Transduction in photoreceptors: Determination of the pigment transition or state coupled to excitation

Several recent reports have shown that light absorption by metarhodopsin does not contribute to the excitation of invertebrate photoreceptors at low intensities. Where the pigment transition scheme is known, this result may be used to exclude some or most of the transitions and states of the pigment as sources of the coupling to excitation. The methodology of this approach is described and illustrated.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Fine structure of the eyes of Onithochiton neglectus (Mollusca: Polyplacophora)

Summary Onithochiton neglectus a common littoral chiton possesses large numbers of small eyes embedded in the outer layer of the shell, the tegmentum. These are arranged in a definite pattern on each shell valve. Each eye lies in a pocket, and is surrounded by pigment laid down in the shell. There is a lens, cup of retina cells and an optic nerve running in an optic canal through the shell. Glial elements are present. The retina cells give rise centrally to a packed array of microvilli, a rhabdom. Cilia are present at the edge of the rhabdom; they have a 9 + 2 arrangement of ciliary filaments and do not appear to be involved in the formation of microvilli. Cells at the periphery of the eye cup give rise to large whorls of membranes, lamellate bodies. These bodies are derived from the membranes of cilia having a 9 + 2 pattern, and form into an extra-cellular space. Nerve processes from the retina cells pass into the optic canal. On the basis of previous work it is thought that the lamellate bodies are also sensory. These structures are discussed in relation to other microvillar and lamellate structures described from photoreceptors.I thank Professor J. E. Morton for his advice in the early stages of this work, and Dr. S. J. Bullivant for the fixation and embedding of material for electron microscopy. To Professor G. A. Horridge I am grateful for advice and the facilities of his laboratory, and to Professor M. S. Laverack, Patricia Holborow and Charles Coleman for much help and encouragement. I am supported by the Science Research Council, and in New Zealand held a Commonwealth Scholarship.     

Visual arrestin modulates gene expression in the retinal pigment epithelium: Implications for homeostasis in the retina

The retinal pigment epithelium (RPE) is essential for maintaining retinal homeostasis by removing and recycling photoreceptor outer segment (POS) in membranes. It also produces and secretes growth factors involved in retinal homeostasis. Arrestin 1 (ARR1) is specifically expressed in photoreceptors (PRs) and a vital molecule for keeping visual cycle between PRs and RPE. In the present study, we showed the expression of ARR1 was decreased by form-deprivation (FD) in retina of rat. The ARR1 was detected in the RPE of the controls but not in the RPE of FD, which indicates RPE phagocytes POS containing ARR1. Furthermore, we overexpressed ARR1 in cultured human RPE and revealed the ARR1 upregulates bFGF expression and downregulates TGF-β1, -β2 and bone morphogenetic protein-2 (BMP-2). The upregulation of bFGF by ARR1 directly works for PR survival and the downregulation of TGF-βs by ARR1 inhibits epithelial mesenchymal transition (EMT) of RPE, which is the underlying mechanism of keeping retinal homeostasis. Our results also indicate the regulation of ARR1 expression in RPE might become a novel therapeutic option for various ocular diseases.     

A new gene (madI) involved in the phototropic response of Phycomyces

Summary Only eight genes are known to be involved in the phototropic response of Phycomyces (madA-H). Mutants affected in these genes have played a major role in the analysis of photosensory transduction processes in this system. A set of new mutants isolated by Alvarez et al. (1989) that are unable to bend towards dim unilateral blue light were studied by complementation and recombination. Two of these mutants have mutations in madE, one has a mutation in madF and one is a double madE madF mutant. The three remaining mutants tested did not complement each other and showed positive complementation with strains carrying mutations in the genes madA, madB, and madC, indicating that they carried mutations in a new gene designated madI. Recombination analysis showed that madI is unlinked to madA, madB and madC.     

