Subcellular localization of type 1 cannabinoid receptors in the rat basal ganglia

AM6527, a neutral CB1 receptor antagonist, suppresses opioid taking and seeking, as well as cocaine seeking in rodents without aversive effects

Preclinical research has demonstrated the efficacy of CB1 receptor (CB1R) antagonists in reducing drug-taking behavior. However, clinical trials with rimonabant, a CB1R antagonist with inverse agonist profile, failed due to severe adverse effects, such as depression and suicidality. As a result, efforts have shifted towards developing novel neutral CB1R antagonists without an inverse agonist profile for treating substance use disorders. Here, we assessed AM6527, a CB1R neutral antagonist, in addiction animal models. Our findings revealed that AM6527 did not affect cocaine self-administration under fixed-ratio reinforcement schedules but dose-dependently inhibited it under progressive-ratio reinforcement schedules. Additionally, AM6527 dose-dependently inhibited heroin self-administration under both fixed-ratio and progressive-ratio reinforcement schedules and oral sucrose self-administration under a fixed-ratio reinforcement schedule, as well as cocaine- or heroin-triggered reinstatement of drug-seeking behavior in rats. However, chronic AM6527 administration for five consecutive days significantly inhibited heroin self-administration only during the initial two days, indicating tolerance development. Notably, AM6527 did not produce rewarding or aversive effects by itself in classical electrical intracranial self-stimulation and conditioned place preference tests. However, in optical intracranial self-stimulation (oICSS) maintained by optogenetic stimulation of midbrain dopamine neurons in DAT-cre mice, both AM6527 and rimonabant dose-dependently inhibited dopamine-dependent oICSS behavior. Together, these findings suggest that AM6527 effectively reduces drug-taking and seeking behaviors without rimonabant-like adverse effects. Thus, AM6527 warrants further investigation as a potential pharmacotherapy for opioid and cocaine use disorders.

Cannabinoid CB1 Receptors Are Expressed in a Subset of Dopamine Neurons and Underlie Cannabinoid-Induced Aversion, Hypoactivity, and Anxiolytic Effects in Mice

Cannabinoids modulate dopamine (DA) transmission and DA-related behavior, which has been thought to be mediated initially by activation of cannabinoid CB1 receptors (CB1Rs) on GABA neurons. However, there is no behavioral evidence supporting it. In contrast, here we report that CB1Rs are also expressed in a subset of DA neurons and functionally underlie cannabinoid action in male and female mice. RNAscope in situ hybridization (ISH) assays demonstrated CB1 mRNA in tyrosine hydroxylase (TH)-positive DA neurons in the ventral tegmental area (VTA) and glutamate decarboxylase 1 (GAD1)-positive GABA neurons. The CB1R-expressing DA neurons were located mainly in the middle portion of the VTA with the number of CB1-TH colocalization progressively decreasing from the medial to the lateral VTA. Triple-staining assays indicated CB1R mRNA colocalization with both TH and vesicular glutamate transporter 2 (VgluT2, a glutamate neuronal marker) in the medial VTA close to the midline of the brain. Optogenetic activation of this population of DA neurons was rewarding as assessed by optical intracranial self-stimulation. Δ⁹-tetrahydrocannabinol (Δ⁹-THC) or ACEA (a selective CB1R agonist) dose-dependently inhibited optical intracranial self-stimulation in DAT-Cre control mice, but not in conditional knockout mice with the CB1R gene absent in DA neurons. In addition, deletion of CB1Rs from DA neurons attenuated Δ⁹-THC-induced reduction in DA release in the NAc, locomotion, and anxiety. Together, these findings indicate that CB1Rs are expressed in a subset of DA neurons that corelease DA and glutamate, and functionally underlie cannabinoid modulation of DA release and DA-related behavior.SIGNIFICANCE STATEMENT Cannabinoids produce a series of psychoactive effects, such as aversion, anxiety, and locomotor inhibition in rodents. However, the cellular and receptor mechanisms underlying these actions are not fully understood. Here we report that CB1 receptors are expressed not only in GABA neurons but also in a subset of dopamine neurons, which are located mainly in the medial VTA close to the midline of the midbrain and corelease dopamine and glutamate. Optogenetic activation of these dopamine neurons is rewarding, which is dose-dependently inhibited by cannabinoids. Selective deletion of CB1 receptor from dopamine neurons blocked cannabinoid-induced aversion, hypoactivity, and anxiolytic effects. These findings demonstrate that dopaminergic CB1 receptors play an important role in mediating cannabinoid action.

Parkinson's disease related alterations in cannabinoid transmission

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic (DAergic) neurons of the substantia nigra pars compacta (SNc) by neurodegeneration. Recent findings in animal models of PD propose tonic inhibition of the remaining DA neurons through GABA release from reactive glial cells. Movement dysfunctions could be ameliorated by promotion of activity in dormant DA cells. The endocannabinoid system (ECS) is extensively present in basal ganglia (BG) and is known as an indirect modulator of DAergic neurotransmission, thus drugs designed to target this system have shown promising therapeutic potential in PD patients. Interestingly, down/up-regulation of cannabinoid receptors (CBRs) varies across the different stages of PD, suggesting that some of the motor/ non-motor deficits may be related to changes in CBRs. Determination of the profile of changes of these receptors across the different stages of PD as well as their neural distribution within the BG could improve understanding of PD and identify pathways important in disease pathobiology. In this review, we focus on temporal and spatial alterations of CBRs during PD in the BG. At present, as inconclusive, but suggestive results have been obtained, future investigations should be conducted to extend preclinical studies examining CBRs changes within each stage in controlled clinical trials in order to determine the potential of targeting CBRs in management of PD.

Cannabinoids and Neurogenesis: The Promised Solution for Neurodegeneration?

The concept of neurons as irreplaceable cells does not hold true today. Experiments and evidence of neurogenesis, also, in the adult brain give hope that some compounds or drugs can enhance this process, helping to reverse the outcomes of diseases or traumas that once were thought to be everlasting. Cannabinoids, both from natural and artificial origins, already proved to have several beneficial effects (e.g., anti-inflammatory, anti-oxidants and analgesic action), but also capacity to increase neuronal population, by replacing the cells that were lost and/or regenerate a damaged nerve cell. Neurogenesis is a process which is not highly represented in literature as neuroprotection, though it is as important as prevention of nervous system damage, because it can represent a possible solution when neuronal death is already present, such as in neurodegenerative diseases. The aim of this review is to resume the experimental evidence of phyto- and synthetic cannabinoids effects on neurogenesis, both in vitro and in vivo, in order to elucidate if they possess also neurogenetic and neurorepairing properties.

Control of exploration, motor coordination and amphetamine sensitization by cannabinoid CB1 receptors expressed in medium spiny neurons

Activation of cannabinoid 1 receptors (CB₁ R) modulates multiple behaviours, including exploration, motor coordination and response to psychostimulants. It is known that CB₁ R expressed by either excitatory or inhibitory neurons mediates different behavioural responses to CB₁ R activation, yet the involvement of CB₁ R expressed by medium spiny neurons (MSNs), the neuronal subpopulation that expresses the highest level of CB₁ R in the CNS, remains unknown. We report a new genetically modified mouse line that expresses functional CB₁ R in MSN on a CB₁ R knockout (KO) background (CB₁ R_(MSN) mice). The absence of cannabimimetic responses measured in CB₁ R KO mice was not rescued in CB₁ R_(MSN) mice, nor was decreased spontaneous locomotion, impaired instrumental behaviour or reduced amphetamine-triggered hyperlocomotion measured in CB₁ R KO mice. Significantly, reduced novel environment exploration of an open field and absence of amphetamine sensitization (AS) measured in CB₁ R KO mice were fully rescued in CB₁ R_(MSN) mice. Impaired motor coordination in CB₁ R KO mice measured on the Rotarod was partially rescued in CB₁ R_(MSN) mice. Thus, CB₁ R expressed by MSN control exploration, motor coordination, and AS. Our study demonstrates a new functional roles for cell specific CB₁ R expression and their causal link in the control of specific behaviors.

