Analgesic effect of etorphine in rats with selective depletions of brain monoamines.

The analgesic effect of etorphine was compared with that of morphine in rats with electrolytic lesions of the nucleus medianus raphe or injected intraventricularly with 6-hydroxydopamine. The effect of both compounds was markedly reduced in animals with raphe lesions, but not significantly modified in those receiving 6-hydroxydopamine. Etorphine and morphine significantly increased the forebrain levels of 5-hydroxyindolacetic acid when administered subcutaneously at doses of 5mg/kg and 2 microgram/kg, respectively. Neither drug significantly affected the forebrain levels of monoamines. It is concluded that, as for morphine the integrity of the serotoninergic system in the brain is important for the full analgesic effect of etorphine.