ANALGESIA PRODUCED BY IMMOBILIZATION STRESS AND AN ENKEPHALINASE INHIBITOR IN AMPHIBIANS

The role of endogenous opioids in modulating pain transmission in amph ibians was examined by two methods known to activate endogenous opioid s in mammals. Analgesia was assessed using the acetic acid test in the Northern grass frog, Rana pipiens. One or 2 h of immobilization produ ced a significant analgesia lasting for at least 90 min. Systemic, but not spinal, administration of naloxone before immobilization prevente d the analgesic effects seen in saline-pretreated controls. Spinal adm inistration of the enkephalinase inhibitor, thiorphan, but not bestati n (both at 100 nmol/frog), produced significant analgesia. The analges ic effect of thiorphan was blocked by coadministration of intraspinal naloxone. These data are the first to suggest a role for endogenous op ioid modulation of noxious stimuli in lower vertebrates by examination of stress-induced analgesia and the action of agents that inhibit enk ephalin degradation.