(???)-Adamantyl-??8-tetrahydrocannabinol (AM-411), a selective cannabinoid CB1 receptor agonist: effects on open-field behaviors and antagonism by SR-141716 in rats

Effects of cannabinoid agonists and antagonists in male rats discriminating the synthetic cannabinoid AM2201

The synthetic forms of delta-9-tetrahydrocannabinol (Δ9-THC), dronabinol or nabilone, have been approved to treat several indications. However, due to safety concerns their clinical utility remains limited. Consequently, there is a need for developing cannabinoid (CB) ligands that display better behavioral pharmacological profiles than Δ9-THC. Here, we utilized drug discrimination methods to compare the interoceptive effects of CB ligands that vary in potency, efficacy, and selectivity at the CB receptors, including two ligands, AM411 and AM4089, that show CB1 partial agonist-like actions in vitro. Male rats were trained to discriminate 0.1 mg/kg AM2201 from saline under a fixed-ratio (FR) 10 response schedule of food reinforcement. After establishing AM2201's discriminative-stimulus effects, pretreatment tests with the CB1 antagonist/inverse agonist rimonabant blocked AM2201's effects, whereas the peripherally-restricted antagonist AM6545 had no effect. Next, the generalization profiles of AM411 and AM4089 with CB1 full agonists (JWH-018, CP-55,940, AM8936), partial agonist (Δ9-THC), and non-cannabinoids (fentanyl, atropine) were compared. The CBs either fully (AM2201, CP-55,940, JWH-018, AM8936, Δ9-THC) or partially (AM411, AM4089) substituted for AM2201, whereas fentanyl and atropine did not produce AM2201-like effects. All CB drugs were more potent than Δ9-THC and correlation analysis confirmed that the relative behavioral potencies of CBs corresponded strongly with their relative affinities at the CB1 but not CB2 receptors. Together, our results further demonstrate that AM411 and AM4089 exhibit better pharmacological profiles compared to Δ9-THC, in that they are more potent and display in vivo partial agonist-like actions that are centrally mediated via CB1 receptors.

Systematic Modification of the Substitution Pattern of the 7-Hydroxy-5-oxopyrazolo[4,3-b]pyridine-6-carboxamide Scaffold Enabled the Discovery of New Ligands with High Affinity and Selectivity for the Cannabinoid Type 2 Receptor

Selective ligands of the CB2 receptor are receiving considerable attention due to their potential as therapeutic agents for a variety of diseases. Recently, 7-hydroxy-5-oxopyrazolo[4,3-b]pyridine-6-carboxamide derivatives were shown to act at the CB2 receptor either as agonists or as inverse agonists/antagonists in vitro and to have anti-osteoarthritic activity in vivo. In this article, we report the synthesis, pharmacological profile, and molecular modeling of a series of twenty-three new 7-hydroxy-5-oxopyrazolo[4,3-b]pyridine-6-carboxamides with the aim of further developing this new class of selective CB2 ligands. In addition to these compounds, seven other analogs that had been previously synthesized were included in this study to better define the structure-activity relationship (SAR). Ten of the new compounds studied were found to be potent and selective ligands of the CB2 receptor, with K_i values ranging from 48.46 to 0.45 nM and CB1/CB2 selectivity indices (SI) ranging from >206 to >4739. In particular, compounds 54 and 55 were found to be high-affinity CB2 inverse agonists that were not active at all at the CB1 receptor, whereas 57 acted as an agonist. The functional activity profile of the compounds within this structural class depends mainly on the substitution pattern of the pyrazole ring.

