Sensory and motivational modulation of immediate and delayed defensive responses under dynamic threat.

BACKGROUND: Despite the wide variations in defensive behaviors displayed by rats during predator encounters, most laboratory studies have focused on a limited number of behaviors, such as immobility in a small enclosure. Other defensive behaviors such as withdrawal and avoidance have been less investigated.

NEW METHOD: Rats repeatedly encountered a fast threatening motion (snapping claws) of a robot predator. The robot was equipped with infrared sensors (millisecond resolution) to detect head entry. A camera placed above arena tracked the body position (sub-second resolution). Sensory and motivational components regulating rats' behaviors were investigated.

RESULTS: The rats exhibited head-withdrawal reflex (HWR). The mean HWR speed depended on both the visual and vibrissal inputs. However, successful escape depended on the vibrissal input. Auditory information later induced conditioned HWR. Slower defensive behaviors such as the stretched posture and freezing were modulated by repeated exposure and motivation level. Introducing a food pellet induced higher rates of approach and elongated posture while reducing time spent hiding and freezing.

COMPARISON WITH EXISTING METHOD(S): An encounter with a fast-striking claw, while foraging for food is more ethologically relevant than dermal shocks, which have been widely used for studying defensive behaviors among rats. More diverse behavioral patterns were observed.

CONCLUSION: A combination of fast and slow defensive behaviors was exhibited in rats when they were exposed to a dynamic threat. Thus, the use of the new dynamic threat model will allow for a more accurate investigation of the various aspects of defensive behavior and emotional regulation in animals.