Fig. 2. The effects of acetylcholine (ACh), atropine (Atr) and neurokinin A (NKA) on gut motility in zebrafish larvae over successive 9 min periods (in vivo experiments). (A) ACh was applied to animals expressing no spontaneous motility (4 d.p.f.). (B–D) Atropine and NKA were applied to animals showing spontaneous motility (B, 4 d.p.f.; C, 5 d.p.f.; D, 6 d.p.f.). Application of saline (NaCl; A–D) did not affect the frequency of anterior anterograde waves (cycles min^-1) compared to the control period. (A) ACh (10^-5 mol l^-1) increased the frequency at 4 d.p.f., indicating the presence of functional muscarinic receptors, but was without effect one stage earlier (not shown). (B–D) Atropine (10^-6 mol l^-1) reduced the frequency of the spontaneous motility from 4 d.p.f. (B), indicating a release of endogenous acetylcholine in the animal. NKA (10^-6 mol l^-1) increased the frequency from 5 (C) to 6–8 d.p.f. (D) after block by atropine, but had no visible effect if applied before atropine (not shown).