Oleoylethanolamide: a novel potential pharmacological alternative to cannabinoid antagonists for the control of appetite

The initial pharmaceutical interest for the endocannabinoid system as a target for antiobesity therapies has been restricted by the severe adverse effects of the CB1 antagonist rimonabant. This study points at oleoylethanolamide (OEA), a monounsaturated analogue, and functional antagonist of anandamide, as a potential and safer antiobesity alternative to CB1 antagonism. Mice treated with equal doses (5 or 10 mg/kg, i.p.) of OEA or rimonabant were analyzed for the progressive expression of spontaneous behaviors (eating, grooming, rearing, locomotion, and resting) occurring during the development of satiety, according to the paradigm called behavioral satiety sequence (BSS). Both drugs reduced food (wet mash) intake to a similar extent. OEA treatment decreased eating activity within the first 30 min and caused a temporary increase of resting time that was not accompanied by any decline of horizontal, vertical and total motor activity. Besides decreasing eating activity, rimonabant caused a marked increase of the time spent grooming and decreased horizontal motor activity, alterations that might be indicative of aversive nonmotivational effects on feeding. These results support the idea that OEA suppresses appetite by stimulating satiety and that its profile of action might be predictive of safer effects in humans as a novel antiobesity treatment.

Behavioral effects of the novel potent cannabinoid CB1 agonist AM 4054

Due to the ubiquity of the CB1 cannabinoid receptor throughout the nervous system, as well as the many potential therapeutic uses of CB1 agonist-based interventions, it is desirable to synthesize novel probes of the CB1 receptor. Here, the acute behavioral effects of systemic (i.p.) administration of the putative novel CB1 full agonist AM 4054 were tested in rats. In Experiment 1, a dose range (0.15625-1.25 mg/kg) of AM 4054 produced effects consistent with CB1 agonism in the cannabinoid tetrad of tasks in rats, including induction of analgesia, catalepsy, hypothermia, and locomotor suppression. These effects were reversed with the CB1-selective inverse agonist AM 251 in Experiment 2, indicating that AM 4054 produced CB1 receptor-mediated effects. Analysis of open-field activity indicated that the reduction in locomotion is more consistent with general motor slowing than anxiogenesis. AM 4054 (0.0625-0.5 mg/kg) also dose-dependently reduced fixed-ratio 5 (FR5) operant responding for food in Experiment 3, and microanalysis of the timing and rate of lever pressing indicated a pattern of suppression similar to other CB1 agonists. Minimum doses of AM 4054 (0.125-0.3125 mg/kg) required to produce significant effects in these behavioral assays were lower than those of many CB1 agonists. It is likely that AM 4054 is a potent pharmacological tool for assessment of cannabinoid receptor function.

Low dose naloxone attenuates the pruritic but not anorectic response to rimonabant in male rats

RATIONALE

Previous research suggests that the acute anorectic effect of cannabinoid CB1 receptor antagonist/inverse agonists may be secondary to response competition from the compulsive scratching and grooming syndrome characteristic of these agents.

OBJECTIVES

As the pruritic effect of rimonabant can be attenuated by the opioid receptor antagonist naloxone, these studies test the prediction that naloxone co-treatment should prevent acute rimonabant anorexia.

METHODS

Two experiments comprehensively profiled the behavioural effects of an anorectic dose of rimonabant (1.5 mg/kg) in the absence or presence of naloxone (experiment 1: 0.01 or 0.1 mg/kg; experiment 2: 0.05 mg/kg).

RESULTS

In both experiments, rimonabant not only significantly suppressed food intake and time spent eating but also induced compulsive scratching and grooming. In experiment 1, although the lower dose of naloxone seemed to weakly attenuate the effects of rimonabant both on ingestive and compulsive behaviours, the higher dose more strongly suppressed the compulsive elements but did not significantly affect the anorectic response. The results of experiment 2 showed that naloxone at a dose which markedly attenuated rimonabant-induced grooming and scratching did not alter the effects of the compound on food intake or time spent feeding. The apparent independence of the ingestive and compulsive effects of rimonabant was confirmed by the observation that despite a 'normalising' effect of naloxone co-treatment on behavioural structure (BSS), the opioid antagonist did not impact the suppressant effect of rimonabant on peak feeding.

