Modulation of NMDA receptor activity by free radicals

The N-methyl-D-aspartate (NMDA) receptor is subject to modulation by oxidizing and reducing agents, which likely act on more than one redox site. Reducing agents have been shown to potentiate, while oxidizing agents inhibit NMDA responses. The effect of the reactive nitrogen oxygen species nitric oxide has however been controversial, with some workers claiming a protective role for nitric oxide through its blocking action on the NMDA receptor. Others have claimed a damaging effect of nitric oxide.;Using patch clamp techniques, we have examined the effects of dithiothreitol (DTT), reactive oxygen species and nitric oxide on different subunit combinations of the NMDA receptor transiently transfected in HEK 293 cells, as well as on cells in young and old cultures of rat cortical neurons. Our results demonstrate a clear subunit-dependence of the effects of reducing and oxidizing agents on heteromeric NMDA receptors, with NR1-NR2A receptors being less sensitive to DTT, superoxide and hydroxyl radical than NR1-NR2B receptors. In addition, NMDA responses in younger cortical neurons were more sensitive to oxidation than those in older cortical neurons. These results may help to explain the greater resistance of young cortical neurons to excitotoxicity. These neurons express NR2B receptors predominantly and may therefore be protected from excitotoxicity by the sensitivity of these receptors to inhibition by ambient levels of oxidizing agents.;We have also shown that the nitric oxide donors (Z)-1-[N-(3-Ammoniopropyl)-N-(n-propyl)amino] (PAPA-NO), diethylamine-NO (DEA-NO) and Angeli's salt (Na2N 2O3-sodium trioxodinitrate), potentiate the glutamate-mediated response of recombinant rat NMDA receptors expressed in HEK 293 cells. The overall effect is an increase in both peak and steady-state whole cell currents induced by glutamate.;Since nitric oxide synthase (NOS) is co-localized with the NMDA receptor at synapses and is activated by Ca2+ influx through the NMDA receptor, these results suggest the involvement of NMDA receptors and nitric oxide in a positive feedback loop. In this model, NMDA receptor activity is enhanced by nitric oxide which is produced as a result of NMDA receptor-mediated activation of NOS. This feedback loop may be important in long term potentiation and excitotoxicity.