Noradrenaline is a neurotransmitter, but also a hormone. It exerts complex effects on both the activity of the brain itself, and norepinephrine affects many different processes in the body, such as an increase in blood pressure and the stimulation of the breakdown of adipose tissue. Norepinephrine and its influence on individual receptors related to it is so significant that doctors use the knowledge about them - drugs affecting the noradrenergic system are used in the treatment of both hypertension and depression.
Noradrenaline(also known asnorepinephrine ) in the human body is primarily one of the main neurotransmitters in the nervous system, this compound - secreted before the adrenal glands - also plays a role in the body as one of the hormones. The name of this molecule comes from the adrenal glands - the word norepinephrine comes from the Latin term that can be translated as "around the kidneys".
Noradrenaline can be found in the structures of the central nervous system, where it is secreted by the so-called noradrenergic neurons. However, this neurotransmitter also plays an important role in the autonomic system itself - next to adrenaline, norepinephrine is the basic neurotransmitter in the sympathetic nervous system.
Noradrenaline: chemical structure and synthesis
Norepinephrine is classified as one of the catecholamines (monoamines). It is formed in a complex cycle of changes in which the primary substrate is the amino acid tyrosine. The first product created in the complex synthesis of norepinephrine is L-DOPA. This compound in turn produces dopamine, which - in a reaction catalyzed by the enzyme dopamine β-hydroxylase - is transformed into norepinephrine.
Noradrenaline: effects depend on the type of stimulated receptors
It cannot be said that norepinephrine always works the same way. Well, the effects of norepinephrine stimulation of cells are closely dependent on the exact noradrenergic receptor the substance binds to. There are at least five noradrenergic receptors and these are the following receptors:
- α1: receptors found mainly in smooth muscles, the activation of which leads to contraction of this type of muscle cells,
- α2: presynaptic receptors (located onthe presynaptic end of the synapse, i.e. the one that releases neurotransmitters to the synaptic cleft), in which the attachment of noradrenaline to them leads to the inhibition of the further release of noradrenaline or other neurotransmitters from a given presynaptic end,
- β1: the main place where these receptors are located are the cells of the heart muscle, stimulating them leads, among others, to increase the heart rate, but also to increase the contractility of cardiomyocytes,
- β2: receptor present on smooth muscle cells in bronchi, gastrointestinal tract or within blood vessels, stimulation of these receptors leads to muscle relaxation; stimulation of β2 receptors also leads to the activation of the enzyme glycogen phosphorylase, which results in the occurrence of glycogenolysis,
- β3: a type of noradrenergic receptors found mainly in adipose tissue cells, their stimulation by noradrenaline leads to lipolysis (i.e. the breakdown of adipose tissue).
Noradrenaline: Norepinephrine actions in the nervous system
In general, norepinephrine - just like adrenaline - can be treated as one of the basic substances that mobilizes the body to be ready and makes it ready to take on various challenges. However, the functions of norepinephrine in the structures of the nervous system are different than in other organs of the human body.
In the central nervous system, the largest clusters of adrenergic neurons (the nerve cells that produce norepinephrine) are found in the bluish area of the pons in the brain. These neurons, however, direct their nerve endings (axons) to numerous areas of the nervous system where adrenergic receptors are located - the combination of norepinephrine with these receptors leads to the appearance of the effects of this neurotransmitter. From the bluish site, axons are directed to such structures as, for example, the thalamus, amygdala or hypothalamus, the ends of adrenergic neurons are also directed to the cerebral cortex, striatum or centers in the spinal cord.
The effects of norepinephrine on the nervous system are at least several, the most important are the effect of this substance on:
- increasing attention and alertness,
- enhancing the processes of remembering new information, but also fostering recalling previously remembered information,
- improving concentration abilities.
Noradrenaline: action on individual organsof the organism
The reactions that occur in the body under the influence of norepinephrine are basically a typical reflection of the function of the sympathetic nervous system, that is, the part of the autonomic nervous system tasked with mobilizing the body and making it ready to fight or flee. Among the various phenomena that appear due to the stimulation of organs by noradrenaline, the following can be mentioned:
- increase in blood pressure (by constricting blood vessels),
- increase in blood glucose (this occurs through several different mechanisms, the increase in blood glucose is due to the aforementioned increased activity of glycogen phosphorylase, but also due to the fact that the pancreas increases the secretion of glucagon under the influence of noradrenaline) ,
- pupil dilation,
- increasing the release of renin by the kidneys, as well as sodium retention in the body,
- increasing the breakdown of adipose tissue,
- slowing down the peristalsis in the digestive tract and reducing the blood supply to the structures involved in digesting food (redistribution of blood in this case is aimed at transferring it even to the muscles, heart or brain - that is, to those structures that are most important in the case of the need to mobilize the body to act).
Noradrenaline: the use of norepinephrine and the effect on adrenergic receptors in medicine
Noradrenaline itself is sometimes used as a drug, it is indicated mainly in life-threatening conditions. The main indication for the administration of norepinephrine is septic shock. In this unit, as a result of the generalized vasodilation, blood pressure drops, and therefore the administration of noradrenaline to the patient (which, after all, constricts the arterial walls) causes an increase in blood pressure.
In the treatment of various diseases, medicine uses not only norepinephrine itself, but also preparations that affect noradrenergic receptors. Examples include:
- preparations from the beta-mimetic group: these drugs (such as salbutamol or fenoterol) are used, inter alia, in in patients with asthma and their use - by relaxing the muscle cells in the airways - leads to bronchodilation,
- agents from the group of beta-blockers (e.g. metoprolol, bisoprolol): blockers of beta-adrenergic receptors are used, among others, in in patients with arterial hypertension, but also in patients with cardiac arrhythmias (e.g. atrial fibrillation),
- drugs from the group of alpha-blockers (e.g. doxazosin):These agents are used, like beta-blockers, in the treatment of hypertension, but also in patients with prostatic hyperplasia,
- preparations from the alpha-agonists group: drugs stimulating α2-adrenergic receptors (i.e. those receptors whose stimulation leads to a reduction in the release of norepinephrine from nerve cells) can be used in the treatment of hypertension - an example of such a drug is methyldopa, which is one of the basic antihypertensive drugs used in pregnant patients.
Preparations influencing noradrenergic transmission in the nervous system also play an important role in psychiatry. An example of the use of these drugs is the treatment of depression - in patients suffering from this disease, for example, SNRI drugs (serotonin and noradrenaline reuptake inhibitors) are used. Preparations increasing the release of norepinephrine (such as, for example, amphetamine derivatives or methylphenidate) are sometimes used in the case of ADHD, in which the concentration and attention deficits may theoretically be related to noradrenaline deficiencies in the structures of the central nervous system.
About the authorBow. Tomasz NęckiA graduate of medicine at the Medical University of Poznań. An admirer of the Polish sea (most willingly strolling along its shores with headphones in his ears), cats and books. In working with patients, he focuses on always listening to them and spending as much time as they need.Read more articles from this author