Dopamine

In the brain, it is secreted by dopaminergic neurons and plays an essential role in movement, motivation, pleasure and reward.

Dopamine is both a neurotransmitter - a chemical substance that acts as a messenger between nerve cells (neurons) in the brain - and a hormone. Synthesised from 3,4-dihydroxyphenylalanine (DOPA), it is produced in dedicated neurons called dopaminergic neurons. The cell bodies of these neurons are located in the brain stem, particularly in the substantia nigra.

Dopamine is released in the synapse, the space between two neurons, allowing nerve impulses to pass from one neuron to the other. After its release, dopamine is rapidly reuptaken by the receptor neurons, which regulates its level in the synaptic space.

Dopamine is a key neurotransmitter in the functioning of the central nervous system, playing a crucial role in motor control and cognitive functions. Balanced levels of dopamine are essential for maintaining optimal neuronal function and good mental health.

What diseases, disorders or symptoms are linked to dopamine levels?

The main mechanisms in which dopamine is involved are :

• Motor control and coordination of voluntary movements: adequate levels of dopamine are necessary to prevent movement disorders such as Parkinson's disease.
• Motivation and reward: when we have a pleasurable or rewarding experience, the brain releases dopamine, creating a sensation of pleasure and satisfaction, hence its name, the pleasure molecule. This reinforces behaviours that lead us to seek out positive experiences. This phenomenon explains its central role in phenomena linked to motivation.
   

The reward circuit is also involved in addictions to tobacco, alcohol, drugs, gambling and so on. Dopamine makes subjects more sensitive to reward and amplifies the prospect of benefits or the desirability of activities. This mechanism is also at the root of the addictions experienced by Parkinson's patients undergoing dopamine treatment.

• Mood regulation: like serotonin, dopamine is also associated with emotional states and mood. Too high a level of dopamine, either through overproduction or too low a level of reuptake, is thought to be at the root of the symptoms associated with schizophrenia.
• Cognitive functions: dopamine plays a role in attention, memory and decision-making. Imbalanced levels of dopamine can lead to motivational disorders, such as the apathy seen in depressive syndromes.

What is the role of dopamine in Parkinson's disease?

Parkinson's disease is characterised by a degeneration of the dopaminergic neurons in the substantia nigra of the brain, located in a particular region of the brain stem. During the course of the disease, a particular protein, a-synuclein, aggregates to form clusters called Lewy bodies in the dopaminergic neurons. The appearance of these Lewy bodies leads to the disappearance of the dopaminergic neurons and a deficit in dopamine production.

The best-known clinical symptoms of the disease are tremors, but the most frequent and characteristic are slowness of movement (akinesia) and muscular rigidity. Patients suffering from progressive supranuclear palsy, a disease also associated with a reduction in dopamine levels, also present these symptoms, although with specific clinical signs.

What role does dopamine play in addiction?

Conversely, high levels of dopamine can be the cause of addiction to alcohol, drugs or gambling. This neurotransmitter is involved in the reward circuit and plays a key role in the sensation of pleasure. Recent data has shown that winning at games or taking drugs increases the release of dopamine in the brain and therefore the feeling of satisfaction. This deregulation of dopamine levels leads gamblers or drug addicts to memorise the association between their behaviour and the pleasure they derive, and thus to become addicted by seeking out this sensation.

In Parkinson's patients, the addition of dopamine to alleviate the symptoms of the disease can lead to the same effects and the same addictive behaviours.