NMDA and Kainate‐evoked Release of Nitric Oxide and Classical Transmitters in the Rat Striatum: In Vivo Evidence that Nitric Oxide May Play a Neuroprotective Role

Abstract

The effects of N‐methy‐d‐aspartate (NMDA), kainate, S‐α‐amino‐3‐hydroxyd‐5‐methyl‐4‐isoxazole propionate (AMPA) and KCI on striatal nitric oxide (NO), acetylcholine (ACh), dopamine (DA), serotonin (5‐HT), aspartate (ASP), glutamate (GLU) and γ‐aminobutyric acid (GABA) release were measured in anaesthetized rats in vivo by microdialysis and in vitro in organotypic slice cultures. Local NMDA (1–100 μM) infusion by retrodialysis dose‐dependently increased levels of classical transmitters, NO₂‐, NO₃‐, ctrulline and arginine at similar thresholds (10 γM) Similar patterns of NMDA‐evoked (50 μM) release were seen in striatal cultures. NMDA‐evoked changes were all calcium‐dependent and blocked by NMDA (APV or MK‐801) but not AMPN/kainate (DNQX) receptor antagonists, excepting DA which could be prevented by both. In vivo, kainate increased NO₂^‐, NO₃^‐, CIT and ARG levels at 50 and 100 μM but was less potent than NMDA. Kainate also evoked significant Ach₁ DA and GLU release dose‐dependently starting at 1–10 μM whereas 5‐HT, ASP and GABA required 50 or 100 μM doses. Kainate effects were inhibited by DNQX, but not by APV, and were calcium‐dependent. AMPA failed to alter NO₂^‐, NO₃^‐, CIT or ARG levels at 50 or 100 μM doses but dose‐dependently increased ACh and DA. Similar results were seen with kainate (50 μM) and AMPA (50 μM) in vitro KCI evoked NO₂^‐, NO₃^‐, CIT and ARG release as well as that of the classical transmitters in vivo and in vitro. In vivo administration of the NO synthase inhibitor L‐nitroarginine (L‐NARG; 100 μM) significantly reduced NO₂^‐, NO₃^‐ and CIT levels and prevented NMDA, kainate or KCI‐evoked increases. It also potentiated ACh, ASP, GLU and GABA release and reduced that of DA in response to 50 μM NMDA whereas treatment with an NO‐donor (SNAP; 10 μM) significantly reduced evoked ACh, ASP and GLU release. The NO synthase inhibitor L‐NARG potentiated kainate‐evoked ACh release and reduced that of DA, although less potently than NMDA, but it had no effect on KCI‐evoked transmitter release. Overall, these results show that both NMDA and kainate increase striatal NO release at similar dose‐thresholds as for classical transmitter release suggesting that NO is dynamically released under physiological and not just pathological conditions. Reduction of striatal NO levels also potentiates calcium‐dependent transmitter release in response to NMDA and, to a lesser extent, kainate, whereas increasing them reduces it. This is consistent with a role for NO as a neuroprotective agent in this region acting to desensitize NMDA receptors.