Influence of early environment on the circadian period of the tau-mutant hamster

The role of the period of the environmental cycle during gestation and infancy on the circadian period of adult hamsters was studied. Tau-mutant hamsters of all three genotypes (+/+, 24-h circadian period; +/tau, 22-h period; tau/tau, 20-h period) were conceived and raised under either a 20-h light-dark cycle or a 24-h cycle. The circadian period in constant darkness was determined at 2 months of age by inspection of records of running-wheel activity. Differences in circadian period of up to 1.2 h were observed. However, changes of the same magnitude were also observed in animals conceived and raised under a 24-h cycle and exposed to a 20-h cycle at 8 months of age. Therefore, it is concluded that the aftereffects of entrainment can account for the apparent influence of the early environment. The free-running period of the circadian pacemaker seems to be under complete genetic control and not to be influenced by the period of the environmental cycle under which the animal is raised.