Phosphatidylinositol 4,5-bisphosphate regulates inspiratory burst activity in the neonatal mouse preBotzinger complex

Neurons of the preBotzinger complex (preBotC) form local excitatory networks and synchronously discharge bursts of action potentials during the inspiratory phase of respiratory network activity. Synaptic input periodically evokes a Ca2+-activated non-specific cation current (I-CAN) postsynaptically to generate 10-30 mV transient depolarizations, dubbed inspiratory drive potentials, which underlie inspiratory bursts. The molecular identity of I-CAN and its regulation by intracellular signalling mechanisms during inspiratory drive potential generation remains unknown. Here we show that mRNAs coding for two members of the transient receptor potential (TRP) family of ion channels, namely TRPM4 and TRPM5, are expressed within the preBotC region of neonatal mice. Hypothesizing that the phosphoinositides maintaining TRPM4 and TRPM5 channel sensitivity to Ca2+ may similarly influence I-CAN and thus regulate inspiratory drive potentials, we manipulated intracellular phosphatidylinositol 4,5-bisphosphate (PIP2) and measured its effect on preBotC neurons in the context of ongoing respiratory-related rhythms in slice preparations. Consistent with the involvement of TRPM4 and TRPM5, excess PIP2 augmented the inspiratory drive potential and diminution of PIP2 reduced it; sensitivity to flufenamic acid (FFA) suggested that these effects of PIP2 were I-CAN mediated. Inositol 1,4,5-trisphosphate (IP3), the product of PIP2 hydrolysis, ordinarily causes IP3 receptor-mediated I-CAN activation. Simultaneously increasing PIP2 while blocking IP3 receptors intracellularly counteracted the reduction in the inspiratory drive potential that normally resulted from IP3 receptor blockade. We propose that PIP2 protects I-CAN from rundown by interacting directly with underlying ion channels and preventing desensitization, which may enhance the robustness of respiratory rhythm.