"A 2008 study suggests that long-term, heavy cannabis use (over five joints daily for more than ten years) are associated with structural abnormalities in the hippocampus and amygdala areas of the brain. The hippocampus, thought to regulate emotion and memory, and the amygdala, involved with fear and aggression, tended to be smaller in heavy and long term cannabis users than in controls (volume was reduced by an average of 12 percent in the hippocampus and 7.1 percent in the amygdala). The study concluded that "heavy daily cannabis use across protracted periods exerts harmful effects on brain tissue and mental health."
"Cannabinoids inhibit the release of several neurotransmitters in the hippocampus, like acetylcholine, norepinephrine, and glutamate, resulting in a major decrease in neuronal activity in that region."