The effect of repeated endotoxin injections on gonadotropin secretion in ewes

Although it is well known that acute inflammation induced by a single injection of the bacterial endotoxin, lipopolysaccharide (LPS), induces a stress response and suppresses luteinizing hormone (LH) secretion, the effects of repeated endotoxin administration on gonadotropin secretion in sheep have not been studied yet. In this work, the influence of inflammation induced by six days of intravenous LPS injection (400 ng.kg(-1) per day) on the release of LH and follicle-stimulating (FSH) hormones was evaluated. Anoestrous ewes were bled twice a day 1 h before and 3 h after LPS injection. Endotoxin injection decreased (P < 0.05) the release of LH from the first day of the experiment. Moreover, on day 5, the suppression of LH secretion was sustained and could also be detected in the samples collected before the next LPS injection. In contrast, elevation (P < 0.05) of FSH release was detected starting from the second dose of the endotoxin. The increased FSH concentration persisted until the end of the experiment. Each LPS injection stimulated (P < 0.05) cortisol release, but from days 3 to 6, this elevation was about three times lower than the level determined during the first two days. The obtained results show that ewes exposed to repeated administration of endotoxin lost their ability to restore proper LH release relatively quickly. This disturbance in gonadotropin secretion could, in part, be associated with the stress induced by LPS, as well as with endotoxin-induced inflammatory challenges.