Potentiating α₂ subunit containing perisomatic GABA_A receptors protects against seizures in a mouse model of Dravet syndrome

Abstract

Key points Dravet syndrome mice (Scn1a+/−) demonstrate a marked strain dependence for the severity of seizures which is correlated with GABAA receptor α2 subunit expression. The α2/α3 subunit selective positive allosteric modulator (PAM) AZD7325 potentiates inhibitory postsynaptic currents (IPSCs) specifically in perisomatic synapses. AZD7325 demonstrates stronger effects on IPSCs in the seizure resistant mouse strain, consistent with higher α2 subunit expression. AZD7325 demonstrates seizure protective effects in Scn1a+/− mice without apparent sedative effects in vivo. Abstract GABAA receptor potentiators are commonly used for the treatment of epilepsy, but it is not clear whether targeting distinct GABAA receptor subtypes will have disproportionate benefits over adverse effects. Here we demonstrate that the α2/α3 selective positive allosteric modulator (PAM) AZD7325 preferentially potentiates hippocampal inhibitory responses at synapses proximal to the soma of CA1 neurons. The effect of AZD7325 on synaptic responses was more prominent in mice on the 129S6/SvEvTac background strain, which have been demonstrated to be seizure resistant in the model of Dravet syndrome (Scn1a+/−), and in which the α2 GABAA receptor subunits are expressed at higher levels relative to in the seizure prone C57BL/6J background strain. Consistent with this, treatment of Scn1a+/− mice with AZD7325 elevated the temperature threshold for hyperthermia‐induced seizures without apparent sedative effects. Our results in a model system indicate that selectively targeting α2 is a potential therapeutic option for Dravet syndrome.

Gabra2 is a genetic modifier of Dravet syndrome in mice

“…This indicates that the majority of GABA A receptors at these synapses are heteromeric combinations of the α1 and α2 subunits, and the α1 subunit is dominant in dictating the properties of synaptic currents. Consistent with our previous study (Nomura et al 2019), AZD7325 treatment had a greater effect in prolonging the decay kinetics of perisomatic IPSCs without affecting the amplitude, demonstrating a differential contribution of α2 activity in these synapses in B6/ edited mice. Interestingly, although there is strong strain-dependence for spontaneous seizures and premature death, strain-dependence does not extend to hyperthermia-induced seizures as Scn1a +/− mice on F1 and 129 strains have similar threshold temperatures.…”

“…This suggests that perisomatic CA1 GABAergic synapses in Edited/B6 mice are enriched in α2-containing receptors compared to those in B6/B6 mice. Consistent with our previous report (Nomura et al 2019), the amplitude of aIPSC was unaffected by genotype (p = 0.12, Mann Whitney) or AZD7325 administration (B6/B6: p = 0.94; B6/Edited: p = 0.70; Wilcoxon) (Fig. 2c) (B6/B6 baseline: 58.1 ± 3.1 pA, n = 13; B6/B6 post: 102 ± 3.6%; and Edited/B6 baseline: 68.5 ± 4.7 pA, n = 11; Edited/B6 post: 100.6 ± 4.2%).…”

Experimental GABA A Receptor Agonists and Allosteric Modulators for the Treatment of Focal Epilepsy

“… 50 However, only recently have several clinical trials been conducted, aiming to investigate the efficacy and safety of clobazam when used as monotherapy in patients with focal epilepsy. The active metabolite of clobazam, N-desmethyl clobazam, shows partial selectivity for GABA A receptors that contain α 2 /α 3 subunits, 39 thus acting as a positive allosteric modulator. A Cochrane review analyzed its effectiveness and safety when used as monotherapy for focal or generalized seizures.…”

“…The analogue KRM-II -81 showed excellent anticonvulsant activity in several animal models of epilepsy (electroshock-induced convulsions in mice, 6 Hz seizure model in mice, pentylenetetrazole (PTZ)-induced seizures in rats, and basolateral amygdala kindling in rats), while its motor-impairing effect was less than that of benzodiazepines. 38 Other substances with positive allosteric modulatory action selective for GABA A receptors with α 2 /α 3 subunits that showed anticonvulsant effects in animal models of epilepsy are: AZD7325 (4-amino-8-(2-fluoro-6-methoxy-phenyl) -N-propyl-cinnoline-3-carboxamide), which increased the threshold for hyperthermia-induced seizures in Scn1a +/− mice, 39 and PF-06372865 (7-ethyl-4-[3-(4-ethylsulfonyl-2-methoxyphenyl)-4-fluorophenyl]imidazo[4,5-c]pyridazine), which efficiently suppressed spike-and-wave discharges in rats genetically prone to absence seizures (Genetic Absence Epilepsy Rats from Strasbourg [GAERS] strain). 40 However, only the latter found its way into clinical trials, and is now known under the name CVL-865.…”

Developing more effective seizure therapies requires more selective drugs

“…Moreover, differences in the expression of GABA A receptor types could in themselves offer some explanation for individual sensitivity to AEDs within the population. In this edition of The Journal of Physiology, Nomura et al (2019) explore the importance of α 2 subunit-containing GABA A receptors in determining seizure severity in a mouse model of Dravet syndrome.…”