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At least 19 recordsLinked to original sources
PubMed2004

Photoreceptor death, trophic factor expression, retinal oxygen status, and photoreceptor function in the P23H rat.

PURPOSE: To relate the oxygen environment of the retina to photoreceptor stability, protection, and function in the P23H rat. METHODS: Heterozygote P23H-3 (Line 3) rats were studied. Photoreceptor death rates were assessed with the TUNEL technique for detection of fragmenting DNA, in a developmental series from postnatal day (P)16 to P105 (adult). In adult retinas, trophic factor status was assessed with immunohistochemistry, intraretinal oxygen environment with O(2)-sensing electrodes, and photoreceptor function by the flash-evoked, dark-adapted electroretinogram (ERG), recorded in anesthetized animals. RESULTS: Photoreceptor death begins by P16; peaks at P25, when the frequency of TUNEL(+) profiles exceeds 70/mm of retina; and then declines to low (<5/mm) adult rates. Compared with that in nondegenerative Sprague-Dawley (SD) rats, the rate of photoreceptor death is abnormally high from P16 and remains several-fold higher than normal into young adulthood. In addition, the outer nuclear layer is reduced to approximately half of control thickness, and the levels of ciliary neurotrophic factor (CNTF), glial fibrillary acidic protein (GFAP), fibroblast growth factor (FGF)-2, and FGF-2/FGFR1 colocalization are markedly upregulated. O(2) tension and uptake are relatively normal in the inner retina, but uptake is considerably reduced, and O(2) tension is significantly raised in the outer retina. Surviving photoreceptors generate an a-wave with normal peak latency but sharply reduced amplitude. CONCLUSIONS: Excess photoreceptor degeneration in the P23H-3 retina begins just after eye opening, peaks in early postnatal life, and then slows, but persists into adulthood. In the adult retina, surviving photoreceptors operate in an environment that is chronically hyperoxic (and therefore toxic) and in which protective factors (CNTF, FGF-2) are chronically upregulated. The net result, slow degeneration and degraded function in an environment that is both toxic and protective, may be representative of adult photoreceptor status in a number of human retinal degenerations. Hyperoxia-induced photoreceptor death may be a self-reinforcing factor that increases oxidative stress in surviving photoreceptors.

Animals↗
PubMed1992

Photoreceptor transplantation: anatomic, electrophysiologic, and behavioral evidence for the functional reconstruction of retinas lacking photoreceptors.

We have investigated the possibility of using transplantation of immature or mature rodent photoreceptors as well as mature human photoreceptors to reconstruct retinas in which photoreceptor degeneration is either inherited or environmentally induced. To this end, we have devised methods for isolating and transplanting the outer nuclear layer (ONL) (e.g., the photoreceptor layer) to the subretinal space of mature rodents. In addition we found that if portions of the inner retina are transplanted along with the intact photoreceptor sheet, photoreceptor organization is better maintained. In ultrastructural studies of the reconstructed retina an outer plexiform-like layer (OPL) is visible at the interface of the transplanted ONL and the host inner nuclear layer, with invaginating ribbon synapses characteristic of those formed by rod photoreceptors evident within this OPL. Ribbon synapses are found only rarely in unreconstructed retina. These results suggest that synaptic connections between transplanted photoreceptors and host cells may be made. Evidence for the potential recovery of function following photoreceptor transplantation is found in visually evoked cortical responses and behavioral responses (pupillary reflex) to light stimulation of the reconstructed eye. These findings suggest the possibility that neural transplantation can reconstruct a sensory end organ--in this case the retina--to restore evoked activity and an appropriate behavioral response to sensory stimulation.

Albinism↗
PubMed2001

A mutation of early photoreceptor development, mikre oko, reveals cell-cell interactions involved in the survival and differentiation of zebrafish photoreceptors.

