Role of the Pineal Gland in Diurnal Endocrine Secretion and Rhythm Regulation
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The effects of the pineal gland on diurnal endocrine function were studied in male rats. Pineal stimulation by exposure to short daily photoperiods (1L:23D) did not alter plasma levels of testosterone, thyroid-stimulating hormone (TSH) or growth hormone (GH) as compared with those of controls maintained under normal (12L:12D) lighting. Pinealectomy lowered diurnal testosterone levels and increased TSH levels in animals maintained under 1L:23D lighting. Pinealectomy also altered the TSH rhythm and shifted the peak in testosterone levels but did not abolish hormonal rhythms. Active immunization against melatonin and its precursor, N-acetylserotonin (NAS), significantly reduced diurnal androgen levels and elevated TSH levels in animals exposed to short photoperiods, but the 24-hour rhythms persisted. Plasma GH exhibited a diurnal rhythm in animals kept under short photoperiods but GH levels were not affected by any of the above treatments. These data suggest that melatonin and/or NAS may be involved in the maintenance of basal testosterone and TSH levels. The pineal may not be involved in generating hormonal rhythms; however, it may have a role in entraining TSH and testosterone rhythms to environmental lighting.
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