Apparent CB1 Receptor Rimonabant Affinity Estimates: Combination with THC and Synthetic Cannabinoids in the Mouse In Vivo Triad Model.

Synthetic cannabinoids (SCs) represent an emerging class of abused drugs associated with psychiatric complications and other substantial health risks. These ligands are largely sold over the internet for human consumption, presumably because of their high cannabinoid 1 receptor (CB1R) affinity and their potency in eliciting pharmacological effects similar to Δ9-tetrahydrocannabinol (THC), as well as circumventing laws illegalizing this plant. Factors potentially contributing to the increased prevalence of SC abuse and related hospitalizations, such as increased CB1R efficacy and non-CB1R targets, highlight the need for quantitative pharmacological analyses to determine receptor mediation of the pharmacological effects of cannabinoids. Accordingly, the present study used pA2 and pKB analyses for quantitative determination of CB1R mediation in which we utilized the CB1R-selective inverse agonist/antagonist rimonabant to elicit rightward shifts in the dose-response curves of five SCs (i.e., A-834,735D; WIN55,212-2; CP55,950; JWH-073; and CP47,497) and THC in producing common cannabimimetic effects (i.e., catalepsy, antinociception, and hypothermia). The results revealed overall similarity of pA2 and pKB values for these compounds and suggest that CB1Rs, and not other pharmacological targets, largely mediated the central pharmacological effects of SCs. More generally, affinity estimation offers a powerful pharmacological approach to assess potential receptor heterogeneity subserving in vivo pharmacological effects of SCs.

Stratification of Cannabinoid 1 Receptor (CB1R) Agonist Efficacy: Manipulation of CB1R Density through Use of Transgenic Mice Reveals Congruence between In Vivo and In Vitro Assays.

Synthetic cannabinoids (SCs) are an emerging class of abused drugs that differ from each other and the phytocannabinoid ∆9-tetrahydrocannabinol (THC) in their safety and cannabinoid-1 receptor (CB1R) pharmacology. As efficacy represents a critical parameter to understanding drug action, the present study investigated this metric by assessing in vivo and in vitro actions of THC, two well-characterized SCs (WIN55,212-2 and CP55,940), and three abused SCs (JWH-073, CP47,497, and A-834,735-D) in CB1 (+/+), (+/-), and (-/-) mice. All drugs produced maximal cannabimimetic in vivo effects (catalepsy, hypothermia, antinociception) in CB1 (+/+) mice, but these actions were essentially eliminated in CB1 (-/-) mice, indicating a CB1R mechanism of action. CB1R efficacy was inferred by comparing potencies between CB1 (+/+) and (+/-) mice [+/+ ED50 /+/- ED50], the latter of which has a 50% reduction of CB1Rs (i.e., decreased receptor reserve). Notably, CB1 (+/-) mice displayed profound rightward and downward shifts in the antinociception and hypothermia dose-response curves of low-efficacy compared with high-efficacy cannabinoids. In vitro efficacy, quantified using agonist-stimulated [35S]GTPγS binding in spinal cord tissue, significantly correlated with the relative efficacies of antinociception (r = 0.87) and hypothermia (r = 0.94) in CB1 (+/-) mice relative to CB1 (+/+) mice. Conversely, drug potencies for cataleptic effects did not differ between these genotypes and did not correlate with the in vitro efficacy measure. These results suggest that evaluation of antinociception and hypothermia in CB1 transgenic mice offers a useful in vivo approach to determine CB1R selectivity and efficacy of emerging SCs, which shows strong congruence with in vitro efficacy.

Assessment of dependence potential and abuse liability of Δ8-tetrahydrocannabinol in mice.

