Behavioural and cellular responses to light of the circadian system of tau mutant and wild-type Syrian hamsters.

The tau mutation shortens the free-running circadian period of the activity rhythm of the Syrian hamster from around 24 h in the wild-type, to 20 h in the homozygous mutant. The aim of this study was to examine the effects of light pulses on the activity rhythms and expression of c-fos in the suprachiasmatic nuclei of wild-type and mutant hamsters. This would make it possible to determine the relative durations of subjective day and night, and thereby characterize further the effect of the mutation upon the circadian cycle. Presentation of light pulses (15 min) to animals kept in dim red light for seven to 10 days induced phase-dependent shifts in the onset of activity. During subjective day (inactive phase) there was no effect of light upon the activity rhythm of either normal or mutant animals. Early in subjective night (i.e. after the start of the active phase), light pulses induced phase delays of approximately one circadian hour in both phenotypes. However, later in subjective night light pulses induced phase advances in activity rhythms. The magnitude of the advance phase shifts differed considerably between the two phenotypes, with the wild-types typically showing shifts of about one circadian hour, while the mutants showed much larger advances of up to 12 circadian hours. In both strains, advances were observed following light at circadian time 22 but not after light at circadian time 24. The relative duration of subjective night was therefore comparable in the mutants and wild-types i.e. around 11 circadian hours. In order to examine the light-induced expression of the immediate early gene c-fos at different circadian phases, animals received a second light pulse (15 min) or a control "dark pulse" at the same circadian time as they had received the first, but seven to 10 circadian days later. They were perfused 1 h after the start of the pulse and brains processed for the immunocytochemical detection of Fos, the protein product of c-fos. Animals receiving dark pulses showed minimal Fos immunoreactivity in the suprachiasmatic nucleus, regardless of the circadian time of presentation of the stimulus. During subjective day (circadian times 4-11), light induced very little Fos immunoreactivity. In contrast, light presented during subjective night (after circadian time 12) induced a dense pattern of Fos immunoreactivity in the ventrolateral, retino-recipient region of the suprachiasmatic nucleus of both wild-type and mutant animals.(ABSTRACT TRUNCATED AT 400 WORDS)