M1 muscarinic receptors inhibit L-type Ca2+ current and M-current by divergent signal transduction cascades.
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, M(1)-muscarinic receptors (M(1)Rs) may inhibit both Ca(2+) (L- and N-) and K(+) (M-) currents by losing a putative interaction between channels and phosphatidylinositol-4,5-bisphosphate (PIP(2)). However, we found previously that M(1)R inhibition of N-current in superior cervical ganglion (SCG) neurons requires loss of PIP(2) and generation of a free fatty acid, probably arachidonic acid (AA) by phospholipase A(2) (PLA(2)). It is not known whether PLA(2) activity and AA also participate in L- and M-current modulation in SCG neurons. To test whether PLA(2) plays a similar role in M(1)R inhibition of L- and M-currents, we used several experimental approaches and found unanticipated divergent signaling. First, blocking resynthesis of PIP(2) minimized M-current recovery from inhibition, whereas L-current recovered normally. Second, L-current inhibition required group IVa PLA(2) [cytoplasmic PLA(2) (cPLA(2))], whereas M-current did not. Western blot and imaging studies confirmed acute activation of cPLA(2) by muscarinic stimulation. Third, in type IIa PLA(2) [secreted (sPLA(2))](-/-)/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 M(1)R-meditated channel modulation that broadens rather than restricts the roles of phospholipids and fatty acids in regulating ion channel activity.
Liu
Liwang
L
Program in Neuroscience and Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
Zhao
Rubing
R
Bai
Yan
Y
Stanish
Lee F
LF
Evans
James E
JE
Sanderson
Michael J
MJ
Bonventre
Joseph V
JV
Rittenhouse
Ann R
AR
D000818:Animals; D000831:Animals, Newborn; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000502:physiology*; D002478:Cells, Cultured; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods; D007150:Immunohistochemistry / Q000379:methods; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D018345:Mice, Knockout; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D018408:Patch-Clamp Techniques / Q000379:methods; D010741:Phospholipases A / Q000172:deficiency; D026902:Potassium Channel Blockers / Q000494:pharmacology; D015221:Potassium Channels / Q000502:physiology*; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D043643:Receptor, Muscarinic M1 / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D017783:Superior Cervical Ganglion / Q000166:cytology
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D004791:Enzyme Inhibitors; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D012333:RNA, Messenger; D043643:Receptor, Muscarinic M1; D010741:Phospholipases A