Interactions among the cannabinoids in the antagonism of the abdominal constriction response in the mouse

The ability of ⁹-tetrahydrocannabinol (THC), cannabinol (CBN), cannabidiol (CBD), 11-OH THC and 8,11-diOH THC to antagonise the abdominal constriction response in the mouse induced by formic acid, phenylquinone, 5-hydroxytryptamine, prostaglandin E₁ (PGE₁) and bradykinin was tested. THC was an effective antagonist against all nociceptive agents with an ED₅₀ in all cases between 1.0 and 2.6 mg/kg. CBN, while also effective against all nociceptive agents, was less potent than THC, with an ED₅₀ range between 46.2 and 112.5 mg/kg. CBD in doses as high as 200 mg/kg was without effect. Using PGE₁ as the nociceptive agent, 11-OH THC was equipotent to THC while 8,11-diOH THC was inactive. Naloxone, while able to antagonise the antinociceptive effect of morphine against formic acid-induced writhing, did not reverse the antinociceptive effects of THC. There were no pharmacological interactions between THC, CBD and CBN.     

Cannabidiol potentiates ��9-tetrahydrocannabinol (THC) behavioural effects and alters THC pharmacokinetics during acute and chronic treatment in adolescent rats

Rationale

The interactions between ??⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD) during chronic treatment, and at equivalent doses, are not well characterised in animal models.

Objectives

The aim of this study is to examine whether the behavioural effects of THC, and blood and brain THC levels are affected by pre-treatment with equivalent CBD doses.

Methods

Adolescent rats were treated with ascending daily THC doses over 21?days (1 then 3 then 10?mg/kg). Some rats were given equivalent CBD doses 20?min prior to each THC injection to allow examination of possible antagonistic effects of CBD. During dosing, rats were assessed for THC and CBD/THC effects on anxiety-like behaviour, social interaction and place conditioning. At the end of dosing, blood and brain levels of THC, and CB₁ and 5-HT_1A receptor binding were assessed.

Results

CBD potentiated an inhibition of body weight gain caused by chronic THC, and mildly augmented the anxiogenic effects, locomotor suppressant effects and decreased social interaction seen with THC. A trend towards place preference was observed in adolescent rats given CBD/THC but not those given THC alone. With both acute and chronic administration, CBD pre-treatment potentiated blood and brain THC levels, and lowered levels of THC metabolites (THC-COOH and 11-OH-THC). CBD co-administration did not alter the THC-induced decreases in CB₁ receptor binding and no drug effects on 5-HT_1A receptor binding were observed.

Conclusions

CBD can potentiate the psychoactive and physiological effects of THC in rats, most likely by delaying the metabolism and elimination of THC through an action on the CYP450 enzymes that metabolise both drugs.     

Cannabidiol reverses the reduction in social interaction produced by low dose Δ-tetrahydrocannabinol in rats

While Δ⁹-tetrahydrocannabinol (THC) is the main psychoactive constituent of the cannabis plant, a non-psychoactive constituent is cannabidiol (CBD). CBD has been implicated as a potential treatment of a number of disorders including schizophrenia and epilepsy and has been included with THC in a 1:1 combination for the treatment of conditions such as neuropathic pain. This study investigated the effect of THC and CBD, alone or in combination, on some objective behaviours of rats in the open field. Pairs of rats were injected with CBD or vehicle followed by THC or vehicle and behaviour in the open field was assessed for 10 min. In vehicle pretreated rats THC (1 mg/kg) significantly reduced social interaction between rat pairs. Treatment with CBD had no significant effect alone, but pretreatment with CBD (20 mg/kg) reversed the THC-induced decreases in social interaction. A higher dose of THC (10 mg/kg) produced no significant effect on social interaction. However, the combination of high dose CBD and high dose THC significantly reduced social interaction between rat pairs, as well as producing a significant decrease in locomotor activity. This data suggests that CBD can reverse social withdrawal induced by low dose THC, but the combination of high dose THC and CBD impairs social interaction, possibly by decreasing locomotor activity.     

