Effects of Perinatal Circadian Programming on Activity and Skeletal Morphology in C57BL/6 Mice

Abstract

The timing and duration of locomotor activity can be impacted by changes in environmental light conditions. In mammals, the circadian system in offspring is initially entrained by that of the mother. Photoentrainment of the biological clock by the perinatal light environment begins at birth, and development of the circadian system is completed postnatally when the individual establishes its own internal circadian rhythm. Because of this perinatal to adult patterning, the perinatal light environment can have important developmental consequences to behavior and physiology in the adult. Here we explore the possible effects of photoentrainment on the skeleton using a mouse model system. Skeletal phenotypes are produced through a combination of an animal's genetic background, its plastic responses to its environment (e.g., diet, activity), and the interactions between genetics and environment. Because light environment can have both immediate and long‐term effects on locomotor behavior patterns, variation in photoperiod may impact bone biology indirectly via altered activity levels. To investigate the influence of perinatal light environment on adult activity patterns and bone morphology, one group of C57BL/6 mice was kept under a standard 12:12 L/D photoperiod (12 L; n = 10) and another group (24 L; n = 8) lived under constant light throughout their prenatal and early postnatal development. After five postnatal weeks, the 24 L group were transitioned into a 12:12 photoperiod. Home cage activity data for each animal was collected at 12, 20, and 28 postnatal weeks. After 31 weeks, mice were sacrificed to allow morphometric analyses on dissected long bones. At each of the three time points, we found that the 24 L mice spent significantly less of their time walking (P = 0.044) and walked lower distances overall than the 12 L group (P = 0.006). However, despite significant differences in activity, comparisons of femur length, AP diameter, ML diameter, proximal width, distal width, head height, and head depth indicated no significant differences between the two treatment groups. Overall, our results suggest that while perinatal light environment does have a lasting influence on the activity levels of adults, neither the differences in perinatal environment nor in adult activity between the two treatment groups had a significant impact on femoral morphology. These results support other recent research which has indicated that genetic background has a greater impact on skeletal phenotype than exercise does. Future studies will include investigating bone mineralization and bending mechanics.Support or Funding InformationAmerican Diabetes Association 1‐14‐BS‐181