Positive and negative coupling of the metabotropic glutamate receptors to a G protein-activated K+ channel, GIRK, in Xenopus oocytes

Metabotropic glutamate receptors (mGluRs) control intracellular signaling cascades through activation of G proteins. The inwardly rectifying K+ channel, GIRK is activated by the beta gamma subunits of G(i) proteins and is widely expressed in the brain. We investigated whether an interaction between mGluRs and GIRK is possible, using Xenopus oocytes expressing mGluRs and a cardiac/brain subunit of GIRK, GIRK1, with or without another brain subunit, GIRK2. mGluRs known to inhibit adenylyl cyclase (types 2, 3, 4, 6, and 7) activated the GIRK channel. The strongest response was observed with mGluR2; it was inhibited by pertussis toxin (PTX). This is consistent with the activation of GIRK by G(i)/G(o)-coupled receptors. In contrast, mGluRla and mGluR5 receptors known to activate phospholipase C, presumably via G proteins of the G(q) class, inhibited the channel's activity. The inhibition was preceded by an initial weak activation, which was more prominent at higher levels of mGluRla expression. The inhibition of GIRK; activity by mGluRla was suppressed by a broad-specificity protein kinase inhibitor, staurosporine, and by a specific protein kinase C (PKC) inhibitor, bis-indolylmaleimide, but not by PTX, Ca2+ chelation, or calphostin C. Thus, mGluRla inhibits the GIRK channel primarily via a pathway involving activation of a PTX-insensitive G protein and, eventually, of a subtype of PKC, possibly PKC-mu. In contrast, the initial activation of GIRK1 caused by mGluRla was suppressed by PTS but not by the protein kinase inhibitors. Thus, this activation probably results from a promiscuous coupling of mGluRla to a G(i)/G(o) protein. The observed modulations may be involved in the mGluRs' effects on neuronal excitability in the brain. Inhibition of GIRK by phospholipase C-activating mGluRs bears upon the problem of specificity of G protein (GIRK interaction) helping to explain why receptors coupled to G(q) are inefficient in activating GIRK.