Amacrine cells in the retina of a cyprinid fish: functional characterization and intracellular labelling with horseradish peroxidase

Summary Forty amacrine cells in retinae of a cyprinid fish, the roach, were intracellularly labelled with horseradish peroxidase following electrophysiological identification as sustained depolarizing, sustained hyperpolarizing or transient units. Labelled cells were analysed by light microscopy and compared with a catalogue of amacrine cells established in a previous Golgi study on the same species. About 30% of the cell types characterized by the Golgi method were encountered in the present study. When intracellularly labelled cells were differentiated on the basis of their dendritic organization in the plane of the retina, a given electrophysiological response pattern was found to be generated by different morphological types, and vice versa. However, examination of the ramification patterns of the dendrites within the inner plexiform layer (i.e. in the radial dimension of the retina), showed that this morphological parameter of a given amacrine cell could be correlated with its light-evoked response. Several amacrine cell types were found to possess special distal dendrites which arose from the main dendritic branches and extended well over a mm in the retina. Distal dendrites were oriented tangentially with respect to the optic nerve papilla, but did not appear to be involved in any synaptic connectivity. It is concluded that the Golgi-based classification is a valuable tool for identifying intracellularly labelled amacrine cells. However, although the correlation between layering of dendrites in the inner plexiform layer and electrophysiology was generally good, additional physiological parameters would be required to determine whether more extensive parallels exist between structural and functional characteristics of amacrine cells. Alternatively, the considerable morphological diversity of amacrine cells may be of limited physiological significance.A preliminary account of the present findings was presented to the Physiological Society (Djamgoz et al. 1984)     

Interconnections between the stereovilli of the fish inner ear

Summary The maculae utriculi and sacculi of the inner ear from the European roach (Rutilus rutilus) were investigated by transmission electron microscopy. The stereovilli of peripherally and centrally located sensory cells differ in several features that suggest a developmental gradient. The stereovilli of the peripheral sensory cells, shown to be differentiating cells by other research groups, are short and less steeply graded in height than in central hair cells. All stereovilli in both kinds of hair bundles are interconnected. In the central bundles of stereovilli basal, tip, and vertical connectors are separated by unconnected regions. In contrast, filaments and sometimes other additional structures connect the stereovilli of peripheral bundles over their entire length, but vertical connectors are usually absent. Osmiophilic material occurring inside peripheral stereovilli is interpreted to be monomeric actin. Central and peripheral hair bundles also differ in their reaction to ruthenium red and cationized ferritin. Only the stereovilli of the central cells can be fused by these polycations. Ruthenium red also discriminates between supporting and sensory cells indicating differences in amount or distribution of extracellular material. Hair bundles, intermediate in properties and position between central and peripheral sensory cells, were also found, so that it became possible to propose a scheme of developmental steps leading from microvilli or microvillus-like stereovilli to the fully differentiated hair bundle.     

Modulation of connexon densities in gap junctions of horizontal cell perikarya and axon terminals in fish retina: effects of light/dark cycles,interruption of the optic nerve and application of dopamine

Summary In the fish retina, connexon densities of gap junctions in the outer horizontal cells are modulated in response to different light or dark adaptation times and wavelengths. We have examined whether the connexon density is a suitable parameter of gap junction coupling under in situ conditions. Short-term light adaptation evoked low connexon densities, regardless of whether white or red light was used. Short-term dark adaptation evoked high connexon densities; this was more pronounced in the axon terminal than in perikaryal gap junctions. Under a 12 h red light/12 h dark cycle, a significant difference in connexon densities between the light and the dark period could be established in the gap junctions of the perikarya and axon terminals. Under a white light/dark cycle, only the gap junctions of axon terminals showed a significant difference. Crushing of the optic nerve resulted in an increase in connexon densities; this was more pronounced in axon terminals than in perikarya. Dopamine injected into the right eye of white-light-adapted animals had no effect. However, dopamine prevented the effect of optic-nerve crushing on connexon density. The reaction of axon-terminal gap junctions to different conditions thus resembles that of perikaryal gap junctions, but is more intense. Axon terminals are therefore thought to play an important role in the adaptation process.     

