Tripping with Synthetic Cannabinoids ("Spice"): Anecdotal and Experimental Observations in Animals and Man

The phenomenon of consuming synthetic cannabinoids ("Spice") for recreational purposes is a fairly recent trend. However, consumption of cannabis dates back millennia, with numerous accounts written on the experience of its consumption, and thousands of scientific reports published on the effects of its constituents in laboratory animals and humans. Here, we focus on consolidating the scientific literature on the effects of "Spice" compounds in various behavioral assays, including assessing abuse liability, tolerance, dependence, withdrawal, and potential toxicity. In most cases, the behavioral effects of "Spice" compounds are compared with those of Δ9-tetrahydrocannabinol. Methodological aspects, such as modes of administration and other logistical issues, are also discussed. As the original "Spice" molecules never were intended for human consumption, scientifically based information about potential toxicity and short- and long-term behavioral effects are very limited. Consequently, preclinical behavioral studies with "Spice" compounds are still in a nascent stage. Research is needed to address the addiction potential and other effects, including propensity for producing tissue/organ toxicity, of these synthetic cannabimimetic "Spice" compounds.

Novel C-Ring-Hydroxy-Substituted Controlled Deactivation Cannabinergic Analogues

In pursuit of safer controlled-deactivation cannabinoids with high potency and short duration of action, we report the design, synthesis, and pharmacological evaluation of novel C9- and C11-hydroxy-substituted hexahydrocannabinol (HHC) and tetrahydrocannabinol (THC) analogues in which a seven atom long side chain, with or without 1'-substituents, carries a metabolically labile 2',3'-ester group. Importantly, in vivo studies validated our controlled deactivation approach in rodents and non-human primates. The lead molecule identified here, namely, butyl-2-[(6aR,9R,10aR)-1-hydroxy-9-(hydroxymethyl)-6,6-dimethyl-6a,7,8,9,10,10a-hexahydro-6H-benzo[c]chromen-3-yl]-2-methylpropanoate (AM7499), was found to exhibit remarkably high in vitro and in vivo potency with shorter duration of action than the currently existing classical cannabinoid agonists.

A high efficacy cannabinergic ligand (AM4054) used as a discriminative stimulus: Generalization to other adamantyl analogs and Δ(9)-THC in rats

In addition to endogenous lipids, the two main cloned receptors (CB1R and CB2R) of the endocannabinoid signaling system (ECS) can be activated (and blocked) by various exogenous ligands. A relatively novel template for CB1R activators contains an adamantyl moiety as a key structural subunit, the first being the cannabinergic AM411. Additional chemical optimization efforts using the classical tricyclic scaffold led to AM4054. Here we explored the in vivo consequences of novel adamantyl analogs in rats trained to recognize the effects of the potent adamantyl cannabinergic AM4054. Rats were trained to discriminate between AM4054 (0.1mg/kg) and vehicle. Three AM4054 analogs and Δ(9)-THC were tested for generalization (substitution) and antagonism was assessed with rimonabant. We found that all cannabinergics resulted in response generalization to the target stimulus AM4054. The order of potency was: AM4054≥AM4083≥AM4050>AM4089>Δ(9)-THC. The CB1R antagonist/inverse agonist rimonabant blocked the discriminative stimulus effects of AM4054. Thus the examined structural modifications affected binding affinities but did not markedly change potencies with the exception of AM4089. In vitro (cAMP assay) functional data have suggested that AM4089 behaves as a partial rather than as a full agonist at CB1R which could explain its lower potency compared to AM4054 (Thakur et al., 2013). The 9β-formyl functionality at C-9 position was identified as an important pharmacophore yielding high in vivo potency. Antagonism by rimonabant suggested CB1R mediation.

