Daily variations in the response of wood mice Apodemus sylvaticus to noradrenaline

Non-shivering thermogenesis (NST) is an important mechanism for heat production in many small rodent species. Daily rhythms of body (rectal) temperature (Tb) reflect the relationship between heat production and heat dissipation. The roles of photoperiod and the time of day at which NST is measured were studied in wood mice Apodemus sylvaticus. Mice of both sexes (N=18) were acclimated to two different photoperiod regimes (16 h:8 h L:D and 8 h:16 h L:D) at a constant ambient temperature (Ta) of 24 °C. Non-shivering thermogenesis was measured as the maximal response of oxygen consumption (V(dot)O2NA) and body temperature (TbNA) to a noradrenaline (NA) injection (1.5 mg kg-1 subcutaneously) in unanaesthetized mice at three different times in the daily rhythm of Tb (N=6 in each group). Mice acclimated to 8 h:16 h L:D had a greater response to noradrenaline at the three different times of the day compared with those acclimated to 16 h:8 h L:D. The extent of the response to noradrenaline within each group varied with time of day; the smallest response was recorded at 18:00±1.5 h, and the greatest was at 01:00 h. These results suggest that photoperiod is an important cue for seasonal acclimatization in this species and that the response to noradrenaline follows a daily rhythm.