Changes in nitric oxide production and cerebral blood flow before development of hyperbaric oxygen-induced seizures in rats

The predictive value of increase in cerebral blood flow (CBF) was examined to detect hyperbaric oxygen (HBO₂)-induced electrical discharge in artificially ventilated rats at three PaCO₂ levels under 5 atmospheric pressures. The possible involvement of NO production in the mechanism of the increase in CBF was also assessed by measurement of major NO metabolites (NO₂^- plus NO₃^-) using a microdialysis technique at the left parietal cortex during HBO₂ exposure. The onset times of electrical discharge, measured in the right frontal region, were significantly prolonged and shortened in the low PaCO₂ group (79±21 min) and high PaCO₂ group (27±7 min), respectively, compared to that in the normal PaCO₂ group (37±5 min). Increase in CBF (200% of the pre-exposure level) was observed in every animal and was sustained until the appearance of electrical discharge. The onset time of increase in CBF was closely related to that of electrical discharge (R²=0.987), and the durations of increase in CBF were almost identical (11-14 min in mean) regardless of the PaCO₂ level. The level of NO₂^- plus NO₃^- was unaffected by the initiation of HBO₂ exposure and simultaneously increased up to 246±59% of control level with the onset of increase in CBF. There was a close relationship between changes in CBF and levels of NO₂^- plus NO₃^- (R²=0.544). These results indicate that monitoring of CBF is useful for the prediction of electrical discharge in artificially ventilated rats regardless of their PaCO₂ levels and that the increase in NO production is related to the mechanism of increase in CBF.