Cannabinoid receptor CB2 modulates axon guidance

Modeling the Effect of Cannabinoid Exposure During Human Neurodevelopment Using Bidimensional and Tridimensional Cultures

Our knowledge about the consumption of cannabinoids during pregnancy lacks consistent evidence to determine whether it compromises neurodevelopment. Addressing this task is challenging and complex since pregnant women display multiple confounding factors that make it difficult to identify the real effect of cannabinoids' consumption. Recent studies shed light on this issue by using pluripotent stem cells of human origin, which can recapitulate human neurodevelopment. These revolutionary platforms allow studying how exogenous cannabinoids could alter human neurodevelopment without ethical concerns and confounding factors. Here, we review the information to date on the clinical studies about the impact of exogenous cannabinoid consumption on human brain development and how exogenous cannabinoids alter nervous system development in humans using cultured pluripotent stem cells as 2D and 3D platforms to recapitulate brain development.

Cannabis, Endocannabinoids and Brain Development: From Embryogenesis to Adolescence

The endocannabinoid signalling system (ECS) plays a critical role from the very beginning of embryogenesis. Accordingly, the ECS is engaged early on in nervous system development, starting from neurulation, supported by the identification of ECS components-both receptors and enzymes controlling endocannabinoid metabolism-at these early stages. In particular, regarding the brain, the ECS is involved in the tightly regulated sequence of events that comprise brain development, from neurogenesis to neuronal migration, morphological guidance for neuronal connectivity, and synaptic circuitry refinement. The importance of this broad role of the ECS across various brain development processes is further underscored by the growing understanding of the consequences of cannabis exposure at different developmental stages. Despite the considerable knowledge we have on the role of the ECS in brain development, significant gaps in our understanding remain, particularly regarding the long-term impact and underlying mechanisms of cannabis exposure at different developmental stages. This review provides an overview of the current state of knowledge on the role of the ECS throughout brain development, from embryogenesis to adulthood, and discusses the impact of cannabis exposure, especially during adolescence-a critical period of circuitry maturation and refinement coinciding with an increased risk of cannabis use.

Evaluating the Role of the Endocannabinoid System in Axon Guidance: A Literature Review

Introduction: The endocannabinoid system (ECS) mediates the actions of cannabis and has been implicated in playing critical roles in key developmental events, including axon guidance. Although several recent studies have demonstrated ECS involvement in neurodevelopment, an emphasis on its putative role in axon guidance has not been reviewed comprehensively. Objective: The purpose of this literature review is to evaluate the interrelationships between the ECS and axon guidance. Methodology: This literature review analyzes existing literature demonstrating the normal role of endocannabinoid (eCB) signaling in axon guidance, with evidence from diverse animal models. Studies were obtained from a search strategy involving terms related to the ECS and axon guidance, and cross-checking cited literature to ensure a complete evaluation. Discussion: Cannabinoid receptors, as well as eCB synthesis and degradation machinery, appear necessary for normal axon guidance during neurodevelopment. Genetic and/or pharmacological disruption of eCB signaling results in axon growth and guidance errors, implying high sensitivity to exogenous cannabinoids. Conclusion: Overall, this review highlights the intricate connections between the ECS and axon guidance in normal neurodevelopment. The mechanistic evidence discussed suggests that alterations of the ECS through genetic and pharmacological interference disrupt its normal functioning and by extension its normal role in regulating neural circuitry formation. A comprehensive understanding of this topic will be valuable in potentially uncovering the mechanisms responsible for the neurodevelopmental defects associated with pre-natal cannabis use.

A simple method for poly-D-lysine coating to enhance adhesion and maturation of primary cortical neuron cultures in vitro

Introduction

Glass coverslips are used as a substrate since Harrison's initial nerve cell culture experiments in 1910. In 1974, the first study of brain cells seeded onto polylysine (PL) coated substrate was published. Usually, neurons adhere quickly to PL coating. However, maintaining cortical neurons in culture on PL coating for a prolonged time is challenging.

Methods

A collaborative study between chemical engineers and neurobiologists was conducted to find a simple method to enhance neuronal maturation on poly-D-lysine (PDL). In this work, a simple protocol to coat PDL efficiently on coverslips is presented, characterized, and compared to a conventional adsorption method. We studied the adhesion and maturation of primary cortical neurons with various morphological and functional approaches, including phase contrast microscopy, immunocytochemistry, scanning electron microscopy, patch clamp recordings, and calcium imaging.

Results

We observed that several parameters of neuronal maturation are influenced by the substrate: neurons develop more dense and extended networks and synaptic activity is enhanced, when seeded on covalently bound PDL compared to adsorbed PDL.

Discussion

Hence, we established reproducible and optimal conditions enhancing maturation of primary cortical neurons in vitro. Our method allows higher reliability and yield of results and could also be profitable for laboratories using PL with other cell types.

Injury-induced activation of the endocannabinoid system promotes axon regeneration

Regeneration after a peripheral nerve injury still remains a challenge, due to the limited regenerative potential of axons after injury. While the endocannabinoid system (ECS) has been widely studied for its neuroprotective and analgesic effects, its role in axonal regeneration and during the conditioning lesion remains unexplored. In this study, we observed that a peripheral nerve injury induces axonal regeneration through an increase in the endocannabinoid tone. We also enhanced the regenerative capacity of dorsal root ganglia (DRG) neurons through the inhibition of endocannabinoid degradative enzyme MAGL or a CB1R agonist. Our results suggest that the ECS, via CB1R and PI3K-pAkt pathway activation, plays an important role in promoting the intrinsic regenerative capacity of sensory neurons after injury.

Effects of Prenatal Cannabinoids Exposure upon Placenta and Development of Respiratory Neural Circuits

Cannabis use has risen dangerously during pregnancy in the face of incipient therapeutic use and a growing perception of safety. The main psychoactive compound of the Cannabis sativa plant is the phytocannabinoid delta-9-tetrahydrocannabinol (A-9 THC), and its status as a teratogen is controversial. THC and its endogenous analogues, anandamide (AEA) and 2-AG, exert their actions through specific receptors (eCBr) that activate intracellular signaling pathways. CB1r and CB2r, also called classic cannabinoid receptors, together with their endogenous ligands and the enzymes that synthesize and degrade them, constitute the endocannabinoid system. This system is distributed ubiquitously in various central and peripheral tissues. Although the endocannabinoid system's most studied role is controlling the release of neurotransmitters in the central nervous system, the study of long-term exposure to cannabinoids on fetal development is not well known and is vital for understanding environmental or pathological embryo-fetal or postnatal conditions. Prenatal exposure to cannabinoids in animal models has induced changes in placental and embryo-fetal organs. Particularly, cannabinoids could influence both neural and nonneural tissues and induce embryo-fetal pathological conditions in critical processes such as neural respiratory control. This review aims at the acute and chronic effects of prenatal exposure to cannabinoids on placental function and the embryo-fetal neurodevelopment of the respiratory pattern. The information provided here will serve as a theoretical framework to critically evaluate the teratogen effects of the consumption of cannabis during pregnancy.

The Endocannabinoid System in Caenorhabditis elegans

The existence of a formal Endocannabinoid System in C. elegans has been questioned due to data showing the absence of typical cannabinoid receptors in the worm; however, the presence of a full metabolism for endocannabinoids, alternative ligands, and receptors for these agents and a considerable number of orthologous and homologous genes regulating physiological cannabinoid-like signals and responses - several of which are similar to those of mammals - demonstrates a well-structured and functional complex system in nematodes. In this review, we describe and compare similarities and differences between the Endocannabinoid System in mammals and nematodes, highlighting the basis for the integral study of this novel system in the worm.

