Effects of various concentrations of CO2 on brain excitability and electrolyte distribution in rats were studied, and also some properties of seizures induced by abrupt withdrawal from high concentrations of CO2. Inhalation of relatively low concentrations of CO2 (5–20%) decreases brain excitability, as measured by an increase in electroshock seizure threshold (EST). In moderately high concentrations (25–40%), CO2 increases brain excitability, as measured by a decrease in EST and the appearance of spontaneous seizures. Inhalation of high concentration of CO2 (40% or higher) markedly decreases brain excitability and causes anesthesia. Thus the effect of CO2 on brain excitability is related to the concentration inhaled. Abrupt removal of rats from high (anesthetic) concentrations of CO2 results in spontaneous clonic seizures within 30 seconds to 1 minute after withdrawal; these seizures last for 1–2 minutes. Inhalation of 50% CO2 decreases brain intracellular Na and K concentrations and produces a marked cellular acidosis. Thirty seconds after abrupt withdrawal of rats from 50% CO2, but prior to the onset of seizures, the concentration of Na in brain cells increases and the concentration of K decreases.

The influence of various concentrations of carbon dioxide and oxygen on the pattern of maximal electroshock seizures (MES), on pentylenetetrazol (Metrazol)-induced seizures, and on recovery time (RT50) from MES has been studied in mice. The major results are summarized as follows: Increasing the concentration of carbon dioxide in 20% oxygen resulted in marked changes in the MES pattern. The duration of the tonic flexor component was increased, and the duration of the tonic extensor component and of the entire tonic phase was decreased; the ratio of flexion to extension was increased markedly. These changes indicate an anticonvulsant effect of carbon dioxide. Carbon dioxide in a concentration of 15.6% protected 50% of mice against a 97% convulsant dose (CD97) of Metrazol. Hypoxia, induced by inhalation of 15 or 10% oxygen, decreased the duration of the tonic flexor component and increased the duration of the tonic extensor component of the MES; the duration of the entire tonic phase was increased; the ratio of flexion to extension was slightly decreased. These changes indicate an excitatory effect of low oxygen concentrations on the brain, under the stated conditions. Hyperoxia, induced by inhalation of 50, 75 or 90% oxygen, in contrast to hypoxia, resulted in effects on MES similar to those produced by inhalation of carbon dioxide. The RT50 was slightly, but not significantly, decreased by hypoxia (15% oxygen), unchanged by hyperoxia (50% oxygen), and markedly increased by an increase in carbon dioxide concentration. Carbon dioxide (5 and 10%) combined with hypoxia (10% oxygen) produced effects on MES pattern and RT50 similar to those produced by carbon dioxide in 20% oxygen.