Potentiation effects of the positive allosteric modulator PTC-174 on NMDA receptors in neonatal male rat substantia nigra dopaminergic neurons.

BACKGROUND AND PURPOSE: Positive allosteric modulators bind to secondary binding sites on the receptor and alter receptor conformation and activation, changing agonist potency and efficacy. PTC-174 is a novel NMDA receptor positive allosteric modulator that particularly enhances activity of receptors containing GluN2C or GluN2D subunits. We have investigated potentiation by PTC-174 of dopaminergic neuron NMDA responses, where 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, PTC-174 potentiated NMDA responses, with a >1.7-fold increase at 200-μM NMDA. The effect 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 NMDA receptor-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 microscopic 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.