Marijuana use has increased across the U.S. following legalization in several states for medical use and similar moves, in a smaller number of states, for recreational use. For example, usage rates in Colorado have reportedly increased substantially following legalization.
Wider use and considered recommendations on medical grounds does not make marijuana “safe” and the issues about the physiological impact of the drug on the body, together with the issue of addiction, remain contested subjects between scientists. However this issue is resolved in the future it remains that the effects of marijuana abuse in the human brain are not well understood.
A relatively new study has raised concerns about the impact of marijuana use on dopamine. Marijuana, as with some other drugs, stimulates brain dopamine levels. Dopamine is a neurotransmitter that helps transmit chemical signals to the brain associated with the brain’s reward and pleasure centers. The chemical also plays a role in controlling movement. Within the brain there are five receptors designed to detect the chemical. Dopamine is also used artificially in some medications, to raise the heart rate and increase blood pressure.
Dopamine signalling is considered to be tied to the rewarding effects of drugs and to be involved with consequential addiction. The processes involved have been supported by brain imaging techniques, such as positron emission tomography (PET) scans.
Earlier studies have shown people with low dopamine activity appear to be more prone to addiction. Building on this, a Harvard University researcher has found dopamine signaling adaptation to occur with marijuana abusers. The effect of this is greater addiction (in a similar way to nicotine), increased stress responses and irritability. These latter two responses of increased anxiety and restlessness were bound up with the feeling of becoming “high” and were linked to same brain regions. There were also some concerns about the effects on heart rhythms.
The research further indicates that with the feeling of becoming “high,” although this effect continues unabated with long-term marijuana use the feelings of “reward” associated with the activity become repressed and “relaxation” associated with the psycho-stimulative effects of the drug are quickly replaced with irritability and enhanced craving for further doses of cannabis. Here is thought that the diminished dopamine response explain why heavy users need to consume even more excessive amounts of marijuana, searching for the same levels of euphoria they experienced during earlier periods of use.
The research, by Bertha Madras, is published in the journal PNAS and is titled “Dopamine challenge reveals neuroadaptive changes in marijuana abusers.” This research built on earlier work by Nora Volkow (National Institute on Alcohol Abuse and Alcoholism) and colleagues which has also been reported to PNAS (“Decreased dopamine brain reactivity in marijuana abusers is associated with negative emotionality and addiction severity.”)