Research reportAntiepileptic drugs prevent seizures in hyperbaric oxygen: A novel model of epileptiform activity
Introduction
Oxygen is a convulsant when breathed for a sufficient time at partial pressures of 2.5 atmospheres absolute (ATA) or above, and latency to onset of epileptiform patterns on the EEG and tonic-clonic neuromotor responses is inversely proportional to inspired Po2. This has been well established in human and animal studies that have been conducted for many years, primarily to devise procedures for protecting personnel exposed to hyperbaric oxygen (HBO2) in certain occupational and therapeutic environments (Balentine, 1982, Behnke et al., 1935, Clark and Thom, 2003, Donald, 1947a, Donald, 1947b, Paton, 1967). Vulnerable groups include military and civilian divers, crew attempting escape from a disabled submarine and patients undergoing hyperbaric oxygen therapy.
Although many drugs have been developed to prevent seizures of various etiologies, particularly those of broad clinical relevance, none have been specifically formulated to protect against seizures in HBO2, and it is unlikely that pharmaceutical companies would undertake the research needed to do so, since the human population at risk is small. Up to now, the only reliable methods for avoiding neurotoxicity in HBO2 have been to limit the dose and duration of hyperbaric oxygenation.
Although the events or conditions that initiate seizures in hyperbaric oxygen and in other seizure disorders may differ, the final common pathways and ultimate molecular targets may be similar. Indeed, some investigators have proposed that oxidative stress plays a contributory role in epilepsy (Patel, 2004, Pearson et al., 2015, Zsurka and Kunz, 2015). We reasoned, therefore, that antiepileptic drugs (AEDs) could prevent or delay seizures of CNS O2 toxicity and as a corollary, AEDs with known mechanisms-of-action might be used as investigative probes to further elucidate events that evoke hyperoxic seizures, although the true mechanisms of action of some of these drugs are not completely understood and may not fit neatly into their nominal functional classes. Furthermore, if a broad range of AEDs are found to prevent or delay seizures in HBO2, and if there are parallels between the ictal events in HBO2 and those seen in other seizure disorders, the hyperbaric model could elucidate mechanisms of various seizure-related disorders, including those due to the acquired and idiopathic epilepsies, traumatic brain injury and other causes. However, it must be understood that most animal models used in epilepsy research only simulate the seizures of epilepsy rather than epilepsy itself (Loscher, 2011), and this would apply to HBO2 as well.
In this study we assessed the ability of nine FDA-approved AEDs, administered individually, to increase seizure latency in mice exposed to 100% oxygen at 5 ATA for 60 min. This level and duration of hyperoxia reliably elicits seizures in this species in a reasonable period of time (60 min or less) without producing a significant degree of direct cardiopulmonary injury (Demchenko et al., 2007).
Since abnormal propagation of excitatory neurotransmission and attenuation of inhibitory neurotransmission are established pathogenic factors in clinical epilepsies (Jefferys, 2010) and are also assumed to be factors in CNS O2 toxicity (Colton and Colton, 1982, Dean et al., 2003, Demchenko and Piantadosi, 2006), we chose to evaluate the protective efficacy in HBO2 of AEDs from two functional classes relevant to excitatory and inhibitory functions: sodium-channel antagonists and GABA transmission enhancers. We compared our findings with published values for the efficacy of the same AEDs in a well-established animal model of epileptic seizures, maximal electroshock (MES).
Section snippets
Seizure activity in vehicle-treated mice
The 24 mice treated with vehicle (0.9% NaCl or DMSO) and exposed to HBO2 at 5 ATA exhibited neuromotor responses that progressed in 2 or 3 stages, comparable to those of the Racine Scale (Racine, 1972). In stage I, restlessness, intensive grooming, slight tremors, twitching vibrissae and transient muscular spasms were observed. In stage II, persistent, rhythmic spasms appeared in the face and body along with bilateral forelimb clonus and escape behaviors. Stage III, if it occurred, was
Discussion
We have demonstrated that a range of FDA-approved AEDs, of two functional classes, can significantly delay seizure onset in extreme hyperoxia. Although other investigators have tested the protective efficacy of two such drugs in HBO2, CBZ and VGB (Bitterman and Halpern, 1995, Hall et al., 2013, Harel et al., 1978, Reshef et al., 1991, Tzuk-Shina et al., 1991), we know of no study in which the anticonvulsant efficacies of multiple AEDs were compared in hyperbaric hyperoxia.
Among the Na+-channel
Conclusions
The findings presented here support our hypothesis and its corollary: a range of FDA-approved AEDs can prevent or delay seizures in extreme hyperoxia, and AEDs with known mechanisms-of-action can serve as investigative probes to elucidate seizure mechanisms in HBO2. Thus, some of the AEDs we tested increased seizure latency to more than triple that observed in vehicle controls. Furthermore, the significant protective efficacy in HBO2 of AEDs known to block Na+-channel function or to enhance
Grants
This work was supported by the Office of Naval Research, United States, Grant N00014-15-1-2072 (to C.A. Piantadosi) and the Russian Federation for Basic Research Grant 15-04-05970 (to I.T. Demchenko).
Methods and materials
Experiments were performed on conscious C57BL/6 mice (weighing 19-25 g and approximately 2.5 months in age) at the Center for Hyperbaric Medicine and Environmental Physiology, Duke University Medical Center (Durham, NC, USA), and at the Institute of Evolutionary Physiology and Biochemistry, Russian Academy Sciences (St. Petersburg, Russia). Animal use protocols were approved independently by the Institutional Animal Care and Use Committee of Duke University and the Ethical Review Board of the
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