Concentration dynamics of nitric oxide in rat hippocampal subregions evoked by stimulation of the NMDA glutamate receptor

doi:10.1073/pnas.0503624102

Concentration dynamics of nitric oxide in rat hippocampal subregions evoked by stimulation of the NMDA glutamate receptor

^*Faculty of Pharmacy and Center for Neurosciences and Cell Biology, University of Coimbra, 3000 Coimbra, Portugal; ^†Department of Anatomy and Neurobiology, University of Kentucky, Lexington, KY 40506-0098; and ^‡Department of Molecular Pharmacology and Toxicology, University of Southern California, Los Angeles, CA 90089-9121

Edited by Salvador Moncada, University of London, London, United Kingdom, and approved October 5, 2005 (received for review May 9, 2005)

Abstract

Nitric oxide ( $\text{[math]}$ ) production in response to stimulation of the NMDA glutamate receptor is implicated not only in the synaptic plasticity in hippocampus but may also participate in excitotoxic cell death. Using $\text{[math]}$ -selective microssensors inserted into the diffusional field of $\text{[math]}$ in acute hippocampal slices, we describe the $\text{[math]}$ concentration dynamics evoked by NMDA receptor activation and report profound differences along the trisynaptic loop of the hippocampus. We measured the oxygen gradient across the slice thickness and conclude that $\text{[math]}$ measurements were performed at cell layers experiencing physiological oxygen tensions. Recordings performed at increasing distances from the point of NMDA receptor stimulation resulted in a progressive decrease of $\text{[math]}$ signals, reaching undetectable levels for distances >400 μm, supporting the notion of a wide diffusional spread of endogenously generated $\text{[math]}$ in the hippocampus. Neither a picoinjection nor a continuous perfusion of NMDA resulted in high steady-state $\text{[math]}$ levels; rather all signals were transient, suggesting that cells are able to efficiently respond to high $\text{[math]}$ concentrations (typically 200-400 nM) bringing it to very low nM levels; the claimed high micromolar $\text{[math]}$ range achieved by excessive stimulation of NMDA receptor may have to be reevaluated. The distinct responses to NMDA receptor stimulation along the trysynaptic loop suggest a differential $\text{[math]}$ activity and/or regulation among the hippocampal subregions. These findings may be relevant for the understanding of the role of $\text{[math]}$ in physiologic mechanisms in the hippocampus and the differential sensitivity of the hippocampal subregions to NMDA receptor-dependent neurodegeneration.

Footnotes

↵ § To whom correspondence should be addressed. E-mail: laranjin{at}ci.uc.pt.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: nNOS, neuronal NO synthase; DG, dentate gyrus; LTP, long-term potentiation; d-AP5, d-(-)-2-amino-5-phosphonopentanoic acid; l-NNA, N_G-nitro-l-arginine; AA, ascorbate; aCSF, artificial cerebrospinal fluid.