Delta-8-tetrahydrocannabinol (Δ8-THC) is a psychotropic cannabinoid produced in low quantities in the cannabis plant. Refinements in production techniques, paired with the availability of inexpensive cannabidiol substrate, have resulted in Δ8-THC being widely marketed as a quasi-legal, purportedly milder alternative to Δ9-THC. Yet, little research has probed the behavioral and physiological effects of repeated Δ8-THC use. The present study aimed to evaluate the effects of acute and repeated exposure to Δ8-THC. We hypothesized that Δ8-THC produces effects similar to Δ9-THC, including signs of drug tolerance and dependence. Adult male and female C57BL/6J mice were treated acutely with Δ8-THC (6.25-100 mg/kg, i.p.) or vehicle and tested in the tetrad battery to quantify cannabimimetic effects (i.e., catalepsy, antinociception, hypothermia, immobility) as compared with a non-selective synthetic cannabinoid (WIN 55,212-2) and Δ9-THC. As previously reported, Δ8-THC (≥12.5 mg/kg) induced cannabimimetic effects. Pretreatment with the CB1 receptor-selective antagonist rimonabant (3 mg/kg, i.p.) blocked each of these effects. In addition, repeated administration of Δ8-THC (50 mg/kg, s.c.) produced tolerance, as well as cross-tolerance to WIN 55,212-2 (10 mg/kg, s.c.) in tetrad, consistent with downregulated CB1 receptor function. Behavioral signs of physical dependence in the somatic signs, tail suspension, and marble burying assays were also observed following rimonabant-precipitated withdrawal from Δ8-THC (≥10 mg/kg BID for 6 days). Lastly, Δ8-THC produced Δ9-THC-like discriminative stimulus effects in both male and female mice. Together, these findings demonstrate that Δ8-THC produces qualitatively similar effects to Δ9-THC, including risk of drug dependence and abuse liability.

Gonadal hormone modulation of ∆9-tetrahydrocannabinol-induced antinociception and metabolism in female versus male rats.

The gonadal hormones testosterone (T) in adult males and estradiol (E2) in adult females have been reported to modulate behavioral effects of ∆9-tetrahydrocannabinol (THC). This study determined whether activational effects of T and E2 are sex-specific, and whether hormones modulate production of the active metabolite 11-hydroxy-THC (11-OH-THC) and the inactive metabolite 11-nor-9-carboxy-THC (THC-COOH). Adult male and female rats were gonadectomized (GDX) and treated with nothing (0), T (10-mm Silastic capsule/100g body weight), or E2 (1-mm Silastic capsule/rat). Three weeks later, saline or the cytochrome P450 inhibitor proadifen (25mg/kg; to block THC metabolism and boost THC's effects) was injected i.p.; 1h later, vehicle or THC (3mg/kg females, 5mg/kg males) was injected i.p., and rats were tested for antinociceptive and motoric effects 15-240min post-injection. T did not consistently alter THC-induced antinociception in males, but decreased it in females (tail withdrawal test). Conversely, T decreased THC-induced catalepsy in males, but had no effect in females. E2 did not alter THC-induced antinociception in females, but enhanced it in males. The discrepant effects of T and E2 on males' and females' behavioral responses to THC suggests that sexual differentiation of THC sensitivity is not simply due to activational effects of hormones, but also occurs via organizational hormone or sex chromosome effects. Analysis of serum showed that proadifen increased THC levels, E2 increased 11-OH-THC in GDX males, and T decreased 11-OH-THC (and to a lesser extent, THC) in GDX females. Thus, hormone modulation of THC's behavioral effects is caused in part by hormone modulation of THC oxidation to its active metabolite. However, the fact that hormone modulation of metabolism did not alter THC sensitivity similarly on all behavioral measures within each sex suggests that other mechanisms also play a role in gonadal hormone modulation of THC sensitivity in adult rats.

Interactions between THC and cannabidiol in mouse models of cannabinoid activity.

RATIONALE: Interest persists in characterizing potential interactions between Delta(9)-tetrahydocannabinol (THC) and other marijuana constituents such as cannabidiol (CBD). Such interactions may have important implications for understanding the long-term health consequences of chronic marijuana use as well as for attempts to develop therapeutic uses for THC and other CB(1) agonists. OBJECTIVES: We investigated whether CBD may modulate the pharmacological effects of intravenously administered THC or inhaled marijuana smoke on hypoactivity, antinociception, catalepsy, and hypothermia, the well characterized models of cannabinoid activity. RESULTS: Intravenously administered CBD possessed very little activity on its own and, at a dose equal to a maximally effective dose of THC (3 mg/kg), failed to alter THC's effects on any measure. However, higher doses of CBD (ED(50)=7.4 mg/kg) dose-dependently potentiated the antinociceptive effects of a low dose of THC (0.3 mg/kg). Pretreatment with 30 mg/kg CBD, but not 3 mg/kg, significantly elevated THC blood and brain levels. No interactions between THC and CBD were observed in several variations of a marijuana smoke exposure model. Either quantities of CBD were applied directly to marijuana, CBD and THC were both applied to placebo plant material, or mice were pretreated intravenously with 30 mg/kg CBD before being exposed to marijuana smoke. CONCLUSIONS: As the amount of CBD found in most marijuana strains in the US is considerably less than that of THC, these results suggest that CBD concentrations relevant to what is normally found in marijuana exert very little, if any, modulatory effects on CB(1)-receptor-mediated pharmacological effects of marijuana smoke.

