Sensory Detection

"Waiter, There’s a Fly in My Beer!"

Science's STKE  04 Sep 2007:
Vol. 2007, Issue 402, pp. tw317
DOI: 10.1126/stke.4022007tw317

There are five well-known, distinct tastes that organisms can detect--sweet, bitter, sour, salty, and umami. Fischler et al. explored the taste neurons of the fruit fly Drosophila melanogaster and discovered a set of neurons that signal detection of a different taste: beer, or, more precisely, carbonated liquids. The authors used immunohistochemistry to detect a set of neurons that expressed a particular gene enhancer probe. These neurons ran from the taste sensors on the proboscis to a region of the brain that processes taste signals. They then expressed a calcium sensor in these neurons that allowed imaging of neuronal activation by confocal microscopy in living flies. The usual taste cues got no response from these neurons, but application of beer or a yeast culture gave strong activation. The authors immediately suspected ethanol or carbon dioxide as potential sensory ligands, and their experiments implicated the latter. Solutions containing dissolved CO2 were the primary stimulants of the cells. The detection of carbonation also influenced the flies’ behavior. They preferred a solution of sodium bicarbonate at pH 6.5 (in which about half the carbonate is converted to CO2) over the same solution at pH 8.5 (in which less than 1% of the carbonate is present as CO2). Overexpression of an inwardly rectifying potassium channel (which hyperpolarizes and inactivates the neurons) in the CO2-detecting taste neurons showed reduced behavioral preference for CO2-containing solutions. Fruit flies have recently been shown to be able to smell CO2, but such detection is mediated by a different system and stimulates avoidance behavior. Thus, the same molecule can produce distinct behaviors in the animal, depending on the mode of detection. The authors point out that these results make good sense when related to the fly’s lifestyle. Carbonated solutions that the fly encounters may well have microorganisms like yeast in them and may be full of nutrients. High concentrations of CO2 in the air, on the other hand, may be associated with unfavorable or stressful conditions and hypoxia. The findings also raise the question whether other organisms might also sense and be attracted to carbonated solutions…Cheers!

W. Fischler, P. Kong, S. Marella, K. Scott, The detection of carbonation by the Drosophila gustatory system. Nature 448, 1054-1057 (2007). [Online Journal]