Abstract
N-methyl-D-aspartate receptor (NMDAR) dysfunction is associated with many central nervous system disorders such as autism, stroke, and neurodegenerative diseases. Thus, regulation of NMDAR function represents a potential therapeutic strategy. A key pathway that regulates NMDARs is through activation of voltage-gated sodium channels (VGSC)s where elevation of intracellular Na+ renders NMDAR more sensitive to Src family kinase phosphorylation with the attendant increase of Ca2+ influx (Yu and Salter, 1999). Elevation of intracellular calcium activates calcineurin and dephosphorylates the NMDAR limiting further calcium entry (Krupp et al., 2002). Mouse hippocampal neuronal culture (HN) and organotypic hippocampal slice culture (OHSC) were used to study the relationship between neuronal intracellular sodium, calcium and structural plasticity. NMDARs and VGSCs were activated using NMDA and brevetoxin-2 (PbTx-2), respectively. In HN, NMDA and PbTx-2 produced concentration-dependent increases of intracellular calcium. Both NMDA and PbTx-2 induced calcium influx through the NMDARs and L-type calcium channels, however, PbTx-2 also recruited the reverse mode of operation of Na+/Ca2+ exchanger (NCX). The effects of NMDA and PbTx-2 on structural plasticity were dependent on Ca2+ influx through identical pathways. These results demonstrated that NMDA and PbTx-2 induce a bidirectional concentration-response curve in neurite outgrowth, dendritic arborization, and spine density. The data revealed that calcineurin was not responsible for the descending phase of NMDA or PbTx-2 on structural plasticity, though it plays an essential role in NMDAR dephosphorylation. Overall, these studies are consistent with the hypothesis that regulation of NMDAR function through VGSC activation may represent a novel pharmacological strategy to promote neuronal structural plasticity.