Expanded characterization of the valproic acid zebrafish larval model of autism spectrum disorder: Insights into dysregulation in the gut

BACKGROUND: Relative lack of data on mechanisms underpinning poor gastrointestinal (GI) health in autism spectrum disorder (ASD) limits patient management strategies. Although the valproic acid (VPA) model of ASD is widely used, a comprehensive profile of the model in zebrafish is still lacking, and almost no data are available on ASD-like GI symptomology or underlying mechanisms. METHODS: ASD-like craniofacial morphology, anxiety-like behavior, GI anatomical and functional dysregulation, and dysregulation of the monoaminergic system in both the brain and the gut were simultaneously assessed in zebrafish larvae at 7 days postfertilization. RESULTS: VPA exposure was associated with increased intraocular distance, neuroligin 3 (NLGN3) dysregulation, decreased muscle tone, and an anxious phenotype (increased thigmotaxis and dark zone preference [light-dark preference test]). In the brain, protein expression levels of tryptophan hydroxylase 2 (TPH2) and serotonin (5-HT), but not serotonin transporter (SERT), were downregulated, as well as tyrosine hydroxylase (TH) and dopamine transporter (DAT), whereas dopamine (DA) was upregulated. In the gut, upregulated serotonergic (increased TPH1 and 5-HT) and downregulated dopaminergic (decreased TH, increased DAT) protein expression was linked to thinner-walled, longer intestinal tracts, and decreased spontaneous peristaltic activity. CONCLUSIONS: Dopaminergic dysregulation aligns with the ASD literature. Novel findings include an association of hyposerotonergic tone with anxiety-like outcomes as core symptomology in ASD. Furthermore, we demonstrate compartment-specific dopaminergic and serotonergic dysregulation in the gut, which provides support for relatively decreased gut motility in ASD.