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In ovariectomized ewes, an injection of oestrogen initially inhibits the tonic secretion of LH, and then induces a large release of LH similar to the preovulatory surge in intact ewes. The pattern of hypothalamic secretion of gonadotrophin-releasing hormone (GnRH) into the pituitary portal blood during this biphasic response to oestrogen was investigated in conscious, unrestrained, ovariectomized adult Ile-de-France ewes during the breeding season. The ewes were ovariectomized and implanted with cannulae for portal blood collection on the same day. Seven days later, portal and peripheral blood samples were collected simultaneously every 5 min for 25 h. The ewes were injected with oestradiol-17β (25 µg i.v. and 25 µg i.m.) 6·25 h after the start of sampling. GnRH and LH were measured by radioimmunoassay in portal and jugular plasma samples respectively.
A clear pulsatile pattern of LH secretion was observed before the oestradiol injection in all ewes, followed by the typical biphasic decrease (negative feedback) and increase (positive feedback) in mean concentrations. The sampling period was divided, for analysis, into pretreatment, negative feedback and positive feedback phases. Before injection with oestradiol, the GnRH pulses were clearly defined in portal blood and always synchronized with LH pulses in the peripheral circulation. The frequency was 5·9 ± 0·6 pulses/6 h (mean ± S.E.M.), and the amplitude was 31·6±7·6 pmol/l. During negative feedback, both the frequency (4·2 ± 0·5 pulses/6 h, P<0·01) and amplitude (15·2 ± 4·6 pmol/l, P<0·05) of the GnRH pulses decreased. During positive feedback, there was a large surge in the concentration of GnRH, due primarily to an increase in pulse frequency (11·0±1·3 pulses/6 h, P<0·01). A change in pulse amplitude was not detected, but there was a large increase in the basal level of GnRH (P<0·05). As a consequence of the changes in frequency and amplitude of the pulses, the mean levels of GnRH before injection with oestradiol (5·3 ± 1·0 pmol/l) differed (P<0·05) from those during negative (3·8±0·5 pmol/l) and positive (18·9±4·7 pmol/l) feedback phases.
These results show that the biphasic pattern of LH secretion induced by oestrogen injection in short-term ovariectomized ewes is caused by parallel changes in the secretion of GnRH as well as changes in pituitary responsiveness to GnRH. An abrupt increase in the frequency of GnRH pulses appears to be a key component of the positive feedback mechanism which elicits the oestradiol-induced surges of both GnRH and LH.
Journal of Endocrinology (1989) 123, 375–382
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