To gain insight into mechanisms involved in photoreceptor development, we characterized a zebrafish mutation in the mikre oko locus that produces early loss of photoreceptor cells. mikre oko photoreceptors lose their elongated morphology at the time of wild-type outer segment formation and undergo cell death within a few days. To investigate whether this phenotype involves cell-cell interaction defects, we performed analysis of genetically mosaic animals. Interactions of mikre oko photoreceptors with wild-type cells rescue several aspects of the mutant phenotype. When placed in a wild-type environment, mikre oko photoreceptor cells retain elongated morphology and survive longer. Moreover, although mutant mikre oko photoreceptor outer segments develop only infrequently and are usually disorganized, mikre oko cone and rod cells in mosaic retinas develop robust outer segments that closely resemble the wild type. In contrast to the outer segments, the proximal regions of mikre oko photoreceptor cells, including their inner segments, the nuclear regions, and the synaptic termini, retain the mutant appearance. mikre oko outer segment rescue is not mediated by interactions with the retinal pigment epithelium. These studies demonstrate that the differentiation of outer segments is surprisingly independent from the more proximal photoreceptor cell features and that outer segment development includes retinal pigment epithelium-independent cell-cell interactions.

Animals↗
PubMed1997

Crx, a novel otx-like homeobox gene, shows photoreceptor-specific expression and regulates photoreceptor differentiation.

We have isolated a novel otx-like homeobox gene, Crx, from the mouse retina. Crx expression is restricted to developing and mature photoreceptor cells. CRX bound and transactivated the sequence TAATCC/A, which is found upstream of several photoreceptor-specific genes, including the opsin genes from many species. Overexpression of Crx using a retroviral vector increased the frequency of clones containing exclusively rod photoreceptors and reduced the frequency of clones containing amacrine interneurons and Müller glial cells. In addition, presumptive photoreceptor cells expressing a dominant-negative form of CRX failed to form proper photoreceptor outer segments and terminals. Crx is a novel photoreceptor-specific transcription factor and plays a crucial role in the differentiation of photoreceptor cells.

3T3 Cells↗
PubMed1991

Central projections of Limulus photoreceptor cells revealed by a photoreceptor-specific monoclonal antibody.

Studies of lateral, median, and ventral eyes of the chelicerate arthropod Limulus polyphemus (the common American horseshoe crab) are providing important basic information about mechanisms for information processing in the peripheral visual system and for the modulation of visual responses by light and circadian rhythms. The processing of visual information in Limulus brain is less well understood in part because the specific central projections of the various classes of visual neurons are not known. This study describes a mouse monoclonal antibody, 3C6A3, which binds to Limulus photoreceptor cell bodies, their axons, and terminals, but not to any other cell type in the central nervous system. This antibody, and intracellular injection of biocytin, are used to demonstrate the central projections of each type of photoreceptor. Our main conclusions are that: 1) the photoreceptors (retinular cells) of the lateral eye project only to the lamina; 2) the photoreceptors of the lateral rudimentary eye project to both the lamina and medulla; 3) the photoreceptors of the median ocellus project only to the ocellar ganglion; and 4) the photoreceptors of the rudimentary median (endoparietal) eye project to the ocellar ganglion and also into the optic tract. These results, along with previous studies, allow us to infer the projections of the secondary cells. The eccentric cells of the lateral eye project to the lamina, medulla, optic tract, and ocellar ganglion. The arhabdomeral cells of the median ocellus project through the ocellar ganglion and to optic tract to the medulla.

Afferent Pathways↗
PubMed1997

Photoreceptor number and outer segment disk membrane surface area in the retina of the rat: stereological data for whole organ and average photoreceptor cell.

A random sampling scheme is employed to obtain stereological estimates of disk membrane surface area in the entire retina and in the average photoreceptor cell. The scheme involves the use of vertical sections with combined light and electron microscopy at several magnification levels. Left and right retinas from six albino animals were analysed. There were no significant lateral differences. On average, the retina had a volume of 16 mm3, thickness of 200 microns and surface area of 80 mm2 (representing about 56% of the external surface of the eyeball). Photoreceptor disk membranes within outer segments amplified total retinal surface by almost 1000-fold (final surface 770 cm2 per retina). The retina contained 3 x 10(7) photoreceptors (packing density 374,000 mm-2) with an average disk membrane surface area of 2600 microns2. Mean nuclear volume in photoreceptor cells was 59 microns3 and the coefficient of variation for the distribution of nuclear volumes was 57%. The data are consistent with an average of 700 disks per photoreceptor cell, a membrane area of 4 microns2 per disk and a convergence ratio of approximately 260 photoreceptors per optic nerve fibre. The basic scheme could be modified for other species and for direct cell counts conducted on rods and cones separately.

