EFFECTS OF SUPERIOR CERVICAL GANGLIONECTOMY AND MELATONIN REPLACEMENTON INTRA-HYPOTHALAMIC LHRH CONTENT AND PULSATILE LUTEINIZING-HORMONE RELEASE IN THE MINK

Long-term effects of subcutaneous melatonin implants on intrahypothala mic LHRH content and on pulsatile luteinizing hormone release have bee n 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 natur al photoperiod environment to short daylengths (LD 4:20), melatonin pe llets 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 impla nts. The intra-hypothalamic LHRH content in ganglionectomized mink was at a basal level similar to that observed during seasonally sexual qu iescence, or after exposure to inhibitory long days (LD 20:4). A signi ficant and transient elevation in LHRH content was observed already af ter fifteen days, and also one month after insertion of melatonin impl ants. This resulted in mean values similar to those observed during th e breeding season, or after exposure to stimulatory short days (LD 4:2 0). 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 co ntent induced by melatonin treatment, and maximum values were reached after two months. Pituitary activity tended to decrease after three mo nths, characterized in particular by a significant decrease in the mea n frequency of LH pulses. In addition, the increase in pulsatile chara cteristics of LH release occurred two months before the peripheral ren ewal of testicular activity. Apparently, the reproductive endocrine fu nction in ganglionectomized mink treated with melatonin implants is re stored more rapidly at the hypothalamic level than at the pituitary or testicular levels.