Characterization of [H-3]Met-enkephalin-Arg(6)-Phe(7) binding to multiple sites in rat and guinea pig cerebellum

[H-3]Met-enkephalin-Arg(6)-Phe(7) (MERF) has been shown to label opioid (ka ppa(2) and delta) and sigma(2) sites in rat and frog brain membrane prepara tions, and no specific binding to kappa(1) opioid receptors could be establ ished (refs. 6 and 8). In this study the binding was examined in rat cerebe llar membranes which are relatively rich in kappa(2)-sites, and in guinea p ig cerebellar preparations where kappa(1) opioid receptors are almost exclu sively present. In accordance with our previous results, [H-3]MERF binding could not be displaced in guinea pig cerebellar membranes neither with U-69 ,593 nor with naloxone or levorphanol suggesting no interaction with opioid sites, nevertheless a K-d of 2.8 nM was calculated in cold saturation expe riments. In rat cerebellar membrane fractions about the half of the specifi c [H-3]MERF binding sites was inhibited by opiate alkaloids such as naloxon e, ethylketocyclazocine, or bremazocine. This portion of the heptapeptide b inding sites was stereoselective as demonstrated by the difference in the a ffinities of the enantiomeric compounds levorphanol and dextrorphan, theref ore it would represent an opioid site. In both tissues (-)N-allyl-normetazo cine (SKF-10,047), which is also considered as sigma(2) ligand, displayed t he highest affinities. Among opioid peptides beta-endorphin and dynorphin(( 1-13)) showed the highest potencies, displacing [H-3]MERF also from its non -opioid sites. It was concluded therefore that [H-3]MERF does not bind to k appa 1 sites, and besides kappa 2-opioid sites substantial binding to pepti de preferring non-opioid sites, and/or sigma(2) receptors also occurs.