Dim Light at Night and Constant Darkness: Two Frequently Used Lighting Conditions That Jeopardize the Health and Well-being of Laboratory Rodents

Abstract

The influence of light on mammalian physiology and behavior is due to the entrainment of circadian rhythms complemented with a direct modulation of light that would be unlikely an outcome of circadian system. In mammals, physiological and behavioral circadian rhythms are regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus. This central control allows organisms to predict and anticipate environmental change, as well as to coordinate different rhythmic modalities within an individual. In adult mammals, direct retinal projections to the SCN are responsible for resetting and synchronizing physiological and behavioral rhythms to the light-dark (LD) cycle. Apart from its circadian effects, light also has direct effects on certain biological functions in such a way that the participation of the SCN would not be fundamental for this network. The objective of this review is to increase awareness, within the scientific community and commercial providers, of the fact that laboratory rodents can experience a number of adverse health and welfare outcomes attributed to commonly-used lighting conditions in animal facilities during routine husbandry and scientific procedures, widely considered as “environmentally friendly.” There is increasing evidence that exposure to dim light at night, as well as chronic constant darkness, challenges mammalian physiology and behavior resulting in disrupted circadian rhythms, neural death, a depressive-behavioral phenotype, cognitive impairment, and the deregulation of metabolic, physiological, and synaptic plasticity in both the short and long terms. The normal development and good health of laboratory rodents requires cyclical light entrainment, adapted to the solar cycle of day and night, with null light at night and safe illuminating qualities during the day. We therefore recommend increased awareness of the limited information available with regards to lighting conditions, and therefore that lighting protocols must be taken into consideration when designing experiments and duly highlighted in scientific papers. This practice will help to ensure the welfare of laboratory animals and increase the likelihood of producing reliable and reproducible results.

Differential Effects of Constant Light and Dim Light at Night on the Circadian Control of Metabolism and Behavior

“…Melanopsin-expressing intrinsically photosensitive retinal ganglion cells transmit information on lighting conditions directly to different brain areas, such as intergeniculate leaflet, the lateral geniculate nucleus of the thalamus [9,14,15], habenular nucleus, nucleus accumbens [9,15,16], medial amygdala, lateral hypothalamus and ventral preoptic area [9,15]. These structures can modulate sleep, mood and cognitive functions, heart rate and glucocorticoid levels [17].…”

Major oscillations in spontaneous home-cage activity with an infraradian periodicity in C57Bl/6 mice housed under constant conditions

“…We do not know the origin of most of these short frequency clusters yet but by using spectral analysis and time series combined with more detailed annotation of parameters in the holding room (staff mobility, cages in/out of the rack, vibrations, noise, spectral or intensity fluctuation of the illumination, etc.) 16,17 we may be able to map their origin in the future. For example, short bursts of recurring activity during daytime in between periods of rest may be a trailing effect, where vibration and noise from one cage trigger activity in the next cage and so on; such patterns being close to synchronized may be possible to track down with the DVC recording technique.…”

“…The global impact of the circadian rhythm on peripheral tissues is mediated through secreted melatonin, endocrine signals and the autonomic nervous system 5,17 . In contrast to the circadian rhythm, the onward signalling to the target organs (ovary and testis in the case of reproduction) of circannual rhythms are not fully understood.…”

“…Here cells of the pineal body translate and feedback the message by supressing the conversion of serotonin (an amine that also serve as a major neurotransmitter in the nervous system) to melatonin 13,14 which is secreted during the dark hours only (idem) 9, [15][16][17][18] .…”

“…The daily oscillation of the key gene products is not exclusive to the SCN where direct interaction occurs with the neuroendocrine (HPA) system within hypothalamus and key regions of the brain 17 , but is expressed in different peripheral tissues of our body 6 explaining the circadian oscillators' profound impact on bodily functions. A number of biorhythms have a higher frequency than the circadian and are collectively referred to as supraradian biorhythms.…”