Densitometric analysis of the VB thalamus at P7 and P21 shows persistent increases in 5-HT axon density in GAP43−/− mice. Values are expressed as a percentage of the WT control mean. The density of 5-HT axons in the VB thalamus of GAP43−/− mice (white bars; 427%) and GAP43+/− mice (gray bars; 158%) is significantly increased compared with WT mice (black bars; 100%) at P7. 5-HT axon density remains elevated in GAP43−/− mice (158%) compared with WT and heterozygote mice at P21. *p ≤ 0.05, ***p ≤ 0.001 relative to WT;⁺p ≤ 0.05,⁺⁺p ≤ 0.01 for GAP43+/− versus GAP43−/− comparisons; not significant, GAP43+/− versus WT at P21.