SPINAL ADMINISTRATION OF ADRENERGIC AGENTS PRODUCES ANALGESIA IN AMPHIBIANS

Direct intraspinal injection of the catecholamines epinephrine and nor epinephrine, and the alpha-adrenergic agents dexmedetomidine and cloni dine, produced a dose-dependent elevation of pain thresholds in the No rthern grass frog, Rana pipiens. Significant analgesic effects were no ted for at least 4 h. The analgesic effect of intraspinal dexmedetomid ine or epinephrine was blocked by systemic pretreatment with the alpha (2)-adrenoceptor antagonists, yohimbine and atipamezole, but not with the alpha(2)-adrenoceptor antagonist, prazosin. Dose-response analyses showed that dexmedetomidine, epinephrine, norepinephrine had similar analgesic potencies, but clonidine was significantly less potent. Anal gesia was observed without accompanying motor or sedative effects. The se results suggest that alpha(2)-adrenoceptor mechanisms ab which medi ate analgesia may have evolved early in vertebrate evolution and that descending epinephrine-containing fibers in the amphibian nervous syst em may be the source of endogenous catecholamines regulating nocicepti ve sensitivity in the amphibian spinal cord.