N-methyl-D-aspartate (NMDA) receptors (NMDARs) are a major class of excitatory neurotransmitter receptors in the central nervous system. They form glutamate-gated ion channels that are highly permeable to calcium and mediate activity-dependent synaptic plasticity. NMDAR dysfunction is implicated in multiple brain disorders, including stroke, various forms of neurodegeneration, chronic pain and schizophrenia. NMDARs are activated by agonists glutamate and glycine, and their activity is modulated by allosteric modulators including endogenous neurosteroids pregnenolone sulfate and 20-oxo-5β-pregnan-3α-yl sulfate and their synthetic analogues.
We have used electrophysiological and molecular biology techniques in combination with molecular modeling to analyze molecular mechanisms of steroid action at NMDARs. In agreement with our theoretical results, the results of our experiments suggest that neurosteroids bind in the extracellular vestibule of the NMDAR channel to prevent the permeation of small mono and divalent ions. In addition, the model of the NMDAR channel opening suggests an explanation for the different contribution of the GluN1 and GluN2B NMDAR subunits to the inhibition by the steroid.