Orchestration of the circadian clock and its association with Alzheimer’s disease: role of endocannabinoid signaling

Abstract

Circadian rhythms are 24-hour natural rhythms regulated by the suprachiasmatic nucleus, also known as the “master clock”. The retino-hypothalamic tract entrains suprachiasmatic nucleus with photic information to synchronise endogenous circadian rhythms with the Earth’s light-dark cycle. However, despite the robustness of circadian rhythms, an unhealthy lifestyle and chronic photic disturbances cause circadian rhythm disruption in the suprachiasmatic nucleus’s TTFL loops via affecting glutamate and γ-aminobutyric acid-mediated neurotransmission in the suprachiasmatic nucleus. Recently, considerable evidence has been shown correlating CRd with the incidence of Alzheimer’s disease. The present review aims to identify the existence and signalling of endocannabinoids in CRd induced Alzheimer’s disease through retino-hypothalamic tract- suprachiasmatic nucleus-cortex. Immunohistochemistry has confirmed the expression of cannabinoid receptor 1 in the suprachiasmatic nucleus to modulate the circadian phases of the master clock. Literature also suggests that cannabinoids may alter activity of suprachiasmatic nucleus by influencing the activity of their major neurotransmitter γ-aminobutyric acid or by interacting indirectly with the suprachiasmatic nucleus’s two other major inputs i.e., the geniculo-hypothalamic tract-mediated release of neuropeptide Y and serotonergic inputs from the dorsal raphe nuclei. Besides, the expression of cannabinoid receptor 2 ameliorates cognitive deficits via reduction of tauopathy and microglial activation. In conclusion, endocannabinoids may be identified as a putative target for correcting CRd and decelerating Alzheimer’s disease.

1. Introduction

The term circadian rhythm (CR) comes from the word “circa” which means “around” and “dies” which means “a day”. CRs are natural rhythms of approximately twenty-four hours. CRs are generated and regulated endogenously by the suprachiasmatic nucleus (SCN), also known as the “master clock”, located in the anteroventral section of the hypothalamus. The SCN governs several biological cycles in numerous species of organisms, such as the sleep-wake cycle, hormonal cycle, and body-temperature cycle. The SCN utilizes specific environmental cues such as light, temperature, and food to synchronize the endogenous CRs with the external light-dark cycle, called Zeitgeber, wherein light is identified as a predominant cue determining day and night time (Panda et al., 2002). In many species, the eye, specifically the retina, is the sole sensory tool that transduces light information into electrochemical signals and facilitates the entrainment of the SCN through the retino-hypothalamic tract (RHT) (Gooley et al., 2001). Stimulation of the SCN via mediation by RHT initiates a cascade of transcriptional–translational feedback loops (TTFL) that eventually construct CRs. Later, these CRs are conveyed to local (in the central nervous system (CNS)) and peripheral oscillators through neurohumoral communication (Sharma et al., 2021b). The SCN-generated CRs are robust in nature. However, age, complete blindness, traumatic injuries to the brain, and several other lifestyle-related factors, such as malnutrition, poor dietary practices, social engagement, education, physical activity, addiction, chronic medication, stress, and disrupted sleep can cause asynchrony in CRs, i.e., CR disruption (CRd). CRds can precipitate severe diseases such as depression, insomnia, and cognitive deficits (Walker et al., 2020; Yu et al., 2020).

Considerable evidence has been accumulated in the past couple of years that correlates CRd with the incidence of Alzheimer’s disease (AD). Recently, Sharma et al. (2021a) demonstrated that photic disturbances to the SCN induced by the chronic light-light condition could lead to dementia of Alzheimer’s type in rodents. They showed the presence of significant oxidative stress, aggregation of amyloid-beta (Aβ₄₂) peptides, and behavioral abnormalities of poor cognition and disorientation in Wistar rats who were subjected to constant light/light interventions for four to five months (Sharma and Goyal, 2020). Bokenberger et al. (2018) showed that individuals who had been subjected to shift or night work for 20 years were at greater risk of developing dementia than the day workers. Also, Jorgensen et al. (2020) showed that female Danish nurses subjected to shift work (n= 6048 working night shifts for ≥six years and n= 8059 working in shifts for ≥six years) have a higher risk of dementia incidence as compared to nurses working in the day for ≥six years. Moreover, chronic jet lag and night shift work-induced CRd expose an individual to prolonged cortisol-mediated CNS insults, eventually leading to cognitive deficits. Gibson et al. (2010) demonstrated that jet lag and shift work are both associated with increased cortisol levels. In addition, they showed that jet lag-mediated activation of the hypothalamic-pituitary-adrenal axis (HPA-axis) results in the suppression of cell proliferation in the hippocampus of Hamster (Kolla and Auger, 2011).

The evidence above correlates the disturbed activity-rest rhythm with the enhanced risk of dementia. However, another important light-related CR, i.e., the sleep-wake cycle, is also strongly associated with dementia. In support, Sabia et al. (2021) showed a 25-year follow-up study in 7959 individuals (age group of 50–70 years) that concluded a 30% higher risk of dementia in those who had ≤six hours of sleep compared to individuals who had seven hours of sleep. The sleep disturbances also prevail with age, and according to various studies, this prevalence varies from 9.1% to 69%. A meta-analysis including 12,926 articles and 246,786 individuals, reviewed multiple sleep disorders such as insomnia, narcolepsy, sleep-related movement disorder, obstructive sleep apnea, and CR sleep disorder and their association with increased risk of dementia (Kessel et al., 2011; Ohayon et al., 2004; Shi et al., 2018). In addition, the growing availability of artificial lights (such as LEDs; 60 lumens/watt (lux = lumens/m²)) and other light-emitting gadgets, such as smartphones, has made sleep problems common among the metropolitan population (Blume et al., 2019; Christensen et al., 2016). These sleep disturbances are related explicitly to CR sleep disorders, such as shift-work disorder, jet-lag disorder, advanced or delayed sleep-wake phase disorder, and irregular or non-24-hour sleep-wake rhythm disorder (Zhu and Zee, 2012).

