REGIONAL DISTRIBUTION OF FOS-LIKE IMMUNOREACTIVITY IN THE RAT-BRAIN AFTER EXPOSURE TO FEAR-INDUCING STIMULI

Fos protein immunohistochemistry was used to identify the neural subst rate of fear/anxiety. The structures activated by exposure of Long Eva ns male rats (280-300 g) to the elevated plus-maze, a widely used anim al model of anxiety, were compared with those activated by chemical st imulation of two aversive areas of the brain, the dorsal periaqueducta l gray matter and the medial hypothalamus. Three different patterns of activation were obtained: Pattern 1 resulted from microinjection of t he excitatory amino acid kainate (60 pmol; N = 5) or of the GABA(A) re ceptor antagonist SR-95531 (16 pmol; N = 3) into the dorsal periaquedu ctal gray matter and consisted mainly of caudal structures; Pattern 2 was observed after kainate injection (60 pmol; N = 4) into the medial hypothalamus and had a predominantly prosencephalic distribution; Patt ern 3 extended from rostral to caudal brain regions and was induced by microinjection of either SR-95531 (16 pmol; N = 1) or kainate (120 pm ol; N = 3) into the medial hypothalamus, as well as by 15-min exposure to the plus-maze (N = 3). Control animals were either injected with s aline into the MH (N = 3) or the PAG (N = 3) or were exposed for 15 s to the elevated plus maze (N = 3) and exhibited no significant labelin g. These results further support the participation of periventricular structures in the regulation of fear and aversion.