Abstracts

Rationale: Childhood absence epilepsy is characterized by frequent, brief losses of consciousness. The stargazer mouse (stg), that has a spontaneous mutation in the gene that encodes stargazin protein, displays absence epilepsy. Stargazin is postulated to be a member of the transmembrane AMPA receptor regulatory protein (TARP) family, acting as a molecular chaperone for AMPA receptors, locating them to the synapse and also contributing to their biophysical properties. 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). The stargazer mouse is of particular importance to understanding the potential role of RT AMPA receptor location and function in the generation of absence seizures. We tested the hypothesis that lack of stargazin can lead to abnormal excitatory transmission in the RT, promoting hyperexcitability and thus absence seizures. Methods: Whole-cell patch-clamp recordings of individual RT neurons were obtained from horizontal brain slices prepared from stg 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 stg mice, we performed current-voltage curves and the NMDA/AMPA receptor ratio was determined. Results: We have demonstrated that sEPSC frequency of stg RT cells is massively reduced (~20% 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 stg RT cells. However, compared to wt, the evoked EPSCs were smaller in amplitude, long in duration and had varying latency. Current-voltage curves demonstrated that stg RT cell AMPA receptor mediated responses have lower conductance compared to wt, whereas the NMDA receptor conductance is similar. Subsequently, the NMDA:AMPA receptor ratio was larger in stg RT cells than in wt. Conclusions: These results provide information on the basic mechanisms regulating excitation of RT neurons and how disruption of such mechanisms may contribute to epilepsy. One explanation for our results is that there are fewer AMPA receptors located at the synapse to mediate excitatory events. Due to the lack of synaptic receptors at glutamatergic synapses onto RT cells, excitatory discharge is more likely to activate extrasynaptic receptors, increasing the duration of the excitatory current and enhancing excitability through an increase in synaptic charge. As the inhibitory output of RT neurons appears to orchestrate the widespread neuronal activity of absence seizures, then increased excitation of RT could play an important role in triggering seizures.