Topography of different photoreceptor cell types in the larval retina of Atlantic halibut (Hippoglossus hippoglossus).
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The identities of single cone cells in the retina of Atlantic halibut (Hippoglossus hippoglossus) larvae were studied by in situ hybridisation using RNA probes for the five different halibut opsins. Four different cone opsins (ultraviolet-, blue-, green- and red-sensitive) are expressed in Atlantic halibut at the end of the yolk-sac period, whereas rod opsin is expressed later in development. Photoreceptor cells expressing ultraviolet-sensitive opsin are found only in the ventral retina, presumably to optimise detection of the downwelling ultraviolet light. The majority of the photoreceptors (approximately 90%) in the retina express green-sensitive opsin and its distribution shows no regional differences. In contrast, blue- and red-sensitive opsins are expressed much less frequently (in approximately 10% of photoreceptors), although these two opsins are also found over the entire retina. The expression patterns of the different visual pigments indicate some form of mosaic expression in the single-coned larval retina, and this is reminiscent of the square mosaic expression found in post-metamorphic Atlantic halibut. These findings suggest plasticity in green-opsin-expressing cells during development, resulting in a square mosaic expression pattern. (+info)
Exchange diffusion effect and euryhalinity in teleosts.
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The sea water (SW)-adapted euryhaline Platichthys flesus, and the marine Serranus exchange about 50% of their internal sodium with the external sodium per hour. This rate of exchange decreases with decreasing salinity of the adaptation medium. When the flounder is transferred from SW to FW an instantaneous 90% reduction of the Na and Cl outflux is observed. About 30 min later a second, progressive, reduction occurs. The outflux reductions appear to result from two types of regulatory mechanisms reducing gill permeability and preventing excessive salt loss. The first reduction corresponds to independent "Na- and Cl-free effects" as shown by transfers to artificial media containing either Na or Cl with an impermeant co-ion. The pattern of simultaneous rapid variations of Na influx and outflux for a range of salinity changes in flounder adapted to SW, 1/2 SW, or 1/4 SW has been studied. The data are compatible with the hypothesis of an exchange diffusion mechanism characterized by a coupling of both unidirectional fluxes. The affinity of the exchange diffusion carrier for sodium has been measured (Km approximately equal to 400 mM). The delayed reduction would result from a progressive diminution of the quantity of carrier available but without modification of its affinity for sodium. When the stenohaline marine perch is transferred from SW to FW, a 40% reduction of the outflux is observed. But it is not the result of an exchange diffusion effect as it is related to the external osmolarity change and not to the NaCl concentration change. Furthermore no delayed reduction is observed after transfer into FW. This transfer is accompanied by a heavy loss of electrolytes resulting in a rapid decline of the plasma electrolyte level and death. A comparative survey of the relative importance of these regulatory mechanisms has been made. (+info)
Bursting properties of caudal neurosecretory cells in the flounder Platichthys flesus, in vitro.
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Bursting activity in type 1 Dahlgren cells was studied using intra- and extracellular recording from an in vitro preparation of the caudal neurosecretory system of the euryhaline flounder. 45% of cells showed spontaneous bursts of approximately 120s duration and 380s cycle period. Similar bursts were triggered by short duration (<5s) depolarising or hyperpolarising pulses. Cells displayed a characteristic depolarising after potential, following either an action potential with associated afterhyperpolarisation, or a hyperpolarising current pulse. This depolarising after potential was related to a 'sag' potential, which developed during the hyperpolarising pulse. Both the depolarising after potential and the sag potential occurred only in cells at more depolarised (<60 mV) holding potentials. In addition, the amplitude of the depolarising after potential was dependent on the amplitude and the duration of the hyperpolarising pulse. The depolarising after potential following action potentials may provide a mechanism for facilitating repetitive firing during a burst. Extracellular recording revealed similar bursting in individual units which was not, however, synchronised between units. Spontaneous bursting activity recorded both intra- and extracellularly was inhibited by application of a known neuromodulator of the system, 5-hydroxytryptamine. This study provides a basis for investigating the relationship between physiological status, Dahlgren cell activity and neuropeptide secretion. (+info)
Vestibuloocular reflex of the adult flatfish. III. A species-specific reciprocal pattern of excitation and inhibition.
