Many substances, termed candidate hormones, are suspected of being hormones in humans but are not considered classical hormones for one of two reasons: Either (1) their functions have not been conclusively documented; or (2) they have well-documented functions as paracrine/autocrine agents, but it is not certain that they ever reach additional target cells via the blood, an essential criterion for classification as a hormone. This second category includes certain of the eicosanoids (Chapter 7) and a large number of what are called growth factors (Chapter 18) that are secreted by multiple cell types and stimulate specific cells to undergo cell division and differentiation.
A substance that fits the first category described above is the amino acid derivative melatonin, which is synthesized from serotonin. This candidate hormone is produced by the pineal gland, an outgrowth from the roof of the diencephalon of the brain (shown but not labeled in Figure 8-38). The exact functions of melatonin in humans are uncertain, but this hormone probably plays an important role in the setting of the body's circadian rhythms and in sleep (Chapter 7). Its secretion is stimulated by sympathetic neurons that constitute the last link in a neuronal chain primarily triggered by receptors in the eyes; darkness stimulates melatonin secretion, and light inhibits it. Melatonin secretion, therefore, undergoes a marked 24-h cycle, being high at night and low during the day. [Environmental lighting does not cause the circadian rhythm, which is of internal origin, but entrains it (see Chapter 7)]. Melatonin's ability to reduce the symptoms of jet lag when administered in small amounts at the
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Principles of Hormonal Control Systems CHAPTER TEN
proper time, its relationship to seasonal affective disorder ("winter depression"), its potential use as a "natural" sleeping pill, its ability to scavenge damaging free radicals, and its possible role in the control of the reproductive system are all being studied.
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