Differential effects of opioid and nonopioid analgesics on conditional discriminations in pigeons.

Under the fixed-consecutive-number schedule (FCN), pigeons were reinforced for responding eight or more times on one response key (work key), and then responding once on a second response key. In one component of this schedule, an external stimulus signalled the completion of the response requirement on the work key (FCN 8-SD), whereas no stimulus change was programmed under the other (FCN 8). Across a range of doses, the mu opioid agonist morphine, the kappa opioid agonist U50,488 and the opioid antagonist naloxone had no consistent effect on accuracy under either FCN schedule. Naloxone and accuracy under either FCN schedule. Naloxone and U50,488 produced a general flattening of the conditional probability functions by decreasing the conditional probability of response runs exceeding the minimum response requirement of eight consecutive responses on the work key. The sigma agonists phencyclidine and (+)N-allylnormetazocine and the nonopioid analgesics clonidine and l-nantradol produced large decreases in accuracy under the FCN 8 and small decreases under the FCN 8-SD. With the exception of (+)N-allylnormetazocine, these drugs consistently increased the conditional probability of responses runs shorter than the minimum response requirement on the work key. These findings indicate that the accuracy-altering effects of some opioid and nonopioid analgesics depend in part on the type of discrimination task.