Background and purpose
M(2), M(3) and/or M(4) muscarinic acetylcholine receptors have been reported to mediate presynaptic inhibition in sympathetic neurons. M(1) receptors mediate an inhibition of K(v)7, Ca(V)1 and Ca(V)2.2 channels. These effects cause increases and decreases in transmitter release, respectively, but presynaptic M(1) receptors are generally considered facilitatory. Here, we searched for inhibitory presynaptic M(1) receptors. Experimental approach: In primary cultures of rat superior cervical ganglion neurons, Ca(2+) currents were recorded via the perforated patch-clamp technique, and the release of [(3)H]-noradrenaline was determined. Key
Purpose
M(2), M(3) and/or M(4) muscarinic acetylcholine receptors have been reported to mediate presynaptic inhibition in sympathetic neurons. M(1) receptors mediate an inhibition of K(v)7, Ca(V)1 and Ca(V)2.2 channels. These effects cause increases and decreases in transmitter release, respectively, but presynaptic M(1) receptors are generally considered facilitatory. Here, we searched for inhibitory presynaptic M(1) receptors. Experimental approach: In primary cultures of rat superior cervical ganglion neurons, Ca(2+) currents were recorded via the perforated patch-clamp technique, and the release of [(3)H]-noradrenaline was determined. Key
Results
The muscarinic agonist oxotremorine M (OxoM) transiently enhanced (3)H 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 M(1) receptors, by opening K(v)7 channels and by preventing action potential propagation. The inhibitory effect was not altered by preventing action potentials or by opening K(v)7 channels, but was reduced by PTX and omega-conotoxin GVIA. The inhibition remaining after PTX treatment was abolished by blockage of M(1) receptors or inhibition of phospholipase C. When [(3)H]-noradrenaline release was triggered independently of voltage-activated Ca(2+) channels (VACCs), OxoM failed to cause any inhibition. The inhibition of Ca(2+) currents by OxoM was also reduced by omega-conotoxin and PTX and was abolished by M(1) antagonism in PTX-treated neurons. Conclusions and implications: These results demonstrate that M(1), in addition to M(2), M(3) and M(4), receptors mediate presynaptic inhibition in sympathetic neurons using phospholipase C to close VACCs.