Nitric oxide synthase inhibitors attenuate phencyclidine-induced disruption of prepulse inhibition.

Glutamate stimulation of N-methyl-D-aspartate (NMDA) receptors results in release of nitric oxide which may mediate the effects of NMDA receptor stimulation and/or may result in feedback inhibition of the presynaptic neuron. Results of a previous study showed that nitric oxide synthase (NOS) inhibitors blocked dizocilpine-induced behavior in mice. In the present study, NOS inhibitors were tested in combination with phencyclidine (PCP), a drug which typically dose-dependently disrupts prepulse inhibition of the acoustic startle response in rats. Alone, NOS inhibitors and promoters do not affect prepulse inhibition; however, when tested in combination with PCP, the NOS inhibitors, L-NOARG, 7-nitroindazole and arcaine--but not the NR2B-selective polyamine site NMDA antagonist, eliprodil--attenuated PCP-induced disruption of prepulse inhibition of the acoustic startle response. These effects are similar to those produced by many atypical antipsychotics and suggests that this class of drugs should be investigated further for their potential utility as antipsychotics and as treatments for PCP abuse.

NMDA antagonists produce site-selective impairment of accuracy in a delayed nonmatch-to-sample task in rats.

Antagonists at the N-methyl-D-aspartate (NMDA) sub-type of glutamate receptor are purported to have detrimental effects on cognitive processes. In order to examine the site selectivity of these effects, phencycline (PCP), dizocilpine, and memantine (PCP-site antagonists), SDZEAA 494 and NPC17742 (competitive NMDA antagonists), ACEA 1021 (glycine-site antagonist), and eliprodil (NR2B-selective polyamine-site selective antagonist) were tested in rats performing a delayed nonmatch-to-sample task. Dizocilpine, PCP and memantine significantly decreased accuracy and discriminability, particularly during brief delay trials. In contrast, the competitive NMDA antagonists, SDZ EAA 494 and NPC 17742, did not affect accuracy or discriminability at any delay. Similarly, ACEA 1021, and eliprodil did not alter behavioral indices in a manner suggesting compromise in information processing at any delay even at doses that decreased the total number of trials completed. These data support previous findings that the effects of NMDA antagonists on accuracy are site-selective, with PCP-site antagonists producing the greatest disruption. Further, while not conclusive, the results are consistent with the hypothesis that NMDA receptor-mediated neurotransmission may be important at early stages of information processing, although further research is necessary to confirm these latter observations.

Discriminative stimulus effects of delta 9-tetrahydrocannabinol and delta 9-11-tetrahydrocannabinol in rats and rhesus monkeys.

Previous reports have suggested that delta 9-11-tetrahydrocannabinol (delta 9-11-THC), an exocyclic analog of delta 9-tetrahydrocannabinol (delta 9-THC), may have weak agonist effects as well as antagonistic properties. The purpose of the present study was to examine the effects of delta 9-11-THC in substitution and antagonism tests in rats and in rhesus monkeys trained to discriminate delta 9-THC from vehicle in two-lever drug-discrimination procedures. The substitution studies showed that delta 9-11-THC generalizes from the training dose of delta 9-THC in rats and in monkeys, although it was less potent in both species. The magnitude of the potency difference was greater in monkeys than in rats. When administered immediately following injection with the training dose of delta 9-THC, delta 9-11-THC failed to block the delta 9-THC cue in rats and showed a lack of dose-responsive inhibition in monkeys. These results suggest that delta 9-11-THC is devoid of antagonistic properties in the drug discrimination paradigm.

Discriminative stimulus effects of CP 55,940 and structurally dissimilar cannabinoids in rats.

