Shift in Adenohypophyseal Activity during Chronic Intermittent Immobilization of Rats

Abstract

Female rats were subjected to 8 h of daily immobilization for 1, 3, 6, 10 or 15 days. Exposure for 3 days inhibited b.w. and induced adrenal enlargement as well as thymus involution; 6 days of stress elicited atrophy of the ovaries and hypophysis. During the later stages of the experiment, the animals showed a positive metabolic balance, and the initial weight alterations of the endocrine glands either tended to regress or were stabilized. On the 6th day, plasma corticosterone reached a peak which was maintained until the end of the experiment, whereas growth hormone (GH), luteinizing hormone (LH) and follicle stimulating hormone (FSH) titers, as determined by radioimmunoassay, fell to 28, 39 and 55% of control values, respectively, following the 1st period of restraint; prolactin (Prl) decreased by 30% after the 3rd day. Thereafter, the plasma concentrations of these hormones remained low, around 40–60% of control levels, although FSH on The 6th day revealed a transient but significant elevation to 180% of initial values. After 15 days of 6-h-daily immobilization, an injection of LH-releasing hormone (LH-RH) and thyrotropin releasing hormone (TRH) induced dramatic increases of LH, FSH, Prl and thyrotropin stimulating hormone (TSH) in plasma. On the basis of the morphologic and hormonal changes, it appears that, in agreement with the concept of a shift in adenohypophyseal activity during severe chronic stress, repeated long-term restraint elicits an inverse relationship between the secretion of ACTH and that of GH, LH, Prl and, to a lesser degree, FSH. Such modifications do not seem to be associated with an inability of the pituitary to secrete these hormones, since it is still responsive to releasing factors.