INHIBITION OF NMDA-GATED ION CHANNELS BY THE P2 PURINOCEPTOR ANTAGONISTS SURAMIN AND REACTIVE-BLUE-2 IN MOUSE HIPPOCAMPAL-NEURONS

1 The action of suramin and reactive blue 2 on N-methyl-D-aspartate (N MDA)-activated ion current was studied in mouse hippocampal neurones i n culture by use of whole-cell patch-clamp recording. 2 Suramin and re active blue 2 inhibited steady-state current activated by 25 mu M NMDA with IC50 values of 68 and 11 mu M, respectively. 3 Reactive blue 2 p roduced a gradual decline of NMDA-activated current to a steady-state, but this slow onset was not an indication of use-dependence, as it co uld be eliminated by exposure to reactive blue 2 before NMDA applicati on In addition, NMDA-activated current recovered completely from inhib ition by reactive blue 2 in the absence of agonist. 4 The slow onset o f inhibition by reactive blue 2 was not apparently due to an action at an intracellular site, as inclusion of 250 mu M reactive blue 2 in th e recording pipette did not alter inhibition by 25 mu M reactive blue 2 applied externally. 5 Reactive blue 2 and suramin inhibited NMDA-gat ed channels in a voltage-independent manner. 6 Reactive blue 2, 25 mu M, decreased the maximal response to NMDA from 1441 to 598 pA without changing its EC50. In contrast, 75 mu M suramin increased the EC50 for NMDA from 13 to 35 mu M, and decreased the maximal response to NMDA f rom 1822 to 1498 pA. Schild analysis of suramin inhibition of NMDA-act ivated current yielded a nonlinear plot. 7 Both agents decreased the m aximal response to glycine without altering its EC50. 8 Suramin and re active blue 2 appear to inhibit NMDA receptor-channels in a manner tha t is noncompetitive with respect to both NMDA and glycine. However, in hibition by suramin differed from that by reactive blue 2, in that sur amin significantly increased the EC50 of NMDA.