CONCLUSION

The acute anorectic response to rimonabant would not appear to be secondary to compulsive scratching and grooming.

Heteroadamantyl cannabinoids

The aliphatic side chain plays a pivotal role in determining the cannabinergic potency of tricyclic classical cannabinoids. We have synthesized a series of analogues in which the C3 position is substituted either directly or through a one-carbon atom linker with an adamantylamine or with an oxa- or an oxazaadamantane. The oxaadamantane pharmacophore in analogue 16 showed the best binding profile for both receptors.

Cannabinoids and novelty investigation: influence of age and duration of exposure

Administration of the synthetic cannabinoid receptor agonist WIN 55,212-2 has been shown to increase indices of noradrenergic activity. Neuroanatomical, neurochemical and behavioral studies have provided evidence supporting a marked impact of cannabinoids on the rat coeruleo-cortical pathway. As activity of this pathway is implicated in setting specific attentional modes, the present study assessed the influence of acute and repeated systemic administration of WIN 55,212-2 on novelty investigation in adolescent and adult male rats by using the hole board behavioral paradigm. Animals were individually acclimated to the hole board for 10-min sessions over 3 days, and novel objects were introduced on the fourth day. Novelty-seeking behavior was measured by comparison of the average number of return visits to a hole containing a novel object versus the average number of return visits to an empty hole. While attenuation of novelty preference was observed in adult rats acutely treated with WIN 55,212-2, both acutely treated adolescent groups retained their preference for novelty. All groups treated with repeated administration of either drug or vehicle demonstrated novelty preference, and no differences were found in the measure of novelty investigation between the groups. Furthermore, this study reproduced findings showing significant differences in locomotor activity that did not coincide with differences in novelty-seeking behavior. These data thus suggest a complex effect of CB1 receptor modulation on novelty preference in the male rat that is modulated by age and treatment.

Synthesis and characterization of 2-substituted bornane pharmacophores for novel cannabinergic ligands

Analogously to the fenchyl and adamantyl groups, the bornyl and epimeric isobornyl groups are compact lipophilic substituents that can be incorporated into drug design to improve pharmacological or physicochemical properties. Methods are reported for the synthesis and characterization of 2-substituted norbornanes and bornanes that can serve as novel cannabinergic ligand intermediates.

Intrinsic effects of AM4113, a putative neutral CB1 receptor selective antagonist, on open-field behaviors in rats

