Growth Hormone-Releasing Hormone-Synthesizing Neurons Are a Subpopulation of Somatostatin Receptor-Labelled Cells in the Rat Arcuate Nucleus: A Combined in situ Hybridization and Receptor Light-Microscopic Radioautographic Study

Abstract

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 ³⁵S-labelled synthetic oligonucleotide (45 mere), and ¹²⁵I-TyrO-DTrp8-somatostatin (¹²⁵I-SRIH) binding sites by light-microscopic radioautography on adjacent 20-µm-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 ¹²⁵I-SRIH binding sites allowed to differentiate three types of cells: (1) GHRH perikarya not associated with pericellular ¹²⁵I-SRIH binding sites around the perimeter of the VMN, (2) ¹²⁵I-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 ^l25I-SRIH-labelled cells. Furthermore, on adjacent sections, the number of both labelled cells were correlated (r = 0.68; p < O.OOl). In this last population, the extent of colocalization of ^l25I-SRIH binding sites on GHRH mRNA-labelled neurons was further investigated in adjacent 5-µm-thick sections. The proportions of cells GHRH mRNA and ¹²⁵I-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 ¹²⁵I-labelled while only 6% of ¹²⁵I-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 ¹²⁵I-SRIH). In the lateral part the extent of colocalization was lower (14 and 4%, respectively, for GHRH mRNA and ¹²⁵I-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.

Co-Localization of Growth Hormone Secretagogue Receptor and NPY mRNA in the Arcuate Nucleus of the Rat

“…Nevertheless, they are thought to regulate GH release. SRIH axon terminals synapse on GHRH neurons in the arcuate nucleus which express SRIH receptors [40]. The SRIH afferents on the arcuate GHRH neurons are inhibitory and will therefore inhibit GHRH release in the portal blood.…”

Expression Analysis of Hypothalamic and Pituitary Components of the Growth Hormone Axis in Fasted and Streptozotocin-Treated Neuropeptide Y (NPY)-Intact (NPY^+/+) and NPY-Knockout (NPY^–/–) Mice

“…Although GHRH and NPY neurons are both located with the arcuate nucleus, to date there is no evidence that GHRH neurons express NPY receptors or form close contacts with NPY neurons. It is possible that NPY-mediated suppression of SRIH expression may play a role in fasting-induced suppression of GHRH, since SRIH receptors (sst1 and sst2) are expressed by GHRH neurons, in the rat [35, 36], and a subpopulation of SRIH neurons located within the arcuate nucleus do appear to contact with GHRH neurons [37, 38]. However, in the present study the directional changes in NPY expression are opposite to those of SRIH, as well as GHRH, an observation that at first seems counterintuitive to the well-known direct inhibitory actions of SRIH on pituitary GH release.…”

Chronic growth hormone (GH) hypersecretion induces reciprocal and reversible changes in mRNA levels from hypothalamic GH-releasing hormone and somatostatin neurons in the rat.

“…Hypothalamic SRIH and GHRH mRNA levels were determined at 6 and 18 wk by in situ hybridization in female rats bearing ectopic GHproducing tumors. As SRIH-specific receptors have recently been located on GHRH neurons (20)(21)(22), we also used quantitative light microscopic autoradiography (23) In situ hybridization was carried out as described elsewhere (25). Briefly, 45-base oligoprobes (bases 31-75 of rat GHRH cDNA [33] and bases 96-111 of rat SRIH cDNA [34] GHRH, respectively.…”