Sexual dimorphism in the distribution of alpha-neoendorphin-like immunoreactivity in the anterior pituitary of the rat.

The localization of the opioid peptide alpha-neoendorphin (alpha-Neo-E) was studied in the anterior pituitary of normal and castrated male and normal female rats. Immunoreactive (ir) cells were noted in both sexes. These alpha-Neo-E-ir cells were further characterized using double immunostaining with an elution-restaining procedure. It was seen that in males, alpha-Neo-E-ir cells corresponded mainly to luteinizing hormone/follicle-stimulating hormone cells and a few thyroid-stimulating hormone (TSH) cells, whereas in females, virtually all alpha-Neo-E-ir cells corresponded to TSH cells. Castration of male rats caused, within 3 to 5 days a dramatic decrease in the number of alpha-Neo-E-ir gonadotrophs, whereas the number of alpha-Neo-E-ir TSH cells tended to increase. Two weeks after castration, however, most alpha-Neo-E-ir cells were also follicle-stimulating hormone-ir. This study demonstrates that in the anterior lobe of the rat, alpha-Neo-E-ir is located within gonadotrophs and/or thyrotrophs, depending on the sex. In addition, results obtained following castration suggest that the expression of this peptide in the anterior pituitary depends upon the steroid environment, possibly indicating that alpha-Neo-E is implicated in the regulation of the pituitary-gonadal axis.