In many animal forms, may it be insects or mammals, a nervous system is present. The reason for such occurrence is to maintain connectivity among various types of tissues and also to response to external stimuli accordingly. A nervous system is built up of nerve cells, nerves, ganglia and many other substituents. Taking in certain messages from inside or outside the body is done by receptors; a sensitive end which excites nerve cells to carry the message and function accordingly. Among many of those receptors, we find Muscarinic receptors and nicotinic receptors. Both these receptors have one thing in common which is the fact that both of them act as Acetylcholine receptors. Depending on the functional mechanism certain differences could be found between the two receptors. Both these receptors are very important as they could be manipulated in drug delivery, acting as selective antagonists and agonists.
Muscarinic receptors commonly noted as mAChRs is a type of acetylcholine receptor. As the name implies, muscarinic receptors are also sensitive to the presence of muscarine. Muscarinic receptors come under the receptor class metabotropic receptors. Metabotropic receptors mean that they use G-proteins as their signaling mechanism. The receptor is located occupying seven transmembrane regions and is connected to intracellular G-proteins at the inside end. When the ligand acetylcholine comes and binds to the receptor G-protein end starts to carry the molecular signaling further to its final destination. The main function of muscarinic receptors is to act as the main end-receptor stimulated by acetylcholine, which is released from the postganglionic fibers in the parasympathetic nervous system.
Nicotinic receptors are commonly noted as nAChRs. It is also a type of acetylcholine receptor. Like muscarinic receptors sensitive to muscarine, nicotinic receptors are sensitive to nicotine. The class of receptors to which nicotinic receptors belong is called ionotropic receptors. Ionotropic receptors have a quite different mechanism compared to metabotropic receptors. These receptors do not use G- proteins. They use gated ion channels. When the ligand acetylcholine or nicotine binds to the gate, the ion channel opens, allowing certain cations (K+ Na+ Ca2+) to diffuse in or out of the cell. Nicotinic receptors bind the neurotransmitter acetylcholine and carryout two main functions. One is to depolarize the plasma membrane, and the other one is to, directly or indirectly, regulate the activity of some genes and release of neurotransmitters.
What is the difference between Muscarinic and Nicotinic Receptors?
• Muscarinic receptors are more sensitive to muscarine while nicotinic receptors are more sensitive to nicotine. However, both are sensitive to acetylcholine.
• Muscarinic receptors belong to the receptor class metabotropic receptors, and nicotinic receptors belong to the receptor class ionotropic receptors.
• Muscarinic receptors use G-proteins and use secondary messengers in the signaling cascade, but nicotinic receptors neither use G- proteins nor the secondary messengers in the signaling cascade.
• Muscarinic receptors do not operate via gated ion channels but via trans-membrane proteins. Nicotinic receptors operate via gated ion channels.
• Muscarinic and nicotinic receptors are found in different locations.