Endocannabinoid system in the neurodevelopment of GABAergic interneurons: implications for neurological and psychiatric disorders

In mature mammalian brains, the endocannabinoid system (ECS) plays an important role in the regulation of synaptic plasticity and the functioning of neural networks. Besides, the ECS also contributes to the neurodevelopment of the central nervous system. Due to the increase in the medical and recreational use of cannabis, it is inevitable and essential to elaborate the roles of the ECS on neurodevelopment. GABAergic interneurons represent a group of inhibitory neurons that are vital in controlling neural network activity. However, the role of the ECS in the neurodevelopment of GABAergic interneurons remains to be fully elucidated. In this review, we provide a brief introduction of the ECS and interneuron diversity. We focus on the process of interneuron development and the role of ECS in the modulation of interneuron development, from the expansion of the neural stem/progenitor cells to the migration, specification and maturation of interneurons. We further discuss the potential implications of the ECS and interneurons in the pathogenesis of neurological and psychiatric disorders, including epilepsy, schizophrenia, major depressive disorder and autism spectrum disorder.

Cannabis sativa: Interdisciplinary Strategies and Avenues for Medical and Commercial Progression Outside of CBD and THC

Cannabis sativa (Cannabis) is one of the world’s most well-known, yet maligned plant species. However, significant recent research is starting to unveil the potential of Cannabis to produce secondary compounds that may offer a suite of medical benefits, elevating this unique plant species from its illicit narcotic status into a genuine biopharmaceutical. This review summarises the lengthy history of Cannabis and details the molecular pathways that underpin the production of key secondary metabolites that may confer medical efficacy. We also provide an up-to-date summary of the molecular targets and potential of the relatively unknown minor compounds offered by the Cannabis plant. Furthermore, we detail the recent advances in plant science, as well as synthetic biology, and the pharmacology surrounding Cannabis. Given the relative infancy of Cannabis research, we go on to highlight the parallels to previous research conducted in another medically relevant and versatile plant, Papaver somniferum (opium poppy), as an indicator of the possible future direction of Cannabis plant biology. Overall, this review highlights the future directions of cannabis research outside of the medical biology aspects of its well-characterised constituents and explores additional avenues for the potential improvement of the medical potential of the Cannabis plant.

Endocannabinoid modulation of dopamine release during reward seeking, interval timing, and avoidance

Endocannabinoids (eCBs) are neuromodulators that influence a wide range of neural systems and behaviors. In the current review, we describe our recent research showing how eCBs, particularly 2-arachidonoylglycerol (2-AG), concurrently shape mesolimbic dopamine (DA) release and associated behavior. We will restrict our discussion by emphasizing three distinct behaviors: reward seeking, interval timing, and active avoidance. During reward seeking we find that 2-AG is necessary to observe cue-evoked DA release events that are thought to represent the value of a rewarding outcome. We then describe data showing that 2-AG modulates unique patterns of DA release and behavior observed under conditions of periodic reinforcement. These data are discussed within the context of interval timing and adjunctive behavior. eCB modulation of DA release is also implicated in defensive behavior, including the avoidance of harm. As in reward seeking, our data suggest that the concentration of DA that is evoked by a warning signal can represent the value of an avoidance outcome. And, disrupting eCB signaling concomitantly reduces the concentration of the avoidance value signal and active avoidance. Disruptions in reward seeking, interval timing, and defensive behavior are commonly observed in a variety of movement disorders (e.g., Parkinson's and Huntington's disease) and disorders of motivation (e.g., addiction). We believe our data on eCB-DA interactions have implications for the development of novel pharmacotherapies to treat these disorders. Thus, we conclude by discussing how eCB pharmacology might be harnessed to treat disorders of movement and motivation.

The modulation of striatonigral and nigrotectal pathways by CB1 signalling in the substantia nigra pars reticulata regulates panic elicited in mice by urutu-cruzeiro lancehead pit vipers

Cannabinoid receptor type 1 (CB₁R) is widely distributed in the substantia nigra pars reticulata (SNpr). However, the role of CB₁R at the SNpr level in threatening situations is poorly understood. We investigated the role of CB₁R in the SNpr on the expression of fear responses in mice confronted with urutu-cruzeiro pit vipers. First, a bidirectional neurotracer was injected into the SNpr; then, immunostaining of the vesicular GABA transporter was conducted at the levels of the striatum (CPu) and deep layers of the superior colliculus (dlSC). In addition, CB₁R immunostaining and GABA labelling were performed in the SNpr. Using a prey-versus-snake paradigm, mice were pretreated with the CB₁R antagonist AM251 (100 pmol) and treated with the endocannabinoid anandamide (AEA, 5 pmol) in the SNpr, followed by bicuculline (40 ng) in the dlSC, and were then confronted with a snake. Bidirectional neural tract tracers associated with immunofluorescence showed the GABAergic striatonigral disinhibitory and nigrotectal inhibitory pathways. Furthermore, we showed that CB₁R labelling was restricted to axonal fibres surrounding SNpr GABAergic cells. We also demonstrated a decrease in the defensive behaviours of mice treated with AEA in the SNpr, but this effect was blocked by pre-treatment with AM251 in this structure. Taken together, our results show that the panicolytic consequences of the AEA enhancement in the SNpr are signalled by CB₁R, suggesting that CB₁R localised in axon terminals of CPu GABAergic neurons in the SNpr modulates the activity of the nigrotectal GABAergic pathway during the expression of defensive behaviours in threatening situations.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

Distinctive Evidence Involved in the Role of Endocannabinoid Signalling in Parkinson's Disease: A Perspective on Associated Therapeutic Interventions

Current pharmacotherapy of Parkinson's disease (PD) is symptomatic and palliative, with levodopa/carbidopa therapy remaining the prime treatment, and nevertheless, being unable to modulate the progression of the neurodegeneration. No available treatment for PD can enhance the patient's life-quality by regressing this diseased state. Various studies have encouraged the enrichment of treatment possibilities by discovering the association of the effects of the endocannabinoid system (ECS) in PD. These reviews delineate the reported evidence from the literature on the neuromodulatory role of the endocannabinoid system and expression of cannabinoid receptors in symptomatology, cause, and treatment of PD progression, wherein cannabinoid (CB) signalling experiences alterations of biphasic pattern during PD progression. Published papers to date were searched via MEDLINE, PubMed, etc., using specific key words in the topic of our manuscript. Endocannabinoids regulate the basal ganglia neuronal circuit pathways, synaptic plasticity, and motor functions via communication with dopaminergic, glutamatergic, and GABAergic signalling systems bidirectionally in PD. Further, gripping preclinical and clinical studies demonstrate the context regarding the cannabinoid compounds, which is supported by various evidence (neuroprotection, suppression of excitotoxicity, oxidative stress, glial activation, and additional benefits) provided by cannabinoid-like compounds (much research addresses the direct regulation of cannabinoids with dopamine transmission and other signalling pathways in PD). More data related to endocannabinoids efficacy, safety, and pharmacokinetic profiles need to be explored, providing better insights into their potential to ameliorate or even regress PD.