Surface charges of the membrane and cell adhesion substances determine the structural integrity of hair bundles from the inner ear of fish

Summary The hairs (stereocilia = stereovilli) of sensory cells from the inner ear of vertebrates are interconnected by several types of connectors, whose role is unknown. They appear to stabilize the hair bundle mechanically, and may be directly involved in mechano-electric transduction. Our transmission electron-microscopical investigation of sensory epithelia from two species of fish (Rutilus rutilus, Scardinius erythrophthalmus, both Leuciscidae) has shown that not only the connectors but also the surface charges of the membrane are important factors for determining the shape of the hair bundle and the spatial interrelation of the stereovilli. A reduction of the ionic strength in the medium leads to an increase in distance between the stereovilli. This may be the result of an extension of the spread of the surface potential of the membrane at low ionic strength. The connectors are not broken by the increase in distance between the stereovilli. They are EDTA (ethylene-diamine-tetra-acetic-acid) resistant as are some cell adhesion molecules such as N-CAM (nerve-cell adhesion molecule) and protein A from Dictyostelium discoideum. The connectors do not prevent polycation-induced fusion of adjacent stereovillar membranes.     

Light-dependent dynamics of gap junctions between horizontal cells in the retina of the crucian carp

Summary The dynamics of gap junctions between outer horizontal cells or their axon terminals in the retina of the crucian carp were investigated during light and dark adaptation by use of ultrathin-section and freeze-fracture electron microscopy. Light adaptation was induced by red light, while dark adaptation took place under ambient dark conditions. The two principal findings were: (1) The density of connexons within an observed gap junction is high in dark-adapted retina, and low in light-adapted retina. This, respectively, may reflect the coupled and uncoupled state of the gap junction. (2) The size of individual gap junctions is larger in light-than in dark-adapted retinae. Whereas the overall area occupied by gap junctions is reduced with dark adaptation, the percentage of small and very small gap junctions increases dramatically. A lateral shift of connexons in the gap junctional membrane is strongly suggested by these reversible processes of densification and dispersion. Two additional possibilities of gap junction modulation are discussed: (1) the de novo formation of very small gap junctions outside the large ones in the first few minutes of dark adaptation, and (2) the rearrangement of a portion of the very large gap junctions. The idea that the cytoskeleton is involved in such modulatory processes is corroborated by thin-section observations.Dedicated to Professor J. Peiffer on the occasion of his 65th birthday     

Distribution of calbindinlike immunoreactive structures in the optic tectum of normal and eye-enucleated cyprinid fish

Summary Using the ABC immunohistochemical method, we investigated the distribution of calbindinlike immunoreactive structures in the optic tectum of normal fish, Tinca tinca, and from normal and unilaterally eye-enucleated fish, Cyprinus carpio. In nonoperated individuals of both species the optic tectum contained numerous immunoreactive neurons with strongly positive somata located in the stratum periventriculare and a thick immunolabeled dendritic shaft ascending radially toward the stratum fibrosum et griseum superficiale. The retinorecipient layers contained many fibrous immunoreactive structures. Some varicose fibers, isolated or in small bundles, were localized to the stratum album centrale, especially in the dorsal tectal half. Unilateral eye removal produced the disappearance of the immunoreactive fibrous structures located in the retinorecipient layers of the tectum contralateral to the enucleation. The present work shows that calbindinlike immunoreactive substances are localized in specific neural circuits of the fish optic tectum and suggests that the calbindin-like immunoreactive fibers in the retinorecipient strata are of retinal origin.     

Indirect effect of planktivorous fish on the growth and reproduction of Daphnia galeata

Both the size of Daphnia galeata primiparae and the average egg volume were smaller in individuals reared from neonata in water inhabited for the preceding 24 hours by Y-O-Y roach when compared with control animals reared in the same water with the exclusion of fish. The smaller size of experimental primiparae resulted from smaller increments in some instars and from the earlier maturation of about two-thirds of those that matured in the 4th instar, while the rest of the experimental animals and all the controls matured in the 5th instar. There were larger clutches and smaller eggs in experimentals when compared with the similar sized controls, but the difference hasn't proved to be statistically significant.     

Gap junction morphology of retinal horizontal cells is sensitive to pH alterations in vitro

Summary Isolated goldfish retinae were incubated in NaHCO₃-reduced solutions, a treatment known to lower intracellular pH and to decrease gap-junction-mediated coupling between cells. The morphology of the gap junctions of horizontal cells examined by means of freezefracture replicas and ultrathin sections displays alterations after such treatment. The gap-junctional particles aggregate into dense clusters or crystalline arrays, whereas controls (pH 7.5) display a loose arrangement of particles. Incubation in NaHCO₃-reduced solution leads to the appearance, in ultrathin sections, of prominent, electron-dense material beneath the gap-junctional membranes. Both effects, the increasing density of particles and the appearance of electron-dense material, are reversible. The application of dopamine, which uncouples horizontal cells, and its antagonist haloperidol produce less clear-cut effects on particle density in vitro.     

