Melanopsin: An opsin in melanophores, brain, and eye

Vertebrate ancient (VA) opsin and extraretinal photoreception in the Atlantic salmon (Salmo salar)

A member of a new photopigment family first isolated from teleost fish, vertebrate ancient (VA) opsin, has recently been shown to form a functional photopigment and to be expressed within a subset of horizontal and amacrine cells of the inner retina. These sites of expression (and structural features) of VA opsin suggest that this photopigment might mediate non-image-forming light-detection tasks. We attempted to gain support for this hypothesis by examining the expression of VA opsin within the central nervous system (CNS) (pineal and deep brain) of the Atlantic salmon Salmo salar. In addition, we examined the sites of rod-opsin, cone-opsin and α -transducin expression within the salmon CNS to provide a more complete description of the extraretinal photoreceptors of a teleost vertebrate. We show that multiple populations of cells within the salmon CNS appear to contain photoreceptors: VA opsin was strongly expressed in the pineal organ and in bilateral columns of subependymal cells in the epithalamus; anti-cone-opsin antibodies labelled cells within the pineal and numerous cells in the anterior hypothalamus (suprachiasmatic nucleus, nucleus preopticus magnocellularis, nucleus preopticus parvocellularis); anti-rod-opsin antibodies labelled cells within the pineal but no other areas within the central brain; and anti- α -transducin antibodies labelled cells within the pineal and the ventral telencephalon. Collectively, our results suggest that VA opsin is a photopigment specialised for irradiance detection tasks within the eye, pineal and central brain, and that the salmon has multiple and varied populations of photoreceptors within the CNS. We review the significance of these findings within the broad context of vertebrate extraretinal photoreception.

Vertebrate ancient-long opsin: a green-sensitive photoreceptive molecule present in zebrafish deep brain and retinal horizontal cells

Nonretinal/nonpineal photosensitivity has been found in the brain of vertebrates, but the molecular basis for such a "deep brain" photoreception system remains unclear. We conducted an extensive search for brain opsin cDNAs of the zebrafish (Danio rerio), a useful animal model for genetic studies, and we have isolated a partial cDNA clone encoding an ortholog of vertebrate ancient (VA) opsin, the function of which is unknown. Subsequent characterization revealed the occurrence of two kinds of mRNAs encoding putative splicing variants, VA and VA-Long (VAL) opsin, the latter of which is a novel variant of the former. Both opsins shared a common core sequence in the membrane-spanning domains, but VAL-opsin had a C-terminal tail much longer than that of VA-opsin. Functional reconstitution experiments on the recombinant proteins showed that VAL-opsin with bound 11-cis-retinal is a green-sensitive pigment (lambdamax approximately 500 nm), whereas VA-opsin exhibited no photosensitivity even in the presence of 11-cis-retinal. Immunoreactivity specific to this functionally active VAL-opsin was localized at a limited number of cells surrounding the diencephalic ventricle of central thalamus, and these cells were distributed over approximately 200 micrometer along the rostrocaudal axis. Taken together with the previous study on the locus of the teleost brain photosensitivity (von Frisch K, 1911), it is strongly suggested that the VAL-positive cells in the zebrafish brain represent the deep brain photoreceptors. The VAL-specific immunoreactivity was also detected in a subset of non-GABAergic horizontal cells in the zebrafish retina. The existence of VAL-opsin, a new member of the rhodopsin superfamily, in these tissues may indicate its multiple roles in visual and nonvisual photosensory physiology.

Vertebrate ancient-long opsin: a green-sensitive photoreceptive molecule present in zebrafish deep brain and retinal horizontal cells

