REPEATED EXPOSURE TO HYPERBARIC-OXYGEN SENSITIZES RATS TO OXYGEN-INDUCED SEIZURES

Repeated exposure to increased partial pressure of oxygen (Po,) is the standard of care for several medical conditions. The side-effects of repeated exposure to hyperbaric oxygen (HBO), however, are not well de fined. Previous studies have demonstrated that acute exposure of rats to HBO causes hypothermia that precedes convulsions. In the present st udies, rats that were repeatedly exposed to 100% oxygen at 4 atmospher es absolute (ATA) pressure developed convulsions earlier than naive co ntrols. There was also trend toward less hypothermia in the rats repea tedly exposed to oxygen. The purpose of this study was to test the hyp othesis that repeated exposure to HBO increases sensitivity to convuls ions induced by HBO and to determine if the time to onset of convulsio ns is affected by the hypothermia caused by exposure to HBO. Rats were repeatedly exposed to 2 ATA oxygen for a total of 10 days. After 72 h , these rats were challenged by exposure to 100% oxygen at 4 ATA press ure. Rats repeatedly exposed to HBO had convulsions significantly earl ier than the naive controls (84+/-8 min compared to 147+/-11 min), and they developed significantly less hypothermia. Control studies sugges ted that the decrease in the degree of hypothermia was caused by both repeated exposure to oxygen and adaptation to the mild restraint used during oxygen re-exposures. Adaptation to restraint eliminated the hyp othermia induced by oxygen but did not change the time to onset of con vulsions. Increased sensitivity to convulsions was present after five exposures to 2 ATA oxygen and persisted for 10 days after the last 2 A TA oxygen re-exposure. Kindling is an animal model of epilepsy which i s caused by repeated exposure to subthreshold doses of convulsant stim uli. This repeated dosing causes an increase in the sensitivity of the animal to the convulsant stimuli that can persist for weeks. The incr eased sensitivity of rats to convulsions during subsequent HBO challen ge that is induced by repeated exposures to subconvulsant 'doses' of H BO (2 ATA) is similar to kindling. At present the mechanism that cause s either kindling or the increased sensitivity to seizures induced by HBO are unknown, and therefore may not be the same.