Enhanced type 1 alpha metabotropic glutamate receptor-stimulated phosphoinositide signaling after pertussis toxin treatment

The regulation of phosphoinositide hydrolysis by the type 1 alpha metabotropic glutamate receptor (mGluR1 alpha) was investigated in stably transfected baby hamster kidney (BHK) cells. Incubation of the cells with L-glutamate, quisqualate, and 1-aminocyclopentane-1S,3R-dicarboxylic acid resulted in a marked accumulation of [H-3]inositol monophosphate (InsP(1)) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P-3] mass in a time- and concentration-dependent manner. Pretreatment of BHK-mGluR1 alpha cells with pertussis toxin [100 ng/ml, 24 hr] led to a dramatic 12-16-fold increase in the accumulation of [H-3]InsP(1) and a 2-fold increase in Ins(1,4,5)P-3 in the absence of added agonist. Although only very low levels (less than or equal to 1 mu M) of L-glutamate could be detected in medium taken from control and PTX-treated cell monolayers, the PTX-elicited effect on basal [H-3]InsP(1) was fully reversed by preincubation of cells in the presence of glutamic-pyruvic transaminase and pyruvate, suggesting that an increased sensitivity to endogenous glutamate was responsible for the apparent agonist-independent activation of phosphoinositidase C (PIC) after PTX treatment. Consistent with this hypothesis, in the presence of glutamic-pyruvic transaminase/pyruvate, the maximal [H-3]InsP(1) response to quisqualate was increased by greater than or equal to 75%, and the EC50 shifted leftward by 65-fold [-log EC50 values (molar), 7.26 +/- 0.23 versus 5.45 +/- 0.07; n = 4) in PTX-treated compared with control cells. In contrast, antagonist effects on agonist-stimulated [H-3]InsP(1) responses were similar in control and PTX-treated BHK-mGluR1 alpha cells. These changes in the concentration-effect curves for mGluR agonists are consistent with a model in which the receptor associates with PTX-sensitive inhibitory (G(i/o)) and PTX-insensitive stimulatory (G(q/11)) G proteins that can each influence PIC activity. The present observations are consistent with a dual regulation of mGluR1 alpha-mediated PIC activity that could be fundamental in controlling the output of phosphoinositide-derived messengers.