Metabotropic receptors: their characteristics and functions.

They are part of the main components of neurons. We explain how they work.

Nowadays, a large part of the population knows that brain information is transmitted by bioelectrical impulses that travel through bundles of neurons or nerves to their destination, thus enabling both perception and action of the internal and external environment.

This transmission depends on the different neurons being able to establish a connection and transmit either voltage or neurotransmitters to each other, requiring some type of mechanism to detect and integrate these elements in the postsynaptic neuron in order to generate or not a reaction in the form of an action potential (or other types of potential). These elements are called receptors. There are two main types of receptors, and metabotropic receptors are one of the most important and well known of these receptors..

Basic definition: what is a receptor?

The term receptor is often used in a large number of contexts and fields, being physics, electronics or the judicial field some of them. Another of these contexts is neuroscience, being this the one we focus on in the present article.

At the neuron level, we call receptors the set of proteins that form part of the neuronal membrane (or glial membrane, since it has been shown that they also have some receptors) and that act as a means of communication with the neuron. act as a means of communication with the outside of the cell.

These are elements that act as a bridge or lock between the inside and the outside of the neuron, and are activated only upon the arrival of certain receptors. is activated only upon the arrival of certain substances (if controlled by neurotransmitters) or in the presence of certain electrical charges in such a way that they open channels through which ions pass that allow the generation of potentials of different types. They are especially important in the generation of excitatory and inhibitory potentials, which facilitate or inhibit the possibility of the appearance of an action potential, and which ultimately allow neuronal communication and the transmission of information.

There are different types of neurochemical receptors, the two main types being ionotropic and metabotropic receptors. It is on the latter that we will focus in this article.

Metabotropic receptors

Metabotropic receptors are among the main and most relevant types of neurochemical receptors, They are activated upon reception with a specific ligand or neurotransmitter.. These receptors are relatively slow acting, since their activation does not generate an immediate opening of the channel but triggers a series of processes that lead to it.

First of all, the neurotransmitter in question must bind to the receptor, which will activate the G protein, an element that can either open the channel so that certain ions can enter and/or exit or activate other elements, known as second messengers. Thus, the action of these receptors is rather indirect.

Although metabotropic receptors are relatively slower than other types of receptors, their action is also longer lasting. Another advantage of these receptors is that they allow the opening of several channels at the same time, since the second messengers can act in cascade. (generating the activation of different proteins and substances) in such a way that the action of the metabotropic receptors can be more multitudinous and allow more easily the generation of some type of potential.

And not only do they make it possible to open channels: the second messengers can have different actions within the neuron, and can even interact with the nucleus without the need to have opened a channel for this purpose.

Some neurotransmitters with metabotropic receptors

Metabotropic receptors are very common in our nervous systeminteracting with different types of neurotransmitters. Below we are going to mention some more specific examples of neurotransmitters that serve as ligands to some of the metabotropic type receptors present in our body.

1. Acetylcholine and muscarinic receptors

Acetylcholine is one of the substances that possesses a specific type of metabotropic receptors, the so-called muscarinic receptors. This type of receptor can be both excitatory and inhibitory, generating different effects depending on its location and function.

This is the predominant type of cholinergic receptor in the central nervous system, as well as in the parasympathetic branch.as well as in the parasympathetic branch of the autonomic nervous system (linked to the heart, intestines and salivary glands).

However, it should be noted that acetylcholine also has other types of receptors, the nicotinic receptors, which are not metabotropic but ionotropic.

2. Dopamine

Dopamine is another substance with metabotropic receptors. In fact, in this case we find that all dopaminergic receptors are metabotropic, with different types depending on whether their action is excitatory or inhibitory and whether they act pre- or postsynaptically.There are different types depending on whether their action is excitatory or inhibitory and whether they act at the pre- or postsynaptic level.

3. Noradrenaline and adrenaline

As with dopamine, from which it is derived, noradrenaline also has all its channels of metabotropic type. Adrenaline, derived from noradrenaline, as well. They are found both inside and outside the nervous system (e.g. in the adipose tissue) and there are different types depending on whether they are excitatory or inhibitory or whether they act pre- or postsynaptically..

4. Serotonin

Serotonin also has metabotropic receptors, this being the major type. However, the 5-HT3 receptor is ionotropic. They are mostly inhibitory type.

5. Glutamate and the metabotropic receptor

Glutamate is one of the main excitatory substances in the brain.but most of its receptors (and the best known, such as NMDA and AMPA) are ionotropic. Only one type of glutamatergic receptor has been identified that is not ionotropic and is simply called metabotropic glutamate receptor.

6. Gamma-aminobutyric acid or GABA

In contrast to glutamate, GABA is the main brain inhibitor. Two basic receptor types have been identified, GABAb being the metabotropic type.

Bibliographic references:

• Gómez, M.; Espejo-Saavedra, J.M. and Taravillo, B. (2012). Psychobiology. Manual CEDE de Preparación PIR, 12. CEDE: Madrid.
• Kandel, E.R.; Schwartz, J.H.; Jessell, T.M. (2001). Principles of Neuroscience. Madrid: McGrawHill.