Topiramate reduces AMPA-induced Ca2+ transients and inhibits GluR1 subunit phosphorylation in astrocytes from primary cultures

Topiramate (TPM) is a structurally novel broad spectrum anticonvulsant known to have a negative modulatory effect on the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate subtypes of glutamate receptors and some types of voltage-gated Na+ and Ca2+ channels, and a positive modulatory effect on some types of gamma-aminobutyric acidA (GABA(A)) receptors and at least one type of K+ channels in neurons. In an earlier work, we showed that the negative modulatory effect of TPM (100 mu M) on AMPA/kainate receptors in neurons is dependent on TPM modulation of the phosphorylation state of these receptors. In this work, we investigated the effect of TPM on AMPA-induced intracellular calcium ([Ca2+](i)) responses in cultured rat cortical astrocytes, with special interest in intracellular mechanisms. Here, we report that the ability of TPM (1-100 mu M) to inhibit AMPA- induced accumulation of Ca2+ in astrocytes is inversely related to the level of protein kinase A (PKA)-mediated phosphorylation of channels activated by AMPA. The level of receptor phosphorylation was further determined with western blot using phosphorylation specific antibodies that recognize the glutamate receptor 1 (GluR1) subunit phosphorylated on Ser845. These results demonstrated that, even in cultured cortical astrocytes, TPM significantly reduced the phophorylation level of GluR1 subunits. Furthermore, it was shown that TPM binds to AMPA receptors in the dephosphorylated state and thereby exerts an allosteric modulatory effect on the ion channel.