Abstract

To investigate differences in agonist affinity, potency, and efficacy across rat brain regions, five representative cannabinoid compounds were investigated in membranes from three different rat brain regions for their ability to maximally stimulate [³⁵S]guanosine-5′-O-(3-thio)triphosphate (GTPγS) binding and bind to cannabinoid receptors (measured by inhibition of [³H]antagonist binding) under identical assay conditions. In all three brain regions, the rank order of potency for the stimulation of [³⁵S]GTPγS binding and the inhibition of [³H]SR141716A binding for these compounds were identical, with CP55940 ≈ levonantradol > WIN55212-2 ≥ Δ⁹-tetrahydrocannabinol (Δ⁹-THC) > methanandamide. The rank order of efficacy was not related to potency, and relative maximal agonist effects varied across regions. Receptor binding fit to a three-site model for most agonists, stimulation of [³⁵S]GTPγS binding fit to a two-site model for all agonists, and high-affinity receptor binding did not appear to produce any stimulation of [³⁵S]GTPγS binding. WIN55212-2, methanandamide, and Δ⁹-THC also were assayed for the inhibition of adenylyl cyclase in cerebellar membranes. The rank orders of potency and efficacy were similar to those for [³⁵S]GTPγS binding, but the efficacies and potencies of methanandamide and Δ⁹-THC compared with WIN55212-2 were higher for adenylyl cyclase inhibition, implying receptor/G-protein reserve.