Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity

Melanopsin (Opn4) requirement for normal light-induced circadian phase shifting

The master circadian oscillator in the hypothalamic suprachiasmatic nucleus is entrained to the day/night cycle by retinal photoreceptors. Melanopsin (Opn4), an opsin-based photopigment, is a primary candidate for photoreceptor-mediated entrainment. To investigate the functional role of melanopsin in light resetting of the oscillator, we generated melanopsin-null mice (Opn4-/-). These mice entrain to a light/dark cycle and do not exhibit any overt defect in circadian activity rhythms under constant darkness. However, they display severely attenuated phase resetting in response to brief pulses of monochromatic light, highlighting the critical role of melanopsin in circadian photoentrainment in mammals.

Mapping quantitative trait loci affecting circadian photosensitivity in retinally degenerate mice

It is known that retinally degenerate C57BL/6J (rd/rd) mice have unattenuated circadian photosensitivity. However, the authors have previously found that CBA/J (rd/rd) mice that carry the same rd mutation have attenuated circadian photosensitivity compared to normal CBA/N (+/+) mice. In the present study, a quantitative trait locus (QTL) analysis using C57BL/6J (rd/rd) and CBA/J (rd/rd) mice was conducted in order to identify the genes affecting circadian photosensitivity of the rd mice. As a result, several putative QTLs onthree separate chromosomes (8, 12, 17) were detected, which indicates that circadian photosensitivity in rd mice is altered by multiple genes. Identification of these genes may provide new insights into the understanding of regulation of circadian photoentrainment and sleep-wake disorders.

Loss of photic entrainment and altered free-running circadian rhythms in math5-/- mice

Mammalian free-running circadian rhythms are entrained to the external light/dark cycle by photic signaling to the suprachiasmatic nuclei via the retinohypothalamic tract (RHT). We investigated the circadian entrainment and clock properties of math5-/- mutant mice. math5 is a critical regulator of retinal ganglion cell development; math5-/- mice show severe optic nerve hypoplasia. By anterograde cholera toxin B tracing, we find that math5-/- mice do not develop an identifiable RHT pathway. This appears to be attributable to agenesis or dysgenesis of the majority of RHT-projecting retinal ganglion cells. math5-/- mice display free-running circadian rhythms with a period approximately 1 hr longer than B6/129 controls (24.43 +/- 0.10 vs 23.62 +/- 0.19 hr; p < 0.00001). The free-running period of heterozygote mice is indistinguishable from that of controls. math5-/- mice show no entrainment to light/dark cycles, whereas heterozygote mice show normal entrainment to both 12 hr light/dark cycles and to a 1 hr skeletal photoperiod. math5-/- mice show reduced ability to entrain their rhythms to the nonphotic time cue of restricted running wheel access but demonstrate both free-running behavior and entrained anticipation of wheel unlocking in these conditions, suggesting the presence of a second diurnal oscillatory system in math5-/- animals. These results demonstrate that retinal ganglion cell input is not necessary for the development of a free-running circadian timekeeping system in the suprachiasmatic nucleus but is important for both photic entrainment and determination of the free-running period.

Time after time: inputs to and outputs from the mammalian circadian oscillators

Oscillating levels of clock gene transcripts in the suprachiasmatic nucleus (SCN) are essential components of the mammalian circadian pacemaker. Their synchronization with daily light cycles involves neural connections from light-sensitive photoreceptor-containing retinal ganglion cells. This clock orchestrates rhythmic expression for approximately 10% of the SCN gene transcripts, of which only 10% are also rhythmically expressed in other tissues. Many of the transcripts expressed rhythmically only in the SCN are involved in neurosecretion, and their secreted products could mediate SCN control over physiological rhythms by coordinating rhythmicity in other nuclei within the brain. The coordination of clock gene transcript oscillations in peripheral tissues could be controlled directly by specific signals or indirectly by rhythmic behavior such as feeding.

