Abstracts

Rationale: Seizures are common in newborns and frequently occur after birth asphyxia. Increased mortality and poor outcome have been associated with a higher seizure burden. Addressing neonatal seizures is a priority, but current drugs are largely ineffective. The molecular and cellular mechanisms underlying birth-asphyxia seizures are unknown, but it is obvious that information on the seizure-triggering mechanisms is of crucial importance in the design of novel therapeutic strategies. Using 6 day-old rat pups, we have recently shown that the post-asphyxia seizure burden is linked to an increase in brain extracellular pH (pH_o) that consists of the recovery from the asphyxia-induced acidosis followed by a subsequent overshoot well above (0.3 pH units) normal baseline level (Helmy et al., 2011). Methods: Our model of birth asphyxia is based on exposing neonatal rat pups to hypoxia (9% O₂) and hypercapnia (20% CO₂) for one hour. Recently (Helmy et al., 2011) we showed that the large post-asphyxia seizure burden is abolished by a graded restoration of normocapnia, which is achieved by gradually reducing CO₂ levels in inhaled air first to 10% for 30 min, and then to 5% for a further 30 min. In the present study, we used 2-photon in vivo imaging to measure intraneuronal pH (pH_i) and pH-sensitive electrodes to monitor pH_o in both brain and body during and after asphyxia. N-methyl-isobutyl-amiloride (MIA), a blocker of Na/H exchange, was administered. Blood-brain barrier (BBB) permeability was assessed by sodium fluorescein. Results: In neocortical neurons in vivo, pH_i showed a biphasic acid-alkaline response resulting in an alkaline plateau level about 0.3 pH units above baseline. The alkaline overshoot of pH_i was strongly suppressed by graded restoration of normocapnia. Electrode recordings of pH in the brain and body demonstrated net efflux of acid equivalents from the brain across the BBB, which was abolished by MIA, or its parent molecule amiloride. Inhibiting Na/H exchange also suppressed the seizures. No increase in BBB permeability was detected by sodium fluorescein extravasation into the brain or in the electrophysiology experiments. Conclusions: The present work shows that post-asphyxia alkalosis in the brain and the consequent seizures are caused by a Na/H exchange-dependent efflux of acid equivalents across the BBB. Our results raise the important and worrying question whether standard resuscitation paradigms where normocapnic conditions are established in a fast manner will, in fact, promote birth-asphyxia seizures. Graded restoration of normocapnia and/or drugs targeting the Na/H exchange in the BBB offer an effective and straightforward means to functionally suppress seizures and ameliorate other neurological sequelae after birth asphyxia, and can be readily used in conjunction with other treatment modalities, such as hypothermia and optimization of blood oxygenation levels. References: Helmy MM, Tolner EA, Vanhatalo S, Voipio J, Kaila K. Brain alkalosis causes birth asphyxia seizures, suggesting therapeutic strategy. Ann Neurol 2011; 69: 493-500.