Endocrine Secretions under Abnormal Light-Dark Cycles and in the Blind

Abstract

Both endogenous and exogenous factors are involved in regulation of endocrine secretions. Among the exogenous ones, light plays an important role both in animals and in humans. Pineal gland mediates light action on the endocrine system, by means of variations of melatonin (MT) secretion. Here we discuss about the influence of abnormal light-dark cycles and in particular of blindness on pineal and pituitary secretions and on those of correlated glands. MT secretion is usually inhibited by light: thus it reaches the highest levels at night. Exposure to short or long photoperiod causes variations in circadian or infradian MT rhythmicity. Blind patients can show higher daytime levels with a phase-advanced or phase-delayed circadian rhythm. Lack of light stimulus affects cortisol rhythm shifting the zenith of secretion and inducing a free-running rhythm. Blindness can abolish nocturnal growth hormone (GH) peak and impair the GH response to some stimuli; moreover it impairs the growth of affected patients. Light stimulus influences favorably gonadal function both in animals and in man. In animals, sexual activity and gonadal function decline during the seasons with reduced luminosity. A similar finding has been described in women living in a region with a strong seasonal contrast in luminosity. Blindness can impair luteinizing hormone, follicle-stimulating hormone, prolactin and testosterone secretion in prepubertal boys causing pubertal delay or more severe hypogonadism; it can affect pubertal development and fertility in women. Light can influence thyroid function in animals. Lack of light stimulus in blind man seems to cause different effects on thyroid function before and after puberty. Increase of free thyroid hormone levels has been found in prepubertal but not in adult blind patients, probably due to a resetting of the treshold for thyrotropin feedback suppression after puberty in these patients.

Shift Work and Circadian Dysregulation of Reproduction

“…Given that melatonin is critical for signaling seasonal day length and the onset of winter-like inhibition of reproductive function in most photoperiodic animals, it is not entirely surprising that melatonin can be inversely related to estrogen levels (61). Specifically, melatonin can interact with gonadal rhythms as evidenced by (i) melatonin-induced augmentation of the LH surge and (ii) reduced ovarian activity in short day-length environments characterized by increased nocturnal melatonin (62). Finally, a clinical trial of melatonin therapy in perimenopausal and menopausal women with low, medium, and high nocturnal melatonin showed that administration of melatonin reduced LH and FSH of women with initial low melatonin (63).…”

Comparison of the validity of three dental methods for the estimation of age at death

“…If the count of cementum annulations is added to the toothspecific eruption age, the result is an estimate of the chronological age. A possible explanation for the layered organization of human cementum is the fact that growth hormone affects cementogenesis [10][11][12] which could be influenced by environmental factors such as the seasonal change [13][14][15].…”

Effect of Light Perception on Menarche in Blind Women

“…There have been reports that endocrine rhythms and secretions among the blind are different than in the general population. [25][26][27][28][29]38 The suggestion that ocular light exposure may influence reproductive development also has important implications for research that suggests exposure to light at night is associated with an increased risk of breast cancer. Recently, the World Health Organization categorized shift-work as a "probable carcinogen."…”