GnRH-stimulated LH and FSH release by perifused anoestrous red fox pituitary cells: gonadal steroid modulation

Abstract

In this study, gonadotropin releasing hormone (GnRH)-stimulated luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretions in the red fox (Vulpes vulpes L.) have been studied during anoestrus using perifused dispersed pituitary cells. The objective was to compare LH and FSH responses to pulses of GnRH in lactating and non-lactating females and to examine the feedback of gonadal steroids. Our results indicate that the pituitary sensitivity to GnRH was greatly reduced in lactating females compared with non-lactating females. Treatment in vitro of cells during 24 h (short-term effect) by progesterone (P) or oestradiol (E2) induced an increase of LH and FSH release, indicating that even during anoestrus, pituitary cells are able to respond positively to gonadal steroids. In contrast, administration of P or E2 in vivo for several days by silastic capsules (long-term effect) induced a reduction of LH and FSH releases by pituitary cells in vitro. Thus, steroids act directly on the pituitary but their action is time- and dose-dependent. During anoestrus in the fox, particularly during lactation, P and E2 might exert a potent negative feedback control on pituitary secretion. In the red fox (Vulpes vulpes L.), the fact that GnRH-stimulated LH and FSH secretions are reduced in lactating females more than in non-lactating females, suggests that P (secreted in vivo during lactation, probably as a consequence of a luteotrophic action of prolactin) and E2 secreted jointly during episodic phases of oestrogenic activity, may be responsible for decreased pituitary responsiveness observed during early anoestrus. This action of gonadal steroids might occur in association with a specific role of prolactin or a suppressive effect of suckling.