Animals↗
PubMed2001

A novel promoter element, photoreceptor conserved element II, directs photoreceptor-specific expression of nocturnin in Xenopus laevis.

Nocturnin is a vertebrate circadian clock-regulated gene, and in Xenopus laevis its mRNA is specifically expressed in retinal photoreceptor cells. We have investigated the transcriptional regulatory mechanism that drives this precise spatial expression pattern of the nocturnin gene. A deletion series of the nocturnin 5'-flanking sequence driving the green fluorescence protein (GFP) reporter was used to generate transgenic Xenopus tadpoles. We found that a construct containing 2.6 kilobase pairs of 5'-flanking sequence targeted high level GFP reporter expression specifically to photoreceptor cells, in a pattern identical to endogenous nocturnin. This photoreceptor-specific expression pattern was maintained with several further deletions of 5'-upstream sequence, including a short 59-base pair fragment. Within this region of 59 base pairs, three perfect repeats of a novel protein binding site were identified by electrophoretic mobility shift assay. Competitions using varying oligonucleotide sequences demonstrated that the sequence required for protein binding is CAGACAGGCTATA, designated photoreceptor-conserved element II (PCE II). The protein complex that binds to this element is enriched in retinal extracts, and mutations of PCE II which fail to bind the protein complex also fail to direct GFP reporter expression to photoreceptors. These results indicate that the PCE II in the proximal promoter of the nocturnin gene is sufficient for driving the photoreceptor-specific expression of nocturnin.

Animals↗
PubMed1999

Lactose promotes organized photoreceptor outer segment assembly and preserves expression of photoreceptor proteins in retinal degeneration.

PURPOSE: We have previously shown that lactose promotes the proper assembly of photoreceptor outer segments in the absence of the retinal pigment epithelium (RPE). The purpose of this study was to determine if the difference between organized and disorganized membranes was a variation in the amounts of two structural proteins, opsin and rds/peripherin. METHODS: Eye rudiments were dissected from Xenopus laevis embryos and the RPE was removed prior to culturing in the following media: Niu-Twitty medium; Niu-Twitty with mannose; Niu-Twitty with lactose. Controls included retinas that matured in vitro with an adherent RPE. Photoreceptor ultrastructure was evaluated with emphasis on outer segment membrane organization. The relative amounts of opsin and rds/ peripherin, two outer segment-specific proteins, were determined, as were their immunolabeling patterns. RESULTS: In control retinas, outer segments were composed of stacked, flattened membranous saccules. Opsin labeling of rod outer segments was very dense, indicative of normally organized disc membranes, and rds/peripherin labeling was heavy at the outer segment disc periphery and incisures. In the absence of the RPE, a whorl-like profile of outer segments is present in what would be the sub-retinal space. Opsin immunolabeling was patchy and disorganized. Immunolabeling of rds/peripherin was present, but in a disorderly array. Mannose showed no protective effect. In contrast, lactose promoted the formation of organized outer segments and allowed for near normal expression of both photoreceptor markers. In retinas with disorganized outer segments, the expression of opsin is downregulated while the expression of rds/peripherin is maintained or upregulated. CONCLUSIONS: Lactose protects against the retinal degeneration induced by RPE removal by preserving the outer segment structure and the photoreceptor immunolabeling patterns. It also maintains constant the relative amounts of opsin and rds/peripherin. It is possible that in degenerating retinas, photoreceptors upregulate rds/peripherin expression in attempt to provide additional support for the proper folding of nascent membranes, however this is insufficient to permit organization of the photoreceptor outer segments. Our results suggest that rescue-effect of lactose is mediated by a non-rds/peripherin related mechanism.