Adolescent Exposure to WIN 55212-2 Render the Nigrostriatal Dopaminergic Pathway Activated During Adulthood

BACKGROUND

During adolescence, neuronal circuits exhibit plasticity in response to physiological changes and to adapt to environmental events. Nigrostriatal dopaminergic pathways are in constant flux during development. Evidence suggests a relationship between early use of cannabinoids and psychiatric disorders characterized by altered dopaminergic systems, such as schizophrenia and addiction. However, the impact of adolescent exposure to cannabinoids on nigrostriatal dopaminergic pathways in adulthood remains unclear. The aim of this research was to determine the effects of repeated activation of cannabinoid receptors during adolescence on dopaminergic activity of nigrostriatal pathways and the mechanisms underlying this impact during adulthood.

METHODS

Male Sprague-Dawley rats were treated with 1.2 mg/kg WIN 55212-2 daily from postnatal day 40 to 65. Then no-net flux microdialysis of dopamine in the dorsolateral striatum, electrophysiological recording of dopaminergic neuronal activity, and microdialysis measures of gamma-aminobutyric acid (GABA) and glutamate in substantia nigra par compacta were carried out during adulthood (postnatal days 72-78).

RESULTS

Repeated activation of cannabinoid receptors during adolescence increased the release of dopamine in dorsolateral striatum accompanied by increased population activity of dopamine neurons and decreased extracellular GABA levels in substantia nigra par compacta in adulthood. Furthermore, perfusion of bicuculline, a GABAa antagonist, into the ventral pallidum reversed the increased dopamine neuron population activity in substantia nigra par compacta induced by adolescent cannabinoid exposure.

CONCLUSIONS

These results suggest that adolescent exposure to cannabinoid agonists produces disinhibition of nigrostriatal dopamine transmission during adulthood mediated by decreased GABAergic input from the ventral pallidum.

Unbalanced Inhibitory/Excitatory Responses in the Substantia Nigra Pars Reticulata Underlie Cannabinoid-Related Slowness of Movements

The substantia nigra pars reticulata (SNr), where the basal ganglia (BG) direct and indirect pathways converge, contains among the highest expression of cannabinoid receptor type 1 (CB1r) in the brain. Hence, SNr is an ideal locus to study pathway interactions and cannabinergic modulations. The objective of this study was to characterize the effects of systemic injections of the CB1r agonist (CP55940) on the balanced activity of the direct/indirect pathways in the SNr and its associated behaviors. To this aim, we recorded somatosensory and pathway-specific representations in the spiking activity of the SNr of male rats under CP55940. CB1r activation mainly decreased the inhibitory, potentially direct pathway component while sparing the excitatory, potentially indirect pathway component of somatosensory responses. As a result, cutaneous stimulation produced unbalanced responses favoring increased SNr firing rates, suggesting a potential locus for cannabinergic motor-related effects. To test this hypothesis, we implemented an ad hoc behavioral protocol for rats in which systemic administration of CP55940 produced kinematic impairments that were completely reverted by nigral injections of the CB1r antagonist (AM251). Our data suggest that cannabinoid-related motor effects are associated with unbalanced direct/indirect pathway activations that may be reverted by CB1r manipulation at the SNr.SIGNIFICANCE STATEMENT The cannabinergic system has been the target of multiple studies to master its potential use as a therapeutic agent. However, significant advances have been precluded by the lack of mechanistic explanations for the variety of its desirable/undesirable effects. Here, we have combined electrophysiological recordings, pharmacological and optogenetic manipulations, and an ad hoc behavioral protocol to understand how basal ganglia (BG) is affected by cannabinoids. We found that cannabinoids principally affect inhibitory inputs, potentially from the direct pathway, resulting in unbalanced responses in the substantia nigra pars reticulata (SNr) and suggesting a mechanism for the cannabinoid-related slowness of movements. This possibility was confirmed by behavioral experiments in which cannabinoid-related slowness of purposeful movements was reverted by cannabinoid receptor type 1 (CB1r) manipulations directly into the SNr.

The role of the endogenous neurotransmitters associated with neuropathic pain and in the opioid crisis: The innate pain-relieving system

Neuropathic pain is a chronic pain caused by central and peripheral nerve injury, long-term diabetes or treatment with chemotherapy drugs, and it is dissimilar to other chronic pain conditions. Chronic pain usually seriously affects the quality of life, and its drug treatment may result in increased costs of social and medical care. As in the USA and Canada, in Europe, the demand for pain-relieving medicines used in chronic pain has also significantly increased, but most European countries are not experiencing an opioid crisis. In this review, the role of various endogenous transmitters (noradrenaline, dopamine, serotonin, met- and leu-enkephalins, β-endorphin, dynorphins, cannabinoids, ATP) and various receptors (α₂, μ, etc.) in the innate pain-relieving system will be discussed. Furthermore, the modulation of pain processing pathways by transmitters, focusing on neuropathic pain and the role of the sympathetic nervous system in the side effects of excessive opioid treatment, will be explained.

Atypical Endocannabinoid Signaling Initiates a New Form of Memory-Related Plasticity at a Cortical Input to Hippocampus

Endocannabinoids (ECBs) depress transmitter release at sites throughout the brain. Here, we describe another form of ECB signaling that triggers a novel form of long-term potentiation (LTP) localized to the lateral perforant path (LPP) which conveys semantic information from cortex to hippocampus. Two cannabinoid CB1 receptor (CB1R) signaling cascades were identified in hippocampus. The first is pregnenolone sensitive, targets vesicular protein Munc18-1 and depresses transmitter release; this cascade is engaged by CB1Rs in Schaffer-Commissural afferents to CA1 but not in the LPP, and it does not contribute to LTP. The second cascade is pregnenolone insensitive and LPP specific; it entails co-operative CB1R/β1-integrin signaling to effect synaptic potentiation via stable enhancement of transmitter release. The latter cascade is engaged during LPP-dependent learning. These results link atypical ECB signaling to the encoding of a fundamental component of episodic memory and suggest a novel route whereby endogenous and exogenous cannabinoids affect cognition.

Potential of Cannabinoid Receptor Ligands as Treatment for Substance Use Disorders

Substance use disorder (SUD) is a major public health crisis worldwide, and effective treatment options are limited. During the past 2 decades, researchers have investigated the impact of a variety of pharmacological approaches to treat SUD, one of which is the use of medical cannabis or cannabinoids. Significant progress was made with the discovery of rimonabant, a selective CB1 receptor (CB1R) antagonist (also an inverse agonist), as a promising therapeutic for SUDs and obesity. However, serious adverse effects such as depression and suicidality led to the withdrawal of rimonabant (and almost all other CB1R antagonists/inverse agonists) from clinical trials worldwide in 2008. Since then, much research interest has shifted to other cannabinoid-based strategies, such as peripheral CB1R antagonists/inverse agonists, neutral CB1R antagonists, allosteric CB1R modulators, CB2R agonists, fatty acid amide hydrolase (FAAH) inhibitors, monoacylglycerol lipase (MAGL) inhibitors, fatty acid binding protein (FABP) inhibitors, or nonaddictive phytocannabinoids with CB1R or CB2R-binding profiles, as new therapeutics for SUDs. In this article, we first review recent progress in research regarding the endocannabinoid systems, cannabis reward versus aversion, and the underlying receptor mechanisms. We then review recent progress in cannabinoid-based medication development for the treatment of SUDs. As evidence continues to accumulate, neutral CB1R antagonists (such as AM4113), CB2R agonists (JWH133, Xie2-64), and nonselective phytocannabinoids (cannabidiol, β-caryophyllene, ∆9-tetrahydrocannabivarin) have shown great therapeutic potential for SUDs, as shown in experimental animals. Several cannabinoid-based medications (e.g., dronabinol, nabilone, PF-04457845) that entered clinical trials have shown promising results in reducing withdrawal symptoms in cannabis and opioid users.

