The nucleus accumbens (NA) is a small part of the brain that is important for motivation, pleasure, and addiction. Sometimes called the brain's “pleasure center,” this cluster of neurons modulates the effects of the neurotransmitter dopamine, on which many neural circuits depend. The nucleus accumbens is a link in the brain pathways that cause addiction and depression. Damage to this region of the brain causes a lack of motivation and inhibits addictive behavior.
There are two nuclei accumbens, one located in each hemisphere of the brain within the striatum, a subcortical region that helps control planned movement of the body. They are composed of an inner core and outer septum. Both are connected to their respective hemisphere's limbic system, the collection of neuronal groups in the temporal lobe that influences emotions and behavioral motivation. One limbic region, the amygdala, modulates strong emotional reactions and habits. The nucleus accumbens is believed to mediate between the amygdala and the various motor responses that accompany habit formation.
Linked to other other parts of the brain that modulate cycles of reward for behavior, the nucleus accumbens is part of a set of neural pathways that facilitate learning new behaviors by pleasurable reinforcement. When the cerebrum processes something that merits a reward — such as a favorite food or a narcotic — the NA releases dopamine and serotonin. The neurotransmitter dopamine induces a feeling of pleasure, while serotonin is a calming influence. A nearby set of nerve fibers, the ventral tegmental area, produces dopamine and sends it to the nucleus accumbens.
Laboratory studies point to a relationship between addiction and the release of seratonin and dopamine in the nucleus accumbens. The surge of these neurotransmitters triggers neural activity correlating with the addict's high and the sensation of reward on which addiction depends. When people crave a substance, neural activity increases in anticipation of future pleasure. Surgical destruction of this region of the brain in drug-addicted rats caused the animals to lose their interest in drugs. A side effect was that the rats also lost general behavioral motivation.
Historically, research on motivated behavior and drug addiction in rodents was the primary source of knowledge about the nucleus accumbens. Other than some isolated research in the 1950s and 1970s, few scientists were able to test the role of the NA in affecting human moods. But in 2005 and 2007, neurosurgeons working on deep brain stimulation for Parkinson's disease offered evidence that depression was improved when electrical pulses were applied to the NA. Surgical data suggested electrical stimulation of the region enhanced the function of dopamine and might relieve major depression.