The visual cells of the corsula mullet

The eye of Rhinomugil corsula has a duplex retina differentiated into dorsal and ventral halves, with the ventral retina 116·4 μm thicker than the dorsal retina. The rods of the ventral retina are relatively longer, with longer outer segments. The nuclei of the outer nuclear layer of the dorsal and ventral halves are in four and six to seven rows respectively. The rod outer segment bears a single incision. The mitochondria of cone and rod inner segments has a vitreal-scleral gradient. Single and double cones are present in both halves, with triple cones in the dorsal half only. The outer segments of double cones are equal and united. The single cones have two connecting cilia. The cone cells are arranged in a square mosaic with four double cones and five single cones to each unit in the dorsal half, and in a rhombic pattern in the ventral half.     

Opsin switch reveals function of the ultraviolet cone in fish foraging

Although several studies have shown that ultraviolet (UV) wavelengths are important in naturally occurring, visually guided behaviours of vertebrates, the function of the UV cone in such behaviours is unknown. Here, I used thyroid hormone to transform the UV cones of young rainbow trout into blue cones, a phenomenon that occurs naturally as the animal grows, to test whether the resulting loss of UV sensitivity affected the animal''s foraging performance on Daphnia magna, a prey zooplankton. The distances and angles at which prey were located (variables that are known indicators of foraging performance) were significantly reduced for UV knock-out fish compared with controls. Optical measurements and photon-catch calculations revealed that the contrast of Daphnia was greater when perceived by the visual system of control versus that of thyroid-hormone-treated fish, demonstrating that the UV cone enhanced the foraging performance of young rainbow trout. Because most juvenile fishes have UV cones and feed on zooplankton, this finding has wide implications for understanding the visual ecology of fishes. The enhanced target contrast provided by UV cones could be used by other vertebrates in various behaviours, including foraging, mate selection and communication.     

Phosphorylation of GRK7 by PKA in cone photoreceptor cells is regulated by light

The retina-specific G protein-coupled receptor kinases, GRK1 and GRK7, have been implicated in the shutoff of the photoresponse and adaptation to changing light conditions via rod and cone opsin phosphorylation. Recently, we have defined sites of phosphorylation by cAMP-dependent protein kinase (PKA) in the amino termini of both GRK1 and GRK7 in vitro. To determine the conditions under which GRK7 is phosphorylated in vivo, we have generated an antibody that recognizes GRK7 phosphorylated on Ser36, the PKA phosphorylation site. Using this phospho-specific antibody, we have shown that GRK7 is phosphorylated in vivo and is located in the cone inner and outer segments of mammalian, amphibian and fish retinas. Using Xenopus laevis as a model, GRK7 is phosphorylated under dark-adapted conditions, but becomes dephosphorylated when the animals are exposed to light. The conservation of phosphorylation at Ser36 in GRK7 in these different species (which span a 400 million-year evolutionary period), and its light-dependent regulation, indicates that phosphorylation plays an important role in the function of GRK7. Our work demonstrates for the first time that cAMP can regulate proteins involved in the photoresponse in cones and introduces a novel mode of regulation for the retinal GRKs by PKA.     

Lrit1, a Retinal Transmembrane Protein,Regulates Selective Synapse Formation in Cone Photoreceptor Cells and Visual Acuity

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Ultrastructure of the retina of two subspecies of Coregonus lavaretus (Teleostei) from Lake Constance (Germany)

In the two studied subspecies of Coregonus lavaretus , the pollan ( C. l. wartmanni ) (which lives deep in the pelagial) and the gangfish ( C. l. macrophthalmus ) (which lives near the slope, closer to the bottom), duplex retinae containing rod and cone photoreceptors are found. Four morphologically different cone types were observed: unequal double cones, short single cones, long single cones and triple cones. The cones are arranged in a square pattern (four double cones around a central short single cone) in the ventral and ventrotemporal and in a row pattern in the nasal and dorsal areas of the retina. Moreover, intermediate patterns can be observed in several regions indicating that double cone twisting occurs, i.e. double cones twist about their longitudinal axis. The highest cone densities are found in the ventrotemporal area. Conversely, the rod photoreceptor density is the highest in the dorsal retina. While the basic morphology of the retina is the same in both subspecies, the distribution of long single and triple cones differs between the studied animals. While these cone types are very rare in the pollan, they are common in the gangfish, though not exhibiting a regular pattern. The findings are discussed with regard to the photic habitat conditions, the systematic position of coregonids and variation of retinal morphology in the two subspecies.     

