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Rats were tested for anticonvulsant responsivity to agents active at central and peripheral-type benzodiazepine receptors before and after they were made tolerant to the anticonvulsant effects of carbamazepine on amygdala-kindled seizures. Tolerance to carbamazepine in this paradigm is a contingent process; it occurs when the drug is administered prior to, but not following the kindled seizure. In animals tolerant to carbamazepine, cross-tolerance was observed to the anticonvulsant effects of PK11195, which is active at peripheral-type benzodiazepine receptors, but not to diazepam, which affects central-type benzodiazepine receptors. In animals treated with carbamazepine after the kindled seizure (not tolerant), no alteration in the anticonvulsant effect of PK11195 was observed. These data extend previous biochemical and pharmacological findings suggesting the importance of peripheral-type benzodiazepine receptor mechanisms in the anticonvulsant effects of carbamazepine and suggest a role for this site in the process of contingent tolerance development.

Carbamazepine's effects on cocaine-induced increases in dopamine overflow in the nucleus accumbens were studied using in vivo microdialysis in anesthetized rats. Rats were chronically treated with a diet containing carbamazepine or no drug for one week prior to microdialysis procedures. The basal levels of dopamine in the nucleus accumbens did not differ between groups; however, the increases in dopamine following cocaine administration (4 mg/kg i.v.) were significantly reduced by carbamazepine. In addition, basal levels of dihydroxyphenyl acetic acid (DOPAC) were significantly diminished in the carbamazepine group and a trend in the same direction was observed for homovanilic acid (HVA). These results are consistent with reports that carbamazepine might decrease dopamine synthesis. Further studies are required to determine the mechanism of carbamazepine's inhibition of cocaine-induced dopamine overflow and its potential clinical implications.

The finding that the development of lidocaine-kindled seizures is blocked by carbamazepine suggests an interaction of carbamazepine with local anesthetic mechanisms. To study the site of interaction, the effects of lidocaine, carbamazepine and another anticonvulsant drug, phenytoin on scorpion venom-enhanced specific binding of [ 3 H]batrachotoxinin A 20-α-benzoate to the sodium channel gating complex were examined in vitro in a rat brain hippocampus preparation. Lidocaine shifted the concentration inhibition curve of carbamazepine to the right and vice versa. Carbamazepine shifted the concentration inhibition curve of phenytoin to the right and vice versa. The experimentally determined apparent dissociation constants were in a good agreement with the dissociation constants calculated for a one-site model, suggesting that the interaction occurs because lidocaine shares a common binding site with carbamazepine and phenytoin in the voltage-dependent sodium channels.