Enhanced response of growth hormone to growth hormone-releasing hormone and a decreased content of hypothalamic somatostatin in a stress-induced rat model of depression.

Abstract

This study was designed to evaluate changes in the hypothalamic somatostatin-growth hormone axis (SRIF-GH axis) in a stress-induced rat model of depression. We exposed male Wistar rats to intermittent walking stress for two weeks, and then measured their spontaneous running activities for 12 days. We divided the rats into the depression-model group and the partial recovery group according to their spontaneous running activities after the termination of exposure to stress. We examined the secretion of GH from the anterior pituitary by injecting human GH-releasing hormone (hGHRH) with intracardiac cannulae or by applying hGHRH or SRIF to isolated anterior pituitaries using a perifusion system. We also determined SRIF content in the stalk-median eminence (SME) and the plasma concentration of GH. In the depression-model group, intracardiac administration of hGHRH caused the enhanced release of GH into plasma, while application of hGHRH or SRIF to the anterior pituitary in vitro had similar effects on GH release in the control and partial recovery groups. Furthermore, the SRIF content was decreased in the SME and the GH concentration was increased in plasma. The partial recovery group gave similar values to the control group. The enhanced response of GH to hGHRH in the depression-model group might have been caused by the reduced content of SRIF in the SME in view of the unchanged response of GH to the infusion of hGHRH or SRIF in the perifusion system.