Brain Research, 201 (1980) 129-141 129 © Elsevier/North-Holland Biomedical Press A STUDY OF THE QUANTAL (ALL-OR-NONE) CHANGE IN REFLEX LATENCY PRODUCED BY OPIATE ANALGESICS JON D. LEVINE*, DENNIS T. MURPHY, DAVID SEIDENWURM, ANNE CORTEZ and HOWARD L. FIELDS Departments of Neurology and Physiology, University of California, San Francisco, Calif. 94143 ( u.s.,4.) (Accepted June 5th, 1980) Key words: pain -- opiates -- reflexes-- analgesia SUMMARY The properties of opiate-induced changes of tail-flick latency were studied in the rat. (1) Morphine and pentazocine produced a stepwise increase in latency which rose from near baseline to cut-off (usually greater than 20 see) in less than 30 sec. Abrupt return to pre-treatment latencies was observed either spontaneously or when the rat was back-titrated with the narcotic antagonist naloxone. (2) The proportion of rats showing this stepwise change increased with in- creasing dose; however, the step itself was independent of dose. The same step was produced by a slow, constant infusion of morphine but was not produced by ice-water stress or barbiturate administration. (3) Increasing heat intensity to the tail shortened the baseline latency and raised the mean dose of morphine required to produce a step latency increase. (4) A step increase in latency was also observed when paw withdrawal instead of tail-fl{ck was measured. We hypothesize that the analgesic behavior described partly defines the oper- ating characterisitics of art intrinsic endorphin-mediated analgesia system which mediates narcotic suppression of withdrawal reflexes. INTRODUCTION Opiates are the most potent analgesic agents presently in use. Great progress has * To whom all correspondence is to be addressed at the Department of Neurology.