Retinoid cycling proteins redistribute in light-/dark-adapted octopus retinas

In cephalopods, the comblex rhodopsin-retinochrome system serves to regenerate metarhodopsin and metaretinochrome after illumination. In the dark, a solubleprotein, retinal-binding protein (RALBP), shuttles 11-cis retinal released from metaretinochrome located in the photoreceptor inner segments to metarhodopsin present in the rhabdoms. While in the rhabdoms, RALBP delivers 11-cis retinal to regenerate rhodopsin and in turn binds the all-trans isomer released by metarhodopsin. RALBP then returns all-trans retinal to the inner segments to restore retinochrome. The conventional interpretation of retinoid cycling is contradicted by immunocytochemical studies showing that, in addition to rhodopsin, retinochrome is present in the rhabdomal compartment, making possible the direct exchange of chromophores between the metapigments with the potential exclusion of RALBP. By using immunofluorescence and laser scanning confocal microscopy, we have precisely located opsin, aporetinochrome, and RALBP in light-/dark-adpated octopus retinas. We found differences in the distribution of all three proteins throughout the retina. Most significantly, comparison of cross sections though light- and dark-adpated rhabdoms showed a dramatic shift in position of the proteins. In the dark, opsin and retinochrome colocalized at the base of the rhabdomal microvilli. In the light, opsin redistributed along the length of the microvillar membranes, and retinochrome retreated to a location that is perhaps extracellular. RALBP was present in the core cytoplasm of the photoreceptor outer segments in the dark, and RALBP moved to the periphery in the light. Because of the colocalization of opsin and retinochrome in the dark, we believe that the two metapigments participate directly in chromophore exchange. RALBP may serve to transport additional chromophore from the inner segments to the rhabdoms and may not be immediately involved in the exchange process. © 1995 Wiley-Liss, Inc.     

Postembryonic development of the dorsal ocellus of the American cockroach

Postembryonic development of the dorsal ocellus of the cockroach Periplaneta americana was examined. Small ocelli (20 micron in diameter) with less than 100 cells in the newly hatched nymph become adult ocelli (more than 500 micron in diameter) with more than 10,000 retinular cells, through ten to 11 nymphal stages. Thus, sequential steps of morphogenesis of rhabdomeres from loose interdigitations of apposed cell membranes to regularly arranged microvilli can be seen in the nymphal ocelli. Some retinular axons appear to extend into the brain as a bundle in the first-instar nymph. Retinular axons differentiated later also extend toward the brain, but there is no evidence that they enter it. The ultrastructure of the ocellus and component analysis of the ocellar electroretinogram suggest that functional connections between retinular axons and ocellar interneurons do not occur in the ocellar capsule until mid (the fifth or sixth)-instar nymphs. The ocellar diameter increases linearly with increase in body length during the nymphal stages, but it increases by a factor of 1.5-2.0 during the final molting: the body length of the adult is almost the same as that of the last-instar nymph. These data suggest that the function of the dorsal ocelli may be closely related to specific adult behavior such as flight.     

Cyclic nucleotide distribution in identified layers of suprafused rabbit retinas

To assess the effects of pharmacologic agents on metabolite levels in identified retinal layers we have devised an apparatus that allows suprafusion of everted eye cups (vitreal surface exposed) or isolated retinas (photoreceptor surface exposed), recording of electroretinograms (ERG), and rapid freezing in a configuration which permits obtaining frozen tangential sections of the retina. Samples from identified layers of the freeze-dried sections can be subsequently weighed and processed for biochemistry by Lowry methods (Lowry and Passonneau , 1972), or cyclic nucleotides can be extracted from weighed samples and assayed. To test the usefulness of the apparatus, we have compared levels of guanosine 3',5'-monophosphate (cGMP) and adenosine 3',5'-monophosphate (cAMP), measured in defined layers of suprafused rabbit retinas, to concentrations reported for retinal layers from eyes removed from light- and dark-adapted rabbits and quickly frozen. For most layers the concentrations of cyclic nucleotides were similar in both the suprafused and whole eye preparations. In experimental applications we suprafused dark-adapted eye cups and retinas with a physiological saline whose free calcium level was sharply reduced by chelation, and observed that cGMP levels were elevated only in those retinal layers containing photoreceptor portions. Ethyleneglycol bis-(beta-amino-ethyl ether)N,N'-tetraacetic acid (EGTA) (3 mM) yielded a threefold increase in cGMP in 10 min when applied to the photoreceptor surface of isolated retinas. However, when the vitreal surfaces of everted eye cups were suprafused with calcium-chelated medium for 10 min, a threefold increase in cGMP required 20 mM EGTA, 3 mM EGTA being ineffective. In another study, vitreal surfaces of eye cups or photoreceptor surfaces of retinas were suprafused with physiological saline containing 3 mM 3-isobutyl-1-methylxanthine (IBMX) in order to alter cAMP retinal levels. In both cases, eightfold increases in cAMP were measured in the inner nuclear and inner plexiform layers, while three- to fourfold increases occurred in the outer nuclear and outer plexiform layers. This agent also caused a large increase in cAMP in the pigment epithelium.     