Probing the carboxyester side chain in controlled deactivation (-)-δ(8)-tetrahydrocannabinols

We recently reported on a controlled deactivation/detoxification approach for obtaining cannabinoids with improved druggability. Our design incorporates a metabolically labile ester group at strategic positions within the THC structure. We have now synthesized a series of (−)-Δ⁸-THC analogues encompassing a carboxyester group within the 3-alkyl chain in an effort to explore this novel cannabinergic chemotype for CB receptor binding affinity, in vitro and in vivo potency and efficacy, as well as controlled deactivation by plasma esterases. We have also probed the chain’s polar characteristics with regard to fast onset and short duration of action. Our lead molecule, namely 2-[(6aR,10aR)-6a,7,10,10a-tetrahydro-1-hydroxy-6,6,9-trimethyl-6H-dibenzo[b,d]pyran-3-yl]-2-methyl-propanoic acid 3-cyano-propyl ester (AM7438), showed picomolar affinity for CB receptors and is deactivated by plasma esterases while the respective acid metabolite is inactive. In further in vitro and in vivo experiments, the compound was found to be a remarkably potent and efficacious CB1 receptor agonist with relatively fast onset/offset of action.

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 Δ(9)-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.

Controlled-deactivation cannabinergic ligands

We report an approach for obtaining novel cannabinoid analogues with controllable deactivation and improved druggability. Our design involves the incorporation of a metabolically labile ester group at the 2'-position on a series of (-)-Δ(8)-THC analogues. We have sought to introduce benzylic substituents α to the ester group which affect the half-lives of deactivation through enzymatic activity while enhancing the affinities and efficacies of individual ligands for the CB1 and CB2 receptors. The 1'-(S)-methyl, 1'-gem-dimethyl, and 1'-cyclobutyl analogues exhibit remarkably high affinities for both CB receptors. The novel ligands are susceptible to enzymatic hydrolysis by plasma esterases in a controllable manner, while their metabolites are inactive at the CB receptors. In further in vitro and in vivo experiments key analogues were shown to be potent CB1 receptor agonists and to exhibit CB1-mediated hypothermic and analgesic effects.

"Herbal incense": designer drug blends as cannabimimetics and their assessment by drug discrimination and other in vivo bioassays

Recently, synthetic cannabinoids originally designed for testing in the laboratory only have found use recreationally in designer herbal blends, originally called "Spice". The myriad of compounds found are for the most part potent full agonists of the cannabinoid receptor 1, producing effects similar to tetrahydrocannabinol (THC) and marijuana. Drug discrimination of these compounds offers a specific behavioral test that can help determine whether these new synthetic compounds share a similar "subjective high" with the effects of marijuana/THC. By utilization of drug discrimination and other behavioral techniques, a better understanding of these new "designer" cannabinoids may be reached to assist in treating both the acute and chronic effects of these drugs. The paper provides a brief exposé of modern cannabinoid research as a backdrop to the recreational use of designer herbal blend cannabimimetics.

AM2389, a high-affinity, in vivo potent CB1-receptor-selective cannabinergic ligand as evidenced by drug discrimination in rats and hypothermia testing in mice

RATIONALE

The endocannabinoid signaling system (ECS) has been targeted for developing novel therapeutics since ECS dysfunction has been implicated in various pathologies. Current focus is on chemical modifications of the hexahydrocannabinol (HHC) nabilone (Cesamet(®)).

OBJECTIVE

To characterize the novel, high-affinity cannabinoid receptor 1 (CB(1)R) HHC-ligand AM2389 [9β-hydroxy-3-(1-hexyl-cyclobut-1-yl)-hexahydrocannabinol in two rodent pre-clinical assays.

MATERIALS AND METHODS

CB(1)R mediation of AM2389-induced hypothermia in mice was evaluated with AM251, a CB(1)R-selective antagonist/inverse agonist. Additionally, two groups of rats discriminated the full cannabinergic aminoalkylindole AM5983 (0.18 and 0.56 mg/kg) from vehicle 20 min post-injection in a two-choice operant conditioning task motivated by 0.1% saccharin/water. Generalization/substitution tests were conducted with AM2389, AM5983, and Δ(9)-tetrahydrocannabinol (Δ(9)-THC).

RESULTS

Δ(9)-THC (30 mg/kg)-induced hypothermia exhibited a faster onset and shorter duration of action compared with AM2389 (0.1 and 0.3 mg/kg). AM251 (3 and 10 mg/kg) attenuated/blocked hypothermia induced by 0.3 mg/kg AM2389. In drug discrimination, the order of potency was AM2389 > AM5983 > Δ(9)-THC with ED(50) values of 0.0025, 0.0571, and 0.2635 mg/kg, respectively, in the low-dose condition. The corresponding ED(50) values in the high-dose condition were 0.0069, 0.1246, and 0.8438 mg/kg, respectively. Onset of the effects of AM2389 was slow with a protracted time-course; the functional, perceptual in vivo half-life was approximately 17 h.

