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  • Adaptive loss of ultraviolet-sensitive/violet-sensitive (UVS/VS) cone opsin in the blind mole rat (Spalax ehrenbergi).

    27 November 2018

    In previous studies, fully functional rod and long-wavelength-sensitive (LWS) cone photopigments have been isolated from the eye of the subterranean blind mole rat (Spalax ehrenbergi superspecies). Spalax possesses subcutaneous atrophied eyes and lacks any ability to respond to visual images. By contrast this animal retains the ability to entrain circadian rhythms of locomotor behaviour to environmental light cues. As this is the only known function of the eye, the rod and LWS photopigments are thought to mediate this response. Most mammals are dichromats possessing, in addition to a single rod photopigment, two classes of cone photopigment, LWS and ultraviolet-sensitive/violet-sensitive (UVS/VS) with differing spectral sensitivities which mediate colour vision. In this paper we explore whether Spalax is a dichromat and has the potential to use colour discrimination for photoentrainment. Using immunocytochemistry and molecular approaches we demonstrate that Spalax is a LWS monochromat. Spalax lacks a functional UVS/VS cone photopigment due to the accumulation of several deleterious mutational changes that have rendered the gene nonfunctional. Using phylogenetic analysis we show that the loss of this class of photoreceptor is likely to have arisen from the visual ecology of this species, and is not an artefact of having an ancestor which lacked a functional UVS/VS cone photopigment. We conclude that colour discrimination is not a prerequisite for photoentrainment in this species.

  • Entrainment of circadian programs.

    27 November 2018

    Of the three defining properties of circadian rhythmicity--persisting free-running rhythm, temperature compensation, and entrainment--the last is often poorly understood by many chronobiologists. This paper gives an overview of entrainment. Where have we been? Where are we now? Whence should we be going? Particular emphasis is given to a discussion of the Discrete vs. Continuous models for entrainment. We provide an integrated mechanism for entrainment from a limit-cycle perspective.

  • Extraretinal photoreceptors and their regulation of temporal physiology.

    27 November 2018

    The extraretinal photoreceptors of non-mammalian vertebrates play an important role in the regulation of temporal physiology. Both the regulation of circadian clocks and the photoperiodic response of many animals depend upon the photic information provided by these receptors. Since their discovery at the beginning of this century, and despite their importance, extraretinal photoreceptors have remained poorly understood. Until recently, their cellular location within the central nervous system, and the nature of the photopigments they use, remained a mystery. Antibodies directed against rod or cone photopigment proteins have been used in immunocytochemical procedures to localize extraretinal photoreceptors. However, findings have been confusing. The use of molecular approaches has led to the identification of several new photopigment gene families. Significantly, these genes are not expressed in the rods and cones of the retina, but in many sites within the central nervous system. Moreover, molecular approaches have proved useful in clarifying some of the earlier immunocytochemical results. Collectively, the recent findings show that non-mammalian vertebrates possess multiple extraocular photoreceptors that may express novel, rod or even cone photopigments. The future challenge is to link these photoreceptors with circadian and photoperiodic physiology.

  • Photopigments and circadian systems of vertebrates.

    27 November 2018

    In the retinal degeneration (rd) mouse the absence of rod cells and the progressive loss of cones does not result in a decrease in circadian phase shifting responses to light. By contrast, rd/rd mice are unable to perform simple visual tasks. In addition, rodless transgenic mice, and mice homozygous for the retinal degeneration slow (rds) mutation, show unattenuated circadian responses to light. Collectively these data suggest that cone cells lacking outer segments are sufficient to maintain normal circadian responses to light, or some unidentified photoreceptor within the retina. An action spectrum for circadian responses to light in rd/rd mice, and molecular analysis of retinally degenerate mice and blind mole rat eyes, suggests the involvement of a mid-to-long wavelength sensitive cone opsin in photoentrainment. Extraocular photoreceptors of non-mammalian vertebrates are currently being analyzed in order to identify functional and evolutionary similarities between visual and non-visual photoreceptor systems.