CP 55,940 is a potent synthetic bicyclic cannabinoid analog that has been used in a number of studies as a radioligand for the cannabinoid receptor. This compound shares behavioral and biochemical properties with naturally occurring cannabinoids such as delta 9-THC. The purpose of the present study was 3-fold: to establish the ability of CP 55,940 to serve as a discriminative stimulus, to determine whether this discriminative stimulus is identical to that of delta 9-THC, and to examine whether a newly developed cannabinoid antagonist, SR141716A, would antagonize the discriminative stimulus effects of CP 55,940. Rats were trained to discriminate 0.1 mg/kg CP 55,940 from vehicle in standard 2-lever operant conditioning chambers. CP 55,940 produced dose-dependent generalization from the training dose in dose-effect determinations conducted before and after testing with other drugs. The effects of the training dose of CP 55,940 were dose-dependently antagonized by co-administration of SR141716A. Results of substitution tests showed that delta 9-THC, WIN 55,212-2, and cannabinol substituted completely for CP 55,940 in a dose-dependent manner; however, CP 55,940 was approx 10-fold more potent than any of the other drugs in producing CP 55,940-like discriminative stimulus effects. Several drugs with CNS depressant properties (phencyclidine, haloperidol and diazepam) failed to produce reliable substitution for CP 55,940. These results demonstrate that CP 55,940 has discriminative stimulus effects and that it shares these effects with structurally dissimilar compounds that, like CP 55,940, bind to the cannabinoid receptor. Further, these effects are blocked by SR141716A, a cannabinoid receptor antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Combinations of clozapine and phencyclidine: effects on drug discrimination and behavioral inhibition in rats.

Phencyclidine (PCP) produces psychotomimetic effects in humans that resemble schizophrenia symptoms. In an effort to screen compounds for antipsychotic activity, preclinical researchers have investigated whether these compounds block PCP-induced behaviors in animals. In the present study, the atypical antipsychotic clozapine was tested in combination with an active dose of PCP in two-lever drug discrimination and mixed signalled-unsignalled differential-reinforcement-of-low-rates (DRL) procedures. PCP produced distinctive effects in each task: it substituted for the training dose in PCP discrimination and it increased the number of responses with short (<3 s) interresponse times as well as increasing overall response rates in the DRL schedule. Acute dosing with clozapine failed to alter the behavioral effects of PCP in either procedure even when tested up to doses that produced pharmacological effects alone. These results suggest that acute dosing with clozapine would not affect behaviors most closely associated with PCP intoxication. Further, they bring into question the utility of using PCP combination procedures in animals to screen for antipsychotic potential. Since chronic dosing is required for therapeutic efficacy of antipsychotics, future studies should focus on investigation of chronic dosing effects of these drugs in combination with PCP.

Effects of modulation of NMDA neurotransmission on response rate and duration in a conflict procedure in rats.

N-Methyl-D-aspartate (NMDA) antagonists and gamma-aminobutyric acid agonists share a number of common pharmacological properties, including motor and anticonvulsant effects. In the present study, site-selective NMDA antagonists were evaluated for potential anxiolytic efficacy and motor impairment in a modified Geller-Seifter conflict procedure, an animal model widely used to screen drugs for anxiolytic effects. Male Sprague-Dawley rats were trained to respond for food reward under a multiple FI 30 s (food only), FR 10 (food + shock) operant schedule. Consistent with the results of previous studies, the benzodiazepines chlordiazepoxide and diazepam selectively increased punished responding and increased response durations at higher doses. The competitive NMDA antagonist CGP 37,849 increased punished responding at some doses, though not selectively, and also increased response duration in both schedule components. The glycine-site modulators milacemide, ACEA 1011 and ACEA 1021, the NR2B-selective polyamine site antagonist eliprodil and NMDA did not produce anticonflict effects at any dose and had inconsistent effects on response durations. These results suggest that the anticonflict effects of NMDA antagonists are not as reliable as those of the benzodiazepines. Further research is needed to clarify the experimental conditions under which the anxiolytic potential of NMDA antagonists is most evident.

Evaluation of agonist-antagonist properties of nitrogen mustard and cyano derivatives of delta 8-tetrahydrocannabinol.

