Biphasic response in the secretion of gonadotrophin-releasing hormone in ovariectomized ewes injected with oestradiol.

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-17 beta (25 micrograms i.v. and 25 micrograms 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 the 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 were 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 less than 0.01) and amplitude (15.2 +/- 4.6 pmol/l, P less than 0.05) of the GnRH pulses decreased.(ABSTRACT TRUNCATED AT 250 WORDS)