Light-dark differences in the effects of ambient temperature on gaseous metabolism in newborn rats

Body temperature (Tb) of rat pups (7-9 days old) raised under a 12:12-h lig ht-dark (L-D) regimen (L: 0700-1900, D: 1900-0700) was consistently higher in D than in L by similar to 1.1 degrees C. We tested the hypothesis that t he L-D differences in Tb were accompanied by differences in the set point o f thermoregulation. Measurements were performed on rat pups at 7-9 clays af ter birth. O-2 consumption ((V) over dot O-2) and CO2 production ((V) over dot CO2) were measured with an open-flow method during air breathing, as am bient temperature (T-a) was decreased from 40 to 15 degrees C at the consta nt rate of 0.5 degrees C/min. At T-a greater than or equal to 33 degrees C, (V) over dot O-2 was not significantly different between L and D, whereas (V) over dot CO2 was higher in L, suggesting a greater ventilation. Over th e 33 to 15 degrees C range the (V) over dot O-2 values in D exceeded those in L by similar to 30%. Specifically, the difference was contributed by dif ferences in thermogenesis at T-a = 30 to 20 degrees C. As T-a was decreased , the critical temperature at which (V) over dot O-2 began to rise was lowe r in L. We conclude that the higher T-b Of rat pups in D is accompanied by a higher set point for thermoregulation and a greater thermogenesis. These results are consistent with the idea that, in newborns, endogenous changes in the set point of thermoregulation contribute to the circadian oscillatio ns of T-b.