Expression of sstr1 and sstr2 in rat hypothalamus: correlation with receptor binding and distribution of growth hormone regulatory peptides.

With the aim of elucidating the role of individual somatostatin receptors in the central control of growth hormone secretion, we have examined the distribution of sstr1 and sstr2 mRNAs in the hypothalamus of the adult rat by in situ hybridization using 35S-labelled antisense riboprobes. Both receptors were expressed strongly in the preoptic area, suprachiasmatic nucleus and arcuate nucleus. High sstr1, but low sstr2, expression was evident in the paraventricular and periventricular nuclei as well as in the ventral premammillary nucleus. Conversely, moderate to high sstr2, but low sstr1, mRNA levels were detected in the anterior hypothalamic nucleus, ventromedial and dorsomedial nuclei and medial tuberal nucleus. Within the arcuate nucleus, the distribution of cells expressing sstr1 and sstr2 was comparable to that of neurons which bind somatostatin-14 selectively, one third of which have been documented to contain growth hormone-releasing hormone. Within the periventricular nucleus, the distribution of cells expressing sstr1 and, to a lesser extent, sstr2 was reminiscent of that of both [125I]somatostatin-labelled and somatostatin-immunoreactive cells. Taken together, these results imply a role for both sstr1 and sstr2 receptors in the central regulation of growth hormone-releasing hormone and somatostatin secretion, and hence of growth hormone release, by somatostatin.