Fos-like immunoreactivity in the periaqueductal gray of rats exposed to a natural predator

IN the present study we examined, in rats exposed to a predator (cat), the distribution of neurons expressing Fos along the continuum formed by the central gray surrounding the caudal pole of the third ventricle and the periaqueductal gray (PAG). After the predatory encounter, a distinct cluster of Fos-immunoreactive cells was observed in the precommissural nucleus. In the rostral two-thirds of the FAG, Fos expression was mostly seen in the dorsomedial and dorsolateral regions. In contrast, at caudal levels of the FAG, most of the Fos-labelled neurons were distributed in the lateral and ventrolateral FAG. These results are discussed and compared with the pattern of the FAG activation after fear conditioned to a context or elicited by aversive foot shock. (C) 1999 Lippincott Williams & Wilkins.