Serotonin N-acetyltransferase (NAT) activity in hen retina and pineal gland: in vivo pharmacological induction at noon and antagonism of the light-evoked suppression at night.

In vivo effective pharmacological procedures are described which markedly increase activity of serotonin N-acetyltransferase (NAT), the key regulatory enzyme in melatonin biosynthesis, during the daytime (in light) and counteract suppressive effects of light on NAI activity at night in the hen retina and pineal gland. Of the tested compounds, and their combinations, the most effective were: "aminophylline + spiroperidol + alpha-methyl-p-tyrosine" for the retina, and "aminophylline + yohimbine (+ alpha-methyl-p-tyrosine)" for the pineal gland. The results give strong support to the concept that the dopaminergic (C(2)-receptor) and noradrenergic (alpha(2)-adrenergic receptor) mechanisms control NAT activity, and melatonin synthesis, in the hen retina and pineal gland, respectively.