Inhibitory interactions between 5-HT subtype 3 (5-HT(3)) and P2X receptors were characterized using whole cell recording techniques. Currents induced by 5-HT (I(5-HT)) and ATP (I(ATP)) were blocked by tropisetron (or ondansetron) and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, respectively. Currents induced by 5-HT + ATP (I(5-HT+ATP)) were only as large as the current induced by the most effective transmitter, revealing current occlusion. Occlusion was observed at membrane potentials of -60 and 0 mV (for inward currents), but it was not present at +40 mV (for outward currents). Kinetic and pharmacological properties of I(5-HT+ATP) indicate that they are carried through 5-HT(3) and P2X channels. Current occlusion occurred as fast as activation of I(5-HT) and I(ATP), was still present in the absence of Ca(2+) or Mg(2+), after adding staurosporine, genistein, K-252a, or N-ethylmaleimide to the pipette solution, after substituting ATP with proportional to, beta-methylene ATP or GTP with GTP-gamma-S in the pipette, and was observed at 35 degrees C, 23 degrees C, and 8 degrees C. These results are in agreement with a model that considers that 5-HT(3) and P2X channels are in functional clusters and that these channels might directly inhibit each other.
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