Food and habitat partitioning among juveniles of three fish species in the pelagial of a mesotrophic lake

An analysis of the diet of 0⁺ perch, smelt and roach caught at night of the pelagial of a mesotrophic lake showed that their food was composed mainly of herbivorous and predatory cladocerans, copepods and Chaoborus larvae during summer, and of herbivorous cladocerans and copepods during October. An analysis of habitat use by juveniles revealed separation among the species: roach occurred in the upper, perch and smelt in the deeper water layers. Food and habitat were alternatively partitioned among the juveniles. In early summer the lowest food overlap between perch and smelt coincided with high habitat overlap. In October high food overlap between perch and smelt corresponded with low habitat overlap. Smelt and roach fry used common food resources throughout the season, but they were segregated in habitat.     

Possible competitive interactions between overwintering tufted duck (Aythya fuligula (L.)) and fish populations of Lough Neagh,Northern Ireland: evidence from diet studies

The overwintering population of tufted duck (Aythya fuligula) on Lough Neagh, Northern Ireland, has varied over the last three decades, with numbers declining in the early 1980s but then increasing to former levels in the late 1980s. Population fluctuations of recently introduced roach (Rutilus rutilus) mirrored these trends. The present study explores the possibility that competition for benthic food resources is responsible for these changes by examining the diets of tufted duck, roach and other major fish species of the lake. Diet overlaps were generally high due to the common consumption of chironomid larvae. The diet of tufted duck overlapped most with that of roach because these two species were the only significant consumers of molluscs. This evidence provides further support for a tufted duck — roach competition hypothesis.     

An interplexiform cell in the goldfish retina: light-evked response pattern and intracellular staining with horseradish peroxidase

Summary The light-evoked response pattern and morphology of one interplexiform cell were studied in the goldfish retina by intracellular recording and staining. The membrane potential of the cell spontaneously oscillated in the dark. In response to a brief light stimulus, the membrane potential initially gave a slow transient depolarization. During maintained light, the oscillations showed a tendency to be suppressed; the response of the cell to the offset of the stimulus was not so prominent. The perikaryon of the interplexiform cell was positioned at the proximal boundary of the inner nuclear layer. The cell had two broad layers of dendrites; one was diffuse in the inner plexiform layer, the other was more sparse in the outer plexiform layer. The morphological and electrophysiological characteristics of the cell are discussed in relation to dopaminergic interplexiform cells and the light-evoked release pattern of dopamine in the teleost retina.     

The connectivity of cones and cone horizontal cells in a mosaic-type teleost retina

Summary A total of 20 Golgi-impregnated cone horizontal cells of Nannacara anomala (Cichlidae) were studied in alternating semi- and ultrathin sections in order to examine their connections with the overlying square mosaic of equal double and central single cones. Cone horizontal cells exhibit three types of processes: (a) the long horizontal axon, (b) short horizontal dendrites with a terminal swelling, and (c) cone contacting processes ascending towards the outer plexiform layer. As seen in tangential sections, the latter processes are arranged in the form of two concentric circles including a central spot. The processes of the inner circle contact the eight double cone pedicles of one square unit: processes of the outer circle contact eight more double cone pedicles which are directly adjacent to the square unit. The central spot represents a process which contacts the central single cone. Processes of the inner circle most often terminate in a dichotomous branching which represents the lateral elements to one ribbon synapse, whereas in the outer circle only a single terminal swelling is observed. Because of the mosaic of the cones and the constancy of this pattern of connectivity a model can be constructed where the dendritic fields of the cone horizontal cells overlap to a considerable extent. From this model, it follows that each double cone pedicle is contacted by four different horizontal cells. The functional significance of these findings for color vision is discussed in the light of recent work with the microspectrophotometer characterizing the cone system of this species as bichromatic. The mosaiclike arrangement of the horizontal cell dendrites supports the conclusion that the parallels between the patterns of receptor and horizontal cells are no coincidence but play an important role in lateral inhibition and neural adaptation of the retina.A preliminary report of this study was given at the international symposium Neural principles in vision held at the University of Munich in September 1975Supported by grant Wa 348/1 of the Deutsche Forschungsgemeinschaft     

Localization of metabotropic glutamate receptors in the outer plexiform layer of the goldfish retina

We studied the localization of metabotropic glutamate receptors (mGluRs) in the goldfish outer plexiform layer by light-and electron-microscopical immunohistochemistry. The mGluR1α antibody labeled putative ON-type bipolar cell dendrites and horizontal cell processes in both rod spherules and cone triads. Immunolabeling for mGluR2/3 was absent in the rod synaptic complex but was found at horizontal cell dendrites directly opposing the cone synaptic ribbon. The mGluR5 antibody labeled Müller cell processes wrapping rod terminals and horizontal cell somata. The mGluR7 antibody labeled mainly horizontal cell dendrites invaginating rods and cones and some putative bipolar cell dendrites in the cone synaptic complex. The finding of abundant expression of various mGluRs in bipolar and horizontal cell dendrites suggests multiple sites of glutamatergic modulation in the outer retina. Financial support for this work was provided by Conselho Nacional de Pesquisa (CNPq), Brazil (grant 200915/98-3 to C.J.)     

