Influence of positive allosteric modulators on GABAB receptor coupling in rat brain:: a scintillation proximity assay characterisation of G protein subtypes

Little is known concerning coupling of cerebral GABA(B) 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 [S-35]GTP gamma S binding to G alpha o and, less markedly, G alpha i(1/3) in cortex, whereas Gq and Gs/olf were unaffected. (R)-baclofen and SKF97581 likewise activated G alpha o and G alpha i(1/3), expressing their actions more potently than GABA. Similar findings were acquired in hippocampus and cerebellum, and the GABA(B) antagonist, CGP55845A, abolished agonist-induced activation of G alpha o and G alpha i(1/3) in all structures. The PAMs, GS39783, CGP7930 and CGP13501, inactive alone, enhanced efficacy and potency of agonist-induced [S-35]GTP gamma S binding to G alpha o in all regions, actions abolished by CGP55845A. In contrast, they did not modify efficacies at G alpha i(1/3). Similarly, in human embryonic kidney cells expressing GABA(B(1a+2)) or GABA(B(1b+2)) receptors, allosteric modulators did not detectably enhance efficacy of GABA at G alpha i(1/3), though they increased its potency. To summarise, GABA(B) receptors coupled both to G alpha o and to G alpha i, but not Gq and Gs/olf, in rat brain. PAMs more markedly enhanced efficacy of coupling to Go versus Gi(1/3). It will be of interest to confirm these observations employing complementary techniques and to evaluate their potential therapeutic significance.