p. 1451).
Yet the debate over whether or not there exists a shared common pathway for depression is complicated by the variety of drugs which can be effectively used as treatment. Two of the main classes of antidepressants, tricyclic and monoamine oxidase inhibitors act on both the serotonergic and norepinephrine systems. Newer drugs, especially the lauded fluoxetine or Prozac act exclusively on serotonin. Scientists have asked how can there be a common pathway if drugs operating on different brain systems achieve the same effect? Yale psychiatrist George Henninger borrowing an analogy from the manner in which anemia operates asserts that there may be a "[final] pathway where both the primary and secondary abnormalities interact to produce the symptom of depression" (Holden, 1991, p. 1451).
The complexity of research surrounding studies of serotonin can be at least partially explained by its own multi-functioning. Neurochemically, 5-HT can be seen operating as if through a cluster effect. Research conducted over the last 20 years discovered and continues to investigate the "multiple populations" of a centralized 5-HT operating as a primary binding site (Cannon, 1991, p. 132). Although many serotonin receptors (5-HT) have been identified, the knowledge of subtypes remains limited. Currently, 5-HT1A agonists are involved in the "treatment of certain anxiety disorders, 5-HT1C and 5-HT2 may be indicated for generalized anxiety disorder, and 5-HT1D receptor agonists are used for migraines" (Bald
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