Photoperiod alters hypothalamic cytokine gene expression and sickness responses following immune challenge in female Siberian hamsters (Phodopus sungorus).

Rodents that live in changing environments display different immune responses mediated in part by photoperiod (day length) cues. Siberian hamsters maintained in winter-like (short) photoperiods display smaller physiological and behavioral responses to immune challenges as compared with hamsters housed in summer-like (long) photoperiods. We hypothesized that these different response patterns are attributable to altered cytokine production in the hypothalamus in response to photoperiod changes. Female hamsters were housed in long or short days for 10 weeks to induce photoperiodic alterations, then injected with either LPS (a bacterial endotoxin) or saline. Fever and food intake were assessed 3 h post-injection; hypothalami and blood were collected 3, 6, and 12 h post-injection. LPS induced lower fever and reduction in food intake responses in short-day hamsters as compared with long-day hamsters. Additionally, short-day hamsters reduced IL-1beta and Tnfalpha expression in the hypothalamus 6 h after LPS injection, as measured by quantitative RT-PCR. Plasma estradiol concentrations did not differ between long- and short-day hamsters. These data suggest that differences in cytokine production in the hypothalamus may underlie the photoperiod-induced differences in sickness responses, and that these changes are not mediated by estradiol.