CONICET | Buscador de Institutos y Recursos Humanos

Social recognition memory was examined at two opposite circadian phases of adult Wistar rats using an intruder-resident paradigm. In this paradigm repeated exposure to the same juvenile intruder leads to a decrease in social investigation by the adult resident rat, as compared to repetitive exposure to different juvenile intruders. The protocol involved a 2x2 schedule, having phase (independent groups of animals were tested in the active and inactive phases) and intertrial interval (30 or 60min) as factors. Wheel-running activity was monitored continuously for identification of circadian phase. To avoid possible masking effects of a light-dark cycle, animals were synchronized to a skeleton photoperiod; this allowed testing them at different circadian phases but in the same lighting conditions. In each trial, the intruder was placed in the adult´s homecage for 5min while social behaviors were registered. Rats were exposed to 5 trials per day along 4 consecutive days; while on days 1 and 2 each resident was exposed to the same intruder, on days 3 and 4 each resident was exposed to a different intruder at each trial. The ANOVA revealed significant phase x intruder x trial x day (F4,116=8.06, P<0.0001), phase x interval x trial (F4,116=2.91, P<0.02), intruder x trial (F4,116=6.7, P<0.001) interaction effects, and a significant intruder effect (F1,29=114.62, P<0.0001). As expected, the resident´s social behavior was greater when different intruders were presented as compared to the corresponding parameter when the same intruder was repeatedly presented, revealing social recognition memory. This effect was stronger when the rats were tested in the inactive phase, particularly when the intertrial interval was 60 min. These results show that social recognition memory, as evaluated by the intruder-resident paradigm, depends upon the phase of the circadian rhythm during which testing is carried out. Financial support by FAPESP, CNPq and CAPES, Brazil.

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