AD is a multifactorial and chronic neurodegenerative disease attributed to the accumulation of Aβ and neurofibrillary tangles (NFTs) in the brain. Although the exact cause of AD’s pathogenesis is still debatable, it is believed that AD pathology begins earlier in a person’s life and simultaneously progresses through distinct stages with varying physical/psychological outcomes. Unfortunately, with insufficient data, it is nearly impossible to estimate the exact number of people who develop dementia due to CRd. However, the evidence does warn about the actual number because numerous studies have correlated the CRd with the preclinical stage of AD (Cordone et al., 2019; Kress et al., 2018). In support of this, the literature indicates increased severity of symptoms, varying protein profile of the hippocampus, and erratic quantitative electroencephalography (q-EEG) readings of AD brains throughout the Braak’s theory defining six stages of progression of pathology involved in Parkinson’s disease and AD (Hondius et al., 2016; Kumar et al., 2020; Kwak, 2006). In addition, studies indicate that sleep deprivation affects the glymphatic pathway and hinders the clearance of Aβ from the CNS (Eugene and Masiak, 2015; Kumar et al., 2020). Further, sleep deprivation also increases the Aβ load in the hippocampus and thalamus (Shokri-Kojori et al., 2018).

The evidence presented above demonstrates the significance of a well-maintained CR in mental health and cognition. As a result, the 1990s were spent proving the importance of some incredibly important chemical messengers that impact the circadian machinery, such as serotonin, Neuropeptide Y (NPY), and GABA. Similarly, just a decade ago, Sanford et al. (2008) discovered a substantial role for the endocannabinoid system (ECS) in modulating the light-mediated phase shifts of SCN generated CRs. Subsequently, Acuna-Goycolea et al. (2010) substantiated the findings of Sanford et al. (2008). In a physiologic state, the ECS acts as an integral and vital part of various brain regions that are predominantly involved in the acquisition, consolidation, and retrieval of memory, such as the hippocampus and cortex (Medina-Vera et al., 2020). In contrast, the normal functioning of the ECS has been found to be disrupted in neurodegeneration. Likewise, CB1 receptor knockout mice were shown to have AD-like symptoms. Also, CB1 receptors were found to be decreased in postmortem cortical brain tissues of AD patients (Maccarrone et al., 2018; Solas et al., 2013).

Based on the evidence presented above, the first half of the article provides an overview of the structural and functional aspects of the circadian machinery and the impact of its disruption on the progression of neurodegeneration. In the second half, we discuss the role of the ECS in neuroprotection and its influence in regulating and resetting CRs.

2. Circadian Rhythm

i). Structural and functional aspects of the circadian machinery

This section will provide an overview of circadian machinery, including the SCN’s inputs and outputs and its morphological and physiological properties. We emphasize the importance of synchronized SCN in health and how its asynchrony can lead to various clinical disorders, most notably neurodegeneration.

a). RHT neurotransmission

The RHT, which comprises the axons of intrinsically photosensitive Retinal Ganglion Cells (ipRGCs), originates from the retina and transmits light information to the SCN, allowing the SCN to be synchronized with our planet’s light-dark cycle. The ipRGCs are classified into six subclasses (M1-M6), with only the M1 subtype innervating the SCN. In addition, M2, M4, and M5 subtypes target several brain areas, such as the lateral geniculate nucleus (LGN), superior colliculus, and olivary pretectal nucleus (OPN) (Brown et al., 2010; Ecker et al., 2010). The M3 and M6 of ipRGCs subtypes are still understudied (Baver et al., 2008; Schmidt et al., 2011). Besides, the M1 subtype of ipRGCs consists of two subpopulations that can be distinguished on the basis of the expression of Brnb3⁻ and Brnb3⁺ transcription factors. The axonal projections of Brnb3⁺ M1 ipRGCs innervate the OPN and facilitate the pupillary light reflex. On the other hand, the axonal projections of Brnb3⁻ M1 ipRGCs projects predominantly to the SCN and photo-entrain it (Qu et al., 2011; Schmidt et al., 2011). Originating from the retina, the axonal fibers of M1 ipRGCs terminate at the ventral core of the SCN (Berson et al., 2002). Without any intermediate synapses (mono-synaptically), these axons transmit non-image-forming photic signals to the SCN core (Czeisler et al., 2005). An action potential generated as a result of the phototransduction cascade in the retina travels down these axons and facilitates the terminal secretion of a principal neurotransmitter, glutamate (Ebling, 1996). In turn, glutamate binds and activates N-methyl-d-aspartate (NMDA) (and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) receptors, leading to the depolarization of SCN neurons (Colwell, 2001; Mizoro et al., 2010). The influx of Ca²⁺ ions initiates the intracellular signalling cascades (Ding et al., 1994). An additional neuropeptide that enhances the synaptic stimulation of SCN is the Pituitary adenylate cyclase-activating peptide (PACAP) (Hannibal, 2006). In this way, RHT conveys photic cues to the SCN and photo-entrains it. Consequently, the SCN cells initiate the period (PER) and cryptochrome (CRY) protein gene-expression-cascade to induce phase-shifts and synchronize the endogenous CRs with the immediate environmental conditions (Buhr and Takahashi, 2013).

b). SCN

The SCN or master circadian pacemaker is an astonishingly complex, steady, robust, and tractable organization of about twenty thousand self-autonomous neurons in the anteroventral section of the hypothalamus (just above the optic chiasm). Moreover, the SCN is a paired neuronal structure, alienated into two sides by the 3^rd ventricle. Each side of SCN consists of about ten thousand neurons that are further distributed in the ventral “core” and dorsal “shell” regions of the SCN. RHT innervates these adjacent ventral cores of SCN and transmits retinal inputs (Abrahamson and Moore, 2001; Reppert and Moore, 1991). Anatomically, the SCN consists of a diverse network of γ-aminobutyric acid (GABA)-ergic neurons that express different neuropeptides, such as Vasoactive intestinal polypeptide (Vip), Arginine vasopressin (Avp), Gastrin-releasing peptide (Grp), Prokineticin-2 (Prok2), and Neuromedin-S (Nms) in a site-specific manner (Figure 1: a) (Dardente et al., 2004). Apart from GABAergic neurons, SCN also consists of glial cells (e.g., astrocytes, oligodendrocytes, and microglia) with multiple roles, such as morphological assistance, modulation of synaptic transmission, and scavenging neurotransmitters from synaptic clefts (Chi-Castaneda and Ortega, 2018). Interestingly, although the SCN is entrained and synchronized by light zeitgeber, it possesses a self-sustained-cell-autonomous characteristic feature where individual neurons generate their respective rhythms. Finally, these individual SCN cell rhythms are integrated into robust and precise CRs (Welsh et al., 2010).

