If you've ever wondered why the same pint of beer makes you feel slightly more buzzed in the summer than it does in the winter, scientists may finally have your answer. Work with drunk fruit flies suggests that the same molecular mechanisms that help control the body's response to temperature are also involved in alcohol tolerance.
Just like in humans, too much alcohol has a toxic effect on a fly. Once imbibed, alcohol--ethanol, actually--makes its way to cell membranes, for example in the nervous system, where it increases their fluidity, much like milk makes cereal soggy. This somehow disrupts the cell's function, translating into what we feel as an alcohol buzz.
A cell's membrane fluidity also depends on temperature, becoming more solid as it cools. To keep things from getting too rigid, the cell cranks up its production of fatty acids, which squeeze into the membrane and loosen it up. Because these fatty acids are regulated by proteins that are also involved in ethanol detoxification, evolutionary geneticist Kristi Montooth of Brown University came up with an idea: The regulatory proteins activated during cold weather might help flies better cope with ethanol.
The team tested the hypothesis on two groups of flies: one from tropical North Australia, raised in temperatures of 26 degrees Celsius, and another from temperate Tasmania, raised at 15 degrees Celsius. When the Tasmanian flies basked in 26-degree climes, their alcohol tolerance--the percentage of ethanol in solution needed to kill half of them within 48 hours--dropped from 13.2% ethanol to 8.8% ethanol. Meanwhile, moving the Australian flies to a chillier 15-degree environment boosted their alcohol tolerance from 12.3% to 15.2%.
Further work with the transplanted tropical Australian flies sheds light on their staying power. The researchers found that when these flies were moved to colder temperatures, they doubled the expression of genes involved in fatty acid synthesis. One of these genes codes for the enzyme acetyl-CoA synthetase, which helps regulate both fatty acid synthesis and ethanol detoxification. Another activated gene codes for the phospholipase D enzyme, which removes ethanol from membranes. So the pathways that spring into action to increase membrane fluidity in cold flies also appear to contribute to increased ethanol tolerance, says Montooth, whose team reports its results today in the Journal of Experimental Biology.
"I found it intriguing that the pathways were related," says physiologist Johannes Overgaard of Aarhus University in Denmark. Flies get their alcohol fix from rotting fruit, and geneticist James Fry of the University of Rochester in New York says that these multiple detox pathways may be needed to help flies deal with particularly "stiff" drinks in the short-term. Still, he says, they probably aren't responsible for long-term changes in how populations adapt to alcohol.
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