Changes in tyrosine hydroxylase activity and melatonin synthesis in rat pineal glands throughout 72 hours of incubation.

Time-dependent modifications of N-acetyltransferase (NAT) activity, melatonin content, and tyrosine hydroxylase (TH) activity within the isolated rat pineal over a 72 h of incubation period were studied. Both pineal NAT activity and melatonin content displayed similar decreases after 12 h of incubation, and reached a maximal reduction of 82% or 93%, respectively, after 48 h. However, a maximal decrease of pineal TH activity was only 47% after 72 h of incubation. Neither the nuclear area of pinealocytes, nor lactate dehydrogenase (LDH) activity in the incubation medium showed any significant variation dependent on the incubation time. Treatment with isoproterenol caused an expected elevation of NAT activity and melatonin content but was unable to modify TH activity. Since a significant degree of pineal TH activity remained following 3 days of incubation, a non-sympathetic source of TH within the rat pineal gland is suggested.