Activation of cannabinoid receptors in rat brain by WIN 55212-2 produces coupling to multiple G protein alpha-subunits with different potencies.

Previous studies had shown that the amplification factors for cannabinoid receptors, defined as the number of total G proteins activated per occupied receptor, differs between several rat brain regions. In this study, we sought to determine which specific Gi/Go(alpha) subunits were activated by CB1 receptors in several rat brain regions and if this coupling might explain the regional differences in receptor/G protein amplification factors. Furthermore, we examined whether cannabinoid agonists might activate different subtypes of G(alpha) subunits with varying degrees of efficacy and/or potency. Activation of specific G proteins by cannabinoid receptors was evaluated by the ability of the agonist WIN 55212-2 to stimulate incorporation of [alpha-(32)P]azidoanilido-GTP into G(alpha) subunits in membranes. Photolabeled G proteins were either directly resolved using urea/SDS-polyacrylamide gel electrophoresis or first immunoprecipitated with specific antisera for different G(alpha) subunits before electrophoresis. Individual G(alpha) subunits were separated into distinct bands on a single gel and the amount of agonist-induced increase in radioactivity was quantified by densitometry. Stimulation of CB1 receptors by WIN 55212-2 resulted in the activation of a distinct pattern of at least five different G(ialpha)/G(oalpha) subunits in several brain regions. Furthermore, although the pattern of G proteins activated by WIN 55212-2 appeared to be similar across brain regions, slight differences were observed in both the percentage of increase and the amount of the individual G(alpha) subunits activated. Most importantly, the amount of WIN 55212-2 required to half-maximally activate individual G proteins in the cerebellum varied over a 30-fold range for different G(alpha) subunits. These results suggest that cannabinoid receptors activate multiple G proteins simultaneously in several brain regions and both the efficacy and potency of cannabinoid agonists to activate individual G(alpha) subunits may vary considerably.