Which neurotransmitter hypothesis is most strongly supported by the mechanism of action of selective serotonin reuptake inhibitors (SSRIs) in Major Depressive Disorder?

Explanation

## Monoamine Hypothesis and SSRI Mechanism in MDD **Key Point:** The **monoamine hypothesis** (also called the catecholamine hypothesis) posits that depression results from a deficiency of monoamine neurotransmitters—primarily **serotonin (5-HT)**, **norepinephrine (NE)**, and **dopamine (DA)**. SSRIs selectively block reuptake of serotonin, increasing synaptic 5-HT levels, and are the first-line pharmacological treatment for MDD. ### Monoamine Hypothesis Framework | Neurotransmitter | Role in Depression | SSRI Effect | |---|---|---| | Serotonin (5-HT) | Mood, sleep, appetite regulation | Blocked reuptake → ↑ synaptic 5-HT | | Norepinephrine (NE) | Arousal, attention, motivation | Indirect modulation via 5-HT-NE interactions | | Dopamine (DA) | Reward, motivation, pleasure | Indirect modulation; some SSRIs have weak DA effects | **High-Yield:** The monoamine hypothesis is the most widely taught and tested neurobiological model of depression in NEET PG. SSRIs' efficacy supports this hypothesis, though the complete pathophysiology is multifactorial. ### Why SSRIs Work (Monoamine Model) 1. Block serotonin reuptake transporter (SERT) 2. ↑ Synaptic serotonin concentration 3. Enhanced 5-HT receptor signaling 4. Downstream neuroplasticity and gene expression changes 5. Symptom improvement over 2–4 weeks **Clinical Pearl:** The lag between SSRI initiation (immediate reuptake blockade) and clinical response (2–4 weeks) suggests that downstream adaptive changes—not just acute neurotransmitter elevation—are critical for antidepressant effect. **Mnemonic:** **MAD** = **M**onoamine **A**ntidepressant **D**eficiency hypothesis.