Dynamic endocannabinoid-mediated neuromodulation of retinal circadian circuitry.

Circadian rhythms are biological rhythms that originate from the "master circadian clock," called the suprachiasmatic nucleus (SCN). SCN orchestrates the circadian rhythms using light as a chief zeitgeber, enabling humans to synchronize their daily physio-behavioral activities with the Earth's light-dark cycle. However, chronic/ irregular photic disturbances from the retina via the retinohypothalamic tract (RHT) can disrupt the amplitude and the expression of clock genes, such as the period circadian clock 2, causing circadian rhythm disruption (CRd) and associated neuropathologies. The present review discusses neuromodulation across the RHT originating from retinal photic inputs and modulation offered by endocannabinoids as a function of mitigation of the CRd and associated neuro-dysfunction. Literature indicates that cannabinoid agonists alleviate the SCN's ability to get entrained to light by modulating the activity of its chief neurotransmitter, i.e., γ-aminobutyric acid, thus preventing light-induced disruption of activity rhythms in laboratory animals. In the retina, endocannabinoid signaling modulates the overall gain of the retinal ganglion cells by regulating the membrane currents (Ca2+, K+, and Cl- channels) and glutamatergic neurotransmission of photoreceptors and bipolar cells. Additionally, endocannabinoids signalling also regulate the high-voltage-activated Ca2+ channels to mitigate the retinal ganglion cells and intrinsically photosensitive retinal ganglion cells-mediated glutamate release in the SCN, thus regulating the RHT-mediated light stimulation of SCN neurons to prevent excitotoxicity. As per the literature, cannabinoid receptors 1 and 2 are becoming newer targets in drug discovery paradigms, and the involvement of endocannabinoids in light-induced CRd through the RHT may possibly mitigate severe neuropathologies.

Unraveling the Endocannabinoid System: Exploring Its Therapeutic Potential in Autism Spectrum Disorder.

The salient features of autism spectrum disorder (ASD) encompass persistent difficulties in social communication, as well as the presence of restricted and repetitive facets of behavior, hobbies, or pursuits, which are often accompanied with cognitive limitations. Over the past few decades, a sizable number of studies have been conducted to enhance our understanding of the pathophysiology of ASD. Preclinical rat models have proven to be extremely valuable in simulating and analyzing the roles of a wide range of established environmental and genetic factors. Recent research has also demonstrated the significant involvement of the endocannabinoid system (ECS) in the pathogenesis of several neuropsychiatric diseases, including ASD. In fact, the ECS has the potential to regulate a multitude of metabolic and cellular pathways associated with autism, including the immune system. Moreover, the ECS has emerged as a promising target for intervention with high predictive validity. Particularly noteworthy are resent preclinical studies in rodents, which describe the onset of ASD-like symptoms after various genetic or pharmacological interventions targeting the ECS, providing encouraging evidence for further exploration in this area.

Late-adolescent onset of prefrontal endocannabinoid control of hippocampal and amygdalar inputs and its impact on trace-fear conditioning behavior.

Prefrontal cortex (PFC) maturation during adolescence is characterized by structural and functional changes, which involve the remodeling of GABA and glutamatergic synapses, as well as changes in the endocannabinoid system. Yet, the way PFC endocannabinoid signaling interacts with local GABA and glutamatergic function to impact its processing of afferent transmission during the adolescent transition to adulthood remains unknown. Here we combined PFC local field potential recordings with local manipulations of 2-AG and anandamide levels to assess how PFC endocannabinoid signaling is recruited to modulate ventral hippocampal and basolateral amygdalar inputs in vivo in adolescent and adult male rats. We found that the PFC endocannabinoid signaling does not fully emerge until late-adolescence/young adulthood. Once present, both 2-AG and anandamide can be recruited in the PFC to limit the impact of hippocampal drive through a CB1R-mediated mechanism whereas basolateral amygdalar inputs are only inhibited by 2-AG. Similarly, the behavioral effects of increasing 2-AG and anandamide in the PFC do not emerge until late-adolescence/young adulthood. Using a trace fear conditioning paradigm, we found that elevating PFC 2-AG levels preferentially reduced freezing behavior during acquisition without affecting its extinction. In contrast, increasing anandamide levels in the PFC selectively disrupted the extinction of trace fear memory without affecting its acquisition. Collectively, these results indicate a protracted recruitment of PFC endocannabinoid signaling, which becomes online in late adolescence/young adulthood as revealed by its impact on hippocampal and amygdalar-evoked local field potential responses and trace fear memory behavior.