Photic resetting of the human circadian pacemaker in the absence of conscious vision

Ocular light exposure patterns are the primary stimuli for entraining the human circadian system to the local 24-h day. Many totally blind persons cannot use these stimuli and, therefore, have circadian rhythms that are not entrained. However, a few otherwise totally blind persons retain the ability to suppress plasma melatonin concentrations after ocular light exposure, probably using a neural pathway that includes the site of the human circadian pacemaker, suggesting that light information is reaching this site. To test definitively whether ocular light exposure could affect the circadian pacemaker of some blind persons and whether melatonin suppression in response to bright light correlates with light-induced phase shifts of thecircadian system, the authorsperformed experiments with 5 totally blind volunteers using a protocol known to induce phase shifts of the circadian pacemaker in sighted individuals. In the 2 blind individuals who maintained light-induced melatonin suppression, the circadian system was shifted by appropriately timed bright-light stimuli. These data demonstrate that light can affect the circadian pacemaker of some totally blind individuals--either by altering the phase of the circadian pacemaker or by affecting its amplitude. They are consistent with data from animal studies demonstrating that there are different neural pathways and retinal cells that relay photic information to the brain: one for conscious light perception and the other for non-image-forming functions.

The extraretinal eyelet of Drosophila: development, ultrastructure, and putative circadian function

Circadian rhythms can be entrained by light to follow the daily solar cycle. In Drosophila melanogaster a pair of extraretinal eyelets expressing immunoreactivity to Rhodopsin 6 each contains four photoreceptors located beneath the posterior margin of the compound eye. Their axons project to the region of the pacemaker center in the brain with a trajectory resembling that of Bolwig's organ, the visual organ of the larva. A lacZ reporter line driven by an upstream fragment of the developmental gap gene Krüppel is a specific enhancer element for Bolwig's organ. Expression of immunoreactivity to the product of lacZ in Bolwig's organ persists through pupal metamorphosis and survives in the adult eyelet. We thus demonstrate that eyelet derives from the 12 photoreceptors of Bolwig's organ, which entrain circadian rhythmicity in the larva. Double labeling with anti-pigment-dispersing hormone shows that the terminals of Bolwig's nerve differentiate during metamorphosis in close temporal and spatial relationship to the ventral lateral neurons (LN(v)), which are essential to express circadian rhythmicity in the adult. Bolwig's organ also expresses immunoreactivity to Rhodopsin 6, which thus continues in eyelet. We compared action spectra of entrainment in different fly strains: in flies lacking compound eyes but retaining eyelet (so(1)), lacking both compound eyes and eyelet (so(1);gl(60j)), and retaining eyelet but lacking compound eyes as well as cryptochrome (so(1);cry(b)). Responses to phase shifts suggest that, in the absence of compound eyes, eyelet together with cryptochrome mainly mediates phase delays. Thus a functional role in circadian entrainment first found in Bolwig's organ in the larva is retained in eyelet, the adult remnant of Bolwig's organ, even in the face of metamorphic restructuring.

Zebrafish melanopsin: isolation, tissue localisation and phylogenetic position

Photoreception is best understood in retinal rods and cones, but it is not confined to these cells. In non-mammals, intrinsically photosensitive cells have been identified within several structures including the pineal, hypothalamus and skin. More recently novel light sensitive cells have been identified in the inner/basal retina of both teleosts and rodents. Melanopsin has been proposed as the photopigment mediating many of these non-rod, non-cone responses to light. However, much about the melanopsin gene family remains to be clarified including their potential role as photopigments, and taxonomic distribution. We have isolated the first orthologue of melanopsin from a teleost fish and show expression of this gene in a sub-set of retinal horizontal cells (type B). Zebrafish melanopsin, and orthologues of this gene, differ markedly from the vertebrate photopigment opsins. The putative counterion is not a glutamate but a tyrosine, the putative G-protein binding domain in the third cytoplasmic loop is not conserved, and they show low levels of amino acid identity (approximately 27%) to both the known photopigment opsins and to other members of the melanopsin family. Mouse melanopsin is only 58% identical to Xenopus, and 68% identical to zebrafish. By contrast, the photosensory opsin families show approximately 75% conservation. On the basis of their structure, genomic organisation, discrete evolutionary lineage, and their co-expression with other opsins, the melanopins are not obvious photosensory opsins. They might represent a separate branch of photopigment evolution in the vertebrates or they may have a non-direct photosensory function, perhaps as a photoisomerase, in non-rod, non-cone light detection.