The neurobiological basis for novel experimental therapeutics in dystonia

Dystonia is a movement disorder characterized by involuntary muscle contractions, twisting movements, and abnormal postures that may affect one or multiple body regions. Dystonia is the third most common movement disorder after Parkinson's disease and essential tremor. Despite its relative frequency, small molecule therapeutics for dystonia are limited. Development of new therapeutics is further hampered by the heterogeneity of both clinical symptoms and etiologies in dystonia. Recent advances in both animal and cell-based models have helped clarify divergent etiologies in dystonia and have facilitated the identification of new therapeutic targets. Advances in medicinal chemistry have also made available novel compounds for testing in biochemical, physiological, and behavioral models of dystonia. Here, we briefly review motor circuit anatomy and the anatomical and functional abnormalities in dystonia. We then discuss recently identified therapeutic targets in dystonia based on recent preclinical animal studies and clinical trials investigating novel therapeutics.

TrkB-dependent disinhibition of the nucleus accumbens is enhanced by ethanol

The nucleus accumbens is a critical integration center for reward-related circuitry and is comprised primarily of medium spiny projection neurons. The dynamic balance of excitation and inhibition onto medium spiny neurons determines the output of this structure. While nucleus accumbens excitatory synaptic plasticity is well-characterized, inhibitory synaptic plasticity mechanisms and their potential relevance to shaping motivated behaviors is poorly understood. Here we report the discovery of long-term depression of inhibitory synaptic transmission in the mouse nucleus accumbens core. This long-term depression is postsynaptically expressed, tropomyosin kinase B (TrkB) receptor-mediated, and augmented in the presence of ethanol. Our findings support the emerging view that TrkB signaling regulates inhibitory synaptic plasticity and suggest this mechanism in the nucleus accumbens as a target for ethanol modulation of reward.

Cannabinoid receptor as a potential therapeutic target for Parkinson's Disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease, characterized by the loss of dopaminergic neurons from substantia nigra pars compacta of basal ganglia caused due to gene mutation, misfolded protein aggregation, reactive oxygen species generation and inflammatory stress. Degeneration of dopaminergic neurons results in muscle stiffness, uncoordinated body movements, sleep disturbance, fatigue, amnesia and impaired voice. Currently, levodopa (L-DOPA) administration is the most widely used therapy for PD. But prolonged administration of L-DOPA is associated with the symptoms of dyskinesia. However, emerging evidences suggest the role of cannabinoid receptors (CBRs) in curtailing the progression of PD by activating neuroprotective pathways. Hence, cannabinoid therapy could be a promising alternative to combat PD in future. In the present review we have discussed the potential role of CBRs in attenuating the key mechanisms of PD and how the existing research gaps needs to be bridged in order to understand the molecular mechanism of CBRs in detail.

Effects of an early life experience on rat brain cannabinoid receptors in adolescence and adulthood

Neonatal handling is an experimental model of early life experience associated with resilience in later life challenges, altering the ability of animals to respond to stress. The endocannabinoid system of the brain modulates the neuroendocrine and behavioral effects of stress, while this system is also capable of being modulated by stress exposure itself. The present study has addressed the question of whether neonatal handling in rats could affect cannabinoid receptors, in an age- and sex-dependent manner, using in situ hybridization and receptor binding techniques. Different effects of neonatal handling were observed in adolescent and adult brain on CB1 receptor mRNA and [3H]CP55,940 binding levels, which in some cases were sexually dimorphic. Neonatal handling interfered in the developmental trajectories of CB1 receptor mRNA levels in striatum and amygdaloid nuclei, as well as of [3H]CP55,940 binding levels in almost all regions studied. Adult handled rats showed reduced [3H]CP55,940 binding levels in the prefrontal cortex, striatum, nucleus accumbens and basolateral amygdala, while binding levels in prefrontal cortex of adolescent handled rats were increased. Finally, handling resulted in decreases in female [3H]CP55,940 binding levels in the striatum, nucleus accumbens, CA3 and DG of dorsal hippocampus and basolateral amygdala. Our results suggest that a brief and repeated maternal separation during the neonatal period induces changes on cannabinoid receptors differently manifested between adolescence and adulthood, male and female brain, which could be correlated to their stress response.

The dose-dependent psychomotor effects of intravenous delta-9-tetrahydrocannabinol (Δ9-THC) in humans

BACKGROUND

Binding studies have demonstrated that levels of the cannabinoid receptor type-1 are highest in the basal ganglia and cerebellum, two areas critical for motor control. However, no studies have systematically examined the dose-related effects of intravenous delta-9-tetrahydrocannabinol, the primary cannabinoid receptor type-1 partial agonist in cannabis, on broad domains of psychomotor function in humans.

AIMS

Therefore, three domains of psychomotor function were assessed in former cannabis users (cannabis abstinent for a minimum of three months; n=23) in a three test-day, within-subject, double-blind, randomized, cross-over, and counterbalanced study during which they received intravenous delta-9-tetrahydrocannabinol (placebo, 0.015 mg/kg, and 0.03 mg/kg).

METHODS

Gross motor function was assessed via the Cambridge Neuropsychological Test Automated Battery Motor Screening Task, fine motor control via the Lafayette Instrument Grooved Pegboard task, and motor timing via a Paced Finger-Tapping Task. In addition, the Cambridge Neuropsychological Test Automated Battery Rapid Visual Processing Task was utilized to determine whether delta-9-tetrahydrocannabinol-induced motor deficits were confounded by disruptions in sustained attention.

RESULTS/OUTCOMES

Delta-9-tetrahydrocannabinol resulted in robust dose-dependent deficits in fine motor control (Grooved Pegboard Task) and motor timing (Paced Finger-Tapping Task), while gross motor performance (Motor Screening Task) and sustained attention (Rapid Visual Processing Task) were unimpaired. Interestingly, despite the observed dose-dependent increases in motor impairment and blood levels of delta-9-tetrahydrocannabinol, subjects reported similar levels of intoxication in the two drug conditions.

CONCLUSIONS/INTERPRETATION

These data suggest that while several domains of motor function are disrupted by delta-9-tetrahydrocannabinol, subjective feelings of intoxication are dissociable from cannabinoid-induced psychomotor effects. Results are discussed in terms of the potential neural mechanisms of delta-9-tetrahydrocannabinol in motor structures.

G protein-coupled receptor heteromers are key players in substance use disorder

G protein-coupled receptors (GPCR) represent the largest family of membrane proteins in the human genome. Physical association between two different GPCRs is linked to functional interactions which generates a novel entity, called heteromer, with specific ligand binding and signaling properties. Heteromerization is increasingly recognized to take place in the mesocorticolimbic pathway and to contribute to various aspects related to substance use disorder. This review focuses on heteromers identified in brain areas relevant to drug addiction. We report changes at the molecular and cellular levels that establish specific functional impact and highlight behavioral outcome in preclinical models. Finally, we briefly discuss selective targeting of native heteromers as an innovative therapeutic option.

