Characterization of alpha-MSH-related peptides released from rat hypothalamic neurons in vitro

Reverse-phase high-performance liquid chromatography analysis, coupled with a sensitive radioimmunoassay for alpha-melanocyte-stimulating hormone (alpha-MSH), was used to characterize the alpha-MSH-related peptides stored in the rat hypothalamus or released from perifused hypothalamic slices. Four peaks of alpha-MSH-like immunoreactivity (alpha-MSH-LI) co-eluting with synthetic des-N alpha-acetyl alpha-MSH, alpha-MSH and their respective sulfoxide derivatives were resolved and quantified. In hypothalamic extract, deacetyl alpha-MSH which was the predominant peptide represented 94.4% of total alpha-MSH-LI content, while the relative amount of alpha-MSH was only 5.6%. Analysis of alpha-MSH-related peptides contained in effluent perifusates showed that deacetyl alpha-MSH and its oxidized form were the major peptides released from neurons in basal conditions or under KCl-induced depolarization (50 mM KCl for 75 min). However, the proportion of acetylated peptide was 3-4 times higher in the perifusion medium than in hypothalamic extracts. Our data indicate that acetylation of des-N alpha-acetyl alpha-MSH may occur during the process of exocytosis. Since acetylation of alpha-MSH markedly increases the behavioural potency of the peptide, these results suggest that regulation of the acetyltransferase activity could be a key mechanism to modulate the bioactivity of alpha-MSH-related peptides in the brain.