Animals↗
PubMed1998

A Ca2+/calmodulin-dependent protein kinase modulates Drosophila photoreceptor K+ currents: a role in shaping the photoreceptor potential.

Light activation of Drosophila photoreceptors leads to the generation of a depolarizing receptor potential via opening of transient receptor potential and transient receptor potential-like cationic channels. Counteracting the light-activated depolarizing current are two voltage-gated K+ conductances, IA and IK, that are expressed in these sensory neurons. Here we show that Drosophila photoreceptors IA and IK are regulated by calcium-calmodulin (Ca2+/calmodulin) via a Ca2+/calmodulin-dependent protein kinase (CaM kinase), with IK being far more sensitive than IA. Inhibition of Ca2+/calmodulin by N-(6 aminohexyl)-5-chloro-1-naphthalenesulfonamide or trifluoperazine markedly reduced the K+ current amplitudes. Likewise, inhibition of CaM kinases by KN-93 potently depressed IK and accelerated its C-type inactivation kinetics. The effect of KN-93 was specific because its structurally related but functionally inactive analog KN-92 was totally ineffective. In Drosophila photoreceptor mutant ShKS133, which allows isolation of IK, we demonstrate by current-clamp recording that inhibition of IK by quinidine or tetraethylammonium increased the amplitude of the photoreceptor potential, depressed light adaptation, and slowed down the termination of the light response. Similar results were obtained when CaM kinases were blocked by KN-93. These findings place photoreceptor K+ channels as an additional target for Ca2+/calmodulin and suggest that IK is well suited to act in concert with other components of the signaling machinery to sharpen light response termination and fine tune photoreceptor sensitivity during light adaptation.

Adaptation, Ocular↗
PubMed1991

Biophysical processes in invertebrate photoreceptors: recent progress and a critical overview based on Limulus photoreceptors.

Limulus ventral nerve photoreceptor, a classical preparation for the study the phototransduction in invertebrate eyes, seems to have a very complex mechanism to transform light energy into a physiological signal. Although the main function of the photoreceptor is to change the membrane conductance according to the illumination, the cell has voltage-activated conductances as well. The voltage-gated conductances are matched to the light-activated ones in the sense that they make the function of the cell more efficient. The complex mechanism of phototransduction and the presence of four different voltage-gated conductance in Limulus ventral nerve photoreceptors indicate that these cells are far less differentiated than the photoreceptor cells of vertebrates. Indications accumulated in recent years support the view that the ventral photoreceptor of Limulus has different light-activated macroscopic current components, ion channels and terminal transmitters. After conclusions from macroscopic current measurements (Payne, 1986; Payne et al. 1986 a, b), direct evidence was presented by single-channel (Nagy & Stieve, 1990 a, b; Nagy, 1990 a, b) and macroscopic current measurements (Deckert et al. 1991 a, b) for three different light-activated conductances. It has been shown that two of these conductances are stimulated by two different excitation mechanisms. The two mechanisms, having different kinetics, release probably two different transmitters. One of them might be the cGMP (Johnson et al. 1986), the other one the calcium ion (Payne et al. 1986 a, b). However, the biochemical processes which link the rhodopsin molecules and the ion channels are not known. The unknown chemical details of the phototransduction result in a delay for the mathematical description of the biophysical mechanisms. More biochemical details are known about the adaptation mechanism. It was found that inositol 1,4,5-trisphosphate is a messenger for the release of calcium ions from the intracellular stores and that calcium ions are the messengers for adaptation (Payne et al. 1986 b; Payne & Fein, 1987). Concerning the mechanism of calcium release, it was revealed that a negative feedback acts on the enzyme cascade to regulate the internal calcium level and to protect the stores against complete emptying (Payne et al. 1988, 1990). Calcium ions also play an important role in the excitation mechanism. (a) In [Ca2+]i-depleted cells the light-induced current was increased after intracellular Ca2+ injection, suggesting that calcium is necessary for the transduction mechanism (Bolsover & Brown, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

L-glutamate-induced depolarization in solitary photoreceptors: a process that may contribute to the interaction between photoreceptors in situ.

