Daily Behavioral Rhythmicity and Organization of the Suprachiasmatic Nuclei in the Diurnal Rodent, Lemniscomys barbarus

Wheel-running activity was recorded in Lemniscomys barbarus exposed to different lighting conditions. This rodent shows rhythmic locomotor activity under natural twilight-light/dark (LD) as well as squared-LD cycles. A mean of 77% of the activity occurred during the light phase. Under different controlled photoperiods, the quantity of daily locomotor activity was relatively stable except for a lower level in the shortest photoperiod tested (LD 06:18). The duration of the active phase tended to increase with the duration of the light phase, especially in the longer photoperiods. Whatever the lighting conditions, Lemniscomys barbarus started running before lights-on and stopped after lights-off. The phase angle of activity offset relative to lights-off was stable in each squared-photoperiod, whereas the phase angle of activity onset relative to lights-on was significantly the highest under the shortest photoperiods. Recording of activity under constant lighting conditions showed that the daily rhythm of locomotor activity is fundamentally circadian. The endogenous period was slightly24h (mean=23.8h) in permanent darkness and24h (mean=24.5h) in continuous light. Re-entrainment of the locomotor activity rhythm after a 6h phase advance or delay requires only four days on average. Moreover, the phase-responses curve to a 30min light pulse (200 lux) in Lemniscomys barbarus kept in constant dark reveals large phase shifts according to circadian times (CT). With CT0 being defined as the onset of daily activity, maximum phase delay and advance shifts were observed at CT11 ( =-5.7h2.3h) and CT21 ( =4.91.2h), respectively. Interestingly, the phase-response curve to light did not show any dead zone. Immunohistochemical staining of the suprachiasmatic nuclei indicates that arginine vasopressin-immunoreactive cell bodies and fibers delimited a dorsal subregion that extends laterally and medially. The ventral subregion is rich in vasoactive intestinal peptide-immunoreactive neurones overlapping a smaller area containing gastrin-releasing peptide-expressing cells and receives numerous fibers labeled with neuropeptide Y antibody. The results of this study clearly demonstrate that Lemniscomys barbarus is a diurnal species highly sensitive to the shifting effects of light. Overall, this rodent can be considered a new and interesting model for circadian rhythm neurobiology.