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Enhanced tonic GABAA inhibition in typical absence epilepsy.

Cope DW, Di Giovanni G, Fyson SJ, Orbán G, Errington AC, Lorincz ML, Gould TM, Carter DA, Crunelli V - Nat. Med. (2009)

Bottom Line: The cellular mechanisms underlying typical absence seizures, which characterize various idiopathic generalized epilepsies, are not fully understood, but impaired gamma-aminobutyric acid (GABA)-ergic inhibition remains an attractive hypothesis.In contrast, we show here that extrasynaptic GABA(A) receptor-dependent 'tonic' inhibition is increased in thalamocortical neurons from diverse genetic and pharmacological models of absence seizures.Increased tonic inhibition is due to compromised GABA uptake by the GABA transporter GAT-1 in the genetic models tested, and GAT-1 is crucial in governing seizure genesis.

View Article: PubMed Central - PubMed

Affiliation: School of Biosciences, Cardiff University, Cardiff, UK.

ABSTRACT
The cellular mechanisms underlying typical absence seizures, which characterize various idiopathic generalized epilepsies, are not fully understood, but impaired gamma-aminobutyric acid (GABA)-ergic inhibition remains an attractive hypothesis. In contrast, we show here that extrasynaptic GABA(A) receptor-dependent 'tonic' inhibition is increased in thalamocortical neurons from diverse genetic and pharmacological models of absence seizures. Increased tonic inhibition is due to compromised GABA uptake by the GABA transporter GAT-1 in the genetic models tested, and GAT-1 is crucial in governing seizure genesis. Extrasynaptic GABA(A) receptors are a requirement for seizures in two of the best characterized models of absence epilepsy, and the selective activation of thalamic extrasynaptic GABA(A) receptors is sufficient to elicit both electrographic and behavioral correlates of seizures in normal rats. These results identify an apparently common cellular pathology in typical absence seizures that may have epileptogenic importance and highlight potential therapeutic targets for the treatment of absence epilepsy.

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Spontaneous absence seizures in GAERS are reduced by intra–thalamic injection of δ subunit–specific antisense ODNs. (a) Graph showing the effect of intra–thalamic injection in GAERS of 1 and 2 nmol site−1 δ subunit–specific antisense ODNs, and 1–2 nmol site−1 non–specific missense ODN, on the time spent in seizure. Values were normalised to the time spent in seizure prior to ODN injection. (b) Comparison of the total number of SWDs following antisense (1 nmol site−1, light grey column; 2 nmol site−1, grey column) and missense (white column) ODN administration. Values were normalised to the number of seizures prior to ODN injection. (c) Effect of 2 nmol site−1 missense (white column) and 2 nmol site−1 antisense (grey column) administration on tonic current amplitude. Values were normalised to the average tonic current amplitude in age–matched, untreated GAERS. (d) Brain section showing that the spread of 2 nmol biotinylated antisense ODN is restricted to the VB thalamus 24 hrs after unilateral injection into the right hemisphere. Arrows indicate the termination of the cannulae in both hemispheres. (b) ‡ and *, P < 0.05 1 and 2 nmol antisense ODN, respectively; ** P < 0.01. (b) and (c) * P < 0.05 and ** P < 0.01. Number of animals in (b) as in (a). Number of recorded neurons in (c) is as indicated.
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Figure 5: Spontaneous absence seizures in GAERS are reduced by intra–thalamic injection of δ subunit–specific antisense ODNs. (a) Graph showing the effect of intra–thalamic injection in GAERS of 1 and 2 nmol site−1 δ subunit–specific antisense ODNs, and 1–2 nmol site−1 non–specific missense ODN, on the time spent in seizure. Values were normalised to the time spent in seizure prior to ODN injection. (b) Comparison of the total number of SWDs following antisense (1 nmol site−1, light grey column; 2 nmol site−1, grey column) and missense (white column) ODN administration. Values were normalised to the number of seizures prior to ODN injection. (c) Effect of 2 nmol site−1 missense (white column) and 2 nmol site−1 antisense (grey column) administration on tonic current amplitude. Values were normalised to the average tonic current amplitude in age–matched, untreated GAERS. (d) Brain section showing that the spread of 2 nmol biotinylated antisense ODN is restricted to the VB thalamus 24 hrs after unilateral injection into the right hemisphere. Arrows indicate the termination of the cannulae in both hemispheres. (b) ‡ and *, P < 0.05 1 and 2 nmol antisense ODN, respectively; ** P < 0.01. (b) and (c) * P < 0.05 and ** P < 0.01. Number of animals in (b) as in (a). Number of recorded neurons in (c) is as indicated.

Mentions: In the second set of experiments, we intra–thalamically injected δ subunit–specific antisense oligodeoxynucleotides (ODNs) (Fig. 5d) in GAERS in order to knock–down δ subunit expression and therefore decrease the number of eGABAARs42. In animals treated with antisense ODNs (1 or 2 nmol site−1), spontaneous seizures were significantly reduced (P < 0.05–0.01) 1–2 days after injection (Fig. 5a,b) compared to control. By comparison, injection of a missense ODN (1–2 nmol site−1) had no effect on spontaneous seizures (Fig. 5a,b). Importantly, injection of the missense ODN (2 nmol site−1) also had no effect on tonic inhibition in P28–32 GAERS, compared to age–matched, untreated animals, whereas the antisense ODN (2 nmol site−1) significantly reduced tonic current amplitude (P < 0.05) (Fig. 5c). Also, neither missense or antisense ODNs had any effect on sIPSCs (Supplementary Table 3). The results from these two sets of experiments demonstrate that enhanced eGABAAR function in the thalamus is important for the appearance of absence seizures in two of the best characterized models of absence epilepsy.


Enhanced tonic GABAA inhibition in typical absence epilepsy.

