In vivo serotonin release and learned helplessness.

Learned helplessness, a behavioral depression caused by exposure to inescapable stress, is considered to be an animal model of human depressive disorder. Like human depression, learned helplessness has been associated with a defect in serotonergic function, but the nature of this relationship is not entirely clear. We have used in vivo microdialysis brain perfusion to measure serotonin (5-hydroxytryptamine, 5HT) in extracellular space of medial frontal cortex in conscious, freely moving rats. Basal 5HT levels in rats perfused before exposure to tail-shock stress did not themselves correlate with subsequent learned helplessness behavior. However, 5HT release after stress showed a significant increase with helpless behavior. These data support the hypothesis that a cortical serotonergic excess is causally related to the development of learned helplessness.