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Open channel block of NMDA receptors by conformationally restricted analogs of milnacipran and their protective effect against NMDA‐induced neurotoxicity

We investigated the blocking effect of the conformationally restricted analogs of milnacipran on NMDA receptors by recording the whole‐cell currents of Xenopus oocytes injected with rat brain mRNA and the single channel currents of cultured hippocampal neurons under voltage‐clamp conditions. Their protective effect against excitotoxicity was also investigated on cultured cortex neurons. All conformationally restricted analogs examined blocked activated NMDA receptors, though their structures were quite different from known NMDA receptor blockers. The analogs with a (1S, 2R, 1′S)‐configuration such as PPDC ((1S, 2R)‐1‐phenyl‐2[(S)‐1‐aminopropyl]‐N,N‐diethylcyclopropanecarboxamide) had lower IC50 values than those with other configurations. The empirical Hill coefficients for each compound were close to unity, indicating a 1:1 stoichiometry for the block. PPDC decreased the maximum responses to both N‐methyl d‐aspartate (NMDA) and glycine without altering their dissociation constants. The blocking effect was enhanced on hyperpolarization. PPDC had no effects on other glutamate receptor subtypes (AMPA, kainate, and metabotropic glutamate receptors) or other neurotransmitter receptors (GABAA, 5HT2C, and AChM1 receptors) produced by the oocytes. PPDC decreased the mean open time of NMDA receptors without decreasing their elementary conductance. The microscopic blocking rate constant was 2.8 × 107 M−1s−1. The macroscopic unblocking rate constant of PPDC was much faster than that of MK‐801. Only the analogs with the (1S, 2R, 1′S)‐configuration protected the cultures against NMDA‐induced neurotoxicity, though they failed to protect against kainate‐induced neurotoxicity. These results show that conformationally restricted analogs, at least PPDC, selectively blocked open channels of NMDA receptors. Synapse 31:87–96, 1999. © 1999 Wiley‐Liss, Inc.