Those interested in medical cannabis will remember that the CB1 receptors, discovered less than 20 years ago, are activated by THC and other cannabinoids in cannabis. This activation provides the psychoactive effects of cannabis and also some of its other health enhancing properties. CB receptors also respond to endocannabinoids produced by own bodies, primarily in our nerve cells. The receptors are part of the endocannabinoid receptor (or regulatory) system, now seen as a major physiological system, with important roles in pain relief, neuroprotection and anti-inflammation, even digestion and vision.
Such CB1 activation by THC from the plant world or anandamide from our own cells, along with other cannabinoids produced by the cannabis plant or our own bodies, can provide profound health benefits. Cannabinoids also work by activating CB2 receptors (primarily found on immune cells). Independent of their actions on receptors, cannabinoids are anti-oxidants, protecting nerve cells and other tissue from oxidation stress.
In the photo below, the CB1 receptors are being marked by the inverse agonist, 18F]MK-9470, a positron emission tomography (PET) tracer for in vivo human PET brain imaging of the cannabinoid-1 receptor. Inverse agonists tend to cause receptors to respond in ways opposite their response to agonists such as THC and anandamide. In the case of cannabinoid receptors, hope that inverse agonists might serve as obesity control agents has faded with problems from nausea and mood disturbances.
The physics of what goes on during such as PET scan it astounding. The process would appear to be highly hazardous to health, yet the procedure is commonplace and apparently without risk. Markers with affinities for certain cell types, such as the compounds used above, MK-9470, emit anti-matter. A positron is the anti-matter equivalent of an electron. When it is emitted from the source, in this case on a CB1 receptor in the brain, it travels only a short distance, a millimeter or so, before encountering its matter equivalent, an electron.
When matter electron and antimatter positron meet, the result is annihilation. Such an encounter releases a short burst of highly energetic photons in the form of gamma rays. Why matter/antimatter annihilation with accompanying gamma ray burst inside the brain is not fatal is not exactly clear. Perhaps a high-energy physicist could comment. Or even a low-energy physicist after coffee.
During this positron emission tomography, sensors detect where the gamma rays are coming from and map these in a 3D representation of brain anatomy and activity. In the images above the patterns of gamma rays being emitted from this matter/antimatter annihilation show the relative distributions of CB1 receptors in various parts of the human brain. See the original research for more detail. Although they are most highly concentrated in the brain, CB1 receptors are also found throughout the entire human body, mainly on nerve cell membranes.