The effects of a number of short-term exogenous stimuli on pineal serotonin-N-acetyltransferase activity in rats.

The present study deals with the question as to what extent the sympathetically innervated rat pineal gland is affected by a number of short-term exogenous stimuli given during day-time, as assessed by measuring pineal serotonin-N-acetyltransferase activity (NAT) which is directly proportional to melatonin formation. In male Sprague-Dawley rats kept under LD 12:12 pineal NAT was statistically significantly depressed by physical immobilization for 2 hours, swimming for 15 min in water of 10 and 30 degrees C, exposure for 2 hours to cold (5 degrees C) or heat (40 degrees C), noise (90 db) for 2 hours and hunger for 17 hours. An increase in NAT was noted after swimming for 15 min in water of 20 degrees C. No effect was detectable after 17 hours of thirst or hunger combined with thirst and in one of 2 experiments involving exposure to heat (40 degrees C, 2 hours) and insulin-induced hypoglycemia. In animals kept under continuous illumination for 48 hours, immobilization resulted in a slightly smaller decrease than under LD 12:12 and insulin-induced hypoglycemia led to a striking increase of NAT. As the changes in pineal NAT are brought about by rather strong exogenous stimuli it is suspected that the rat pineal gland during day-time is not very susceptible to ambient factors of normal range.