Presence of mother prompts dissociation of sickness behavior, fever, and hypothalamic gene expression in lipopolysaccharide-injected guinea pig pups

Early Attachment Disruption Increases Central Sensitivity of Depressive-Like Behavior to Stimulation by PGE-2

Early disruption of filial attachment appears to sensitize underlying threat-related processes to produce vulnerability for developing depression when encountering stressors later in life. Evidence indicates neuroinflammatory signaling mediates this effect, though exact mechanisms are poorly understood. In a guinea pig model, early periods of isolation from the mother produce depressive-like behavior and fever, which sensitize with repeated isolation. Because cyclooxygenase inhibitors reduce these effects, the sensitization appears to involve prostaglandins such as PGE-2. We asked if isolation increased central sensitivity to PGE-2, which could underlie the sensitization process. Experiment 1 established that intraventricular (ICV) infusion of 1 or 3 µg of PGE-2 increased depressive-like behavior in pups. The 3 µg dose also produced an initial suppression and then a rise in core temperature. In Experiment 2, repeated isolation of pups sensitized depressive-like behavior. Days later (early adolescence), 1 µg of ICV-PGE-2 increased depressive-like behavior in previously isolated guinea pigs, but not in non-previously isolated controls. Core temperature was unaffected. Thus, early isolation from the mother increased sensitivity of depressive-like behavior, but not fever, to PGE-2. Results suggest that increased sensitivity to PGE-2 could play a role in the enhanced vulnerability to depression in adolescents previously exposed to early attachment disruption.

Social Buffering as a Tool for Improving Rodent Welfare

The presence of a conspecific can be calming to some species of animal during stress, a phenomenon known as social buffering. For rodents, social buffering can reduce the perception of and reaction to aversive experiences. With a companion, animals may be less frightened in conditioned fear paradigms, experience faster wound healing, show reduced corticosterone responses to novelty, and become more resilient to everyday stressors like cage-cleaning. Social buffering works in diverse ways across species and life stages. For example, social buffering may rely on specific bonds and interactions between individuals, whereas in other cases, the mere presence of conspecific cues may reduce isolation stress. Social buffering has diverse practical applications for enhancing rodent wellbeing (some of which can be immediately applied, while others need further development via welfare-oriented research). Appropriate social housing will generally increase rodents' abilities to cope with challenges, with affiliative cage mates being the most effective buffers. Thus, when rodents are scheduled to experience distressing research procedures, ensuring that their home lives supply high degrees of affiliative, low stress social contact can be an effective refinement. Furthermore, social buffering research illustrates the stress of acute isolation: stressors experienced outside the cage may thus be less impactful if a companion is present. If a companion cannot be provided for subjects exposed to out-of-cage stressors, odors from unstressed animals can help ameliorate stress, as can proxies such as pieces of synthetic fur. Finally, in cases involving conditioned fear (the learned expectation of harm), newly providing social contact during exposure to negative conditioned stimuli (CS) can modify the CS such that for research rodents repeatedly exposed to aversive stimuli, adding conspecific contact can reduce their conditioned fear. Ultimately, these benefits of social buffering should inspire the use of creative techniques to reduce the impact of stressful procedures on laboratory rodents, so enhancing their welfare.

How Does the Social Grouping of Animals in Nature Protect Against Sickness? A Perspective

Sickness behavior is broadly represented in vertebrates, usually in association with the fever response in response to acute infections. The reactions to sickness behavior in a group member or potential group member in humans is quite variable, depending upon circumstances. In animals, the reactions to sickness behavior in a group member or potential group member evoke a specific response that reflects the species-specific lifestyle. Groups of animals can employ varied strategies to reduce or address exposure to sickness. Most of these have scarcely been studied in nature from a disease perspective: (1) adjusting exposure to sick conspecifics or contaminated areas; (2) caring for a sick group member; (3) peripheralization and agonistic behaviors to strange non-group conspecifics; and (4) using special strategies at parturition when newborn are healthy but vulnerable. Unexplored in this regard is infanticide, where newborn that are born with very little immunity until they receive antibody-rich colostrum, could be a target of maternal infanticide if they manifest signs of sickness and could be infectious to littermates. The strategies used by different species are highly specific and dependent upon the particular circumstances. What is needed is a more general awareness and consideration of the possibilities that avoiding or adapting to sickness behavior may be driving some social behaviors of animals in nature.

The Legacy of Sickness Behaviors

Systemic infections of all types lead to a syndrome known as sickness behaviors. Changes in the behavior of febrile humans and animals formed the original basis for this concept. Body temperature is behaviorally regulated in both endotherms and ectotherms. However, infections cause other changes in body functions, including sleep disruption, anorexia, cognitive and memory deficits and disorientation. The brain mediates this entire cluster of symptoms, even though most major infections occur outside the brain. The true importance of sickness behaviors is not the numerous discoveries of symptoms that affect all of us when we get sick. Instead, the legacy of 30 years of research in sickness behaviors is that it established the physiologic importance of reciprocal communication systems between the immune system and the brain. This conceptual advance remains in its infancy.