'When a rat smells a cat': The distribution of Fos immunoreactivity in ratbrain following exposure to a predatory odor

Wistar rats were exposed to a fabric collar that had been worn by a domesti c 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 ti me in the hide box during a single 20-min exposure session, whereas rats ex posed to a control odor (an unworn collar) spent less than 20% of their tim e hiding. One hour following this session, rats were killed and Fos immunor eactivity was compared between cat odor-exposed rats, control odor-exposed rats and an additional group that had remained in their home cages. Cat odo r-exposed rats showed greater Fos expression than controls in many brain re gions, particularly in the medial amygdala, medial hypothalamus and periaqu eductal gray. Significant findings included strong and selective induction of Fos in the posteroventral medial amygdaloid nucleus, the premamillary nu cleus (dorsal part). ventromedial hypothalamic nucleus (dorsomedial part), dorsomedial hypothalamic nucleus, periaqueductal gray (dorsomedial, dorsola teral and ventrolateral parts) and the cuneiform nucleus. Robust Fos expression in the ventromedial hypothalamus. premamillary nucleu s 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 draw s attention to this subregion as a possible interface between olfactory inp ut and emotional output. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.