Background and Purpose : Positive allosteric modulators bind to secondary binding sites (allosteric sites) on the receptor and alter receptor conformation and activation by changing agonist potency and efficacy. PTC-174 is a novel NMDA receptor positive allosteric modulator that particularly enhances the activity of receptors containing GluN2C or GluN2D subunits. In this study we investigated the potentiation effects of PTC-174 on dopaminergic neuron NMDA responses where the receptors are likely to be GluN1/GluN2B/GluN2D triheteromeric receptors. Experimental Approach : Whole-cell and outside-out patch-clamp recordings from substantia nigra dopaminergic neurons in acute mid-brain slices from 6- to 8-day-old rats were used to investigate the effects of PTC-174 on NMDA responses and on NMDA receptor-mediated excitatory post-synaptic currents (EPSCs). A new ‘hypercube’ model with explicit receptor subunit-dependent binding steps for agonists and allosteric modulators was used to describe the effects of PTC-174. Key Results : As expected from previous work, PTC-174 potentiated NMDA responses, with a >1.7-fold increase at 200 μM NMDA. The effect of PTC-174 was independent of inhibition by the GluN2B-selective NAM ifenprodil when the two drugs were co-applied. In outside-out patches, PTC-174 increased channel open probability without altering the unitary current amplitude. PTC-174 prolonged the decay of NMDAR-mediated EPSCs and increased EPSC charge transfer. The results were described by a two-binding site subunit-specific ‘hypercube’ model that suggests PTC-174 reduces agonist affinity but enhances channel-gating efficacy. Conclusion and Implications : The main implication of the results in this paper is that PAMs like PTC-174 have the potential to enhance NMDA receptor signalling at substantia nigra dopaminergic neurons in vivo and so could enhance basal ganglia dopamine signalling. Drugs in this class are also of potential interest for pre-clinical studies relevant to NMDA receptor hypofunction.

Background and Purpose : Radiprodil is a GluN2B subunit selective NMDA receptor negative allosteric modulator. It is currently under clinical investigation as a possible anti-epileptic drug for paediatric use. The goal of this study 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 6- to 8-day-old rats were used to investigate the concentration-dependence of radiprodil action. Radiprodil action was analysed using a new ‘hypercube’ model with explicit receptor subunit-dependent binding steps for agonists and antagonists and allosteric constants describing the effect of radiprodil on agonist binding and receptor channel gating. Key Results : As expected from previous work with the GluN2B selective inhibitor ifenprodil, radiprodil produced a concentration-dependent partial inhibition of the NMDA response with about 40% maximum inhibition in the concentration range from 30 – 100 nM. Surprisingly, at higher radiprodil concentrations, the inhibition decreased. Radiprodil (30 nM) also caused a 12% decrease in single channel current amplitude. Conclusion and Implications : The results can be described by a two-binding site subunit-specific model that assumes radiprodil inhibits receptor activation by binding to the GluN2B subunit while reducing inhibition by binding to the GluN2D subunit of a GluN1/GluN2B/GluN2D triheteromeric receptor. We conclude that NMDA receptor allosteric modulators like radiprodil can display a combination of inhibitory and potentiating 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.