The Effects of Cannabinoids on Executive Functions: Evidence from Cannabis and Synthetic Cannabinoids-A Systematic Review

Background—Cannabis is the most popular illicit drug in the Western world. Repeated cannabis use has been associated with short and long-term range of adverse effects. Recently, new types of designer-drugs containing synthetic cannabinoids have been widespread. These synthetic cannabinoid drugs are associated with undesired adverse effects similar to those seen with cannabis use, yet, in more severe and long-lasting forms. Method—A literature search was conducted using electronic bibliographic databases up to 31 December 2017. Specific search strategies were employed using multiple keywords (e.g., “synthetic cannabinoids AND cognition,” “cannabis AND cognition” and “cannabinoids AND cognition”). Results—The search has yielded 160 eligible studies including 37 preclinical studies (5 attention, 25 short-term memory, 7 cognitive flexibility) and 44 human studies (16 attention, 15 working memory, 13 cognitive flexibility). Both pre-clinical and clinical studies demonstrated an association between synthetic cannabinoids and executive-function impairment either after acute or repeated consumptions. These deficits differ in severity depending on several factors including the type of drug, dose of use, quantity, age of onset and duration of use. Conclusions—Understanding the nature of the impaired executive function following consumption of synthetic cannabinoids is crucial in view of the increasing use of these drugs.

The cannabinoid-1 receptor is abundantly expressed in striatal striosomes and striosome-dendron bouquets of the substantia nigra

Presynaptic cannabinoid-1 receptors (CB₁-R) bind endogenous and exogenous cannabinoids to modulate neurotransmitter release. CB₁-Rs are expressed throughout the basal ganglia, including striatum and substantia nigra, where they play a role in learning and control of motivated actions. However, the pattern of CB₁-R expression across different striatal compartments, microcircuits and efferent targets, and the contribution of different CB₁-R-expressing neurons to this pattern, are unclear. We use a combination of conventional techniques and novel genetic models to evaluate CB₁-R expression in striosome (patch) and matrix compartments of the striatum, and in nigral targets of striatal medium spiny projection neurons (MSNs). CB₁-R protein and mRNA follow a descending dorsolateral-to-ventromedial intensity gradient in the caudal striatum, with elevated expression in striosomes relative to the surrounding matrix. The lateral predominance of striosome CB₁-Rs contrasts with that of the classical striosomal marker, the mu opioid receptor (MOR), which is expressed most prominently in rostromedial striosomes. The dorsolateral-to-ventromedial CB₁-R gradient is similar to Drd2 dopamine receptor immunoreactivity and opposite to Substance P. This topology of CB₁-R expression is maintained downstream in the globus pallidus and substantia nigra. Dense CB₁-R-expressing striatonigral fibers extend dorsally within the substantia nigra pars reticulata, and colocalize with bundles of ventrally extending, striosome-targeted, dendrites of dopamine-containing neurons in the substantia nigra pars compacta (striosome-dendron bouquets). Within striatum, CB₁-Rs colocalize with fluorescently labeled MSN collaterals within the striosomes. Cre recombinase-mediated deletion of CB₁-Rs from cortical projection neurons or MSNs, and MSN-selective reintroduction of CB₁-Rs in knockout mice, demonstrate that the principal source of CB₁-Rs in dorsolateral striosomes is local MSN collaterals. These data suggest a role for CB₁-Rs in caudal dorsolateral striosome collaterals and striosome-dendron bouquet projections to lateral substantia nigra, where they are anatomically poised to mediate presynaptic disinhibition of both striosomal MSNs and midbrain dopamine neurons in response to endocannabinoids and cannabinomimetics.

Long lasting effects of chronic WIN55,212-2 treatment on mesostriatal dopaminergic and cannabinoid systems in the rat brain

Cannabinoid administration modulates dopamine transmission via an indirect, multisynaptic mechanism that includes the activation of cannabinoid type-1 receptor (CB1R). The present study evaluated in rodents, the effects of acute and chronic (20 days) WIN55,212-2 administration, a non-selective CB1R agonist, on dopamine uptake and synthesis in the mesolimbic and nigrostriatal dopaminergic pathways and associate them to its effects on the endocannabinoid system. The effect of spontaneous withdrawal, after different abstinence periods (7 days, 20 days), was also assessed. Acute and chronic administration of WIN55,212-2 decreased dopamine transporter (DAT) binding and mRNA levels, as well as tyrosine hydroxylase (TH) mRNA expression in the substantia nigra (SN) and ventral tegmental area (VTA). In the striatum, chronic WIN55,212-2 administration led to decreased protein expression of DAT and TH, whereas no alterations were observed after acute administration, suggesting a diminished dopamine uptake and synthesis after chronic agonist treatment. Furthermore, after chronic agonist treatment, we observed reduced CB1R binding and mRNA levels in SN and striatum, providing evidence for a possible regulatory role of the endocannabinoid system on dopaminergic function. Seven days after WIN55,212-2 cessation, we observed a rebound increase in mRNA, binding and total protein levels of DAT and TH in VTA, SN and striatum proposing the existence of a biphasic expression pattern, which was also observed in CB1R binding levels. Within the 20-day period of abstinence, TH mRNA and protein levels and CB1R binding levels remain increased. The above results indicate that chronic CB1R agonist treatment induces long-lasting control of the mesostriatal dopaminergic activity.

Identifying novel members of the Wntless interactome through genetic and candidate gene approaches

Wnt signaling is an important pathway that regulates several aspects of embryogenesis, stem cell maintenance, and neural connectivity. We have recently determined that opioids decrease Wnt secretion, presumably by inhibiting the recycling of the Wnt trafficking protein Wntless (Wls). This effect appears to be mediated by protein-protein interaction between Wls and the mu-opioid receptor (MOR), the primary cellular target of opioid drugs. The goal of this study was to identify novel protein interactors of Wls that are expressed in the brain and may also play a role in reward or addiction. Using genetic and candidate gene approaches, we show that among a variety of protein, Wls interacts with the dopamine transporter (target of cocaine), cannabinoid receptors (target of THC), Adenosine A2A receptor (target of caffeine), and SGIP1 (endocytic regulator of cannabinoid receptors). Our study shows that aside from opioid receptors, Wntless interacts with additional proteins involved in reward and/or addiction. Future studies will determine whether Wntless and WNT signaling play a more universal role in these processes.

Perspectives on the Role of Endocannabinoids in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) are diagnosed on the basis of three behavioral features, namely, (1) deficits in social communication, (2) absence or delay in language and (3) stereotypy. The consensus regarding the neurological pathogenesis of ASDs is aberrant synaptogenesis and synapse function. Further, it is now widely accepted that ASD is neurodevelopmental in nature, placing emphasis on derangements occurring at the level of intra- and intercellular signaling during corticogenesis. At present, there is an ever-growing list of mutations in putative susceptibility genes in affected individuals, preventing effective transformation of knowledge gathered from basic science research to the clinic. In response, the focus of ASD biology has shifted toward the identification of cellular signaling pathways that are common to various ASD-related mutations in hopes that these shared pathways may serve as more promising treatment targets than targeting individual genes or proteins. To this end, the endogenous cannabinoid (endocannabinoid, eCB) system has recently emerged as a promising therapeutic target in the field of ASD research. The eCB system is altered in several neurological disorders, but the role of these bioactive lipids in ASD etiology remains poorly understood. In this perspective, we review current evidence linking eCB signaling to ASDs and put forth the notion that continued focus on eCBs in autism research may provide valuable insight into pathophysiology and treatment strategies. In addition to its role in modulating transmitter release at mature synapses, the eCB signaling system plays important roles in many aspects of cortical development, and disruption of these effects of eCBs may also be related to ASD pathophysiology.

