Light receptor feels the heat

Compiled by BioPhotonics staff

The light-sensing protein rhodopsin, packed inside the eye’s photoreceptor cells, is critical for sensing temperatures, scientists report.

Rhodopsin, one of the most studied sensory receptors, had been thought to function solely in the eye as a light receptor, but now investigators at Johns Hopkins University have found that it is used by fruit fly larvae and other organisms to distinguish between slight temperature differences.

The research team identified this role for rhodopsin while studying the process that results in the activation of a temperature-sensor protein called TRPA1, one of the many “trip” channels abundant on sensory cells that receive communication from the outside world.

Earlier discoveries determined that TRPA1 enables fruit fly larvae to detect minuscule changes in temperature to enhance their survival. Although temperature-sensor protein channels function to avoid hot and cold temperatures, they found that TRPA1 was not directly turned on by changes in the comfortable 18 to 24 °C temperature range.

Studies on fruit flies have shown that rhodopsin, a photoreceptor in the eye, is critical for sensing temperatures.

To determine what receptor responds to temperature and activates TRPA1, the scientists conducted a series of tests on fruit fly larvae missing the gene that codes for rhodopsin, using normal, wild-type larvae as a control group. The team released about 75 larvae on a plate with two temperature zones: Half the plate was kept at a desirable 18 °C, while the other side ranged from 14 to 32 °C.

After 10 minutes, the larvae that lacked rhodopsin, just like the larvae lacking TRPA1, could not discriminate between temperatures in the comfortable range, whereas the wild-type larvae and the rhodopsin-mutant larvae chose the desirable 18 °C.

Their findings, which appeared in the March 11, 2011, issue of Science (doi: 10.1126/science.1198904), concluded that rhodopsin is activated by temperature, which then activates TRPA1.

The surprise is that rhodopsin’s thermo-sensing role has no connection to light, the investigators reported. The wild-type fruit fly larvae chose the 18 °C temperature over 24 °C when they were kept in a dark box.