Differential Effects of Δ9-Tetrahydrocannabinol and Methanandamide in CB1 Knockout and Wild-Type Mice

Effects of cannabinoids on ligand-gated ion channels

Phytocannabinoids such as Δ⁹-tetrahydrocannabinol and cannabidiol, endocannabinoids such as N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol, and synthetic cannabinoids such as CP47,497 and JWH-018 constitute major groups of structurally diverse cannabinoids. Along with these cannabinoids, CB1 and CB2 cannabinoid receptors and enzymes involved in synthesis and degradation of endocannabinoids comprise the major components of the cannabinoid system. Although, cannabinoid receptors are known to be involved in anti-convulsant, anti-nociceptive, anti-psychotic, anti-emetic, and anti-oxidant effects of cannabinoids, in recent years, an increasing number of studies suggest that, at pharmacologically relevant concentrations, these compounds interact with several molecular targets including G-protein coupled receptors, ion channels, and enzymes in a cannabinoid-receptor independent manner. In this report, the direct actions of endo-, phyto-, and synthetic cannabinoids on the functional properties of ligand-gated ion channels and the plausible mechanisms mediating these effects were reviewed and discussed.

Carbon-silicon switch led to the discovery of novel synthetic cannabinoids with therapeutic effects in a mouse model of multiple sclerosis

Cannabinoids are widely studied as therapeutic agents for the treatment of various diseases. Among them, THC and CBD are two important phytocannabinoids which have served as structural templates for the design of synthetic analogs. In this study, we designed and synthesized a variety of novel cannabinoids based on the structural backbones of THC and CBD using the carbon-silicon switch strategy. A dimethyl silyl group was introduced as the tail group and two series of novel compounds were designed and synthesized, which showed a wide range of binding affinity for CB1 and CB2 receptors. Among them, compound 15b was identified as a non-selective CB1 and CB2 agonist and 38b as a selective agonist for the CB2 receptor. Preliminary screening showed that both compounds have improved metabolic stability than their carbon analogs and good in vivo pharmacokinetic profiles. Furthermore, both 15b and 38b significantly alleviated the phenotype of experimental autoimmune encephalomyelitis (EAE) in mice.

Methanandamide diminish the Porphyromonas gingivalis lipopolysaccharide induced response in human periodontal ligament cells

BACKGROUND

The endocannabinoid system is involved in the regulation of periodontal tissue homeostasis. Synthetic cannabinoid methanandamide (Meth-AEA) has improved stability and affinity to cannabinoid receptors compared to its endogenous analog anandamide. In the present study, we investigated the effect of methanandamide on the production of pro-inflammatory mediators in primary human periodontal ligament cells (hPdLCs).

METHODS

hPdLCs were treated with Meth-AEA for 24 h, and the resulting production of interleukin (IL)-6, IL-8, and monocyte chemotactic protein (MCP)-1 was measured in the absence or the presence of Porphyromonas gingivalis lipopolysaccharide (LPS). Additionally, the effect of Meth-AEA on the proliferation/viability of hPdLCs was measured by the MTT method.

RESULTS

Methanandamide at a concentration of 10 μM significantly inhibited P. gingivalis LPS induced production of IL-6, IL-8, and MCP-1. Basal production of IL-6 and IL-8 was slightly enhanced by 10 μM Meth-AEA. No effect of Meth-AEA on the basal production of MCP-1 was observed. Meth-AEA in concentrations up to 10 μM did not affect the proliferation/viability of hPdLCs, but significantly inhibited it at a concentration of 30 μM.

CONCLUSION

Our study suggests that the inflammatory response in periodontal ligament cells could be influenced by the activation of the cannabinoid system, which might be potentially involved in the progression of periodontal disease.

Cannabinoid1 (CB-1) receptor antagonists: a molecular approach to treating acute cannabinoid overdose

The legalization of cannabis for both recreational and medical use in the USA has resulted in a dramatic increase in the number of emergency department visits and hospital admissions for acute cannabinoid overdose (also referred to as cannabis intoxication and cannabis poisoning). Both “edibles” (often sold as brownies, cookies, and candies) containing large amounts of Δ⁹-tetrahydrocannabinol and synthetic cannabinoids (many possessing higher potencies and efficacies than Δ⁹-tetrahydrocannabinol) are responsible for a disproportionate number of emergency department visits relative to smoked cannabis. Symptoms of acute cannabinoid overdose range from extreme lethargy, ataxia, and generalized psychomotor impairment to feelings of panic and anxiety, agitation, hallucinations, and psychosis. Treatment of acute cannabinoid overdose is currently supportive and symptom driven. Converging lines of evidence indicating many of the symptoms which can precipitate an emergency department visit are mediated through activation of cannabinoid₁ receptors. Here, we review the evidence that cannabinoid₁ receptor antagonists, originally developed for indications ranging from obesity to smoking cessation and schizophrenia, provide a molecular approach to treating acute cannabinoid overdose.

