preparing cd-1 mice for aggressive behaviour towards adolescent c57bl&#47;6 mice for accelerated social defeat stress

Adolescent Social Stress Paradigm in C57BL&amp;#47;6 Mice for Behavioral Research

The chronic social defeat stress paradigm is widely used to model social stress in adult postnatal day (PND) &#62;65 male rodents. This paradigm is based on the natural aggressive behavior of an adult male rodent when an intruder invades its territory. This model is used in a variety of rodent species, including rats, hamsters, and mice1,2,3,4,5,6,7,8,9, and consists of a combination of physical aggression and psychological stress lasting about 10 days, during which an intruder rodent experiments a few min of physical aggression from the resident rodent. The two rodents remain in the resident&#39;s home cage, separated by a divider that allows sensory but not physical contact7. In mice experiments, the most commonly used resident/aggressor mice are retired breeders Swiss CD-1 mice, which show robust territorial behavior against intruder mice6,7. For the intruder mice, the best characterized strain is the inbreed C57BL/6 strain2,4,5. The defeated mouse is exposed to a new aggressor every day to prevent habituation to the aggressor. Control mice are housed with a different conspecific each day. At 24 h after the last defeat session, experimental mice are tested in a social interaction test (SIT) in which they can explore an open field in the absence (no target), or presence of a novel CD-1 mouse (target). Control mice spend more time in the interaction zone with the target versus the non-target part of the task. Defeated mice are classified as susceptible (ratio&#60;1) or resilient (ratio&#62;1) according to a social interaction ratio (time spent in the interaction zone with aggressor present/time spent in the interaction zone with aggressor absent). This procedure provides a useful tool to study individual differences in response to stress.
Until recent years, the chronic social defeat stress model has been used mostly in adult male mice because the accentuated male dominance hierarchies involve fighting against males but not against females6,7. Furthermore, male rodents typically do not attack females; instead, they engage in mating behaviors10. Notwithstanding these obstacles, different strategies have been developed to adapt the chronic social defeat stress paradigm for adult female mice. For example, the California mouse model of social defeat is based on the natural aggression of this monogamous species from both sexes when defending their territory9,11. Other approaches focus on inducing aggressive behavior of the CD-1 mice by stimulating their ventromedial hypothalamus to have consistent aggressive behavior10,12, or by applying male urine in the experimental adult female mice to receive attacks from CD-1 aggressors13. This heightened and consistent aggression of the CD-1 mice is critical for the experimental intruder mouse to show clear behavioral signs of subordination towards the repeated attacks by the aggressor during length of the interaction6.
Adapting the chronic social defeat model to use in adolescent C57BL/6mice 
Adolescence is a period marked by substantial psychosocial maturation, which unfolds in parallel with changes in the micro and macro architecture of the brain, particularly in the prefrontal cortex. In both humans and rodents, there is little consensus regarding the specific onset and end of the adolescent period14,15. Furthermore, there are critical windows of vulnerability within adolescence for experience-induced disruption of ongoing brain and cognitive development16,17,18,19. Puberty and adolescence occur at the same time, but these terms are not synonymous. Puberty signifies the onset of sexual maturation, whereas adolescence represents a broader phase characterized by the gradual shift from a juvenile state to achieving independence20. Different groups have suggested that adolescence in mice spans from weaning (PND 21) until adulthood (PND 60)21. Particularly, early adolescence can be referred to as the first and second weeks postweaning (PND 21-34) and mid-adolescence as the PND 35-48 period. These ranges encompass discrete developmental periods regarding for example the development of the dopamine system22,23,24, vulnerability to drug effects on developing neuronal networks17,25,26,27, and distinct behavioral characteristics16,20,28,29,30.
Fighting behavior from the resident mouse is required for the social defeat protocol. However, as with female mice, males do not typically engage in aggressive interactions with early adolescent mice, possibly because they do not perceive them as a threat. Most studies exploring the effects of chronic social defeat in adolescent C57BL/6 mice have been performed during the mid-adolescent period31,32,33,34,35; others do not specify the postnatal day of adolescent exposure36,37, or extend the days for the defeat to early adulthood38 or do not allow sensory contact39; other studies on adolescent mice use different strains40,41. The characteristics of these studies using chronic social defeat stress in adolescent male mice are summarized in Table 1.
Our research group is interested in targeting specific adolescent windows of exposure, including early adolescence, in C57BL/6 mice. Because of the short duration of the different adolescent periods, a modified version of the accelerated version of the chronic social defeat stress paradigm was designed42. This model was termed accelerated social defeat stress (AcSD) for adolescent mice. Previous work shows that there are significant sex differences in sensitivity to social stress in adolescence in rats8,26,43,44,45, as well as in the harmful effects of social stress on mental health trajectories in humans46,47,48,49,50,51,52,53,54,55,56. The AcSD model works effectively for adolescent female mice, too, allowing them to investigate potential sex-specific consequences as well as explore the neurobiological foundation.
Table 1: Studies using social defeat stress paradigms in adolescent male mice. Strain and species: California Mouse: Peromyscus californicus. C57BL/6: Mus musculus black 6 inbred mouse strain. C57BL/6J: M. musculus black 6 inbred mouse model provided by Jackson&#39;s laboratories. CD-1: M. musculus from Swiss outbred albino mouse strain. ICR: M. musculus Institute of Cancer Research outbred albino mouse strain. OF1: M. musculus Oncins France 1 outbred albino mouse strain. 
Abbreviations: wk = week; PND = Postnatal day; res = resilient; sus = susceptible; unsus = unsusceptible. <a href="https://www.jove.com/files/ftp_upload/66455/Table 1_RE.xlsx" target="_blank">Please click here to download this Table.</a>

Author Spotlight: Understanding Adolescent Social Adversity Effects on Neurodevelopment in Mice

Social Defeat Stress Model for Adolescent C57BL&#47;6 Male and Female Mice

We investigate the impact of adolescent social adversity on neurodevelopment and behavioral outcomes, specifically focusing on male and female C57BL/6 mice. We model social distress using an adapted version of the chronic social defeated stress paradigm. Our model called accelerated social defeated stress is tailored for its use in adolescent C57BL/6 mice.The social defeated stress protocol requires aggressive behavior from the resident mouse. Existing studies are often conducted in the middle adolescent period, or lack a specificity in terms of postnatal day of exposure. Some studies extend into early adulthood or employ the strains of mice that are not commonly used.Implementing the social defeat model in adolescent C57BL/6 mice has been challenging because adult or adolescent mice do not typically attack early adolescent male or female mice and adolescent is a short period of life that encompasses discreet temporal windows of vulnerability. The ACSD model successfully induces social avoidance 24 hours later in both males and females. It is short enough to allow the exposure during discreet periods within adolescence.It allows the segregation of resilient and susceptible mice and in the first model available to study social defeated stress in adolescent C57BL/6 female mice. Our model was generated to be used in adolescent male and female C57BL/6 mouse. This four day stress paradigm with two physical attack sessions per day uses an adult C57BL/6 male mouse to prime the C1 mouse for aggressiveness, ensuring that the C1 mouse readily attacks the experimental male or female mouse.

Preparing CD-1 Mice for Aggressive Behaviour Towards Adolescent C57BL&#47;6 Mice for Accelerated Social Defeat Stress

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