Photoresponses of the compound eye of the sandhopper Talitrus saltator (Crustacea, Amphipoda) in the ultraviolet-blue range.

The semi-terrestrial sandhopper Talitrus saltator uses celestial visual cues to orient along the sea-land axis of the beach. Previous spectral-filtering experiments suggested that it perceives directional information from wavelengths in the ultraviolet (UV)-blue range. Binary choice experiments between dark and UV (380-nm) light carried out on dark-adapted individuals of T. saltator showed photopositive movement to UV. Morphologically, each ommatidium in the eye consists of five retinula cells, four large and one small. In electroretinogram experiments, sensitivity of the dark-adapted eye is dominated by a receptor maximally sensitive at about 390-450 nm and secondarily sensitive at about 500-550 nm. Selective light-adaptation experiments at 580 nm showed the apparent sensitivity decreasing at around the secondary sensitive range, thus disclosing the existence of UV-blue photoreceptor cells. Here the existence of UV-blue detection is confirmed, and evidence is provided that green and UV-blue visual pigments are located in the large and small retinula cells, respectively.

Seasonal variation of chromophore composition in the eye of the Japanese dace, Tribolodon hakonensis.

The relationship between seasonal variation and the effect of several different environmental factors on chromophore composition was investigated in the eye of the Japanese dace, Tribolodon hakonensis which lives either in rivers or in the sea. Eyes obtained from river and sea populations had both retinal (A1) and 3,4-didehydroretinal (A2) all through the year but the ratio of these chromophores showed seasonal variation the relative amount of A2 was higher in winter and lower in summer. Besides seasonal variation, A2 showed marked differences depending on habitat: the highest proportion of A2 was 67% in January and the lowest 13% in July, in the river population, whereas in the sea population the highest and the lowest values were only 30 and 6%, respectively, during the same months. The seasonal variation in gonadosomatic index showed no correlation to variations in A2 proportion, and the maximum difference in water temperature between summer and winter was ca. 15 degrees C for both habitats. Because spectral conditions at the locations of capture of both river and sea populations were similar, we conclude that Japanese dace eyes are affected by exogenous factors related to differences between freshwater and seawater environments.

Microanatomical characteristics of marginal ommatidia in three different size-classes of the semi-terrestrial isopod Ligia exotica (Crustacea / Isopoda)

The aims of this paper have been (a) to characterize marginal ommatidia from different eye regions through a detailed description of their distinct ultrastructural features in three different size-classes of L. exotica, and (b) to compare microanatomical characteristics of the marginal ommatidia with those of ommatidia of the same eye, but located further centrally. On the basis of transverse as well as longitudinal sections we conclude that new ommatidia are added from a crescentic dorso-anterio-ventral edge of the eye and that maturing ommatidia go through a sequence in which originally the nuclei of cone-, pigment-, and retinula cells are arranged in three separate layers. At the beginning of the microvillar development, the organization of the corresponding rhabdomeres is still quite different (much less regular) from that of those rhabdomeres that make up the mature rhabdom. Marginal ommatidia always possess smaller diameters than more centrally located ones and retinal screening pigment granules are most apparent in the retinula cells only after the first microvilli have appeared. The diameters of rhabdom microvilli (approx. 55 nm) do not differ in ommatidia from the five investigated eye regions in small specimens (< 1.5 cm body length), but show a tendency to be slightly wider in the anterior (= frontal or rostral) regions of the eye (approx. 65 nm) in larger specimens (> 2.0 cm body length).

Microanatomical characteristics of marginal ommatidia in three different size-classes of the semi-terrestrial isopod Ligia exotica (Crustacea / Isopoda)

The aims of this paper have been (a) to characterize marginal ommatidia from different eye regions through a detailed description of their distinct ultrastructural features in three different size-classes of L. exotica, and (b) to compare microanatomical characteristics of the marginal ommatidia with those of ommatidia of the same eye, but located further centrally. On the basis of transverse as well as longitudinal sections we conclude that new ommatidia are added from a crescentic dorso-anterio-ventral edge of the eye and that maturing ommatidia go through a sequence in which originally the nuclei of cone-, pigment-, and retinula cells are arranged in three separate layers. At the beginning of the microvillar development, the organization of the corresponding rhabdomeres is still quite different (much less regular) from that of those rhabdomeres that make up the mature rhabdom. Marginal ommatidia always possess smaller diameters than more centrally located ones and retinal screening pigment granules are most apparent in the retinula cells only after the first microvilli have appeared. The diameters of rhabdom microvilli (approx. 55 nm) do not differ in ommatidia from the five investigated eye regions in small specimens (< 1.5 cm body length), but show a tendency to be slightly wider in the anterior (= frontal or rostral) regions of the eye (approx. 65 nm) in larger specimens (> 2.0 cm body length). (AU)

The process of cell adhesion among dissociated single cells of Hydra: morphological observations.