delta 8-Tetrahydrocannabinol (delta 8-THC) is a naturally occurring cannabinoid with a characteristic pharmacological profile of in vivo effects. Previous studies have shown that modification of the structure of delta 8-THC by inclusion of a nitrogen-containing functional group alters this profile and may alkylate the cannabinoid receptor, similar to the manner in which beta-funaltrexamine (beta-FNA) alkylates the micro-opioid receptor. Two novel analogs of delta 8-THC were synthesized: a nitrogen mustard analog with a dimethylheptyl side chain (NM-delta 8-THC) and a cyano analog with a dimethylpentyl side chain (CY-delta 8-THC). Both analogs showed high affinity for brain cannabinoid receptors and when administered acutely, produced characteristic delta 9-THC-like effects in mice, including locomotor suppression, hypothermia, antinociception and catalepsy. CY-delta 8-THC shared discriminative stimulus effects with CP 55,940; for NM-delta 8-THC, these effects also occurred, but were delayed. Although both compounds attenuated the effects of delta 9-THC in the mouse behavioral tests, evaluation of potential antagonist effects of these compounds was complicated by the fact that two injections of delta 9-THC produced similar results, suggesting that acute tolerance or desensitization might account for the observations. NM-delta 8-THC, but not CY-delta 8-THC, attenuated the discriminative stimulus effects of CP 55,940 in rats several days following injection. Hence, addition of a nitrogen-containing functional group to a traditional cannabinoid structure does not eliminate agonist effects and may produce delayed attenuation of cannabinoid-induced pharmacological effects.

Potent anandamide analogs: the effect of changing the length and branching of the end pentyl chain.

To examine the effect of changing the length and branching of the end pentyl chain (C5H11) of anandamide (AN), various analogs 1a-h and 2a-f were synthesized from either the known aldehyde ester 6a or from the alcohol 6b and tested for their pharmacological activity. A reproducible procedure was developed for the conversion of arachidonic acid to 6a or 6b in gram quantities (overall yield 15%). The appropriate tetraene esters 7 were prepared by carrying out a Witting reaction, between 6a and the ylide generated from the phosphonium salt of the appropriate alkyl halide or between the ylide of 6d (prepared from 6a-->6b-->6c-->6d) and the appropriate alkyl aldehydes. They were then hydrolyzed to the corresponding acids and transformed into AN analogs 1 via their acid chlorides then treated with excess ethanolamine. alpha-Alkylation of esters 7 gave compounds 8 which were hydrolyzed to the corresponding acids. These acids via their acid chlorides and subsequent treatment with excess fluoroethylamine gave the target compounds 2. In this way analogs 1e and 2a-c were synthesized from 6d while all the remaining analogs were prepared from 6a. In order to assess the optimal length of the alkyl terminus, analogs 1a-d were prepared and showed moderately high affinities (18-55 nM). However analogs 1a-c failed to produce significant pharmacological effects at doses up to 30 mg/kg. Analog 1d was found to be a weak partial agonist. The reason for the lack of activity in 1a-c is presently not clear. Like the THCs, the branching of the end pentyl chain in AN (1e-h) increased potency both in in vitro and in vivo activities; the dimethylheptyl (DMH) analog 1e was the most potent in the series. Similar alkyl substitutions were carried out in the fluoro-2-methylanandamide series (2a-f), and all of these analogs had high receptor affinities (1-14 nM), the DMH analog 2a being the most potent. With a few exceptions they showed robust pharmacological effects, and AN-like profiles. It was shown that the SAR of the end pentyl chain in AN is very similar to that of THCs. However, the magnitude of enhanced potency observed when the side chain of THC was changed from straight to branched was not observed when the end chain of AN was similarly changed.

Long-term behavioral and neurodegenerative effects of perinatal phencyclidine administration: implications for schizophrenia.

