Diurnal CRH mRNA Rhythm in the Hypothalamus: Decreased Expression in the Evening Is Not Dependent on Endogenous Glucocorticoids

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Abstract
Levels of hypothalamic corticotropin-releasing hormone (CRH) mRNA and plasma glucocorticoids vary diurnally as a result of circadian influences on the hypothalamopituitary-adrenal axis. CRH mRNA expression increases from morning to afternoon in rats but decreases rapidly near the onset of dark as glucocorticoids reach peak concentrations in plasma. Since glucocorticoids are normally inhibitory on hypothalamic CRN mRNA expression, we determined whether the glucocorticoid secretion at the diurnal peak reduced CRH mRNA concentration in the evening. We found that adrenalectomy did not prevent the decrease in CRH mRNA levels near the onset of dark. It appears that the drop in CRH mRNA expression occurs via a steroid-independent mechanism. While the mean CRH mRNA level increased after adrenalectomy, the shape of the CRH mRNA rhythm remained unchanged except in the morning. Interestingly, adrenalectomy increased CRH mRNA levels disproportionately in the morning, producing a sharp rise followed by a plateau during the light phase instead of the gradual rise observed in intact animals. We subsequently treated adrenalectomized animals with corticosterone pellets to determine whether a constant steroid signal was sufficient in restoring the normal shape of the mRNA rhythm dring the lihgt phase. Results indicate that the endogenous steroid rhythm is not necessary for generating the normal CRH mRNA rhythm during the light phase. Instead, a constant exposure to corticosterone at approximately 50% of the daily mean (2.4-3 µg/dl) appears to be sufficient for regulation of the mRNA rhythm. Additionally, corticosterone replacement achieving a lower concentration of 1.5 µg/dl seems effective in regulating basal ACTH secretion in the morning despite being insufficient for regulating CRH or POMC mRNA levels at this time. We conclude that the shape of the diurnal CRH mRNA rhythm in the hypothalamus is largely independent of steroid regulation, although a steroid-dependent component appears to exist during the circadian nadir. A constant steroid level in the range primarily for type I corticosteroid receptor binding can reinstate CRH mRNA variation which occurs during the light phase, indiating that corticosterone oscillation is not necessary for diurnal CRH mRNA regulation. Finally, corticosterone may regulate basal ACTH secretion more effectively than CRH mRNA expression at the diurnal nadir.

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