We examined open-field effects in rats of the cannabinoid 1 receptor (CB1R) agonist WIN55,212-2 (WIN; 3 mg/kg) and its interaction with the CB1R putative neutral antagonist AM4113 (0.3 to 3 mg/kg). Separate studies examined AM4113 alone (0.3 to 5.6 mg/kg). Unlike the CB1R antagonist rimonabant, in vitro (e.g., [Sink K.S., McLaughlin P.J., Wood J.A., Brown C., Fan P., Vemuri V.K., Pang Y., Olzewska T., Thakur G.A., Makriyannis A., Parker L.A., Salamone J.D. The novel cannabinoid CB(1) receptor neutral antagonist AM4113 suppresses food intake and food-reinforced behavior but does not induce signs of nausea in rats. Neuropsychopharmacology 2008a; 33: 946-955.; Sink K.S., Vemuri V.K., Olszewska T., Makriyannis A., Salamone J.D. Cannabinoid CB1 antagonists and dopamine antagonists produce different effects on a task involving response allocation and effort-related choice in food-seeking behavior. Psychopharmacology (Berl) 2008b; 196: 565-574.]) AM4113 produced no change in cAMP accumulation (neutral antagonism vis-a-vis inverse agonism). Recorded behaviors were: ambulation, rearing, circling, latency, scratching, grooming, defecation, urination and vocalization/squeaking. WIN reduced ambulation and rearing; AM4113 completely (ambulation) or partially (rearing) antagonized these behaviors. WIN alone resulted in circling and an increased latency to leave the start area; effects blocked by AM4113. AM4113 increased scratching and grooming, effects attenuated but not abolished by WIN. AM4113 alone tended to reduce ambulation and rearing and had no effect on latency or circling. AM4113 alone increased scratching and grooming. Effects on defecation, urination and vocalization were non-significant. The open-field effects of AM4113 are similar to those reported for rimonabant in rats. Yet, unlike the inverse agonists rimonabant and AM251, the putative neutral CB1R antagonist AM4113 did not produce signs of nausea in ferrets and rats ([Chambers A.P., Vemuri V.K., Peng Y., Wood J.T., Olszewska T., Pittman Q.J., Makriyannis A., Sharkey K.A. A neutral CB1 receptor antagonist reduces weight gain in rat. Am J Physiol Regul Integr Comp Physiol 2007; 293: R2185-2193.; Sink K.S., McLaughlin P.J., Wood J.A., Brown C., Fan P., Vemuri V.K., Pang Y., Olzewska T., Thakur G.A., Makriyannis A., Parker L.A., Salamone J.D. The novel cannabinoid CB(1) receptor neutral antagonist AM4113 suppresses food intake and food-reinforced behavior but does not induce signs of nausea in rats. Neuropsychopharmacology 2008a; 33: 946-955.; Sink K.S., Vemuri V.K., Olszewska T., Makriyannis A., Salamone J.D. Cannabinoid CB1 antagonists and dopamine antagonists produce different effects on a task involving response allocation and effort-related choice in food-seeking behavior. Psychopharmacology (Berl) 2008b; 196: 565-574.]).

Endogenous opioids and cannabinoids: system interactions in the regulation of appetite, grooming and scratching

Growing evidence suggests substantial crosstalk between endogenous opioid and cannabinoid systems in the regulation of appetite. Not only is cannabinoid-induced hyperphagia abolished by opioid receptor antagonists (and vice versa), but several laboratories have reported supra-additive anorectic responses following co-administration of opioid and CB1 receptor antagonists. In the present study, videoanalysis has been used to characterise the acute effects of sub-anorectic doses of rimonabant (0.25, 0.75 mg/kg) and naloxone (0.1 mg/kg), alone and in combination, on mash intake, ingestive and non-ingestive behaviour, and post-treatment weight gain in male rats. The results confirmed that, when administered alone, none of these treatments significantly altered mash consumption, various measures of feeding behaviour, or weight gain. Although most non-ingestive behaviours were also unaffected, 0.75 mg/kg rimonabant induced compulsive scratching and grooming. However, when naloxone was given in combination with either dose of rimonabant, both food intake and time spent feeding were significantly decreased while the behavioural satiety sequence (BSS) was accelerated. On further analysis, the co-treatment reductions in food intake and feeding behaviour were found to be of an additive rather than supra-additive nature. Intriguingly, the co-administration of naloxone also virtually abolished the compulsive scratching response to the higher dose of rimonabant. Findings are discussed in relation to current views on the molecular bases of opioid-cannabinoid system interactions and the unexpected 'dual' advantage (reduction in appetite plus attenuation of side-effect) of low-dose combinations of opioid and cannabinoid CB1 receptor antagonists.

Grooming, scratching and feeding: role of response competition in acute anorectic response to rimonabant in male rats

RATIONALE

Although the CB1 receptor antagonist/inverse agonist rimonabant acutely suppresses food intake in rodents, the behavioural specificity of this effect remains unclear.

OBJECTIVES

To profile the behavioural effects of rimonabant in a free-feeding context.

MATERIALS AND METHODS

Videoanalysis was employed to characterise the effects of acute rimonabant (1.5 and 3.0 mg/kg, IP) on the behaviour of non-deprived male rats exposed to palatable mash. Data were also collected on post-treatment weight gain, and, as prolonged appetite suppression has been found after single dosing with compounds of this series, rats were reassessed (drug-free) for food intake 7 days after initial testing.