Retrograde endocannabinoid signaling at inhibitory synapses in vivo.

Endocannabinoid (eCB)-mediated suppression of inhibitory synapses has been hypothesized, but this has not yet been demonstrated to occur in vivo because of the difficulty in tracking eCB dynamics and synaptic plasticity during behavior. In mice navigating a linear track, we observed location-specific eCB signaling in hippocampal CA1 place cells, and this was detected both in the postsynaptic membrane and the presynaptic inhibitory axons. All-optical in vivo investigation of synaptic responses revealed that postsynaptic depolarization was followed by a suppression of inhibitory synaptic potentials. Furthermore, interneuron-specific cannabinoid receptor deletion altered place cell tuning. Therefore, rapid, postsynaptic, activity-dependent eCB signaling modulates inhibitory synapses on a timescale of seconds during behavior.

Circadian Influence on Acute Stress-induced Changes in Cortico-limbic Endocannabinoid Levels in Adult Male Rats.

The endocannabinoid (eCB) system plays an important role in regulating the stress response, including glucocorticoid release and the generation of avoidance behaviour. Its two major ligands, 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (anandamide; AEA), are dynamically influenced by psychological stress to gate the generation of the stress response and facilitate recovery upon stress termination. Many biological systems exhibit circadian "daily" rhythms, including glucocorticoids and endocannabinoids, and the behavioural and endocrine impact of stress is modulated by the time of day. Nonetheless, most preclinical experiments investigating the interaction between stress and endocannabinoids occur in the light, "inactive" phase. We therefore tested if circadian phase influences stress-induced changes in eCB levels in the hippocampus (HIP), prefrontal cortex (PFC), and amygdala (AMY). Adult male rats were exposed to 15 min swim stress or immediately euthanized, and brains were collected. Testing occurred either early in the light or early in the dark phase of their cycle to compare circadian effects. We found that overall, stress decreased AEA in the AMY and HIP, with an effect in the PFC dependent on the time of day. Conversely, stress increased 2-AG in the AMY, with an effect in the PFC and HIP dependent on the time of day. This suggests that stress has a similar overall impact on eCB levels regardless of circadian phase, but that subtle differences may occur depending on the brain region, especially the PFC.

A cortico-amygdala neural substrate for endocannabinoid modulation of fear extinction.

Preclinical and clinical studies implicate endocannabinoids (eCBs) in fear extinction, but the underlying neural circuit basis of these actions is unclear. Here, we employed in vivo optogenetics, eCB biosensor imaging, ex vivo electrophysiology, and CRISPR-Cas9 gene editing in mice to examine whether basolateral amygdala (BLA)-projecting medial prefrontal cortex (mPFC) neurons represent a neural substrate for the effects of eCBs on extinction. We found that photoexcitation of mPFC axons in BLA during extinction mobilizes BLA eCBs. eCB biosensor imaging showed that eCBs exhibit a dynamic stimulus-specific pattern of activity at mPFC→BLA neurons that tracks extinction learning. Furthermore, using CRISPR-Cas9-mediated gene editing, we demonstrated that extinction memory formation involves eCB activity at cannabinoid CB1 receptors expressed at vmPFC→BLA synapses. Our findings reveal the temporal characteristics and a neural circuit basis of eCBs' effects on fear extinction and inform efforts to target the eCB system as a therapeutic approach in extinction-deficient neuropsychiatric disorders.

