Endogenous Oscillator and Photoreceptor for Serotonin N-Acetyltransferase Rhythm in Chicken Pineal Gland
The pineal gland of various species of vertebrates synthesizes a specific indoleamine, melatonin, in a circadian fashion with a high production during nighttime and a low content during daytime. This circadian rhythm of melatonin synthesis is regulated by the change in the activity of serotonin N-acetyltransferase that converts serotonin to N-acetylserotonin (Weissbach et al. 1961). The day-night changes of N-acetyltransferase activity in the pineal gland persist in blinded rats (Deguchi 1975) and in blinded chickens (Binkley and Geller 1975), indicating that the enzyme rhythm is generated by an endogenous circadian oscillator. The neural mechanism that controls the circadian rhythm of N-acetyltransferase activity has been well established in rat pineal gland (Klein and Weiler 1970, Klein et al. 1971, Deguchi and Axelrod 1972a, Moore and Klein 1974). The neural impulse originating in the suprachiasmatic nuclei in hypothalamus is transmitted through superior cervical ganglion to the pineal gland. The nerve endings in the pineal gland release more transmitter norepinephrine at night compared to daytime, which increases the intracellular cyclic AMP levels and stimulates the synthesis of N-acetyltransferase molecules in pinealocytes. A similar mechanism has been recently proposed for the pineal glands of other mammals.
KeywordsCircadian Rhythm Pineal Gland Circadian Oscillator Pineal Organ Lighting Schedule
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