Mg(2+) block properties of triheteromeric GluN1–GluN2B–GluN2D NMDA receptors on neonatal rat substantia nigra pars compacta dopaminergic neurones

Native NMDA receptors (NMDARs) are tetrameric channels formed by two GluN1 and two GluN2 subunits. So far, seven NMDARs subunits have been identified and they can form diheteromeric or triheteromeric NMDARs (more than one type of GluN2 subunit). Extracellular Mg(2+) is an important regulator of NMDARs, and particularly the voltage dependence of Mg(2+) block is crucial to the roles of NMDARs in synaptic plasticity and the integration of synaptic activity with neuronal activity. Although the Mg(2+) block properties of diheteromeric NMDARs are fully investigated, properties of triheteromeric NMDARs are still not clear. Our previous data suggested that dopaminergic neurones expressed triheteromeric GluN1–GluN2B–GluN2D NMDARs. Here, using NMDARs in dopaminergic neurones from postnatal day 7 (P7) rats as a model system, we characterize the voltage-dependent Mg(2+) block properties of triheteromeric NMDARs. In control conditions, external Mg(2+) significantly inhibits the whole cell NMDA-evoked current in a voltage-dependent manner with IC(50) values of 20.9 μm, 53.3 μm and 173 μm at −90 mV, −70 mV and −50 mV, respectively. When the GluN2B-selective antagonist ifenprodil was applied, the Mg(2+) sensitivity of the residual NMDA-mediated currents (which is mainly carried by GluN1–GluN2B–GluN2D NMDARs) is reduced to IC(50) values of 45.9 μm (−90 mV), 104 μm (−70 mV) and 276 μm (−50 mV), suggesting that triheteromeric GluN1–GluN2B–GluN2D NMDARs have less affinity for external Mg(2+) than GluN1–GluN2B receptors. In addition, fitting I(NMDA)–V curves with a trapping Mg(2+) block model shows the triheteromeric GluN1–GluN2B–GluN2D NMDARs have weaker voltage-dependent Mg(2+) block (δ = 0.56) than GluN1–GluN2B NMDARs. Finally, our concentration jump and single channel recordings suggest that GluN1–GluN2B–GluN2D rather than GluN1–GluN2D NMDARs are present. These data provide information relevant to Mg(2+) block characteristics of triheteromeric NMDARs and may help to better understand synaptic plasticity, which is dependent on these triheteromeric NMDARs.