Probing the circadian oscillator of a mammal by two-pulse perturbations

When organisms are maintained under constant conditions of light and temper ature, their endogenous circadian rhythms free run, manifesting their intri nsic period. The phases of these free-running rhythms can be shifted by sti muli of light, temperature, and drugs. The change from one free-running ste ady state to another following a perturbation often involves several transi ent cycles (cycles of free-running rhythm drifting slowly to catch up with the postperturbation steady state). Although the investigation of oscillato r kinetics in circadian rhythms of both insects and mammals has revealed th at the circadian pacemaker phase shifts instantaneously, the phenomenon of transient cycles has remained an enigma. We probed the phases of the transi ent cycles in the locomotor activity rhythm of the field mouse Mus booduga, evoked by a single light pulse (LP), using LPs at critically timed phases. The results of our experiments indicate that the transient cycles generate d during transition from one steady state to another steady state do not re present the state of the circadian pacemaker (basic oscillator) controlling the locomotor activity rhythm in Mus booduga.