DIURNALLY CHANGING EFFECTS OF LOCOMOTOR-ACTIVITY ON BODY-TEMPERATURE IN LABORATORY MICE

In mice circadian body temperature curves are masked due to the effect of motor activity. However, body temperature will not immediately ref lect activity, but rather the integrated activity over IT minutes (int egration time) and after a certain delay (lag), and the sensitivity to such masking may change throughout the circadian cycle. The aims of t he present investigation were to estimate IT and lag, to quantify the effect of motor activity on body temperature at different times of the day, and, using these results, to draw temperature curves that are cl oser to the endogenous one. Activity and body temperature of adult mal e laboratory mice were recorded telemetrically at 10-min intervals. An imals were housed in air-conditioned rooms (T = 22 +/- 2 degrees C; re lative humidity: 55-65%) with a light-dark cycle of 12 h:12 h (light f rom 0700 to 1900 hours) and food and water available ad lib. The diurn al activity and body temperature rhythms were similar with a main maxi mum during the dark time and a secondary maximum immediately following lights-on. Nearly all changes of activity were reflected in body temp erature. IT and lag were established on the basis of the best correlat ion between body temperature and activity (overlapping 4-h sections of 12 days) for all combinations of IT from 10 to 90 min and lag from 0 to 50 min (10-min steps each). The overall means of IT and lag were 40 and 0 min, respectively. During the dark time the values were somewha t larger, but not significantly so. The correlation between activity a nd body temperature was significantly better in the light time compare d to the dark time. The sensitivity of the body temperature to changes in activity was investigated by linear regression analysis for every hour over 12 days (IT = 40 min, lag = 0 min). The gradients assessed b y regression analysis showed a diurnal pattern with maximal values dur ing the light time (p < 0.01). Thus, body temperature was raised by ac tivity more during the light time (minimum of body temperature and act ivity) than during the dark time. The intercepts showed a nearly sinus oidal diurnal pattern with maximal values in the middle of the dark ti me. Accepting that the intercepts correspond to zero activity at a cer tain time of day, one might use them to get a curve that is closer to the endogenous body temperature rhythm. Mechanisms (circadian and ther moregulatory) that might cause the diurnally changing sensitivity of b ody temperature to activity are discussed. (C) 1998 Elsevier Science I nc.