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Antidepressants: From Basics to Research

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PART 1 - UNDERSTANDING DEPRESSION (The "Why" Before the "What")

What is Depression?

• Depressed mood most of the day
• Loss of interest or pleasure (anhedonia)
• Sleep disturbances (insomnia or hypersomnia)
• Weight/appetite changes
• Psychomotor agitation or retardation
• Fatigue or loss of energy
• Feelings of guilt or worthlessness
• Executive dysfunction / poor concentration
• Suicidal ideation

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PART 2 - PATHOPHYSIOLOGY (The Neuroscience)

The Monoamine Hypothesis

• Reserpine (depletes monoamines) causes depression in a subset of patients
• Tryptophan-free diets cause rapid relapse in patients responding to serotonergic antidepressants
• A functional polymorphism in the SERT gene promoter (short "s" allele) increases vulnerability to depression under stress and reduces SSRI response
• All currently available antidepressants enhance synaptic monoamine availability - this is the strongest indirect evidence

Beyond Monoamines

• Monoamine changes happen within hours, but antidepressant effects take 2-4 weeks - suggesting downstream changes matter more
• The neurotrophic hypothesis: antidepressants upregulate BDNF (Brain-Derived Neurotrophic Factor), which promotes neuroplasticity and neurogenesis in the hippocampus
• The neuroplasticity/neuroprogression hypothesis (Stahl's): depression is associated with progressive structural changes in the brain; effective treatment may halt or reverse this
• The NMDA/glutamate hypothesis (newer): ketamine's rapid antidepressant effect via NMDA receptor blockade points to glutamate as a critical player

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PART 3 - CLASSIFICATION OF ANTIDEPRESSANTS

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CLASS 1: SSRIs - Selective Serotonin Reuptake Inhibitors

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CLASS 2: SNRIs - Serotonin-Norepinephrine Reuptake Inhibitors

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CLASS 3: TCAs - Tricyclic Antidepressants

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CLASS 4: MAOIs - Monoamine Oxidase Inhibitors

• Phenelzine - irreversible, non-selective (MAO-A + B); oral
• Tranylcypromine - irreversible, non-selective; oral; stimulant-like
• Isocarboxazid - irreversible, non-selective; oral
• Selegiline - selective MAO-B at low doses, becomes non-selective at antidepressant doses; available as transdermal patch (avoids dietary tyramine restriction at lowest dose)

• Tyramine crisis ("cheese reaction"): Normally, gut MAO-A breaks down tyramine in aged cheese, wine, cured meats. MAOIs block this → tyramine floods circulation → hypertensive crisis. Patients must follow a tyramine-restricted diet.
• Serotonin syndrome: Cannot combine with any serotonergic drug (SSRIs, SNRIs, TCAs, triptans, meperidine, tramadol). Washout period: 14 days after stopping MAOI before starting another serotonergic drug; 14 days after stopping most SSRIs, but 5 weeks after stopping fluoxetine (due to long half-life of norfluoxetine).

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CLASS 5: Atypical / Newer Antidepressants

5-HT2 Receptor Modulators

• Trazodone: 5-HT2 antagonist + weak SERT inhibitor. Highly sedating. Rarely used as antidepressant - now used primarily as a hypnotic (off-label). Risk of priapism. No dependence/tolerance. - Katzung's, p. 841
• Nefazodone: Related to trazodone. Black box warning for hepatotoxicity (lethal hepatic failure reported). Rarely prescribed.

Tetracyclic/Unicyclic

• Bupropion: Unicyclic aminoketone. Unique mechanism - inhibits DAT and NET (dopamine + norepinephrine reuptake). No sexual dysfunction. No weight gain. Lowers seizure threshold (dose-dependent). Used for MDD, smoking cessation, ADHD. Contraindicated in eating disorders and seizure history. Resembles amphetamine structurally.
• Mirtazapine: Alpha-2 adrenergic antagonist (increases NE/5-HT release) + blocks 5-HT2 and 5-HT3 + strong H1 antagonist. Very sedating. Significant weight gain. Good for depression with insomnia, weight loss, or severe nausea. "NaSSA" = noradrenergic and specific serotonergic antidepressant.
• Vilazodone: SSRI + 5-HT1A partial agonist. Minimal weight gain and sexual dysfunction.
• Vortioxetine: Multimodal - SERT inhibitor + 5-HT3/5-HT7/5-HT1D antagonist + 5-HT1B partial agonist + 5-HT1A agonist. Evidence for improving cognitive dysfunction associated with depression. Not simply classified as SSRI. - Katzung's, p. 842

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PART 4 - PHARMACODYNAMICS SUMMARY

