Increases in Plasma Growth Hormone Concentration and Naso-Anal Length in Rats Following Isolation of the Medial Basal Hypothalamus

Abstract

The effects of surgical isolation of the medial basal hypothalamus (MBH) on growth were studied in adult female rats. Isolation of MBH caused a gradual increase in naso-anal (N-A) length. N-A length was significantly increased 92 days after surgery and the augmented rate of growth continued until autopsy at 139 days. The increase in N-A length was accompanied by a marked gain in body weight and by conspicuous obesity. Non-stress and stress levels of plasma growth hormone (GH) were determined by radio-immunoassay at 7–11 weeks after surgery. Rats with MBH isolation had circulating non-stress levels of GH that were approximately 6 times higher than those of controls (p < 0.01) at both 7 and 11 weeks. Whereas intact controls demonstrated a marked decrease in GH levels following ether stress, those with MBH isolation showed variable responses: some demonstrated normal decreases; others showed no responses; while yet another group showed marked increases. These results confirm and extend prior findings and suggest that the MBH receives neural connections that normally inhibit growth and plasma GH levels.