Differential interactions between chlordiazepoxide, pentobarbital and benzodiazepine antagonists Ro 15-1788 and CGS 8216 in a drug discrimination procedure

Carisoprodol tolerance and precipitated withdrawal

AIMS

Carisoprodol is a muscle relaxant that acts at the GABA(A) receptor. Concerns about the abuse liability of carisoprodol are increasing, but evidence that carisoprodol produces tolerance and a significant withdrawal syndrome has yet to be established. The purpose of the current study was to determine if repeated administration of carisoprodol produces tolerance and withdrawal signs in a mouse model.

METHODS

Carisoprodol (0, 100, 200, 300, or 500 mg/kg bid, i.p.) was administered to Swiss-Webster mice for 4 days and loss-of-righting reflex was measured 20-30 min following each administration. On the fourth day, bemegride (20 mg/kg), flumazenil (20 mg/kg), or vehicle was administered following carisoprodol and withdrawal signs were measured. Separate groups of mice receiving the same treatment regimen and dose range were tested for spontaneous withdrawal at 6, 12 and 24 h after the last dose of carisoprodol.

RESULTS

The righting reflex was dose-dependently impaired following the first administration of carisoprodol. A 75-100% decrease in the magnitude of the impairment occurred over the four days of exposure, indicating the development of tolerance to the carisoprodol-elicited loss-of-righting reflex. Withdrawal signs were not observed within 24h following spontaneous withdrawal; however, bemegride and flumazenil each precipitated withdrawal within 15-30 min of administration.

CONCLUSIONS

Carisoprodol treatment resulted in tolerance and antagonist-precipitated withdrawal, suggesting it may have an addiction potential similar to that of other long-acting benzodiazepine or barbiturate compounds.

Carisoprodol-mediated modulation of GABAA receptors: in vitro and in vivo studies

Carisoprodol is a frequently prescribed muscle relaxant. In recent years, this drug has been increasingly abused. The effects of carisoprodol have been attributed to its metabolite, meprobamate, a controlled substance that produces sedation via GABA(A) receptors (GABA(A)Rs). Given the structural similarities between carisoprodol and meprobamate, we used electrophysiological and behavioral approaches to investigate whether carisoprodol directly affects GABA(A)R function. In whole-cell patch-clamp studies, carisoprodol allosterically modulated and directly activated human alpha1beta2gamma2 GABA(A)R function in a barbiturate-like manner. At millimolar concentrations, inhibitory effects were apparent. Similar allosteric effects were not observed for homomeric rho1 GABA or glycine alpha1 receptors. In the absence of GABA, carisoprodol produced picrotoxin-sensitive, inward currents that were significantly larger than those produced by meprobamate, suggesting carisoprodol may directly produce GABAergic effects in vivo. When administered to mice via intraperitoneal or oral routes, carisoprodol elicited locomotor depression within 8 to 12 min after injection. Intraperitoneal administration of meprobamate depressed locomotor activity in the same time frame. In drug discrimination studies with carisoprodol-trained rats, the GABAergic ligands pentobarbital, chlordiazepoxide, and meprobamate each substituted for carisoprodol in a dose-dependent manner. In accordance with findings in vitro, the discriminative stimulus effects of carisoprodol were antagonized by a barbiturate antagonist, bemegride, but not by the benzodiazepine site antagonist, flumazenil. The results of our studies in vivo and in vitro collectively suggest the barbiturate-like effects of carisoprodol may not be due solely to its metabolite, meprobamate. Furthermore, the functional traits we have identified probably contribute to the abuse potential of carisoprodol.

