Researchers identify drug-tolerance mechanism in flies

A protein found on the surface of nerve cells makes fruit flies tolerant to a drug after just a single, brief exposure, which may reveal ways to address this early step toward addiction in humans.

Neuroscientist Nigel Atkinson at The University of Texas at Austin and his laboratory determined this by studying the response of fruit flies (Drosophila) to a 15-minute exposure to benzyl alcohol coated on the inner walls of test tubes. Flies that had had one previous exposure to the organic solvent recovered more quickly from being knocked out by the drug than flies that were first-timers. The flies that developed tolerance also had increased activity of the slo gene. The gene produces the surface protein, which helps stimulate signaling between nerve cells in the brain.

Genetically modified flies that lacked the slo gene failed to develop tolerance, while flies modified to have increased slo activity were more drug resistant than normal, providing added proof of the gene’s importance for tolerance. Because the human slo gene is almost identical to the one in fruit flies, Atkinson said, “If we could describe the series of steps involved in changing slo gene expression, then all the components involved in producing that change could become potential targets for anti-addiction drugs.”

The findings will be published online the week of Monday, Nov. 29, by the Proceedings of the National Academy of Sciences.

Benzyl alcohol (BA) is usually used as an anesthetic. Although few people seek it as a drug, the same behavioral and genetic effects were found when flies were exposed to the addictive chemical in glue or other addictive organic solvents. Initially, flies placed inside BA-laden test tubes began moving quickly and climbing the walls. Within minutes, this excited phase ended and they started falling off the walls and stumbling, before passing out. “They start tripping more and taking longer and longer to get up, basically acting like a drunk at a college party,” Atkinson said.

The reverse behavior happened during recovery from the drug. The flies began stumbling around and then climbing the test-tube walls within minutes. The flies that had previously been exposed to BA recovered much more rapidly than their drug-free counterparts. The researchers have since found that this tolerance lasts for more than seven days. “These findings show that very short exposures to drugs have very long-term effects,” he said.

Genetic analyses of BA-exposed flies revealed that the slo gene behaved differently during the excitement and sedative phases of drug intoxification. During excitement, the level of slo activity dropped by 75 percent, whereas its level was more than twice as high as normal when flies became sedated.

Atkinson and his co-authors suggest that the protein produced by slo serves as a thermostat to control how actively nerve cells transmit signals to other nerve cells. The slo protein serves as a portal, or channel, on the surface of nerve cells that permits potassium to enter the cells in a way that modifies electrical signaling. The researchers think that when a drug causes nerve cells to fire too rapidly, slo levels are decreased to produce fewer portals and dampen the drug’s effect. Having the slo gene instead become more active when the nervous system is sedated by a drug would help overcome that sedation, and underlies the development of tolerance.

“We think that producing more channels is a way to make the nervous system fire more excitedly, and decreasing the number of channels would make the nervous system less excited,” Atkinson said, noting that the slo channel responses likely act to compensate for other genes that are directly excited or sedated by drugs. “It’s not going to be a simple story where there’s just one protein involved in a response to a drug,” he said.

Because slo is central to tolerance development, Atkinson’s laboratory plans to identify the factors that regulate the slo gene in response to drug intoxification. Fruit flies are easy to genetically manipulate, so the work could rapidly identify which of those factors would be good targets for future drug research.

“We will use Drosophila to answer these regulatory questions,” Atkinson said. “Once we understand what happens in flies we can more easily ask if the same thing happens in mammals.”