Photoreceptor development in the pineal organ and the eye of Plecoglossus altivelis and Paralichthys olivaceus (Teleostei)

Summary The ontogenetic apperance of pineal photo-receptors was compared with that of retinal photoreceptors in the ayu Plecoglossus altivelis and the lefteye flounder Paralichthys olivaceus, which hatched 10 days and 3 days after fertilization, respectively. Despite the disparity in incubation time, the outer segments (containing membranous lamellae) of the pineal photoreceptors first appeared from 3 to 4 days after fertilization in both species. In contrast, the outer segments of the retinal photoreceptors first became visible 5 to 6 days after fertilization, although a characteristic retinal stratification and the optic tract leaving the ganglion cell layer were already found 4 days after fertilization in both species. The functional significance of these temporal disparities and/or similarities in photoreceptor development are discussed with special reference to the timing of daily rhythmic activities during the early developmental period of the teleosts.The results were presented at the Annual Meeting of the Japanese Society of Scientific Fisheries on April 2, 1990 (Tokyo)     

The Mammalian Pineal Expresses the Cone but Not the Rod Cyclic GMP Phosphodiesterase

Abstract: To determine the presence of cone or rod cyclic GMP phosphodiesterase (EC 3.1.4.17) in the mammalian pineal, extracts from adult rat and bovine pineals were injected onto a Mono Q anion-exchange HPLC column and eluted with an NaCl linear gradient. Fractions were immunoadsorbed with monoclonal antibodies specific to rod and cone phosphodiesterases (ROS-1) and to calmodulin-phosphodiesterase complexes (ACC). Profiles were assayed with 10 µmol/L [³H]cyclic GMP in the presence of calcium-calmodulin, histone, or trypsin. Rat and bovine pineals displayed a single peak of activity recognized by ROS-1, which corresponded to the activity of the cone but, not to the rod in bovine retina. ROS-1 immunoadsorbed ∼80% of the activity in the 60-day-old rat pineal but only 26% of the activity in bovine pineal. ACC immunoadsorbed the remaining activity in both species. Western blot analysis of rat pineal extracts revealed three polypeptides of ∼87, 15, and 10 kDa when probed with a rod/cone phosphodiesterase-specific antiserum. The specific activity of the cone-like phosphodiesterase in 10-day-old rat pineals was twice that of this isozyme in the bovine retina and 150 times that in the bovine pineal. The specific activity of phosphodiesterase in rat pineals decreased with age. We conclude that an enzyme with biochemical and antigenic characteristics similar to cone, but distinct from rod phosphodiesterase, is present in bovine and rat pineals.     

The common white sucker (Catostomus commersoni ): a fish with ultraviolet sensitivity that lacks polarization sensitivity

Two families of fishes, the Cyprinidae and Salmonidae, exhibit ultraviolet sensitivity and polarization sensitivity (i.e., differential sensitivity to the orientation of the electric field of polarized light). Both of these families possess a square arrangement of double cones and/or their dividing partitions in the centro-temporal retina, an area where polarization sensitivity has been tested for and found. To correlate the presence of an ordered cone mosaic in the centro-temporal retina with polarization sensitivity in ultraviolet-sensitive fishes, we examined the visual system of the common white sucker (Catostomus commersoni) and compared it to those of the above-mentioned families. We found that the common white sucker possesses four cone-mediated neural mechanisms similar to those in cyprinids and salmonids, but it does not exhibit polarization sensitivity. In addition, unlike cyprinids and salmonids, the common white sucker shows a random cone mosaic in the centro-temporal retina. These results suggest that polarization sensitivity in ultraviolet-sensitive fishes requires an ordered double-cone mosaic in this area of the retina. Accepted: 19 July 1997     

Naturally occurring genetic variation affects Drosophila photoreceptor determination

 The signal transduction pathway controlling determination of the identity of the R7 photoreceptor in the Drosophila eye is shown to harbor high levels of naturally occurring genetic variation. The number of ectopic R7 cells induced by the dosage-sensitive Sev ^S11.1 transgene that encodes a mildly activated form of the Sevenless tyrosine kinase receptor is highly sensitive to the wild-type genetic background. Phenotypes range from complete suppression to massive overproduction of photoreceptors that exceeds reported effects of known single gene modifiers, and are to some extent sex-dependent. Signaling from the dominant gain-of-function Drosophila Epidermal Growth Factor Receptor (DER-Ellipse) mutations is also sensitive to the genetic backgrounds, but there is no correlation with the effects on Sev ^S11.1 . This implies that different genes and/or alleles modify the two activated receptor genotypes. The evolutionary significance of the existence of high levels of genetic variation in the absence of normal phenotypic variation is discussed. Received: 20 September 1997 / Accepted: 10 November 1997