The antinociceptive effect of Delta9-tetrahydrocannabinol in the arthritic rat involves the CB(2) cannabinoid receptor

Cannabinoid CB₂ receptors have been implicated in antinociception in animal models of both acute and chronic pain. We evaluated the role both cannabinoid CB₁ and CB₂ receptors in mechanonociception in non-arthritic and arthritic rats. The antinociceptive effect of Δ⁹-tetrahydrocannabinol (Δ⁹THC) was determined in rats following administration of the cannabinoid CB₁ receptor-selective antagonist, SR141716A, the cannabinoid CB₂ receptor-selective antagonist, SR144528, or vehicle. Male Sprague–Dawley rats were rendered arthritic using Freund’s complete adjuvant and tested for mechanical hyperalgesia in the paw-pressure test. Arthritic rats had a baseline paw-pressure of 83 ± 3.6g versus a paw-pressure of 177 ± 6.42g in non-arthritic rats. SR144528 or SR141716A (various doses mg/kg; i.p.) or 1:1:18 (ethanol:emulphor:saline) vehicle were injected 1 h prior to Δ⁹THC (4mg/kg; i.p) or 1:1:18 vehicle and antinociception determined 30min post Δ⁹THC. AD₅₀'s for both antagonists were calculated with 95% confidence limits. In addition, midbrain and spinal cord were removed for determination of cannabinoid CB₁ and CB₂ receptor protein density in the rats. SR144528 significantly attenuated the antinociceptive effect of Δ⁹THC in the arthritic rats [AD₅₀ = 3.3 (2.7–4) mg/kg], but not in the non-arthritic rats at a dose of 10/mg/kg. SR141716A significantly attenuated Δ⁹THC-induced antinociception in both the non-arthritic [AD₅₀ = 1.4 (0.8–2) mg/kg] and arthritic rat [AD₅₀ = 2.6 (1.8–3.1) mg/kg]. SR141716A or SR144528 alone did not result in a hyperalgesic effect as compared to vehicle. Our results indicate that the cannabinoid CB₂ receptor plays a critical role in cannabinoid-mediated antinociception, particularly in models of chronic inflammatory pain.     

Intravenous self-administration of the cannabinoid CB1 receptor agonist WIN 55,212-2 in rats

RATIONALE: Delta9-tetrahydrocannabinol (Delta9-THC), the main psychoactive ingredient of marijuana, as well as synthetic cannabinoid (CB1) receptor agonists, has led to negative or equivocal results when tested with the intravenous self-administration procedure, the best validated behavioural model for evaluating abuse liability of drugs in experimental animals. We recently reported, however, that the synthetic CB1 receptor agonist WIN 55,212-2 is intravenously self-administered by drug-naive mice and that its self-administration is blocked by the cannabinoid CB1 receptor antagonist SR 141716A. OBJECTIVE: To assess a reliable model of cannabinoid intravenous self-administration in rats. Long Evans male rats were allowed the opportunity to self-administer WIN 55,212-2 at doses ranging from 6.25 to 50 microg/kg per injection, under a fixed-ratio 1 (FR1) schedule of reinforcement and nose-pokes as the operant responses. The effect of either a change in the unit drug dose available or a pretreatment with the specific CB1 receptor antagonist SR 141716A were then investigated (maintenance phase). Finally, the extinction of the self-administration behaviour was evaluated. RESULTS: Response rate depended on the drug dose available, with maximum rates occurring at 12.5 microg/kg per injection. Response rate increased following pretreatment with the specific CB1 receptor antagonist, SR 141716A. Moreover, operant behaviour rapidly extinguished following both the substitution of saline or vehicle for cannabinoid and the disconnection of the drug delivery pumps. CONCLUSION: Rats will intravenously self-administer the synthetic CB1 receptor agonist WIN 55,212-2 under specific experimental conditions, thus allowing further investigation of the neurobiological mechanisms underlying cannabinoid-taking behaviour.     

