The muscarinic receptors

The muscarinic receptors share little similarity in their tertiary structure and physiological function with the nicotinic receptors.

The muscarinic receptors are metabotropic receptors, which means that they are coupled to G proteins and thus belong to the superfamily of G proteincoupled receptors.

The muscarinic receptors are monomers of 440-540 amino acids with seven membrane-spanning domains, the N-terminus residing on the extracellular side and the C-terminus on the intracellular side (Fig. 3.5).

Selective agonists of the muscarinic receptors are substances like arecholine, betanechol, carbachol, etacholine, oxotrimorin and pilocarpin. Selective antagonists are atropine, gallamine, pirenzepin, scopolamine, telenzepin and 4-DAMP.

Based on their pharmacological properties, the muscarinic receptors have been divided into two classes, which are designated Ml and M2. This classifica-

Fig. 3.5 Schematic drawing of a muscarinic acetylcholine receptor (mAChR). In contrast to the nicotinic receptor, this receptor does not form an ion channel. The receptor consists of seven transmembrane-spanning domains and belongs to the G protein-coupled receptor superfamily (GPCRs).

tion corresponds to their different selectivity to the antagonists pirenzepine (M1) and gallamine (M2). Because of the discovery of a competitive cardioselec-tive antagonist, AF-DX 116, the M2 receptors have been subdivided into two further subclasses: M2a andM2^ (or M3) receptors. The different subtypes are also present in the central nervous system of mammals.

The genes of five different subtypes of muscarinic receptors have been identified, cloned and sequenced. The five subtypes are labeled as m1, m2, m3, m4 and m5 (Table 3.1). They differ in their distribution and their signal transduction pathways. The amino acid sequence of the membrane-spanning regions is highly conserved among the five subtypes.

The actylcholine-binding side of the muscarinic receptor consists of a pocket-shaped indentation which is formed by the transmembrane domains and which exhibits a site for allosteric regulation by several compounds.

The binding of acetylcholine to muscarinic receptors activates different signal transduction pathways, depending on the type of muscarinic receptor. The two main pathways are:

• inhibition of adenylate cyclase activity with subsequent reduction of intracellular cAMP levels;

• activation of phosphatidyl inositol to form diacylglycerol and inositol trispho-sphate.

The M1 receptor group (m1, m3, m5) is coupled to a pertussis toxin-insensitive G protein of the Gq family, which activates phospholipase C. Activation of phos-pholipase C leads to the generation of the second messengers diacylglycerol (DAG) and inositol trisphosphate (IP3) from phosphatidylinositol. Inositol tri-

54 I 3 Neurotransmitters Table 3.1 Essential properties and antagonists of the different muscarinergic receptor subtypes.

Properties Subtype

ml

m2

m3

m4

m5

Molecular mass

51240

51715

66127

53 058

60186

(kDa)

Amino acids

460

466

590

479

532

G protein

Gq, Gll

Gi, G0

Gq, G11

G„ Go

Gq, G11

coupling

Effectors

IP3/DAG

cAMP(-)

IP3/DAG

cAMP(-)

IP3/DAG

Tissue

Brain, autonomic

Brain, heart,

Brain, secretory

Brain, lung

?

ganglia, vas

sympatic ganglia,

glands, smooth

deferens, secretory

lung, Uterus,

muscles

glands, sympatic

smooth muscles

ganglia

Brain areas

Cerebral cortex,

Basal forebrain,

Cerebral cortex,

Cortex,

a)

hippocampus,

bulbus olfactorius,

piriform cortex,

hippocampus,

amygdala,

thalamus,

bulbus olfactorius,

olfactory tubercle,

striatum, bulbus

cerebellum,

thalamus,

thalamus, striatum

olfactorius,

brain stem

striatum,

olfactory tubercle,

brain stem

nucleus

accumbens

Subtype-selective

Pirenzepine,

Gallamine,

4-DAMP

Tropicamide,

?

antagonists

telenzepine

himbacine

hexocyclium

a) Only small amounts of the m5 subtype have as yet been discovered and its distribution the CNS is not fully understood.

a) Only small amounts of the m5 subtype have as yet been discovered and its distribution the CNS is not fully understood.

sphosphate initiates the release of calcium from intracellular stores, while DAG activates protein kinase C.

Muscarinic receptors of type M2 (m2, m4) are coupled to Gj proteins. Activation of the M2 group inhibits the activity of adenylate cyclase.

This classification has been extended by recent studies, indicating that additional pathways are involved in signal transduction:

• stimulation of the phospholipase A2 by receptors of type Ml (ml, m3, m5);

• stimulation of the phospholipase D (ml and m3);

• stimulation of adenylate cyclase mediated by the /5-subunit of G proteins;

• inhibition of phosphodiesterases.

Activation of the muscarinic receptors initiates a number of depolarizing and hyperpolarizing currents through direct or indirect mechanisms. These effects include:

• stimulation of the inwardly rectifying potassium conductance by muscarinic receptors of type M2 (m2 and m4);

• inhibition of calcium conductance by muscarinic receptors of type M2 (an effect mediated either directly via G proteins or indirectly via a reduction in cAMP levels);

• activation of calcium-dependent potassium, chloride and cation conductance by muscarinic receptors of type M1 (m1, m3, m5);

• inhibition of voltage and time-dependent potassium conductance (M-current) by m1 and m3.

The muscarinic receptors of type M1 are preferentially expressed in the cortex, the hippocampus (including the dentate gyrus), the nucleus accumbens, the striatum and the amygdala.

Type M2 receptors occur in cholinergic nuclei of the thalamus as well as in the superior colliculus, olfactory bulb and in the brain stem. Muscarinic receptors of type M3 (or the subtype M2/5) are found in the hippocampus and in the cerebral cortex.

In situ hybridization studies revealed that mRNA of m1 is formed in cortical and striatal brain areas and in the amygdala, the hippocampus, the nucleus accumbens, the olfactory bulb and the olfactory tubercle. By the same technique, mRNA of the m2 subtype has been demonstrated in cholinergic neurons of the cortex and some subcortical areas. These data are in accordance with the distribution of muscarinic receptors of type M2.

Messenger RNA of m3 has been found in the cerebral cortex, the piriform cortex, the hippocampus, the thalamus and in the caudate-putamen, while mRNA of m4 was demonstrated in high concentrations in the cortex, the hippocampus, the thalamus, the caudate-putamen complex and the olfactory tubercle. Messenger RNA of m5 is widely distributed throughout the central nervous system, though the expression of the corresponding protein is rather low (Table 3.1).

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