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Neurotransmitter: Types of neurotransmitters (functions and classification)

 Neurotransmitter: Types of neurotransmitters (functions and classification)
What are neurotransmitters and what function do they perform in our brain?

A neurotransmitter ( neuromediator or second messenger ) is a biomolecule allowing neurotransmission , ie the transmission of information from one neuron (a type of cell of the nervous system ) to another neuron, one muscle cell or gland , through the synapses that separates it.

The neurotransmitter is released from the synaptic vesicles at the limb of the presynaptic neuron, towards the synapse, crosses the synaptic space and acts on cellular receptorsspecific to the target cell.

Definition of neurotransmitter

The neurotransmitters are biomolecules that meet the following three criteria:
  • The substance must be present inside the neurons . A chemical cannot be secreted from a presynaptic neuron unless it is present there.
  • Enzymes that allow the synthesis of the substance must be present in the neurons of the area where said neurotransmitter is located . Since complex biochemical pathways are needed to produce neurotransmitters, the demonstration that the enzymes and precursors necessary to synthesize the substance are present in presynaptic neurons provides additional evidence that the substance is used as a neurotransmitter.
  • The effect of the neurotransmitter must be reproduced if the same substance is applied exogenously . A neurotransmitter acts on its white cell, by the presence in these of specific receptors for the neurotransmitter. The effect must be identical (action identity) to that of presynaptic stimulation.

Difference between neurotransmitter and hormone

A neurotransmitter when released only communicates to an immediate neuron, through the synapse. Instead, a hormone communicates with another cell no matter how far it is, traveling through the bloodstream.

Although some neurotransmitters usually act as hormones, they are called neurohormones .

Strictly speaking, according to a definition of hormone , that of Roger Guillemin , a neurotransmitter would be a “hormone” (of paracrine secretion ), released by neurons. Although due to its specific characteristics, the neurotransmitter is often considered a form of cellular communication other than hormones, the distinction between one and the other is diffuse.

A hormone is any substance that is released by a cell acting on another cell, both near and far, and regardless of the uniqueness or ubiquity of its origin and regardless of the route used for transport, whether blood circulation, axoplasmic flow or space interstitial. "Roger Guillemin, Nobel Prize in Medicine 1977"

Types of neurotransmitters: functions and classification

The neurotransmitters are chemicals created by the body that transmit signals (ie, information) from one neuron to the next through contact points called  synapses.

When this occurs, the chemical is released by the vesicles of the pre-synaptic neuron, crosses the synaptic space and acts by changing the action potential in the post-synaptic neuron.

There are different types of neurotransmitters, each with different functions . In fact, the study of this class of substances is essential to understand how the human mind works.

In this article we will review some of the different classes of neurotransmitters, the most significant.

Main neurotransmitters and their functions

The list of known neurotransmitters has been increasing since the 1980s, and today more than 60 have been counted .

This is not strange, considering the complexity and versatility of the human brain. It produces all kinds of mental processes, from the management of emotions to the planning and creation of strategies, through the realization of involuntary movements and the use of language.

All this variety of tasks is behind many neurons coordinating with each other  to make the different parts of the brain work in a coordinated way, and for this it is necessary that they have a communication mode capable of adapting to many situations.

The use of different types of neurotransmitters allows to regulate in many different ways the way in which one or other groups of nerve cells are activated. For example, a certain occasion may require serotonin levels to drop and dopamine levels to rise, and that will have a certain consequence on what happens in our mind. Thus, the existence of the wide variety of neurotransmitters allows the nervous system to have a wide range of behaviors, which is necessary to adapt to a constantly changing environment.

In short, having more neurotransmitters involved in the functioning of the nervous system (and their corresponding receptors in nerve cells) means that there is more variety of possible interactions between groups of neurons. But what are the most important neurotransmitters in the human organism and what functions do they play? The main neurochemicals are mentioned below.

1. Serotonin

This neurotransmitter is synthesized from tryptophan , an amino acid that is not manufactured by the body, so it must be provided through the diet. Serotonin (5-HT) is commonly known as the happiness hormone , because low levels of this substance are associated with depression and obsession.

