Role of the Pineal Gland in Diurnal Endocrine Secretion and Rhythm Regulation

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.