Scientists have measured increased dopamine levels in the synapses of the reward pathway in rats self-administering cocaine. Just as they did for heroin, rats will press a bar to receive injections of cocaine directly into areas of the reward pathway such as the nucleus accumbens and the VTA. Again, if the injection needle is placed near these regions (but not in them), the rat will not press the bar to receive the cocaine. The ability of rats to self-administer cocaine is an excellent predictor of the addictive potential of this drug.