Entrainment properties of the circadian locomotor activity rhythm of the field mouse mus booduga under complete and skeleton photoperiodic regimes

Abstract

Predictions for the phase angle differences (Ψ) between the activity rhythm and the zeitgeber for different skeleton photoperiods based on the phase response curve (PRC) and the free-running period (Τ) of the field mouse Mus booduga were made. These predictions were based on two assumptions: (i) The PRC for light pulses of 1 h duration and ca 45 lx intensity should resemble the PRC for pulses of 15 min duration and 1000 lx intensity. (ii) One of the two light pulses (LP) constituting the skeleton photoperiod should always impinge upon that zone of the PRC which has a slope of < -2. Experiments were performed to compare Ψ under skeleton and complete photoperiods and also to test the assumptions made in predicting Ψ. The results show that the basic oscillation underlying the activity rhythm of the field mouse Mus booduga undergoes a "phase-jump" when two brief light pulses (of 1 h duration) were used to mimic a photoperiod of 20 h. The Ψ values obtained for skeleton photoperiods closely match the predicted values. Under complete photoperiods, the experimentally obtained values match the predictions only up to 16 h. We conclude therefore that beyond this photoperiod, two discrete light pulses may not be sufficient to simulate the effect of a complete photoperiod.