INCREASING SENSITIVITY OF THE PITUITARY TO GNRH FROM EARLY TO LATE ANESTRUS IN THE BEAGLE BITCH

The sensitivity of the pituitary to GnRH in early and late anoestrus a nd the indirect response of the ovary were investigated in six adult b eagle bitches. Plasma concentrations of: LH and oestradiol were determ ined after i.v. injection of graded doses of GnRH (0, 0.01, 0.1, 1, 10 and 100 mu g kg(-1)). The responses were measured by the LH and oestr adiol concentration profiles over time. The responses of LH and oestra diol were significantly dose dependent (P = 0.002 and P < 0.001, respe ctively). The responses of LH and oestradiol were significantly higher (P = 0.02 and P = 0.001, respectively) in late anoestrus than in earl y anoestrus. The responses of LH and the responses of oestradiol were positively correlated (v = 0.97, P = 0.001). It is concluded that duri ng the course of anoestrus in the bitch pituitary sensitivity to GnRH increases while the ovary responds accordingly.