M1 muscarinic receptors inhibit L-type Ca2+ current and M-current by divergent signal transduction cascades

Abstract

Ion channels reside in a sea of phospholipids. During normal fluctuations in membrane potential and periods of modulation, lipids that directly associate with channel proteins influence gating by incompletely understood mechanisms. In one model, M1-muscarinic receptors (M1Rs) may inhibit both Ca2+ (L- and N-) and K+ (M-) currents by losing a putative interaction between channels and phosphatidylinositol-4,5-bisphosphate (PIP2). However, we found previously that M1R inhibition of N-current in superior cervical ganglion (SCG) neurons requires loss of PIP2 and generation of a free fatty acid, probably arachidonic acid (AA) by phospholipase A2 (PLA2). It is not known whether PLA2 activity and AA also participate in L- and M-current modulation in SCG neurons. To test whether PLA2 plays a similar role in M 1R inhibition of L- and M-currents, we used several experimental approaches and found unanticipated divergent signaling. First, blocking resynthesis of PIP2 minimized M-current recovery from inhibition, whereas L-current recovered normally. Second, L-current inhibition required group IVa PLA2 [cytoplasmic PLA2 (cPLA2)], whereas M-current did not. Western blot and imaging studies confirmed acute activation of cPLA2 by muscarinic stimulation. Third, in type IIa PLA2 [secreted (sPLA2)]-/-/cPLA 2-/- double-knock-out SCG neurons, muscarinic inhibition of L-current decreased. In contrast, M-current inhibition remained unaffected but recovery was impaired. Our results indicate that L-current is inhibited by a pathway previously shown to control M-current over-recovery after washout of muscarinic agonist. Our findings support a model of M1R-meditated channel modulation that broadens rather than restricts the roles of phospholipids and fatty acids in regulating ion channel activity. Copyright © 2006 Society for Neuroscience.