"When a rat smells a cat": the distribution of Fos immunoreactivity in rat brain following exposure to a predatory odor.

Wistar rats were exposed to a fabric collar that had been worn by a domestic cat. Exposure took place in an open rectangular arena containing a small wooden "hide box". Rats exposed to cat odor spent more than 87% of their time in the hide box during a single 20-min exposure session, whereas rats exposed to a control odor (an unworn collar) spent less than 20% of their time hiding. One hour following this session, rats were killed and Fos immunoreactivity was compared between cat odor-exposed rats, control odor-exposed rats and an additional group that had remained in their home cages. Cat odor-exposed rats showed greater Fos expression than controls in many brain regions, particularly in the medial amygdala, medial hypothalamus and periaqueductal gray. Significant findings included strong and selective induction of Fos in the posteroventral medial amygdaloid nucleus, the premamillary nucleus (dorsal part), ventromedial hypothalamic nucleus (dorsomedial part), dorsomedial hypothalamic nucleus, periaqueductal gray (dorsomedial, dorsolateral and ventrolateral parts) and the cuneiform nucleus. Robust Fos expression in the ventromedial hypothalamus, premamillary nucleus and periaqueductal gray confirms previous suggestions of a role for these areas in predator-induced defensive behavior. Fos immunoreactivity in the medial, but not central or basolateral amygdala is a novel finding and draws attention to this subregion as a possible interface between olfactory input and emotional output.