Cognitive, Behavioral, and Systems NeuroscienceResearch PaperDim nighttime illumination alters photoperiodic responses of hamsters through the intergeniculate leaflet and other photic pathways
Section snippets
Breeding and initial husbandry
Male Siberian hamsters (Phodopus sungorus) were selected from a colony established at University of California, San Diego since 1994 and maintained under a 24 h light:dark cycle with 14 h light and 10 h darkness (LD 14:10, lights on: 0600 PST, lights off: 2000 PST; photophase: ∼100 lx, scotophase: 0 lx). After weaning, hamsters were group-housed inside polypropylene cages (48×27×20 cm3) on open racks. Ambient temperature was maintained at 22±2 °C with ad libitum access to water and food (Purina
Results
Overall, α lengthened progressively during the 8-week exposure to the short day photoperiod (Week: F(8,52)=12.51, P<0.0001; Fig. 2, Fig. 3A). Scotophase condition significantly influenced changes in α under the short day photoperiod (Week*SC: F(8,52)=2.97, P<0.01), in a manner that tended to differ between IGL-Intact and IGL-Lesioned animals (Week*SC*IGL Status: F(8,52)=2.07, P<0.06). When partitioned by IGL Status, scotophase condition significantly influenced changes in activity duration of
Discussion
Dim nighttime illumination in IGL-intact animals accelerated expansion of α under short day lengths, as previously demonstrated (Gorman and Elliott, 2004). Extending prior research establishing that the IGL is not necessary for photo-entrainment (Pickard et al., 1987, Johnson et al., 1989), ablating the IGL did not influence circadian entrainment with completely dark nights in short day photoperiods. Most notably, IGL-lesioned hamsters provided with dimly lit nights failed to display the
Conclusions
In conclusion, the present study adds to a growing body of evidence that highlights the importance of IGL input under a variety of conditions that incorporate naturalistic lighting elements. In the wild, day lengths change gradually with the seasons; nocturnal rodents commonly minimize daytime light exposure by resting in darkened burrows; and nocturnal activity occurs under dim illumination from the moon and stars (Biberman et al., 1966, Thorington, 1980, Brainard et al., 1984, but see Daan et
Acknowledgments
This work was supported by NSF grant IBN-0346391 and NIH grant NICHD-36460. We thank Antonio Mora and Robert Sundberg for providing excellent animal care. We are also grateful to our research assistants Lindsey Stewart and Yasasvi Vasili.
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Present address: Neuroscience Institute, Morehouse School of Medicine, 720 Westview Drive South West, Atlanta, GA 30310, USA.