Modulation of neurotransmission by cannabinoids in the basal ganglia

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 Stimulate the TRP Channel-Dependent Release of Both Serotonin and Dopamine to Modulate Behavior in C. elegans

Cannabis sativa alters sensory perception and exhibits potential medicinal benefits. In mammals, cannabinoids activate two canonical receptors, CB₁/CB₂, as well additional receptors/ion channels whose overall contributions to cannabinoid signaling have yet to be fully assessed. In Caenorhabditis elegans, the endogenous cannabinoid receptor agonist, 2-arachidonoylglycerol (2-AG) activates a CB₁ ortholog, NPR-19, to modulate behavior (Oakes et al., 2017). In addition, 2-AG stimulates the NPR-19 independent release of both serotonin (5-HT) and dopamine (DA) from subsets of monoaminergic neurons to modulate locomotory behaviors through a complex monoaminergic signaling pathway involving multiple serotonin and dopamine receptors. 2-AG also inhibits locomotion in remodeled monoamine receptor mutant animals designed to measure the acute release of either 5-HT or DA, confirming the direct effects of 2-AG on monoamine release. 2-AG-dependent locomotory inhibition requires the expression of transient receptor potential vanilloid 1 (TRPV1) and TRPN-like channels in the serotonergic or dopaminergic neurons, respectively, and the acute pharmacological inhibition of the TRPV1-like channel abolishes both 2-AG-dependent 5-HT release and locomotory inhibition, suggesting the 2-AG may activate the channel directly. This study highlights the advantages of identifying and assessing both CB₁/CB₂-dependent and independent cannabinoid signaling pathways in a genetically tractable, mammalian predictive model, where cannabinoid signaling at the molecular/neuronal levels can be correlated directly with changes in behavior.SIGNIFICANCE STATEMENT This study is focused on assessing CB₁/CB₂-independent cannabinoid signaling in a genetically tractable, whole-animal model where cannabinoid signaling at the molecular/neuronal levels can be correlated with behavioral change. Caenorhabditis elegans contains a cannabinoid signaling system mediated by a canonical cannabinoid receptor, NPR-19, with orthology to human CB₁/CB₂ (Oakes et al., 2017). The present study has characterized an NPR-19-independent signaling pathway that involves the cannabinoid-dependent release of both serotonin and dopamine and the expression of distinct TRP-like channels on the monoaminergic neurons. Our work should be of interest to those studying the complexities of CB₁/CB₂-independent cannabinoid signaling, the role of TRP channels in the modulation of monoaminergic signaling, and the cannabinoid-dependent modulation of behavior.

Molecular Imaging in Huntington's Disease

Huntington's disease (HD) is a rare monogenic neurodegenerative disorder caused by a trinucleotide CAG repeat expansion in the huntingtin gene resulting in the formation of intranuclear inclusions of mutated huntingtin. The accumulation of mutated huntingtin leads to loss of GABAergic medium spiny neurons (MSNs); subsequently resulting in the development of chorea, cognitive dysfunction and psychiatric symptoms. Premanifest HD gene expansion carriers, provide a unique cohort to examine very early molecular changes, occurring before the development of overt symptoms, to elucidate disease pathophysiology and identify reliable biomarkers of HD progression. Positron emission tomography (PET) is a non-invasive molecular imaging technique allowing the evaluation of specific molecular targets in vivo. Selective PET radioligands provide invaluable tools to investigate the role of the dopaminergic system, brain metabolism, microglial activation, phosphodiesterase 10A, and cannabinoid, GABA, adenosine and opioid receptors in HD. PET has been employed to monitor disease progression aiming to identify a reliable biomarker to predict phenoconversion from premanifest to manifest HD.

Prefrontal endocannabinoids, stress controllability and resilience: A hypothesis

Stressor exposure is a predisposing risk factor for many psychiatric conditions such as PTSD and depression. However, stressors do not influence all individuals equally and in response to an identical stressor some individuals may be vulnerable while others are resilient. While various biological and behavioral factors contribute to vulnerability versus resilience, an individual's degree of control over the stressor is among the most potent. Even with only one experience with control over stress, behavioral control has been shown to have acute and long-lasting stress-mitigating effects. This suggests that control both blunts the response to acute stress and prepares the subject to be resilient to future stressors. In this review, we first summarize the evidence which suggests the ventromedial prefrontal cortex (vmPFC) is a critical component of stressor controllability circuits and a locus of neuroplasticity supporting the acute and long-lasting consequences of control. We next review the central endocannabinoid (eCB) system as a possible mediator of short and long-term synaptic transmission in the vmPFC, and offer a hypothesis whereby eCBs regulate vmPFC circuits engaged when a subject has control over stress and may contribute to the encoding of acute stress coping into long lasting stressor resilience.

Striatal molecular alterations in HD gene carriers: a systematic review and meta-analysis of PET studies

BACKGROUND

Over the past years, positron emission tomography (PET) imaging studies have investigated striatal molecular changes in premanifest and manifest Huntington's disease (HD) gene expansion carriers (HDGECs), but they have yielded inconsistent results.

OBJECTIVE

To systematically examine the evidence of striatal molecular alterations in manifest and premanifest HDGECs as measured by PET imaging studies.

METHODS

MEDLINE, ISI Web of Science, Cochrane Library and Scopus databases were searched for articles published until 7 June 2017 that included PET studies in manifest and premanifest HDGECs. Meta-analyses were conducted with random effect models, and heterogeneity was addressed with I² index, controlling for publication bias and quality of study. The primary outcome was the standardised mean difference (SMD) of PET uptakes in the whole striatum, caudate and putamen in manifest and premanifest HDGECs compared with healthy controls (HCs).

RESULTS

Twenty-four out of 63 PET studies in premanifest (n=158) and manifest (n=191) HDGECs and HCs (n=333) were included in the meta-analysis. Premanifest and manifest HDGECs showed significant decreases in dopamine D₂ receptors in caudate (SMD=-1.233, 95% CI -1.753 to -0.713, p<0.0001; SMD=-5.792, 95% CI -7.695 to -3.890, p<0.0001) and putamen (SMD=-1.479, 95% CI -1.965 to -0.992, p<0.0001; SMD=-5.053, 95% CI -6.558 to -3.549, p<0.0001), in glucose metabolism in caudate (SMD=-0.758, 95% CI -1.139 to -0.376, p<0.0001; SMD=-3.738, 95% CI -4.880 to -2.597, p<0.0001) and putamen (SMD=-2.462, 95% CI -4.208 to -0.717, p=0.006; SMD=-1.650, 95% CI -2.842 to -0.458, p<0.001) and in striatal PDE10A binding (SMD=-1.663, 95% CI -2.603 to -0.723, p=0.001; SMD=-2.445, 95% CI -3.371 to -1.519, p<0.001).

CONCLUSIONS

PET imaging has the potential to detect striatal molecular changes even at the early premanifest stage of HD, which are relevant to the neuropathological mechanisms underlying the development of the disease.

Progress toward pharmacotherapies for cannabis-use disorder: an evidence-based review

Cannabis is the most widely used and variably regulated drug in the world, with increasing trends of use being reported in the US, Australia, Asia, and Africa. Evidence has shown a decrease in the age of commencement of cannabis use in some developed countries and a prolongation of risk of initiation to cannabis use beyond adolescence among more recent users. Cannabis use is associated with numerous health risks and long-term morbidity, as well as risk of developing cannabis-use disorders. Cannabis users infrequently seek professional treatment, and normally do so after a decade of use. Cannabis-use disorders are currently treated using a selection of psychosocial interventions. Severity of withdrawal is a factor that increases the risk of relapse, and is the target of pharmacotherapy studies. Currently, there is no approved pharmacotherapy for cannabis-use disorders. A number of approaches have been examined, and trials are continuing to find a safe and effective medication with little abuse liability.

