Upregulated NMDAR-mediated GABAergic transmission underlies autistic-like deficits in Htr3a knockout mice

Mutations in serotonin pathway genes, especially the serotonergic receptor subunit gene HTR3A, are associated with autism. However, the association of HTR3A deficiency with autism and the underlying mechanisms remain unknown. Methods: The Htr3a knockout (KO) mice were generated using transcription activator-like effector nuclease technology. Various behavior tests, including social interaction, social approach task, olfactory habituation/dishabituation, self-grooming, novel object recognition, contextual fear conditioning, elevated plus maze, open field and seizure susceptibility, were performed to assess the phenotypes. Transcriptome sequencing was carried out to search for molecular network and pathways underlying the phenotypes. Electrophysiological recordings, immunoblotting, immunofluorescence staining, immunoprecipitation, and quantitative real-time PCR were performed to verify the potential mechanisms. The N-methyl-D-aspartate receptor (NMDAR) antagonist memantine was used to treat the KO mice for rescuing the phenotypes. Results: The Htr3a KO mouse model showed three phenotypic domains: autistic-like behaviors (including impaired social behavior, cognitive deficits, and increased repetitive self-grooming), impaired memory, and attenuated susceptibility to pentylenetetrazol-induced seizures. We observed enhanced action potential-driven γ-aminobutyric acid-ergic (GABAergic) transmission in pyramidal neurons and decreased excitatory/inhibitory (E/I) ratio using the patch-clamp recording. Transcriptome sequencing on the hippocampus revealed the converged pathways of the dysregulated molecular networks underlying three phenotypic domains with upregulation of NMDAR. We speculated that Htr3a KO promotes an increase in GABA release through NMDAR upregulation. The electrophysiological recordings on hippocampal parvalbumin-positive (PV⁺) interneuron revealed increased NMDAR current and NMDAR-dependent excitability. The NMDAR antagonist memantine could rescue GABAergic transmission in the hippocampus and ameliorate autistic-like behaviors of the KO mice. Conclusion: Our data indicated that upregulation of the NMDAR in PV⁺ interneurons may play a critical role in regulating GABAergic input to pyramidal neurons and maybe involve in the pathogenesis of autism associated with HTR3A deficiency. Therefore, we suggest that the NMDAR system could be considered potential therapeutic target for autism.