Which neurotransmitter hypothesis of Major Depressive Disorder proposes that depression results from deficiency in monoamines, particularly serotonin, norepinephrine, and dopamine?

Explanation

## The Monoamine Hypothesis of Depression **Key Point:** The **monoamine hypothesis** (also called the catecholamine hypothesis or biogenic amine hypothesis) is the foundational neurochemical model of MDD. It posits that depression arises from insufficient synaptic availability of serotonin (5-HT), norepinephrine (NE), and dopamine (DA). ### Historical Context and Evidence 1. **Discovery:** Emerged in the 1960s when tricyclic antidepressants (TCAs) and monoamine oxidase inhibitors (MAOIs) were observed to increase monoamine levels. 2. **Mechanism:** These drugs block reuptake or inhibit degradation of monoamines, leading to clinical improvement in depression. 3. **Validation:** The hypothesis explains why SSRIs (selective serotonin reuptake inhibitors) are effective — they increase 5-HT availability. ### The Three Key Monoamines in MDD | Neurotransmitter | Brain Region | Function in Mood | Deficit Manifestation | |------------------|--------------|------------------|----------------------| | **Serotonin (5-HT)** | Prefrontal cortex, limbic system | Mood regulation, impulse control | Depressed mood, anxiety | | **Norepinephrine (NE)** | Locus coeruleus, prefrontal cortex | Attention, arousal, motivation | Anhedonia, fatigue, poor concentration | | **Dopamine (DA)** | Ventral tegmental area, nucleus accumbens | Reward, motivation, pleasure | Anhedonia, lack of motivation | **High-Yield:** This hypothesis is the rationale behind all first-line antidepressants (SSRIs, SNRIs, TCAs). It is tested extensively in NEET PG because it underpins pharmacological treatment selection. **Mnemonic:** **SAD** = **S**erotonin, **A**drenaline (norepinephrine), **D**opamine — the three monoamines implicated in depression. **Clinical Pearl:** While the monoamine hypothesis has limitations (e.g., drugs increase monoamines within hours, but clinical improvement takes weeks), it remains the most widely used model for understanding antidepressant action and guiding drug selection. **Warning:** The monoamine hypothesis is not the complete picture — neuroinflammation, HPA axis dysfunction, neuroplasticity, and genetic factors also contribute to MDD pathogenesis. However, for NEET PG purposes, the monoamine model is the primary framework. [cite:Harrison 21e Ch 397]