Feedback basis of seasonal breeding: test of an hypothesis

Abstract

It has long been recognized that the seasonal pattern of reproduction in many species is controlled by environmental photoperiod (Yeates, 1949;Hafez, 1952;Menaker, 1971;Follett, 1978;Turek & Campbell, 1979). Insight into the mechanisms whereby photoperiodic cues are transduced into endocrine messages which govern gonadal function, however, has been gained more recently. It has been proposed that in ewes this process of transduction includes a marked change in the hypothalamo\p=n-\hypophysialsystem which controls the tonic mode of gonadotrophin secretion (Legan, (Baird, 1978). The rising levels of LH stimulate ovarian oestradiol secretion, and the two hormones increase in a parallel fashion until oestradiol reaches the threshold for eliciting oestrous behaviour and the LH surge which causes ovulation. The parallel rises in LH and oestradiol are sustained, typically continuing for 48-60 h and ending with the preovulatory LH surge. A crucial requisite for this progression of events is a low response to oestradiol

Suppressed release of LH in ovariectomized post-partum ewes

“…If the pulsatile release of LH reflects the pulsatile release of LH-RH (Lincoln & Short, 1980) Goodman & Legan, 1980), could in part reflect a reduction in hypothalamo-pituitary activity independent of the influence of ovarian hormones, as well as an increased inhibitory effect of oestrogen on LH release (Legan, Karsch & Foster, 1977). A time effect would suggest that ovarian hormones have a role in the control of the neuroendocrine mechanisms responsible for the rhythmic release of LH.…”

Season influences FSH concentration in ovariectomized Ile-de-France ewes

“…Acting through the pineal gland, photoperiod determines the activity of the LH pulse generating mechanism. Seasonal changes occur both in the ability of oestradiol to inhibit the frequency of LH pulses (Legan et al, 1977;Goodman et al, 1982;Martin et al, 1983) and direct effects on the frequency of LH pulses in the absence of gonadal steroids (Karsch et al, 1980;Goodman et al, 1982;Montgomery et al, 1985;Robinson et al, 1985).…”

Control of pulsatile LH secretion during seasonal anoestrus in the ewe

“…The seasonality of reproductive activity in the ewe is controlled by environmental factors [53], mainly photoperiod, which organize the annual sexual cycle [50]. Ewes [38,44]. It should also be mentioned that a direct, non-oestradiol-dependent effect of photoperiod on LHRH/LH pulsatility has been clearly demonstrated, but up to now, it appears quantitatively minor [58,92].…”