On this post I will continue writing about neurotransmitters (see this post for a quick explanation of the term neurotransmitter) and today I will focus on dopamine, a neurotransmitter involved in movement, or better, motor control (unfortunately, defects in the dopaminergic systems are well known by Parkinson patients and their families) as well as executive function, motivation, reward and addiction.
Dopamine is produced by dopaminergic neurons in the brain, located mostly in the substantia nigra and ventral tegmental area (VTA) (both located in the midbrain). In the substantia nigra, the dopaminergic neurons play a primary role in the control of motor function and learning motor skills. When the dopaminergic neurons in this pathway degenerate, this causes a dysregulation of motor control, as we know well from Parkinson’s Disease symptoms. In the VTA, the dopaminergic neurons project to many different areas, such as the prefrontal cortex, amygdala (see this post to get an idea of what the role of amygdala is) and hippocampus (the area of the brain involved in memory formation). The dopaminergic projections in the prefrontal cortex, the main part of the brain associated with higher-ordered thinking, help to improve working memory (the memory involved in storing, focusing attention on and manipulating information for a relatively short period of time, such as the carry-over operation in mental arithmetic). In the amygdala, these dopaminergic projections are responsible for emotion formation and processing, while in the hippocampus, the presence of dopaminergic neurons is associated with learning, once more working memory but also long-term memory formation. Dopamine produced from the VTA is released during pleasurable situations, causing arousal and influencing motivations to seek out the pleasurable activity or occupation (see picture below for a schematic of the brain areas producing dopamine and their projections in the brain).
As we can well infer from the dopamine production areas and projections, dopamine is the key force behind most of our actions and interpersonal relationships. When not enough dopamine is produced by the substantia nigra, voluntary motions may become delayed or uncoordinated, which is common in Parkinson’s disease. On the other hand, if there is too much dopamine in the substantia nigra projections, the body will make unnecessary movements, like the repetitive tics that are a common symptom of Gilles de la Tourette’s syndrome.
As I have mentioned above, dopamine affects memory and consequently learning processes and information retaining: when dopamine is present during an event or experience, we will remember it, otherwise, if dopamine is absent, we will not remember a thing. Dopamine is tied to the reward center, so if we do not feel interested in specific activities or learning certain subjects, then dopamine levels will decrease in the prefrontal cortex and we will not feel the motivation to remember the facts presented to us.
Moreover, dopamine is the chemical that regulates how we perceive and experience pleasure. This neurotransmitter is released during pleasurable moments or situations and it causes a person to seek out a desirable activity over and over again. Indeed, eating (especially sugary foods) and having sexual intercourse are stimulants of dopamine release. On the other hand, most addiction-causing drugs work by targeting the dopamine neurotransmitters in the brain: for example, cocaine and amphetamines inhibit the re-uptake of dopamine (a mechanism that controls the amount of the said neurotransmitter available for the brain’s usage so that an excessive amount is not present) in each synapse available. The repetitive exposure to addictive substances causes nerve cells in the nucleus accumbens and the prefrontal cortex (the part of the brain that is used in planning and decision making) communicate with each other. This causes us to associate liking something and wanting it, making us go after it. This is the process that motivates us to search for the cause of pleasure.
This mechanism was originally beneficial for humans, as it helped our ancestors relate pleasure to essential substances and behaviours that they needed to live, such as food and sex. The accumulation of dopamine in the brain makes it continuing to create new dopamine receptors. Over time, the brain adapts and dopamine stops having the same effect as it did originally, which causes the individual to search for more of what it crave, an effect called tolerance.