Cholinergic receptors

Cholinergic receptors are of two types:

• Muscarinic
• Nicotinic

Muscarine preferentially binds to muscarinic receptors

Nicotine preferentially binds to nicotinic receptors

Acetylcholine binds equally to both.

Muscarinic receptors

All postganglionic parasympathetic neurons synapses with muscarinic receptors.

Subclasses M1, M2, M3, M4, and M5.

Mainly on neurons, but:

M1 also found on gastric parietal cells,

M2 also found on cardiac cells and smooth muscle

M3 also found on exocrine glands and smooth muscle

Drugs with muscarinic activities preferentially stimulate muscarinic receptors, but may show some activity at nicotinic receptors at high concentration.

Nicotinic receptors

Found in:

• CNS
• Adrenal medulla (which synapses with preganglionic sympathetic system)
• Autonomic ganglia
• Neuromuscular junction

The nicotinic receptors in autonomic ganglia differ from those in neuromuscular junction. Thus the drugs blocking one may not block the other.