Role of a novel photopigment,melanopsin, in behavioral adaptation to light

Adaptation to changes in the ambient light is of critical importance to life. In mammals, three principal photoadaptation mechanisms depend on ocular photoreception and exhibit spectral sensitivity suggestive of the opsin class of photopigment(s). These include rapid adaptation of the visual system to the ambient light by pupil constriction, direct modulation of neuroendocrine function and entrainment of the circadian clock to the day:night cycle. Surprisingly, these processes can largely function independent of classical rod/cone photoreceptors, suggesting a novel opsin-based signaling mechanism. They appear to involve a recently discovered network of intrinsically photosensitive retinal ganglion cells that make direct or indirect axonal connections to brain centers regulating photoadaptive behaviors. The discovery of a novel opsin, melanopsin, in these cells has offered an exciting entry point to explore, at the molecular level, how mammals adapt to their light environment. There is now genetic proof of a principal role for melanopsin in all three major photoadaptation processes.     

Photoreceptors in the pineal of lower vertebrates: Functional aspects

Summary The pineal of lower vertebrates characteristically contains true and modified photoreceptors with functional und structural homologies to retinal photoreceptors. Afferent nerves convey photic information from the pineal to sensory areas of the brain stem. Light also influences synthetic activity within the organ, controlling the rhythm in melatonin production which is generated endogenously. The molecular mechanisms underlying this rhythmic event are described and the hypothesis advanced that the pineal transduces several forms of environmental stimulus involved in the regulation of rhythmic function.     

Structures and molecules involved in generation and regulation of biological rhythms in vertebrates and invertebrates

Melatonin from the retina and the pineal gland functions in neuroendocrine hierarchies. Photoreceptors — eyes and extraretinal — detect light. Oscillators — pineal and suprachiasmatic nuclei — act as pacemakers. Driven neuroendocrine rhythms carry temporal hormone signals throughout the body. Light controls melatonin: light sets the phase of the melatonin rhythm and determines the duration of melatonin synthesis. By these means, circadian rhythms (e.g. in locomotor activity and body temperature) and seasonal rhythms (e.g. in reproduction) are controlled.     

Some reflections on the phylogeny and function of the pineal

The pineal gland is a universal feature of vertebrate organization and has been implicated in the control of rhythmic adaptations to daily and seasonal cycles. This paper considers three aspects of pineal function; the generation of a rhythmical endocrine signal (the nocturnal synthesis of melatonin) and the use of the signal in the regulation of circadian and photoperiodic functions. The shape of the nocturnal signal is determined by an interaction of afferent neural control and biochemical processes intrinsic to the pinealocyte. The nature of the effect of the signal upon circadian systems is unclear, and in adult mammals may not be a specific, direct influence upon the entrainment pathways of the oscillator. In the foetus, strong evidence exists for a physiological role of the maternal melatonin signal as a true internal zeitgeber, remnants of which may persist in the adult. Photoperiodic time measurement in adult and foetal mammals is critically dependent upon the melatonin signal. Indirect evidence indicates that several neural systems may be involved in the response to melatonin and consistent with this, a variety of central melatonin binding sites have been identified in the brain and pituitary. The intra-cellular actions of melatonin and the properties of melatonin responsive neural systems have yet to be identified, but in the context of photoperiodic time measurement, it is clear that the neural responses to melatonin are not dependent upon the circadian clock. The two central effects of melatonin; photoperiodic time measurement and circadian entrainment are probably mediated through completely separate mechanisms.     

