On the Action of the Anti-Absence Drug Ethosuximide in the Rat and Cat Thalamus

Abstract

The action of ethosuximide (ETX) on Na⁺, K⁺, and Ca²⁺ currents and on tonic and burst-firing patterns was investigated in rat and cat thalamic neurons in vitro by using patch and sharp microelectrode recordings. In thalamocortical (TC) neurons of the rat dorsal lateral geniculate nucleus (LGN), ETX (0.75–1 mm) decreased the noninactivating Na⁺ current,I_NaP, by 60% but had no effect on the transient Na⁺ current. In TC neurons of the rat and cat LGN, the whole-cell transient outward current was not affected by ETX (up to 1 mm), but the sustained outward current was decreased by 39% at 20 mV in the presence of ETX (0.25–0.5 mm): this reduction was not observed in a low Ca²⁺ (0.5 mm) and high Mg²⁺ (8 mm) medium or in the presence of Ni²⁺ (1 mm) and Cd²⁺(100 μm). In addition, ETX (up to 1 mm) had no effect on the low-threshold Ca²⁺ current,I_T, of TC neurons of the rat ventrobasal (VB) thalamus and LGN and in neurons of the rat nucleus reticularis thalami nor on the high-threshold Ca²⁺current in TC neurons of the rat LGN.

Sharp microelectrode recordings in TC neurons of the rat and cat LGN and VB showed that ETX did not change the resting membrane potential but increased the apparent input resistance at potentials greater than −60 mV, resulting in an increase in tonic firing. In contrast, ETX decreased the number of action potentials in the burst evoked by a low-threshold Ca²⁺ potential. The frequency of the remaining action potentials in a burst also was decreased, whereas the latency of the first action potential was increased. Similar effects were observed on the burst firing evoked during intrinsic δ oscillations.

These results indicate an action of ETX onI_NaP and on the Ca²⁺-activated K⁺ current, which explains the decrease in burst firing and the increase in tonic firing, and, together with the lack of action on low- and high-threshold Ca²⁺ currents, the results cast doubts on the hypothesis that a reduction of I_T in thalamic neurons underlies the therapeutic action of this anti-absence medicine.