In the mammalian retina, besides the conventional rod-cone system, a melanopsin-associated photoreceptive system exists that conveys photic information for accessory visual functions such as pupillary light reflex and circadian photo-entrainment [1][2][3][4][5][6][7] . On ablation of the melanopsin gene, retinal ganglion cells that normally express melanopsin are no longer intrinsically photosensitive 8 . Furthermore, pupil reflex 8 , light-induced phase delays of the circadian clock 9,10 and period lengthening of the circadian rhythm in constant light 9,10 are all partially impaired. Here, we investigated whether additional photoreceptive systems participate in these responses. Using mice lacking rods and cones, we measured the action spectrum for phase-shifting the circadian rhythm of locomotor behaviour. This spectrum matches that for the pupillary light reflex in mice of the same genotype 11 , and that for the intrinsic photosensitivity of the melanopsin-expressing retinal ganglion cells 7 . We have also generated mice lacking melanopsin coupled with disabled rod and cone phototransduction mechanisms. These animals have an intact retina but fail to show any significant pupil reflex, to entrain to light/dark cycles, and to show any masking response to light. Thus, the rod-cone and melanopsin systems together seem to provide all of the photic input for these accessory visual functions. © 2003 Nature Publishing GroupCorrespondence and requests for materials should be addressed to K.-W.Y. (kwyau@mail.jhmi.edu). Supplementary Information accompanies the paper on www.nature.com/nature. Competing interests statementThe authors declare that they have no competing financial interests. 8 . In independently produced melanopsin-knockout mice, others have found that the ability of light to phase-delay and lengthen the period of the circadian rhythm is also diminished 9,10 . For the pupil reflex, this photic response can be quantitatively accounted for by a functional complementarity between the rod-cone system and the melanopsin system, without the need to invoke any additional light-detection system 8 . Nonetheless, the proposal has persisted that cryptochromes-flavoproteins reported to have a direct light-detecting role in Drosophila 12,13 -may have the same function in mammals [14][15][16] despite earlier evidence to the contrary 17 . To settle this question, we first examined the action spectrum for phase-shifting the circadian rhythm in mice lacking rod and cone photoreceptors (rd/rd cl) 18 . Next, we generated triple-knockout mice lacking all confirmed photodetection systems-Opn4 −/− Gnat1 −/− Cnga3 −/− (melanopsin (also known as opsin 4), guanine nucleotide-binding protein α-transducin 1 (also known as rod transducin α-subunit, or Trα) and cyclic GMP-gated channel A-subunit 3, respectively)-and tested these animals for pupil reflex, circadian photo-entrainment and the masking response to light. NIH Public AccessIrradiance-response relations for the light-induced phase shifting of the circadian rhythm of l...

Masking and entrainment are two different processes that result in rhythms in physiology and behavior. Considering its functional importance, lamentably little study has been devoted to masking compared to that lavished on entrainment. This paper discusses the origin and definition of terminology (positive and negative masking, paradoxical masking) and methods of quantifying masking. How masking is scored can greatly influence the interpretation of results.

Some of the main themes in this review are as follows. 1. The notion that non-photic zeitgebers are weak needs re-examining. Phase-shifts to some non-photic manipulations can be as large as those to light pulses. 2. As well as being able to phase-shift and entrain free-running rhythms, non-photic events have a number of other effects: these include after-effects of entrainment, period changes, and promotion of splitting. 3. The critical variable for non-photic shifting is unknown. Locomotor activity is more likely to be an index of some other necessary state rather than being causal itself. This index may be better when tendencies to move are channelled into easily measured behaviours like wheel-running. 4. Given ignorance about the critical variable, quantification of activity may be the best presently available measure of zeitgeber intensity. Therefore, the behaviour during non-photic manipulations must be examined as carefully as the shifts themselves. When no phase-shifting follows manipulations such as IGL lesions or serotonin depletion, if the animals are inactive, then little can be inferred. 5. Lack of information on the critical variable(s) for non-photic shifting makes it problematic to compare data from studies using different non-photic manipulations. However, the presence of locomotor activity (or its correlate) does appear to be necessary for triazolam to produce shifts. 6. Novelty-induced wheel-running in hamsters depends on the NPY projection from the IGL to SCN. It remains to be determined how important NPY is in other species or in clock-resetting by other manipulations, but methods are now available to study this. 7. Interactions between photic and non-photic zeitgebers remain virtually unexplored, but it is evident that photic and non-photic stimuli can attenuate the phase-shifting effects of each other. Interactions are not purely additive or predictable from PRCs. 8. The circadian system does more than synchronize free-running rhythms to the solar day. Its non-photic functions and their interactions with photic inputs probably account for some of the anatomical complexity of circadian circuitry.

