Measurement of receptor-mediated functional activation of G proteins in postmortem human brain membranes

Guanine nucleotide-binding regulatory proteins (G proteins) play a pivotal role in receptor-mediated transmembrane signal transduction, and have been implicated in modes of action of psychotropic drugs as well as in pathogenesis of psychiatric disorders. In the present investigation, functional activation of G proteins coupled with several receptors, in particular with GABAB receptors, was assessed by agonist-induced stimulation of high-affinity GTPase, an enzyme that is intrinsic to alpha subunit of G protein, in postmortem human frontal cortical membranes. High-affinity GTPase activity was stimulated by GABA as well as (+/-)-baclofen, a selective GABAB receptor agonist, with EC50 values of 60-150 and 10-40 microM, respectively, in a Mg(2+)-dependent manner. The (+/-)-baclofen-stimulated response was antagonized by the selective GABAB receptor antagonist, 2-hydroxy-saclofen, in a competitive manner with a KB value of 59 microM. Although the maximal percent increase above basal value (% Emax) for GABAB receptor-mediated high-affinity GTPase activity was varied from subject to subject, % Emax values for both agonists were highly correlated with each other, and replicable and stable in a given subject, indicating that this measure is trustworthy as an index of functional coupling between receptors and G proteins in future studies at the aim of elucidating possible alteration of receptor/G protein interaction in psychiatric disorders. The % Emax values for GABAB receptor-mediated responses were correlated inversely with brain storage duration, which should be critically considered in postmortem studies. The increases in high-affinity GTPase activity stimulated by several agonists other than GABAB receptor agonists seemed too low to quantify for making a comparison in future studies.