Altered levels of β-endorphin fragments after chronic morphine treatment of guinea-pig ileum in vivo and in vivo

Abstract The isolated myenteric plexus-longitudinal muscle of the guinea-pig ilem (GPI) was used as testsystem to study the influence of chronic morphine treatment on the levels of enkephalins, β-endorphin and some of its fragments. The peptides were assayed by means of a combination of high pressure liquid chromatography and radioimmunoassays. It was found that the levels of methionine- and leucine-enkephalin and β-endorphin were not altered by chronic morphine treatment of guinea-pigs in vivo nor in GPI exposed to morphine in vitro . However, the levels of some β-endorphin fragments i.c. γ-endorphin and des-tyrosine-γ-endorphin were elevated after morphine treatment in vitro and in vivo respectively. It is suggested that β-endorphin and its fragments are involved in homeostatic processes during development of opiate tolerance.