Glucocorticoid administration into the dorsolateral but not dorsomedial striatum accelerates the shift from a spatial toward procedural memory

Glucocorticoid stress hormones are known to enhance the consolidation of hippocampus-dependent spatial and contextual memory. Recent findings indicate that glucocorticoids also enhance the consolidation of procedural memory that relies on the dorsal striatum. The dorsal striatum can be functionally subdivided into the dorsolateral striatum (DLS), which is primarily implicated in shaping procedural memories, and the dorsomedial striatum (DMS), which is engaged in spatial memory. Here, we investigated the hypothesis that posttraining glucocorticoid administration into the DLS promotes the formation of a procedural memory that will normally take place only with extensive training. Male Wistar rats were trained to find a reward in a cross maze that can be solved through either place or response learning. Rats received four trials per day for 5days, a probe trial on Day 6, further training on Days 7-13, and an additional probe trial on Day 14. On Days 2-4 of training, they received posttraining infusions of corticosterone (10 or 30ng) or vehicle into either the DLS or DMS. Rats treated with vehicle into either the DLS or DMS displayed place learning on Day 6 and response learning on Day 14, indicating a shift in control of learned behavior toward a habit-like procedural strategy with extended training. Rats administered corticosterone (10ng) into the DLS displayed response learning on both Days 6 and 14, indicating an accelerated shift to response learning. In contrast, corticosterone administered posttraining into the DMS did not significantly alter the shift from place to response learning. These findings indicate that glucocorticoid administration into the DLS enhances memory consolidation of procedural learning and thereby influences the timing of the switch from the use of spatial/contextual memory to habit-like procedural memory to guide behavior.

CB1 cannabinoid receptor-mediated anandamide signaling mechanisms of the inferior colliculus modulate the haloperidol-induced catalepsy

The inferior colliculus (IC), a midbrain structure that processes acoustic information of aversive nature, is distinguished from other auditory nuclei in the brainstem by its connections with structures of the motor system. Previous evidence relating the IC to motor behavior shows that glutamatergic and GABAergic mechanisms in the IC exert influence on systemic haloperidol-induced catalepsy. There is substantial evidence supporting a role played by the endocannabinoid system as a modulator of the glutamatergic neurotransmission, as well as the dopaminergic activity in the basal nuclei and therefore it may be considered as a potential pharmacological target for the treatment of movement disorders. The present study evaluated if the endocannabinoid system in the IC plays a role in the elaboration of systemic haloperidol-induced catalepsy. Male Wistar rats received intracollicular microinjection of either the endogenous cannabinoid anandamide (AEA) at different concentrations (5, 50 or 100pmol/0.2μl), the CB₁ cannabinoid receptor antagonist AM251 at 50, 100 or 200pmol/0.2μl or vehicle, followed by intraperitoneal (IP) administration of either haloperidol at 0.5 or 1mg/kg or physiological saline. Systemic injection of haloperidol at both doses (0.5 or 1mg/kg, IP) produced a cataleptic state, compared to vehicle/physiological saline-treated group, lasting 30 and 50min after systemic administration of the dopaminergic receptors non-selective antagonist. The midbrain microinjection of AEA at 50pmol/0.2μl increased the latency for stepping down from the horizontal bar after systemic administration of haloperidol. Moreover, the intracollicular administration of AEA at 50pmol/0.2μl was able to increase the duration of catalepsy as compared to AEA at 100pmol/0.2-μl-treated group. Intracollicular pretreatment with AM251 at the intermediate concentration (100pmol/0.2μl) was able to decrease the duration of catalepsy after systemic administration of haloperidol. However, neither the intracollicular microinjection of AM251 at the lowest (50pmol/0.2μl) nor at the highest (200pmol/0.2μl) concentration was able to block the systemic haloperidol-induced catalepsy. Furthermore, the intracollicular administration of AM251 at 100pmol/0.2μl was able to decrease the duration of catalepsy as compared to AM251 at 50pmol/0.2μl- and AM251 at 200pmol/0.2-μl-treated group. The latency for stepping down from the horizontal bar - induced by haloperidol administration - was decreased when microinjection of AEA at 50pmol/0.2μl was preceded with blockade of CB1 receptor with AM251 (100pmol/0.2μl). Our results strengthen the involvement of CB1-signaled endocannabinoid mechanisms of the IC in the neuromodulation of catalepsy induced by systemic administration of the dopaminergic receptors non-selective antagonist haloperidol.

A Primary Cortical Input to Hippocampus Expresses a Pathway-Specific and Endocannabinoid-Dependent Form of Long-Term Potentiation

The endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG), a key modulator of synaptic transmission in mammalian brain, is produced in dendritic spines and then crosses the synaptic junction to depress neurotransmitter release. Here we report that 2-AG-dependent retrograde signaling also mediates an enduring enhancement of glutamate release, as assessed with independent tests, in the lateral perforant path (LPP), one of two cortical inputs to the granule cells of the dentate gyrus. Induction of this form of long-term potentiation (LTP) involved two types of glutamate receptors, changes in postsynaptic calcium, and the postsynaptic enzyme that synthesizes 2-AG. Stochastic optical reconstruction microscopy confirmed that CB₁ cannabinoid receptors are localized presynaptically to LPP terminals, while the inhibition or knockout of the receptors eliminated LPP-LTP. Suppressing the enzyme that degrades 2-AG dramatically enhanced LPP potentiation, while overexpressing it produced the opposite effect. Priming with a CB₁ agonist markedly reduced the threshold for LTP. Latrunculin A, which prevents actin polymerization, blocked LPP-LTP when applied extracellularly but had no effect when infused postsynaptically into granule cells, indicating that critical actin remodeling resides in the presynaptic compartment. Importantly, there was no evidence for the LPP form of potentiation in the Schaffer-commissural innervation of field CA1 or in the medial perforant path. Peripheral injections of compounds that block or enhance LPP-LTP had corresponding effects on the formation of long-term memory for cues conveyed to the dentate gyrus by the LPP. Together, these results indicate that the encoding of information carried by a principal hippocampal afferent involves an unusual, regionally differentiated form of plasticity.

Endocannabinoid Modulation of Orbitostriatal Circuits Gates Habit Formation

Everyday function demands efficient and flexible decision-making that allows for habitual and goal-directed action control. An inability to shift has been implicated in disorders with impaired decision-making, including obsessive-compulsive disorder and addiction. Despite this, our understanding of the specific molecular mechanisms and circuitry involved in shifting action control remains limited. Here we identify an endogenous molecular mechanism in a specific cortical-striatal pathway that mediates the transition between goal-directed and habitual action strategies. Deletion of cannabinoid type 1 (CB1) receptors from cortical projections originating in the orbital frontal cortex (OFC) prevents mice from shifting from goal-directed to habitual instrumental lever pressing. Activity of OFC neurons projecting to dorsal striatum (OFC-DS) and, specifically, activity of OFC-DS terminals is necessary for goal-directed action control. Lastly, CB1 deletion from OFC-DS neurons prevents the shift from goal-directed to habitual action control. These data suggest that the emergence of habits depends on endocannabinoid-mediated attenuation of a competing circuit controlling goal-directed behaviors.

Hierarchical glucocorticoid-endocannabinoid interplay regulates the activation of the nucleus accumbens by insulin

Here we asked if insulin activation of the nucleus accumbens in vitro is reflected by an increase in (3)H-deoxyglucose ([(3)H]DG) uptake, thus subserving a new model to study molecular mechanisms of central insulin actions. Additionally, we investigated the dependence of this insulin effect on endocannabinoids and corticosteroids, two major culprits in insulin resistance. We found that in acute accumbal slices, insulin (3 and 300nM but not at 0.3nM) produced an increase in [(3)H]DG uptake. The synthetic cannabinoid agonist, WIN55212-2 (500nM) and the glucocorticoid dexamethasone (10μM), impaired insulin (300nM) action on [(3)H]DG uptake. The glucocorticoid receptor (GcR) antagonist, mifepristone (10μM) prevented dexamethasone from inhibiting insulin's action. Strikingly, this anti-insulin action of dexamethasone was also blocked by two CB1 cannabinoid receptor (CB1R) antagonists, O-2050 (500nM) and SR141716A (500nM), as well as by tetrahydrolipstatin (10μM), an inhibitor of diacylglycerol lipases-the enzymes responsible for the synthesis of the endocannabinoid, 2-arachidonoyl-glycerol (2-AG). On the other hand, the blockade of the post-synaptic 2-AG metabolizing enzymes, α,β-serine hydrolase domain 6/12 by WWL70 (1μM) also prevented the action of insulin, probably via increasing endogenous 2-AG tone. Additionally, an anti-insulin receptor (InsR) antibody immunoprecipitated CB1Rs from accumbal homogenates, indicating a physical complexing of CB1Rs with InsRs that supports their functional interaction. Altogether, insulin stimulates glucose uptake in the nucleus accumbens. Accumbal GcR activation triggers the synthesis of 2-AG that in turn binds to the known CB1R-InsR heteromer, thus impeding insulin signaling.

