Between-subject transfer of emotional information evokes specific pattern of amygdala activation.

Emotional states displayed by an animal or a human can seriously affect behavior of their conspecifics. The amygdala plays a crucial role in the processing of emotions. In this study, we describe an experimental rat model of between-subject transfer of emotional information and its effects on activation of the amygdala. The rats were kept in pairs, and one animal (designated as "demonstrator") was treated to specific behavioral training of either foot-shock-reinforced context conditioning or just exposure to a novel context. We next examined the influence of the demonstrators on the exploratory behavior of their cagemates (called "observers") and the observers' performance of the acoustic startle response. We report that we can distinguish both groups of observers from the control animals (as shown by startle-response measure) and distinguish between observers (by means of indexing the exploration), with respect to whether they were paired with demonstrators treated to different experimental conditions. Furthermore, we show that the observers have most of their amygdala activated (as revealed by c-Fos mapping) to the same level as the demonstrators and, in the case of the central amygdala, to an even higher level. Moreover, the level of c-Fos expression in the observers reflected the specific behavioral treatment of the demonstrators with whom they were paired. Thus, in this study, we have shown that undefined emotional information transferred by a cohabitant rat can be evaluated and measured and that it evokes very strong and information-specific activation of the amygdala.