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

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          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.

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          Most cited references 4

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          Effects of melatonin on vertebrate circadian systems.

          In many species of vertebrates the pineal gland and its indoleamine hormone melatonin play central roles in the control of circadian rhythms, whereas in some species, the pineal gland appears to hold little importance. However, recent research indicates that the circadian rhythms of many species of reptiles, birds and mammals, including humans, are synchronized by the administration of exogenous melatonin. These studies have led to questions concerning the role of this hormone in circadian organization in general. Studies of the sites and mechanisms of melatonin action further indicate that melatonin may be an excellent pharmacological tool for research on the cellular mechanisms of circadian clock function and have pointed to the possibility that melatonin or melatonin analogues may be therapeutically useful for the control of circadian clock dysfunctions such as jet lag, shift-work syndrome and sleep disorders.
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            Circadian rhythm abnormalities in totally blind people: incidence and clinical significance

             R Sack (1992)
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              Blindness and menarche.


                Author and article information

                Horm Res Paediatr
                Hormone Research in Paediatrics
                S. Karger AG
                March 1998
                16 March 1998
                : 49
                : 3-4
                : 153-157
                Institute of Endocrinology, Second University of Naples, Italy
                23163 Horm Res 1998;49:153–157
                © 1998 S. Karger AG, Basel

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                Page count
                References: 72, Pages: 5


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