Current status of PET imaging in Huntington's disease

Purpose

To review the developments of recent decades and the current status of PET molecular imaging in Huntington’s disease (HD).

Methods

A systematic review of PET studies in HD was performed. The MEDLINE, Web of Science, Cochrane and Scopus databases were searched for articles in all languages published up to 19 August 2015 using the major medical subject heading “Huntington Disease” combined with text and key words “Huntington Disease”, “Neuroimaging” and “PET”. Only peer-reviewed, primary research studies in HD patients and premanifest HD carriers, and studies in which clinical features were described in association with PET neuroimaging results, were included in this review. Reviews, case reports and nonhuman studies were excluded.

Results

A total of 54 PET studies were identified and analysed in this review. Brain metabolism ([¹⁸F]FDG and [¹⁵O]H₂O), presynaptic ([¹⁸F]fluorodopa, [¹¹C]β-CIT and [¹¹C]DTBZ) and postsynaptic ([¹¹C]SCH22390, [¹¹C]FLB457 and [¹¹C]raclopride) dopaminergic function, phosphodiesterases ([¹⁸F]JNJ42259152, [¹⁸F]MNI-659 and [¹¹C]IMA107), and adenosine ([¹⁸F]CPFPX), cannabinoid ([¹⁸F]MK-9470), opioid ([¹¹C]diprenorphine) and GABA ([¹¹C]flumazenil) receptors were evaluated as potential biomarkers for monitoring disease progression and for assessing the development and efficacy of novel disease-modifying drugs in premanifest HD carriers and HD patients. PET studies evaluating brain restoration and neuroprotection were also identified and described in detail.

Conclusion

Brain metabolism, postsynaptic dopaminergic function and phosphodiesterase 10A levels were proven to be powerful in assessing disease progression. However, no single technique may be currently considered an optimal biomarker and an integrative multimodal imaging approach combining different techniques should be developed for monitoring potential neuroprotective and preventive treatment in HD.

Neuroimaging in Huntington’s disease

Huntington’s disease (HD) is a progressive and fatal neurodegenerative disorder caused by an expanded trinucleotide CAG sequence in huntingtin gene (HTT) on chromosome 4. HD manifests with chorea, cognitive and psychiatric symptoms. Although advances in genetics allow identification of individuals carrying the HD gene, much is still unknown about the mechanisms underlying the development of overt clinical symptoms and the transitional period between premanifestation and manifestation of the disease. HD has no cure and patients rely only in symptomatic treatment. There is an urgent need to identify biomarkers that are able to monitor disease progression and assess the development and efficacy of novel disease modifying drugs. Over the past years, neuroimaging techniques such as magnetic resonance imaging (MRI) and positron emission tomography (PET) have provided important advances in our understanding of HD. MRI provides information about structural and functional organization of the brain, while PET can detect molecular changes in the brain. MRI and PET are able to detect changes in the brains of HD gene carriers years ahead of the manifestation of the disease and have also proved to be powerful in assessing disease progression. However, no single technique has been validated as an optimal biomarker. An integrative multimodal imaging approach, which combines different MRI and PET techniques, could be recommended for monitoring potential neuroprotective and preventive therapies in HD. In this article we review the current neuroimaging literature in HD.

Hyperactivity induced by the dopamine D2/D3 receptor agonist quinpirole is attenuated by inhibitors of endocannabinoid degradation in mice

The present study was designed to investigate the effect of pharmacological inhibition of endocannabinoid degradation on behavioural actions of the dopamine D2/D3 receptor agonist quinpirole in male C57Bl/6J mice. In addition, we studied the effects of endocannabinoid degradation inhibition on both cocaine-induced psychomotor activation and behavioural sensitization. We analysed the effects of inhibition of the two main endocannabinoid degradation enzymes: fatty acid amide hydrolase (FAAH), using inhibitor URB597 (1 mg/kg); monoacylglycerol lipase (MAGL), using inhibitor URB602 (10 mg/kg). Administration of quinpirole (1 mg/kg) caused a temporal biphasic response characterized by a first phase of immobility (0-50 min), followed by enhanced locomotion (next 70 min) that was associated with the introduction of stereotyped behaviours (stereotyped jumping and rearing). Pretreatment with both endocannabinoid degradation inhibitors did not affect the hypoactivity actions of quinpirole. However, this pretreatment resulted in a marked decrease in quinpirole-induced locomotion and stereotyped behaviours. Administration of FAAH or MAGL inhibitors did not attenuate the acute effects of cocaine. Furthermore, these inhibitors did not impair the acquisition of cocaine-induced behavioural sensitization or the expression of cocaine-induced conditioned locomotion. Only MAGL inhibition attenuated the expression of an already acquired cocaine-induced behavioural sensitization. These results suggest that pharmacological inhibition of endocannabinoid degradation might exert a negative feedback on D2/D3 receptor-mediated hyperactivity. This finding might be relevant for therapeutic approaches for either psychomotor disorders (dyskinesia, corea) or disorganized behaviours associated with dopamine-mediated hyperactivity.

Increased brain metabolism after acute administration of the synthetic cannabinoid HU210: a small animal PET imaging study with 18F-FDG

Cannabis use has been shown to alter brain metabolism in both rat models and humans although the observations between both species are conflicting. In the present study, we examined the short term effects of a single-dose injection of the synthetic cannabinoid agonist HU210 on glucose metabolism in the rat brain using small animal (18)F-2-fluoro-deoxyglucose (FDG) Positron Emission Tomography (PET) 15 min (Day 1) and 24h (Day 2) post-injection of the agonist in the same animal. Young adult male Wistar rats received an intra-peritoneal injection of HU210 (100 μg/kg, n=7) or vehicle (n=5) on Day 1. Approximately 1mCi of (18)F-FDG was injected intravenously into each animal at 15 min (Day 1) and 24h (Day 2) post-injection of HU210. A 5-min Computer Tomography (CT) scan followed by a 20-min PET scan was performed 40 min after each (18)F-FDG injection. Standardised Uptake Values (SUVs) were calculated for 10 brain regions of interest (ROIs). Global increased SUVs in the whole brain, hence global brain metabolism, were observed following HU210 treatment on Day 1 compared to the controls (21%, P<0.0001), but not in individual brain regions. On Day 2, however, no statistically significant differences were observed between the treated and control groups. At the 24h time point (Day 2), SUVs in the HU210 treated group returned to control levels (21-30% decrease compared to Day 1), in all ROIs investigated (P<0.0001). In the control group, SUVs did not differ between the two acquisition days in all brain regions. The present results suggest that high-dose HU210 increases brain glucose metabolism in the rat brain shortly after administration, in line with normalised human in vivo studies, an effect that was no longer apparent 24 h later.

The Dopamine and Cannabinoid Interaction in the Modulation of Emotions and Cognition: Assessing the Role of Cannabinoid CB1 Receptor in Neurons Expressing Dopamine D1 Receptors

Although cannabinoid CB1 receptors (CB1Rs) are densely expressed in neurons expressing dopamine D1 receptors (D1Rs), it is not fully understood to what extent they modulate emotional behaviors. We used conditional CB1R knock-out animals lacking CB1Rs in neurons expressing D1R (D1–CB1^−/−) in order to answer this question. To elucidate the behavioral effects of CB1R deficiency in this specific neuronal subpopulation, we subjected D1–CB1^−/− mice to a battery of behavioral tests which included exploration-based tests, depressive-like behavioral tests, social behavior, and fear-related memory paradigms. D1–CB1^−/− did not show any difference in the exploration-based paradigms such as open field, elevated plus maze, or novel object investigation test, except for an increase in novelty-induced grooming. By contrast, they showed a mild anhedonia-like state as described by the slightly decreased preference for sweet solution, as compared to wild-type control group. This decrease, however, could be observed only during the first day of exposure, thus suggesting increased neophobia as an alternative explanation. Accordingly, mutant mice performed normally in the forced swim test, a procedure widely used for evaluating behavioral despair in rodents. However, weak- to moderate anxiety-like phenotypes were evident when D1–CB1^−/− mice were tested for social behavior. Most strikingly, D1–CB1^−/− mice exhibited significantly increased contextual and auditory-cued fear, with attenuated within session extinction, suggesting that a specific reduction of endocannabinoid signaling in neurons expressing dopamine D1Rs is able to affect acute fear adaptation. These results provided first direct evidence for a cross-talk between dopaminergic D1Rs and endocannabinoid system in terms of controlling negative affect.

