Assessment of social behavior directed toward sick partners and its relation to central cytokine expression in rats

Abstract

Acute illness not only reduces the expression of social behavior by sick rodents, but can also lead to avoidance responses when detected by healthy, would-be social partners. When healthy animals interact with a sick partner, an intriguing question arises: does exposure to a sick conspecific elicit an anticipatory immune response that would facilitate defense against future infection? To address this question, healthy adult male Sprague-Dawley rats (N=64) were given a brief social interaction (30 min) with a partner that was either sick (250 μg/kg injection with lipopolysaccharide [LPS] 3 h prior to test) or healthy (sterile saline injection). During this exposure, social behavior directed toward the healthy or sick conspecific was measured. Additionally, the impact of housing condition was assessed, with rats group- or isolate-housed. Immediately after social interaction, brains were harvested for cytokine assessments within socially-relevant brain structures (olfactory bulb, amygdala, hippocampus and PVN). As expected, behavioral results demonstrated that (i) there was a robust suppression of social interaction directed against sick conspecifics; and (ii) isolate-housing generally increased social behavior. Furthermore, examination of central cytokine expression in healthy experimental subjects revealed a modest increase in TNF-α in rats that interacted with a sick social partner, but only in the olfactory bulb. Among the LPS-injected partners, expected increases in IL-1β, IL-6, and TNF-α expression were observed across all brain sites. Moreover, IL-1β and IL-6 expression was exacerbated in LPS-injected partners that interacted with isolate-housed experimental subjects. Together, these data replicate and extend our prior work showing that healthy rats avoid sick conspecifics, and provide preliminary evidence for an anticipatory cytokine response when rats are exposed to a sick partner. These data also provide new evidence to suggest that recent housing history potently modulates cytokine responses evoked by LPS.

The role of social cognition in parasite and pathogen avoidance

“…The roles of other neuromodulators, including immune components that can affect behaviour and are influenced by oestrogens, and pathogen threat also need to be further considered. Results of recent studies have indicated that male rats exposed to LPS-treated conspecifics display avoidance responses and a modest increase in the cytokine tumour necrosis factor a [133]. This avoidance is consistent with activation of the behavioural immune system, the display of disgust and associated cognitive responses.…”

Familiarity modulates social approach toward stressed conspecifics in female rats

“…The transmission of affect allows for the communication of impending threat between conspecifics [ 2 ] and exposure to a stressed animal alters the physiological state and behavior of an observer [ 3 ]. Observers may appraise situations to make decisions that can either protect them from harm, such as avoiding a sick or aggressive individual [ 4 ].…”

Social neuroscience of disgust

“…These protective responses in humans have been coined as the “behavioral immune system” and are associated with the elicitation of anticipatory neuroimmune responses, appropriate affective and cognitive responses, and relevant avoidance responses that may be the initial defense against pathogens. Similar aversive and avoidance responses to parasites and pathogens are evident in non‐human animals and have similarly been interpreted as reflecting disgust and perhaps a comparable behavioral immune system . Although not explicitly stated these aversive and avoidance may be considered in the context of a behavioral immune system.…”

“…Similar aversive and avoidance responses to parasites and pathogens are evident in non-human animals and have similarly been interpreted as reflecting disgust and perhaps a comparable behavioral immune system. 16,24,25 Although not explicitly stated these aversive and avoidance may be considered in the context of a behavioral immune system.…”