Evaluation of the effects of opioid agonists and antagonists under a delayed matching-to-sample procedure in pigeons

Abstract

The effects of several opioid agonists and antagonists were examined in pigeons performing under a delayed matching-to-sample procedure. The mu agonists morphine and l-methadone, the kappa agonists U 50,488 and ethylketocyclazocine, and the opioid antagonist naloxone had no effect on the accuracy of responding. These drugs were, however, behaviorally active as evidenced by the dose-dependent decreases in rates of responding associated with their administration. In contrast, the sigma agonists (+) N-allylnormetazocine and phencyclidine decreased the accuracy of responding in a dose-dependent fashion. The relative magnitude of these drug-induced decreases in accuracy were similar across the no delay (0-s), short (2-s), and long (8-s) delay intervals. For these drugs, accuracy-decreasing effects were obtained only at doses that reduced rates of responding. The results of the present investigation parallel those reported in pigeons responding under drug discrimination tasks, in which the discriminative stimulus properties produced by the mu and kappa agonists are similar to each other but distinguishable from those produced by the sigma agonists.

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

“…1 Across a range of doses, the opioid agonists morphine and U50,488 as well as the opioid antagonist naloxone had no consistent effect on accuracy (percent of reinforced runs) under either variant of the FCN schedule. These results confirm and extend those reported previously in pigeons responding under DMTS procedures (McMillan 1981;Picker et al 1987b). These Findings are, however, in contrast to the accuracy-decreasing effects associated with the administration of morphine in pigeons responding under repeated acquisition (Thompson and Moerschbaecher 1981), brightness (West et al 1982) and delayed color (Nielsen and Appel 1983) discrimination procedures.…”

“…Nevertheless, the present investigation supports previous findings which indicate that the effects of mu agonists on accuracy are not due solely to the nondrug level of accuracy or the presence or absence of internal or external stimuli. (Picker et al 1987b). In that study, U50,488 and ethylketocyclazocine had no effect on the accuracy of matching behavior at short, intermediate and long delay intervals.…”

“…Assuming that accuracy levels reflect task difficulty, the level of difficulty under the FCN 8 (see Table 1) was comparable to, and in some instances more difficult than, those obtained in investigations where morphine was reported to decrease accuracy (West et al 1982;Nielsen and Appel 1983). Moreover, in pigeons responding under the DMTS procedure, where accuracy levels were varied by increasing the length of the delay interval, even doses of morphine and U50,488 that decreased response rates failed to alter accuracy (McMillan 1981 ;Picker et al 1987b). It is unlikely, then, that the absence of opioidinduced decreases in accuracy under the FCN and DMTS procedures can be attributed to the failure of these discrimination tasks to achieve sufficient levels of difficulty.…”

“…Accuracy-decreasing effects have, however, been reported in pigeons responding under delayed color discriminations, brightness discriminations and repeated acquisition procedures (Thompson and Moerschbaecher 1981;West et al 1982;Nielsen and Appel 1983). Although systematic investigation of the effects of kappa agonists are lacking, recent reports indicate that ethylketocyclazocine and the highly selective kappa agonist U50,488 do not affect accuracy under the DMTS procedure (Picker et al 1987b).…”

“…Under a delayed matching-to-sample (DMTS) procedure, for example, morphine and/-methadone have no effect on accuracy up to and including doses that markedly decrease response rates (McMillan 1981;Picker et al 1987b). Accuracy-decreasing effects have, however, been reported in pigeons responding under delayed color discriminations, brightness discriminations and repeated acquisition procedures (Thompson and Moerschbaecher 1981;West et al 1982;Nielsen and Appel 1983).…”