sentences:  The study gave six of the twelve "teenage" rats a small dose of THC, reportedly equivalent to one joint smoked by a human, every three days.The rats were allowed to administer heroin by pushing a lever and the study found the rats given THC took larger doses of heroin.The institute examined the brain cells in the rats and found THC alters the opioid system that is associated with positive emotions, which lessens the effects of opiates on rat's brain and thus causes them to use more heroin.