Increasing sensitivity of the pituitary to GnRH from early to late anoestrus in the beagle bitch.

The sensitivity of the pituitary to GnRH in early and late anoestrus and the indirect response of the ovary were investigated in six adult beagle bitches. Plasma concentrations of LH and oestradiol were determined after i.v. injection of graded doses of GnRH (0, 0.01, 0.1, 1, 10 and 100 micrograms kg-1). The responses were measured by the LH and oestradiol concentration profiles over time. The responses of LH and oestradiol were significantly dose dependent (P = 0.002 and P < 0.001, respectively). The responses of LH and oestradiol were significantly higher (P = 0.02 and P = 0.001, respectively) in late anoestrus than in early anoestrus. The responses of LH and the responses of oestradiol were positively correlated (r = 0.97, P = 0.001). It is concluded that during the course of anoestrus in the bitch pituitary sensitivity to GnRH increases while the ovary responds accordingly.