Growth Hormone-Releasing Activity in the Hypothalamus and Plasma of Rats Subjected to Stress. ∗

Summary

Experiments were performed in the rat to determine whether the increased secretion of growth hormone (GH) during stress involves augmentation of synthesis and/or release of hypothalamlc GH-releasing factor (GRF). Two different stimuli were used as stress: injection of formalin, which was previously shown to be ineffective in inducing GH release; and cold exposure, which evoked depletion of pituitary GH. Exposure of rats to cold (4°C) for 1 hr resulted in a significant depletion of pituitary GH, which was accompanied by disappearance of hypothalamic GRF and appearance of GRF activity in plasma. Cold exposure for 5 minutes was ineffective. Injection of 10% formalin did not induce pituitary GH depletion, or affect hypothalamic and plasma GRF activity. These results indicate that some stressful stimuli, such as cold exposure, capable of releasing GH, induce this effect via the hypothalamus; other stresses, like formalin injection, which do not release GH, exert little effect on the GH-release mechanism of the hypothalamus.