Self-administration of Δ<Superscript>9</Superscript>-tetrahydrocannabinol (THC) by drug naive squirrel monkeys

Rationale Interest in therapeutic activities of cannabinoids has been restrained by the fact that they are most often mediated through activation of cannabinoid CB₁ receptors, the same receptors that mediate the effects of ⁹-tetrahydrocannabinol (THC) and are responsible for the abuse liability of marijuana. Persistent intravenous self-administration of THC by animals was first demonstrated in squirrel monkeys and shown to be mediated by CB₁ receptors, but monkeys in the study had a history of cocaine self-administration, raising the possibility that persistent neurobiological adaptations might subsequently predispose animals to self-administer THC.Objectives To demonstrate persistent intravenous self-administration of THC in drug-naive squirrel monkeys.Methods Monkeys with no history of exposure to other drugs learned to press a lever for intravenous injections (0.2 ml in 0.2 s) of THC under a 10-response, fixed-ratio schedule with a 60-s time-out after each injection. Acquisition of THC self-administration was rapid and the final schedule was reached in 11–34 sessions. Dose of THC was then varied from 1 to 16 µg/kg per injection with vehicle extinction following each dose of THC.Results THC maintained significantly higher numbers of self-administered injections per session and higher rates of responding than vehicle at doses of 2, 4 and 8 µg/kg per injection, with maximal rates of responding at 4 µg/kg per injection. Response rates, injections per session and total THC intake per session were two- to three-fold greater in monkeys with no history of exposure to other drugs compared to previous findings in monkeys with a history of cocaine self-administration.Conclusions THC can act as an effective reinforcer of drug-taking behavior in monkeys with no history of exposure to other drugs, suggesting that self-administration of THC by monkeys provides a reliable animal model of human marijuana abuse.     

Effect of cannabinoids on lithium-induced vomiting in the<Emphasis Type="Italic"> Suncus murinus</Emphasis> (house musk shrew)

Rationale Marijuana has been reported to interfere with nausea and vomiting in chemotherapy patients. The principal cannabinoids found in marijuana include the psychoactive compound -9-tetrahydrocannabinol (THC) and the non-psychoactive compound cannabidiol (CBD). The experiments reported here evaluated the potential of THC and CBD to interfere with vomiting in the Suncus murinus (house musk shrew) produced by lithium chloride (LiCl), which is the most commonly employed unconditioned stimulus for taste avoidance.Objectives To evaluate the potential of the principal components of marijuana, THC and CBD, to suppress Li-induced vomiting in the house musk shrew.Methods Shrews were injected with vehicle or one of two cannabinoids [-9-THC (1–20 mg/kg), or CBD (2.5–40 mg/kg)] 10 min prior to an injection of LiCl (390 mg/kg of 0.15 M) and were then observed for 45 min. The frequency of vomiting episodes and the latency to the first episode were measured. The role of the CB1 receptor in these effects was also evaluated by pretreatment with SR-141716.Results -9-THC produced a dose-dependent suppression of Li-induced vomiting, with higher doses producing greater suppression than lower doses. CBD produced a biphasic effect with lower doses producing suppression and higher doses producing enhancement of Li-induced vomiting. The suppression of Li-induced vomiting by THC, but not by CBD, was reversed by SR-141716.Conclusions These results indicate that two major cannabinoid compounds found in marijuana, THC and CBD, are effective treatments for Li-induced vomiting; however, only THC acts by the CB1 receptor. The effects of THC and CBD on vomiting were dose dependent; with THC the effect was linear, but with CBD the effect was biphasic.     

Divergent effects of cannabidiol on the discriminative stimulus and place conditioning effects of Delta(9)-tetrahydrocannabinol

