Hatching barely four days after their eggs are fertilized, the humble Danio rerio has become the workhorse of many developmental biology labs; their transparent eggs and larvae allow scientists to follow the development of embryonic tissues in the rapidly growing young. Anna Holmberg,Catharina Olsson and Susanne Holmgren from Göteborg University, Sweden,have taken full advantage of this to scrutinise the development of gut motility in tiny zebrafish larvae. Holmberg explains that the team had already discovered that the neurotransmitter acytylcholine appears to stimulate gut contractions. Keen to discover other neurotransmitters that regulate gut activity in the developing fish, the team turned their attention to a molecule that is known to inhibit gut contractions in adults, nitric oxide, to see if the youngsters produce the neurotransmitter and whether their embryonic intestines are able to respond to it(p. 2472).
Gently immobilising the embryos and larvae in soft agarose, the team were able to monitor the microscopic animals' gut contractions as they exposed the developing fish to compounds that stimulated and suppressed nitric oxide synthesis. Holmberg explains that the developing youngsters intestines only begin pulsating about 3-4 days after fertilization, so it wasn't clear whether or not the embryos' intestines responded to nitric oxide at 3 days. However,by 4 days development, Holmberg could clearly see that the embryonic fishes'guts were affected by the presence and absence of nitric oxide; nitric oxide appeared to inhibit gut contraction even before the youngsters begin feeding. And the neurotransmitter's effects became stronger as the youngsters developed.
Curious to know whether the 3-day-old youngsters were unable to synthesise nitric oxide or simply lacked the equipment to respond to it, the team probed the intestines of the developing fish, ranging from 2-day-old embryos to 7-day-old larvae, for nitric oxide synthase, the enzyme that synthesizes nitric oxide, to see if the tiny fish were capable of producing the neurotransmitter. Surprisingly, even the 2-day-old embryos expressed nitric oxide synthase. The team suspect that even though the developing fish can produce nitric oxide, their intestines are too immature to propagate the gut's rippling contractions.
According to Holmberg, the guts of zebrafish larvae begin responding to the inhibitory effects of nitric oxide at about the same time that they become responsive to acetylcholine, and she suspects that `there is probably a co-functionality between these two pathways to balance sweeping gut contractions at a desired frequency'.