Final answer:
Tricyclic antidepressants work by blocking the reuptake of norepinephrine and serotonin into the presynaptic neuron, increasing their concentration in the synaptic cleft and enhancing their effect on the post-synaptic receptors.
Step-by-step explanation:
The question asks about the mechanism by which tricyclic antidepressants achieve their therapeutic effect. The correct answer is D. Tricyclic antidepressants work by prolonging the duration of norepinephrine and serotonin at the receptor sites. By blocking the reuptake of these neurotransmitters into the presynaptic neuron, these drugs increase the levels of norepinephrine and serotonin remaining in the synaptic cleft, which in turn enhances their effect on post-synaptic receptor sites. This action counteracts the neurotransmitter deficit seen in depression. Unlike selective serotonin reuptake inhibitors (SSRIs), which specifically target serotonin, tricyclic antidepressants affect both norepinephrine and serotonin and thus have a broader mechanism of action.