Dopamine (neurotransmitter): what it is, functions and characteristics

Brain substances (also called neurotransmitters have an essential role in the body's functions.

One of them is dopamine, known for its involvement in reinforcement systems, in the regulation of memory, in emotions and in the execution of movements.

This substance is also related to schizophrenia; that is why antipsychotics act on it, blocking its receptors. In this article we will know its brain locations, functions, receptors and substances that inhibit or enhance it. In addition, we will see how it is related to some disorders such as ADHD or schizophrenia itself.

• Recommended article: "What is oxytocin and what functions does it have?"

Dopamine: characteristics

Dopamine is a very important brain neurotransmitter, which is related to functions such as the movement (motor functions), executive functions, emotions, motivation and reinforcement.

This brain substance is highly implicated in psychotic disorders, especially schizophrenia, since it has been observed that in these patients dopamine concentrations are higher than normal.

What's more, antipsychotics used to treat these disorders, are mainly based on reducing dopamine levels in the brain (they are dopamine antagonists). This reduction in dopamine has been shown to help alleviate the positive symptoms of schizophrenia (delusions, hallucinations ...).

Location and functions

Dopamine is found in significant amounts in four brain pathways or systems: the nigrostriatal pathway (substantia nigra and basal ganglia), the mesolimbic pathway, the mesocortical pathway and the tuberoinfundibular pathway.

Let's see what functions are related to these four ways or systems:

1. Nigrostriatal system

Within this system (located in the midbrain), dopamine is found primarily in the basal ganglia and substantia nigra areas. In the nigrostriatal system, dopamine has a function related to movement.

On the other hand, it has been observed that in patients with Parkinson's disease, dopamine is deficient in this area. This makes sense, since in Parkinson's disease movement is especially affected (it is its most characteristic symptom).

2. Mesolimbic system

The second location of dopamine is the mesolimbic system, which is found, like the previous one, in the cerebral midbrain. Specifically, in the limbic system and the nucleus accumbens (areas involved in reinforcement and emotions). Thus, in the mesolimbic system, dopamine is especially related to emotions and positive reinforcement; They are areas that are activated when we experience pleasure or pleasant sensations.

This system is involved in the positive symptoms of schizophrenia (the high concentration of dopamine in the mesolimbic has been linked to these symptoms). Remember that positive symptoms include "excess" symptoms, such as hallucinations, disorganized or extravagant behavior, delusions, and so on.

3. Mesocortical system

Dopamine is also found in the mesocortical system, located in the prefrontal midbrain. That is why (its prefrontal location) that the presence of dopamine in said system is related to executive functions: planning, attention, cognition ...

Contrary to the previous one, the mesocortical system is related to the negative symptoms of schizophrenia (abulia, affective flattening, anhedonia, apathy…); that is, the “default” symptoms.

4. Tuberoinfundibular system

The fourth system where we find dopamine is located in the hypothalamus and pituitary gland. (These structures are connected through the infundibulum). Dopamine in the tuberoinfundibular system inhibits prolactin, a hormone related to the secretion of breast milk during pregnancy. That is, dopamine here exerts hormonal control.

When taking antipsychotics (which decrease the concentration of dopamine in the four mentioned pathways), in this specific system prolactin increases, producing side effects such as galactorrhea (secretion of milk in people who are not breastfeeding) and enlargement mammary.

Receivers

Receptors are structures that are found in the cell membrane and that allow the connection of neurotransmitters; that is, they allow the transmission of information, and the increase of certain brain substances.

Generally, drugs (for example antipsychotics, antidepressants ...) act on cell receptors, increasing or inhibiting the secretion of certain substances (depending on whether their mechanism of action is agonist [increase] or antagonist [decrease or inhibition]).

Each type of neurotransmitter has specific receptors; in the case of dopamine, there are two types: presynaptic and postsynaptic. As dopamine receptors we find the D1 and D5 receptors (postsynaptic), and the D2, D3 and D4 receptors (pre or post-synaptic).

The altered receptors in schizophrenia are D2; these are involved in reinforcement and addictions. In schizophrenia, there is an overactivation of these receptors, and an increase in the dopaminergic substance (dopamine). Antipsychotics, as we have mentioned, reduce the concentration of this substance.

Agonists

Agonist substances or drugs increase the concentration of "X" substance in the brain. In other words, it can be said that agonists increase the effect of said substance. Each brain neurotransmitter (such as norepinephrine, serotonin ...) has its own agonist substances. These substances can be natural substances, drugs, drugs ...

In the case of dopamine, we find four main agonist substances (stimulant substances):

1. Apomorphine

Apomorphine, curiously, is a dopamine agonist, but at high doses; at low doses, however, it acts as an antagonist (inhibiting its effect). It is a synthetic derivative of another substance, morphine. Apomorphine is used to treat Parkinson's disease.

2. Amphetamines

Amphetamines are drugs that act on dopamine (DA) and norepinephrine (NA). They are powerful stimulants of the CNS (Central Nervous System), and their mechanism of action is based on reversing the reuptake pumps of these substances; that is, they increase their release and inhibit their reuptake.

3. Cocaine

Another dopamine agonist is cocaine, another known drug, which is extracted from coca leaves (a type of bush), and can also be synthesized in the laboratory. Cocaine works by inhibiting the reuptake of dopamine, causing its levels to increase.

4. Methylphenidate

Finally, methylphenidate, a drug known to be indicated and used in cases of ADHD (Deficiency Disorder). Attention and Hyperactivity), also inhibits dopamine reuptake, increasing its concentration in the brain.

Paradoxically, although methylphenidate is a stimulant, it is a drug that has been shown to improve attention and reduce hyperactivity (and impulsivity) in children with ADHD. In children with ADHD, deficient levels of dopamine have been found in the prefrontal area of ​​the frontal lobe (since it is reuptained very quickly).

Antagonists

In contrast, antagonist substances inhibit the action of substance "X", decrease its concentration or reduce its effect.. The main dopamine antagonists are antipsychotic drugs, which can be classic or typical (first generation) or atypical (second generation).

What antipsychotics do, as we have already mentioned, is to block dopamine D2 receptors, to reduce or inhibit the effect of this substance; that is, they act as antagonists of it.

Antipsychotics are used especially in psychotic disorders, although they also have indications for cases of OCD (Obsessive Compulsive Disorder), chronic pain, movement and tic disorders, agitation, confusion, delirium, alcohol deprivation (alcohol)... The indications will always depend on the type of antipsychotic and its properties.

Bibliographic references

• Carlson, N.R. (2005). Physiology of behavior. Madrid: Pearson Education.

• Netter, F. (1989). Nervous system. Anatomy and physiology. Barcelona: Salvat.

• Stahl, S.M. (2002). Essential psychopharmacology. Neuroscientific bases and clinical applications. Barcelona: Ariel.