UC San Francisco researchers have identified an enzyme that could prove to be a target for reducing the craving for, and excessive use of, alcohol - a hallmark of alcoholism.
In the November issue of Nature Neuroscience, they report that mice genetically engineered to lack the enzyme, known as PKCe, were 75 percent less apt to imbibe than their normal brethren.
Mice lacking the enzyme also responded as if they were pre-treated with a class of drugs that are commonly used to treat seizures, anxiety and epilepsy, but showed now sign of being sleepy or sedate, normally a side effect of the drugs. The finding suggests that inhibiting PKCe activity could also improve, or supplant, these existing drugs, the researchers said.
The explanation for the findings is that the absence of PKCe enhances the effects of alcohol and the other drugs, known as benzodiazepines, on a molecular receptor in the brain known as GABA-A. The GABA-A receptor is the molecular channel through which most signals telling the brain to feel gratified or relaxed or sedated - all messages of "inhibition" -- are communicated. Normally, the GABA-A receptor is regulated by a chemical messenger known as the GABA-A neurotransmitter. When the neurotransmitter is fired from one neuron, it latches onto the receptor site in the membrane of a target cell and there, like a latchkey, opens the receptor's channel, allowing chloride ions containing signals of inhibition to flow through to a receptive neuron. The result is a flood of gratification, relaxation or sedation (depending on the signals) in the central nervous system.
Alcohol and benzodiazepines, which latch on to an adjacent spot on the GABA-A receptor, act by enhancing the response of the receptor to the GABA neurotransmitter, prompting the receptor to keep its channel open longer than normal, which leads to the release of more signals of gratification, relaxation or sedation.
The UCSF study demonstrates that, without PKCe, but in the presence of alcohol or the other drugs, the GABA-A receptor becomes even more sensitive to the GABA neurotransmitter, responding, in a sense, like a hair trigger. The outcome is that the receptor keeps its channel open even longer, allowing still more signals of inhibition to flow into the central nervous system.
The increased sensitivity to alcohol in the absence of PKCe was dramatized in the animals' behavior after they took a nip. Mice lacking PKCe became twice as active in response to low doses of alcohol (dashing around their cages) than normal mice. And they became more sedated in response to high doses (taking longer to awaken) than normal animals. (These findings were in keeping with the curiously contrasting effects of alcohol at low and high doses.) The findings were also observed in brain cells, with the GABA-A receptor responding to lower doses of alcohol and benzodiazepines in genetically altered mice than in normal mice.
And it is this increased sensitivity, said Clyde Hodge, PhD, a UCSF assistant professor of neurology and an investigator at the Ernest Gallo Clinic and Research Center at UCSF, that decreases the animals' craving for alcohol. "These mice support the concept emerging in alcohol research that increased sensitivity to alcohol intoxication lessens the likelihood that a person will become an alcoholic," Hodge said.
A long-running UC San Diego study of sons of alcoholics illustrates the theory in humans. The ongoing trial, conducted by UCSD researcher Mark A. Schuckit, MD, UCSD professor of psychiatry, shows that those sons who were more sensitive to alcohol's effects as teenagers were less likely to become alcoholics as adults.
The UCSF results may suggest, said the researchers, that mutations in the gene for the PKCe enzyme may influence the susceptibility of certain individuals to becoming alcoholics. However, it's just as likely, they said, that they have simply identified a mechanism for disrupting normally occurring molecular activity at the GABA receptor.
"It's possible that PKCe's impact on the GABA receptor in response to alcohol and other drugs is simply a biochemical phenomenon that will give us a handle on making new drugs to regulate the GABA receptor," said co-author Robert Messing, MD, UCSF associate professor of neurology and an investigator at the Ernest Gallo Clinic and Research Center at UCSF.
"Pharmacological agents that inhibit PKCe might prove useful for treating alcoholism and provide a non-sedating alternative for enhancing GABA-A receptor function in therapy of other disorders, such as anxiety and epilepsy," Messing said.
On a more fundamental level, the UCSF finding offers an important insight into the complex regulation of the key receptor for signals of gratification, relaxation and sedation in the central nervous system. Scientists have known for some time that alcohol and benzodiazepines act by enhancing the effect of the GABA neurotransmitter on its receptor and that these so-called "allosteric modulators" do so by latching on to a site adjacent to the key activation site on the GABA receptor. The new finding suggests that PKCe, one of at least 10 known forms of the ubiquitous PKC enzyme, plays a role in restraining the GABA receptor's sensitivity to these inciting factors.
And this, said Messing, represents "a unique kind of modulation." PKC enzymes act by "phosphorylating," or adding a phosphate group, to receptors. In this case, the impact of phosphorylation appears to be dampening the brain's response to alcohol.
Other co-authors of the UCSF study were Kristin K. Mehmert, BS, Stephen P. Kelley, MS, Thomas McMahon, BS, and Ashley Haywood, BA, all staff research associates; M. Foster Olive, PhD, senior scientist, Dan Wang, MD, MS, staff research associate, and Ana Maria Sanchez-Perez, PhD, post-doctoral fellow, all investigators at the Ernest Gallo Clinic and Research Center at UCSF. The study was funded by the State of California for Alcohol and Drug Abuse Research, and the Alcoholic Beverage Medical Research Foundation.
Journal
Nature Neuroscience