Ligand-induced regulation and localization of cannabinoid CB1 and dopamine D2L receptor heterodimers

The cannabinoid CB(1) (CB(1)) and dopamine D(2) (D(2)) receptors are coexpressed in the basal ganglia, an area of the brain involved in such processes as cognition, motor function, and emotional control. Several lines of evidence suggest that CB(1) and D(2) receptors may oligomerize, providing a unique pharmacology in vitro and in vivo. However, limited information exists on the regulation of CB(1) and D(2) receptor dimers. We used a novel technique, multicolor bimolecular fluorescence complementation (MBiFC) to examine the subcellular localization of CB(1)-D(2L) heterodimers as well as D(2L)-D(2L) homodimers in a neuronal cell model, Cath. a differentiated cells. MBiFC was then used to explore the effects of persistent ligand treatment on receptor dimerization at the plasma membrane and intracellularly. Persistent (20-h) agonist treatment resulted in increased formation of CB(1)-D(2L) heterodimers relative to the D(2L)-D(2L) homodimers. The effects of the D(2) agonist quinpirole were restricted to the intracellular compartment and may reflect increased D(2L) receptor expression. In contrast, treatment with the CB(1) receptor agonist (2)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol (CP55, 940) produced increases in both membrane and intracellular CB(1)-D(2L) heterodimers independently of alterations in CB(1) receptor expression. The effects of CB(1) receptor activation were attenuated by the CB(1) antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM281) and were both time- and dose-dependent. The effects of CB(1) activation were examined further by combining MBiFC with a constitutively active CB(1) receptor mutant, CB(1)T210I. These studies demonstrated that the expression of CB(1)T210I increased intracellular CB(1)-D(2L) heterodimer formation. In summary, agonist-induced modulation of CB(1)-D(2L) oligomerization may have physiological implications in diseases such as Parkinson's disease and drug abuse.

Acute and non-acute effects of cannabis on brain functioning and neuropsychological performance

Cannabis has an ancient history of human use and is currently one of the most commonly used drugs worldwide. Understanding its impact on neurobehavioral functioning is of significant public health concern. In recent decades, substantial progress has been made in understanding the impact of cannabis use on neurobehavioral functioning. This has been fueled, in part, by characterization of an endocannabinoid signaling system in the brain through which cannabis exerts its psychoactive effects. Acute intoxication with cannabis causes marked changes in subjective mental status, brain functioning, and neuropsychological performance. Some of these changes are consistently detected and well characterized, yet others are not. Changes in brain functioning and neuropsychological performance are also reported after abstinence, but appear to be mild, circumscribed, and transient. On the other hand, functional neuroimaging often reveals subtle differences in the brain functioning of abstinent cannabis users compared with controls. The persistence and clinical significance of these differences, however, remains to be determined. Neuropsychological deficits and differences in brain functioning are most consistently observed only among frequent, heavy users, who are those most likely addicted to cannabis. The dire impact of drug addiction on a person's life and everyday functioning suggests that the large number of individuals addicted to cannabis experience substantial negative effects from its use. This manuscript reviews the scientific literature on the aforementioned topics in detail, providing evidence for converging findings, and highlighting areas in need of further investigation.

Motor effects of delta 9 THC in cerebellar Lurcher mutant mice

The present study evaluated the effects of the principal active component of marijuana (delta 9 THC) on motor abilities and motor learning in mice with cerebellar dysfunction. For this purpose, spontaneous locomotor activity, equilibrium abilities, muscular tone, motor coordination and motor learning were investigated in Lurcher mutant and non-mutant B6/CBA mice 20 min after i.p. administration of 4 or 8 mg kg(-1) of delta 9 tetra hydro cannabinol (delta 9 THC). The performances were compared to those obtained by Lurcher and non-mutant mice injected with vehicle (Tween 80). The results showed that at the dose of 4 mg kg(-1) but not at the dose of 8 mg kg(-1), the cannabinoid (CB) substance reduced deficits in motor coordination, equilibrium and muscular tone and facilitated motor learning in Lurcher mice. On the other hand, only a muscular strength decrease was observed in control B6/CBA mice injected with the dose of 8 mg kg(-1) of delta 9 THC. These results suggested that cannabinoid derivative could represent a new field of investigation concerning the treatment of cerebellar ataxic syndrome in humans.

Chronic use of marijuana decreases cannabinoid receptor binding and mRNA expression in the human brain

Chronic exposure to Cannabis sativa (marijuana) produced a significant down-regulation of cannabinoid receptor in the postmortem human brain. The significant decrease in maximal binding capacity was not accompanied by changes in the affinity constant. [3H]SR141716A binding was reduced in the caudate nucleus, putamen and in the accumbens nucleus. A significant decrease of binding sites was seen in the globus pallidus. Also in the ventral tegmental area and substantia nigra pars reticulata quantitative analysis of the density of receptors shows a significant reduction in [3H]SR141716A binding. In Cannabis sativa user brains, compared with normal brains [3H]SR141716A binding was reduced only in the hippocampus. The density of cannabinoid receptor 1 mRNA-positive neurons was significantly lower in Cannabis sativa users than in control brains for the caudate nucleus, putamen, accumbens nucleus and hippocampal region (CA1-CA4, areas of Ammon's horn). No hybridization was seen in the mesencephalon and globus pallidus.

Translation of nondopaminergic treatments for levodopa-induced dyskinesia from MPTP-lesioned nonhuman primates to phase IIa clinical studies: keys to success and roads to failure

Studies in MPTP-lesioned nonhuman primates have demonstrated the potential of nondopaminergic drugs in reducing the problems of levodopa-induced dyskinesia (LID). Here we review the process of translating findings from the monkey to man. Agents targeting glutamate, adensosine, noradrenaline, 5-hydroxytryptamine, cannabinoid, and opioid transmitter systems have been assessed for antidyskinetic potential in human studies. Eleven nondopaminergic drugs with antidyskinetic efficacy in the MPTP primate have been advanced to proof-of-concept phase IIa trials in PD patients (amantadine, istradefylline, idazoxan, fipamezole, sarizotan, quetiapine, clozapine, nabilone, rimonabant, naloxone, and naltrexone). For all six nondopaminergic transmitter systems reviewed, the MPTP-lesioned primate correctly predicted phase II efficacy of at least one drug. Of the 11 specific molecules tested in both monkeys and humans, 8 showed clear antidyskinetic properties in both human and monkey. In the instances where the primate studies did not, or did not consistently, predict the outcome of the human studies, the discrepancy may reflect limitations in the validity of the model or limitations in the design of either the clinical or the preclinical studies. We find that the major determinant of success in predicting efficacy is to ensure that primate studies are conducted in a statistically rigorous way and incorporate designs and outcome measures with clinical applicability. On the other hand, phase IIa trials should strive to replicate the preclinical study, especially in terms of protocol, drug dose equivalence, and outcome measure, so as to test the same hypothesis. Failure to meet these criteria carries the risk of false negative conclusions in phase IIa trials.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

Cannabinoids and dopamine receptors' action on calcium current in rat neurons

OBJECTIVE

To study the effects of cannabinoid, glutamate, and dopamine agonists and antagonists on the calcium current rat sympathetic neurons.

