Reevaluation of CNS Oxygen Toxicity Seizures in Male Sprague‐Dawley Rats

Abstract

Prolonged breathing of hyperbaric oxygen (HBO2) results in central nervous system O2 toxicity (CNS‐OT; i.e., seizures, Sz's). The threat of CNS‐OT is what limits the use of HBO2 in hyperbaric, diving and submarine medicine. Past studies have identified CNS‐OT as the onset of visibly detectable tonic‐clonic Sz's preceded by increased cortical EEG activity. It has been our experience, however, that onset of 1st Sz in Sprague‐Dawley (SD) rats is variable between animals and, in some cases, difficult to confirm in real time. This is problematic because the latency time to seizures (LSz) is how the risk of CNS‐OT is evaluated in hyperbaric research. Consequently, we have begun using longer exposures to HBO2 to capture recurring changes in EEG activity and behaviors that can be analyzed offline to redefine the criteria for 1st Sz. We also wanted to know if subsequent Sz's increased in intensity. Accordingly, cortical EEGs were recorded in 27 adult male SD rats that were implanted with a DSI 4‐ET radio transmitter. At least 1 week following surgery, each rat was placed in a hyperbaric chamber and “dived” to 5 ATA O2. 71 Sz's were identified in 27 rats based on quantified changes in cortical EEG activity (NeuroScore 2.1) and our modified Racine scale of six Sz‐related behaviors. A cortical seizure (CSz) was defined as a series of EEG polyspikes that crossed a threshold of 200μV, lasted ≥15 seconds, and contained an increase in theta wave activity coupled with at least one of the Sz‐related behaviors (65% of the Sz were classified as CSz). Two other Sz types were also recorded: neurological seizures (NSz) that included only the EEG profile described above, but without any detectable behavior changes (14%); and behavioral seizure (BSz) that included one or more of the behavioral components without any detectable changes in cortical EEG activity (21%). LSz was calculated from the moment the chamber pressure reached 5 ATA O2 until the onset of CSz, NSz or BSz. The LSz to 1st seizure was 7.2 ± 5.3 min (n= 27), while the 2nd and 3rd Sz's occurred at 26.2 ± 29.88 min (n=25) and 40 ± 40.86 min (n=13), respectively. Remarkably, in between bouts of Sz's, animals exhibited normal behaviors (grooming, exploring, chewing, etc.) and normal EEG activity in 5 ATA O2. There was also a shift in the type of Sz between the 1st and 2nd Sz such that the fraction of BSz + NSz significantly increased from 22% to 44%. Moreover, intensity of the Sz increased progressively with each successive Sz based on our modified Racine score. We conclude that the majority of HBO2 Sz's in SD rats can be readily identified using a combination of defined EEG and behavioral criteria. However, not all Sz's during HBO2 fit both criteria. The occurrence of BSz suggests that neurological Sz's originate outside the cerebral cortex. The occurrence of NSz indicate that some Sz's will be missed in studies that do not measure EEG activity. The fact that 1st Sz is often underwhelming compared to subsequent Sz's suggests the LSz may be overestimated in some animals. To better understand the SD rat model of CNS‐OT, we recommend that future studies need to 1) implant telemetric leads into multiple cortical and subcortical areas to localize the site of Sz‐genesis; and 2) determine the degree of performance impairment during 1st, 2nd & 3rd Sz's.Support or Funding InformationONR N000141310405 & N000141612537