Circadian temperature and activity rhythms in mice under free-running and entrained conditions; Assessment after purification of the temperature rhythm

Six female mice were studied separately for six weeks, first in constant li ght (3001x), and then on a 12:12 L:D schedule (light on 07:00-19:00 h). Foo d and water were available ad libitum. Abdominal temperature and spontaneou s locomotor activity were measured every 10 min. In constant light, the animals free-ran with both temperature and activity records showing circadian rhythms that were significantly greater than 24 h ; by contrast, in the LD schedule, the circadian rhythms had become entrain ed and showed a stable phase relation to this schedule. The direct masking effects upon raw temperatures caused by bursts of activity were clearly see n, and could be removed by a process of 'purification'. A comparison of the activity profiles during the entrained and free-running phases showed that the imposed light-dark cycle resulted in decreased acti vity in the light, increased activity in the dark, and bursts of activity a t the light-dark and dark-light transitions. Masking effects due to the act ivity profile were present in the raw temperature profile, and many could b e removed by purification using the activity profile; however, there was ev idence that other masking effects, independent of activity, were present al so. The efficacy of thermoregulatory compensation, as assessed from the rise of core temperature produced by spontaneous locomotor activity, was, in compa rison with the free-running condition, increased in the dark phase and decr eased in the light phase; this would appear to be one way to limit the temp erature rise that occurs in the active phase of the rest-activity cycle.