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Evans, J. A., Carter, S. N., Freeman, D. A., & Gorman, M. R. (2012). Dim nighttime illumination alters photoperiodic responses of hamsters through the intergeniculate leaflet and other photic pathways. Neuroscience, 202, 300–308.
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.
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Evans, J. A., Elliott, J. A., & Gorman, M. R. (2007). Circadian effects of light no brighter than moonlight. J Biol Rhythms, 22(4), 356–367.
Abstract: In mammals, light entrains endogenous circadian pacemakers by inducing daily phase shifts via a photoreceptor mechanism recently discovered in retinal ganglion cells. Light that is comparable in intensity to moonlight is generally ineffective at inducing phase shifts or suppressing melatonin secretion, which has prompted the view that circadian photic sensitivity has been titrated so that the central pacemaker is unaffected by natural nighttime illumination. However, the authors have shown in several different entrainment paradigms that completely dark nights are not functionally equivalent to dimly lit nights, even when nighttime illumination is below putative thresholds for the circadian visual system. The present studies extend these findings. Dim illumination is shown here to be neither a strong zeitgeber, consistent with published fluence response curves, nor a potentiator of other zeitgebers. Nevertheless, dim light markedly alters the behavior of the free-running circadian pacemaker. Syrian hamsters were released from entrained conditions into constant darkness or dim narrowband green illumination (~0.01 lx, 1.3 x 10(-9) W/cm(2), peak lambda = 560 nm). Relative to complete darkness, constant dim light lengthened the period by ~0.3 h and altered the waveform of circadian rhythmicity. Among animals transferred from long day lengths (14 L:10 D) into constant conditions, dim illumination increased the duration of the active phase (alpha) by ~3 h relative to complete darkness. Short day entrainment (8 L:16 D) produced initially long alpha that increased further under constant dim light but decreased under complete darkness. In contrast, dim light pulses 2 h or longer produced effects on circadian phase and melatonin secretion that were small in magnitude. Furthermore, the amplitude of phase resetting to bright light and nonphotic stimuli was similar against dimly lit and dark backgrounds, indicating that the former does not directly amplify circadian inputs. Dim illumination markedly alters circadian waveform through effects on alpha, suggesting that dim light influences the coupling between oscillators theorized to program the beginning and end of subjective night. Physiological mechanisms responsible for conveying dim light stimuli to the pacemaker and implications for chronotherapeutics warrant further study.
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Evans, J. A., Elliott, J. A., & Gorman, M. R. (2009). Dim nighttime illumination accelerates adjustment to timezone travel in an animal model. Curr Biol, 19(4), R156–7.
Abstract: Jetlag reflects a mismatch between local and circadian time following rapid timezone travel [1]. Appropriately timed bright light can shift human circadian rhythms but recovery is slow (e.g., 1-2 days per timezone). Most symptoms subside after resynchronization, but chronic jetlag may have enduring negative effects [2], including even accelerated mortality in mice [3]. Melatonin, prescription drugs, and/or exercise may help shift the clock but, like bright light, require complex schedules of application [1]. Thus, there is a need for more efficient and practical treatments for addressing jetlag. In contrast to bright daytime lighting, nighttime conditions have received scant attention. By incorporating more naturalistic nighttime lighting comparable in intensity to dim moonlight, we demonstrate that recovery after simulated jetlag is accelerated when nights are dimly lit rather than completely dark.
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Fonken, L. K., Finy, M. S., Walton, J. C., Weil, Z. M., Workman, J. L., Ross, J., et al. (2009). Influence of light at night on murine anxiety- and depressive-like responses. Behav Brain Res, 205(2), 349–354.
Abstract: Individuals are increasingly exposed to light at night. Exposure to constant light (LL) disrupts circadian rhythms of locomotor activity, body temperature, hormones, and the sleep-wake cycle in animals. Other behavioural responses to LL have been reported, but are inconsistent. The present experiment sought to determine whether LL produces changes in affective responses and whether behavioural changes are mediated by alterations in glucocorticoid concentrations. Relative to conspecifics maintained in a light/dark cycle (LD, 16:8 light/dark), male Swiss-Webster mice exposed to LL for three weeks increased depressive-like behavioural responses as evaluated by the forced swim test and sucrose anhedonia. Furthermore, providing a light escape tube reversed the effects of LL in the forced swim test. LL mice displayed reduced anxiety as evaluated by the open field and elevated-plus maze. Glucocorticoid concentrations were reduced in the LL group suggesting that the affective behavioural responses to LL are not the result of elevated corticosterone. Additionally, mice housed in LD with a clear tube displayed increased paired testes mass as compared to LL mice. Taken together, these data provide evidence that exposure to unnatural lighting can induce significant changes in affect, increasing depressive-like and decreasing anxiety-like responses.
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Fonken, L. K., Kitsmiller, E., Smale, L., & Nelson, R. J. (2012). Dim nighttime light impairs cognition and provokes depressive-like responses in a diurnal rodent. J Biol Rhythms, 27(4), 319–327.
Abstract: Circadian disruption is a common by-product of modern life. Although jet lag and shift work are well-documented challenges to circadian organization, many more subtle environmental changes cause circadian disruption. For example, frequent fluctuations in the timing of the sleep/wake schedule, as well as exposure to nighttime lighting, likely affect the circadian system. Most studies of these effects have focused on nocturnal rodents, which are very different from diurnal species with respect to their patterns of light exposure and the effects that light can have on their activity. Thus, the authors investigated the effect of nighttime light on behavior and the brain of a diurnal rodent, the Nile grass rat. Following 3 weeks of exposure to standard light/dark (LD; 14:10 light [~150 lux] /dark [0 lux]) or dim light at night (dLAN; 14:10 light [~150 lux] /dim [5 lux]), rats underwent behavioral testing, and hippocampal neurons within CA1, CA3, and the dentate gyrus (DG) were examined. Three behavioral effects of dLAN were observed: (1) decreased preference for a sucrose solution, (2) increased latency to float in a forced swim test, and (3) impaired learning and memory in the Barnes maze. Light at night also reduced dendritic length in DG and basilar CA1 dendrites. Dendritic length in the DG positively correlated with sucrose consumption in the sucrose anhedonia task. Nighttime light exposure did not disrupt the pattern of circadian locomotor activity, and all grass rats maintained a diurnal activity pattern. Together, these data suggest that exposure to dLAN can alter affective responses and impair cognition in a diurnal animal.
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