What Is the Reward System of the Brain?
The brain reward system or mesocorticolimbic circuit is a brain circuit that controls addictive behaviors, mood, feeding behaviors, positive emotions (e.g., pleasure), and learning [1].
1. What Activates the Reward System?
The reward system is activated every time we experience something rewarding. For instance, the reward system is triggered when eating something we really enjoy or meeting people we like. The result is a sensation of pleasure and joy.
2. What Part of the Brain Processes Rewards?
There are several parts of the brain that processes rewards, however, the main ones involve the ventral tegmental area (VTA), the nucleus accumbens, the prefrontal cortex, the amygdala, and the hippocampus.
Rewards are controlled by the reward system through the neurotransmitter dopamine.
2.1. What Is the Ventral Tegmental Area (VTA) and Its Role in the Reward System?
The ventral tegmental area (VTA) is a brain structure located in the midbrain that contains the cell bodies of the dopaminergic neurons where dopamine is released.
2.2. What Is the Nucleus Accumbens and Its Role in the Reward System?
The nucleus Accumbens is a brain structure that is part of the ventral striatum in the basal ganglia. It contains nerve cells that are sensitive to dopamine.
The nerve cells of the nucleus accumbens express on their surface the dopamine receptors, the D1-like (D1, and D5) and the D2-like (D2, D3, and D4) family of receptors where dopamine binds. Once activated by dopamine, these cells cause feelings of pleasure.
2.3. What Is the Amygdala and Its Role in the Reward System?
The amygdala is an almond-shaped brain structure located deep in the cerebrum. It is made of a cluster of neurons (Nucleus) responsible for memory including reward-related memory.
For instance, if we had a good experience that led to a feeling of pleasure, this information is stored in the amygdala in the form of desirable memory.
2.5. What Is the Hippocampus and Its Role in the Reward System?
The hippocampus is a brain structure located in the thalamus which is part of the diencephalon (forebrain).
It is made of several clusters of neurons (Nuclei) responsible for coordinating interactions between the nervous system and the endocrine system through the pituitary gland.
Like the amygdala, the hippocampus is also responsible for memory including reward-related memory.
2.6. What Is the Prefrontal Cortex and Its Role in the Reward System?
The prefrontal cortex is the part of the brain that covers up the front of the frontal lobe. It is involved in executive functions such as planning, decision making, and short-term memory.
Within the reward system, the prefrontal cortex coordinates all reward-related information and decides on the type of behavior in response to the information available.
3. What Is Dopamine in the Brain?
Dopamine is a neurotransmitter that is released by dopaminergic neurons to control movement and reward-motivated behavior, such as pleasure, craving, rewards-related decision making, associative learning (learning new things), and short-term memory [2].
Dopamine is synthesized through the metabolism of an essential amino acid known as Phenylalanine. Phenylalanine first generates tyrosine (another amino acid), then tyrosine produces DOPA which finally produces dopamine.
After dopamine synthesis by the dopaminergic neurons, it is transported from the cytoplasm of the neurons all the way to the synapses by a cellular transporter named VMAT2 (Vesicular Monoamine transporter 2), where it is released through vesicles known as synaptic vesicles.
The release of dopamine vesicles in the synaptic cleft is triggered by action potentials, and once released, dopamine binds to the dopamine receptors, the D1-like (D1, and D5) and the D2-like (D2, D3, and D4) family of receptors, found on the surface of dendrites of postsynaptic neurons.
The binding of dopamine to its receptors initiates action potentials in the receiving neurons known as neurotransmission.
After the binding activation, dopamine is recycled back to the presynapse by the dopamine transporter until the next activity.
4. What Are the Disorders of the Reward System?
They are several disorders associated with dysfunctions of the reward system, including addiction, mood disorders, schizophrenia, and attention deficit hyperactivity disorder (ADHD).
4.1. What Is Addiction?
Addiction is a compulsive engagement in rewarding stimuli that have adverse consequences. The affected individual is incapable of stopping using a substance or engaging in a behavior. An example of this is drugs addiction.
4.2. Mood Disorders
Mood disorders are emotional states or moods characterized by inconsistency or swings in expressing emotions such as sadness, irritability, aggressivity, impulsivity, and excessive happiness.
Substance abuse such as alcoholism, cocaine, opioids (e.g., heroin) can cause mood disorders through addiction [3]. For instance, depression can be induced by substance abuse associated with an imbalance of neurotransmitters such as dopamine.
Besides addiction, mood disorders are also characterized by reduced motivation which may be associated with reduced activity of dopamine in the striatum [4].
4.3. Schizophrenia
Schizophrenia is a complex and severe mental health disorder that is classified as a type of psychosis. It is characterized by significant alterations in thoughts, perception, mood, and behaviors [5].
Like mood disorders, schizophrenia is also associated with a lack of motivation which may be due to abnormalities in the orbitofrontal cortex and striatum.
4.4. Attention Deficit Hyperactivity Disorder (ADHD)
Attention deficit hyperactivity disorder (ADHD) is a brain disorder that affects the behavior of affected individuals and is characterized by an impulsive and constant pattern of inattention and hyperactivity.
In ADHD individuals, there are alterations in motivation and reward which may be associated with the levels of dopamine [6].
These levels are high during the hyperactive phase which increases motivation and reward, while during the inattention phase the levels of dopamine decrease resulting in a lack of motivation and reward.
Conclusion
The reward system controls addictive behaviors, mood, feeding behaviors, positive emotions (e.g., pleasure), and learning.
It is a complex system that involves several brain structures, including the ventral tegmental area (VTA), the nucleus accumbens, the prefrontal cortex, the amygdala, and the hippocampus.
The neurotransmission of information with the reward system is ensured by the neurotransmitter, dopamine.
Dopamine is released by the dopaminergic neurons in the ventral tegmental and excites neurons in the nucleus accumbens. Memory related to reward is stored in the amygdala and the hippocampus. Finally, the behavior associated with motivation and reward is coordinated by the prefrontal cortex.
Anomalies of the reward system result in disorders including addiction, mood disorders, schizophrenia, and attention deficit hyperactivity disorder (ADHD).
