GROWTH HORMONE-RELEASING HORMONE-SYNTHESIZING NEURONS ARE A SUBPOPULATION OF SOMATOSTATIN RECEPTOR-LABELED CELLS IN THE RAT ARCUATE NUCLEUS - A COMBINED INSITU HYBRIDIZATION AND RECEPTOR LIGHT-MICROSCOPIC AUTORADIOGRAPHIC STUDY

Distribution of growth-hormone-releasing hormone (GHRH) cell bodies and somatostatin binding sites were compared in the mediobasal hypothalamus of the rat. GHRH-synthesizing neurons were visualized by in situ hybridization, using as S-35-labelled synthetic oligo-nucleotide (45 mere), and I-125-Tyr0-DTrp8-somatostatin (I-125-SRIH) binding sites by light-microscopic radioautography on adjacent 20-mu-m-thick frozen mirror sections. GHRH mRNA hybridizing cells were detected mostly in the ventrolateral portion of the arcuate nucleus (ARC) and around the perimeter of the ventromedial nucleus (VMN). Comparison with the distribution of pericellular I-125-SRIH binding sites allowed to differentiate three types of cells: (1) GHRH perikarya not associated with pericellular I-125-SRIH binding sites around the perimeter of the VMN, (2) I-125-SRIH-labelled cells, not associated with GHRH perikarya in the periventricular zone along the dorsal part of the third ventricle, and (3) in the ventrolateral portion of the ARC, GHRH mRNA-labelled neurons had the same distribution as I-125-SRIH-labelled cells. Furthermore, on adjacent sections, the number of both labelled cells were correlated (r = 0.68; p < 0.001). In this last population, the extent of colocalization of I-125-SRIH binding sites on GHRH mRNA-labelled neurons was further investigated in adjacent 5-mu-m-thick sections. The proportions of cells GHRH mRNA and I-125-SRIH allowed to differentiate three subdivisions of the arcuate: the periventricular (PV), ventrobasal (VB) and lateral portions. In the PV-ARC, 27% of GHRH-synthesizing cells were coidentified as I-125-labelled while only 6% of I-125-labelled cells contained GHRH mRNA. In the VB-ARC the proportion of double-labelled cells was equivalent (31 and 26%, respectively for GHRH mRNA and I-125-SRIH). In the lateral part the extent of colocalization was lower (14 and 4%, respectively, for GHRH mRNA and I-125-SRIH). These results confirm by in situ hybridization the previous immunocytochemical description of hypothalamic GHRH-producing cells and provide,an anatomical substrate for a direct SRIH regulation of GHRH neurons in the ARC. The subpopulation of arcuate neurons bearing SRIH receptors in the periventricular part of the nucleus and not associated with GHRH-synthesizing cells remains to be identified and suggests that SRIH controls other hypothalamic functions at this level.