Pretraining inactivation of the caudal pontine reticular nucleus impairs the acquisition of conditioned fearpotentiated startle to an odor, but not light

Recent data from developing rats suggest that structures downstream from the amygdala are involved in the acquisition of conditioned fear-potentiated startle (FPS). The authors tested this idea in adult rats by temporarily inactivating the structure critical for FPS, the caudal pontine reticular nucleus (PnC), during fear conditioning. When the conditioned stimulus (CS) was an odor, rats displayed freezing, but not FPS, at test. This effect was not due to a decrease in footshock sensitivity. Further, no savings were evident on retraining. When the CS was a light, inactivation of the PnC had no effect on the acquisition of FPS. Thus, the PnC may be crucial for the acquisition of conditioned FPS to an odor, but not a light. The dominant view of classically conditioned fear proposes that a conditioned stimulus (CS) elicits a central state of fear, as opposed to a series of specific stimulus–response associations. Further, both the acquisition and activation of this central state of fear are controlled by the amygdala (Davis, 1992; LeDoux, 2000). Various neural structures are then targeted by the amygdala for the instigation of species-specific defense reactions. For example, in rats, axons from the amygdala to the midbrain periaqueductal gray matter mediate conditioned freezing (Carrive, 1993; DeOca, De-Cola, Maren, & Fanselow, 1998), and projections to the caudal pontine reticular nucleus (PnC) are critical for potentiating the acoustic startle reflex (Davis, Falls, Campeau, & Kim, 1993; Koch, 1999). Evidence for this view comes from findings that a particular fear response can be attenuated by lesions of (or inactivating) its corresponding brain site, while leaving other fear behaviors intac