Antinociceptive and behavioral effects of leu-enkephalin (LENK) or D-Ala2-met-enkephalinamide (DAMEA) in rats after central chemical sympathectomy or serotoninectomy.

In rats with central chemical sympathectomy induced by two injections into the lateral brain ventricle (icv) of 250 micrograms of 6-hydroxydopamine or with central chemical serotoninectomy induced by 75 micrograms of 5,6-dihydroxytryptamine icv the antinociceptive and behavioral effects of LENK (200 micrograms icv) or DAMEA (10 or 20 micrograms icv) were examined. The chemical lesion of central noradrenaline or dopamine neurons or specific lesion of serotonin neurons abolished antinociceptive effects of LENK and DAMEA. Both peptides inhibited exploratory activity of rats. Central chemical serotoninectomy but not sympathectomy reversed some behavioral effects induced by LENK and DAMEA. These results suggest modulatory influence of the central noradrenaline and serotonin neurons in the antinociceptive mechanism of action of enkephalins and the participation of the central serotonin neurons on the behavior of rats induced by enkephalins.