Nonretinal/nonpineal photosensitivity has been found in the brain of vertebrates, but the molecular basis for such a "deep brain" photoreception system remains unclear. We conducted an extensive search for brain opsin cDNAs of the zebrafish (Danio rerio), a useful animal model for genetic studies, and we have isolated a partial cDNA clone encoding an ortholog of vertebrate ancient (VA) opsin, the function of which is unknown. Subsequent characterization revealed the occurrence of two kinds of mRNAs encoding putative splicing variants, VA and VA-Long (VAL) opsin, the latter of which is a novel variant of the former. Both opsins shared a common core sequence in the membrane-spanning domains, but VAL-opsin had a C-terminal tail much longer than that of VA-opsin. Functional reconstitution experiments on the recombinant proteins showed that VAL-opsin with bound 11-cis-retinal is a green-sensitive pigment (lambdamax approximately 500 nm), whereas VA-opsin exhibited no photosensitivity even in the presence of 11-cis-retinal. Immunoreactivity specific to this functionally active VAL-opsin was localized at a limited number of cells surrounding the diencephalic ventricle of central thalamus, and these cells were distributed over approximately 200 micrometer along the rostrocaudal axis. Taken together with the previous study on the locus of the teleost brain photosensitivity (von Frisch K, 1911), it is strongly suggested that the VAL-positive cells in the zebrafish brain represent the deep brain photoreceptors. The VAL-specific immunoreactivity was also detected in a subset of non-GABAergic horizontal cells in the zebrafish retina. The existence of VAL-opsin, a new member of the rhodopsin superfamily, in these tissues may indicate its multiple roles in visual and nonvisual photosensory physiology.

Ectopic pigmentation in Xenopus in response to DCoH/PCD, the cofactor of HNF1 transcription factor/pterin-4alpha-carbinolamine dehydratase

DCoH, the dimerization cofactor of the HNF-1 homeodomain proteins (hepatocyte nuclear factor-1alpha and beta), is involved in gene expression by associating with these transcription factors. The protein also called PCD for pterin-4alpha-carbinolamine dehydratase is a bifunctional factor as it catalyzes also the regeneration of tetrahydrobiopterin. This coenzyme is used by the enzyme phenylalanine hydroxylase, which generates tyrosine, the precursor of catecholamines and melanin. DCoH/PCD presumably cooperates with other partners, because it is expressed earlier than HNF1 and phenylalanine hydroxylase (PAH) in early vertebrate development. It is also found in cells lacking HNF1 and PAH like skin, brain and the pigmented epithelium of the eye suggesting a yet unknown function. We show that the overexpression of DCoH/PCD in Xenopus induces the formation of ectopic pigment cells in the epidermis, that are visible earlier than the endogenous pigmentation and broader distributed. This ectopic pigmentation is accompanied by an increase in tyrosinase activity and the amount of melanin. Overexpression of DCoH/PCD induces the appearance of pigment cells also in animal cap explants, that normally differentiate into atypical epidermis. DCoH/PCD mutants with impaired carbinolamine dehydratase activity retain the potential to induce pigmentation and we propose therefore that DCoH/PCD is not simply an essential enzyme for melanin biosynthesis, but also a regulator for the differentiation of pigment producing cells.

Irregular S-cone mosaics in felid retinas. Spatial interaction with axonless horizontal cells, revealed by cross correlation

In most mammals short-wavelength-sensitive (S) cones are arranged in irregular patterns with widely variable intercell distances. Consequently, mosaics of connected interneurons either may show some type of correlation to photoreceptor placement or may establish an independent lattice with compensatory dendritic organization. Since axonless horizontal cells (A-HC's) are supposed to direct all dendrites to overlying cones, we studied their spatial interaction with chromatic cone subclasses. In the cheetah, the bobcat, and the leopard, anti-S-opsin antibodies have consistently colabeled the A-HC's in addition to the S cones. We investigated the interaction between the two cell mosaics, using autocorrelation and cross-correlation procedures, including a Voronoi-based density probe. Comparisons with simulations of random mosaics show significantly lower densities of S cones above the cell bodies and primary dendrites of A-HC's. The pattern results in different long-wavelength-sensitive-L- and S-cone ratios in the central versus the peripheral zones of A-HC dendritic fields. The existence of a related pattern at the synaptic level and its potential significance for color processing may be investigated in further studies.

