This article describes the use of chronic resident intruder social stress as an ethologically relevant paradigm to model postpartum depression and anxiety in lactating rodents.
Exposure to chronic stress is a reliable predictor of depressive disorders, and social stress is a common ethologically relevant stressor in both animals and humans. However, many animal models of depression were developed in males and are not applicable or effective in studies of postpartum females. Recent studies have reported significant effects of chronic social stress during lactation, an ethologically relevant and effective stressor, on maternal behavior, growth, and behavioral neuroendocrinology. This manuscript will describe this chronic social stress paradigm using repeated exposure of a lactating dam to a novel male intruder, and the assessment of the behavioral, physiological, and neuroendocrine effects of this model. Chronic social stress (CSS) is a valuable model for studying the effects of stress on the behavior and physiology of the dam as well as her offspring and future generations. The exposure of pups to CSS can also be used as an early life stress that has long term effects on behavior, physiology, and neuroendocrinology.
The etiology of mood disorders, including postpartum depression, often involves exposure to chronic stress1,2. Although depression in mothers has negative effects on both parenting 3 and psychological development of the offspring 4 ,the vast majority of studies on depression and anxiety focus on male animals and humans. Although there are a few animal models of altered maternal behavior using chronic stress, most have substantial physiological components which may not pertain to chronic psychosocial stressors commonly experienced by humans or focus mostly on the prepartum period. Chronic restraint stress during pregnancy in rats increases maternal care and anxiety and decreases maternal aggression in some studies 5, but decreases maternal care in others 6. Restraint stress is a potent physical stressor 7, and some reports are inconsistent with the attenuated maternal care observed in human mothers with postpartum depression 4. Prenatal ultramild stress does not affect pup care, but does reduce maternal aggression 8. Ultramild stress paradigms use a variety of randomized stressors (such as 30 degree cage tilt, reversed light cycle, confinement, overnight illumination, and soiled cage) over several days, and the effect of individual components is often unclear. It is likely that some stressors used in ultramild stress paradigms, such as reversed light cycle and overnight illumination directly affect maternal care independent of any stress effects, as maternal care in rats has a circadian rhythm. Exposure to wet bedding is a significant thermal stress to the pups, and can result in pup mortality in less than 24 hr in young litters (personal observations).
Recent rodent studies have been focusing on the use of ethologically relevant stressors, such as chronic social stress (CSS), in the study of the effects of stress on offspring behavior and mood disorders9-13. In an effort to develop an ethological model for postpartum depression, reduce the physiological component of the stressor used, and generate consistent behavioral and physiological results, our lab has focused on the use of social stress. While chronic social defeat is effectively used as a model for social stress-induced depression in male rodents 14, this is not an ecologically relevant behavioral model for many maternal rodents if male intruders are used, as a single social defeat of a maternal rodent usually results in cannibalism of the litter by the male intruder. However, acute exposure to a novel male intruder, without social defeat of the dam, elicits robust behavioral and endocrine stress responses in lactating females15-18. There is also evidence that chronic stress during lactation, including exposure to a male intruder, directly impairs milk production 19,20, which may impact pup growth. These results in maternal rats are supported by studies in virgin females, where chronic social instability (varying isolation and crowding) increases hypothalamic–pituitary–adrenal axis activity 11,21, alters body temperature regulation, reduces sucrose and food intake, and has been proposed as an animal model for female depression11. Self grooming, a measure of anxiety 22,23 is also sensitive to social stress manipulations 24,25.
Chronic social stress (CSS) during lactation can disrupt maternal behavior and attenuate growth of the dam and offspring. The daily presentation of a novel male intruder during lactation decreases maternal care, increases maternal aggression towards a novel male intruder and self grooming, and inhibits growth in both the dam and her offspring 10. These effects indicate that CSS is an ethologically relevant and effective model for mood disorders that involve impairments in maternal care, anhedonia, elevated anxiety, and attenuated offspring growth, such as postpartum depression. Both the maternal care and offspring data indicate that CSS may have enduring effects on the future generations, and research on the effects of chronic stress on maternal behavior support the hypothesis that the effects of chronic stress on maternal behavior are hormonally mediated (Murgatroyd and Nephew 2012). Furthermore, recent data from the adult female offspring of stressed dams supports the hypothesis that CSS as a form of early life stress causes changes in central oxytocin (OXT) and prolactin (PRL) which are associated with inefficient nursing behavior 12. The CSS model complements other effective models of stress induced disruptions in maternal behavior, such as limiting nesting material26,27 and maternal separation28,29. Unique aspects of the CSS model are that it is ethologically relevant in both rodents and humans, has a relatively minor physiological component, and can be combined with similar stressors at other life stages (social defeat, chronic social instability).
