Midazolam cue in rats: effects of Ro 15-1788 and picrotoxin

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 properties of a new anxiolytic, DN-2327, in rats

In an operant learning lever-pressing procedure on an FR10 schedule of milk reinforcement, male Wistar rats were trained to discriminate between saline and 3 mg/kg IP DN-2327, a new anxiolytic which acts on benzodiazepine receptors, 3 mg/kg IP diazepam or 15 mg/kg IP pentylenetetrazol (PTZ). More than 80% appropriate lever responding was established after 27, 38 and 44 daily training sessions with DN-2327, diazepam and PTZ, respectively, as the training drug. Although rats trained with DN-2327 dose-dependently generalized to various doses of DN-2327 and diazepam, the cue of DN-2327 was more potent than that of diazepam: ED50 values of DN-2327 and diazepam for stimulus generalization were 0.30 and 0.66 mg/kg, respectively. These animals partially generalized to pentobarbital (1-10 mg/kg) but did not generalize to buspirone (0.1-10 mg/kg). Rats trained with diazepam dose-dependently generalized to various doses of DN-2327, diazepam and pentobarbital with ED50 values of 0.51, 0.47 and 4.5 mg/kg, respectively, but did not generalize to buspirone. In rats trained with PTZ, DN-2327 and diazepam antagonized the discriminative stimulus produced by 15 mg/kg PTZ in a dose-dependent manner with ED50 values of 0.27 and 0.83 mg/kg, respectively, but buspirone neither antagonized nor was able to substitute for the PTZ-induced stimulus. The cue of DN-2327 was antagonized by flumazenil dose-dependently as was that of diazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentobarbital-like discriminative stimulus effects of direct GABA agonists in rats

The discriminative stimulus effects of direct and indirect-acting GABAergic drugs were investigated in rats trained to discriminate 5 mg/kg pentobarbital (PB) from saline under a two-lever fixed ratio (FR) 32 schedule of food reinforcement. PB and diazepam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Muscimol, thiomuscimol and 4,5,6,7-tetrahydroisoxazolo [5,4-c]-pyridin-3-ol (THIP) produced intermediate levels of pentobarbital-lever responding (40-60%), accompanied by dose-dependent decreases in rates of responding following THIP and muscimol administration. The GABAA agonist progabide and its metabolite 4-([(4-chlorophenyl) (5-fluoro-2-hydroxyphenyl)methylene]amino)] butyric acid (SL 75102) also partially substituted for PB, producing means of 39-73% PB-lever responding. The GABAB agonist, baclofen, completely failed to substitute for PB even at doses that decreased rates of responding. These results show that the discriminative stimulus effects of indirect GABAA agonists, PB and diazepam, although similar to one another, differ from those of direct GABAA receptor agonists, which produced only partial substitution for PB. The GABAB agonist, baclofen, can be distinguished by lacking any ability to substitute for PB. These results contribute to a further understanding of the similarities and differences in the behavioral effects of different types of GABA agonists.

Dissociation between the effects of benzodiazepine receptor agonists on behavioral vigilance and responsitivity

The effects of benzodiazepine receptor (BZR) full agonists chlordiazepoxide and midazolam, and the partial agonist beta-carboline ZK 91,296 on the rat's performance in a simple reaction time paradigm were examined. This task required the animals to respond to a rarely and unpredictably occurring brief (50 ms) visual stimulus. Non-parametric measures of signal sensitivity and response bias derived from signal-detection theory were used as a basis for the dissociation between the effects of these drugs on attentional abilities and general responsivity. The dose-dependent effects of midazolam (0.1-3.13 mg/kg) on signal sensitivity and general responsivity occurred in parallel. In contrast, the effects of chlordiazepoxide (1.56-12.5 mg/kg) on signal sensitivity were largely independent from effects on response bias. The partial agonist ZK 91,296 (0.39-25 mg/kg) in general had little effect on performance. The effects of the highest doses of chlordiazepoxide and midazolam were reversed by the co-administration of the BZR antagonist Ro15-1788 (15 mg/kg). Additionally, extension of the stimulus presentation time to 500 ms decreased the magnitude of the effect of chlordiazepoxide on signal sensitivity. These results support the hypothesis that BZR agonist-induced disruption of attentional abilities is not necessarily confounded by effects on general responsivity or sedation, and thus may represent a discrete pharmacological property of BZR-agonists.

