Circadian Variation of Immunoreactive CRF in Rat Hypothalamus
The activity of the hypothalamic-pituitary-adrenal axis is characterized by episodic secretion and circadian variations. In normal human subjects, ACTH and cortisol secretion is pulsatile in nature; pulse amplitude and frequency dramatically increase during the second half of the night, giving high plasma concentrations of those hormones at 0800 hr, and then gradually decrease during the day, giving low plasma concentrations at 2400-0200 hr. This diurnal rhythmicity seems synchronized with the environment day-night cycles, which condition the sleep-activity cycles. In rats, whose normal activity occurs during the night, the ACTH rhythm does not resemble that in humans; instead, plasma ACTH and corticostrone concentrations are high in the evening (1600–2000 hr) and low in the morning (0800-1200 hr). Although hypothalamus-lesioned rats still have significant circadian variations in corticosterone production (Nicholson et al., 1985), the hypothalamus seems to be the main regulator of the rhythmic activity of the hypothalamic-pituitary-adrenal axis.
KeywordsAnterior Pituitary Circadian Variation Interassay Variation Hypothalamic Extract Significant Circadian Rhythm
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