Cannabinoid receptor 2 (Cb2r) mediates cannabinol (CBN) induced developmental defects in zebrafish

Of the three primary cannabinoids in cannabis: Δ⁹-Tetrahydrocannabinol (Δ⁹-THC), cannabidiol (CBD) and cannabinol (CBN), very little is known about the actions of CBN, the primary oxidative metabolite of THC. Our goal was to determine if CBN exposure during gastrulation alters embryonic development, and if so, does it act via the canonical cannabinoid receptors. Zebrafish embryos were exposed to CBN during gastrulation and exhibited dose-dependent malformations, increased mortality, decreased locomotion and a reduction in motor neuron branching. Moreover, larva showed a significant reduction in the response to sound stimuli. CBN exposure altered the development of hair cells associated with otic vesicles and the lateral line. Pharmacological block of Cb2rs with AM 630 or JTE 907 prevented many of the CBN-induced developmental defects, while block of Cb1rs with AM 251 or CP 945598 had little or no effect. Altogether we show that embryonic exposure to CBN results in alterations in embryonic growth, neuronal and hair cell development, physiology and behavior via Cb2r-mediated mechanisms.

Animal evidence considered in determination of cannabis smoke and Δ9 -tetrahydrocannabinol as causing reproductive toxicity (developmental endpoint): Part III. Proposed neurodevelopmental mechanisms of action

This review summarizes the most common potential pathways of neurodevelopmental toxicity due to perinatal exposure to Δ⁹ -tetrahydrocannabinol (Δ⁹ -THC) that lead to behavioral and other adverse outcomes (AOs). This is Part III in a set of reviews highlighting the animal-derived data considered by California's Developmental and Reproductive Toxicant Identification Committee (DARTIC) in 2019. The Hazard Identification Document (HID) provided to the DARTIC included a summary of human, whole animal, and mechanistic data on the neurodevelopmental toxicity of cannabis smoke and Δ⁹ -THC. The literature search for mechanistic data has been updated through 2020. We focus on mechanistic pathways relating to behavioral and other neurodevelopmental outcomes of perinatal exposure to Δ⁹ -THC. The endocannabinoid system (EC system) plays a crucial role in many processes involved in neurodevelopment and exposure to Δ⁹ -THC can alter these processes. Whole animal studies report changes in cognitive ability, behavior, and motor function after prenatal exposure to Δ⁹ -THC. Findings from mechanistic studies add to this evidence and further provide information regarding the pathways leading to these outcomes. Neuromechanistic studies can bridge the gaps between molecular initiating events and apical neurodevelopmental endpoints caused by a chemical. They offer insight into potential alterations in the same pathways by other chemicals that can also result in AOs. Studies of cannabinoid receptor agonist-induced molecular alterations and provide deep biological plausibility at the mechanistic level for the cognitive, behavioral, and motor impairments observed in animal studies after perinatal exposure to Δ⁹ -THC.

Cannabinoid Receptor 2 (CB2) Inverse Agonist SMM-189 Induces Expression of Endogenous CB2 and Protein Kinase A That Differentially Modulates the Immune Response and Suppresses Experimental Colitis

The causes of Crohn’s disease (CD) and ulcerative colitis (UC), the two most common forms of inflammatory bowel disease (IBD), are multi-factorial and include dysregulation of immune cells in the intestine. Cannabinoids mediate protection against intestinal inflammation by binding to the G-protein coupled cannabinoid receptors 1 and 2 (CB1 and CB2). Here, we investigate the effects of the CB2 inverse agonist SMM-189 on dextran sodium sulfate (DSS)-induced experimental colitis. We observed that SMM-189 effectively attenuated the overall clinical score, reversed colitis-associated pathogenesis, and increased both body weight and colon length. Treatment with SMM-189 also increased the expression of CB2 and protein kinase A (PKA) in colon lamina propria lymphocytes (LPLs). We noticed alterations in the percentage of Th17, neutrophils, and natural killer T (NKT) cells in the spleen, mesenteric lymph nodes (MLNs), and LPLs of mice with DSS-induced colitis after treatment with SMM-189 relative to DSS alone. Further, myeloid-derived suppressor cells (MDSCs) during colitis progression increased with SMM-189 treatment as compared to DSS alone or with control cohorts. These findings suggest that SMM-189 may ameliorate experimental colitis by inducing the expression of endogenous CB2 and PKA in LPLs, increasing numbers of MDSCs in the spleen, and reducing numbers of Th17 cells and neutrophils in the spleen, MLNs, and LPLs. Taken together, these data support the idea that SMM-189 may be developed as a safe novel therapeutic target for IBD.

The Vertical and Horizontal Pathways in the Monkey Retina Are Modulated by Typical and Atypical Cannabinoid Receptors

The endocannabinoid (eCB) system has been found in all visual parts of the central ner-vous system and plays a role in the processing of visual information in many species, including monkeys and humans. Using anatomical methods, cannabinoid receptors are present in the monkey retina, particularly in the vertical glutamatergic pathway, and also in the horizontal GABAergic pathway. Modulating the eCB system regulates normal retinal function as demonstrated by electrophysiological recordings. The characterization of the expression patterns of all types of cannabinoid receptors in the retina is progressing, and further research is needed to elucidate their exact role in processing visual information. Typical cannabinoid receptors include G-protein coupled receptor CB1R and CB2R, and atypical cannabinoid receptors include the G-protein coupled receptor 55 (GPR55) and the ion channel transient receptor potential vanilloid 1 (TRPV1). This review focuses on the expression and localization studies carried out in monkeys, but some data on other animal species and humans will also be reported. Furthermore, the role of the endogenous cannabinoid receptors in retinal function will also be presented using intraocular injections of known modulators (agonists and antagonists) on electroretinographic patterns in monkeys. The effects of the natural bioactive lipid lysophosphatidylglucoside and synthetic FAAH inhibitor URB597 on retinal function, will also be described. Finally, the potential of typical and atypical cannabinoid receptor acti-vity regulation in retinal diseases, such as age-related macular degeneration, diabetic retinopathy, glaucoma, and retinitis pigmentosa will be briefly explored.

Physiological Rules of Endocannabinoid Action During Fetal and Neonatal Brain Development

The endocannabinoid system is chiefly recognized as a homeostatic regulator of synaptic neurotransmission, primarily through the modulation of presynaptic CB₁ cannabinoid neurons. Accordingly, the use of plant-derived cannabinoids received significant attention recently given the broad spectrum of physiological and pathobiological processes the endocannabinoid system is involved in. Nevertheless, a parallel line of research from a number of developmental biology groups has uncovered fundamental, evolutionarily conserved, and molecularly unique processes that endocannabinoids drive during development of the central nervous system. This lecture transcript is a concise summary of nearly 20 years of research on endocannabinoid-gated mechanisms of neurogenic specification events, which particularly define the numbers, placement, and connectivity of cortical neurons. A summary of both CB₁ and alternative cannabinoid receptor contributions to neural differentiation is also discussed. Besides, insights are given into how phytocannabinoids can bypass physiologically timed and pivoted endocannabinoid action to inflict developmental errors that can significantly compromise the adaptive and computational ability of neurocircuits. By discussing specific subcellular targets of phytocannabinoid action and inferring errant glia versus neuron fate decisions and communication, a cellular basis is outlined for lifelong psychiatric phenotypes in offspring that associate with maternal cannabis seeking during pregnancy.

Participation of L-Lactate and Its Receptor HCAR1/GPR81 in Neurovisual Development

During the development of the retina and the nervous system, high levels of energy are required by the axons of retinal ganglion cells (RGCs) to grow towards their brain targets. This energy demand leads to an increase of glycolysis and L-lactate concentrations in the retina. L-lactate is known to be the endogenous ligand of the GPR81 receptor. However, the role of L-lactate and its receptor in the development of the nervous system has not been studied in depth. In the present study, we used immunohistochemistry to show that GPR81 is localized in different retinal layers during development, but is predominantly expressed in the RGC of the adult rodent. Treatment of retinal explants with L-lactate or the exogenous GPR81 agonist 3,5-DHBA altered RGC growth cone (GC) morphology (increasing in size and number of filopodia) and promoted RGC axon growth. These GPR81-mediated modifications of GC morphology and axon growth were mediated by protein kinases A and C, but were absent in explants from gpr81-/- transgenic mice. Living gpr81-/- mice showed a decrease in ipsilateral projections of RGCs to the dorsal lateral geniculate nucleus (dLGN). In conclusion, present results suggest that L-lactate and its receptor GPR81 play an important role in the development of the visual nervous system.

