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

Numerous studies have investigated the behavioural effects of β‐endorphin, both endogenous and exogenously applied. However, the potential for biotransformation of β‐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 β‐endorphin biotransformation in the striatum of rats. We infused 1.0 nmol β‐endorphin into the striatum of adult male Fischer rats and observed rapid cleavage resulting in β‐endorphin 1–18, as well as several fragments resulting from further N‐terminal degradation. In vitro studies with incubation of full‐length β‐endorphin, with and without protease inhibitors, in the incubation fluid of isolated striatal slices indicate that β‐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 β‐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 β‐endorphin. The binding affinities of full‐length β‐endorphin, as well as previously identified β‐endorphin fragments α‐endorphin (β‐endorphin 1–16) and γ‐endorphin (β‐endorphin 1–17), and the fragment identified in the present study, β‐endorphin 1–18, at heterologously expressed μ, δ and κ‐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 β‐endorphin.