Evaluation of central serotonergic function in affective and related disorders by the fenfluramine challenge test: a critical review

Plasma prolactin levels following oral administration of the serotonin (5-HT) releasing agent, fenfluramine hydrochloride, have been extensively used to evaluate central serotonergic function in affective and related disorders. Cortisol responses to fenfluramine have generally been a less informative measure. In healthy subjects, prolactin release by fenfluramine is dose-dependent, blocked by antagonists of serotonin receptors of the 5-HT-2a/2c type, negatively correlated with age and increased in young females. In major depression a preponderance of studies have found blunted prolactin responses compared to matched normal controls. Although a significant minority of studies have not found blunting, increased prolactin release has not been observed. The blunted prolactin release is not due to a deficient secretory capacity of pituitary lactotrophs and is congruent with other evidence for reduced central serotonergic function in major depression. Blunting of the prolactin response may be associated with severity of depression and with elevated baseline cortisol levels. Treatment with antidepressant drugs and electroconvulsive therapy has been reported to increase the prolactin response but this has not been replicated in all studies. Blunted prolactin responses to fenfluramine have been fairly consistently associated with impulsive aggression in different personality disorders and with severity of suicide attempts in depressed patients. A number of studies employing the fenfluramine challenge test (FCT) have been conducted in obsessive compulsive disorder but their results have been variable. Prolactin responses to fenfluramine may be enhanced in panic disorder and chronic fatigue syndrome but the number of studies in these conditions is small as is the case for seasonal affective disorder. Since the therapeutic administration of fenfluramine as an appetite suppressant has been suspended because of reports of cardiac complications, further use of this compound as a challenge agent is not anticipated. Future studies are likely to employ agents acting on specific serotonin receptors and should apply methodological insights from the use of the FCT, which are considered in this review. Use of concomitant brain imaging to evaluate the central effects of challenge agents directly is likely to become more prevalent and may supplant neuroendocrine challenge paradigms such as the FCT which have been remarkably heuristic but are limited in scope and methodologically complex.