||It has been suggested that Nucleus accumbens core and Nucleus accumbens shell be merged into this article. (Discuss) Proposed since January 2013.|
|Brain: Nucleus accumbens|
Nucleus accumbens visible in red.
|Latin||nucleus accumbens septi|
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 in the basal forebrain rostral to the preoptic area of the hypothalamus. The nucleus accumbens and the olfactory tubercle collectively form the ventral striatum, which is part of the basal ganglia.
Each cerebral 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 pleasure including laughter, reward, and reinforcement learning, as well as fear, aggression, impulsivity, addiction, and the placebo effect.
The nucleus accumbens is an aggregate of neurons which is described as having an outer shell and an inner core.
The core of the NAcc is made up mainly of medium spiny neurons. Compared to the neurons in the shell, those in the core have an increased density of dendritic spines, branch segments, and terminal segments. From the core, the neurons project to other sub-cortical areas such as the globus pallidus and the substantia nigra. GABA is one of the main neurotransmitters in the NAcc, and GABA receptors are also abundant. These neurons are also the main projection or output neurons of the nucleus accumbens.
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 reticular formation of the pons.
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 modulate the activity of neurons within the nucleus accumbens. These neurons are activated directly or indirectly by euphoriant drugs (e.g., amphetamine, opiates, nicotine, etc.) and by participating in rewarding experiences (e.g., sex, music, exercise, etc.).
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.
Chronic drug use and addiction
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 cocaine, heroin, nicotine, or alcohol. This increase in dopamine is believed to be responsible for the reinforcing effects that provoke substance dependency. Functional-imaging studies in humans have shown that environmental cues associated with addictive drugs release dopamine in the nucleus accumbens. However, when given methylphenidate, addicted subjects showed a much smaller release of dopamine in this area than non-addicted subjects. These findings suggest that the nucleus accumbens is associated with the beginnings of substance dependency, and that the dorsal striatum is responsible for reinforcing dependency.
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 increased activation (i.e., increased fMRI BOLD signal-change) was observed among heterosexual males viewing pictures of attractive women.
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. Nucleus accumbens has also been used as a target to treat small groups of patients with therapy-refractory obsessive-compulsive disorder.
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.
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- Nucleus Accumbens
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|Wikimedia Commons has media related to Nucleus accumbens.|
- The role of the nucleus accumbens in the reward circuit. Part of "The Brain From Top to Bottom." at thebrain.mcgill.ca
- Nucleus Accumbens - Cell Centered Database
- Stained brain slice images which include the "nucleus%20accumbens" at the BrainMaps project