Inhibition of transmitter release from rat sympathetic neurons via presynaptic M1 muscarinic acetylcholine receptors

Background and purpose:  M2, M3 and/or M4 muscarinic acetylcholine receptors have been reported to mediate presynaptic inhibition in sympathetic neurons. M1 receptors mediate an inhibition of Kv7, CaV1 and CaV2.2 channels. These effects cause increases and decreases in transmitter release, respectively, but presynaptic M1 receptors are generally considered facilitatory. Here, we searched for inhibitory presynaptic M1 receptors. Experimental approach:  In primary cultures of rat superior cervical ganglion neurons, Ca2+ currents were recorded via the perforated patch-clamp technique, and the release of [3H]-noradrenaline was determined. Key results:  The muscarinic agonist oxotremorine M (OxoM) transiently enhanced 3H outflow and reduced electrically evoked release, once the stimulant effect had faded. The stimulant effect was enhanced by pertussis toxin (PTX) and was abolished by blocking M1 receptors, by opening Kv7 channels and by preventing action potential propagation. The inhibitory effect was not altered by preventing action potentials or by opening Kv7 channels, but was reduced by PTX and ω-conotoxin GVIA. The inhibition remaining after PTX treatment was abolished by blockage of M1 receptors or inhibition of phospholipase C. When [3H]-noradrenaline release was triggered independently of voltage-activated Ca2+ channels (VACCs), OxoM failed to cause any inhibition. The inhibition of Ca2+ currents by OxoM was also reduced by ω-conotoxin and PTX and was abolished by M1 antagonism in PTX-treated neurons. Conclusions and implications:  These results demonstrate that M1, in addition to M2, M3 and M4, receptors mediate presynaptic inhibition in sympathetic neurons using phospholipase C to close VACCs.