Phospholipid metabolism is required for M1 muscarinic inhibition of N-type calcium current in sympathetic neurons.

The signal transduction cascade mediating muscarinic receptor modulation of N-type Ca2+ channel activity by the slow pathway has remained incompletely characterized despite focused investigation. Recently we confirmed a role for the G-protein Gq and identified phospholipase C (PLC), phospholipase A2 (PLA2), and arachidonic acid (AA) as additional molecules involved in N-current inhibition in superior cervical ganglion (SCG) neurons by the slow pathway. We have further characterized this signal transduction cascade by testing whether additional molecules downstream of phosphatidylinositol-4,5-bisphosphate (PIP2) are required. The L-channel antagonist nimodipine was bath-applied to block L-current. Pretreating cells with pertussis toxin (PTX) minimized M2/M4 muscarinic receptor inhibition of N-current by the membrane-delimited pathway. Consistent with our previous studies, pharmacologically antagonizing M1 muscarinic receptors (M1Rs), Gqalpha, PLC, PLA2, and AA minimized N-current inhibition by the muscarinic agonist oxotremorine-M (Oxo-M). When cells were left untreated with PTX, leaving the membrane-delimited pathway intact and the same antagonists retested, Oxo-M decreased whole cell currents. Moreover, inhibited currents displayed slowed activation kinetics, indicating intact N-current inhibition by the membrane-delimited pathway. These findings indicate that the antagonists used to block the slow pathway acted selectively. PLA2 cleaves AA from phospholipids, generating additional metabolites. We tested whether the metabolite lysophosphatidic acid (LPA) mimicked the inhibitory actions of Oxo-M. In contrast to AA, applying LPA did not inhibit whole cell currents. Taken together, these findings suggest that the slow pathway requires M1Rs, Gqalpha, PLC, PIP2, PLA2, and AA for N-current inhibition.