Little is known concerning coupling of cerebral GABAB receptors to G protein subtypes, and the influence of positive allosteric modulators (PAMs) has not been evaluated. These questions were addressed by an antibody‐capture/scintillation proximity assay strategy. GABA concentration‐dependently enhanced the magnitude of [35S]GTPγS binding to Gαo and, less markedly, Gαi1/3 in cortex, whereas Gq and Gs/olf were unaffected. (R)‐baclofen and SKF97581 likewise activated Gαo and Gαi1/3, expressing their actions more potently than GABA. Similar findings were acquired in hippocampus and cerebellum, and the GABAB antagonist, CGP55845A, abolished agonist‐induced activation of Gαo and Gαi1/3 in all structures. The PAMs, GS39783, CGP7930 and CGP13501, inactive alone, enhanced efficacy and potency of agonist‐induced [35S]GTPγS binding to Gαo in all regions, actions abolished by CGP55845A. In contrast, they did not modify efficacies at Gαi1/3. Similarly, in human embryonic kidney cells expressing GABAB(1a+2) or GABAB(1b+2) receptors, allosteric modulators did not detectably enhance efficacy of GABA at Gαi1/3, though they increased its potency. To summarise, GABAB receptors coupled both to Gαo and to Gαi, but not Gq and Gs/olf, in rat brain. PAMs more markedly enhanced efficacy of coupling to Go versus Gi1/3. It will be of interest to confirm these observations employing complementary techniques and to evaluate their potential therapeutic significance.

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