Periodicity of Wheel-Running Activity in Mice in Relation to Light-Dark Cycle : II. Effects of Genotype and Constant Condition of Illumination

Abstract

Male mice of two inbred C57BL/6 and DBA/2 strains, and their Fl hybrid were individually run in cages with a wheel for 5 consecutive days under 12 hr lightdark cycle (LD) which was in light from 8 : 30 a. m. to 8 : 30 p. m., alternating with 12 hr of dark. Following this LD session, mice were maintained for successive 10 days under the constant condition of light (EXP. A) and dark (EXP. B). Finally the condition was returned to the original LD session for 5 days.The main results of the present experiments were as follows : (1) DBA show-ed higher level of 24 hr activity than those of C57BL throughout all sessions both in EXP. A and B. The Fl mice exceeded their parental strains in the activity level, which indicates heterosis. The finding is in agreement with that of BRUELL (6, 7). (2) As can be seen in Table 1 and 3, the nocturnal rhythms of the wheel activity were cleary evident during the LD session. In addition, it was shown that during the constant illuminations the ratio of activity during 8 : 30 p. m.-8 : 30 a. m. to total 24 hr activity decreased gradually with days. The rates of these daily decrement were significantly different among groups both in the constant light (F=2.05, df=18/216) and dark (F=2.46, df=18/252) session. (3) Table 2 shows that periodicities of three groups were longer than 24 hours under the constant light and shorter under the constant dark condition. From Figure 1 and 2, average free-running rhythm (FRR) under the constant light for C57BL, DBA and Fl were 25.11, 25.63 and 25.26 hours, respectively. Under the constant dark, their means were 23.96, 23.38 and 23.27 hours with a significant difference of 0.001 level (F=9.26, df=2/19). This result is in agreement with the findings of ASCHOFF (3 4).It was discussed from the above results that the genetic differences of FRR in the constant condition of dark may relate to the mechanism of some neurochemical rhythms which appear to persist in the constant dark but disappear in the constant light condition.