High-affinity cannabinoid binding sites in brain membranes labeled with [³H]-5'-trimethylammonium Δ⁸-tetrahydrocannabinol

The binding of [³H]5'-trimethylammonium Δ⁸-tetrahydrocannabinol (THC) ([³H]TMA) to rat neuronal membranes was studied. TMA is a positively charged analog of Δ⁸THC modified on the 5' carbon, a portion of the molecule not important for its psychoactivity. Unlabeled TMA inhibits field-stimulated contractions of the guinea-pig ileum (IC₅₀ = 1 μM) in the same presynaptic manner as Δ⁹THC. [³H]TMA binds saturably and reversibly to brain membranes with high affinity (K(D) = 89 nM) to apparently one class of site (Hill coefficient, 1.1). Highest binding site density occurs in the brain, but several peripheral organs also display specific binding. Detergent solubilizes the sites without affecting their pharmacological properties. Molecular sieve chromatography reveals a bimodal peak of [³H]TMA binding activity of approximately 60,000 daltons apparent molecular weight. Δ⁹THC competitively inhibits [³H]TMA binding potently (K(i) = 27 nM) and stereoselectively. For some cannabinoids potency in behavioral and physiological tests parallels their affinity for the [³H]TMA binding sites. However, several nonpsychotropic cannabinoids are active at the binding site.