Endocannabinoid Signaling from 2-Arachidonoylglycerol to CB1 Cannabinoid Receptor Facilitates Reward-based Learning of Motor Sequence

The endocannabinoid system modulates synaptic transmission, controls neuronal excitability, and is involved in various brain functions including learning and memory. 2-arachidonoylglycerol, a major endocannabinoid produced by diacylglycerol lipase-α (DGLα), is released from postsynaptic neurons, retrogradely activates presynaptic CB₁ cannabinoid receptors, and induces short-term or long-term synaptic plasticity. To examine whether and how the endocannabinoid system contributes to reward-based learning of a motor sequence, we subjected male CB₁-knockout (KO) and DGLα-KO mice to three types of operant lever-press tasks. First, we trained mice to press one of three levers labeled A, B, and C for a food reward (one-lever task). Second, we trained mice to press the three levers in the order of A, B, and C (three-lever task). Third, the order of the levers was reversed to C, B, and A (reverse three-lever task). We found that CB₁-KO mice and DGLα-KO mice exhibited essentially the same deficits in the operant lever-press tasks. In the one-lever task, both strains of knockout mice showed a slower rate of learning to press a lever for food. In the three-lever task, both strains of knockout mice showed a slower rate of learning of the motor sequence. In the reverse three-lever task, both strains of knockout mice needed more lever presses for reversal learning. These results suggest that the endocannabinoid system facilitates reward-based learning of a motor sequence by conferring the flexibility with which animals can switch between strategies.

Discriminative Stimulus Properties of Phytocannabinoids, Endocannabinoids, and Synthetic Cannabinoids

Psychoactive cannabinoids from the marijuana plant (phytocannabinoids), from the body (endocannabinoids), and from the research lab (synthetic cannabinoids) produce their discriminative stimulus effects by stimulation of CB1 receptors in the brain. Early discrimination work with phytocannabinoids confirmed that Δ9-tetrahydrocannabinol (Δ9-THC) is the primary psychoactive constituent of the marijuana plant, with more recent work focusing on characterization of the contribution of the major endocannabinoids, anandamide and 2-arachidonoylglycerol (2-AG), to Δ9-THC-like internal states. Collectively, these latter studies suggest that endogenous increases in both anandamide and 2-AG seem to be optimal for mimicking Δ9-THC's discriminative stimulus effects, although suprathreshold concentrations of anandamide also appear to be Δ9-THC-like in discrimination assays. Recently, increased abuse of synthetic cannabinoids (e.g., "fake marijuana") has spurred discrimination studies to inform regulatory authorities by predicting which of the many synthetic compounds on the illicit market are most likely to share Δ9-THC's abuse liability. In the absence of a reliable model of cannabinoid self-administration (specifically, Δ9-THC self-administration), cannabinoid discrimination represents the most validated and pharmacologically selective animal model of an abuse-related property of cannabinoids - i.e., marijuana's subjective effects. The influx of recent papers in which cannabinoid discrimination is highlighted attests to its continued relevance as a valuable method for scientific study of cannabinoid use and abuse.

Preclinical studies on the reinforcing effects of cannabinoids. A tribute to the scientific research of Dr. Steve Goldberg

RATIONALE

The reinforcing effects of most abused drugs have been consistently demonstrated and studied in animal models, although those of marijuana were not, until the demonstration 15 years ago that delta-9-tetrahydrocannabinol (THC) could serve as a reinforcer in self-administration (SA) procedures in squirrel monkeys. Until then, those effects were inferred using indirect assessments.

OBJECTIVES

The aim of this manuscript is to review the primary preclinical procedures used to indirectly and directly infer reinforcing effects of cannabinoid drugs.

METHODS

Results will be reviewed from studies of cannabinoid discrimination, intracranial self-stimulation (ICSS), conditioned place preference (CPP), as well as change in levels of dopamine assessed in brain areas related to reinforcement, and finally from self-administration procedures. For each procedure, an evaluation will be made of the predictive validity in detecting the potential abuse liability of cannabinoids based on seminal papers, with the addition of selected reports from more recent years especially those from Dr. Goldberg's research group.

RESULTS AND CONCLUSIONS

ICSS and CPP do not provide consistent results for the assessment of potential for abuse of cannabinoids. However, drug discrimination and neurochemistry procedures appear to detect potential for abuse of cannabinoids, as well as several novel "designer cannabinoid drugs." Though after 15 years transfer of the self-administration model of marijuana abuse from squirrel monkeys to other species remains somewhat problematic, studies with the former species have substantially advanced the field, and several reports have been published with consistent self-administration of cannabinoid agonists in rodents.

Genetic Manipulation of the Endocannabinoid System

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

Dissimilar cannabinoid substitution patterns in mice trained to discriminate Δ(9)-tetrahydrocannabinol or methanandamide from vehicle

Δ(9)-Tetrahydrocannabinol (THC) discrimination in rodents is a behavioral assay that has been used to probe differences among classes of cannabinoids in rats. The purpose of this study was to determine whether traditional and anandamide-like cannabinoids were distinguishable in cannabinoid discrimination procedures in mice. Male mice were trained to discriminate 30 mg/kg THC or 70 mg/kg methanandamide from vehicle in a two-lever milk-reinforced drug discrimination procedure. After acquisition, agonist tests with THC, methanandamide, CP 55940, and anandamide were conducted, as were antagonism tests with rimonabant. Substitution (agonism) and antagonism tests were also carried out in female mice trained to discriminate THC. THC and CP 55940 fully substituted in THC-trained mice of both sexes. Further, THC substitution was rimonabant reversible. In contrast, mice injected with methanandamide or anandamide failed to respond substantially on the THC lever, even up to doses that decreased overall responding. In methanandamide-trained mice, methanandamide fully generalized to the methanandamide training dose. Rimonabant did not reverse this generalization. Although THC, CP 55940, and anandamide also increased responding on the methanandamide lever, the magnitude of substitution was less than for methanandamide. These results suggest incomplete overlap in the underlying mechanisms mediating endocannabinoid pharmacology and marijuana intoxication. Further, they suggest that methanandamide discrimination may involve a non-CB(1) receptor mechanism that is particularly prominent at higher doses.

