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

The regulation of phosphoinositide hydrolysis by the type 1alpha metabotropic glutamate receptor (mGluR1alpha) 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 [3H]inositol monophosphate (InsP1) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] mass in a time- and concentration-dependent manner. Pretreatment of BHK-mGluR1alpha cells with pertussis toxin [ 100 ng/ml, 24 hr] led to a dramatic 12-16-fold increase in the accumulation of [3H]InsP1 and a 2-fold increase in Ins(1,4,5)P3 in the absence of added agonist. Although only very low levels (/=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 [3H]InsP1 responses were similar in control and PTX-treated BHK-mGluR1alpha 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 (Gi/o) and PTX-insensitive stimulatory (Gq/11) G proteins that can each influence PIC activity. The present observations are consistent with a dual regulation of mGluR1alpha-mediated PIC activity that could be fundamental in controlling the output of phosphoinositide-derived messengers.