Expression of c-Fos and CRF in the brains of rats differing in the strength of a fear response.

The aim of the study was to examine the neurochemical background of differences in the individual responses to conditioned aversive stimuli, using the strength of a rat conditioned freezing response (the contextual fear test), as a discriminating variable. It was shown that low responders (LR), i.e. rats with duration of a freezing response one standard error, or more, below the mean value, had a higher activity of the M2 cortical area, and the median raphe nucleus (c-Fox expression), in comparison to the high responders (HR), i.e. rats with the duration of a freezing response one standard error, or more, above the mean value. These animals had also stronger 5-HT- and CRF-related immunostaining in the M2 area, and increased concentration of GABA in the basolateral nucleus of amygdala (in vivo microdialysis). The LR group vocalized more during test session in the aversive band, and had higher serum levels of corticosterone, examined 10 min after test session. It was shown that different natural patterns of responding to conditioned aversive stimuli are associated with different involvement of brain structures and with dissimilar neurochemical mechanisms.