Intra-accumbal NMDA but not AMPA/kainate receptor antagonist attenuates WIN55,212-2 cannabinoid receptor agonist-induced antinociception in the basolateral amygdala in a rat model of acute pain

Previous studies showed the role of basolateral amygdala (BLA) in cannabinoid-induced antinociception. Several lines of evidence indicated that the nucleus accumbens (NAc) receives excitatory glutamatergic inputs primarily from limbic-related structures, including the hippocampus, BLA, and various thalamic nuclei. Additionally, it has been shown that the NAc plays an important role in mediating the suppression of animal models of pain. In the present study, we examined the role of NMDA and AMPA/kainate receptors within the NAc in antinociception induced by intra-BLA cannabinoid receptor agonist WIN55,212-2 in rats. 126 adult male albino Wistar rats weighing 230-280 g were unilaterally implanted by two separate cannulae into the BLA and NAc. Dose-response antinociceptive effects of different doses of intra-BLA WIN55,212-2 (5, 10 and 15 μg/0.3 μl/rat) were evaluated in this study. Moreover, animals received intra-accumbal infusions of either NMDA receptor antagonist, AP5 (0.5, 2.5 and 5 μg/0.5 μl saline) or AMPA/kainate receptor antagonist, CNQX (0.1, 0.5 and 2.5 μg/0.5 μl DMSO), just 2 min before microinjection of WIN55,212-2 into the BLA. Antinociceptive responses of drugs were obtained by tail-flick analgesiometer and represented as maximal possible effect (%MPE) at 5, 15, 30, 45 and 60 min after their administrations. Results showed that intra-accumbal AP5 dose-dependently prevented antinociception induced by intra-BLA administration of WIN55,212-2 (15 μg/rat) in time set intervals. Nonetheless, administration of AMPA/kainate receptor antagonist, CNQX, could not affect WIN-induced analgesia. Additionally, solely administration of intra-accumbal injection of CNQX (2.5 μg/0.5 μl DMSO), but not AP5 (5 μg/0.5 μl saline), could significantly change the baseline tail-flick latencies in the rats. It seems that NMDA receptors located in the NAc, in part, mediate the antinociceptive responses of cannabinoid within the BLA in acute model of pain.     

Recent advances in cannabinoid receptor agonists and antagonists

This review is an overview of the recent advances in cannabinoid chemistry with a special emphasis on the patent literature. The term cannabinoid includes analogues of the natural components of cannabis, endocannabinoids and a wide array of chemical structures such as 1,5-diarylpyrazoles, indoles, quinolines and arylsulphonamide derivatives capable of acting as cannabinoid receptor agonists and antagonists. These receptors, discovered in the early nineties, seem to be involved in different biochemical processes and thus represent interesting therapeutic targets for drug research.     

Will the new CB1 cannabinoid receptor antagonist SR-147778 have advantages over rimonabant?

Obesity and alcoholism are two common modern-day diseases. The cannabinoid CB1 receptor antagonist rimonabant is in Phase III clinical trial for the treatment of obesity with preliminary results showing that it decreases appetite and body weight. Animal studies have shown that rimonabant is effective in the treatment of alcoholism. SR-147778 is a new potent and selective CB1 receptor antagonist. In animals, SR-147778 has been shown to inhibit CB1 receptor-mediated hypothermia, analgesia and slowing of gastro-intestinal transit. In rats trained to drink sucrose, the oral administration of SR-147778 3 mg/kg, before the presentation of sucrose, decreased the consumption of sucrose. SR-147778 3mg/kg also reduced spontaneous feeding in rats deprived of food and also in non-deprived rats. In Sardinian alcohol--preferring (sP) rats, in the alcohol-naive state, SR-147778 slowed the development of a preference for alcohol. In alcohol-experienced sP rats SR-147778 (2.5, 5 and 10 mg/kg p.o.) reduced the alcohol intake. When alcohol-experienced sP rats are deprived of alcohol for 15 days, there is a large intake of alcohol on reintroduction of alcohol, and this response was almost abolished by treatment with SR-147778. From the preclinical studies published to date, there is no obvious major point of difference between rimonabant and SR-147778, and both are promising agents for the treatmentof obesity and alcoholism.     

