CLONIDINE IN-VIVO MIMICS THE ACUTE SUPPRESSIVE BUT NOT THE PHASE-SHIFTING EFFECTS OF LIGHT ON CIRCADIAN-RHYTHM OF SEROTONIN N-ACETYLTRANSFERASE ACTIVITY IN CHICK PINEAL-GLAND

Comparative in vivo studies on effects of pulses of light and clonidin e, a selective agonist of alpha(2)-adrenergic receptors, on the circad ian rhythm of serotonin N-acetyltransferase (NAT) activity in chick pi neal gland were performed. Six-hour pulses of white light caused an ac ute suppression of the nocturnal NAT activity and phase-dependent phas e shifts of the circadian rhythm of the enzyme activity relative to co ntrols. Systemic administration of clonidine acutely, suppressed NAT a ctivity of chick pineal gland, but did not affect the phase of subsequ ent cycles in constant darkness. These results give further support to the concept based on in vitro studies that ol,-adrenergic receptors a re involved in regulation of melatonin biosynthesis in chick pineal gl and by a mechanism distal to the pacemaker that generates the circadia n melatonin rhythm.