L-Glutamate is a leading candidate for the vertebrate photoreceptor transmitter. In addition to the signal transmission to second-order neurons, photoreceptors communicate with each other not only electrically but also chemically. In the present study, by using solitary turtle photoreceptors, we examined the possibility that L-glutamate mediates interreceptor communication. L-Glutamate evoked an inward current in all subtypes of photoreceptors voltage-clamped to the resting potential. The highest glutamate sensitivity was located at the axon terminal. Both stereoisomers of aspartate were effective, whereas kainate, quisqualate, N-methyl-D-aspartate, and D-glutamate were ineffective. The presence of Na+ was essential to response generation; even Li+ could not substitute for Na+. The relation between L-glutamate-induced current and the membrane voltage was strongly inward-rectifying. These results favor the hypothesis that the L-glutamate-induced response is generated by an electrogenic uptake carrier. However, L-glutamate-induced current was always accompanied by an increase in current fluctuations, a phenomenon commonly observed in ion channels but not expected for an uptake carrier. Although the underlying mechanism needs further elucidation, it seems likely that L-glutamate is a transmitter for communication between photoreceptors.

Animals↗
PubMed2002

Distinct functions of photoreceptor cell-specific nuclear receptor, thyroid hormone receptor beta2 and CRX in one photoreceptor development.

PURPOSE: To clarify the functions of a specific subtype of thyroid hormone receptor (TR), TRbeta2, and photoreceptor cell-specific nuclear receptor (PNR) in the development of cone photoreceptors. METHODS: The expression of short (S)- and medium (M)-wavelength cone opsins was analyzed by reverse transcription polymerase chain reaction (RT-PCR) and Northern blot analysis in mice without a functional PNR (rd7/rd7 mice), and levels of plasma thyroid hormones and expression of TRbeta2 were also examined. Concomitantly, by means of reporter assays, the roles of PNR and TRbeta2 in the S- and M-cone opsin expression were explored at the transcriptional level. RESULTS: In rd7/rd7 mice, an abnormal increase in cone photoreceptors was observed immediately before retinal maturation normally occurs. Although an increase in S-cone opsin in the retina was observed during and after retinal development, the expression of M-cone opsin expression was not perturbed during retinal maturation. Plasma concentrations of thyroid hormone and levels of TRbeta2 expression in the rd7/rd7 mouse retina over the developmental period were normal. Transcriptional studies demonstrated that TRbeta2, but not PNR, activated the M-cone opsin gene promoter function, while suppressing the S-cone opsin promoter function enhanced by CRX in a thyroid hormone-dependent manner. CONCLUSIONS: The results indicate that PNR may suppress proliferation of cone photoreceptor progenitor cells and that the regulation of S- and M-cone opsin gene expression is mediated by TRbeta2 and CRX, but not by PNR. Thus, our results partly disclosed the molecular mechanism of cone photoreceptor development, highlighting the distinct functions of PNR and TRbeta2.

Animals↗
PubMed2004

Photoreceptor-specific nuclear receptor NR2E3 functions as a transcriptional activator in rod photoreceptors.

NR2E3, a photoreceptor-specific orphan nuclear receptor, is believed to play a pivotal role in the differentiation of photoreceptors. Mutations in the human NR2E3 gene and its mouse ortholog are associated with enhanced S-cones and retinal degeneration. In order to gain insights into the NR2E3 function, we performed temporal and spatial expression analysis, yeast two-hybrid screening, promoter activity assays and co-immunoprecipitation studies. The Nr2e3 expression was localized preferentially to the rod, and not to the cone, photoreceptor nuclei in rodent retina. The yeast two-hybrid screening of a retinal cDNA library, using NR2E3 as the bait, identified another orphan nuclear receptor NR1D1 (Rev-erbalpha). The interaction of NR2E3 with NR1D1 was confirmed by glutathione S-transferase pulldown and co-immunoprecipitation experiments. In transient transfection studies using HEK 293 cells, both NR2E3 and NR1D1 activated the promoters of rod phototransduction genes synergistically with neural retina leucine zipper (NRL) and cone-rod homeobox (CRX). All four proteins, NR2E3, NR1D1, NRL and CRX, could be co-immunoprecipitated from the bovine retinal nuclear extract, suggesting their existence in a multi-protein transcriptional regulatory complex in vivo. Our results demonstrate that NR2E3 is involved in regulating the expression of rod photoreceptor-specific genes and support its proposed role in transcriptional regulatory network(s) during rod differentiation.