Cannabis sativa (marijuana plant) contains myriad cannabinoid compounds; yet, investigative attention has focused almost exclusively on Delta(9)-tetrahydrocannabinol (THC), its primary psychoactive substituent. Interest in modulation of THC's effects by these other cannabinoids (e.g., cannabidiol (CBD)) has been stimulated anew by recent approval by Canada of Sativex (a 1:1 dose ratio combination of CBD:THC) for the treatment of multiple sclerosis. The goal of this study was to determine the degree to which THC's abuse-related effects were altered by co-administration of CBD. To this end, CBD and THC were assessed alone and in combination in a two-lever THC discrimination procedure in Long-Evans rats and in a conditioned place preference/aversion (CPP/A) model in ICR mice. CBD did not alter the discriminative stimulus effects of THC at any CBD:THC dose ratio tested. In contrast, CBD, at CBD:THC dose ratios of 1:1 and 1:10, reversed CPA produced by acute injection with 10mg/kg THC. When administered alone, CBD did not produce effects in either procedure. These results suggest that CBD, when administered with THC at therapeutically relevant ratios, may ameliorate aversive effects (e.g., dysphoria) often associated with initial use of THC alone. While this effect may be beneficial for therapeutic usage of a CBD:THC combination medication, our discrimination results showing that CBD did not alter THC's discriminative stimulus effects suggest that CBD:THC combination medications may also produce THC-like subjective effects at these dose ratios.     

Differentiation between low- and high-efficacy CB1 receptor agonists using a drug discrimination protocol for rats

Rationale

The “subjective high” from marijuana ingestion is likely due to Δ⁹-tetrahydrocannabinol (THC) activating the central cannabinoid receptor type 1 (CB₁R) of the endocannabinoid signaling system. THC is a weak partial agonist according to in vitro assays, yet THC mimics the behavioral effects induced by more efficacious cannabinergics. This distinction may be important for understanding similarities and differences in the dose–effect spectra produced by marijuana/THC and designer cannabimimetics (“synthetic marijuana”).

Objective

We evaluated if drug discrimination is able to functionally detect/differentiate between a full, high-efficacy CB₁R agonist [(±)AM5983] and the low-efficacy agonist THC in vivo.

Materials and methods

Rats were trained to discriminate between four different doses of AM5983 (0.10 to 0.56 mg/kg), and vehicle and dose generalization curves were determined for both ligands at all four training doses of AM5983. The high-efficacy WIN55,212-2 and the lower-efficacy (R)-(+)-methanandamide were examined at some AM5983 training conditions. Antagonism tests involved rimonabant and WIN55,212-2 and AM5983. The separate (S)- and (R)-isomers of (±)AM5983 were tested at one AM5983 training dose (0.30 mg/kg). The in vitro cyclic adenosine monophosphate (cAMP) assay examined AM5983 and the known CB₁R agonist CP55,940.

Results

Dose generalization ed₅₀ values increased as a function of the training dose of AM5983, but more so for the partial agonists. The order of potency was (R)-isomer > (±)AM5983 > (S)-isomer and AM5983 > WIN55,212-2 ≥ THC > (R)-(+)-methanandamide. Surmountable antagonism of AM5983 and WIN55,212-2 occurred with rimonabant. The cAMP assay confirmed the cannabinergic nature of AM5983 and CP55,940.

Conclusions

Drug discrimination using different training doses of a high-efficacy, full CB₁R agonist differentiated between low- and high-efficacy CB₁R agonists.     

The cannabinoid CB1 receptor antagonist SR141716A attenuates the memory impairment produced by Δ9-tetrahydrocannabinol or anandamide

 The administration of Δ⁹-tetrahydrocannabinol (THC), the principle psychoactive ingredient in marijuana, or the endogenous cannabinoid anandamide, has been shown to impair recent memory. The purpose of the present investigation was to determine if the cannabinoid CB₁ receptor antagonist SR141716A could attenuate THC- or anandamide-induced memory impairment, and to assess the effects on memory of SR141716A alone. Memory was assessed in rats well-trained in a two-component instrumental discrimination task, consisting of a conditional discrimination, and a non-match-to-position to assess recent or working memory. SR141716A (0.0–2.0 mg/kg) had no effect on either the conditional discrimination or the non-match-to-position. However, SR141716A (0.0–2.0 mg/kg) attenuated the memory impairment produced by THC (2.0 or 4.0 mg/kg) as indexed by an enhancement of performance in the non-match-to-position. When administered to rats pretreated with anandamide (2.0 mg/kg), SR141716A (0.0–2.5 mg/kg) impaired performance in the conditional discrimination at the highest dose. This was interpreted as a deficit in some capacity unrelated to memory (e.g., motor impairment). However, lower doses of SR141716A (0.1 and 0.5 mg/kg) attenuated the anandamide-induced impairment of performance in the non-match-to-position without affecting the conditional discrimination. This is the first report that the memory impairment produced by anandamide can be attenuated by a cannabinoid antagonist; results suggest that anandamide-induced memory disruption is mediated by CB₁ receptors. Received: 25 June 1997 / Final version: 7 February 1998     

