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Guinea-pig sympathetic neurons express varying proportions of two distinct P2X receptors

Yu Zhong, Philip M. Dunn and Geoffrey Burnstock

1. Characterization of P2X receptors on neurons of guinea-pig superior cervical ganglion (SCG) has been carried out using a whole-cell voltage-clamp technique.

2. Application of ATP and ,-methylene ATP (-MeATP) produced fast activating inward currents, which desensitized slowly. The maximum response to -MeATP was 36 ± 23 % (range 0·1-100 %) of that evoked by ATP in the same cell.

3. Co-application of -MeATP (300 µM) with ATP (300 µM) produced a response that was 97 ± 1 % of that given by ATP alone. Following desensitization with -MeATP, the decrease in response to ATP was equal to the absolute reduction in response to -MeATP in the same cell.

4. The concentration-response curve for -MeATP had an EC₅₀ of 42 µM and a Hill coefficient of 1·17. For cells where the ratio of -MeATP/ATP currents at 100 µM was < 0·1, the ATP concentration-response curve had an EC₅₀ of 56 µM and a Hill coefficient of 1·95. However, in cells where the ratio was > 0·7, the curve had an EC₅₀ of 60 µM and a Hill coefficient of 0·97.

5. The response to 100 µM -MeATP was inhibited by 2' (or 3')-O-trinitrophenyl-ATP (TNP-ATP) with an IC₅₀ of 70 nM. However, on cells where the ratio of -MeATP/ATP currents was < 0·1, ATP was inhibited by TNP-ATP with an IC₅₀ of 522 nM.

6. Immunohistochemical staining with antibodies raised against rat P2X₂ and P2X₃ epitopes suggested that both subunits were expressed by guinea-pig SCG neurons.

7. We conclude that varying proportions of two distinct P2X receptors coexist on the cell bodies of individual guinea-pig SCG neurons, which may correspond to homomeric P2X₂ and heteromeric P2X_2/3 receptors.

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