Potential and Limits of Cannabinoids in Alzheimer's Disease Therapy

Simple Summary

This review was aimed at exploring the potentiality of drugging the endocannabinoid system as a therapeutic option for Alzheimer’s disease (AD). Recent discoveries have demonstrated how the modulation of cannabinoid receptor 1 (CB1) and receptor 2 (CB2) can exert neuroprotective effects without the recreational and pharmacological properties of Cannabis sativa. Thus, this review explores the potential of cannabinoids in AD, also highlighting their limitations in perspective to point out the need for further research on cannabinoids in AD therapy.

Abstract

Alzheimer’s disease (AD) is a detrimental brain disorder characterized by a gradual cognitive decline and neuronal deterioration. To date, the treatments available are effective only in the early stage of the disease. The AD etiology has not been completely revealed, and investigating new pathological mechanisms is essential for developing effective and safe drugs. The recreational and pharmacological properties of marijuana are known for centuries, but only recently the scientific community started to investigate the potential use of cannabinoids in AD therapy—sometimes with contradictory outcomes. Since the endocannabinoid system (ECS) is highly expressed in the hippocampus and cortex, cannabis use/abuse has often been associated with memory and learning dysfunction in vulnerable individuals. However, the latest findings in AD rodent models have shown promising effects of cannabinoids in reducing amyloid plaque deposition and stimulating hippocampal neurogenesis. Beneficial effects on several dementia-related symptoms have also been reported in clinical trials after cannabinoid treatments. Accordingly, future studies should address identifying the correct therapeutic dosage and timing of treatment from the perspective of using cannabinoids in AD therapy. The present paper aims to summarize the potential and limitations of cannabinoids as therapeutics for AD, focusing on recent pre-clinical and clinical evidence.

Cannabinoid Signaling in Auditory Function and Development

Plants of the genus Cannabis have been used by humans for millennia for a variety of purposes. Perhaps most notable is the use of certain Cannabis strains for their psychoactive effects. More recently, several biologically active molecules within the plants of these Cannabis strains, called phytocannabinoids or simply cannabinoids, have been identified. Furthermore, within human cells, endogenous cannabinoids, or endocannabinoids, as well as the receptors and secondary messengers that give rise to their neuromodulatory effects, have also been characterized. This endocannabinoid system (ECS) is composed of two primary ligands-anandamide and 2-arachidonyl glycerol; two primary receptors-cannabinoid receptors 1 and 2; and several enzymes involved in biosynthesis and degradation of endocannabinoid ligands including diacylglycerol lipase (DAGL) and monoacylglycerol lipase (MAGL). Here we briefly summarize cannabinoid signaling and review what has been discerned to date with regard to cannabinoid signaling in the auditory system and its roles in normal physiological function as well as pathological conditions. While much has been uncovered regarding cannabinoid signaling in the central nervous system, less attention has been paid to the auditory system specifically. Still, evidence is emerging to suggest that cannabinoid signaling is critical for the development, maturation, function, and survival of cochlear hair cells (HCs) and spiral ganglion neurons (SGNs). Furthermore, cannabinoid signaling can have profound effects on synaptic connectivity in CNS structures related to auditory processing. While clinical cases demonstrate that endogenous and exogenous cannabinoids impact auditory function, this review highlights several areas, such as SGN development, where more research is warranted.

Linking mPFC circuit maturation to the developmental regulation of emotional memory and cognitive flexibility

The medial prefrontal cortex (mPFC) and its abundant connections with other brain regions play key roles in memory, cognition, decision making, social behaviors, and mood. Dysfunction in mPFC is implicated in psychiatric disorders in which these behaviors go awry. The prolonged maturation of mPFC likely enables complex behaviors to emerge, but also increases their vulnerability to disruption. Many foundational studies have characterized either mPFC synaptic or behavioral development without establishing connections between them. Here, we review this rich body of literature, aligning major events in mPFC development with the maturation of complex behaviors. We focus on emotional memory and cognitive flexibility, and highlight new work linking mPFC circuit disruption to alterations of these behaviors in disease models. We advance new hypotheses about the causal connections between mPFC synaptic development and behavioral maturation and propose research strategies to establish an integrated understanding of neural architecture and behavioral repertoires.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

Cannabinoid Receptor Modulation of Neurogenesis: ST14A Striatal Neural Progenitor Cells as a Simplified In Vitro Model

The endocannabinoid system (ECS) is involved in the modulation of several basic biological processes, having widespread roles in neurodevelopment, neuromodulation, immune response, energy homeostasis and reproduction. In the adult central nervous system (CNS) the ECS mainly modulates neurotransmitter release, however, a substantial body of evidence has revealed a central role in regulating neurogenesis in developing and adult CNS, also under pathological conditions. Due to the complexity of investigating ECS functions in neural progenitors in vivo, we tested the suitability of the ST14A striatal neural progenitor cell line as a simplified in vitro model to dissect the role and the mechanisms of ECS-regulated neurogenesis, as well as to perform ECS-targeted pharmacological approaches. We report that ST14A cells express various ECS components, supporting the presence of an active ECS. While CB1 and CB2 receptor blockade did not affect ST14A cell number, exogenous administration of the endocannabinoid 2-AG and the synthetic CB2 agonist JWH133 increased ST14A cell proliferation. Phospholipase C (PLC), but not PI3K pharmacological blockade negatively modulated CB2-induced ST14A cell proliferation, suggesting that a PLC pathway is involved in the steps downstream to CB2 activation. On the basis of our results, we propose ST14A neural progenitor cells as a useful in vitro model for studying ECS modulation of neurogenesis, also in prospective in vivo pharmacological studies.

Beta-Caryophyllene, a CB2R Selective Agonist, Protects Against Cognitive Impairment Caused by Neuro-inflammation and Not in Dementia Due to Ageing Induced by Mitochondrial Dysfunction

BACKGROUND

Dementia is a neurodegenerative disorder majorly evidenced by cognitive impairment. Although there are many types of dementia, the common underlying etiological factors in all the types are neuro-inflammation or aging induced apoptosis. β-caryophyllene, a cannabinoid type-2 receptor agonist, has been reported to have promising neuroprotective effects in cerebral ischemia and neuro-inflammation.

OBJECTIVE

In the present study, we evaluated the effects of β-caryophyllene against animal models of dementia whose etiology mimicked neuro-inflammation and aging.

METHODS

Two doses (50 and 100 mg/kg of body weight) of β-caryophyllene given orally were tested against AlCl₃-induced dementia in male Sprague Dawley (SD) rats using the Morris water maze test. Subsequently, the effect of the drug was assessed for episodic memory in female SD rats using novel object recognition task in doxorubicin-induced neuro-inflammation and chemobrain model. Moreover, its effects were evaluated in D-galactose-induced mitochondrial dysfunction leading to dementia.

RESULTS

β-caryophyllene, at both doses, showed significant improvement in memory when assessed using parameters like target quadrant entries, escape latency and path efficiency in the Morris water maze test for spatial memory. In the doxorubicin-induced chemobrain model, β-caryophyllene at 100 mg/kg significantly elevated acetylcholinesterase and catalase levels and lowered lipid peroxidation compared to the disease control. In the novel object recognition task, β-caryophyllene at 100 mg/kg significantly improved recognition index and discrimination index in the treated animals compared to the disease control, with a significant increase in catalase and a decrease in lipid peroxidation in both hippocampus and frontal cortex. However, in the D-galactose-induced mitochondrial dysfunction model, β-caryophyllene failed to show positive effects when spatial memory was assessed. It also failed to improve D-galactose-induced diminished mitochondrial complex I and II activities.

CONCLUSION

Hence, we conclude that β-caryophyllene at 100 mg/kg protects against dementia induced by neuro-inflammation with no effect on neuronal aging induced by mitochondrial dysfunction.

