The effect of hypophysectomy and growth hormone replacement on sst1 and sst2 somatostatin receptor subtype messenger ribonucleic acids in the arcuate nucleus

Abstract

Although considerable evidence indicates that somatostatin (SRIF) exerts direct actions on GH-releasing hormone-containing arcuate neurons within the hypothalamus to modulate hypophyseal GH secretion, the underlying mechanism(s) remains to be elucidated. We recently demonstrated high levels of expression of the messenger RNAs (mRNAs) coding for two prototypic receptors of the recently cloned SRIF receptor (sst) family, sst1 and sst2, in the arcuate nucleus of the rat hypothalamus. However, information on the biological roles of these receptor subtypes and the factors regulating their expression is lacking. In the present study, we hypothesized that perturbations in GH would influence sst mRNA levels in cells of the arcuate nucleus in vivo. To test this hypothesis, we examined the effects of hypophysectomy (HPX) and HPX with GH replacement, on sst1 and sst2 mRNA levels in the brains of adult male rats by in situ hybridization using 35S-labeled antisense riboprobes. The number of labeled cells and the density of silver grains per cell were quantified using a computer-assisted image analysis system. Two weeks after HPX, there was a 50-60% reduction in both the number and labeling density of sst1 and sst2 mRNA-expressing cells in the arcuate nucleus compared to those in sham-operated control rats. Administration of recombinant human GH (200 micrograms/day for 7 days by continuous sc infusion using osmotic minipumps) to HPX rats augmented both the cell number (P < 0.05) and labeling density (P < 0.01) of sst1 mRNA in the arcuate nucleus, but did not significantly alter sst2 mRNA levels compared to those in HPX rats infused with H2O. There were no significant changes in sst1 and sst2 mRNA levels in extra-arcuate areas, including the cerebral cortex and medial habenula, or in suprachiasmatic, medial preoptic, and magnocellular preoptic nuclei after either HPX or GH replacement. These results indicate that the expression of both sst1 and sst2 SRIF receptor subtypes in brain is under the regulatory influence of pituitary hormones. They further suggest that GH may participate in the regulation of its own secretion by influencing the expression of the sst1 receptor gene on arcuate neurons. Such a mechanism may be important in the feedback regulation of GH secretion by the arcuate nucleus.