Figure 1:

Anatomical elucidation of SCN and TTFL:

Afferents

The SCN receives photic and non-photic information from different locations through direct and indirect pathways. For instance, afferents from the retinal ipRGCs travel through RHT and transmit photic information directly into the ventral core of the SCN (Abrahamson and Moore, 2001). SCN neurons that express AVP, VIP, and GRP neuropeptides are targeted as a direct recipient of photic information from the retina (Abrahamson and Moore, 2001; Antle and Silver, 2005; Fernandez et al., 2016). Besides, the SCN core receives indirect photic inputs from the thalamic intergeniculate leaflet (IGL), which colocalizes GABAergic neurons with neurons expressing other neuropeptides such as NPY. In rats and hamsters, NPY immunoreactive IGL neurons travel into the SCN core via the geniculo-hypothalamic tract (GHT) (Moore and Card, 1994; Moore and Speh, 1993; Morin and Blanchard, 1995; Morin and Blanchard, 2001). Further, serotonergic inputs from the dorsal raphe nuclei (DRN) transmit non-photic information to the SCN’s core region (Figure 2) (Hay-Schmidt et al., 2003). On the other hand, the SCN shell receives its afferents from the SCN core. However, evidence also indicates afferents from sources other than the SCN core. These afferents arise from various nuclei of the hypothalamus. Similarly, afferents from the hippocampus, cerebral cortex, basal forebrain, brainstem cholinergic nuclei, and medullary noradrenergic areas innervate the SCN shell (Yan et al., 2007).

Figure 2:

Structural and functional aspects of circadian machinery and its influence on biological rhythms: SCN primarily receives its afferents from the retina through RHT, from the IGL through geniculohypothalamic, and from raphe nuclei through serotonergic projections. Subsequently, synchronized SCN entrains various other local and peripheral circadian clocks and thus regulates multiple physiological cycles in the body.

Efferents

Several efferents protrude from the SCN core as well as shell and arborize into distinct regions of the brain, such as the medial preoptic area, ventrolateral preoptic area, subparaventricular zone, the dorsomedial nucleus of the hypothalamus, IGL, periaqueductal gray, paraventricular nucleus of the thalamus, and lateral septum (Abrahamson and Moore, 2001; Kriegsfeld et al., 2004). The local circadian clocks (also known as central circadian clocks) in these regions regulate the feeding-fasting cycle, sleep-wake cycle, etc., and modulate the rhythmicity of autonomic and neuroendocrine systems (Hastings et al., 2018). Interestingly, these central circadian clocks further entrain the local molecular clocks of the peripheral system and, thereby, regulate several physiological rhythms, e.g., rhythms related to metabolism, alertness, blood pressure, and renal activity (Hastings et al., 2003; Morris et al., 2015; Santhi et al., 2016; Voogel et al., 2001). This way, the SCN regulates both behavioral and physiological rhythms (Figure 2).

Transcriptional–translational feedback loops in SCN

SCN maintains its rhythm in the absence of environmental cues, and this robustness is regulated by the positive and negative feedback loop system of clock components, called TTFL (Takahashi, 2017). TTFL exists in individual cells of SCN and comprises various genes and their proteins. In the absence of external light, inside the nucleus of SCN cells, at circadian dawn (circadian time (CT) 0), the positive regulators of TTFL, i.e., circadian locomotor output cycles protein kaput (CLOCK) and brain and muscle ARNT- like 1 (BMAL1), form a heterodimer and bind to the enhancer-box (E-box) regulatory sequences. This interaction promotes the transcription of the PER and CRY genes. Later, mRNAs of PER and CRY translocate into the cytoplasm and get translated by ribosomes into PER (PER 1, 2, 3) and CRY (1, 2) proteins (Lee et al., 2001). Sequentially, PER and CRY proteins form dimer complexes in the cytoplasmic area, and these complexes, by circadian dusk (CT 12), migrate back to the nucleus. Eventually, nuclear accumulation of PER-CRY protein complexes represses their own transcription and serves as the loop’s negative regulators. During the upcoming circadian night (i.e., CT12–CT24), the degradation of PER-CRY complexes occurs. By the time CT0, PER-CRY complexes fully degrade, allowing CLOCK and BMAL1 to reinitiate the transcription of PER and CRY genes and begin the circadian cycle anew. In this way, SCN maintains the ~24-hours circadian cycle. Two additional regulatory loops support the robustness of crucial TTFL. These loops include CLOCK-BMAL1 heterodimers mediated transcription of Rev-erbα and Rorα, where RORs trigger transcription of BMAL1 and REV-ERBs repress it (Figure 1: b) (Buhr and Takahashi, 2013; Cho et al., 2012; Guillaumond et al., 2005; Takahashi, 2017).

ii). Disrupted circadian rhythm and neurodegeneration

A complex relationship exists between CRd and neurodegenerative disorders such as AD (Leng et al., 2019). Recent studies reveal that chronic CRd plays a pivotal role in the onset and progression of AD; however, they also suggest that CRd is common among AD patients. Although, it is still unknown if CRd induces AD or vice versa, evidence indicates a bidirectional relationship (Homolak et al., 2018; Kress et al., 2018; Musiek, 2017). Besides, both CRd and AD share identical pathologies and symptoms, such as cognitive disturbances, oxidative stress, disturbed sleep-wake cycle, and metabolic abnormalities (Cai et al., 2012; Zimmet et al., 2019). The evidence suggests that CRd and AD intersect pathophysiologically in Aβ production, glymphatic clearance, oxidative stress, neuroinflammation, secretion of melatonin, and metabolic dysfunction (Homolak et al., 2018; Leng et al., 2019).

Considering the role of the CR-regulated sleep-wake cycle in Aβ regulation, it appears that both factors are bidirectionally related. Over the last few years, our lab has successfully established an animal model displaying CRd-mediated manifestation of an AD-like phenotype in adult male Wistar rats subjected to chronic light/light conditions (Sharma et al., 2021a). The animals exhibit dysregulated gene expression of Per2. Also, a significant dysregulation in hyperoxidized peroxiredoxins (PRX-SO2/3) and peroxiredoxin-1 (PRX1) protein expression was noted. Further, increased oxidative stress and dysregulated glutamate and GABA dynamics were also noted in the SCN. Furthermore, upregulation of Bace1, Mgat3, and Aβ42 levels in the hippocampus was observed (Sharma et al., 2021a). On the other hand, Bero et al. (2011) showed that a rise in Aβ levels in interstitial fluid is caused by endogenous neuronal activity, which eventually leads to local Aβ aggregation.

The Blood-Brain-Barrier (BBB) and the glymphatic pathway clear this increased Aβ concentration from the brain during sleep (Tarasoff-Conway et al., 2015). Also, the glymphatic pathway clears tau from the brain. However, sleep deprivation alters this physiological process and hinders the Aβ clearance through BBB and glymphatic pathway (Wu et al., 2019).

