A novel mechanism for the control of circadian clock period by light

The free-running period expressed by circadian clocks in constant environmental conditions is history dependent, and one effect of entrainment of circadian clocks by light cycles is to cause long-lasting changes in the free-running period that are termed aftereffects. It has been suggested that aftereffects are a consequence of the particular phase relationships among constituent oscillators of the circadian system that are established by the entrainment. In a test of this hypothesis, it is shown that aftereffects of entrainment of the free-running rhythm of nerve impulse activity from the eye of Bulla gouldiana are completely unaffected by treatment with 12-, 24-, or 48-h pulses of the translation inhibitor cycloheximide or with 24-h pulses of the transcription inhibitor DRB (5,6-dichlorobenzimidazole riboside). These treatments reset the phase of circadian oscillators generally and those in the eye of B. gouldiana specifically. The absence of any effect of the treatments indicates that aftereffects are independent of oscillator phase. These results suggest that history-dependent changes in period result from a novel, long-lasting, and previously unrecognized mechanism of action of light on the circadian pacemaking system. Furthermore, the data indicate that aftereffects can persist in the absence of translation, transcription, or the continued cycling of the circadian system.