Medical marijuana is proliferating across the country due to the ability of cannabis ingestion to treat important clinical problems such as chronic pain. However, negative side effects and the development of tolerance limit the widespread therapeutic use of Δ9-tetrahydrocannabinol (Δ9-THC), the major psychoactive ingredient in cannabis.
THC’s side effects are produced via its actions at cannabinoid CB1 receptors in the brain. Thus, scientists theorized that an agent with similar mechanistic actions, but that activate CB2 receptors instead, may eliminate the unwanted side effects while maintaining an equivalent level of efficacy.
Dr. Andrea Hohmann and her colleagues at Indiana University tested this strategy and found that, unlike Δ9-THC, repeated dosing with the cannabinoid CB2 agonist AM1710 suppresses chemotherapy-induced pain in mice without producing tolerance, physical withdrawal, motor dysfunction, or hypothermia. Moreover, the therapeutic effects of AM1710 were preserved in mice lacking CB1 receptors but absent in mice lacking CB2 receptors.
Their findings are reported in the current issue of Biological Psychiatry.
“Our study is important because it demonstrates beyond doubt that activation of cannabinoid CB2 receptors suppresses neuropathic pain without producing signs of physical dependence (i.e., a withdrawal syndrome) or other unwanted side effects associated with activation of CB1 receptors in the brain,” said Hohmann.
Their studies used animals that were treated with a chemotherapeutic agent (paclitaxel) to produce pain. When animals were given AM1710, a CB2 agonist, its pain-suppressive effects were fully preserved and its therapeutic effects were maintained even after repeated dosing.
The marijuana plant contains several chemicals that may prove useful for treating a range of illnesses or symptoms, leading many people to argue that it should be made legally available for medical purposes. In fact, a growing number of states (20 as of March 2014) have legalized marijuana’s use for certain medical conditions.
The term “medical marijuana” is generally used to refer to the whole unprocessed marijuana plant or its crude extracts, which are not recognized or approved as medicine by the U.S. Food and Drug Administration (FDA). But scientific study of the active chemicals in marijuana, called cannabinoids, has led to the development of two FDA-approved medications already, and is leading to the development of new pharmaceuticals that harness the therapeutic benefits of cannabinoids while minimizing or eliminating the harmful side effects (including the “high”) produced by eating or smoking marijuana leaves.
Alternatively, and as expected, when animals were given Δ9-THC, they developed complete tolerance to the pain-suppressing effects of THC and with repeated dosing, THC was no longer effective in suppressing neuropathic pain.
When the THC-treated animals were challenged with a drug that blocks CB1 receptors in the brain, the animals showed a prominent withdrawal syndrome, indicating signs of physical dependence following removal of THC. Strikingly, this was not the case with the CB2 agonist; blocking either CB1 or CB2 receptors produced no signs of withdrawal in animals treated chronically with the CB2 agonist.
Hohmann added, “We think our data suggests that CB2 receptors are an important target for suppressing chronic pain without unwanted side effects (e.g. psychoactivity, addiction).”
“It is important to know whether the benefits of cannabis ingestion for pain could be attributed in large part to the stimulation of CB2 receptors,” commented Dr. John Krystal, Editor of Biological Psychiatry. “CB2 agonists, in theory, would present less risk regarding addiction and intoxication than the ingestion of cannabis or THC.”
What Are Cannabinoids and How Might They Be Useful Medically?
Cannabinoids are a large family of chemicals related to delta-9-tetrahydrocannabinol (THC), marijuana’s main psychoactive (mind-altering) ingredient. Besides THC, the marijuana plant contains over 100 other cannabinoids. Scientists and manufacturers of “designer” drugs have also synthesized numerous cannabinoids in the laboratory (some of which are extremely potent and, when abused, have led to serious health consequences). The body also produces its own cannabinoid chemicals (called endocannabinoids), which play a role in regulating pleasure, memory, thinking, concentration, movement, coordination, sensory and time perception, appetite, and pain.
Currently the two main cannabinoids of interest therapeutically are THC and cannabidiol (CBD), found in varying ratios in the marijuana plant. THC stimulates appetite and reduces nausea (and there are already approved THC-based medications for these purposes), but it may also decrease pain, inflammation, and spasticity. CBD is a non-psychoactive cannabinoid that may also be useful in reducing pain and inflammation, controlling epileptic seizures, and possibly even treating psychosis and addictions.
National Institute on Drug Abuse (NIDA)
More work will be necessary before CB2 receptor agonists could be prescribed for use in humans, but for now, these data support the therapeutic potential of CB2 agonists for managing pain without the adverse effects associated with cannabis.
The article is “Chronic Cannabinoid Receptor 2 Activation Reverses Paclitaxel Neuropathy Without Tolerance or Cannabinoid Receptor 1-Dependent Withdrawal” by Liting Deng, Josée Guindon, Benjamin L. Cornett, Alexandros Makriyannis, Ken Mackie, Andrea G. Hohmann (doi: 10.1016/j.biopsych.2014.04.009). The article appears in Biological Psychiatry, Volume 77, Issue 5 (March 1, 2015), published by Elsevier.
Notes for editors
The authors’ affiliations, and disclosures of financial and conflicts of interests are available in the article.
John H. Krystal, M.D., is Chairman of the Department of Psychiatry at the Yale University School of Medicine, Chief of Psychiatry at Yale-New Haven Hospital, and a research psychiatrist at the VA Connecticut Healthcare System. His disclosures of financial and conflicts of interests are available here.
About Biological Psychiatry
Biological Psychiatry is the official journal of the Society of Biological Psychiatry, whose purpose is to promote excellence in scientific research and education in fields that investigate the nature, causes, mechanisms and treatments of disorders of thought, emotion, or behavior. In accord with this mission, this peer-reviewed, rapid-publication, international journal publishes both basic and clinical contributions from all disciplines and research areas relevant to the pathophysiology and treatment of major psychiatric disorders.
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