What is the Difference Between Ionotropic and Metabotropic Receptors

The main difference between ionotropic and metabotropic receptors is that ionotropic receptors are linked with ion channels whereas metabotropic receptors are coupled with G proteins.

Ionotropic and metabotropic receptors are two types of membrane-bound receptors that are activated by the binding of neurotransmitters. Ionotropic receptors are transmembrane ion channels while metabotropic receptors are G protein-coupled receptors.

Key Areas Covered

1. What are Ionotropic Receptors
   • Definition, Characteristics, Importance
2. What are Metabotropic Receptors
   • Definition, Characteristics, Importance
3. Similarities Between Ionotropic and Metabotropic Receptors
   • Outline of Common Features
4. Difference Between Ionotropic and Metabotropic Receptors
   • Comparison of Key Differences

Key Terms

Ionotropic Receptors, Metabotropic Receptors

What are Ionotropic Receptors

Ionotropic receptors are ion channels that open in response to a conformational change in the receptor upon binding of a chemical messenger (ligand) such as a neurotransmitter. Generally, They are transmembrane proteins with allosteric binding sites: the binding site for the ligand is in a different portion of the receptor. The ion conduction pore occurs in another portion of the receptor. The binding of the ligand causes conformational changes in the receptor, making the ion channel selective to a particular type of ions including Na+, K+, Ca2+, or Cl–.

Figure 1: Inotropic and Metabotropic Receptors

One example of a ligand-gated ion channel is the nicotinic acetylcholine receptor. It typically consists of a pentamer of protein subunits. It also contains two binding sites for acetylcholine, which alter the conformation of the receptor that leads to the opening of an internal pore. Normally, the opening of the pore allows Na+ ions to flow down their electrochemical gradient into the cell. The opening of a sufficient number of channels at once depolarizes the postsynaptic membrane due to the inward flow of positive charges carried by Na+ ions. Generally, this initiates the action potential.

What are Metabotropic Receptors

Metabotropic receptors are a subtype of membrane receptors that use signal transduction mechanisms, often G proteins, to activate a series of intracellular events by second messenger chemicals. Therefore, metabotropic receptors do not form an ion channel pore. Glutamate receptors, muscarinic acetylcholine receptors, and GABAB receptors are examples of metabotropic receptors. In general, metabotropic receptors have seven hydrophobic transmembrane domains. With the binding of the neurotransmitter to the receptor, G-protein undergoes activation and it later activates the secondary messengers. 

Figure 2: G Protein Activation

Moreover, opening channels by metabotropic receptors involves activating a number of molecules in the intracellular mechanism. These receptors take a long time to open in comparison to inotropic receptors. But they have a much longer effect than ionotropic receptors. In general, ionotropic receptors open quickly but only remain open for a few milliseconds.

Similarities Between Ionotropic and Metabotropic Receptors

• Ionotropic and metabotropic receptors are two types of receptors on the cell membrane.
• Neurotransmitters bind and activate these receptors.
• They are post-synaptic receptors and their main function is signal transduction.

Difference Between Ionotropic and Metabotropic Receptors

Definition

Ionotropic receptors refer to the transmembrane ion channels that open upon the binding of a chemical messenger (ligand) such as a neurotransmitter to the allosteric binding site while metabotropic receptors refer to a type of membrane receptors that use signal transduction mechanisms, often G proteins, to activate a series of intracellular events using second messenger chemicals.

Significance

Usually, ionotropic receptors are linked with ion channels while metabotropic receptors are coupled with the G protein.

Examples

Nicotinic acetylcholine receptor is an example of an ionotropic receptor while glutamate receptors, muscarinic acetylcholine receptors, GABAB receptors, most serotonin receptors, and receptors for norepinephrine, epinephrine, histamine, dopamine, neuropeptides, and endocannabinoids are the examples of metabotropic receptors.  

Conclusion

In brief, ionotropic and metabotropic receptors are two types of membrane-bound receptors that are activated upon the binding of ligands such as neurotransmitters. Significantly, ionotropic receptors are linked to an ion channel that is very sensitive to either Na+, K+, Ca2+, or Cl–. In general, metabotropic receptors, on the other hand, are responsible for the activation of G-protein upon binding the ligand to the receptor. This binding involves the activation of a cascade of metabolic pathways that finally activates an ion channel. Therefore, the main difference between ionotropic and metabotropic receptors is the pathway of activation.

References:

1. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Two Families of Postsynaptic Receptors. 

Image Courtesy:

1. “1226 Receptor Types.” By OpenStax – Own work (CC BY 4.0) via Commons Wikimedia
2. “G-Protein” By Llt001 at English Wikibooks – Own Work (CC BY-SA 3.0) via Commons Wikimedia