Light reception: discovering the clock-eye in mammals

Classical and melanopsin photoreception in irradiance detection: negative masking of locomotor activity by light

Studies in mice lacking either classical or melanopsin photoreception have been useful in describing the photoreceptor contribution to irradiance detection in accessory visual responses. However, application of these findings to irradiance detection in intact animals is problematical because retinal degeneration or manipulation can induce secondary changes in the retina. Among responses dependent on irradiance detection, the suppression of activity by light (negative masking) has had limited study. To further understand the function of classical and melanopsin photoreceptors we studied irradiance and spectral sensitivity of masking by light, primarily in mice with intact retinae. The sensitivity of negative masking was equivalent for medium ( approximately 500 nm) and short wavelengths ( approximately 365 nm) in three strains of wild-type mice, identifying a marked short-wavelength-sensitive-cone input. At medium wavelengths, spectral sensitivity above 500 nm had closest fit to the nomogram for the medium-wavelength-sensitive-cone, but a combined input of cone and melanopsin photoreceptors in wild-type mice seems likely. Under white light a decompression of the irradiance range of masking in C3H rd/rd cl mice, lacking rods and cones, identified a functional deficiency presumably resulting from the absence of classical photoreceptor input. Together the evidence demonstrates a pronounced and sustained classical photoreceptor input to irradiance detection for negative masking, and suggests one role of classical photoreceptor input is to constrain dynamic range.

Melanopsin-dependent nonvisual responses: evidence for photopigment bistability in vivo

In mammals, nonvisual responses to light have been shown to involve intrinsically photosensitive retinal ganglion cells (ipRGC) that express melanopsin and that are modulated by input from both rods and cones. Recent in vitro evidence suggests that melanopsin possesses dual photosensory and photoisomerase functions, previously thought to be a unique feature of invertebrate rhabdomeric photopigments. In cultured cells that normally do not respond to light, heterologous expression of mammalian melanopsin confers light sensitivity that can be restored by prior stimulation with appropriate wavelengths. Using three different physiological and behavioral assays, we show that this in vitro property translates to in vivo, melanopsin-dependent nonvisual responses. We find that prestimulation with long-wavelength light not only restores but enhances single-unit responses of SCN neurons to 480-nm light, whereas the long-wavelength stimulus alone fails to elicit any response. Recordings in Opn4-/- mice confirm that melanopsin provides the main photosensory input to the SCN, and furthermore, demonstrate that melanopsin is required for response enhancement, because this capacity is abolished in the knockout mouse. The efficiency of the light-enhancement effect depends on wavelength, irradiance, and duration. Prior long-wavelength light exposure also enhances short-wavelength-induced phase shifts of locomotor activity and pupillary constriction, consistent with the expression of a photoisomerase-like function in nonvisual responses to light.

Circadian Clocks: Translation Lost

One of the big questions in biological rhythms research is how a stable and precise circa-24 hour oscillation is generated on the molecular level. While increasing complexity seemed to be the key, a recent report suggests that circa-24 hour rhythms can be generated by just four molecules incubated in a test tube.

Entrainment dissociates transcription and translation of a circadian clock gene in neurospora

Circadian systems coordinate the daily sequence of events in cells, tissues, and organisms. In constant conditions, the biological clock oscillates with its endogenous period, whereas it is synchronized to the 24 hr light:dark cycle in nature. Here, we investigate light entrainment of Neurospora crassa to photoperiods that mimic seasonal changes. Clock gene (frequency, or frq) RNA levels directly reflect the light environment in all photoperiods, whereas the FRQ protein follows neither RNA levels nor light transitions. Induction of frq RNA and protein can be dissociated by as much as 6 hr, depending on photoperiod. The phase of entrainment at the physiological level (e.g., asexual spore development) correlates with FRQ protein. Thus, a dissociation of transcription, translation, and protein stability is fundamental to circadian entrainment of Neurospora. Our findings suggest that simple feedback models are insufficient to explain the molecular circadian mechanisms under entrained conditions and that clock control of light input pathways involves posttranscriptional regulation. The regulators mediating the dissociation between RNA and protein levels are still unknown and will be the key to understanding both circadian timing at the molecular level and how the clock exerts control over many cellular processes.