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.

The extraretinal eyelet of Drosophila: development, ultrastructure, and putative circadian function

Circadian rhythms can be entrained by light to follow the daily solar cycle. In Drosophila melanogaster a pair of extraretinal eyelets expressing immunoreactivity to Rhodopsin 6 each contains four photoreceptors located beneath the posterior margin of the compound eye. Their axons project to the region of the pacemaker center in the brain with a trajectory resembling that of Bolwig's organ, the visual organ of the larva. A lacZ reporter line driven by an upstream fragment of the developmental gap gene Krüppel is a specific enhancer element for Bolwig's organ. Expression of immunoreactivity to the product of lacZ in Bolwig's organ persists through pupal metamorphosis and survives in the adult eyelet. We thus demonstrate that eyelet derives from the 12 photoreceptors of Bolwig's organ, which entrain circadian rhythmicity in the larva. Double labeling with anti-pigment-dispersing hormone shows that the terminals of Bolwig's nerve differentiate during metamorphosis in close temporal and spatial relationship to the ventral lateral neurons (LN(v)), which are essential to express circadian rhythmicity in the adult. Bolwig's organ also expresses immunoreactivity to Rhodopsin 6, which thus continues in eyelet. We compared action spectra of entrainment in different fly strains: in flies lacking compound eyes but retaining eyelet (so(1)), lacking both compound eyes and eyelet (so(1);gl(60j)), and retaining eyelet but lacking compound eyes as well as cryptochrome (so(1);cry(b)). Responses to phase shifts suggest that, in the absence of compound eyes, eyelet together with cryptochrome mainly mediates phase delays. Thus a functional role in circadian entrainment first found in Bolwig's organ in the larva is retained in eyelet, the adult remnant of Bolwig's organ, even in the face of metamorphic restructuring.

Programming of the fetal suprachiasmatic nucleus and subsequent adult rhythmicity

The suprachiasmatic nucleus (SCN) is the site of the generation and entrainment of circadian rhythms. Similar to other structures, it develops throughout gestation but is still immature for some time after. This suggests that the SCN could be vulnerable to maternal influences, such as poor nutrition, stress and drugs, all of which can affect neuronal development. Evidence is accumulating that suggests that this is the case, with body size at birth influencing melatonin production in adult humans and maternal malnutrition, and stress affecting sleep in rodents. Interestingly, the maternal environment affects the phase of rhythms and the response of the circadian timing system to light pulses. The nature of these changes in adult rhythmicity is similar to those commonly associated with depression in humans. Thus, abnormal fetal programming might predispose adults to depressive illness.

Circadian phototransduction and the regulation of biological rhythms

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Melatonin synthesis enzymes in Macaca mulatta: focus on arylalkylamine N-acetyltransferase (EC 2.3.1.87)

