TY - JOUR T1 - Enhanced Type 1α Metabotropic Glutamate Receptor-Stimulated Phosphoinositide Signaling after Pertussis Toxin Treatment JF - Molecular Pharmacology JO - Mol Pharmacol SP - 406 LP - 414 DO - 10.1124/mol.52.3.406 VL - 52 IS - 3 AU - Alan M. Carruthers AU - R. A. John Challiss AU - Rajendra Mistry AU - Ruth Saunders AU - Christian Thomsen AU - Stefan R. Nahorski Y1 - 1997/09/01 UR - http://molpharm.aspetjournals.org/content/52/3/406.abstract N2 - The regulation of phosphoinositide hydrolysis by the type 1α metabotropic glutamate receptor (mGluR1α) 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-mGluR1α 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 (≤1 μm) of l-glutamate could be detected in medium taken from control and PTX-treated cell monolayers, the PTX-elicited effect on basal [3H]InsP1 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 [3H]InsP1 response to quisqualate was increased by ≥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-mGluR1α 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 mGluR1α-mediated PIC activity that could be fundamental in controlling the output of phosphoinositide-derived messengers. ER -