N-acetyltransferase activity in the pineal glands of immature rats (12--14 days old) shows a spontaneous rise and fall when the glands are placed in organ culture. The peak of the 28-fold change occurs 5--16 hr after the cultures are initiated. This observation can be interpreted in two ways: (1) the pup pineal gland is responding to norepinephrine released when nerve endings degenerate in culture, or (2) the pup pineal gland has an innate ability which is responsible. Whatever the mechanism, the phenomenon is associated with the development of pineal gland function since pineal glands of adult rats do not show a spontaneous rise and fall in identical experiments.
In rats, the pineal gland has a rhythm in the activity of the enzyme, N-acetyltransferase (NAT), which is thought responsible for daily cycles of melatonin synthesis. Neonatal rat pineal glands, but not those of adult rats, have a single cycle that is observed in vitro during the first day of organ culture. The neural regulation of the cycle was investigated using neonatal rats with adult rats used for comparison. Prior treatment of rat pups with constant light did not abolish the cycle in vitro though it did abolish the in vivo rhythm. Removal of the superior cervical ganglia did not abolish the in vivo rhythm that was measured the first day after surgery, but ablation of the ganglia did abolish the rhythm if several days or more elapsed after surgery. Extirpation of the superior cervical ganglia abolished the in vitro NAT cycle in pup pineal glands as did the pharmacological equivalent, injection of 6-hydroxydopamine. Propranolol, a beta blocking agent, prevented the occurrence of the cycle in vitro.
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