Decrease in PIP2-channel interactions is the final common mechanism involved in PKC- and arachidonic acid-mediated inhibitions of GABA(B)-activated K+ current

Abstract

We showed in our previous study that in hippocampal CA1 neurons the stimulation of muscarinic receptors inhibited the GIRK current (I-GIRK) via a PLC/PKC pathway, whereas group I metabotropic glutamate receptors (mGluR) inhibited I-GIRK via a PLA(2)/arachidonic acid pathway. In this study, we present evidence that receptor-mediated signalling pathways activated by the two G(q)-coupled receptors (G(q)PCRs) converge on the inhibition of GIRK channel-PIP2 interaction. I-GIRK was activated in acutely isolated hippocampal CA1 neurons by repetitive application of baclofen, a GABA(B) receptor agonist, with a 2-3 min interval. When both CCh and DHPG were pretreated before the second I-GIRK activation, the magnitude of the second I-GIRK was 52.2 +/- 2.5% of the first I-GIRK, which was not significantly different from the magnitude of inhibition by CCh or DHPG alone. This result shows that the effects of muscarinic receptor and group I mGluR stimulation on I-GIRK are not additive but occlusive, suggesting that each pathway may converge to a common mechanism that finally regulates I-GIRK. To test the involvement Of PIP2 in this mechanism, the effect of CCh and DHPG on I-GIRK was tested in cells loaded with exogenous PIP2. The inhibition Of I-GIRK by CCh or DHPG was almost completely abolished in PIP2-loaded cells. We confirmed that the inhibition of I-GIRK by direct application of phorbol ester or arachidonic acid was also completely reversed in PIP2-loaded cells. These results indicate that the decrease in PIP2-channel interactions is the final common mechanism responsible for G(q)PCR-induced inhibitions Of I-GIRK mediated by PKC and arachidonic acid.