Source 1:
Abusing marijuana blunts the brain's ability to respond to dopamine, the neurotransmitter that's responsible for our feelings of pleasure, motivation, and reward. Many abused substances stimulate brain dopamine signaling - the mechanism underlying the rewarding effects of drugs, food, and sex. Both groups produced just as much extra dopamine after taking the drug, but the marijuana abusers had significantly blunted behavioral, cardiovascular, and brain dopamine responses to the stimulant. Unlike cocaine and alcohol abusers, marijuana abusers appear to produce the same amount of dopamine with methylphenidate as people who aren't user's, Science explains, but their brains don't know what to do with it. (iflscience.com)…