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In juvenile flatfish the vestibuloocular reflex (VOR) circuitry that underlies compensatory eye movements adapts to a 90 degrees relative displacement of vestibular and oculomotor reference frames during metamorphosis. VOR pathways are rearranged to allow horizontal canal-activated second-order vestibular neurons in adult flatfish to control extraocular motoneurons innervating vertical eye muscles. This study describes the anatomy and physiology of identified flatfish-specific excitatory and inhibitory vestibular pathways. In antidromically identified oculomotor and trochlear motoneurons, excitatory postsynaptic potentials (EPSPs) were elicited after electrical stimulation of the horizontal canal nerve expected to provide excitatory input. Electrotonic depolarizations (0.8-0.9 ms) preceded small amplitude (<0.5 mV) chemical EPSPs at 1.2-1.6 ms with much larger EPSPs (>1 mV) recorded around 2.5 ms. Stimulation of the opposite horizontal canal nerve produced inhibitory postsynaptic potentials (IPSPs) at a disynaptic latency of 1.6-1.8 ms that were depolarizing at membrane resting potentials around -60 mV. Injection of chloride ions increased IPSP amplitude, and current-clamp analysis showed the IPSP equilibrium potential to be near the membrane resting potential. Repeated electrical stimulation of either the excitatory or inhibitory horizontal canal vestibular nerve greatly increased the amplitude of the respective synaptic responses. These observations suggest that the large terminal arborizations of each VOR neuron imposes an electrotonic load requiring multiple action potentials to maximize synaptic efficacy. GABA antibodies labeled axons in the medial longitudinal fasciculus (MLF) some of which were hypothesized to originate from horizontal canal-activated inhibitory vestibular neurons. GABAergic terminal arborizations were distributed largely on the somata and proximal dendrites of oculomotor and trochlear motoneurons. These findings suggest that the species-specific horizontal canal inhibitory pathway exhibits similar electrophysiological and synaptic transmitter profiles as the anterior and posterior canal inhibitory projections to oculomotor and trochlear motoneurons. Electron microscopy showed axosomatic and axodendritic synaptic endings containing spheroidal synaptic vesicles to establish chemical excitatory synaptic contacts characterized by asymmetrical pre/postsynaptic membrane specializations as well as gap junctional contacts consistent with electrotonic coupling. Another type of axosomatic synaptic ending contained pleiomorphic synaptic vesicles forming chemical, presumed inhibitory, synaptic contacts on motoneurons that never included gap junctions. Altogether these data provide electrophysiological, immunohistochemical, and ultrastructural evidence for reciprocal excitatory/inhibitory organization of the novel vestibulooculomotor projections in adult flatfish. The appearance of unique second-order vestibular neurons linking the horizontal canal to vertical oculomotor neurons suggests that reciprocal excitation and inhibition are a fundamental, developmentally linked trait of compensatory eye movement circuits in vertebrates. (+info)
Cationic amino acids involved in dicarboxylate binding of the flounder renal organic anion transporter.
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Three conserved cationic amino acids in predicted transmembrane domains 1, 8, and 11, respectively, of the flounder renal organic anion transporter, fROAT, were changed by site-directed mutagenesis and the resulting mutants functionally characterized in Xenopus laevis oocytes. Uptake of p-aminohippurate (PAH) in oocytes that expressed mutant H34I, K394A, or R478D was markedly reduced compared with oocytes that expressed wild-type fROAT, but was still several-fold higher than that in water-injected control oocytes. Immunocytochemically, no decrease in cell surface expression of the mutants could be detected. Only mutant R478D appeared to have a lower PAH affinity than the wild type. Similar to wild-type-dependent PAH transport, uptake induced by mutant H34I was sensitive to glutarate (GA) cis-inhibition. In contrast, mutants K394A and R478D could not be significantly affected by up to 10 mM GA, although the cRNA-dependent PAH uptake could still be almost completely suppressed by probenecid. Moreover, again in contrast to the wild type, neither PAH influx nor PAH efflux mediated by these two mutants could be trans-stimulated by GA, nor did they induce GA transport. These data suggest that amino acids K394 and R478 in fROAT are required for dicarboxylate binding and PAH/dicarboxylate exchange. (+info)
Intestinal iron uptake in the European flounder (Platichthys flesus).
