In the brain, pleasure has a universal signature: the release of the neurotransmitter dopamine in a part of the brain called the nucleus accumbens. Dopamine release in this part of the brain is so consistently tied with pleasure that neuroscientists refer to the region as the pleasure center.
What dopamine does once it's released is not fully understood. Scientists used to believe that it alone was responsible for the joy and pleasure that comes with rewarding behaviors. That belief stemmed from studies that linked the amount of dopamine released with the degree of the high that drugs produced. It now appears, however, that dopamine has a much more sophisticated role. While dopamine in the brain might coincide with pleasure, it does not necessarily produce pleasure. Studies of the neural effects of nicotine show, for example, that nicotine causes a surge of dopamine but does not produce euphoria that smokers would consider a high. Meanwhile, events that are unpleasant and stressful also prompt the release of dopamine in the nucleus accumbens. Consequently, dopamine cannot simply be the brain's pleasure switch, though it clearly has an important role in pleasure.
A growing body of evidence suggests that—in at least some contexts—dopamine is the switch for "wanting," rather than "liking," which would explain its ability to reinforce behaviors. Another body of evidence points to a role for dopamine in learning and memory. Those studies suggest that dopamine release allows the brain to compare expected outcomes with actual outcomes. In that scenario, dopamine surges tell the brain that an outcome is "better than expected." Conversely, the interruption of dopamine release tells the brain that an outcome is "worse than expected."