Cope DW, Di Giovanni G, Fyson SJ, Orbán G, Errington AC, Lorincz ML, Gould TM, Carter DA, Crunelli V - Nat. Med. (2009)

Spontaneous absence seizures in GAERS are reduced by intra–thalamic injection of δ subunit–specific antisense ODNs. (a) Graph showing the effect of intra–thalamic injection in GAERS of 1 and 2 nmol site−1 δ subunit–specific antisense ODNs, and 1–2 nmol site−1 non–specific missense ODN, on the time spent in seizure. Values were normalised to the time spent in seizure prior to ODN injection. (b) Comparison of the total number of SWDs following antisense (1 nmol site−1, light grey column; 2 nmol site−1, grey column) and missense (white column) ODN administration. Values were normalised to the number of seizures prior to ODN injection. (c) Effect of 2 nmol site−1 missense (white column) and 2 nmol site−1 antisense (grey column) administration on tonic current amplitude. Values were normalised to the average tonic current amplitude in age–matched, untreated GAERS. (d) Brain section showing that the spread of 2 nmol biotinylated antisense ODN is restricted to the VB thalamus 24 hrs after unilateral injection into the right hemisphere. Arrows indicate the termination of the cannulae in both hemispheres. (b) ‡ and *, P < 0.05 1 and 2 nmol antisense ODN, respectively; ** P < 0.01. (b) and (c) * P < 0.05 and ** P < 0.01. Number of animals in (b) as in (a). Number of recorded neurons in (c) is as indicated.
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Related In: Results  -  Collection

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Figure 5: Spontaneous absence seizures in GAERS are reduced by intra–thalamic injection of δ subunit–specific antisense ODNs. (a) Graph showing the effect of intra–thalamic injection in GAERS of 1 and 2 nmol site−1 δ subunit–specific antisense ODNs, and 1–2 nmol site−1 non–specific missense ODN, on the time spent in seizure. Values were normalised to the time spent in seizure prior to ODN injection. (b) Comparison of the total number of SWDs following antisense (1 nmol site−1, light grey column; 2 nmol site−1, grey column) and missense (white column) ODN administration. Values were normalised to the number of seizures prior to ODN injection. (c) Effect of 2 nmol site−1 missense (white column) and 2 nmol site−1 antisense (grey column) administration on tonic current amplitude. Values were normalised to the average tonic current amplitude in age–matched, untreated GAERS. (d) Brain section showing that the spread of 2 nmol biotinylated antisense ODN is restricted to the VB thalamus 24 hrs after unilateral injection into the right hemisphere. Arrows indicate the termination of the cannulae in both hemispheres. (b) ‡ and *, P < 0.05 1 and 2 nmol antisense ODN, respectively; ** P < 0.01. (b) and (c) * P < 0.05 and ** P < 0.01. Number of animals in (b) as in (a). Number of recorded neurons in (c) is as indicated.
Mentions: In the second set of experiments, we intra–thalamically injected δ subunit–specific antisense oligodeoxynucleotides (ODNs) (Fig. 5d) in GAERS in order to knock–down δ subunit expression and therefore decrease the number of eGABAARs42. In animals treated with antisense ODNs (1 or 2 nmol site−1), spontaneous seizures were significantly reduced (P < 0.05–0.01) 1–2 days after injection (Fig. 5a,b) compared to control. By comparison, injection of a missense ODN (1–2 nmol site−1) had no effect on spontaneous seizures (Fig. 5a,b). Importantly, injection of the missense ODN (2 nmol site−1) also had no effect on tonic inhibition in P28–32 GAERS, compared to age–matched, untreated animals, whereas the antisense ODN (2 nmol site−1) significantly reduced tonic current amplitude (P < 0.05) (Fig. 5c). Also, neither missense or antisense ODNs had any effect on sIPSCs (Supplementary Table 3). The results from these two sets of experiments demonstrate that enhanced eGABAAR function in the thalamus is important for the appearance of absence seizures in two of the best characterized models of absence epilepsy.

Bottom Line: The cellular mechanisms underlying typical absence seizures, which characterize various idiopathic generalized epilepsies, are not fully understood, but impaired gamma-aminobutyric acid (GABA)-ergic inhibition remains an attractive hypothesis.In contrast, we show here that extrasynaptic GABA(A) receptor-dependent 'tonic' inhibition is increased in thalamocortical neurons from diverse genetic and pharmacological models of absence seizures.Increased tonic inhibition is due to compromised GABA uptake by the GABA transporter GAT-1 in the genetic models tested, and GAT-1 is crucial in governing seizure genesis.

View Article: PubMed Central - PubMed

Affiliation: School of Biosciences, Cardiff University, Cardiff, UK.

ABSTRACT
The cellular mechanisms underlying typical absence seizures, which characterize various idiopathic generalized epilepsies, are not fully understood, but impaired gamma-aminobutyric acid (GABA)-ergic inhibition remains an attractive hypothesis. In contrast, we show here that extrasynaptic GABA(A) receptor-dependent 'tonic' inhibition is increased in thalamocortical neurons from diverse genetic and pharmacological models of absence seizures. Increased tonic inhibition is due to compromised GABA uptake by the GABA transporter GAT-1 in the genetic models tested, and GAT-1 is crucial in governing seizure genesis. Extrasynaptic GABA(A) receptors are a requirement for seizures in two of the best characterized models of absence epilepsy, and the selective activation of thalamic extrasynaptic GABA(A) receptors is sufficient to elicit both electrographic and behavioral correlates of seizures in normal rats. These results identify an apparently common cellular pathology in typical absence seizures that may have epileptogenic importance and highlight potential therapeutic targets for the treatment of absence epilepsy.

Show MeSH
Related in: MedlinePlus