Effects of the cannabinoid CB<Subscript>1</Subscript> receptor antagonist rimonabant on distinct measures of impulsive behavior in rats

Rationale Pathological impulsivity is a prominent feature in several psychiatric disorders, but detailed understanding of the specific neuronal processes underlying impulsive behavior is as yet lacking. Objectives As recent findings have suggested involvement of the brain cannabinoid system in impulsivity, the present study aimed at further elucidating the role of cannabinoid CB₁ receptor activation in distinct measures of impulsive behavior. Materials and methods The effects of the selective cannabinoid CB₁ receptor antagonist, rimonabant (SR141716A) and agonist WIN55,212-2 were tested in various measures of impulsive behavior, namely, inhibitory control in a five-choice serial reaction time task (5-CSRTT), impulsive choice in a delayed reward paradigm, and response inhibition in a stop-signal paradigm. Results In the 5-CSRTT, SR141716A dose-dependently improved inhibitory control by decreasing the number of premature responses. Furthermore, SR141716A slightly improved attentional function, increased correct response latency, but did not affect other parameters. The CB₁ receptor agonist WIN55,212-2 did not change inhibitory control in the 5-CSRTT and only increased response latencies and errors of omissions. Coadministration of WIN55,212-2 prevented the effects of SR141716A on inhibitory control in the 5-CSRTT. Impulsive choice and response inhibition were not affected by SR141716A at any dose, whereas WIN55,212-2 slightly impaired response inhibition but did not change impulsive choice. Conclusions The present data suggest that particularly the endocannabinoid system seems involved in some measures of impulsivity and provides further evidence for the existence of distinct forms of impulsivity that can be pharmacologically dissociated.     

Synergistic efects of opioid and cannabinoid antagonists on food intake

RATIONALE: Central cannabinoid systems have been implicated in appetite regulation through the hyperphagic effects of exogenous and endogenous cannabinoids. These effects may involve activation of reward systems and be mediated in part by opioidergic processes. OBJECTIVE: Cannabinoid-opioid interactions in feeding were examined by testing the combined effects on food intake of sub-anorectic doses of selective antagonists for CB1 and opioid receptors. METHODS: Male rats (n = 8) received subcutaneous injections of naloxone (0, 0.1, 0.5, 1.0 mg/kg) and SR141716 (0, 0.1, 0.5, 1.0 mg/kg) before l-h, nocturnal food (chow) intake tests. RESULTS: Neither naloxone nor SR141716 reliably affected feeding when administered alone. By contrast, combined administration of the two antagonists significantly suppressed chow intake at each dose combination. Joint administration of the highest doses of each antagonist suppressed intake by 73%, a significantly greater effect than produced by either naloxone (32%) or SR141716 alone (17%). CONCLUSION: The data reveal a synergistic interaction between the effects of naloxone and SR141716 on feeding, provide further evidence of important functional relationships between endogenous cannabinoid and opioid systems, and strengthen the postulated role for endocannabinoids in reward processes contributing to the normal control of appetite.     

Age-dependent effects of the cannabinoid CB1 antagonist SR141716A on food intake, body weight change, and pruritus in rats

Rationale  The cannabinoid CB1 selective antagonist SR141716A (Rimonabant) has been shown to decrease body weight in laboratory animals and humans. Furthermore, SR141716A can elicit scratching behavior in rodents, a behavior that has been hypothesized to contribute to SR141716A-induced decrease in food intake. Although childhood obesity is a rising health issue, it is unknown whether SR141716A is equipotent at modulating food intake and other CB1-mediated behaviors in younger subjects. Objective  To determine whether CB1 receptor blockade is equipotent at modulating food and water intake, body weight, and scratching behavior, the effect of a range of SR141716A doses on these behaviors in food-restricted postnatal day (P) 18, 28, and 60 male rats was investigated. Brain concentrations of SR141716A were determined in each age group. Results  SR141716A dose- and age-dependently suppressed food and water intake and body weight gain and elicited head scratching, with the most potent effects observed in P18 and P28 rats. Brain concentrations of SR141716A were significantly elevated in P18 rats relative to P28 and P60 rats. SR141716A-elicited head scratching was attenuated by the 5-HT_2A/2C antagonist ketanserin. Conclusions  SR141716A is more potent at modulating food intake and head scratching in very young animals; these differences can be attributed to an increase in brain penetration of SR141716A for P18 but not for P28 and P60 rats. In addition, SR141716-elicited head scratching is modulated by 5HT receptor antagonism and is not a contributing factor to SR141716A's anorectic effects.     

