Studies of circadian patterns in CRF secretion fron the rat hypothalamus isolated and incubated in vitro.

An in vitro bioassay which discriminates between vasopressin and CRF was used in an investigation of the secretion of CRF from the isolated rat hypothalamus incubated in vitro for varying periods of time at different times in the 24-hour cycle. A porcine hypothalamic CRF extract was also tested in the assay and elicited responses similar to those elicited by rat hypothalamic incubation media. During continuous incubations sampled at a series of times (up to 120 minutes) the pattern of CRF secretion is seen to be phasic. The secretion of CRF, when the hypothalamus is incubated for two consecutive 15 minute incubation periods, is shown to be calcium ion-dependent and to display a circadian rhythm with peaks occurring in the afternoon when studied at different times in the 24-hour cycle. From this, the secretion of CRF by the hypothalamus isolated and incubated in vitro is considered not to represent non-specific diffusion or "leakage" of CRF, but a phenomenon of physiological significance which reflects a circadian rhythm in CRF secretion in vivo. The secretion of CRF during two consecutive 15 minute incubations measured systematically at different times of the day shows a circadian rhythm which parallels the peaks and troughs in the circadian rhythm in plasma corticosterone levels, but which shows a phase delay with the increase in hypothalamic CRF content in the afternoon. Reversal of the light/dark cycle results in a reversal of the morning/evening difference in the pattern of CRF secretion during the first 15 minutes of incubation and a concomitant phase-reversal of the circadian rhythm in plasma corticosterone levels. These findings imply that CRF secretion is involved in the. regulation of circadian rhythmicity in plasma corticosterone levels, and that circadian rhythmicity in CRF secretion may be entrained by photoperiodic cues.