Short-day stimulation of testicular activity and immunoreactivity of the hypothalamic GnRH system in mink following deafferentation of the pineal body by bilateral superior cervical ganglionectomy and melatonin replacement

Abstract

The effects of superior cervical ganglionectomy on testicular function (testis volume and plasma testosterone levels) and the immunocytochemical activity of the GnRH hypothalamic system were studied in the mink, a short-day breeder. Animals reared in a natural photo-period were (i) ganglionectomized at four different times during the period extending from the end of summer to the end of autumn (September 15, October 20, October 28, and December 1), and (ii) reared for 50 days in a short gonadostimulatory photoperiod (4L:20D). Lastly, an attempt was made to overcome the effects of superior cervical ganglion removal by administering melatonin to mink reared in a natural photoperiod. In mink reared in a natural photoperiod, deafferentation of the pineal on September 15 ( L= 12.5h) or October 20 ( L= 10.5h) resulted in consistently low values of testicular volume and plasma testosterone until the end of the experiment (February). When the operation was performed on October 28 ( L= 10h) testicular activity was initiated but only lasted a short time and did not allow maximal gonadal development. When superior cervical ganglionectomy was carried out on December 1 ( L= 8.5h), during the phase of renewed testicular activity, the increases in testicular volume and testosterone levels were not affected by the operation and the subsequent variation of these parameters was identical to that observed in intact animals. Similarly, in mink reared for 50 days in a photoperiod of 4L:20D before superior cervical ganglionectomy, deafferentation of the pineal did not prevent gonadostimulation induced by short days. These results show that once the hypothalamic system secreting GnRH is triggered by short days, its activity is no longer controlled by the photoperiod. The immunohistochemical study of GnRH-secreting hypothalamic neurons, carried out in December without prior colchicine treatment in animals ganglionectomized in September or October, showed the presence of only a small number of GnRH-positive perikarya. Similarly, positive axonal endings in the external layer of the median eminence were scarce and weakly immunoreactive. On the other hand, in intact animals, in those ganglionectomized on December 1, and those exposed to the short photoperiod of 4L:20D, perikarya and immunoreactive endings were numerous and intensely labeled. In mink ganglionectomized in October ( L= 10.5h) and thereby maintained in a phase of sexual quiescence, insertion of melatonin implants in January triggered testicular activity 50 days later.