A novel human opsin in the inner retina

Here we report the identification of a novel human opsin, melanopsin, that is expressed in cells of the mammalian inner retina. The human melanopsin gene consists of 10 exons and is mapped to chromosome 10q22. This chromosomal localization and gene structure differs significantly from that of other human opsins that typically have four to seven exons. A survey of 26 anatomical sites indicates that, in humans, melanopsin is expressed only in the eye. In situ hybridization histochemistry shows that melanopsin expression is restricted to cells within the ganglion and amacrine cell layers of the primate and murine retinas. Notably, expression is not observed in retinal photoreceptor cells, the opsin-containing cells of the outer retina that initiate vision. The unique inner retinal localization of melanopsin suggests that it is not involved in image formation but rather may mediate nonvisual photoreceptive tasks, such as the regulation of circadian rhythms and the acute suppression of pineal melatonin. The anatomical distribution of melanopsin-positive retinal cells is similar to the pattern of cells known to project from the retina to the suprachiasmatic nuclei of the hypothalamus, a primary circadian pacemaker.

Non-visual photoreception by a variety of vertebrate opsins

Extraretinal photoreceptors in animals are involved in a variety of physiological functions such as photo-entrainment of circadian rhythm, photoperiodicity and body colour change. We have identified pinopsin in the chicken pineal gland as a typical 'non-visual' photoreceptive molecule. Pinopsin with bound 11-cis-retinal shows a blue-light sensitivity (lambda max = 468 nm), and it may play a role in synchronizing the phase of the endogenous circadian oscillator with an environmental dark-light cycle. Pinopsin is not a unique pineal opsin in animals. In the zebrafish, we have detected expression of two rhodopsin genes, the nucleotide sequences of which are very similar but distinct from each other. One is canonical rhodopsin expressed in the retina, and the other is expressed in the pineal gland. The latter gene is widely distributed among teleosts, and we named it 'exo-rhodopsin' after extraocular rhodopsin. On the other hand, our effort to identify the 'deep brain opsin' responsible for the photoperiodic gonadal response resulted in the identification of two kinds of opsins; pinopsin in the toad anterior preoptic nucleus and rhodopsin in the pigeon lateral septum. Both of these opsins are localized in the cerebrospinal fluid-contacting neurons in the brain of the two animals. We also identified VAL opsin in zebrafish retinal horizontal cells, which have not been considered as photoreceptive cells. It has become evident that animals employ a wide variety of photoreceptive molecules for 'non-visual' purposes.

Diversity of opsin immunoreactivities in the extraretinal tissues of four anuran amphibians

The pineal complex, deep brain, and skin have been known to function as extraretinal photoreceptors in non-mammalian vertebrates. To see the diversity of localization of extraretinal photoreceptors in lower vertebrates having different habitats, we analyzed the opsin-like immunoreactivities in anuran amphibians, Xenopus laevis, Rana catesbeiana, Rana nigromaculata, and Bufo japonicus. An antiserum (toad Rh-AS) was raised against rhodopsin purified from the retinas of Japanese toad, B. japonicus. In the retina of all the anurans examined, the outer segments of rods were immunopositive to toad Rh-AS. The outer segments of most pinealocytes were immunopositive in R. catesbeiana, R. nigromaculata, and B. japonicus. The outer segments of photoreceptor-like cells within the frontal organ of R. nigromaculata were immunostained. Interestingly, toad Rh-AS immunostained many secretory cells of mucous glands in the head skin of B. japonicus, implying the presence of a novel photoreceptive molecule. Within the hypothalamus, toad Rh-AS immunostained many cells in the magnocellular preoptic nucleus of R. catesbeiana and B. japonicus. Toad Rh-AS also labeled cerebrospinal fluid (CSF)-contacting cells in the anterior preoptic nucleus of R. nigromaculata and those adjacent to the lateral ventricle within the septum of R. catesbeiana. Thus the distribution patterns of the rhodopsin-like immunoreactivities among the anurans were highly diverged, and there was no relationship between the distribution patterns and their habitats. J. Exp. Zool. 286:136-142, 2000.