Animals↗
PubMed1995

The sequence of arrestins from rod and cone photoreceptors in the frogs Rana catesbeiana and Rana pipiens. Localization of gene transcripts by reverse-transcription polymerase chain reaction on isolated photoreceptors.

Members of the arrestin protein family are known to participate in the inactivation of rhodopsin and other heptahelical receptors. Arrestins bind to the activated and phosphorylated state of these receptors, consequently blocking the ability of the receptors to activate the guanine-nucleotide-binding protein (G protein). We have determined the sequences of four retinal arrestins from two species of frog, Rana catesbeiana and Rana pipiens. Using polymerase chain reaction on reverse-transcribed mRNA isolated from single photoreceptor cells, we show that two of these arrestins are from rod photoreceptors and two rod photoreceptors and two are from cone photoreceptors. Comparison of these arrestins with the twenty known arrestin sequences identifies three regions of the protein that are well conserved across all phylogenetic groups. These regions may function in the binding of the arrestin to the heptahelical receptors. In addition, the Rana arrestins contain a uniquely acidic C-terminal sequence.

Amino Acid Sequence↗
PubMed2002

Cloning and characterization of the canine photoreceptor specific cone-rod homeobox (CRX) gene and evaluation as a candidate for early onset photoreceptor diseases in the dog.

PURPOSE: The cone-rod homeobox protein (CRX) is a member of the homeodomain-containing protein family expressed in the retinal photoreceptors and pinealocytes; it is involved in the regulation of the coordinate expression of multiple photoreceptor specific genes during retinal development. Mutations in the CRX gene are causally associated with retinal degeneration phenotypes in man. To clone the full length cDNA, characterize the genomic organization of canine CRX, map the gene in a radiation hybrid (RH) panel, and evaluate it as a candidate for canine inherited retinal degenerations. METHODS: cDNA representational difference analysis (RDA) was done using normal and cone degeneration (cd) affected retinas. Exonic primers designed from consensus sequences of mammalian CRX cDNA were used to amplify and sequence dog genomic DNA. Canine specific primers were used for RH mapping of CRX on the RH3000 cell line. Linkage, sequencing and/or mapping the disease locus was used to evaluate CRX as a disease associated candidate gene. RESULTS: The gene comprises three exons and two introns and codes for a transcript with a 900 bp open reading frame (ORF). In agreement with human map data, RH mapping placed canine CRX on the proximal end of CFA1, in a region of synteny with HSA19q13-q13.3. Based on RH mapping, meiotic linkage or sequencing data, we excluded CRX as the cause of canine early onset photoreceptor degenerations affecting Alaskan malamutes (cd), collies (rod-cone dysplasia 2, rcd2), American Staffordshire terriers, and Tibetan terriers. CONCLUSIONS: Canine CRX has a high level of nucleotide and amino acid sequence identity with orthologous sequences reported for other species. The gene is excluded from causal association with 4 early onset photoreceptor diseases affecting cones (cd) or rods and cones (rcd2, PRA in American Staffordshire terriers, and Tibetan terriers).

Amino Acid Sequence↗
PubMed2004

Guanylyl cyclase-activating proteins (GCAPs) are Ca2+/Mg2+ sensors: implications for photoreceptor guanylyl cyclase (RetGC) regulation in mammalian photoreceptors.

