Somatostatin Content of the Median Eminence in Female Rats with Lesion-Induced Disruption of the Inhibitory Control of Growth Hormone Secretion*

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Abstract
These experiments were designed to determine whether brain lesions which elevate nonstress plasma levels of GH and disrupt stress-induced suppression of GH secretion in female rats affect the median eminence content of somatostatin. Some rats received lesions in the medial or lateral preoptic-anterior hypothalamic area (PO-AHA), while others received anterior hypothalamic cuts. Sham-operated and intact rats served as controls. GH and somatostatin were measured by RIA. Medial but not lateral PO-AHA lesions caused elevated nonstress plasma GH levels at 2,14, 17, and 23 weeks after surgery, but normal levels were obtained at autopsy at 27 weeks. These lesions compromised GH responses to stress at 14 and 23 weeks. Rats with anterior hypothalamic cuts showed elevated nonstress GH levels at 17, 23, and 27 weeks after surgery and loss of the GH response to stress at 14 and 23 weeks. Median eminence content of somatostatin was reduced approximately 80% in rats with medial PO-AHA lesions or anterior cuts. Whereas medial PO-AHA lesions were associated with normal body length and weight and evidence of estrogen secretion, anterior hypothalamic cuts produced increased linear growth and body weight and signs of functional castration. These results suggest that the effects of lesions which cause prolonged elevation of nonstress GH levels and disruption of the GH stress response are due to interference with somatostatin neurons located in the medial PO-AHA. Somatostatin content of the median eminence seems to depend largely on connections originating in or traversing the medial PO-AHA.