Muscarinic acetylcholine receptor

Muscarinic receptors, or mAChRs, are acetylcholine receptors that form metabotropic ion channels in the plasma membranes of certain neurons. They are called "muscarinic" because they are more sensitive to muscarine than to nicotine. Those for which the contrary is true are known as nicotinic acetylcholine receptors (nAChRs). Many drugs and other substances (for example pilocarpine and scopolamine) act as selective agonists or antagonists of muscarinic or nicotinic receptors, making this distinction useful.

The mAChRs are a type of G protein-coupled receptor (i.e. they use a second messenger) which together with the nAchRs, mediate the actions of acetylcholine in the nervous system.

Pharmacological application

Ligands targeting the mAChR that are currently approved for clinical use include non-selective antagonists for the treatment of Parkinson's disease, atropine (to dilate the pupil), scopolomine (used to prevent motion sickness), and ipratropium (used in the treatment of asthma).

Function

Acetylcholine (ACh) is a neurotransmitter found extensively in the brain and autonomous nervous system. It is also the neurotransmitter used to cause voluntary muscle contraction. Muscarinic receptors are used in the following roles:

Sympathetic and parasympathetic postganglionic: recovery receptors

ACh is always used as the transmitter within the autonomic ganglion. Nicotinic receptors on the postganglionic neuron are responsible for the initial fast depolarization (Fast EPSP) of that neuron. As a consequence of this, nicotinic receptors are often cited as the receptor on the postganglionic neurons at the ganglion. However, the subsequent hyperpolarization (IPSP) and slow depolarization (Slow EPSP) which represent the recovery of the postganglionic neuron from stimulation are actually mediated by muscarinic receptors, types M₂ and M₁ respectively (discussed later).

Presynaptically within the postganglionic neurons

Another role for these receptors is at the junction of the innervated tissue and the postganglionic neuron in the parasympathetic division of the autonomous nervous system. Here acetylcholine is again used as a neurotransmitter, and muscarinic receptors form the principal receptors on the innervated tissue. In addition, muscarinic acetylcholine receptors pre-synaptically on the post-ganglionic neuron bind to the released acetylcholine and regulate the response of the postganglionic neuron.

Between the postganglionic neurons and the innervated tissue

By contrast, this junction in the sympathetic division does not tend to use acetylcholine as a neurotransmitter (instead, norepinephrine is used), and therefore neither muscarinic nor nicotinic receptors are involved, but rather adrenergic α₁ and β₁ receptors. A very few parts of the sympathetic system use cholinergic receptors (sweat glands being one of the few exceptions). In these cases, the receptors are of the muscarinic type. The sympathetic nervous system also has single nerves terminating at the chromaffin cells in the adrenal medulla, which secrete epinephrine and norepinephrine into the bloodstream. Acetylcholine is used as a neurotransmitter, and the receptor is of the nicotinic type. The somatic nervous system uses acetylcholine at the junction between its one peripheral nerve and the innervated tissue, also of the nicotinic type.

In the higher central nervous system

Muscarinic acetylcholine receptors are also present and distributed throughout the central nervous system, in post-synaptic and pre-synaptic positions. There is also some evidence for postsynaptic receptors on sympathetic neurons allowing the parasympathetic nervous system to inhibit sympathetic effects.

On the presynaptic membrane of the neuromuscular junction

It's now known they also appear on the pre-synaptic membrane of somatic neurons in the neuro-muscular junction, where they are involved in the regulation of acetylcholine release.

The form of muscarinic receptors

Muscarinic acetylcholine receptors belong to a class of metabotropic receptors which use G proteins as their signalling mechanism. There are known to be a large number of these G-protein-coupled receptors for neurotransmitters, hormones, and other substances. G proteins are also present in taste, and odour detecting cells, in the retina, and in many other systems.

In such receptors, the signalling molecule (the ligand) binds to a receptor which has seven transmembrane regions, in this case the ligand is ACh. This receptor is bound to intracellular proteins, known as G proteins, which begin the information cascade within the cell.

By contrast nicotinic receptors use an ion-gated mechanism for signalling. Sufficient ligands cause an ion channel to open, filling (or evacuating) a cell of a particular ion.

