Scientists at UCSF's Ernest Gallo Clinic and Research Center have identified a single brain protein that can account for most of the intoxicating effects of alcohol. The finding pinpoints perhaps the best target yet for a drug to block alcohol's effect and potentially treat alcoholism, the scientists say.
The mechanisms by which alcohol acts on the brain are thought to be similar throughout the animal kingdom, since species from worms and fruit flies to mice and humans all become intoxicated at similar alcohol concentrations. But although studies have identified a number of genes that can partially influence how alcohol affects behavior, this is the first finding that a single gene and the brain protein it codes for - known as an ion channel - are responsible for the intoxicating effects of alcohol in a living organism, according to the researchers.
The discovery was made in a six-year research effort focusing on Caenorhabditis elegans, the roundworm widely studied because about half of its approximately 20,000 genes have counterparts in the human genome.
"We have found that alcohol acts on this channel in nerve cells to cause neural depression and intoxication," said Steven McIntire, MD, PhD. "We would expect that the same process functions in humans, who also have this type of channel." McIntire is senior author of a report on the discovery in the December 12 issue of the journal CELL. He is assistant professor of neurology at UCSF and principal investigator at the UCSF-affiliated Gallo Clinic and Research Center.
Researchers already knew that the gene known as slo-1 codes for a channel-like protein in the brain that can allow potassium ions to pour out of neurons, a normal process that temporarily slows down the neuron's activity. In the study, the scientists discovered that alcohol makes the channel open more frequently, depressing neuron activity and leading to sluggish, uncoordinated movement typical of intoxication.
The same kind of channel - known as the BK channel - is found in the human brain, the researchers say, suggesting that a drug that modifies alcohol's effect on the channel could quickly sober someone up after a bout of drinking, or weaken the taste for alcohol among people vulnerable to alcoholism.
"Until we conduct human studies, we can't say for sure whether this channel or the pathways involving this channel are defective in alcoholics, but this is a highly attractive target. We now know it is central to the intoxicating effect of alcohol," McIntire said. Studies of BK channels in cell culture suggest that the human BK channel is affected in the same way, he added.
After identifying the role of the slo-1 gene, the scientists were able to show that worms lacking this one gene were virtually unaffected by alcohol, often behaving normally even when exposed to alcohol doses that would leave normal animals comatose.
"Alcohol has a diffuse effect, and it certainly acts at other sites as well," McIntire said. "Studies have shown that altering one gene or another can partially affect behavioral responses to ethanol. But this is the first study to demonstrate that a single gene mutation can create such strong resistance to the behavioral effect of ethanol."
The channel studied by McIntire and his colleagues, technically known as the BK potassium channel, is one of more than 200 known channels that regulate cell activities by controlling the flow of charged atoms, or ions, in and out of the cell. BK channels are active in nerve, muscle and gland tissue in mammals, where they control neurotransmitter release, muscle contraction and hormonal secretion, the scientists report. The channel may be involved in hormonal or non-behavioral effects of ethanol as well.
If alcohol activation of the BK channel is the major cause of intoxication, then artificially activating the channel without alcohol should also produce the intoxicating effect, the scientists reasoned. They identified mutants in which the BK channels opened more often than usual - without alcohol exposure - just as normal channels do when exposed to alcohol. As they predicted, worms with these abnormal BK channels acted just as intoxicated - without any exposure to alcohol - as normal worms did under the influence, confirming the new finding.
The research began with a search for genes that account for alcohol's intoxicating effect. The scientists treated the microscopic worms with chemicals that cause mutations and result in mutant offspring. They then screened thousands of different mutants for how they moved when exposed to alcohol, and thousands of others for their ability to lay eggs in the presence of ethanol. From this process, they identified eight genetic variants that showed varying degrees of resistance to the effects of alcohol on locomotion. Only those with mutated slo-1 genes were extremely resistant to alcohol.
The research showed that the alcohol-resistance trait was due to slo-1 genes in neurons, as opposed to other cells such as muscle. Then, using standard electrophysiological techniques, the scientists determined that alcohol activates the BK potassium channel in the living animals, inhibiting neuronal action and leading to the decreased speed and motor control. BK channel mutants, on the other hand - worms that either lacked the channels or had non-functional ones - were virtually immune to the effects of alcohol.
The scientists conclude that the BK channel is the major physiological mediator of ethanol intoxication in C. elegans. More important, if BK channels mediate alcohol effects, they write, then the "near-ubiquitous" presence of BK channels in mammals may explain the varied effects of alcohol on people as well.
"Identifying the molecules in the brain that alcohol acts on to change behavior will allow us to pursue a direct approach to develop drugs or other therapies to treat alcohol addiction," McIntire says.
Collaborators in the research and co-authors on the CELL paper are Andrew G. Davies, PhD, senior scientist; Jonathan T. Pierce-Shimomura, PhD, and Hongkyun Kim, PhD, both post-doctoral scientists; and Tod R. Thiele, BS, research assistant, all in McIntire's lab; Antonello Bonci, MD, UCSF assistant professor of neurology; Cornelia I. Bargmann, PhD, UCSF professor of anatomy; and Miri K. VanHoven, a graduate student in Bargmann's lab.
The Gallo Center research was supported by the National Institutes of Health, the Department of Defense and by funds provided by the State of California for medical research on alcohol and substance abuse through the University of California, San Francisco.
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