Allosteric modulation of adenosine A1 receptor coupling to G-proteins in brain

Amino-4,5,6,7-tetrahydrobenzo( b) thiophen-3-yl 4-chlorophenylmethanone (T62) is a member of a group of allosteric modulators of adenosine A(1) receptors tested in animal models of neuropathic pain to increase the efficacy of adenosine. To determine its mechanisms at the level of receptor-G-protein activation, the present studies examined the effect of T62 on A(1)-stimulated [S-35] guanosine-5'-O-(gamma-thio)-triphosphate ([S-35] GTP gamma S) binding in brain membranes, and by [S-35] GTP gamma S autoradiography using the A(1) agonist, phenyl-isopropyladenosine ( PIA), to activate G-proteins. In hippocampal membranes, T62 increased both basal and PIA-stimulated [S-35] GTP gamma S binding. The effect of T62 was non-competitive in nature, since it increased the maximal effect of PIA, with no effect on agonist potency. GTP gamma S saturation analysis showed that T62 increased the number of G-proteins activated by agonist but had no effect on the affinity of activated G-proteins for GTP gamma S. [S-35] GTPcS autoradiography showed that the neuroanatomical localization of T62-stimulated [S-35] GTPcS binding was identical to that of PIA-stimulated activity. The increase in PIA-stimulated activity by T62 varied between brain regions, with areas of lower A(1) activation producing the largest percent modulation by T62. These results suggest a mechanism of allosteric modulators to increase the number of activated G-proteins per receptor, and provide a neuroanatomical basis for understanding potential therapeutic effects of such drugs.