Ultrastructural observations were made on the initial adhesion process at the adherent region of Hydra endodermal cell pairs brought into contact (following dissociation) using a three-dimensional laser manipulator. Total contact length across the diameter of the adherent region decreased during the period 10-60 min after initial adhesion. However, the mean numbers of closest (<4 nm) and medium (5-25 nm) separation distances between membranes (thought to be important in total cell adhesion) were not significantly different. These data indicate that adherent cell pairs maintain a constant adhesiveness during the first 60 min of the adhesion process, despite membrane rearrangements. The relative length of each separation distance in adherent cell pairs approached that reported previously for intact Hydra. The sums of lengths in both the closest and medium categories (as a proportion of total contact length) increased because the length of cleavages (distances >25 nm) decreased significantly during the same time period. These results suggest that adherent cell pairs undergo rapid, active membrane changes in the adherent region, which might be associated with cell sorting. The possible significance of these changes for active rearrangement are discussed.

Diurnal changes in retinula cell sensitivities and receptive fields (two-dimensional angular sensitivity functions) in the apposition eyes of Ligia exotica (Crustacea, Isopoda).

The structural organization of the retinula cells in the eye of Ligia exotica changes diurnally. At night, the microvilli elongate, losing the regular and parallel alignment characteristic of the day condition. Crystalline cones and distal rhabdom tips are not pushed into each other during the day, but at night the rhabdoms protrude into the crystalline cones by up to 5 microm. Screening pigment granules in the retinula cells disperse during the night, but migrate radially towards the vicinity of the rhabdom during the day. No such displacements of the pigment granules of either distal or proximal screening pigment cells were observed. The sensitivity of the eye, monitored by electroretinogram (ERG) recordings, changes diurnally: values at midnight are, on average, 10 times those occurring during the day. However, intracellular recordings from single retinula cells (50 during the day and 50 at night) indicate that the difference between night and day sensitivities is only 2.5-fold. Two-dimensional angular sensitivity curves, indicative of a single unit's spatial sensitivity, had considerably less regular outlines at night than during the day. If based on the 50 % sensitivity level, day and night eyes possessed receptive fields of almost identical width (approximately 2 degrees), but if sensitivities below the 50 % limit were included, then receptive fields at night were significantly more extensive. We suggest that the morphological adaptations and diurnal changes in chromophore content seen in the apposition eye of L. exotica allow this animal to improve its photon capture at night while preserving at least some of the spatial resolving power characteristic of the light-adapted state. This would explain why this animal is capable of performing complex escape behaviours in the presence of predators both in bright and in very dim light.

The brain as a photoreceptor: intracerebral ocelli in the firefly.

This paper deals with the structure and function of the intracerebral ocelli in the caudal area of the brain of the Japanese firefly. A pair of epilaterally placed specialized pigmented organs was found at the caudal ends of the brains of the fireflies Luciola cruciata and L. lateralis. On the basis of light and transmission electron micrographs of both male and female individuals these organs seemed photoreceptive in nature. Intracellular and extracellular recordings were obtained from the intracerebral ocelli of the fireflies with microelectrodes. The physiological evidence revealed that the cells found in the brain were, indeed, photoreceptors.

Initial homotypic cell pair adhesion in regenerating Hydra facilitates subsequent adhesion of homotypic cells.

In Hydra vulgaris at the level of dissociated single cells endodermal cells adhere to each other more readily than to ectodermal cells at the initial adhesion. The time required for adhesion to occur between two adjacent cells is shorter for both endodermal and ectodermal homotypic cell adhesions once the initial adhesion of the first pair of cells has been established. It is confirmed that contact of an aggregated pair with additional homotypic cells facilitates the occurrence of homotypic adhesions; heterotypic adhesions are discouraged. This suggests that adhesion of homotypic cells contributes to an increased readiness for subsequent homotypic cells to adhere.

Ultrastructural observations of adherent cell pairs in hydra vulgaris

Previous morphological studies of cell sorting in Hydra vulgaris have considered only clusters of cells. Here, we present ultrastructural observations on the adherent region of cell pairs brought into contact (following dissociation) using a three-dimensional laser manipulator. There was a much larger area of close membrane contact between endodermal cell pairs in comparison with ectodermal cell pairs. Separation distances between membranes were categorized into three classes: closest distance (<4 nm); medium distance (5-25 nm); and cleavage (>25 nm). The sum of distances in the closest and medium categories as a proportion of total contact length was significantly greater (P<0.01) for endodermal cells (49.0+/-6.5 %) than for ectodermal cells (26.7+/-4. 4 %). In intact Hydra, this sum of distances was also significantly greater for endodermal cells, indicating that newly adherent cells, even after adhesion for only 10 min, display similar morphological characteristics to cells in intact Hydra. This suggests that close membrane contacts contribute to differential cell adhesion, which may form the basis of the cell sorting process.

Light-regulated localization of the beta-subunit of Gq-type G-protein in the crayfish photoreceptors.