METHODS

Calcium current was recorded using the whole-cell variant of the patch-clamp technique. After expression in neuronal membranes of the cannabinoid CB1, glutamate mGluR2, or dopamine D1 receptor (by microinjection of the levant receptor's cDNA into the neuron's nucleus) agonists' and antagonists' effects were observed.

RESULTS

Applications of agonists of the expressed receptor (0.1-10 microM) decreased the calcium current. The calcium current was increased after application of cannabinoid antagonists (AM251 and AM630); these compounds thus act as inverse agonists in this preparation. Glutamate and dopamine antagonists had no effects on the calcium current by themselves. Combined application of cannabinoids and dopamine, but not glutamate, agonists produced a decrement in the calcium current that was bigger than either of the effects seen when one agonist was applied alone.

CONCLUSIONS

These results suggest that cannabinoid with dopamine receptors have an interactive inhibitory effect on the calcium current in this preparation, indicating that within the nervous system, receptor interactions may be important in the regulation of ion-channel functions.

Behavioural effects of quinpirole following withdrawal of chronic treatment with the CB1 agonist, HU-210, in rats

The present study investigated spontaneous and quinpirole-induced motor responses of in rats, following withdrawal from chronic treatment with the potent cannabinoid agonist HU-210. Withdrawal from chronic HU-210 (20 microg/kg daily, 14 days) produced a decrease in spontaneous activity at 1 and 2 days and enhanced the hyperactivity induced by acute administration of the dopamine D2 agonist quinpirole (0.5 mg/kg) at 4 days after the end of HU-210 treatment. Administration of quinpirole on day 4 of withdrawal from chronic HU-210 enhanced stereotyped responses and induced jumping behaviour. These results suggest that withdrawal from chronic exposure to cannabinoid agonists could induce a time-dependent alteration in dopamine D2 psychomotor function, leading to a behavioural disorganization, comparable to acute psychotic episodes after continuous cannabinoids.

A role for endocannabinoids in the generation of parkinsonism and levodopa-induced dyskinesia in MPTP-lesioned non-human primate models of Parkinson's disease

Endocannabinoids and cannabinoid CB1 receptors play a role in the control of movement by modulating GABA, glutamate, and other neurotransmitters throughout the basal ganglia. Roles for abnormalities in endocannabinoid signaling in Parkinson's disease (PD) and the major side effect of current treatments, levodopa-induced dyskinesia (LID), have been suggested by rodent studies. Here we show that signaling by endocannabinoids contributes to the pathophysiology of parkinsonism and LID in MPTP-lesioned, non-human primate models of Parkinson's disease. In MPTP-lesioned marmosets previously treated with levodopa to establish LID, attenuation of CB1 signaling by systemic administration of rimonabant (1 and 3 mg/kg) had anti-parkinsonian actions, equivalent to a 71% increase in motor activity at 3 mg/kg. Rimonabant did not elicit dyskinesia. Co-administration of levodopa (8 mg/kg) and rimonabant (1 and 3 mg/kg) resulted in significantly less dyskinesia than levodopa alone, without significantly affecting the anti-parkinsonian action of levodopa. These data suggest that enhanced endocannabinoid signaling may be involved in the pathophysiology of both parkinsonism and LID. To define potential mechanisms by which such a role might be mediated, we determined the levels of the endocannabinoids anandamide and 2-arachidonyl glycerol (2-AG) throughout the basal ganglia in normal and three groups of MPTP-lesioned cynomolgus monkeys (untreated; acutely treated with L-DOPA, non-dyskinetic; long-term treated, with levodopa-induced dyskinesia). In the untreated, MPTP-lesioned primate, parkinsonism was associated with increases in both 2-AG (+88%) and anandamide (+49%) in the striatum, and of 2-AG (+97%) in the substantia nigra, changes that are consistent with the previously suggested role for endocannabinoids in mechanisms attempting to compensate for loss of dopamine in untreated parkinsonism. Increased levels of anandamide (+34%) in the external globus pallidus of MPTP-lesioned animals were normalized by levodopa treatment and may contribute to the generation of parkinsonian symptoms. However, no clear alteration in endocannabinoid levels could be correlated with the expression of LID. These data highlight the potential roles played by endocannabinoids and CB1 in PD and LID and suggest the need for further research to pursue the multiple therapeutic opportunities for manipulating this system in movement disorders.

Cannabinoid CB1 antagonists possess antiparkinsonian efficacy only in rats with very severe nigral lesion in experimental parkinsonism

We have observed that systemic administration of cannabinoid CB1 antagonists exerts antiparkinsonian effects in rats with very severe nigral lesion (>95% cell loss), but not in rats with less severe lesion (85-95% cell loss). Local injections into denervated striatum and corresponding globus pallidus reduced parkinsonian asymmetry. Infusions into lesioned substantia nigra enhanced motor asymmetries, but this effect was absent after very severe nigral lesion. At the striatal level, CB1 antagonists act enhancing dopamine D1 receptor function and reducing D2 receptor function. Striatal dopaminergic denervation did not affect cannabinoid CB1 receptor coupling to G proteins. These results suggest that (i) systemic administration of CB1 antagonists in rats with severe nigral degeneration is ineffective because striatopallidal-mediated motor effects are antagonized by nigra-mediated activity, and (ii) CB1 antagonists exert antiparkinsonian effects after very severe nigral degeneration because nigra-mediated inhibition disappears. CB1 receptor antagonists that lack psychoactive effects might be of therapeutic value in the control of very advanced stage of Parkinson's disease in humans.

Effects on turning of microinjections into basal ganglia of D1 and D2 dopamine receptors agonists and the cannabinoid CB1 antagonist SR141716A in a rat Parkinson's model

Brain cannabinoid CB(1) receptors are expressed in neural areas that contribute to movement such as basal ganglia, where they co-localize with dopamine D(1) and D(2) receptors. The objective of the present study was to further study the functional role of CB(1) receptors along with D(1) and D(2) dopamine receptors of basal ganglia by local injections of SR141716A (CB(1) receptor antagonist), SKF-38393 (D(1) agonist), and quinpirole (D(2) agonist), in a rat Parkinson's model. Turning response after amphetamine was considered as the parkinsonian variable for quantifying motor effects of drugs. The findings indicated that, after intrastriatal infusions, both D(1) or D(2) dopamine receptor agonists alone reduced turning in parkinsonian rats. At the pallidal and subthalamic levels, D(1) (not D(2)) receptor stimulation also reduced rotation. Regarding SR141716A-induced effects, CB(1) antagonism reduced motor asymmetry in parkinsonian rats after injections into striatum, globus pallidus, and to a lesser extent, subthalamic nucleus. At the level of dorsal striatum, effects of SR141716A were mediated through an opposite modulation of D(1) and D(2) dopamine receptor function. At the pallidal and subthalamic nucleus levels, motor effects after SR14716A are not associated to modulation of D(1) and D(2) receptor function.

Experimental parkinsonism alters anandamide precursor synthesis, and functional deficits are improved by AM404: a modulator of endocannabinoid function

Modulation of the endocannabinoid system might be useful in treating Parkinson's disease. Here, we show that systemic administration of N-(4-hydroxyphenyl)-arachidonamide (AM404), a cannabinoid modulator that enhances anandamide (AEA) availability in the biophase, exerts antiparkinsonian effects in 6-hydroxydopamine-lesioned rats. Local injections of AM404 into denervated striata reduced parkinsonian motor asymmetries, these effects being associated with the reduction of D2 dopamine receptor function together with a positive modulation of 5-HT(1B) serotonin receptor function. Stimulation of striatal 5-HT(1B) receptors alone was observed to ameliorate parkinsonian deficits, supporting the fact that AM404 exerts antiparkinsonian effects likely through stimulation of striatal 5-HT(1B) serotonin receptor function. Hence, modulation of cannabinoid function leading to enhancement of AEA in the biophase might be of therapeutic value in the control of symptoms of Parkinson's disease. On the other hand, reduced levels of N-acyl-transferase (AEA precursor synthesizing enzyme), without changes in fatty acid amidohydrolase (AEA degradative enzyme), were detected in denervated striata in comparison with intact striata. This finding reveals the presence of a homeostatic striatal mechanism emerging after dopaminergic denervation likely tending to enhance low dopamine tone.

