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
- M1 receptor
- M2 receptor
- M3 receptor
- M4 receptor
- M5 receptor.
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.