Effects of light on the circadian activity rhythm of Djungarian hamsters (Phodopus sungorus) with delayed activity onset.

A number of Djungarian hamsters (Phodopus sungorus) of our institute show activity patterns that seem incompatible with proper adjustment to a periodic environment. The activity onset of those animals is continuously delayed, whereas the activity offset is stably coupled to "lights-on", leading to compression of activity time. A series of experiments was conducted to evaluate the possible causes of the deteriorated ability of DAO (delayed activity onset) hamsters to synchronize. Thus, we investigated the properties of the endogenous circadian rhythm plus parametric and non-parametric light effects on hamsters of DAO and Wild type (WT) phenotypes. Free-running rhythms were studied in constant darkness (DD) or constant light (LL) of different intensities (1, 10, 100 lux). To investigate photic phase responses, hamsters were kept in DD and exposed to light pulses (100 lux, 15 min), at circadian time (CT) CT14 and CT22. Differences were verified statistically by ANOVA. Light intensity exerted significant effect on the free-running period (tau). In DD, tau was significantly longer in DAO than WT hamsters. With increasing light intensity, tau lengthened in both phenotypes, though not at a similar rate. In 10 and 100 lux LL, however, tau did not differ between the two phenotypes. The robustness of the circadian activity rhythm was highest in DD and decreased in LL. No differences between phenotypes were noted. The percentage of arrhythmic animals was low in DD, but remarkably high in LL, and always higher in WT hamsters. The total amount of activity/day was highest in DD; DAO hamsters were less active than WT hamsters under each lighting condition. Light pulses induced phase delays when applied at CT14 and phase advances at CT22, with advances being stronger than delays. Also at CT14, the response of the activity onset was stronger than the activity offset. The opposite was observed at CT22. At CT14, the phase response did not differ between the phenotypes. However, at CT22 the phase advance was significantly weaker in DAO than WT hamsters despite their longer tau. The results provide further evidence that the distinct activity pattern of DAO hamsters is due to an altered interaction between the circadian clock and photic zeitgeber.