_Mid-Brain Light Sensor helps birds respond to seasons Sarah Loftus, 30th October 2011
Recent research published in Biology Letters by Davies et al.
(open access) has identified a light-sensing pigment in the mid-brain
of chickens thought to be involved their response to seasons.
The work, carried out at Oxford University’s Nuffield Department of Opthalmology,
shows that a pigment called vertebrate ancient (VA) opsin, which is
produced in the hypothalamus of birds, is sensitive to light of the
appropriate wavelength for natural daylight detection and therefore may
be involved in a bird’s response to seasons, for example knowing the
appropriate time to mate. The diagrams below summarise the method used.
Chicken hypothalamus (which naturally produces VA opsin) or cells cultured to produce VA opsin (by providing them with the gene to make the pigment) are broken up to release their contents into solution. This solution is poured over a matrix that specifically binds VA opsin so that it is purified from the cell solution. (Image by Sarah Loftus)
Purified VA opsin is then put through a spectrophotometer (a machine that measures light absorbance) and a profile showing the wavelengths of light absorbed by VA opsin is produced. (Image by Sarah Loftus)
Mammals are only able to detect light in their retina, but it was found that birds could additionally detect light in their mid-brains as exposure of this region to light for lengths of time similar to Spring daylight hours stimulated testicular growth in ducks, but when only exposed for shorter lengths of time, similar to Winter daylight hours, there was no effect on reproduction. Light is able to pass through the avian skull but the range of wavelengths able to be detected in the hypothalamus is narrow due to light being scattered as it passes through feathers, skin, skull and brain and the absorption of some wavelengths by haemoglobin. VA opsin, however, is particularly sensitive to light in this narrow range making it an ideal candidate for a mid-brain light sensor. Further research will now be required to directly link this photopigment and the neurones that produce it to the way birds respond to seasons.