Efferent pathway of the amygdala involved in conditioned fear as measured with the fear-potentiated startle paradigm.

Fear-potentiated startle in the rat is a measure of conditioned fear that is blocked by lesions of the central nucleus of the amygdala. In a companion study, Rosen, Hitchcock, Sananes, Miserendino, and Davis (1991) demonstrated a direct anatomical projection from the central nucleus to the brainstem startle reflex circuit. In the present study, fear-potentiated startle was blocked by lesions that interrupted this pathway at 3 different levels or by a crossed lesion that interrupted the pathway at its source on one side and at a more caudal level on the other side. Although synaptic relays have not been ruled out entirely, the data suggest that the direct projection from the central nucleus of the amygdala to the startle circuit mediates the expression of fear-potentiated startle. These findings are consistent with the literature indicating that efferent projections from the central nucleus to various brainstem structures are involved in the expression of several conditioned fear responses.