A 38-year-old woman presents to the psychiatry outpatient department with persistent low mood, loss of interest in activities, and sleep disturbance for the past 3 months. She denies any history of mood elevation or psychotic symptoms. On examination, she appears withdrawn and has poor eye contact. Which of the following is the most common neurotransmitter dysfunction implicated in Major Depressive Disorder?
A. Acetylcholine deficiency
B. Serotonin deficiency
C. Glutamate excess
D. Dopamine excess
Explanation
Neurochemical Basis of Major Depressive Disorder
Key Point
The monoamine hypothesis, particularly serotonin deficiency, is the most widely accepted neurochemical model of Major Depressive Disorder and forms the basis of first-line pharmacological treatment.
The Monoamine Hypothesis
The classical understanding of MDD involves dysfunction of three monoamine neurotransmitter systems:
Table
Neurotransmitter
Role in MDD
Clinical Correlate
Serotonin
Most common deficiency; regulates mood, sleep, appetite
Core depressive symptoms
Norepinephrine
Deficiency contributes to anhedonia and fatigue
Loss of motivation and energy
Dopamine
Deficiency linked to anhedonia and psychomotor retardation
Loss of pleasure and motivation
High-YieldNEET PG
Serotonin dysfunction is the most frequently cited and most common neurochemical abnormality in MDD. This is why selective serotonin reuptake inhibitors (SSRIs) are the first-line pharmacological agents.
Evidence for Serotonin Deficiency
1.
Tryptophan depletion studies — acute reduction of serotonin precursor worsens mood in vulnerable individuals
2.
Cerebrospinal fluid (CSF) studies — reduced 5-hydroxyindoleacetic acid (5-HIAA), a serotonin metabolite, in depressed patients
3.
Receptor binding studies — abnormalities in serotonin transporter (SERT) and 5-HT receptors
4.
SSRI efficacy — SSRIs block serotonin reuptake and are the most effective and tolerated first-line agents
Clinical Pearl
While serotonin is the most common dysfunction, MDD is increasingly recognized as a disorder of multiple neurotransmitter systems and neural circuits (prefrontal cortex, amygdala, hippocampus, anterior cingulate). However, for examination purposes, serotonin deficiency remains the "most common" answer.
Dopamine excess — Dopamine is typically deficient (not in excess) in MDD; dopamine agonists may help anhedonia but are not first-line.
Acetylcholine deficiency — While anticholinergic effects can worsen mood, primary acetylcholine dysfunction is not a major mechanism in MDD.
Glutamate excess — Emerging research suggests glutamate dysregulation, but this is a secondary or supplementary mechanism, not the most common primary abnormality.
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