[Therapeutic use of cannabinoids in neurology]

This review gives insight into the potential therapeutical role of cannabinoids in neurology. Preclinical data are presented which could give a rationale for the clinical use of cannabinoids in the fields of multiple sclerosis, spasticity, epilepsy, movement disorders, and neuroprotection after traumatic head injury or ischemic stroke. Besides, clinical data (case reports, open-label and randomised controlled studies) dealing with the therapeutical use of cannabinoids in these fields are reported and discussed. At present, clinical data are insufficient to recommend the use of cannabinoids in any neurological disease as standard therapy. Several questions still have to be answered (which cannabinoid? which way of administration? stimulation of endogenous cannabinoids? separation between desired and undesired effects?), and controlled studies are still needed to clarify the potential therapeutical role of cannabinoids in neurology.

Neuroprotective strategies in Parkinson's disease : an update on progress

In spite of the extensive studies performed on postmortem substantia nigra from Parkinson's disease patients, the aetiology of the disease has not yet been established. Nevertheless, these studies have demonstrated that, at the time of death, a cascade of events had been initiated that may contribute to the demise of the melanin-containing nigro-striatal dopamine neurons. These events include increased levels of iron and monoamine oxidase (MAO)-B activity, oxidative stress, inflammatory processes, glutamatergic excitotoxicity, nitric oxide synthesis, abnormal protein folding and aggregation, reduced expression of trophic factors, depletion of endogenous antioxidants such as reduced glutathione, and altered calcium homeostasis. To a large extent, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) animal models of Parkinson's disease confirm these findings. Furthermore, neuroprotection can be afforded in these models with iron chelators, radical scavenger antioxidants, MAO-B inhibitors, glutamate antagonists, nitric oxide synthase inhibitors, calcium channel antagonists and trophic factors. Despite the success obtained with animal models, clinical neuroprotection is much more difficult to accomplish. Although the negative studies obtained with the MAO-B inhibitor selegiline (deprenyl) and the antioxidant tocopherol (vitamin E) may have resulted from an inappropriate choice of drug (selegiline) or an inadequate dose (tocopherol), the niggling problem that still remains is why these drugs, and others, do work in animals while they fail in the clinic. One reason for this may be related to the fact that in normal human brains the number of dopaminergic neurons falls by around 3-5% every decade, while in Parkinson's disease this decline is greater. Brain autopsy studies have shown that by the time the disease is identified, some 70-75% of the dopamine-containing neurons have been lost. More sensitive reliable methods and clinical correlative markers are required to discern between confoundable symptomatic effects versus a possible neuroprotective action of drugs, namely, the ability to delay or forestall disease progression by protecting or rescuing the remaining dopamine neurons or even restoring those that have been lost.A number of other possibilities for the clinical failure of potential neuroprotectants also exist. First, the animal models of Parkinson's disease may not be totally reflective of the disease and, therefore, the chemical pathologies established in the animal models may not cause, or contribute to, the progression of the disease clinically. Second, because of the series of events occurring in neurodegeneration and our ignorance about which of these factors constitutes the primary event in the pathogenic process, a single drug may not be adequate to induce neuroprotection and, as a consequence, use of a cocktail of drugs may be more appropriate. The latter concept receives support from recent complementary DNA (cDNA) microarray gene expression studies, which show the existence of a gene cascade of events occurring in the nigrostriatal pathway of MPTP, 6-OHDA and methamphetamine animal models of Parkinson's disease. Even with the advent of powerful new tools such as genomics, proteomics, brain imaging, gene replacement therapy and knockout animal models, the desired end result of neuroprotection is still beyond our current capability.

The endocannabinoid system in the basal ganglia and in the mesolimbic reward system: implications for neurological and psychiatric disorders

To date, N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol are the best studied endocannabinoids and are thought to act as retrograde messengers in the central nervous system (CNS). By activating presynaptic cannabinoid CB1 receptors, they can reduce glutamate release in dorsal and ventral striatum (nucleus accumbens) and alter synaptic plasticity, thereby modulating neurotransmission in the basal ganglia and in the mesolimbic reward system. In this review, we will focus on the role of the endocannabinoid system within these neuronal pathways and describe its effect on dopaminergic transmission and vice versa. The endocannabinoid system is unlikely to directly affect dopamine release, but can modify dopamine transmission trough trans-synaptic mechanisms, involving gamma-aminobutyric acid (GABA)-ergic and glutamatergic synapses, as well as by converging signal transduction cascades of the cannabinoid and dopamine receptors. The dopamine and endocannabinoid systems exert a mutual control on each other. Cannabinergic signalling may lead to release of dopamine, which can act via dopamine D1-like receptors as a negative feedback mechanism to counteract the effects of activation of the cannabinoid CB1 receptor. On the other hand, dopaminergic signalling via dopamine D2-like receptors may lead to up-regulation of cannabinergic signalling, which is likely to represent a negative feedback on dopaminergic signalling. The consequences of these interactions become evident in pathological conditions in which one of the two systems is likely to be malfunctioning. We will discuss neurological and psychiatric disorders such as Parkinson's and Huntington's disease, drug addiction and schizophrenia. Furthermore, the possible role of the endocannabinoid system in disorders not necessarily depending on the dopaminergic system, such as eating disorders and anxiety, will be described.

Effects of CB1 cannabinoid receptor modulating compounds on the hyperkinesia induced by high-dose levodopa in the reserpine-treated rat model of Parkinson's disease

The present study was designed to determine the potential of CB1 cannabinoid receptor modulating compounds in the treatment of L-3,4-dihydroxyphenylalanine (L-dopa)-induced dyskinesia in Parkinson's disease. In the reserpine-treated rat model of parkinsonism, administration of a high dose of L-dopa (150 mg/kg) but not of Cl-APB (0.5 mg/kg) or quinpirole (0.5 mg/kg) produced a hyperkinetic state characterised by an increase in horizontal and vertical activity, which likely represent correlates of antiparkinsonian and dyskinetic activity, respectively. Injection of the CB1 cannabinoid receptor antagonist SR141716 (0.1-3 mg/kg) reduced the increase in vertical activity elicited by L-dopa without affecting the increase in horizontal activity. Injection of the CB1 cannabinoid receptor agonist WIN55,212-2 (0.1-3 mg/kg) reduced the L-dopa-induced increase in vertical activity and, at the highest dose only (3 mg/kg), also reduced horizontal activity elicited by L-dopa. WIN55,212-2 (1 mg/kg) reduced motor activity induced by both the D1 receptor agonist Cl-APB (0.5 mg/kg) and the D2 receptor agonist quinpirole (0.5 mg/kg) in the reserpine-treated rat. SR141716 (1 mg/kg) had no effects on motor activity induced by Cl-APB (0.5 mg/kg) nor quinpirole (0.5 mg/kg) in the reserpine-treated rat. Injection of the inhibitor of endocannabinoid transport AM404 (0.1-1 mg/kg) did not affect the increase in horizontal or vertical activity elicited by L-dopa (150 mg/kg) in the reserpine-treated rat. The data suggest that both CB1 cannabinoid receptor antagonists and agonists can modulate the behavioural effects of L-dopa and may be useful for the treatment of the dyskinesia associated with long-term L-dopa treatment of Parkinson's disease.

