Abstracts

Rationale:
Status epilepticus (SE) results in complex pathophysiological alterations in the brain and long-term sequelae in survivors. Focal neocortical SE in mice results in neuronal loss, increases in functional excitatory synapses and connectivity and hyperexcitability in neocortical networks in vitro (1). Previous reports have shown that prolonged neocortical seizures can induce injury to downstream targets, such as hippocampus, that might contribute to long-term consequences of SE.  We hypothesized that the intense cortical output during SE would induce chronic structural and functional changes in excitatory connectivity within downstream targets, such as the thalamic nucleus reticularis (nRt), a major target of neocortical output, where enhanced excitatory connectivity would result in significant alterations in thalamocortical circuit function.
Method:
We induced focal status epilepticus (FSE) with epidural application of gabazine (150 µM) and 4-AP (150 µM) over the somatosensory cortex of anesthetized mice (Perez-Ramirez et al, Neurobiol. Dis, 2020).Electrographic FSE was accompanied by contralateral focal myoclonic activity and was allowed to persist for ~ 2h prior to perfusion. Ten days later we obtained confocal images of the central nRt area from sections immunoreacted for pre- and postsynaptic markers of excitatory synapses. We measured colocalized pixel areas for VGLUT1-PSD95 and VGLUT2-PSD95, putative excitatory synapses of presumed neocortical and thalamic origins, respectively. Counts of NeuN labeled neurons within ROIs were obtained from confocal sections and analyzed using Cell Profiler software. Whole cell voltage clamp recordings were used to measure spontaneous EPSC frequency. Statistical significance was tested with Wilcoxon test with a p < 0.05.
Results:
Results showed long-lasting effects of FSE  including: 1. Decreased numbers of presumed Gabaergic neurons in central nRt, indicated by decreased NeuN profiles; 2. Structural evidence for increased excitatory synapses in nRt, likely of both thalamic and cortical origins; and 3. Increases in functional  excitation of nRt neurons from cortical and thalamic sources.
Conclusion:
These results indicate that a single episode of neocortical FSE can induce morphological and physiological changes in nRT 10 days post-FSE, that would presumably alter the function of intra-thalamic and cortico-thalamic excitatory and inhibitory circuits. The increases in PSD95-VGLUT1 and PSD95-VGLUT2 puncta in close apposition suggest an increase in putative excitatory synapses onto nRt neurons from both cortex and thalamus (relay nucleus). These results are significant in that they suggest that thalamic alterations occur post-FSE that could result in long term sequelae such as disruption of normal thalamocortical functions and contribute to behavioral abnormalities and post-SE epileptogenesis.
Funding:
:Supported by NS082644 and NS039579 from the National Institutes of Neurological Disorders and Stroke (D.A.P), the George and Irene Pimley Research Fund (D. A.P.) and an AES postdoctoral research fellowship (M-B P-R).