Are cannabidiol and Δ9-tetrahydrocannabivarin negative modulators of the endocannabinoid system? A systematic review

Based upon evidence that the therapeutic properties of Cannabis preparations are not solely dependent upon the presence of Δ⁹-tetrahydrocannabinol (THC), pharmacological studies have been recently carried out with other plant cannabinoids (phytocannabinoids), particularly cannabidiol (CBD) and Δ⁹-tetrahydrocannabivarin (THCV). Results from some of these studies have fostered the view that CBD and THCV modulate the effects of THC via direct blockade of cannabinoid CB₁ receptors, thus behaving like first-generation CB₁ receptor inverse agonists, such as rimonabant. Here, we review in vitro and ex vivo mechanistic studies of CBD and THCV, and synthesize data from these studies in a meta-analysis. Synthesized data regarding mechanisms are then used to interpret results from recent pre-clinical animal studies and clinical trials. The evidence indicates that CBD and THCV are not rimonabant-like in their action and thus appear very unlikely to produce unwanted CNS effects. They exhibit markedly disparate pharmacological profiles particularly at CB₁ receptors: CBD is a very low-affinity CB₁ ligand that can nevertheless affect CB₁ receptor activity in vivo in an indirect manner, while THCV is a high-affinity CB₁ receptor ligand and potent antagonist in vitro and yet only occasionally produces effects in vivo resulting from CB₁ receptor antagonism. THCV has also high affinity for CB₂ receptors and signals as a partial agonist, differing from both CBD and rimonabant. These cannabinoids illustrate how in vitro mechanistic studies do not always predict in vivo pharmacology and underlie the necessity of testing compounds in vivo before drawing any conclusion on their functional activity at a given target.     

Effect of low doses of Δ<Superscript>9</Superscript>-tetrahydrocannabinol and cannabidiol on the extinction of cocaine-induced and amphetamine-induced conditioned place preference learning in rats

Rationale Using the place-preference conditioning paradigm, we evaluated the potential of the two most prominent cannabinoids found in marijuana, the psychoactive component ⁹-tetrahydrocannabinol (⁹-THC) and the nonpsychoactive component cannabidiol (CBD), to potentiate extinction of a cocaine-induced and an amphetamine-induced conditioned place preference in rats.Methods To determine the effects of pretreatment with ⁹-THC or CBD on extinction, a cocaine-induced and amphetamine-induced place preference was first established. Rats were then given an extinction trial, during which they were confined to the treatment-paired floor for 15 min. Thirty minutes prior to the extinction trial, they were injected with a low dose of ⁹-THC (0.5 mg/kg), CBD (5 mg/kg) or vehicle. The potential of the CB₁ receptor antagonist, SR141716, to reverse the effects of ⁹-THC or CBD was also evaluated. To determine the hedonic effects of CBD, one distinctive floor was paired with CBD (5 mg/kg) and another with saline. Finally, to determine the effect of ⁹-THC or CBD on the establishment and/or the expression of a place preference during four cycles of conditioning trials, rats were injected with ⁹-THC (0.25–1 mg/kg), CBD (5 mg/kg) or vehicle 25 min prior to receiving an injection of amphetamine followed by placement on the treatment floor; on alternate days, they received injections of vehicle followed by saline and placement on the nontreatment floor. The rats then received two test trials; on one trial they were pretreated with the cannabinoid and on the other trial with vehicle.Results ⁹-THC and CBD potentiated the extinction of both cocaine-induced and amphetamine-induced conditioned place preference learning, and this effect was not reversed by SR141716. The cannabinoids did not affect learning or retrieval, and CBD was not hedonic on its own.Conclusions These results are the first to show that both ⁹-THC, which acts on both CB1 and CB2 receptors, and CBD, which does not bind to CB₁ or CB₂ receptors, potentiate the extinction of conditioned incentive learning.     

