Muscarinic Receptors| Their Effects, Function, Agonists and Antagonists

Muscarinic receptors are the ones present in parasympathetic system.

They are stimulated by acetylcholine a neurotransmitter in the nervous system.

Hence they are called as cholinergic receptors beside nicotinic receptors.

All the muscarnic receptors are G-protein coupled receptor types.

They show their response in few seconds to minutes unlike nicotinic receptors which show response in milliseconds.

Muscarinic Receptors types

There are 5 types of muscarnic receptors. They are divided so based their mechanism of response and also their specific function. The list of muscarnic receptors include

  1. M1 receptor
  2. M2 receptor
  3. M3 receptor
  4. M4 receptor
  5. M5 receptor.
Muscarinic Receptors

Of the five types M1, M2 and M3 are present in large proportion while other two are limited to nerve endings in the brain.

M1 Receptor: This is selectively stimulate by oxytremorine and antagonized by pirenzepine. This is distributed in regions like cortex, hippocampus, striatum of the brain and also in the ganglionic cells.

Functions: Control motor functions, learning and memory, gastric secretion etc. This is mediated by release of histamine and stimulation H2 receptors in the stomach to secrete the acid.

M2 Receptors: These are selectively stimulate by methacholine and antagonized by tripitramine and methoctramine. Present majorly in the heart and few smooth muscles.

Function: They regulate bradycardia i.e. decrease in heart rate. Also responsible for analgesia and tremor in Parkinsonism.

M3 Receptors: These are selectively stimulated by bethanecol and antagonized by solifenacin and darifenacin.

Function: They control the constriction of iris in the eye, blood vessels and also dilation of blood vessels by release of nitric oxide an endothelial relaxation factor.

M4 Receptors: These are located only in the brain and have limited function.

M5 Receptors: Theses also have limited function and are confined to only brain in distribution.

All the muscarnic receptors are G-protein coupled receptors. They have small differences among the mechanism of G-protein function for individual receptor type. Receptor M1 and M2 function by rise in intracellular calcium while M2 functions by influx of potassium.

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