Fig. 2. The development of a nitrergic control system in the gut of zebrafish embryos and larvae. L-NAME (10^–3 mol l^–1), which blocks NOS activity, or the nitric oxide donor SNP (10^–4 mol l^–1) were applied, and changes in anterograde and retrograde contraction frequencies were calculated. The effect of L-NAME was calculated in relation to the first saline application, while effects of SNP were calculated as changes in activity compared to the last 3 min of the preceding L-NAME period (referred to as L-NAME before SNP). (A,B) L-NAME increased the anterograde contraction frequencies in comparison to saline at 4 d.p.f. (A; N=10) and 5–6 d.p.f. (B; N=12), indicating endogenous formation of NO. SNP reduced the gut frequency at 4 d.p.f. (N=7) and 5–6 d.p.f. (N=11), confirming the presence of an inhibitory NO pathway. (C,D) Retrograde contraction waves were not affected by L-NAME (N=9) or SNP (N=4) at 4 d.p.f. (C), but at 5–6 d.p.f. L-NAME (N=12) increased and SNP (N=6) decreased the contraction frequency (D). Results are presented as cycles min^–1 (mean ± s.e.m.). *P<0.05. d.p.f., days post fertilization; L-NAME, N^G-nitro-L-arginine methyl ester; NO, nitric oxide; NOS, nitric oxide synthase; SNP, sodium nitroprusside.