Reproduction (2003) 125, 347–355 Research Effects of pregnancy on pulsatile secretion of LH and gonadotrophin-releasing hormone-induced LH release in sheep: a longitudinal study K. H. Al-Gubory 1 , J. Hervieu 2 and P. A. Fowler 3 1 Unit´ e de Recherches de Physiologie Animale, 78352 Jouy-en-Josas Cedex, France; 2 Domaine Exp´ erimental de Brou¨ essy, 78114 Magny-les-Hameaux, Institut National de la Recherche Agronomique, France; and 3 Department of Obstetrics and Gynaecology, University of Aberdeen, Aberdeen AB25 2ZD, UK Pulsatile LH secretion and its control throughout pregnancy have not been fully determined in sheep. Expt 1 determined the patterns of LH secretion in five ewes on days 10, 20, 60 and 120 of pregnancy and on day 10 postpartum, compared with those on day 10 of the oestrous cycle. Mean ( ± SEM) concentrations of LH declined steadily throughout pregnancy (ANOVA, P < 0.01) and were lower (P < 0.01) on day 60 (0.19 ± 0.3 ng ml −1 ) and on day 120 (0.18 ± 0.4 ng ml −1 ) of pregnancy than on day 10 of the oestrous cycle (0.55 ± 0.04 ng ml −1 ). This decrease was due to a significant reduction in the number and the amplitude of LH pulses. Only on day 120 of pregnancy were progesterone concentrations higher (P < 0.01) than on day 10 of the oestrous cycle. Although concentrations of progesterone on day 10 postpartum were barely detectable, mean LH concentration (0.45 ± 0.09 ng ml −1 ) was not different from that on day 10 of the oestrous cycle. Expt 2 examined the LH responses in a separate group of four ewes to a physiological dose of GnRH (0.2 g) on days 10, 20, 60 and 120 of pregnancy and on day 10 postpartum, compared with those on day 10 of the oestrous cycle. The area under the LH response curve and the maximum LH concentrations induced by GnRH declined steadily throughout pregnancy (ANOVA, P < 0.01) and were lower (P < 0.01) on days 60 and 120 of pregnancy than on day 10 of the oestrous cycle, but these parameters were not different between day 10 postpartum and day 10 of the oestrous cycle. Expt 3 examined the LH responses in a separate group of four ewes to a potent GnRH agonist, buserelin (0.5 g), on days 10, 60 and 120 of pregnancy. The area under the LH response curve and the maximum LH concentrations induced by GnRH were lower (P < 0.01) on days 60 and 120 than on day 10 of pregnancy, but were not different between days 60 and 120. This longitudinal study demonstrates that the pulsatile LH release and pituitary responsiveness to GnRH decreases progressively as pregnancy advances, but does not support the hypothesis that high concentrations of progesterone are solely responsible for the inhibition of pulsatile LH secretion and GnRH-induced LH release during pregnancy in sheep. Introduction LH is released by the pituitary gland in a pulsatile manner, resulting in peripheral blood LH patterns characterized by peaks. Pulsatile LH secretion in sheep is driven by the episodic discharges of the hypothalamic peptide, GnRH (Caraty et al., 1982; Clarke and Cummins, 1982; Levine et al., 1982). Variations in pulsatile LH release occur during different stages of the oestrous cycle in sheep (Hauger et al., 1977) and reflect changes in pulsatile GnRH release (Clarke et al., 1987). A great deal of information is available regarding pituitary LH respon- siveness to GnRH (Chamley et al., 1974; Jenkin and Heap, 1974; Jenkin et al., 1977; Wright et al., 1981), Email: algubory@jouy.inra.fr pituitary content of LH (Chamley et al., 1976; Jenkin et al., 1977; Crowder et al., 1982), the amount of mRNA encoding pituitary LH and  subunits (Wise et al., 1985; Fowler and McNeilly, 1997) and GnRH receptor (Fowler and McNeilly, 1997), and the quantity of pituitary GnRH receptors and the stores of GnRH in the median eminence, the preoptic area and the hypothalamus (Crowder et al., 1982) in pregnant ewes. Despite these investigations, the pulsatile secretion of LH and its control throughout pregnancy have not been elucidated. Previous efforts to characterize the pattern of LH secretion by intensive blood sampling have failed to show changes in pulsatile LH release in advancing pregnancy in ewes (Al-Gubory et al., 1994). In this investigation, the study of pulsatile LH release was limited because blood sampling was not performed during the oestrous c  2003 Society for Reproduction and Fertility 1470-1626/2003