Arylalkylamine N-acetyltransferase (AANAT; serotonin N-acetyltransferase, EC 2.3.1.87) plays a unique transduction role in vertebrate physiology as the key interface between melatonin production and regulatory mechanisms. Circulating melatonin is elevated at night in all vertebrates, because AANAT activity increases in the pineal gland in response to signals from the circadian clock. Circadian regulation of melatonin synthesis is implicated in a variety of human problems, including jet lag, shift work, insomnia, and abnormal activity rhythms in blind persons. In this report AANAT was studied in the rhesus macaque to better understand human melatonin regulation. AANAT mRNA is abundant in the pineal gland and retina, but not elsewhere; AANAT mRNA is uniformly distributed in the pineal gland, but is limited primarily to the photoreceptor outer segments in the retina. Day and night levels of pineal and retinal AANAT mRNA are similar. In contrast, AANAT activity and protein increase more than 4-fold at night in both tissues. The activity of hydroxyindole-O-methyltransferase, the last enzyme in melatonin synthesis, is tonically high in the pineal gland, but is nearly undetectable in the retina; hydroxyindole O-methyltransferase mRNA levels exhibited a similar pattern. This supports the view that the source of circulating melatonin in primates is the pineal gland. The discovery in this study that rhesus pineal AANAT mRNA is high at all times is of special importance because it shows that posttranscriptional control of this enzyme plays a dominant role in regulating melatonin synthesis.

Opsin-G11-mediated signaling pathway for photic entrainment of the chicken pineal circadian clock

Light is a major environmental signal for entrainment of the circadian clock, but little is known about the intracellular phototransduction pathway triggered by light activation of the photoreceptive molecule(s) responsible for the phase shift of the clock in vertebrates. The chicken pineal gland and retina contain the autonomous circadian oscillators together with the photic entrainment pathway, and hence they represent useful experimental models for the clock system. Here we show the expression of G11alpha, an alpha subunit of heterotrimeric G-protein, in both tissues by cDNA cloning, Northern blot, and Western blot analyses. G11alpha immunoreactivity was colocalized with pinopsin in the chicken pineal cells and also with rhodopsin in the outer segments of retinal photoreceptor cells, suggesting functional coupling of G11alpha with opsins in the clock-containing photosensitive tissues. The physical interaction was examined by coimmunoprecipitation experiments, the results of which provided evidence for light- and GTP-dependent coupling between rhodopsin and G11alpha. To examine whether activation of endogenous G11 leads to a phase shift of the oscillator, Gq/11-coupled m1-type muscarinic acetylcholine receptor (mAChR) was ectopically expressed in the cultured pineal cells. Subsequent treatment of the cells with carbamylcholine (CCh), an agonist of mAChR, induced phase-dependent phase shifts of the melatonin rhythm in a manner very similar to the effect of light. In contrast, CCh treatment induced no measurable effect on the rhythm of nontransfected (control) cells or cells expressing G(i/o)-coupled m2-type mAChR, indicating selectivity of the G-protein activation. Together, our results demonstrate the existence of a G11-mediated opsin-signaling pathway contributing to the photic entrainment of the circadian clock.

Melatonin MT-1-receptor immunoreactivity in the human eye

AIM

To examine the distribution of melatonin 1a (MT1) receptors in the human eye.

METHODS

Seven normal human eyes were examined by immunohistochemical staining of paraffin sections, using an anti-MT1 primary antibody and an ABC detection system.

RESULTS

MT1 receptor immunoreactivity (MT1-IR) was detected primarily in the inner segments of rods and cones and in retinal ganglion cells. In addition, MT1-IR was present in the adventitia of retinal arteries and veins, including the papillary region, but absent in ciliary and choroidal vessels. Mild staining of corneal endothelial cells and keratocytes was observed in all but two eyes.

CONCLUSION

MT1-IR is present in various ocular tissues with the highest density in photoreceptor cells and ganglion cells. The physiological function of these receptors deserves further investigation.