Characterization of the endocannabinoid system, CB(1) receptor signalling and desensitization in human myometrium

BACKGROUND AND PURPOSE

The endocannabinoid plays vital roles in several aspects of reproduction, including gametogenesis, fertilization and parturition. However, little is known regarding the presence or role of the endocannabinoid system in myometrial function. Here the presence of the endocannabinoid system and signalling properties of cannabinoid receptors were characterized.

EXPERIMENTAL APPROACH

Components of the endocannabinoid system were identified using qRT-PCR, immunohistochemical, immunoblotting and radioligand binding experiments. Cannabinoid receptor signalling pathways were characterized using standard MAPK and second messenger assays.

KEY RESULTS

Primary myometrium expresses the endocannabinoid synthesizing enzyme N-acyl-phosphatidyl ethanolamine-specific phospholipase D, endocannabinoid degrading enzyme fatty acid amide hydrolase and cannabinoid CB₁, but not CB₂ receptors or transient receptor potential vanilloid-type-1 channels. The CB₁ receptor ligand anandamide caused a Gα_i/o-dependent inhibition of adenylate cyclase reducing intracellular cAMP levels, and Gα_i/o, phosphoinositide-3-kinase, Src-kinase-dependent ERK activation. CB₁ receptor-generated signals declined following continual anandamide stimulation, possibly due to ligand metabolism since free anandamide concentrations declined during the experiment from 2.5 µM initially, to 500 nM after >30 min. However, identical loss of CB₁ receptor responsiveness occurred in the presence of the metabolically stable derivative methanandamide. Moreover, RNAi-mediated depletion of arrestin3 (a negative regulator of receptor signalling) prevented loss of CB₁ receptor activity, enhancing and prolonging ERK signals.

CONCLUSIONS AND IMPLICATIONS

The myometrium has the capacity to synthesize, respond to and degrade endocannabinoids. Furthermore, reduced CB₁ receptor responsiveness occurs as a consequence of receptor desensitization, not agonist depletion and we identify a key role for arrestin3 in this process.     

Cannabinoids modulate neuronal firing in the rat basolateral amygdala: evidence for CB1- and non-CB1-mediated actions

Recent evidence indicates that the basolateral amygdala (BLA) may be involved in behavioural effects induced by cannabinoids. High levels of CB1 cannabinoid receptors have been shown in this region, where they modulate excitatory and inhibitory synaptic transmission. However, the neurophysiological effects of these opposing synaptic actions have not been investigated in vivo. To this purpose, single-unit extracellular recordings were performed in urethane anaesthetized rats in order to determine whether exogenously applied cannabinoids influenced the spontaneous or evoked electrical activity of neurons in the BLA. The effects of cannabinoids were found to be dependent on the characteristics of the neurons examined and on the properties of the agents used. We tested and compared two structurally different synthetic cannabinoid receptor agonists, the highly potent HU-210 (0.125-1.0 mg/kg, i.v.) and WIN55212-2 (WIN, 0.125-1.0 mg/kg, i.v.). With a CB1 cannabinoid receptor-dependent mechanism, HU-210 potently inhibited the firing rate of BLA interneurons whereas WIN modulated the discharge rate in a biphasic manner. By contrast, BLA projection neurons, antidromically identified from the shell of the nucleus accumbens, were significantly inhibited by WIN at all doses tested, while HU-210 administration led to less consistent effects, since only 1.0 mg/kg inhibited firing rate in the majority of recorded neurons. Additionally, WIN, but not HU-210, significantly attenuated short-latency spiking activity in BLA projection neurons evoked by electrical stimulation of the medial prefrontal cortex. In these neurons, WIN-induced effects were antagonised by the non-selective cannabinoid receptor antagonist SR141716A and by the vanilloid receptor antagonist capsazepine, but not by the selective CB1 antagonist AM-251. Taken together, our findings indicate that the overall excitability of efferent neurons in the BLA is strongly reduced by WIN in a non-CB1-dependent manner. In this effect, the contribution of a novel cannabinoid-vanilloid-sensitive putative non-CB1 receptors, the existence of which was postulated in recent reports, might play a role.     