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PART 5 - PHARMACOKINETICS

• Most antidepressants are highly protein bound (except venlafaxine/desvenlafaxine at 27-30%)
• Most undergo hepatic CYP450 metabolism
• Fluoxetine + paroxetine: potent CYP2D6 inhibitors (many drug interactions)
• Fluvoxamine: potent CYP3A4 + CYP1A2 inhibitor
• Citalopram/escitalopram/sertraline: modest CYP interactions - preferred when polypharmacy is a concern
• Fluoxetine has the longest half-life (norfluoxetine t1/2 ~5-7 days) - least discontinuation syndrome, slowest washout needed before MAOI

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PART 6 - CLINICAL USE

When to Use Which Drug

Duration of Treatment

• Acute phase: 6-12 weeks (assess response)
• Continuation phase: 4-9 months after remission (prevents relapse)
• Maintenance: Long-term if 2+ serious MDD episodes in 5 years, or 3+ lifetime episodes - Katzung's, p. 851

Bipolar Depression - Important Warning

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PART 7 - SIDE EFFECTS COMPARISON

• Paroxetine has the highest sexual dysfunction among SSRIs
• Bupropion has no anticholinergic or sexual side effects
• Citalopram has the most QTc prolongation among SSRIs

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PART 8 - SEROTONIN SYNDROME

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PART 9 - NOVEL AND RESEARCH-FRONTIER TREATMENTS

Ketamine and Esketamine (NMDA Antagonists)

• Racemic IV ketamine (off-label): produces antidepressant effects within hours - vs. weeks for conventional drugs
• Esketamine (Spravato) intranasal: FDA-approved for Treatment-Resistant Depression (TRD) and for depression with acute suicidal ideation
• Mechanism: NMDA receptor (glutamate) blockade, rapid AMPA activation, BDNF release, synaptogenesis
• Esketamine: 48% bioavailable intranasally, t1/2 of 7-12 hours; administered in clinic under observation due to dissociation risk
• Dextromethorphan + Bupropion (Auvelity): FDA-approved 2022; bupropion inhibits CYP2D6 to increase dextromethorphan levels (another NMDA antagonist); faster onset than SSRIs - Katzung's, p. 847

Brexanolone and Zuranolone (GABA-A Modulators)

• Brexanolone (Zulresso): IV neuroactive steroid; positive allosteric modulator of GABA-A; FDA-approved for postpartum depression; 60-hour infusion; 99% protein bound
• Zuranolone (Zurzuvae): Oral version; FDA-approved 2023 for postpartum depression and MDD; rapid onset (days)
• These represent a completely new mechanism - no monoamine involvement

Psilocybin (Research Stage)

• Classical psychedelic; 5-HT2A agonist
• Phase 2/3 trials showing rapid, durable antidepressant effects in treatment-resistant depression
• A 2025 systematic review (PMID: 39788410) explored neurobiological blood biomarker mechanisms

Vortioxetine - Cognitive Enhancement

• 2025 systematic review (PMID: 39808348) confirmed vortioxetine's expanding role in neurological conditions beyond depression, consistent with its cognitive effects

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PART 10 - TREATMENT-RESISTANT DEPRESSION (TRD)

1. Optimize current drug (dose increase, duration check, adherence)
2. Switch to different class
3. Combine two antidepressants (e.g., SSRI + bupropion, SSRI + mirtazapine)
4. Augment: Add lithium, atypical antipsychotics (aripiprazole, quetiapine, brexpiprazole, cariprazine), thyroid hormone
5. Ketamine/Esketamine (FDA-approved specifically for TRD)
6. ECT (Electroconvulsive Therapy): Most effective treatment for severe/refractory depression; 70-90% response rate
7. TMS (Transcranial Magnetic Stimulation): Non-invasive, FDA-approved

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PART 11 - THE ANTIDEPRESSANT DELAY PARADOX

1. Receptor downregulation theory: Initial 5-HT increase activates somatodendritic 5-HT1A autoreceptors, which suppress firing. Over weeks, these autoreceptors desensitize, allowing full 5-HT output.
2. Neuroplasticity theory: The therapeutic effect depends on downstream gene expression changes (BDNF, synaptic remodeling), which take weeks.
3. Glucocorticoid receptor normalization: Chronic stress causes glucocorticoid receptor resistance; antidepressants gradually restore sensitivity.