Discriminative stimulus properties of the benzodiazepine receptor antagonist flumazenil

Rats were trained to discriminate the stimulus properties of the benzodiazepine (BZ) receptor antagonist flumazenil using a conditioned taste aversion procedure. On drug trials, fluid-restricted rats were injected with flumazenil (32 mg/kg), given access to a 0.25% saccharin solution for 30 min, and injected with LiCl (1.8 mEq/kg IP). On saline trials, injections of saline bracketed the period of saccharin consumption. Acquisition of the discriminated taste aversion, as measured by differential effects on drinking between saline and drug trials, developed after only five pairings of flumazenil with the LiCl injections. Flumazenil did not alter saccharin consumption in unconditioned controls (N = 9) that never received LiCl. The discrimination was also measured by flumazenil's ability to reduce the preference for saccharin over tap water using two-bottle choice tests. Flumazenil demonstrated dose-dependent generalization upon decreasing the training dose as low as 1 mg/kg. Two other BZ receptor antagonists of different chemical structure, CGS 8216 and ZK 93426, substituted completely for the flumazenil stimulus. Partial generalization was exhibited to the partial inverse agonists FG 7142 and beta-CCE, while the full inverse agonists DMCM and PTZ failed to substitute for the flumazenil stimulus. The BZ receptor agonists diazepam and alprazolam failed to substitute for the flumazenil stimulus, although partial generalization was shown with CDP. The results suggest that the BZ receptor antagonist flumazenil may produce intrinsic discriminative stimulus effects that are independent from those of BZ receptor agonists or inverse agonists.

Comparison of the discriminative stimulus effects of midazolam after intracranial and peripheral administration in the rat

Rats were trained in a two-lever drug discrimination paradigm to discriminate midazolam (0.32 mg/kg, i.p. or 1.0 mg/kg, i.p.) from the no-drug condition. After completion of i.p. and s.c. midazolam generalization gradients (0.032-1.0 mg/kg), rats were surgically implanted with unilateral cannulae into the lateral ventricles. Intracerebroventricular (i.c.v.) doses of 1.1-44.2 micrograms midazolam were delivered to unrestrained rats. Midazolam produced dose-dependent increases in drug-appropriate responding by all three routes of administration, but was 2.4- to 4.3-fold more potent when given i.c.v. than when given s.c. or i.p. Midazolam, over the dose range tested, did not produce substantial decreases in response rate by any route of administration. The discriminative-stimulus effect of i.c.v. midazolam was blocked by peripherally administered flumazenil, and such antagonism was surmounted by a 2- to 5-fold increase in the i.c.v. midazolam dose. Taken together, these data suggest that the discriminative-stimulus effects of midazolam are mediated via central benzodiazepine (BZ) receptors.

Motor performance decrement by midazolam: antagonism by Ro 15-1788 and CGS 8216

Rats were trained in a fine motor control performance that required operation by a paw of a force transducer so that it remained between upper and lower limits of a force band for a continuous 1.5-sec period to deliver each food pellet. Acute doses of midazolam (0.75-3.0 mg/kg, SC) impaired indices of motor performance in a graded, dose-related fashion. When administered alone, Ro 15-1788 (0.1-5.0 mg/kg, SC) had no effect on motor behavior while CGS 8216 (0.5 and 1.0 mg/kg, IP) alone had small effects. In general, the motor performance decrements produced by midazolam were antagonized in a dose-related fashion by both Ro 15-1788 and CGS 8216.

Discriminative stimulus properties of midazolam: comparison with other benzodiazepines

Rats (N = 12) were trained to discriminate midazolam (1 mg/kg, IP) from vehicle in a food reinforced operant conditioning procedure. Midazolam, flunitrazepam, diazepam, chlordiazepoxide and pentobarbital showed dose-dependent substitution for midazolam. Buspirone and Ro 15-1788 did not substitute for midazolam. The midazolam cue was dose-dependently antagonized by Ro 15-1788. In rats (N = 12) trained to discriminate chlordiazepoxide (3 mg/kg, IP) from vehicle midazolam, flunitrazepam, diazepam and chlordiazepoxide substituted completely and dose dependently for chlordiazepoxide. The relative potency of chlordiazepoxide and diazepam was three times less in the midazolam-trained animals than in the chlordiazepoxide-trained animals. Response rate and latency data further support the main finding that the midazolam cue is similar, but not identical to the cue of classical benzodiazepines.