Effect of cannabis on pentobarbital-induced sleeping time and pentobarbital metabolism in the rat

The effects of two cannabis extracts with different cannabinoid compositions, as well of as pure Δ¹-tetrahydrocannabinol (THC), cannabinol (CBN) and cannabidiol (CBD), on pentobarbital metabolism were studied in the rat. Extract I, with high proportions of CBN and CBD relative to THC, when given by gavage 21·5, 40 or 63 hr before pentobarbital (30 mg/kg, i.p.), prolonged the sleeping time by 53, 42 and 21 per cent respectively. This effect was paralleled by decreases in the rate of disappearance of [¹⁴C]pentobarbital from the blood, and of pentobarbital metabolism by liver microsomal preparations in vitro. Extract II, with low relative proportions of CBN and CBD, did not have any significant effect on penobarbital metabolism or sleeping time. CBD alone, in the same dose as that given in Extract I, had very similar effects, while a dose of CBD equivalent to that given in Extract II had no effect. THC, CBN and CBD added to normal rat liver microsomes in vitro inhibited pentobarbital metabolism competitively, CBD being a much more potent inhibitor than THC and CBN. The CBD content may, therefore, be a significant factor in interactions between marijuana and other drugs.     

Regional enhancement of cannabinoid CB1 receptor desensitization in female adolescent rats following repeated ��9-tetrahydrocannabinol exposure

BACKGROUND AND PURPOSE

Disruption of the substantial re-organization of the brain during adolescence may be induced by persistent abuse of marijuana. The aim of this study was to determine whether adolescent and adult rats exhibit differential adaptation of brain cannabinoid (CB₁) receptors after repeated exposure to Δ⁹-tetrahydrocannabinol (THC).

EXPERIMENTAL APPROACH

Rats of both ages and sexes were dosed with 10 mg kg⁻¹ THC or vehicle twice daily for 9.5 days. Subsequently, CB₁ receptor function and density were assessed.

KEY RESULTS

In all brain regions, THC treatment produced desensitization and down-regulation of CB₁ receptors. While the magnitude of down-regulation did not differ across groups, greater desensitization was evident in the brains of THC-treated female adolescent rats for most regions. Adolescent females showed greater desensitization than adult females in the prefrontal cortex, hippocampus, periaqueductal gray (PAG) and ventral midbrain. In contrast, adolescent males exhibited less desensitization in the prefrontal cortex, hippocampus and PAG, an effect opposite to that seen in females. With the exception of the PAG, sex differences were seen only in adolescents, with greater desensitization in the prefrontal cortex, striatum, hippocampus, PAG, and ventral midbrain of females.

CONCLUSIONS AND IMPLICATIONS

These results suggest that the brains of adolescent females may be particularly vulnerable to disruption of CB₁ receptor signalling by marijuana abuse. Alternatively, increased desensitization may reflect protective adaptation. Given the extensive re-organization of the brain during adolescence, this disruption has potential long-term consequences for maturation of the endocannabinoid system.     

Inhalation exposure to smoke from synthetic “marijuana” produces potent cannabimimetic effects in mice

Background

Use of synthetic “marijuana” has increased in recent years, produced adverse effects and prompted the temporary DEA ban of five specific cannabinoid analogs, including JWH-018. The objectives of the current study include determining the chemical content of the herbal product, Buzz, assessing its behavioral effects upon inhalation exposure to mice, determining whether CB₁ receptors mediate its pharmacological activity, and ascertaining its biodisposition in blood and various organs.

Methods

Using a nose-only exposure system, mice were exposed to smoke produced from combustion of an herbal incense product, Buzz, which contained 5.4% JWH-018. Cannabimimetic effects following smoke exposure were evaluated using the tetrad procedure, consisting of the following indices: hypomotility, antinociception, catalepsy, and hypothermia. Additionally, blood and tissues were collected for JWH-018 quantification.

