Decrease in PIP 2 –channel interactions is the final common mechanism involved in PKC‐ and arachidonic acid‐mediated inhibitions of GABA B ‐activated K + current

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–PIP 2 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 PIP 2 in this mechanism, the effect of CCh and DHPG on I GIRK was tested in cells loaded with exogenous PIP 2 . The inhibition of I GIRK by CCh or DHPG was almost completely abolished in PIP 2 ‐loaded cells. We confirmed that the inhibition of I GIRK by direct application of phorbol ester or arachidonic acid was also completely reversed in PIP 2 ‐loaded cells. These results indicate that the decrease in PIP 2 –channel interactions is the final common mechanism responsible for G q PCR‐induced inhibitions of I GIRK mediated by PKC and arachidonic acid.