Endocannabinoids and cannabinoid receptor genetics

This review presents the remarkable advances that have been achieved in marijuana (cannabinoid) research, with the discovery of specific receptors and the existence of naturally occurring cannabis-like substances in the human body and brain. The last decade has seen more rapid progress in marijuana research than any time in the thousands of years that marijuana has been used by humans, particularly in cannabinoid genomics. The cDNA and genomic sequences encoding G protein-coupled cannabinoid receptors (Cnrs) from several species have now been cloned. Endogenous cannabinoids (endocannabinoids), synthetic and hydrolyzing enzymes and transporters that define neurochemically-specific cannabinoid brain pathways have been identified. Endocannabinoid lipid signaling molecules alter activity at G protein-coupled receptors (GPCR) and possibly at anandamide-gated ion channels, such as vanilloid receptors. Availability of increasingly-specific CB1 and CB2 Cnr antagonists and of CB1 and CB2 Cnr knockout mice have increased our understanding of these cannabinoid systems and provides tantalizing evidence for even more G protein-coupled Cnrs. Initial studies of the Cnr gene structure, regulation and polymorphisms whet our appetite for more information about these interesting genes, their variants and roles in vulnerabilities to addictions and other neuropsychiatric disorders. Behavioral studies of cannabinoids document the complex interactions between rewarding and aversive effects of these drugs. Pursuing cannabinoid-related molecular, pharmacological and behavioral leads will add greatly to our understanding of endogenous brain neuromodulator systems, abused substances and potential therapeutics. This review of CB1 and CB2 Cnr genes in human and animal brain and their neurobiological effects provide a basis for many of these studies. Therefore, understanding the physiological cannabinoid control system in the human body and brain will contribute to elucidating this natural regulatory mechanism in health and disease.

Striatal Cannabinoid CB1 Receptor mRNA Expression Is Decreased in the Reserpine-Treated Rat Model of Parkinson's Disease

High levels of both endocannabinoids and endocannabinoid receptors are present in the basal ganglia. Attention has recently focused on the role of endocannabinoids in the control of movement and in movement disorders of basal ganglia origin such as Parkinson's disease. We investigated CB1 cannabinoid receptor mRNA expression in the reserpine-treated rat model of Parkinson's disease using in situ hybridization. Reserpine treatment caused a topographically organized reduction in CB1 receptor mRNA expression in the striatum (ranging from 11.6% medially to 53.6% laterally and dorsally). No change in CB1 receptor mRNA expression was observed in the cerebral cortex or septum. This reduction in CB1 receptor mRNA expression may be secondary to increased endocannabinoid stimulation of the receptor as increased basal ganglia endocannabinoid levels have been shown to occur in this model of Parkinson's disease. The data support the idea that cannabinoid receptor antagonists may provide a useful treatment for the symptoms of Parkinson's disease.

The role of cannabinoids in neurodegenerative diseases

An understanding of the actions of Cannabis (Marijuana) has evolved from folklore to science over the previous hundred years. This progression was spurred by the discovery of an endogenous cannabinoid system consisting of two receptors and two endogenous ligands. This system appears to be intricately involved in normal physiology, specifically in the control of movement, formation of memories and appetite control. As we are developing an increased understanding of the physiological role of endocannabinoids it is becoming clear that they may be involved in the pathology of several neurological diseases. Furthermore an array of potential therapeutic targets is being determined--including specific cannabinoid agonists and antagonists as well as compounds that interrupt the synthesis, uptake or metabolism of the endocannabinoids. This article reviews the recent progress in understanding the contribution of endocannabinoids to the pathology and therapy of Huntington's disease. Parkinson's disease, schizophrenia and tremor.

Role of the endogenous cannabinoid system as a modulator of dopamine transmission: implications for Parkinson's disease and schizophrenia

The endogenous cannabinoid system is a new signaling system composed by the central (CB1) and the peripheral (CB2) receptors, and several lipid transmitters including anandamide and 2-arachidonylglycerol. This system is the target of natural cannabinoids, the psychoactive constituents of Cannabis sativa preparations (marijuana, hashish). Acute and chronic cannabis exposure has been associated with subjective feelings of pleasure and relaxation, but also to the onset of psychiatric syndromes, a decrease of the efficacy of neuroleptics and alterations in the extrapyramidal system regulation of motor activity. These actions point to a tight association of the cannabinoid system with the brain dopaminergic circuits involved in addiction, the clinical manifestation of positive symptoms of schizophrenia and Parkinson's disease. The present work discusses anatomical, biochemical and pharmacological evidences supporting a role for the endogenous cannabinoid system in the modulation of dopaminergic transmission. Cannabinoid CB1 receptors are present in dopamine projecting brain areas. In primates and certain rat strains it is also located in dopamine cells of the A8, A9 and A10 mesencephalic cell groups, as well as in hypothalamic dopaminergic neurons controlling prolactin secretion. CB1 receptors co-localize with dopamine D1/D2 receptors in dopamine projecting fields. Manipulation of dopaminergic transmission is able to alter the synthesis and release of anandamide as well as the expression of CB1 receptors. Additionally, CB1 receptors can switch its transduction mechanism to oppose to the ongoing dopamine signaling. Acute blockade of CB1 receptor potentiates the facilitatory role of dopamine D2 receptor agonists on movement. CB1 stimulation results in sensitization to the motor effects of indirect dopaminergic agonists. The dynamics of these changes indicate that the cannabinoid system is an activity-dependent modulator of dopaminergic transmission, an hypothesis relevant for the design of new therapeutic strategies for dopamine-related diseases such as the psychosis and Parkinson's disease.

CB1 cannabinoid receptor inhibits synaptic release of glutamate in rat dorsolateral striatum

CB1 cannabinoid receptors in the neostriatum mediate profound motor deficits induced when cannabinoid drugs are administered to rodents. Because the CB1 receptor has been shown to inhibit neurotransmitter release in various brain areas, we investigated the effects of CB1 activation on glutamatergic synaptic transmission in the dorsolateral striatum of the rat where the CB1 receptor is highly expressed. We performed whole cell voltage-clamp experiments in striatal brain slices and applied the CB1 agonists HU-210 or WIN 55,212-2 during measurement of synaptic transmission. Excitatory postsynaptic currents (EPSCs), evoked by electrical stimulation of afferent fibers, were significantly reduced in a dose-dependent manner by CB1 agonist application. EPSC inhibition was accompanied by an increase in two separate indices of presynaptic release, the paired-pulse response ratio and the coefficient of variation, suggesting a decrease in neurotransmitter release. These effects were prevented by application of the CB1 antagonist SR141716A. When Sr(2+) was substituted for Ca(2+) in the extracellular solution, application of HU-210 (1 microM) significantly reduced the frequency, but not amplitude, of evoked, asynchronous quantal release events. Spontaneous release events were similarly decreased in frequency with no change in amplitude. These findings further support the interpretation that CB1 activation leads to a decrease of glutamate release from afferent terminals in the striatum. These results reveal a novel potential role for cannabinoids in regulating striatal function and thus basal ganglia output and may suggest CB1-targeted drugs as potential therapeutic agents in the treatment of Parkinson's disease and other basal ganglia disorders.

