Evidence for central a -adrenergic modulation of rat pineal melatonin

This study was performed to distinguish central and peripheral a -adrenoceptors in the inhibition of rat pineal melatonin synthesis. The 2 rats received lipoor hydrophilic a -adrenoceptor ligand injections at middark; after 1 or 2 h the pineal melatonin contents were 2 measured. The lipophilic agonist medetomidine (100 mg/kg s.c.) suppressed the melatonin contents significantly, while the hydrophilic agonists ST-91 and p-aminoclonidine (10 or 100 mg/kg i.v.) did not. The suppression by medetomidine was counteracted by the lipophilic antagonist yohimbine (0.3–3.0 mg/kg i.p.) but not by the hydrophilic antagonist L-659,066 (1–10 mg/kg i.v.). In conclusion, the suppression of nocturnal melatonin synthesis by a -adrenoceptor agonists is mainly of central origin.  2000 Elsevier Science B.V. 2 All rights reserved. Theme: Endocrine and autonomic regulation Topic: Neuroendocrine regulation: other