A novel human opsin in the inner retina

Here we report the identification of a novel human opsin, melanopsin, that is expressed in cells of the mammalian inner retina. The human melanopsin gene consists of 10 exons and is mapped to chromosome 10q22. This chromosomal localization and gene structure differs significantly from that of other human opsins that typically have four to seven exons. A survey of 26 anatomical sites indicates that, in humans, melanopsin is expressed only in the eye. In situ hybridization histochemistry shows that melanopsin expression is restricted to cells within the ganglion and amacrine cell layers of the primate and murine retinas. Notably, expression is not observed in retinal photoreceptor cells, the opsin-containing cells of the outer retina that initiate vision. The unique inner retinal localization of melanopsin suggests that it is not involved in image formation but rather may mediate nonvisual photoreceptive tasks, such as the regulation of circadian rhythms and the acute suppression of pineal melatonin. The anatomical distribution of melanopsin-positive retinal cells is similar to the pattern of cells known to project from the retina to the suprachiasmatic nuclei of the hypothalamus, a primary circadian pacemaker.

Photic induction of mPer1 and mPer2 in cry-deficient mice lacking a biological clock

Mice lacking mCry1 and mCry2 are behaviorally arrhythmic. As shown here, cyclic expression of the clock genes mPer1 and mPer2 (mammalian Period genes 1 and 2) in the suprachiasmatic nucleus and peripheral tissues is abolished and mPer1 and mPer2 mRNA levels are constitutively high. These findings indicate that the biological clock is eliminated in the absence of both mCRY1 and mCRY2 (mammalian cryptochromes 1 and 2) and support the idea that mammalian CRY proteins act in the negative limb of the circadian feedback loop. The mCry double-mutant mice retain the ability to have mPer1 and mPer2 expression induced by a brief light stimulus known to phase-shift the biological clock in wild-type animals. Thus, mCRY1 and mCRY2 are dispensable for light-induced phase shifting of the biological clock.

Retinopathy and attenuated circadian entrainment in Crx-deficient mice

Crx, an Otx-like homeobox gene, is expressed specifically in the photoreceptors of the retina and the pinealocytes of the pineal gland. Crx has been proposed to have a role in the regulation of photoreceptor-specific genes in the eye and of pineal-specific genes in the pineal gland. Mutations in human CRX are associated with the retinal diseases, cone-rod dystrophy-2 (adCRD2; refs 3, 4, 5), retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA), which all lead to loss of vision. We generated mice carrying a targeted disruption of Crx. Crx-/- mice do not elaborate photoreceptor outer segments and lacked rod and cone activity as assayed by electroretinogram (ERG). Expression of several photoreceptor- and pineal-specific genes was reduced in Crx mutants. Circadian entrainment was also affected in Crx-/- mice.

Exo-rhodopsin: a novel rhodopsin expressed in the zebrafish pineal gland

The zebrafish, a useful animal model for genetic studies, has a photosensitive pineal gland, which has an endogenous circadian pacemaker entrained to environmental light-dark cycles [G.M. Cahill, Brain Res. 708 (1996) 177-181]. Although pinopsin has been found in the pineal glands of birds and reptiles, the molecular identity responsible for fish pineal photosensitivity remains unclear. This study reports identification of a novel opsin gene expressed in the zebrafish pineal gland. The deduced amino acid sequence is similar to, but not identical (74% identity) with that of canonical rhodopsin in the zebrafish retina. This novel rhodopsin is expressed in the majority of pineal cells but not in retinal cells, and hence named exo-rhodopsin after extra-ocular rhodopsin. This study first shows that two different rhodopsin genes are expressed in an individual animal each within a unique location. A phylogenetic analysis indicated that the exo-rhodopsin gene was produced by a duplication of the rhodopsin gene at an early stage in the ray-finned fish lineage. As expected, the exo-rhodopsin gene was found in the medakafish and European eel genomes, suggesting strongly that exo-rhodopsin is a pineal opsin common to teleosts. Identification of exo-rhodopsin in the zebrafish provides an opportunity for studying the role of pineal photoreceptive molecules by using genetic approaches.

