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

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. Suramin and reactive blue 2 inhibited steady‐state current activated by 25 μM NMDA with IC50 values of 68 and 11 μM, respectively. 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. 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. Reactive blue 2 and suramin inhibited NMDA‐gated channels in a voltage‐independent manner. 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. Both agents decreased the maximal response to glycine without altering its EC50. 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.