Mean girls: social stress models for female rodents

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• Abstract: Social stressors are known to have strong negative impacts on mental health. There is a long history of preclinical social defeat stress studies in rodents focusing on males that has produced important insights into the neural mechanisms that modulate depression- and anxiety-related behavior. Despite these impressive results, a historical weakness of rodent social stress models has been an under-representation of studies in females. This is problematic because rates of depression and anxiety are higher in women versus men. Recently there has been a surge of interest in adapting social stress methods for female rodents. Here we review new rodent models that have investigated numerous facets of social stress in females. The different models have different strengths and weaknesses, with some model systems having stronger ethological validity with other models having better access to molecular tools to manipulate neural circuits. Continued use and refinement of these complementary models will be critical for addressing gaps in understanding the function of neural circuits modulating depression- and anxiety-related behavior in females.

  What is a pair bond?

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• Abstract: Pair bonding is a psychological construct that we attempt to operationalize via behavioral and physiological measurements. Yet, pair bonding has been both defined differently in various taxonomic groups as well as used loosely to describe not just a psychological and affective phenomenon, but also a social structure or mating system (either social monogamy or just pair living). In this review, we ask the questions: What has been the historical definition of a pair bond? Has this definition differed across taxonomic groups? What behavioral evidence do we see of pair bonding in these groups? Does this observed evidence alter the definition of pair bonding? Does the observed neurobiology underlying these behaviors affect this definition as well? And finally, what are the upcoming directions in which the study of pair bonding needs to head?

  Sex differences in responses to aggressive encounters among California mice

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• Abstract: Despite how widespread female aggression is across the animal kingdom, there remains much unknown about its neuroendocrine mechanisms, especially in females that engage in aggression outside the peripartum period. Although the impact of aggressive experience on steroid hormone responses have been described, little is known about the impact of these experiences on female behavior or the subsequent neuropeptide responses to performing aggression. In this study, we compared behavioral responses in both male and female adult California mice based on if they had 0, 1, or 3 aggressive encounters using a resident intruder paradigm. We measured how arginine vasopressin and oxytocin cells in the paraventricular nucleus responded to aggression using c-fos immunohistochemistry. We saw that both sexes disengaged from intruders with repeated aggressive encounters, but that on the final day of testing females were more likely to freeze when they encountered intruders compared to no aggression controls – which was not significant in males. Finally, we saw that percent of arginine vasopressin and c-fos co-localizations in the posterior region of the paraventricular nucleus increased in males who fought compared to no aggression controls. No difference was observed in females. Overall, there is evidence that engaging in aggression induces stress responses in both sexes, and that females may be more sensitive to the effects of fighting.