There are two ways in which an animal can confine its behavior to a nocturnal or diurnal niche. One is to synchronize an endogenous clock that in turn controls the sleep-wake cycle. The other is to respond directly to illumination with changes in activity. In mice, high illumination levels suppress locomotion (negative masking) and low illumination levels enhance locomotion (positive masking). To investigate the role of the newly discovered opsin-like protein melanopsin in masking, we used 1 h and 3 h pulses of light given in the night, and also a 3.5:3.5 h light-dark (LD) cycle. Mice lacking the melanopsin gene had normal enhancement of locomotion in the presence of dim lights but an impaired suppression of locomotion in the presence of bright light. This impairment was evident only with lights in the order of 10 lux or brighter. This suggests that melanopsin in retinal ganglion cells is involved in masking, as it is in pupil contraction and phase shifts. Melanopsin is especially important in maintaining masking responses over long periods.

The pervasive role of circadian clocks in regulating physiology and behavior is widely recognized. Their adaptive value is their ability to be entrained by environmental cues such that the internal circadian phase is a reliable predictor of solar time. In mammals, both light and nonphotic behavioral cues can entrain the principal oscillator of the hypothalamic suprachiasmatic nuclei (SCN). However, although light can advance or delay the clock during circadian night, behavioral events trigger phase advances during the subjective day, when the clock is insensitive to light. The recent identification of Period (Per) genes in mammals, homologues of dperiod, which encodes a core element of the circadian clockwork in Drosophila, now provides the opportunity to explain circadian timing and entrainment at a molecular level. In mice, expression of mPer1 and mPer2 in the SCN is rhythmic and acutely up-regulated by light. Moreover, the temporal relations between mRNA and protein cycles are consistent with a clock based on a transcriptional͞translational feedback loop. Here we describe circadian oscillations of Per1 and Per2 in the SCN of the Syrian hamster, showing that PER1 protein and mRNA cycles again behave in a manner consistent with a negative-feedback oscillator. Furthermore, we demonstrate that nonphotic resetting has the opposite effect to light: acutely down-regulating these genes. Their sensitivity to nonphotic resetting cues supports their proposed role as core elements of the circadian oscillator. Moreover, this study provides an explanation at the molecular level for the contrasting but convergent effects of photic and nonphotic cues on the clock.

Phase shifts in free-running activity rhythms of male golden hamsters, Mesocricetus auratus, often occur when they establish a new territory and home after a cage change. Similar shifts also often occur after pairs of animals interact with each other for half an hour. When these events take place during the middle of the hamsters' subjective day, they produce phase advances: when late in the subjective night, they produce phase delays. Repeated social interactions at the same time of day can entrain activity rhythms in a way consistent with the shape of the phase response curves. Not all individuals become entrained, as is predictable from the modest amplitude of the phase response curve. The effects of social interactions and of other disturbances may be mediated through an oscillator phased by general arousal. The present findings have implications for the interpretation of drug-induced changes in biological rhythms.

A method of estimating natural sex ratios of hatchlings of species with temperature-dependent sexual differentiation from data on incubation durations is described. The method was applied to loggerhead turtles (Caretta caretta) nesting in Brazil. Data on incubation durations were collected from 11 nesting beaches monitored for up to six seasons. It was estimated that 82.5% of the loggerhead hatchlings produced were female. The strongly female-biased sex ratio in Brazil is similar to that found. previously for loggerheads using beaches in the eastern U.S.A. This suggests that a female-biased hatchling sex ratio may be a feature of loggerhead populations.RCsumC : On trouvera ici la description d'une mCthode d'estimation des rapports m2les : femelles i partir de donnCes sur la durCe de l'incubation chez des tortues nConates appartenant i des espkces i differentiation sexuelle dCterminCe par la tempkrature. La methode a Ct C utilisCe chez des Caouanes (Caretta caretta) du BrCsil. La durCe de l'incubation a Ct C mesurCe dans les nids de tortues sur 11 plages de reproduction ob les donnCes ont Ct C relevCes pendant plusieurs saisons, parfois jusqu'i six. Nos estimations ont rCvClC que 82,5% des tortues i 1'Cclosion Ctaient des femelles. Ce rapport m2les : femelles trks faible mesurC au BrCsil est semblable i celui qui a Ct6 enregistre sur des plages de reproduction de cette tortue dans 1'Est des ~t a t s -~n i s .Ce nombre dCmesurC de femelles chez les tortues fraichement Ccloses est donc probablement une caractkristique des populations de caouanes. [Traduit par la RCdaction]

Estimates of sex ratios of hatchling sea turtles range from approximately 50% female (Chelonia mydas and Dermochelys coriacea in Suriname) to approximately 90% female (Caretta caretta in Florida). Seasonal sex production profiles (SSPPs) show how similar overall sex ratios can be achieved in dissimilar ways. Possible explanations of the data include sampling error, constraints on evolutionary adjustment of pivotals or behavior to local thermal conditions, and violations of assumptions required by classical Fisherian theory. o