NMDA Receptors Of The Nervous System: What They Are And What Functions They Have

We know that our neurons communicate with each other through synapses, where neurotransmitters intervene. The main excitatory neurotransmitter in the brain is glutamate, which has different types of receptors. Here we will talk about one of them: NMDA receptors

In this article we will learn what this type of receptors consist of, what characteristics they have, how they work and how they are linked to memory, learning and brain plasticity. However, first we will do a brief introduction about the types of neurotransmitters that exist, to understand where glutamate is located.

What are neurotransmitters and how are they classified?

Neurotransmitters are biomolecules that enable the transmission of information between neurons (that is, neurotransmission), through a chemical or electrical process (depending on the case) called neuronal synapses.

There are many types of neurotransmitters; The most accepted classification is the one that divides them into three large groups:

1. Amines

Amines, in turn, are divided into quaternary amines (acetylcholine) and monoamines (which, in turn, are divided into: catecholamines and indolamines).

2. Amino acids

They include glutamate, GABA, glycine and histamine.

3. Neuropeptides

For their part, neuropeptides include endorphins, enkephalins, dynorphins and vasopressin

Glutamate and its NMDA receptors

As we have seen, glutamate, also called glutamic acid, is an amino acid-type brain neurotransmitter. Glutamate is the brain’s excitatory neurotransmitter par excellence, and is related to multiple functions, especially learning. It is located throughout the brain, and also in the spinal cord.

Like all neurotransmitters, glutamate has different types of receptors, which are structures located in cells (for example in neurons) where neurotransmitters bind, allowing synapsis (which can be electrical or chemical).

To understand it in a simple and, broadly speaking, way, Synapses are those connections between neurons that keep these nerve cells in constant communication and that allow the transmission of information, which makes it possible to achieve different processes: thinking, making decisions, paying attention, reasoning, speaking…

In this way, glutamate has four types of receptors: NMDA receptors (which we will talk about in this article), AMPA receptors, kainate and a type of metabotropic receptor.

NMDA receptors: general characteristics

NMDA receptors are very complex proteins that act as glutamate receptors. At a functional level, NMDA receptors, together with AMPA glutamate receptors, are fundamentally related to two cognitive processes: learning and memory. Specifically, NMDA receptors are essential, above all, for memory. Besides, They are also strongly linked to neuronal or synaptic plasticity

On the other hand, NMDA receptors have also been related to the origin of different pathologies or diseases, such as: epilepsy, certain neurodegenerative diseases (such as Alzheimer’s, Parkinson’s and Huntington’s diseases), schizophrenia or strokes.

Functioning

What do the acronym NMDA mean? They are the acronym for “N-methyl D-aspartate”, which is a selective agonist responsible for specifically binding this type of glutamate receptors, but not others. When these types of receptors are activated, non-selective ion channels open for all kinds of cations (ions with a positive electrical charge).

The receptors are activated by a power differential, when Magnesium ions (Mg2+) come into contact. This step It allows sodium ions (Na+), calcium ions (Ca2+) (these in smaller quantities) and potassium ions (K+) to flow

The flow of calcium ions, specifically, is essential to enhance the processes of synaptic plasticity or brain plasticity. This type of plasticity consists of the fact that external stimuli cause the strengthening of certain synapses, and the weakening of others.

Thus, synaptic, cerebral or neuronal plasticity, allows neurons to function correctly, communicate with each other and modulate their activity depending on the environment and environmental stimuli. In short, it allows the brain to adapt to changes and, in addition, allows its functions to be maximized.

A type of ionotropic receptor

At a structural and functional level, NMDA receptors, also called NMDAr, are ionotropic receptors But let’s back up a bit; There are three types of brain receptors: ionotropic (such as NMDA receptors), metabotropic and autoreceptors. Compared to the other two, ionotropic receptors are faster.

Their main characteristic is that they function as specific ion channels for certain ions, that is, the receptor itself acts as a channel.

Features

NMDA receptors, together with glutamate, are related to a multitude of functions of the nervous system (SN). They are mainly responsible for regulating the postsynaptic excitatory potential of cells Furthermore, as we have seen, NMDA receptors play an essential role in processes such as: neuronal plasticity, memory and learning.

On the other hand, some studies also mention the role played by the binding of glutamate to NMDA receptors in cell emigration processes.

1. Neuronal (or synaptic) plasticity

Neuronal plasticity and its relationship with NMDA receptors has been widely studied. It is known that the activation and consolidation of certain synapses, especially during development (although also in adults), they enable the maturation of the SN circuits, that is, they promote their functional connections.

All this occurs thanks to neuronal plasticity, which largely depends on NMDA receptors.

More specifically, NMDA receptors are activated by a very specific type of synaptic plasticity, called long-term potentiation (LTP). Most memory and learning processes are based on this form of plasticity.

2. Memory

Regarding its link with memory, it has been shown how NMDA receptors play an essential role in the processes that involve the formation of memory; this includes a type of memory called episodic memory (that which allows us to remember lived experiences and that shapes our autobiography).

3. Learning

Finally, NMDA receptors are also linked to learning processes, and it has been seen how their activation occurs in the face of this type of process, which is, in turn, related to memory and brain plasticity.