Seasonal variations of pulsatile luteinizing hormone release in the mink (Mustela vison)

The pulsatile secretion of the hypophyseal luteinizing hormone (LH) is induced by the pulsatile secretion of the hypothalamic neurons secreting gonadotrophin-releasing hormone (GnRH). Seasonal variations in the pulsatility of LH were studied in the adult male mink (Mustela vison), reared under natural environmental conditions. Twenty-one animals were studied according to five critical phases in the breeding season: (1) the terminal phase of sexual quiescence, which precedes renewal of gonadal activity (October-November); (2) renewal of gonadal activity (December); (3) maximum gonadal activity at the height of the breeding season (February); (4) reduction of testicular activity (April); and (5) the initial phase of testicular quiescence (June). Levels of gonadal growth and activity were used to define each phase. A second animal group was studied after being reared for 2 months in an experimental gonado-inhibitory photoperiod, which, necessarily for the mink, was of the "long-day" type: 20L:4D regimen in the present study. Results, obtained with fully conscious animals, provide evidence for the pulsatile secretion of gonadotrophic hormone in this species. In spite of inter-individual differences in pulse patterns, particularly in phases 1 and 2, the pulsatile character of LH secretion is seen to vary markedly as a function of gonadal activity. The variations reflect an increase of hypophyseal activity as early as the preparative phase to the breeding season, and a decrease of activity during the testicular regression phase, which is followed by the onset of gonadotrophic quiescence in June. The main parameter affected statistically by these seasonal fluctuations is pulse frequency; variations in pulse frequency correlated with variations in mean plasma concentrations of LH. In the experimental gonado-inhibitory photoperiod, which led to a severe reduction in gonadal activity, all hormonal pulsatility parameters were statistically reduced; this confirms the importance of photoperiodic control of reproduction in Mustela vison. Several possible mechanisms are proposed for photoperiodic control.