Abstracts

Children with Down Syndrome (DS, trisomy of chromosome 21) are at high risk of seizures, including infantile spasms (IS) and generalized tonic-clonic seizures. We previously reported increased IS susceptibility and neocortical excitation-inhibition ratio in a mouse DS model that carries human chromosome 21, TcMAC21 (Shao et al., 2023). Here we further examined their susceptibility to tonic-clonic type seizures and epileptiform network activity and investigated cellular mechanisms specific to DS pathophysiology – triplicated genes that critically regulate synaptic functions, GRIK1, GIRK2, and SYNJ1, which encode kainate receptors, GABA_B receptor, and synaptojanin (a protein regulates synaptic vesicle recycling and trafficking), respectively. 

Seizure susceptibility was assessed by exposing animals to flurothyl (a GABA_A-R antagonist) inhalation. Epileptiform activity was examined in neocortical-hippocampal slices in two ways: (1) spontaneous epileptiform bursts in 0 Mg²⁺ and (2) evoked responses. Potential cellular mechanisms were investigated in neocortical layer V neurons using whole-cell patch-clamp recordings. KA current was evoked by puff application of KA (100 uM) to recorded neurons held at -70 mV. GABA_AR-mediated sIPSCs were recorded by holding neurons at 0 mV. GABA_B-mediated potentials were induced by bath-application of baclofen (50 uM, 2 min) in current clamp mode, holding at -60 mV. SYNJ function was assessed by the efficacy of transmitter depletion (decay of EPSCs) during repetitive synaptic stimulation.

Flurothyl inhalation consistently induced tonic-clonic seizures in both euploid (EU) controls and TcMAC21 mice. Latency to seizure onset was significantly shorter in TcMAC21 than EU in P9-11 mice. At P17-18, TcMAC21 mice showed both shorter seizure latencies and longer seizure durations than euploid mice, suggesting an increased propensity to flurothyl. The frequency and duration of spontaneous epileptiform bursts were similar in slices from EU and TcMAC21 mice; however, the evoked responses were significantly greater in TcMAC21 than euploid controls. KA current in layer V neurons was significantly larger and the frequency of sIPSCs was significantly lower in the TcMAC21 vs EU mice, suggesting increased KAR-mediated excitation and reduced GABA_A-R mediated inhibition. GABA_B-R mediated potentials were similar in EU and TcMAC21 mice. During repetitive synaptic stimulation, EPSCs decay faster in euploid than TcMAC21 neurons, suggesting slower neurotransmitter depletion (i.e., more efficient synaptic vesicle recycling and trafficking) in TcMAC21 neurons.

In addition to their enhanced propensity to IS, TcMAC21 mice also exhibited increased susceptibility to flurothyl-induced tonic-clonic seizures in vivo and to epileptiform activity in vitro. Moreover, these mice express increased KAR-mediated excitation and less neurotransmitter depletion (consistent with triplication of GRIK1 and SYNJ1) and reduced GABA_A-R inhibition, which may contribute to increased susceptibility to seizures. Together, these results provide new insight into DS-specific mechanisms of pathogenesis of seizures.