Role of NMDA receptor and nitric oxide pathway in intracellular calcium elevation in rat hippocampal CA1 region during hypoxia

Abstract

The aim of our study is to examine the hypothesis that the endogenous production of nitric oxide (NO) and the activation of NMDA receptors play an important role in intracellular calcium ([Ca2+]i) response to hypoxia in rat hippocampal CA1 region. The NO concentration and [Ca2+]i levels in the CA1 region of hippocampal slices were measured, respectively, by electrochemical method and calcium spectrofluorometry. Both nitric oxide synthase (NOS) inhibitor L-NMMA (100 lM) and neuronal NOS (nNOS) inhibitor 7-NINA (200 lM) significantly reduced the elevation of NO and [Ca2+]i levels in hypoxia. The inhibitory effect of L-NMMA was concentration-dependent. In addition, soluble cyclic guanyl cyclase inhibitor ODQ (10 lM) attenuated the elevation of [Ca2+]i by 45%. These results suggest that hypoxia elevates the endogenous NO production by nNOS and the cGMPdependent pathway plays a role in the [Ca2+]i elevation. To examine the involvement of NMDA receptors in the NO/cGMP pathway, we found that NMDA receptor inhibitor MK801 (100 lM) markedly reduced the hypoxiainduced [Ca2+]i elevation by 65%. L-arginine enhanced the [Ca2+]i elevation by 27% and it attenuated the effect of MK801 on the [Ca2+]i level. Moreover, L-NMMA slightly reduced the inhibitory effect of MK801. Therefore these results are consistent with the idea that the activation of NO-mediated cGMPdependent pathway and NMDA receptors are involved in the calcium influx in hippocampal CA1 neurons during hypoxia. Keywords: cyclic cGMP, hypoxia, nitric oxide, nitric oxide synthase, NMDA. ACKNOWLEDGEMENT: Supported by grants N_HKU711/02, HKU7184/01M.