Results

Inhalation exposure to Buzz produced dose-related tetrad effects similar to marijuana as well as dose-related increased levels of JWH-018 in the blood, brain, heart, kidney, liver, lung, and spleen. The behavioral effects were blocked by rimonabant, a CB₁ receptor antagonist. Effects produced by Buzz were similar in magnitude and time-course to those produced by marijuana, though equipotent doses of Buzz and marijuana yielded considerably lower brain levels of JWH-018 than THC for the respective materials.

Conclusions

Inhalation exposure to a product containing JWH-018 penetrates into the brain and other organs and produces CB₁ receptor-mediated behavioral pharmacological effects in mice. The increased potency of JWH-018 compared to THC, the variable amount of drug added to various herbal products, and unknown toxicity, undoubtedly contribute to public health risks of synthetic cannabinoids.     

Discriminative stimulus effects in rats of SR-141716 (rimonabant), a cannabinoid CB1 receptor antagonist

Objective To examine the discriminative stimulus effects of (i) the cannabinoid CB₁ receptor antagonist SR-141716 (SR, 5.6 mg/kg) and vehicle, and (ii) the cannabinoid receptor agonist ⁹-THC (THC, 1.8 mg/kg) and vehicle using a discriminated taste aversion (DTA) procedure.Methods Two groups of rats (n=6) were trained to discriminate between these drugs and vehicle in DTA (t=20 min). The 30-min drinking bout of tap water following drug (SR or THC) treatment was followed by an injection of lithium chloride (LiCl, 120 mg/kg) in the experimental animals. When offered water after vehicle pretreatment, experimental animals subsequently were given IP saline (NaCl, 10 ml/kg). Post-drinking treatment for controls (n=6) was NaCl, irrespective of the pretreatment condition (SR, THC or vehicle). Additional water was provided during the afternoon (30 min) with no other manipulations. Food was available ad lib at all times. When the discriminations were established other doses and drugs were examined (t=20 min). In testing there were no post-drinking treatments.Results The SR-related analog AM-251 (dose range: 1–5.6 mg/kg) substituted for SR, whereas other drugs such as the cannabinoid CB₂ receptor antagonist SR-144528 (3 and 10 mg/kg), THC (1–10 mg/kg), flumazenil (1–10 mg/kg), naloxone (1–10 mg/kg), morphine (10 and 18 mg/kg) and d-amphetamine (1 and 3 mg/kg) did not. There was a dose-related attenuation of SR-induced suppression of drinking when THC (1.8–10 mg/kg) was given together with SR (5.6 mg/kg). In the THC trained rats, SR (1–10 mg/kg), morphine (10 and 18 mg/kg) and d-amphetamine (1 and 3 mg/kg) did not substitute for THC. SR (1 mg/kg) attenuated the THC (1.8 mg/kg) induced suppression of drinking. Together with 3 mg/kg SR and 1.8 mg/kg THC, drinking was roughly equally suppressed in both the experimental group and the controls.Conclusion SR-141716 induces a discriminative stimulus complex in DTA that shows potential for further examination of cannabinoid receptor antagonism.     

Cannabidiol fails to reverse hypothermia or locomotor suppression induced by ù9-tetrahydrocannabinol in Sprague-Dawley rats

Background and Purpose

Growing evidence shows cannabidiol (CBD) modulates some of the effects of Δ⁹-tetrahydrocannabinol (THC). CBD is a constituent of some strains of recreational cannabis but its content is highly variable. High CBD strains may have less memory-impairing effects than low-CBD strains and CBD can reverse behavioural effects of THC in monkeys. CBD/THC interactions in rodents are more complicated as CBD can attenuate or exacerbate the effects of THC. This study was undertaken to determine if CBD could reverse hypothermia or hypolocomotor effects caused by THC in rats.

Experimental Approaches

Male Sprague-Dawley rats were prepared with radiotelemetry devices and then given doses of THC (10–30 mg·kg⁻¹, i.p.) with or without CBD. Experiments determined the effect of simultaneous or 30 min pretreatment with CBD in a 1:1 ratio with THC, as well as the effect of CBD in a 3:1 ratio. Additional experiments determined the effects of pretreatment with the cannabinoid CB₁ receptor antagonist SR141716 (rimonabant).

