Contributions of the amygdala central nucleus and ventrolateral periaqueductal grey to freezing and instrumental suppression in Pavlovian fear conditioning

In Pavlovian fear conditioning animals receive pairings of a neutral cue and an aversive stimulus such as an electric foot-shock. Through such pairings, the cue will come to elicit a central state of fear that produces a variety of autonomic and behavioral responses, among which are conditioned freezing and suppression of instrumental responding, termed conditioned suppression. The central nucleus of the amygdala (CeA) and the ventrolateral periaqueductal grey (vlPAG) has been strongly implicated in the acquisition and expression of conditioned fear. However, previous work suggests different roles for the CeA and vlPAG in fear learning maybe revealed when fear is assessed with conditioned freezing or conditioned suppression. To further explore this possibility we gave rats selective lesions of either the CeA or vlPAG and trained them in Pavlovian first-order fear conditioning as well as Pavlovian second-order fear conditioning. We concurrently assessed the acquisition of conditioned freezing and conditioned suppression. We found that vlPAG and CeA lesions impaired both first- and second-order conditioned freezing. VlPAG lesions did not impair, and CeA lesions only transiently impaired, first-order conditioned suppression. However, both vlPAG and CeA lesions impaired second-order conditioned suppression. These results suggest that the CeA and vlPAG are critically important to expressing fear through conditioned freezing but play different and less critical roles in expressing fear through conditioned suppression.