The emerging role of the endocannabinoidome-gut microbiome axis in eating disorders.

Among the sources of chemical signals regulating food intake, energy metabolism and body weight, few have attracted recently as much attention as the expanded endocannabinoid system, or endocannabinoidome (eCBome), and the gut microbiome, the two systems on which this review article is focussed. Therefore, it is legitimate to expect that these two systems also play a major role in the etiopathology of eating disorders (EDs), in particular of anorexia nervosa, bulimia nervosa and binge-eating disorder. The major mechanisms through which, also via interactions with other endogenous signaling systems, the eCBome, with its several lipid mediators and receptors, and the gut microbiome, via its variety of microbial kingdoms, phyla and species, and armamentarium of metabolites, intervene in these disorders, are described here, based on several published studies in either experimental models or patients. Additionally, in view of the emerging multi-faceted cross-talk mechanisms between these two complex systems, we discuss the possibility that the eCBome-gut microbiome axis is also involved in EDs.

Neuropharmacological Modulation of N-methyl-D-aspartate, Noradrenaline and Endocannabinoid Receptors in Fear Extinction Learning: Synaptic Transmission and Plasticity.

Learning to recognize and respond to potential threats is crucial for survival. Pavlovian threat conditioning represents a key paradigm for investigating the neurobiological mechanisms of fear learning. In this review, we address the role of specific neuropharmacological adjuvants that act on neurochemical synaptic transmission, as well as on brain plasticity processes implicated in fear memory. We focus on novel neuropharmacological manipulations targeting glutamatergic, noradrenergic, and endocannabinoid systems, and address how the modulation of these neurobiological systems affects fear extinction learning in humans. We show that the administration of N-methyl-D-aspartate (NMDA) agonists and modulation of the endocannabinoid system by fatty acid amide hydrolase (FAAH) inhibition can boost extinction learning through the stabilization and regulation of the receptor concentration. On the other hand, elevated noradrenaline levels dynamically modulate fear learning, hindering long-term extinction processes. These pharmacological interventions could provide novel targeted treatments and prevention strategies for fear-based and anxiety-related disorders.

Endocannabinoids at the synapse and beyond: implications for neuropsychiatric disease pathophysiology and treatment.

Endocannabinoids (eCBs) are lipid neuromodulators that suppress neurotransmitter release, reduce postsynaptic excitability, activate astrocyte signaling, and control cellular respiration. Here, we describe canonical and emerging eCB signaling modes and aim to link adaptations in these signaling systems to pathological states. Adaptations in eCB signaling systems have been identified in a variety of biobehavioral and physiological process relevant to neuropsychiatric disease states including stress-related disorders, epilepsy, developmental disorders, obesity, and substance use disorders. These insights have enhanced our understanding of the pathophysiology of neurological and psychiatric disorders and are contributing to the ongoing development of eCB-targeting therapeutics. We suggest future studies aimed at illuminating how adaptations in canonical as well as emerging cellular and synaptic modes of eCB signaling contribute to disease pathophysiology or resilience could further advance these novel treatment approaches.

Role of CB1 receptors in the acute regulation of small intestinal permeability: effects of high-fat diet.