Key Results

CBD did not attentuate THC-induced hypothermia or hypolocomotion but instead exaggerated these effects in some conditions. The antagonist SR141716 blocked hypolocomotor effects of THC for the first hour after injection and the hypothermia for 6 h; thus validating the pharmacological model.

Conclusions and Implications

There is no evidence from this study that elevated CBD content in cannabis could provide protection from the physiological effects of THC, in rats.     

Effect of cannabinoids on the turnover rate of acetylcholine in rat hippocampus,striatum and cortex

Summary The effects of ⁹-tetrahydrocannabinol, (⁹THC) the major psychoactive compound of marijuana, and cannabidiol (CBD), a non-psychoactive component, on the acetylcholine (ACh) concentration and the turnover rate of ACh (TR_ACh) have been studied in various regions of the rat brain. Neither ⁹THC doses from 0.2 to 10 mg/kg nor CBD (10 or 20 mg/kg) alter the ACh concentration in the brain areas examined 30 min, after the intravenous injection. However, ⁹-THC (doses from 0.2 to 10 mg/kg) causes a marked dose-related decrease in the TR_ACh in hippocampus whereas CBD is without effect in this brain region even when 20 mg/kg is given. Furthermore, high doses of ⁹-THC (5 mg/kg) and CBD (20 mg/kg) that produce a significant decrease in the TR_ACh of striatum fail to change the TR_ACh in parietal cortex. The low doses of ⁹-THC required to reduce hippocampal TR_ACh suggest that an action on these cholinergic mechanisms may play a role in the psychotomimetic activity of ⁹-THC.     

Chronic Δ9-Tetrahydrocannabinol Exposure Induces a Sensitization of Dopamine D2/3 Receptors in the Mesoaccumbens and Nigrostriatal Systems

Δ⁹-Tetrahydrocannabinol (THC), through its action on cannabinoid type-1 receptor (CB₁R), is known to activate dopamine (DA) neurotransmission. Functional evidence of a direct antagonistic interaction between CB₁R and DA D₂-receptors (D₂R) suggests that D₂R may be an important target for the modulation of DA neurotransmission by THC. The current study evaluated, in rodents, the effects of chronic exposure to THC (1 mg/kg/day; 21 days) on D₂R and D₃R availabilities using the D₂R-prefering antagonist and the D₃R-preferring agonist radiotracers [¹⁸F]fallypride and [³H]-(+)-PHNO, respectively. At 24 h after the last THC dose, D₂R and D₃R densities were significantly increased in midbrain. In caudate/putamen (CPu), THC exposure was associated with increased densities of D₂R with no change in D₂R mRNA expression, whereas in nucleus accumbens (NAcc) both D₃R binding and mRNA levels were upregulated. These receptor changes, which were completely reversed in CPu but only partially reversed in NAcc and midbrain at 1 week after THC cessation, correlated with an increased functionality of D_2/3R in vivo, based on findings of increased locomotor suppressive effect of a presynaptic dose and enhanced locomotor activation produced by a postsynaptic dose of quinpirole. Concomitantly, the observations of a decreased gene expression of tyrosine hydroxylase in midbrain together with a blunted psychomotor response to amphetamine concurred to indicate a diminished presynaptic DA function following THC. These findings indicate that the early period following THC treatment cessation is associated with altered presynaptic D_2/3R controlling DA synthesis and release in midbrain, with the concurrent development of postsynaptic D_2/3R supersensitivity in NAcc and CPu. Such D_2/3R neuroadaptations may contribute to the reinforcing and habit-forming properties of THC.     

