Functional interactions between nicotinic and P2X channels in short-term cultures of guinea-pig submucosal neurons

1. Functional interactions between nicotinic and P2X receptors in submucosa l neurons were investigated. Whole-cell currents induced by ACh (I-ACh) and ATP (I-ATP) were blocked by hexamethonium and pyridoxalphosphate-6-azophen yl-2',4'-disulfonic acid (PPADS), respectively. Currents induced by simulta neous application of the two transmitters (IACh+ATP) were only as large as the current induced by the most effective of these substances. This current occlusion indicates that activation of nicotinic and P2X channels is not i ndependent. 2. Kinetic parameters of IACh+ATP indicate that they are carried through ch annels activated by either substance. In agreement with this interpretation , both I-ACh and I-ATP amplitudes were decreased when ATP and ACh were appl ied simultaneously, whereas no cross-desensitization was observed when nico tinic and P2X receptors were desensitized individually. 3. Current occlusion was observed at membrane potentials of -60 and +10 mV, when I-ACh and I-ATP were inward. However, when these currents were outwar d (at +40 mV), current occlusion was not observed. Current occlusion was st ill observed at +40 mV in experiments in which the reversal potential of th ese currents had been adjusted to more positive values. 4. Current occlusion occurred as soon as currents were detected (<5 ms), wa s still present in the absence of Ca2+, Na+ or Mg2+, and after adding staur osporine, genistein, K-252a, or N-ethylmaleimide to the pipette solution. S imilar observations were noted after substituting alpha,beta-methylene ATP for ATP, or GTP for GTP-gamma-S in the pipette and in experiments carried o ut at 36, 23 and 9 degrees C. 5. We propose that nicotinic and P2X channels are in functional clusters of at least two, and that the influx of ions through one activates (through a llosteric interactions) a mechanism that inhibits the other channel.