Modulation of a (+)amphetamine discriminative stimulus in rats by 8-hydroxy-2-(N,N-di-n-propylamino)tetralin (8-OH DPAT).

It is well established that the discriminative stimulus (DS) effect of amphetamine involves a dopaminergic and/or noradrenergic mechanism. These catecholamines can be modulated by the 5-HT(1A) serotonin receptor agonist 8-hydroxy-2-(N,N-di-n-propylamino)tetralin (8-OH DPAT). The present study was conducted to determine whether 8-OH DPAT could influence the DS effects of (+)amphetamine. Administration of 8-OH DPAT doses to Sprague-Dawley rats trained to discriminate 1 mg/kg of (+)amphetamine (ED(50)=0.33 mg/kg) using a two-lever operant paradigm (VI-15 s schedule of reinforcement for appetitive reward) failed to result in stimulus generalization when administered alone, and failed to antagonize the stimulus effect when administered in combination with the training dose of (+)amphetamine. However, administration of 8-OH DPAT doses that produced saline-like responding (i.e., 0.01-0.1 mg/kg; <20% amphetamine-appropriate responding) in combination with the ED(50) dose of (+)amphetamine resulted in the animals' making a progressively greater number of responses on the drug-appropriate lever such that a combination of 0.1 mg/kg of 8-OH DPAT plus (+)amphetamine (0.33 mg/kg) elicited 91% (+)amphetamine-appropriate responding. In a separate study, administration of (+)amphetamine doses in combination with fixed doses of 8-OH DPAT (either 0.01 or 0.1 mg/kg) resulted in an apparent leftward shift of the dose-response curve. The results indicate that (+)amphetamine can be more effective as a discriminative stimulus in the presence of 8-OH DPAT than in its absence.