Nonshivering thermogenesis (NST) is a main source of heat for many small mammals. It undergoes seasonal changes, being the highest in winter and the lowest in summer. Such acclimatization can ensure winter survival for species living in moderate or cold climates. Nevertheless, not only seasonal, but also daily changes in the capacity for NST seem to be of great importance. In this study, the effects of season and time of day on the temperature of brown adipose tissue (T(BAT)), preferred ambient temperature (PT(a)) and activity after noradrenaline (NA) injections in golden hamsters (Mesocricetus auratus) housed under semi-natural conditions were investigated. Animals were kept in outdoor enclosures and experienced natural changes in both, photoperiod and ambient temperature (T(a)). NA-induced hyperthermia was the largest during autumn (mean increase in T(BAT) by 0.74+/-0.04 degrees C), while during summer increase in T(BAT) was similar to that recorded in control (saline-injected) animals (0.16+/-0.05 degrees C and 0.24+/-0.04 degrees C, respectively). In spring hyperthermia was intermediary (0.57+/-0.05 degrees C). Daily variations in the response to NA depended on the season. In summer, the largest increase in T(BAT) (0.45+/-0.1 degrees C) was recorded during the first part of the day, while in autumn-in the middle of the day and night (1.1+/-0.1 degrees C and 0.9+/-0.1 degrees C, respectively). In spring, all NA injections induced large increase in T(BAT) except for the injection in the middle of the night. The largest decrease in PT(a) after NA administration was recorded in autumn (mean decrease by 1.5+/-0.3 degrees C). Both, seasonal and daily changes in the capacity for NST reflect different demands for heat dependently on the time of the year and time of the day. It can be concluded that although long history of breeding in captivity, golden hamsters preserved ability to survive in natural environment.
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