Low Basal CB2R in Dopamine Neurons and Microglia Influences Cannabinoid Tetrad Effects

There are two well-characterized cannabinoid receptors (CB1R and CB2R and other candidates): the central nervous system (CNS) enriched CB1R and peripheral tissue enriched CB2R with a wide dynamic range of expression levels in different cell types of human tissues. Hepatocytes and neurons express low baseline CB1R and CB2R, respectively, and their cell-type-specific functions are not well defined. Here we report inducible expression of CB1R in the liver by high-fat and high sugar diet and CB2R in cortical neurons by methamphetamine. While there is less controversy about hepatocyte CB1R, the presence of functional neuronal CB2R is still debated to date. We found that neuron CB2R basal expression was higher than that of hepatocyte CB1R by measuring mRNA levels of specific isoform CB2A in neurons isolated by fluorescence-activated cell sorting (FACS) and CB1A in hepatocytes isolated by collagenase perfusion of liver. For in vivo studies, we generated hepatocyte, dopaminergic neuron, and microglia-specific conditional knockout mice (Abl-Cnr1Δ, Dat-Cnr2Δ, and Cx3cr1-Cnr2Δ) of CB1R and CB2R by crossing Cnr1f/f and Cnr2f/f strains to Abl-Cre, Dat-Cre, and Cx3cr1-Cre deleter mouse strains, respectively. Our data reveals that neuron and microglia CB2Rs are involved in the "tetrad" effects of the mixed agonist WIN 55212-2, CB1R selective agonist arachidonyl-2'-chloroethylamide (ACEA), and CB2R selective agonist JWH133. Dat-Cnr2Δ and Cx3cr1-Cnr2Δ mice showed genotypic differences in hypomobility, hypothermia, analgesia, and catalepsy induced by the synthetic cannabinoids. Alcohol conditioned place preference was abolished in DAT-Cnr2Δ mice and remained intact in Cx3cr1-Cnr2Δ mice in comparison to WT mice. These Cre-loxP recombinant mouse lines provide unique approaches in cannabinoid research for dissecting the complex endocannabinoid system that is implicated in many chronic disorders.

Divergent Effects of Systemic and Intracollicular CB Receptor Activation Against Forebrain and Hindbrain-Evoked Seizures in Rats

Cannabinoid (CB) receptor agonists are of growing interest as targets for anti-seizure therapies. Here we examined the effect of systemic administration of the CB receptor agonist WIN 55,212-2 (WIN) against audiogenic seizures (AGSs) in the Genetically Epilepsy Prone Rat (GEPR)-3 strain, and against seizures evoked focally from the Area Tempestas (AT). We compared these results to the effect of focal administration of the CB1/2 receptor agonist CP 55940 into the deep layers of the superior colliculus (DLSC), a brain site expressing CB1 receptors. While systemic administration of WIN dose-dependently decreased AGS in GEPR-3s, it was without effect in the AT model. By contrast, intra-DLSC infusion of CP 55940 decreased seizures in both models. To determine if the effects of systemic WIN were dependent upon activation of CB1 receptors in the DSLC, we next microinjected the CB1 receptor antagonist SR141716, before WIN systemic treatment, and tested animals for AGS susceptibility. The pretreatment of the DLSC with SR141716 was without effect on its own and did not alter the anti-convulsant action of WIN systemic administration. Thus, while CB receptors in the DLSC are a potential site of anticonvulsant action, they are not necessary for the effects of systemically administered CB agonists.

Heterogeneity of Neuroinflammatory Responses in Amyotrophic Lateral Sclerosis: A Challenge or an Opportunity?

Amyotrophic Lateral Sclerosis (ALS) is a complex pathology: (i) the neurodegeneration is chronic and progressive; it starts focally in specific central nervous system (CNS) areas and spreads to different districts; (ii) multiple cell types further than motor neurons (i.e., glial/immune system cells) are actively involved in the disease; (iii) both neurosupportive and neurotoxic neuroinflammatory responses were identified. Microglia cells (a key player of neuroinflammation in the CNS) attracted great interest as potential target cell population that could be modulated to counteract disease progression, at least in preclinical ALS models. However, the heterogeneous/multifaceted microglia cell responses occurring in different CNS districts during the disease represent a hurdle for clinical translation of single-drug therapies. To address this issue, over the past ten years, several studies attempted to dissect the complexity of microglia responses in ALS. In this review, we shall summarize these results highlighting how the heterogeneous signature displayed by ALS microglia reflects not only the extent of neuronal demise in different regions of the CNS, but also variable engagement in the attempts to cope with the neuronal damage. We shall discuss novel avenues opened by the advent of single-cell and spatial transcriptomics technologies, underlining the potential for discovery of novel therapeutic targets, as well as more specific diagnostic/prognostic not-invasive markers of neuroinflammation.

Cannabinoid receptor subtype influence on neuritogenesis in human SH-SY5Y cells

Human SH-SY5Y neuroblastoma cells stably expressing exogenous CB₁ (CB₁XS) or CB₂ (CB₂XS) receptors were developed to investigate endocannabinoid signaling in the extension of neuronal projections. Expression of cannabinoid receptors did not alter proliferation rate, viability, or apoptosis relative to parental SH-SY5Y. Transcripts for endogenous cannabinoid system enzymes (diacylglycerol lipase, monoacylglycerol lipase, α/β-hydrolase domain containing proteins 6 and 12, N-acyl phosphatidylethanolamine-phospholipase D, and fatty acid amide hydrolase) were not altered by CB₁ or CB₂ expression. Endocannabinoid ligands 2-arachidonoylglycerol (2-AG) and anandamide were quantitated in SH-SY5Y cells, and diacylglycerol lipase inhibitor tetrahydrolipstatin decreased 2-AG abundance by 90% but did not alter anandamide abundance. M3 muscarinic agonist oxotremorine M, and inhibitors of monoacylglycerol lipase and α/β hydrolase domain containing proteins 6 &12 increased 2-AG abundance. CB₁ receptor expression increased lengths of short (<30 μm) and long (>30 μm) projections, and this effect was significantly reduced by tetrahydrolipstatin, indicative of stimulation by endogenously produced 2-AG. Pertussis toxin, Gβγ inhibitor gallein, and β-arrestin inhibitor barbadin did not significantly alter long projection length in CB₁XS, but significantly reduced short projections, with gallein having the greatest inhibition. The rho kinase inhibitor Y27632 increased CB₁ receptor-mediated long projection extension, indicative of actin cytoskeleton involvement. CB₁ receptor expression increased GAP43 and ST8SIA2 mRNA and decreased ITGA1 mRNA, whereas CB₂ receptor expression increased NCAM and SYT mRNA. We propose that basal endogenous production of 2-AG provides autocrine stimulation of CB₁ receptor signaling through Gi/o, Gβγ, and β-arrestin mechanisms to promote neuritogenesis, and rho kinase influences process extension.

Neural Stem Cells and Cannabinoids in the Spotlight as Potential Therapy for Epilepsy

Epilepsy is one of the most common brain diseases worldwide, having a huge burden in society. The main hallmark of epilepsy is the occurrence of spontaneous recurrent seizures, having a tremendous impact on the lives of the patients and of their relatives. Currently, the therapeutic strategies are mostly based on the use of antiepileptic drugs, and because several types of epilepsies are of unknown origin, a high percentage of patients are resistant to the available pharmacotherapy, continuing to experience seizures overtime. Therefore, the search for new drugs and therapeutic targets is highly important. One key aspect to be targeted is the aberrant adult hippocampal neurogenesis (AHN) derived from Neural Stem Cells (NSCs). Indeed, targeting seizure-induced AHN may reduce recurrent seizures and shed some light on the mechanisms of disease. The endocannabinoid system is a known modulator of AHN, and due to the known endogenous antiepileptic properties, it is an interesting candidate for the generation of new antiepileptic drugs. However, further studies and clinical trials are required to investigate the putative mechanisms by which cannabinoids can be used to treat epilepsy. In this manuscript, we will review how cannabinoid-induced modulation of NSCs may promote neural plasticity and whether these drugs can be used as putative antiepileptic treatment.

