Central administration of corticotrophin releasing hormone but not arginine vasopressin stimulates the secretion of luteinizing hormone in rams in the presence and absence of testosterone.

Abstract

This study tested the hypothesis that central administration of corticotrophin-releasing hormone (CRH) and/or arginine vasopressin (AVP) will affect the secretion of LH in rams and that testosterone is necessary for these actions to occur. Plasma LH levels were measured in castrated rams during 1 h infusion of either 100 microliter vehicle/mock cerebrospinal fluid (CSF) or mock CSF containing 25 microgram CRH, 25 microgram AVP or 25 microgram of each peptide through guide cannulae into the third cerebral ventricle. These intracerebroventricular (i.c.v.) infusions were given to the castrated rams following injections (i.m.) each 12 h of oil or 8 mg testosterone propionate for 7 days. Blood samples were collected every 10 min for 4 h before i.c.v. infusion, during infusion and for 4 h following the infusion. Infusion of vehicle did not affect any endocrine parameters. In contrast, the plasma concentrations of LH and the amplitude of LH pulses were increased significantly during and following infusion of CRH, and this effect was not influenced by whether the castrated rams were treated with testosterone propionate or whether the CRH was administered in combination with AVP. Infusion of AVP alone did not affect LH secretion. The frequency of LH pulses and the plasma concentrations of FSH did not change with any of the i.c.v. treatments. The plasma concentrations of cortisol were significantly increased by CRH and AVP infusions. The plasma concentrations of cortisol achieved during and following i.c.v. infusion of CRH and AVP combined were greater than the concentrations achieved as a result of treatment with AVP alone but were similar to those with CRH. There was no effect of testosterone propionate on cortisol levels. These results show that CRH, but not AVP, is capable of acting either centrally or at the pituitary level to increase the secretion of LH in rams and these actions are not affected by testosterone. The stimulatory effects of CRH on LH secretion are to increase the amplitude of GnRH pulses and/or the responsiveness of the pituitary to the actions of GnRH with no effect on the frequency of GnRH pulses. The secretion of FSH in rams is not influenced by either CRH or AVP. The effect of CRH to increase LH pulse amplitude occurs in the face of increased cortisol levels, further reinforcing our belief that this adrenal steroid does not affect the reproductive axis in this species.