Race, genes and cannabis

Scientists have found a switch in the brain which may explain why smoking cannabis causes psychosis and addiction in more than one-in-ten users. The team, at Aberdeen University found a genetic difference in the switch, probably inherited from early humans who smoked the drug in prehistoric times. The difference may also explain why some people could be more susceptible to conditions such as obesity. The researchers, at the university’s Kosterlitz Center for Therapeutics, studied genetic differences around a gene called CNR1, which produces what are known as cannabinoid receptors in the brain which control parts of the brain involved in memory, mood, appetite and pain. Cannabinoid receptors activate these areas of the brain when they are triggered by naturally-occurring chemicals in the body known as endocannabinoids. Chemicals found in the cannabis and "skunk" mimic the action of endocannabinoids. It is known that cannabis has pain-relieving and anti-inflammatory properties which can help treat diseases such as multiple sclerosis and arthritis. However, developing drugs from cannabis to treat these conditions is hampered by the fact that such drugs will have psychoactive side effects - and smoked cannabis can cause addiction and psychosis in up to 12% of users. Dr Alasdair MacKenzie, who led the research, said: "We looked at one specific genetic difference in CNR1 because we know it is linked to obesity and addiction. What we found was a mutation that caused a change in the genetic switch for the gene itself - a switch that is very ancient and has remained relatively unchanged in over three hundred million years of evolution, since before the time of the dinosaurs. These genetic "switches" regulate the gene itself, ensuring that it is turned on or off in the right place at the right time and in the right amount. It is normally thought that mutations cause disease by reducing the function of the gene, or the switch that controls it. In this case however, the mutation actually increased the activity of the switch in parts of the brain that control appetite and pain, and also, and most especially, in the part of the brain called the hippocampus, which is affected in psychosis. We know that this overactive switch is relatively rare in Europeans, but is quite common in African populations. But we were all once African, so something must have decreased it in our early ancestors who left Africa and migrated through Central Asia towards Europe and the north. One possibility we are keen to explore is that once in Central Asia these early migrants came into contact with the cannabis plant, which we know was endemic across that area at that time. It is possible that the side effects of taking cannabis were such that people with the mutation were not so effective in producing and raising children. Therefore, over the generations the numbers of people with the mutation decreased."