Effects of heroin, methadone, LAAM and cyclazocine on acquisition and performance of response sequences in monkeys.

In each of three components of a multiple schedule, monkeys were required to emit a different sequence of four responses in a predetermined order on four levels. Sequence completions produced food on a fixed-ratio schedule. Errors produced a brief timeout. One component of the multiple schedule was a repeated-acquisition task where the four-response sequence changed each session (learning). The second component of the multiple schedule was also a repeated-acquisition task, but acquisition was supported through the use of a stimulus fading procedure (faded learning). In a third component of the multiple schedule, the sequence of responses remained the same from session to session (performance). At high doses, each drug tested produced essentially the same effect. In all three components, response rate was substantially decreased and percent errors increased. At lower doses, however, their effects differed. Heroin and methadone produced dose-dependent sporadic periods of pausing, but had little or no effect on the accuracy of responding. LAAM also produced sporadic periods of pausing, but its effects on accuracy were variable. In contrast, cyclazocine produced no such pauses in responding but rather decreased the local rates of correct responding in a dose-related manner. These same doses of cyclazocine increased percent errors in the learning component, but not in the faded learning or performance components. The results are generally consistent with the view that putative mu opioid agonists do not affect the accuracy of a discrimination in monkeys except at those doses which produce a substantial decrease in the overall rate of responding.