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

Abstract

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 °C, exposure for 2 hours to cold (5 °C) or heat (40 °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 °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 °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.