Drug discrimination: stimulus control during repeated testing in extinction

Rats were trained, under a two-lever drug-discrimination procedure, to respond differentially depending upon whether lorazepam (1.0 mg/kg) or no injection had been administered before the session. Responses on the appropriate lever produced a food pellet under a modified fixed-ratio (FR) 10 schedule, in which the 10 responses had to be emitted consecutively. In reinforcement tests, completing an FR 10 on either lever produced a pellet. In extinction tests, stimulus changes paired with reinforcement occurred but no pellet was delivered. Training sessions were conducted between test sessions. Each of four extinction phases consisted of six tests preceded by one stimulus (e.g., lorazepam). Repeated exposures to extinction reduced response rates for all rats, but stimulus control, as inferred from either percentage of total responses or percentage of total FR 10s on the drug-appropriate lever, remained high. The percentage of total FR 10s measure was less subject to skewing under low-rate conditions than was the percentage of total responses measure and provided an evaluation of stimulus control in terms of meeting the consecutive response contingency. These results demonstrate a level of independence between response rate and stimulus control in drug discrimination, which has positive implications for the validity of interpreting discriminative effects of novel test conditions in well-trained animals, even when overall response rates are low.