Basal and gonadotropin releasing hormone stimulated gonadotropin levels in the female red fox (Vulpes vulpes L.). Negative feedback of ovarian hormones during anoestrus

Abstract

In the red fox, Vulpes vulpes L., an inhibition of gonadotropic function is observed in early anoestrus, particularly during lactation. During this period, secretion of progesterone as a result of the persistent corpora lutea after parturition and episodic releases of estradiol signify ovarian activity, suggesting involvement of these hormones in the modulation of pituitary hormones (luteinizing hormone (LH), follicle-stimulating hormone (FSH)). Effects of ovariectomy and (or) progesterone or estradiol treatments in vivo upon basal and gonadotropin releasing hormone (GnRH)-stimulated LH and FSH were observed. After ovariectomy, a great increase in the basal level of both gonadotropins and in GnRH-stimulated LH release, but not GnRH-stimulated FSH release, were observed. Progesterone treatment induced a decrease in GnRH-stimulated LH and FSH secretions and a decrease in basal LH and FSH levels in ovariectomized females. Estradiol treatment abolished basal secretions and GnRH responses for both hormones. These results suggest a negative feedback of both ovarian steroids at the hypothalamopituitary level on LH and FSH secretions during early anoestrus.