weed grows brain cells

Weed grows brain cells

One synthetic cannibinoid has the same effect on part of the brain as antidepressants. © Punchstock

Expanding the mind

Amelia Eisch
University of Texas Southwestern Medical Centre, Dallas

Most addictive drugs inhibit the growth of new brain cells. But injections of a cannabis-like chemical seem to have the opposite effect in mice, according to new research. Experts say that the results, if borne out by further studies, could have far-reaching implications for addiction research and the application of marijuana in medicine.

They found that giving rats high doses of HU210 twice a day for 10 days increased the rate of nerve cell formation, or neurogenesis, in the hippocampus by about 40%.

Jacobs says it could be that HU210 and THC do not have the same effect on cell growth. It could also be the case that cannabinoids behave differently in different rodent species – which leaves open the question of how they behave in humans.

Just like Prozac?

A synthetic chemical similar to the active ingredient in marijuana makes new cells grow in rat brains. What is more, in rats this cell growth appears to be linked with reducing anxiety and depression. The results suggest that marijuana, or its derivatives, could actually be good for the brain.

When the rats who had received the cannabinoid were placed under stress, they showed fewer signs of anxiety and depression than rats who had not had the treatment. When neurogenesis was halted in these rats using X-rays, this effect disappeared, indicating that the new cell growth might be responsible for the behavioural changes.

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A previous study showed that the antidepressant fluoxetine (Prozac) also increases new cell growth, and the results indicated that it was this cell growth that caused Prozac’s anti-anxiety effect. Zhang wondered whether this was also the case for the cannabinoid, and so he tested the rats for behavioural changes.

Weed grows brain cells

Also, the ability to draw definitive conclusions about marijuana’s long-term impact on the human brain from past studies is often limited by the fact that study participants use multiple substances, and there is often limited data about the participants’ health or mental functioning prior to the study. Over the next decade, the National Institutes of Health is funding the Adolescent Brain Cognitive Development (ABCD) study—a major longitudinal study that will track a large sample of young Americans from late childhood (before first use of drugs) to early adulthood. The study will use neuroimaging and other advanced tools to clarify precisely how and to what extent marijuana and other substances, alone and in combination, affect adolescent brain development.

A large longitudinal study in New Zealand found that persistent marijuana use disorder with frequent use starting in adolescence was associated with a loss of an average of 6 or up to 8 IQ points measured in mid-adulthood. 43 Those who used marijuana heavily as teenagers and quit using as adults did not recover the lost IQ points. People who only began using marijuana heavily in adulthood did not lose IQ points. Two shorter-duration prospective longitudinal twin studies found that youth who used marijuana showed significant declines in verbal ability (equivalent to 4 IQ points) and general knowledge between the preteen years (ages 9 to 12, before use) and late adolescence/early adulthood (ages 17 to 20); however those who went on to use marijuana at older ages already had lower scores on these measures at the start of the study, before they started using the drug. Also, no predictable difference was found between twins when one used marijuana and one did not. 44

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Several studies, including two large longitudinal studies, suggest that marijuana use can cause functional impairment in cognitive abilities but that the degree and/or duration of the impairment depends on the age when a person began using and how much and how long he or she used. 41

Marijuana, Memory, and the Hippocampus

Substantial evidence from animal research and a growing number of studies in humans indicate that marijuana exposure during development can cause long-term or possibly permanent adverse changes in the brain. Rats exposed to THC before birth, soon after birth, or during adolescence show notable problems with specific learning and memory tasks later in life. 32–34 Cognitive impairments in adult rats exposed to THC during adolescence are associated with structural and functional changes in the hippocampus. 35–37 Studies in rats also show that adolescent exposure to THC is associated with an altered reward system, increasing the likelihood that an animal will self-administer other drugs (e.g., heroin) when given an opportunity (see “Is marijuana a gateway drug?”).

Distribution of cannabinoid receptors in the rat brain. Brain image reveals high levels (shown in orange and yellow) of cannabinoid receptors in many areas, including the cortex, hippocampus, cerebellum, and nucleus accumbens (ventral striatum).

Imaging studies of marijuana’s impact on brain structure in humans have shown conflicting results. Some studies suggest regular marijuana use in adolescence is associated with altered connectivity and reduced volume of specific brain regions involved in a broad range of executive functions such as memory, learning, and impulse control compared to people who do not use. 38,39 Other studies have not found significant structural differences between the brains of people who do and do not use the drug. 40

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Memory impairment from marijuana use occurs because THC alters how the hippocampus, a brain area responsible for memory formation, processes information. Most of the evidence supporting this assertion comes from animal studies. For example, rats exposed to THC in utero, soon after birth, or during adolescence, show notable problems with specific learning/memory tasks later in life. Moreover, cognitive impairment in adult rats is associated with structural and functional changes in the hippocampus from THC exposure during adolescence.

More research will be needed to answer definitively whether marijuana use causes long-term IQ losses and whether factors that weren’t measured in the prior research, such as the increasing amounts of THC in cannabis and the emergence of new cannabis products, are relevant.