A new set of experiments in mice confirms that a brain receptor associated with the reinforcing effects of marijuana also helps to stimulate the rewarding and pleasurable effects of alcohol.
The research, which was conducted at the U.S. Department of Energy's Brookhaven National Laboratory and was published online September 2, 2005, by the journal Behavioural Brain Research, confirms a genetic basis for susceptibility to alcohol abuse and also suggests that drugs designed to block these receptors could be useful in treatment.
"These findings build on our understanding of how various receptors in the brain's reward circuits contribute to alcohol abuse, help us understand the role of genetic susceptibility, and move us farther along the path toward successful treatments," said Brookhaven's Panayotis (Peter) Thanos, lead author of this study and many others on "reward" receptors and drinking (see: http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=05-49 and , http://www.bnl.gov/thanoslab).
Earlier studies in animals and humans have suggested that so-called cannabinoid receptors known as CB1 -- which are directly involved in triggering the reinforcing properties of marijuana -- might also stimulate reward pathways in response to drinking alcohol. Thanos' group investigated this association in two experiments.
In the first experiment, they measured alcohol preference and intake in mice with different levels of CB1 receptors: wild type mice with normal levels of CB1; heterozygous mice with approximately 50 percent levels; and so-called knockout (KO) mice that lack the gene for CB1 and therefore have no CB1 receptors. All mice were given a choice of two drinking bottles, one with pure water and one with a 10 percent alcohol solution -- approximately equivalent to the alcohol content of wine. Mice with the normal levels of CB1 receptors had a stronger preference for alcohol and drank more than the other two groups, with the CB1-deficient mice showing the lowest alcohol consumption.
After establishing each group's level of drinking, the scientists treated animals with a drug known to block CB1 receptors (SR141716A) and tested them again. (These animals were also compared with animals injected with plain saline to control for the effect of the injection.) In response to the CB1 receptor-blocking drug, mice with normal and intermediate levels of receptors drank significantly less alcohol compared to their pre-treatment levels, while KO mice showed no change in drinking in response to the treatment.
In the second experiment, the scientists compared the tendency of wild type and KO mice to seek out an environment in which they had previously been given alcohol. Known as "conditioned place preference," this is an established technique for determining an animal's preference for a drug.