Date Published: September 5, 2011
Publisher: Hindawi Publishing Corporation
Author(s): Adam C. Errington, David W. Cope, Vincenzo Crunelli.
It is well established that impaired GABAergic inhibition within neuronal networks can lead to hypersynchronous firing patterns that are the typical cellular hallmark of convulsive epileptic seizures. However, recent findings have highlighted that a pathological enhancement of GABAergic signalling within thalamocortical circuits is a necessary and sufficient condition for nonconvulsive typical absence seizure genesis. In particular, increased activation of extrasynaptic GABAA receptors (eGABAAR) and augmented “tonic” GABAA inhibition in thalamocortical neurons have been demonstrated across a range of genetic and pharmacological models of absence epilepsy. Moreover, evidence from monogenic mouse models (stargazer/lethargic) and the polygenic Genetic Absence Epilepsy Rats from Strasbourg (GAERS) indicate that the mechanism underlying eGABAAR gain of function is nonneuronal in nature and results from a deficiency in astrocytic GABA uptake through the GAT-1 transporter. These results challenge the existing theory that typical absence seizures are underpinned by a widespread loss of GABAergic function in thalamocortical circuits and illustrate a vital role for astrocytes in the pathology of typical absence epilepsy. Moreover, they explain why pharmacological agents that enhance GABA receptor function can initiate or exacerbate absence seizures and suggest a potential therapeutic role for inverse agonists at eGABAARs in absence epilepsy.
Typical absence epilepsy is characterised by the regular occurrence of nonconvulsive seizures that result in periods of sudden and brief (average ≈10 seconds, range ≈4–40 seconds) loss of consciousness. In the electroencephalogram (EEG), human absence seizures are typified by the appearance of generalized, synchronous, and bilateral “spike (or polyspike) and slow wave discharges” (SWD) occurring at frequencies between 2.5–4 Hz [1, 2]. Although typical absence seizures are significant clinical features of many generalized idiopathic epilepsies (IGEs), as defined by the classification of the International League Against Epilepsy (ILAE) , they are the only neurological symptom presented in childhood absence epilepsy (CAE). CAE has an annual incidence of approximately 2–8 per 100,000 children under 16 years of age, with seizure onset typically occurring between 3 and 8 years of age and seizure frequency often as high as several hundred events per day . Absence seizures associated with CAE are not triggered by visual or other sensory stimuli and are not usually associated with neurometabolic or neurophysiological deficits, a factor which is thought to contribute to ~70% spontaneous remission rates in adolescence [2, 4]. Nonetheless, in this pure absence epilepsy phenotype, there is a consensus, based upon older invasive studies and more recent imaging investigations, that seizure genesis and propagation occur as a result of aberrant electrical activity in reciprocally connected thalamic and cortical regions (i.e., thalamocortical circuits) without significant involvement of other brain areas including hippocampus and limbic regions which are often associated with convulsive seizures [5–9]. In fact, recent observations in humans suggest that seizure genesis occurs due to paroxysmal activation of discrete frontal and parietal cortical territories prior to spread into other cortical and thalamic regions [5–8]. This review will, therefore, focus on the key cellular elements of thalamocortical circuits and in particular upon thalamocortical neurons.
It has been demonstrated in vitro using several different genetic animal models of absence seizures that the tonic GABAA current in TC neurons of the VB thalamus is enhanced in animals displaying an epileptic phenotype compared to their respective nonepileptic control animals (Figure 1) . This was first shown in the polygenic GAERS model but has subsequently also been demonstrated for various mice models with known, but divergent, spontaneous monogenic mutations, including stargazer and lethargic mice. In GAERS animals, there is a clear developmental profile for this increased GABAergic function (Figure 1(a)). Up to postnatal day sixteen, the tonic current in VB of GAERS is similar to that of the nonepileptic control (NEC) strain. However, in the 24 hour period between the postnatal day 16-17, there is a significant (almost doubling) increase in the amplitude of the tonic current in VB TC neurons of the epileptic animals  that remains elevated well past the time of seizure onset (around the postnatal day 30 in this strain). These data suggest that, rather than occurring as a consequence of seizure onset, the pathological enhancement of tonic GABA inhibition during development in GAERS may be proepileptogenic. Moreover, despite the full developmental profile for the monogenic lethargic and stargazer mice being unknown, it is clear in these models that a significant enhancement of tonic current in TC neurons is present after seizure onset, (Figure 1(b)) . In contrast, no tonic GABAA current is detected in the GABAergic NRT neurons of GAERS or NEC animals (unpublished observation) as is indeed the case in normal Wistar rats .
