Targeting thalamic nuclei is not sufficient for the full anti-absence action of ethosuximide in a rat model of absence epilepsy

Abstract

Absence epilepsy is characterised by recurrent periods of physical and mental inactivity coupled to bilateral, synchronous spike and wave discharges (SWDs) on the electroencephalogram. The mechanism of action of ethosuximide (ETX), a drug specific for absence seizures, is believed to involve a reduction in the low threshold T-type Ca²⁺ current in thalamocortical and nucleus reticularis thalami (NRT) neurones, although other electrophysiological data have questioned this. Here, we employed a genetic rat model of absence seizures to investigate the effects of directly administering ETX to the thalamus.

SWDs were immediately and substantially reduced (∼90%) by systemic administration of ETX (177–709 μmol/kg), or by bilateral microinfusion into the thalamus of the GABA_B antagonist, CGP 36742 (5–27 nmol per side). However, infusion of ETX (1–200 nmol per side) into the ventrobasal complex or the NRT resulted in a reduction of SWDs that was delayed (30–60 min) and less marked (∼50%). Administration of ETX (0.2 mM to 1 M) to a greater volume of thalamus by reverse microdialysis also produced significant but delayed reduction of SWDs at concentrations >1 mM. Only at 5 mM were seizures significantly reduced (∼70%) within 30 min of administration.

These results suggest that targeting of the thalamus alone may be insufficient for an immediate and full anti-absence action for ETX. Concomitant or exclusive actions in the cortex remain a possibility.

Introduction

Absence epilepsy is characterised by brief recurrent periods of impairment of consciousness associated with symmetrical, bilateral spike and wave discharges (SWDs) on the electroencephalogram (EEG; Panayiotopoulos, 1997). SWDs are believed to be generated by aberrant thalamocortical rhythms (Crunelli and Leresche, 2002b), and a role for GABA_B IPSPs and low-threshold Ca²⁺ potentials (LTCP), generated by the T-type Ca²⁺ current (I_T), has been suggested to underlie the firing of thalamocortical (TC) neurones during SWDs (Crunelli and Leresche, 1991, Bal et al., 2000, Blumenfeld and McCormick, 2000). Indeed, elevations of the α_1G and α_1H subunits of T-type Ca²⁺ channel mRNA levels have been demonstrated in thalamic regions of the Genetic Absence Epilepsy Rat from Strasbourg (GAERS; Talley et al., 2000), a validated model of absence epilepsy (Vergnes et al., 1982, Marescaux et al., 1992, Danober et al., 1998). Furthermore, the inability of γ-hydroxybutyrate to elicit SWDs in knockout mice lacking the α_1G subunit of T-type Ca²⁺ channels (Kim et al., 2001) again supports a role for I_T in absence seizures, though the selective interpretation of these results in support of an LTCP-mediated firing in thalamic neurones during SWDs is premature. These data, in fact, may also reflect the lack either of LTCPs in cortical neurones (Sohal and Huguenard, 2001) and/or of the depolarising influence exerted by the ‘window’ component of I_T (Hughes et al., 2002) in thalamic and cortical neurones.

Ethosuximide (ETX) is a drug specifically indicated for the control of absence seizures (Sherwin, 1989). Studies in isolated TC and nucleus reticularis thalami (NRT) neurones have demonstrated a 40% reduction in the amplitude of I_T by therapeutic concentrations of ETX, leading to the proposal that the reduction of I_T in thalamic neurones may be the mechanism by which ETX exerts its anti-absence effect (Coulter et al., 1989, Huguenard and Prince, 1994, Huguenard, 2002). While a similar action has recently been shown in cloned α_1G and α_1I channels (Lacinova et al., 2000, Gomora et al., 2001), other studies have failed to demonstrate an effect of ETX on native I_T in different neurones (Thompson and Wong, 1991, Sayer et al., 1993), including TC and NRT cells (Pfrieger et al., 1992, Leresche et al., 1998). Furthermore, ETX has been observed to decrease the persistent Na⁺ and sustained K⁺ currents in TC and cortical neurones (Leresche et al., 1998, Crunelli and Leresche, 2002a). Recently, a novel T-channel antagonist, U-92032, has also been shown to potently block intrathalamic rhythms (Porcello et al., 2003). Although their study reported no effects on Na⁺ currents in thalamic slices, inhibition of voltage-dependent Na⁺ channels by U-92032 has been reported in the hippocampus (Avery and Johnston, 1997).

To establish whether ETX exerts its anti-absence action via a direct effect on thalamic neurones in vivo, we have compared the effects of this drug administered either by systemic injection, or by microinfusion and reverse microdialysis into the ventrobasal thalamus (VB) or NRT of GAERS. We have compared these data with the anti-SWD action of intrathalamic microinfusion of a GABA_B antagonist, as intrathalamic injection of GABA_B antagonists has been previously reported to abolish SWDs in different absence models (Liu et al., 1992, Snead, 1992, Hosford et al., 1995). Some of these data have been published in preliminary form (Richards et al., 2001, Manning et al., 2002).