Cannabinoids affect the mouse visual acuity via the cannabinoid receptor type 2

Recently, there have been increasing indications that the endocannabinoid (eCB) system is involved in vision. Multiple research teams studied the cannabinoid receptor type 2 (CB2R) expression and function in the mouse retina. Here, we examined the consequence of CB2R modulation on visual acuity using genetic and pharmacologic tools. We found that Cnr2 knockout mice show an enhanced visual acuity, CB2R activation decreased visual acuity while CB2R blockade with the inverse agonist AM630 increased it. The inhibition of 2-arachidonylglycerol (2-AG) synthesis and degradation also greatly increased and decreased visual acuity, respectively. No differences were seen when the cannabinoid receptor type 1 (CB1R) was deleted, blocked or activated implying that CB2R exclusively mediates cannabinoid modulation of the visual acuity. We also investigated the role of cannabinoids in retinal function using electroretinography (ERG). We found that modulating 2-AG levels affected many ERG components, such as the a-wave and oscillatory potentials (OPs), suggesting an impact on cones and amacrine cells. Taken together, these results reveal that CB2R modulates visual acuity and that eCBs such as 2-AG can modulate both visual acuity and retinal sensitivity. Finally, these findings establish that CB2R is present in visual areas and regulates vision-related functions.

Developmentally Transient CB1Rs on Cerebellar Afferents Suppress Afferent Input, Downstream Synaptic Excitation, and Signaling to Migrating Neurons

The endocannabinoid system plays important roles in brain development, but mechanistic studies have focused on neuronal differentiation, migration, and synaptogenesis, with less attention to transcellular interactions that coordinate neurodevelopmental processes across developing neural networks. We determined that, in the developing rodent cerebellar cortex (of both sexes), there is a transient window when the dominant brain cannabinoid receptor, CB1R, is expressed on afferent terminals instead of output neuron Purkinje cell synapses that dominate the adult cerebellum. Activation of these afferent CB1Rs suppresses synaptic transmission onto developing granule cells, and consequently also suppresses excitation of downstream neurons in the developing cortical network, including nonsynaptic, migrating neurons. Application of a CB1R antagonist during afferent stimulation trains and depolarizing voltage steps caused a significant, sustained potentiation of synaptic amplitude. Our data demonstrate that transiently expressed afferent CB1Rs regulate afferent synaptic strength during synaptogenesis, which enables coordinated dampening of transcortical developmental signals.SIGNIFICANCE STATEMENT The endogenous cannabinoid system plays diverse roles in brain development, which, combined with the rapidly changing legal and medical status of cannabis-related compounds, makes understanding how exogenous cannabinoids affect brain development an important biomedical objective. The cerebellum is a key brain region in a variety of neurodevelopmental disorders, and the adult cerebellum has one of the highest expression levels of CB1R, but little is known about CB1R in the developing cerebellum. Here we report a developmentally distinct expression and function of CB1R in the cerebellum, in which endogenous or exogenous activation of CB1Rs modifies afferent synaptic strength and coordinated downstream network signaling. These findings have implications for recreational and medical use of exogenous cannabinoids by pregnant and breastfeeding women.

Interference with the Cannabinoid Receptor CB1R Results in Miswiring of GnRH3 and AgRP1 Axons in Zebrafish Embryos

The G protein-coupled cannabinoid receptors type 1 (CB1R) and type 2 (CB2R), and their endocannabinoid (eCBs) ligands, have been implicated in several aspects of brain wiring during development. Here we aim to assess whether interfering with CB1R affects development, neuritogenesis and pathfinding of GnRH and AgRP neurons, forebrain neurons that control respectively reproduction and appetite. We pharmacologically and genetically interfered with CB1R in zebrafish strains with fluorescently labeled GnRH3 and the AgRP1 neurons. By applying CB1R antagonists we observed a reduced number of GnRH3 neurons, fiber misrouting and altered fasciculation. Similar phenotypes were observed by CB1R knockdown. Interfering with CB1R also resulted in a reduced number, misrouting and poor fasciculation of the AgRP1 neuron’s axonal projections. Using a bioinformatic approach followed by qPCR validation, we have attempted to link CB1R functions with known guidance and fasciculation proteins. The search identified stathmin-2, a protein controlling microtubule dynamics, previously demonstrated to be coexpressed with CB1R and now shown to be downregulated upon interference with CB1R in zebrafish. Together, these results raise the likely possibility that embryonic exposure to low doses of CB1R-interfering compounds could impact on the development of the neuroendocrine systems controlling sexual maturation, reproduction and food intake.

The Acute Activation of the CB1 Receptor in the Hippocampus Decreases Neurotoxicity and Prevents Spatial Memory Impairment in Rats Lesioned with β-Amyloid 25-35

Given their anti-inflammatory properties, cannabinoids have been shown to be neuroprotective agents and to reduce excitotoxicity, through the activation of the Cannabinoid receptor type 1 (CB1r). These properties have led to CB1r being proposed as pharmacological targets for the treatment of various neurodegenerative diseases. Amyloid-β 25-35 (Aβ_25-35) induces the expression of inducible nitric oxide synthase (iNOS) and increases nitric oxide (NO●) levels. It has been observed that increased NO● concentrations trigger biochemical pathways that contribute to neuronal death and cognitive damage. This study aimed to evaluate the neuroprotective effect of an acute activation of CB1r on spatial memory and its impact on iNOS protein expression, NO● levels, gliosis and the neurodegenerative process induced by the injection of Aβ_(25-35) into the CA1 subfield of the hippocampus. ACEA [1 μM/1 μL] and Aβ_(25-35) [100 μM/1 μL] and their respective vehicle groups were injected into the CA1 subfield of the hippocampus. The animals were tested for spatial learning and memory in the eight-arm radial maze, with the results revealing that the administration of ACEA plus Aβ_(25-35) improves learning and memory processes, in contrast with the Aβ_(25-35) group. Moreover, ACEA plus Aβ_(25-35) prevented both the increase in iNOS protein and NO● levels and the reactive gliosis induced by Aβ_(25-35). Importantly, neurodegeneration was significantly reduced by the administration of ACEA plus Aβ_(25-35) in the CA1 subfield of the hippocampus. The data obtained in the present research suggest that the acute early activation of CB1r is crucial for neuroprotection.

Altered Swimming Behaviors in Zebrafish Larvae Lacking Cannabinoid Receptor 2

Background and Objectives: The cannabinoid receptor 2 (CB2) was previously implicated in brain functions, including complex behaviors. Here, we assessed the role of CB2 in selected swimming behaviors in zebrafish larvae and developed an in vivo upscalable whole-organism approach for CB2 ligand screening.

Experimental Approach: Using CRISPR-Cas9 technology, we generated a novel null allele (cnr2^upr1) and a stable homozygote-viable loss-of-function (CB2-KO) line. We measured in untreated wild-type and cnr2^upr1/upr1 larvae, photo-dependent (swimming) responses (PDR) and center occupancy (CO) to establish quantifiable anxiety-like parameters. Next, we measured PDR alteration and CO variation while exposing wild-type and mutant animals to an anxiolytic drug (valproic acid [VPA]) or to an anxiogenic drug (pentylenetetrazol [PTZ]). Finally, we treated wild-type and mutant larvae with two CB2-specific agonists (JWH-133 and HU-308) and two CB2-specific antagonists, inverse agonists (AM-630 and SR-144528).

Results: Untreated CB2-KO showed a different PDR than wild-type larvae as well as a decreased CO. VPA treatments diminished swimming activity in all animals but to a lesser extend in mutants. CO was strongly diminished and even more in mutants. PTZ-induced inverted PDR was significantly stronger in light and weaker in dark periods and the CO lower in PTZ-treated mutants. Finally, two of four tested CB2 ligands had a detectable activity in the assay.

Conclusions: We showed that larvae lacking CB2 behave differently in complex behaviors that can be assimilated to anxiety-like behaviors. Mutant larvae responded differently to VPA and PTZ treatments, providing in vivo evidence of CB2 modulating complex behaviors. We also established an upscalable combined genetic/behavioral approach in a whole organism that could be further developed for high-throughput drug discovery platforms.

