Classification By the use of selective radioactively labeled agonist and antagonist substances, five subtypes of muscarinic receptors have been determined, named M1–M5 (using an upper case M and subscript number).
M1,
M3,
M5 receptors are coupled with
Gq proteins, while
M2 and
M4 receptors are coupled with Gi/o proteins. There are other classification systems. For example, the drug
pirenzepine is a muscarinic antagonist (decreases the effect of ACh), which is much more potent at M1 receptors than it is at other subtypes. The acceptance of the various subtypes proceeded in numerical order, therefore, earlier sources may recognize only M1 and M2 subtypes, while later studies recognize M3, M4, and most recently M5 subtypes.
Genetic differences Meanwhile,
geneticists and
molecular biologists have characterised five genes that appear to encode muscarinic receptors, named m1-m5 (lowercase m; no subscript number). They code for pharmacologic types M1-M5. The receptors m1 and m2 were determined based upon partial sequencing of M1 and M2 receptor proteins. The others were found by searching for homology, using
bioinformatic techniques.
Difference in G proteins G proteins contain an alpha-subunit that is critical to the functioning of receptors. These subunits can take a number of forms. There are four broad classes of form of G-protein: Gs, Gi, Gq, and G12/13. Muscarinic receptors vary in the G protein to which they are bound, with some correlation according to receptor type. G proteins are also classified according to their susceptibility to
cholera toxin (CTX) and
pertussis toxin (PTX, whooping cough). Gs and some subtypes of Gi (Gαt and Gαg) are susceptible to CTX. Only Gi is susceptible to PTX, with the exception of one subtype of Gi (Gαz) which is immune. Also, only when bound with an agonist, those G proteins normally sensitive to PTX also become susceptible to CTX. The various G-protein subunits act differently upon secondary messengers, upregulating Phospholipases, downregulating cAMP, and so on. Because of the strong correlations to muscarinic receptor type, CTX and PTX are useful experimental tools in investigating these receptors. The muscarinic acetylcholine receptor subtype
selectivities of a large number of antimuscarinic drugs have been reviewed.
M1 receptor This receptor is found mediating slow
EPSP at the ganglion in the postganglionic nerve, is common in
exocrine glands and in the CNS. It is predominantly found bound to G proteins of class
Gq, which use upregulation of
phospholipase C and, therefore,
inositol trisphosphate and intracellular calcium as a signaling pathway. A receptor so bound would not be susceptible to CTX or PTX. However, Gi (causing a downstream decrease in
cAMP) and Gs (causing an increase in cAMP) have also been shown to be involved in interactions in certain tissues, and so would be susceptible to PTX and CTX, respectively.
M2 receptor The M2 muscarinic receptors are located in the heart and lungs. In the heart, they act to slow the
heart rate down below the normal baseline
sinus rhythm, by slowing the speed of
depolarization. In humans, under resting conditions, vagal activity dominates over sympathetic activity. Hence, inhibition of M2 receptors (e.g. by atropine) will cause a raise in heart rate. They also moderately reduce contractile forces of the
atrial cardiac muscle, and reduce conduction velocity of the
atrioventricular node (AV node). It also serves to slightly decrease the contractile forces of the
ventricular muscle. M2 muscarinic receptors act via a
Gi type receptor, which causes a decrease in cAMP in the cell, inhibition of voltage-gated Ca2+ channels, and increasing efflux of K+, in general, leading to inhibitory-type effects.
M3 receptor The M3 muscarinic receptors are located at many places in the body. They are located in the smooth muscles of the blood vessels, as well as in the lungs. Because the M3 receptor is Gq-coupled and mediates an increase in intracellular calcium, it typically causes contraction of smooth muscle, such as that observed during
bronchoconstriction and
bladder voiding. However, with respect to vasculature, activation of M3 on vascular endothelial cells causes increased synthesis of
nitric oxide,
which diffuses to adjacent vascular smooth muscle cells and causes their relaxation, thereby explaining the paradoxical effect of
parasympathomimetics on vascular tone and bronchiolar tone. Indeed, direct stimulation of vascular smooth muscle, M3 mediates vasoconstriction in diseases wherein the vascular endothelium is disrupted. The M3 receptors are also located in many glands, which help to stimulate secretion in, for example, the salivary glands, as well as other glands of the body. Like the M1 muscarinic receptor, M3 receptors are G proteins of class
Gq that upregulate
phospholipase C and, therefore,
inositol trisphosphate and intracellular calcium as a signaling pathway. M4 receptors work via
Gi receptors to decrease cAMP in the cell and, thus, produce generally inhibitory effects. Possible
bronchospasm may result if stimulated by
muscarinic agonists.
M5 receptor Location of M5 receptors is not well known. Like the M1 and M3 muscarinic receptor, M5 receptors are coupled with G proteins of class
Gq that upregulate phospholipase C and, therefore, inositol trisphosphate and intracellular calcium as a signaling pathway. ==Pharmacological application==