How Heavy Marijuana Use May Hurt the Developing Brain: An Animal Study
Joel Yager, MD reviewing Verrico CD et al. Am J Psychiatry 2014 Feb 28. Corcoran C. Am J Psychiatry 2014 Feb 28.
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How Heavy Marijuana Use May Hurt the Developing Brain: An Animal Study - NEJM Journal Watch
With legalization of recreational marijuana use accomplished in two states and likely in many more, concerns about how it may affect youth are becoming more prominent than ever. To examine cognitive issues, investigators intravenously administered vehicle or Δ9-tetrahydrocannabinol (THC) at a dose equivalent to one or two marijuana cigarettes 5 days per week for 6 months to seven matched pairs of male adolescent rhesus monkeys.
The monkeys were repeatedly assessed for spatial memory (i.e., recalling the quadrant of a computer touch screen in which a square appeared) and object memory (i.e., recalling the color and shape of an object presented on the screen). With THC, improvement in accuracy (expected because of age and practice effects) was impaired both acutely and chronically on the spatial working memory task, with neither tolerance nor increased sensitivity to the marijuana effects seen over time. Object memory was not affected.
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In rodent research, THC has been shown to decrease developmental neuroplasticity through glutamatergic pathways. The present study clearly demonstrates
acute and persistent effects of chronic marijuana use on developing brains in adolescent monkeys — findings that are consistent with reports of a decline in IQ among human teenagers who persistently smoke marijuana. An editorialist notes that IQ declines might be reversible in even heavy marijuana users who stop early enough in their teens. However, a recent large cohort study from childhood to middle adulthood showed declines in adult IQ with persistent marijuana use that did not reverse with cessation of marijuana, especially if heavy marijuana use began in adolescence (NEJM JW Psychiatry Sep 17 2012). Clinicians, public health officials, and users: Be advised.