Abstract
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.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
We edited the following: Introduction: Small typos and clarification on GluN2 and GluN3 subunit binding. Methods: We added details on exclusion of a subset of rats displaying abnormal seizure number compared to colony. We added methodology on intrathalamic injections. We included details on the automated spike and wave discharge scorer. Results: We performed more experiments to assess GRIN2B expression. Analysis of spike and wave discharges and sleep-wake were performed in the light and dark phase. We normalised spectral power data to average REM, NREM and wake. We added data from intrathalmic injections of ethosuximide and memantine. We also edited the text to include clear hypotheses in each section. Discussion: Details were added on patient populations with GRIN2B mutations, findings in other animal models, mechanisms of spike and wave discharge generation and sleep-wake associated with NMDAR function
Abbreviations
- ASD
- autism spectrum disorder
- NMDAR
- N-methyl-D-aspartate receptor
- SWD
- spike and wave discharge
- REM
- rapid eye movement sleep
- NREM
- non-rapid eye movement sleep
- nRT
- reticular thalamic nucleus.
