G-PROTEIN-MEDIATED INHIBITION OF PHOSPHOINOSITIDE METABOLISM EVOKED BY METABOTROPIC GLUTAMATE RECEPTORS IN FROG OOCYTES

1. Metabotropic glutamate receptor subtype 1 (mGluR1), when expressed in Xenopus oocytes, activates phosphoinositide-specific phospholipase C (PLC) in a G protein-dependent manner. This reaction results in the activation of chloride channels in the oocytes, and can be monitored electrophysiologically. We expressed different G protein alpha-subunits together with mGluR1. in oocytes, and examined the effects of these G protein subunits on the PLC-mediated reaction. 2. The expression of the alpha-subunit of G(L2), a bovine version of G(11), which is a member of the G(q) subgroup, potentiated the mGluR1-evoked reaction, whereas the alpha-subunit of G(L1), a bovine G(14), which is also a member of the G(q) subgroup, strongly suppressed it. The expression of G(s) alpha, also suppressed this reaction. 3. We then expressed G beta(1) gamma(2)-subunits in addition to the G alpha-subunits, and examined the mGluR1-evoked reactions. Both the potentiation and suppression by G(L2)alpha and G(L1)alpha, respectively, were more pronounced in the presence of the G beta(1) gamma(2)-subunits. In contrast, the suppression by G(s) alpha: was completely reversed by G beta(1) gamma(2). 4. The direct activation of G proteins by the intracellular injection of either fluoride ions or guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) causes similar PLC-mediated reactions. The expression of G(L2)alpha, G(L1)alpha or G(s) alpha, a caused potentiation, suppression and no change, respectively, on the fluoride- (or GTP gamma S-) evoked reactions. 5. We therefore concluded that G(L2) is able to stimulate PLC, whereas there are at least two different mechanisms of inhibition for the PLC-mediated reaction by the G protein alpha-subunits. One is the competition between the endogenous and exogenous G alpha-subunits for the endogenous G beta gamma to form functional heterotrimers, as exemplified by G(s) alpha. The other mechanism was observed for G(L1), in which an activated G protein interacts with PLC and thereby inhibits its activity or its activation.