Abstract
Homeostatic plasticity stabilizes neuronal activity during development in part by adjusting intrinsic excitability in response to sensory perturbations. This mechanism is called intrinsic homeostatic plasticity (IHP), characterized by the recruitment or removal of voltage-gated sodium (Nav) or potassium (Kv) channels, changes in conductance, and modifications of the axon initial segment (AIS). The AIS is the main site of action potential initiation and is highly populated with Nav and Kv channels. However, it remains unknown whether autism spectrum disorder (ASD) associated genes may affect AIS remodeling during homeostatic perturbation. To answer this question, we used rats with a 50% loss of the SCN2A gene (SCN2A+/-), a high-confidence ASD-associated gene that encodes for Nav1.2 channels, as our animal model. Experiments were performed during the critical period (P23-P26) in the binocular visual cortex (V1b). To induce homeostatic changes, we used inhibitory DREADDS and measured AIS changes by staining for AnkG, a scaffolding protein that anchors Nav and Kv channels. Our baseline results showed that AIS structure and morphology did not change in L2/3 and L5, indicating that SCN2A haploinsufficiency does not alter AIS size or location. After activity suppression in L2/3, WT neurons showed expected compensatory structural modifications in the
AIS via shortening, while SCN2A neurons displayed no such compensatory mechanisms. However, in L5, WT neurons showed a lengthening of the AIS, and a reduced fluorescence, while SCN2A neurons exhibited an exceptionally large fluorescence increase. This indicates a potential compensatory dysregulation of SCN2A animals to adjust their AIS properties compared to WT animals. Given that AIS plasticity can be influenced by neuromodulatory input, we next examined cholinergic activity in the horizontal diagonal band (HDB), a basal forebrain region that projects to V1, and is known to be necessary for proper IHP. Results showed that SCN2A animals exhibited reduced and more variable recruitment of HDB cholinergic neurons, independent of sleep/wake state. This suggests impaired neuromodulation, which may contribute to disrupted AIS restructuring mechanisms. Together, these results demonstrate that at baseline, AIS structure is preserved, but upon induction of homeostatic plasticity, WT neurons may preserve a stable circuit via modulation of intrinsic excitability, whereas SCN2A neurons are unable to properly compensate for activity perturbations, which may be due to impaired cholinergic activity.