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

Abstract

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.