Functional anatomy of the fiddler crab compound eye (Uca vomeris: Ocypodidae,Brachyura, Decapoda)

We describe the structural organization of the ommatidium in the compound eye of the fiddler crab, Uca vomeris, at both the light‐ and the electron‐microscopy levels. We pay particular attention to the organization of the optical system, the retinular cells, the rhabdom, and of pigment cells. Although the fiddler crab compound eye is of the apposition type, typical for Brachyuran crabs, we identify a number of novel, functionally relevant aspects of ommatidial organization that have not previously been described. The flat corneal facet lenses provide the main focusing power and therefore must contain a gradient of refractive index. Each ommatidium has the typical set of eight retinular cells, with a distal retinular cell R8 lying close to the proximal tip of the crystalline cone. R8 is shaped into four lobes, which are separated by proximal extensions of the four crystalline cone cells and of distal extensions of retinular cells R1–R7. The microvilli in the R8 rhabdom are not aligned in a uniform direction, while the microvilli of the main rhabdom show the typical crustacean pattern of alternating bands of horizontally (R3, R4, R7) and vertically aligned microvilli (R1, R2, R5, R6). We describe in detail the distribution and structural properties of screening pigment granules in the two types of pigment cells and in the retinular cells in the equatorial eye. We discuss the functional significance of this fine‐structural organization of the fiddler crab compound eye in relation to visual processing and visual ecology. J. Comp. Neurol. 522:1264–1283, 2014. © 2013 Wiley Periodicals, Inc.     

Ascending and descending projections from the superior olivary complex in guinea pigs: different cells project to the cochlear nucleus and the inferior colliculus

Protein kinase C (PKC) desensitizes the light response in photoreceptors from the ventral optic nerve of the horseshoe crab Limulus. Photoisomerization of Limulus rhodopsin leads to phosphoinositide hydrolysis, resulting in the production of inositol trisphosphate and diacylglycerol (DAG). Inositol trisphosphate mobilizes intracellular stores of Ca(2+), resulting in photoreceptor excitation in Limulus, while DAG may activate PKC. We investigated whether PKC-mediated desensitization of the photoresponse is accompanied by ultrastructural changes in the rhodopsin-bearing photosensitive membrane (rhabdom) in Limulus ventral photoreceptors. PKC activation by (-)-indolactam V in darkness induces disorganization and swelling of the rhodopsin-containing microvilli and endocytosis of rhabdomeral membrane. The effects of (-)-indolactam V on dark-adapted photoreceptor ultrastructure are reversible, are stereospecific, are blocked by coapplication of PKC inhibitors, and closely match those induced by continuous, bright light. Rhabdom disorganization and endocytosis via PKC activation may, therefore, contribute to desensitization of the light-adapted photoreceptor.     

Immunocytochemical localization of opsin, visual arrestin, myosin III, and calmodulin in Limulus lateral eye retinular cells and ventral photoreceptors

The photoreceptors of the horseshoe crab Limulus polyphemus are classical preparations for studies of the photoresponse and its modulation by circadian clocks. An extensive literature details their physiology and ultrastructure, but relatively little is known about their biochemical organization largely because of a lack of antibodies specific for Limulus photoreceptor proteins. We developed antibodies directed against Limulus opsin, visual arrestin, and myosin III, and we have used them to examine the distributions of these proteins in the Limulus visual system. We also used a commercial antibody to examine the distribution of calmodulin in Limulus photoreceptors. Fixed frozen sections of lateral eye were examined with conventional fluorescence microscopy; ventral photoreceptors were studied with confocal microscopy. Opsin, visual arrestin, myosin III, and calmodulin are all concentrated at the photosensitive rhabdomeral membrane, which is consistent with their participation in the photoresponse. Opsin and visual arrestin, but not myosin III or calmodulin, are also concentrated in extra-rhabdomeral vesicles thought to contain internalized rhabdomeral membrane. In addition, visual arrestin and myosin III were found widely distributed in the cytosol of photoreceptors, suggesting that they have functions in addition to their roles in phototransduction. Our results both clarify and raise new questions about the functions of opsin, visual arrestin, myosin III, and calmodulin in photoreceptors and set the stage for future studies of the impact of light and clock signals on the structure and function of photoreceptors.