The neuroendocrine-immune axis in Siberian hamsters: Proximate mechanisms underlying seasonal changes in sickness behavior and immune function (Phodopus sungorus)

The immune, endocrine, and nervous systems interact to maintain homeostasis, and to protect against and overcome pathogenic invasion. Responses to infection include both physiological and behavioral changes, such as fever, lethargy, and reductions in food and water intake. Rather than nonspecific manifestations of illness, these “sickness behaviors” are organized, adaptive strategies that are often critical to host survival. Mounting an immune response, however, is energetically costly. Animals have evolved to maintain a balanced energy budget. Often, energy directed towards immune function must be balanced against competing demands such as reproduction, growth, and basic cellular maintenance. For many animals, a predictable energy shortage arrives each winter; low food availability often coincides with high thermoregulatory demands in low temperatures. Thus, energy availability for immune function waxes and wanes across the year.; Specific adaptations have evolved among animals to maximize energy conservation and prepare them for winter. Siberian hamsters (Phodopus sungorus ) stop reproducing during winter by responding to photoperiodic cues that signal its arrival. The following dissertation experiments were designed to test the hypothesis that hamsters may also adjust energetically demanding sickness behaviors and immune responses prior to winter in response to alterations in day length. The first set of experiments investigated photoperiodic influences on the expression of fever and anorexia, and the underlying neuroendocrine mechanisms. The durations of fever and anorexia are truncated in hamsters housed in short, winter-like, days, and long-term exposure to long duration melatonin appears to organize these responses. The next set of experiments tested whether photoperiod and experimental stressors interact to influence immune responses in male and female hamsters. Glucocorticoids are released in response to virtually all immune or stressful stimuli, and immune responses share common mechanisms with stress responses. Short-day hamsters have higher baseline immune responses, respond more quickly to acute stress, and enhance immune function following stress compared to long-day hamsters. Furthermore, some, but not all responses vary according to sex and reproductive status. Taken together, the complex interactions that occur among the nervous, endocrine, and immune systems appear to allow flexible expression of systemic responses to infection in a manner that is consistent with the specific physiological, environmental, or energetic requirements of the individual.