CONTROL OF PULSATILE LH-SECRETION DURING SEASONAL ANESTRUS IN THE EWE

The seasonality of reproductive activity in the ewe in temperate latit udes is controlled by photoperiod. Its annual variations control the t emporal organization of the sexual cycle by changing the activity of t he gonadotrophic axis. Cyclic oestrous behaviour usually appears in th e 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 chara cterized by the absence of ovulation and sexual behaviour. During seas onal anoestrus, a decrease in LH pulse frequency is observed. The inhi bition of pulsatile LH secretion is maintained throughout the anoestro us season and is responsible for the low reproductive activity during this period. Variation in the seasonal inhibition of LH pulsatility re sults from an increase in the negative feedback by oestradiol on LH pu lse frequency during the long days of spring and summer. The inhibitio n of LH secretion involves increased action of dopamine in the hypotha lamus on the chain of nervous elements which controls gonadotrophic ac tivity, Among the various dopaminergic structures, the retrochiasmatic A15 nucleus is involved in the inhibitory control of LH pulsatility b y oestradiol during the long day period. Oestradiol increases the dopa minergic tone of the A15 nucleus in ovariectomized ewes during the lon g day period. In this structure, the effect of oestradiol on the dopam inergic metabolism probably results from a direct, local activation. I n the sheep, dopamine might also participate in the inhibition of gona dotrophin activity during other periods of reproductive life. (C) Inra /Elsevier, Paris.