Effects of mu, kappa and sigma opioids on fixed consecutive number responding in rats.

Responding in rats was maintained under a fixed consecutive number 20 schedule. Under this schedule, at least 20 consecutive responses were required on one lever before a response on a second lever produced food. Morphine, buprenorphine, ethylketocyclazocine (ECK), N-allylnormetazocine (NANM) and d-cyclazocine all caused dose-dependent decreases in response rate. With the exception of buprenorphine and EKC, each drug also produced a decrease in the percent of reinforced runs. Differences among the drugs were more apparent, however, on the basis of the conditional probability of switching to the second lever after any given run length on the first lever. Morphine increased the probability of premature switching and decreased the probability of switching after run lengths greater than 20. Buprenorphine decreased the probability of switching at all run lengths and EKC produced occasional increases in premature switching. In sharp contrast to the other opioids tested, NANM and d-cyclazocine generally increased the probability of switching at all run lengths. Thus, it appears that the fixed consecutive number schedule may be a sensitive procedure for distinguishing among the behavioral effects of various opioid agonists.