In addition, the literature suggests that CRd, AD, and oxidative stress form a triangular intersection in which one component fuels pathogenesis in the other. Considerable evidence supports this hypothesis, e.g., in the master circadian clock, components of TTFL such as Bmal1 and CLOCK act as a positive regulator of glucocorticoids (e.g., cortisol, a stress hormone) secretion, whereas CRY represses their production (Phan and Malkani, 2018). Further, deletion of Bmal1 and CRY1 and/or CRY2 results in the abnormal regulation of glucocorticoids (Lamia et al., 2011; Leliavski et al., 2014). This indicates that a key stress regulator, i.e., the HPA axis, is strongly governed by the CR (Girotti et al., 2009). Besides, an array of evidence suggests that insufficient sleep raises cortisol levels and enhances the risk of AD (Girotti et al., 2009; Leproult et al., 1997; Ouanes and Popp, 2019). Likewise, a meta-analysis of 17,245 participants found that, in the morning, AD patients possess higher cortisol levels in their saliva, blood, and cerebrospinal fluid (Zheng et al., 2020).

Further, microglia in the CNS has an endogenous CR that regulates immune responses. However, Fonken et al. (2016) discovered that the clock genes of hippocampal microglia exhibit erratic expression with age. They found that, in aged rats, Per1 and Per2 mRNAs express in an aberrant manner. Also, the microglial expression of cytokines such as TNF-α and IL-1 was elevated throughout the day. This study clearly suggests that aging alters the circadian control of neuroinflammatory functions.

SCN regulated secretion of melatonin plays a critical role in brain homeostasis. Melatonin is an endogenous sleep promoter, and with pharmacological interventions, melatonin can induce phase shifts in CR to reset the sleep-wake cycle as desired (Burgess et al., 2010). Besides, melatonin has long been known to exhibit anti-amyloidal and anti-oxidant properties (Vincent, 2018). Additionally, melatonin is also believed to play a vital role in metabolic regulation. For instance, Nishida et al. (2003) showed that pinealectomy in rats results in a significant insulin surge, a condition that resembles type-2 diabetes (Korkmaz et al., 2009; Nishida et al., 2003). These physiological benefits of melatonin appear to be associated with improved sleep rhythms, resulting in reduced Aβ levels and neuroinflammation (Zisapel, 2018). However, melatonin production declines gradually with age, leading to CRd and related complications like an arrhythmic sleep cycle (Karasek, 2004).

Interestingly, the pathologies mentioned above are highly complicated and intricately intertwined, with one influencing the other and being influenced by it. However, they eventually cause substantial disruptions in normal brain homeostasis, leading to neurodegeneration over time (Figure 3).

Figure 3:

Diagrammatic representation of the bidirectional relationship between CRd and AD: CRd and AD share a bidirectional relationship where one factor potentiates the risks for the other. Risk factors for chronic CRd include unnatural lighting conditions, shift-work, and jet lag. In addition, malnutrition hinders the inter-oscillator-linking between the master clock and peripheral oscillators. Also, malnutrition reduces the ability of SCN to align and lock the endogenous circadian phase with external light-dark conditions in lab animals (Aguilar-Roblero et al., 1997). Besides, after the manifestation of CRd, a cascade of pathologies initiates inside the brain which eventually results in AD. The key components of such pathologies include dysfunction of metabolic homeostasis, dysregulation of brain’s clearance systems (glymphatic clearance and BBB clearance), elevation in neuronal activity and Aβ production, amplified oxidative stress, and dysregulation of melatonin secretion.

3. Endocannabinoid system (ECS)

The ECS is a highly distributed biological system that is an essential and integral part of various biological operations throughout the CNS and the peripheral nervous system (PNS), e.g., neuromodulatory actions. Also, its widespread distribution across the CNS implies that it plays an essential role in our physiological and psychological behaviour. Over the last few decades, several studies have demonstrated ECS stimulation with synthetic and natural cannabinoids. The results, however, are dose-dependent, conflicting, and debatable. In addition, the ECS facilitates the development of the CNS and modulates synaptic plasticity and helps alleviate endogenous and exogenous insults to the body. The ECS consists of endocannabinoids (ECs) and their receptors, i.e., cannabinoid receptors (CBRs). ECs are lipid-based retrograde neurotransmitters that primarily bind to CBRs (Lu and Mackie, 2016). This section of the article includes the general pharmacology of the ECS and its localization in the brain. Further, we discuss the role of the ECS in memory, neurodegeneration, and neuroprotection. Also, we addressed a few studies that suggest both detrimental and positive effects of cannabinoid treatment in memory. Finally, we discuss the impact of the ECS on brain homeostasis and CRs regulation.

i). Localization of Cannabinoids

Devane et al. (1992) initially discovered the endocannabinoid anandamide (AEA), whereas two CBRs, i.e., CB1 and CB2, were first identified in 1990 and 1993, respectively (Matsuda et al., 1990; Munro et al., 1993). Despite these two key CBRs, ECs also binds to several other orphan receptors, such as G-protein-coupled receptors (GPR18, GPR55, GPR119) and transient receptor potential channels (TRPV1, TRPM8) (Morales and Reggio, 2017). However, CB1 and CB2 are primary molecular targets of two ECs, AEA and 2-arachidonoylglycerol (2-AG). AEA act as a partial agonist of CB1 receptors, although 2-AG is a potent agonist of both the CBRs. Also, 2-AG is a major regulator of various patho/physiological processes in the body, such as cognition, emotion, pain sensation, neuroinflammation, and energy balance (Baggelaar et al., 2018; Casarotto et al., 2015). Conversely, phyto-/exocannabinoids such as tetrahydrocannabinol (THC) act as an agonist of CBRs (Baggelaar et al., 2018). Cannabidiol (CBD), another exocannabinoid, weakly antagonizes both the CBRs. Conversely, CBD is a potent agonist of the TRPV1 receptor and strongly antagonizes TRPM8. Besides, CBD also neutralizes certain side effects of THC (De Petrocellis et al., 2011; Hudson et al., 2019). The expression of ECs and CBRs is documented throughout the CNS and PNS (Freitas et al., 2018). In CNS, CB1 is expressed in the amygdala, hippocampus, several nuclei of thalamus and hypothalamus, cerebellar cortex, olfactory and visual system, etc. (Heinbockel and Straiker, 2021; Straiker et al., 1999; Zou and Kumar, 2018). CB2 expression in the CNS is limited and not as densely as compared to CB1 receptors. CB2 receptors are expressed in the brain stem, hippocampus, and certain other CNS regions (Stempel et al., 2016; Van Sickle et al., 2005). Also, the expression of both CB1 and CB2 receptors is found in the peripheral part of the CNS, i.e., the retina (Lu et al., 2000; Straiker et al., 1999). Moreover, the CB1 receptor is strongly expressed in the SCN, although no evidence of CB2 receptor expression in the SCN has been found yet (Acuna-Goycolea et al., 2010).