CONCLUSIONS

This potent cannabinergic HHC exhibited a slow onset of action with a protracted time-course. The AM2389 chemotype appears well suited for further drug development, and AM2389 currently is used to probe behavioral consequences of sustained ECS activation.

Cannabinergic aminoalkylindoles, including AM678=JWH018 found in 'Spice', examined using drug (Δ(9)-tetrahydrocannabinol) discrimination for rats

We examined four different cannabinergic aminoalkylindole ligands, including one drug (AM678=JWH018) found in herbal 'Spice' concoctions, for their ability to substitute for Δ(9)-tetrahydrocannabinol (THC), and the ability of the cannabinoid receptor 1-selective antagonist/inverse agonist rimonabant to block the substitution, 30 and 90 min after intraperitoneal injection. Rats trained to discriminate the effects of vehicle from those produced by 3 mg/kg of THC were used. The order of potency was: AM5983≥AM678>AM2233>WIN55212-2 at both test intervals. AM5983 and AM678 appeared eight times more potent than THC, followed by AM2233 (about twice as potent as THC), and WIN55212-2 approximately THC at the 30-min test interval. The aminoalkylindoles showed reduced potency (i.e. an increased ED50 value) at the longer injection-to-test interval of 90 min compared with testing at 30 min. The rightward shifts by coadministration of rimonabant were approximately 8-fold to 12-fold for AM5983 and AM678, compared with an approximately 3-fold rightward shift for the WIN55212-2 curve. AM2233 (1.8 mg/kg) substitution was also blocked by 1 mg/kg of rimonabant. In conclusion, AM5983 and AM678=JWH018 are potent cannabimimetics derived from an aminoalkylindole template. WIN55212-2 seemed to interact differently with rimonabant, compared with either AM5983 or AM678, indicating potential differences in the mechanism(s) of action among cannabinergic aminoalkylindoles.

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Transfer of the discriminative stimulus effects of Δ9-THC and nicotine from one operant response to another in rats

OBJECTIVE

Transfer of the discriminative stimulus effects of two drugs from one operant (original-response) to a topographically different response (transfer-response) that was spared drug discrimination training was investigated.

MATERIALS

Eight rats were trained in a counterbalanced one manipulandum (lever press and nose poke) drug discrimination procedure. Counterbalanced IP administered nicotine (0.3 mg/kg) or Δ(9)-tetrahydrocannabinol (3.0 mg/kg) functioned as discriminative stimuli. S(D) drugs occasioned sessions of food-reinforcement (variable-interval 30-s schedule); S(Δ) drugs occasioned non-reinforcement. The original-response (lever-pressing or nose-poking) was initially reinforced during 30-min S(D) drug sessions, and non-reinforced on the other alternating S(Δ)-drug sessions.

RESULTS

Two separate 5-min non-reinforcement tests, counterbalanced by drug order, revealed stimulus control over the original-response by both drugs, which transferred to the transfer-response. Subsequent extinction training of the transfer-response attenuated the original-response response rates with the S(D) drug conditions but had little impact on discriminative control. Discriminative control was reversed for the transfer-response but had little impact on the original-response but, again, reduced response rate.

CONCLUSION

These data demonstrate that (a) discriminative control by two distinct drug states can transfer and modulate a topographically different free-operant response and, (b) as is true for exteroceptive stimuli, drug states that function as antecedents embedded within the operant three-term contingency have differing relationships with the response and the primary reinforcer.

Discriminative stimulus functions of methanandamide and delta(9)-THC in rats: tests with aminoalkylindoles (WIN55,212-2 and AM678) and ethanol

OBJECTIVE

The aim of the study was to characterize in vivo the aminoalkylindoles WIN55,212-2 (WIN) and AM678 (naphthalen-1-yl(1-pentyl-1H-indol-3-yl)methanone) as cannabinoid receptor (CB(1)R) ligands using drug discrimination. Tests also involved delta(9)-tetrahydrocannabinol (THC) and R-(+)-methanandamide (mAEA), a metabolically stable analog of the endogenous ligand anandamide, as well as the CB(1)R selective antagonist/inverse agonist rimonabant; tests with ethanol assessed pharmacological specificity. We used two different drug discriminations (mAEA and THC) allowing us to explore potential differences in CB(1)R activation which could be attributed to variations in their respective CB(1)R signaling mechanisms.

