Females are often less aggressive than males, but they exhibit high levels of agonistic behavior against an intruder in the area of the nest during lactation. This behavior is referred to as maternal aggression. In rats, maternal aggressive behavior occurs more often from postpartum day 3 (PPD 3) to PPD 12. Social instigation is an experimental protocol used to increase the levels of aggression that are typical of the species. In the present study we used social instigation to analyze the expression of a marker of neuronal activity, c-fos. Lactating rats on PPD 5, in the presence of their pups, were divided into four groups: (1) no social instigation and no aggressive behavior, (2) social instigation and no aggressive behavior, (3) no social instigation and aggressive behavior, and (4) social instigation and aggressive behavior. Sixty minutes after the aggression test we used immunohistochemistry to detect Fos in two brain regions, the ventral-orbital region of the prefrontal cortex (VO PFC) and dorsal raphe nucleus (DRN). Our results showed that rats with aggressive behavior that were provoked exhibited an increase in Fos expression in the VO PFC compared with the control group (i.e., no social instigation and no aggressive behavior). No change in Fos expression was found in the DRN. These results complement previous findings with microinjection of serotonin 5-hydroxytryptamine-1B receptor agonists into the same region, demonstrating that the VO PFC is an important region in the modulation of maternal aggressive behavior.

Among rodents, maternal aggression in the postpartum period represents a species-typical adaptation, but when aggressive behavior increases beyond this adaptive level, it can represent a model of excessive aggression. This study assessed the neuroendocrine response of lactating rats and socially instigated male rats. The aim of the present study was to assess neuroendocrine responses and the behavioral pattern of lactating rats and males that were subjected to an emotional stressor using the social instigation protocol. We measured plasma corticosterone levels as the key hormonal parameter of the hypothalamic-pituitary-adrenal (HPA) axis and oxytocin, prolactin, and progesterone, which are released in response to several types of stressors. Our results showed that lactating rats that were subjected to only social instigation or aggressive confrontation in the presence of their pups had lower plasma corticosterone levels, and this response was similar to oxytocin, prolactin, and progesterone levels. By contrast, male rats showed increased corticosterone levels after being subjected only to social instigation. Male rats also engaged in aggressive behavior compared with the control group. In conclusion, this study demonstrated that lactating rats subjected to social instigation exhibited an attenuation of the HPA axis response, which is considered to be crucial to the dam's welfare so that it can care for its offspring. Thus, we can infer that lactation is a relevant factor in neuroendocrine responses to stress because of the increased levels of corticosterone in males.