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 clonidine, a selective agonist of alpha 2-adrenergic receptors, on the circadian rhythm of serotonin N-acetyltransferase (NAT) activity in chick pineal gland were performed. Six-hour pulses of white light caused an acute suppression of the nocturnal NAT activity and phase-dependent phase shifts of the circadian rhythm of the enzyme activity relative to controls. Systemic administration of clonidine acutely suppressed NAT activity of chick pineal gland, but did not affect the phase of subsequent cycles in constant darkness. These results give further support to the concept based on in vitro studies that alpha 2-adrenergic receptors are involved in regulation of melatonin biosynthesis in chick pineal gland by a mechanism distal to the pacemaker that generates the circadian melatonin rhythm.