Effects of neonatal testosterone administration on anterior pituitary levels of Met5-enkephalin in adult rats.

Neonatal injection of testosterone to male rats significantly increased the anterior pituitary level of Met5-enkephalin-like immunoreactivity (ME-LI) in adulthood. Neonatal castration resulted in a decrease of the ME-LI level, but neonatal testosterone administration failed to restore the level. Combined administration of androgens neonatally and in adulthood was required to restore the level to that of unoperated controls. On the other hand, neonatal testosterone injection to female neonates diminished the elevation of ME-LI level which occurred after ovariectomy in adulthood. These results suggest that neonatal testosterone plays some role in priming or modifying the responsiveness of the anterior pituitary ME system to gonadal hormones present during adulthood.