Putative cholinergic elements in the photosensory pineal organ and retina of a teleost,Phoxinus phoxinus L. (Cyprinidae)

Summary Putative cholinergic neurons in the photosensory pineal organ of a cyprinid teleost, the European minnow, were studied by use of choline acetyltransferase (ChAT) immunocytochemistry and acetylcholinesterase (AChE) histochemistry. Pinealofugally projecting neurons were visualized using retrograde HRP-filling through their cut axons. For comparison, the distribution of choline acetyltransferase immunoreactivity (ChAT-IR) and AChE-positive elements in the retina was investigated.While the distributional patterns of ChAT-IR and strongly AChE-positive perikarya in the retina are similar and may represent the same neuronal population, ChAT-IR and AChE-positive elements in the pineal organ appear to belong to separate populations. In the retina, small- to medium-sized perikarya in the inner nuclear layer, and small perikarya in the ganglion cell layer are ChAT-IR and AChE positive. The entire inner plexiform layer is AChE positive, while only sublaminae 1, 2 and 4 are ChAT-IR. No indication of cholinergic activity was observed in the optic axon layer.In the pineal organ, ChAT-IR is restricted to small perikarya situated rostrally and dorsally in the pineal end-vesicle. AChE-positive neurons are present throughout the pineal end-vesicle and the pineal stalk. The pineal tract (the pinealofugally projecting axons of intrapineal neurons) is strongly AChE positive, but displays no ChAT-IR. The distribution of pinealofugally projecting neurons, labeled with retrogradely transported HRP, is markedly dissimilar to that of the ChAT-IR elements. It is proposed that the photosensory pineal organ transmits photic information to the brain via a non-cholinergic pathway. The possibility that the ChAT-IR neurons represent small local interneurons is discussed in the light of comparative physiological and anatomical findings.     

Injection of xenogeneic cells into teleost fish elicits systemic and local cellular innate immune responses

The early innate immune response of the teleost gilthead seabream (Sparus aurata L.) against xenogeneic cells was studied. Fish received a single intraperitoneal injection of xenogeneic cells (tumour cell line), following which leucocyte mobilization, degranulation, peroxidase content, respiratory burst and phagocytic and cytotoxic activities were determined in both peritoneal exudate leucocytes (PELs) and head-kidney leucocytes (HKLs). The total number of PELs increased from 4 h post-injection until the end of the experiment (3 days). Interestingly, flow cytometric analysis of PEL and HKL suspensions revealed variations in the proportion of cell types. The percentage of HK acidophilic granulocytes significantly increased after 72 h, whereas PE acidophils increased after 4 h. Moreover, numbers of PE lymphocytes and monocyte-macrophages significantly increased during the experiment. The peroxidase content of the leucocytes was unaffected, although PEL degranulation was largely enhanced. This liberation of peroxidases correlated well with the enhancement of the oxidative respiratory burst activity in PELs, reflecting leucocyte activation. However, phagocytosis only increased in PELs 4 h after intraperitoneal injection, whereas the cytotoxic activity of HKLs increased 1 and 2 days post-injection but, in general, decreased in the PELs. Our data thus demonstrate that the appearance of xenogeneic cells involves leucocyte mobilization and innate immune-response activation at the site of invasion and in the head-kidney. Involvement of the various leucocyte types and potential modes of activation are discussed.This work was partially funded by the European Commission (QLRT-2001-00722). A. Cuesta and I. Salinas are fellows of Fundación CajaMurcia and Fundación Séneca, respectively.     

Patterns of routine swimming and metabolic rate in juvenile cyyprinids at three temperatures: analysis with a respirometer-activity-monitoring system

Summary The spontaneous swimming activity and oxygen consumption of juvenile roach (Rutilus rutilus (L.)), were monitored for 48–72 h at 8, 15 and 20°C and a photoperiod (L:D) of 11:13. At low levels of activity metabolic cost is constant and thus corresponds to the standard rate of metabolism (low cost activity). At higher levels of activity metabolic cost increases in proportion with the degree of activity. The slope for this high cost activity indicates an expenditure of 0.4–1.3 mol O₂·g^-1·h^-1 per arbitrary activity unit at the three experimental temperatures. Extrapolation of this relationship to zero activity would underestimate the measured standard metabolic rate. High cost activity occurred in only one experiment out of three at 8°C, low cost activity hardly at all at 20°C, whereas at 15°C both forms of activity were present in five of the seven experiments conducted. Thus, not only the intensity but also the pattern of activity is affected by environmental temperature.     