RESULTS

Both doses of rimonabant not only decreased mash consumption (44-55%) but also reduced 24-h weight gain. Although videoanalysis confirmed the inhibitory effects of rimonabant on feeding behaviour, it also revealed concurrent reductions in locomotion, rearing and sniffing as well as substantial (up to tenfold) and dose-dependent increases in grooming and scratching. Timecourse analyses further revealed that rimonabant dose-dependently induced frequent episodes of atypical scratching that waned over the test but which were succeeded by prolonged and behaviourally disruptive grooming. Finally, as groups did not differ in mash consumption on retest, any prolonged anorectic effect of acute rimonabant dissipates within 7 days of treatment.

CONCLUSIONS

The anorectic response to rimonabant in male rats would appear to be due largely to response competition. This parsimonious conclusion is supported by the less profound (although still significant) increases in scratching and grooming observed in rats treated with a sub-anorectic dose (0.5 mg/kg) of the compound.

Acute anorectic response to cannabinoid CB1 receptor antagonist/inverse agonist AM 251 in rats: indirect behavioural mediation

Despite a large and consistent literature on the suppressant effects of cannabinoid CB1 receptor antagonists/inverse agonists (e.g. rimonabant, AM 251) on food intake and weight gain in rodents, surprisingly little is known about the behavioural selectivity of such effects. In this study, ethological scoring was used to characterize the acute behavioural effects of the rimonabant analogue AM 251 (1.5 and 3.0 mg/kg, intraperitoneally) in nondeprived male rats during a 1-h test with palatable mash. Data were also collected on daily weight gain and on retest food intake 7 days after dosing. Results showed that the higher dose of AM 251 significantly inhibited mash consumption (32% decrease relative to vehicle control), reduced time spent feeding during the test and suppressed body weight gain over the 48-h period that followed acute dosing. No effects on mash consumption were observed when the animals were retested drug-free 1 week after drug treatment. Detailed video analysis of the test sessions showed that, over the dose range tested, AM 251 did not significantly interfere with the vast majority of noningestive behaviours. Both doses of the compound, however, significantly increased the incidence of and the time spent on scratching, whereas the higher dose additionally increased both the number and duration of grooming episodes. The latter effect in particular disrupted the normal structure of behaviour (behavioural satiety sequence) with atypically high levels of grooming displacing feeding during the middle part of the test session. Overall, the behavioural profile of AM 251 in a free-feeding context is very similar to (but approximately two-fold less potent than) that recently reported for the parent molecule, rimonabant. Together, these data strongly suggest that the acute anorectic response to CB1 receptor antagonists/inverse agonists is indirectly mediated via major alterations to other components of the behavioural repertoire.

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

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

The effect of the palmitoylethanolamide analogue, palmitoylallylamide (L-29) on pain behaviour in rodent models of neuropathy

BACKGROUND AND PURPOSE

Cannabinoids are associated with analgesia in acute and chronic pain states. A spectrum of central cannabinoid (CB(1)) receptor-mediated motor and psychotropic side effects limit their therapeutic potential. Here, we investigate the analgesic effect of the palmitoylethanolamide (PEA) analogue, palmitoylallylamide (L-29), which via inhibition of fatty acid amide hydrolase (FAAH) may potentiate endocannabinoids thereby avoiding psychotropic side effects.

EXPERIMENTAL APPROACH

The in vivo analysis of the effect of L-29 on measures of pain behaviour in three rat models of neuropathic pain.