Tolerance to the discriminative stimulus effects of ethanol following chronic inhalation exposure to ethanol in C57BL/6J mice

A significant consequence of chronic ethanol (EtOH) exposure is the development of tolerance. The present study was designed to investigate tolerance to the discriminative stimulus properties of EtOH following chronic EtOH exposure. Adult male C57BL/6J mice were trained to discriminate EtOH (1.00 g/kg; i.p.) from saline, using a food-reinforced two-lever operant task. Following acquisition and establishment of criterion discrimination performance, a series of generalization tests were conducted to generate a baseline EtOH dose-response curve with a calculated ED50 dose of 0.42 g/kg. Mice were then placed into control (air) or EtOH inhalation chambers for 64 h. In Experiment 1, discriminative stimulus generalization tests with the EtOH ED50 dose conducted 24 h following chronic EtOH (or air) exposure did not yield significantly different EtOH responding, although a trend towards reduced sensitivity to the EtOH cue (tolerance) was evident. In Experiment 2, a cumulative dosing procedure (ED50=0.37 g/kg) was employed, yielding a baseline EtOH dose-response function with a calculated ED50 dose of 0.37 g/kg. At 24 h following chronic EtOH exposure, re-determination of the EtOH dose-response curve revealed a significant shift to the right, with more than a twofold increase in the ED50 value (ED50=1.09 g/kg) compared to the control air exposure condition (ED50=0.49 g/kg). This apparent tolerance to the EtOH cue dissipated in chronic EtOH-exposed mice tested 48 h following the inhalation treatment (ED50=0.51 g/kg). These results demonstrate tolerance to the discriminative stimulus effects of EtOH in C57BL/6J mice following chronic EtOH exposure in inhalation chambers.

Effects of modulators of N-methyl-D-aspartate receptor-mediated neurotransmission on diazepam discrimination in rats

N-methyl-D-aspartate (NMDA) antagonists share a number of pharmacological effects with GABA(A) agonists, including anxiolytic and anticonvulsant effects. This study evaluated the effects of site-selective NMDA antagonists in rats trained to discriminate the benzodiazepine diazepam from vehicle. As expected, diazepam produced robust discriminative stimulus effects and dose-dependently substituted for the training dose. Mixed results were obtained with competitive NMDA antagonists: whereas NPC 17742 partially substituted for diazepam, SDZ EAA 494 did not elicit responding on the diazepam-associated lever. Other site-selective NMDA antagonists, including the open channel blocker phencyclidine, the glycine-site antagonists ACEA 1021 and MDL 102,288, the polyamine-site antagonist arcaine, and the glutamate release inhibitor riluzole, failed to substitute for diazepam. Agonists at nonbenzodiazepine sites of the GABA(A) receptor complex were also tested for comparison purposes. The barbiturate pentobarbital and the neurosteroid Co 2-1068 partially substituted for diazepam. In contrast, the anticonvulsant carbamazepine failed to substitute even at a dose that substantially reduced response rates. These results suggest that substitution of NMDA antagonists for GABA(A) agonists is dependent upon the site at which the NMDA antagonist binds. Further, they suggest that similarities between the stimulus properties of GABA(A) agonists and NMDA antagonists are at least as strong as similarities among agonists acting at different sites on GABA(A) receptors.

Prenatal protein malnutrition enhances stimulus control by CDP, but not a CDP/THIP combination in rats

In the present study, the effects of prenatal protein malnutrition on stimulus control exerted by the benzodiazepine (BZ), chlordiazepoxide (CDP) and the GABA-A receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) were characterized. The adult, male offspring of female Sprague-Dawley rats fed either low (6% casein) or adequate (25% casein) protein diets 5 weeks prior to mating and throughout pregnancy served as subjects. Subjects were first trained to discriminate CDP (8.0 mg/kg ip) from saline using drug discrimination procedures. Once a criterion level of performance was achieved, generalization tests were performed to lower doses of CDP (4.0, 2.0, 1.0, 0.5 and 0.25 mg/kg) and then to several doses of THIP (10.0, 7.5, 5.6 and 3.2 mg/kg). Lastly, the ability of a single dose of THIP (3.0 mg/kg) to enhance discriminative control by several low doses of CDP (4.0, 2.0, 1.0 and 0.5 mg/kg) was assessed. Although both diet groups acquired the original CDP/saline discrimination at the same rate, malnourished rats exhibited significantly more generalization to low doses of CDP than their well-nourished counterparts. Neither diet group exhibited significant generalization to THIP nor a difference in THIP's ability to enhance the CDP cue. These results suggest that a subject's sensitivity to the stimulus properties of drugs can be selectively modified by prenatal malnutrition.

