Biphasic actions of topiramate on monoamine exocytosis associated with both soluble N-ethylmaleimide-sensitive factor attachment protein receptors and Ca2+-induced Ca2+-releasing systems

To explore the pharmacological mechanisms of topiramate (TPM), we determined the effects of TPM on monoamine (dopamine and serotonin) exocytosis associated with N-ethylmaleimide-sensitive factor attachment protein receptors and Ca2+-induced Ca2+-releasing systems, including inositol-triphosphate receptor and ryanodine receptor in freely moving rat pre-frontal cortex using in vivo microdialysis. During resting stage, Ca2+ output from endoplasmic reticulum Ca2+ store via inositol-triphosphate receptor regulates syntaxin-associated monoamine exocytosis mechanism, whereas during neuronal hyperexcitable stage, Ca2+ output via ryanodine receptor regulates synaptobrevin-associated monoamine exocytosis mechanism. Basal monoamine releases were increased and decreased by therapeutically relevant and supratherapeutic concentration of TPM, respectively. The therapeutic-relevant concentration of TPM increased Ca2+-evoked release concentration-dependently; however, its stimulatory effect was attenuated in the supratherapeutic range. The K+-evoked releases were reduced by TPM concentration-dependently (from therapeutic to supratherapeutic ranges). The therapeutic-relevant concentration of TPM-induced elevation of basal release was reduced by cleavage with syntaxin and inhibition of inositol-triphosphate receptor predominantly, by cleavage with SNAP-25 and synaptobrevin weakly, but not by ryanodine receptor inhibitor. The therapeutic-relevant concentration of TPM-induced elevation of Ca2+-evoked release was reduced by cleavage with syntaxin and inositol-triphosphate receptor inhibitor selectively. The therapeutic-relevant concentration of TPM-induced reduction of K+-evoked monoamine release was abolished by cleavage with synaptobrevin, but was not affected by cleavage with SNAP-25 or synaptobrevin. The stimulatory effect of ryanodine receptor agonist on K+-evoked monoamine release was reduced by TPM, whereas that of inositol-triphosphate receptor agonist was not affected by TPM. Therefore, these results indicate that the combination of the effects of TPM on exocytosis mechanisms associated with SNARE and Ca2+-induced Ca2+-releasing systems, enhancement of inositol-triphosphate receptor/syntaxin and inhibition of ryanodine receptor/synaptobrevin in pre-frontal cortex, may be involved in clinical actions of TPM.