Extracellular Biotransformation of β‐Endorphin in Rat Striatum and Cerebrospinal Fluid

Abstract

Numerous studies have investigated the behavioural effects of beta-endorphin, both endogenous and exogenously applied. However, the potential for biotransformation of beta-endorphin in the extracellular space of the brain has not been previously directly addressed in vivo. Utilising microinfusion/microdialysis and matrix-assisted laser desorption/ionisation mass spectrometry, we investigated beta-endorphin biotransformation in the striatum of rats. We infused 1.0 nmol beta-endorphin into the striatum of adult male Fischer rats and observed rapid cleavage resulting in beta-endorphin 1-18, as well as several fragments resulting from further N-terminal degradation. In vitro studies with incubation of full-length beta-endorphin, with and without protease inhibitors, in the incubation fluid of isolated striatal slices indicate that beta-endorphin is initially cleaved predominantly at the Phe(18)-Lys(19), position, as well as at the Leu(17)-Phe(18) position. Investigations of cerebrospinal fluid revealed similar enzymatic cleavage of beta-endorphin. The observed pattern of cleavage sites (Phe(18)-Lys(19) and Leu(17)-Phe(18)) is consistent with published in vitro studies of purified insulin-degrading enzyme cleavage of beta-endorphin. The binding affinities of full-length beta-endorphin, as well as previously identified beta-endorphin fragments alpha-endorphin (beta-endorphin 1-16) and gamma-endorphin (beta-endorphin 1-17), and the fragment identified in the present study, beta-endorphin 1-18, at heterologously expressed mu, delta and kappa-opioid receptors, respectively, were determined; the affinity of the truncation fragments is reduced at each of the receptors compared to the affinity of full length beta-endorphin.