STIMULATION OF LH-SECRETION IN SHEEP BY CENTRAL ADMINISTRATION OF CORTICOTROPIN-RELEASING HORMONE

Corticotrophin-releasing hormone (CRH) has been proposed as a mediator of the anti-reproductive effects of stress through an action within t he hypothalamus to inhibit GnRH secretion. This hypothesis was tested in sheep by studying the responses to central administration of CRH in both sexes and in both seasons. Sexually mature, Ile-de-France ewes a nd Romanov rams that had been,gonadectomized and implanted with a perm anent guide cannula into the third cerebral ventricle were used. Ewes were studied in the presence and absence of exogenous oestradiol plus progesterone, in both the breeding and anoestrous seasons. All rams we re treated with testosterone and were studied only during the breeding season. Each observation involved serial samples (every 10 min) of ju gular blood for 5 h before (control) and 5 h after an intracerebrovent ricular (icy) injection of either saline (vehicle) or 5 nmoles CRH in 20 mu l vehicle. The saline injections did not affect any of the endoc rine variables measured; however, CRH always increased cortisol concen trations in jugular plasma. In the absence of treatment with replaceme nt sex steroids, icy injection of CRH had no effect on pulsatile LH se cretion in females either during the breeding season or during anoestr us. However, LH pulse frequency and mean LH concentrations increased s ignificantly on every occasion on which animals were treated with sex steroids. Treatment with CRH also increased LH secretion in the testos terone-treated rams. It is concluded that, contrary to the hypothesize d role of CRH as an inhibitor of reproductive activity, this neuropept ide stimulates pulsatile LH (and thus GnRH) secretion, at least in thi s species. The fact that gonadal steroids seem to be obligatory for th e expression of this effect suggests that the protocols used in past s tudies need to be reassessed.