Regarding SSRIs, which statement best explains their delayed therapeutic onset (2–4 weeks) despite rapid serotonin reuptake inhibition?
- A Initial inhibition of serotonin reuptake activates somatodendritic 5-HT1A autoreceptors that dampen serotonergic firing; desensitisation of these autoreceptors over 2–4 weeks allows full synaptic serotonin enhancement ✓
- B SSRIs must undergo hepatic conversion to active metabolites before reaching therapeutic CNS levels
- C SSRIs require gradual upregulation of postsynaptic 5-HT3 receptors over weeks before mood improvement is possible
- D SSRIs are highly protein-bound and require weeks to reach steady-state plasma concentrations
Correct answer: A. Initial inhibition of serotonin reuptake activates somatodendritic 5-HT1A autoreceptors that dampen serotonergic firing; desensitisation of these autoreceptors over 2–4 weeks allows full synaptic serotonin enhancement
Explanation
When SSRIs are first administered, elevated synaptic serotonin activates presynaptic/somatodendritic 5-HT1A autoreceptors on raphe neurons, causing a compensatory reduction in serotonergic cell firing that limits net serotonin increase. Over 2–4 weeks of continued SSRI use, these autoreceptors downregulate and desensitise, releasing the brake on serotonin neurotransmission. This gradual adaptive change, not simply plasma levels or receptor upregulation, accounts for the delayed antidepressant response.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.