Endocannabinoids synthesis, release, and degradation

ECs have an exclusive and specific synthesis mechanism. Both the key ECs are synthesized on-site, separately, and on-demand. Moreover, the synthesis of ECs occurs in the postsynaptic neurons. After synthesis, instead of getting stored in vesicles, ECs undergo enzymatic degradation. Diacylglycerol (DAG) acts as a precursor for the synthesis of 2-AG, which is catalyzed by diacylglycerol lipase-α (DAGLα). Besides, AEA gets synthesized from N-acyl-phosphatidylethanolamine (NAPE) by NAPE-specific phospholipase D (NAPE-PLD). ECs are lipophilic; hence they easily traverse the cell membrane. Later in the synaptic cleft, ECs retrogradely activate CBRs that are positioned over presynaptic neurons. Interestingly, AEA is known to activate both presynaptic CBRs and intracellular-postsynaptic non-CBR targets (e.g., TRPV1) (Ulugöl, 2014). Furthermore, ECs also activate the CBRs that are localized in astrocytes (Hablitz et al., 2020). Eventually, fatty acid amide hydrolase (FAAH) degrades AEA into arachidonic acid and ethanolamine in postsynaptic neurons. Whereas, at presynaptic neurons, monoacylglycerol lipase (MAGL) facilitates the hydrolysis of 2-AG into arachidonic acid and glycerol (Figure 4) (Ulugöl, 2014).

Figure 4:

Simplified scheme representing endocannabinoids modulated synaptic transmission.

4. Role of Endocannabinoid system in CRs and memory

The ECs mentioned above and CBRs play a critical role in processing memory and maintaining CRs, although the work done in this area is quite limited. Nevertheless, evidence indicates ECS as an integral part of memory and the circadian system.

i). Endocannabinoids in memory

Multiple brain areas are involved in memory acquisition, consolidation, and retrieval. For example, the hippocampus primarily processes declarative memory, including semantic and episodic memory (Roediger et al., 2008). The cerebellum regulates implicit memories such as procedural memory, classical conditioning, and motor learning (Cooke et al., 2004). Also, the amygdala is critical in the processing of fear memories (Josselyn, 2010). Interestingly, the ECS exists throughout the aforementioned brain’s key memory processing areas, and there is substantial evidence that it influences cognitive processes. For instance, Lunardi et al. (2020) investigated the influence of the ECS on memory updation and deletion/extinction. They found that hippocampal infusion of CB1 antagonists/inverse agonist AM251 and rimonabant significantly reduce memory updation, impair behavioral flexibility, and enhance the forgetting of fear memory in Wistar rats (Lunardi et al., 2020). Further, Nedaei et al. (2016) demonstrated that microinjecting the CB1 receptor agonist arachidonylcyclopropylamide (1–4 ng/rat) into the basolateral amygdala of the rat could attenuate the scopolamine-induced impairment in memory consolidation. Furthermore, glucocorticoids appear to recruit ECs signaling in the dorsal striatum to promote memory consolidation in Wistar rats by enhancing retention avoidance after inhibitory avoidance training (Siller-Pérez et al., 2019). Alarcon et al. (2020) further strengthen the idea of ECs participation in memory acquisition and consolidation. They concluded that the CB agonist WIN55,212–2 impairs spatial memory acquisition in the Morris water maze test but does not affect its consolidation. ECs signaling also aids in the reward-based learning of a motor sequence, which is impaired in CB1 and DGLα knockout mice (Tanigami et al., 2019).

ii). Endocannabinoids in neurodegeneration and neuroprotection

Given its significance in cognitive processes, the ECs is also involved in various neurodegenerative processes such as AD, although the evidence appears to argue for its beneficial role. Recently, several studies have attempted to investigate the role of ECs in AD and found that the normal functioning of the ECs is altered in the neurodegenerative state of the brain. Also, Maccarrone et al. (2018) demonstrated that overexpression of full-length human mutant Amyloid precursor protein (APP) in the hippocampus of Tg2576 mice interferes with the membrane localization of CB1 receptors and influences their inhibitory signaling activity. Further, failure of the ECS system in laboratory animals results in increased morbidity and mortality. According to Aso et al. (2018), crossing APP/PS1 (APP-presenilin-1) transgenic mice with CB1 knockout mice resulted in offsprings having imbalanced excitatory/inhibitory neurotransmission in the hippocampus with extremely low survival. In addition, AD-like symptoms such as impaired memory were also observed by the authors. The expression of CBRs and their endogenous ligands are also affected by AD. For instance, CB1 receptors were found to be significantly reduced in the postmortem cortical brain tissues of AD patients. In contrast, the expression of CB2 receptors was found to be significantly enhanced (Solas et al., 2013).

Several studies have presented a contradictory role of the ECS in the CNS during the progression of AD. For instance, Galán-Ganga et al. (2021) demonstrated the overexpression of CB2 receptors as an early event in tauopathies. Also, a significant increase in the AEA levels induced by overexpression of hTAU^P301L was observed in the hippocampus. Further, the authors observed a reduction in the levels of degrading enzyme FAAH and concluded that deficiency of CB2 receptors could ameliorate hTAU^P301L induced neurodegeneration and cognitive deficits. Interestingly, this study denies any relevant role of CB2 receptors in overexpressed hTAU^P301L induced neuroinflammation. However, multiple studies showed cannabinoids effectively ameliorate the Aβ-induced indirect neurotoxicity, such as microglial activation in in-vitro and in-vivo studies. Martín-Moreno et al. (2012) found that chronic oral administration of cannabinoids for four months in Tg APP 2576 mice can attenuate microglial activation, reduce the release of TNF-α, suppress COX-2 expression, decrease cortical Aβ deposition, and ameliorate cognitive deficits. Moreover, cannabinoids also reduce the expression of TNF-α and the production of nitric oxide (Ramírez et al., 2005). Ehrhart et al. (2005) corroborated these findings. They further added that JWH-015 mediated selective activation of CB2 receptors causes a considerable reduction in Interferon-γ induced phosphorylation of JAK/STAT1 and expression of CD40 receptor. CD40 signaling promotes pathological activation of microglial cells and inhibits microglial phagocytosis of Aβ₄₂ peptides. Therefore, cannabinoid-mediated suppression of CD40 appears to be a viable approach of protecting critical brain regions against Aβ-induced neuroinflammation. Further, nuclear peroxisome proliferator-activated receptor-γ (PPAR-γ) signaling facilitates the neuroprotective effect of cannabinoid agonist WIN55,212–2 and reduces the elevated levels of TNF-α, active caspase 3, nuclear NF-kB, and TUNEL-positive neurons in the hippocampus (Fakhfouri et al., 2012).

