Administration of gonadal steroids to neonatal rats affects beta-endorphin levels in the adult

Administration of gonadal steroids to neonatal rats has a profound effect on the function of the neuroendocrine system in the adult animal. Considering that gonadal steroids modulate hypothalamic and pituitary levels of beta-endorphin (BE) in adult male and female rats, the effects of neonatal gonadal steroid treatment on BE levels in the adult animal were investigated. Neonatal male rats were administered testosterone and neonatal female rats were treated with estrogen. Matched control littermates received vehicle. All animals were sacrificed at 90 days of age. Neonatal gonadal steroid treatment did not affect the level of immunoreactive beta-endorphin (IR-BE) in the anterior pituitary (AP) of male rats but did result in a significant increase in IR-BE in the AP of female rats. Neonatal administration of gonadal steroids produced a significant decrease in IR-BE in the neurointermediate lobe of the pituitary (NIL) of both male and female rats, with the magnitude of the decrease being greater in the NIL of the female rats. IR-BE levels in the hypothalamus of male or female rats were not altered by the treatments. Column chromatography indicated that the increase in IR-BE in the AP represented a proportional increase in BE and beta-lipotropin, while the reduction in IR-BE in the NIL of the treated rats represented a reduction in BE. These findings suggest that gonadal steroids may influence the development of the neurotransmitter systems which regulate BE levels in the adult pituitary, the development of the biosynthetic mechanisms of the adult pituitary, or both.