Central mediation and differential blockade by cannabinergics of the discriminative stimulus effects of the cannabinoid CB1 receptor antagonist rimonabant in rats

RATIONALE

Discovery of an endocannabinoid signaling system launched the development of the blocker rimonabant, a cannabinoid CB1 receptor (CB(1)R) antagonist/inverse agonist. Due to untoward effects, this medication was withdrawn and efforts have been directed towards discovering chemicals with more benign profiles.

OBJECTIVE

This study aims to comparatively evaluate new ligands using a rimonabant discriminated drinking aversion procedure.

METHODS

Rats discriminated between rimonabant (5.6 mg/kg) and vehicle. The 30 min saccharin (0.1%) drinking after rimonabant pretreatment was followed by injection of lithium chloride (120 mg/kg) in the experimental animals. After vehicle pretreatment, experimental animals were given i.p. NaCl (10 ml/kg). Postdrinking treatment for controls was NaCl, irrespective of pretreatment condition (rimonabant or vehicle).

RESULTS

The centrally acting neutral CB(1)R antagonist AM4113, but not the limited brain penetrating CB(1)R neutral antagonist AM6545, substituted for rimonabant. The CB(1)R agonists THC (1-10 mg/kg), AM1346 (1-10 mg/kg) did not substitute. The rimonabant-induced conditioned suppression of saccharin drinking was attenuated when CB(1)R agonists AM5983 (0.01-1 mg/kg) and THC (10 mg/kg), but not the CB(1)R agonist AM1346 (0.1-18 mg/kg), were combined with rimonabant (5.6 mg/kg). By varying the injection-to-test interval, we gauged the relative duration of the cueing effects of rimonabant, and the in vivo functional half-life was estimated to be approximately 1.5 h.

CONCLUSION

A neutral CB(1)R antagonist (AM4113) produced cueing effects similar to those of rimonabant and generalization likely was centrally mediated. The functional cueing effects of rimonabant are relatively short-acting, pharmacologically selective, and differentially blocked by cannabinergics.

Antagonism of ∆⁹-THC induced behavioral effects by rimonabant: time course studies in rats

The objective was to examine the time course of the cannabinoid 1 receptor antagonist/inverse agonist rimonabant's ability to antagonize in vivo cannabinergic agonist effects. We used two behavioral procedures sensitive to the effects of ∆⁹-tetrahydrocannabinol (∆⁹-THC): rat drug discrimination (EXP-1) and suppression of fixed-ratio responding (FR) for food reinforcement (EXP-2). Two training doses of ∆⁹-THC (1.8 and 3 mg/kg) served as discriminative cues in two groups discriminating ∆⁹-THC from vehicle; injections were i.p. 20 min before session onset. Tests assessed the dose-response functions of ∆⁹-THC and the time course for rimonabant in its ability to block the discriminative stimulus effects of ∆⁹-THC. For antagonism testing, the training doses of ∆⁹-THC were used and the rimonabant dose was 1mg/kg. Tests were 20, 60, 120, and 240 min post rimonabant administration; ∆⁹-THC was always administered 20 min prior to testing. For EXP-2, only one response lever was activated and every 10th (FR-10) press on that lever resulted in food delivery. Once the response rate stabilized, tests occurred with ∆⁹-THC, rimonabant and combinations of the drugs. The ED(50) estimates for the dose-response functions were 0.38 (±0.28-0.51) and 0.50 (±0.40-0.63) mg/kg for the training doses of 1.8 and 3 mg/kg ∆⁹-THC, respectively. The time course studies suggested functional half-life estimates of 128.4 (±95.7-172.2) and 98.4 (±64.2-150.7) min by rimonabant for the two groups in EXP-1, respectively. Similarly, the functional half-life of rimonabant was 118.9 (±66.1-213.9) min in EXP-2. Thus, antagonism of ∆⁹-THC by rimonabant is relatively short lasting.

Cannabinoids and experimental models of multiple sclerosis

The inflammatory response is a hallmark in the development of autoimmune-mediated neurodegenerative diseases of the central nervous system (CNS). Research on these pathological phenomena is being extensively undertaken and experimental autoimmune encephalomyelitis (EAE) serves as a valuable animal model. Studies from this model have generated interesting insights into biological effects of cannabinoids and may, at least to a certain extent, reflect the cannabinoid-mediated protective mechanisms also in human diseases with similar characteristics, such as multiple sclerosis (MS). Cannabinoids are involved in regulation of the immune system. These effects comprise modulation of inflammatory reaction through components of the innate and adaptive immune responses. Cannabinoids also confer neuroprotection and assist neuroregeneration, thus maintaining a balance within the delicate CNS microenvironment and restoring function following pathological condition, commonly driven by neuroinflammation. Continued studies of cannabinoid actions in EAE pathogenesis should be beneficial for the better understanding of the mechanisms governing such a vast array of physiological effects and in development of new therapeutic strategies for the treatment of human neuroinflammatory and neurodegenerative diseases.