Involvement of CB1 cannabinoid receptors in emotional behaviour

RATIONALE: Endogenous and exogenous cannabinoids acting through the CB1 cannabinoid receptors are implicated in the control of a variety of behavioural and neuroendocrine functions, including emotional responses, and learning and memory processes. Recently, knockout mice deficient in the CB1 cannabinoid receptor have been generated, and these animals result in an excellent tool to evaluate the neurophysiology of the endogenous cannabinoid system. OBJECTIVES: To establish the role of the CB1 cannabinoid receptor in several emotional-related behavioural responses, including aggressiveness, anxiety, depression and learning models, using CB1 knockout mice. METHODS: We evaluated the spontaneous responses of CB1 knockout mice and wild-type controls under different behavioural paradigms, including the light/dark box, the chronic unpredictable mild stress, the resident-intruder test and the active avoidance paradigm. RESULTS: Our findings showed that CB1 knockout mice presented an increase in the aggressive response measured in the resident-intruder test and an anxiogenic-like response in the light/dark box. Furthermore, a higher sensitivity to exhibit depressive-like responses in the chronic unpredictable mild stress procedure was observed in CB1 knockout mice, suggesting an increased susceptibility to develop an anhedonic state in these animals. Finally, CB1 knockout mice showed a significant increase in the conditioned responses produced in the active avoidance model, suggesting an improvement of learning and memory processes. CONCLUSIONS: Taken together these findings demonstrate that endogenous cannabinoids through the activation of CB1 receptors are implicated in the control of emotional behaviour and participate in the physiological processes of learning and memory.     

Syntheses of two isotopically labeled CB1 receptor antagonists

Synthesis of deuterium‐labeled CB₁ receptor antagonist 2‐d₉ was accomplished in three steps by alkylation of 2‐nitrophenylacetonitrile with cyclopentyl‐d₉ bromide, reductive cyclization of the resulting secondary nitrile into the 3‐cyclopentyl indole‐d₉ and its N‐sulfonylation with corresponding p‐amidosulfonyl chloride. Another, structurally related, CB₁ receptor antagonist 1 was radiolabeled with carbon‐14 by oxidative cleavage of 3‐cyclopentyl indole followed by the ring closure of o‐acyl substituted N‐formylaniline with potassium cyanide‐[¹⁴C], in situ reduction‐elimination of the intermediate amino alcohol, and N‐sulfonylation of the resulting 3‐cyclopentyl indole‐2‐[¹⁴C].     

Effects of ketamine on acute somatic nociception in wild-type and N-methyl-D-aspartate (NMDA) receptor epsilon1 subunit knockout mice

Although the properties of ketamine appear to be well characterized, there is a lot of ambiguity in the literature regarding its analgesic effects. After careful selection of proper experimental conditions and drug doses, we systematically characterized the effects of systemic ketamine on acute somatic nociception in mice and examined the role of the NMDA receptor epsilon1 subunit in mediating its analgesia. Intraperitoneal administration of ketamine was not analgesic in any of the phasic pain assays (thermal, mechanical, electrical) applied to C57BL/6 (wild-type) and NMDA receptor epsilon1 subunit knockout (mutant) mice. Surprisingly, rather than being analgesic for thermal nociception, ketamine showed pronociceptive properties in case of low-intensity heat stimulation in wild-type mice. In the formalin test (tonic pain), ketamine significantly reduced phase 2 nociceptive behavior in both wild-type and mutant mice. These data indicate that in wild-type mice ketamine has no analgesic effect on phasic pain in normal somatic tissues, but alleviates tonic pain after inflammation. Such analgesic spectrum of ketamine can be fully explained by its NMDA receptor antagonist properties. The results for the mutant mice suggest that the epsilon1 subunit of the NMDA receptor does not mediate the analgesic effects of ketamine in tonic pain.     

