receptors of automatic Ns Mascarinic and Nicotinic

By : Sara Sami Yucuncu Yil University

2015

Cholinergic Receptors (Cholinoceptors)

• Two families of cholinoceptors, designated: 1. muscarinic and 2. nicotinic receptors, can be distinguished from each other on the basis of their different affinities for agents that mimic the action of acetylcholine (cholinomimetic agents or parasympathomimetics).

Types of cholinergic receptors

A. Muscarinic receptors• There are five subclasses of

muscarinic receptors: M1, M2, M3, M4, and M5.

• Only M1, M2 and M3, receptors have been functionally characterized.

• These receptors, in addition to binding acetylcholine, also recognize muscarine.

• Muscarine is an alkaloid that is present in certain poisonous mushrooms.

1. Locations of muscarinic receptors: • These receptors have been found on ganglia of the peripheral

nervous system and on the autonomic effector organs, such as the heart, smooth muscle, brain, and exocrine glands.

Although all five subtypes have been found on neurons, M1 receptors are also found on gastric parietal cells, M2 receptors on cardiac cells and smooth muscle, and M3 receptors on the bladder, exocrine glands, and smooth muscle.

Drugs with muscarinic actions preferentially stimulate muscarinic receptors on these tissues, but at high concentration they may show some activity at nicotinic receptors

When M1 and M3 receptors are activated, the receptor undergoes a conformational change and interacts with a G protein, designated Gq, which in turn activates phospholipase C (PC).1 This leads to the hydrolysis of phosphatidylinositol-(4,5)-bisphosphate-P2 to yield diacylglycerol and inositol (1,4,5)-trisphosphate (formerly called inositol (1,4,5)-triphosphate), which cause an increase in intracellular Ca2+ .

This acation can then interact to stimulate or inhibit enzymes, or cause hyperpolarization, secretion, or contraction.

2.Mechanisms of acetylcholine signal transduction

2.Mechanisms of acetylcholine signal transduction on M2 Receptor:

M2 subtype on the cardiac muscle stimulates a G protein, designated Gi, that inhibits adenylyl cyclase2 and increases K+ conductance, to which the heart responds with a decrease in rate and force of contraction.

Nicotinic Receptor

B. Nicotinic receptorsThe nicotinic receptor is composed of five subunits, and it

functions as a ligand-gated ion channel.

B. Nicotinic receptors.

Binding of two acetylcholine molecules elicits a conformational change that allows the entry of sodium ions, resulting in the depolarization of the effector cell.

• Nicotinic receptors are located in the CNS, adrenal medulla, autonomic ganglia, and the neuromuscular junction. Those at the neuromuscular junction are sometimes designated NM and the others NN.

• The nicotinic receptors of autonomic ganglia differ from those of the neuromuscular junction. For example, ganglionic receptors are selectively blocked by hexamethonium, whereas neuromuscular junction receptors are specifically blocked by tubocurarine.

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