ULTRASTRUCTURAL-LOCALIZATION OF GALANIN AND GALANIN RECEPTORS IN THE GUINEA-PIG MEDIAN-EMINENCE

The aim of this work was to compare the localization of galanin and ga lanin receptors in the guinea pig median eminence at the light and ele ctron microscopic level. Concerning galanin the highest labeling was s hown in the external part of the median eminence. At the ultrastructur al level, galanin immunoreactivity was observed only in nerve terminal s containing granular vesicles of approximately 120 nm in diameter. Li ght microscopic autoradiographs of semithin sections exhibited a moder ate labeling in the external part of the median eminence. Galanin rece ptors were labeled in vitro on semithin sections (2 mu m) using the hi ghly specific radioligand [I-125]galanin. Ultrastructural data showed that most of galanin binding sites overlaid membrane appositions betwe en nerve terminals and also between nerve terminal and tanycyte. By co nsidering the percentages in the distribution of the binding it appear ed that galanin receptors were located on some nerve ending membranes. Our observations were not really in favor of a presence of receptors in tanycytes. The presence of galanin nerve endings in the external pa rt suggests that like in the rat the peptide may have a direct hypophy siotrophic role. In contrast, the occurrence of numerous binding sites gives additional arguments in favor of a local action (paracrine and/ or autocrine) of galanin occurring via galanin receptors located essen tially on the pericapillary nerve terminals in the guinea pig median e minence. (C) 1997 Elsevier Science B.V.