Decreased alcohol self-administration and increased alcohol sensitivity and withdrawal in CB1 receptor knockout mice

Recent advances in the understanding of the neurobiological basis of alcohol dependence suggest that the endocannabinoid system may play a key role in the reinforcing effects of ethanol. In the present study, disruption of CB1 receptors in mice generated on a CD1 background decreased both ethanol consumption and preference. This decreased ethanol self-administration was associated with increased sensitivity to the acute intoxicating effects of ethanol. Mutant mice were more sensitive to the hypothermic and sedative/hypnotic effects of acute ethanol administration (1.5-4.0 g/kg), although plasma ethanol concentrations did not differ from those of controls. Moreover, wild-type mice exhibited normal locomotor activation caused by 1.0-2.5 g/kg injection of ethanol, whereas mutant mice displayed sedation in response to the injection of the same ethanol doses. The severity of alcohol withdrawal-induced convulsions was also increased in CB1(-/-) mice. Our results suggest that CB1 receptors participate in the regulation of ethanol drinking and demonstrate that their disruption lead to increased ethanol sensitivity and withdrawal severity.     

Targeting the cannabinoid CB2 receptor: modelling and structural determinants of CB2 selective ligands

Recent developments indicate that CB2 receptor ligands have the potential to become therapeutically important. To explore this potential, it is necessary to develop compounds with high affinity for the CB2 receptor and little affinity for the CB1 receptor. This review will discuss structure-activity relations at both receptors for classical cannabinoids and cannabimimetic indoles. Examples of CB2 selective ligands from both classes of compounds are presented and the structural features leading to selectivity are described. Two approaches, receptor mutations and molecular modelling, have been employed to investigate the interaction of ligands with both cannabinoid receptors. These results obtained from these techniques are discussed.     

Effects of electrical lesion of basolateral amygdala nucleus on rat anxiety‐like behaviour under acute,sub‐chronic,and chronic stresses

Stress contributes, as a risk factor, to such psychological disorders as anxiety. The effects of electrical lesions in the basolateral amygdala nucleus (BLA) were investigated on the locomotor activity and anxiety‐like behaviour in different stress durations. For this purpose, rats were randomly allocated to control, sham, and experimental groups, the latter including groups with and without BLA nucleus subjected to acute, sub‐chronic, and chronic stress conditions for 1, 7, and 21 days, respectively, applied 6 h/d. The induced anxiety behaviour was evaluated using the open field test (OFT) while other variables were measured. Findings revealed that sub‐chronic stress led to significantly reduced (P<.05) anxiety behaviours as measured by entries into and the time spent in the centre area while it also led to significant impairments in exploratory and locomotor activities, indicating intensified anxiety‐like behaviour. BLA lesion affected rat behaviour differently such that it significantly (P<.05) decreased fear under sub‐chronic and chronic stress conditions as evidenced by the subjects’ greater tendency to enter the centre area in the open field test and their increased number of rearing events (P<.01). However, BLA lesion led to no significant decrease in the locomotor activity of subjects exposed to sub‐chronic or chronic stress conditions as compared with those in similar groups but without BLA lesion. Finally, BLA lesion was found not only to decrease significantly (P<.01) adrenal gland and body weights, particularly under sub‐chronic stress, but also to play a critical role in modulating adrenal functions by decreasing adrenal gland weight, and thereby reducing depression‐like symptoms.     

Comparison of acute and repeated treatment with the 5-HT1A receptor ligands 8-OH-DPAT and ipsapirone in animal models of anxiety and depression

The present study compared the 5-HT_1A receptor ligands 8-OH-DPAT and ipsapirone with diazpepam and imipramine in the shock induced ultrasonic vocalization anxiety test and the forced swimming depression test in the rat. Acutely, 8-OH-DPAT induced anxiolytic and antidepressive effects (ED₅₀: 0.12 and 1.4 mg/kg, i.p., respectively), whereas ipsapirone induced anxiolytic (ED₅₀: 0.6 mg/kg) and moderate antidepressive effects (33% at 3-10 mg/kg). Virtually no tolerance developed for the anxiolytic effects after 2 weeks of treatment with 0.03-1 mg/kg 8-OH-DPAT or 0.1-10 mg/kg ipsapirone (i.p., b.i.d.), with 10 mg/kg/day ipsapirone (s.c., mini-pumps), or with 1.5 μg/rat/hr 8-OH-DPAT (local infusion in the dorsal raphe nucleus, mini-pumps). However, some tolerance developed for the antidepressive effects of 8-OH-DPAT (ED₅₀: 0.6, 1.4, 2.5 and >3 mg/kg, after 2 weeks of pretreatment with vehicle, 0.3, 1, and 3 mg/kg 8-OH-DPAT, respectively, i.p., b.i.d.). In the case of ipsapirone, the dose-effect curve in the forced swimming test was shifted to the left after 2 weeks of pretreatment with ipsapirone (0.3-10 mg/kg, i.p., b.i.d.). Acutely, diazepam induced an anxiolytic effect (ED₅₀: 3.6 mg/kg, i.p.), but failed to induce an antidepressive effect; whereas imipramine induced an antidepressive effect (ED₅₀: 20.5 mg/kg) and a moderate anxiolytic effect (max. efficacy: 47% at 30 mg/kg). Upon repeated administration (2 weeks), diazepam (5 mg/kg) showed t0olerance for its anxiolytic effects and weak antidepressive effects emerged, whereas imipramine (20 mg/kg) showed weak sensitization for both effects. It is concluded that (a) with all compounds, tolerance, as well as sensitization can be observed, depending on the behavioral test, the dose and the type of compound; and (b) compared with the other compounds tested, relatively low doses of 5-HT_1A drugs offer the most attractive profile of mixed anxiolytic/antidepressive activity. © 1993 wiley-Liss, Inc.     

