**Core Concept:** Tricyclic antidepressants are a class of antidepressant medications that work by inhibiting reuptake of neurotransmitters, primarily serotonin, norepinephrine, and dopamine. They have been widely used in the past but are now mostly replaced by selective serotonin reuptake inhibitors (SSRIs) due to better safety profiles.

**Why the Correct Answer is Right:** The correct answer, imipramine (option D), is a tricyclic antidepressant. It inhibits the reuptake of serotonin, norepinephrine, and dopamine, leading to increased neurotransmitter levels in the synaptic cleft and contributing to its antidepressant effect.

**Why Each Wrong Option is Incorrect:**
A. Amitriptyline (option A) is also a tricyclic antidepressant, inhibiting the reuptake of neurotransmitters but is not as selective as imipramine or SSRIs.
B. Doxepin (option B) is a tricyclic antidepressant, working similarly to amitriptyline and imipramine.
C. Nortriptyline (option C) is another tricyclic antidepressant, acting through neurotransmitter reuptake inhibition.

**Why Each Wrong Option is Incorrect:**
D. Desipramine (option D) is also a tricyclic antidepressant, inhibiting neurotransmitter reuptake but is less potent than imipramine or SSRIs.
E. Propranolol (option E) is a beta-blocker and does not belong to the tricyclic antidepressant class.

**Clinical Pearl:** Tricyclic antidepressants have several side effects, including orthostatic hypotension, anticholinergic effects, and cardiotoxicity, which contribute to their narrow therapeutic index and limited use compared to modern antidepressants like SSRIs.