An Introduction to the Endogenous Cannabinoid System

The endocannabinoid system (ECS) is a widespread neuromodulatory system that plays important roles in central nervous system development, synaptic plasticity, and the response to endogenous and environmental insults. The ECS comprises cannabinoid receptors, endogenous cannabinoids (endocannabinoids), and the enzymes responsible for the synthesis and degradation of the endocannabinoids. The most abundant cannabinoid receptors are the CB1 cannabinoid receptors; however, CB2 cannabinoid receptors, transient receptor potential channels, and peroxisome proliferator activated receptors are also engaged by some cannabinoids. Exogenous cannabinoids, such as tetrahydrocannabinol, produce their biological effects through their interactions with cannabinoid receptors. The best-studied endogenous cannabinoids are 2-arachidonoyl glycerol and arachidonoyl ethanolamide (anandamide). Despite similarities in chemical structure, 2-arachidonoyl glycerol and anandamide are synthesized and degraded by distinct enzymatic pathways, which impart fundamentally different physiologic and pathophysiologic roles to these two endocannabinoids. As a result of the pervasive social use of cannabis and the involvement of endocannabinoids in a multitude of biological processes, much has been learned about the physiologic and pathophysiologic roles of the ECS. This review provides an introduction to the ECS with an emphasis on its role in synaptic plasticity and how the ECS is perturbed in schizophrenia.

Immunohistochemical distribution of the cannabinoid receptor 1 and fatty acid amide hydrolase in the dog claustrum

Cannabinoid receptor 1 (CB1R) and fatty acid amide hydrolase (FAAH) are part of the endocannabinoid system (ECB) which exerts a neuromodulatory activity on different brain functions and plays a key role in neurogenesis. Although many studies have reported FAAH and CB1R expression in the brain of different animal species, to the best of our knowledge they have never been described in the canine claustrum. Claustrum samples, obtained from necropsy of four neurologically normal dogs, were formalin fixed for paraffin embedding. Sections were either stained for morpho-histological analysis or immunostained for CB1R and FAAH. Analysis of adjacent sections incubated with the two antisera showed a complementary labeling pattern in the claustrum, with CB1R antibody staining fibers while anti-FAAH antibody stained cell bodies and the proximal portion of dendrites; this particular anatomical relationship suggests a retrograde endocannabinoid action via CB1R. CB1R and FAAH complementary immunostaining and their cellular localization reported here provide the first anatomical evidence for existence of the ECB in the dog claustrum.

Loss of Striatonigral GABAergic Presynaptic Inhibition Enables Motor Sensitization in Parkinsonian Mice

Degeneration of dopamine (DA) neurons in Parkinson's disease (PD) causes hypokinesia, but DA replacement therapy can elicit exaggerated voluntary and involuntary behaviors that have been attributed to enhanced DA receptor sensitivity in striatal projection neurons. Here we reveal that in hemiparkinsonian mice, striatal D1 receptor-expressing medium spiny neurons (MSNs) directly projecting to the substantia nigra reticulata (SNr) lose tonic presynaptic inhibition by GABAB receptors. The absence of presynaptic GABAB response potentiates evoked GABA release from MSN efferents to the SNr and drives motor sensitization. This alternative mechanism of sensitization suggests a synaptic target for PD pharmacotherapy.

Endocannabinoid signalling in reward and addiction

Brain endocannabinoid (eCB) signalling influences the motivation for natural rewards (such as palatable food, sexual activity and social interaction) and modulates the rewarding effects of addictive drugs. Pathological forms of natural and drug-induced reward are associated with dysregulated eCB signalling that may derive from pre-existing genetic factors or from prolonged drug exposure. Impaired eCB signalling contributes to dysregulated synaptic plasticity, increased stress responsivity, negative emotional states and cravings that propel addiction. Understanding the contributions of eCB disruptions to behavioural and physiological traits provides insight into the eCB influence on addiction vulnerability.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Activation of cannabinoid system in anterior cingulate cortex and orbitofrontal cortex modulates cost-benefit decision making

Despite the evidence for altered decision making in cannabis abusers, the role of the cannabinoid system in decision-making circuits has not been studied. Here, we examined the effects of cannabinoid modulation during cost-benefit decision making in the anterior cingulate cortex (ACC) and orbitofrontal cortex (OFC), key brain areas involved in decision making. We trained different groups of rats in a delay-based and an effort-based form of cost-benefit T-maze decision-making task. During test days, the rats received local injections of either vehicle or ACEA, a cannabinoid type-1 receptor (CB1R) agonist in the ACC or OFC. We measured spontaneous locomotor activity following the same treatments and characterized CB1Rs localization on different neuronal populations within these regions using immunohistochemistry. We showed that CB1R activation in the ACC impaired decision making such that rats were less willing to invest physical effort to gain high reward. Similarly, CB1R activation in the OFC induced impulsive pattern of choice such that rats preferred small immediate rewards to large delayed rewards. Control tasks ensured that the effects were specific for differential cost-benefit tasks. Furthermore, we characterized widespread colocalizations of CB1Rs on GABAergic axonal ends but few colocalizations on glutamatergic, dopaminergic, and serotonergic neuronal ends. These results provide first direct evidence that the cannabinoid system plays a critical role in regulating cost-benefit decision making in the ACC and OFC and implicate cannabinoid modulation of synaptic ends of predominantly interneurons and to a lesser degree other neuronal populations in these two frontal regions.

Components of the endocannabinoid and dopamine systems are dysregulated in Huntington's disease: analysis of publicly available microarray datasets

The endocannabinoid system (ECS) and the dopaminergic system (DAS) are two major regulators of basal ganglia function. During Huntington's disease (HD) pathogenesis, the expression of genes in both the ECS and DAS is dysregulated. The purpose of this study was to determine the changes that were consistently observed in the ECS and DAS during HD progression in the central nervous system (CNS) and in the periphery in different models of HD and human HD tissue. To do this, we conducted a meta-analysis of differential gene expression in the ECS and DAS using publicly available microarray data. The consolidated data were summarized as observed changes in gene expression (OCGE) using a weighted sum for each gene. In addition, consolidated data were compared to previously published studies that were not available in the gene expression omnibus (GEO) database. The resulting data confirm gene expression changes observed using different approaches and provide novel insights into the consistency between changes observed in human tissue and various models, as well as disease stage- and tissue-specific transcriptional dysregulation in HD. The major implication of the systems-wide data presented here is that therapeutic strategies targeting the ECS or DAS must consider the dynamic changes in gene expression over time and in different body areas, which occur during HD progression and the interconnectedness of the two systems.