The network of time: understanding the molecular circadian system

The circadian clock provides a temporal structure that modulates biological functions from the level of gene expression to performance and behaviour. Pioneering work on the fruitfly Drosophila has provided a basis for understanding how the temporal sequence of daily events is controlled in mammals. New insights have come from work on mammals, specifically from studying the daily activity profiles of clock mutant mice; from more detailed recordings of clock gene expression under different experimental conditions and in different tissues; and from the discovery and analysis of a growing number of additional clock genes. These new results are moving the model paradigm away from a simple negative feedback loop to a molecular network. Understanding the coupling and interactions of this network will help us to understand the evolution of the circadian system, advance medical diagnosis and treatment, improve the health of shift workers and frequent travellers, and will generally enable the treatment of clock-related pathologies.

Life between clocks: daily temporal patterns of human chronotypes

Human behavior shows large interindividual variation in temporal organization. Extreme "larks" wake up when extreme "owls" fall asleep. These chronotypes are attributed to differences in the circadian clock, and in animals, the genetic basis of similar phenotypic differences is well established. To better understand the genetic basis of temporal organization in humans, the authors developed a questionnaire to document individual sleep times, self-reported light exposure, and self-assessed chronotype, considering work and free days separately. This report summarizes the results of 500 questionnaires completed in a pilot study individual sleep times show large differences between work and free days, except for extreme early types. During the workweek, late chronotypes accumulate considerable sleep debt, for which they compensate on free days by lengthening their sleep by several hours. For all chronotypes, the amount of time spent outdoors in broad daylight significantly affects the timing of sleep: Increased self-reported light exposure advances sleep. The timing of self-selected sleep is multifactorial, including genetic disposition, sleep debt accumulated on workdays, and light exposure. Thus, accurate assessment of genetic chronotypes has to incorporate all of these parameters. The dependence of human chronotype on light, that is, on the amplitude of the light:dark signal, follows the known characteristics of circadian systems in all other experimental organisms. Our results predict that the timing of sleep has changed during industrialization and that a majority of humans are sleep deprived during the workweek. The implications are far ranging concerning learning, memory, vigilance, performance, and quality of life.

Non-rod, non-cone photoreception in the vertebrates

When reflected from a surface, light can provide a representation of the spatial environment, whilst gross changes in environment light can signal the time of day. The differing sensory demands of using light to detect environmental space and time appear to have provided the selection pressures for the evolution of different photoreceptor systems in the vertebrates, and probably all animals. This point has been well recognised in the non-mammals, which possess multiple opsin/vitamin A-based photoreceptor populations in a variety of sites distributed both within and outside the CNS. By contrast, eye loss in mammals abolishes all responses to light, and as a result, all photoreception was attributed to the rods and cones of the retina. However, studies over the past decade have provided overwhelming evidence that the mammalian eye contains a novel photoreceptor system that does not depend upon the input from the rods and cones. Mice with eyes but lacking rod and cone photoreceptors can still detect light to regulate their circadian rhythms, suppress pineal melatonin, modify locomotor activity, and modulate pupil size. Furthermore, action spectra for some of these responses in rodents and humans have characterised at least one novel opsin/vitamin A-based photopigment, and molecular studies have identified a number of candidate genes for this photopigment. Parallel studies in fish showing that VA opsin photopigment is expressed within sub-sets of inner retina neurones, demonstrates that mammals are not alone in having inner retinal photoreceptors. It therefore seems likely that inner retinal photoreception will be a feature of all vertebrates. Current studies are directed towards an understanding of their mechanisms, determining the extent to which they contribute to physiology and behaviour in general, and establishing how they may interact with other photoreceptors, including the rods and cones. Progress on each of these topics is moving very rapidly. As a result, we hope this review will serve as an introduction to the cascade of papers that will emerge on these topics in the next few years. We also hope to convince the more casual reader that there is much more to vertebrate photoreceptors than the study of retinal rods and cones.