Effect of the photoperiod and administration of melatonin on folliculostellate cells of the pituitary pars distalis of adult male viscacha (Lagostomus maximus maximus).

Numerous reports have shown the effect of photoperiod and melatonin administration on the different hormone secreting cell types in the pituitary pars distalis. The viscacha (Lagostomus maximus maximus) is a rodent with photoperiod-dependent seasonal reproduction. The aim of this study was to examine the effect of photoperiod seasonal variations and melatonin administration on the folliculostellate cells in pituitary pars distalis of viscacha. Immunohistochemistry and image analysis were used to measure the percentage of S-100-positive area (total, cellular and colloidal) and the number of folliculostellate cells. The S-100 protein was immunolocalized at intracellular (folliculostellate cells) and extracellular (follicular colloid) levels. The morphometric parameters analyzed exhibited seasonal variations with highest values in the summer (long photoperiod) and lowest values in the winter (short photoperiod). The administration of melatonin caused a significant decrease of immunostaining. Results suggest that the natural photoperiod might be the most important environmental signal causing the decrease in folliculostellate cells immunostaining observed in the winter. These findings agree with seasonal changes previously reported in endocrine cells and suggest that folliculostellate cells may be involved in the paracrine regulation of the secretory activity of pituitary pars distalis through S-100 protein production.