Dual modulation of CNS voltage-gated calcium channels by cannabinoids: Focus on CB1 receptor-independent effects

The neuromodulatory effects of cannabinoids in the central nervous system have mainly been associated with G-protein coupled cannabinoid receptor (CB1R) mediated inhibition of voltage-gated calcium channels (VGCCs). Numerous studies show, however, that cannabinoids can also modulate VGCCs independent of CB1R activation. Nevertheless, despite the fact that endocannabinoids have a nearly equal efficacy for direct and CB1R-mediated effects on VGCC, the role of the direct cannabinoid-VGCC interaction has been largely underestimated. In this review, we summarize recent studies on the modulation of different types of VGCCs by cannabinoids, highlight the evidence for and implications of the CB1R-independent modulation, and put forward the concept, that direct interaction of cannabinoids and VGCCs is as important in regulation of VGCCs function as the CB1R-mediated effects.

In vivo pharmacology of endocannabinoids and their metabolic inhibitors: therapeutic implications in Parkinson's disease and abuse liability

This review focuses on the behavioral pharmacology of endogenous cannabinoids (endocannabinoids) and indirect-acting cannabinoid agonists that elevate endocannabinoid tone by inhibiting the activity of metabolic enzymes. Similarities and differences between prototype cannabinoid agonists, endocannabinoids and inhibitors of endocannabinoid metabolism are discussed in the context of endocannabinoid pharmacokinetics in vivo. The distribution and function of cannabinoid and non-CB(1)/CB(2) receptors are also covered, with emphasis on their role in disorders characterized by dopamine dysfunction, such as drug abuse and Parkinson's disease. Finally, evidence is presented to suggest that FAAH inhibitors lack the abuse liability associated with CB(1) agonists, although they may modify the addictive properties of other drugs, such as alcohol.

Discriminative stimulus functions in rats of AM1346, a high-affinity CB1R selective anandamide analog

OBJECTIVE

To characterize in vivo the high-affinity CB(1) cannabinoid receptor (CB(1)R) selective anandamide analog AM1346 [alkoxyacid amide of N-eicosa-tetraenylamine] using drug discrimination. Substitution tests involved Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and R-(+)-methanandamide (mAEA), a metabolically stable analog of anandamide (AEA), as well as the CB(1)R antagonist/inverse agonist rimonabant; D: -amphetamine and morphine were also examined to assess pharmacological specificity.

MATERIALS AND METHODS

Rats were initially trained to discriminate between i.p.-injected vehicle and 3 mg/kg AM1346 (group 3 mg/kg; t' = 20 min); subsequently, the rats were retrained with 5.6 mg/kg AM1346 (group 5.6 mg/kg; t' = 20 min).

RESULTS

Dose-generalization curves of AM1346, Delta(9)-THC, and mAEA suggested the following order of potency: Delta(9)-THC > AM1346 > mAEA both for rats discriminating between 3 and 5.6 mg/kg AM1346 from vehicle. In group 3 mg/kg, challenge by 1 mg/kg rimonabant resulted in parallel shifts to the right of the dose-generalization curves for Delta(9)-THC and AM1346, suggesting surmountable antagonism. Surmountable antagonism was not demonstrated with rimonabant-mAEA combinations. A long duration of effect was indicated when 3 mg/kg AM1346 was examined after different time intervals following i.p. administration (group 3 mg/kg). The in vivo half-life was close to 5 h. Neither D: -amphetamine nor morphine generalized in either of groups 3 mg/kg and 5.6 mg/kg, suggesting pharmacological specificity.

CONCLUSION

Unlike mAEA, the surmountable antagonism between rimonabant and AM1346 showed that the structural features of AEA can be modified to produce novel ligands that reduce the dissociation between the discriminative stimulus and rate decreasing effects of CB(1)R agonists derived from an AEA template.

Cannabinoid agonists differentially substitute for the discriminative stimulus effects of Delta(9)-tetrahydrocannabinol in C57BL/6J mice

RATIONALE

A variety of behavioral procedures have been developed to assess cannabinoid activity in mice; however, the feasibility of establishing Delta(9)-THC as a discriminative stimulus in mice has not been documented.

OBJECTIVE

One goal was to establish Delta(9)-THC as a discriminative stimulus in mice; after having done so, another goal was to examine the in vivo mechanism of action of Delta(9)-THC with other cannabinoids and noncannabinoids.

MATERIALS AND METHODS

C57BL/6J mice (n = 8) were trained to discriminate Delta(9)-THC (10 mg/kg i.p.) from vehicle while responding under a fixed ratio 30 schedule of food presentation.

RESULTS

Mice satisfied the discrimination criteria in 18-98 (median = 67) sessions and the discriminative stimulus effects of Delta(9)-THC were dose-dependent (ED(50) = 2.6 mg/kg). CP 55940 and WIN 55212-2 dose-dependently increased Delta(9)-THC-appropriate responding to 100% (ED(50) = 0.032 and 0.45 mg/kg, respectively), whereas methanandamide and a variety of noncannabinoids (cocaine, ethanol, and ketamine) produced a maximum of 34% Delta(9)-THC-appropriate responding. The cannabinoid CB(1) antagonist SR 141716A (rimonabant) surmountably antagonized the discriminative effects of Delta(9)-THC, CP 55940, and WIN 55212-2; methanandamide did not significantly modify the Delta(9)-THC discriminative stimulus.

