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This study was performed to explain how the molecular processes governing the biosynthesis of gonadotropinreleasing hormone (GnRH) and GnRH receptor (GnRHR) in the hypothalamic-pituitary unit are reflected by luteinizing hormone (LH) secretion in sheep during anoestrous period and during luteal and follicular phases of the oestrous cycle. Using an enzyme-linked immunosorbent assay (ELISA), we analyzed the levels of GnRH and GnRHR in preoptic area (POA), anterior (AH) and ventromedial hypothalamus (VM), stalk-median eminence (SME), and GnRHR in the anterior pituitary gland (AP). Radioimmunoassay has also been used to define changes in plasma LH concentrations. The study provides evidence that the levels of GnRH in the whole hypothalamus of anoestrous ewes were lower than that in sheep during the follicular phase of the oestrous cycle (POA: p < 0.001, AH: p < 0.001, VM: p < 0.01, SME: p < 0.001) and not always than in luteal phase animals (POA: p < 0.05, SME: p < 0.05). It has also been demonstrated that the GnRHR amount in the hypothalamus-anterior pituitary unit, as well as LH level, in the blood in anoestrous ewes were significantly lower than those detected in animals of both cyclic groups. Our data suggest that decrease in LH secretion during the long photoperiod in sheep may be due to low translational activity of genes encoding both GnRH and GnRHR. Key words: ewe, seasonality, hypothalamus, pituitary, GnRH, GnRHR, LH. Cette etude visait a expliquer comment les processus moleculaires a la base de la biosynthese du GnRH et du R-GnRH dans l'unite constituee de l'hypothalamus et de l'hypophyse se traduisent par la secretion de LH pendant la periode anrestrale et les phases luteale et folliculaire du cycle restral chez la brebis. A l'aide de la methode ELISA (immunoenzymatique), nous avons analyse les taux de GnRH et de R-GnRH dans l'aire previsuelle (APV), dans l'hypothalamus anterieur (HA) et ventro-median (HVM) ainsi que dans la tige pituitaire et l'eminence mediane (TEM), et les taux de R-GnRH dans le lobe anterieur de l'hypophyse (LAH). Nous avons aussi determine les variations des concentrations plasmatiques de LH a l'aide de la technique de RIA (radioimmunoessai). L'etude apporte des donnees selon lesquelles les taux de GnRH dans l'ensemble de l'hypothalamus des brebis en periode anrestrale sont plus bas que pendant la phase folliculaire du cycle restral (APV : p < 0,001, HA : p < 0,001, HVM : p < 0,01, TME : p < 0,001). Par contre, ces memes taux de GnRH ne sont pas toujours plus bas que chez les animaux en phase luteale (APV : p < 0,05, TEM : p < 0,05). Nous avons aussi montre que pendant la periode anrestrale chez la brebis, de fa^on analogue aux taux de LH dans le sang, les taux de R-GnRH sont plus bas dans l'unite constituee de l'hypothalamus et du lobe anterieur de l'hypothalamus que ceux qui sont mesures chez les animaux des deux groupes du cycle. Nos donnees laissent entendre que la diminution de la secretion de LH pendant la longue photoperiode chez la brebis pourrait etre le fait d'une faible activite de traduction des genes codant le GnRH et le R-GnRH. [Traduit par la Redaction] Mots-cles: brebis, saisonnalite, hypothalamus, hypophyse, GnRH, R-GnRH, LH., Introduction Seasonal and cycle stage-related changes in the innervations of hypothalamic gonadotropin-releasing hormone (GnRH) cells and in the functions of some neural systems affecting GnRH release in sheep pointed to [...]

Contents The effects of prolonged, intermittent infusion of β-endorphin or naloxone into the third cerebral ventricle of follicular-phase ewes on the expression of genes encoding Gn RH and Gn RHR in the hypothalamus and Gn RHR in the anterior pituitary gland (AP) were examined by an enzyme-linked immunoabsorbent assay. Activation or blockade of μ-opioid receptors significantly decreased or increased the Gn RH concentration and Gn RHR abundance in the hypothalamus, respectively, and affected in the same way Gn RHR quantity in the AP gland. The changes in the levels of Gn RH and Gn RHR after treatment with β-endorphin as well as following action of naloxone were reflected in fluctuations of plasma LH concentrations. On the basis of these results, it is suggested that β-endorphinergic system in the hypothalamus of follicular-phase ewes affects directly or via β-endorphin-sensitive interneurons Gn RH and Gn RHR biosynthesis leading to suppression in secretory activity of the hypothalamic-pituitary axis. [ABSTRACT FROM AUTHOR]