Age of First Marijuana Use May Have Long-term Effects on Brain Development — Here’s Why

According to a recent article published on ScienceDaily, researchers at the University of Texas say that the age at which adolescents using marijuana can alter the typical course of  brain development, compromising brain structures that are responsible for higher order thinking.

A research team from The University of Texas found that adolescents who begin using marijuana prior to age 16, and those who begin using the drug after that age, suffer different kinds of neurological harms with regular use. They also discovered that the  amount of marijuana ingested over time affects the extent of neurological change that occurs. The Texas study was published in Developmental Cognitive Neuroscience  and is described online in an article posted by ScienceDaily in February 2016.

To understand the importance of this finding, it is helpful to understand some things about the process of cortical “pruning” that occurs as a normal part of brain development. Pruning refers to a process in which unnecessary synaptic connections are eliminated and white matter (myelin) is wrapped around the remaining ones, stabilizing and strengthening them. The pruning process serves to consolidate learning.

An interesting article on pbs.org likens the process of pruning connections between brain cells, which occurs in early childhood and again in adolescence, to the pruning of a tree. That is, “By cutting back weak branches, others flourish.” The first period of pruning, which begins when children are about three, occurs because the brains of babies grow by over-producing connections. Scientists now know that there is a second burst of synaptic formation that occurs in the frontal cortex just before puberty and that this is also followed by a round of pruning during adolescence. Frontline producer Sarah Spinks explains in the article on  the PBS website that this second round of growth and pruning is considered to be a very critical period of development for the brain because the cortex is associated with a variety of important cognitive and emotional processes. These include problem solving, memory, language, judgment, sexual behavior and emotional expression. In fact, the pre-frontal cortex is often called the “chief executive officer of the brain.”

Ms. Spinks cites the work of Dr. Jay Giedd of the National Institute Mental Health, who observes  that what teens do, and do not do during the period of synaptic growth and pruning in adolescence can affect them for the rest of their lives. He calls this the “use it or lose it principle,” and he said on  Frontline’s documentary entitled  “Inside the Teenage Brain”, that, “If a teen is doing music or sports or academics, those are the cells and connections that will be hardwired. If they’re lying on the couch or playing video games or MTV, those are the cells and connections that are going to survive.”

Dr. Nora Volkow, Director of the National Institute on Drug Abuse (NIDA) recently spoke to Medscape Medical News and she, too, highlighted the vulnerability of adolescent brains to the toxic effects of psychoactive substances. Like Dr. Giedd, Dr. Volkow stressed that adolescence is a period in which the brain undergoes a great deal  of “brain programming,” so that adolescent activities have enormous impact on the architecture and connectivity of the adult brain. Dr. Volkow explained that all psychoactive drugs “directly interfere with the process of neural pruning and interregional connectivity”. Dr. Volkow said, in her interview with Mescape, “Young brains and drugs shouldn’t mix”.Period.”

As the ScienceDaily article points out, The University of Texas team knew that successful pruning during adolescence results in “reduced cortical thickness and greater gray and white matter contrast…(and also leads to) increased gyrification, which is the addition of wrinkles or folds on the brain’s surface”. So they used magnetic resonance imaging (MRI) technology to examine the brains of 42 heavy marijuana users. Twenty of their subjects were “early onset users”, who began marijuana use, on average, between the ages of 13-14. The other 22 were “late onset”, and began using, on average, at 16 years 9 months of age. The subjects, who ranged in age betweeen 21-50, all reported beginning marijuana use during adolescence and said that they continued using the substance throughout adulthood, at least one time a week.

MRI results from the Texas study showed that “the more marijuana early onset users consumed, the greater their cortical thickness, the less gray and white matter contrast, and the less intricate the gyrification, as compared to late onset users.” On the other hand, “those who began using marijuana after age 16 showed brain change that would normally manifest later in life: thinner cortical thickness (and) stronger gray and white matter contrast.

The Texas researchers stressed that,

“In the early onset group, we found that how many times an individual uses and the amount of marijuana used strongly relates to the degree to which brain development does not follow the normal pruning pattern. The effects observed were above and beyond effects related to alcohol use and age. These findings are in line with the current literature that suggest that cannabis use during adolescence can have long-term consequences.”

Francesca Filbey, the lead investigator for the Texas study said that future research is planned in order to determine what cognitive and behavioral phenomena may be associated with  brain changes that the researchers observed. Dr. Volkow, in her discussion with the Medscape interviewers said that we already know that when adolescents use psychoactive drugs, the way in which substances interfere with brain development  negatively affects their academic performance . Dr. Volkow stated that, with respect to cannabis , “There is both preclinical and clinical evidence supporting the view (that its use by adolescents is associated with an) ‘amotivational’ state,” characterized by both apathy and poor concentration.  The Medscape article pointed out that any  drug use that begins in adolescence and continues into adulthood “can impair behavioral adaptability, mental health, and life trajectories“.

Click here to read more about the impact of marijuana and other drugs on the brain.

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