CONCLUSIONS

The discriminative stimulus effects of Delta(9)-THC, CP 55940, and WIN 55212-2 are mediated by the same (i.e., CB(1)) receptors, whereas the effects of methanandamide or a metabolite of methanandamide are mediated at least in part by non-CB(1) receptors. The discriminative stimulus effects of Delta(9)-THC in mice could be used to evaluate mechanisms of cannabinoid activity with approaches (e.g., inducible knockouts) currently unavailable in nonmurine species.

Discriminative stimulus effects of the cannabinoid CB1 receptor antagonist rimonabant in rats

OBJECTIVE

To examine the discriminative stimulus effects of the cannabinoid CB(1) receptor (CB(1)R) antagonist/inverse agonist rimonabant (SR141716A) using a discriminated taste aversion (DTA) procedure.

MATERIALS AND METHODS

Groups of rats were trained to discriminate between drug (5.6 or 3 mg/kg) and vehicle in DTA (t' = 20 min). The 30-min drinking opportunity after rimonabant pretreatment was followed by injection of lithium chloride (120 mg/kg) in the experimental (EXP) animals. When offered fluid after vehicle pretreatment, EXP animals subsequently were given intraperitoneal saline (NaCl, 10 ml/kg). Post-drinking treatment for controls (CONT) was NaCl irrespective of the pretreatment condition (rimonabant or vehicle). Tests examined other doses and drugs (t' = 20 min).

RESULTS

The rimonabant analog AM251 (1 to 5.6 mg/kg) substituted for rimonabant. AM281 also appeared to substitute, but interpretation is complicated by unconditioned effects (drinking suppressed also in the CONT group). The CB(2)R antagonists SR144528 (18 and 30 mg/kg), AM630 (1 to 10 mg/kg), and the CB(1)R agonist methanandamide (mAEA, 3 and 10 mg/kg) did not substitute. There was a dose-related attenuation of the rimonabant-induced suppression of saccharin drinking when Delta9-tetrahydrocannabinol (Delta9-THC; 0.3 to 5.6 mg/kg), but not mAEA (1 to 10 mg/kg), was given together with rimonabant (3 mg/kg). Unconditioned effects occurred with the mAEA-rimonabant combination, not evident for combinations of rimonabant and Delta9-THC. mAEA (10 mg/kg) plus AM251 (5.6 mg/kg) resulted in strong unconditioned effects.

CONCLUSION

Rimonabant induces a discriminative stimulus in DTA that continues to show potential for further examination of cannabinoid receptor antagonism.

Effects of AM1346, a high-affinity CB1 receptor selective anandamide analog, on open-field behavior in rats

AM1346 is a cannabinoid receptor type 1 (CB1R) anandamide analog [alkoxyacid amide of N-eicosa-(5Z, 8Z, 11Z, 14Z)-tetraenylamine] with high affinity and selectivity for the CB1 vs. CB2 receptor [Ki (CB1)=1.5 nmol/l; Ki (CB2)=152 nmol/l]. The present study characterized the effects of AM1346 (5.6-18 mg/kg) and its interaction with the CB1R antagonist/inverse agonist SR141716 (1-5.6 mg/kg) on open-field behaviors of rats. AM1346 reduced ambulation (horizontal activity), rearing (vertical activity) and increased the degree of circling and the latency to leave the central area of the open-field arena. AM1346 also tended to reduce defecation and to increase vocalization in a dose-dependent manner. In pretreatment studies, SR141716 completely blocked the effects of AM1346 on circling and latency and partially antagonized the effects of 18 mg/kg AM1346 on ambulation and rearing. SR141716 also tended to decrease AM1346-induced vocalization in a dose-dependent manner. Earlier studies have shown that SR141716, given alone, can increase grooming behavior and, as well, produces dose-related increases in scratching. In the present studies, these effects were attenuated in a dose-related manner by AM1346. The present profile of behavioral effects for AM1346 is consistent with its designation as a CB1R agonist. When combined with drug discrimination data (surmountable antagonism of effects of SR141716 by Delta(9)-THC and AM1346 but not by methanandamide, i.e. AM356), these data indicate that the anandamide analog AM1346 may be more behaviorally similar to cannabinoids like Delta(9)-THC than to other anandamide-based molecules such as methanandamide.

The lack of CB1 receptors prevents neuroadapatations of both NMDA and GABA(A) receptors after chronic ethanol exposure

As the contribution of cannabinoid (CB1) receptors in the neuroadaptations following chronic alcohol exposure is unknown, we investigated the neuroadaptations induced by chronic alcohol exposure on both NMDA and GABA(A) receptors in CB1-/- mice. Our results show that basal levels of hippocampal [(3)H]MK-801 ((1)-5-methyl-10,11-dihydro-5Hdibenzo[a,d]cyclohepten-5,10-imine) binding sites were decreased in CB1-/- mice and that these mice were also less sensitive to the locomotor effects of MK-801. Basal level of both hippocampal and cerebellar [(3)H]muscimol binding was lower and sensitivity to the hypothermic effects of diazepam and pentobarbital was increased in CB1-/- mice. GABA(A)alpha1, beta2, and gamma2 and NMDA receptor (NR) 1 and 2B subunit mRNA levels were altered in striatum of CB1-/- mice. Our results also showed that [(3)H]MK-801 binding sites were increased in cerebral cortex and hippocampus after chronic ethanol ingestion only in wild-type mice. Chronic ethanol ingestion did not modify the sensitivity to the locomotor effects of MK-801 in both genotypes. Similarly, chronic ethanol ingestion reduced the number of [(3)H]muscimol binding sites in cerebral cortex, but not in cerebellum, only in CB1+/+ mice. We conclude that lifelong deletion of CB1 receptors impairs neuroadaptations of both NMDA and GABA(A) receptors after chronic ethanol exposure and that the endocannabinoid/CB1 receptor system is involved in alcohol dependence.

