A history of postponing shock does not appear to alter the discriminative stimulus effects of cocaine

Previous research has shown that the rate of punished lever pressing of monkeys is typically decreased by cocaine administration. However, cocaine increases punished responding in monkeys with a history of responding maintained by the postponement of shock presentation. This raises the question of whether other behavioral effects of cocaine differ following a history of postponing shock. The present experiment examined whether a history of postponing shock alters the discriminative stimulus effects of cocaine. Three squirrel monkeys were trained to discriminate cocaine (0.56 mg/kg, intramuscular) from saline. Presses on the left lever produced food following saline injections whereas presses on the right lever were reinforced following administration of cocaine. Occasional test sessions were conducted in which cocaine (0.1-0.56 mg/kg), midazolam (0.03-0.56 mg/kg) or pentobarbital (0.3-5.6 mg/kg) was injected prior to the session and responding on either lever was reinforced. Discrimination training was discontinued during a second experimental phase in which responding was maintained by shock postponement. Pulling a chain postponed mild shocks for 25 s, whereas shocks occurred every 5 s in the absence of responding. Next, the drug discrimination dose-response curves were redetermined. The dose-response curves for all drugs before and after the shock postponement history were similar. This outcome suggests that the influence of a history of shock postponement is specific to punished responding.