Discriminative stimulus effects of pentobarbital in rhesus monkeys: tests of stimulus generalization and duration of action.

Rhesus monkeys were trained to emit 20 or 30 consecutive responses on one lever following an IM injection of pentobarbital (10 or 18 mg/kg) and the same number of consecutive responses on another lever following an injection of saline. The required number of correct consecutive responses in both cases resulted in food delivery. When responding was reliably under the control of the presession injection, the ability of a variety of other compounds to produce pentobarbital-appropriate responding was examined. Diazepam, clobazam, methohexital, pentobarbital, and phenobarbital, given 10 or 20 min before the session, produced dose-related pentobarbital-appropriate responding in each monkey. Ethylketazocine and dextromethorphan produced responding primarily on the saline-appropriate lever, whereas codeine, cyclazocine, dextrorphan, and ketamine resulted in responding that was, on the average, intermediate between that appropriate for pentobarbital and that appropriate for saline. When tested at various times after their injection, methohexital (3.2 mg/kg) and pentobarbital (10 mg/kg) produced pentobarbital-appropriate responding within 10 min. Barbital (56 mg/kg) resulted in pentobarbital-appropriate responding only if at least 1 h intervened between the injection and the experimental session. The discriminative effects of methohexital, pentobarbital, and barbital lasted approximately 20-60, 120-240, and 480-720 min, respectively. The time-course of the discriminative stimulus effects of barbiturates in the rhesus monkey appears to parallel closely other pharmacological actions of these compounds.