A patient with major depressive disorder develops urinary retention and constipation. What neurotransmitter blockade is most likely responsible for these effects?

• A. Acetylcholine
• B. Dopamine
• C. Histamine
• D. Norepinephrine

Answer: (A)

In the context of major depressive disorder and the side effects of urinary retention and constipation, the blockade of acetylcholine is most relevant. Acetylcholine is a key neurotransmitter that plays several roles, including regulation of the autonomic nervous system which controls involuntary bodily functions such as bladder control and gastrointestinal motility.

When acetylcholine signaling is inhibited, as can occur with certain antidepressant medications that have anticholinergic properties, patients can experience urinary retention due to the impact on bladder function, as well as constipation from reduced gastrointestinal motility. The blockade of acetylcholine disrupts the normal function of the parasympathetic nervous system, leading to these specific symptoms.

Dopamine primarily influences mood and motivation but is less directly tied to urinary and bowel function in this context. Histamine is more involved in sleep regulation and allergic responses, and norepinephrine mainly affects arousal and attention and does not typically cause urinary retention or constipation directly like acetylcholine does. Therefore, the presence of urinary retention and constipation points towards acetylcholine blockade as the cause of these side effects.