The primary objectives in developing the CSS paradigm were to use an ethologically and clinically relevant stressor, induce anhedonia (as measured by maternal care and saccharin preference), increase behavioral measures of anxiety, and impair pup physiological, behavioral, and or endocrine development. The purpose of the model is to test current and novel treatments and practices for postpartum depression and anxiety.
The data obtained from the CSS paradigm is most likely to be consistent and repeatable in a stable behavioral research environment. It is argued that the housing and husbandry of lab animals is a component that needs to be more carefully considered in any type of study, especially behavioral studies. Housing and husbandry practices which may introduce confounds include: starting with pregnant dams shipped from the supplier, moving dams between rooms daily for behavioral testing, variation in the time of day of behavioral testing, excessive exposure of dams to novel sights, sounds, or odors, not using home cage for behavioral testing, a lack of or inconsistent nesting material, and poor routine animal care (e.g. exposure to wet and soiled bedding). In addition, the inclusion of other behavioral tests during lactation (e.g. forced swim test or elevated plus maze) may introduce confounds. However, the use of a novel cage environment or other acute stressor may be useful to reveal behavioral differences not apparent with home cage testing due to potential interactions between chronic stress, acute stress, and behavior.
The currently described protocol is effective with Sprague Dawley rats, but modifications of the protocol may be needed for other strains and species. For example, pups must be protected with a barrier from highly aggressive Long Evans males to prevent injury. Critical practices for the social stress administration include the use of smaller or similarly sized males and having the first interaction of a male intruder with a highly aggressive day 2 dam to ensure defeat and consistently submissive behavior in the males. If males do attack a litter, they should be removed from the study, as it is likely that they will attack subsequent litters. One option for reducing the likelihood of overly aggressive males is to have them defeated by a larger male prior to the start of the study. For the behavioral scoring, the recording and analysis of both frequency and duration of most behaviors is necessary to provide a complete picture of the behavior, and the use of shorter recording intervals or spot checks may result in the collection of insufficient data to thoroughly test the hypotheses. Thorough and consistent training of scorers can be accomplished with the use previously scored behavioral videos.
The main application of the CSS model is the testing of current and novel drug treatments for postpartum depression and anxiety. An additional use of this model is to investigate the effects of common practices and procedures on maternal care, such as cesarean section births, induced labor, and various lactation schedules. Further investigation of potential effects of stress on lactation will be valuable given the lack of studies on this topic. While milk samples from the dams would be useful, the need for anesthesia and exogenous OXT in the collection of these samples introduces several behavioral and physiological confounds, so studies on the effects of stress on lactation may not be conducive to simultaneous behavioral analysis. The potential for transgenerational studies with the CSS model is great, and these investigations may also include various stress treatments of the adult offspring to investigate potential interactions between early life stress and later stress exposure.
The significance of the CSS model and associated behavioral testing centers on ethological and translational relevance. It is postulated that the lack of effectiveness of current treatments for depression and anxiety is due to a lack of ethological overlap between animal and human studies. Animal models of depression and anxiety often rely on robust physical stressors or highly selected strains which may explain the finding that current antidepressant treatments are most effective with severe depression. Physical stressors such as restraint and exposure to a wide variety of randomized stressors do not simulate common human experiences. The rationale for the randomized exposure used on chronic mild stress paradigms is to prevent acclimation. This is a significant concern, but the ideal situation may be to use a protocol which animals do not acclimate to that more closely resembles the etiology of depression and anxiety in humans, such as social stress. In addition to promoting the use of more ethological stressors, another advantage of the CSS model is the use of a relevant measure of anhedonia, maternal behavior. This is a robust reward mediated behavior which directly mediates the adverse effects of depression in mothers on offspring. The combined incorporation of ethological stressors and measures of anhedonia in depression and anxiety research will be especially beneficial in the identification of safe and effective preventative measures and treatments.
Social stress has been used successfully in males and nulliparous females for several years, and the expansion of this work into maternal females is likely to produce a wealth of novel data for postpartum depression as well as early life stress associated disorders in both males and females. It is postulated that the increased use of ethological animal models in depression and anxiety research will lead to more effective treatments due to the collection of behavioral data that is ethologically and translationally relevant, less variable, and more repeatable.
The authors have nothing to disclose.
We would like to thank Chantal Lau for advice on assessing milk intake, Klaus Miczek for advice on the use of social stress, and Sheryl Goodman for feedback on the clinical relevance of chronic social stress. The development of this protocol was supported by NICHD K99/R00 HDO59943 to BCN.
Material Name | Company | Catalogue Number | Comments |
ODlog video analysis | Macropod software | Version 2.7.2 | Available for PC or Mac |
Video recorder | Sony | HDR-CX130 |