Opsin-G11-mediated signaling pathway for photic entrainment of the chicken pineal circadian clock

Light is a major environmental signal for entrainment of the circadian clock, but little is known about the intracellular phototransduction pathway triggered by light activation of the photoreceptive molecule(s) responsible for the phase shift of the clock in vertebrates. The chicken pineal gland and retina contain the autonomous circadian oscillators together with the photic entrainment pathway, and hence they represent useful experimental models for the clock system. Here we show the expression of G11alpha, an alpha subunit of heterotrimeric G-protein, in both tissues by cDNA cloning, Northern blot, and Western blot analyses. G11alpha immunoreactivity was colocalized with pinopsin in the chicken pineal cells and also with rhodopsin in the outer segments of retinal photoreceptor cells, suggesting functional coupling of G11alpha with opsins in the clock-containing photosensitive tissues. The physical interaction was examined by coimmunoprecipitation experiments, the results of which provided evidence for light- and GTP-dependent coupling between rhodopsin and G11alpha. To examine whether activation of endogenous G11 leads to a phase shift of the oscillator, Gq/11-coupled m1-type muscarinic acetylcholine receptor (mAChR) was ectopically expressed in the cultured pineal cells. Subsequent treatment of the cells with carbamylcholine (CCh), an agonist of mAChR, induced phase-dependent phase shifts of the melatonin rhythm in a manner very similar to the effect of light. In contrast, CCh treatment induced no measurable effect on the rhythm of nontransfected (control) cells or cells expressing G(i/o)-coupled m2-type mAChR, indicating selectivity of the G-protein activation. Together, our results demonstrate the existence of a G11-mediated opsin-signaling pathway contributing to the photic entrainment of the circadian clock.

Central and peripheral circadian oscillator mechanisms in flies and mammals

Circadian oscillators are cell-autonomous time-keeping mechanisms that reside in diverse tissues in many organisms. In flies and mice, the core molecular components that sustain these oscillators are highly conserved, but the functions of some of these components appear to have diverged significantly. One possible reason for these differences is that previous comparisons have focused primarily on the central oscillator of the mouse and peripheral oscillators in flies. Recent research on mouse and Drosophila peripheral oscillators shows that the function of the core components between these organisms may be more highly conserved than was first believed, indicating the following: (1) that central and peripheral oscillators in flies do not necessarily have the same molecular mechanisms; (2) that mammalian central oscillators are regulated differently from peripheral oscillators; and (3) that different peripheral oscillators within and across species show striking similarities. The core feedback loop in peripheral oscillators might therefore be functionally well conserved, and central oscillators could be specialized versions of a basic oscillator design.

Coordination of circadian timing in mammals

Time in the biological sense is measured by cycles that range from milliseconds to years. Circadian rhythms, which measure time on a scale of 24 h, are generated by one of the most ubiquitous and well-studied timing systems. At the core of this timing mechanism is an intricate molecular mechanism that ticks away in many different tissues throughout the body. However, these independent rhythms are tamed by a master clock in the brain, which coordinates tissue-specific rhythms according to light input it receives from the outside world.

The circadian photopigment melanopsin is expressed in the blind subterranean mole rat, Spalax

Despite severe degeneration of its eyes, the blind subterranean mole rat, Spalax, is able to adjust circadian rhythms to the environmental light/dark cycle due to a conserved retinohypothalamic tract (RHT). The photopigment mediating the circadian photoreception and it cellular localisation is unknown in the Spalax retina. Here we show, using in situ hybridization and immunohistochemistry, that melanopsin, a recently identified opsin, is expressed in retinal ganglion cells which also co-store PACAP, a neurotransmitter of the RHT. The melanopsin-component of retinal ganglion cells in the Spalax retina is well conserved resulting in a relatively higher density of melanopsin positive cells per area compared to the rat. The results show that the Spalax, as sighted animals expresses melanopsin in ganglion cells projecting to the circadian clock supporting a role of melanopsin as a circadian photopigment.

White Collar-1, a circadian blue light photoreceptor, binding to the frequency promoter

In the fungus Neurospora crassa, the blue light photoreceptor(s) and signaling pathway(s) have not been identified. We examined light signaling by exploiting the light sensitivity of the Neurospora biological clock, specifically the rapid induction by light of the clock component frequency (frq). Light induction of frq is transcriptionally controlled and requires two cis-acting elements (LREs) in the frq promoter. Both LREs are bound by a White Collar-1 (WC-1)/White Collar-2 (WC-2)-containing complex (WCC), and light causes decreased mobility of the WCC bound to the LREs. The use of in vitro-translated WC-1 and WC-2 confirmed that WC-1, with flavin adenine dinucleotide as a cofactor, is the blue light photoreceptor that mediates light input to the circadian system through direct binding (with WC-2) to the frq promoter.

