Seasonal variations of gonadotropins in the pars distalis male Viscacha pituitary. Effect of chronic melatonin treatment

The gonadotropes, LH and FSH cells, were immunohistochemically identified in the pituitary pars distalis of the adult male viscacha (Lagostomus maximus maximus) using specific antibodies against hLH‚ and hFSH‚ with the streptavidinbiotin- peroxidase complex. The distribution, size and percentage immunopositive area of these cells were analyzed by image analysis in viscachas captured during the annual reproductive cycle and after the chronic administration of melatonin. The LH‚ and FSHb cells showed seasonal changes in the distribution, size and percentage immunopositive area. The LHb cells were found widely distributed throughout the pars distalis during the reproductive period, and they were found in the ventro-medial region in the pars distalis during the gonadal regression and gonadal recovery periods. The LHb cells reached the largest size and immunopositive area during the reproductive period and the smallest size and immunopositive area during the gonadal regression period. The FSHb cells were found in the ventro-medial region during reproductive and gonadal regression periods. The FSHb cells were found widely distributed throughout the pars distalis during the gonadal recovery period when they showed the maximum percentage immunopositive area. A decrease in the size of LH‚ and FSHb cells was observed after the chronic administration of melatonin. Moreover, it produces a decrease in the immunopositive area occupied by the LH‚ cells but not in the immunopositive area occupied by the FSHb cells. Our results show great activity of LHb and FSHb cells in different moments of the annual reproductive cycle demonstrating that these cells are not secrete in parallel. Moreover, melatonin acts differentially on the activity of the gonadotrope cells.