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.     

Adolescent peer-rejection persistently alters pain perception and CB1 receptor expression in female rats

Peer-interactions are particularly important during adolescence and teenagers display enhanced sensitivity toward rejection by peers. Social rejection has been shown to induce alterations in pain perception in humans. However, the neurobiological consequences of adolescent social rejection have yet to be extensively characterized, and no appropriate animal model is available. Here, we propose inadequate playful interactions in adolescent rats as a novel animal model for social peer-rejection and examine potential long-term consequences into adulthood. Acute social pairing of female adolescent Wistar rats with an age-matched rat from the less playful Fischer344 strain was found to alter social play and decrease pain reactivity, indicating Fischer rats as inadequate social partners for Wistar animals. Therefore, in a second experiment, adolescent female Wistar rats were either reared with another Wistar rat (adequate social rearing; control) or with a Fischer rat (inadequate social rearing; play-deprived). Beginning on day 50, all Wistar rats were group housed with same-strain partners and tested for behavioral, neurobiological and endocrine differences in adulthood. Playful peer-interactions were decreased during adolescence in play-deprived animals, without affecting social contact behavior. Consequently, adult play-deprived rats showed decreased pain sensitivity and increased startle reactivity compared to controls, but did not differ in activity, anxiety-related behavior or social interaction. Both groups also differed in their endocrine stress-response, and expression levels of the cannabinoid CB1 receptor were increased in the thalamus, whereas FAAH levels were decreased in the amygdala. The present animal model therefore represents a novel approach to assess the long-term consequences of peer-rejection during adolescence.     

Yohimbine produces antinociception in the formalin test in rats: involvement of serotonin1A receptors

Rationale: Previous studies have suggested that the α₂-adrenergic receptor antagonist yohimbine produced antinociceptive effects in the formalin test in rats. However, yohimbine is also an agonist at serotonin (5-HT)_1A receptors, suggesting the possibility that the antinociceptive effects of yohimbine might be mediated via these receptors. Objective: The purpose of the present studies was to evaluate the potential role of 5-HT_1A receptors in mediating the antinociceptive effects of yohimbine. Methods: The antinociceptive effects of yohimbine were evaluated using the formalin test in rats. Results: Yohimbine (2.5–10 mg/kg s.c.) produced dose-related antinociception during both phase I and phase II of the formalin test, and was approximately equipotent and equiefficacious to morphine. The selective 5-HT_1A receptor antagonist WAY 100,635 (0.03–3.0 mg/kg s.c.) produced a partial reversal of yohimbine. In comparison, the selective 5-HT_1A receptor agonist (±)8-hydroxy- dipropylaminotetralin HBr (8OH-DPAT; 1.0 mg/kg s.c.) also produced a dose-related antinociception in the formalin test, although 8OH-DPAT was completely reversed by WAY 100,635 (3.0 mg/kg s.c.). The antinociceptive effects of yohimbine were not antagonized by the 5-HT_1B/1D antagonist GR 127935 (1.0 mg/kg and 3.0 mg/kg s.c.), the 5-HT₂ antagonist LY53857 (1.0 mg/kg s.c.), or the 5-HT₃ antagonist zatosetron (3.0 mg/kg s.c.). Conclusions: The present results demonstrate that yohimbine produces a dose-related antinociception in the formalin test in rats which is mediated in part by the agonistic actions at 5-HT_1A receptors. Received: 10 September 1999 / Final version: 5 November 1999     