The ordered visual transduction complex of the squid photoreceptor membrane

The study of visual transduction has given invaluable insight into the mechanisms of signal transduction by heptahelical receptors that act via guanine nucleotide binding proteins (G-proteins). However, the cyclic-GMP second messenger system seen in vertebrate photoreceptor cells is not widely used in other cell types. In contrast, the retina of higher invertebrates, such as squid, offers an equally accessible transduction system, which uses the widespread second messenger chemistry of an increase in cytosolic calcium caused by the production of inositol-(1,4,5)-trisphosphate (InsP3) by the enzyme phospholipase C, and which may be a model for store-operated calcium influx. In this article, we highlight some key aspects of invertebrate visual transduction as elucidated from the combination of biochemical techniques applied to cephalopods, genetic techniques applied to flies, and electrophysiology applied to the horseshoe crab. We discuss the importance and applicability of ideas drawn from these model systems to the understanding of some general processes in signal transduction, such as the integration of the cytoskeleton into the signal transduction process and the possible modes of regulation of store-operated calcium influx.

Spatio-temporal analysis of light-induced Fos expression in the retina of rd mutant mice

Rd mutant mice are visually blind but they maintain the ability of synchronising their circadian rhythms to the external light-dark cycles. We used immunocytochemical procedures to detect light-induced Fos expression in the rd mice retina. We found that Fos is expressed in the rd retina in an unattenuated pattern through the entire life of the animal. Furthermore, we have found that cells expressing Fos are distributed throughout the whole retina, while opsin expression takes place only in the dorsal half of the retina in the 1-year old rd mice. Finally, we found that light induces Fos expression in the rd retina at the same levels during the subjective day as during the subjective night, whereas in the suprachiasmatic nucleus (SCN), Fos is stimulated by light only during the subjective night. Our results support the hypothesis that new, undiscovered photoreceptors are implicated in light perception for the circadian system.

Encephalopsin: a novel mammalian extraretinal opsin discretely localized in the brain

We have identified a mammalian opsin, encephalopsin, that shows strong and specific expression in the brain. Encephalopsin defines a new family of opsins and shows highest homology to vertebrate retinal and pineal opsins. Encephalopsin is highly expressed in the preoptic area and paraventricular nucleus of the hypothalamus, both regions implicated in encephalic photoreception in nonmammalian vertebrates. In addition, encephalopsin shows highly patterned expression in other regions of the brain, being enriched in selected regions of the cerebral cortex, cerebellar Purkinje cells, a subset of striatal neurons, selected thalamic nuclei, and a subset of interneurons in the ventral horn of the spinal cord. Rostrocaudal gradients of encephalopsin expression are present in the cortex, cerebellum, and striatum. Radial stripes of encephalopsin expression are seen in the cerebellum. In the cortex and cerebellum, encephalopsin expression is considerably higher and more highly patterned in the adult than in the neonate. Encephalopsin is the first putative extraocular opsin identified in mammals and may play a role in encephalic photoreception.

Encephalopsin: a novel mammalian extraretinal opsin discretely localized in the brain

We have identified a mammalian opsin, encephalopsin, that shows strong and specific expression in the brain. Encephalopsin defines a new family of opsins and shows highest homology to vertebrate retinal and pineal opsins. Encephalopsin is highly expressed in the preoptic area and paraventricular nucleus of the hypothalamus, both regions implicated in encephalic photoreception in nonmammalian vertebrates. In addition, encephalopsin shows highly patterned expression in other regions of the brain, being enriched in selected regions of the cerebral cortex, cerebellar Purkinje cells, a subset of striatal neurons, selected thalamic nuclei, and a subset of interneurons in the ventral horn of the spinal cord. Rostrocaudal gradients of encephalopsin expression are present in the cortex, cerebellum, and striatum. Radial stripes of encephalopsin expression are seen in the cerebellum. In the cortex and cerebellum, encephalopsin expression is considerably higher and more highly patterned in the adult than in the neonate. Encephalopsin is the first putative extraocular opsin identified in mammals and may play a role in encephalic photoreception.

