Influence of light-dark cycle on liver temperature in the rat.

I. Light-dark cycle, feeding, activity and liver temperature.

Abstract

A copper-constantan thermocouple was implanted in the livers of male Wistar rats, weighing 150-300g, and the liver temperature was measured for 3-4 weeks in unanaesthetized, relatively unrestrained rats.
Under lighting conditions of LD 12:12, liver temperature was high in the dark period and low in the light period. The temperature curves were of three types, and each rat did not necessarily have the same type of curve during the experimental period.
When the light-dark cycle was reversed, the circadian rhythm of liver temperature adjusted to the new lighting regimen in 5-7 days, during which time each period of the rhythm was prololonged.
It was observed that a temporary rise of liver temperature occurred at feeding time, when the latter was restricted to a few hours. After the controlled feeding time was maintained for several days, the liver temperature began to rise just before feeding time even during a fast of 48 hours.
Variations of liver temperature, on the whole, corresponded to the activity of the animal, but the coincidence was not quite complete when the temperature and the activity were observed more frequently. Moreover, after a phase shift in the light-dark cycle the liver temperature no longer paralleled physical activity.
It was concluded that feeding, activity and the light-dark cycle per se were not direct causes of the circadian rhythm of liver temperature; the rhythm must be endogeneous, the light-dark cycle being a Zeitgeber.