BEHAVIORAL AND CELLULAR-RESPONSES TO LIGHT OF THE CIRCADIAN SYSTEM OFTAU MUTANT AND WILD-TYPE SYRIAN-HAMSTERS

The tau mutation shortens the free-running circadian period of the act ivity 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 su bjective 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 rhyth m of either normal or mutant animals. Early in subjective night (i.e. after the start of the active phase), light pulses induced phase delay s of approximately one circadian hour in both phenotypes. However, lat er in subjective night light pulses induced phase advances in activity rhythms. The magnitude of the advance phase shifts differed considera bly between the two phenotypes, with the wild-types typically showing shifts of about one circadian hour, while the mutants showed much larg er advances of up to 12 circadian hours. In both strains, advances wer e observed following light at circadian time 22 but not after light at circadian time 24. The relative duration of subjective night was ther efore comparable in the mutants and wild-types i.e. around 11 circadia n hours. In order to examine the light-induced expression of the immed iate 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 circadi an days later. They were perfused 1 h after the start of the pulse and brains processed for the immunocytochemical detection of Fos, the pro tein product of c-fos. Animals receiving dark pulses showed minimal Fo s immunoreactivity in the suprachiasmatic nucleus, regardless of the c ircadian 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 ventr olateral, retino-recipient region of the suprachiasmatic nucleus of bo th wild-type and mutant animals. The phase of Fos-induction extended t hroughout subjective night and was closely linked both to activity ons et and offset. During late subjective night/early subjective day (circ adian times 24-3) there was a significant correlation between the timi ng of the light pulse relative to activity offset and the number of Fo s immunoreactive nuclei in the suprachiasmatic nucleus. Overall, the r elative duration of the Fos-inductive phase was not significantly diff erent between wild-types and mutants. These findings suggest that the relative lengths of subjective day and night as defined by two criteri a, behavioural phase-shifting and expression of c-fos are the same,in circadian hours, in the wild-type and mutant animals. This is consiste nt with the view that the tau mutation accelerates the circadian pacem aker, such that in the homozygote it consists of 24 circadian hours, e ach of only 50 solar min duration.