Midbrain and Feedback Control of Adrenocorticotrophin Secretion

Abstract

The reaction of the pituitaryadrenal axis of normal and of midbrainsectioned animals has been studied after having artificially decreased (unilateral adrenalectomy) or increased (dexamethasone administration) the blood titers of adrenal steroids. In normal animals plasma corticosterone levels remain quite constant for at least 4 hr following unilateral adrenalectomy. On the contrary, a progressive drop of blood corticosterone levels which starts immediately after the extirpation of one adrenal has been observed in midbrain-sectioned animals. Three μg/100 g body wt of dexamethasone induces a significant drop of plasma corticosterone levels in normal animals, while 100 μg/100 g body wt of dexamethasone is needed in order to significantly reduce plasma corticosterone titers in the midbrain-sectioned animals. The midbrain-sectioned animal does not realize then that its blood levels of corticoids are changed until extremely high or extremely low levels of blood corticoids are reached; only when such values are attained can the servomechanisms responding to these fluctuations thus be brought into play in the midbrain-sectioned animals. The hypothesis is put forward that, in addition to an ACTHactivating, an ACTH-inhibiting and a stressfacilitating system, the midbrain might participate in ACTH control by acting as a sort of modulator. (Endocrinology75: 765, 1964)