Novel opioid peptides endomorphin-1 and endomorphin-2 are present in mammalian immune tissues

Endomorphin (EM)-1 and EM-2 are opioid tetrapeptides, reported within the central nervous system, which have very high specificity and affinity for the μ-opioid receptor. We have used newly developed and well-characterised radioimmunoassays (RIAs) in combination with reversed-phase high-performance liquid chromatography (HPLC) to detect EM-1 and EM-2 immunoreactivity (ir) in rat immune tissues. Endomorphins were detectable in extracts of rat spleen (total EM-1-ir/spleen: 440±73 pg, mean±SEM, a=group of eight rats; EM-2-ir: 150±12 pg) and thymus (EM-1-ir: 152±18 pg, mean±SEM n=8; EM-2-ir: 156±28 pg). EM-2-ir was detectable in extracts of human spleen (338±196 pg/g tissue, n=3). Multiple peaks of EM-1-ir and EM-2-ir were observed in rat spleen and thymus extracts, and multiple peaks of EM-2-ir were observed in extracts of human spleen, following reversed-phase HPLC and RIAs. This is the first report of endomorphin immunoreactivity in tissues of the rat and human immune systems.