Altered Expression of the δ Subunit of the GABA_AReceptor in a Mouse Model of Temporal Lobe Epilepsy

Abstract

␦ Subunit-containing GABA A receptors are located predominantly at nonsynaptic sites in the dentate gyrus where they may play important roles in controlling neuronal excitability through tonic inhibition and responses to GABA spillover. Immunohistochemical methods were used to determine whether ␦ subunit expression was altered after pilocarpine-induced status epilepticus in C57BL/6 mice in ways that could increase excitability of the dentate gyrus. In pilocarpine-treated animals, the normal diffuse labeling of the ␦ subunit in the dentate molecular layer was decreased by 4 d after status epilepticus (latent period) and remained low throughout the period of chronic seizures. In contrast, diffuse labeling of ␣4 and ␥2 subunits, potentially interrelated GABA A receptor subunits, was increased during the chronic period. Interestingly, ␦ subunit labeling of many interneurons progressively increased after pilocarpine treatment. Consistent with the observed changes in ␦ subunit labeling, physiological studies revealed increased excitability in the dentate gyrus of slices obtained from the pilocarpine-treated mice and demonstrated that physiological concentrations of the neurosteroid tetrahydrodeoxycorticosterone were less effective in reducing excitability in the pilocarpine-treated animals than in controls. The findings support the idea that alterations in nonsynaptic ␦ subunit-containing GABA A receptors in both principal cells and interneurons could contribute to increased seizure susceptibility in the hippocampal formation in a temporal lobe epilepsy model.

δ Subunit Susceptibility Variants E177A and R220H Associated with Complex Epilepsy Alter Channel Gating and Surface Expression of α4β2δ GABAAReceptors

“…␦ subunits preferentially coassemble with ␣4 subunits in the thalamus, a structure critical for pathogenesis of IGEs (Wisden et al, 1992;Sur et al, 1999), and GABRD knock-out animals exhibit spontaneous seizures (Spigelman et al, 2002), reinforcing our finding that impaired ␣4␤␦ receptor function is associated with complex epilepsy in humans (Dibbens et al, 2004). Also, ␣4 and/or ␦ subunit expression was changed in experimental seizures (Clark et al, 1994;Schwarzer et al, 1997;Brooks-Kayal et al, 1998;Peng et al, 2004), and reduction of ␦ subunits may contribute to epileptogenesis in experimental epilepsy (Peng et al, 2004). All of these link ␣4␤␦ receptor function to epileptogenesis.…”

“…However, it is not known whether the reduction in whole-cell current was attributable to a decrease in receptor expression and/or a defect in channel gating. Furthermore, ␣4 and ␦ subunits preferentially coassemble in the thalamus (Sur et al, 1999), a structure critical for expression of IGEs, and alterations in ␣4 and ␦ subunit proteins were reported in experimental temporal lobe epilepsy and proposed to contribute to epileptogenesis (Peng et al, 2004). Therefore, whole-cell current amplitude and time course, surface expression, and gating properties of h␣4␤2␦ receptors containing each of the variant ␦ subunits were examined to determine the mechanistic basis for these susceptibility variants contributing to the pathogenesis of complex IGEs.…”

Decrease in Tonic Inhibition Contributes to Increase in Dentate Semilunar Granule Cell Excitability after Brain Injury

“…As in granule cells (Stell et al, 2003;Wei et al, 2003;Peng et al, 2004), we find that SGC tonic inhibition is mediated by GABA A R with ␦ subunits. The post-traumatic decrease in SGC tonic GABA currents occurs simultaneously with unchanged THIP modulation and reduction in SGC sIPSC frequency, indicating that both decreases in GABA A R ␦ subunits and reduced synaptic spillover (Glykys and Mody, 2007) may contribute to the decrease.…”

“…Apart from the classical synaptic inhibitory currents, granule cells express tonic GABA currents mediated by extrasynaptic and perisynaptic GABA A R containing ␣4 and ␦ subunits (Stell et al, 2003;Wei et al, 2003;Peng et al, 2004;Mtchedlishvili and Kapur, 2006). Given the similarities between SGCs and granule cells, and the evidence for injury-induced changes in granule cell tonic inhibition (Mtchedlishvili et al, 2010), we examined SGCs for the presence and postinjury changes in tonic GABA currents.…”

“…Tonic inhibition can contribute substantially to resting membrane conductance and regulate neuronal gain and excitability (Mitchell and Silver, 2003;Ruiz et al, 2003;Chadderton et al, 2004;Farrant and Nusser, 2005). Receptors underlying granule cell tonic GABA currents are known to be altered in models of acquired epilepsy (Peng et al, 2004;Zhang et al, 2007;Zhan and Nadler, 2009;Rajasekaran et al, 2010), including cortical impact injury (Mtchedlishvili et al, 2010). However, whether the source of inhibitory inputs to SGCs is the same as that of granule cells, whether SGCs have tonic GABA currents, and if brain injury modifies synaptic and tonic inhibition in SGCs, is not known.…”