Abstracts

Rationale: Childhood absence epilepsy is characterized by frequent, brief losses of consciousness. The GluR4 knock-out mouse (KO) that lacks the GluR4 AMPA-receptor subunit, displays absence epilepsy. Ionotropic AMPA receptors (formed in heterotetramers from different combinations of GluR1, GluR2, GluR3 and GluR4 subunits) mediate fast excitatory transmission. The GluR4 subunit is prominent in the thalamus, particularly at corticothalamic synapses in the RT. Absence seizures arise from disturbances of the thalamocortical circuit that connects the excitatory neurons of the cortex and dorsal thalamus and the inhibitory neurons of the thalamic reticular nucleus (RT). We tested the hypothesis that lack of GluR4 AMPA-receptor subunit can lead to abnormal excitatory transmission in the RT, paradoxically promoting hyperexcitability and thus absence seizures. Methods: Whole-cell patch-clamp recordings of individual RT neurons were obtained from horizontal brain slices prepared from KO and wild-type (wt) littermate mice (postnatal day 18+). Spontaneous excitatory postsynaptic currents (sEPSCs) were isolated and recorded. In order to mimic excitatory input from the cortex or dorsal thalamus onto RT cells, extracellular stimuli were delivered using a concentric bipolar electrode positioned in the internal capsule, and subsequent evoked responses were recorded. In order to study whether NMDA receptor mediated activity is altered in KO mice, we performed current-voltage (I/V) curves and the NMDA/AMPA receptor ratio was determined. Results: We have demonstrated that sEPSC frequency of KO RT cells is massively reduced (~50% of wt). Furthermore, the remaining events are lower in amplitude and have slower decay kinetics. Despite the reduction in sEPSC events, EPSCs were reliably evoked in KO RT cells. However, compared to wt, the evoked EPSCs were smaller in amplitude and longer in duration. The I/V curves demonstrated that KO RT cell AMPA receptor mediated responses have lower conductance compared to wt. Surprisingly the NMDA receptor conductance is also reduced at positive holding potentials, possibly due to compensatory changes in glutamate release or receptor number. Indeed when trains of stimuli were applied, KO cells had less failures than wt cells, suggesting changes in glutamate release. There were no changes in NMDA/AMPA receptor ratio. Conclusions: These results provide information on the basic mechanisms regulating excitation of RT neurons and how disruption of such mechanisms contributes to epilepsy. Due to the lack of GluR4-containing receptors at glutamatergic synapses onto RT cells, excitatory discharge is more likely to activate slower kinetic subunit containing receptors, increasing the duration of the excitatory current and enhancing excitability. As the inhibitory output of RT neurons appears to orchestrate the widespread neuronal activity of absence seizures, then increased duration of excitatory currents in RT, as well as potential compensatory pre- and post-synaptic changes, could play an important role in triggering seizures.