The future of type 1 cannabinoid receptor allosteric ligands

Abstract

Allosteric modulation of the type 1 cannabinoid receptor (CB1R) holds great therapeutic potential. This is because allosteric modulators do not possess intrinsic efficacy, but instead augment (positive allosteric modulation) or diminish (negative allosteric modulation) the receptor’s response to endogenous ligand. Consequently, CB1R allosteric modulators have an effect ceiling which allows for the tempering of CB1R signaling without the desensitization, tolerance, dependence, and psychoactivity associated with orthosteric compounds. Pain, movement disorders, epilepsy, obesity are all potential therapeutic targets for CB1R allosteric modulation. Several challenges exist for the development of CB1R allosteric modulators, such as receptor subtype specificity, translation to in vivo systems, and mixed allosteric/agonist/inverse agonist activity. Despite these challenges, elucidation of crystal structures of CB1R and compound design based on structure–activity relationships will advance the field. In this review, we will cover recent progress for CB1R allosteric modulators and discuss the future promise of this research.     

Milk intake and survival in newborn cannabinoid CB1 receptor knockout mice: evidence for a "CB3" receptor

Cannabinoids, whether plant-derived, synthetic or endogenous, have been shown to stimulate appetite in the adult organism. We have reported previously that cannabinoid receptors play a critical role during the early suckling period: The selective cannabinoid CB(1) receptor antagonist N-(piperidiny-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141617A) permanently prevented milk ingestion in a dose-dependent manner, when administered to (Sabra, albino) mouse pups, within 1 day of birth. As a consequence, these pups died within the first week of life. We now generalize this finding to a different strain of mice (C57BL/6). Further, we show that cannabinoid CB(1) receptor blockade (20 mg/kg SR141716A) must occur within 24 h after birth as injection of SR141716A into 2- or 5-day-old pups had a much smaller effect or no effect at all, respectively. Cannabinoid CB(1) receptor knockout mice did not ingest milk on the first day of life, similarly to SR141716A-treated normal pups, as measured by the appearance of "milkbands". However, the knockout pups started to display milkbands from day 2 of life. Survival rates of cannabinoid CB(1) receptor knockout mice were affected significantly, but to a lesser extent than normal pups, by the administration of SR141716A. Daily administration of the endocannabinoid 2-arachidonoyl glycerol, or the synthetic agonists (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN55,212-2, 5 mg/kg) or (-)-cis-3-[2-Hydroxy4-(1,1-dimethylheptyl) phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP55,940, 5 or 20 mg/kg) did not promote survival or weight gain in CB(1)(-/-) pups. Our data support previous evidence for a critical role of cannabinoid CB(1) receptors for the initiation of suckling. Further, the present observations support the existence of an unknown cannabinoid receptor, with partial control over milk ingestion in newborns. Our data also suggest that the CB(1)(-/-) neonates possess a compensatory mechanism which helps them overcome the lack of cannabinoid CB(1) receptors.     

Modulation of morphine-induced Fos-immunoreactivity by the cannabinoid receptor antagonist SR 141716