Keeping an eye on retinal clocks

Circadian pacemakers that drive rhythmicity in retinal function are found in both invertebrates and vertebrates. They have been localized to photoreceptors in molluscs, amphibians, and mammals. Like other circadian pacemakers, they entrain to light, oscillate based on a negative feedback between transcription and translation of clock genes, and control a variety of physiological and behavioral rhythms that often includes rhythmic melatonin production. As a highly organized and accessible tissue, the retina is particularly well suited for the study of the input-output pathways and the mechanism for rhythm generation. Impressive advances can now be expected as researchers apply new molecular techniques toward looking into the eye's clock.

Expression of the medaka (Oryzias latipes) Ol-Rx3 paired-like gene in two diencephalic derivatives, the eye and the hypothalamus

Here we report the expression pattern of the homeobox Ol-Rx3 gene, a medaka gene homologous to the mouse, Xenopus, zebrafish and Drosophila Rx genes. Ol-Rx3 starts to be expressed, at late gastrula stages, in the presumptive territories of the anterior brain. Subsequently, transcripts are localised in an antero-ventral region of the prosencephalon and in the primordia of the optic vesicles. During organogenesis, distribution of Ol-Rx3 transcripts are gradually restricted to the floor of the diencephalon, the prospective territory of the hypothalamus and the neurohypophysis. During late development and in adult, Ol-Rx3 expression is maintained in hypothalamic nuclei bordering the third ventricle. In the optic vesicles, Ol-Rx3 expression is temporarily switched off when the eye cup morphogenesis is complete, but it is turned on again in the inner nuclear layer of the retina. Thus, the early expression pattern of Ol-Rx3 is in agreement with a conserved role in the specification of the ventral forebrain and eye field. Putative functions linked to late expression domains are discussed in light of the different hypothesis concerning the involvement of vertebrate Rx genes in the maintenance of particular cell fate.

Regulation of mammalian circadian behavior by non-rod, non-cone, ocular photoreceptors

Circadian rhythms of mammals are entrained by light to follow the daily solar cycle (photoentrainment). To determine whether retinal rods and cones are required for this response, the effects of light on the regulation of circadian wheel-running behavior were examined in mice lacking these photoreceptors. Mice without cones (cl) or without both rods and cones (rdta/cl) showed unattenuated phase-shifting responses to light. Removal of the eyes abolishes this behavior. Thus, neither rods nor cones are required for photoentrainment, and the murine eye contains additional photoreceptors that regulate the circadian clock.

The endogenous chromophore of retinal G protein-coupled receptor opsin from the pigment epithelium

The recent identification of nonvisual opsins has revealed an expanding family of vertebrate opsin genes. The retinal pigment epithelium (RPE) and Müller cells contain a blue and UV light-absorbing opsin, the RPE retinal G protein-coupled receptor (RGR, or RGR opsin). The spectral properties of RGR purified from bovine RPE suggest that RGR is conjugated in vivo to a retinal chromophore through a covalent Schiff base bond. In this study, the isomeric structure of the endogenous chromophore of RGR was identified by the hydroxylamine derivatization method. The retinaloximes derived from RGR in the dark consisted predominantly of the all-trans isomer. Irradiation of RGR with 470-nm monochromatic or near-UV light resulted in stereospecific isomerization of the bound all-trans-retinal to an 11-cis configuration. The stereospecificity of photoisomerization of the all-trans-retinal chromophore of RGR was lost by denaturation of the protein in SDS. Under the in vitro conditions, the photosensitivity of RGR is at least 34% that of bovine rhodopsin. These results provide evidence that RGR is bound in vivo primarily to all-trans-retinal and is capable of operating as a stereospecific photoisomerase that generates 11-cis-retinal in the pigment epithelium.

