Enhanced social motivation in briefly isolated male rats.

Loneliness and anxiety are associated with psychiatric disorders in humans. Although brief social isolation in adult rats and mice has been proposed as a rodent model of loneliness, its socioemotional characteristics are not well known. In this study, we evaluated the social and emotional behaviors of adult male rats subjected to brief social isolation. Isolated rats frequently showed sniffing behavior toward empty cylinders where conspecifics had previously existed, as well as conspecifics themselves. Furthermore, social motivation correlated with anxiety levels, as indicated by the elevated plus-maze test performance in isolated but not in non-isolated rats. These results suggest that high social motivation is associated with anxiety in briefly isolated rats.

Socially Transferred Stress Experience Modulates Social Affective Behaviors in Rats.

Social communication of affective states between individuals, as well as actual experiences, influences their internal states and behaviors. Although prior stress experiences promote empathy-like behaviors, it remains unclear whether the social transmission of stress events modulates these behaviors. Here, we provide evidence that transferred stress experiences from cage mates modulate socioaffective approach-avoidance behaviors in rats. Male Wistar-Imamichi rats were assigned to one of five experimental groups (Control (n = 15); no shock with shocked cage mates (n = 15); low (0.1 mA, n = 15), middle (0.5 mA, n = 14), and high shock (1.0 mA, n = 14)). Except for the naïve and housed with stressed mate groups, rats received two foot-shocks (5 s for each). The next day, the subjects were allowed to explore two unfamiliar conspecifics; one was a naïve, while the other was a distressed conspecific that received two foot-shocks (1.0 mA, 5 s) immediately before the test. Rats that were housed with stressed mates, as well as those that experienced a higher intensity of foot-shocks, were more likely to approach, while naïve rats avoided, a distressed conspecific. These results suggest that socially transferred stress shifts socioaffective response styles from avoidance to approach toward a stressed conspecific in rats.

Prior stress experience modulates social preference for stressed conspecifics in male rats.

Adult male rats tend to avoid adult conspecifics in distress. In this study, we asked whether prior stress experience would modulate social preference for a stressed conspecific using a social affective preference (SAP) test. Male Long-Evans adult rats were assigned to the shocked and non-shocked groups. In the shocked group, rats were acutely subjected to foot shocks (1.0 mA, 5 s duration × 2) 24 h before the SAP test. During the SAP test, the experimental rats were placed in an arena where two adult conspecific stimuli, one of which received the foot shocks immediately before the SAP test, were presented at both ends and allowed to explore freely for 5 min. We measured sniffing behavior toward each conspecific as an index of social preference. Non-shocked adult rats avoided, while shocked rats approached, the stressed conspecifics more than the non-stressed ones. These results suggest that prior stress promotes social preference for a stressed conspecific in adult male rats.

Enhanced methamphetamine-induced conditioned place preference in risk-taking rats.

Patients with psychiatric disorders, such as gambling and substance use, tend to exhibit maladaptive decision-making. In this study, we assessed individual differences in risk-taking behaviors using a rat gambling task (GT) and investigated the relationship between risk-taking behaviors and vulnerability to drug dependence using methamphetamine (METH)-induced conditioned place preference (CPP). In the GT using a radial arm maze, male Long-Evans rats were trained to choose one of three choice arms (a low-risk/low reward (L-L), a high-risk/high reward (H-H), and an empty arm) in 16 trials per day for 14 days. METH-induced CPP consisted of 6 sessions: habituation, conditioning, preference test (Test I), extinction, extinction test (Test II), and reinstatement test (Test III). Results demonstrated that the percentage of choosing the H-H arm was significantly positively correlated with the percentage of time spent in the METH-paired compartment in the preference test, but not with the extinction and reinstatement tests, suggesting that risk-taking rats are more vulnerable to drug dependence.

Social enrichment improves social recognition memory in male rats.

The social environment is thought to have a strong impact on cognitive functions. In the present study, we investigated whether social enrichment could affect rats' memory ability using the "Different Objects Task (DOT)," in which the levels of memory load could be modulated by changing the number of objects to be remembered. In addition, we applied the DOT to a social discrimination task using unfamiliar conspecific juveniles instead of objects. Animals were housed in one of the three different housing conditions after weaning [postnatal day (PND) 21]: social-separated (1 per cage), standard (3 per cage), or social-enriched (10 per cage) conditions. The object and social recognition tasks were conducted on PND 60. In the sample phase, the rats were allowed to explore a field in which 3, 4, or 5 different, unfamiliar stimuli (conspecific juveniles through a mesh or objects) were presented. In the test phase conducted after a 5-min delay, social-separated rats were able to discriminate the novel conspecific from the familiar ones only under the condition in which three different conspecifics were presented; social-enriched rats managed to recognize the novel conspecific even under the condition of five different conspecifics. On the other hand, in the object recognition task, both social-separated and social-enriched rats were able to discriminate the novel object from the familiar ones under the condition of five different objects. These results suggest that social enrichment can enhance social, but not object, memory span.