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

This study was designed to evaluate changes in the hypothalamic somato statin-growth hormone axis (SRIF-GH axis) in a stress-induced rat mode l of depression. We exposed male Wistar rats to intermittent walking s tress for two weeks, and then measured their spontaneous running activ ities 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 t he secretion of GH from the anterior pituitary by injecting human GH-r eleasing hormone (hGHRH) with intracardiac cannulae or by applying hGH RH or SRIF to isolated anterior pituitaries using a perifusion system. We also determined SRIF content in the stalk-median eminence (SME) an d the plasma concentration of GH, In the depression-model group, intra cardiac 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 SM E and the GH concentration was increased in plasma. The partial recove ry group gave similar values to the control group. The enhanced respon se 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 re sponse of GH to the infusion of hGHRH or SRIF in the perifusion system .