The emergence of pigment cell biology: a personal view

This is a semi-autobiographical coverage of my research career in pigment cell biology presented in the context of the emergence and growth of the discipline. This anecdotal presentation tells about some historical personages in the field. My undergraduate studies at the University of Rochester are related to my graduate work at the University of Iowa. I tell how my dissertation research was derived from a marriage between my interests in experimental embryology and the new field of comparative endocrinology. My early years of research at Iowa and as a young faculty member in Zoology at the University of Arizona were much concerned with the evolution of our knowledge of the chemistry and biology of melanocyte-stimulating hormone (MSH), especially concerning the pigment cells of lower vertebrates. Our developmental, structural, functional, and biochemical characterization of vertebrate chromatophores is described, as is our elucidation of the dermal chromatophore unit. The direct effects of light on changes in pigmentation are considered in descriptions of both the tail-darkening reaction and the role of the pineal gland in melanophore control. Emphasis is placed on the developmental biology of pigmentation, especially on the concept that all pigment cells are derived in common from a stem cell of neural-crest origin, whose expression is influenced by factors, such as melanization-inhibiting factor (MIF), localized in specific areas of the skin to thus produce specific pigmentation patterns. This research is considered in light of what is known about the agouti locus and MSH in the expression of mammalian pigmentation patterns. Part of my work has included ecological considerations, and some of this is touched upon. My role as founder of the journal 'Pigment Cell Research', is presented briefly, as is my involvement in the XIIIth International Pigment Cell Conference and in the establishment of both the International Pigment Cell Society and the International Federation of Pigment Cell Societies. Finally, I comment on the future of research in pigmentation.

Photoentrainment in mammals: a role for cryptochrome?

There is growing evidence in support of the hypothesis that, in mammals, photoreceptive tasks are segregated into those associated with creating a detailed visual image of the environment and those involved in the photic regulation of temporal biology. The hypothesis that this segregation extends to the use of different photoreceptors remains unproven, but published reports from several mammalian species that circadian photoentrainment survives a degree of retinal degeneration sufficient to induce visual blindness suggest that this may be so. This has lead to speculation that mammals might employ a dedicated 'circadian photoreceptor' distinct from the rod and cone cells of the visual system. The location and nature of this putative circadian photoreceptor has become a matter of conjecture. The latest candidates to be put forward as potential circadian photopigments are the mammalian cryptochrome proteins (CRY1 and 2), putative vitamin-B2 based photopigments. To date, published experimental evidence falls short of a definitive demonstration that these proteins form the basis of circadian photoreception in mammals. Consequently, this review aims to assess their suitability for this task in light of what we know regarding the biology of the cyrptochromes and the nature of mammalian photoentrainment.

Symmetry breaking in the zygotes of the fucoid algae: controversies and recent progress

Despite its many advantages as an experimental system for the study of the epigenesis of polarity, it is obvious that the fucoid zygote also presents many problems. The development of polarity proceeds largely independently of direct gene action and thus may be considered a problem in cellular physiology. Ca2+ appears to play an important role in the process, but the optical properties of the zygotes (opacity and autofluorescence) hamper the use of modern methods of visualizing the distribution of Ca2+ and other ions. Likewise, other approaches, such as injection of fluorescent-labeled G-actin, in order to study the dynamics of actin filaments, are subject to the same limitations. It may be that the application of two-photon microscopy will enable experimenters to avoid some of these problems. This technique uses excitation wavelengths that are twice the wavelength of maximum absorption by fluorophores, and sufficient photon density for absorption is achieved only in a thin section. The fucoid zygotes are considerably more transparent to longer wavelengths, so attenuation of the exciting light and autofluorescence should be significantly reduced. Perhaps we will then be able to see further into these opaque cells. Another problem concerns the use of different species and genera. This may be unavoidable; for example, those of us who are land-locked tend to rely on Pelvetia, as it travels and stores better than the various species of Fucus and is less seasonal. Our colleagues fortunate enough to work near the ocean prefer to use the species that are locally available. Nevertheless, it is important to be careful about cross-genus and cross-species generalizations. While it is unlikely, based on what we know, that there are fundamental differences in physiological mechanisms among species, there may be small but still important differences in details. Obviously, investigators should directly compare results in more than one species whenever possible. The area of greatest disagreement, perhaps, concerns the mechanism of polarity formation, as opposed to its overt manifestation, germination. Are Ca2+ and actin involved or not? Assuming Ca2+ is involved, is the source internal or external? One basis for the different findings may be the differences in the strength of the polarizing signal provided to the zygotes. Clearly, the cells have powerful mechanisms for amplifying a faint asymmetry and developing an axis in response to an external signal. Furthermore, the fucoids generally develop in the intertidal zone and thus must be adapted to meeting the challenge of a widely varying external environment. They may have alternate mechanisms for responding to unilateral light. We have adopted the approach of presenting the cells with a fairly weak light signal--the minimum required to induce a considerable degree of organization of a population of zygotes. We then determine the effects of various inhibitors on photopolarization. One advantage of this approach is that it has allowed us to find treatments that increase the sensitivity of the zygotes to light, something that would not be possible if the untreated controls were fully polarized. Some of the differences between our results and those of others may be related to their use of a stronger light stimulus. It may be that if given a strong stimulus, a sufficient trace is left in the cells so that they can organize an axis when an inhibitor is removed. Careful consideration of this point may help to reconcile apparently contradictory findings. Despite these difficulties, the fucoid zygotes are likely to continue to be an important experimental system. Technology, including the development of more specific inhibitory reagents, may allow some of the shortcomings of the system to be overcome, and careful consideration of experimental conditions may resolve some of the points of disagreement.

