Inhibition of NMDA-gated ion channels by the P2 purinoceptor antagonists suramin and reactive blue 2 in mouse hippocampal neurones

1. The action of suramin and reactive blue 2 on N-methyl-D-aspartate (NMDA)-activated ion current was studied in mouse hippocampal neurones in culture by use of whole-cell patch-clamp recording.
2. Suramin and reactive blue 2 inhibited steady-state current activated by 25 μM NMDA with IC₅₀ values of 68 and 11 μM, respectively.
3. Reactive blue 2 produced a gradual decline of NMDA-activated current to a steady-state, but this slow onset was not an indication of use-dependence, as it could be eliminated by exposure to reactive blue 2 before NMDA application. In addition, NMDA-activated current recovered completely from inhibition by reactive blue 2 in the absence of agonist.
4. The slow onset of inhibition by reactive blue 2 was not apparently due to an action at an intracellular site, as inclusion of 250 μM reactive blue 2 in the recording pipette did not alter inhibition by 25 μM reactive blue 2 applied externally.
5. Reactive blue 2 and suramin inhibited NMDA-gated channels in a voltage-independent manner.
6. Reactive blue 2, 25 μM, decreased the maximal response to NMDA from 1441 to 598 pA without changing its EC₅₀. In contrast, 75 μM suramin increased the EC₅₀ for NMDA from 13 to 35 μM, and decreased the maximal response to NMDA from 1822 to 1498 pA. Schild analysis of suramin inhibition of NMDA-activated current yielded a nonlinear plot.
7. Both agents decreased the maximal response to glycine without altering its EC₅₀.
8. Suramin and reactive blue 2 appear to inhibit NMDA receptor-channels in a manner that is noncompetitive with respect to both NMDA and glycine. However, inhibition by suramin differed from that by reactive blue 2, in that suramin significantly increased the EC₅₀ of NMDA.