OVARIAN MODULATION OF THE ESTRADIOL-INDUCED LH SURGE IN PREPUBERTAL AND SEXUALLY MATURE GILTS

The role of ovarian secretions in modulating positive oestrogen feedba ck on LH release in late prepubertal and sexually mature,gilts was stu died. Gilts were either ovariectomized (OVX) at 230 days of age (sexua lly mature, OVX 230, control), ovariectomized at 160 days of age (pre- pubertal, OVX 160) or ovariectomized at 160 days of age and given eith er low, high or very high oestradiol substitution therapy (two or thre e Silastic implants, 5 or 8 cm in length) from 160 days to 230 days (O VX 160 = 2E(5), OVX 160 + 2E(8) and OVX 160 + 3E(8), respectively). Th e LH surge responses to three i.m. injections of oestradiol benzoate g iven at intervals of 12 h tin total 10 mu g oestradiol benzoate kg(-1) bodyweight) were compared at 260 days. An additional group of sexuall y mature gilts was ovariectomized at 260 days (OVX 260) and challenged with oestradiol benzoate at 360 days. LH surges with peak LH concentr ations below pretreatment values (resembling LH responses to oestradio l benzoate in immature gilts) were classified as immature, and those w ith peak LH concentrations above pretreatment values as mature. LH con centrations before treatment with oestradiol benzoate were reduced (P < 0.05) in OVX 160 + 2E(5) and OVX 160 + 2E(8) gilts compared with OVX 230 (control). When compared with the control group, the time to LH s urge peak was longer in OVX 260,oilts, LH peak amplitude was reduced i n the OVX 160 group and the area under the curve of the LH surge (ng L H ml(-1) plasma (48 h)(-1)) was less in all other groups (P < 0.05). C lassification of LH surges by mature:immature criteria indicated a hig h mature:immature ratio in OVX 230 gilts (6/1) and a low ratio in OVX 160 (1/7) and OVX 260 (1/6), gilts. The long-term effect of ovariectom y was partially overcome in the OVX 160 + 2E(5) group (mature:immature = 3/3). However, previous exposure to supraphysiological concentratio ns of oestradiol las in groups OVX 160 + 2E(8) and OVX 160 + 3E(8)) re sulted in a high percentage of animals (66% and 100%, respectively) th at did not respond to the oestradiol benzoate challenge with an LH sur ge. Immediately before oestradiol benzoate administration (30 days aft er removal of implants) plasma oestradiol concentrations in these two groups were still high (P < 0.05) compared with all other groups. GnRH -induced (0.2 mu g kg(-1) bodyweight) LH secretion, evaluated 10 days after treatment with oestradiol benzoate, was depressed (P < 0.05) in OVX 160 + SE, gilts, but not affected by age at ovariectomy. We sugges t that continuous ovarian secretion is necessary for the final maturat ion of the LH surge mechanism in late prepubertal gilts and also for m aintaining the full functionality of this mechanism in sexually mature gilts. The range over which ovarian oestrogens tune the final maturat ion of this process appears to be rather narrow; however, other ovaria n factors may contribute to the final maturation. Furthermore, continu ed exposure to supraphysiological concentrations of oestradiol was fou nd to be detrimental to the development of the LH surge mechanism.