Evolution of vertebrate retinal photoreception

Recent findings shed light on the steps underlying the evolution of vertebrate photoreceptors and retina. Vertebrate ciliary photoreceptors are not as wholly distinct from invertebrate rhabdomeric photoreceptors as is sometimes thought. Recent information on the phylogenies of ciliary and rhabdomeric opsins has helped in constructing the likely routes followed during evolution. Clues to the factors that led the early vertebrate retina to become invaginated can be obtained by combining recent knowledge about the origin of the pathway for dark re-isomerization of retinoids with knowledge of the inability of ciliary opsins to undergo photoreversal, along with consideration of the constraints imposed under the very low light levels in the deep ocean. Investigation of the origin of cell classes in the vertebrate retina provides support for the notion that cones, rods and bipolar cells all originated from a primordial ciliary photoreceptor, whereas ganglion cells, amacrine cells and horizontal cells all originated from rhabdomeric photoreceptors. Knowledge of the molecular differences between cones and rods, together with knowledge of the scotopic signalling pathway, provides an understanding of the evolution of rods and of the rods'' retinal circuitry. Accordingly, it has been possible to propose a plausible scenario for the sequence of evolutionary steps that led to the emergence of vertebrate photoreceptors and retina.     

Calcium Modulation of Ligand Affinity in the Cyclic GMP–gated Ion Channels of Cone Photoreceptors

To investigate modulation of the activation of cGMP-gated ion channels in cone photoreceptors, we measured currents in membrane patches detached from the outer segments of single cones isolated from striped bass retina. The sensitivity of these channels to activation by cGMP depends on the history of exposure to divalent cations of the membrane''s cytoplasmic surface. In patches maintained in 20 μM Ca⁺⁺ and 100 μM Mg⁺⁺ after excision, the current amplitude dependence on cGMP is well described by a Hill equation with average values of K _1/2, the concentration necessary to activate half the maximal current, of 86 μM and a cooperativity index, n, of 2.57. Exposing the patch to a solution free of divalent cations irreversibly increases the cGMP sensitivity; the average value of K _1/2 shifts to 58.8 μM and n shifts to 1.8. Changes in cGMP sensitivity do not affect other functional parameters of the ion channels, such as the interaction and permeation of mono- and divalent cations. Modulation of cGMP activation depends on the action of an endogenous factor that progressively dissociates from the channel as Ca⁺⁺ concentration is lowered below 1 μM. The activity of the endogenous modulator is not well mimicked by exogenously added calmodulin, although this protein competes with the endogenous modulator for a common binding site. Thus, the modulation of cGMP affinity in cones depends on the activity of an unidentified molecule that may not be calmodulin.     

Circadian rhythms in the retina of rats with photoreceptor degeneration

Previous studies have demonstrated that the mammalian retina contains a circadian clock system that controls several retinal functions. In mammals the location of the retinal circadian clock is unknown whereas, in non-mammalian vertebrates, earlier work has demonstrated that photoreceptor cells contain the circadian clock. New experimental evidence has suggested that in mammals the retinal circadian clock may be located outside the photoreceptor cells. In this study we report that circadian rhythms in Aa-nat mRNA (in vivo) and melatonin synthesis (in vitro) are still present in the retina of rats lacking photoreceptors. The circadian pacemaker(s) controlling such rhythms is probably located in kainic acid sensitive neurons in the inner retina since kainic acid injections abolished the rhythmicity. These data are the first direct demonstration that circadian rhythmicity in the mammalian retina can be generated independently from the photoreceptors and the suprachiasmatic nuclei of the hypothalamus.     

Recoverin depletion accelerates cone photoresponse recovery

The neuronal Ca²⁺-binding protein Recoverin has been shown to regulate phototransduction termination in mammalian rods. Here we identify four recoverin genes in the zebrafish genome, rcv1a, rcv1b, rcv2a and rcv2b, and investigate their role in modulating the cone phototransduction cascade. While Recoverin-1b is only found in the adult retina, the other Recoverins are expressed throughout development in all four cone types, except Recoverin-1a, which is expressed only in rods and UV cones. Applying a double flash electroretinogram (ERG) paradigm, downregulation of Recoverin-2a or 2b accelerates cone photoresponse recovery, albeit at different light intensities. Exclusive recording from UV cones via spectral ERG reveals that knockdown of Recoverin-1a alone has no effect, but Recoverin-1a/2a double-knockdowns showed an even shorter recovery time than Recoverin-2a-deficient larvae. We also showed that UV cone photoresponse kinetics depend on Recoverin-2a function via cone-specific kinase Grk7a. This is the first in vivo study demonstrating that cone opsin deactivation kinetics determine overall photoresponse shut off kinetics.