- A drug like alcohol has the potential to interact with many proteins in the brain.
- New research examines alcohol's interaction with proteins called N-methyl-D-aspartic acid (NMDA) receptors.
- Alcohol appears to inhibit or block NMDA receptor activity within specific regions of the brain.
Despite numerous problems associated with alcohol abuse, and a history of use that extends back thousands of years, researchers still know little about alcohol's actions in the brain. Unlike 'simple' drugs such as cocaine, marijuana and heroin that interact with specific proteins, a more 'complex' drug such as alcohol has the potential to interact with many proteins in the brain. In the November issue of Alcoholism: Clinical & Experimental Research, researchers examine alcohol's interaction with proteins called N-methyl-D-aspartic acid (NMDA) receptors.
"NMDA receptors in the brain are a key site of action of the neurotransmitter glutamate, which increases the activity of brain neurons," explained Darin J. Knapp, research assistant professor of psychiatry at the University of North Carolina School of Medicine and lead author of the study. "Alcohol is known to interfere with the function of NMDA receptors in the brain. In fact, investigators have shown that alcohol-NMDA interactions influence many features of alcohol exposure, including effects on fetal development, seizures, gene expression in brain, intoxication, tolerance to ethanol, and alcohol dependence. A significant part of the motivation for our work here at the Bowles Center for Alcohol Studies comes from the understanding that alcoholism is a brain disease with a neurobiological basis and not a moral failure or a lack of willpower."
"NMDA receptors are one of the proteins that have been suggested to be sensitive to alcohol," added Andrey Ryabinin, assistant professor of behavioral neuroscience at the Oregon Health Sciences University. "They are also considered to be very important for various functions of the brain. Recently, for example, they gained special attention when their importance for memory formation was shown." Ryabinin was referring to an experiment in which mice were genetically modified to have an increased concentration of NMDA receptors in the hippocampus (an area very important for formation of complex types of memory). Seemingly as a result of the modifications, the mice performed better at learning standard laboratory memory tasks.
"One of the features of alcoholism and drug addiction is the formation of habits," said Ryabinin. "An alcoholic is used to drinking alcohol frequently or, whenever he/she drinks, he/she has the habit of drinking a lot, or he/she is used to going to a bar when there is free time, etc. There could therefore be a common link between becoming an alcoholic and developing, or learning, an alcohol-related habit, and NMDA receptors could be involved in this."
The objective of the Knapp study was to induce and to block Fos protein in the brain as measured with Fos-like Immunohistochemistry (Fos-LI). Measurement of Fos-LI is a form of "brain mapping." This technique allows researchers to make maps of brain regions that change their activity after different challenges, such as administration of alcohol. The Fos protein belongs to a family of proteins known to reflect changes in cellular activity and participate in regulating the activity of genes. For this experiment, Sprague-Dawley rats were given NMDA and/or alcohol injections (intra-abdominally) or infusions (intragastric ethanol or intravenous NMDA). When both doses were administered, alcohol preceded NMDA by 15 minutes.
The major effect of alcohol was to inhibit or prevent NMDA-induced Fos protein induction. That is, Fos protein induction occurred in specific regions of the brain following administration of alcohol and NMDA. NMDA binds to specific glutamate receptors in brain. "Our findings provide new evidence for the interaction of alcohol with specific neurotransmitter receptors of the living brain," said Knapp.
Ryabinin agrees with Knapp's assessment. "A big task in alcohol research is to find proteins sensitive to alcohol," said Ryabinin. "Finding brain regions where alcohol and NMDA interact suggests that in these brain regions NMDA receptors are very sensitive to alcohol. So here it seems that by finding this interaction, Knapp and colleagues have confirmed that NMDA receptors are very sensitive to alcohol, which would suggest that alcohol acts in the brain by modifying these receptors."
Knapp and Ryabinin point out that one potential alternative explanation for the findings is that rodents injected with high doses of alcohol are sedated. The sedation itself, rather than specific effects of alcohol at glutamate receptors, may account for the effect of alcohol in some brain regions. Knapp notes, however, that nonspecific effects such as sedation are not likely to explain the findings.
"We administered both alcohol and NMDA through a variety of routes in these studies," he says. "The striking consistency in the effects of alcohol across various administration routes and experimental protocols suggests that nonspecific effects such as sedation do not explain the findings. Furthermore, our results are consistent with work from several other laboratories which have shown that alcohol blocks NMDA effects at both sedating and non-sedating doses."
Knapp indicates that finding those brain regions where alcohol's effects are mediated is a key step in determining how specific symptoms and possibly one's susceptibility to alcoholism may develop.
"Individuals who may be more susceptible to the intoxicating or addictive effects of alcohol may have important differences in a given neurotransmitter function in specific regions of brain," said Knapp. "In this regard, the NMDA-type receptor for the glutamate neurotransmitter system is clearly a logical target for scientists in our field to study further. Understanding how and where these effects are mediated in alcoholics and non-alcoholics would be an important step in finding a cure for the disease."
This article was co-authored by Darin Knapp.
Co-authors of the Alcoholism: Clinical & Experimental Research paper included: Christopher J. Braun and Alda Fernandes of the Bowles Center for Alcohol Studies; Gary E. Duncan of the Department of Psychiatry; Ying Qian of the University of North Carolina Neurosciences Center; Fulton T. Crews of the Bowles Center for Alcohol Studies, and the Departments of Psychiatry and Pharmacology; and George R. Breese of the Bowles Center for Alcohol Studies, the University of North Carolina Neurosciences Center, and the Departments of Psychiatry, Pharmacology and Anesthesiology - all at the University of North Carolina at Chapel Hill.
The study was funded by the National Institute on Alcohol Abuse and Alcoholism.