Short photoperiods attenuate central responses to an inflammogen.

In most parts of the world, environmental conditions vary in a predictable seasonal manner. Thus, seasonal variation in reproductive timing and immune function has emerged in some species to cope with disparate seasonal demands. During the long days of spring and summer when food availability is high and thermoregulatory demands low, Siberian hamsters invest in reproduction, whereas during the harsh short days of winter hamsters divert energy away from reproductive activities and modify immune capabilities. Many seasonal adaptations can be recapitulated in a laboratory setting by adjusting day length (photoperiod). Early-life photoperiods are important sources of seasonal information and can establish an individual's developmental trajectory. Siberian hamsters housed under short days (SD; 8 h light/day) recover more rapidly than long-day (LD; 16 h light/day) hamsters from immune activation with lipopolysaccharide (LPS). SD hamsters attenuate fever response, reduce cytokine production, and abrogate behavioral responses following LPS injection. The mechanism by which SD Siberian hamsters attenuate febrile response remains unspecified. It is possible that periphery-to-brain communication of inflammatory signals is altered by exposure to photoperiod. Rather than testing photoperiod effects on each of the multiple routes by which immunological cues are communicated to the CNS, we administered LPS intracerebroventricularly (i.c.v.) following adolescent exposure to either 6 weeks of SD or LD. Injection of LPS i.c.v. led to a similar immune reaction in SD hamsters as previously reported with intraperitoneal injection. Short days attenuated the response to LPS with diminished fever spike and duration, as well as decreased locomotor inactivity. Furthermore, only LD hamsters demonstrated anhedonic-like behavior following LPS injection as evaluated by decreased preference for a milk solution. These results suggest that photoperiodic differences in response to infection are due in part to changes in central immune activation.