Evidence for non-independent gating of P2X2 receptors expressed in Xenopus oocytes

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Abstract
P2X2 receptor is an ATP-activated ion channel which is widely expressed in the nervous system, and mediates synaptic transmission. We recorded currents of P2X2 receptors expressed in Xenopus oocytes from outside-out patches and have found that currents recorded from patches containing a single or multiple P2X2 channels differ in a manner suggesting positive cooperativity. First, the currents from multichannel patches exhibit simultaneous transitions more frequently than predicted from the activity of independent channels. Second, the mean open lifetime at the current level of a single channel in a multichannel burst is about six times longer than the open time of currents from single channel patches, a trend opposite to what is expected of independent channels. These results indicate that the channels have positive cooperativity and that the longer opening is due to a slower closing rate. Third, from kinetic analysis the likelihood of the cooperative model is significantly larger than that of the independent model. Fourth, the open channel noise of currents from patches containing multiple channels is less than half that from a single channel, which is consistent with the channel properties being different when they are active in groups. Taken together, our results suggest that P2X2 receptors are non-independent, but interact with positive cooperativity.