Submitted on February 5, 2009
Revised on March 18, 2009
Accepted on March 27, 2009
Cross-inhibition Between Nicotinic Acetylcholine Receptors and P2X Receptors in Myenteric Neurons and HEK-293 cells
Dima A. Decker1 and James J. Galligan1*
1 Michigan State University
* To whom correspondence should be addressed. E-mail: galliga1{at}msu.edu.
The enteric nervous system (ENS) controls gut function. P2X and nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that mediate fast synaptic excitation in the ENS. Close molecular coupling in enteric neuronal membranes contributes to a mutually inhibitory interaction between these receptors; this effect is called cross-inhibition. We studied the molecular mechanisms responsible for cross-inhibition. Whole-cell patch clamp techniques were used to measure P2X- and nAChR-mediated currents in cultured enteric neurons and HEK-293 cells. In cultured myenteric neurons, ACh (3 mM) and ATP (1 mM) co-application evoked an inward current that was only 57 ± 6% (P < 0.05) of the predicted current that would have occurred if the two populations of channels were activated independently. In HEK-293 cells co-expressing 
3
4 nAChR/P2X2 receptors, co-application of ATP and ACh caused a current that was 58 ± 7% of the predicted current (P < 0.05). To test the importance of P2X subunit C-terminal tail length on cross-inhibition, P2X3 and P2X4 subunits, which have shorter C-terminal tails were studied. Cross-inhibition with 34 nAChRs and P2X3 or P2X4 subunits was similar to that occurring with P2X2 subunits. P2X receptor or 34 nAChR desensitization did not prevent receptor cross-inhibition. These data indicate that the 34-P2X receptor interaction is not restricted to P2X2 subunits. In addition, active and desensitized conformations of the P2X receptor inhibit nAChR function. These molecular interactions may modulate the function of synapses that use ATP and ACh as fast synaptic transmitters in the ENS.
