Which of the following is NOT a Tricyclic antidepressant?

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Gabapentin is not a tricyclic antidepressant because it does not share the three-ring chemical structure or the usual mechanism of action of TCAs. Tricyclic antidepressants such as amitriptyline, imipramine, and nortriptyline work mainly by inhibiting the reuptake of norepinephrine and serotonin, which increases these neurotransmitters in the synapse and helps alleviate depressive symptoms. They also commonly cause anticholinergic and sedative effects due to receptor antagonism.

Gabapentin, on the other hand, is an anticonvulsant often used for neuropathic pain and seizures. It acts differently—binding to the alpha-2-delta subunit of voltage-gated calcium channels to modulate neurotransmitter release—rather than inhibiting monoamine reuptake. This fundamental difference in structure and mechanism is why gabapentin is not classified as a tricyclic antidepressant.