Social recognition memory: influence of age, sex, and ovarian hormonal status

Abstract

Social recognition memory underlies many forms of rodent interaction and can be easily tested in the laboratory. Sex differences in aspects of this memory have been reported among young adults, and some studies indicate an age-related decline among male rats. In contrast, neither the impact of natural fluctuations in ovarian hormones nor the performance of aged female rats on social recognition memory has been previously evaluated. In experiments 1 and 2, the social recognition memory of young adult female Long-Evans rats (age 3-5 months) was compared during proestrus and estrus, and performance was found to be stable across estrous cycle phases. In experiment 3, the social recognition memory of young adults as compared to aged (16.5-19.5 months) rats was tested using the social discrimination procedure, following delays of 15, 45, 90 or 120 min. The estropausal status of aged female rats was tracked during the experiment but was not found to influence memory ability. Males of both ages investigated juveniles (both novel and familiar) more than did females, although despite this difference, both sexes demonstrated robust memory. Interestingly, only young adult females were capable of demonstrating memory following the longest delay. Collectively, our findings indicate that the pattern of age-related changes in social recognition memory is subtle and that aging does not greatly alter the behavioral sex differences observed among young adults.

Figure 1

Experiment 1 (n=7). A Young adult females spent less time investigating when introduced to a juvenile female during proestrus compared to estrus (*p<.03). B. Young adult females (n=7) demonstrated equivalently robust social recognition memory of juvenile females during proestrus and estrus (*main effect of familiarity p<.01). Time spent investigating during the recognition portion is presented as a proportion of time spent investigating during the introduction.

Figure 2

Experiment 2 (n=16). A. Estrous cycle phase did not influence the amount of time taken by young adult females to achieve 15 seconds of investigation when introduced to a juvenile female. B. When investigation of the juvenile during the introduction was held constant at 15 seconds, young adult females still demonstrated equivalently robust social recognition memory during proestrus and estrus (*main effect of familiarity p<.001).

Figure 3

Experiment 3 (n=42; young adult males (n=10) and females (n=10), aged males (n=6) and females n=12). A. Both sex and age influenced the amount of time taken by animals to obtain 30 seconds of investigation when introduced to a juvenile of the same sex: females took longer than males (p<.001) and aged animals took longer than young adults (p<.06). * malesfamiliar comparison following the indicated delay. +over white bars indicates a statistical trend towards novel>familiar comparison following the indicated delay.