Preliminary research links habitual marijuana use with inhibited mental development

By Valerie Lapointe

Habitual marijuana use may inhibit the brain from maturing properly, according to on-going research conducted by a team of scientists at Rosalind Franklin University of Medicine and Science in North Chicago.

Kuei Tseng, associate professor of molecular pharmacology,  reported Tuesday on preliminary findings from the research with rats. He stressed the need for continued research at the presentation  hosted by the Chicago Council on Science and Technology.

The onset of schizophrenia and addiction-related syndromes has historically been observed during late adolescence and early adulthood. While the neurodevelopmental process underlying the conditions remain largely unknown, the ongoing studies being done by Tseng and his team at Rosalind Franklin suggest that the habitual use of cannabis may be linked to a three-fold increase in the risk of onset for these conditions.

Tseng was awarded a $1.95 million grant from the National Institutes of Health for his five-year study of “Adolescent Maturation of the Prefrontal Cortex: Modulation by Cannabinoids.” Preliminary research was published in the journal of Molecular Psychiatry in March 2014.

“It’s clear that in teenagers there is an association with smoking marijuana and increased risk of developing schizophrenia. This has been validated across the last 15-20 years,” said Tseng. “But the scientific community is still wondering why this is happening, why is there an age-dependent factor for cannabis.” He added that a certain percentage of people believe that cannabis exposure is related to schizophrenia more as a self-medicating tool for people with this condition.

Tseng and his team have used rats in testing the effects of cannabinoid exposure during narrow age windows that parallel human adolescence and then analyzed the way information is later processed in the adult prefrontal cortex. They discovered that when CB1 receptors are habitually activated by cannabis use during adolescence, the development of the prefrontal cortex stalls.

Due to the inherent ethical issues with human trials, Tseng’s studies have been conducted only with rats so far. Researchers presented rats – some exposed to cannabis and some not – with a tone as an auditory cue.  At the end of the tone, rats receive a light shock – more of a tickle, Tseng said. After one or two trials the animal will freeze at the tone, having learned that there is an association between the sound and the shock, a classic Pavlovian response. Then on day two  they repeat the test with the tone, only this time they do not administer the shock. The next step is for researchers to see how many presentations it takes before a rat stops freezing in preparation for the shock once the shock is no longer administered.

“This is basic fear conditioning,” said Tseng. “We don’t see differences initially but on day two there are differences between rats treated with cannabis and rats not treated with cannabis. With the [control] rats, it takes five or six presentations before the animal realizes there is no longer a shock and stops freezing up. But with the animals that were treated with cannabis they have a lot harder time extinguishing the behavior – the freezing behavior. They have difficulty altering or changing the context of something that was learned the day before.”

What this demonstrates is a decreased cognitive flexibility, an inability to adapt to circumstances, Tseng said. A similar effect is often observed in people suffering from PTSD. With marijuana use – since there is no standard dose or method of ingestion – and THC (tetrahydrocannabinol, the active ingredient in cannabis) concentration varies depending on the strain of the plant, it is difficult for researchers to pinpoint the amount that represents a tipping point in cerebral function impairment.

Tseng’s research suggests that chronic cannabis use by teens may be linked to persistent behavioral deficits in adulthood, including problems with attention span and impulse control. The findings add to prior research that draws a causal correlation between adolescent marijuana use and the risk of developing schizophrenia, he said.

At the monthly speaker series sponsored by the Chicago Council on Science and Technology, audience members raised concerns and questions about his work.

“The main area of discussion when it comes to cannabis these days seems to be legalization,” said Leslie Oldhom, a neuroscience fellow and researcher. “Do you think that in the rush to legalize this substance we’ve jumped over some scientific questions about what health effects we might see from its widespread use in the population?”

Tseng acknowledged that there is still much research to be done and that continued study is the only way to answer the lingering questions involving the potential effects of habitual marijuana use. “Future research will tell us what other mechanisms can be triggered to avoid this type of impairment of the frontal lobe,” Tseng said.

Another audience member voiced concerns over the growing use of medical marijuana therapies for seizure disorders, sometimes in children, and asked if this is a practice to be concerned about.

“Use of THC-related compounds in children should be very carefully and sparingly administered because the brain is still changing, not just in the frontal lobe, but throughout, And there is still so much research to be done. We just don’t know what the long term effects of this could be.”

While the research is far from conclusive Tseng closed his presentation with a word of warning, “Don’t take cannabis before the age of 17, because if you do there is a risk that the development of the frontal lobe will be delayed or impaired.”

Habitual  marijuana use may be linked to mental disorders or inhibited mental development. (Tanjila Ahmed/Flickr)
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