Stimulation of NMDA and AMPA glutamate receptors elicits distinct concentration dynamics of nitric oxide in rat hippocampal slices

Abstract

Nitric oxide ((*)NO) is an intercellular messenger implicated in memory formation and neurodegeneration in the hippocampus. Owing to its physical and chemical properties, the concentration dynamics of (*)NO is a critical issue in determining its bioactivity as a signaling molecule. Its production is closely related to glutamate N-methyl-D-aspartate (NMDA) receptors, following a rise in intracellular calcium levels. However, that dependent on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors remains elusive and controversial, despite reports describing a role for these receptors in other brain regions, largely because of lack of quantitative and dynamic measurements of (*)NO. Using a (*)NO-selective microsensor inserted in the diffusional spread of (*)NO in the CA1 region of rat hippocampal slices, we measured its real-time endogenous production, following activation of ionotropic glutamate receptors and under tissue physiological oxygen tension. Both NMDA and AMPA stimulation resulted in a concentration-dependent (*)NO production but encompassing distinct kinetics for lag phases and slower rates of (*)NO production were observed for AMPA stimulation. Robustness of the results was achieved instrumentally and pharmacologically, by means of nitric oxide synthase (NOS) inhibitors and antagonists of NMDA (D-(-)-2-amino-5-phosphonopentanoic acid, AP5) and AMPA (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide, NBQX) receptors. When using glutamate as a stimulus, (*)NO production was of lower magnitude in the presence of AP5 plus NBQX than with AP5 alone, suggesting that even when NMDA receptors are inhibited Ca(2+) rises to levels to induce a peak of (*)NO from the background. Whereas extracellular Ca(2+) was required for the (*)NO signals, Philanthotoxin-4,3,3 (PhTX-4,3,3) a toxin used to target Ca(2+)-permeable AMPA receptors, attenuated (*)NO production. These observations are interpreted on basis of a distinct coupling between the glutamate receptors and neuronal NOS. A role for Ca(2+)-permeable AMPA receptors in the Ca(2+) activation of neuronal NOS is suggested.