A growing body of evidence suggests the existence of a functional interaction between opioid and cannabinoid systems. The present study further investigated this functional interaction by examining the combined effects of morphine and the cannabinoid receptor antagonist SR 141716 on Fos-immunoreactivity (Fos-IR), a marker for neural activation. Male albino Wistar rats were treated with SR 141716 (3 mg/kg, intraperitoneally), morphine HCl (10 mg/kg, subcutaneously), vehicle, or SR 141716 and morphine combined (n=6 per group). Rats were injected with morphine or its vehicle 30-min after administration of SR 141716 or its vehicle and perfused 3 h later. Locomotor activity and body temperature were both increased in the morphine-treated group and SR 141716 significantly inhibited these effects. Morphine increased Fos-IR in several brain regions including the caudate-putamen (CPu), cortex (cingulate, insular and piriform), nucleus accumbens (NAS) shell, lateral septum (LS), bed nucleus of the stria terminalis (BNST), median preoptic nucleus (MnPO), medial preoptic nucleus (MPO), hypothalamus (paraventricular, dorsomedial and ventromedial), paraventricular thalamic nucleus (PV), amygdala (central and basolateral nuclei), dorsolateral periaqueductal gray, ventral tegmental area (VTA), and Edinger–Westphal nucleus. SR 141716 alone increased Fos-IR in the cortex (cingulate, insular and piriform), NAS (shell), LS, BNST, hypothalamus (paraventricular, dorsomedial and ventromedial), PV, amygdala (central, basolateral and medial nuclei), VTA, and Edinger–Westphal nucleus. SR 141716 attenuated morphine-induced Fos-IR in several regions including the CPu, cortex, NAS (shell), LS, MnPO, MPO, paraventricular and dorsomedial hypothalamus, PV, basolateral amygdala, VTA, and Edinger–Westphal nucleus (EW). These results provide further support for functional interplay between the cannabinoid and opioid systems. Possible behavioural and physiological implications of the interactive effects of SR 141716 on morphine-induced Fos-IR are discussed.     

CB<Subscript>1</Subscript> receptor mediation of cannabinoid behavioral effects in male and female rats

Rationale Cannabinoids have been shown to produce greater behavioral effects in female than male rats. Although central nervous system CB₁ receptors are known to mediate cannabinoid-induced behavioral effects in male rats, it is not known whether the same is true for females.Objective To determine if cannabinoid-induced antinociception and catalepsy are similarly mediated by central CB₁ receptors in male and female rats.Methods The ability of SR141716A, a CB₁ receptor selective antagonist, administered ICV (1–1000 g) or IT (1–600 g) to block 10 mg/kg IP ⁹-THC-induced antinociception (paw pressure) and catalepsy (bar test), was compared in male and female rats.Results ⁹-THC alone produced slightly greater antinociception, and significantly greater catalepsy in females than males. When administered ICV, SR141716A partially antagonized ⁹-THC-induced antinociception in both females and males. IT SR141716A also antagonized ⁹-THC-induced antinociception in both sexes; it was slightly more potent in males but equally effective in males and females. SR141716A antagonized ⁹-THC-induced catalepsy in a similar manner in males and females when given ICV or IT.Conclusions These results confirm that ⁹-THC-induced behavioral effects are mediated by central CB₁ receptors in male and female rats.     

Antiobesity effects of the novel in vivo neutral cannabinoid receptor antagonist 5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-3-hexyl-1H-1,2,4-triazole--LH 21

The present study evaluates the pharmacological profile of the new neutral cannabinoid CB1 receptor antagonist 5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-3-hexyl-1H-1,2,4-triazole -LH-21- on feeding behavior and alcohol self-administration in rats, two behaviors inhibited by cannabinoid CB1 receptor antagonists. Administration of LH-21 (0.03, 0.3 and 3 mg/kg) to food-deprived rats resulted in a dose-dependent inhibition of feeding. Subchronic administration of LH-21 reduced food intake and body weight gain in obese Zucker rats. Acute effects on feeding were not associated with anxiety-like behaviors, or induction of complex motor behaviors such as grooming or scratching sequences, usually observed after central administration of cannabinoid receptor blockers with inverse agonist properties. LH-21 did not markedly reduce alcohol self-administration (30% reduction observed only at a high dose of 10 mg/kg). This pharmacological pattern partially overlaps that of the reference cannabinoid CB1 receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide, SR141716A, (0.3, 1 and 3 mg/kg) that reduced feeding and alcohol self-administration with similar efficacy. In vitro analysis of blood-brain barrier permeability using a parallel artificial membrane permeation assay demonstrated that LH-21 has lower permeation through membranes than SR141716A. That was confirmed in vivo by studies showing lower potency of peripherally injected LH-21 when compared to SR141716A to antagonize motor depression induced by intracerebroventricular administration of the CB1 agonist CP55,940. The neutral antagonist profile and the lower penetration into the brain of LH-21 favour this class of antagonists with respect to reference inverse agonists for the treatment of obesity because they potentially will display reduced side effects.     

