Evaluation of changes in the secretion of corticotrophin releasing activity using the isolated rat hypothalamus incubated in vitro

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Abstract
The secretion of corticotrophin releasing activity (CRA) from the isolated rat hypothalamus incubated in vitro was investigated under various conditions of incubation and of pretreatment of donor animals providing hypothalami. Media from hypothalamic incubations were assayed for CRA by a validated double in-vitro bioassay technique which differentiates CRA from vasopressin. A circadian rhythm was found in the secretion of CRA in vitro from isolated hypothalami obtained from animals killed at different times of the day. Secretion of CRA increased significantly at 19.00 h (dusk) compared with the secretion rate at 07.00 h, in synchrony with a rise in plasma corticosterone levels. In addition, both plasma corticosterone concentrations and CRA secretion in vitro were higher at 07.00 h than at 19.00 h after exposure of the donor animals to a reversed light cycle for 7–10 days. Hypothalami obtained from animals chronically treated with betamethasone in the drinking water showed a diminished secretion of CRA in vitro. Exposure of untreated animals to ether vapour for 2 min immediately before death significantly increased the subsequent secretion of CRA in vitro. Ether exposure did not significantly affect the secretion of CRA in vitro from hypothalami of betamethasone-treated rats. There was a close correlation between plasma corticosterone levels and in-vitro CRA release after these treatments. The results suggest that the secretion of CRA examined in this way is a phenomenon which can reflect the changes which occur in the activity of the hypothalamo-pituitary-adrenal system in vivo during the 24-h cycle, after glucocorticoid treatment and after ether stress.