Abstract

Cannabinoid receptor activation of G-proteins can be measured by WIN 55212–2-stimulated [³⁵S]GTPγS 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]GTPγS binding to the B _maxof 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]GTPγS binding, some regions displayed different relative levels of agonist-stimulated [³⁵S]GTPγS 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]GTPγS, [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]SR141716AB _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.