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Antidepressant Pharmacology: SSRIs, SNRIs, and Beyond

Mechanism of Action

The Monoamine Hypothesis

Classic theory: depression results from deficiency of serotonin (5-HT), norepinephrine (NE), and/or dopamine (DA) in key brain regions. While overly simplistic, it remains a useful framework for understanding antidepressant pharmacology. The delayed onset of therapeutic effect (2-4 weeks) despite rapid reuptake inhibition points to downstream neuroplastic changes as the true mechanism.

SSRIs (Selective Serotonin Reuptake Inhibitors)

Agents: fluoxetine, sertraline, paroxetine, citalopram, escitalopram, fluvoxamine. Block the serotonin transporter (SERT) with varying degrees of selectivity. Increase synaptic 5-HT availability, leading to desensitization of presynaptic 5-HT1A autoreceptors over weeks. Downstream effects include increased BDNF expression and hippocampal neurogenesis. Unique pharmacologic properties by agent:. Fluoxetine: longest half-life (2-6 days, active metabolite norfluoxetine 4-16 days), 5-HT2C antagonism (activating), CYP2D6 inhibitor. Sertraline: mild dopamine transporter (DAT) inhibition, sigma-1 receptor binding, relatively fewer drug interactions. Paroxetine: anticholinergic properties, potent CYP2D6 inhibitor, short half-life (withdrawal-prone), NE reuptake inhibition at higher doses. Citalopram: QTc prolongation concern at doses >40 mg (FDA warning), most serotonin-selective. Escitalopram: S-enantiomer of citalopram, considered most selective SSRI, dose-dependent QTc effect less pronounced than citalopram. Fluvoxamine: strong CYP1A2 and CYP2C19 inhibitor, FDA-approved for OCD (not MDD in US)

SNRIs (Serotonin-Norepinephrine Reuptake Inhibitors)

Agents: venlafaxine, desvenlafaxine, duloxetine, levomilnacipran, milnacipran. Block both SERT and norepinephrine transporter (NET) Venlafaxine acts primarily as an SSRI at low doses (<150 mg); NE reuptake inhibition emerges at higher doses. Duloxetine: balanced SERT/NET inhibition across dose range; FDA-approved for chronic pain conditions (diabetic neuropathy, fibromyalgia, chronic musculoskeletal pain) Levomilnacipran: preferential NET over SERT inhibition (3:1 ratio) Side effects include dose-dependent hypertension (particularly venlafaxine), sweating, and urinary hesitancy.

Atypical Antidepressants

Bupropion: norepinephrine-dopamine reuptake inhibitor (NDRI); no sexual dysfunction or weight gain; seizure risk dose-dependent; useful for smoking cessation; contraindicated in eating disorders and seizure history. Mirtazapine: noradrenergic and specific serotonergic antidepressant (NaSSA); alpha-2 adrenergic antagonist, 5-HT2A/2C and 5-HT3 antagonist, potent H1 antihistamine; sedating and appetite-stimulating at lower doses; less sedation at higher doses (more NE activation) Trazodone: serotonin antagonist and reuptake inhibitor (SARI); primarily used as a hypnotic at low doses (25-100 mg); antidepressant at higher doses (150-400 mg); risk of priapism; alpha-1 antagonism causes orthostatic hypotension. Vilazodone: SSRI plus 5-HT1A partial agonist; may have faster onset; take with food for absorption. Vortioxetine: multimodal serotonergic antidepressant; SERT inhibition plus 5-HT1A agonism, 5-HT1B partial agonism, 5-HT3/5-HT7/5-HT1D antagonism; may improve cognitive symptoms of depression.

Tricyclic Antidepressants (TCAs)

Older generation agents: amitriptyline, nortriptyline, imipramine, desipramine, clomipramine. Block SERT and NET with varying ratios; also block histaminergic (H1), muscarinic, and alpha-1 adrenergic receptors. Tertiary amines (amitriptyline, imipramine) are more serotonergic and have more side effects. Secondary amines (nortriptyline, desipramine) are more noradrenergic and better tolerated. Therapeutic drug monitoring available (nortriptyline therapeutic window: 50-150 ng/mL) Lethal in overdose (cardiac sodium channel blockade, wide QRS, seizures)

MAOIs (Monoamine Oxidase Inhibitors)

Irreversible: phenelzine, tranylcypromine; reversible: moclobemide (not available in US), selegiline transdermal patch. Inhibit monoamine oxidase A (metabolizes 5-HT, NE) and/or B (metabolizes DA, phenylethylamine) Highly effective, especially for atypical depression and treatment-resistant depression. Dietary tyramine restriction required for irreversible oral MAOIs (hypertensive crisis risk) Selegiline transdermal patch at 6 mg/24hr: selective MAO-B inhibition, no dietary restrictions needed at this dose. Absolutely contraindicated with serotonergic drugs (risk of serotonin syndrome); wash-out periods required (2 weeks for most, 5 weeks after fluoxetine)

Clinical Selection of Antidepressants

First-Line Treatment Principles

SSRIs and SNRIs are first-line due to favorable therapeutic index, tolerability, and broad evidence base. Choice among first-line agents guided by side effect profile, comorbidities, drug interactions, and patient preference. All antidepressants have roughly equivalent efficacy for moderate-severe MDD (~60-70% response rate) Cipriani et al. (2018) network meta-analysis: escitalopram, sertraline, vortioxetine, and mirtazapine showed best combination of efficacy and acceptability.

