Brain Blood Flow Modulates the Neurotoxic Action of Hyperbaric Oxygen via Neuronal and Endothelial Nitric Oxide

Studies on conscious rats with inhibition of NO synthase were used to assess the dynamics of brain blood flow and EEG traces during hyperbaric oxygenation at 4 or 5 atm. Oxygen at a pressure of 4 atm induced cerebral vasoconstriction in intact animals and decreased blood flow by 11-18% (p < 0.05) during 60-min exposure to hyperbaric oxygenation. Paroxysmal EEG activity and oxygen convulsions did not occur in rats at 4 atm of O₂. At 5 atm, convulsive activity appeared on the EEG at 41 ± 1.9 min, and blood flow decreased significantly during the first 20 min; blood flow increased by 23 ± 9%, as compared with controls, (p < 0.01) before the appearance of convulsions on the EEG. Prior inhibition of NO synthase I (NOS I) and NO synthase III (NOS III) with Nω-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg) or inhibition only of NOS I with 7-nitroindazole (7-NI, 50 mg/kg) prevented the development of hyperoxic hyperemia and paroxysmal spikes on the EEG during hyperbaric oxygenation at 5 atm. These results show that hyperbaric oxygen induces changes in cerebral blood flow which modulate its neurotoxic action via nitric oxide synthesized both in neurons and in cerebral vessels.