Abstracts

Rationale: Absence seizures are characterized by bilaterally synchronous spike-and-wave discharges in the electroencephalogram, which reflect abnormal oscillations in thalamocortical (TC) networks. The GluR4 knock-out mouse (KO) that lacks the GluR4 AMPA-receptor subunit, which is particularly abundant in the thalamus, displays absence epilepsy. Absence seizures arise from disturbances of the TC circuit that connects the excitatory neurons of the cortex and dorsal thalamus and the inhibitory neurons of the thalamic reticular nucleus (RT). Altered inhibition of TC cells from RT is a key contributor to the generation of absence seizures. Thus it was important to determine how intra-thalamic inhibition is altered in the GluR4 KO epileptic mice. Therefore, we examined if lack of GluR4 subunit leads to abnormal output of RT GABAergic neurons onto TC neurons, altering inhibition in the intra-thalamic network which may lead to absence seizures. Methods: Whole-cell patch-clamp recordings of TC neurons were obtained from horizontal brain slices prepared from KO and wild-type (wt) littermate mice (postnatal day 18+) in the presence of excitatory neurotransmission blockers in all the experiments. Spontaneous inhibitory postsynaptic currents (sIPSCs) were recorded. Miniature IPSCs were recorded in TC cells in presence of TTX. In order to mimic inhibitory input from the RT nucleus onto TC cells, extracellular stimuli were delivered using a concentric bipolar electrode positioned in the RT nucleus, and subsequent evoked responses were recorded. Results: We have demonstrated that sIPSC frequency of KO TC cells is increased compared to wt. Moreover, the miniature IPSCs frequency is also increased suggesting a higher probability of release from RT GABAergic terminals onto TC cells. Interestingly, the burstiness of the sIPSCs was significantly increased in the KO TC cells, and was not altered by TTX application, and thus did not depend on the firing of RT GABAergic neurons. Furthermore, stimulation of RT nucleus produced a larger amplitude IPSC response in TC cells, as well as a larger propensity for a secondary long-lasting (> 150 ms) oscillatory-like GABAA receptor mediated current. Indeed, we have shown with local field potential recordings in the TC nuclei that oscillatory activity, both spontaneous and evoked by internal capsule stimulation, is more robust in the KOs. Conclusions: Our results suggest that in the KO mice the GABAergic transmission from RT to TC cells is strongly enhanced, which is unexpected because of the reduction of excitatory drive onto RT cells in these mice. The striking result of our study is that the increased frequency of sIPSCs as well as their burstiness in the TC cells didn’t depend on the firing of RT cells, suggesting a higher probability of GABA release from RT to TC cells. As the inhibitory output of RT neurons appears to orchestrate the widespread neuronal activity of absence seizures, then increased GABAergic transmission from RT to TC cells could play an important role in triggering seizures.