DIFFERENCES IN RESPONSES OF THE CIRCADIAN SYSTEM TO LIGHT IN THE SYRIAN-HAMSTER

The circadian pacemakers, controlling rhythms of wheel-running activity of male Syrian hamsters (M. auratus) from 2 breeding colonies, responded differently to single and repeated light pulses, as well as to various constant light intensities. These differences were examined further to test the principal features of Pittendrigh and Daan''s entrainment model and to test whether the differences were of genetic origin or determined by preexperimental conditions. In accordance with the discrete entrainment model, the circadian rhythms were entrained by light-dark cycles, consisting of repetitive short light pulses, when each pulse fell at that phase of the rhythm which is characterized by a response that equals the difference between the period of the rhythm (t) and that of the entraining light cycle (Zeitgeber). According to this implication, the different amplitudes of the phase response curves (PRC), derived from effects of single pulses in the 2 stocks of hamsters, were correlated with different ranges of entrainment of the activity rhythms by repeated light pulses of the same intensity and duration. The differences observed in hamsters of the 2 stocks were not genetically determined, but could be interpreted as being based on different physiological states of the circadian pacemaker system associated with different phase relations between 2 coupled oscillators.