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.

Cueing effects of anxiolytic benzodiazepines (DZP and Ro 11-3128): stereospecificity and antagonism by the convulsant benzodiazepine Ro 5-3663

Two groups of gerbils were trained in a T-maze to discriminate between the vehicle condition (4 ml/kg) and either of the benzodiazepine (BDZ) agonists diazepam (DZP) and Ro 11-3128; administration (5.6 mg/kg) was IP 5 min prior to training onset. Once trained, novel doses and drugs were assessed in test sessions interposed between the regular training days. A dose-related generalization effect occurred with both compounds (range 0.1-5.6 mg/kg), the effect being similar at both the 5 and 15 min test intervals; the two intervals were evaluated after a single injection. The lack of generalization of Ro 11-3624 (range 5.6-56 mg/kg) indicates a stereoisomeric separation of BDZ agonist activity. Ro 5-4864 (range 17.5-56 mg/kg), an agent chemically/structurally related to DZP, did not produce DZP responding at either of the two test inverals; clear-cut convulsant activity occurred at the 15 min interval. The convulsant BDZ compound Ro 5-3663 (3 and 10 mg/kg) antagonized the DZP stimulus irrespective of whether Ro 5-3663 was given either prior to, simultaneously with, or shortly after the DZP injection.

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 chlordiazepoxide and zolpidem. Agonist and antagonist effects of CGS 9896 and ZK 91296

In previous studies the effects of CGS 9896, a pyrazoloquinoline ligand at benzodiazepine receptors, in rats trained to discriminate benzodiazepines from vehicle, have been variable. The present experiment confirmed that this compound produced responding on the drug-lever in rats trained to discriminate 5 mg/kg of chlordiazepoxide from saline, and showed that CGS 9896 did not antagonise the effect of chlordiazepoxide in this test. In contrast, CGS 9896 antagonised the stimulus properties of zolpidem (2 mg/kg), a non-benzodiazepine hypnotic, which displaces benzodiazepines from their binding sites. The drug CGS 9896 also antagonised responding on the drug-lever produced by chlordiazepoxide in rats trained with zolpidem. The beta-carboline, ZK 91296, produced effects similar to those of CGS 9896, giving rise to responding on the drug-lever in rats trained with chlordiazepoxide and antagonising the zolpidem cue. These results demonstrate the mixed agonist-antagonist effects of CGS 9896 and ZK 91296 and suggest that the stimulus properties of chlordiazepoxide and zolpidem may be mediated by different sub-types of benzodiazepine receptors.

Midazolam cue in rats: effects of drugs acting on GABA and 5-hydroxytryptamine systems, anticonvulsants and sedatives

The discriminative stimulus effect of midazolam, a short-acting benzodiazepine, was used for testing the effects of related drugs including agents thought to act at different sites in the proposed benzodiazepine receptor complex. Rats were trained in a standard two- bar operant conditioning procedure with food reinforcers delivered on a tandem schedule. The 0.4 mg/kg dose of midazolam used for training was well discriminated, typically yielding about 95% correct responding. There was no generalization to the GABA agonists muscimol and THIP, to the 5-HT antagonists cyproheptadine and methergoline, to buspirone, CGS 9896, ethanol, Ro 5-4864, promethazine, phenytoin sodium or sodium valproate. Muscimol and THIP also failed to potentiate the effects of midazolam. The GABA antagonist bicuculline weakly attenuated the discriminative effect of midazolam without impairing generalization to pentobarbitone, whereas the benzodiazepine inverse agonist FG 7142 did not attenuate the effect of midazolam. The results provide additional evidence for the notable specificity of the midazolam cue but do little to link the behavioural effects of benzodiazepines to GABA or 5- HT systems. Perhaps the potency, efficacy or selectivity of the GABA agonists was inadequate to produce the expected results. Only the effects of bicuculline, and those reported previously for picrotoxin, provided some support for the hypothesis that midazolam cue is mediated by the GABA system.