Effects of pH on the actions of dizocilpine at the N-methyl-D-aspartate receptor complex

1. The effects of varying pH from 6.5 to 7.4 and 8.0 on the actions of dizocilpine (MK801) on the N-methyl-D-aspartate (NMDA) receptor were investigated by use of a [3H]-dizocilpine binding assay and NMDA-induced intracellular free Ca2+ ([Ca2+]i) increases in cultured forebrain neurones. 2. Increasing pH from 6.5 to 8.0 significantly increased the rate of association and dissociation of [3H]-dizocilpine. The association process was better described by two rate constants under each condition, while only dissociation of [3H]-dizocilpine at pH 8.0 required two rate constants adequately to describe the curve. Equilibrium affinity of [3H]-dizocilpine was not altered by changing pH from 6.5 to 8.0. 3. NMDA and glycine together increased [Ca2+]i measured by fura-2 microspectrofluorimetry in single cultured neurones from rat forebrain. Compared to control response measured at pH 7.4, the combined effects of NMDA and glycine were reduced to 38.9% of control values by lowering pH to 6.5 and increased to 148.9% by raising pH to 8.0. 4. Dizocilpine (200 nM) effectively reversed increases in [Ca2+]i produced by NMDA together with glycine. The rate of reversal produced by this concentration of dizocilpine was considerably slower than that required for cells to recover to baseline following agonist removal. The rate at which dizocilpine reversed NMDA- and glycine-induced [Ca2+]i increases was not altered by raising pH from 6.5 to 8.0. 5. These data support the hypothesis that the rates associated with [3H]-dizocilpine binding are controlled by the level of activation of the receptor. In addition, these data confirm previous findings that NMDA responses are sensitive to small changes in pH.However, the lack of sensitivity to pH changes of dizocilpine inhibition of NMDA responses show that other factors are also important in controlling the action of channel-blocking NMDA receptor antagonists.