PINEAL MELATONIN IN RATS - SUPPRESSION BY THE SELECTIVE ALPHA(2)-ADRENOCEPTOR AGONIST MEDETOMIDINE

This study was done to clarify the role of alpha(2)-adrenoceptors in t he regulation of pineal melatonin synthesis. A selective alpha(2)-adre noceptor agonist, medetomidine, and antagonist, atipamezole, were inje cted subcutaneously into rats and their pineal melatonin contents were measured by radioimmunoassay. Medetomidine (120 mu g/kg) suppressed m elatonin at night to a similar extent during the rising and descending phase of melatonin synthesis, but it did not affect the daytime level . A dose of 12 mu g/kg was ineffective; doses of 30-180 mu g/kg suppre ssed nocturnal melatonin levels close to the daytime levels. Significa nt suppression was reached within 15 min and the effect started to fad e 3 h after the injection (120 mu g/kg). At midday, medetomidine did n ot inhibit isoproterenol-stimulated synthesis of melatonin. Atipamezol e (0.4 or 1.2 mp/kg) had no effect alone, but it counteracted the mede tomidine-induced suppression. The effects of alpha(2)-adrenoceptor lig ands on melatonin synthesis depend on the rime of day and/or on the ac tivity of the pineal sympathetic nerves.