Feb. 2, 2009 -- The molecular answer to a 30-year puzzle over what triggers birds to breed in spring has been solved by U.K. researchers.
Russell Foster of the University of Oxford unveiled his team's latest findings at a meeting of the Australian Neuroscience Society being held in Canberra.
Scientists have long been puzzled how birds know when it's spring breeding time, said Foster, who is a circadian neuroscientist, studying how body clocks are regulated by light.
The main way birds could know that it is spring is by detecting the changes in the number of daylight hours. But experiments in the 1930s showed that birds who had their eyes covered or removed still knew when to breed, said Foster.
Later experiments implanted fiber optics in birds to simulate a spring-like day and tested the responses of various parts of the brain to this.
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"These studies found the birds' reproductive system was triggered to become active when the hypothalamus was illuminated," said Foster.
Although the hypothalamus is deep in the base of the brain, it can still detect light because, as any child who has ever shone a torch through their hand knows, light penetrates tissue.
"Huge amounts of light actually reach the base of a brain of a bird and we've measured that empirically," said Foster. "It's useless of course for any image analysis, but it's perfectly good for telling you whether the light is on or off and therefore how long the day is."
Foster and colleagues are the first to identify the biochemical basis for this light sensitivity. They have found a light-sensitive pigment, called VA opsin, in the bird's hypothalamus.
"We now have an understanding at a molecular level of how that light is detected and ultimately how that light signal is being turned into a reproductive response," said Foster.
VA opsin is absent in all mammals including humans, probably because our ancestors were nocturnal and a photoreceptor buried deep into the brain would not be sensitive enough to detect day length.
These animals relied on specialized receptors in their eyes rather than in their brains, he said.
Foster's team previously discovered specialized cells containing the light-sensitive pigment melanopsin in the mammalian eye that are responsible for detecting day length.
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