Enhancing GABAergic tonic inhibition reduces seizure-like activity in the neonatal mouse hippocampus and neocortex.

Around one-third of neonatal seizures do not respond to first-line anticonvulsants, including phenobarbital, which enhances phasic inhibition. Whether enhancing tonic inhibition decreases seizure-like activity in the neonate when GABA is mainly depolarizing at this age is unknown. We evaluated if increasing tonic inhibition using THIP (4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol, Gaboxadol), a δ-subunit-selective GABAA receptor agonist, decreases seizure-like activity in neonatal C57BL/6J mice (postnatal day P5-8, both sexes) using acute brain slices. Whole-cell patch-clamp recordings showed that THIP enhanced GABAergic tonic inhibitory conductances in layer V neocortical and CA1 pyramidal neurons and increased their rheobase without altering sEPSCs characteristics. Two-photon calcium imaging demonstrated that enhancing the activity of extrasynaptic GABAARs decreased neuronal firing in both brain regions. In the 4-aminopyridine and the Low-Mg2+ model of pharmacoresistant seizures, THIP reduced epileptiform activity in the neocortex and CA1 hippocampal region of neonatal and adult brain slices in a dose-dependent manner. We conclude that neocortical layer V and CA1 pyramidal neurons have tonic inhibitory conductances, and when enhanced, they reduce neuronal firing and decrease seizure-like activity. Therefore, augmenting tonic inhibition could be a viable approach for treating neonatal seizures.Significance Statement Neonatal seizures occur in roughly 1-3/1000 term births and 10-fold more frequently in preterm neonates. Around 30-50% of neonatal seizures resist current treatments, and drug resistance often develops if diagnosis and treatment are delayed, for example, in underserved areas. There has been no significant improvement in treating neonatal seizures for nearly 50 years, especially for patients in whom pharmacoresistance develops. Our results show that enhancing tonic inhibition in neonatal mice, a developmental age when GABA mainly depolarizes, decreases neuronal firing and epileptiform activity in vitro Our results suggest that augmenting tonic inhibition could be an alternative treatment for neonatal seizures, especially in pharmacy-resistant ones.