Muscarinic acetylcholine receptor

Muscarinic receptors are those membrane-bound acetylcholine receptors that are more sensitive to muscarine than to nicotine. Those for which the contrary is true are known as nicotinic acetylcholine receptors. Muscarine and nicotine are both alkaloids. Many drugs and other substances (for example pilocarpine and scopolamine) act as agonists or antagonists of only muscarinic or only nicotinic receptors, making this distinction useful.

Uses
Acetylcholine (ACh) is a neurotransmitter extensively found in the brain and autonomic nervous system. 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 depolarisation (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 hyperpolarisation (IPSP) and slow depolarisation (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 autonomic nervous system. Here acetylcholine is again used as a neurotransmitter, and muscarinic receptors form the principal receptors on the innervated tissue. In addition, muscinaric 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, noradrenaline is used), and therefore neither muscarinic nor nicotinic receptors are involved. A very few parts of the sympathetic system (sweating, for example), do use acetylcholine as a neurotransmitter at this position. 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 adrenaline and noradrenaline 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 neuroreceptors, 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 our 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.

Classification strategies
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). 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.

Meanwhile, geneticists and molecular biologists have characterised five genes which appear to encode muscarinic receptors, named m1-m5 (lower case m). 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.

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, Gq and G12. 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 succeptible to CTX. Only G_i is succeptible 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.