The Binding of Corticosterone to Brain Proteins: Diurnal Variation1

Abstract

The quantity of 3H-corticosterone bound to protein(s) in rat brain cytosol derived from animals subjected to controlled lighting was studied at 4-hr intervals throughout the day. The binding of ;3H-corticosterone to brain proteins was measured by gel chromatography on Sephadex G-25 and by precipitation of the proteins with protamine. In control (nonadrenalectomized, nonstressed) animals, the greatest amount of :3H-corticosterone binding occurred at 1200 and the lowest at 2000. The binding of 3Hcorticosterone was inversely related to the circulating amounts of endogenous corticosterone which indicated that the amount of endogenous steroid bound in brain appears to be related to the circulating blood levels. The results obtained with protamine and with the perfusion experiments demonstrate that the alterations in binding are not due to contamination of cytosol with CBG. When the hippocampus, septum and cerebral cortex from control animals were studied separately, the results were qualitatively the same as those obtained with whole brain. The diurnal variation in corticosterone binding observed in control animals was not apparent in adrenalectomized animals. The amount of corticosterone bound to proteins from adrenalectomized animals was equivalent to the amount bound in control animals when blood corticosterone was minimal. Multiple stresses (pentobarbital anesthesia and laparotomy) of nonadrenalectomized animals significantly reduced the amount of 3H-corticosterone bound relative to the lowest value seen in nonstressed animals. Thus it is evident that the amount of corticosterone bound by proteins in rat brain cytosol is related to the circulating levels of endogenous corticosterone. These events may be involved in the negative feedback mechanism for regulating ACTH release. (Endocrinology93: 1152, 1973)