Natural 'high' could avoid chronic marijuana use
December 1, 2014
Science Daily/Vanderbilt University Medical Center
Replenishing the supply of a molecule that normally activates cannabinoid receptors in the brain could relieve mood and anxiety disorders and enable some people to quit using marijuana, a Vanderbilt University study suggests.
Cannabinoid receptors are normally activated by compounds in the brain called endocannabinoids, the most abundant of which is 2-AG. They also are "turned on" by the active ingredient in marijuana.
Sachin Patel, M.D., Ph.D., and his colleagues developed a genetically modified mouse with impaired ability to produce 2-AG in the brain. The mice exhibited anxiety-like behaviors, and female mice also displayed behaviors suggestive of depression.
When an enzyme that normally breaks down 2-AG was blocked, and the supply of the endocannabinoid was restored to normal levels, these behaviors were reversed, the researchers reported on Nov. 26 in the online edition of the journal Cell Reports.
If further research confirms that some people who are anxious and depressed have low levels of 2-AG, this method of "normalizing 2-AG deficiency could represent a viable ... therapeutic strategy for the treatment of mood and anxiety disorders," they concluded.
However, this approach has not been tested in humans, they cautioned.
Relief of tension and anxiety is the most common reason cited for chronic marijuana use. Thus, restoring depleted levels of 2-AG also "could be a way to help people using marijuana," added Patel, the paper's senior author and professor of Psychiatry and of Molecular Physiology and Biophysics.
Chronic use of marijuana down-regulates cannabinoid receptors, and thus paradoxically increases anxiety. This can lead to a "vicious cycle" of increasing marijuana use that in some cases leads to addiction.
Patel and his colleagues previously have found cannabinoid receptors in the central nucleus of the amygdala of the mouse. The amygdala is a key emotional hub in the brain involved in regulating anxiety and the flight-or-fight response.
They also have found that chemically modified inhibitors of the COX-2 enzyme they developed relieve anxiety behaviors in mice by activating natural "endocannabinoids" without gastrointestinal side effects. Clinical trials of some of these potential drugs could begin in the next several years.
Cyclooxygenase (COX) enzymes produce pro-inflammatory prostaglandins and are the target of aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs), used to relieve pain and inflammation. It has been known for several years that COX-2 inhibition also activates endocannabinoids.
https://www.sciencedaily.com/releases/2014/12/141201113253.htm
Painkilling system in brain: Too much of a good thing?
August 25, 2010
Science Daily/Scripps Research Institute
Repeatedly boosting brain levels of one natural painkiller soon shuts down the brain cell receptors that respond to it, so that the painkilling effect is lost, according to a surprising new study led by Scripps Research Institute and Virginia Commonwealth University scientists. The study has important implications for drug development.
The natural painkiller, 2-AG, is one of the two major "endocannabinoid" neurotransmitters. The other, anandamide, can be kept at high levels in the brain without losing its therapeutic effects, and researchers had hoped that the same would be true for 2-AG.
"One implication is that maximally elevating 2-AG levels in the brain might not provide a straightforward path to new pain drugs," says Benjamin F. Cravatt III, PhD, professor and chair of the Department of Chemical Physiology and member of the Skaggs Institute for Chemical Biology at Scripps Research in La Jolla, California, who led the study with Aron Lichtman, PhD, a professor of pharmacology and toxicology at Virginia Commonwealth University in Richmond, Virginia. "But we remain optimistic that more modest elevations in 2-AG could produce sustained pain relief. Perhaps more importantly, on a basic science level, we've been able to tease apart a key difference between the two major endocannabinoid signaling pathways, since one can maximally elevate anandamide without observing tolerance."
The report appears in the August 22, 2010 issue of Nature Neuroscience.
A Better Chill Pill
Like the opioid system, the endocannabinoid system was discovered as a result of humans identifying a plant -- in this case marijuana (cannabis sativa) -- that artificially boosts its activity. Marijuana's main active ingredient, THC, typically reduces pain and anxiety. Researchers have sought to develop drugs that reproduce such therapeutic effects while leaving out THC's unwanted side effects -- which include memory impairment, locomotor dysfunction, and possibly addiction.
Cannabinoid research received a boost in 1990 with the description of the main cannabinoid receptor in the brain, CB1, and a few years later with the discoveries of the body's own (endo-) cannabinoids, anandamide and 2-AG, which exert most of their effects by binding to CB1. Cannabinoid receptors are now known to be widely distributed in the brain, and when activated by anandamide or 2-AG, tend to calm the activity of the neurons where they reside. However, researchers so far have been unable to develop artificial cannabinoids that bind to CB1 without producing unwelcome THC-like side effects.
An alternative strategy has been to boost levels of the body's own cannabinoids by inhibiting the enzymes that normally break them down. And so far this has worked for anandamide. Inhibitors of its breakdown enzyme, fatty acid amide hydrolase (FAAH), have been shown to boost anandamide levels and reduce pain and inflammation without adverse side effects in animal tests and early clinical trials.
A similar strategy for boosting 2-AG may be promising, too, especially since 2-AG levels in the brain are naturally higher than anandamide's. Two years ago, the Cravatt and Lichtman laboratories jointly reported the development of an inhibitor of 2-AG's breakdown enzyme, monoacylglycerol lipase (MAGL). When administered to mice, it boosted their brain levels of 2-AG on average by a factor of eight, and produced a pain-killing effect comparable to that of FAAH inhibitors.
Diminishing Returns
Now the two labs report that 2-AG's pain-killing effect disappears after six days of treatment. "When you continually stimulate the endocannabinoid system by maximally raising 2-AG levels, you effectively desensitize the system," says Cravatt.
In one experiment, an injection of the MAGL inhibitor into mice showed evidence of pain relief on standard tests, but after six consecutive daily injections the drug could no longer achieve this effect. These chronically treated mice also lost much of their sensitivity to THC and to a synthetic CB1-binding compound, and showed a classic sign of drug dependency�when abruptly withdrawn from 2-AG's influence by having their CB1 receptors blocked, they developed paw flutters -- a murine version of the shakes.
"When we investigated at the molecular level, we found that the number of CB1 receptors in the mouse brains had been reduced," says Jacqueline Blankman, a graduate student at the Scripps Research Kellogg School of Science and Technology who was co-first-author on the paper with Joel Schlosburg of the Lichtman lab. This receptor "downregulation" occurred in some brain areas but not others
To confirm this effect, the researchers utilized another experimental mouse model where the gene for MAGL was inactivated. This lifelong genetic disruption of MAGL also resulted in high 2-AG levels as well as a reduced and desensitized CB1 system.
"Because we're seeing downregulation of the whole cannabinoid system and tolerance to the anti-pain effects, it does raise some concern about whether MAGL would be a suitable pain target," says Blankman.
"If you are going to inhibit MAGL, you probably wouldn't want to produce a complete inactivation of the enzyme," Cravatt adds.
By contrast with the 2-AG experiments, chronically boosting anandamide had none of these effects on the CB1 system. Cravatt doesn't yet know why these two molecules have such different impacts when delivered chronically. He notes, however, that anandamide may be produced selectively under stress conditions, and perhaps for that reason is less likely to trigger a brain-wide CB1 downregulation.
"The question of why anandamide and 2-AG have such different effects when given chronically is certainly going to be motivating us from now on," says Cravatt. "But already with this finding and the development of these models we've taken a significant step forward in understanding and being able to manipulate this important neurotransmitter system."
This study was supported by the National Institutes of Health.
https://www.sciencedaily.com/releases/2010/08/100824151036.htm