Myelin Basic Protein Is an Endogenous Inhibitor of the High‐Affinity Cannabinoid Binding Site in Brain

: Radioligand binding studies with the water‐soluble cannabinoid [3H]5′‐trimethylammonium A8‐tetrahydrocannabinol ([3H]TMA) have revealed a saturable highaffinity site in brain that is specific for cannabinoids. To determine whether endogenous compounds of brain might act upon the site physiologically, we sought inhibitors in extracts of brain. An endogenous inhibitor has been purified to homogeneity by acid extraction of rat brain followed by adsorption to a reverse‐phase matrix and gel filtration chromatography. The purified inhibitor has a subunit molecular mass of 14,500 daltons by sodium dodecyl sulfatepolyacrylamide gel electrophoresis (SDS‐PAGE). Inhibition of [3H]TMA binding by the purified inhibitor occurs with a Ki of about 4 nM in a noncompetitive manner. The molecular weight, abundance, and extraction properties are the same as a species of myelin basic protein (MBP). The MBPs of rat, rabbit, pig, and cow also inhibit [3H]TMA binding noncompetitively with similar potencies. The purified inhibitor comigrates with rat MBP‐small form on SDS‐PAGE, has a similar amino acid composition, and is recognized by antibody directed against MBP. Studies of fragments of rabbit MBP suggest that the determinants of affinity for the [3H]TMA site are contained primarily within the C‐terminal half of the rabbit MBP. Synthetic polycationic peptides such as polylysine and polyarginine mimic the effects of MBP, suggesting that the high‐affinity cannabinoid binding site recognizes large polycations. The identification of the endogenous inhibitor of [3H]TMA binding as MBP suggests that MBP interacts physiologically with the high‐affinity cannabinoid site.