Matching Agent to Patient

Side Effect Management

Common SSRI/SNRI Side Effects

GI: nausea (most common early side effect, typically transient), diarrhea. CNS: headache, insomnia or sedation, activation/anxiety in first 1-2 weeks. Sexual dysfunction: decreased libido, anorgasmia, erectile dysfunction; affects 30-60% of patients; does not typically resolve spontaneously. Weight gain: modest with most agents; more significant with paroxetine and mirtazapine. Bleeding risk: SSRI-mediated platelet serotonin depletion; increased risk with concurrent NSAIDs or anticoagulants. Hyponatremia/SIADH: especially in elderly; monitor sodium if symptomatic. Apathy/emotional blunting: dose-dependent; may be misinterpreted as incomplete response.

Managing Sexual Dysfunction

Dose reduction if clinically feasible. Switch to bupropion, mirtazapine, or vilazodone/vortioxetine. Augmentation with bupropion (evidence for benefit) PDE5 inhibitors (sildenafil) for erectile dysfunction. Drug holidays not generally recommended (withdrawal risk, inconsistent evidence)

Discontinuation Syndrome

More common with short half-life agents (paroxetine, venlafaxine, desvenlafaxine) Symptoms: dizziness, nausea, paresthesias ("brain zaps"), irritability, insomnia, flu-like symptoms. Onset 1-3 days after abrupt cessation; can last weeks. Prevention: gradual taper over 2-4 weeks minimum; consider cross-taper to fluoxetine for severe cases. Not a sign of "addiction" -- important to educate patients.

The Delayed Onset of Therapeutic Effect

Acute reuptake inhibition occurs within hours. Clinical improvement requires 2-4 weeks (sometimes 6-8 weeks for full effect) Proposed mechanisms for delay: autoreceptor desensitization, downstream gene expression changes, BDNF-mediated neuroplasticity, hippocampal neurogenesis. Early improvement by week 2 is a positive prognostic indicator. Adequate trial: minimum 4-6 weeks at therapeutic dose.

<image> A detailed diagram showing the serotonergic synapse with labeled presynaptic and postsynaptic neurons. Show the serotonin transporter (SERT) being blocked by an SSRI molecule, with accumulation of serotonin in the synaptic cleft. Label the presynaptic 5-HT1A autoreceptor, postsynaptic 5-HT1A, 5-HT2A, 5-HT2C, and 5-HT3 receptors. Include arrows showing the cascade from acute SERT blockade to downstream effects: autoreceptor desensitization, increased BDNF expression, and enhanced neuroplasticity. Medical illustration style with clean lines and color coding. </image>

<image> A comparative pharmacology chart of antidepressant classes arranged as a visual table. Columns represent receptor/transporter targets: SERT, NET, DAT, 5-HT2A, 5-HT2C, H1, M1, alpha-1. Rows represent drug classes: SSRIs, SNRIs, bupropion, mirtazapine, trazodone, TCAs, MAOIs. Use color-coded circles (size proportional to binding affinity) to show relative receptor activity for each class. Include a legend mapping circle size to affinity (Ki values). Clean medical reference style. </image>

<image> A clinical decision algorithm flowchart for antidepressant selection in MDD. Start with "Moderate-to-Severe MDD confirmed" at the top. Branch based on comorbidities: chronic pain (leads to duloxetine/venlafaxine), insomnia (leads to mirtazapine/trazodone), weight/sexual concern (leads to bupropion), no specific drivers (leads to SSRI). Show a second tier for inadequate response at 4-6 weeks: dose optimization, then switch within or between classes, then augmentation strategies. Use clean boxes and arrows with color coding for each pathway. </image>

Clinical Pearls

Start SSRIs/SNRIs at the lowest effective dose and titrate based on response and tolerability; "start low, go slow" especially in the elderly and anxious patients. Always ask about sexual dysfunction proactively -- patients rarely volunteer this information. Fluoxetine's long half-life makes it the most forgiving for missed doses and the least likely to cause discontinuation syndrome. Paroxetine and venlafaxine are the most likely to cause discontinuation syndrome -- taper slowly. Bupropion is the only antidepressant with no serotonergic activity; it will not help anxiety-predominant presentations and may worsen anxiety. Citalopram doses above 40 mg/day are associated with dose-dependent QTc prolongation (FDA 2011 safety communication); limit to 20 mg/day in hepatic impairment, elderly, or CYP2C19 poor metabolizers. MAOIs are underutilized but remain the most effective agents for atypical depression; consider for treatment-resistant cases. The 2018 Cipriani meta-analysis is the largest comparative efficacy study (522 trials, 116,477 patients) and is an essential reference for evidence-based antidepressant selection. Document an adequate trial as 4-6 weeks at therapeutic dose before declaring treatment failure.

References

• Cipriani A, et al. Comparative efficacy and acceptability of 21 antidepressant drugs for the acute treatment of adults with major depressive disorder: a systematic review and network meta-analysis. Lancet. 2018;391(10128):1357-1366.
• Stahl SM. Stahl's Essential Psychopharmacology: Neuroscientific Basis and Practical Applications. 5th ed. Cambridge University Press; 2021.
• APA Practice Guidelines for the Treatment of Major Depressive Disorder. 3rd ed. American Psychiatric Association; 2010 (with 2019 guideline watch update).
• FDA Drug Safety Communication: Revised recommendations for Celexa (citalopram) related to dose-dependent QT prolongation. March 2012.
• Hirschfeld RM. History and evolution of the monoamine hypothesis of depression. J Clin Psychiatry. 2000;61(Suppl 6):4-6.

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