Ionotropic Receptor

A receptor is a biological molecule that binds to a ligand and produces effects. Many types of receptors are present in human body. Some receptors are coupled with enzymes. Others are enzymes themselves. Some receptors are linked to some GTP coupled proteins.

All these receptors require some second messenger to carry out their response. In midst of all these receptors is a special type of receptors that do not require any second messenger system. They are the ionotropic receptors.

Ionotropic receptors are coupled with some ion channels. When they bind a ligand, activation of the receptor results in a conformational change, rendering the ion channel open. In other words, they are the ligand-gated ion channels. Binding of the ligand results in the opening of ion channels.

In this article, we will discuss the general structure of an ionotropic receptor. We will also discuss some important types of ionotropic receptors in the body, their location, and clinical significance, along with their role in the body. We will discuss drugs affecting different ionotropic receptors and their clinical significance.

General Structure

The ionotropic receptors are the transmembrane proteins often referred to as ligand-gated ion channels. These are the ion channels that allow the diffusion of ions down the concentration gradient. This diffusion is facilitated by some ligand that binds to the receptor and opens the ion channels.

These receptors are often composed of more than one subunit. The ligand binds to one or more subunits, resulting in a conformational change that opens the ion channels. As a result, ions rush in or out of the cell down their concentration gradient.

Now
we will talk about some important ionotropic receptors present in the body.

Nicotinic Acetylcholine Receptors

These are the ionotropic receptors present in the neuromuscular junction. They are made up of five subunits arranged in the form of staves around a barrel. These are two α, one β, one ɛ, and one δ subunits. The acetylcholine binds to one of the α subunit, which increases the affinity of other α subunit.

When both α subunits are occupied by the acetylcholine, it causes a conformational change that results in the opening of ion channels. These ion channels allow the sodium and potassium ions to diffuse down the concentration gradient. They are therefore called the cation channels. 

Location

They
are present in the postjunctional fold of sarcolemma, a part of neuromuscular
junction.

Function

They cause the excitation of the skeletal muscle. They allow the nerve impulse to be passed from the motor neuron to the skeletal muscle fibers.

Drugs

The activity of nicotinic receptors is affected by cholinergic drugs and neuromuscular blockers.

The
cholinergic drugs enhance the activity by increasing the amount of
acetylcholine available to activate the receptor. These include direct-acting
drugs such as methacholine and indirect-acting drugs such as nerve gas.

Neuromuscular blockers, as the name suggests, block the activity of these receptors. They do so either by antagonizing the receptors or by causing increased depolarization and A1 block.

Clinical Significance

Myasthenia Gravis is an important clinical condition associated with nicotinic receptors. It is an autoimmune disease in which antibodies are formed against the nicotinic receptors. It results in generalized muscle weakness, which most severely affects the muscles of eyes and pharynx.

GABA Receptors

These are important ionotropic receptors present in the CNS. They are the main inhibitory receptors in the CNS. They are made up of five subunits. Binding of GABA neurotransmitter to one of the subunit results in the opening of the ion channels. These receptors are classified into two types on the basis of ionic specificity. These are GABA­_A and GABA_B receptors.

GABA_A Receptors

GABA_A
receptors are linked to Cl^- channels. When a molecule of GABA
binds to this receptor, it causes the opening of the Cl^- channels.
As the concentration of Cl^- ions is higher in the extracellular
fluid, the Cl^- ions flow inside the cell decreasing the membrane
potential, thus causing hyperpolarization.

GABA_B Receptor

Although
these receptors cause opening of K⁺ channels,  GABA_B receptors are linked to
G-proteins and are not purely ionotropic receptors.  

Location

These
receptors are present on the neurons within the brain.

Function

These
are the main inhibitory receptors present in the brain. Activation of these
receptors decreases neuronal firing. They play an important role in calming a
person, inducing sleep and relieving anxiety.

Drugs

Most of the CNS depressants act via GABA receptors. Most of these drugs potentiate the effects of GABA on these receptors.

Barbiturates such as secobarbital, phenobarbital, etc. act on GABA_A receptor and increase the duration of open Cl^- channel.

Benzodiazepines,
used to relieve anxiety and insomnia, also act on GABA_A receptor and
increase the frequency of opening of Cl^- channels.