Cannabinoid effects on behaviors maintained by ethanol or food: a within-subjects comparison

The cannabinoid CB1 antagonist rimonabant (SR141716A) has been proposed as a therapeutic agent for several addictive disorders, including alcoholism. Rimonabant may selectively reduce responding for an ethanol solution compared with an alternative. While this could represent a specific effect of CB1 inhibition on ethanol reinforcement, this could also result from differences in the baseline rates of behavior or experiences between comparison groups. We developed a procedure in rats that allows a within-subject comparison of ethanol and food-maintained responding and provides well matched baseline response rates. We determined the effects of acute doses of rimonabant (0.3-5.6 mg/kg, intraperitoneal) and the CB1 agonist Delta-9-tetrahydrocannabinol (1.0-5.6 mg/kg, intraperitoneal) on responding for food and ethanol under a multiple fixed-ratio schedule. To confirm that rimonabant blocked cannabinoid receptors, the ability of rimonabant to antagonize Delta-9-tetrahydrocannabinol effects in the same subjects under the same reinforcement schedule was also determined. In contrast with previous reports, rimonabant did not significantly alter responding for ethanol or food. The effects of Delta-9-tetrahydrocannabinol on responding for food were completely antagonized by rimonabant, whereas Delta-9-tetrahydrocannabinol effects on responding for ethanol were not. These results suggest that there may be neuroadaptation of the cannabinoid system following aging or chronic self-administration of ethanol.

Discriminative stimulus functions of AM-1346, a CB1R selective anandamide analog in rats trained with Delta9-THC or (R)-methanandamide (AM-356)

OBJECTIVE

To characterize in vivo the high-affinity cannabinoid CB1 receptor (CB1R) selective anandamide analog AM-1346 [alkoxyacid amide of N-eicosa-tetraenylamine] using drug discrimination procedures. D-amphetamine and also morphine in the (R)-methanandamide-trained group (see below) were examined to assess pharmacological specificity.

METHODS

Three groups of rats were trained to discriminate between vehicle and (1) 1.8 mg/kg Delta9-tetrahydrocannabinol (Delta9-THC); (2) 5.6 mg/kg Delta9-THC; and (3) 10 mg/kg (R)-methanandamide (AM-356; a metabolically stable analog of anandamide). Delta9-THC was given i.p. 30 min and (R)-methanandamide 15 min before training.

RESULTS

AM-1346 generalized to all three training conditions, both at 15 and 30 min after administration. The rank order potency was: Delta9-THC > AM-1346 > (R)-methanandamide. AM-1346 appeared slightly more potent 30 min compared to 15 min postadministration. In the presence of 0.3 mg/kg of the CB1R antagonist/inverse agonist SR-141716A, the dose generalization curves of Delta9-THC and AM-1346 resulted in parallel shifts to the right in the 1.8 mg/kg Delta9-THC-trained group. A long duration of action for AM-1346 (relative to AM-356) was indicated in tests where AM-1346 was examined in the 5.6 mg/kg Delta9-THC-trained group. Neither D-amphetamine, nor morphine generalized in either of the groups, suggesting pharmacological specificity.

CONCLUSION

Unlike (R)-methanandamide, the surmountable antagonism between SR-141716A and AM-1346 shows that the structural features of anandamide can be modified in ways that reduce the dissociation between the discriminative stimulus and rate decreasing effects of CB1R agonists derived from an anandamide template.

Alcohol-related genes: contributions from studies with genetically engineered mice

Since 1996, nearly 100 genes have been studied for their effects related to ethanol in mice using genetic modifications including gene deletion, gene overexpression, gene knock-in, and occasionally by studying existing mutants. Nearly all such studies have concentrated on genes expressed in brain, and the targeted genes range widely in their function, including most of the principal neurotransmitter systems, several neurohormones, and a number of signaling molecules. We review 141 published reports of effects (or lack thereof) of 93 genes on responses to ethanol. While most studies have focused on ethanol self-administration and reward, and/or sedative effects, other responses studied include locomotor stimulation, anxiolytic effects, and neuroadaptation (tolerance, sensitization, withdrawal). About 1/4 of the engineered mutations increase self-administration, 1/3 decrease it, and about 40% have no significant effect. In many cases, the effects on self-administration are rather modest and/or depend on the specific experimental procedures. In some cases, genes in the background strains on which the mutant is placed are important for results. Not surprisingly, review of the systems affected further supports roles for serotonin, gamma-aminobutyric acid, opioids and dopamine, all of which have long been foci of alcohol research. Novel modulatory effects of protein kinase C and G protein-activated inwardly rectifying K+ (GIRK) channels are also suggested. Some newer research with cannabinoid systems is promising, and has led to ongoing clinical trials.

