a, Axon outgrowth from ABX thalamic explants proximal to ABX striatal explant, and supplemented with i) vehicle, ii) trimethylamine N-oxide (TMAO, 1 nM) iii) 5-aminovalerate (5-AV, 1 nM), iv) imidazole propionate (IP, 1 nM), v) 3-indoxyl-sulfate (3-IS, 1 nM) or vi) hippurate (HIP, 1 nM). Scale= 250μm. b, Number of axons per 200 μm2 surface area proximal to striatal explant (One-way ANOVA+Tukey’s, n=14, 13, 7, 7, 7, 7, 8). c, Netrin-G1a (magenta) and L1 (cyan) in E14.5 brain sections from ABX dams treated with vehicle or 4-microbial metabolites (4-MM: TMAO, 5-AV, IP, and HIP). Scale= 500 μm. d, Netrin-G1a per matched ROI (yellow lines) in E14.5 brain sections of vehicle- or 4-MM-treated ABX dams. (Two-way ANOVA+Tukey’s, n= 8 offspring from different dams). e, L1 per matched ROI in E14.5 brain sections from vehicle or 4-MM-treated ABX dams. (Two-way ANOVA+Tukey’s, n=8 offspring from different dams). f, Force filament to induce 50% paw withdrawal in adult offspring of SPF, ABX, ABX+Veh, and ABX+4-MM dams (One-way ANOVA+Tukey’s, n=5, 7, 6, 7 dams). g, Latency to contact the adhesive in adult offspring of SPF, ABX, ABX+Veh, ABX+4-MM dams (top) (One-way ANOVA+Tukey’s, n = 6, 6, 6, 7 dams). Latency to remove the adhesive after first contact in offspring of SPF, ABX, and ABX+Veh, ABX+4-MM dams (bottom) (One-way ANOVA+Tukey’s, n = 6, 6, 6, 7 dams). Behavioral data for SPF and ABX as reference controls are as in Figures 2 and 3. Mean±SEM. *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001, n.s.=not statistically significant.