iii). CBs modulation in SCN circuitry for AD

a). CBs modulation in CRd

As previously stated, in the SCN, GABA act as a principal neurotransmitter. However, several other neuropeptides, including serotonin and NPY, have neuromodulatory effects in the SCN. Initially, researchers looked into the role of these neuropeptides in the modulation of SCN-generated CRs. Eventually, they focussed on identifying other probable neuropeptides that can modulate the master clock (Mintz et al., 1999; Moore and Speh, 1993; Sanford et al., 2008). With time, a growing body of evidence suggested that the ECS is somehow associated with the circadian system. For instance, in the rat brain, both the key ligands of ECS, i.e., 2-AG and AEA, exhibit diurnal variations (Valenti et al., 2004); for review, see Vaughn et al. (2010). In addition, studies found that not only does sleep modulate the expression of the CB1 gene in the rat pons, but CBRs expressed at the medial septum nuclei also regulate the sleep-wake cycle. Further, CB1 receptor agonists were found to increase the total percentage of rapid eye movement sleep and enhanced total sleep time. Furthermore, chronic treatment with exogenous Δ9-tetrahydrocannabinol was found to invert the CR of brain temperature (Martin et al., 2003; Perron et al., 2001; Puskar et al., 2021). In light of the above evidence, it was postulated that the ECS stayed under the influence of the circadian system or vice versa. It was hypothesized that the ECS is involved in modulating the inputs of various afferents and the internal activity of the SCN. Sanford et al. (2008) initially reported the role of CB1 receptors in modulating the circadian phases of the master clock. This study verifies the role of the CB1 receptor agonist in attenuating the light-induced phase advances of locomotor activity in hamsters. The authors reported a significant phase advanced activity of CB1 agonist CP55940 when administered (i.p.) at CT 18.25, prior to light exposure (20lux) at CT 19.00, for 10 minutes, for ten days. In addition, they also observed the maximal inhibitory effect of CP55940 at an i.p. dose of 0.5 mg/kg. Further, based on immunohistochemical findings, which confirmed the expression of CBRs at SCN, IGL, and DRN, the authors suggest that cannabinoids may alter the activity of SCN cells by influencing the activity of their major neurotransmitter GABA or by interacting indirectly with the SCN’s two other major afferents, i.e., GHT and serotonergic inputs from the DRN.

The proposed hypothesis of how CBRs influence the activity mechanism of SCN by Sanford et al. (2008) correlates with retrospective studies. For instance, Barbacka-Surowiak and Gut (2001) investigated the significance of intact DRN signaling in maintaining a wheel-running activity rhythm in mice. They found that electrical lesions in the DRN result in reduced and fragmented locomotor activity in mice subjected to D/D light conditions. They found that mice subjected to L/D conditions exhibit an early onset of activity, whereas mice subjected to L/L conditions exhibit a longer period of activity. This evidence established a potential influence of DRN signaling in circadian rhythmicity. Later, serotonergic inputs from the DRN were also reported to inhibit RHT mediated activation of SCN neurons (Glass et al., 2003). Activation of NMDA receptors by glutamate, released from the RHT terminals, induces light-like phase shifts in the SCN and appears essential for transmitting the light information from the retina to the SCN (Mintz et al., 1999). Studies suggest that 5HT_1B receptors are located presynaptically on RHT axon terminals, and application of 5HT_1B agonists (TFMPP and CGS 12066A) inhibit an NMDA-induced phase-shifting mechanism in SCN (Boschert et al., 1994; Pickard et al., 1996). Similarly, Quipazine, a non-selective serotonin (5-HT) receptor agonist, was found to inhibit light-induced c-fos protein (an activity marker) expression in the SCN (Selim et al., 1993). The evidence provided above infers the impact of DRN signaling in CRs. Interestingly, on the other hand, certain evidence supports the speculations of Sanford et al. (2008), which state that ECs signaling influences the SCN’s serotonergic inputs from the DRN. For instance, CB receptor agonist CP 55,940 increases the firing rate of serotonergic dorsal raphe neurons and increases the serotonin level in various nuclei of the hypothalamus (Figure 5) (Arévalo et al., 2001; Gobbi et al., 2005; Sanford et al., 2008). In addition, Lydic et al. (1984) showed a correlation between the activity rhythm, the sleep cycle, and the firing pattern of DRN neurons. According to this study, in single-cell recording, the typical firing pattern of a cat’s DRN during wakefulness is 2–3 spikes/s; however, during slow-wave or synchronized sleep, a reduction in spikes (1 spike/s) was observed, which was further reduced to 0.05 spikes/s during desynchronized sleep rhythm. Interestingly, similar observations were made by Mendiguren and Pineda (2009) , who showed that CB antagonists AM251 (1 µM) and rimonabant (1 µM) diminished the firing rate of DRN’s 5-HT cells, which suggests that both ECs and DRN signaling are related to a critical component of CRs, i.e., sleep. Further, the role of CB agonists in potentiating the synchronized sleep rhythm support the idea that the ECS indirectly influences the activity of NMDA receptors in the SCN by stimulating DRN inputs to the SCN (Puskar et al., 2021; Vaughn et al., 2010).

Figure 5:

A proposed hypothesis for cannabinoid mediated modulation of CR: Light-mediated retinal stimulation triggers action potentials in RHT, eventually leading to glutamate release in the SCN core from presynaptic RHT terminals. As a result, activation of SCN neurons causes endogenous CR to synchronize with external light-dark conditions. However, light-mediated activation of SCN in the early morning or late evening resulted in advanced and delayed phases, respectively. GABA is primarily responsible for information transmission throughout the SCN, and its modulation appears to affect the regular CR. Cannabinoid agonists affect GABA activity and aid in the resetting of CR. Furthermore, cannabinoid agonists impede the SCN’s ability to entrain to light information. Additionally, serotonergic inputs from raphe nuclei activate presynaptic 5HT1B receptors at RHT, inhibiting glutamate release. Cannabinoid agonists were shown to increase the firing rate of serotonergic dorsal raphe neurons. Furthermore, non-photic cues are transmitted from the IGL to the SCN via NPY, and NPY agonists were found to aid in resetting CR. Interestingly, Cannabinoid agonists are also believed to enhance this physiology.