As well as resulting from genetic modifications, SWDs can be generated in genetically “normal” animals through administration of various pharmacological agents. The best-established pharmacological model of typical absence seizures is achieved by the systemic administration of γ-hydroxybutyric acid (GHB) [48–50]. However, it has been known for some time that systemic administration of THIP, a selective agonist at δ subunit-containing extrasynaptic GABAARs, also elicits SWDs in normal animals, (Figure 2(a)) . In the context of the involvement of enhanced thalamic tonic GABAA inhibition in several genetic models of absence epilepsy, the pharmacological induction of seizures by THIP becomes more readily explainable. This is because, as previously disclosed, THIP can potently enhance tonic GABAA currents of TC neurons in nonepileptic rats, (Figure 2(b)),  and mice [15, 16], thus mimicking the enhanced thalamic tonic inhibition observed in genetic models. On the other hand, the effects of GHB, which does not bind to GABAARs and is believed to elicit absence seizures by activation of GABABRs , become more difficult to interpret in light of the apparent necessity for enhanced eGABAAR signalling during SWDs. However, it has now been demonstrated in brain slices of Wistar rats that GHB enhances tonic GABAA currents in TC neurons, (Figure 2(c)) . The effects on tonic GABAergic inhibition in vitro are dose dependent with concentrations used reflecting those that are required to elicit absence seizures in vivo, (Figure 2(d)) . Moreover, the effects of GHB are not due to nonspecific binding interactions since the GHB-mediated enhancement of tonic current is negated by the GABABR antagonist CGP55845, (Figure 2(d)). In fact, application of CGP55845 alone significantly reduces the tonic GABAA current amplitude in TC neurons of Wistar rats to 74% of the control values, indicating that facilitation of extrasynaptic GABAARs by GABABRs contributes approximately one quarter of the tonic GABAA current in normal rats. Importantly, CGP55845 also reduces the tonic current in GAERS, stargazer, and lethargic mice to about 55, 65, and 57% of control, (Figure 2(e)), respectively,  suggesting that facilitation of extrasynaptic GABAAR function by GABABR activation contributes up to half of the pathologically enhanced tonic current in these genetic models.
As previously described, SWDs of typical absence epilepsy appear to be initiated in deep layers (V/VI) of the cortex where intracellular recordings show rhythmic paroxysmal depolarisations occurring in phase with the EEG spike [61–63]. The action potentials associated with these synchronous depolarisations in turn provide strong rhythmic input to thalamic nuclei. In NRT neurons in vivo, the strong converging corticothalamic input that result from cortical volleys during SWDs produces bursts of excitatory postsynaptic potentials (EPSPs) that trigger T-type Ca2+-channel-mediated LTS and bursts of action potentials. In contrast, TC neurons receive both monosynaptic excitation directly from corticothalamic inputs and disynaptic inhibition via the NRT. In vivo intracellular recordings made in GAERS have shown that during ictal activity TC neurons typically receive sequences of one EPSP plus four to six IPSPs arriving in phase with each EEG spike and that action potential firing is rare [62, 64]. This is likely due to the much stronger corticothalamic excitatory inputs into NRT neurons compared to TC neurons  and the robust nature of the LTS-driven action potential bursts of NRT neurons [62, 64]. Thus, it is highly probable although it remains to be directly demonstrated that strong GABAergic input into TC neurons during SWDs produces activation of eGABAARs and that the corresponding increase in tonic current contributes to the observed downregulation of TC neuron output during ictal activity.
Augmented tonic GABAA inhibition in TC neurons represents the first potential molecular mechanism that is common to both well-established pharmacological and genetic models of typical absence seizures. Despite having a range of divergent genetic mutations, GAERS (polygenic), stargazer (Ca2+ channel γ2 subunit, TARP-γ2), lethargic (Ca2+ channel β4 subunit), SSADH−/− and GAT-1−/− mice all display SWDs characteristic of typical absence epilepsy, whereas in δ−/− mice drugs that commonly produce SWDs are ineffective. Importantly, because powerful GABAA IPSPs can be recorded in the vast majority of TC neurons during absence seizures in vivo [64, 66], these findings also indicate that model systems that aim to reproduce typical absence seizures by blocking GABAARs of TC neurons are inherently flawed.