A psychiatrist prescribes venlafaxine (an SNRI) for a patient. What is the MECHANISM that explains why SNRIs may cause more sexual side effects and urinary hesitancy at higher doses compared to SSRIs?
- A Greater 5-HT2A blockade at high SNRI doses
- B Direct dopamine blockade in the tuberoinfundibular pathway causing hyperprolactinaemia
- C Norepinephrine reuptake inhibition causing alpha-1 adrenergic stimulation (vasoconstriction) and urethral sphincter contraction, plus combined serotonergic effects on sexual function ✓
- D Anticholinergic effect of venlafaxine on the bladder detrusor muscle
Correct answer: C. Norepinephrine reuptake inhibition causing alpha-1 adrenergic stimulation (vasoconstriction) and urethral sphincter contraction, plus combined serotonergic effects on sexual function
Explanation
SNRIs like venlafaxine inhibit both serotonin and norepinephrine reuptake. The additional noradrenergic effects at higher doses contribute to urinary hesitancy/retention through alpha-1 adrenergic stimulation causing urethral sphincter contraction. Sexual side effects in SNRIs are mediated by serotonergic effects (5-HT2 receptor activation delaying/inhibiting orgasm and libido) combined with noradrenergic effects. Venlafaxine does not have significant anticholinergic activity or dopamine blockade; it does not raise prolactin through dopaminergic mechanisms.
Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.
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