Behavioral effects of peripheral interleukin-1 administration in adult CD-1 mice: specific inhibition of the offensive components of intermale agonistic behavior

Abstract

Peripheral administration of interleukin-1 β (IL-1 β) in rodents reduces exploratory behavior in a novel environment while decreasing social investigation of a juvenile conspecific. In this study we wanted to test the effects of peripherally administered IL-1 β on another aspect of the mouse social repertoire, namely intraspecific fighting towards an adult male intruder. In the first experiment, sickness behavior induced by IL-1 β (1 μg/mouse) in adult CD-1 mice was assessed by direct observation of behavioral changes following placement into a novel environment. Three hours after injection, subjects were individually introduced for 20 min in a cage with clean sawdust and a number of behavioral items recorded. Blood samples were collected at the end of the testing session. Body temperature was measured right before, 1 h and 3.5 h following injection. In IL-1 β treated mice, exploration (assessed by measuring duration and frequency of Wall Rearing and Rearing behaviors) was nearly totally suppressed, while duration and frequency of behaviors such as Grooming, Bar Holding, and Digging were also markedly reduced. Administration of IL-1 β significantly elevated CORT secretion above basal levels and, as previously reported for mice, induced hypothermia (about 2°C). In the second experiment, we assessed mice receiving IL-1 β (0.25; 0.5 or 1 μg/mouse or saline solution) in a social context. Three hours after injection, subjects were placed into a neutral cage for 20 min with a non-injected adult male conspecific and aggressive behavior scored. Overall, IL-1 β administration affected the social repertoire of treated mice in a dose-dependent fashion. Specifically, agonistic components of aggressive behavior were nearly totally suppressed, while the defensive elements, such as Upright Defensive posture, Upright Submissive posture, Crouching, or Flee were not affected by IL-1 β. Overall these data support the notion that sickness behavior induced by IL-1 β administration represents an organized behavioral strategy and is not an aspecific response to an illness-type of condition.