Antidepressant-like effects of Δ⁹-tetrahydrocannabinol and rimonabant in the olfactory bulbectomised rat model of depression

The endocannabinoid signalling system is widely accepted to play a role in controlling the affective state. Plant cannabinoids are well known to have behavioural effects in animals and humans and the cannabinoid CB₁ receptor antagonist rimonabant has recently been shown to precipitate depression-like symptoms in clinical trial subjects. The aim of the present study was to investigate the behavioural and neurochemical effects of chronic administration of Δ⁹-tetrahydrocannabinol (THC) and rimonabant on intact and olfactory bulbectomised (OB) rats used as a model of depression.As expected, OB rats were hyperactive in the open field. Repeated THC (2 mg/kg, i.p. once every 48 h for 21 days) and rimonabant (5 mg/kg, i.p. once every 48 h for 21 days) reduced this hyperactivity, which is typical of clinically effective antidepressant drugs. In intact animals, chronic THC increased brain derived neurotrophic factor (BDNF) expression levels in the hippocampus and frontal cortex but rimonabant had no effect. Rimonabant increased the levels of phosphorylated extracellular signal regulated kinases (p-ERKs_1/2) in the hippocampus and prefrontal cortex and THC also increased expression in frontal cortex. OB did not affect BDNF or p-ERK_1/2 expression in the hippocampus or frontal cortex and in, contrast to the intact animals, neither THC nor rimonabant altered expression in the OB rats.These findings indicate antidepressant-like behavioural properties of both THC and rimonabant in OB rats although additional studies are required to clarify the relationship between the chronic effects of cannabinoids in other pre-clinical models and in human depression.     

The interaction between prostaglandin E1 andΔ 9-Tetrahydrocannabinol on intestinal motility and on the abdominal constriction response in the mouse

The interaction between ⁹-tetrahydrocannabinol (THC) and PGE₁ was studied using two pharmacological parameters-the rate of passage of a charcoal meal through mouse small intestine and the abdominal constriction response in the mouse. PGE₁ administered intraperitoneally produced a dose-dependent decrease in intestinal motility, and this effect was antagonized by low (0.25 mg/kg) doses of THC and potentiated by higher doses of THC (1 mg/kg). Kinetic analysis suggested that the interaction was of a mixed but predominantly competitive type. PGF₂ produced an increase in intestinal motility but this was not dose-dependent. THC antagonized the effect of PGF₂ in a dose-dependent manner suggestive of a physiological antagonism.THC (0.25–2 mg/kg) antagonized the dosedependent PGE₁ abdominal constriction response in a fashion which suggested a mixed (though mainly competitive) antagonism.It would seem, therefore, that on the two pharmacological parameters studied THC appears to be interacting with PGE₁ at the same receptor site.Although the doses of THC used are within the range of those used in man, it is not implied that these results are necessarily implicated in the psychoactivity of the drug.     

The relationship of in vivo central CB1 receptor occupancy to changes in cortical monoamine release and feeding elicited by CB1 receptor antagonists in rats

Rationale Cannabinoid type 1 (CB₁) receptor antagonists are reportedly effective in reducing food intake both preclinically and clinically. This may be due in part to their effects on monoamine release in the brain. The level of central CB₁ receptor occupancy underlying these neurobiological effects is unclear. Objectives We explored the relationship between in vivo CB₁ receptor occupancy in the frontal cortex and changes in both monoamine release in the medial prefrontal cortex (mPFC) and feeding behavior in rats in response to two orally administered CB₁ receptor antagonists presently in clinical trials, SR141716A (rimonabant) and SLV319. Methods CB₁ receptor occupancy was measured using [³H] SR141716A, and these occupancies were related to potencies to mediate increases in dopamine (DA) and norepinephrine (NE) release measured with microdialysis and decreases in consumption of a highly palatable diet (HP). Results High receptor occupancy levels (>65%) were required to detect increases in monoamine release that were achieved with 3 and 10 mg/kg of SR141716A and 10 mg/kg of SLV319 for DA and 10 mg/kg of SR141716A for NE. Decreases in HP consumption were seen at occupancies higher than 65% for SR141716A that were achieved with 3 and 10 mg/kg. In contrast, decreases in HP consumption were seen at relatively low CB₁ receptor occupancies (11%) for SLV319. Conclusions The occupancy method described here is an effective tool for interrelating central CB₁ receptor occupancy with neurobiological actions of CB₁ receptor antagonists and for furthering our understanding of the role of CB₁ receptors in central nervous system physiology and pathology.