Endogenous regulators of G protein signaling proteins regulate presynaptic inhibition at rat hippocampal synapses

Presynaptic inhibition mediated by G protein-coupled receptors (GPCRs) can develop and decay in a few seconds. This time course is too rapid to be accounted for by the intrinsic GTPase activity of G alpha subunits alone. Here, we test the hypothesis that endogenous regulators of G protein signaling (RGS proteins) are required for rapid, brief presynaptic inhibition. Endogenous G protein alpha subunits were uncoupled from GPCRs by treating cultures with pertussis toxin (PTX). Adenoviral expression of mutant PTX-insensitive (PTX-i) G alpha (i1-3) or G alpha (o) subunits rescued adenosine-induced presynaptic inhibition in cultured hippocampal neurons. Expression of double mutant G alpha (i1) or G alpha (o) subunits that were both PTX-insensitive and unable to bind RCS proteins (PTX/RGS-i) also rescued presynaptic inhibition. Presynaptic inhibition mediated by PTX/RGS-i subunits decayed much more slowly after agonist removal than that mediated by PTX-i subunits or native G proteins. The onset of presynaptic inhibition mediated by PTX/RGS-i G alpha (o) was also slower than that mediated by PTX-i G alpha (o). In contrast, the onset of presynaptic inhibition mediated by PTX/RCS-i G alpha (i1) was similar to that mediated by PTX-i Ga-i1. These results suggest that endogenous RGS proteins regulate the time course of G protein signaling in mammalian central nervous system presynaptic terminals.