Input- and cell-type-specific endocannabinoid-dependent LTD in the striatum

Changes in basal ganglia plasticity at the corticostriatal and thalamostriatal levels are required for motor learning. Endocannabinoid-dependent long-term depression (eCB-LTD) is known to be a dominant form of synaptic plasticity expressed at these glutamatergic inputs; however, whether eCB-LTD can be induced at all inputs on all striatal neurons is still debatable. Using region-specific Cre mouse lines combined with optogenetic techniques, we directly investigated and distinguished between corticostriatal and thalamostriatal projections. We found that eCB-LTD was successfully induced at corticostriatal synapses, independent of postsynaptic striatal spiny projection neuron (SPN) subtype. Conversely, eCB-LTD was only nominally present at thalamostriatal synapses. This dichotomy was attributable to the minimal expression of cannabinoid type 1 (CB1) receptors on thalamostriatal terminals. Furthermore, coactivation of dopamine receptors on SPNs during LTD induction re-established SPN-subtype-dependent eCB-LTD. Altogether, our findings lay the groundwork for understanding corticostriatal and thalamostriatal synaptic plasticity and for striatal eCB-LTD in motor learning.

Detection of cannabinoid receptors CB1 and CB2 within basal ganglia output neurons in macaques: changes following experimental parkinsonism

Although type 1 cannabinoid receptors (CB₁Rs) are expressed abundantly throughout the brain, the presence of type 2 cannabinoid receptors (CB₂Rs) in neurons is still somewhat controversial. Taking advantage of newly designed CB₁R and CB₂R mRNA riboprobes, we demonstrate by PCR and in situ hybridization that transcripts for both cannabinoid receptors are present within labeled pallidothalamic-projecting neurons of control and MPTP-treated macaques, whereas the expression is markedly reduced in dyskinetic animals. Moreover, an in situ proximity ligation assay was used to qualitatively assess the presence of CB₁Rs and CB₂Rs, as well as CB₁R–CB₂R heteromers within basal ganglia output neurons in all animal groups (control, parkinsonian and dyskinetic macaques). A marked reduction in the number of CB₁Rs, CB₂Rs and CB₁R–CB₂R heteromers was found in dyskinetic animals, mimicking the observed reduction in CB₁R and CB₂R mRNA expression levels. The fact that chronic levodopa treatment disrupted CB₁R–CB₂R heteromeric complexes should be taken into consideration when designing new drugs acting on cannabinoid receptor heteromers.

L-DOPA disrupts adenosine A(2A)-cannabinoid CB(1)-dopamine D(2) receptor heteromer cross-talk in the striatum of hemiparkinsonian rats: biochemical and behavioral studies

Long-term therapy with L-3,4-dihydroxyphenylalanine (L-DOPA), still the most effective treatment in Parkinson's disease (PD), is associated with severe motor complications such as dyskinesia. Experimental and clinical data have indicated that adenosine A2A receptor antagonists can provide symptomatic improvement by potentiating L-DOPA efficacy and minimizing its side effects. It is known that the G-protein-coupled adenosine A2A, cannabinoid CB1 and dopamine D2 receptors may interact and form functional A2A-CB1-D2 receptor heteromers in co-transfected cells as well as in rat striatum. These data suggest that treatment with a combination of drugs or a single compound selectively acting on A2A-CB1-D2 heteromers may represent an alternative therapeutic treatment of PD. We investigated the expression of A2A-CB1-D2 receptor heteromers in the striatum of both naïve and hemiparkinsonian rats (HPD-rats) bearing a unilateral 6-hydroxydopamine (6-OHDA) lesion, and assessed how receptor heteromer expression and biochemical properties were affected by L-DOPA treatment. Radioligand binding data showed that A2A-CB1-D2 receptor heteromers are present in the striatum of both naïve and HPD-rats. However, behavioral results indicated that the combined administration of A2A (MSX-3 or SCH58261) and CB1 (rimonabant) receptor antagonists, in the presence of L-DOPA does not produce a response different from administration of the A2A receptor antagonist alone. These behavioral results prompted identification of heteromers in L-DOPA-treated animals. Interestingly, the radioligand binding results in samples from lesioned animals suggest that the heteromer is lost following acute or chronic treatment with L-DOPA.

Temporal changes of CB1 cannabinoid receptor in the basal ganglia as a possible structure-specific plasticity process in 6-OHDA lesioned rats

The endocannabinoid system has been implicated in several neurobiological processes, including neurodegeneration, neuroprotection and neuronal plasticity. The CB1 cannabinoid receptors are abundantly expressed in the basal ganglia, the circuitry that is mostly affected in Parkinson’s Disease (PD). Some studies show variation of CB1 expression in basal ganglia in different animal models of PD, however the results are quite controversial, due to the differences in the procedures employed to induce the parkinsonism and the periods analyzed after the lesion. The present study evaluated the CB1 expression in four basal ganglia structures, namely striatum, external globus pallidus (EGP), internal globus pallidus (IGP) and substantia nigra pars reticulata (SNpr) of rats 1, 5, 10, 20, and 60 days after unilateral intrastriatal 6-hydroxydopamine injections, that causes retrograde dopaminergic degeneration. We also investigated tyrosine hydroxylase (TH), parvalbumin, calbindin and glutamic acid decarboxylase (GAD) expression to verify the status of dopaminergic and GABAergic systems. We observed a structure-specific modulation of CB1 expression at different periods after lesions. In general, there were no changes in the striatum, decreased CB1 in IGP and SNpr and increased CB1 in EGP, but this increase was not sustained over time. No changes in GAD and parvalbumin expression were observed in basal ganglia, whereas TH levels were decreased and the calbindin increased in striatum in short periods after lesion. We believe that the structure-specific variation of CB1 in basal ganglia in the 6-hydroxydopamine PD model could be related to a compensatory process involving the GABAergic transmission, which is impaired due to the lack of dopamine. Our data, therefore, suggest that the changes of CB1 and calbindin expression may represent a plasticity process in this PD model.

Interleukin-1β alters the sensitivity of cannabinoid CB1 receptors controlling glutamate transmission in the striatum

Proinflammatory cytokines such as tumor necrosis factor-α and interleukin-1β (IL1β) regulate both excitatory and inhibitory synaptic transmission in the central nervous system. The interaction between IL1β and endocannabinoid system (ECS) is also emerging, based on the evidence that IL1β effects on striatal spontaneous excitatory and inhibitory postsynaptic currents are regulated by transient receptor potential vanilloid 1 (TRPV1) channels, members of the ECS. Furthermore, IL1β has also been shown to control the sensitivity of cannabinoid CB1 receptors controlling GABA transmission (CB1Rs(GABA)) in the striatum. To better detail the synaptic action of IL1β, and to clarify its complex interaction with the ECS, here we investigated the possible interplay between IL1β and CB1Rs controlling glutamate transmission (CB1Rs(glu)), other critical elements of the ECS. Our results show that the sensitivity of CB1Rs(glu) is fully blocked in the presence of IL1β in corticostriatal brain slices, and that the protein kinase C/TRPV1 pathway is involved in this effect. IL1β failed to modulate the sensitivity of glutamate synapses to the stimulation of GABAB receptors. We also provided evidence that IL1β-CB1Rs(GABA) but not IL1β-CB1Rs(glu) interaction is under the control of the brain-derived neurotrophic factor (BDNF)/trkB signaling and of lipid raft composition, because BDNF gene partial deletion, pharmacological blockade of trkB and membrane cholesterol removal with methyl-β-cyclodextrin all blocked IL1β-mediated inhibition of CB1Rs(GABA) but left unaltered the sensitivity of CB1Rs(glu) to this cytokine. Our results provide further evidence that synaptic transmission and the ECS are regulated by IL1β in the striatum.