The effect of various anaesthetics and of the manipulations of the hypothalamo-pituitary-adrenocortical system (hypophysectomy, adrenalectomy and lesion of the mediobasal hypothalamus) was studied on immunoreactive-ACTH levels in the plasma and in the cerebrospinal fluid in the rat. The anaesthetics used (Hypnorm, pentobarbital, urethan and Ketanest) were without effect on immunoreactive-ACTH concentration in the cerebrospinal fluid. Immunoreactive-ACTH was significantly decreased after hypophysectomy and elevated after adrenalectomy in both cerebrospinal fluid and plasma. Destruction of the mediobasal hypothalamus resulted in reduced immunoreactive-ACTH content in the cerebrospinal fluid (about 20% of control) in both experiments, whereas immunoreactive-ACTH levels in the plasma of the lesioned rats were lower only in one of the two experiments performed. These data suggest that the main source of the immunoreactive-ACTH in the cerebrospinal fluid of the rat is the hypothalamus; the contribution of the pituitary gland being less than 50% of the radioimmunoassayable ACTH.
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