Inhibitory feedback action of oestradiol on tonic secretion of luteinizing hormone in pre-and postpubertal gilts

Abstract

Experiments were performed in gilts to test whether the inhibitory feedback action of oestradiol-17β on tonic luteinizing hormone (LH) secretion decreases around puberty and whether a stimulus to tonic LH secretion occurs which is independent of ovarian steroid control. Fifty-eight German Landrace gilts were either ovariectomized (OVX), OVX and treated with oestradiol-17β via subcutaneous silastic implants to maintain chronic low levels of oestradiol-17β, or served as controls. Treatments began at either 130, 180 or 210 days of age. All 130- and 180-day-old gilts providing data used for analysis were prepubertal, while 210-day-old gilts were OVX between Days 9–15 of their first oestrous cycle. Blood samples for analysis of LH were taken over a 4 h period at 10 min intervals 1 day before OVX and on Days 2, 7, 14, 21 and 28 after OVX. Intact controls were sampled on corresponding days. Mean LH, nadir LH levels, LH pulse frequency and amplitude were calculated for each 4 h sampling period. Mean plasma LH levels were not affected by age in intact controls. Within 2 days of ovariectomy plasma LH concentrations increased rapidly and then further increased only marginally without any difference between the three age groups. In the youngest age group which received a 3 cm oestradiol-17β implant resulting in oestradiol levels and uterine weights below those of respective controls ( P < 0.05), plasma LH was suppressed until 145 days of age when circulating LH increased ( P < 0.01). Between 180 and 210 days of age oestradiol-17β implants of 6 cm length (producing oestradiol levels and uterine weights similar to those of respective controls) suppressed LH to levels of controls. In postpubertal gilts oestradiol-17β implants of the same length and with similar effects on plasma oestradiol levels were unable to suppress LH until 21 days after ovariectomy when LH returned to pretreatment levels. Changes in mean LH in all age and reproductive status groups reflected changes in LH pulse frequency and nadir LH levels, whereas LH pulse amplitude was not affected. Since the postovariectomy levels of LH did not differ between pre- and postpubertal animals, it appears that in the gilt a stimulus to tonic LH secretion which is independent of ovarian steroid control does not occur during puberty. A major part of the reduction in the sensitivity to oestradiol negative feedback became apparent after first ovulation, which does not support the view that a resetting of the “gonadostat” initiates ovarian cyclicity. The transition from high to low oestradiol feedback sensitivity may be temporary or not complete even after first ovulation and a return to the hypersensitive state may occur.