Effects of salinity on chloride cells in the euryhaline cyprinodontid fish Rivulus marmoratus

Summary The ultrastructure and density of chloride cells in the gill, opercular epithelium, and opercular skin of the euryhaline self-fertilizing fish Rivulus marmoratus (Cyprinodontidae) were studied with electron and fluorescence microscopy. R. marmoratus raised from birth in 1, 50, 100, and 200% seawater were compared. Chloride cells from fish raised in each of the four salinities exhibited an invaginated pit structure at the apical crypt. Multicellular complexes were present in the 1% seawater group and in those fish raised in higher salinities where elaborate interdigitations were seen between cells. Chloride cells from gills of fish raised in 200% seawater had a significantly higher percentage of their cytoplasmic volume composed of mitochondria than did those from fish raised in 1% seawater (69.9% vs 37.4%). The opercular skin and opercular epithelium had the same density of chloride cells (4.2×10⁴-4.5×10⁴ chloride cells/cm²), and this number did not vary significantly with increased salinity. The opercular skin thus appears far more responsive to environmental salinity than the opercular epithelium. Chloride cells from the opercular epithelium of fish raised in 200% seawater were found to be 39% larger than those from fish raised in 1% seawater, whereas the chloride cells from the opercular skin of the 200% seawater group were 107% larger than those from the 1% seawater group.     

Pineal-retinal relationships: rhythmic biosynthesis and immunocytochemical localization of melatonin in the retina of the pike (Esox lucius)

Summary The levels of melatonin and the activities of two enzymes of the melatonin biosynthetic pathway, serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were measured throughout the light-dark cycle in the retina of a teleost fish, the pike. HIOMT activity did not display significant variations, whereas NAT activity and melatonin content showed a daily rhythm, high levels occurring during the night. The profiles of the latter two rhythms did not closely match one another and differed from those previously described in the pineal organ of the same species. These results are discussed with respect to a possible paracrine role of retinal melatonin. Melatonin-like immunoreactivity was found in the photoreceptor cell layer and in the Müller cells of the inner nuclear layer. The intensity of the melatonin-like immunoreactivity varied throughout the 24 h light-dark cycle, in good correlation with the variations in the melatonin level as measured by radioimmunoassay.This article is dedicated to the memory of Dr. Klaus Hoffmann     

Retino-fugal projections in congenitally monophthalmic fish and anurans

Summary Intra-ocular deposition of horseradish peroxidase was used to visualize optic tract projections in normal and congenitally monophthalmic catfish and Xenopus. In neither species was evidence for an increased ipsilateral visual component found in congenitally one-eyed specimens. This indicates that competition between axons from both eyes is not an important mechanism for fiber distribution in the chiasm during ontogeny. Furthermore, it suggests that enhanced ipsilateral components, previously noted in unilaterally enucleated fish and anurans, are caused by debris of degenerated axons.     

Effects of cortisol on gill chloride cell morphology and ionic uptake in the freshwater trout,Salmo gairdneri

Summary Daily intramuscular injection of cortisol (4 mg kg^–1 body weight) in rainbow trout,Salmo gairdneri, for 10 days caused significant increases in the number and individual apical surface area of gill chloride cells per mm² of filament epithelium. Concomitantly, whole body influxes of sodium (Na⁺) and chloride (Cl^–) increased. Acute (3 h) intra-arterial infusion of cortisol did not affect whole body Na⁺ or Cl^– influx. A significant correlation was observed between both Na⁺ and Cl^– influxes and the fractional apical surface area of filament chloride cells in control, sham (saline-injected) and experimental (cortisol-injected) fish. The chloride cells displayed similar ultrastructural modifications in trout undergoing cortisol treatment as in trout transferred to ion-deficient water. These findings suggest the existence of structure/function relationships in which branchial chloride cell morphology is an important determinant of Na⁺ and Cl^– transport capacity. We conclude that chronic cortisol treatment enhances whole body Na⁺ and Cl^– influxes by promoting proliferation of branchial chloride cells. The results of correlation analysis indicate that the chloride cell is an important site of NaCl uptake in freshwater rainbow trout.