The anticonvulsant zonisamide positively modulates recombinant and native glycine receptors at clinically relevant concentrations

GABAA and glycine receptors mediate fast synaptic inhibitory neurotransmission. Despite studies showing that activation of cerebral glycine receptors could be a potential strategy in the treatment of epilepsy, few studies have assessed the effects of existing anticonvulsant therapies on recombinant or native glycine receptors. We, therefore, evaluated the actions of a series of anticonvulsants at recombinant human homo-oligomeric glycine receptor alpha 1, alpha 2 and alpha 3 subtypes expressed in Xenopus oocytes using two-electrode voltage-clamp methods, and then assessed the most effective drug at native glycine receptors from entorhinal cortex neurons using whole-cell voltage-clamp recordings. Ganaxolone, tiagabine and zonisamide positively modulated glycine induced currents at recombinant homomeric glycine receptors. Of these, zonisamide was the most efficacious and exhibited an EC50 value ranging between 450 and 560 mu M at alpha 1, alpha 2 and alpha 3 subtypes. These values were not significantly different indicating a non-selective modulation of glycine receptors. Using a therapeutic concentration of zonisamide (100 mu M), the potency of glycine was significantly shifted from 106 to 56 mu M at alpha 1, 185 to 112 mu M at alpha 2, and 245 to 91 mu M at alpha 3 receptors. Furthermore, zonisamide (100 mu M) potentiated exogenous homomeric and heteromeric glycine mediated currents from layer II pyramidal cells of the lateral or medial entorhinal cortex. As therapeutic concentrations of zonisamide positively modulate recombinant and native glycine receptors, we propose that the anticonvulsant effects of zonisamide may, at least in part, be mediated via this action.

Automated summary

The study evaluated the effects of anticonvulsant drugs on recombinant and native glycine receptors, with zonisamide showing positive modulation on glycine receptors and potentially mediating anticonvulsant effects through this action.