Extrahypothalamic control of stress-induced inhibition of growth hormone secretion in the rat

The aim of these experiments was to determine whether the telencephalon is essential for stress-induced suppression of plasma GH levels and, if so, which telencephalic structures are involved. In the first study, the telencephalon was completely removed from adult female rats by suction; intact and sham-operated rats served as controls. At 24 h after surgery, blood samples were collected at the initiation of and 15 min after a 3-min immobilization-blood withdrawal stress procedure and used for determining non-stress and stress plasma levels of GH, respectively. GH was measured by radioimmunoassay. In contrast to the normal stress-induced decrements in GH levels observed in controls, this response was blocked and stress caused a paradoxical increase (P less than 0.01) in GH levels in telencephalon-ablated rats. These results suggested that the telencephalon is essential for stress-induced inhibition of GH secretion. Therefore, the several components of the telencephalon were ablated individually in additional experiments to determine the structure(s) involved. Placement of large, bilateral lesions in the amygdala or complete removal by suction of hippocampus, septum or striatum did not interfere with the normal GH response to stress at 24 h after surgery. However, removal of the preoptic region duplicated the effects of complete telencephalon ablation i.e., the normal stress-induced decrement was not only blocked but also reversed. These findings suggest that the preoptic region is essential for mediating inhibition of GH secretion in response to immobilization-blood withdrawal stress and that other telencephalic structures are not essential for this neuroendocrine response.