Evaluation of the role of the arachidonic acid cascade in anandamide's in vivo effects in mice

The pharmacological profiles of the endocannabinoid anandamide and exogenous cannabinoids (e.g., Delta9-tetrahydrocannabinol) are similar, but not exactly the same. One notable difference is that anandamide's in vivo effects in mice are not blocked by the brain cannabinoid (CB1) receptor antagonist SR141716A. The degree to which the rapid metabolism of anandamide to arachidonic acid might be involved in this unexpected lack of effect was the focus of this study. Mice were tested in a tetrad of tests sensitive to cannabinoids, consisting of spontaneous locomotion, ring immobility, rectal temperature and tail flick nociception. Anandamide and arachidonic acid produced a similar profile of effects, but neither drug was blocked by SR141716A. When hydrolysis of anandamide was inhibited by an amidase inhibitor (phenylmethyl sulfonyl fluoride; PMSF), however, SR141716A significantly attenuated anandamide's effects but did not completely block them. Similarly, the effects of the metabolically stable anandamide analog O-1812 were attenuated by SR141716A. The role of oxidative metabolism in anandamide's effects in the tetrad was also investigated through pharmacological modulation of cyclooxygenase and lipoxygenase, two major classes of enzymes that degrade arachidonic acid. Whereas the non-selective cyclooxygenase inhibitor ibuprofen blocked the in vivo effects of arachidonic acid, it did not alter anandamide's effects. Other modulators of the cyclooxygenase and lipoxygenase pathways also failed to block anandamide's effects. Together, these results offer partial support for a pharmacokinetic explanation of the failure of SR141716A to antagonize the effects of anandamide; however, they also suggest that non-CB1, non-CB2 receptors may be involved in mediation of anandamide's in vivo actions, particularly at higher doses.

Synthesis of a potential photoactivatable anandamide analog

A potential photoreactive analog of anandamide was synthesized via selective hydrogenation of a skipped tetrayne intermediate. This compound might be a useful tool to search for new cannabinoid receptors.

Receptor-independent actions of cannabinoids on cell membranes: Focus on endocannabinoids

Cannabinoids are a structurally diverse group of mostly lipophilic molecules that bind to cannabinoid receptors. In fact, endogenous cannabinoids (endocannabinoids) are a class of signaling lipids consisting of amides and esters of long-chain polyunsaturated fatty acids. They are synthesized from lipid precursors in plasma membranes via Ca(2+) or G-protein-dependent processes and exhibit cannabinoid-like actions by binding to cannabinoid receptors. However, endocannabinoids can produce effects that are not mediated by these receptors. In pharmacologically relevant concentrations, endocannabinoids modulate the functional properties of voltage-gated ion channels including Ca(2+) channels, Na(+) channels, various types of K(+) channels, and ligand-gated ion channels such as serotonin type 3, nicotinic acetylcholine, and glycine receptors. In addition, modulatory effects of endocannabinoids on other ion-transporting membrane proteins such as transient potential receptor-class channels, gap junctions and transporters for neurotransmitters have also been demonstrated. Furthermore, functional properties of G-protein-coupled receptors for different types of neurotransmitters and neuropeptides are altered by direct actions of endocannabinoids. Although the mechanisms of these effects are currently not clear, it is likely that these direct actions of endocannabinoids are due to their lipophilic structures. These findings indicate that additional molecular targets for endocannabinoids exist and that these targets may represent novel sites for cannabinoids to alter either the excitability of the neurons or the response of the neuronal systems. This review focuses on the results of recent studies indicating that beyond their receptor-mediated effects, endocannabinoids alter the functions of ion channels and other integral membrane proteins directly.

Cannabinoids modulate the P-type high-voltage-activated calcium currents in purkinje neurons

Endocannabinoids released by postsynaptic cells inhibit neurotransmitter release in many central synapses by activating presynaptic cannabinoid CB1 receptors. In particular, in the cerebellum, endocannabinoids inhibit synaptic transmission at granule cell to Purkinje cell synapses by modulating presynaptic calcium influx via N-, P/Q-, and R-type calcium channels. Using whole cell patch-clamp techniques, we show that in addition to this presynaptic action, both synthetic and endogenous cannabinoids inhibit P-type calcium currents in isolated rat Purkinje neurons independent of CB1 receptor activation. The IC50 for the anandamide (AEA)-induced inhibition of P-current peak amplitude was 1.04 +/- 0.04 microM. In addition, we demonstrate that all the tested cannabinoids in a physiologically relevant range of concentrations strongly accelerate inactivation of P currents. The effects of AEA cannot be attributed to the metabolism of AEA because a nonhydrolyzing analogue of AEA, methanandamide inhibited P-type currents with a similar efficacy. All effects of cannabinoids on P-type Ca2+ currents were insensitive to antagonists of CB1 cannabinoid or vanilloid TRPV1 receptors. In cerebellar slices, WIN 55,212-2 significantly affected spontaneous firing of Purkinje neurons in the presence of CB1 receptor antagonist, in a manner similar to that of a specific P-type channel antagonist, indicating a possible functional implication of the direct effects of cannabinoids on P current. Taken together these findings demonstrate a functionally important direct action of cannabinoids on P-type calcium currents.