KEY RESULTS

Systemically administered L-29 (10 mg kg(-1)) reduced hypersensitivity to mechanical and thermal stimuli in the partial sciatic nerve injury (PSNI) model of neuropathic pain; and mechanical hypersensitivity in a model of antiretroviral (ddC)-associated hypersensitivity and a model of varicella zoster virus (VZV)-associated hypersensitivity. The effects of L-29 were comparable to those of gabapentin (50 mg kg(-1)). The CB(1) receptor antagonist SR141716a (1 mg kg(-1)) and the CB(2) receptor antagonist SR144528 (1 mg kg(-1)) reduced the effect of L-29 on hypersensitivity in the PSNI and ddC models, but not in the VZV model. The peroxisome proliferator-activated receptor-alpha antagonist, MK-886 (1 mg kg(-1)), partially attenuated the effect of L-29 on hypersensitivity in the PSNI model. L-29 (10 mg kg(-1)) significantly attenuated thigmotactic behaviour in the open field arena without effect on locomotor activity.

CONCLUSIONS AND IMPLICATIONS

L-29 produces analgesia in a range of neuropathic pain models. This presents L-29 as a novel analgesic compound that may target the endogenous cannabinoid system while avoiding undesirable side effects associated with direct cannabinoid receptor activation.

Irt>t schedule controlled behavior in ‘learned-helpless’ rats: Effects from a cannabinoid agonist

Human depression is partly a congenital disorder. Aspects of the behavior accompanying depression can be magnified by genetic manipulation of bred animal species. Learned Helplessness (LH) is a trait-mark behavior that successfully breeds in rodents. Here, 'congenital' LH (cLH) rats were trained to recognize and respond to 12s long interval cues (irt>12s schedule). Rats compliant to an irt>t schedule will space responses evenly and respond rhythmically. Irt>t schedule derived data are plotted in histograms showing irt (interresponse time) frequencies. A pause response peak emerges, for outbred rats, at irt values approximating the minimum interval for reinforcement. cLH rats [n=9] complied poorly to schedule contingencies when diluent (vehicle) was injected before testing. Moderate and high dose injections of a CB 1 receptor selective agonist drug (AM 411), however, increased operant schedule compliance and normalized the cLH rats' irt>t histogram distributions. Performance indicators for cLH rats are presented alongside coordinate measures from a comparison group [n=5] of normally bred Sprague-Dawley (SD) rats. In both cLH and SD rats, treatment session histograms revealed shifts of the pause response peak not accompanied by a change in motor responsiveness. The irt>12s histogram shifts were absent when AM 411 dosages were arranged to follow pre-medication injections of a CB 1 receptor selective antagonist drug (AM 251). In short, AM 411 increased timing acuity in rats prone to behavioral despair but had opposite timing effects in normally bred SD rats.

Structural modifications of the cannabinoid side chain towards C3-aryl and 1',1'-cycloalkyl-1'-cyano cannabinoids

The compounds reported in this study are Delta(8)-THC analogues in which the C3 five-carbon linear side chain of Delta(8)-THC was replaced with aryl and 1',1'-cycloalkyl substituents. Of the compounds described here analogues 2d (CB(1), K(i)=11.7 nM. CB(2), K(i)=9.39 nM) and 2f (CB(1), K(i)=8.26 nM. CB(2), K(i)=3.86 nM) exhibited enhanced binding affinities for CB(1) and CB(2), exceeding that of Delta(8)-THC. Efficient procedures for the synthesis of these novel cannabinoid analogues are described.

Antagonism of discriminative stimulus effects of delta(9)-THC and (R)-methanandamide in rats

RATIONALE

In previous drug discrimination studies we observed surmountable antagonism by Delta(9)-tetrahydrocannabinol (THC) in the presence of constant doses of SR-141716 [N-(piperidin-1-yl)-5-(4-chloro-phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] (0.3 and 1 mg/kg), but there was only marginal evidence for surmountable antagonism with combinations of SR-141716 and (R)-methanandamide, a chiral analog of the endocannabioid anandamide.

OBJECTIVE

Here we examine antagonism where the cannabinoid CB1 receptor agonist [Delta(9)-THC and (R)-methanandamide] dose is held constant (i.e., the training dose) and the antagonist {i.e., SR-141716 and AM-251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; 2 ml/kg]} dose varied. We also tested the cannabinoid CB2 receptor antagonist SR-144528 {N-[(1S)-endo-1,3,3-trimethylbicyclo(2.2.1)heptan-2-yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide}.