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Quick Reference Summary

DEPRESSION
    ↓ 5-HT, NE, DA in synapses
    
ANTIDEPRESSANT STRATEGIES:
    Block reuptake: SSRI → ↑5-HT
                    SNRI → ↑5-HT + ↑NE
                    TCA  → ↑5-HT + ↑NE + many side effects
                    Bupropion → ↑DA + ↑NE
    
    Block degradation: MAOI → ↑all three
    
    Receptor modulation: Mirtazapine → ↑NE + ↑5-HT release
                         Vortioxetine → multimodal 5-HT
    
    Novel targets:
                    Ketamine/Esketamine → NMDA block → hours
                    Brexanolone/Zuranolone → GABA-A modulation
                    Psilocybin → 5-HT2A agonism (research)

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• Katzung's Basic and Clinical Pharmacology 16e, Chapter 30 (Antidepressant Agents)
• Stahl's Essential Psychopharmacology, Chapter 6 (Mood Disorders) and Chapter 7 (Antidepressants)
• Kaplan & Sadock's Comprehensive Textbook of Psychiatry
• The Maudsley Prescribing Guidelines 15e
• Harrison's Principles of Internal Medicine 22e

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Antidepressants: From Basics to Research

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PART 1 - UNDERSTANDING DEPRESSION (The Foundation)

DSM-5 Criteria for Major Depressive Episode

1. Depressed mood most of the day
2. Anhedonia - loss of interest or pleasure
3. Sleep disturbance (insomnia or hypersomnia)
4. Weight/appetite change
5. Psychomotor agitation or retardation
6. Fatigue or loss of energy
7. Feelings of guilt or worthlessness
8. Poor concentration / executive dysfunction
9. Suicidal ideation

About 85% of patients with a single MDD episode will have at least one recurrence. Many progress to chronic, treatment-resistant episodes. - Katzung's, p. 851

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PART 2 - NEUROSCIENCE OF DEPRESSION

The Monoamine Hypothesis

• Reserpine (depletes monoamines) causes depression in a subset of patients
• Tryptophan-free diet causes rapid relapse in patients responding to serotonergic antidepressants
• Catecholamine depletion triggers relapse in patients on noradrenergic agents
• SERT gene promoter polymorphism: The "s" (short) allele → more vulnerable to depression under stress, less likely to respond to SSRIs. The "l" (long) allele → stress-resistant, better SSRI response
• Every antidepressant discovered so far has significant monoamine system effects - Katzung's, p. 837-838

The Delayed Response Paradox

1. Autoreceptor desensitization theory: Initial 5-HT surge activates inhibitory 5-HT1A somatodendritic autoreceptors, dampening firing. Over weeks these desensitize, allowing full output.
2. Neurotrophic/BDNF theory: The real therapeutic effect depends on downstream upregulation of BDNF (Brain-Derived Neurotrophic Factor), hippocampal neurogenesis, and synaptic remodeling - these take weeks.
3. Glucocorticoid receptor theory: Chronic stress impairs glucocorticoid receptor sensitivity; antidepressants gradually restore this.

Beyond Monoamines

• Neuroplasticity hypothesis (Stahl's): Depression involves progressive structural brain changes (hippocampal atrophy, reduced synaptic density). Antidepressants may arrest or reverse neuroprogression if started early.
• Glutamate/NMDA hypothesis: Ketamine's near-instant antidepressant effect via NMDA receptor blockade opened an entirely new understanding - glutamate dysregulation may be a core driver.
• Neuroinflammation hypothesis: Elevated pro-inflammatory cytokines (IL-6, TNF-α, CRP) correlate with depression, especially treatment-resistant forms. Some antidepressants have anti-inflammatory properties.

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PART 3 - COMPLETE CLASSIFICATION OF ANTIDEPRESSANTS

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CLASS 1: SSRIs (Selective Serotonin Reuptake Inhibitors)

• GI: nausea, diarrhea (most common at initiation - usually resolves in 1-2 weeks)
• Sexual dysfunction: reduced libido, delayed orgasm, anorgasmia (30-40% of patients - often persists)
• CNS: insomnia/agitation or sedation (drug-dependent), headache
• Weight gain: minimal short-term; can be significant long-term
• Discontinuation syndrome (especially paroxetine, venlafaxine): flu-like symptoms, "brain zaps," dizziness on abrupt cessation - always taper

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CLASS 2: SNRIs (Serotonin-Norepinephrine Reuptake Inhibitors)

SNRIs bind the serotonin and norepinephrine transporters as do TCAs, but unlike TCAs do not have significant affinity for other receptors. - Katzung's, p. 840

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CLASS 3: TCAs (Tricyclic Antidepressants)

• Treatment-resistant depression (failed SSRIs/SNRIs)
• Chronic neuropathic pain
• Migraine prevention
• Enuresis (imipramine)
• Insomnia (doxepin low dose)

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CLASS 4: MAOIs (Monoamine Oxidase Inhibitors)

• Cannot combine with any serotonergic drug → serotonin syndrome
• Cannot combine with sympathomimetics (amphetamine, cocaine, ephedrine) → hypertensive crisis
• Cannot combine with meperidine → fatal interaction (serotonin syndrome + respiratory depression)
• Washout: Stop MAOI 14 days before starting SSRI. Stop fluoxetine 5 weeks before starting MAOI.