Characterization of the pupil light reflex, electroretinogram and tonometric parameters in healthy rat eyes

PURPOSE

To characterize the pupil light reflex (PLR), electroretinographic (ERG) and tonometric parameters in healthy rat eyes.

METHODS

Brown Norway rats were used for experiments. The PLR was evaluated with a computerized pupillometer (n = 27), ERGs were recorded simultaneously from both eyes (n = 27) and IOP was measured with a Tonopen (n = 15).

RESULTS

The analysis of the PLR parameters confirmed that the consensual PLR had a significantly smaller amplitude (0.1-0.2 mm; p = 0.03) and an increased latency time (p = 0.001) compared to the direct PLR. Electroretinography revealed an a-wave amplitude of 207.2 +/- 13 microV with a latency of 25.6 +/- 0.7 ms, and a b-wave 554.3 +/- 24.5 microV with a latency of 21.4 + 1.8 ms. The flicker ERG recording revealed amplitudes of 40.6 +/- 2.4 microV. Tonometry measurements revealed that isoflurane, but not halothane, anesthesia suppressed the IOP (non-anesthetized: 25.3 +/- 1.0 mmHg; 1% halothane + 30% NO: 26.2 +/- 1.1 (p > 0.1); 1% isoflurane + 30% NO: 20.1 + 1.6 (p < 0.05)).

CONCLUSIONS

Consensual PLR in rats has a relative deficit compared to the direct PLR. Isoflurane anesthesia has a suppressive effect on the IOP in healthy rat eyes.

Effect of 192 IgG-saporin on circadian activity rhythms, expression of P75 neurotrophin receptors, calbindin-D28K, and light-induced Fos in the suprachiasmatic nucleus in rats

Photic entrainment of circadian rhythms in mammals is mediated through a direct retinal projection to the core region of the suprachiasmatic nucleus (SCN), the circadian clock. A proportion of this projection contains the low-affinity p75 neurotrophic receptor (p75NTR). Neonatal monosodium glutamate (MSG) treatment, which dramatically reduces p75NTR immunoreactivity in the SCN has no impact on photic entrainment. In order to clarify the contribution of p75NTR fibers in photic entrainment, targeted lesions of the p75NTR-immunoreactive SCN plexus were performed using intracerebroventricular (ICV) or intrahypothalamic injections of the immunotoxin 192 IgG-saporin (SAP) in rats. SAP treatment effectively abolished p75NTR immunoreactivity within the SCN core. ICV SAP treatment produced three different behavioral activity patterns: Animals became arrhythmic, displayed a shorter free-running period, or remained rhythmic following the lesion. Arrhythmic animals had large hypothalamic lesion which encompassed the entire SCN. In rhythmic rats, ICV-SAP significantly reduced immunostaining for calbindin-D28k (CaBP) in the SCN, and rats with shortened free-running periods had the lowest number of CaBP immunoreactive cells. ICV SAP also attenuated light-induced Fos expression in the SCN core. Despite lack of p75NTR and reduced CaBP and Fos expression in the SCN, SAP-treated rhythmic rats displayed normal photic entrainment. Intrahypothalamic SAP treatment reduced CaBP expression in the SCN but had no effect on light-induced Fos expression, free-running rhythms, or photic entrainment. The data show that p75NTR-immunoreactive elements in the SCN are not required for photic entrainment.