METHODS

Different groups of rats were trained to discriminate between vehicle and three different doses of Delta(9)-THC (1.8, 3, and 5.6 mg/kg, presumably reflecting different efficacy demands) as well as 10 mg/kg (R)-methanandamide. Dose-generalization tests involved different doses of the cannabinoid CB1 receptor agonists. Antagonist tests varied the dose of the antagonist (range: 0.1 and 3 mg/kg for SR-141716 and AM-251, and 1 to 10 mg/kg for SR-144528).

RESULTS

SR-141716 and AM-251 doses dependently blocked the agonist-induced discriminative stimulus effects. SR-141716 tended to be slightly more potent than AM-251. The effective dose 50 (ED(50)) of SR-141716 was higher in the 5.6 mg/kg Delta(9)-THC-trained group relative to the two other Delta(9)-THC-trained groups. The cannabinoid CB2 receptor antagonist SR-144528 combined with the training dose of 1.8 mg/kg Delta(9)-THC, as well as when combined with the training dose of 10 mg/kg (R)-methanandamide, did not markedly change drug-appropriate (agonist) responses.

CONCLUSION

Data support that the discriminative stimulus effects of (R)-methanandamide and its overlap with the Delta(9)-THC cue are, indeed, CB1 receptor mediated events as revealed in antagonism tests with the selective central CB1 receptor antagonists SR-141716 and AM-251. The activation of cannabinoid CB2 receptors appears to be insignificant for these discriminations.

Natural cannabinoids: templates for drug discovery

Recent studies have elucidated the biosynthetic pathway of cannabinoids and have highlighted the preference for a C-3 n-pentyl side chain in the most prominently represented cannabinoids from Cannabis sativa and their medicinally important decarboxylation products. The corresponding C-3 n-propyl side chain containing cannabinoids are also found, although in lesser quantities. Structure-activity relationship (SAR) studies performed on Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the key psychoactive ingredient of Cannabis, and its synthetic analogues have identified the C-3 side chain as the key pharmacophore for ligand affinity and selectivity for the known cannabinoid receptors and for pharmacological potency. Interestingly, the terminal n-pentyl saturated hydrocarbon side chain of endocannabinoids also plays a corresponding crucial role in conferring similar properties. This review briefly summarizes the biosynthesis of cannabinoids and endocannabinoids and focuses on their side chain SAR.

Enhancement of inhibitory avoidance and conditioned taste aversion memory with insular cortex infusions of 8-Br-cAMP: involvement of the basolateral amygdala

There is considerable evidence that in rats, the insular cortex (IC) and amygdala are involved in the learning and memory of aversively motivated tasks. The present experiments examined the effects of 8-Br-cAMP, an analog of cAMP, and oxotremorine, a muscarinic agonist, infused into the IC after inhibitory avoidance (IA) training and during the acquisition/consolidation of conditioned taste aversion (CTA). Posttraining infusion into the IC of 0.3 microg oxotremorine and 1.25 microg 8-Br-cAMP enhanced IA retention. Infusions of 8-Br-cAMP, but not oxotremorine, into the IC enhanced taste aversion. The experiments also examined whether noradrenergic activity in the basolateral amygdala (BLA) is critical in enabling the enhancement of CTA and IA memory induced by drug infusions administered into the IC. For both CTA and IA, ipsilateral infusions of beta-adrenergic antagonist propranolol administered into the BLA blocked the retention-enhancing effect of 8-Br-cAMP or oxotremorine infused into the IC. These results indicate that the IC is involved in the consolidation of memory for both IA and CTA, and this effect requires intact noradrenergic activity into the BLA. These findings provide additional evidence that the BLA interacts with other brain regions, including sensory cortex, in modulating memory consolidation.