The nucleus accumbens (NAcc) is a region in the brain’s basal forebrain that plays a critical role in the reward circuit. It is involved in pleasure, reinforcement learning, and the processing of motivation and reward. Dysregulation of this area is often implicated in addiction, depression, and other psychiatric disorders.
Functions
Primary Function: The Reward Circuit
The nucleus accumbens (NAcc) is a component of the mesolimbic dopamine pathway, which is stimulated during rewarding experiences.
When someone does something rewarding, such as eating a favorite food or engaging in an enjoyable activity, dopamine neurons in the ventral tegmental area (VTA) are activated.
These VTA neurons project to the NAcc, resulting in increased dopamine levels. This activation tells us to repeat the rewarding experience.
However, research has shown that dopamine levels in the NAcc increase in response to both rewarding and aversive stimuli, making its role more complex than simply processing rewards.
Additional Functions
The NAcc is involved in learning about rewards and the stimuli associated with them. It helps stimulate reward-seeking behaviors while suppressing actions that are less likely to obtain rewards.
Through its connection to the amygdala, it attributes feelings towards rewarding or aversive experiences.
Its connection to the hippocampus helps attribute memories and learning to these experiences.
Studies have shown the NAcc has greater activation for rewarding stimuli when they are unpredictable. The structure also plays crucial roles in:
- Locomotion
- Learning
- Avoidance
- Impulsivity
- Risk-taking behaviors
- Sexual motivation
Due to input from the limbic system and output similarity to the motor nuclei of the basal ganglia, the NAcc functions as an interface between the limbic and motor systems.
Location
The NAcc is situated in the basal forebrain, with one nucleus in each cerebral hemisphere.
It sits at the intersection of the caudate (a neural region involved in learning and memory) and putamen (an area that regulates movement and motor learning), functioning as a specialized part of the striatal complex.
The structure is round but flattened dorsally and consists of two distinct regions: a central core surrounded by a shell.
Each region serves different but complementary functions. The shell maintains stronger connections to the limbic system, processing emotional and motivational information, while the core connects more strongly to the motor system, influencing physical responses to stimuli.
This specialized architecture allows the NAcc to act as a crucial interface between emotional processing and motor control.
The primary cellular components are medium spiny neurons, which can be influenced by environmental factors.
These neurons are particularly responsive to dopamine, a neurotransmitter central to motivation, reward, and motor control.
The NAcc’s position and connections make it a key modulator of information flow between the amygdala, basal ganglia, and prefrontal cortex, allowing it to integrate emotional, motivational, and executive functions.
Conditions associated with the nucleus accumbens
Substance use disorders
Typically, with repeated exposure to a reward, the rewarding feeling will wane. However, with drugs of abuse, the functioning of the reward system can become overwhelming since the effects of these drugs do not wane after repeated exposure.
This can be one of the reasons why people develop substance use disorders. Drugs of abuse tend to increase dopamine in the NAcc or change synaptic plasticity, whereas non-abused drugs generally do not affect dopamine in the NAcc or synaptic plasticity.
The mesolimbic dopamine pathway can become hypofunctional in the addicted brain, resulting in decreased interest in non-drug-related stimuli and increased dependence on the drug to experience rewarding feelings.
Mood disorders
As the NAcc is involved with cognitive and emotional functions, it may be involved in psychiatric conditions such as mood disorders. People with mood disorders have been found to have reduced NAcc activation (Heller et al., 2009).
People with mood disorders were also found to have reduced NAcc volume in comparison to those without mood disorders (Baumann et al., 1999). A study used deep brain stimulation (a therapy for severe depression that proves resistant to other treatment methods) on the NAcc of those with depression.
They found that after 12 months, 50% of the people had significant reductions in their depressive symptoms (Bewernick et al., 2010). This implies that the NAcc may play a role in symptoms of depression.
Anxiety disorders
Due to connections to emotional regions of the brain, specifically the amygdala, the NAcc may be linked to anxiety conditions, which are also thought to be related to emotional regions.
It was found that fear memories and anxiety induced by stress appear to be influenced by mechanisms in the NAcc (Du et al., 2019).
Deep brain stimulation has been applied to the NAcc of people with obsessive-compulsive disorder (OCD), an anxiety disorder, resulting in significant improvements in their symptoms and quality of life (Denys et al., 2010).
This further implies that anxiety disorders may be related to dysfunctions of the NAcc.
Parkinson’s Disease
Apathy, which is associated with a lack of interest and loss of initiative, is a complication of Parkinson’s Disease.
It has been found that these symptoms were associated with atrophy of the left NAcc in people with this disease (Carriere et al., 2014), suggesting that the NAcc may have some involvement with this condition.
Alzheimer’s Disease
The NAcc is believed to be a brain region that is associated with cognitive impairments in people with Alzheimer’s Disease. Alterations of dopaminergic systems are frequently reported in those with Alzheimer’s, and these alterations are commonly linked with cognitive and non-cognitive symptoms.
It has been suggested that the VTA dopaminergic neuron degeneration results in lower dopamine outflow in the NAcc shell and hippocampus. This is said to be linked to impaired memory performance and dysfunctions of reward processing (Nobili et al., 2017).
Tourette Syndrome
The NAcc may also play a role in Tourette Syndrome. A study found that deep brain stimulation treatment has proved to be beneficial for reducing some of the syndrome’s motor manifestations, including alleviating tics and compulsions with the person’s self-injurious behavior (Zabek et al., 2008).
Chronic pain
It has been suggested that those with chronic pain may have smaller NAcc volumes compared to those without chronic pain.
It has been suggested that the smaller NAcc may play a part in the risk of developing chronic pain (Makary et al., 2020).
References
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Carriere, N., Besson, P., Dujardin, K., Duhamel, A., Defebvre, L., Delmaire, C., & Devos, D. (2014). Apathy in Parkinson’s disease is associated with nucleus accumbens atrophy: a magnetic resonance imaging shape analysis. Movement disorders, 29(7), 897-903.
Denys, D., Mantione, M., Figee, M., Van Den Munckhof, P., Koerselman, F., Westenberg, H., Bosch, A. & Schuurman, R. (2010). Deep brain stimulation of the nucleus accumbens for treatment-refractory obsessive-compulsive disorder. Archives of general psychiatry, 67(10), 1061-1068.
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