METHODS

There were two concurrently trained groups of rats. One group discriminated between i.p. injected vehicle and 10 mg/kg mAEA. The other group was trained to discriminate between vehicle and 1.8 mg/kg THC.

RESULTS

Dose generalization curves for AM678, WIN55,212-2, THC, and mAEA suggested the following rank order of potency: AM678 > WIN55,212-2 > or = THC > mAEA in both drug discrimination groups. Challenge by 1 mg/kg rimonabant resulted in shifts to the right of the generalization curves for the two aminoalkylindoles (4.4-fold for AM678 and 11.3-fold for WIN in the mAEA group, whereas for the THC group, the corresponding values were 13 and 2.6, respectively), suggesting surmountable antagonism. Ethanol did not generalize in either of the two groups, suggesting pharmacological specificity.

CONCLUSION

Data are congruent with the general observation that there is substantial overlap in the discriminative stimulus effects of CB(1)R ligands across different chemical classes. However, the quantitative differences in the interactions between the two aminoalkylindoles and rimonabant in the two discrimination groups suggest subtle variations in the ligand-receptor activation(s).

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

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.]).

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.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Effects of the CB1R agonist WIN-55,212-2 and the CB1R antagonists SR-141716 and AM-1387: Open-field examination in rats

This study examined the open-field (O-F) effects in rats of the cannabinoid 1 receptor (CB1R) agonist WIN-55,212-2 (WIN; 1 to 5.6 mg/kg) and its interaction with the CB1R antagonist/inverse agonist SR-141716 (1 to 5.6 mg/kg). Additionally, separate studies examined the O-F effects of SR-141716 (1 to 10 mg/kg) and a newly synthesized CB1R selective antagonist/inverse agonist AM-1387 (3 and 10 mg/kg) when these ligands were administered alone. Both antagonists are characterized in vitro by decreased of GTPgammaS binding and increased cAMP accumulation (inverse agonism). WIN dose dependently reduced ambulation (horizontal activity) and rearing (vertical activity); SR-141716 completely (WIN 3 mg/kg) or partially (WIN 5.6 mg/kg) normalized these behaviors. WIN alone resulted in circling and in an increased latency to leave the start area of the O-F, effects blocked by all doses of SR-141716. Both the increased scratching and grooming, associated with SR-141716 administration, were attenuated but not abolished by WIN. SR-141716 alone tended to reduce ambulation (significant at 10 mg/kg) and rearing (non-significant), had no effect on latency, and increased scratching and grooming (both frequency and duration), at doses of 3 mg/kg and up. At the doses examined, AM-1387 had no effect on ambulation, rearing, latency but significantly increased scratching (10 mg/kg); there was also a trend for increased grooming (both frequency and duration). The O-F profile of WIN suggests more similarity with the effects of THC rather than methanandamide (and presumably also anandamide). Intrinsic activity (scratching and grooming) by SR-141716 was re-affirmed and seemed to be associated with administration of AM-1387 as well. AM-1387 was less potent than SR-141716.

Delta9-THC induced hyperphagia and tolerance assessment: interactions between the CB1 receptor agonist delta9-THC and the CB1 receptor antagonist SR-141716 (rimonabant) in rats

