cPLA2α-/- sympathetic neurons exhibit increased membrane excitability and loss of N-Type Ca2+ current inhibition by M1 muscarinic receptor signaling

Group IVa cytosolic phospholipase A(2) (cPLA(2)alpha) mediates GPCR-stimulated arachidonic acid (AA) release from phosphatidylinositol 4,5-bisphosphate (PIP2) located in plasma membranes. We previously found in superior cervical ganglion (SCG) neurons that PLA(2) activity is required for voltage-independent N-type Ca2+ (N-) current inhibition by M-1 muscarinic receptors (M(1)Rs). These findings are at odds with an alternative model, previously observed for M-current inhibition, where PIP2 dissociation from channels and subsequent metabolism by phospholipase C suffices for current inhibition. To resolve cPLA(2)alpha's importance, we have investigated its role in mediating voltage-independent N-current inhibition (similar to 40%) that follows application of the muscarinic agonist oxotremorine-M (Oxo-M). Preincubation with different cPLA(2)alpha antagonists or dialyzing cPLA(2)alpha antibodies into cells minimized N-current inhibition by Oxo-M, whereas antibodies to Ca2+-independent PLA(2) had no effect. Taking a genetic approach, we found that SCG neurons from cPLA(2)alpha(-/-) mice exhibited little N-current inhibition by Oxo-M, confirming a role for cPLA(2)alpha. In contrast, cPLA(2)alpha antibodies or the absence of cPLA(2)alpha had no effect on voltage-dependent N-current inhibition by M-2/M(4)Rs or on M-current inhibition by M(1)Rs. These findings document divergent M1R signaling mediating M-current and voltage-independent N-current inhibition. Moreover, these differences suggest that cPLA(2)alpha acts locally to metabolize PIP2 intimately associated with N- but not M-channels. To determine cPLA(2)alpha's functional importance more globally, we examined action potential firing of cPLA(2)alpha(+/+) and cPLA(2)alpha(-/-) SCG neurons, and found decreased latency to first firing and interspike interval resulting in a doubling of firing frequency in cPLA(2)alpha(-/- ) neurons. These unanticipated findings identify cPLA(2)alpha as a tonic regulator of neuronal membrane excitability.