What is the mechanism of action for tricyclic antidepressants?

Tricyclic antidepressants (TCAs) primarily work by blocking the reuptake of norepinephrine and serotonin, although their most characteristic action is the inhibition of norepinephrine reuptake. By doing this, they increase the levels of norepinephrine in the synaptic cleft, which enhances neurotransmission and contributes to their antidepressant effects. The balanced inhibition of both norepinephrine and serotonin reuptake allows for a broader therapeutic effect, but the emphasis on norepinephrine in the question highlights the importance of this neurotransmitter in the mechanism of action of TCAs.

The other mechanisms described in the other options, such as inhibition of serotonin reuptake, enhancing dopamine levels, and increasing GABA activity, are more characteristic of other classes of psychiatric medications or contribute to the effects of varied drugs rather than serving as the primary mechanism for tricyclic antidepressants. Therefore, the focus on norepinephrine reuptake inhibition directly relates to how tricyclic antidepressants exert their therapeutic impact in the treatment of depression and related disorders.