Cannabinoid receptor messenger RNA levels decrease in a subset of neurons of the lateral striatum, cortex and hippocampus of transgenic Huntington's disease mice

One of the earliest changes, at the molecular level, that occurs in human Huntington's disease patients is reduction in cannabinoid receptor ligand binding in the substantia nigra pars reticulata compared to neurologically normal controls. The loss of cannabinoid receptor binding is thought to occur early in or prior to the development of Huntington's disease neuropathology. We wish to determine whether cannabinoid receptor messenger RNA levels were altered in a mouse model of Huntington's disease. Transgenic mice hemizygous for the promoter sequence and exon 1 of the human Huntington's disease gene exhibit a progressive neurological phenotype with many of the features of Huntington's disease. This neurological phenotype develops in the absence of neural degeneration making these mice a model system to dissociate changes related to cell dysfunction from changes related to cell loss. We examine the steady-state levels and cellular distribution of the brain-specific cannabinoid receptor messenger RNA by northern blot and in situ hybridization. The cannabinoid receptor messenger RNA was expressed throughout the striatum, cortex and hippocampus of wild-type mice. At four and five weeks of age, there was no difference in the distribution of the cannabinoid receptor messenger RNA between the wild-type and transgenic Huntington's disease mice. At six, seven, eight and 10 weeks of age, however, the Huntington's disease mice exhibit reduced levels of cannabinoid receptor messenger RNA in the lateral striatum compared to age-matched controls. The Huntington's disease mice also showed a loss of cannabinoid receptor messenger RNA within a subset of neurons in the cortex and hippocampus. We did not observe any difference in the expression of cannabinoid receptor between the wild-type and Huntington's disease mice throughout Ammon's horn of the hippocampus or in the medial striatum. The decrease in cannabinoid receptor messenger RNA levels preceded the development of the Huntington's disease phenotype and neuronal degeneration and, therefore, these transgenic mice model early cellular changes observed in human patients. Our results demonstrate that the single copy cannabinoid receptor gene is subjected to cell-specific and time-dependent regulation of the steady-state level of its gene product as a result of the expression of the Huntington's disease gene. As the endogenous cannabinoid receptor agonist, anandimide, has been shown to modulate dopamine neurotransmission within the basal ganglia, the loss of cannabinoid receptors may contribute to the development of motor symptoms or cognitive decline or both seen in Huntington's disease patients.

Enhancement of anandamide formation in the limbic forebrain and reduction of endocannabinoid contents in the striatum of delta9-tetrahydrocannabinol-tolerant rats

Recent studies have shown that the pharmacological tolerance observed after prolonged exposure to synthetic or plant-derived cannabinoids in adult rats is accompanied by down-regulation/desensitization of brain cannabinoid receptors. However, no evidence exists on possible changes in the contents of the endogenous ligands of cannabinoid receptors in the brain of cannabinoid-tolerant rats. The present study was designed to elucidate this possibility by measuring, by means of isotope dilution gas chromatography/mass spectrometry, the contents of both anandamide (arachidonoylethanolamide; AEA) and its biosynthetic precursor, N-arachidonoylphosphatidylethanolamine (NArPE), and 2-arachidonoylglycerol (2-AG) in several brain regions of adult male rats treated daily with delta9-tetrahydrocannabinol (delta9-THC) for a period of 8 days. The areas analyzed included cerebellum, striatum, limbic forebrain, hippocampus, cerebral cortex, and brainstem. The same regions were also analyzed for cannabinoid receptor binding and WIN-55,212-2-stimulated guanylyl-5'-O-(gamma-[35S]thio)-triphosphate ([35S]GTPgammaS) binding to test the development of the well known down-regulation/desensitization phenomenon. Results were as follows: As expected, cannabinoid receptor binding and WIN-55,212-2-stimulated [35S]GTPgammaS binding decreased in most of the brain areas of delta9-THC-tolerant rats. The only region exhibiting no changes in both parameters was the limbic forebrain. This same region exhibited a marked (almost fourfold) increase in the content of AEA after 8 days of delta9-THC treatment. By contrast, the striatum exhibited a decrease in AEA contents, whereas no changes were found in the brainstem, hippocampus, cerebellum, or cerebral cortex. The increase in AEA contents observed in the limbic forebrain was accompanied by a tendency of NArPE levels to decrease, whereas in the striatum, no significant change in NArPE contents was found. The contents of 2-AG were unchanged in brain regions from delta9-THC-tolerant rats, except for the striatum where they dropped significantly. In summary, the present results show that prolonged activation of cannabinoid receptors leads to decreased endocannabinoid contents and signaling in the striatum and to increased AEA formation in the limbic forebrain. The pathophysiological implications of these findings are discussed in view of the proposed roles of endocannabinoids in the control of motor behavior and emotional states.

In vivo characterization of the specific cannabinoid receptor antagonist, SR141716A: behavioral and cellular responses after acute and chronic treatments

To characterize the behavioral and biochemical effects of the cannabinoid CB1 antagonist SR141716A, we injected the compound intraperitoneally (ip) at doses from 0.625 mg/kg to 5 mg/kg in rats. SR141716A per se induced a dose-dependent increase of some behavioral signs such as wet dog and head shakes, forepaw fluttering, grooming, and facial rubbing. When the highest dose of SR141716A (5 mg/kg ip) was injected once a day for four days, tolerance developed to most of the behavioral signs, although with different time courses, except for grooming behavior, which was still significantly different from controls after the fourth injection although reduced by 38% from the first. To characterize the biochemical mechanism underlying these effects, we designed a series of biochemical studies on specific cerebral areas from rats treated with the highest dose of SR141716A (5 mg/kg ip). Thirty minutes after SR141716A injection, cAMP accumulation in the cortex, striatum, hippocampus, mesencephalon, and cerebellum was the same as in controls, whereas protein kinase A (PKA) activity was significantly increased in the hippocampus (65%) and striatum (87%). To explain this difference, we performed a cAMP assay at an early time (10 min) and found a significant increase in the striatum and hippocampus, suggesting that the change in cAMP level is the earliest event in the G protein-coupled receptor transduction pathway ending in a pharmacological effect after 30 min. When the same assays were done in tolerant animals, no change was seen in either cAMP levels or PKA activity in the brain areas considered. To conclude, we found in vivo that SR141716A acts through activation of the cAMP cascade and our results represent an important point for developing potential therapeutic application for SR141716A.

Role of endocannabinoids in brain development

In addition to those functions that have been extensively addressed in this special issue, such as nociception, motor activity, neuroendocrine regulation, immune function and others, the endogenous cannabinoid system seems to play also a role in neural development. This view is based on a three-fold evidence. A first evidence emerges from neurotoxicological studies that showed that synthetic and plant-derived cannabinoids, when administered to pregnant rats, produced a variety of changes in the maturation of several neurotransmitters and their associated-behaviors in their pups, changes that were evident at different stages of brain development. A second evidence comes from studies that demonstrated the early appearance of elements of the endogenous cannabinoid system (receptors and ligands) during the brain development. The atypical location of these elements during fetal and early postnatal periods favours the notion that this system may play a role in specific molecular events related to neural development. Finally, a third evidence derives from studies using cultures of fetal glial or neuronal cells. Cannabinoid receptors are present in some of these cultured cells and their activation produced a set of cellular effects consistent with a role of this system in the process of neural development. All this likely supports that endocannabinoids, early synthesized in nervous cells, play a role in events related to development, by acting through the activation of second messenger-coupled cannabinoid receptors.

Anandamide and WIN 55212-2 inhibit cyclic AMP formation through G-protein-coupled receptors distinct from CB1 cannabinoid receptors in cultured astrocytes

The effects of anandamide and the cannabinoid receptor agonists WIN 55212-2 and CP 55940 on the evoked formation of cyclic AMP were compared in cultured neurons and astrocytes from the cerebral cortex and striatum of mouse embryos. The three compounds inhibited the isoproterenol-induced accumulation of cyclic AMP in neuronal cells, and these responses were blocked by the selective CB1 receptor antagonist SR 141716A. The three agonists were more potent in cortical than striatal neurons. Interestingly, WIN 55212-2, CP 55940 and anandamide also inhibited the isoproterenol-evoked accumulation of cyclic AMP in astrocytes but, in contrast to WIN 55212-2 and CP 55940, anandamide was much more potent in striatal than cortical astrocytes. Inhibition was prevented by pertussis toxin pretreatment, but not blocked by SR 141716A. Therefore, G-protein-coupled receptors, distinct from CB1 receptors, are involved in these astrocytic responses. Moreover, specific binding sites for [3H]-SR 141716A were found in neurons but not astrocytes. Furthermore, using a polyclonal CB1 receptor antibody, staining was observed in striatal and cortical neurons, but not in striatal and cortical astrocytes. Taken together, these results suggest that glial cells possess G-protein-coupled receptors activated by cannabinoids distinct from the neuronal CB1 receptor, and that glial cells responses must be taken into account when assessing central effects of cannabinoids.

