NR2 subunits and their role in striatal N-methyl-D-aspartate receptor function

Converging glutamatergic and dopaminergic inputs from cortex, thalamus and substantia nigra are received by medium-sized spiny neurons (MSNs), the main cell population of the striatum. N-methyl-D-aspartate receptors (NMDARs) are a glutamate receptor subtype expressed by MSNs and these receptors consist of NR1, NR2A and/or NR2B subunits. Additionally, MSNs express dopamine receptors and subpopulations of MSNs primarily express either D1 (D1R) or D2 (D2R) dopamine receptors. The present study used genetic (transgenic mice) and pharmacological (NR2 antagonists) approaches to determine if NR2 subunits affect normal NMDA and dopamine receptor function in MSNs. In particular, questions regarding the topographic distribution of NR2A- and NR2B-expressing NMDARs were addressed along with the functional consequences of eliminating specific subtypes of NR2-expressing receptors. The most prominent contribution of NR2A-containing NMDARs was observed at synaptic sites while NR2B-containing receptors were found at both synaptic and extrasynaptic receptor locations. NMDAR-mediated current amplitudes were decreased and decay times were altered by deletion or blockade of NR2 subunits. The distribution of NR2-expressing NMDARs was also examined between D1R- and D2R-expressing MSNs. NR2 subunit distribution was different with NR2A contributing a larger component to NMDAR-mediated currents in D1R MSNs while NR2B did the opposite, contributing a larger component in D2R MSNs. Finally, NR2 subunits affect dopamine modulation of NMDAR-mediated currents. D1R potentiation was shown to be differentially affected by NR2 subunits with enhanced potentiation when the NR2A subunit was genetically deleted and there was a trend toward reduced potentiation when NR2B-expressing NMDARs were pharmacologically blocked. In contrast, D2R modulation was largely unaffected. Together, these data suggest that D2R-expressing MSNs of the indirect pathway are more vulnerable to excitotoxicity while dopamine modulation in direct pathway D1R-expressing MSNs is exclusively affected by shifts in NR2 subunit composition. Balance of the direct and indirect pathways is vital to normal basal ganglia function. Imbalances in these pathways occur in pathological conditions such as Huntington's and Parkinson's diseases and could involve alterations in NMDA and dopamine receptor interactions. Therefore, a better understanding of NR2 subunits and their contribution to these specific receptor interactions could lead to improved therapeutic approaches for these conditions.