DISTRIBUTION OF GLUCAGON-LIKE PEPTIDE-1 AND OTHER PREPROGLUCAGON-DERIVED PEPTIDES IN THE RAT HYPOTHALAMUS AND BRAIN-STEM

Central administration of the preproglucagon-derived peptide glucagon- like peptide-1 significantly inhibits ingestion of food and water, and glucagon-like peptide-1 binding sites are present in a multitude of c entral areas involved in the regulation of ingestional behaviour. To e valuate further the neuroanatomical organization of central glucagon-l ike peptide-1 containing neuronal circuits with potential implications on ingestional behaviour, we carried out a series of experiments in t he rat demonstrating the topographical sites of synthesis and processi ng of the preproglucagon precursor followed by a chromatographic analy sis of the processed fragments. In situ hybridization histochemistry r evealed that preproglucagon encoding messenger RNA was expressed in a single population of neurons in the caudal portion of the nucleus of t he solitary tract. Gel chromatographic analysis of hypothalamic and br ainstem tissue extracts revealed that the preproglucagon precursor is processed in a fashion similar to that seen in the small intestine, pr eferentially giving rise to glicentin, glucagon-like peptide-1 and glu cagon-like peptide-2. This single brain site of glucagon-like peptide- 1 synthesis was subsequently confirmed by immunohistochemical demonstr ation of glucagon-like peptide-l-immunoreactive perikarya in the centr al and caudal parts of the nucleus of the solitary tract. Numerous sit es containing glucagon-like peptide-1 immunoreactive fibres were, howe ver, discovered in the forebrain including hypothalamic, thalamic and cortical areas. The densest innervation by glucagon-like peptide-1 imm unoreactive nerve fibres was seen in the hypothalamic dorsomedial and paraventricular nuclei, but numerous glucagon-like peptide-1 immunorea ctive fibres were also seen throughout the periventricular strata of t he third ventricle. Dual-labelling immunohistochemistry for tyrosine h ydroxylase and glucagon-like peptide-1 gave no evidence for co-localiz ation of catecholamines and glucagon-like peptide-1 in neurons of the lower brainstem. To identify neurons of the nucleus of the solitary tr act that project to the hypothalamic paraventricular nucleus, the retr ograde tracer FluoroGold was injected into this hypothalamic target an d dual immunocytochemical identification of glucagon-like peptide-1 an d tyrosine hydroxylase-positive neurons was performed on brainstem sec tions containing retrogradely labelled perikarya. From this experiment it was seen that many of the retrogradely labelled neurons in the cen tral portion of the nucleus of the solitary tract are catecholaminergi c, while none is glucagon-like peptide-1 immunoreactive. In contrast, most of the retrogradely labelled neurons of the caudal portion of the nucleus of the solitary tract contain glucagon-like peptide-1. These observations further substantiate that glucagon-like peptide-1 neurons of the solitary tract constitute a distinct non-catecholaminergic cel l group which projects to many targets, one of which is the hypothalam ic paraventricular nucleus. Copyright (C) 1997 IBRO.