Valproic acid in amygdala-kindled rats: alterations in anticonvulsant efficacy, adverse effects and drug and metabolite levels in various brain regions during chronic treatment.

Amygdala-kindled rats were treated with valproic acid (VPA; administered as its sodium salt) 3 times daily at 200 mg/kg i.p. for 6 weeks, and anticonvulsant and adverse effects during this period were studied. Groups of nonkindled rats were treated in parallel for determination of VPA and its major active metabolites in various brain regions after different durations of treatment. After the first injection of VPA, 200 mg/kg, seizure severity, seizure duration and duration of electrical afterdischarges recorded from the stimulated amygdala were reduced significantly, but only one of nine animals was protected completely from kindled seizures. At day 3 of chronic treatment, the anticonvulsant activity of VPA had increased markedly so that seven of nine animals were totally protected from seizures. However, this potent anticonvulsant effect was only transitory so that after 1 week of treatment the anticonvulsant effect of the medication was similar to that obtained after the first dosing. The effect of VPA remained at this level for the subsequent weeks, but there was a second, more permanent increase in the number of protected animals after 4 to 6 weeks. Plasma and brain levels of VPA and its metabolites remained relatively constant throughout the chronic treatment although there was a moderate accumulation of some metabolites, e.g., trans isomer of 2-propyl-2-pentenoic acid, in specific brain nuclei. The most prominent adverse effects of VPA were ataxia, muscle relaxation, wet-dog shake behavior and an increase in body temperature. Except for body temperature, tolerance developed to these adverse effects, but escape from wet-dog shake behavior occurred much more rapidly than reduction of other adverse effects. Pathohistological examination of liver sections from animals treated with VPA for 6 weeks showed no indication of any hepatotoxic effects. After drug withdrawal, kindled seizure parameters returned toward control values without evidence of significant carry-over effects. Five days after termination of treatment, only minute amounts of VPA and trans isomer of 2-propyl-2-pentenoic acid were determined in some brain regions, indicating that there was no persistence of active drug or metabolite concentrations in the brain.