Effects of cannabinoid CB1 receptor antagonist rimonabant in consolidation and reconsolidation of methamphetamine reward memory in mice

Rationale  Previous studies have shown that cannabinoid CB1 receptors play an important role in specific aspects of learning and memory, yet there has been no systematic study focusing on the involvement of cannabinoid CB1 receptors in methamphetamine-related reward memory. Objectives  The purpose of this study was to examine whether rimonabant, a cannabinoid CB1 receptor antagonist, would disrupt the consolidation and reconsolidation of methamphetamine-related reward memory, using conditioned place preference paradigm (CPP). Materials and methods  Separate groups of male Kunming mice were trained to acquire methamphetamine CPP. Vehicle or rimonabant (1 mg/kg or 3 mg/kg, i.p.) was given at different time points: immediately after each CPP training session (consolidation), 30 min before the reactivation of CPP (retrieval), or immediately after the reactivation of CPP (reconsolidation). Methamphetamine CPP was retested 24 h and 1 and 2 weeks after rimonabant administration. Results  Rimonabant at doses of 1 and 3 mg/kg significantly inhibited the consolidation of methamphetamine CPP. Only high-dose rimonabant (3 mg/kg) disrupted the retrieval and reconsolidation of methamphetamine CPP. Rimonabant had no effect on methamphetamine CPP in the absence of methamphetamine CPP reactivation. Conclusions  Our findings suggest that cannabinoid CB1 receptors play a major role in methamphetamine reward memory, and cannabinoid CB1 receptor antagonists may be a potential pharmacotherapy to manage relapse associated with drug-reward-related memory.     

Arsenic reduces the antipyretic activity of paracetamol in rats: Modulation of brain COX-2 activity and CB1 receptor expression

We examined whether subacute arsenic exposure can reduce paracetamol-mediated antipyretic activity by affecting COX pathway and cannabinoid CB₁ receptor regulation. Rats were preexposed to elemental arsenic (4 ppm) as sodium arsenite through drinking water for 28 days. Next day pyrexia was induced with lipopolysaccharide and paracetamol's (200 mg/kg, oral) antipyretic activity was assessed. The activities of COX-1 and COX-2, the levels of PGE₂, TNF-α and IL-1β and expression of CB₁ receptors were assessed in brain. Arsenic inhibited paracetamol-mediated antipyretic activity. COX-1 activity was not affected by any treatments. Paracetamol decreased COX-2 activity, levels of PGE₂, TNF-α and IL-1β and caused up-regulation of CB₁ receptors. Arsenic caused opposite effects on these parameters. In the arsenic-preexposed rats, paracetamol-mediated effects were attenuated, while CB₁ receptor up-regulation was reversed to down-regulation. Results suggest that elevated COX-2 activity and reduced CB₁ expression could be involved in the arsenic-mediated attenuation of the antipyretic activity of paracetamol.     

Effects of cannabinoid receptor activation on rabbit bisected vas deferens strips

1. In the present study, the effects of anandamide and WIN 55,212-2, cannabinoid receptor agonists, were investigated on electrical field stimulation (EFS)-induced biphasic twitch responses obtained from the epididymal and prostatic portions of rabbit vas deferens strips. 2. Anandamide and WIN 55,212-2 dose-dependently inhibited both the first and second phases of the EFS-induced twitch responses recorded from epididymal and prostatic portions of the vas deferens over the concentration range 10(-9) to 3 x 10(-6) mol/L. 3. The cannabinoid CB1 receptor antagonist AM 251 (10(-6) mol/L) and the cannabinoid CB2 receptor antagonist AM 630 (10(-6) mol/L) had no effect on the inhibitory action of anandamide on the biphasic twitch responses in the prostatic and epididymal portions of the rabbit vas deferens. 4. In both the prostatic and epididymal portions of the rabbit vas deferens, AM 251 significantly, but not completely, reversed the inhibitory effect of WIN 55,212-2 on the first phase of the twitch response. In contrast, AM 630 did not have any effect on the inhibitory action of WIN 55,212-2 in the rabbit vas deferens strips. 5. The inhibitory effects of anandamide or WIN 55,212-2 on EFS-induced twitch responses of both the prostatic and epididymal portions of the rabbit vas deferens were not altered in the presence of 10(-5) mol/L naloxone. 6. These results suggest that cannabinoid receptors may have a modulatory role in the regulation of sympathetic transmission in the rabbit vas deferens. However, further investigation is required to characterize the receptors involved.     

