Catecholamines: types and functions of these neurotransmitters

Dopamine, adrenaline, and norepinephrine, the three main catecholamines, are some of the most relevant neurotransmitters for our nervous system. In this article we will analyze the chemical properties and functions of each of these catecholamines, as well as the common characteristics between the three neurotransmitters.

• Related article: "Types of neurotransmitters: functions and classification"

What are catecholamines?

Catecholamines are a group of neurotransmitters from the class of monoamines, to which tryptamines (serotonin and melatonin), histamine or phenethylamines also belong. Dopamine, adrenaline and noradrenaline they are the three main catecholamines.

At a chemical level, these neurotransmitters are characterized by the presence of a catechol (a compound organic containing a benzene ring and two hydroxyl groups) and an amine in the side chain. They are derived from the amino acid tyrosine, which we obtain through protein-rich foods such as dairy, bananas, avocados or nuts.

The main site of catecholamine synthesis is the chromaffin cells of the adrenal medulla, as well as the postganglionic fibers of the sympathetic nervous system. We will describe in more detail the characteristics of the synthesis of these neurotransmitters in the following sections.

The role of these neurotransmitters is fundamental in processes such as cognition, emotion, memory, and learning, motor control and regulation of the endocrine system. Also the noradrenaline and adrenaline are key players in the stress response.

Increases in catecholamine levels are associated with increased heart rate and glucose levels and with activation of the parasympathetic nervous system. Catecholaminergic dysfunctions can cause alterations in the nervous system, and consequently neuropsychiatric disorders such as psychosis or Parkinson's disease.

The 3 main catecholamines

Dopamine, adrenaline, and norepinephrine are very similar from a chemical point of view, but each of them has distinctive peculiarities that make a detailed description necessary for such from understand the functions of each of these catecholamines.

1. Dopamine

Our body transforms tyrosine into another amino acid, levodopa or L-DOPA, and this in turn becomes dopamine. In turn, dopamine is the most basic catecholamine, and both adrenaline and norepinephrine are made from this neurotransmitter.

When found in the brain, dopamine plays a role as a neurotransmitter; this means that it participates in the sending of electrochemical signals between neurons. Instead, in the blood it functions as a chemical messenger and contributes to vasodilation and the inhibition of the activity of the digestive, immune and pancreas systems.

The brain pathways in which dopamine is involved, mainly the nigrostriatal and mesolimbic, relate to reinforcement-motivated behavior: their levels increase when we get rewards. In this way dopamine is important for processes such as learning, motor control and addictions to psychoactive substances.

Alterations in these two neural pathways cause psychotic symptoms. Positive symptoms such as hallucinations have been linked to dysfunctions in the nigrostriatal pathway (which connects the substantia nigra striatum, a structure of the basal ganglia), and negative ones, such as emotional deficits, with dysfunctions in the mesocortical.

Destruction of dopaminergic neurons in the substantia nigra of the midbrain is the cause of Parkinson's disease. This degenerative neurological disorder is characterized above all by the presence of deficits and alterations of a motor nature, in particular the tremors at rest.

• Related article: "Parkinson's: causes, symptoms, treatment and prevention""

2. Adrenalin

Epinephrine is generated from the oxidation and methylation of dopamine, mainly in the locus coeruleus, located in the brainstem. The synthesis of this neurotransmitter is stimulated by the release of adrenocorticotropic hormone in the sympathetic nervous system.

Adrenaline and noradrenaline, which we will talk about below, are considered the hormones of the stress, since when they act outside the nervous system they do not act as neurotransmitters but as hormones. They are related to cardiac and respiratory regulation and to consumption of body resources to meet environmental challenges.

Both adrenaline and norepinephrine are essential in the response to multiple types of stressors and other processes related to activation of the body, such as physical exercise, exposure to heat and reduction of blood levels of oxygen or glucose.

• You may be interested: "Adrenaline, the hormone that activates us"

3. Noradrenaline

The oxidation of adrenaline gives rise to norepinephrine, in the same way that that of dopamine converts it to adrenaline and that of tyrosine to dopamine. Like adrenaline, it plays the role of a neurotransmitter in the nervous system and a hormone in the rest of the body.

Among the functions of norepinephrine we can highlight brain alertness, maintaining wakefulness, focusing attention, the formation of memories and the appearance of feelings of anxiety, as well as the increase in blood pressure and the release of glucose stores.

The reduction in norepinephrine levels is associated with alterations in different types of learning, particularly the consolidation of long-term memories and latent learning. This function is probably due to the control of neuronal activity by norepinephrine in regions of the brain involved in learning, such as the amygdala.

At the psychopathological level this neurotransmitter is implicated in stress and anxiety disorders, on the major depression, in Parkinson's disease and in attention deficit hyperactivity disorder.

Bibliographic references:

• Kobayashi, K. (2001). Role of catecholamine signaling in brain and nervous system functions: new insights from mouse molecular genetic study. Journal of Investigative Dermatology Symposium Proceedings, 6 (1): 115-21.

• Zouhal, H., Jacob, C., Delamarche, P. & Gratas-Delamarche, A. (2008). Catecholamines and the effects of exercise, training and gender. Sports Medicine, 38 (5): 401-23.