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Iron is an essential element because it is a key constituent of the metalloproteins involved in cellular respiration and oxygen transport. There is no known regulated excretory mechanism for iron, and homeostasis is tightly controlled via its uptake from the diet. This study assessed in vivo intestinal iron uptake and in vitro iron absorption in a marine teleost, the European flounder Platichthys flesus. Ferric iron, in the form (59)FeCl(3), was reduced to Fe(2+) by ascorbate, and the bioavailability of Fe(3+) and Fe(2+) were compared. In vivo Fe(2+) uptake was significantly greater than Fe(3+) uptake and was reduced by the iron chelator desferrioxamine. Fe(2+) was also more bioavailable than Fe(3+) in in vitro studies that assessed the temporal pattern and concentration-dependency of iron absorption. The posterior region, when compared with the anterior and mid regions of the intestine, was the preferential site for Fe(2+) uptake in vivo. In vitro iron absorption was upregulated in the posterior intestine in response to prior haemoglobin depletion of the fish, and the transport showed a Q(10) value of 1.94. Iron absorption in the other segments of the intestine did not correlate with haematocrit, and Q(10) values were lower. Manipulation of the luminal pH had no effect on in vitro iron absorption. The present study demonstrates that a marine teleost absorbs Fe(2+) preferentially in the posterior intestine. This occurs in spite of extremely high luminal bicarbonate concentrations recorded in vivo, which may be expected to reduce the bioavailability of divalent cations as a result of the precipitation as carbonates (e.g. FeCO(3)). (+info)
An outbreak of VHSV (viral hemorrhagic septicemia virus) infection in farmed Japanese flounder Paralichthys olivaceus in Japan.
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A rhabdoviral disease occurred in farmed populations of market sized Japanese flounder (hirame) Paralichthys olivaceus in the Seto Inland Sea of Japan in 1996. The causative agent was identified as viral hemorrhagic septicemia virus (VHSV) based on morphological, immunological, and genetic analyses. Diseased fish that were artificially injected with a representative virus isolate showed the same pathological signs and high mortality as observed in the natural outbreak. This is the first report of an outbreak of VHSV infection in cultured fish in Japan. Clinical signs of diseased fish included dark body coloration, an expanded abdomen due to ascites, congested liver, splenomegaly, and a swollen kidney. Myocardial necrosis was most prominent and accompanied by inflammatory reactions. Necrotic lesions also occurred in the liver, spleen and hematopoietic tissue, and were accompanied by circulatory disturbances due to cardiac failure. Hemorrhagic lesions did not always appear in the lateral musculature. Transmission electron microscopy revealed many rhabdovirus particles and associated inclusion bodies containing nucleocapsids in the necrotized myocardium. The histopathological findings indicated that the necrotizing myocarditis could be considered a pathognomonic sign of VHSV infection in Japanese flounder. (+info)
Morphology and biology of parasite responsible for scuticociliatosis of cultured olive flounder Paralichthys olivaceus.
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Based on the arrangement and shape of the buccal structure, scuticociliates isolated from cultured olive flounder Paralichthys olivaceus belonged to the family Uronematidae and showed many characteristics of Uronema marinum. There was variation in the morphometry of clinical isolates taken from different organs of infected flounder. However, the isolates did not show any significant difference in morphometry under cultured conditions. The ciliates were easily maintained in in vitro medium to which antibiotic agents had been added and which had been enriched with the raw brain tissue of a healthy olive flounder. The ciliates propagated in a wide range of both temperature (6 to 30 degrees C) and salinity (10 to 35 ppt). (+info)