The endocannabinoid system of the gastrointestinal tract is involved in the control of intestinal barrier function. Whether the cannabinoid 1 (CB1) receptor is expressed on the intestinal epithelium and acutely regulates barrier function has not been determined. Here, we tested the hypothesis that ligands of the CB1 receptor acutely modulate small intestinal permeability and that this is associated with altered distribution of tight junction proteins. We examined the acute effects of CB1 receptor ligands on small intestinal permeability both in chow-fed and 2-wk high-fat diet (HFD)-fed mice using Ussing chambers. We assessed the distribution of CB1 receptor and tight junction proteins using immunofluorescence and the expression of CB1 receptor using PCR. A low level of CB1 expression was found on the intestinal epithelium. CB1 receptor was highly expressed on enteric nerves in the lamina propria. Neither the CB1/CB2 agonist CP55,940 nor the CB1 neutral antagonist AM6545 altered the flux of 4kDa FITC dextran (FD4) across the jejunum or ileum of chow-fed mice. Remarkably, both CP55,940 and AM6545 reduced FD4 flux across the jejunum and ileum in HFD-fed mice that have elevated baseline intestinal permeability. These effects were absent in CB1 knockout mice. CP55,940 reduced the expression of claudin-2, whereas AM6545 had little effect on claudin-2 expression. Neither ligand altered the expression of ZO-1. Our data suggest that CB1 receptor on the intestinal epithelium regulates tight junction protein expression and restores barrier function when it is increased following exposure to a HFD for 2 wk.NEW & NOTEWORTHY The endocannabinoid system of the gastrointestinal tract regulates homeostasis by acting as brake on motility and secretion. Here we show that when exposed to a high fat diet, intestinal permeability is increased and activation of the CB1 receptor on the intestinal epithelium restores barrier function. This work further highlights the role of the endocannabinoid system in regulating intestinal homeostasis when it is perturbed.

Circadian regulation of memory under stress: Endocannabinoids matter.

Organisms ranging from plants to higher mammals have developed 24-hour oscillation rhythms to optimize physiology to environmental changes and regulate a plethora of neuroendocrine and behavioral processes, including neurotransmitter and hormone regulation, stress response and learning and memory function. Compelling evidence indicates that a wide array of memory processes is strongly influenced by stress- and emotional arousal-activated neurobiological systems, including the endocannabinoid system which has been extensively shown to play an integral role in mediating stress effects on memory. Here, we review findings showing how circadian rhythms and time-of-day influence stress systems and memory performance. We report evidence of circadian regulation of memory under stress, focusing on the role of the endocannabinoid system and highlighting its circadian rhythmicity. Our discussion illustrates how the endocannabinoid system mediates stress effects on memory in a circadian-dependent fashion. We suggest that endocannabinoids might regulate molecular mechanisms that control memory function under circadian and stress influence, with potential important clinical implications for both neurodevelopmental disorders and psychiatric conditions involving memory impairments.

Endocannabinoid and Nitric Oxide-Dependent IGF-I-Mediated Synaptic Plasticity at Mice Barrel Cortex.

Insulin-like growth factor-I (IGF-I) signaling plays a key role in learning and memory. IGF-I increases the spiking and induces synaptic plasticity in the mice barrel cortex (Noriega-Prieto et al., 2021), favoring the induction of the long-term potentiation (LTP) by Spike Timing-Dependent Protocols (STDP) (Noriega-Prieto et al., 2021). Here, we studied whether these IGF-I effects depend on endocannabinoids (eCBs) and nitric oxide (NO). We recorded both excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) evoked by stimulation of the basal dendrites of layer II/III pyramidal neurons of the Barrel Cortex and analyzed the effect of IGF-I in the presence of a CB1R antagonist, AM251, and inhibitor of the NO synthesis, L-NAME, to prevent the eCBs and the NO-mediated signaling. Interestingly, L-NAME abolished any modulatory effect of the IGF-I-induced excitatory and inhibitory transmission changes, suggesting the essential role of NO. Surprisingly, the inhibition of CB1Rs did not only block the potentiation of EPSCs but reversed to a depression, highlighting the remarkable functions of the eCB system. In conclusion, eCBs and NO play a vital role in deciding the sign of the effects induced by IGF-I in the neocortex, suggesting a neuromodulatory interplay among IGF-I, NO, and eCBs.

The Autism-Psychosis Continuum Conundrum: Exploring the Role of the Endocannabinoid System.

