NMDA glutamate receptors (NMDARs) on substantia nigra pars compacta (SNc) dopamine cells possibly contribute to excitotoxic neurodegeneration of dopamine cells in Parkinson’s disease and are therefore potential therapeutic targets. We have used patch-clamp recording methods to identify NMDAR subtypes present in excised, outside-out patches from dopamine cells in 300 µm coronal slices of SNc from humanely killed rats (postnatal days 14-16).
Single-channel recordings of NMDAR channels activated by 100 nM NMDA and 10 µM glycine were analysed to determine which subunits form functional NMDARs in SNc dopamine cells. Distributions of channel amplitudes identified four conductance levels (18, 27, 37 and 49 pS) in 11 of 13 patches. Direct transitions between 37 and 18 pS conductance levels exhibited asymmetry in the frequency of transitions between the two conductance states, indicating that NR2D subunits, rather than NR2C subunits contribute to these receptors (Wyllie et al. 1996). The NR2B antagonist ifenprodil (1 µM) reduced Popen by 41.1 ± 8.8 % (mean ± S.E.M., n = 7 patches) but, unexpectedly, this effect was not selective for only high conductance channels. Ifenprodil reduced equally the activity of all conductance levels, suggesting that the NR2B subunit contributes to both high and low conductance receptors. Channel activity was not sensitive to 50 nM Zn2+ or 1 µM TPEN (N, N, NÔ, NÔ-tetrakis(2-pyridyl-methyl)ethylenediamine), a high-affinity zinc chelating agent, suggesting that NR2A subunits do not contribute to these receptors. These data suggest that functional NMDARs on SNc dopamine cells in 2-week-old rats are heterotrimers composed of NR1, NR2B and NR2D subunits.
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This work was supported by the MRC and Wellcome Trust. S.J. is supported by the Parkinson’s Disease Foundation and the National Parkinson’s Foundation.