Response curve, free-running period, and activity time in circadian locomotor rhythm of rats.

Phase response curves (PRC) for the spontaneous locomotor rhythm were constructed by applying short light pulses to rats in constant darkness (DD). The offset of locomotor activity as well as the onset was taken as a phase reference (offset PRC vs. onset PRC). The amount of phase shift yielded by light pulses was evaluated on the next day of pulse treatment (immediate PRC) and also after completion of a new steady state (steady state PRC). Significant differences in shape were observed between the onset and offset PRCs as well as between the immediate and steady state PRCs. In the immediate onset PRC, an area under the phase advance part (A) was absent, while it was present in the immediate offset PRC. In contrast, the steady state PRCs for activity onset and offset were essentially the same. The shape of steady state PRC depended on the free-running period in DD (tau). In the PRCs of long tau rhythms, the range covered by the phase delay area (D) was lengthened without changing its amplitude, resulting in a larger D/A ratio. A strong positive correlation was detected between tau and activity time (alpha). The steady state PRC shapes also depended on alpha; the D/A ratio was larger in a long alpha rhythm than in a short alpha. These results are in good agreement with the hypothesis that the circadian locomotor rhythm of nocturnal rodents is regulated by two coupled oscillators.