By Christine Kielhorn PHD
The activation of CB1 Receptors reduces the sensation of pain in the animal body; THC is able to reduce pain.
Chronic pain is the most common reason for taking medical cannabis. Neuropathic pain is a form of chronic pain relating to injury or disease of the nerves. There are a number of studies, including large, human clinical trials, on the effectiveness of cannabis for treating and managing chronic neuropathic pain. So, what is the science behind cannabis as a treatment for neuropathic pain? How does it work?
The human body has an endocannabinoid system that consists of several receptors and molecules that circulate through the body and activate or inhibit the action of these receptors. There are two cannabinoid receptors, CB1and CB2, which are classified as G-protein coupled receptors that send signals to the cell to regulate a large variety of cellular processes. The endocannabinoid system plays a role in the body’s detection and transmission of pain, although the exact mechanism is complex and not fully known.
The CB1 receptors are found primarily in the central nervous system, but do exist in other kinds of cells throughout the body. This receptor is the one primarily activated by delta-9-tetrahydrocannabinol (THC) and triggers the high that follows taking THC. The CB1 receptor appears to be involved in pain transduction, specifically in helping the body control the experience of pain. For example, mice with no CB1 receptors in their peripheral nervous system seem to feel pain more strongly.
The CB2 receptors also seem to be involved in this process because mice with no CB2 receptors also appear to feel pain more strongly. These receptors are primarily located in the cells of the immune system, but like CB1receptors, can be found in other kinds of cells as well. CB2 receptors have been found in tissues that are responsible for transmitting pain signals, such as dorsal root ganglion sensory neurons and in the spinal cord.
The effects of cannabinoids like THC, cannabidiol (CBD), and others have been studied in various animal models of pain to discover their efficacy and mechanism of action. An animal model of nerve injury is one way researchers can simulate neuropathic pain in animals. Studies on different nerve injury models have found cannabinoids appear to suppress the nerve stimulation that leads to pain sensations. Activation of CB receptors appears to prevent pain development after injury. In addition, using CBD reduced sensitivity to pain, but this is perhaps through a different kind of receptor, called the transient receptor potential cation channel, or vanilloid receptor (TRPV1).
Diabetic pain models have also been developed in mice to represent a different kind of neuropathic pain. THC dampened the pain signals in mice experiencing diabetic neuropathic pain. Also, researchers found that THC works better than morphine alone, but a low dose of THC together with morphine enhanced the pain relief of morphine. This could allow both THC and morphine to be taken together at lower doses to reduce unwanted side effects.
There have been a number of human clinical studies on the use of cannabis for treating pain. In general, these studies have found a dose dependent effect of THC on pain reduction. As in the animal studies, cannabinoids taken in combination with opioids (like morphine) also appear to be more effective while requiring a lower dose of opioids. These studies also found that quality of sleep was improved in patients with neuropathic pain while taking cannabis, and muscle spasms were reduced by taking a combination of THC and CBD (in a drug called Sativex). Combining THC with CBD can help to minimize the psychotropic effects of THC, which is not always desired in pain patients.
Neuropathic pain can be reduced by the use of cannabis. The mechanism of pain relief is complex, but involves the endocannabinoid receptors present in the peripheral nervous system. Activating the endocannabinoid receptors CB1 and CB2 reduces the sensation of pain, allowing patients suffering from neuropathic pain to enjoy a higher quality of life.