This study examined effects of the CB1 receptor antagonist/inverse agonist SR-141716 and the CB1 receptor agonist delta9-tetrahydrocannabinol (delta9-THC) on feeding behavior in male Sprague-Dawley rats. Rats were housed individually with free access to regular pelletized laboratory chow [after a 2 weeks handling phase, animals had access to regular chow for 21 h (Study 1) or 22 h (Study 2); high-fat powder food for 3 h in Study 1 and 2 h in Study 2, respectively], and free access to water. Animals were maintained on a reversed 12-h light/dark cycle (dark beginning at noon). Rats were habituated to this type of feeding and light/dark schedule for 3 weeks until a stable baseline for food intake was achieved. In Study 1, animals were examined after administration of delta9-THC alone (dose range 0.1-1.8 mg/kg), SR-141716 alone (dose range 0.03-0.3 mg/kg), and the two drugs combined; injections were given i.p. at the beginning of the second hour after presenting the high-fat diet and drugs were given twice weekly. There was a dose-related increase in high-fat diet intake, peaking at 0.56-1 mg/kg delta9-THC. SR-141716 alone suppressed the high-fat diet intake below control levels. A combination of 0.3 mg/kg SR-141716 and 0.56 mg/kg delta9-THC counteracted the effects on consumption of either drug alone. In Study 2, experimental rats were treated initially with 0.56 mg/kg delta9-THC for six consecutive days; controls received vehicle. Attenuation of the hyperphagia (high-fat diet) was evident after the second injection. Increasing doses of delta9-THC (1 and 1.8 mg/kg, for two and three consecutive days, respectively) did not reinstate the initial hyperphagia. In conclusion, low-to-moderate doses of delta9-THC produced hyperphagia (to a high-fat food source), which was antagonized by SR-141716. SR-141716 singly suppressed intake of the high-fat diet. Delta9-THC-induced hyperphagia dissipated rapidly upon chronic treatment; however, it is unclear whether this reflects pharmacological tolerance or the emergence of a conditioned taste aversion in Study 2.

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.

Adamantyl cannabinoids: a novel class of cannabinergic ligands

Structure-activity relationship studies have established that the aliphatic side chain plays a pivotal role in determining the cannabinergic potency of tricyclic classical cannabinoids. We have now synthesized a series of analogues in which a variety of adamantyl substituents were introduced at the C3 position of Delta(8)-THC. Our lead compound, (-)-3-(1-adamantyl)-Delta(8)-tetrahydrocannabinol (1a, AM411), was found to have robust affinity and selectivity for the CB1 receptor as well as high in vivo potency. The X-ray crystal structure of 1a was determined. Exploration of the side chain conformational space using molecular modeling approaches has allowed us to develop cannabinoid side chain pharmacophore models for the CB1 and CB2 receptors. Our results suggest that although a bulky group at the C3 position of classical cannabinoids could be tolerated by both CB1 and CB2 binding sites, the relative orientation of that group with respect to the tricyclic component can lead to receptor subtype selectivity.

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 CB(1) receptor antagonist SR-141716 (SR, 5.6 mg/kg) and vehicle, and (ii) the cannabinoid receptor agonist Delta(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 CB(2) 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.

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

(-)-Adamantyl-Delta8-tetrahydrocannabinol (AM-411) is a 'classical' tricyclic cannabinoid CB1 receptor agonist in which the C-3 alkyl side-chain has been replaced with an adamantyl group. The compound is cannabinoid CB1 receptor subtype selective (CB1 Ki=6.86 nmol/l, CB2 Ki=52.0 nmol/l). We examined the effects of AM-411 alone and in combination with the cannabinoid CB1 receptor antagonist/inverse agonist, SR-141716, on open-field behaviors of rats. The lowest effective dose of AM-411, 3 mg/kg, suppressed ambulation (horizontal activity) and rearing (vertical activity) and increased circling frequency compared to vehicle control levels. Co-administration of SR-141716 normalized these changes. SR-141716 (3 and 5.6 mg/kg) also produced significant increases in scratching and grooming (both frequency and duration), effects that were not eliminated in the presence of AM-411. Coupled with previous drug discrimination data, the open-field profile of AM-411 suggests that this high-affinity CB1 cannabinoid receptor agonist induces behavioral effects similar to the natural cannabinoid Delta9-tetrahydrocannabinol and different from (R)-methanandamide, a chiral analog of the endogenous ligand anandamide.

The cannabinoid receptor antagonist SR-141716 does not readily antagonize open-field effects induced by the cannabinoid receptor agonist (R)-methanandamide in rats

