TCA mechanism – serotonin and norepinephrine reuptake inhibition with receptor blockade

Mechanism of Action of Tricyclic Antidepressants (TCAs)

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Introduction

Tricyclic antidepressants (TCAs) are one of the oldest classes of antidepressant medications. Although largely replaced by SSRIs due to their improved safety profiles, TCAs are still widely used for treatment-resistant depression, neuropathic pain, and other off-label conditions. They work by inhibiting the reuptake of norepinephrine and serotonin, thereby increasing the levels of these neurotransmitters in the synaptic cleft.

Common TCAs include:

  • Amitriptyline
  • Nortriptyline
  • Imipramine
  • Desipramine
  • Clomipramine
  • Doxepin

These drugs are frequently covered in USMLE, GPAT, NCLEX, and NEET-PG pharmacology sections.

Stepwise Mechanism of Action of TCAs

  1. Inhibition of reuptake transporters
    TCAs inhibit both the norepinephrine transporter (NET) and the serotonin transporter (SERT) on presynaptic neurons.
  2. Increased synaptic levels of NE and 5-HT
    This inhibition leads to an accumulation of norepinephrine (NE) and serotonin (5-HT) in the synaptic cleft.
  3. Enhanced postsynaptic neurotransmission
    The increased availability of NE and 5-HT leads to improved neurotransmission, which contributes to mood elevation.
  4. Delayed therapeutic response
    Like SSRIs, TCAs exhibit a delayed clinical effect, typically seen after 2–4 weeks, due to neuroadaptive changes in receptor sensitivity.
  5. Additional receptor blockade
    TCAs also block muscarinic (anticholinergic), histaminergic (H1), and alpha-1 adrenergic receptors, contributing to both therapeutic and adverse effects.

Pharmacokinetic Parameters of TCAs

DrugHalf-lifeMetabolismCYP InteractionExcretion
Amitriptyline~10–28 hoursHepatic (CYP2D6)Strong CYP2D6 substrateRenal
Nortriptyline~18–44 hoursHepaticModerateRenal
Imipramine~12–25 hoursHepatic (CYP2D6)ModerateRenal
Desipramine~15–24 hoursHepaticModerateRenal

Clinical Uses of TCAs

  • Major depressive disorder (MDD)
  • Neuropathic pain (e.g., diabetic neuropathy)
  • Migraine prophylaxis
  • Chronic tension-type headaches
  • Nocturnal enuresis (imipramine)
  • Obsessive-compulsive disorder (clomipramine)
  • Insomnia (off-label)
  • Fibromyalgia

Adverse Effects of TCAs

  • Sedation – due to H1 blockade
  • Dry mouth, blurred vision, constipation, urinary retention – anticholinergic effects
  • Orthostatic hypotension – alpha-1 blockade
  • Weight gain – H1 and 5-HT receptor modulation
  • Sexual dysfunction
  • Arrhythmias and QT prolongation – sodium channel blockade
  • Seizures – especially with overdose
  • High lethality in overdose – caution in suicidal patients

Comparative Analysis: TCAs vs SSRIs

FeatureTCAsSSRIs
Reuptake inhibitionNE + 5-HTSelective for 5-HT
Receptor blockingMuscarinic, alpha-1, H1Minimal
SedationCommonLess common
Cardiovascular riskHigh (arrhythmias)Low
Overdose safetyDangerousSafer

Practice MCQs

Q1. TCAs primarily work by inhibiting:
a. Monoamine oxidase
b. Dopamine reuptake
c. Serotonin and norepinephrine reuptake ✅
d. GABA reuptake

Q2. Which TCA is commonly used in OCD?
a. Amitriptyline
b. Clomipramine ✅
c. Nortriptyline
d. Desipramine

Q3. TCA overdose can lead to:
a. Hepatitis
b. QT prolongation and arrhythmia ✅
c. Parkinsonism
d. Pancreatitis

Q4. Common anticholinergic side effect of TCAs is:
a. Polyuria
b. Diarrhea
c. Dry mouth ✅
d. Tinnitus

Q5. Which receptor blockade causes orthostatic hypotension with TCAs?
a. D2 receptor
b. H1 receptor
c. Muscarinic receptor
d. Alpha-1 adrenergic receptor ✅

Q6. Amitriptyline is useful in:
a. Acute psychosis
b. Migraine prophylaxis ✅
c. Schizophrenia
d. ADHD

Q7. Which TCA is a metabolite of amitriptyline?
a. Imipramine
b. Nortriptyline ✅
c. Clomipramine
d. Desipramine

Q8. Why should TCAs be used cautiously in cardiac patients?
a. Cause bradycardia
b. Cause AV block
c. Block sodium channels and prolong QT ✅
d. Increase renin secretion

Q9. Which symptom is LEAST likely with TCAs?
a. Sedation
b. Constipation
c. Insomnia ✅
d. Blurred vision

Q10. Which CYP enzyme metabolizes many TCAs?
a. CYP3A4
b. CYP2E1
c. CYP2D6 ✅
d. CYP1A2

FAQs

Q1: Are TCAs first-line drugs today?
No. Due to their adverse effect profile and overdose risk, they are usually reserved for treatment-resistant cases.

Q2: Can TCAs cause seizures?
Yes. Especially in high doses or overdose situations.

Q3: Why are TCAs dangerous in overdose?
Because of cardiotoxicity, arrhythmias, and CNS effects. Even a 2-week supply can be lethal.

Q4: Do TCAs cause weight gain?
Yes, especially amitriptyline and doxepin due to histaminergic and serotonergic modulation.

Q5: Can TCAs be used in children?
Not commonly. Imipramine may be used for nocturnal enuresis in select pediatric cases.

References

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