Selective activation of G-protein subtypes by vertebrate and invertebrate rhodopsins

We have quantitatively investigated specificities in activating G-protein subtype by bovine and squid rhodopsins to examine whether or not the phototransduction cascade in each of the photoreceptor cells is determined by the colocalization of a large amount of G-protein subtype (Gt or Gq). In contrast to the efficient activation of respective Gt and Gq, bovine and squid rhodopsins scarcely activated G-protein counterparts. Exchange of alpha- and betagamma-subunits of Gt and Gq indicated the critical role of the alpha-subunit in specific binding to respective rhodopsins. Thus the specific recognition of G-protein subtype by each rhodopsin is a major mechanism in determining the phototransduction cascade.

Circadian rhythms: eyes of the clock

The daily light-dark cycle synchronizes the internal circadian clock with the outside world. Blind organisms maintain this light-induced entrainment, suggesting the existence of a non-visual phototransduction pathway. The photoreceptor is unknown, but several intriguing candidates have recently come to light.

Light-dependent activation of rod transducin by pineal opsin

The pineal gland expresses a unique member of the opsin family (P-opsin; Max, M., McKinnon, P. J., Seidenman, K. J., Barrett, R. K., Applebury, M. L., Takahashi, J. S., and Margolskee, R. F. (1995) Science 267, 1502-1506) that may play a role in circadian entrainment and photo-regulation of melatonin synthesis. To study the function of this protein, an epitope-tagged P-opsin was stably expressed in an embryonic chicken pineal cell line. When incubated with 11-cis-retinal, a light-sensitive pigment was formed with a lambdamax at 462 +/- 2 nm. P-opsin bleached slowly in the dark (t1/2 = 2 h) in the presence of 50 mM hydroxylamine. Purified P-opsin in dodecyl maltoside activated rod transducin in a light-dependent manner, catalyzing the exchange of more than 300 mol of GTPgammaS (guanosine 5'-O-(3-thiotriphosphate))/mol of P-opsin. The initial rate for activation (75 mol of GTPgammaS bound/mol of P-opsin/min at 7 microM) increased with increasing concentrations of transducin. The addition of egg phosphatidylcholine to P-opsin had little effect on the activation kinetics; however, the intrinsic rate of decay in the absence of transducin was accelerated. These results demonstrate that P-opsin is an efficient catalyst for activation of rod transducin and suggest that the pineal gland may contain a rodlike phototransduction cascade.

Neural substrates of Drosophila rhythms revealed by mutants and molecular manipulations

In the fruit-fly Drosophila, rhythmic expression of the clock gene period is detected in cells throughout the body. Whereas these cells could be pacemakers for circadian rhythms of unknown physiological processes, the brain pacemakers are known to be responsible for circadian behavior. Recent progress in genetic and molecular studies of clock genes in Drosophila has permitted the identification of brain pacemakers at the cellular level and their output pathways to rhythmic behavior. Similar studies in other insect species have suggested considerable diversity in the anatomical and neurochemical properties of pacemaker cells, as well as in the mechanisms of clock-gene regulation.