Cntnap2-KO Autism Model Mice Exhibit Perturbed Parvalbumin-Positive Interneuron Input and Output in Primary Visual Cortex V1

Poster Presentation
Friday, May 3, 2019: 11:30 AM-1:30 PM
Room: 710 (Palais des congres de Montreal)
M. Bridi and S. Huang, Hussman Institute for Autism, Baltimore, MD
Background: Sensory processing dysfunction is a key feature and common challenge in autism. The refinement and maintenance of sensory processing depends on GABAergic inhibition. Studies in humans and animals suggest that altered excitatory/inhibitory balance or impaired inhibition could underlie the development of autism, and altering inhibitory circuitry can change the neuronal response to sensory stimulation. Low-level sensory processing alterations could exert larger downstream effects on higher-level behaviors relying sensory input. Changes in genes related to synaptic function and connectivity are risk factors for autism. Cntnap2 encodes CASPR2, a neurexin-family cell-adhesion molecule expressed throughout the brain. Cntnap2 knockout (KO) mice are an established model of autism that exhibit phenotypes including repetitive/stereotyped behavior, hyperactivity, reduced social interaction, seizures, synaptic spine instability, and changes in excitatory and inhibitory transmission, but the role of Cntnap2 in visual processing remains unknown.

Objectives: We sought to investigate the effects of Cntnap2 deletion on inhibitory circuitry in primary visual cortex, V1. Because we have previously demonstrated that Cntnap2 deletion leads to a developmentally-dependent reduction in tonic inhibitory currents and decreased spontaneous IPSC frequency in layer 2/3 pyramidal cells, we set out to investigate the impact of Cntnap2 deletion on the function of fast-spiking parvalbumin-positive (PV+) interneurons within V1.

Methods: We used whole-cell patch-clamp electrophysiology to record from L2/3 pyramidal cells and PV+ interneurons in slices made from V1 of Cntnap2-WT and -KO mice at 8 weeks of age. AAV injection was used to express fluorophores and/or ChR2 in PV+ interneurons in Cntnap2-/-×PVCre mice, for identification and optogenetic stimulation.

Results: We have previously shown that L2/3 pyramidal cells from 8-week-old Cntnap2-KO mice exhibit lower spontaneous IPSC frequency and reduced tonic inhibitory conductance mediated by δ-subunit containing GABAARs. Stimulation of the feed-forward L4 to L2/3 pathway to evoke monosynaptic EPSCs and disynaptic IPSCs in pyramidal neurons resulted in a higher E/I ratio in KO mice compared to WT controls, indicating reduced inhibition. We did not observe an effect of genotype on intrinsic somatic properties of PV+ interneurons of V1, but optogenetic stimulation of PV+ terminals to evoke IPSC trains in pyramidal cells in L2/3 revealed increased paired-pulse depression at these inhibitory synapses. We again used stimulation of feed-forward L4 to L2/3 synapses to measure the NMDA/AMPA ratio in PV+ interneurons, and found that KO cells exhibited a significantly reduced NMDA/AMPA ratio.

Conclusions: Attenuated GABAAR tonic currents and spontaneous IPSC frequency indicate fewer GABAergic synapses in Cntnap2-KO animals. Changes in evoked E/I balance and PV+ interneuron-generated IPSCs in V1 pyramidal cells suggests deficits in inhibition by fast-spiking interneurons. While PV+ interneuron properties were largely unaffected by the deletion of Cntnap2, the reduction of the NMDA/AMPA ratio suggests major deficits in neuronal information processing and integration in the cortical inhibitory circuit. Increased paired-pulse depression at PV-IN inhibitory synapses suggests higher initial GABA release and could indicate axonal dysfunction. Future studies should investigate the cellular/molecular mechanisms that bridge the gap between Cntnap2 mutation and inhibitory dysfunction, as well as the effects of diminished inhibition on the function of the V1 cortical circuit.

See more of: Animal Models
See more of: Animal Models