NMDA receptor subunit‐dependent analysis of radiprodil inhibition in neonatal male rat substantia nigra dopaminergic neurons

BACKGROUND AND PURPOSE: Radiprodil is a GluN2B subunit selective NMDA receptor negative allosteric modulator which is currently under clinical investigation as a possible anti-epileptic drug for paediatric use. Our goal was to investigate the inhibition of dopaminergic neuron NMDA responses by radiprodil. EXPERIMENTAL APPROACH: Whole-cell and outside-out patch-clamp recordings from substantia nigra dopaminergic neurons in acute mid-brain slices from neonatal male rats were used to investigate radiprodil action. Radiprodil action was analysed using a 'hypercube' model with explicit receptor GluN2B subunit-dependent binding steps for agonist and antagonist and allosteric constant describing the effect of radiprodil on agonist binding and receptor channel gating and a 2nd binding site for radiprodil that may mediate partial reversal of the radiprodil inhibitory effect. KEY RESULTS: Radiprodil produced a concentration-dependent partial inhibition with about 40% maximum at concentrations between 30 and 100 nM. Surprisingly, at higher radiprodil concentrations, the inhibition decreased. Radiprodil (30 nM) also caused a 12% decrease in single channel current. CONCLUSION AND IMPLICATIONS: The results are described by a two-binding site model assuming radiprodil inhibits receptor activation by binding to the GluN2B subunit whilst reducing inhibition by binding to a secondary site, which may or may not be on the receptor. We conclude that NMDA receptor allosteric modulators such as radiprodil can display a combination of inhibitory and reversal of inhibitory effects, depending on drug concentration and receptor subunit combinations. These properties may give some allosteric drugs a unique profile of action, which could be advantageous in some clinical circumstances.