Conformational characteristics of the interaction of SR141716A with the CB1 cannabinoid receptor as determined through the use of conformationally constrained analogs

Interest in cannabinoid pharmacology increased dramatically upon the identification of the first cannabinoid receptor (CB1) in 1998 and continues to expand as additional endocannabinoids and cannabinoid receptors are discovered. Using CB1 receptor (CB1R) systems, medicinal chemistry programs began screening libraries searching for cannabinoid ligands, ultimately leading to the discovery of the first potent cannabinoid receptor antagonist, SR141716A (Rimonabant). Its demonstrated efficacy in treating obesity and facilitating smoking cessation, among other impressive pharmacological activities, has furthered the interest in cannabinoid receptor antagonists as therapeutics, such that the number of patents and publications covering this class of compounds continues to grow at an impressive rate. At this time, medicinal chemistry approaches including combinatorial chemistry, conformational constraint, and scaffold hopping are continuing to generate a large number of cannabinoid antagonists. These molecules provide an opportunity to gain insight into the 3-dimensional structure-activity relationships that appear crucial for CB1R-ligand interaction. In particular, studies in which conformational constraints have been imposed on the various pyrazole ring substituents of SR141716A provide a direct opportunity to characterize changes in conformation/conformational freedom within a single class of compounds. While relatively few conformationally constrained molecules have been synthesized to date, the structure-activity information is often more readily interpreted than in studies where entire substituents are replaced. Thus, it is the focus of this mini-review to examine the structural properties of SR141716A, and to use conformationally constrained molecules to illustrate the importance of conformation and conformational freedom to CB1R affinity, selectivity, and efficacy.     

Detection of cannabinoid agonist evoked increase in BOLD contrast in rats using functional magnetic resonance imaging

BOLD-contrast functional magnetic resonance imaging (fMRI) was used to investigate the effects of the synthetic cannabinoid agonist HU210 on the rat brain in order to determine potential CNS sites of action for the functional effects of cannabinoids. After obtaining basal data, rats (n=8) were given the cannabinoid agonist HU210 (10 microg/kg i.v.) and volume data sets collected for 85 mins. Significant increases in functional BOLD activity were observed in specific brain regions including those important in pain (PAG), reward (VTA and accumbens) and motor function (striatum). In order to confirm cannabinoid receptor involvement in the HU210 evoked functional BOLD activity, rats (n=8) were pre-treated with the CB1 cannabinoid receptor antagonist SR141716A (100 microg/kg i.v.) prior to HU210. Pretreatment with SR141716A abolished all significant evoked HU210 functional BOLD activity. To exclude the involvement of potential systemic effects induced by the cannabinoid agonist administration on the observed evoked functional BOLD activity a separate experiment investigated the effect of HU210 (10 microg/kg i.v.) on mean arterial pressure and showed that HU210 had no significant effect on pressure under chloral hydrate anaesthesia. In summary, this study demonstrates that the cannabinoid agonist HU210 evokes a significant increase in BOLD functional activity in specific regions and that this was cannabinoid receptor mediated. Furthermore the study indicates the potential value of fMRI in rodents to delineate pharmacologically induced changes in regional brain function.     

