The **5-HT4 receptor (5-HT4R)** is a member of the serotonin receptor family. It is a **G protein-coupled receptor (GPCR)** primarily found in the gastrointestinal tract, the central nervous system, and the heart. --- ### 1. Basic Characteristics | Property | Description | | :--- | :--- | | **Type** | G Protein-Coupled Receptor (GPCR) | | **G-Protein Link** | Gs (Stimulatory) | | **Primary Mechanism** | Activation of Adenylyl Cyclase $\rightarrow$ Increase in cAMP | | **Key Locations** | GI Tract (myenteric plexus), Brain (striatum, hippocampus), Heart (atria) | --- ### 2. Signal Transduction Mechanism The "electronic" or biochemical pathway of the 5-HT4 receptor follows a stimulatory cascade: 1. **Ligand Binding:** Serotonin (5-HT) binds to the extracellular loops and transmembrane domains of the receptor. 2. **Conformational Change:** This triggers the exchange of GDP for **GTP** on the alpha subunit of the $G_s$ protein. 3. **Adenylyl Cyclase Activation:** The $\alpha_s$ subunit dissociates and activates the enzyme **Adenylyl Cyclase**. 4. **Second Messenger Production:** ATP is converted into **cyclic AMP (cAMP)**. 5. **Protein Kinase A (PKA):** Rising cAMP levels activate PKA, which phosphorylates various targets (like ion channels), leading to excitatory effects (e.g., muscle contraction in the gut or neurotransmitter release in the brain). --- ### 3. Functional Roles * **Gastrointestinal:** Enhances the **peristaltic reflex** by stimulating the release of acetylcholine. * **Central Nervous System:** Influences **memory and learning** by modulating the release of glutamate and acetylcholine in the hippocampus. * **Cardiovascular:** Can increase heart rate and contractility (atrial) under certain conditions. --- ### 4. Pharmacological Classification Drugs acting on 5-HT4 receptors are categorized by their electronic interaction (affinity and efficacy): | Drug Class | Example | Clinical Use | | :--- | :--- | :--- | | **Agonists** | Prucalopride | Chronic Constipation | | **Partial Agonists** | Tegaserod | IBS-C (Irritable Bowel Syndrome) | | **Antagonists** | Piboserod | Research (Heart failure studies) |