Environmental context differentially affects behavioral, leukocyte, cortisol, and interleukin-6 responses to low doses of endotoxin in the rhesus monkey

Abstract

Bacterial infections and proinflammatory cytokines induce behavioral and physiological responses associated with withdrawal and sickness behavior. These effects in rodents are often exacerbated by stressful environmental contexts. To model this synergistic effect of arousal and stress, low doses of lipopolysaccharide (LPS), 4 or 40ng/kg, were administered to rhesus monkeys in different environmental contexts. Activity, emotional and social behaviors, cortisol, interleukin-6 (IL-6), and peripheral leukocyte trafficking were assessed in 4 experiments: an initial 3h time-course in a novel cage (Experiment 1); an extended 24h time-course (Experiment 2); a highly arousing context in which an animal was engaged in the Human Intruder Paradigm (HIP, Experiment 3); and finally in an undisturbed context in their Home Cage (Experiment 4). The moderately arousing novel cage potentiated leukocyte, neutrophilic, IL-6, and cortisol changes as compared to the Home Cage. Unlike the social withdrawal seen in rodents, monkeys engaged in a marked increase in social behavior. IL-6 levels were positively correlated with Proximal Contact, which was induced to a greater degree by the higher dose of LPS. In contrast, the high arousal HIP condition appeared to obviate the effects of LPS. Thus, the rhesus monkey provides an excellent animal model for investigating the behavioral and physiological actions of endotoxemia, which are profoundly influenced by the situational context in which the individual is evaluated.

Figures

Figure 1

Physiological effects of 4 or 40 ng/kg LPS as compared to saline in juvenile rhesus macaques (n = 4, 5, and 4, respectively) at 3 h post-injection in a novel cage. Monkeys were tested while living as a pair after being recently transferred to a novel cage. (A) Total leukocyte number per μl. (B) Number of neutrophils per μl. (C) Plasma cortisol levels (μg/dl). (D) Plasma IL-6 (pg/ml). Data are expressed as Mean ± SEM. *p<.05.

Figure 2

Percentage of time spent engaged in affiliative behavior over a 3h period after saline, 4, or 40 ng/kg LPS (n = 4, 4, and 5, respectively) in a novel cage. LPS-treated animals expressed more affiliation, with a marginal decrease among 40 ng/kg subjects compared to 4 ng/kg. Data are expressed as Mean ± SEM. *p<.05>

Figure 3

Affiliative behavior observed between focal animal and untreated companion over 24 h time course after either saline or 4ng/kg LPS (n = 4/group) in a novel cage. Data are expressed as Mean ± SEM. Monkeys showed significantly more affiliative behavior throughout the 6 h during the LPS time course (see Figure 2). *p<.05.>

Figure 4

Effect of saline or 4 ng/kg LPS on physiological responses during stress of relocation and exposure to the Human Intruder Paradigm (HIP) (n = 4/group). Data are expressed as Mean ± SEM. (A) Total leukocyte count per μl. (B) Number of neutrophils per μl. (C) Plasma cortisol levels (μg/dl). (D) Plasma levels of IL-6 (pg/ml). LPS did not significantly augment the arousing effects of the HIP.

Figure 5

Physiological effects of saline or LPS in the home cage at 3 h post-injection (n = 10). Data are expressed as Mean ± SEM. (A) Total leukocyte number per μl. (B) Number of neutrophils per μl. (C) Plasma cortisol (μg/dl). (D) Plasma IL-6 levels (pg/ml). There was a significant shift in the cell differential due to LPS, but the resulting neutrophilia and IL-6 changes were more modest than in Experiment 1. Cortisol levels were not elevated above baseline. *p<.05.

Figure 6

Comparison of the physiological effects of 4 ng/kg LPS at 3 h post-injection during different environmental contexts (Novel + Companion and Alone + Intruder, n = 4; Home Cage, n = 10). Data are expressed as Mean ± SEM. (A) Total leukocyte number per μl. (B) Number of neutrophils per μl. (C) Plasma cortisol (μg/dl). (D) Plasma IL-6 levels (pg/ml). *p<.05.