Cholinoceptors

CHOLINOCEPTORS

Two classes of receptors for ACh are recognised —muscarinic and nicotinic; the former is a G protein coupled receptor, while the latter is a ligand gated cation channel.

Muscarinic

These receptors are selectively stimulated by muscarine and blocked by atropine. They are located primarily on autonomic effector cells in heart, blood vessels, eye, smooth muscles and glands of gastrointestinal, respiratory and urinary tracts, sweat glands, etc. and in the CNS. Subsidiary muscarinic receptors are also present in autonomic ganglia where they appear to play a modulatory role by inducing a longlasting late EPSP.

Muscarinic autoreceptors are present prejunctionally on postganglionic cholinergic nerve endings: their activation inhibits further ACh release. Similar ones have been demonstrated on adrenergic terminals: their activation inhibits NA release (may contribute to vasodilator action of injected ACh). All blood vessels have muscarinic receptors (though most of them lack cholinergic innervation) located on endothelial cells whose activation releases EDRF which diffuses to the smooth muscle to cause relaxation.

Subtypes of Muscarinic Receptor

By pharmacological as well as molecular cloning techniques, muscarinic receptors have been divided into 5 subtypes M₁, M₂, M₃, M₄ and M₅. The first 3 are the major subtypes (Table 7.3) that are present on effector cells as well as on prejunctional nerve endings, and are expressed both in peripheral organs as well as in the CNS. The M₄ and M₅ receptors are present mainly on nerve endings in certain areas of the brain and regulate the release of other neurotransmitters. Functionally, M₁, M₃ and M₅ fall in one class while M₂ and M ₄ fall in another class. Muscarinic agonists have shown little subtype selectivity, but antagonists (pirenzepine for M₁, tripitramine for M₂ and darifenacin for M₃) are more selective. Most organs have more than one subtype, but usually one subtype predominates in a given tissue.

M₁: The M₁ is primarily a neuronal receptor located on ganglion cells and central neurones, especially in cortex, hippocampus and corpus striatum. It plays a major role in mediating gastric secretion, relaxation of lower esophageal sphincter (LES) on vagal stimulation and in learning, memory, motor functions, etc.

M₂: Cardiac muscarinic receptors are predominantly M₂ and mediate vagal bradycardia. Autoreceptors on cholinergic nerve endings are also of M₂ subtype. Smooth muscles express some M₂ receptors as well which, like M_3, mediate contraction.

M₃: Visceral smooth muscle contraction and glandular secretions are elicited through M₃ receptors, which also mediate vasodilatation through EDRF release. Together the M₂ and M₃ receptors mediate most of the wellrecognized muscarinic actions including contraction of LES.

The muscarinic receptors are G-protein coupled receptors having the characteristic 7 membrane traversing amino acid sequences. The M₁ and M₃ (also M₅) subtypes function through Gq protein and activate membrane bound phospholipase C (PLc)—generating inositol trisphosphate (IP₃) and diacylglycerol (DAG) which in turn release Ca²⁺ intracellularly—cause depolarization, glandular secretion and raise smooth muscle tone. They also activate phospholipase A₂ resulting in enhanced synthesis and release of prostaglandins and leucotrienes in certain tissues. The M₂ (and M₄) receptor opens K⁺ channels (through βγ subunits of regulatory protein Gi) and inhibits adenylyl cyclase (through α subunit of Gi) resulting in hyperpolarization, reduced pacemaker activity, slowing of conduction and decreased force of contraction in the heart. The M₄ receptor has been implicated in facilitation/ inhibition of transmitter release in certain areas of the brain, while M₅ has been found to facilitate dopamine release and mediate reward behaviour.

Nicotinic

These receptors are selectively activated by nicotine and blocked by tubocurarine or hexamethonium. They are rosettelike pentameric structures (see Fig. 4.4) which enclose a ligand gated cation channel: their activation causes opening of the channel and rapid flow of cations resulting in depolarization and an action potential. On the basis of location and selective agonists and antagonists two subtypes N_M and N_N (previously labelled N₁ and N₂) are recognized (Table 7.4).

N_M: These are present at skeletal muscle endplate: are selectively stimulated by phenyl trimethyl ammonium (PTMA) and blocked by tubocurarine. They mediate skeletal muscle contraction.

N_N: These are present on ganglionic cells (sympathetic as well as parasympathetic), adrenal medullary cells (embryologically derived from the same site as ganglionic cells) and in spinal cord and certain areas of brain. They are selectively stimulated by dimethyl phenyl piperazinium (DMPP), blocked by hexamethonium, and constitute the primary pathway of transmission in ganglia.