Cytochemical evidence for different routes of gonadotropin-releasing hormone processing by large gonadotropes and granulosa cells

The route and rate of internalization of GnRH were compared in studies of dispersed ovarian granulosa cells and large pituitary gonadotropes from fractions enriched by centrifugal elutriation. GnRH receptors were localized with the use of a biotinylated [D-Lys⁶]GnRH analog, followed by avidingold or avidin-biotin-peroxidase complex stains. Both target cell types bound the [biotinyl-D-Lys⁶]GnRH (Bio-GnRH) in 1 min, and there were multiple patches of label on microvilli and coated or uncoated pits by 3 min. Quantification of the avidin-gold stains showed a significant increase in the number of labeled sites per cell profile at 3 min, followed by a decrease 15 min after exposure. No staining was seen in cells treated with medium only or with Bio-GnRH competing with a 100-fold excess of unlabeled [D-Lys⁶]GnRH. Internalization of the Bio-GnRH occurred during the first 3 min in both target cell types. However, the initial sites of processing appeared to be different. In granulosa cells, label was in vesicles and receptosomes (endosomes) and a few small multivesicular bodies. No stain was seen in the Golgi region for at least 15 min, at which time the stain was of low intensity. At later times (15–30 min), most of the label appeared in large multivesicular bodies. In contrast, gonadotropes exhibited labeling in Golgi complex cisternae, condensing vesicles, and immature granules as early as 3 min after exposure. Label was also seen on a subpopulation of granules in the cytoplasm and in a few multivesicular bodies. These comparative studies of two different target cells suggest that whereas the rates of internalization of GnRH are similar, the initial sites of processing may be different. Granulosa cells may degrade or separate the ligand from its receptor in multivesicular bodies. Large pituitary gonadotropes appear to use the Golgi complex route, and the processing may be associated with the formation of granules. The staining pattern correlates with early immunocytochemical studies that showed staining for GnRH on gonadotrope granules. We hypothesize that the granules may be sites for degradation of the ligand, separation of the ligand from its receptor, recycling of the receptor to the plasma membrane, or all three.