This study examined the effects of the cannabinoid CB(1) receptor agonist (R)-methanandamide and the CB(1) receptor antagonist SR-141716 on open-field behaviors in rats. Animals were examined after administration of (R)-methanandamide (dose range 10 to 30 mg/kg) plus vehicle, and the two drugs in combination; the dose range of SR-141716 was 0.3 to 5.6 mg/kg. Injections were given intraperitoneally 20 min prior to initial testing. Additional exposures to the open-field arena occurred for the groups treated with 30 mg/kg (R)-methanandamide at 60 and 120 min post administration. There was a dose-related suppression of ambulation (horizontal activity) and rearing (vertical activity) after (R)-methanandamide administration. Coadministration of SR-141716 did not counteract the suppression induced by 10 and 18 mg/kg (R)-methanandamide but rather tended to augment it, especially with regard to ambulation using SR-141716 doses of 1 mg/kg and up. The latency to leave the starting area in the center of the field was significantly elevated by 30 mg/kg (R)-methanandamide. This effect was completely blocked by SR-141716. With increasing doses of SR-141716, there was an increase in grooming and scratching. Generally, the strongest effects occurred 20 min post administration with the exception of grooming, which reached maximum at 60 min post. Differences in the number of circlings, vocalizations, urination, and defecation generally did not differ clearly among treatments. These results coupled with previous open-field data examining combinations of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and SR-141716 [Pharmacol. Biochem. Behav. 73 (2002) 911] underscore pharmacological differences between (R)-methanandamide and Delta(9)-THC revealed by their interactions with SR-141716.

(R)-Methanandamide and delta9-tetrahydrocannabinol-induced operant rate decreases in rats are not readily antagonized by SR-141716A

The current study examined the interaction between the cannabinoid CB(1) receptor agonists Delta(9)-tetrahydrocannabinol and (R)-methanandamide in combination with the cannabinoid CB(1) receptor antagonist SR-141716A (N-(piperidin-1-yl)-5-(4-chloro-phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide HCl) in rats responding for food on a fixed ratio (FR-10) schedule of food reinforcement. The study provided only limited evidence for antagonism by SR-141716A (at 1 mg/kg but not with 0.3, 3 and 10 mg/kg) of the rate suppressant effects induced by the cannabinoid CB(1) receptor agonist Delta(9)-tetrahydrocannabinol (and only at the single dose of 5.6 mg/kg Delta(9)-tetrahydrocannabinol). (R)-Methanandamide in combination with SR-141716A resulted in a greater rate suppression compared to that induced by (R)-methanandamide alone. Thus, SR-141716A augmented the rate-decreasing effects of (R)-methanandamide and only minimally altered the rate-decreasing effects of Delta(9)-tetrahydrocannabinol. Additionally, high doses (10 and 30 mg/kg) of SR-141716 singly consistently suppressed the rate of responding. The current results coupled with our previous data examining combinations of Delta(9)-tetrahydrocannabinol or (R)-methanandamide and SR-141716 (see text) underscore pharmacological/behavioral differences (whether quantitative or qualitative) between the cannabinoid CB(1) agonists (R)-methanandamide and Delta(9)-tetrahydrocannabinol revealed by their interactions with the cannabinoid CB(1) antagonist SR-141716.

Interactions between the CB1 receptor agonist Delta 9-THC and the CB1 receptor antagonist SR-141716 in rats: open-field revisited

This study examined the effects of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and the CB1 antagonist SR-141716 on open-field behaviors in male Sprague-Dawley rats. Animals were examined after administration of Delta(9)-THC alone (dose range: 0.3-5.6 mg/kg), SR-141716 alone (dose range: 1-5.6 mg/kg) and the two drugs in combination; injections were given intraperitoneally 30 min prior to testing. There was a dose-related suppression of ambulation (horizontal activity) and rearing (vertical activity) after Delta(9)-THC administration. Co-administration of SR-141716 counteracted this suppression; however, antagonism was only partial for rearing. Interestingly, 1 mg/kg SR-141716 was as effective as 3 and 5.6 mg/kg SR-141716 in this antagonist action. Increasing doses of Delta(9)-THC produced an increase in circling behavior; latency to leave the starting area in the center of the field was significantly elevated by 5.6 mg/kg Delta(9)-THC. Those effects were completely blocked by SR-141716. Grooming and scratching showed a dose-related increase following administration of SR-141716 (1-5.6 mg/kg), which were only partially blocked by co-administration of Delta(9)-THC (3 and 5.6 mg/kg). When given alone, only the highest dose of SR-141716 (5.6 mg/kg) depressed ambulation; rearing and latency were not significantly changed, and circling was absent. Differences in the number of vocalizations, urination and defecation generally did not differ clearly among the treatment conditions. These results may show that SR-141716 is acting as (i) an inverse agonist and/or (ii) that the endogenous cannabinoid system is tonically active under certain conditions.