- 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 |
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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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