How Heavy Marijuana Use May Hurt the Developing Brain: An Animal Study

Joel Yager, MD

In adolescent monkeys, 6 months of daily intravenous THC impaired spatial working memory, but not object memory, and did not increase tolerance to its acute effects.

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.

Citation(s):

Verrico CD et al. Repeated Δ9-tetrahydrocannabinol exposure in adolescent monkeys: Persistent effects selective for spatial working memory. Am J Psychiatry 2014 Feb 28; [e-pub ahead of print].

Corcoran C.Marijuana and adolescence: What can we learn from primates? Am J Psychiatry 2014 Feb 28; [e-pub ahead of print].