Effect of stress-like concentrations of cortisol on follicular developmentand the preovulatory surge of LH in sheep

Stress-Like concentrations of cortisol Increase the negative feedback poten cy of oestradiol in castrated male sheep. A similar cortisol-dependent resp onse in female sheep might be expected to suppress gonadotrophin secretion and impair follicular development and ovulation. The oestrous activity of 2 1 female sheep was synchronized using progestogen-treated vaginal pessaries to test this hypothesis. Stress-like concentrations of cortisol (60-70 ng ml(-1)) were established by continuous infusion of cortisol (80 mu g kg(-1) h(-1); n = 13) beginning 5 days before, and continuing for 5 days after, p essary removal. Control animals (n = 8) received a comparable volume of veh icle (50% ethanol-saline) over the 10 day infusion period. Serum concentrat ions of oestradiol increased progressively in control sheep during the 48 h immediately after pessary removal. This increase in serum oestradiol was b locked or significantly attenuated in sheep receiving stress-like concentra tions of cortisol. Preovulatory surge-like secretion of LH was apparent in control animals 58.5 +/- 2.1 h after pessary removal. In contrast, surge-li ke secretion of LH was not observed during the 5 days after pessary removal in 54% (7 of 13) of sheep receiving cortisol. Moreover, the onset of the s urge was significantly delayed in the cortisol-treated ewes that showed sur ge-like secretion of LH during the infusion period. The ability of episodic pulses of exogenous GnRH to override the anti-gonadal effect of cortisol w as examined in a second study. Oestrous activity of 12 ewes was synchronize d using progestogen-containing pessaries as described above. Ewes were rand omly assigned to one of three treatment groups (n = 4 ewes per group). Anim als received cortisol (100 mu g kg(-1) h(-1); groups 1 and 2) or a comparab le volume of vehicle (group 3) beginning 5 days before, and continuing for 2 days after, pessary removal. Pulses of GnRH (4 ng kg(-1) h(-1), i.v.; gro up 1) or saline (groups 2 and 3) at 1 h intervals were initiated at pessary removal and continued for 48 h. Serum concentrations of oestradiol were no t significantly increased after pessary removal in sheep receiving cortisol alone. Conversely, serum concentrations of oestradiol increased progressiv ely during the 48 h after pessary removal in control ewes and in ewes recei ving cortisol and GnRH. At the end of infusion, serum concentrations of oes tradiol did not differ (P > 0.05) between control (7.7 +/- 0.8 pg ml(-1)) e wes and ewes receiving cortisol and episodic GnRH (6.4 +/- 1.3 pg ml(-1)). Moreover, these values were significantly greater (P < 0.05) than the serum concentrations of oestradiol in animals receiving cortisol (1.0 +/- 0.4 pg ml(-1)) alone. Collectively, these data indicate stress-like concentration s of cortisol block or delay follicular development and the preovulatory su rge of LH in sheep. In addition, episodic GnRH overrides cortisol-induced d elay in follicular maturation.