IDENTIFICATION OF SPECIFIC BINDING-SITES FOR GLUCAGON-LIKE PEPTIDE-1 ON THE POSTERIOR LOBE OF THE RAT PITUITARY

Glucagon-like peptide-1 (GLP-1) immunoreactivity has been found in aut onomic and neuroendocrine brain regions, whereas only limited data are available regarding the characterization and localization of brain GL P-1 receptors. In the present study, using quantitative in vitro autor adiography, a high density of specific binding sites for GLP-1 was cha racterized on sections of the posterior pituitary lobe of the rat. Low specific binding of radiolabeled GLP-1 was found in the anterior lobe and no specific binding in the intermediate lobe. To examine the spec ificity of GLP-1 binding sites, sections of the posterior lobe were in cubated with radiolabeled GLP-1 in the presence of various peptides. R adiolabeled [Tyr(39)]exendin-4, a specific GLP-1 agonist, bound to the se receptor sites with the same affinity as GLP-1, while glucagon and vasoactive intestinal peptide (VIP) were unable to displace I-125-GLP- 1. Both unlabeled exendin-4 and GLP-1 inhibited this binding with equa lly high affinity. Using I-125-[Tyr(39)]exendin-4 as radiolabel, the c oncentration of binding sites was found to be 7.8 +/- 0.4 fmol/mg tiss ue. Further analysis of the binding data from experiments with tissue slices revealed the presence of high and low affinity binding sites. I n experiments with unlabeled [Tyr(39)]exendin-4, the K(d)s were 6.2 +/ - 1.4 x 10-(12) and 9.3 +/- 1.5 x 10-(10) M, respectively, and in expe riments with unlabeled GLP-1, 3.4 +/- 1.8 x 10-(12) and 5.9 +/- 1.5 x 10-(10) M, respectively. Since both GLP-1 and [Tyr(39)]exendin-4 revea l the same binding characteristics, it is concluded that both peptides bind to the same receptor sites in the rat posterior pituitary lobe. I-125-GLP-1 and I-125-[Tyr(39)]exendin-4 also specifically bound to th e nucleus of the solitary tract, the subfornical organ, the median emi nence, the lateral mammillary nucleus and the lateral subnucleus of th e interpeduncular nucleus. The presence of GLP-1-binding sites in diff erent circumventricular organs suggests that this hormone may serve to modulate autonomic and neuroendocrine functions in the rat brain.