Colocalization of luteinizing hormone and serotonin in secretory granules of mammalian gonadotrophs

Publisher Website
Google Scholar
Abstract
Previous studies have demonstrated the uptake of exogenous and storage of endogenous 5‐hydroxytryptamine (5‐HT) in parenchymal cells of the anterior pituitary. The present experiments were undertaken to test the hypothesis that the endogenous 5‐HT of the anterior lobe of the pituitary is costored with β‐luteinizing hormone (β‐LH) within the same secretory granules of gonadotrophs. Electron microscope immunocytochemistry was used to detect 5‐HT and β‐LH immunoreactivities in the anterior pituitary glands of mice and bats. Primary antisera generated in different species of animals to these two antigens were localized with appropriate species‐specific secondary antisera coupled to colloidal gold particles of different sizes. This enabled 5‐HT and β‐LH immunoreactivities to be demonstrated simultaneously on ultrathin sections of fixed anterior lobe tissue mounted on electron microscope (EM) grids. In both bats and mice 5‐HT immunoreactivity, identified by immunostaining of β‐LH, was found in gonadotrophs, and in no other cell type. Within gonadotrophs about 25% of the secretory granules were labeled by antisera to both 5‐HT and β‐LH, although 100% of granules reacted with the antiserum to B‐LH. No secretory granules were found that were immunostained only by the antiserum to 5‐HT. It is concluded that endogenous 5‐HT may be a normal constituent of mammalian gonadotrophs and that it is colocalized with β‐LH in at least a subset of the secretory granules of these cells. It cannot yet be concluded that gonadotrophs synthesize 5‐HT as well as taking it up from the ambient medium. The function of 5‐HT in the pituitary remains unknown; however, the costorage of 5‐HT and β‐LH in the same secretory granules would seem to be a mechanism to ensure the simultaneous release of the two substances by exocytosis.