Involvement of opioid system in antidepressant‐like effect of the cannabinoid CB1 receptor inverse agonist AM‐251 after physical stress in mice

Cannabinoid inverse agonists possess antidepressant‐like properties, but the mechanism of this action is unknown. Numerous studies have reported the interaction between opioid and cannabinoid pathways. In this study, acute foot‐shock stress was used in mice to investigate the involvement of the opioid pathway in the antidepressant‐like effect of the cannabinoid CB₁ receptor inverse agonist AM‐251. Stress was induced by intermittent foot‐shock stimulation for 30 min. Then, using the forced swimming test (FST) and tail suspension test (TST), the immobility time was measured. Results show that the immobility time was significantly prolonged in animals subjected to foot‐shock stress, compared with non‐stressed controls (P < 0.01). Also, the serum corticosterone level was significantly increased after stress induction (P < 0.001). Administration of AM‐251 (0.5 and 0.3 mg/kg, intraperitoneally (i.p.)), significantly decreased the immobility time of stressed mice in the FST (P < 0.001 and P < 0.01, respectively) and TST (P < 0.01 and P < 0.05, respectively). The lowest dose of AM‐251 (0.1 mg/kg), naltrexone (0.3 mg/kg), and morphine (1.0 mg/kg) did not show any significant effect on stressed animals (P > 0.05). Co‐administration of AM‐251 with sub‐effective dose of naltrexone decreased the effective dose of this cannabinoid inverse agonist, to 0.1 mg/kg (P < 0.01). On the other hand, administration of the sub‐effective dose of morphine reversed the anti‐immobility effect of AM‐251 (0.5 mg/kg; P < 0.001). In conclusion, the present study for the first time reveals the possible role of opioid signalling in the antidepressant‐like properties of AM‐251 in a foot‐shock stress model.     

Stimulation of brain cannabinoid CB1 receptors can ameliorate hypertension in spontaneously hypertensive rats

Excessive activation of the sympatho-adrenomedullary system plays a pathogenic role in triggering and sustaining essential hypertension. We previously reported that, in normotensive rats, intracerebroventricularly (i.c.v.) administered neuropeptides, corticotropin-releasing factor and bombesin induced activation of the sympatho-adrenomedullary system, and that brain cannabinoid CB₁ receptors negatively regulated this activation. In this study, we investigated the effects of brain CB₁ receptor stimulation on blood pressure and the sympatho-adrenomedullary outflow in spontaneously hypertensive rats (SHRs), commonly used animal models of essential hypertension, and in Wistar-Kyoto (WKY) rats, normotensive controls of SHRs. In 18-week-old SHRs and WKY rats under urethane anaesthesia (1.0 g/kg, i.p.), SHRs exhibited significantly higher systolic, mean and diastolic blood pressures and plasma noradrenaline and adrenaline, and a lower heart rate than WKY rats. Single administration of arachidonyl 2′-chloroethylamide (ACEA, CB₁ agonist, 1.4 µmol/animal, i.c.v.) significantly but partially reduced mean and diastolic blood pressures and the plasma level of noradrenaline in SHRs compared to vehicle (N,N-dimethylformamide)-treated SHRs. These ACEA-induced reductions were abolished by central pretreatment with rimonabant (CB₁ antagonist, 300 nmol/animal, i.c.v.), which alone showed no significant effect on blood pressures or plasma noradrenaline and adrenaline levels of SHRs. On the other hand, ACEA had no significant effect on blood pressure or plasma noradrenaline and adrenaline levels in WKY rats. These results suggest that stimulation of brain CB₁ receptors can ameliorate hypertension accompanied by enhanced sympathetic outflow without affecting blood pressure under normotensive conditions.