Regional differences in cannabinoid receptor/G-protein coupling in rat brain.

Cannabinoid receptor activation of G-proteins can be measured by WIN 55212-2-stimulated [35S]GTPgammaS binding. Receptor/transducer amplification factors, interpreted as the number of G-proteins activated per occupied receptor, are the ratio of the apparent B(max) of net agonist-stimulated [35S]GTPgammaS binding to the B(max) of receptor binding. The present study examined whether amplification factors for cannabinoid receptors differ among various rat brain regions. In autoradiographic studies with [3H]WIN 55212-2 and WIN 55212-2-stimulated [35S]GTPgammaS binding, some regions displayed different relative levels of agonist-stimulated [35S]GTPgammaS binding than receptor binding. To quantify amplification factors, membranes from different brain regions were assayed by saturation binding analysis of net WIN 55212-2-stimulated [35S]GTPgammaS, [3H]SR141716A (antagonist) and [3H]WIN 55212-2 (agonist) binding. For [3H]SR141716A binding, amplification factors varied significantly from 2.0 (frontal cortex) to 7.5 (hypothalamus). For [3H]WIN 55212-2 binding, amplification factors ranged from 2.4 (hippocampus) to 5.5 (thalamus). Comparison of receptor binding and G-protein activation at subsaturating concentrations of WIN 55212-2 indicated that amplification factors may vary with receptor occupancy in some regions like cerebellum. Ratios between high-affinity [3H]WIN 55212-2 B(max) and [3H]SR141716A B(max) also differed significantly among brain regions. These results demonstrate that G-protein coupling by cannabinoid receptors differs among brain regions, and therefore depends on the cellular environment.