Growth hormone receptor messenger ribonucleic acid distribution in the adult male rat brain and its colocalization in hypothalamic somatostatin neurons.

The activity of both somatostatin (SS) and GH-releasing hormone (GHRH) neurons within several hypothalamic nuclei is regulated, in part, by the feedback effects of GH. However, whether GH, or its intermediate, insulin-like growth factor I, acts on these neurons to alter the synthesis and release of SS and GHRH is unknown. We argued that if GH itself acts directly on the brain to govern its own secretion, then regions of the brain containing SS and GHRH neurons may express the GH receptor gene. We tested this hypothesis by performing in situ hybridization for GH receptor messenger RNA (mRNA) and mapping its distribution in the brain. We observed GH receptor mRNA-containing cells in various brain regions including the thalamus, septal region, hippocampus, dentate gyrus, amygdala, and hypothalamus. Next we sought evidence for expression of the GH receptor mRNA by SS neurons in the hypothalamus. We addressed this by performing a double-label in situ hybridization to identify neurons expressing both SS mRNA and GH receptor mRNA. We report that SS neurons in the periventricular nucleus and in the paraventricular nucleus coexpress the GH receptor gene, whereas few, if any, of the SS neurons in the cortex express detectable amounts of the GH receptor mRNA. These findings suggest that GH acts directly on the brain and participates in the regulation of its own secretion through a direct action on hypothalamic SS neurons.