Effect of levetiracetam on molecular regulation of hippocampal glutamate and GABA transporters in rats with chronic seizures induced by amygdalar FeCl 3 injection

Abstract

Enhancement of the glutamatergic excitatory synaptic transmission efficacy in the FeCl 3 induced epilepsy model is associated with changes in the levels of glutamate and GABA transporter proteins. This study examined the effect of levetiracetam (LEV) on glutamate overflow and glutamate/GABA transporters expression in rats with epileptogenesis induced by the amygdalar injection of 1.0 μl of 100 mM FeCl 3 (epileptic rat) and in control rats receiving amygdalar acidic saline injection (non-epileptic rat). In amygdalar acidic saline injected rats, 40 mM KCl-evoked glutamate overflow was significantly suppressed by both 32 and 100 μM LEV co-perfusion. In unilateral amygdalar FeCl 3 injected rats, 32 μM LEV was ineffective, but the 100 μM LEV statistically suppressed glutamate overflow. Western blotting was employed to determine the hippocampal expression of glutamate/GABA transporters in epileptic or non-epileptic rats. The rats were treated for 14 days with 54 mg/kg LEV or vehicle intraperitoneally injection. Following 14 days of treatment, the ipsilateral hippocampus was removed for a Western blot analysis. In non-epileptic rats, the expression increased for all of the glutamate and GABA transporters (GLAST, GLT-1, EAAC-1, GAT-1 and GAT-3) while the glutamate transporter regulating protein (GTRAP3–18) decreased in comparison to those of normal rats that were treated with the vehicle. In epileptic rats receiving LEV, the EAAC-1 and GAT-3 levels increased while GTRAP3–18 (89%) decreased in comparison to those of the epileptic rats treated with the vehicle. GTRAP3–18 inhibitor regulates glutamate-binding affinity to EAAC-1. The anti-epileptic action of LEV may be partially due to a reduction of glutamate-induced excitotoxicity and an enhancement of the GABAergic inhibition as observed with the inhibitory effect on the 40 mM KCl-evoked glutamate overflow. These conclusions are supported by the increase in the expression of glial glutamate transporters (GLAST and GLT-1), and the increase in the expression of EAAC-1 and GAT-3 associated with a decrease in GTRAP3–18. The increased expression of EAAC-1 and the decreased expression of GTRAP3–18 in association with the up-regulation of GAT-3 due to such continual LEV administration was thus found to enhance GABA synthesis and reverse the transport of GABA both in non-epileptic and epileptic rats. The suppression of glutamate excitation and the enhancement of GABA inhibition in the rats with continual LEV administration is a result of the up-regulation of glutamate and GABA transporters with the down-regulation of GTRAP3–18. These observations together demonstrated the critical molecular mechanism of the anti-epileptic activity of LEV.