Some reflections on the phylogeny and function of the pineal

Summary The pineal gland is a universal feature of vertebrate organization and has been implicated in the control of rhythmic adaptations to daily and seasonal cycles. This paper considers three aspects of pineal function; the generation of a rhythmical endocrine signal (the nocturnal synthesis of melatonin) and the use of the signal in the regulation of circadian and photoperiodic functions. The shape of the nocturnal signal is determined by an interaction of afferent neural control and biochemical processes intrinsic to the pinealocyte. The nature of the effect of the signal upon circadian systems is unclear, and in adult mammals may not be a specific, direct influence upon the entrainment pathways of the oscillator. In the foetus, strong evidence exists for a physiological role of the maternal melatonin signal as a true internal zeitgeber, remnants of which may persist in the adult. Photoperiodic time measurement in adult and foetal mammals is critically dependent upon the melatonin signal. Indirect evidence indicates that several neural systems may be involved in the response to melatonin and consistent with this, a variety of central melatonin binding sites have been identified in the brain and pituitary. The intra-cellular actions of melatonin and the properties of melatonin responsive neural systems have yet to be identified, but in the context of photoperiodic time measurement, it is clear that the neural responses to melatonin are not dependent upon the circadian clock. The two central effects of melatonin; photoperiodic time measurement and circadian entrainment are probably mediated through completely separate mechanisms.The Editors wish to thank Dr M. Hastings for coordinating this multi-author review.     

The absence of melanopsin alters retinal clock function and dopamine regulation by light

The retinal circadian clock is crucial for optimal regulation of retinal physiology and function, yet its cellular location in mammals is still controversial. We used laser microdissection to investigate the circadian profiles and phase relations of clock gene expression and Period gene induction by light in the isolated outer (rods/cones) and inner (inner nuclear and ganglion cell layers) regions in wild-type and melanopsin-knockout (Opn ₄ ^?/? ) mouse retinas. In the wild-type mouse, all clock genes are rhythmically expressed in the photoreceptor layer but not in the inner retina. For clock genes that are rhythmic in both retinal compartments, the circadian profiles are out of phase. These results are consistent with the view that photoreceptors are a potential site of circadian rhythm generation. In mice lacking melanopsin, we found an unexpected loss of clock gene rhythms and of the photic induction of Per1-Per2 mRNAs only in the outer retina. Since melanopsin ganglion cells are known to provide a feed-back signalling pathway for photic information to dopaminergic cells, we further examined dopamine (DA) synthesis in Opn ₄ ^?/? mice. The lack of melanopsin prevented the light-dependent increase of tyrosine hydroxylase (TH) mRNA and of DA and, in constant darkness, led to comparatively high levels of both components. These results suggest that melanopsin is required for molecular clock function and DA regulation in the retina, and that Period gene induction by light is mediated by a melanopsin-dependent, DA-driven signal acting on retinal photoreceptors.     

Melanopsin and inner retinal photoreception

Over the last ten years there has been growing acceptance that retinal photoreception among mammals extends beyond rods and cones to include a small number of intrinsically photosensitive retinal ganglion cells (ipRGCs). These ipRGCs are capable of responding to light in the absence of rod/cone input thanks to expression of an opsin photopigment called melanopsin. They are specialised for measuring ambient levels of light (irradiance) for a wide variety of so-called non-image-forming light responses. These include synchronisation of circadian clocks to light:dark cycles and the regulation of pupil size, sleep propensity and pineal melatonin production. Here, we provide a review of some of the landmark discoveries in this fast developing field, paying particular emphasis to recent findings and key areas for future investigation.     

Biogenic amine acetylation: No detectable circadian rhythm in whole brain homogenates of the insectOstrinia nubilalis

Summary With the biogenic amines tryptamine, dopamine, and octopamine as substrates, N-acetyltransferase activity shows no detectable circadian rhythm in homogenates of whole brains of the European corn borerOstrinia nubilalis (Lepidoptera: Pyralidae). The circadian clock of this insect may be fundamentally different from the N-acetyltransferase pacemaker in the pineal gland of vertebrates.     

Adaptation of pineal expressed teleost exo-rod opsin to non-image forming photoreception through enhanced Meta II decay

