Control of pulsatile LH secretion during seasonal anoestrus in the ewe.

The seasonality of reproductive activity in the ewe in temperate latitudes is controlled by photoperiod. Its annual variations control the temporal organization of the sexual cycle by changing the activity of the gonadotrophic axis. Cyclic oestrous behaviour usually appears in the ewe at the end of summer or the beginning of autumn and finishes in winter or at the very beginning of spring. Seasonal anoestrus is characterized by the absence of ovulation and sexual behaviour. During seasonal anoestrus, a decrease in LH pulse frequency is observed. The inhibition of pulsatile LH secretion is maintained throughout the anoestrous season and is responsible for the low reproductive activity during this period. Variation in the seasonal inhibition of LH pulsatility results from an increase in the negative feedback by oestradiol on LH pulse frequency during the long days of spring and summer. The inhibition of LH secretion involves increased action of dopamine in the hypothalamus on the chain of nervous elements which controls gonadotrophic activity. Among the various dopaminergic structures, the retrochiasmatic A15 nucleus is involved in the inhibitory control of LH pulsatility by oestradiol during the long day period. Oestradiol increases the dopaminergic tone of the A15 nucleus in ovariectomized ewes during the long day period. In this structure, the effect of oestradiol on the dopaminergic metabolism probably results from a direct, local activation. In the sheep, dopamine might also participate in the inhibition of gonadotrophin activity during other periods of reproductive life.