When tiny Helisoma trivolvis embryos tumble and roll in their transparent egg cases, the young snails aren't simply working out, they are mixing the fluids around them to maximise their oxygen supply during development. Most of the time, the youngsters gently roll inside their clear compartments, but occasionally they burst into violent action, before slowing back to a less frantic speed. Jeffrey Goldberg was intrigued, and wondered how the tiny molluscs control this sophisticated behaviour, even before they develop their central nervous system. Over the years, he has discovered many key elements of the embryo's simple control system, and now he has added nitric oxide to the list of neurotransmitters that keep the embryos rolling(p. 3143).
During the last ten years, Goldberg's team discovered that a pair of embryonic neurones, ENC1, develop around the newly formed cilia at the time that the embryos begin tumbling. Since then, they have found that the pair of nerves sense oxygen levels inside the egg to control the gyrating behaviour that delivers oxygen to the developing embryo. Having isolated the mini-neurosystem, Goldberg wondered how the neurons sense and drive the tumbling motion. Could the neurochemical, nitric oxide, be involved?
Goldberg needed to find evidence of the nitric oxide generating enzyme,nitric oxide synthase (NOS), before he could discover whether the neurochemical regulated the tumbling behaviour. Using NADPH diapharose, a stain that binds tightly to NOS, Goldberg and Alison Cole began searching for NOS in the newly formed neurons and cilia. Goldberg remembers the moment when he looked down the microscope and clearly saw the tiny neural structures highlighted with the NOS stain. He says `it was fantastic [seeing] that the circuit was full of nitric oxide!'
Having found the nitric oxide source, Goldberg wanted to know how the unstable chemical affected the embryos. While videoing their sedate rotations,Goldberg's team fed the embryos extra nitric oxide, and in turn, switched off the embryo's own source of nitric oxide to see how the neurochemical affected the tumbling behaviour. This time, Goldberg was astonished to find that cutting off the embryo's nitric oxide supply cut their sedate rolling behaviour by 50%! He explains that this means that the nerves and cilia are constantly producing nitric oxide to drive the embryo's continual gentle tumbling. Goldberg believes that this is the first occurrence of a nitric oxide synthase that is permanently trapped in the on position.
But which part of the simple system is driven by nitric oxide? Does the neurochemical stimulate the neurone to trigger short bouts of violent tumbling, or are the neurones gently releasing nitric oxide into the cilia to continually regulate their gentle beating? Goldberg analysed the embryo's behavioural responses as he altered the nitric oxide supply, and discovered that the neurochemical works at both the neural and the ciliary levels.
Goldberg hopes that the transparent snail embryos will help him see through every aspect of the embryo's early neurological function, including nitric oxide.