Both acute and chronic administration of N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine and dizocilpine have been proposed to mimic some of the symptoms of schizophrenia. The purposes of the present study were first, to characterize the long-term behavioral and neurodegenerative effects of subchronic administration of phencyclidine to perinatal rats and second, to determine whether pretreatment with olanzapine could attenuate these effects. On postnatal days 7, 9 and 11 rat pups were pretreated with either vehicle or olanzapine prior to administration of either saline or phencyclidine (10 mg/kg). Some pups were killed on postnatal day 12 for biochemical determinations and others were tested on postnatal days 24-28 for prepulse inhibition of acoustic startle, on postnatal day 42 for phencyclidine-induced locomotor activity and between postnatal days 33 and 70 for acquisition of a delayed spatial learning task. Phencyclidine treatment resulted in a substantial increase in fragmented DNA in the frontal and olfactory cortices consistent with neurodegeneration by an apoptotic mechanism. An increase in the NMDA receptor NR1 subunit mRNA was also observed in the cortex. Gel shift assays showed that phencyclidine also increased the nuclear translocation of nuclear factor-kappaB proteins in the prefrontal cortex. In tissue from the frontal cortex, western blot analysis revealed that phencyclidine treatment increased Bax and decreased Bcl-X(L) proteins. Later in development, it was observed that perinatal phencyclidine treatment significantly retarded baseline prepulse inhibition of acoustic startle measured shortly after weaning. In 42-day-old rats, it was found that challenge with 2 mg/kg phencyclidine increased locomotor activity to a significantly greater extent in the rats that had been pretreated with phencyclidine. Similarly, perinatal phencyclidine treatment significantly delayed the acquisition of a delayed spatial alternation task. Each of the aforementioned changes (except for the spatial learning task, which was not tested) was significantly inhibited by olanzapine pretreatment, an antipsychotic drug known to be effective against both positive and negative symptoms of schizophrenia. Further, olanzapine treatment for 12 days following the administration of phencyclidine was also able to reverse the phencyclidine-induced deficit in baseline prepulse inhibition. Together these data suggest that perinatal administration of phencyclidine results in long-term behavioral changes that may be mechanistically related to the apoptotic neurodegeneration observed in the frontal cortex. It is postulated that these deficits may model the hypofrontality observed in schizophrenia and that this model may be helpful in designing appropriate pharmacotherapy.

Effects of competitive and non-competitive N-methyl-D-aspartate (NMDA) antagonists in squirrel monkeys trained to discriminate D-CPPene (SDZ EAA 494) from vehicle.

Drug discrimination studies have proven useful for comparing and contrasting the behavioral effects of site-selective N-methyl-D-aspartate (NMDA) antagonists. This study examined the effects of competitive and non-competitive NMDA antagonists in squirrel monkeys trained to discriminate 1 mg/kg D-CPPene [D-3-(2-carboxypiperazine-4-yl)-1- propenyl-1-phosphonic acid; SDZ EAA 494] from vehicle in a two-lever drug discrimination procedure. Results show that D-CPPene and several other competitive NMDA antagonists (NPC 17742, CGS 19755, and CGP 37849) completely substituted for D-CPPene in a dose-dependent manner. In contrast, phencyclidine (PCP) and ketamine produced only partial substitution at doses that severely suppressed response rates. These results are consistent with results of earlier studies with rats and monkeys showing differences in the discriminative stimulus effects of competitive and PCP-like non-competitive NMDA antagonists. The data support the predictions (1) that D-CPPene and the other competitive NMDA antagonists tested would have similar subjective effects in humans and (2) that some differences would be found in the subjective effects of competitive NMDA antagonists and PCP-like non-competitive antagonists.

Discriminative stimulus effects of site-selective N-methyl-D-aspartate antagonists in NPC 17742-trained rats and squirrel monkeys.

Drug discrimination studies in rats and monkeys with competitive N-methyl-D-aspartate. (NMDA) antagonists as training drugs have shown that these drugs typically cross-substitute for each other, whereas phencyclidine (PCP)-like NMDA channel blockers produce partial, if any, substitution. In the present study, rats and squirrel monkeys were trained to discriminate the competitive NMDA antagonist, NPC 17742, from vehicle in a two-lever drug discrimination procedure for food reinforcement. The competitive NMDA antagonists, NPC 12626, SDZ EAA 494 (D-CPPene), and MDL 100,453 fully substituted for NPC 17742 in monkeys or in rats. The relative potencies of these compounds were similar across species. Open channel blockers, PCP and dizocilpine, and the tricyclic antidepressant and low affinity PCP-site ligand, desipramine, produced minimal responding on the NPC 17742-associated lever in rats or monkeys. The glycine-site modulators, (+)-HA-966, ACEA 1021 and milacemide, and the polyamine/NR2B-selective antagonist, eliprodil, also failed to substitute fully for NPC 17742 in rats and monkeys. These data complement and extend results of previous studies which have shown a lack of PCP-like discriminative stimulus effects of these non-competitive NMDA antagonists by further showing that they also do not share discriminative stimulus effects with those produced by many competitive NMDA antagonists. These observations would support a prediction that differences in side-effect profiles should emerge among types of NMDA antagonists.

Preclinical evaluation of the reinforcing and discriminative stimulus effects of agomelatine (S-20098), a melatonin agonist.

