Hypothalamic sites responding to predator threats--the role of the dorsal premammillary nucleus in unconditioned and conditioned antipredatory defensive behavior.

In this study we provide a comprehensive analysis of the hypothalamic activation pattern during exposure to a live predator or an environment previously associated with a predator. Our results support the view that hypothalamic processing of the actual and the contextual predatory threats share the same circuit, in which the dorsal premammillary nucleus (PMd) plays a pivotal role in amplifying this processing. To further understand the role of the PMd in the circuit organizing antipredatory defensive behaviors, we studied rats with cytotoxic PMd lesions during cat exposure and examined the pattern of behavioral responses as well as how PMd lesions affect the neuronal activation of the systems engaged in predator detection, in contextual memory formation and in defensive behavioral responses. Next, we investigated how pharmacological blockade of the PMd interferes with the conditioned behavioral responses to a context previously associated with a predator, and how this blockade affects the activation pattern of periaqueductal gray (PAG) sites likely to organize the conditioned behavioral responses to the predatory context. Behavioral observations indicate that the PMd interferes with both unconditioned and conditioned antipredatory defensive behavior. Moreover, we have shown that the PMd influences the activation of its major projecting targets, i.e. the ventral part of the anteromedial thalamic nucleus which is likely to influence mnemonic processing, and PAG sites involved in the expression of antipredatory unconditioned and conditioned behavioral responses. Of particular relevance, this work provides evidence to elucidate the basic organization of the neural circuits integrating unconditioned and contextual conditioned responses to predatory threats.

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.