Research report Mapping the neural circuit activated by alarm pheromone perception by c-Fos immunohistochemistry Yasushi Kiyokawa, Takefumi Kikusui T , Yukari Takeuchi, Yuji Mori Laboratory of Veterinary Ethology, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan Accepted 23 February 2005 Available online 1 April 2005 Abstract We previously reported that the alarm pheromones released from stressed male rats exaggerated both behavioral and autonomic (stress- induced hyperthermia) responses in recipient rats that were introduced into a novel environment. Subsequent experiments provided evidence that these alarm pheromones could be divided into two functionally different categories based on the site specificity and testosterone dependency of their production. However, the neural mechanisms underlying these behavioral and physiological responses remain unknown. In the present study, we examined Fos expression in 26 brain sites of the recipient rat 60 min after the exposure to the pheromone that aggravated stress-induced hyperthermia. The alarm pheromone-exposed rats showed a concurrent increase in Fos expression, in contrast to control odor-exposed rats in the anterior division lateral and medial group of the bed nucleus of the stria terminalis, paraventricular nucleus, dorsomedial hypothalamic nucleus, anterodorsal medial, lateral and basolateral amygdaloid nucleus, ventrolateral periaqueductal gray, laterodorsal tegmental nucleus, and locus coeruleus. These results provide information about the neural mechanisms in response to a non- sexual pheromone, i.e., an alarm pheromone, and suggest that the perception of the alarm pheromone is related to stress-responsive brains structures, including the hypothalamus and brainstem, as well as to the amygdaloid nuclei. D 2005 Elsevier B.V. All rights reserved. Theme: Other systems of the CNS Topic: Limbic system and hypothalamus Keywords: Alarm pheromone; Stress-induced hyperthermia; Brain mapping; Rat; Medial amygdala; The bed nucleus of the stria terminalis 1. Introduction Chemical communication plays important roles in various social interactions among mammals, including sexual [52], territorial [20], and maternal behaviors [33]. Alarm chemo- signals, which alert animals to the proximity of conspecific individuals [1,4,53], are considered to be used widely in the animal kingdom. In rodent species, it was reported that rats could distinguish between the odors released from stressed and non-stressed conspecifics [51]. These odors were shown to change the behavior [1,34,35] and immune responses of the recipient [11]. We previously reported that the alarm pheromones released from male rats receiving foot shocks augmented both behavioral (increased sniffing, walking and rearing, and decreased resting behavior) and autonomic (stress- induced hyperthermia, SIH) responses in recipient male rats [26]. We then found that these alarm pheromones could be divided into two functionally different categories based on the androgen-dependent production and the area of the body surface from which they were released, namely, one category of alarm pheromone modifying recipient behavior is released from the whisker pad of the male in a testosterone-dependent manner, whereas the other category of alarm pheromone influencing the autonomic response is released from the perianal region in a testosterone-inde- pendent manner [27,28]. Considering that the importance of propagating the notice of a dangerous situation to family or 0006-8993/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2005.02.061 T Corresponding author. Fax: +81 3 5841 8190. E-mail address: akikus@mail.ecc.u-tokyo.ac.jp (T. Kikusui). Brain Research 1043 (2005) 145 – 154 www.elsevier.com/locate/brainres