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Photoperiodic Adjustments in Immune Function Protect Siberian Hamsters from Lethal Endotoxemia

Departments of Psychology and Neuroscience, The Ohio State University, Columbus, OH 43210, USA

Andrew K. Hotchkiss

Departments of Psychology and Neuroscience, The Ohio State University, Columbus, OH 43210, USA

Staci D. Bilbo

Department of Psychology, Johns Hopkins University, Baltimore, MD 21218, USA

Steven G. Kinsey

Departments of Psychology and Neuroscience, The Ohio State University, Columbus, OH 43210, USA

Randy J. Nelson

Departments of Psychology and Neuroscience, The Ohio State University, Columbus, OH 43210, USA

Seasonal changes in day length enhance or suppress components of immune function in individuals of several mammalian species. Siberian hamsters (Phodopus sungorus) exhibit multiple changes in neuroendocrine, reproductive, and immune function after exposure to short days. The manner in which these changes are integrated into the host response to pathogens is not well understood. The present experiments tested the hypothesis that short-day changes in immune function alter the pathogenesis of septic shock and survival after challenge with endotoxin. Male and female Siberian hamsters raised in long-day photoperiods were transferred as adults to short days or remained in their natal photoperiod. Six to 8 weeks later, hamsters were injected i.p. with 0, 1, 2.5, 10, 25, or 50 mg/kg bacterial lipopolysaccharide (LPS) (the biologically active constituent of endotoxin), and survival was monitored for 96 h. Short days significantly improved survival of male hamsters treated with 10 or 25 mg/kg LPS and improved survival in females treated with 50 mg/kg LPS. Transfer from long to short days shifted the LD₅₀ in males by approximately 90%, from 5.3 to 9.9 mg/kg, and in females from 11.1 to 15.0 mg/kg ([#002B]35%). Long-day females were more resistant than were males to lethal endotoxemia. In vitro production of the proinflammatory cytokine TNF in response to LPS stimulation was significantly lower in macrophages extracted from short-day relative to long-day hamsters, as were circulating concentrations of TNF in vivo after i.p. administration of LPS, suggesting that diminished cytokine responses to LPS in short days may mitigate the lethality of endotoxemia. Adaptation to short days induces changes in immune parameters that affect survival in the face of immune challenges.

Key Words: photoperiodism • host defense • seasonality • LPS • TNF • proinflammatory cytokine

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