Abstract
Return of fear after successful exposure therapy calls for a better understanding of the mechanisms of relapse. Classical conditioning research provides a useful framework for conceptualising the acquisition, extinction and reappearance of fear. The present paper focuses on reinstatement, the return of extinguished conditioned responses due to the experience of one or more unconditioned stimuli (USs) after extinction. This phenomenon illustrates that unpredictable USs can lead to a return of fear after successful exposure. The data we present is one of the first demonstrations that conditioned suppression of instrumental behaviour can be used as an index of classical conditioning in laboratory mice. The procedure proves to be a promising instrument for assessing fear in mice, both in the context of research aimed at unravelling the functional characteristics of learning and memory in healthy mice and in the context of research aimed at unravelling the neurobiological substrate of psychiatric disorders, e.g., in studies with transgenic and knockout mice. Using this procedure, we report the first observation of reinstatement of conditioned suppression in this species.