The effects of endogenous and exogenous nitric oxide on gut motility in zebrafish Danio rerio embryos and larvae

Using motion analysis, the ontogeny of the nitrergic control system in the gut was studied in vivo in zebrafish Danio rerio embryos and larvae. For the first time we show the presence of a nitrergic tonus, modulating both anterograde and retrograde contraction waves in the intestine of developing zebrafish. At 4 d.p.f. (days post fertilisation), the nitric oxide synthase (NOS) inhibitor L-NAME (three boluses of 50-100 nl, 10(-3) mol l(-1)) increased the anterograde contraction wave frequency by 0.50 +/- 0.10 cycles min(-1). Subsequent application of the NO donor sodium nitroprusside (SNP; three boluses of 50-100 nl, 10(-4) mol l(-1)) reduced the frequency of propagating anterograde waves (-0.71 +/- 0.20 cycles min(-1)). This coincided with the first appearance of an excitatory cholinergic tonus, observed in an earlier study. One day later, at 5 d.p.f., in addition to the effect on anterograde contraction waves, application of L-NAME increased (0.39 +/- 0.15 cycles min(-1)) and following SNP application reduced (-1.61 +/- 0.36 cycles min(-1)) the retrograde contraction wave frequency. In contrast, at 3 d.p.f., when no spontaneous motility is observed, application of L-NAME did not induce contraction waves in either part of the gut, indicating the lack of a functional inhibitory tonus at this early stage. Gut neurons expressing NOS-like immunoreactivity were present in the distal and middle intestine as early as 2 d.p.f., and at 1 day later in the proximal intestine. In conclusion, the present study suggests that a nitrergic inhibitory tonus develops shortly before or at the time for onset of exogenous feeding.