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Effects of NMDA administration in the substantia nigra pars compacta on the striatal dopamine release before and after repetitive exposures to nitrogen narcosis in rats.

Hyperbaric nitrogen-oxygen exposure developed in rats a decrement of the striatal dopamine release, which was reversed by repetitive exposures. This dopamine decrease could be the result of the antagonistic effect of nitrogen on NMDA receptors. The increment of the dopamine release, following repetitive exposures to nitrogen, could be attributed to a desensitisation of NMDA receptors to the effects of nitrogen. To test these hypotheses, male Sprague-Dawley rats were implanted with electrodes in the striatum to measure dopamine release by voltammetry and cannula in the substantia nigra pars compacta for NMDA injection. Free-moving rats were exposed up to 3MPa of nitrogen-oxygen mixture before and after 5 exposures to 1MPa. At the first exposure to 3MPa, the dopamine level decreased (-15%) but is counteracted by NMDA administration. In contrast, after repetitive exposure, the second exposure to 3MPa, induces a 10% dopamine increase. NMDA administration significantly potentiated this increase. Our results neither support the hypothesis of an antagonist effect of nitrogen on NMDA receptors at the first exposure, nor that of a NMDA receptor desensitization following repetitive exposures to hyperbaric nitrogen.