  • Photic entrainment of the circadian clock: from Drosophila to mammals.

    27 November 2018

    Entrainment is as fundamental to an organism's circadian timing as are the molecular mechanisms involved in the functioning of the intracellular clock oscillator. In nature, one of the principle, although not the only, circadian entraining stimulus (Zeitgeber) is provided by the daily light--dark cycles. In animals, the visual processing apparatus alone is inadequate to accomplish the task of transducing circadian photic signals to the clockwork machinery. In fact, it is ever more appreciated by circadian biologists that organisms as divergent as plants and mammals have evolved a wonderfully complex array of partly redundant specializations which can guarantee the precise alignment of biological and environmental time. Research in circadian biology is cruising at such a rate that attempts to review the state of the art can only hope, at best, to provide a snapshot of the speeding cruiser from its wake. This paper will hopefully provide a reasonably sharp portrayal of what is at hand.

  • Expression of developmentally defined retinal phenotypes in the histogenesis of retinoblastoma.

    27 November 2018

    Retinoblastoma, the most common intraocular tumor of childhood, is a malignant neoplasm that arises during retinal development. The embryonal cell target for neoplastic transformation is not yet clearly defined. To better understand the histogenetic potential of this tumor, the expression of photoreceptor and glial cell-associated proteins were examined in 22 primary retinoblastomas. Interphotoreceptor retinol-binding protein (IRBP), cone and rod opsins were selected as the photoreceptor specific proteins due to their different temporal patterns of expression during normal retinal development. Neoplastic Müller cell differentiation, and non-neoplastic reactive astrocytes were identified using cellular retinaldehyde binding-protein (CRAlBP), and glial fibrillary acidic protein (GFAP), respectively. Photoreceptor proteins were present in 16 cases and showed different cellular patterns of expression. IRBP and cone opsin were usually abundant. Although rod opsin was clearly identified in eight tumors, its expression was more restricted than either IRBP or cone opsin. This differential pattern of expression, opposite to the normal pattern of photoreceptor gene expression in the adult retina, corresponded to a marked decrease in mRNA for rod opsin. Cone opsin and IRBP colocalized in fleurettes demonstrating that neoplastic human cone cells are capable of IRBP synthesis. Müller cell differentiation was present in 12 of the 16 cases in which photoreceptor proteins were detected. In contrast, GFAP was only present in reactive, stromal astrocytes associated with blood vessels. Our data suggest that the retinoblastoma has the histogenetic potential of the immature neural retinal epithelium which can give rise to both photoreceptor and Müller cell lineages. The differential expression of cone and rod phenotypes in retinoblastoma is consistent with the "default" mechanism of cone cell differentiation.

  • A comparison of some photoreceptor characteristics in the pineal and retina. II. The Djungarian hamster (Phodopus sungorus).

    27 November 2018

    A rod-specific antiserum was used to immunolabel elements within the retina and pineal of the adult Djungarian hamster and Welsh Mountain sheep. In the retina immunostaining was localized to the outer segments and perikarya of photoreceptor cells, while in the pineal limited numbers of labelled pinealocytes were scattered throughout the gland. An enzyme-linked immunosorbent assay (ELISA) was then used to obtain a quantitative measure of rod opsin in total eye and pineal extracts from the Djungarian hamster. Total rod opsin (+/- SEM) in the eye was measured by absorbance spectroscopy (1.88 +/- 0.10 nmoles opsin/eye) and by using the ELISA (1.75 +/- 0.02 nmoles opsin/eye). The opsin content from a total of 56 pineals gave a mean value of 0.34 +/- 0.01 pmoles opsin/pineal. Since a functional photopigment should be coupled in a 1:1 ratio to a chromophore, we investigated whether we could identify 11-cis and/or all-trans retinaldehydes in the pineal extracts by quantitative extraction and HPLC analysis as the oximes. No evidence of 11-cis or all-trans retinaloxime could be found, the chromatograms were indistinguishable from those produced by extracts of cortical brain tissue. We conclude that the opsin present within the adult hamster pineal is not coupled to the common vertebrate retinaldehyde chromophore, and as a result, is unlikely to be part of a functional photopigment.