SR141716, a CB1 receptor antagonist, decreases the sensitivity to the reinforcing effects of electrical brain stimulation in rats

RATIONALE: The endogenous cannabinoid system is thought to play a role in reinforcement processes. OBJECTIVES: We tested the effects of five doses of the cannabinoid receptor 1 (CB1) antagonist SR141716 [0, 0.3, 1, 3 and 10 mg/kg intraperitoneal (IP)] on intracranial self-stimulation at the level of the median forebrain bundle (MFB). Self-stimulation was assessed 30 min and 210 min after SR141716 administration. We compared the effect of SR141716 with the effect of a decrease in the magnitude of stimulation (-100 microA) and the effects of a cocaine injection (1, 5 and 10 mg/kg IP). METHODS: a protocol of rate-frequency curve for self-stimulation was applied. Two rate-frequency curves were established daily, 3 h apart. The frequency required to produce half-maximal performance (M50) and the maximal performance (RMax) were used as the parameters to characterize the rate-frequency functions. RESULTS: SR141716 decreased the sensitivity to the electrical brain stimulation. SR141716 induced a shift to the right of the rate-frequency curve. This effect depended on the dose administered and the time after injection. Thirty minutes after the injection, 1, 3 and 10 mg/kg SR141716 induced a significant decrease in sensitivity to electrical stimulation, as shown by an elevation in the M50 value. RMax showed a tendency to decrease with increasing doses. At 210 min after administration, 3 and 10 mg/kg SR141716 maintained their decreasing effect on the sensitivity to the stimulation as shown by the significant increase of the M50, however, the maximal response was restored to the basal value. A decrease in self-stimulation intensity produced an effect comparable to the one observed 30 min after either 3 or 10 mg/kg SR141716, while cocaine (5 and 10 mg/kg) produced the opposite effect. Neither condition affected the rate-frequency curve measured 3 h later. CONCLUSIONS: In accordance with recent observations, these experiments suggest that the endogenous cannabinoid system facilitates the perception or the effects of positive reinforcers. They also suggest that this neurochemical system could be a target of interest for treating psychopathologies implicating the reinforcing system.     

Cannabinoid receptors and reward in the rat: a conditioned place preference study

RATIONALE: We wished to investigate further the hypothesis of an endogenous cannabinoid 'aversive counter-rewarding system, as the rewarding properties of cannabinoids using standard procedures remain ambiguous. OBJECTIVES: The purpose of this study was to confirm the behavioural effects of a highly potent synthetic cannabinoid agonist (HU210) and the selective cannabinoid antagonist SR 141716A using conditioned place preference (CPP). METHODS: HU210 (20, 60 and 100 microg kg(-1), SR141716A (0.25, 0.5, 2 and 3 mg kg(-1)), cocaine (15 mg kg(-1) and delta9-THC (1.5 mg kg(-1)) were given to male Lister hooded rats using an unbiased CPP design. RESULTS: SR141716A and cocaine produced place preference at all doses tested, whereas HU210 and delta9-THC produced aversion as expressed by time spent in the drug-paired compartment of the CPP apparatus. CONCLUSIONS: The aversive effects of cannabinoid agonists and the rewarding effect of the cannabinoid antagonist are suggestive of a cannabinergic tone in the rat brain. Further research is needed to determine the precise relationship of that tone with the reward pathways of the brain.     

The cannabinoid receptor antagonist SR 141716 prevents acquisition of drinking behavior in alcohol-preferring rats

The cannabinoid CB₁ receptor antagonist, N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide) (SR 141716); 0.3–3 mg/kg, i.p., twice daily for 10 days), prevented the acquisition of alcohol drinking behavior in rats genetically selected for alcohol preference (Sardinian alcohol-preferring (sP) rats), having the free choice between alcohol (10%, v/v) and water. The results suggest that activation of cannabinoid CB₁ receptors is essential for the acquisition of alcohol drinking behavior in animals with a genetically determined alcohol preference.     

