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The discussion in perspective

The appreciation and respect of the audience for a series of papers which were all very clearly presented and which complemented one another very well was evident throughout. The direction of the discussion tended to reverse the order of presentation and started with the control of seasonal variation in sheep. Cumming pointed out that there was more to seasonality than a sudden change in sensitivity to negative feedback of oestrogen on LH secretion as had been deduced from the rapid changes in LH concentration seen at the beginning and end of anoestrus in ovariectomized ewes implanted with oestrogen. The progressive build-up in the rate of ovulation towards the middle of the breeding season and its subsequent decline, routinely observed in Australia, suggested another dimension, possibly involving FSH. Cumming also drew attention to the time lags involved and observed that one really had to think in terms of long- and short-term effects. Lamming suggested that the control of seasonal breeding might not be quite as simple or as general as the study of sheep of one breed in one environment might imply. He acknowledged that the long-term ovariectomized ewe used by his group was not an appropriate model to study seasonal changes in sensitivity to negative feedback but wondered whether the Karsch models were adequate in terms of positive feedback. He asked if oestrogen given shortly after the regression of the last corpus luteum of the breeding season would lead to ovulation (inferring that exogenous administration would overcome the inadequate build up of preovulatory oestrogen due to inadequate LH) but this had not been tried. Secondly, why do sheep ovulate following the withdrawal of progestagen sponges during anoestrus? LH must then be able to overcome negative feedback and stimulate oestrogen production to a sufficient extent to lead to positive feedback and ovulation. Karsch suggested that this may be a characteristic of the progestagen, the dose used, or the stage of anoestrus; in Michigan, when they had used implants to raise physiological levels of progesterone, their removal did not lead to similar events. Reference was then made to the 3 sheep of the 7 infused with LH during anoestrus that did not secrete enough oestrogen to trigger positive feedback. Did this imply a seasonal decline in sensitivity to positive feedback? It was, however, pointed out that the concentrations of oestrogen achieved in peripheral plasma in 2 of these 3 ewes was particularly low, possibly due to a carry-over effect of depression by severe negative feedback before the start of the infusion. Baird commented that high concentrations of LH are known to terminate oestrogen secretion and that the infusion of 4 ng LH/min could have been on the border line since 3 ng/min stimulated and 5 ng/min terminated oestrogen secretion. (The 3 ewes which failed to ovulate may, however, give us as much information as the 4 which did.) The role of variation in positive feedback in anoestrus was also discussed earlier in the meeting when Friman and others reported that they, like Karsch, could not detect seasonal differences. Cumming stated that he and his colleagues have found photoperiodic effects on positive feedback in support of the original observation of seasonal differences made by workers at A.B.R.O. The balance between the dose of oestrogen and the seasonal characteristics of the breed studied were agreed to be relevant; the Suffolk sheep used in Michigan would, for example, ovulate in response to stress at any time of anoestrus; the Finnish Landrace in the original study of Land and his colleagues at A.B.R.O. have been postulated to be particularly insensitive to oestrogen. There are two separate questions, "does a seasonal decline in sensitivity to positive feedback contribute to the cessation of oestrous cycles?" (to which the Karsch data say no) and secondly, "might such variation nevertheless exist and contribute to our understanding of the basic changes in hypothalamic activity throughout the year?" (to which Karsch has not yet addressed himself and to which the answer might well be yes). Cumming pointed out that the first ovulation at the beginning of the breeding season is not preceded by increased progesterone. This can be extrapolated to suggest that, although failure of positive feedback may not precipitate the end of one breeding season, it may delay the onset of the next.