Abstracts

Rationale: Angelman syndrome (AS) is a genetical neurodevelopmental disorder caused by loss-of-function of the UBE3A gene; main manifestation consisted of intellectual disability, ataxia and epilepsy accompanied with characteristic diffuse high voltage slow waveforms on EEG. As a mechanism of ataxia, we previously reported that extrasynaptic GABAA receptor-mediated tonic inhibition is decreased in cerebellar granule cells (CGCs) of Ube3a deficient mice (AS mice). As its mechanisms, we showed that UBE3A controls degradation of neuronal GABA transporter 1 (GAT1) and its deficiency induces a surplus of GAT1 that results in decreased GABA levels. As GAT1 expression is not restricted to the cerebellum, deregulation of tonic inhibition may be involved in pathophysiology of various symptoms including epilepsy. Here, we investigated tonic inhibition of cortical pyramidal neuron and thalamic relay neurons in AS mice and estimated the contribution of deregulated tonic inhibition on epilepsy by evaluating effects of selective positive ionotropic modulators for extracynaptic GABAA receptors.  Methods: Tonic inhibitions of cortical/hippocampal pyramidal neurons and thalamic relay neurons were evaluated using patch-clamp recording in an acute brain slice prepared from mice postnatal days 21-28. EEG recording and analysis for seizure susceptibility induced by flurothyl inhalation was conducted using 6-12 months old mice. Effects of two compounds for reinforcing tonic inhibition, MP-Ⅲ-022 or Gaboxadol on epileptic features were independently assessed by acute i.p. addministration. Results: Tonic inhibition was decreased in pyramidal neurons of the hippocampus and the cortex, but not in thalamo-cortical relay neurons in the thalamus. As the mechanisms of this region-specific dysregulation, we illustrated that GAT1 expression was increased in the hippocampus and the cortex in AS mice but was faint and comparable between AS and WT mice in the thalamus. AS mice showed significantly higherδ-θwave power in intracranial EEG during awake and higher seizure susceptibility induced by flurothyl inhalation. These epileptic features were improved by acute administration of low dose MP-Ⅲ-022, positive allosteric modulator forα5 subunit-containing GABAa receptors. In contrast, both EEG and seizure susceptibility were aggravated by higher dose of Gaboxadol, the other enhancer for tonic inhibition by activatingδsubunit-containing GABAa receptor.  Conclusions: Because α5 and δ subunit are dominantly expressed in extrasynapse of the cortical/hippocampal pyramidal neurons and thalamic relay neurons respectively, our contradict results of their selective modulators indicate that not a global change in tonic inhibition, but a relative decrease in tonic inhibition in the cortex/hippocampus compared with the thalamus can disturb the regular thalamo-cortical networks, resulting in epilepsy of AS. We proposed that administration of low dose MP-Ⅲ-22 could be potenital theurapietic strategy for improving epilepsy in patients with AS. Funding: Grant-in-Aid for Scientific Research (B), 18H02777.Grant-in-Aid from Angelman Syndrome FoundationGrant-in-Aid from The Japan Epilepsy Research Foundation