ESTROGEN FEEDBACK IN THE PIG - SEXUAL-DIFFERENTIATION AND THE EFFECT OF PRENATAL TESTOSTERONE TREATMENT

We studied the stimulatory estrogen feedback on LH release in newborn (6-day-old) and pre-pubertal (160-day-old) male and female Landrace pigs and the effect of exposure to testosterone in fetal life on sexual differentiation of the stimulatory estrogen feedback. In newborn gonadectomized male pigs neither 60 nor 600 μg estradiol benzoate (EB)/kg BW i.m. had an effect on the level of LH in the plasma for up to 120 h following treatment. In female neonates the plasma concentrations of LH were significantly (p<0.05) suppressed only at 24 h after 600 μg EB/kg BW. Gonadectomized prepubertal male pigs responded to 50 μg EB/kg BW with a significant (p<0.01) decrease of plasma LH at 24 h and a return to pretreatment levels at 72 h in the face of still elevated estradiol-17β levels. Prepubertal gilts responded to 60 μg EB/kg BW with a surge of LH 48-60 h following treatment. The stimulatory estrogen feedback was significantly (p<0.05) impaired in females of the same age that had been exposed to testosterone-propionate (TP) via their mother (i.m. injections on 3 occasions separated by 2 day intervals, 5 mg/kg BW) starting on Day 30, 50 or 70 of fetal life. The number of gilts that had ovulated by 250 days was reduced and the weight of the ovaries and the uterus was significantly (p<0.01 and P<0.001, respectively) depressed. TP treatment of sows starting on Day 90 or 106 of pregnancy had no effect on the functioning of the stimulatory estrogen feedback mechanism or the ovaries of their female offspring. We conclude that the stimulatory estrogen feedback mechanism of the female pig gradually matures and suggest that with respect to the effect of prenatal or neonatal testosterone treatment on the estrogen feedback the pig is similar to the rat, guinea pig and sheep, but different from the rhesus monkey. The observed sexual dimorphism in the LH response to estradiol benzoate might be quantitative rather than qualitative.