Stimulation of voluntary ethanol intake by cannabinoid receptor agonists in ethanol-preferring sP rats

RATIONALE: Recent studies have shown that the cannabinoid CB1 receptor antagonist, SR 141716, is capable of reducing voluntary ethanol intake in rodents, suggesting the involvement of the CB1 receptor in the neural circuitry mediating the positive reinforcing properties of ethanol. OBJECTIVES: The present study extended to the agonists the investigation on the pharmacological manipulation of ethanol intake by cannabinoid agents. METHODS: Selectively bred, Sardinian alcohol-preferring (sP) rats were offered ethanol and water under the two-bottle free choice procedure with unlimited access for 24 h/day. RESULTS: The acute administration of WIN 55,212-2 (0.5-2 mg/kg; IP) and CP 55,940 (3-30 microg/kg; IP) induced a significant, dose-dependent increase in ethanol intake. Conversely, water consumption and intake of regular food and a highly palatable sucrose solution were not affected by treatment with WIN 55,212-2 and CP 55,940. The stimulatory effect of WIN 55,212-2 and CP 55,940 on ethanol intake was completely prevented by administration of SR 141716 (0.3 mg/kg; IP) and the opioid receptor antagonist, naloxone (0.1 mg/kg; IP). CONCLUSIONS: Administration of WIN 55,212-2 and CP 55,940 promoted voluntary ethanol intake in sP rats. This effect was mediated by stimulation of the cannabinoid CB1 receptor and required the activation of the endogenous opioid system. The results of the present study add further support to the hypothesis that the cannabinoid CB1 receptor is part of the neural substrate regulating ethanol intake. These results are also discussed in terms of WIN 55,212-2 and CP 55,940 administration possibly fixing to a higher level the hedonic set-point mechanism regulating ethanol drinking behavior in sP rats.     

CB<Subscript>1</Subscript> cannabinoid receptor agonists increase intracranial self-stimulation thresholds in the rat

Rationale Addictive drugs have a number of commonalities in animal behavioral models. They lower intracranial self-stimulation (ICSS) thresholds, support self-administration, and produce conditioned place preference (CPP). However, cannabinoids appear atypical as drugs of abuse, since there are controversial data in the literature concerning their reinforcing properties.Objectives The aim of the present study was to examine the effects of cannabinoids on brain reward using the rate–frequency curve shift paradigm of ICSS.Methods Male Sprague–Dawley rats were implanted with electrodes into the medial forebrain bundle (MFB). Rate–frequency functions were determined by logarithmically decreasing the number of cathodal pulses in a stimulation train from a value that sustained maximal responding to one that did not sustain responding. After brain stimulation reward thresholds stabilized rats received intraperitoneal (IP) injections of the potent CB₁ receptor agonists WIN 55,212-2 (graded doses 0.1, 0.3, 1 and 3 mg/kg), CP 55,940 (graded doses 10, 30, 56 and 100 g/kg), or HU-210 (graded doses 10, 30, 100 g/kg).Results With the exception of the highest dose of all cannabinoid agonists tested, which significantly increased the threshold frequency required for MFB ICSS, all other doses of the tested drugs did not affect ICSS thresholds. The CB₁ receptor antagonist SR141716A reversed the actions of WIN 55,212-2 and CP 55,940, but not HU-210. However, the selective CB₁ cannabinoid receptor antagonist AM 251 counteracted the effect of HU-210. Both CB₁ receptor antagonists, at the doses used in the present study, did not affect reward thresholds by themselves.Conclusions The present results indicate that cannabinoid agonists do not exhibit reinforcing properties in the ICSS paradigm, but rather have an inhibitory influence on reward mechanisms. The results suggest that the anhedonic effects of cannabinoids are probably mediated by cannabinoid CB₁ receptors.     

Improvement of memory in rodents by the selective CB1 cannabinoid receptor antagonist,SR 141716

