Regulation of allosteric modulation of GABAa receptor function: interaction between recombininant GABAa receptors and the novel anticonvulsant drug topiramate.

GABA [A] receptors are the main source of fast inhibitory synaptic transmission in the brain, and are therefore of great interest as a target of pharmacological modulation. Numerous exogenous chemical compounds that affect GABA A receptor function result in reduced neuronal excitability and hyperexcitability, and lead to seizure control and/or sedation. However, high concentrations of many allosteric modulators result in detrimental side effects including coma and death. It is important to understand the factors that not only regulate receptor function itself, but also those factors that may regulate the allosteric modulation of receptor function. Examining the interactions of GABA A receptors and known allosteric modulators will lead to improvement in rational drug design. An allosteric modulator of GABA A receptor function that reportedly elicits comparatively minor side effects is the novel anticonvulsant drug topiramate (TPM). Discovering the mechanism of TPM action on GABA A receptors will provide insight useful for understanding receptor function and tailoring site-specific drugs. TPM was introduced to the market in 1996. Basic research studies have demonstrated that TPM affects the function of multiple voltage-gated and ligand-gated channels, including GABA A receptors, but it is unknown if one or more of its modulatory actions are responsible for its anticonvulsant activity. TPM modulation of ligand- and voltage-gated ion channel function is variable and often inconsistent. Indeed, TPM has been found to produce enhancement, inhibition, and no effect on GABA-evoked currents of mammalian neurons in primary neuronal cultures. The purpose of this study was to provide an explanation for the variability of TPM modulation of GABA-evoked currents . The overall goals of this study were (1) to identify the determinants of TPM modulatory interactions with GABA A receptors, (2) to provide new information related to GABA A receptor function, and (3) to provide new insights useful for rational drug design targeting specific types of GABA A receptors. The Xenopus oocyte expression model permitted the determination of the GABA A receptor subunit-specificity of TPM, the subunit location of the binding site(s) for TPM, and the contribution of protein kinase activity to TPM allosteric modulation.