Critical period for cycloheximide blockade of light-induced phase advances of the circadian locomotor activity rhythm in golden hamsters.

Recent studies indicate that the protein products of genes involved in transcriptional regulation play an important role in light-induced phase-shifting of the circadian clock, and suggest that protein synthesis may be necessary for some critical steps in the process by which light can induce a phase shift. In order to determine if this protein-dependent step occurs before, during or at some time after the light pulse is presented, golden hamsters were treated with the protein synthesis inhibitor, cycloheximide (65 mg/kg, injected subcutaneously), at various times relative to the presentation of a light pulse that normally induces an advance in the circadian locomotor activity rhythm. When hamsters were treated with cycloheximide 1 h or 30 min before, as well as 0, 1, 2, 3 or 4 h after the onset of the light pulse; the phase advancing effects of light were completely blocked and delay phase shifts were observed. The observed phase shifts in these groups of animals were not significantly different from those observed in animals treated with cycloheximide without exposure to light. Treatment with cycloheximide 6 or 3 h before the light pulse partially blocked light-induced phase advances. The phase advancing effects of light were not altered in those animals injected with cycloheximide 6 or 9 h after the light pulse. These results support the hypothesis that protein synthesis is necessary for light-induced phase advances in the mammalian circadian clock, and indicate that such synthesis is necessary for at least 4 h after the light pulse for phase shifts to occur. Thus, light may not induce an instantaneous (< 1 h) phase shift in the circadian clock of hamsters, but instead depends on the synthesis of proteins for about 4 h after light stimulation.