Abstract
The circadian system in mammals entrains to several oscillating factors in the environment. Among them, a light-dark cycle is the common as well as most potent factor so far examined. Entraining light signals enter the brain through the retinohypothalamic tract and reach the suprachiasmatic nucleus (SCN) where the master clock is located. The circadian system in humans is able to entrain to non-photic factors as best exemplified by entrainment of totally blind persons. However, it is not known whether the non-photic entrainment is achieved by the clock in the SCN or other clocks located somewhere outside the SCN. Methamphetamine, a CNS stimulant, is known to produce robust activity rhythms in bilaterally SCN lesioned rats and aperiodic clock mutant or Cry1/Cry2 double knockout mice, indicating the existence of a behavior related oscillator(s) outside the SCN. Interestingly, the activity rhythm produced by methamphetamine shows similar characteristics to those observed in the human sleep rhythm, such as internal desynchronization from the circadian rhythms, a long endogenous period, circabidian (ca. 48 h) rhythm and entrainment to non-photic factors. The underlying mechanism is regarded as a non-SCN clock. A feeding-associated oscillator is another example of non-SCN clock. Activity rhythms in the circadian domain were developed in rats and mice, when feeding is restricted to a fixed time of day. The feeding-associated oscillation persists for several weeks after the termination of restricted feeding. Using these model animals, we are attempting to find out the site of oscillation of non-SCN clock. [J Physiol Sci. 2006;56 Suppl:S7]