Influence of training paradigm on specificity of drug mixture discriminations

Generalization to different drugs and drug mixtures has been examined in rats trained to discriminate a mixture of amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) from saline (AND discrimination, n = 8) or to discriminate the same mixture from its component drugs alone (AND-OR discrimination, n = 9). The studies used two-lever operant procedures with a tandem variable interval 1-min fixed-ratio 10 schedule of food reinforcement. There was partial generalization to nicotine and midazolam and no generalization to cocaine, caffeine, or ethanol under AND-discrimination conditions and no generalization to any of these drugs in the AND-OR discrimination. Nicotine or midazolam coadministered with the training doses of pentobarbitone and amphetamine, respectively, produced full generalization in the AND discrimination and partial generalization under AND-OR conditions. Cocaine coadministered with pentobarbitone generalized fully under both procedures, but at larger doses in the AND-OR than in the AND discrimination. Mixtures of either nicotine plus midazolam or caffeine plus ethanol produced very marked generalization under AND-discrimination conditions, but were without significant effect in the AND-OR procedure. The results consistently supported the hypothesis that the AND-OR discrimination procedure increases the specificity of discriminations based on drug mixtures.

Secobarbital in humans discriminating triazolam under two-response and novel-response procedures

Humans were trained to discriminate the benzodiazepine triazolam (0.32 mg/70 kg) from placebo under a two-response (drug vs. placebo) drug discrimination procedure. Dose-effect curves for several drugs were then determined in a crossover design using the two-response procedure and a 'novel-response procedure' that provided a novel-appropriate response for drugs unlike triazolam or placebo. Three subjects were tested with triazolam (0.1-0.32 mg/70 kg), the barbiturate secobarbital (56-177 mg/70 kg), and caffeine (320 and 560 mg/70 kg). Triazolam dose dependently increased triazolam-appropriate responding under both procedures and generally did not occasion novel-appropriate responding under the novel-response procedure. Secobarbital substituted for triazolam in the two-response procedure and dose-dependently increased novel-appropriate responding as well as occasioned some triazolam-appropriate responding in the novel-response procedure. Caffeine generally occasioned placebo-appropriate responding under the two-response procedure and a mix of novel- and placebo-appropriate responding under the novel-response procedure. Triazolam and secobarbital produced qualitatively similar self-reported drug effects. These results suggest that the novel-response procedure for human drug discrimination may enhance the pharmacological selectivity of triazolam- and placebo-appropriate responding.

Discriminative stimulus effects of melatonin in the rat

To provide initial information on the potential mechanisms underlying the discriminative stimulus effects of melatonin, rats were trained to discriminate melatonin (150 mg/kg, IP) from saline in a two-choice discrete-trial avoidance paradigm. Stimulus generalization curves for melatonin were steep; complete generalization with melatonin occurred at 100-150 mg/kg. Triazolam generalized completely with melatonin (n = 7). Flurazepam generalized completely with melatonin in only two out of six rats; however, partial generalization was produced in the remaining four animals. The melatonin-appropriate responding produced by triazolam was antagonized completely (in six out of seven rats) by 0.3-10 mg/kg flumazenil (Ro 15-1788). In contrast, the dose of flumazenil sufficient to block completely the melatonin-like discriminative effects of triazolam failed to block the stimulus effects of the training dose of melatonin. Pentobarbital produced primarily melatonin-appropriate responding, with complete generalization with melatonin in five out of seven rats. Diphenhydramine generalized completely with melatonin in two out of seven rats; however, little or no partial generalization was observed in the remaining five rats. These results suggest that melatonin may produce its discriminative effects through sites on the GABAA-benzodiazepine receptor complex distinct from the benzodiazepine binding sites.