Moreover, Johnson et al. (1989) observed altered CRs in IGL lesioned hamsters, concluding that GHT influences SCN entrainment. This evidence also potentiates the idea that ECs signaling influences GHT to modulate SCN’s activity. For instance, the NPY is believed to transmit non-photic cues from the IGL to the SCN via GHT. Albers and Ferris (1984) investigated the circadian phase-shifting properties of NPY by microinjecting it into the SCN of hamsters that were exposed to constant light conditions. They reported that animals injected before the activity onset showed an advanced phase; however, animals injected after 12 hours of activity onset showed the opposite effect, i.e., delayed phase. Similarly, NPY-mediated phase shifts were also observed in animals that were subjected to constant darkness. However, both in-vitro and in-vivo studies suggest that NPY-mediated phase shifts occurred when administered only during subjective days (Harrington and Schak, 2000; Huhman and Albers, 1994; Johnson et al., 1989).

Interestingly, NPY treatment prevents NMDA-mediated light-like phase shifts in the lab animals. Several in-vitro and in-vivo studies showed that NPY-mediated inhibition of light-induced phase shifts in laboratory animals occurs via the NPY Y1, Y2, and Y5 receptors. In addition, studies suggest that even though NPY inhibits light-induced phase shifts in NPY Y1−/− mice, it fails to reset the CRs in Y2−/− animals. Further, NPY Y5 receptor antagonists were found to potentiate the influence of light on CRs (Lall and Biello, 2003; Soscia and Harrington, 2004, 2005; Yannielli et al., 2004; Yannielli and Harrington, 2001). The evidence presented above strongly suggests that, alike CB agonists, NPY plays a critical role in preventing the SCN from photic disturbances. Interestingly, CB1 receptor stimulation appears to surge NPY release in the rat hypothalamus. On the other hand, the CB1 receptor antagonist AM251 reduces NPY immunoreactivity in the arcuate nucleus. As stated above, Sanford et al. (2008) identified CB1 receptor immunoreactivity in the IGL, implying that CB agonists may play a role in regulating GHT signaling to the SCN; however, no clear evidence has been provided yet. Nonetheless, the strong association between NPY and CBR ligands and their similarity in adjusting CRs suggest a need for dedicated study (Bakkali-Kassemi et al., 2011; Gamber et al., 2005).

In 2010, Acuna-Goycolea et al. corroborated the findings of Sanford et al. (2008) and showed a significant reduction of light-induced phase delays by a CB1 receptor agonist. They reported that cannabinoids attenuate the SCN’s ability to get entrained to light; however, they also mention no effect of cannabinoids on glutamate release from the RHT inputs. This study was performed on adult (8–12 week old) male C57BL/6J mice, and dose administration was done using an intraventricular cannula. After the free-running period of 14 days in D/D light conditions, mice were infused at CT 15.5 (under 15 W dim red light: < 1 lux at cage level) with WIN55,212,2 (3 nm/μl: 3 μl total volume) and AM251 (6 nm/μl: 1.5 μl total volume) along with DMSO as a vehicle. Thirty minutes later, at CT 16, the mice were exposed to light (50 lux for 10 minutes) and returned to their cage. With cell-attached patch-clamp electrophysiological recordings, the authors found that activation of CB1R by WIN55,212,2 results in reduced GABA release from the presynaptic axon terminals of SCN cells. Further, based on the fact that GABA exhibits dual inhibitory and excitatory roles in the SCN, the authors speculated that the effect of CB1R stimulation on activation or inhibition of SCN’s circuit might depend upon the timing of the subjective day (Acuna-Goycolea et al., 2010). Various studies have shown the importance of GABA-mediated neurotransmission in the SCN. For instance, GABA antagonists were found to be particularly involved in blocking circadian phase delays (Ralph and Menaker, 1985). Also, agents responsible for potentiating GABA activity, such as benzodiazepine diazepam, inhibit light-induced phase advances (Ralph and Menaker, 1989). The evidence mentioned above advocates for the role of GABA in circadian phase shifts but fails to explain whether GABA exhibits inhibitory or stimulatory effects in the SCN. Certain studies suggest its dual role, e.g., Albus et al. (2005) demonstrate that GABA act as a transporter of phase information between ventral and dorsal SCN. Moreover, it excites neurons of the dorsal SCN and imposes inhibitory effects on the ventral part of the SCN. By contrast, in fetal mice, GABA plays a role in refining the circadian output rhythm. However, it appears to exhibit no role in integrating the overall CRs of individual SCN neurons (Ono et al., 2019). Moreover, the ventral SCN is highly vulnerable to light, and therefore alteration in its electrical activity and phase shifts (gene expression) can be observed rapidly. The ventral SCN significantly affects the dorsal SCN and causes a phase shift. However, phase shifting and resynchronization in the dorsal SCN take several days. Besides, the transmission of phase information between ventral and dorsal SCN can be prevented by bicuculline (a competitive antagonist of GABA_A receptors) (Albus et al., 2005). Acuna-Goycolea et al. (2010) confirm the possibility of CB1 receptor-mediated alterations of GABAergic communication between the SCN cells, as activation of CB1 receptors alleviates the discharge of GABA from presynaptic axon terminals in the SCN. In another study, Hablitz et al. (2020) corroborated the findings of Acuna-Goycolea et al. (2010) and by using electrophysiological recordings (recording time ZT7– ZT10; lights off at ZT12) showed that CB1 receptors agonist WIN 55,212–2 (3 μM) treatment reduces the frequency (but not amplitude) of GABA receptor-mediated postsynaptic current (mGPSC). They further corroborated their finding by treating SCN coronal slice preparations with CB1 receptors antagonist AM2515 (μM), which completely blocked the activity of WIN 55,212–2. In addition, the authors reported that the activation of astrocytes’ Ca²⁺ signaling intermediates the CBRs mediated modulation of the presynaptic GABA release from SCN cells. To test this hypothesis, the authors first monitored the Ca²⁺ levels in the presence of WIN 55,212–2. For this purpose, a transgenic mouse line, GFAP-Cre⁺ (B6.Cg-Tg(Gfap-cre)73.12Mvs/J), was injected bilaterally into the SCN (coordinates: x, –0.4, y, ±0.2, and z, –5.8 from bregma) with a viral construct that encodes for Cre-recombinase-activated ultrasensitive fluorescent calcium indicator (GCaMP6) for real-time imaging of Ca²⁺ signaling in SCN astrocytes. The authors found that WIN 55,212–2 (3 μM) treatment significantly increased the Ca²⁺ levels in the astrocytes. The authors then attempted to examine the mechanism by which postsynaptic SCN neurons use ECs signaling to recruit astrocytes in order to control presynaptic GABA release. Authors hypothesized that ATP released from astrocytes gets converted into adenosine and activates adenosine receptors necessary for the WIN 55,212–2 mediated alterations in the SCN. To test this hypothesis, they demonstrated that in the presence of CGS15943 (50 μM) (a potent adenosine 1 and adenosine 2A receptor antagonist) in the bath, WIN 55,212–2 fails to show its activity, i.e., inhibition of postsynaptic mGPSC. Next, they treated SCN slices with the adenosine 1 receptor-specific antagonist DPCPX and found similar results. Finally, the authors concluded that postsynaptic SCN neurons release ECs into the synaptic region, which then bind to adjacent CB1 receptors on astrocytes and increase intracellular Ca²⁺ levels in astrocytes, resulting in the release of adenosine into the synaptic region, which then activates the adenosine 1 receptor on presynaptic neurons, resulting in the decrease of GABA release from presynaptic SCN neurons (Hablitz et al., 2020).