Receptor isoforms

Classification

By the use of selective radioactively-labelled agonist and antagonist substances, four subtypes of muscarinic receptors have been determined, named M₁-M₄ (using an upper case M and subscript number). 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 has proceeded in numerical order: therefore, sources exist which only recognise the M₁/M₂ distinction, more recent studies tend to recognise M₃, and the most recent M₄.

Genetic differences

Meanwhile, geneticists and molecular biologists have characterised five genes which appear to encode muscarinic receptors, named m1-m5 (lower case m; no subscript number). The first four code for pharmacologic types M₁-M₄. The fifth, m5, corresponds to a subtype of receptor which has not been detected pharmacologically. 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 which 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, G_s, G_i, G_q and G_12/13. 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). G_s and some subtypes of G_i (G_αt and G_αg) are succeptible to CTX. Only G_i is succeptible to PTX, with the exception of one subtype of G_i (G_αz) 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.

Comparison of types

Type	Gene	Function	PTX	CTX	Effectors	Agonists	Antagonists
M₁		EPSP in autonomic ganglia secretion from salivary glands and stomach In CNS (memory?)	no (yes)	no (yes)	G_q (G_i) (G_s): Slow EPSP. ↓ K⁺ conductance	acetylcholine oxotremorine carbachol McNA343	atropine scopolamine dicycloverine tolterodine oxybutynin ipratropium mamba toxin MT7 pirenzepine telenzepine
M₂		slow heart rate reduce contractile forces of atrium reduce conduction velocity of AV node In CNS homotropic inhibition	yes	no	G_i ↑ K⁺ conductance ↓ Ca²⁺ conductance	acetylcholine methacholine carbachol oxotremorine	atropine dicycloverine tolterodine oxybutynin ipratropium methoctramine tripitamine gallamine
M₃		smooth muscle contraction increased endocrine and exocrine gland secretions, e.g. salivary glands and stomach In CNS Eye accommodation vasodilation induce emesis	no	no	G_q	acetylcholine bethanechol carbachol oxotremorine pilocarpine (in eye)	atropine dicycloverine tolterodine oxybutynin ipratropium darifenacin tiotropium
M₄		Enhanced locomotion In CNS	yes	?	G_i ↑ K⁺ conductance ↓ Ca²⁺ conductance	acetylcholine carbachol oxotremorine	atropine dicycloverine tolterodine oxybutynin ipratropium mamba toxin MT3
M₅	CHRM5	In CNS	no	?	G_q	acetylcholine carbachol oxotremorine	atropine dicycloverine tolterodine oxybutynin ipratropium

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 G_q which use upregulation of phospholipase C and therefore inositol trisphosphate and intracellular calcium as a signalling pathway. A receptor so bound would not be susceptible to CTX or PTX. However, G_i (causing a downstream decrease in cAMP) and G_s (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, where they act to slow the heart rate down to normal sinus rhythm after stimulatory actions of the sympathetic nervous system, by slowing the speed of depolarization. They also reduce contractile forces of the atrial cardiac muscle, and reduce conduction velocity of the atrioventricular node (AV node). However, they have no effect on the contractile forces of the ventricular muscle.

M₂ muscarinic receptors act via a G_i type receptor, which causes a decrease in cAMP in the cell, generally 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 G_q-coupled and mediates an increase in intracellular calcium, it typically causes constriction of smooth muscle, such as that observed during bronchoconstriction. 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 vasconstriction in pathologies whereby the vascular endothelium is disrupted.

The M₃ receptors are also located in many glands which help to stimulate secretion in salivary glands and other glands of the body.

Like the M₁ muscarinic receptor, M₃ receptors are G proteins of class G_q which upregulate phospholipase C and therefore inositol trisphosphate and intracellular calcium as a signalling pathway.

M₄ receptor

M₄ receptors are found in the CNS.

Receptors work via G_i receptors to decrease cAMP in the cell and thus produce generally inhibitory effects.

M₅ receptor

Location of M₅ receptors are not well known.

Like the M₁ and M₃ muscarinic receptor, M₅ receptors are coupled with G proteins of class G_q which upregulate phospholipase C and therefore inositol trisphosphate and intracellular calcium as a signalling pathway.