Glycine receptors in CNS neurons as a target for nonretrograde action of cannabinoids

At many central synapses, endocannabinoids released by postsynaptic cells act retrogradely on presynaptic G-protein-coupled cannabinoid receptors to inhibit neurotransmitter release. Here, we demonstrate that cannabinoids may directly affect the functioning of inhibitory glycine receptor (GlyR) channels. In isolated hippocampal pyramidal and Purkinje cerebellar neurons, endogenous cannabinoids anandamide and 2-arachidonylglycerol, applied at physiological concentrations, inhibited the amplitude and altered the kinetics of rise time, desensitization, and deactivation of the glycine-activated current (I(Gly)) in a concentration-dependent manner. These effects of cannabinoids were observed in the presence of cannabinoid CB1/CB3, vanilloid receptor 1 antagonists, and the G-protein inhibitor GDPbetaS, suggesting a direct action of cannabinoids on GlyRs. The effect of cannabinoids on I(Gly) desensitization was strongly voltage dependent. We also demonstrate that, in the presence of a GABA(A) receptor antagonist, GlyRs may contribute to the generation of seizure-like activity induced by short bursts (seven stimuli) of high-frequency stimulation of inputs to hippocampal CA1 region, because this activity was diminished by selective GlyR antagonists (strychnine and ginkgolides B and J). The GlyR-mediated rhythmic activity was also reduced by cannabinoids (anandamide) in the presence of a CB1 receptor antagonist. These results suggest that the direct inhibition of GlyRs by endocannabinoids can modulate the hippocampal network activity.

SR 141716A differentially attenuates the behavioral effects of delta9-THC in rhesus monkeys

The prototypic cannabinoid CB1 antagonist SR 141716A is one important pharmacologic tool for examining CB1 receptors that mediate the behavioral and physiologic effects of delta9-tetrahydrocannabinol (delta9-THC). This study examined the effects of SR 141716A on the rate-decreasing, hypothermic and discriminative stimulus effects of delta9-THC in rhesus monkeys. In monkeys (n=4) responding under a multiple fixed ratio (FR-10:FR-10) schedule of food presentation and stimulus-shock termination, the potency of i.m. delta9-THC to decrease responding in the food component (ED50=0.64 mg/kg) was threefold greater than its potency in the stimulus-shock termination component (ED50=2.14 mg/kg). In the same monkeys, hypothermia was induced by delta9-THC at a dose (e.g. 0.32 mg/kg) that did not alter responding in either schedule component; the maximum decrease was 2.1 degrees C at a dose of 3.2 mg/kg. A dose of 0.32 mg/kg of SR 141716A, significantly attenuated delta9-THC-induced hypothermia without attenuating the rate-decreasing effects of delta9-THC in either component of the multiple schedule. The largest dose of i.m. SR 141716A that was studied, 1.0 mg/kg, significantly decreased rectal temperature and responding in the food component but did not significantly decrease responding in the stimulus-shock termination component of the multiple schedule. In a separate group of monkeys (n=3) that discriminated i.v. delta9-THC (0.1 mg/kg) while responding under an FR-5 schedule of stimulus-shock termination, SR 141716A (0.32 and 1 mg/kg) significantly increased the ED50 of the delta9-THC by 2.3- and 3.7-fold, respectively. Collectively, these results demonstrate that the behavioral effects of delta9-THC are not equally attenuated by SR 141716A.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

CB1 receptor knockout mice display reduced ethanol-induced conditioned place preference and increased striatal dopamine D2 receptors

Cannabinoids and ethanol activate the same reward pathways, and recent advances in the understanding of the neurobiological basis of alcoholism suggest that the CB1 receptor system may play a key role in the reinforcing effects of ethanol and in modulating ethanol intake. In the present study, male CB1 receptors knockout mice generated on a CD1 background displayed decreased ethanol-induced conditioned place preference (CPP) compared to wild-type (CB1(+/+)) mice. Ethanol (0.5, 1.0, 1.5, and 2.0 g/kg) induced significant CPP in CB1(+/+) mice at all doses tested, whereas it induced significant CPP only at the highest dose of ethanol (2.0 g/kg) in CB1(-/-) mice. However, there was no genotypic difference in cocaine (20 mg/kg)-induced CPP. There was also no genotypic difference, neither in cocaine (10-50 mg/kg) nor in D-amphetamine (1.2-5 mg/kg)-induced locomotor effects. In addition, mutant and wild-type mice did not differ in sensitivity to the anxiolytic effects of ethanol (1.5 g/kg) when tested using the elevated plus maze. Interestingly, this decrease in ethanol efficacy to induce CPP in CB1(-/-) mice was correlated with an increase in D2/D3 receptors, as determined by [3H]raclopride binding, whereas there was no difference in D1-like receptors, as determined by [3H]SCH23390 binding, measured in the striatum from drug-naive mice. This increase in D2/D3 binding sites observed in CB1 knockout mice was associated with an altered locomotor response to the D2/D3 agonist quinpirole (low doses 0.02-0.1 mg/kg) but not to an alteration of quinpirole (0.1-1.0 mg/kg)-induced CPP compared to wild-type mice. Altogether, the present results indicate that lifelong deletion of CB1 receptors reduced ethanol-induced CPP and that these reduced rewarding effects of ethanol are correlated to an overexpression of striatal dopamine D2 receptors.

Efficacy in CB1 receptor-mediated signal transduction

CB(1) receptor cellular signal transduction is dependent on the expression of G proteins to which the receptor couples, the potential for precoupling of particular G proteins to the receptors either by scaffolding mechanisms or colocalization in lipid raft domains, and the effector mechanisms that these transducer molecules regulate. This discourse will evaluate studies of efficacy for CB(1) receptor-Gi/o activation at the molecular level. Evidence for brain regional differences in CB(1) receptor signal transduction efficacy and agonist selectivity for G proteins will be summarized. The possibility that CB(1) receptors interact with Gs or Gq will be evaluated, and questions with regard to the constitutive activity and G protein sequestration will be posed.