Elsevier

Neuroscience

Volume 202, 27 January 2012, Pages 300-308
Neuroscience

Cognitive, Behavioral, and Systems Neuroscience
Research Paper
Dim nighttime illumination alters photoperiodic responses of hamsters through the intergeniculate leaflet and other photic pathways

https://doi.org/10.1016/j.neuroscience.2011.11.037Get rights and content

Abstract

In mammals, light entrains the central pacemaker within the suprachiasmatic nucleus (SCN) through both a direct neuronal projection from the retina and an indirect projection from the intergeniculate leaflet (IGL) of the thalamus. Although light comparable in intensity to moonlight is minimally effective at resetting the phase of the circadian clock, dimly lit and completely dark nights are nevertheless perceived differentially by the circadian system, even when nighttime illumination is below putative thresholds for phase resetting. Under a variety of experimental paradigms, dim nighttime illumination exerts effects that may be characterized as enhancing the plasticity of circadian entrainment. For example, relative to completely dark nights, dimly lit nights accelerate development of photoperiodic responses of Siberian hamsters transferred from summer to winter day lengths. Here we assess the neural pathways underlying this response by testing whether IGL lesions eliminate the effects of dim nighttime illumination under short day lengths. Consistent with previous work, dimly lit nights facilitated the expansion of activity duration under short day lengths. Ablation of the IGL, moreover, did not influence photoperiodic responses in animals held under completely dark nights. However, among animals that were provided dimly lit nights, IGL lesions prevented the short-day typical expansion of activity duration as well as the seasonally appropriate gonadal regression and reduction in body weight. Thus, the present data indicate that the IGL plays a central role in mediating the facilitative effects of dim nighttime illumination under short day lengths, but in the absence of the IGL, dim light at night influences photoperiodic responses through residual 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.

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