The pleasure center is the general term used to refer to parts of the brain that deal with pleasure. Some such areas are the nucleus accumbens, the septum pellucidium and the hypothalamus.
The pleasure center was discovered in the 1950s by two brain scientists, James Olds and Peter Milner. They were trying to find out if rats would be made uncomfortable by electrically stimulating certain areas of the brain, particularly the limbic system.
During the experiment, rats were given an electric current if they entered a certain area of the cage. The idea was that if the current was uncomfortable, they would not return.
However, the rats returned again and again. The brain area that was being stimulated was labeled 'the pleasure center.' They were eventually allowed to press the lever that controlled the current themselves.
The rats came back again and again to press the lever--sometimes as often as 700 times per hour. The rats chose the current over eating and drinking and eventually died from exhaustion.