Photoreception by vertebrates enables both image-forming vision and non-image-forming responses such as circadian photoentrainment. Over the recent years, distinct non-rod non-cone photopigments have been found to support circadian photoreception in diverse species. By allowing specialization to this sensory task a selective advantage is implied, but the nature of that specialization remains elusive. We have used the presence of distinct rod opsin genes specialized to either image-forming (retinal rod opsin) or non-image-forming (pineal exo-rod opsin) photoreception in ray-finned fish (Actinopterygii) to gain a unique insight into this problem. A comparison of biochemical features for these paralogous opsins in two model teleosts, Fugu pufferfish (Takifugu rubripes) and zebrafish (Danio rerio), reveals striking differences. While spectral sensitivity is largely unaltered by specialization to the pineal environment, in other aspects exo-rod opsins exhibit a behavior that is quite distinct from the cardinal features of the rod opsin family. While they display a similar thermal stability, they show a greater than tenfold reduction in the lifetime of the signaling active Meta II photoproduct. We show that these features reflect structural changes in retinal association domains of helices 3 and 5 but, interestingly, not at either of the two residues known to define these characteristics in cone opsins. Our findings suggest that the requirements of non-image-forming photoreception have lead exo-rod opsin to adopt a characteristic that seemingly favors efficient bleach recovery but not at the expense of absolute sensitivity.     

Vision in the ultraviolet

Sensitivity to ultraviolet light (UV) is achieved by photoreceptors in the eye that contain a class of visual pigments maximally sensitive to light at wavelengths <400 nm. It is widespread in the animal kingdom where it is used for mate choice, communication and foraging for food. UV sensitivity is not, however, a constant feature of the visual system, and in many vertebrate species, the UV-sensitive (UVS) pigment is replaced by a violet-sensitive (VS) pigment with maximal sensitivity between 410 and 435 nm. The role of protonation of the Schiff base-chromophore linkage and the mechanism for tuning of pigments into the UV is discussed in detail. Amino acid sequence analysis of vertebrate VS/UVS pigments indicates that the ancestral pigment was UVS, with loss of UV sensitivity occurring separately in mammals, amphibia and birds, and subsequently regained by a single amino acid substitution in certain bird species. In contrast, no loss of UV sensitivity has occurred in the UVS pigments of insects.     

Ultrastructure and biochemistry of the pineal organ in deep-sea lanternfishes (Myctophidae)

Summary Pineal structural and biochemical adaptations in lanternfishes included: 1) few photoreceptor outer segment discs; 2) conventional synapses between photoreceptors and pineal neurons; and 3) low levels (0–60 pg/pineal) of serotonin compared to those (>1.0 ng/pineal) in the goldfish pineal organ. These findings suggest reduced photosensory and/or neuroendocrine functions in these deep-sea fishes.     

The pineal and melatonin: regulators of circadian function in lower vertebrates

The pineal has been identified as a major circadian pacemaker within the circadian system of a number of lower vertebrates although other pacemaking sites have been implicated as well. The rhythmic synthesis and secretion of the pineal hormone, melatonin, is suggested as the mechanism by which the pineal controls circadian oscillators located elsewhere. Both light and temperature cycles can entrain the pineal melatonin rhythm. The pineal, therefore, acts as a photo and thermoendocrine transducer which functions to synchronize internal cycle with cycles in the environment. A model is presented which portrays the pineal as a major component of a 'multioscillator' circadian system and which suggests how these multiple circadian clocks are coupled to each other and to cycles of light and temperature in the external world.     

The pineal and melatonin: Regulators of circadian function in lower vertebrates

Summary The pineal has been identified as a major circadian pacemaker within the circadian system of a number of lower vertebrates although other pacemaking sites have been implicated as well. The rhythmic synthesis and secretion of the pineal hormone, melatonin, is suggested as the mechanism by which the pineal controls circadian oscillators located elsewhere. Both light and temperature cycles can entrain the pineal melatonin rhythm. The pineal, therefore, acts as a photo and thermoendocrine transducer which functions to synchronize internal cycle with cycles in the environment. A model is presented which portrays the pineal as a major component of a multioscillator circadian system and which suggests how these multiple circadian clocks are coupled to each other and to cycles of light and temperature in the external world.     

The pineal and melatonin: Regulators of circadian function in lower vertebrates

Summary The pineal has been identified as a major circadian pacemaker within the circadian system of a number of lower vertebrates although other pacemaking sites have been implicated as well. The rhythmic synthesis and secretion of the pineal hormone, melatonin, is suggested as the mechanism by which the pineal controls circadian oscillators located elsewhere. Both light and temperature cycles can entrain the pineal melatonin rhythm. The pineal, therefore, acts as a photo and thermoendocrine transducer which functions to synchronize internal cycle with cycles in the environment. A model is presented which portrays the pineal as a major component of a multioscillator circadian system and which suggests how these multiple circadian clocks are coupled to each other and to cycles of light and temperature in the external world.     

