Cannabis Harms Brain, Imaging Shows
In their study, the team compared 11 heavy cannabis users with 12 healthy control subjects, all approximately 28 years of age.
“Our average cannabis user started using in adolescence, at around 16 years, had been using for the previous 11 years, had been dependent before age 21, and in the previous month had been using almost every day,” explained Dr Weinstein.
Cannabis users had no medical or psychiatric comorbidities and used no other drugs, including nicotine, she said.
No relationship between moderate cannabis use and IQ for teenagers - UK study
Recent and extensive research carried out in the UK, has shown no connection between occasional adolescent marijuana use and teenagers’ poorer educational performance.
The long-term study, which has been following the health of children born in the early 1990s, argues that other previous findings - suggesting lower IQ in cannabis users - may have resulted from social habits and lifestyle typically associated with pot abuse, rather than marijuana itself.
“It’s hard to know what causes what - do kids do badly at school because they are smoking weed, or do they smoke weed because they’re doing badly? This study suggests it is not as simple as saying cannabis is the problem,” said Claire Mokrysz of University College London.
However, the study warned that heavier cannabis use by teenagers may lead to “slightly poorer” exam performance.
Another study, also carried out in Britain, has shown the bright side of the matter, saying cannabis could be used to prevent cancer spreading. According to scientists, marijuana’s main psychoactive ingredient - THC, could be useful for shrinking tumors.
After an inpatient abstinence period of 5 to 7 days, the study participants underwent a baseline PET scan with [11C]PHNO, followed by a second scan 3 hours after the oral administration of amphetamine.
Blunts Dopamine Release
Baseline scans did not differ significantly between the two groups, but dopamine release capacity was significantly different after amphetamine administration.
In the striatum as a whole, cannabis users had significantly less dopamine release than the control subjects (18.4% vs 24.9%; P = .002). Dopamine release was also significantly lower in the cannabis users than in the control subjects in the associative striatum (14.6% vs 21.1%; P = .003), the sensory motor striatal subregions (24.6% vs 32.3%; P = .003), and the pallidum (12.9% vs 22.6%; P = .012). However, there was no difference in dopamine release between the groups in the thalamus or midbrain.
“Our study provides definitive evidence that in heavy cannabis users, there is a detectable deficit of striatal dopamine release using an amphetamine challenge,” said Dr Weinstein. “Within the striatum, the subdivisions seem to have a different pattern, in contrast to reports of other substance abuse. And our exploratory analysis suggests that the deficits we are seeing in dopamine release in the striatum have a functional significance - that lower dopamine release is associated with lower working memory and learning performance.”
For schizophrenia, this study suggests “a potential mechanism by which cannabis may be contributing to the risk for psychosis or the severity of psychosis, specifically by interfering with dopamine transmission in one key brain region involved in psychosis -the head of the caudate,” added Dr Abi-Dargham. “If this is repeatedly occurring at a young age, it could result in abnormal salience and could have offsite effects on the circuitry. These comments obviously are a little speculative, but can be further tested now that we have this information,” she pointed out.
“This study is striking in that it provides further evidence that long-term cannabis use is associated with alteration in the function of key brain chemical messengers - in this case dopamine, a key chemical messenger for motivation and attention,” said Oliver Howes, MD, from the Institute of Psychiatry at King’s College London, United Kingdom.
“The blunting of dopamine release that they find fits with other studies showing reduced dopamine synthesis in cannabis users. This could be linked to the addictive potential of cannabis and other problems, such as lack of motivation, seen in regular users,” Dr Howes told Medscape Medical News.
Although “previous studies, including a study from the same group, did not show significant differences in striatal dopamine function between chronic cannabis users and controls, those studies used a more traditional tracer, raclopride, which binds to both dopamine D₂ and D₃ receptors,” said Matthijs Bossong, PhD, from the Brain Center Rudolf Magnus at University Medical Center Utrecht, the Netherlands.
This study “used a specific dopamine D₃ receptor tracer, which suggests that striatal dopamine D₃ receptors are particularly involved in cannabis dependence,” he explained. It also suggests that “[11C]PHNO may actually be a better tracer to measure acute dopamine fluctuations in the striatum. It has a better sensitivity.”
###
Kate Johnson
Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2015 Annual Meeting: Abstract 32. Presented June 7, 2015.