CNS excitability changes during altitude acclimatization and deacclimatization in rats

Abstract

Electroshock convulsions in rats at sea level and at natural and simulated high altitudes demonstrated a central nervous system (CNS) excitation during acclimatization and depression during deacclimatization. Excitation, shown by decreased minimal seizure threshold and by increased duration of tonic extension during maximal seizures, was evident after 1 or 2 days exposure to 12,500 ft and after 2 hr to 20,000 ft. Changes in maximal seizures disappeared after 2 months at 12,500 ft, but seizure thresholds were significantly lower than in sea level controls even after 8 months. Recovery from maximal seizures remained prolonged and variable at altitude. During deacclimatization after 8 months at 12,500 ft or 6 days at 20,000 ft, CNS depression was shown by absence or shorter duration of tonic extension during maximal seizures and by a rapid rise in seizure thresholds. Increased excitability at altitude is probably due to hypoxia and hyperventilation-induced hypocapnia. The depression during deacclimatization is similar to that produced by hypercapnia and hyperoxia and suggests that CNS sensitivity to CO₂ and O₂ becomes altered during acclimatization.