An alternative experimental procedure for studying predator-related defensive responses.

In the present study, we introduce an experimental procedure to study, in rats, a wide range of natural defensive reactions. Animals were tested in an experimental apparatus that consisted of a home cage (25 x 25 x 25 cm) connected to another chamber (25 x 25 x 25 cm-the food compartment) by a hallway (12.5 cm wide and 100 cm long, with 25-cm high walls). During 10 days before the testing procedures, each animal was isolated in the home cage, and, at the beginning of the dark phase, allowed to explore the rest of the apparatus and obtain food pellets stored in the food compartment. The testing consisted of three phases: exploring a familiar and safe environment (phase 1, on the 10th day), cat exposure (phase 2, on the 11th day), and, on the following day, exposure to the environment where the predator had been previously encountered (phase 3). These three conditions thus provided a low-defense baseline; a high level of freezing during cat exposure; and a high level of risk assessment to the hostile environment condition. An important feature of the present experimental procedure was that the behavioral responses were very stable among the animals tested within each individual phase of the testing schedule. In each phase of the testing schedule, we have also examined the Fos immunoreactivity in pontine periventricular sites related to controlling behavioral activation (i.e. the nucleus incertus) or attentional status (i.e. the locus coeruleus). Animals actively exploring a safe and familiar environment presented an increased activation of the nucleus incertus; the locus coeruleus, in turn, was particularly activated during cat exposure, and also, to lesser degree, during exposure to the hostile environment. These results give further support to the view that the animals present quite distinct behavioral states during each one of the testing situations. Taken together, the evidence suggests the present experimental procedure as particularly suitable for analyzing the neural basis of a number of specific defensive responses.