Abstract
A modified version of the intruder-resident paradigm was used to investigate if social recognition memory lasts at least 24 h. One hundred and forty-six adult male Wistar rats were used. Independent groups of rats were exposed to an intruder for 0.083, 0.5, 2, 24, or 168 h and tested 24 h after the first encounter with the familiar or a different conspecific. Factor analysis was employed to identify associations between behaviors and treatments. Resident rats exhibited a 24-h social recognition memory, as indicated by a 3- to 5-fold decrease in social behaviors in the second encounter with the same conspecific compared to those observed for a different conspecific, when the duration of the first encounter was 2 h or longer. It was possible to distinguish between two different categories of social behaviors and their expression depended on the duration of the first encounter. Sniffing the anogenital area (49.9% of the social behaviors), sniffing the body (17.9%), sniffing the head (3%), and following the conspecific (3.1%), exhibited mostly by resident rats, characterized social investigation and revealed long-term social recognition memory. However, dominance (23.8%) and mild aggression (2.3%), exhibited by both resident and intruders, characterized social agonistic behaviors and were not affected by memory. Differently, sniffing the environment (76.8% of the non-social behaviors) and rearing (14.3%), both exhibited mostly by adult intruder rats, characterized non-social behaviors. Together, these results show that social recognition memory in rats may last at least 24 h after a 2-h or longer exposure to the conspecific.
Highlights
Recognition of a conspecific is advantageous because members of the group can spend time on activities related to group management and protection rather than on vigorous investigation of a previously met non-hazardous individual
The results of the present study show for the first time that male rats exhibit social recognition memory that lasts at least 24 h
This effect was revealed by a significantly longer social investigation by the resident rats exposed to a different adult intruder conspecific as compared to exposure to the adult familiar intruder conspecific (Figure 1A,B)
Summary
Recognition of a conspecific is advantageous because members of the group can spend time on activities related to group management and protection rather than on vigorous investigation of a previously met non-hazardous individual. The introduction of the familiar intruder within the resident’s home cage for another 5-min encounter elicits far less social investigation compared to that which occurred during the prior encounter and that seen towards a novel intruder. This effect is only seen when the intertrial interval (ITI) is about 30-60 min but not when it is about 2 h [1,5,7,10]; that is, at longer ITIs, the effect vanishes. The intruder-resident paradigm has been used as a research tool to investigate short-term memory in rats [5,7,10,11,12], and may potentially provide an animal model for investigating social interaction dysfunctions such as autism [13]
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