Characterization of the forms of [beta]-endorphin and [alpha]-MSH in the caudal medulla of the rat and guinea pig

Abstract

The post-translational processing of pro-opiomelanocortin (POMC) in brain remains controversial. Classically, there was thought to be a single cell group in the arcuate nucleus with long projections through limbic structures. More recently, a second cell group was discovered in the caudal medulla. This study addresses the question of POMC processing in this region. Steady-state analysis of acid extracts of dorsal caudal medulla from rat and guinea pig CNS by gel filtration chromatography and radioimmunoassay indicated that in both species the major POMC-related end products are [alpha]-MSH-sized material and [ss]-endorphin-sized. In this tissue [ss]-LPH and ACTH represent minor end products. Analysis of the [alpha]-MSH-sized material from both species by reverse-phase HPLC indicated that in the rat caudal medulla approximately 79% of the [alpha]-MSH-related material is acetylated, and in the guinea pig caudal medulla approximately 85% of the [alpha]-MSH-related material is acetylated. Analysis of the forms of [ss]-endorphin isolated from the rat caudal medulla by cation exchange chromatography revealed that acetylated and non-acetylated forms of [ss]-endorphin are present in this region of the rat CNS. Approximately 65% of the [ss]-endorphin in the rat caudal medulla is N-acetylated. Analysis of the forms of [ss]-endorphin isolated from the guinea pig caudal medulla indicated that approximately 63% of the [ss]-endorphin is N-acetylated in this region of the guinea pig CNS. These data indicate that the post-translational processing of POMC in the dorsal caudal medulla, the site of the nucleus tractus solitarius POMC cell group, is distinct from the processing patterns that have been reported for POMC systems in the mammalian anterior pituitary, intermediate pituitary and arcuate nucleus.