Opioid drugs and timeout from avoidance.

The effects of µ agonists fentanyl, methadone and morphine and kappa agonist U50,488 on behavior maintained by negative reinforcement were determined. Rats were trained on concurrent schedules in which pressing one lever postponed shock on a Sidman avoidance schedule and pressing the other lever produced signaled periods of timeout from avoidance on variable-ratio schedules. All of the µ agonists decreased responding maintained by timeout from avoidance at doses that increased or did not affect avoidance rates. The kappa agonist U50,488 decreased response rates in some rats, but increased responding in others. In no case was a selective reduction in responding on the timeout lever produced by U50,488. Thus, the previously reported selective decreases in timeout responding by morphine are also produced by µ agonists fentanyl and methadone, but not by kappa agonist U50, 488.