Interactions of diazepam and pentobarbital with RO 15-4513 on intracranial self-stimulation discrimination behavior in rats

Rats implanted with electrodes in the lateral hypothalamus were trained in a discrete trial procedure to make a differential response (right or left lever press) in the presence or absence of brain stimulation. When a high level of accuracy (95% correct) was attained in the discrimination, testing with vehicle, RO 15-4513, diazepam (1.0-10 mg/kg), diazepam plus RO 15-4513 (1.0 mg/kg), pentobarbital (1.0-17.5 mg/kg) and pentobarbital plus RO 15-4513 began. Diazepam, at 10 mg/kg, disrupted the discrimination behavior, and it also decreased the total number of lever-presses and increased the time to complete the session. These effects were blocked by the coadministration of 1.0 mg/kg RO 15-4513. Pentobarbital produced effects similar to those of diazepam, but these effects were only reversed to a limited extent by RO 15-4513. By itself, however, RO 15-4513 also decreased the total number of lever presses and increased the time to complete the session. Results were consistent with our previous findings with alcohol and RO 15-4513, and supported the notion that diazepam and alcohol have some similar effects at the GABA-benzodiazepine receptor complex.

Discriminative stimulus properties of CGS 9896: interactions within the GABA/benzodiazepine receptor complex

Male rats were trained to discriminate the stimulus effects of CGS 9896 (30.0 mg/kg) from its vehicle. Once trained, discriminative performance was observed to be dose-responsive in the 3.75-30.0 mg/kg range and analysis of the dose-response curve generated an ED50 of 6.44 mg/kg. Generalization testing with chlordiazepoxide and pentobarbital produced CGS 9896-appropriate responding, whereas administration of the GABA agonists SL 75 102 resulted in 75% (intermediate) generalization to the CGS 9896 discriminative stimulus. Although full antagonism of the CGS 9896 cue was obtained following administration of Ro15-1788 and pentylenetetrazole, the inverse agonist DMCM failed to provide complete antagonism. These results suggest that the discriminative properties of CGS 9896 are consistent with its activity as a benzodiazepine receptor agonist.

Ro 15-4513 does not antagonize the discriminative stimulus- or rate-depressant effects of ethanol in rats

Female rats, trained to discriminate between IP administered 1.2 g/kg ethanol (ETOH) and the saline vehicle (12 ml/kg), did not press the nondrug associated lever in tests with ETOH (0.9 and 1.2 g/kg) plus the purported amethystic imidazo benzodiazepine Ro 15-4513 (3 and 10 mg/kg) as examined at two intervals after ETOH administrations viz. 7.5 and 15 min. The two doses of Ro 15-4513 were administered 5 min prior to ETOH. Response times were increased in tests with the combination. ETOH in expired air was not different in the two drug conditions, i.e., ETOH singly and together with Ro 15-4513, irrespective of the dose combinations examined. Rats trained to press a bar (FR-10 operant behavior) for sweetened water disclosed increases in the time used to obtain the reinforcer after treatments with ETOH and Ro 15-4513. Thus, Ro 15-4513 did not seem to reverse any of the behaviors examined in this study.

Discriminative stimulus properties of anxiolytic and sedative drugs: pharmacological specificity

In the first set of experiments rats were trained to discriminate a dose of 5 mg/kg chlordiazepoxide from saline. The chlordiazepoxide cue was antagonized by flumazepil (Ro 15-1788) and by CGS 8216, and generalized to a variety of anxiolytic and sedative drugs including the benzodiazepine receptor ligands zopiclone, suriclone, CL 218,872, CGS 9896, and ZK 91296. The novel imidazopyridine hypnotic, zolpidem, which also displaces benzodiazepines from their binding sites, failed to produce high levels of responding on the chlordiazepoxide-associated level except at a dose which greatly reduced rates of lever pressing. In further experiments rats were trained to discriminate a dose of 2 mg/kg zolpidem from saline. This dose produced reductions in response rates but an attempt to establish a lower dose of zolpidem as a discriminative stimulus was largely unsuccessful. Zolpidem-appropriate responding was produced by pentobarbital, chlordiazepoxide, triazolam, CL 218,872, clorazepate, lorazepam, quazepam, and zopiclone but only at doses which reduced response rates. The zolpidem cue was antagonized by flumazepil, CGS 9896, and ZK 91296. While the discriminative stimulus produced by chlordiazepoxide may be related to its anxiolytic action, the zolpidem stimulus is probably more closely associated with sedation. It was also tentatively concluded that the stimulus properties of chlordiazepoxide and zolpidem are produced by activity at different subtypes of benzodiazepine receptors.

