 It seemed that stimulation of those parts of the brain gave the animals pleasure, and in later work humans reported pleasurable sensations from such stimulation.When rats were tested in Skinner boxes where they could stimulate the reward system by pressing a lever, the rats pressed for hours.Research in the next two decades established that dopamine is one of the main chemicals aiding neural signaling in these regions, and dopamine was suggested to be the brain's "pleasure chemical".