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CLASS 5: Atypical Antidepressants

Bupropion (Wellbutrin, Zyban)

• Mechanism: Inhibits DAT and NET (dopamine + norepinephrine reuptake inhibitor). No serotonergic activity.
• Unique advantages: No sexual dysfunction. No weight gain (often weight-neutral or slight loss). No anticholinergic effects.
• Additional uses: Smoking cessation (Zyban), ADHD, seasonal affective disorder
• Side effects: Insomnia, agitation, dry mouth, dose-dependent seizures (contraindicated in eating disorders - electrolyte disturbances lower seizure threshold; avoid in seizure history)
• Note: Resembles amphetamine structurally; activating rather than sedating

Mirtazapine (Remeron)

• Mechanism: Alpha-2 adrenergic antagonist (blocks presynaptic autoreceptors → increases NE and 5-HT release) + blocks 5-HT2 and 5-HT3 receptors + potent H1 antagonist (sedation and appetite)
• NaSSA: noradrenergic and specific serotonergic antidepressant
• Advantages: Good for depression with insomnia, poor appetite, weight loss, nausea, or severe anxiety
• Side effects: Significant sedation and weight gain; low sexual dysfunction

Trazodone

• Mechanism: 5-HT2A antagonist + weak SERT inhibitor
• Primary use now: Off-label hypnotic - very sedating, no tolerance or dependence risk. Rarely used as primary antidepressant.
• Unique risk: Priapism (prolonged erection - up to 1 in 6,000 male patients)

Nefazodone

• Related to trazodone. Black box warning: rare but fatal hepatotoxicity. Rarely prescribed.

Vilazodone (Viibryd)

• SSRI + 5-HT1A partial agonist
• Minimal weight gain and sexual dysfunction
• Requires food for adequate absorption

Vortioxetine (Trintellix)

• Multimodal: SERT inhibitor + 5-HT3/5-HT7/5-HT1D antagonist + 5-HT1B partial agonist + 5-HT1A agonist
• Not simply an SSRI - actions not primarily from SERT inhibition
• Key advantage: Improves cognitive dysfunction associated with depression (approved for this in Europe and USA)
• FDA-approved for MDD; favorable sexual dysfunction profile
• Katzung's, p. 842

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PART 4 - PHARMACODYNAMICS AT A GLANCE

The increased synaptic monoamine availability triggers a cascade: downstream gene transcription via CREB → production of BDNF, glucocorticoid receptors, beta-adrenoreceptors → this protein-level change is what actually determines the therapeutic benefit and the 2-4 week delay. - Katzung's, p. 848

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PART 5 - KEY PHARMACOKINETICS

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PART 6 - SIDE EFFECT COMPARISON TABLE

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PART 7 - CLINICAL DECISION-MAKING

Choosing an Antidepressant

Duration Rules

• Acute phase: 6-12 weeks to assess response (at least 4 weeks at adequate dose before declaring failure)
• Continuation: 4-9 months after achieving remission (prevents relapse)
• Maintenance: Long-term if 2+ serious episodes in 5 years, OR 3+ lifetime episodes - Katzung's, p. 851

Antidepressant Failure / Treatment-Resistant Depression

1. Optimize current drug (dose, duration, adherence check)
2. Switch within class (different SSRI) or across class (SSRI → SNRI or bupropion)
3. Combine: SSRI + bupropion, SSRI + mirtazapine ("California rocket fuel")
4. Augment: Add lithium, atypical antipsychotics (aripiprazole, quetiapine, brexpiprazole), T3 thyroid hormone
5. Esketamine (Spravato) - FDA-approved specifically for TRD
6. ECT (70-90% response rate - most effective treatment for severe/refractory depression)
7. TMS (transcranial magnetic stimulation) - non-invasive, FDA-approved
8. Psilocybin-assisted therapy (clinical trials/research)

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PART 8 - DANGEROUS DRUG INTERACTIONS

Serotonin Syndrome

MAOI-Specific Interactions

• Tyramine-containing foods → hypertensive crisis
• Sympathomimetics (amphetamine, pseudoephedrine) → hypertensive crisis
• Meperidine → serotonin syndrome + respiratory depression (particularly dangerous)

