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 secret ing gonadotrophin-releasing hormone (GnRH). Seasonal variations in the pulsatility of LH were studied in the adult male mink (Mustela vision ), reared under natural environmental conditions. Twenty-one animals w ere 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 gonad al 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 pulsat ile secretion of gonadotrophic hormone in this species. In spite of in ter-individual differences in pulse patterns, particularly in phases 1 and 2, the pulsatile character of LH secretion is seen to vary marked ly as a function of gonadal activity. The variations reflect an increa se of hypophyseal activity as early as early as the preparative phase to the breeding season, and a decrease of activity during the testicul ar regression phase, which is followed by the onset of gonadotrophic q uiescence in June. The main parameter affected statistically by these seasonal fluctuations is pulse frequency; variations in pulse frequenc y correlated with variations in mean plasma concentrations of LH. In t he experimental gonado-inhibitory photoperiod, which led to a severe r eduction 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.