Prenatal cannabinoid exposure and altered neurotransmission

Marijuana is one of the most commonly used illicit drugs worldwide. In addition, use of synthetic cannabinoids is increasing, especially among adolescents and young adults. Although human studies have shown that the use of marijuana during pregnancy leads to adverse behavioral effects, such as deficiencies in attention and executive function in affected offspring, the rate of marijuana use among pregnant women is steadily increasing. Various aspects of human behavior including emotion, learning, and memory are dependent on complex interactions between multiple neurotransmitter systems that are especially vulnerable to alterations during the developmental period. Thus, exploration of neurotransmitter changes in response to prenatal cannabinoid exposure is crucial to develop an understanding of how homeostatic imbalance and various long-term neurobehavioral deficits manifest following the abuse of marijuana or other synthetic cannabinoids during pregnancy. Current literature confirms that vast alterations to neurotransmitter systems are present following prenatal cannabinoid exposure, and many of these alterations within the brain are region specific, time-dependent, and sexually dimorphic. In this review, we aim to provide a summary of observed changes to various neurotransmitter systems following cannabinoid exposure during pregnancy and to draw possible correlations to reported behavioral alterations in affected offspring.

Increased expression of cannabinoid CB2 and serotonin 5-HT1A heteroreceptor complexes in a model of newborn hypoxic-ischemic brain damage

Preclinical work shows cannabidiol as a promising drug to manage neonatal hypoxic-ischemic brain damage (NHIBD). The molecular mechanism is not well defined but the beneficial effects of this phytocannabinoid are blocked by antagonists of both cannabinoid CB₂ (CB₂R) and serotonin 5-HT_1A (5-HT1AR) receptors that, in addition, may form heteromers in a heterologous expression system. Using bioluminescence energy transfer, we have shown a direct interaction of the two receptors that leads to a particular signaling in a heterologous system. A property attributed to the heteromer, namely cross-antagonism, was found in primary cultures of neurons thus indicating the occurrence of the receptor heteromer in the CNS. Oxygen-glucose deprivation to neurons led to an increase of CB₂R-mediated signaling and an upregulation of CB₂-5-HT_1A heteroreceptor complex expression. In situ proximity ligation assays in brain cortical sections were performed to compare the expression of CB₂-5-HT_1A complexes in rat E20 fetuses and at different postnatal days. The expression, which is elevated in fetus and shortly after birth, was sharply reduced at later ages (even at P7). The expression of heteromer receptors was more marked in a model of NHIBD and, remarkably, the drop in expression was significantly delayed with respect to controls. These results indicate that CB₂-5-HT_1A heteroreceptor complex may be considered as a target in the therapy of the NHIBD. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Activation of the Cannabinoid Type 2 Receptor by a Novel Indazole Derivative Normalizes the Survival Pattern of Lymphoblasts from Patients with Late-Onset Alzheimer's Disease

BACKGROUND

Alzheimer's disease is a multifactorial disorder for which there is no disease-modifying treatment yet. CB2 receptors have emerged as a promising therapeutic target for Alzheimer's disease because they are expressed in neuronal and glial cells and their activation has no psychoactive effects.

OBJECTIVE

The aim of this study was to investigate whether activation of the CB2 receptor would restore the aberrant enhanced proliferative activity characteristic of immortalized lymphocytes from patients with late-onset Alzheimer's disease. It is assumed that cell-cycle dysfunction occurs in both peripheral cells and neurons in patients with Alzheimer's disease, contributing to the instigation of the disease.

METHODS

Lymphoblastoid cell lines from patients with Alzheimer's disease and age-matched control individuals were treated with a new, in-house-designed dual drug PGN33, which behaves as a CB2 agonist and butyrylcholinesterase inhibitor. We analyzed the effects of this compound on the rate of cell proliferation and levels of key regulatory proteins. In addition, we investigated the potential neuroprotective action of PGN33 in β-amyloid-treated neuronal cells.

RESULTS

We report here that PGN33 normalized the increased proliferative activity of Alzheimer's disease lymphoblasts. The compound blunted the calmodulin-dependent overactivation of the PI3K/Akt pathway, by restoring the cyclin-dependent kinase inhibitor p27 levels, which in turn reduced the activity of the cyclin-dependent kinase/pRb cascade. Moreover, this CB2 agonist prevented β-amyloid-induced cell death in neuronal cells.

CONCLUSION

Our results suggest that the activation of CB2 receptors could be considered a useful therapeutic approach for Alzheimer's disease.

Receptors of intermediates of carbohydrate metabolism, GPR91 and GPR99, mediate axon growth

During the development of the visual system, high levels of energy are expended propelling axons from the retina to the brain. However, the role of intermediates of carbohydrate metabolism in the development of the visual system has been overlooked. Here, we report that the carbohydrate metabolites succinate and α-ketoglutarate (α-KG) and their respective receptor-GPR91 and GPR99-are involved in modulating retinal ganglion cell (RGC) projections toward the thalamus during visual system development. Using ex vivo and in vivo approaches, combined with pharmacological and genetic analyses, we revealed that GPR91 and GPR99 are expressed on axons of developing RGCs and have complementary roles during RGC axon growth in an extracellular signal-regulated kinases 1 and 2 (ERK1/2)-dependent manner. However, they have no effects on axon guidance. These findings suggest an important role for these receptors during the establishment of the visual system and provide a foundational link between carbohydrate metabolism and axon growth.

Distinct roles of neuronal and microglial CB2 cannabinoid receptors in the mouse hippocampus

The effects of cannabinoids are primarily mediated by type-1 cannabinoid receptors in the brain and type-2 cannabinoid receptors (CB2Rs) in the peripheral immune system. However, recent evidence demonstrates that CB2Rs are also expressed in the brain and implicated in neuropsychiatric effects. Diverse types of cells in various regions in the brain express CB2Rs but the cellular loci of CB2Rs that induce specific behavioral effects have not been determined. To manipulate CB2R expression in specific types of cells in the dorsal hippocampus of adult mice, we used Cre-dependent overexpression and CRISPR-Cas9 genome-editing techniques in combination with adeno-associated viruses and transgenic mice. Elevation and disruption of CB2R expression in microglia in the CA1 area increased and decreased, respectively, contextual fear memory. In CA1 pyramidal neurons, disruption of CB2R expression enhanced spatial working memory, whereas their overexpression reduced anxiety levels assessed asan increase in the exploration time in the central area of open field. Interneuronal CB2Rs were not involved in the modulation of cognitive or emotional behaviors tested in this study. The targeted manipulation of CB2R expression in pyramidal neurons and microglia suggests that CB2Rs in different types of cells in the mature hippocampus play distinct roles in the regulation of memory and anxiety.

Cannabinoid CB1 and CB2 receptors differentially modulate L- and T-type Ca2+ channels in rat retinal ganglion cells

Endocannabinoid signaling system is involved in regulating multiple neuronal functions in the central nervous system by activating G-protein coupled cannabinoid CB1 and CB2 receptors (CB1Rs and CB2Rs). Growing evidence has shown that CB1Rs and CB2Rs are extensively expressed in retinal ganglion cells (RGCs). Here, modulation of L- and T-types Ca²⁺ channels by activating CB1Rs and CB2Rs in RGCs was investigated. Triple immunofluorescent staining showed that L-type subunit Ca_V1.2 was co-localized with T-type subunits (Ca_V3.1, Ca_V3.2 and Ca_V3.3) in rat RGCs. In acutely isolated rat RGCs, the CB1R agonist WIN55212-2 suppressed both peak and steady-state Ca²⁺ currents in a dose-dependent manner, with IC₅₀ being 9.6 μM and 8.4 μM, respectively. It was further shown that activation of CB1Rs by WIN55212-2 or ACEA, another CB1R agonist, significantly suppressed both L- and T-type Ca²⁺ currents, and shifted inactivation curve of T-type one toward hyperpolarization direction. While the effect on L-type Ca²⁺ channels was mediated by intracellular cAMP/protein kinase A (PKA), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways, only CaMKII signaling pathway was involved in the effect on T-type Ca²⁺ channels. Furthermore, CB65 and HU308, two specific CB2R agonists, significantly suppressed T-type Ca²⁺ channels, which was mediated by intracellular cAMP/PKA and CaMKII signaling pathways, but had no effect on L-type channels. These results imply that endogenous cannabinoids may modulate the excitability and the output of RGCs by differentially suppressing the activity of L- and T-type Ca²⁺ channels through activation of CB1Rs and CB2Rs. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

CB2 Cannabinoid Receptor As Potential Target against Alzheimer's Disease

The CB2 receptor is one of the components of the endogenous cannabinoid system, a complex network of signaling molecules and receptors involved in the homeostatic control of several physiological functions. Accumulated evidence suggests a role for CB2 receptors in Alzheimer's disease (AD) and indicates their potential as a therapeutic target against this neurodegenerative disease. Levels of CB2 receptors are significantly increased in post-mortem AD brains, mainly in microglia surrounding senile plaques, and their expression levels correlate with the amounts of Aβ42 and β-amyloid plaque deposition. Moreover, several studies on animal models of AD have demonstrated that specific CB2 receptor agonists, which are devoid of psychoactive effects, reduce AD-like pathology, resulting in attenuation of the inflammation associated with the disease but also modulating Aβ and tau aberrant processing, among other effects. CB2 receptor activation also improves cognitive impairment in animal models of AD. This review discusses available data regarding the role of CB2 receptors in AD and the potential usefulness of specific agonists of these receptors against AD.

