A single reminder trial updates fear memory and affects extinction in rats housed under different living conditions

Abstract

The aim of the present study was to investigate the influence of housing of rats in the enriched environment (EE), social isolation (SI), overcrowding (CROW) and standard (STAND) conditions on the updating, reconsolidation and subsequent extinction of fear memory. Male and female rats were placed in different conditions from 30 to 120 postnatal days (PND). At PND 90 in all rats, the fear conditioning was elaborated. 24 hours later, half of the rats were re-exposed to the same context and cue presentation (Re+) and the other half did not receive re-exposure (Re-). After a further 24 hours, the retention test was used to assess reconsolidation of contextual and cue memory. For the next 2 days, the extinction of the conditioned fear response by applying 10 non-reinforced trials each day was carried out. In the retention test, the freezing time in response to the context or tone in Re+ groups housed in the STAND, EE or CROW conditions was substantially less than in Re- groups, which indicated memory updating. The freezing time to the tone in the SI Re+ vs. Re- groups were about the same and the freezing time in the SI Re+ was significantly longer than in the Re+ groups in STAND and CROW conditions. The decrease of the freezing time in response to the cue presentation in Re+ vs. Re- rats was maximal in the CROW group. The changes in fear memory elicited by reactivation persisted during the extinction trials. The results obtained indicate that the reactivation procedure in the SI rats does not update fear memory, and the memory of the CROW rats was the least stable. Two types of psychosocial stresses, social isolation and overcrowding, have multidirectional effects on the reconsolidation of fear memory.