Agomelatine (S-20098), an analog of melatonin, has shown promise as a chronobiotic in animal models of sleep phase disorders and is being developed for clinical use. Previous research has shown that the pharmacological profile of melatonin-like drugs overlaps that of gamma-amino butyric acid (GABA) agonists. Given the potential of drugs within the latter class for recreational abuse in humans, evaluation of this potential for melatonin analogs that target similar therapeutic indications is important. In the present study, agomelatine was tested in animal models of the subjective and reinforcing effects of CNS depressant drugs; i.e., diazepam discrimination in rats and IV methohexital self-administration in rhesus monkeys, respectively. Neither agomelatine nor melatonin substituted for diazepam in rats trained to discriminate 2.5 mg/kg diazepam from vehicle. Further, agomelatine was not self-administered by rhesus monkeys. These results suggest that agomelatine would not produce diazepam-like intoxication in humans, nor would it likely be subject to abuse.

Pharmacological specificity of delta 9-tetrahydrocannabinol discrimination in rats.

While many previous studies have shown that a variety of cannabinoids substitute and cross-substitute for delta 9-tetrahydrocannabinol (THC) in drug discrimination procedures, few have systematically examined potential THC-like effects of non-cannabinoid compounds. The purpose of the present study was to delineate further the pharmacological specificity of THC discrimination. Rats were trained to discriminate THC (3.0 mg/kg) from vehicle. Following determination of a dose-effect curve with THC, substitution tests with selected compounds from a variety of pharmacological classes, including l-phenylisopropyl adenosine, dizocilpine, dextromethorphan, clozapine, buspirone, MDL 72222, muscimol, midazolam and chlordiazepoxide, were performed. Whereas THC produced full dose-dependent substitution, substitution tests with non-cannabinoid drugs resulted in less than chance (50%) levels of responding on the THC-appropriate lever, with the exception of (+)-MDMA (2.5 mg/kg, 50%) and diazepam (3.0 mg/kg, 67%). These results are consistent with those of previous studies and suggest that the discriminative stimulus effects of THC exhibit pharmacological specificity.

Reduced sensorimotor reactivity following traumatic brain injury in rats.

The present study examined sensorimotor reactivity in rats following traumatic brain injury (TBI). Moderate injury was induced with midline fluid percussion in some of the rats. Others received identical surgery, but were not injured (sham-injured rats), or received neither surgery nor injury (naive rats). All rats were evaluated in acoustic and/or tactile startle procedures. At 8 days post-injury, the sensorimotor reactivity of TBI rats to acoustic stimuli was severely reduced compared to that of sham-injured rats. This TBI-induced deficit was enduring (> 30 days). In a separate experiment, greater sensorimotor reactivity was observed with tactile (vs. acoustic) stimulation in both TBI and naive rats although startle amplitudes for the TBI rats were lower than control levels for both types of stimuli. These results suggest that sensorimotor reactivity is altered by TBI and that the startle procedure is a promising method for investigation of information processing alterations following TBI.

Effects of site-selective NMDA receptor antagonists in an elevated plus-maze model of anxiety in mice.

NMDA receptor antagonists have been shown to be anxiolytic in animal models of anxiety, although they have not been tested extensively. These compounds bind to several specific sites within the NMDA-receptor complex, including the NMDA site itself, the phencyclidine site, and the strychnine-insensitive glycine site. The purpose of the present study was to examine potential anxiolytic effects of site-selective NMDA receptor antagonists in the elevated plus-maze. Drug-naive albino mice were placed in the center of an elevated maze shaped like a plus sign. Two opposing arms were enclosed by high walls; the crossing arms were open. Following injection with drug or vehicle, the number of entries and time spent in each type of arm were measured during 5-min tests. Analysis of results showed that the benzodiazepine, diazepam, and the competitive NMDA receptor antagonist, NPC 17742 (2R,4R,5S 2-amino-4,5-(1,2-cyclohexyl)-7-phosphono-heptanoic acid), increased number of open arm entries and open arm time. N-Nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor which may interfere with the transduction of NMDA receptor activation, also increased open arm entries and time; however, the magnitude of these increases was small. The phencyclidine-site NMDA receptor antagonist, phencyclidine, increased open arm entries, but failed to significantly increase open arm time. ACEA 1021 (5-nitro-6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione), a putative glycine-site antagonist, had significant effects only on open arm entries at the highest dose tested. These results suggest that NMDA receptor antagonists show promise as potential anxiolytic agents, but that differences among antagonists acting at different cellular sites may be expected.