TCA Overdose

CYP2D6 Interactions

• Tricyclic antidepressants
• Antipsychotics (haloperidol, risperidone)
• Opioids (codeine → cannot convert to morphine; tramadol → reduced analgesia)
• Beta-blockers (metoprolol)
• Antiarrhythmics (flecainide, propafenone)

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PART 9 - NOVEL TREATMENTS (The Research Frontier)

Ketamine and Esketamine - The Paradigm Shift

• NMDA receptor antagonist → rapid glutamate-mediated synaptogenesis
• Antidepressant effects within hours (vs. weeks for SSRIs)
• Single dose can relieve depression for 1-2 weeks
• Used for treatment-resistant depression and acute suicidality
• t1/2 ~3 hours; given as subanesthetic infusion (0.5 mg/kg over 40 min)

• S-enantiomer of ketamine; intranasal spray
• 48% bioavailable intranasally; Tmax 20-40 minutes; t1/2 7-12 hours
• Must be administered in a certified clinic under supervision (dissociation, dizziness)
• FDA-approved for: (1) Treatment-Resistant Depression + oral antidepressant, (2) MDD with acute suicidal ideation
• Katzung's, p. 847; The Maudsley Prescribing Guidelines 15e

• Bupropion inhibits CYP2D6 → raises dextromethorphan levels → NMDA antagonist effect
• Oral tablet; faster onset than SSRIs (1-2 weeks)
• t1/2 22 hours; peak in 3 hours

Brexanolone and Zuranolone (Neuroactive Steroids)

• IV neuroactive steroid; positive allosteric modulator of GABA-A receptors
• 60-hour continuous IV infusion; must be administered in hospital
• 99% protein bound; t1/2 ~9 hours
• FDA-approved for postpartum depression (PPD)
• Response within 24-48 hours (unprecedented for depression)

• Oral neuroactive steroid (GABA-A modulator)
• 14-day course treatment
• FDA-approved for PPD and MDD
• Rapid onset: significant improvement by Day 3 in some trials

Psilocybin - Active Clinical Research

• 5-HT2A agonist (classical psychedelic)
• Mechanism likely involves rapid synaptogenesis, default mode network reset
• Phase 2/3 trials in treatment-resistant depression, MDD, PTSD, addiction
• FDA granted "Breakthrough Therapy" designation for TRD
• A 2025 systematic review on psilocybin blood biomarker mechanisms (PMID 39788410) is mapping the neurobiological underpinnings
• Not yet FDA-approved for depression (regulatory uncertainty as of 2026)

Vortioxetine - Cognitive Frontier

• 2025 systematic review (PMID 39808348) confirmed expanding neurological applications for vortioxetine, consistent with its well-documented pro-cognitive effects in depression

Other Emerging Targets

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PART 10 - THE "BIG PICTURE" SUMMARY

ROOT CAUSE OF DEPRESSION (current understanding):
↓ Synaptic 5-HT, NE, DA  +  Neuroplasticity failure  +  HPA axis dysregulation
+  Glutamate dysfunction  +  Neuroinflammation

↓
HOW ANTIDEPRESSANTS WORK:

BLOCK REUPTAKE:
  SSRIs  → ↑5-HT (selective)
  SNRIs  → ↑5-HT + ↑NE
  TCAs   → ↑5-HT + ↑NE + many side effects
  Bupropion → ↑DA + ↑NE

BLOCK DEGRADATION:
  MAOIs → ↑5-HT + ↑NE + ↑DA

RECEPTOR MODULATION:
  Mirtazapine → ↑NE + ↑5-HT release via alpha-2 blockade
  Vortioxetine → multimodal 5-HT (most complex)
  Trazodone → 5-HT2 blockade (primarily sedation/hypnotic)

NOVEL MECHANISMS:
  Ketamine/Esketamine → NMDA blockade → synaptogenesis → hours
  Brexanolone/Zuranolone → GABA-A modulation (postpartum)
  Psilocybin → 5-HT2A → circuit reset (research)

↓
DOWNSTREAM (common final pathway):
↑BDNF → Neuroplasticity → Restored hippocampal circuits → Remission

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• Katzung's Basic and Clinical Pharmacology, 16th Edition - Chapter 30 (Antidepressant Agents)
• Stahl's Essential Psychopharmacology - Chapters 6 & 7
• Kaplan & Sadock's Comprehensive Textbook of Psychiatry
• The Maudsley Prescribing Guidelines in Psychiatry, 15th Edition
• Harrison's Principles of Internal Medicine, 22nd Edition (2025)