The inhibition of basal channel activity at high levels of receptor expression could be due to enough receptor molecules being spontaneously active without agonist, thereby continually activating the inhibitory cascade at a significant level (cf. a specific protein kinase C (PKC) inhibitor, bis-indolylmaleimide, but not by PTX, Ca2+ chelation, or calphostin C. Thus, mGluR1a inhibits the GIRK channel primarily via a pathway including activation of a PTX-insensitive G protein and, eventually, of a subtype of PKC, possibly PKC-. In contrast, the initial activation of GIRK1 caused by mGluR1a was suppressed by PTX but not by the protein kinase inhibitors. Thus, this activation probably results from a promiscuous coupling of mGluR1a to a Gi/Go protein. The observed modulations may be involved in the mGluRs’ effects on neuronal excitability in the brain. Inhibition of GIRK by phospholipase CCactivating mGluRs bears upon the problem of specificity of G protein (GIRK conversation) helping to explain why receptors coupled to Gq are inefficient in activating GIRK. oocytes, these receptors activate a large endogenous Ca2+-dependent chloride current, a fact that enabled molecular cloning by functional expression of the first mGluR, mGluR1 (Masu et al., 1991; Houamed et al., 1991). Group II and group III receptors inhibit adenylyl cyclase (AC) activity, suggesting that they couple to G proteins of the Gi/Go class (Gilman, 1987). The molecular mechanisms by which mGluRs exert their physiological effects are not yet fully comprehended. Their known effects include direct mediation of glutamatergic synaptic transmission at some synapses, both hyperpolarizing and depolarizing. Presynaptic group II and III autoreceptors inhibit transmitter release. All three groups have been shown to inhibit L-type voltage-gated Ca2+ channels, and groups I and II also inhibit N-type Diphenyleneiodonium chloride channels. mGluRs also modulate the ionotropic AMPA, Rabbit Polyclonal to BAIAP2L2 NMDA, and GABA-A receptors (examined by Nakanishi, 1994; Pin and Duvoisin, 1995). mGluRs inhibit several types of K+ currents: the Diphenyleneiodonium chloride voltage-dependent M-type current, the Ca2+-activated current (IKAHP), a voltage-dependent K+ current IK,slow, and resting K+ currents (Schwartz, 1993; Guerineau et al., 1994; Ikeda et al., 1995; Luthi et al., 1996). Activation of K+ currents by mGluRs has been shown in cerebellar granule cells (Fagni et al., 1991). GIRK1 (KGA, Kir3.1; Kubo et al., 1993; Dascal et al., Diphenyleneiodonium chloride 1993oocytes (Hedin et al., 1996). Functional inward rectifier channels are believed to be heterooligimers created by GIRK1 with the other subunits (Lesage et al., 1995; Kofuji et al., 1995; Krapivinsky et al., 1995oocytes. In addition, a negative coupling exists between the PLC-coupled mGluRs (types 1 and 5) and GIRK, most probably mediated by activation of the GqCphospholipase C pathway and a PKC subtype. materials and methods Preparation of RNAs and Oocytes DNA plasmids made up of the various clones were linearized with the appropriate restriction enzymes using a standard protocol (Dascal and Lotan, 1992): GIRK1 (Dascal et al., 1993= 5). Vacant bars, Po in cells unexposed to glutamate (= 5). Po was averaged over periods of 3 min. The abscissa shows time after the start of the record. Glutamate was added at = 3 min (= quantity of cells tested. Comparisons between two groups were carried out using two-tailed Student’s test. Comparisons between more than two groups were carried out using one-way nonparametric ANOVA followed by Dunn’s test, using the SigmaStat software (Jandel Scientific, Corte Madera, CA). results Gi/Go-coupled mGluRs Activate GIRK via PTX-sensitive G Proteins The GIRK channels were expressed by injecting RNA of GIRK1 alone or with RNA of GIRK2. In oocytes injected with GIRK1 RNA alone, the channels are most probably created by GIRK1 and the endogenous subunit, GIRK5 (Hedin et al., 1996), and they will be termed GIRK1/GIRK5 channels. In oocytes injected with RNAs of GIRK1 and GIRK2 (a combination especially.