ABSTRACT
The serotonin (5-HT) neuronal system originates primarily from two groups of brainstem nuclei: the median and dorsal raphe nuclei (33). There are extensive cortical and subcortical projections from these nu clei. Earlier thinking that 5-HT neurons might mediate fear responses in a unitarian fashion has been rejected. In general, the recent animal literature suggests that the 5-HT system responds acutely to threat with increases in 5-HT function. This pattern of response to stress by the 5-HT system (particularly in cortical areas) has been postulated to lead to enhancement of information processing concerning the threatening stimulus (34). There is also evidence that 5-HT neurons may have a restraining effect on brainstem structures implicated in panic responses (e.g., the periaqueductal gray area) (reviewed in Ref. 35). Disruption of this restraining influence may precipitate a panic episode. Another important characteristic of the 5-HT system is the many 5-HT-receptor types now known to exist. Several of these (5-HT 3, 5-HT 1 A, and 5HT 2 A ) have been closely implicated in fear behaviors in animals and, as a consequence, have been the target for recent anxiolytic drug devel opment efforts (36).