Entrainment impaired, masking spared: an apparent genetic abnormality that prevents circadian rhythm entrainment to 24-h lighting cycles in California mice

Lighting cycles can influence the expression of daily activity rhythms in two ways: by entraining the circadian pacemaker that normally drives this rhythm, and by directly affecting the expression of activity itself, thereby 'masking' the influence of the pacemaker. We describe a California mouse (Peromyscus californicus) in which these processes are dissociated. Circadian rhythms of wheel-running activity were recorded continuously while the animal was housed in a standard light/dark cycle and in constant darkness. This animal expressed a normal circadian rhythm that failed to entrain to the light/dark cycle, but was completely masked during the light phase. This animal's phenotype appears to have a genetic basis, since the progeny of selective crosses of his descendants showed similar abnormalities. These mice are the first example of animals expressing apparently normal circadian rhythms that are not entrained by light, but that still show potent masking responses to light exposure.

Light induction of a vertebrate clock gene involves signaling through blue-light receptors and MAP kinases

The signaling pathways that couple light photoreception to entrainment of the circadian clock have yet to be deciphered. Two prominent groups of candidates for the circadian photoreceptors are opsins (e.g., melanopsin) and blue-light photoreceptors (e.g., cryptochromes). We have previously showed that the zebrafish is an ideal model organism in which to study circadian regulation and light response in peripheral tissues. Here, we used the light-responsive zebrafish cell line Z3 to dissect the response of the clock gene zPer2 to light. We show that the MAPK (mitogen-activated protein kinase) pathway is essential for this response, although other signaling pathways may also play a role. Moreover, action spectrum analyses of zPer2 transcriptional response to monochromatic light demonstrate the involvement of a blue-light photoreceptor. The Cry1b and Cry3 cryptochromes constitute attractive candidates as photoreceptors in this setting. Our results establish a link between blue-light photoreceptors, probably cryptochromes, and the MAPK pathway to elicit light-induced transcriptional activation of clock genes.

Phototransduction by retinal ganglion cells that set the circadian clock

Light synchronizes mammalian circadian rhythms with environmental time by modulating retinal input to the circadian pacemaker-the suprachiasmatic nucleus (SCN) of the hypothalamus. Such photic entrainment requires neither rods nor cones, the only known retinal photoreceptors. Here, we show that retinal ganglion cells innervating the SCN are intrinsically photosensitive. Unlike other ganglion cells, they depolarized in response to light even when all synaptic input from rods and cones was blocked. The sensitivity, spectral tuning, and slow kinetics of this light response matched those of the photic entrainment mechanism, suggesting that these ganglion cells may be the primary photoreceptors for this system.

Molecular genetics of timing in intrinsic circadian rhythm sleep disorders

Recent advances in circadian biology are identifying key genes and the molecular clockworks they command. These biochemical systems provide new tools for evaluating clinically observed, intrinsic circadian rhythm sleep disorders. A striking example was last year's discovery of a point mutation in a human clock gene that produces a sleep phase syndrome. This finding suggested that other intrinsic sleep disorders may have genetic underpinnings, and that less debilitating variations in sleep/wake behavior may be revealed by molecular screening of known clock genes in broader human populations.

Melanopsin: a novel photopigment involved in the photoentrainment of the brain's biological clock?

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms of physiology and behaviour of approximately 24 hours. The clock needs, however, like a watch that runs too fast or too slow, daily adjustment and the most important stimulus for this adjustment is the environmental light/dark cycle, a process know as photoentrainment. It is well established that the eye contains a separate anatomical and functional system mediating light information to the clock. Until recently, the photopigment responsible for light entrainment of the circadian system has been elusive but recent studies have provided evidence that melanopsin, a recently identified opsin, could be the circadian photopigment. This conclusion is based on the observation that melanopsin is expressed exclusively in retinal ganglion cells projecting to the SCN, a projection known as the retinohypothalamic tract (RHT) and that these ganglion cells are intrinsically photosensitive. Melanopsin is present in the plasma membrane of soma, dendrites and axons forming an extensive photoreceptive network in the entire retina. Although these findings make melanopsin a strong candidate as a circadian photopigment, a number of functional experiments are needed before the role of melanopsin is finally proven.