Chronic (-)-delta9-tetrahydrocannabinol treatment induces sensitization to the psychomotor effects of amphetamine in rats

Clinical and basic research studies have linked cannabinoid consumption to the onset of psychosis, specially schizophrenia. In the present study we have evaluated the effects of the natural psychoactive constituent of Cannabis (-)-delta9-tetrahydrocannabinol on the acute actions of the psychostimulant, D-amphetamine, on behaviour displayed by male rats on a hole-board, a proposed animal model of amphetamine-induced psychosis. Cannabinoid-amphetamine interactions were studied (1) 30 min after acute injection of (-)-delta9-tetrahydrocannabinol (0.1 or 6.4 mg/kg, i.p.); (2) 30 min after the last injection of 14-daily treatment with (-)-delta9-tetrahydrocannabinol (0.1 or 6.4 mg/kg) and 3) 24 h after the last injection of 14-daily treatment with (-)-delta9-tetrahydrocannabinol (6.4 mg/kg). Acute cannabinoid exposure antagonized the amphetamine-induced dose-dependent increase in locomotion, exploration and the decrease in inactivity. Chronic treatment with (-)-delta9-tetrahydrocannabinol resulted in tolerance to this antagonistic effect on locomotion and inactivity but not on exploration, and potentiated amphetamine-induced stereotypies. Lastly, 24 h of withdrawal after 14 days of cannabinoid treatment resulted in sensitization to the effects of D-amphetamine on locomotion, exploration and stereotypies. Since (-)-delta9-tetrahydrocannabinol is a cannabinoid CB1 receptor agonist, densely present in limbic and basal ganglia circuits, and since amphetamine enhances monoaminergic inputs (i.e., dopamine, serotonin) in these brain areas, the present data support the hypothesis of a role for the cannabinoid CB1 receptor as a regulatory mechanism of monoaminergic neuron-mediated psychomotor activation. These findings may be relevant for the understanding of both cannabinoid-monoamines interactions and Cannabis-associated psychosis.

Brain cannabinoid systems as targets for the therapy of neurological disorders

Unprecedented developments in cannabinoid research within the past decade include discovery of a brain (CB1) and peripheral (CB2) receptor; endogenous ligands, anandamide, and 2-arachidonylglycerol; cannabinoid drug-induced partial and inverse agonism at CB1 receptors, antagonism of NMDA receptors and glutamate, and antioxidant activity; and preferential CB1 receptor localization in areas subserving spasticity, pain, abnormal involuntary movements, seizures, and amnesia. These endogenous structures and chemicals and mechanisms are potentially new pathophysiologic substrates, and targets for novel cannabinoid treatments, of several neurological disorders.

Role of the endogenous cannabinoid system in the regulation of motor activity

One of the prominent pharmacological features of drugs acting at the brain cannabinoid receptor (CB1) is the induction of alterations in motor behavior. Catalepsy, immobility, ataxia, or the impairment of complex behavioral acts are observed after acute administration of either natural and synthetic cannabinoid receptor agonists or the endogenous CB1 ligand anandamide. The dense presence of CB1 receptors in the cerebellum and in the basal ganglia, especially at the outflow nuclei (substantia nigra and the internal segment of the globus pallidus), supports the existence of an endogenous cannabinoid system regulating motor activity. In the basal ganglia, the functionality of the anandamide-CB1 system is poorly understood. Dual effects are often observed after the administration of CB1 ligands in animal models of pharmacological manipulation of basal ganglia transmitter systems, indicating that the activity of the anandamide-CB1 system depends on the ongoing activation of the different elements of the basal ganglia. This finding is in agreement with the proposed activity-dependent release of anandamide from a plasmalemma precursor. Additionally, a potential state-dependent bidirectional coupling of the CB1 receptor to the adenylate cyclase transduction system has also been described. From this perspective, the endogenous cannabinoid system can be proposed as a local regulator of neurotransmission processes within the basal ganglia. This system may serve as a counterregulatory homeostatic mechanism preserving the functional role of basal ganglia circuits in coding the serial order of events that constitute movement.

Role of cannabinoid receptors in memory storage

Studies have shown that delta 9-tetrahydrocannabinol, the principal psychoactive ingredient in marijuana, produces memory deficits similar to those produced by neurochemical lesions of the hippocampus. Such lesions impair performance in short-term spatial memory tasks learned prior to the lesion. Animals trained in the behavioral task following the lesion can still perform the task, but learn a different behavioral strategy. Cannabinoid agonists impair behavioral performance in a delay-dependent manner similar to that produced by lesions, but also shift the behavioral response strategy. A possible role for cannabinoid receptors and endogenous cannabinoids may thus be to regulate the storage (i.e., encoding) of information, as well as the means by which that information is retrieved.

Endocannabinoids: endogenous cannabinoid receptor ligands with neuromodulatory action

The existence of an endogenous cannabinoid system was demonstrated conclusively with the discovery of endogenous brain constituents capable of activating the cannabinoid receptors functionally. These compounds are synthesized by neuronal cells and inactivated through re-uptake and enzymatic hydrolysis by both neurons and astrocytes. In analogy with the endorphins they can be referred to as endocannabinoids. Apart from the identification of their metabolic pathways, research carried out in the past six years has focused on the possible cellular and molecular targets for the actions of endocannabinoids. These studies have confirmed a similarity between the endocannabinoids and the psychoactive substance in marijuana, delta9(-)-tetrahydrocannabinol, and have suggested a role for endocannabinoids in the modulation of neurotransmitter action and release.

Cannabinoid transmission and reward-related events

The reward/reinforcement circuitry of the mammalian brain consists of synaptically interconnected neurons associated with the medial forebrain bundle, linking the ventral tegmental area, nucleus accumbens, and ventral pallidum. Electrical stimulation of this circuit supports intense self-stimulation in animals and, in humans, produces intense pleasure or euphoria. This circuit is strongly implicated in the neural substrates of drug addiction and in such addiction-related phenomena as withdrawal dysphoria and craving. This circuit is also implicated in the pleasures produced by natural rewards (e.g., food, sex). Cannabinoids are euphorigenic in humans and have addictive liability in vulnerable persons, but were long considered "anomalous" drugs of abuse, lacking pharmacological interaction with these brain reward substrates. It is now clear, however, that cannabinoids activate these brain substrates and influence reward-related behaviors. From these actions, presumably, derive both the abuse potential of cannabinoids and the possible clinical efficacy in dysphoric states.

Correlation between cannabinoid mediated effects on paired pulse depression and induction of long term potentiation in the rat hippocampal slice

Cannabinoids cause an increase in synaptic transmission via gamma-aminobutyric acid (GABA) receptors and this may be the mechanism by which activation of CB1 receptors blocks the induction of long-term potentiation (LTP). To test this hypothesis, we used paired pulse depression (PPD) of CA1 population spike responses recorded in the rat hippocampal slice as an index of GABA-ergic feedback inhibition, to establish whether the effects of a stereoselective CB1 receptor agonist on GABA-ergic transmission and LTP were correlated. The active isomer, WIN55212-2, blocked the induction of LTP and suppressed PPD over the concentration range 250 nM-5 microM, whereas the inactive isomer, WIN55212-3, was inactive at 5 microM. The effects of 5 microM WIN55212-2 on both LTP and PPD were completely blocked by the CB1 receptor antagonist SR141716A (5 microM). The results show that the effects are correlated in that both suppression of PPD and blockade of induction of LTP are probably mediated by CBI receptors. However, the suppression in PPD suggests that WIN55212-2 caused a decrease in GABA-ergic feedback transmission which would be expected to facilitate, rather than block, the induction of LTP. We therefore conclude that the blockade of LTP by cannabinoids is not via upregulation of GABA-ergic synaptic transmission.