepinephrine transmission. Trials using a norepinephrine transporter inhibitor in subjects with mild cognitive impairment showed significant impact on cognition (Levey, 2022).
An exhaustive review of Alzheimer’s theory would be expansive. Instead, given our hypothesis of the LC-NE system as a central manager of consciousness, evolutionary drive, and biological age, we would then attempt to describe dementia primarily as a result of LC-NE insufficiency and AD as exhaustion of the LC nuclei itself.
In this model, we would propose the following:
Amyloid is a result of either healthy neuronal function or in response to the incidence of gamma oscillations or successful network activation. The role of this protein might be just as waste or, in certain forms, to remain in the local neurons to mark them for activation during sleep and glymphatic clearance. During sleep, LC-NE activation would then participate in reactivation of memory engrams, transfer of information to long-term memory storage, and glymphatic clearance of amyloid in relevant regions to reset encoding neurons and promote input pathways involved using previously discussed mechanisms.
Various issues can arise along this pathway. Lack of sleep or network function during sleep is shown to have a significant impact on memory encoding and consciousness. LC-NE activation and function decreases with age as neurons within the LC are depleted. Cardiovascular or glymphatic dysfunction could impair the ability of the system to respond to LC-NE activation or successfully clear amyloid. Infection or inflammation within brain tissue would also stress this system. Finally, alterations in neuromodulatory tone or network and circuit organization could result in an inability to properly activate the LC-NE for a particular region or stimulus.
Amyloid accumulation without glymphatic clearance would cause changes in both local neurogliavascular signaling and LC response to this condition. The propensity of cleavage into different amyloid isomers may be a response to impaired glymphatic amyloid clearance. Early compensation mechanisms may include packaging amyloid into plaques by local glia to mitigate impact on circuit function (impaired by TREM2 mutation), as well as hyperactivation of LC-NE systems to correct the error (impacted by APOE). The initial amyloid accumulation period without cognitive dysfunction likely reflects this compensation and compartmentalization process.
Alzheimer’s disease, which is first observed as neuronal death and tau tangle accumulation within the locus coeruleus, could be described as exhaustion of the LC-NE nuclei. Tau accumulation would then likely progress along primary noradrenergic inputs initially, including downstream regions such as the basal forebrain cholinergic neurons. The concept of cognitive reserve, individuals with high amyloid burden but few cognitive deficits, is likely to correlate with improved ability of glia to tightly package amyloid and of the LC to maintain its cognitive function or compensate in the face of impaired clearance. Individuals that have either appropriately exercised this circuit through socialization, novelty-seeking, and emotional fulfillment or that have not had its function depleted by stress, trauma, infection, cardiovascular dysfunction, or diet across lifespan would maintain function longer in the setting of plaque accumulation.
While in certain situations, we may see amyloid plaque accumulation on its own disrupt acute/short-term cognitive function, early LC-NE insufficiency or development of AD are likely to be observed in cognitive functions involving long-term encoding of memory (due to disruption of memory engram formation and subsequent reactivation during sleep), learning, and recall. Later changes in attention, executive function, and encoding would occur as a result of decreased LC-NE input to or depletion of the basal forebrain, dopaminergic nuclei, and serotonergic nuclei, respectively.
Other dementias occur when LC input to an ‘arm’ of its influence or LC ability to interact with a given region is impaired. Parkinson’s would be failure of dopaminergic nuclei (specifically subthalamic nucleus (STN)) due to exhaustion or depletion that inhibits the ability to switch away from default mode network and initiate external center of awareness or movement. Lewy-Body dementia (LBD) and Parkinson’s disease with dementia (PDD) may be associated with dopaminergic depletion in other regions (ventral tegmental area) or could be related to exhaustion of the LC-NE as it attempts to compensate or function without the dopaminergic arm of the circuit. Some early indications have promoted a role for serotonin depletion in Frontotemporal Dementia (Herrmann, 2012). Vascular dementia would occur when noradrenaline input can no longer enact a functional response given the altered cardiovascular system (no blood flow, reduced hormone/neuromodulator input, no functional glymphatic pumping, amyloid plaque interference). Once a significant amount of amyloid load occurs without rescue OR if LC fails early because it is not as functional OR if LC can no longer provide input to a region, we see initiation of neuronal death pathway, tau phosphorylation, neurodegeneration, and neurofibrillary tangles as a biological pathway response – essentially, a shutting down of a neuronal circuit to mitigate energy expenditure or as no longer having utility for survival. Likely to occur across all late-stage dementias.
The factors that reduce risk of AD are the things that exercise or flex this allostatic load /novelty circuit. Exercise, language learning, socialization, novel experience, functional memory recall and reevaluation of old memories with additional context/ideas. Reduced stress, reduced oxidative damage, low wear and tear from viral/bacterial infection, diet, cardiovascular stress.
We propose that the primary function of the LC is not just in managing consciousness, awareness, and attention acutely, and not just its role in neuropsychiatric disorders, but operates as the central location for learning, evolutionary drive/fulfillment, and biological age through its influence on inflammation, plasticity and pluripotency, neuroendocrine system, cardiovascular function, and cognition. Additionally, we would then propose that dementia may be able to be primarily viewed as LC insufficiency and AD as depletion of the LC and its projections.
Treatments focused on gamma oscillation stimulation, noradrenaline transport, reception, and reuptake, or direct interaction with the LC – particularly the application of LC-programmed stem cells (Yao, 2024), are likely to provide disease-modifying benefits across dementia patient populations. However, while LC-based interventions are likely to improve cognitive phenotypes and prevent neurodegeneration, restoration of LC-NE input and glymphatic flow may not remove densely packed plaques.
Apolipoprotein E4 (APOE4)
APOE4 is the largest genetic risk factor for the development of AD. Recently, it has been shown that ApoE4 selectively binds to the vesicular monoamine transporter 2 (VMAT2) and inhibits neurotransmitter uptake. The exclusion of norepinephrine (NE) from synaptic vesicles leads to its oxidation into the toxic metabolite 3,4-dihydroxyphenyl glycolaldehyde (DOPEGAL), which subsequently activates cleavage of Tau at N368 by asparagine endopeptidase (AEP) and triggers LC neurodegeneration (Kang, 2021). We would hypothesize that the primary mechanism of APOE4 on AD development is due to its impairment of norepinephrine transport and its role in early LC depletion and neurodegeneration.
A Note Towards Multiple Sclerosis (MS), Chronic Pain, and Autoimmune Conditions
Recent research has indicated a central role for Epstein-Barr virus in the development of multiple sclerosis (Bjornevik, 2023). Stress related disorders, depression, and anxiety have all been shown to significantly correlate with later development of autoimmune disorders (Song, 2018; Andersson, 2015; Siegmann, 2018). The development of chronic pain is often associated with trauma (Beard, 2011). We would suggest that the influence of the LC-NE system on inflammation and brain-body interactions may play a common role in these conditions and others related to autoimmunity. That is, in MS, it may be that viral infection disrupts signaling between local regions and the information provided to the LC, resulting in delayed-onset activation of autoimmunity which may sporadically recur during similar vagal-mediated immune responses. Additionally, some instances of autoimmunity and chronic pain may be due to the interactions between those conscious experiences (chronic or acute), reactivation of those memory engrams, and the autonomic/vagal/inflammatory/bodily response (conscious or subconscious) to those experiences.