Mechanisms of Tolerance to the Anticonvulsant Effects of Acetazolamide in Mice: Relation to the Activity and Amount of Carbonic Anhydrase in Brain

Abstract

The mechanism by which tolerance develops to the anticonvulsant effects of acetazolamide (AZM) was investigated in Swiss-Webster mice. The effects of single and six daily doses of 40 mg or 200 mg/kg AZM on electroshock seizure threshold (EST), maximal electroshock (MES) seizure pattern, and on the activity and total amount of carbonic anhydrase II (CAII) in various subcellular fractions (cytosol, microsomes, and myelin) of cerebral cortex, cerebellum, and brainstem were assessed. The activity of CAII was measured by microassay, and the total amount was measured by immunoassay methods developing in this laboratory. From the activity (units per microgram of protein) and total amount (nanograms per microgram protein) data, the specific activity (units per nanogram CAII) of the enzyme was calculated. With multiple doses, tolerance developed to both elevation of the EST and modification of the MES pattern noted with the single doses of AZM. Accompanying the development of tolerance of the anticonvulsant effects of AZM was an increase in both the activity and specific activity of CAII in the various subcellular fractions and areas of the brain. The effects were dose dependent. Tolerance to the EST-elevating effects of AZM correlated with increases in the activity, total amount, and specific activity of CAII in the various subcellular fractions and areas of the brain. The effects were dose dependent. Tolerance to the EST-elevating effects of AZM correlated with increases in the activity, total amount, and specific activity of CAII in the myelin fraction of the cerebral cortex. Tolerance to the anti-MES effect correlated with increases in the activity, total amount, and specific activity of CAII in cystolic fraction of the brainstem. Development of tolerance to AZM proceeds by two mechanisms that are dose-dependent: (a) increase in the total amount of enzyme, changes that suggest increased synthesis of CAII; and (b) activation of existing molecules of CAII, a process that appears to involve phosphorylation. The data suggest that activity of CAII is regulated by several mechanisms in brain tissues and that preservation of its activity is vital to the normal functioning of the brain.