Alcohol
also potentiate GABA at GABA_A receptors.

Glutamate Receptors

These are excitatory neurotransmitters present in CNS. They are made up of four subunits. Each subunit further consists of three domains; amino-terminal domain, ligand-binding domain, and a transmembrane domain.

The amino-terminal domain is important for the structural stability of the receptor. The ligand-binding domain, as the name suggests, binds the ligand or glutamate neurotransmitter. Both these domains are extracellular domains. The third one is a transmembrane domain that forms the ion channel. Together, all these four subunits form a tetramer.

As the ligand-binding domain is a part of every subunit, only two of these domains are needed to be occupied by the glutamate neurotransmitter in order to open the ion channel.

The ion channel, when open, allows the entry of calcium ions into the cell. This diffusion of positive ions within the cell causes depolarization resulting in action potential. This is the reason why glutamate receptors are excitatory receptors.

Depending
on the type of agonists they bind, glutamate receptors are of two types, AMPA
receptors and NMDA receptors.

Function

The
glutamate receptors are the most commonly found receptors in the brain. They
are the main excitatory receptors of the CNS. The NMDA type receptors are also
responsible for regulating synaptic plasticity and memory.

Drugs

The drugs acting on glutamate receptors can be divided into two categories, agonists and antagonists.

Agonists

The
agonists at the APMA and NMDA receptors are different.

The
agonists at APMA include glutamate, AMPA, 5-Fluorowillardiine, Domoic acid and
Quisqualic acid, etc.

The agonists at NMDA receptors include glutamate, D-cycloserine, glycine, and quinolinate, etc.

Antagonists

The
antagonists at two types of receptors are also different.

The
antagonists at NMDA include ketamine, PCP, tramadol, kynurenic acid, etc.

The
antagonists at the AMPA receptors are Kynurenic acid, Perampanel, etc.

Serotonin 5-HT­­₃ Receptors

These are the only serotonin ionotropic receptors present in the chemoreceptor trigger zone. The bind to endogenous serotonin and cause opening of the ion channels. These ion channels are permeable to sodium, potassium and calcium ions.

The
activation of these channels initiates the process of depolarization and an
action potential is generated. The resultant effect is activation of CTZ
neurons and emesis occurs.

Function

These
receptors trigger the process of emesis.

Drugs

The
clinically significant drugs are the serotonin 5-HT₃ selective antagonist. These include ondansetron,
granisetron, and other “setrons” drugs. These are important anti-emetic drugs.

Clinical Significance

The antagonists of these drugs are used as anti-emetic
drugs. They are commonly used to prevent vomiting in chemotherapy and post-radiotherapy
medication.

ATP Gated Receptors

These are also called P2X receptors. They consist of two transmembrane domains that form the ion channels and an extracellular domain. The extracellular domain acts as a ligand-binding domain.

The binding of ATP to this domain results in the opening of the ion channel. These ion channels allow the entry of calcium channels within the cell. As a result, depolarization occurs, and action potential is generated.

Functions

The ATP or P2X receptors perform several important functions. These include the following;

• Regulation of cardiac rhythm and
  heart rate
• Regulation of blood vessel tone
• Platelet aggregation
• Contraction of Vas Deferens for
  ejaculation
• Contraction of bladder for
  micturition
• Apoptosis

Conclusion/Summary

Ionotropic receptors are actually ligand-gated ion channels. They are the only receptors that do not require a second messenger and show immediate effects. The effects of such receptors are immediate and are very short-lived.

Depending upon their subunits and location, they allow
different ions to diffuse down the concentration gradient. They may be
excitatory or inhibitory receptors.

The important ionotropic receptors are the following.

Nicotinic receptors: They are present in the neuromuscular junction. They are the excitatory receptors responsible for skeletal muscle contraction.

GABA receptors: They are the most common inhibitory receptors present in the CNS. They are responsible for sedation, sleep and anesthesia.

Glutamate receptors: They are the most abundant excitatory receptors in the CNS.

5-HT₃ receptors: These are the ionotropic
serotonin receptors present in the chemoreceptor trigger zone. They are
responsible for causing emesis.

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