In addition to its relationship with mood, 5-HT performs different functions within the body, among which stand out: its fundamental role in digestion, control of body temperature, its influence on sexual desire or its role in the sleep-wake cycle regulation.

Too much serotonin can cause a set of symptoms of varying severity.

2. Dopamine

Dopamine is another of the best known neurotransmitters, because it  is involved in addictive behaviors and is the cause of pleasurable sensations . However, among its functions we also find the coordination of certain muscle movements, the regulation of memory, the cognitive processes associated with learning and decision making.

3. Endorphins

Have you noticed that after going for a run or exercising, you feel better, more lively and energetic? Well, this is mainly due to endorphins, a natural drug that is released by our body and that produces a feeling of pleasure and euphoria.

Some of its functions are: promote calm, improve mood, reduce pain , delay the aging process or enhance the functions of the immune system.

4. Adrenaline (epinephrine)

Adrenaline is a neurotransmitter that triggers survival mechanisms , as it is associated with situations in which we have to be alert and activated because it allows us to react in stressful situations.

In short, adrenaline fulfills both physiological functions (such as the regulation of blood pressure or respiratory rate and pupil dilation) and psychological functions (keeping us alert and being more sensitive to any stimulus).

5. Noradrenaline (norepinephrine)

Adrenaline is involved in different functions of the brain and is related to motivation, anger or sexual pleasure. Noradrenaline mismatch is associated with depression and anxiety.

6. Glutamate

Glutamate is the most important excitatory neurotransmitter in the central nervous system . It is especially important for memory and its recovery, and is considered as the main mediator of sensory, motor, cognitive, emotional information. Somehow, it stimulates several mental processes of essential importance.

Research states that this neurotransmitter is present in 80-90% of brain synapses. Excess glutamate is toxic to neurons and is related to diseases such as epilepsy, stroke or amyotrophic lateral disease.

7. GABA

GABA (gamma-aminobutyric acid) acts as an inhibitory messenger, so it slows down the action of excitatory neurotransmitters . It is widely distributed in the neurons of the cortex, and contributes to motor control, vision, regulates anxiety , among other cortical functions.

On the other hand, this is one of the types of neurotransmitters that do not cross the blood brain barrier , so it must be synthesized in the brain. Specifically, it is generated from glutamate.

8. Acetylcholine

Curiously,  e ste is the first neurotransmitter to be discovered . This happened in 1921 and the discovery took place thanks to Otto Loewi, a German biologist who won the Nobel Prize in 1936. Acetylcholine widely distributed by synapses of the central nervous system, but is also found in the peripheral nervous system.

Some of the most outstanding functions of this neurochemist are: it participates in the stimulation of the muscles, in the transition from sleep to wakefulness and in the processes of memory and association .

Classification of neurotransmitters

The types of neurotransmitters can be classified from these categories, each of which includes several substances:

1. Amines

They are neurotransmitters that are derived from different amino acids such as tryptophan. In this group are: Norepinephrine, epinephrine, dopamine or serotonin.

2. Amino acids

Unlike the previous ones (which are derived from different amino acids), these are amino acids. For example: Glutamate, GABA, aspartate or glycine.

3. Purines

Recent research indicates that purines such as ATP or adenosine also act as chemical messengers .

4. Gases

Nitric oxide is the main neurotransmitter in this group.

5. Peptides

Peptides are widely distributed throughout the brain. For example: endorphins, dinorphins and takinins .

6. Esters

Within this group is acetylcholine.

Its operation

It should not be forgotten that, although each type of neurotransmitter can be associated with certain functions in the nervous system (and, therefore, with certain effects at the psychological level), these are not elements with intentions and an objective to follow, so that its repercussions on us are purely circumstantial and depend on the context.

In other words, neurotransmitters have the effects that they have because our body has evolved to make this exchange of substances something that helps us survive, by allowing the coordination of different cells and organs of the body.

