Effects of pregnant mare serum gonadotropin (PMSG) on pituitary responsiveness to progesterone and LHRH in the hen, Gallus domesticus

Laying hens were primed once daily with either pregnant mare serum gonadotropin (PMSG) or saline until spontaneous ovulations were suppressed (PMSG-treated birds) or for 5-7 days (controls). PMSG-primed hens and saline-primed hens were subsequently injected with vehicle, 500 μg progesterone or 20 μg [Arg⁸]-LHRH. Twelve hr following the injection, PMSG-treated hens were found to lack a normal follicular hierarchy and to have a significantly greater number of follicles larger than 2.5 cm compared to saline-primed hens. Treatment with PMSG also increased basal plasma concentrations of estradiol-17β and decreased plasma LH compared to controls. Finally, challenge with progesterone or LHRH failed to either stimulate LH release or to overcome the ovulation-suppressing effect of PMSG. Results suggest that the action of PMSG in the hen can be attributed, at least in part, to the unresponsiveness of the pituitary to the normal LH-stimulating hormone(s).