  • Immunocytochemical studies on the LHRH system of the Japanese quail: influence by photoperiod and aspects of sexual differentiation.

    27 November 2018

    Immunocytochemistry was used to determine if photoperiod and/or sex have any effect on the pattern of the luteinizing hormone-releasing hormone (LHRH) system in the brain of the Japanese quail. Immunopositive perikarya were found within three major areas of the brain: the rostral paraolfactory lobe, the preoptic, and the septal region. A quantitative analysis of LHRH cell numbers was performed on male and female quail after two photoperiodic treatments: sexually mature birds exposed to 24 weeks of 20 h light: 4 h darkness (20L:4D), and birds with a regressed reproductive system (induced by transfer from a photoregime of 20L:4D to 25 short days of 8L:16D). Two-way analysis of variance showed that short-day males display significantly (p less than 0.05) more immunopositive perikarya (607 +/- 134) than long-day males (291 +/- 114), short-day females (293 +/- 103) or long-day females (330 +/- 92). The density of LHRH-immunoreactive nerve fibres and the intensity of the immunostaining in the median eminence were always greater in long-day sexually mature quail (male and female) than in animals exposed to 25 days of 8L:16D. These results demonstrate that the LHRH system of the quail is influenced by photoperiod and mirrors sexual differentiation.

  • Evidence for a daily rhythmicity in the acute release of luteinizing hormone in response to electrical stimulation in the Japanese quail.

    27 November 2018

    This study was undertaken to examine the effect of electrical stimulation of the hypothalamus at different times of day on luteinizing hormone (LH) secretion in male castrated quail on short days (8L:16D). The posterior hypothalamus was stimulated with square-wave pulses of 80 microA for 2 min through chronically-implanted platinum microelectrodes. Stimulation was carried out on each quail at 4 (treatment A), 10 (B), or 14 h (C) after dawn. Plasma LH levels were increased markedly within 2 min of ending the stimulation but reached basal levels again over the next 20 min or so. The absolute increase was significantly greater in treatment B (10 h after lights on) than at the other times tested. This is consistent with a rhythm in hypothalamic responsivity. The results are discussed in the context of the rhythm of photoinducibility which occurs early in the night and which is used by quail as a photoperiodic clock to regulate seasonal reproduction.

  • Opsins and mammalian photoentrainment.

    27 November 2018

    Research over the past decade has provided overwhelming evidence that photoreception in the vertebrate eye is not confined to the rod and cone photoreceptors. It appears that photoreceptor cells within the inner retina provide irradiance information to a wide variety of different photosensory tasks including photoentrainment, pupillary constriction and masking behaviour. Action spectra in mice lacking all rod and cone photoreceptors ( rd/rd cl) have demonstrated the existence of a previously uncharacterised, opsin/vitamin-A-based photopigment with peak sensitivity at 479 nm (opsin photopigment/OP(479)). The review addresses the question: has the gene encoding OP(479) already been isolated, and if not, what type of gene should we be seeking and where in the eye might this gene be expressed? On the basis of available data, the gene that encodes OP(479) remains unidentified, and two broad possibilities exist. On the assumption that OP(479) will be like all of the other vertebrate photopigments (ocular and extraocular) and share a close phylogenetic relationship based upon amino acid identity and a conserved genomic structure, then the gene encoding OP(479) has yet to be isolated. Alternatively, there may have been a separate line of photopigment evolution in the vertebrates that has given rise to the melanopsin family. If true then the mammalian melanopsin gene may encode OP(479). Only when melanopsin and other candidates for OP(479) have been functionally expressed, and shown to encode a photopigment that matches the action spectrum of OP(479), can firm conclusions about the identity of the non-rod, non-cone ocular photoreceptor of mammals be made.