Evidence indicates shared physiopathological mechanisms between autism and psychosis. In this regard, the endocannabinoid system has been suggested to modulate neural circuits during the early stage of neurodevelopment, with implications for both autism and psychosis. Nevertheless, such potential common markers of disease have been investigated in both autism and psychosis spectrum disorders, without considering the conundrum of differentiating the two groups of conditions in terms of diagnosis and treatment. Here, we systematically review all human and animal studies examining the endocannabinoid system and its biobehavioral correlates in the association between autism and psychosis. Studies indicate overlapping biobehavioral aberrancies between autism and schizophrenia, subject to correction by modulation of the endocannabinoid system. In addition, common cannabinoid-based pharmacological strategies have been identified, exerting epigenetic effects across genes controlling neural mechanisms shared between autism and schizophrenia. Interestingly, a developmental and transgenerational trajectory between autism and schizophrenia is supported by evidence that exogenous alteration of the endocannabinoid system promotes progression to inheritable psychosis phenotypes in the context of biobehavioral autism vulnerability. However, evidence for a diametral association between autism and psychosis is scant. Several clinical implications follow from evidence of a developmental continuum between autism and psychosis as a function of the endocannabinoid system dysregulation.

Molecular Alterations of the Endocannabinoid System in Psychiatric Disorders.

The therapeutic benefits of the current medications for patients with psychiatric disorders contrast with a great variety of adverse effects. The endocannabinoid system (ECS) components have gained high interest as potential new targets for treating psychiatry diseases because of their neuromodulator role, which is essential to understanding the regulation of many brain functions. This article reviewed the molecular alterations in ECS occurring in different psychiatric conditions. The methods used to identify alterations in the ECS were also described. We used a translational approach. The animal models reproducing some behavioral and/or neurochemical aspects of psychiatric disorders and the molecular alterations in clinical studies in post-mortem brain tissue or peripheral tissues were analyzed. This article reviewed the most relevant ECS changes in prevalent psychiatric diseases such as mood disorders, schizophrenia, autism, attentional deficit, eating disorders (ED), and addiction. The review concludes that clinical research studies are urgently needed for two different purposes: (1) To identify alterations of the ECS components potentially useful as new biomarkers relating to a specific disease or condition, and (2) to design new therapeutic targets based on the specific alterations found to improve the pharmacological treatment in psychiatry.

Modulation of Endocannabinoid System Components in Depression: Pre-Clinical and Clinical Evidence.

Depression is characterized by continuous low mood and loss of interest or pleasure in enjoyable activities. First-line medications for mood disorders mostly target the monoaminergic system; however, many patients do not find relief with these medications, and those who do suffer from negative side effects and a discouragingly low rate of remission. Studies suggest that the endocannabinoid system (ECS) may be involved in the etiology of depression and that targeting the ECS has the potential to alleviate depression. ECS components (such as receptors, endocannabinoid ligands, and degrading enzymes) are key neuromodulators in motivation and cognition as well as in the regulation of stress and emotions. Studies in depressed patients and in animal models for depression have reported deficits in ECS components, which is motivating researchers to identify potential diagnostic and therapeutic biomarkers within the ECS. By understanding the effects of cannabinoids on ECS components in depression, we enhance our understanding of which brain targets they hit, what biological processes they alter, and eventually how to use this information to design better therapeutic options. In this article, we discuss the literature on the effects of cannabinoids on ECS components of specific depression-like behaviors and phenotypes in rodents and then describe the findings in depressed patients. A better understanding of the effects of cannabinoids on ECS components in depression may direct future research efforts to enhance diagnosis and treatment.

Effects of endocannabinoid system modulation on social behaviour: A systematic review of animal studies.

