Modifications induced by neonatal steroids in reproductive organs and behavior of male rats.

Male rats injected on Day 3 neonatally with .01, .1, 1, 10, 100, or 1,000 micrograms of estradiol benzoate (EB), 10,000 microgram of testosterone propionate (TP), or sesame oil were subsequently examined for testicular, penile, and accessory organ development. Sexual behavior was evaluated during therapy with fluoxymesterone (FM) and then with TP. Estradiol benzoate in dosages greater than 1.0 micrograms delayed testicular descent, reduced the size and hormone responsiveness of reproductive organs, and decreased sexual behavior in a dose-dependent manner. The 10,000-microgram dosage of neonatal TP delayed testicular descent and reduced sexual behavior to levels near those of the 10--100 micrograms EB groups, but it produced no significant penile or accessory organ changes. Neither reduced peripheral organ development nor inhibited neonatal testicular secretions fully explain reductions in male behavior following large dosages of neonatal TP. Neonatal androgen may reduce the responsiveness of central nervous system neurons governing male sexual behavior after being converted to estrogen or by directly altering steroid receptor systems.