Therefore, when we use drugs that emulate the functioning of these neurotransmitters, they often have side effects that may even be the opposite of the expected effect, if they interact abnormally with the substances that are already in our nervous system. The balance that is maintained in the functioning of our brain is somewhat fragile, and neurotransmitters do not learn to adapt their influence on us to fulfill what is supposed to be "their function"; We should worry about that.

On the other hand, reducing the behavior of a human being to the existence of types of neurotransmitters is to fall into the error of excessive reductionism, since the behavior does not arise spontaneously from the brain , but appears from the interaction between the living being and the surroundings.

Biochemical processes associated with neurotransmission

  • Synthesis of the neurotransmitter by presynaptic neurons. Glial cells participate . Depending on the nature of the neurotransmitter, it can be synthesized in the neuronal soma or nerve endings. Some neurotransmitters are synthesized directly in nerve endings thanks to enzymes that have been synthesized in the soma and transported to these endings. Through the interior of the axon flows a stream of free substances or enclosed in vesicles, which can be precursors of both neurotransmitters or their enzymes, called axonic flow.
  • Neurotransmitter storage in vesicles of synaptic termination.
  • Release of the neurotransmitter by exocytosis , which is calcium dependent. When a nerve impulse reaches the presynaptic neuron, it opens the calcium channels , entering the ion into the neuron and releasing the neurotransmitter into the synaptic space. The calcium in addition to initiating exocytosis enable the transfer of the vesicles to the places of his release with the help of plasma membrane proteins and vesicle membrane. When calcium enters the neuron, an enzyme called calmodulin is activated , which is a protein kinase , responsible for phosphorylating synapsin I, located in the membrane of the vesicles and that binds them to the actin filaments . When synapsin I is phosphorylated, synaptic vesicles detach from actin and move to the sites where they must be emptied. The fusion of the vesicular membrane with the plasma membrane is a complex process involving several proteins such as synaptobrevin, synaptotagmin, rab-3 (of the vesicular membrane) syntaxin, SNAP-25, n-sec 1 (of the plasma membrane ) and N-ethylmaleimide (NSF) sensitive factor with ATPase activity . This set of proteins, form the SNARE complex that forms a pore in the plasma membrane and allows the fusion of both membranes and the exit of the substance as the vesicular content to the synaptic space.
  • Activation of the neurotransmitter receptor located in the plasma membrane of the postsynaptic neuron. The postsynaptic receptor is a protein structure that triggers a response. The neuroreceptors can be:
  • Ionotropic receptors: They produce a rapid response by opening or closing ion channels , which produce depolarizations, generating action potentials, excitational responses, produce hyperpolarizations or inhibitory responses. In the first case, monoionic cation channels such as those of Sodium and Potassium act, while in the second case, it is the Chloride channels that are activated.
  • Metabotropic receptors: They release intracellular messengers, such as cyclic AMP , Calcium, and phospholipids by the signal transduction mechanism . These second messengers activate protein kinases, which phosphorylate by activating or deactivating channels inside the cell. In the case of a depolarization, it is the potassium channels that close, in case of hyperpolarization, the same channels are opened producing the increase of intracellular cations.
  • Initiation of the actions of the second messenger .
  • Inactivation of the neurotransmitter , either by chemical degradation or by resorption in the membranes. In the synaptic space, there are specific enzymes that inactivate the neurotransmitter. In addition, presynaptic neurons have receptors for the neurotransmitter that recapture it by introducing it and storing it again in vesicles for later discharge.
  • In the nervous system there are two receptor superfamils for neurotransmitters, depending on the number of transmenbranary regions they have to receive information. There is a selectivity of a family of receptors for a single neurotransmitter that is only possible by connecting to the appropriate membrane.

These two families are:
  • The first family: Share the fact of having seven transmenbrannial regions, using the G protein for and making use of the second messenger (see " G-protein coupled receptor ")
  • The second family: Share the common molecular makeup of each member with five transmenbranary regions and with several versions of each receptor configured around an ion channel .
  • The drugs of cerebral action act in any or some of these stage / s.