There is a clear link between psychiatric disorders and social behaviour, and evidence suggests the involvement of the endocannabinoid system (ECS). A systematic review of preclinical literature was conducted using MEDLINE (PubMed) and PsychINFO databases to examine whether pharmacological and/or genetic manipulations of the ECS alter social behaviours in wildtype (WT) animals or models of social impairment (SIM). Eighty studies were included. Risk of bias (RoB) was assessed using SYRCLE's RoB tool. While some variability was evident, studies most consistently found that direct cannabinoid receptor (CBR) agonism decreased social behaviours in WT animals, while indirect CBR activation via enzyme inhibition or gene-knockout increased social behaviours. Direct and, more consistently, indirect CBR activation reversed social deficits in SIM. These CBR-mediated effects were often sex- and developmental-phase-dependent and blocked by CBR antagonism. Overall, ECS enzyme inhibition may improve social behaviour in SIM, suggesting the potential usefulness of ECS enzyme inhibition as a therapeutic approach for social deficits. Future research should endeavour to elucidate ECS status in neuropsychiatric disorders characterized by social deficits.

A narrative review of molecular mechanism and therapeutic effect of cannabidiol (CBD).

Cannabidiol (CBD) is an abundant non-psychoactive phytocannabinoid in cannabis extracts which has high affinity on a series of receptors, including Type 1 cannabinoid receptor (CB1), Type 2 cannabinoid receptor (CB2), GPR55, transient receptor potential vanilloid (TRPV) and peroxisome proliferator-activated receptor gamma (PPARγ). By modulating the activities of these receptors, CBD exhibits multiple therapeutic effects, including neuroprotective, antiepileptic, anxiolytic, antipsychotic, anti-inflammatory, analgesic and anticancer properties. CBD could also be applied to treat or prevent COVID-19 and its complications. Here, we provide a narrative review of CBD's applications in human diseases: from mechanism of action to clinical trials.

(Wh)olistic (E)ndocannabinoidome-Microbiome-Axis Modulation through (N)utrition (WHEN) to Curb Obesity and Related Disorders.

The discovery of the endocannabinoidome (eCBome) is evolving gradually with yet to be elucidated functional lipid mediators and receptors. The diet modulates these bioactive lipids and the gut microbiome, both working in an entwined alliance. Mounting evidence suggests that, in different ways and with a certain specialisation, lipid signalling mediators such as N-acylethanolamines (NAEs), 2-monoacylglycerols (2-MAGs), and N-acyl-amino acids (NAAs), along with endocannabinoids (eCBs), can modulate physiological mechanisms underpinning appetite, food intake, macronutrient metabolism, pain sensation, blood pressure, mood, cognition, and immunity. This knowledge has been primarily utilised in pharmacology and medicine to develop many drugs targeting the fine and specific molecular pathways orchestrating eCB and eCBome activity. Conversely, the contribution of dietary NAEs, 2-MAGs and eCBs to the biological functions of these molecules has been little studied. In this review, we discuss the importance of (Wh) olistic (E)ndocannabinoidome-Microbiome-Axis Modulation through (N) utrition (WHEN), in the management of obesity and related disorders.

Approaches to studying injury-induced sensitization and the potential role of an endocannabinoid transmitter.

Endocannabinoids are traditionally thought to have an analgesic effect. However, it has been shown that while endocannabinoids can depress nociceptive signaling, they can also enhance non-nociceptive signaling. Therefore, endocannabinoids have the potential to contribute to non-nociceptive sensitization after an injury. Using Hirudo verbana (the medicinal leech), a model of injury-induced sensitization was developed in which a reproducible piercing injury was delivered to the posterior sucker of Hirudo. Injury-induced changes in the non-nociceptive threshold of Hirudo were determined through testing with Von Frey filaments and changes in the response to nociceptive stimuli were tested by measuring the latency to withdraw to a nociceptive thermal stimulus (Hargreaves apparatus). To test the potential role of endocannabinoids in mediating injury-induced sensitization, animals were injected with tetrahydrolipstatin (THL), which inhibits synthesis of the endocannabinoid transmitter 2-arachidonoylglycerol (2-AG). Following injury, a significant decrease in the non-nociceptive response threshold (consistent with non-nociceptive sensitization) and a significant decrease in the response latency to nociceptive stimulation (consistent with nociceptive sensitization) were observed. In animals injected with THL, a decrease in non-nociceptive sensitization in injured animals was observed, but no effect on nociceptive sensitization was observed.