The evidence mentioned above supports the notion that EC signaling affects the photic inputs of SCN by modulating various chemical messengers of non-photic afferents of the SCN and interferes with its internal activity (Figure 5). Further, CB receptor activation facilitates the resetting of phase shifts in the SCN. However, the complete picture of the underlying mechanism is yet to be drawn.

b). CBs modulating AD induced by CRd

As discussed previously, CRd-induced AD is a well-established fact (Sharma and Goyal, 2020; Sharma et al., 2021a; Sharma et al., 2021b); however, there is currently no evidence to support the involvement of CBs in the prevention of CRd-induced AD. Nevertheless, we can look at the other side of the picture, where multiple studies advocate for CBs in the prevention of AD-induced CRd; for review, see Verma et al. (2021). The repertoire of AD-mediated disturbances in CRs includes sundowning syndrome (which is an abrupt worsening of confusion, agitation, and aggression at dusk), disturbed sleep-wake cycle and its fluctuating duration, excessive somnolence, eating disturbances, activity-rest rhythm disruption, dysregulated secretion of the stress hormone, altered metabolism and so on (Butterfield and Halliwell, 2019; Hatfield et al., 2004; Kai et al., 2015; McCleery et al., 2016; Peter-Derex et al., 2015; Volicer et al., 2001). CB treatment has been shown to be advantageous in reducing the severity of the aforementioned AD-induced CRd (Verma et al., 2021). Therefore, based on the facts presented above, we emphasize the evaluation of ECS participation in CRd-induced AD.

Based on the evidence, we emphasize the need for investigating the ECS-mediated indirect control of glutamate release in the SCN core by increasing serotonin release in the SCN and activating the 5HT1B receptors at the terminal ends of RHT. Another probable idea is that ECS-mediated activation of IGL neurons potentiates NPY release in SCN, which ultimately inhibits phase shifts of CRs via an unknown mechanism, or that acting directly on intracellular GABAergic communication of SCN and altering it to prevent phase shifts of CRs.

Conclusion

According to the literature discussed in the review, there is ample evidence that a mutual participation of CRd and neurodegeneration exists in exacerbating each other’s severity. An unhealthy lifestyle and chronic photic disturbances are noted to cause CRd in SCN’s TTFL loops via affecting neurotransmission of glutamate and GABA. A widespread distribution of ECs across the CNS and the expression of cannabinoid receptor, CB1, in the SCN modulate the circadian phases of the master clock. The specific modulatory role of CB1 receptors in the SCN associated with AD pathology warrants additional investigation. Furthermore, cannabinoid CB2 receptors have been reported to express in the brain glial cells, including microglia that are involved with inflammatory cytokines, suggesting an important role in the inflammatory pathology of AD. Thus, given the expression and role of CB1 and CB2 receptors mediated signalling, manipulation of the ECs in the SCN may offer novel pharmacological approach to the novel treatment of AD. The present article summarizes evidences for presenting ECs as a putative target for correcting CRd and deaccelerating AD progression. It may be concluded that the cannabinoids, CB1 may alter SCN’s cells’ activity via geniculohypothalamic tract-mediated release of neuropeptide Y, GABA in SCN and serotonergic inputs from raphe nuclei.

Highlights.

• SCN generated CRs are robust in nature; however, lifestyle-related factors, such as chronic photic disturbances, can cause asynchrony in CRs, i.e., CR disruption (CRd).

• Numerous evidence correlates CRd with the enhanced risk of dementia.

• CRd (e.g., CR sleep disorders) increases the Aβ load in the hippocampus and thalamus. CRd also affects the glymphatic pathway and hinders the clearance of Aβ from the CNS.

• Literature suggests the Endocannabinoid system (ECS) as a mutual link between CRd and AD, as the normal functioning of the ECS has been found to be disrupted in neurodegeneration. Likewise, CB1 knockout mice were shown to have AD-like symptoms.

• Multiple studies suggest that ECS plays an essential role in modulating the light-mediated phase shifts of SCN generated CRs.

Abbreviation

2-AG

2-Arachidonoylglycerol

AVP

Arginine vasopressin

BMAL1

Brain and muscle ARNT- like 1

CB 1, 2

Cannabinoid receptor 1, 2

CNS

Central nervous system

CLOCK

Circadian locomotor output cycles protein kaput

CR

Circadian rhythm

CT

Circadian time

CRd

CR disruption

CRY

Cryptochrome

CRY 1, 2

Cryptochrome 1, 2

DAGLα

Diacylglycerol lipase-α

DRN

Dorsal raphe nucleus

ECS

Endocannabinoid system

ECs

endocannabinoids

FAAH

Fatty acid amide hydrolase

Grp

Gastrin-releasing peptide

GHT

Geniculohypothalamic tract

GPR55, 119

G-protein coupled receptor 55, 119

GPCRs

G-protein-coupled receptors

HPA-axis

hypothalamic-pituitary-adrenal axis

IGL

Intergeniculate leaflet

ipRGCs

Intrinsically photosensitive Retinal Ganglion Cells

LED

Light-emitting diode

MAGL

Monoacylglycerol lipase

NAPE-PLD

NAPE-specific phospholipase D

Nms

Neuromedin-S

NPY

Neuropeptide Y

NMDA

N-methyl-d-aspartate

OPN

Olivary pretectal nucleus

PER

Period

PER 1, 2

Period 1, 2

PNS

Peripheral nervous system

PLC

Phospholipase C

Prok2

Prokineticin-2

PLR

Pupillary light reflex

RGCs

Retinal Ganglion Cells

RHT

Retino-hypothalamic tract

SCN

Suprachiasmatic nucleus

TTFL

Transcriptional–translational feedback loops

TRPM8

Transient receptor potential cation channel subfamily M (melastatin) member 8

TrpV1

Transient receptor potential cation channel subfamily V member 1

TRPA1

Transient receptor potential cation channel, subfamily A, member 1

Vip

Vasoactive intestinal polypeptide

GABA

γ-aminobutyric acid