- Cholinergic receptors are specialized protein receptors located on the surface of cells that bind acetylcholine (ACh), a key neurotransmitter in the autonomic and somatic nervous systems.
- Acetylcholine acts on two types of receptors:
- Nicotinic receptors (ionotropic)
- Muscarinic receptors (metabotropic)
Muscarinic Cholinergic receptors
Type
- G-protein-coupled receptors (GPCRs)
Subtypes and Signal Pathways
| Subtype |
G-protein |
Pathway |
Effect |
| M1 |
Gq |
Activates phospholipase C → IP₃ & DAG → ↑Ca²⁺ |
CNS excitation, gastric acid secretion |
| M2 |
Gi |
Inhibits adenylate cyclase → ↓cAMP, opens K⁺ channels |
Slows heart rate (SA node), decreases atrial contraction |
| M3 |
Gq |
Same as M1 |
Smooth muscle contraction, glandular secretion |
| M4, M5 |
Less well understood, mainly in CNS |
|
CNS effects, modulation of dopaminergic pathways |
Distribution and Effects
| Receptor |
Location |
Function |
| M1 |
CNS (brain), gastric glands |
Memory, cognition, gastric acid secretion |
| M2 |
Heart (SA node, atria), CNS |
Decreases heart rate and atrial contractility |
| M3 |
Smooth muscle (GI, bronchi), exocrine glands, eye (iris, ciliary muscle) |
Smooth muscle contraction (e.g., bronchoconstriction), increased secretions, pupil constriction |
| M4/M5 |
CNS |
Modulation of dopamine, learning, memory |
Nicotinic Cholinergic receptors
Type
- Ligand-gated ion channels (ionotropic receptors)
Structure
- Pentameric (5 subunits forming a central pore)
- Binding of ACh opens the channel, allowing Na⁺ and Ca²⁺ influx, causing depolarization.
Subtypes and Distribution
| Subtype |
Location |
Function |
| Nn (Neuronal nicotinic) |
Autonomic ganglia (both sympathetic and parasympathetic), adrenal medulla, CNS |
Mediates transmission in autonomic ganglia; catecholamine release in adrenal medulla |
| Nm (Muscle nicotinic) |
Neuromuscular junction (skeletal muscles) |
Initiates muscle contraction |
Key Features
- Fast-acting
- Mediates excitatory postsynaptic potentials (EPSPs)
- Desensitization occurs with prolonged exposure
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