Acute and chronic stress-like levels of cortisol inhibit the oestradiol stimulus to induce sexual receptivity but have no effect on sexual attractivity or proceptivity in female sheep

Stress-like levels of cortisol inhibit sexual receptivity in ewes but the mechanism of this action is not understood. One possibility is that cortisol interferes with the actions of oestradiol to induce sexual receptivity. We tested this hypothesis in 2 experiments with ovariectomised ewes that were artificially induced into oestrus by 12days of i.m. injections of progesterone followed by an i.m. injection of oestradiol benzoate (ODB) 48h later. In Experiment 1, ewes were randomly allocated to the following groups: saline infusion+25mug ODB, saline infusion+50mug ODB, cortisol infusion+25mug ODB or cortisol infusion+50mug ODB (n=5 per group). Saline or cortisol was infused i.v. for 40h beginning at the ODB injection. In Experiment 2, ewes were infused with saline or cortisol (n=5 per group) for 5h beginning 1h before ODB injection. In both experiments, ewe sexual behaviour (attractivity, proceptivity and receptivity) was quantified every 6h. Blood samples were also collected. The cortisol infusion yielded plasma concentrations of cortisol similar to those seen during psychosocial stress. In both experiments, cortisol suppressed receptivity index (number of immobilisations by ewe/courtship displays by ram) and the number of times ewes were mounted but had no effect attractivity or proceptivity, irrespective of the dose of ODB (Experiment 1). Cortisol also suppressed LH pulse amplitude. These results suggest that both an acute (5h) and chronic (40h) infusion of cortisol inhibit oestradiol-induced sexual receptivity in ewes and that increasing the dose of ODB does not overcome the inhibitory effects of cortisol.