Absence seizures and sleep-wake abnormalities in a rat model of GRIN2B neurodevelopmental disorder

Pathogenic mutations in GRIN2B are an important cause of severe neurodevelopmental disorders resulting in epilepsy, autism and intellectual disability. GRIN2B encodes the GluN2B subunit of N-methyl-D-aspartate receptors (NMDARs), which are ionotropic glutamate receptors critical for normal development of the nervous system and synaptic plasticity. Here, we characterized a novel Grin2b heterozygous knockout rat model with 24-hour EEG recordings. We found rats heterozygous for the deletion (Grin2b+/-) had a higher incidence of spontaneous spike and wave discharges, the electrographic correlate of absence seizures, than wild-type rats (Grin2b+/+). Spike and wave discharges were longer in duration and displayed higher overall spectral power in Grin2b+/- when compared to those in Grin2b+/+ animals. Heterozygous mutant rats also had abnormal sleep-wake brain state dynamics over the circadian cycle. Specifically, we identified a reduction in total rapid eye movement sleep and, altered distributions of non-rapid eye movement sleep and wake epochs, when compared to controls. This was accompanied by an increase in overall spectral power during non-rapid eye movement sleep in Grin2b+/-. The sleep-wake phenotypes were largely uncorrelated to the incidence of spike and wave discharges. We then tested the antiseizure efficacy of ethosuximide, a T-type voltage-gated calcium channel blocker used in the treatment of absence seizures, and memantine, a noncompetitive NMDAR antagonist currently explored as a mono or adjunctive treatment option in NMDAR related neurodevelopmental disorders. Ethosuximide reduced the number and duration of spike and wave discharges, while memantine did not affect the number of spike and wave discharges but reduced their duration. These results highlight two potential therapeutic options for GRIN2B related epilepsy. Our data shows the new rat Grin2b haploinsufficiency model exhibits clinically relevant phenotypes. As such, it could prove crucial in deciphering underlying pathological mechanisms and developing new therapeutically translatable strategies for GRIN2B neurodevelopmental disorders.