Therapeutic effects of KRM-II-81, positive allosteric modulator for 2/3 subunit containing GABA_A receptors, in a mouse model of Dravet syndrome

Introduction: Dravet syndrome (DS) is an intractable epilepsy syndrome concomitant with neurodevelopmental disorder that begins in infancy. DS is dominantly caused by mutations in the SCN1A gene, which encodes the alpha subunit of a voltage-gated Na channel. Pre-synaptic inhibitory dysfunction is regarded as the pathophysiological mechanism, but an effective strategy for ameliorating seizures and behavioral problems is still under development. Here, we evaluated the effects of KRM-II-81, a newly developed positive allosteric modulator for alpha 2/3 subunit containing GABA(A) receptors (alpha 2/3-GABA(A)R) in a mice model of DS both in vivo and at the neuronal level.Methods: We used knock-in mice carrying a heterozygous, clinically relevant SCN1A mutation (background strain: C57BL/6 J) as a model of the DS (Scn1a( WT/A1783V) mice), knock-in mouse strain carrying a heterozygous, clinically relevant SCN1A mutation (A1783V). Seizure threshold and locomotor activity was evaluated by using the hyperthermia-induced seizure paradigm and open filed test, respectively. Anxiety-like behavior was assessed by avoidance of the center region in locomotor activity. We estimated a sedative effect by the total distance traveled in locomotor activity and grip strength. Inhibitory post synaptic currents (IPSCs) were recorded from a hippocampal CA1 pyramidal neuron in an acutely prepared brain slice.Results: KRM-II-81 significantly increased the seizure threshold of Scn1a(WT/A1783V )mice in a dose-dependent manner. A low dose of KRM-II-81 specifically improved anxiety-like behavior of Scn1a(WT/A1783V) mice. A sedative effect was induced by relatively high dose of KRM-II-81 in Scn1a(WT/A1783V )mice, the dose of which was not sedative for WT mice. KRM-II-81 potentiated IPSCs by increasing its decay time kinetics. This effect was more prominent in Scn1a(WT/A1783V) mice.Discussion: Higher activation of alpha 2/3-GABA(A)R by KRM-II-81 suggests a compensatory modification of post synaptic inhibitory function against presynaptic inhibitory dysfunction in Scn1a(WT/A1783V). The increased sensitivity for KRM-II-81 may be relevant to the distinct dose-dependent effect in each paradigm of Scn1a(WT/A1783V) mice.Conclusion: Selective activation for alpha 2/3-GABA(A)R by KRM-II-81 could be potential therapeutic strategy for treating seizures and behavioral problems in DS.