Sex-related difference in the rat pituitary [Met5]-enkephalin level--altered by gonadectomy

Ontogenetic study of the rat pituitary [Met5]-enkephalin-like immunoreactivity (ME-LI) revealed that a sex-related difference in the level of this peptide began at 35 days of age. By 70 days of age, the level of male ME-LI was twice that of the female. Castration reduced the ME-LI in male rats and increased the ME-LI in female rats. These results suggest a possible role of sex hormones in regulating the pituitary enkephalin system.