On serotonin and experimental anxiety

Background: This review describes the development of a research line on the role of serotonin (5-HT) in experimental anxiety that was initiated in 1969, in the laboratory founded by P.B. Dews, W.H. Morse and R.T. Kelleher at the Harvard Medical School, and has evolved until this date. Results: Initially, it was found that two non-selective 5-HT receptor antagonists released punished responding in pigeons with a magnitude comparable to that of benzodiazepine anxiolytics. This result was one of the key evidences that led to the concept that 5-HT enhanced anxiety by acting both in the forebrain and in the periaqueductal gray matter (PAG). Further evidence supported this hypothesis regarding the forebrain, but results with electrical stimulation and intracerebral drug injection into the PAG indicated that 5-HT inhibited aversive behavior evoked from this area. As a result, it has been suggested that 5-HT has a dual role in the regulation of defense, namely enhancing learned responses to potential or distal threat through actions in the forebrain while inhibiting unconditioned responses to proximal threat by acting on the PAG. The former would be related to generalized anxiety and the latter to panic disorder. To test this hypothesis, a new animal model, named the elevated T-maze, has been designed. It consists of one arm enclosed by walls that is perpendicular to two open arms elevated from the floor. The same rat performs two tasks, namely inhibitory avoidance of the elevated open arms, representing conditioned anxiety and one-way escape from one of the open arms, representative of unconditioned fear. Conclusion: The differential effects of drugs acting on 5-HT observed in the two tasks of the ETM generally support the hypothesis under scrutiny.