Effects of Superior Cervical Ganglionectomy and Melatonin Replacement on Intra-hypothalamic LHRH Content and Pulsatile Luteinizing Hormone Release in the Mink

Abstract

Long-term effects of subcutaneous melatonin implants on intrahypothalamic LHRH content and on pulsatile luteinizing hormone release have been investigated in ganglionectomized male mink. Animals were submitted to bilateral removal of the superior cervical ganglion in mid-April. A preliminary study revealed that plasma LH concentrations remain at a basal level throughout the year following ganglionectomy. In a second experiment, one month after ganglionectomy and transfer from the natural photoperiod environment to short daylengths (LD 4:20), melatonin pellets were subcutaneously implanted to overcome deafferentation of the pineal. Progressive effects of treatment were studied 7 days, 15 days, and one, two and three months after insertion of the melatonin implants. The intra-hypothalamic LHRH content in ganglionectomized mink was at a basal level similar to that observed during seasonally sexual quiescence, or after exposure to inhibitory long days (LD 20:4). A significant and transient elevation in LHRH content was observed already after fifteen days, and also one month after insertion of melatonin implants. This resulted in mean values similar to those observed during the breeding season, or after exposure to stimulatory short days (LD 4:20). A decrease in hypothalamic LHRH content started after two months. No pattern of pulsatile LH secretion was recorded in ganglionectomized untreated mink. A significant increase in all parameters of pulsatile LH secretion was observed fifteen days after the elevation of LHRH content induced by melatonin treatment, and maximum values were reached after two months. Pituitary activity tended to decrease after three months, characterized in particular by a significant decrease in the mean frequency of LH pulses. In addition, the increase in pulsatile characteristics of LH release occurred two months before the peripheral renewal of testicular activity. Apparently, the reproductive endocrine function in ganglionectomized mink treated with melatonin implants is restored more rapidly at the hypothalamic level than at the pituitary or testicular levels.

Testicular activity is restored by melatonin replacement after suprachiasmatic nucleus lesion or superior cervical ganglionectomy in mink

“…In mink, SCNx or pineal deafferentation following SCGx prevented the stimulation of testicular activity generally induced by autumnal or experimental exposure to short days. These results are consistent with previous results btained in this species (Maurel et al., 1991a, 1992a, 1997). The SCN has been shown to be the major circadian pacemaker generating rhythms and, in mink, SCN destruction affects the general locomotor activity rhythm (Maurel et al., 1991b).…”

“…Numerous studies on pinealectomized hamster and sheep have stressed the importance of using a timed infusion paradigm (TIP) for melatonin delivery (reviewed in Bartness et al., 1993), showing the importance of the frequency of signal presentation and of a period of low to zero circulating melatonin concentration. However, it has been shown that the continuous release of melatonin by subcutaneous implants or intrahypothalamic microimplants, without daily rhythmicity may induce gonad stimulation not only at the testicular level but also by reinducing the pulsatile secretion of luteinizing hormone (LH) in various animals including the mink (Maurel et al., 1997). In both sheep and hamster experiments, the duration of the nocturnal melatonin peak and not its circadian phase is most important for determining the reproductive response (see Goldman, 1999).…”