In crayfish photoreceptor cells, Gq-type G-protein plays a central role in the phototransduction pathway, and the translocation of Gq alpha has been proposed as one of the molecular mechanisms to control photoreceptor sensitivity. We here investigated beta subunit of Gq and its localization profiles under various light conditions in the crayfish photoreceptor cells to understand the functional characteristic of visual Gq in the phototransduction pathway. An immunoprecipitation experiment was performed using an anti-Gq alpha antibody and a thiol-cleavable crosslinker. A 39 kDa protein was co-immunoprecipitated with Gq alpha, but not by irradiation, in the presence of GTP gamma S. The partial amino acid sequence of the 39 kDa protein was similar to G beta e in Drosophila photoreceptors, indicating that the crayfish G beta which combines with Gq alpha is a G beta e homologue. Immunohistochemical and immunoblot analyses revealed that the amount of the G beta decreased in the rhabdomeric membranes and increased in the cytoplasm in the light, compared with that in the dark. The profile of the translocation was similar to that reported for Gq alpha. Since both alpha and beta gamma subunits are necessary for G-proteins to be activated by rhodopsin in the rhabdom, the light-modulated translocation of a G beta e homologue possibly controls the amount of Gq which can be activated by light-stimulated rhodopsin.

The effect of background luminance on visual responses to strong flashes: perceived brightness and the early rise of photoreceptor responses.

The threshold intensity for large-long incremental stimuli rises proportionally to adapting background luminance IB (Weber adaptation), but the intensity required to evoke a criterion high-brightness sensation rises much less steeply. We propose that this difference originates in the very first stage of visual processing, in the phototransduction and adaptation properties of the retinal photoreceptor cells. A physiological model previously found to account for visual latency and brightness as functions of stimulus intensity in the dark-adapted state [Donner, K. (1989). Visual Neuroscience, 3, 39-51] is extended to cover different states of adaptation. It is assumed that the neural coding of high intensities is based on the rate of rise (quasi-derivative) of the photoreceptor response just after it reaches a small threshold amplitude. The shallow background adaptation functions for high-brightness criteria emerge as a consequence of the relative constancy of the leading edge of large responses under backgrounds, a phenomenon that can be formally described by compensating changes in photoreceptor sensitivity and time scale. We first test the model on supra-threshold responses in the frog retina, where the discharge rate of ganglion cells (a possible neural code for brightness) and the primary rod hyperpolarizations can be recorded under identical conditions. The two are related as predicted over at least 3 log units of background intensity. We then show that published data on the background adaptation of human foveal high brightness judgments conform to the same model, assuming that human cones accelerate as IB-b with b = 0.14-0.15.

Light-modulated subcellular localization of the alpha-subunit of GTP-binding protein Gq in crayfish photoreceptors.

Gq-type GTP-binding protein (Gq) plays an important role in invertebrate visual phototransduction. The subcellular localization of the alpha subunit of visual Gq in crayfish photoreceptor was investigated immunocytochemically and biochemically to demonstrate the details of the rhodopsin-Gq interaction. The localization of Gq(alpha) changed depending on the light condition. In the dark, Gq(alpha) was localized in the whole rhabdoms as the membrane-bound form. In the light, half of the Gq(alpha) was localized in the cytoplasm as the soluble form. The translocation of Gq(alpha) was reversible. The light-modulated translocation possibly controls the amount of Gq that can be activated by rhodopsin. In vitro hydroxylamine treatment of rhabdomeric membranes suggested that the translocation was regulated by the fatty-acid modification of Gq(alpha).

Light-induced binding of proteins to rhabdomeric membranes in the retina of crayfish (Procambarus clarkii).

Light-induced protein interaction as part of the process of visual transduction in arthropods with rhabdomeric photoreceptors was investigated biochemically by using crayfish retina. Two kinds of retinal buffer soluble proteins (one of 40 kDa and the other of 46 kDa) were found to bind to the irradiated rhabdomeric membranes both in vitro and in vivo. The proteins bound to the membranes in the presence of metarhodopsin. An antibody against mouse arrestin (S-antigen) cross-reacted with the 40 kDa protein. These results suggest that the binding of the proteins to the membranes is caused by the formation of metarhodopsin, and that the 40 kDa protein has a similar structure to arrestin.

Interaction of GTP-binding protein Gq with photoactivated rhodopsin in the photoreceptor membranes of crayfish.

Interaction of G-protein with photoactivated rhodopsin (Rh*) in crayfish photoreceptor membranes was investigated by immunoprecipitation using an antibody against rhodopsin. Two kinds of protein were co-precipitated with rhodopsin. One is an alpha subunit of class-q G-protein (42 kDa, CGq alpha) which showed light-induced, dose-dependent binding to rhodopsin, and the other is an actin-like protein (44 kDa) with light-independent binding. Most of the CGq alpha was available for binding to Rh* but was dissociated from Rh* in the presence of GTP gamma S. These findings demonstrate that, in the crayfish photoreceptor, a Gq class of G-protein is activated by Rh*.

Primary structure of crayfish visual pigment deduced from cDNA.

The primary structure of opsin of the crayfish Procambarus clarkii has been deduced from the cDNA sequence. The opsin is composed of 376 amino acid residues including all the conservative residues characteristic of other members of the rhodopsin family. Comparison of sequences of all known opsins reveals that the major Drosophila rhodopsin is more similar to the crayfish rhodopsin than to the Drosophila UV-sensitive pigments. The phylogenetic trees of invertebrate opsins are constructed.