Nitric oxide holds the key to firefly glow

LONDON The mysterious secret of how fireflies control when they glow is starting to be unravelled. A study published in 29 June Science, has discovered that the key chemical that fireflies need to switch on and off could be nitric oxide.Scientists have known for some time that the spectacular lightshow put on by fireflies as part of their courting display starts from bursts of neural activity stimulating the release of the primary neurotransmitter, octopamine. This triggers the firefly's light producing organ in the abdomen, called the lantern, which contains thousands of photocytes subdivided into an inner region rich in organelles housing luceriferin and luciferase. These react to generate light when oxygen is available, which is provided by thousands of mitochondria packed along the photocyte edges.But it remained unclear how this process was being switched on, as the firefly abdominal nerve endings are not in direct contact with the photocytes, and are separated by a gap of 17 μm. Based on similarities they observed between lantern cells and other cells that produce nitric oxide (NO), researchers from Tufts University and Brigham and Women's Hospital wondered if the likely biochemical trigger could be NO.To test this, they placed fireflies into an observation chamber in the dark with a steady flow of gaseous nitrogen and oxygen for five minutes. When they introduced NO at a dose commonly used for inhalation anaesthesia in human patients, the fireflies glowed or flashed almost continuously. They stopped once the nitric oxide supply was turned off.The researchers then wanted to discover whether NO was acting in the lantern or affecting the nerves that triggered the flashing. So the team devised a way of removing the nerves leading to the lantern while leaving the abdomen intact. When they added NO to the isolated lantern, it glowed. This was stopped by adding NO-scavenging chemicals — even when the lantern was stimulated directly with octopamine. Activity staining and immunocytochemical analysis of the microscopic structures within the lantern revealed that the enzyme producing NO, nitric oxide synthase, was located between the neuron endings and the lantern cells.Based on their findings he researchers have proposed a theory to explain how the firefly flash control machinery might work. The firefly's lantern is normally turned off and the photocytes are de-activated. Oxygen moving into the photocytes is continuously consumed by the mitochondria, which prevents oxygen from reaching the light-producing chemicals sequestered inside the photocytes. The arrival of a nerve signal simulates cells to produce NO, which inhibits mitochondrial respiration and allows oxygen to pass freely through the photocytes and so activate light production."We haven't shown the exact mechanism but our model seems plausible," said lead author Barry Trimmer, Associate Professor of Neurobiology at the Department of Biology, Tufts University. "We know that mitochondria can be switched off by nitric oxide in the test tube and we know the light flash in fireflies is triggered by oxygen. Our next task is to prove our hypothesis works."The findings could also be important in terms of setting a new direction for research into nitric oxide, suggested Trimmer. Although nitric oxide is one of the simplest biological molecules in nature it is thought to play a role in many processes in biology and medicine, from headaches to erectile dysfunction. "We know it is important in blood flow and it is a signalling molecule in the brain. But we may have found a novel mechanism for nitric oxide where it acts on the mitochondria, which occur every cell in the body. And as nature does not work by doing something brand new, it is reasonable to suggest this mechanism may be occurring in other species," he concluded.

Nitric oxide holds the key to firefly glow

Nitric oxide was once regarded simply as a noxious pollutant. But its ever expanding functional repertoire may now include generation of firefly luminescence.

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