title: A study of the quantal (all-or-none) change in reflex latency produced by opiate analgesics.
abstract: 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 sec) 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 increasing 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-flick was measured. We hypothesize that the analgesic behavior described partly defines the operating characteristics of an intrinsic endorphin-mediated analgesia system which mediates narcotic suppression of withdrawal reflexes.
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009433:Neural Inhibition / Q000187:drug effects; D009619:Nociceptors / Q000187:drug effects*; D010423:Pentazocine / Q000494:pharmacology*; D051381:Rats; D011957:Receptors, Opioid / Q000187:drug effects; D012018:Reflex / Q000187:drug effects*
chemical_list: D011957:Receptors, Opioid; D009270:Naloxone; D009020:Morphine; D010423:Pentazocine