Neuroanatomy and neurochemistry of sleep

Sleep is regulated by homeostatic and circadian factors, and the regulation of sleep of mammals shares many molecular properties with the rest state of submammalian species. Several brain structures take part in waking: the basal forebrain, posterior and lateral hypothalamus, and nuclei in the tegmentum and pons. Active sleep mechanisms are located to the preoptic/anterior hypothalamic area. In addition to acetylcholine and monoamines, glutamate and hypocretin/orexin are important waking factors. Gamma-aminobutyric acid and several peptide factors, including cytokines, growth hormone-releasing hormone and prolactin, are related to sleep promotion. Adenosine is an important homeostatic sleep factor acting in basal forebrain and preoptic areas through A1 and A2A receptors. Prolonged waking activates inducible nitric oxide synthase in the basal forebrain, which through energy depletion causes adenosine release and recovery sleep. Numerous genes have been found differentially displayed in waking compared with sleep, and they relate to neural transmission, synaptic plasticity, energy metabolism and stress protection. The genetic background of a few sleep disorders has been solved.     

Sex chromosomes and sex-determining genes: insights from marsupials and monotremes

Comparative studies of the genes involved in sex determination in the three extant classes of mammals, and other vertebrates, has allowed us to identify genes that are highly conserved in vertebrate sex determination and those that have recently evolved roles in one lineage. Analysis of the conservation and function of candidate genes in different vertebrate groups has been crucial to our understanding of their function and positioning in a conserved vertebrate sex-determining pathway. Here we review comparisons between genes in the sex-determining pathway in different vertebrates, and ask how these comparisons affect our views on the role of each gene in vertebrate sex determination.     

Visual input to rat pineal

Summary Electrophysiological recordings from freely behaving rats, previously implanted stereotaxically with permanent electrodes in the pineal, ventromedial hypothalamus, caudate nucleus, lateral geniculate body and medial geniculate body were obtained. The pineal photic responses revealed 5 sequential components. Injection of a neuronal blocker at the level of the superior cervical ganglion did not alter the earlier photic responses, but did eliminate the late components (N₂–P₃) for 60–90 min after the injection. All of the other responses were unchanged during the experiment. The present experiments demonstrated that photic input travels to the pineal through two pathways.The author is grateful to Dr C.M. Prashad and Ms. Marjorie Brown for technical assistance. Supported in part by grant NS 16596.     

Ultrastructure and biochemistry of the pineal organ in deep-sea lanternfishes (Myctophidae)

Pineal structural and biochemical adaptations in lanternfishes included: 1) few photoreceptor outer segment discs; 2) conventional synapses between photoreceptors and pineal neurons; and 3) low levels (0-60 pg/pineal) of serotonin compared to those (greater than 1.0 ng/pineal) in the goldfish pineal organ. These findings suggest reduced photosensory and/or neuroendocrine functions in these deep-sea fishes.     

Die α-Glycerophosphat-Oxydation des Heuschreckenbrustmuskels (Locusta migratoria)

Summary The conditions were studied for the glycerophosphate oxidation by homogenate from locust flight muscle, and the O₂-consumption was measured. Maximal oxidation rates were found with 0.087m glycerophosphate, 8 × 10^–6 m cytochromec, 7 × 10^–6 m DPN and pH 7.5. The production of dihydroxyacetone phosphate is followed by further oxidation steps, as could be shown by estimation of the different fractions of acid-soluble phosphate. Comparative studies were made on different insects and vertebrates. The rate of succinate oxidation by insect muscle was found to be ten times higher than that of vertebrate muscle. The relation of glycerophosphate oxidation to succinate oxidation is quite different in insects and vertebrates, but no difference could be detected between the two types of muscle metabolism of insects: the carbohydrate and the fatutilizing type.

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