DIFFERENTIAL PHOTOPERIODIC CONTROL OF SEASONAL-VARIATIONS IN PULSATILE LUTEINIZING-HORMONE RELEASE IN LONG-DAY (FERRET) AND SHORT-DAY (MINK) MAMMALS

The aim of this study was to elucidate the pituitary adjustments impli cated in photoperiodic control of reproduction in two mammalian specie s, the ferret and the mink. In the ferret, which displays renewed test icular activity when the days lengthen, we observed a stimulation of t he pulsatile liberation of luteinizing hormone (LH) reflecting the pul satile activity of the gonadotropin-releasing hormone (GnRH) system wh en daylight was equal to or in excess of LD 8:16. This photoperiod coi ncides precisely with the phase of photosensitivity known to be essent ial for gonadal stimulation in this species. In the mink, which displa ys renewed testicular activity when the days shorten, pituitary stimul ation was activated when daylight was reduced to LD 11:13, and gonadal stimulation occurred only when daylight was further reduced to LD 10: 14. In addition to requiring different photoperiodic stimuli to trigge r the seasonal reproductive process, the two species display variation s in the ''timing'' of pituitary control of renewed gonadal activity.