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 i nvestigated by use of a [H-3]-dizocilpine binding assay and NMDA-induc ed intracellular free Ca2+ ([Ca2+]i) increases in cultured forebrain n eurones. 2 Increasing pH from 6.5 to 8.0 significantly increased the r ate of association and dissociation of [H-3]-dizocilpine. The associat ion process was better described by two rate constants under each cond ition, while only dissociation of [H-3]-dizocilpine at pH 8.0 required two rate constants adequately to describe the curve. Equilibrium affi nity of [H-3]-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 m icrospectrofluorimetry 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 loweri ng pH to 6.5 and increased to 148.9% by raising pH to 8.0. 4 Dizocilpi ne (200nm) effectively reversed increases in [Ca2+]i produced by NMDA together with glycine. The rate of reversal produced by this concentra tion of dizocilpine was considerably slower than that required for cel ls 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 hy pothesis that the rates associated with [H-3]-dizocilpine binding are controlled by the level of activation of the receptor. In addition, th ese data confirm previous findings that NMDA responses are sensitive t o small changes in pH. However, the lack of sensitivity to pH changes of dizocilpine inhibition of NMDA responses show that other factors ar e also important in controlling the action of channel-blocking NMDA re ceptor antagonists.