FREE-RADICAL GENERATION IN THE BRAIN PRECEDES HYPERBARIC OXYGEN-INDUCED CONVULSIONS

We tested the hypothesis that hyperbaric oxygenation (HBO) generates free radicals in the brain before the onset of neurological manifestations of central nervous system (CNS) oxygen poisoning. Chronically cannulated, conscious rats were individually placed in a transparent pressure chamber and exposed to (1) 5 atmospheres absolute (ATA) oxygen for 15 min (n = 4); (2) 5 ATA oxygen for 30 min (n = 5), during which no visible convulsions occurred; (3) 5 ATA oxygen for 30 min with recurrent convulsions (n = 6); (4) 5 ATA oxygen until the appearance of the first visible convulsions (n = 5); (5) 4 ATA oxygen for 60 min during which no convulsions occurred (n = 5); and (6) 5 ATA air for 30 min (n = 5, controls). Immediately before compression, 1 mL of 0.1 M of alpha-phenyl-N-tert-butyl nitrone (PBN) was administered intravenously (iv) for spin trapping. At the termination of each experiment, rats were euthanized by pentobarbital iv and decompressed within 1 min. Brains were rapidly removed for preparation of lipid extracts (Folch). The presence of PBN spin adducts in the lipid extracts was examined by electron spin resonance (ESR) spectroscopy. ESR spectra from unconvulsed rats exposed to 5 ATA oxygen for 30 min revealed both oxygen-centered and carbon-centered PBN spin adducts in three of the five brains. One of the five rats in this group showed an ascorbyl signal in the ESR spectrum. In contrast, only one of the six rats with recurrent convulsions and one of five which were sacrificed immediately after the first visible convulsions showed a low-intensity ESR signal due to PBN spin adducts. Unconvulsed rats exposed to 5 ATA oxygen for 15 min or 4 ATA oxygen for 60 min showed no PBN spin adducts but had an ascorbyl signal in two of four and three of five ESR spectra, respectively. Lipid extracts from control rats exposed to 5 ATA air showed no ascorbyl ESR signal. The preconvulsive appearance of PBN spin adducts and ascorbyl radical ESR signals is consistent with the free radical theory of the CNS oxygen toxicity. However, the reason for diminution or disappearance of PBN spin adducts after HBO-induced convulsions remains to be investigated.