Social short-term memory in rodents is based on the recognition of a juvenile by an adult conspecific when the juvenile is presented on two successive occasions. Cannabimimetics are claimed to induce memory deficits in both humans and animals. In the brain, they mainly bind to CB1 receptors for which anandamide is a purported endogenous ligand. SR 141716, a specific antagonist of CB1 receptors, dose-dependently reverses biochemical and pharmacological effects of cannabimimetics. More particularly, it antagonizes the inhibition of hippocampal long-term potentiation induced by WIN 55,212-2 and anandamide, and it increases arousal when given alone. The present experiments study the ability of SR 141716 (from 0.03 to 3 mg/kg SC) to facilitate short-term olfactory memory in the social recognition test in rodents. SR 141716 improved social recognition in a long intertrial paradigm with a threshold dose of 0.1 mg/kg SC. At 1 mg/kg, it antagonized the memory disturbance elicited by retroactive inhibition. Scopolamine (0.06 mg/kg IP) partially reversed its memory-enhancing effect. Moreover, SR 141716 reduced memory deficit in aged rats (0.03–0.1 mg/kg) and mice (0.3–1 mg/kg). As SR 141716 is not known to exhibit any pharmacological activity which is not mediated by CB1 receptors, the results strongly support the concept that blockade of CB1 receptors plays an important role in consolidation of short-term memory in rodents and suggest there may be a role for an endogenous cannabinoid agonist tone (anandaminergic) in forgetting.     

Inhibition of long-term potentiation in rat hippocampal slices by anandamide and WIN55212-2: reversal by SR141716 A,a selective antagonist of CB1 cannabinoid receptors

It has been reported previously that ⁹-tetrahydrocannabinol and the synthetic cannabinoid agonist HU-210 [(–)-11-OH-⁸-dimethylheptyl tetrahydrocannabinol] prevent long-term potentiation (LTP) induction in rat hippocampal slices. In this study we confirm that both WIN55212-2 {R-(+)-(2,3-dihydro-5-methyl-3-[{4-morpholinyl} methyl] pyrol [1,2,3-de]-1,4-benzoxazin-6-yl) (1-naphtalenyl) methanone monomethanesulphonate} (3 and 10 M), another synthetic cannabinoid agonist, and anandamide (10 M), considered to be the endogenous ligand of cannabinoid receptors, inhibit LTP formation in the Schaffer collateral-CA1 field complex. In addition, we show that SRt417l6A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-lH-pyrazole-3-carboxamide hydrochloride] at 0.1–10 M, a potent and selective antagonist of CB1 cannabinoid receptors, concentration-dependently reversed the inhibition of LTP induced by both WIN55212-2 and anandamide. These data indicate that cannabinoid receptor agonists inhibit hippocampal LTP formation through CB1 receptor activation and that anandamide could be a candidate for an endogenous neuromessenger involved in memory processes.     

The actions of some cannabinoid receptor ligands in the rat isolated mesenteric artery

1. The actions of a number of cannabinoid receptor ligands were investigated using the myograph-mounted rat isolated mesenteric artery. Anandamide, CP 55,940, HU-210, palmitoylethanolamide and WIN 55,212-2 all caused concentration-dependent relaxations of methoxamine-precontracted vessels which were not affected by removal of the endothelium.
2. Precontracting vessels with 60 mM KCl instead of methoxamine greatly reduced the vasorelaxant effects of anandamide and palmitoylethanolamide. High K⁺ solution caused a modest decrease in the relaxant potency of CP 55,940 and HU-210, and had no effect on relaxations induced by WIN 55,212-2.
3. Relaxations of methoxamine-induced tone by anandamide, CP 55,940 and HU-210, but not palmitoylethanolamide and WIN 55,212-2, were attenuated by the cannabinoid receptor antagonist, SR 141716A. Relaxation of vessels contracted with 60 mM KCl by CP 55,940 was also sensitive to SR 141716A.
4. Anandamide and CP 55,940 caused small but concentration-dependent contractions in resting vessels in the absence of extracellular calcium. These were not sensitive to SR 141716A. Palmitoylethanolamide and WIN 55,212-2 produced smaller contractions only at higher concentrations.
5. Anandamide and CP 55,940, but not palmitoylethanolamide and WIN 55,212-2, caused concentration-dependent inhibition of the phasic contractions induced by methoxamine in calcium-free conditions, but only anandamide caused inhibition of contractions to caffeine under such conditions. These inhibitory effects were not antagonised by SR 141716A.
6. The present study provides the first detailed investigation of the actions of cannabinoid agonists on vascular smooth muscle. Our results show that these compounds exert both receptor-dependent and -independent effects on agonist-induced calcium mobilization in the rat isolated mesenteric artery.