Barbiturate reinforced responding in rhesus monkeys: comparisons of drugs with different durations of action

The rate and pattern of pressing a lever was studied in rhesus monkeys under conditions where each press led to the intravenous injection of a barbiturate or of saline. The reinforcing effect of the barbiturate was indicated by higher rates of responding on the lever that delivered drug than on the lever than did not and by lower rates of responding when saline was delivered instead of the barbiturate. Responding increased and was maintained by the following ranges of dose/injection: barbital (2.5-10.0 mg/kg), pentobarbital (0.25-4.0 mg/kg), amobarbital (0.25-4.0 mg/kg), thiopental (0.50-4.0 mg/kg) and methohexital (0.125-2.0 mg/kg). Responses rates were inversely related to the dose delivered with each injection for each of the barbiturates over these ranges. Within 3 hour access periods, injections tended to occur in bursts followed by pauses.