TrkB agonists prevent post-ischemic BDNF-TrkB mediated emergence of refractory neonatal seizures in CD-1 pups

Kipnis, P.A.; Sullivan, B.J.; Carter, B.M.; Kadam, S.D.

Refractory neonatal seizures do not respond to first-line anti-seizure medications (ASMs) like phenobarbital (PB), a positive allosteric modulator for GABAA receptors, the most widely used ASM to treat neonatal seizures. GABAA receptor-mediated inhibition is dependent upon neuronal chloride regulation. The electroneutral cation-chloride transporter KCC2 mediates neuronal chloride extrusion; an age-dependent increase of KCC2 expression enables the shift of GABAergic signaling from depolarizing to hyperpolarizing. BDNF-TrkB activation following excitotoxic injury recruits downstream targets like PLCγ1, leading to KCC2 hypofunction. This study investigated the efficacy of partial and full TrkB agonists; LM22A-4 (LM), HIOC and Deoxygedunin (DG) respectively, on PB-refractory seizures, post-ischemic TrkB-pathway activation, and KCC2 membrane stability in a P7 CD-1 mouse model of refractory neonatal seizures. Anti-seizure efficacy was determined by quantifying seizure burdens with continuous video-EEG. LM rescued PB-refractory seizures in a sexually dimorphic manner. LM anti-seizure efficacy was associated with a significant reduction in the post-ischemic phosphorylation of TrkB at Y816, a site known to mediate post-ischemic KCC2 hypofunction via PLCγ1 activation. LM additionally rescued ischemia-induced pKCC2-S940 dephosphorylation preserving its membrane stability. HIOC and DG, two novel full TrkB agonists, also rescued PB-refractoriness and post-ischemic TrkB-PLCγ1 pathway activation. Additionally, chemogenetic inactivation of TrkB significantly reduced post-ischemic neonatal seizure burdens at P7. Developmental expression profiles of TrkB and KCC2 in naïve pups identified developmental differences that may underlie the sex-dependent variance in anti-seizure efficacy. These results support a novel role for the TrkB receptor in the emergence of age-dependent refractory neonatal seizures.