The Effect of Hypophysectomy on Somatostatin-Like Immunoreactivity in Discrete Hypothalamic and Extrahypothalamic Nuclei*

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Abstract
Several hypothalamic and extrahypothalamic sites that have high concentrations of somatostatin-positive nerve terminals and/or cell bodies are important in the regulation of GH secretion. GH is capable of inhibiting its own secretion under certain prescribed conditions, and a short loop feedback regulatory mechanism may involve somatostatinergic pathways. The purpose of this investigation was to determine the effect of removal of GH by hypophysectomy on the content of somatostatin- like immunoreactivity (SLI) in discrete hypothalamic and extrahypothalamic nuclei. Individual nuclei were removed from frozen brain sections of hypophysectomized and sham-operated male rats. The tissue content of somatostatin was determined by a specific RIA. The content of SLI in the median eminence of hypophysectomized animals was significantly reduced by 38%, compared to shamoperated controls (278 ± 53.2 vs. 447.0 ± 57.4 pg/μg protein, respectively). Significant reductions of SLI in the medial preoptic (50%), arcuate (33%), and periventricular (30%) nuclei were also observed in hypophysectomized animals when compared to controls (10.2 ± 1.6 vs. 20.0 ± 3.0; 60.2 ± 8.2 vs. 89.8 ± 13.3; and 19.4 ± 1.8 vs. 27.8 ±3.1 pg/μg protein, respectively). No significant changes were detected in the ventromedial, suprachiasmatic, medial, central, or cortical amygdaloid nuclei nor in the nucleus interstitialis striae terminalis. These data suggest that GH may exert a feedback effect on specific hypothalamic nuclei that involves somatostatin-containing systems.