Discriminative stimulus properties of pentobarbital and progesterone in male and female rats

Intact male and ovariectomized female rats were trained to discriminate 12 mg/kg pentobarbital from physiological saline. Generalization tests with different doses of pentobarbital did not reveal significant sex differences in the pentobarbital generalization gradient. Different doses of progesterone produced a generalization gradient to pentobarbital in ovariectomized females, but not in intact males. The results of the present experiment thus suggests that systemic administration of progesterone produces a "pentobarbital-like" stimulus in ovariectomized female rats, but not in intact males.

Effects of chlordiazepoxide training dose on the mixed agonist-antagonist properties of benzodiazepine receptor antagonist Ro 15-1788, in a drug discrimination procedure

In experiment 1, rats (n = 12) were trained to discriminate the benzodiazepine (BDZ) compound chlordiazepoxide (CDP, 20 mg/kg, IP) from saline in a two-lever food-reinforced procedure, and subsequently were tested for stimulus control with different doses of CDP, Ro 15-1788 (a proposed BDZ receptor antagonist) and Ro 15-1788 plus 20 mg/kg CDP. Ro 15-1788 (0.63-40 mg/kg) dose-dependently antagonized CDP, and induced predominantly saline appropriate responding when administered alone. Thereafter, the same rats were retrained by progressively decreasing the training dose, to discriminate 2.5 mg/kg CDP from saline, and were tested again with the same compounds. Ro 15-1788 (0.16-40 mg/kg) now failed to antagonize CDP (2.5 mg/kg) and increased the percentage of drug-appropriate responding in a dose-related manner when administered alone. In experiment 2, separate groups of rats (n = 10) were similarly trained to discriminate either 15 or 3 mg/kg CDP from saline. Tests with CDP, Ro 15-1788 and Ro 15-1788 plus CDP (either 15 or 3 mg/kg) yielded similar results to experiment 1, suggesting that the training dose effects on generalization and antagonism of Ro 15-1788 were not affected by the manner in which the lower CDP dose acquired drug stimulus control. It is concluded that mixed agonist-antagonist properties are apparent after variations of the BDZ training dose in a drug discrimination procedure.

Effects of training dose on discrimination and cross-generalization of chlordiazepoxide, pentobarbital and ethanol in the rat

Six groups of rats (N = 8), trained to discriminate chlordiazepoxide (5 or 20 mg/kg), pentobarbital (5 or 15 mg/kg) or ethanol (750 or 1500 mg/kg) from saline in a two-lever food-reinforced procedure, were tested for stimulus generalization with the three drugs. Training drug, but not training dose, affected the extent of generalization to a test drug; symmetrical generalization between chlordiazepoxide and pentobarbital and asymmetrical generalization between chlordiazepoxide and ethanol and between pentobarbital and ethanol was observed. Training dose level affected slope and ED50 of the generalization gradients of training drugs and substitution drugs, discriminative performance, response bias and threshold dose for response suppression. Indices of lever selection and percentage drug-appropriate lever responses yielded similar generalization maxima, slopes and ED50S. The potency of chlordiazepoxide relative to the potency of pentobarbital to induce drug stimulus generalization varied across the experimental groups. The results indicate differences between the discriminative effects of chlordiazepoxide, pentobarbital and ethanol. It is suggested that the discriminative effects of chlordiazepoxide, pentobarbital and ethanol are not based on their response rate modulating effects and that training dose is not a determinant for the extent of cross-generalization between these compounds.