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The nucleus accumbens (NAcc), also known as the accumbens nucleus or as the
nucleus accumbens septi (Latin for nucleus adjacent to the septum) is a region
of the human brain in the basal forebrain rostral to the preoptic area. The
nucleus accumbens and the olfactory tubercle collectively form the ventral
striatum, which is part of the basal ganglia.
Each brain hemisphere has its own nucleus accumbens. It is located where the
head of the caudate and the anterior portion of the putamen meet just lateral to
the septum pellucidum. The nucleus accumbens can be divided into two structures—the
nucleus accumbens core and the nucleus accumbens shell. These structures have
different morphology and function.
Research has indicated the nucleus accumbens has an important role in reward,
pleasure, reinforcement learning, laughter, addiction, aggression, fear,
impulsivity and the placebo effect.
Cell types
The principal neuronal cell type found in the nucleus accumbens is the medium
spiny neuron. The neurotransmitter produced by these neurons is gamma-aminobutyric
acid (GABA), one of the main inhibitory neurotransmitters of the central nervous
system. These neurons are also the main projection or output neurons of the
nucleus accumbens.
While 95% of the neurons in the nucleus accumbens are medium spiny GABA-ergic
projection neurons, other neuronal types are also found such as large aspiny
cholinergic interneurons.
Output and input
Output
The output neurons of the nucleus accumbens send axon projections to the basal
ganglia and the ventral analog of the globus pallidus, known as the ventral
pallidum (VP). The VP, in turn, projects to the medial dorsal nucleus of the
dorsal thalamus, which projects to the prefrontal cortex as well as the striatum.
Other efferents from the nucleus accumbens include connections with the
substantia nigra, and the pontine reticular formation.
Input
Major inputs to the nucleus accumbens include prefrontal association cortices,
basolateral amygdala, and dopaminergic neurons located in the ventral tegmental
area (VTA), which connect via the mesolimbic pathway. Thus the nucleus accumbens
is often described as one part of a cortico-striato-thalamo-cortical loop.
Dopaminergic input from the VTA is thought to modulate the activity of neurons
within the nucleus accumbens. These terminals are also the site of action of
highly-addictive drugs such as *** and amphetamine, which cause a manifold
increase in dopamine levels in the nucleus accumbens.
Another major source of input comes from the CA1 and ventral subiculum of the
hippocampus to the dorsomedial area of the Nucleus accumbens. The neurons of the
hippocampus have a noteworthy correlation to slight depolarizations of cells in
the nucleus accumbens, which makes them more positive and therefore more
excitable. The correlated cells of these excited states of the medium spiny
neurons in the Nucleus accumbens are shared equally between the subiculum and CA1.
The subiculum neurons are found to hyperpolarize (increase negativity) while the
CA1 neurons "ripple" (fire > 50 Hz) in order to accomplish this priming.
Research
Addiction and drug use
Research using microdialysis has shown that the levels of dopamine in the
extracellular fluid of the nucleus accumbens increase when rats are injected
with addictive drugs such as ***, ***, nicotine, or alcohol. This
increase in dopamine is believed to be responsible for the reinforcing effects
that later stimulate drug-taking behavior. Functional-imaging studies in humans
have shown that environmental cues associated with addictive drugs releases
dopamine in the nucleus accumbens. However, when administered methylphenidate,
drug addicted subjects had a much smaller release of dopamine in this area than
non-addicted subjects. These findings suggest the notion that the nucleus
accumbens is associated with the beginnings of drug addiction and the dorsal
striatum is responsible for the augmentation of the drug habit.
The nucleus accumbens has been targeted by stereotactic surgery for ablation as
a treatment in China for alcoholism.
Pleasure and reinforcement
Although the nucleus accumbens has traditionally been studied for its role in
addiction, it plays an equal role in processing many rewards such as food and
sex. The nucleus accumbens is selectively activated during the perception of
pleasant, emotionally arousing pictures and during mental imagery of pleasant,
emotional scenes. A 2005 study found that it is involved in the
regulation of emotions induced by music, perhaps consequent to its role in
mediating dopamine release. The nucleus accumbens plays a role in rhythmic
timing and is considered to be of central importance to the limbic-motor
interface (Mogensen).
In the 1950s, James Olds and Peter Milner implanted electrodes into the septal
area of the rat and found that the rat chose to press a lever which stimulated
it. It continued to prefer this even over stopping to eat or drink. This
suggests that the area is the "pleasure center" of the brain and is involved in
reinforcement learning. In rats, stimulation of the ventral tegmental area
causes the release of dopamine in the nucleus accumbens much in the same way as
addictive drugs and natural reinforcers, such as water or food, initiate the
release of dopamine in the nucleus accumbens. The same results have been
seen in human subjects in functional imaging studies. For example, increased
dopamine concentration is seen in the extracellular fluid of the nucleus
accumbens when subjects believed they were being given money, and when
heterosexual males were presented pictures of attractive women.
Maternal behavior
An fMRI study conducted in 2005 found that when mother rats were in the presence
of their pups the regions of the brain involved in reinforcement, including the
nucleus accumbens, were highly active. Levels of dopamine increase in the
nucleus accumbens during maternal behavior, while lesions in this area upset
maternal behavior. When human mothers are presented pictures of their
children, fMRIs show an increased brain activity in the nucleus accumbens and
other reinforcing brain regions and a decrease in activity in areas of the brain
involved with negative emotions.
Deep brain stimulation
In April 2007, two research teams reported on having inserted electrodes into
the nucleus accumbens in order to use deep brain stimulation to treat severe
depression. In 2010 experiments reported that deep brain stimulation of the
nucleus accumbens was successful in decreasing depression symptoms in 50% of
patients who did not respond to other treatments such as electroconvulsive
therapy.
Placebo effect
One research team found a correlation between the activation of the NAcc and the
anticipation of effectiveness of a placebo, indicating a central role of the
nucleus accumbens in the placebo effect.