| The vast majority of information processing in the central nervous system (CNS) is mediated by excitatory and inhibitory chemical synaptic transmission. N-methyl-D-aspartate receptors (NMDARs) are a class of glutamate-gated cation channels that mediate fast excitatory neurotransmission and play critical role in most aspects of nervous system function, including plasticity and cognition. NMDARs are heterotetrameric channels assembled from a combination of different NR1 splice variants (of which there are seven) and NR2 subunits (NR2A, B, C and D). NMDAR function is modulated by a number of endogenous molecules such as polyamines and neuroactive steroids as well as cations such as H+ and Zn2+. Pregnenolone sulfate (PS), an endogenous neurosteroid, differentially modulates the activity of recombinant NMDARs containing different NR2 subunits through a steroid modulatory domain (SMD1) that is critical for both the PS action and H+ sensitivity.; Here we report that [H+] differentially modulates PS action at recombinant NMDARs expressed in Xenopus laevis oocytes. PS modulation by [H+] is both NR1 splice variant and NR2 subunit dependent. The presence of the alpha-exon in the NR1 splice variant enhances PS, but not pregnanolone sulfate (3alpha5betaS) modulation. Furthermore, we identify critical residues at the junction of M4-S2 and M3-S2 extracellular domains of the NR2A subunit that are essential for either PS potentiation or 3alpha5betaS inhibition, respectively. The increase in PS induced potentiation of the NMDA response at low pH suggests a potential pathophysiological role of PS during hypoxic-ischemic conditions as occurs during stroke. In addition we report a second, delayed phase of NMDAR modulation by which low concentrations (nM) of PS regulate the insertion of functional NR1-1a/NR2A receptors into the plasma membrane via a G-protein coupled and Ca2+/PKC dependent mechanism. It is known that NMDARs mediate fast synaptic transmission and influence processes of plasticity underlying learning and memory. Our results suggest that PS and PS-like molecules may alter plasticity and represent an alternative molecular target for the development of potential therapeutics for the treatment of multiple neurological disorders, including Alzheimer's disease. |
