Circadian rhythm hormonal secretion

Poor Timing of Neurotransmission. The activity of some brain circuits, like the secretion of certain hormones, varies at certain times of the day. Called circadian rhythms, the timing of these rhythms may be disrupted in some illnesses. Examples include sleep disorders such as insomnia and narcolepsy, as well as other conditions such as nighttime binge-eating disorder. 

Studies of mammals subjected to SCN destruction and transplantation have revealed that the hypothalamic SCN contains a master circadian oscillator which is involved in a number of behaviours and hormonal secretions. The circadian oscillatory activity of SCN neurons is directly demonstrated by the measurement of [ " C] glucose metabolic activity and field potentials assessed by electro-physiological devices. The clock oscillatory genes mPer1 and mPer2 are expressed rhythmically in most neurons in the SCN. Thus, thousands of clock cells in the SCN might generate the rhythm. 

Melatonin (lV-acetyl-5-methoxytryptamine) is a neuro-hormone secreted by the pineal gland from the amino acid precursor [,-tryptophan. Its endogenous secretion is photosensitive and has a circadian rhythm—plasma melatonin concentrations are highest at night in both diurnal and nocturnal animals, and fall with age (1). The nocturnal melatonin peak coincides with a drop in body temperature and increased sleepiness in healthy humans. Oral melatonin has a short half-life (30-50 minutes) and extensive first-pass metabolism. Its clearance is reduced in severe liver disease (2). 

Many hormonal secretion processes also exhibit strong circadian components. This is true, for instance, for cortisol, antidiuretic hormone, and growth hormone. The secretion of growth hormone is markedly increased during the early periods of sleep, and the secretion of antidiuretic hormone also reflects the sleep-wake cycle. The mechanisms underlying these oscillations can often be traced back to cyclical variations in the activity of the central nervous system. At the same time, the circadian rhythm modulates the above-mentioned ultradian oscillations. 

For linear systems, the principle of superposition applies, and different oscillatory modes can evolve independently of one another. However, biological systems in general are not linear, and separation of different regulatory mechanisms may not be justified, even when they involve different time scales. One type of phenomenon that can arise from the interaction between two oscillatory modes is modulation of the amplitude and frequency of the faster mode in dependence of the phase of the slower mode. This type of phenomenon was demonstrated in Fig. 12.2c where the frequency of the myogenic mode fjast changes in step with the amplitude of the TGF-mediated mode. Similar modulation phenomena can be expected to occur in many other biological systems such as, for instance, the interaction between the circadian and the ultradian rhythms of hormone secretion [25]. 

Q4 Glucocorticoid secretion is controlled by the hypothalamus and anterior pituitary gland. Corticotrophin releasing factor (CRF) is produced in the hypothalamus and travels in the hypophyseal portal blood vessels to the anterior pituitary to release ACTH (adrenocorticotrophic hormone). There is a daily (circadian) rhythm in CRF and ACTH secretion, with a peak in the morning between 7 and 9 a.m. and a low point during the night. 

Weitzman ED. Circadian rhythms and episodic hormone secretion. Ann Rev Med 1976 27 225-43. 

Depressive states can alter significantly the basal rates of pituitary hormone secretion and their circadian rhythms. In fact, depression may be related to periods when hormonal rhythms are out of phase with other rhythms in the body. Circulating levels of sleep-inducible hormones (GH, PRL, LH) are lower in depressive states, whereas that of ACTH is elevated. The basal level of ACTH in a depressed individual is elevated to the extent that it flattens the circadian oscillations of the hormone. These changes resemble those seen during disruptive phase shifts (e.g., east-bound trip, altered work schedule, etc.) and emphasize the importance of CNS influence on the release of anterior pituitary hormones. 

I Adverse Effects. Side effects (see Table 54—6) of carbamazepine may fluctuate daily, paralleling the rise and decline of serum concentrations. The side-effect profile also may follow a circadian rhythm. Neurosensory side effects (e.g., diplopia, blurred vision, nystagmus, ataxia, unsteadiness, dizziness, and headache) are the most common, occurring in 35% to 50% of patients. These side effects are more common during initiation of therapy and may dissipate with continued treatment. Patients have variable threshold concentrations for the occurrence of CNS side effects. If the carbamazepine serum concentration is kept below the individual threshold, the CNS side effects can be minimized. Dosage manipulation, including the use of the controlled- or sustained-release preparations, should be tried before the patient is considered to be intolerant of carbamazepine. Carbamazepine may induce a hyponatremic hyposmolar condition that is similar to the syndrome of inappropriate antidiuretic hormone secretion. The incidence may increase with age. Periodic determinations of serum sodium concentration are recommended, especially in the elderly." ... 

Melatonin is a hormone secreted mainly by the pineal gland. It is synthesized from tryptophan, and its characteristic circadian rhythm is ruled by light. Touitou et al.86 have reviewed the relationship of melatonin and aging. Since the circulating levels of melatonin decrease with aging, questions arise regarding its origin and/or the consequences of this condition, which may be related to the availability of tryptophan. 

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