At the Tip of an Iceberg: Prenatal Marijuana and Its Possible Relation to Neuropsychiatric Outcome in the Offspring

Endocannabinoids regulate brain development via modulating neural proliferation, migration, and the differentiation of lineage-committed cells. In the fetal nervous system, (endo)cannabinoid-sensing receptors and the enzymatic machinery of endocannabinoid metabolism exhibit a cellular distribution map different from that in the adult, implying distinct functions. Notably, cannabinoid receptors serve as molecular targets for the psychotropic plant-derived cannabis constituent Δ(9)-tetrahydrocannainol, as well as synthetic derivatives (designer drugs). Over 180 million people use cannabis for recreational or medical purposes globally. Recreational cannabis is recognized as a niche drug for adolescents and young adults. This review combines data from human and experimental studies to show that long-term and heavy cannabis use during pregnancy can impair brain maturation and predispose the offspring to neurodevelopmental disorders. By discussing the mechanisms of cannabinoid receptor-mediated signaling events at critical stages of fetal brain development, we organize histopathologic, biochemical, molecular, and behavioral findings into a logical hypothesis predicting neuronal vulnerability to and attenuated adaptation toward environmental challenges (stress, drug exposure, medication) in children affected by in utero cannabinoid exposure. Conversely, we suggest that endocannabinoid signaling can be an appealing druggable target to dampen neuronal activity if pre-existing pathologies associate with circuit hyperexcitability. Yet, we warn that the lack of critical data from longitudinal follow-up studies precludes valid conclusions on possible delayed and adverse side effects. Overall, our conclusion weighs in on the ongoing public debate on cannabis legalization, particularly in medical contexts.

Stimulation of brain glucose uptake by cannabinoid CB2 receptors and its therapeutic potential in Alzheimer's disease

Cannabinoid CB2 receptors (CB2Rs) are emerging as important therapeutic targets in brain disorders that typically involve neurometabolic alterations. We here addressed the possible role of CB2Rs in the regulation of glucose uptake in the mouse brain. To that aim, we have undertaken 1) measurement of (3)H-deoxyglucose uptake in cultured cortical astrocytes and neurons and in acute hippocampal slices; 2) real-time visualization of fluorescently labeled deoxyglucose uptake in superfused hippocampal slices; and 3) in vivo PET imaging of cerebral (18)F-fluorodeoxyglucose uptake. We now show that both selective (JWH133 and GP1a) as well as non-selective (WIN55212-2) CB2R agonists, but not the CB1R-selective agonist, ACEA, stimulate glucose uptake, in a manner that is sensitive to the CB2R-selective antagonist, AM630. Glucose uptake is stimulated in astrocytes and neurons in culture, in acute hippocampal slices, in different brain areas of young adult male C57Bl/6j and CD-1 mice, as well as in middle-aged C57Bl/6j mice. Among the endocannabinoid metabolizing enzymes, the selective inhibition of COX-2, rather than that of FAAH, MAGL or α,βDH6/12, also stimulates the uptake of glucose in hippocampal slices of middle-aged mice, an effect that was again prevented by AM630. However, we found the levels of the endocannabinoid, anandamide reduced in the hippocampus of TgAPP-2576 mice (a model of β-amyloidosis), and likely as a consequence, COX-2 inhibition failed to stimulate glucose uptake in these mice. Together, these results reveal a novel general glucoregulatory role for CB2Rs in the brain, raising therapeutic interest in CB2R agonists as nootropic agents.

A Comparative Analysis of the Endocannabinoid System in the Retina of Mice, Tree Shrews, and Monkeys

The endocannabinoid (eCB) system is widely expressed in various parts of the central nervous system, including the retina. The localization of the key eCB receptors, particularly CB1R and CB2R, has been recently reported in rodent and primate retinas with striking interspecies differences. Little is known about the distribution of the enzymes involved in the synthesis and degradation of these eCBs. We therefore examined the expression and localization of the main components of the eCB system in the retina of mice, tree shrews, and monkeys. We found that CB1R and FAAH distributions are well-preserved among these species. However, expression of NAPE-PLD is circumscribed to the photoreceptor layer only in monkeys. In contrast, CB2R expression is variable across these species; in mice, CB2R is found in retinal neurons but not in glial cells; in tree shrews, CB2R is expressed in Müller cell processes of the outer retina and in retinal neurons of the inner retina; in monkeys, CB2R is restricted to Müller cells. Finally, the expression patterns of MAGL and DAGLα are differently expressed across species. Overall, these results provide evidence that the eCB system is differently expressed in the retina of these mammals and suggest a distinctive role of eCBs in visual processing.

Expression and Function of the Endocannabinoid System in the Retina and the Visual Brain

Endocannabinoids are important retrograde modulators of synaptic transmission throughout the nervous system. Cannabinoid receptors are seven transmembrane G-protein coupled receptors favoring G_i/o protein. They are known to play an important role in various processes, including metabolic regulation, craving, pain, anxiety, and immune function. In the last decade, there has been a growing interest for endocannabinoids in the retina and their role in visual processing. The purpose of this review is to characterize the expression and physiological functions of the endocannabinoid system in the visual system, from the retina to the primary visual cortex, with a main interest regarding the retina, which is the best-described area in this system so far. It will show that the endocannabinoid system is widely present in the retina, mostly in the through pathway where it can modulate neurotransmitter release and ion channel activity, although some evidence also indicates possible mechanisms via amacrine, horizontal, and Müller cells. The presence of multiple endocannabinoid ligands, synthesizing and catabolizing enzymes, and receptors highlights various pharmacological targets for novel therapeutic application to retinal diseases.

CB2 Cannabinoid Receptor Knockout in Mice Impairs Contextual Long-Term Memory and Enhances Spatial Working Memory

Neurocognitive effects of cannabinoids have been extensively studied with a focus on CB1 cannabinoid receptors because CB1 receptors have been considered the major cannabinoid receptor in the nervous system. However, recent discoveries of CB2 cannabinoid receptors in the brain demand accurate determination of whether and how CB2 receptors are involved in the cognitive effects of cannabinoids. CB2 cannabinoid receptors are primarily involved in immune functions, but also implicated in psychiatric disorders such as schizophrenia and depression. Here, we examined the effects of CB2 receptor knockout in mice on memory to determine the roles of CB2 receptors in modulating cognitive function. Behavioral assays revealed that hippocampus-dependent, long-term contextual fear memory was impaired whereas hippocampus-independent, cued fear memory was normal in CB2 receptor knockout mice. These mice also displayed enhanced spatial working memory when tested in a Y-maze. Motor activity and anxiety of CB2 receptor knockout mice were intact when assessed in an open field arena and an elevated zero maze. In contrast to the knockout of CB2 receptors, acute blockade of CB2 receptors by AM603 in C57BL/6J mice had no effect on memory, motor activity, or anxiety. Our results suggest that CB2 cannabinoid receptors play diverse roles in regulating memory depending on memory types and/or brain areas.