Guanylyl cyclase-activating proteins (GCAP) are EF-hand Ca(2+)-binding proteins that activate photoreceptor guanylyl cyclase (RetGC) in the absence of Ca(2+) and inhibit RetGC in a Ca(2+)-sensitive manner. The reported data for the RetGC inhibition by Ca(2+)/GCAPs in vitro are in disagreement with the free Ca(2+) levels found in mammalian photoreceptors (Woodruff, M. L., Sampath, A. P., Matthews, H. R., Krasnoperova, N. V., Lem, J., and Fain, G. L. (2002) J. Physiol. (Lond.) 542, 843-854). We have found that binding of Mg(2+) dramatically affects both Ca(2+)-dependent conformational changes in GCAP-1 and Ca(2+) sensitivity of RetGC regulation by GCAP-1 and GCAP-2. Lowering free Mg(2+) concentrations ([Mg](f)) from 5.0 mm to 0.5 mm decreases the free Ca(2+) concentration required for half-maximal inhibition of RetGC ([Ca]((1/2))) by recombinant GCAP-1 and GCAP-2 from 1.3 and 0.2 microm to 0.16 and 0.03 microm, respectively. A similar effect of Mg(2+) on Ca(2+) sensitivity of RetGC by endogenous GCAPs was observed in mouse retina. Analysis of the [Ca]((1/2)) changes as a function of [Mg](f) in mouse retina shows that the [Ca]((1/2)) becomes consistent with the range of 23-250 nm free Ca(2+) found in mouse photoreceptors only if the [Mg](f) in the photoreceptors is near 1 mm. Our data demonstrate that GCAPs are Ca(2+)/Mg(2+) sensor proteins. While Ca(2+) binding is essential for cyclase activation and inhibition, Mg(2+) binding to GCAPs is critical for setting the actual dynamic range of RetGC regulation by GCAPs at physiological levels of free Ca(2+).

Animals↗
PubMed2000

Photoreceptor classes and transmission at the photoreceptor synapse in the retina of the clawed frog, Xenopus laevis.

The photoreceptor population in Xenopus consists of a green-sensitive rod (lambda(max) = 523 nm), a blue-sensitive rod (lambda(max) = 445 nm) and three classes of cone. The largest cone is red-sensitive (lambda(max) = 611 nm). The intermediate cone is presumed to be blue-sensitive based on physiological criteria, whereas the miniature cone may be UV-sensitive. Horizontal cells (HC) are of two sorts: axon-bearing and axonless. The axon-bearing HC is of the luminosity type and probably contacts all types of photoreceptor. The axonless HC is of the chromaticity type and contacts only intermediate (blue) cones and at least one type of rod. During development dendrites of HCs and bipolar neurons penetrate photoreceptor bases. A progressive maturation of HC and bipolar synapses with rods and cones occurs between tadpoles stages 37/8 and 46. Neighboring rods and cones are joined by gap junctions. During this same period, the outer segments are laid down and photopigments synthesized. A linear relation was found between the quantum capturing ability of the rod and its absolute threshold. Mature rods of the Xenopus retina release glutamate in a calcium-dependent manner. Glutamate release was found to be a linear function of calcium influx through L-type calcium channels. Both types of HC possess ionotropic glutamate receptors of the AMPA subtype.

Animals↗
PubMed1990

Telodendrial contact of HRP-filled photoreceptors in the turtle retina: pathways of photoreceptor coupling.

Synaptic contacts of photoreceptors in the turtle retina were studied by intracellular injection of horseradish peroxidase (HRP) and electron microscopy. Both cone and rod photoreceptors radiated basal processes (telodendria) from their terminal endings. These telodendria ran laterally in the outer plexiform layer. The telodendria of cones gave rise to many fine branches that penetrated synaptic cavities of several neighboring cones. Tips of these branches terminated near the walls of synaptic cavities. Some of the telodendrial contact formed two types of basal junction: symmetrical and punctate. The distribution of cones that made telodendrial contacts with the HRP-filled cone were quantitatively investigated. Green-sensitive cones (n = 3) made telodendrial contacts with neighboring red- and blue-sensitive cones, blue-sensitive cones (n = 4) with red- and green-sensitive cones, and red-sensitive cones (n = 9) with red- and green-sensitive cones. In contrast to these cone connections, rod telodendria did not penetrate neighboring photoreceptors. Direct synaptic contacts were not found between rods and cones. Our results clarify the variety of cone couplings in turtle retina: the three chromatic classes of cones are selectively coupled by the basal junctions at the ends of telodendrial processes.

Animals↗