Fear response of rat pups to a non-aversive social stimulus: Evidence for the involvement of memory processes.

Learning processes in rats during early development are importantly mediated by the mother, which represents the primary source of environmental information. This study aimed to determine whether aversive early experiences can induce the expression of pups' fear responses toward a non-aversive stimulus as a consequence of a memory process. First, we determined pups' fear responses toward an anesthetized female after being exposed to this stimulus or an empty cage together with their mothers from Postnatal Day (PNDs) 1 to 4. Second, we evaluated if the administration of the protein synthesis inhibitor cycloheximide (CHX; 0.2 mg/kg, subcutaneously (sc).) disrupted the reconsolidation processes and abolished the fear response on PND 9. Only female pups previously exposed to the female intruder expressed fear responses toward an anesthetized female on PND 8. CHX administration to female pups immediately after exposure to an anesthetized female on PND 8 suppressed fear responses on PND 9, indicating that the fear expression was the result of a memory process, probably mediated by the mother. These findings demonstrated that early experiences can shape responses to social stimuli in a sex-dependent manner and emphasize the critical role of the mother in influencing fear learning in a social context.