Palmitoylethanolamide, a Natural Retinoprotectant: Its Putative Relevance for the Treatment of Glaucoma and Diabetic Retinopathy

Retinopathy is a threat to the eyesight, and glaucoma and diabetes are the main causes for the damage of retinal cells. Recent insights pointed out a common pathogenetic pathway for both disorders, based on chronic inflammation. Palmitoylethanolamide (PEA) is an endogenous cell protective lipid. Since its discovery in 1957 as a biologically active component in foods and in many living organisms, around 500 scientific papers have been published on PEA's anti-inflammatory and neuron-protective properties. PEA has been evaluated for glaucoma, diabetic retinopathy, and uveitis, pathological states based on chronic inflammation, respiratory disorders, and various pain syndromes in a number of clinical trials since the 70s of 20th century. PEA is available as a food supplement (PeaPure) and as diet food for medical purposes in Italy (Normast, PeaVera, and Visimast). These products are notified in Italy for the nutritional support in glaucoma and neuroinflammation. PEA has been tested in at least 9 double blind placebo controlled studies, among which two studies were in glaucoma, and found to be safe and effective up to 1.8 g/day, with excellent tolerability. PEA therefore holds a promise in the treatment of a number of retinopathies. We discuss PEA as a putative anti-inflammatory and retinoprotectant compound in the treatment of retinopathies, especially related to glaucoma and diabetes.

Role of GPR55 during Axon Growth and Target Innervation

Guidance molecules regulate the navigation of retinal ganglion cell (RGC) projections toward targets in the visual thalamus. In this study, we demonstrate that the G-protein-coupled receptor 55 (GPR55) is expressed in the retina during development, and regulates growth cone (GC) morphology and axon growth. In vitro, neurons obtained from gpr55 knock-out (gpr55^-/-) mouse embryos have smaller GCs, less GC filopodia, and have a decreased outgrowth compared with gpr55^+/+ neurons. When gpr55^+/+ neurons were treated with GPR55 agonists, lysophosphatidylinositol (LPI) and O-1602, we observed a chemo-attractive effect and an increase in GC size and filopodia number. In contrast, cannabidiol (CBD) decreased the GC size and filopodia number inducing chemo-repulsion. In absence of the receptor (gpr55^-/-), no pharmacologic effects of the GPR55 ligands were observed. In vivo, compared to their wild-type (WT) littermates, gpr55^-/- mice revealed a decreased branching in the dorsal terminal nucleus (DTN) and a lower level of eye-specific segregation of retinal projections in the superior colliculus (SC) and in the dorsal lateral geniculate nucleus (dLGN). Moreover, a single intraocular injection of LPI increased branching in the DTN, whereas treatment with CBD, an antagonist of GPR55, decreased it. These results indicate that GPR55 modulates the growth rate and the targets innervation of retinal projections and highlight, for the first time, an important role of GPR55 in axon refinement during development.

An endocannabinoid system is present in the mouse olfactory epithelium but does not modulate olfaction

Endocannabinoids modulate a diverse array of functions including progenitor cell proliferation in the central nervous system, and odorant detection and food intake in the mammalian central olfactory system and larval Xenopus laevis peripheral olfactory system. However, the presence and role of endocannabinoids in the peripheral olfactory epithelium have not been examined in mammals. We found the presence of cannabinoid type 1 (CB1) and cannabinoid type 2 (CB2) receptor protein and mRNA in the olfactory epithelium. Using either immunohistochemistry or calcium imaging we localized CB1 receptors on neurons, glia-like sustentacular cells, microvillous cells and progenitor-like basal cells. To examine the role of endocannabinoids, CB1- and CB2- receptor-deficient (CB1(-/-)/CB2(-/-)) mice were used. The endocannabinoid 2-arachidonylglycerol (2-AG) was present at high levels in both C57BL/6 wildtype and CB1(-/-)/CB2(-/-) mice. 2-AG synthetic and degradative enzymes are expressed in wildtype mice. A small but significant decrease in basal cell and olfactory sensory neuron numbers was observed in CB1(-/-)/CB2(-/-) mice compared to wildtype mice. The decrease in olfactory sensory neurons did not translate to impairment in olfactory-mediated behaviors assessed by the buried food test and habituation/dishabituation test. Collectively, these data indicate the presence of an endocannabinoid system in the mouse olfactory epithelium. However, unlike in tadpoles, endocannabinoids do not modulate olfaction. Further investigation on the role of endocannabinoids in progenitor cell function in the olfactory epithelium is warranted.

Long-term consequences of perinatal and adolescent cannabinoid exposure on neural and psychological processes

Marihuana is the most widely consumed illicit drug, even among adolescents and pregnant women. Given the critical developmental processes that occur in the adolescent and fetal nervous system, marihuana consumption during these stages may have permanent consequences on several brain functions in later adult life. Here, we review what is currently known about the long-term consequences of perinatal and adolescent cannabinoid exposure. The most consistent findings point to long-term impairments in cognitive function that are associated with structural alterations and disturbed synaptic plasticity. In addition, several neurochemical modifications are also evident after prenatal or adolescent cannabinoid exposure, especially in the endocannabinoid, glutamatergic, dopaminergic and opioidergic systems. Important sexual dimorphisms are also evident in terms of the long-lasting effects of cannabinoid consumption during pregnancy and adolescence, and cannabinoids possibly have a protective effect in adolescents who have suffered traumatic life challenges, such as maternal separation or intense stress. Finally, we suggest some future research directions that may encourage further advances in this exciting field.

Genetic Manipulation of the Endocannabinoid System

The physiological and pathophysiological functions of the endocannabinoid system have been studied extensively using transgenic and targeted knockout mouse models. The first gene deletions of the cannabinoid CB(1) receptor were described in the late 1990s, soon followed by CB(2) and FAAH mutations in early 2000. These mouse models helped to elucidate the fundamental role of endocannabinoids as retrograde transmitters in the CNS and in the discovery of many unexpected endocannabinoid functions, for example, in the skin, bone and liver. We now have knockout mouse models for almost every receptor and enzyme of the endocannabinoid system. Conditional mutant mice were mostly developed for the CB(1) receptor, which is widely expressed on many different neurons, astrocytes and microglia, as well as on many cells outside the CNS. These mouse strains include "floxed" CB(1) alleles and mice with a conditional re-expression of CB(1). The availability of these mice made it possible to decipher the function of CB(1) in specific neuronal circuits and cell populations or to discriminate between central and peripheral effects. Many of the genetic mouse models were also used in combination with viral expression systems. The purpose of this review is to provide a comprehensive overview of the existing genetic models and to summarize some of the most important discoveries that were made with these animals.

Localization of diacylglycerol lipase alpha and monoacylglycerol lipase during postnatal development of the rat retina

In recent decades, there has been increased interest in the physiological roles of the endocannabinoid (eCB) system and its receptors, the cannabinoid receptor types 1 (CB1R) and 2 (CB2R). Exposure to cannabinoids during development results in neurofunctional alterations, which implies that the eCB system is involved in the developmental processes of the brain. Because of their lipophilic nature, eCBs are synthesized on demand and are not stored in vesicles. Consequently, the enzymes responsible for their synthesis and degradation are key regulators of their physiological actions. Therefore, knowing the localization of these enzymes during development is crucial for a better understanding of the role played by eCBs during the formation of the central nervous system. In this study, we investigated the developmental protein localization of the synthesizing and catabolic enzymes of the principal eCB, 2-arachidonoylglycerol (2-AG) in the retinas of young and adult rats. The distribution of the enzymes responsible for the synthesis (DAGLα) and the degradation (MAGL) of 2-AG was determined for every retinal cell type from birth to adulthood. Our results indicate that DAGLα is present early in postnatal development. It is highly expressed in photoreceptor, horizontal, amacrine, and ganglion cells. MAGL appears later during the development of the retina and its presence is limited to amacrine and Müller cells. Overall, these results suggest that 2-AG is strongly present in early retinal development and might be involved in the regulation of the structural and functional maturation of the retina.

Programming of neural cells by (endo)cannabinoids: from physiological rules to emerging therapies

Among the many signalling lipids, endocannabinoids are increasingly recognized for their important roles in neuronal and glial development. Recent experimental evidence suggests that, during neuronal differentiation, endocannabinoid signalling undergoes a fundamental switch from the prenatal determination of cell fate to the homeostatic regulation of synaptic neurotransmission and bioenergetics in the mature nervous system. These studies also offer novel insights into neuropsychiatric disease mechanisms and contribute to the public debate about the benefits and the risks of cannabis use during pregnancy and in adolescence.