Complex involvement of pertussis toxin-sensitive G proteins in the regulation of type 1 alpha metabotropic glutamate receptor signaling in baby hamster kidney cells

Previously, we demonstrated that the coupling of the metabotropic glutamate receptor mGlu1 alpha to phosphoinositide hydrolysis is enhanced by pertuss is toxin (PTX) in stably transfected baby hamster kidney cells (BHK). Here, we show that the PTX effect on agonist-stimulated [H-3]inositol phosphate accumulation can be resolved into two components: an immediate increase in agonist potency, and a more slowly developing increase in the magnitude of the response observed at maximally effective agonist concentrations. Using G(q)/(11)alpha- and G(i/o)alpha-selective antibodies to immunoprecipitate [ S-35]guanosine-5'-O-(3-thio)triphosphate-bound G alpha proteins, we also sh ow that agonist stimulation of mGlu1a in BHK membranes increases specific [ S-35]guanosine-5'-O-(3-thio)triphosphate binding to both G(q/11) and G(i/o) proteins. Preincubation of BHK-mGlu1 alpha with L-glutamate (300 mu M) res ults in a progressive loss (60% in 30 min) of L-quisqualate-induced [H-3]in ositol phosphate accumulation (without a change in potency), providing evid ence for agonist-induced receptor desensitization. Although such desensitiz ation of mGlu receptor signaling was mimicked by a phorbol ester, agonist-i nduced phosphorylation of the receptor was not observed and protein kinase C inhibition by Ro 31-8220 did not prevent L-glutamate-mediated desensitiza tion. In contrast, PTX treatment of the cells almost completely prevented L -glutamate-mediated desensitization. Together, these data provide evidence for a multifunctional coupling of mGlu1a to different types of G proteins, including PTX-sensitive G(i)-type G proteins. The latter are involved in th e negative control of phospholipase C activity while also influencing the r ate of desensitization of the mGlu1 alpha receptor.