Differential Regulation of Brain and Pituitary Corticotropin-Releasing Factor Receptors by Corticosterone*

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Abstract
The regulatory actions of CRF during the neuroendocrine response to stress are mediated by specific receptors within the nervous system and the anterior pituitary gland. Glucocorticoids exert negative feedback inhibition on ACTH secretion by interacting at the pituitary corticotrophs and the central nervous system. To determine whether glucocorticoids influence ACTH secretion by regulating the concentration of CRF receptor sites, binding of [125I]Tyr-oCRF to pituitary and brain membrane-rich particles was studied after glucocorticoid treatment. Corticosterone administration (0.5-150 mg/day) for 1-4 days in adult male rats caused a dose-dependent decrease in the number of CRF receptors in the anterior pituitary in parallel with the reduction in ACTH secretion. In the brain, binding studies in membrane-rich fractions or by autoradiography in slide-mounted frozen sections revealed no changes in CRF receptors in the cortex, hippocampus, amygdala, septal area, and olfactory bulb, although circulating corticosterone levels were higher than during stress. The selective down-regulation of anterior pituitary CRF receptors after corticosterone administration, without alterations in brain CRF receptors, is similar to the change in CRF receptors previously reported after adrenalectomy and indicates that receptor regulatory mechanisms in secretory cells differ from those in neural tissue. Furthermore, the decrease in pituitary CRF receptors after physiological increases in circulating glucocorticoids may contribute to the inhibitory effects of adrenal steroids on ACTH secretion.