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J E Dowling

Publications and source records attributed to J E Dowling.

At least 19 recordsLinked to original sources
PubMed1992

Retinoic acid-induced duplication of the zebrafish retina.

Exogenous treatment of zebrafish embryos with retinoic acid induces a duplication of the retinas during development. These effects occur only when retinoic acid is applied within a 2-hr period prior to and during the initial formation of the optic primordia, and they are concentration-dependent. Light microscopic examination reveals that the second retina derives from cells in the ventral region of the developing eyecup that normally become pigment epithelial cells. Two distinct ganglion cell fields are usually observed in eyes with duplicated retinas. Bundles of axons from each ganglion cell field join as they leave the eye and innervate the contralateral tectum.

Animals↗
PubMed1991

Nematosomes in external horizontal cells of white perch (Roccus americana) retina: changes during dark and light adaptation.

Cytoplasmic inclusions resembling nucleoli and termed nematosomes have been described in several areas of the nervous system but not in the retina. Here we report the presence of these structures in the external horizontal cells of white perch (Roccus americana). Nematosomes were larger and more numerous in dark-adapted retinas than in light-adapted ones. An inverse relation of horizontal cell spinules was found in these cells, suggesting that electron-dense material observed in spinules may originate from nematosomes.

Adaptation, Ocular↗
PubMed1991

Effects of light stimuli on the release of dopamine from interplexiform cells in the white perch retina.

Interplexiform cells are centrifugal neurons in the retina carrying information from the inner to the outer plexiform layers. In teleost fish, interplexiform cells appear to release dopamine in the outer plexiform layer after prolonged darkness that modulates the receptive-field size and light responsiveness of horizontal cells (Mangel & Dowling, 1985; Yang et al., 1988a, b). It has been proposed that interplexiform cells may also release dopamine upon steady illumination because horizontal cells' receptive fields shrink in the light (Shigematsu & Yamada, 1988). Here, we report the shrinkage of the receptive fields of horizontal cells seen in the presence of background illumination is not blocked by dopamine antagonists, indicating that dopamine does not underlie the receptive-field size changes observed during steady illumination. Flickering light, however, does appear to stimulate the release of dopamine from the interplexiform cells, resulting in a marked reduction of horizontal cell receptive-field size. Taken together, experiments on horizontal cells indicate that dopamine is released from interplexiform cells in the teleost retina after prolonged darkness and during flickering light, but that dopamine release from interplexiform cells during steady retinal illumination is minimal.

Animals↗
PubMed1991

Dopamine release from interplexiform cells in the retina: effects of GnRH, FMRFamide, bicuculline, and enkephalin on horizontal cell activity.

In teleost fish, dopaminergic interplexiform cells provide an intraretinal centrifugal pathway from the inner to the outer plexiform layer, where they make abundant synapses on cone-related horizontal cells. The interplexiform cells receive all their input in the inner plexiform layer from centrifugal fibers and amacrine cells. In fish, centrifugal fibers contain gonadotropin hormone-releasing hormone (GnRH)-like and FMRFamide-like peptides (Munz et al., 1982; Stell et al., 1984), whereas amacrine cells contain a variety of neuroactive substances, including a number of peptides. In this study, we examined the effects of GnRH, FMRFamide, bicuculline, and enkephalin on horizontal cell activity in the white perch retina in an attempt to understand the synaptic inputs to the interplexiform cells. When the retina was superfused with Ringer's solution containing GnRH, horizontal cells depolarized (approximately 10 mV), and their responses to small spots increased, whereas their responses to full-field lights decreased. Thus, GnRH closely mimicked the effects of dopamine on horizontal cells. The GnRH antagonist [D-Phe2, Pro3, D-Phe6]-GnRH blocked the effects of GnRH, as did haloperidol. GnRH also had no effect on horizontal cells in retinas treated with 6-hydroxydopamine. The results indicate that GnRH acts by stimulating the release of dopamine from interplexiform cells. FMRFamide alone produced no changes on either the membrane potential or light responses of horizontal cells, but it did suppress the effects of GnRH on horizontal cells in some experiments. FRMFamide also reversed the effects of prolonged darkness on horizontal cell responses. When bicuculline was applied to the retina, horizontal cells also depolarized (approximately 10 mV), responses to full-field illumination decreased, and responses to small spots increased. Most of the effects of bicuculline were suppressed by haloperidol, indicating that bicuculline also stimulates the release of dopamine from interplexiform cells. Similar results were obtained when [D-Ala2]-met-enkephalinamide was applied to the retina; horizontal cells depolarized (approximately 10 mV), responses to full-field stimuli decreased, and responses to the light spots increased. On the other hand, [D-Ala2]-leuenkephalinamide and [D-Ala2, D-Leu5]-enkephalin had no effects on horizontal cells. Both haloperidol and naloxone blocked the effects of [D-Ala2]-met-enkephalinamide on horizontal cells, indicating that [D-Ala2]-met-enkephalinamide stimulates dopamine release from interplexiform cells via specific opiate receptors.

Animals↗
PubMed1990

Structural features and adaptive properties of photoreceptors in the skate retina.

In the duplex (rod/cone) retinae of most vertebrates, electrical responses associated with nocturnal vision can be recorded in relative isolation only over a limited intensity range before there is encroachment by signals from the cone system; e.g., at mesopic levels of illumination, the rods begin to saturate, and the cone mechanism is brought into play. On the other hand, the retinae of some sub-species of skate appear to contain only one class of photoreceptors, namely rods. Nevertheless, skate photoreceptors, as well as other retinal neurons, are able to respond to incremental stimuli presented on background fields so intense that 90% or more of the available rhodopsin is bleached during the exposure. These and other findings raise some doubts as to whether the skate has, in fact, an all-rod retina. In this paper, we present a body of evidence--based on the results of photochemical, anatomical, and electrophysiological studies--to support the view that only a single class of photoreceptor subserves vision in the skate retina. In addition, recent findings will be described that: a) demonstrate how the visual cells transform from sluggishly responding rods to brisk, cone-like elements, and b) may provide some insight into the functional significance of this unusual adaptive property.

Adaptation, Ocular↗
PubMed1990

On the duplex nature of the skate retina.

The skate retina contains only one type of photoreceptor which has rod-like properties in the dark-adapted state. In the presence of background illumination, the receptors take on cone-like properties, i.e., their photoresponses become much faster, are less sensitive to light, and can be elicited in the presence of very bright backgrounds. Although the transformation is an extremely slow process, the skate retina performs like that of animals with mixed, rod-cone retinae. In this report we examine some postreceptoral features which may relate to this remarkable behavior. We show, for example, that the skate photoreceptor terminals make two kinds of junctions (ribbon synapses and basal junctions) that appear analogous to the two kinds of synaptic contacts made by cone terminals in other species. Furthermore, two types of horizontal cells are seen in the skate retina. Although the light-evoked responses of these horizontal cells are similar, there are differences in their current profiles recorded under voltage clamp, and in the nature of their dendritic processes. We have also observed several unusual postreceptoral structural features that may have some bearing on the response properties of the skate retina. In addition, a comparison of the adaptive properties of the receptor potential with those of intraretinal responses (b-wave, PNR) reveals differences that suggest strongly the presence of a network adaptive mechanism originating within the proximal retina. The network mechanism appears to be controlled to a large extent by the extracellular concentration of potassium [K+]0; i.e., changes in [K+]0 affect significantly the sensitivities of the b-wave and PBR, but have little effect on responses arising in the distal retina.

Adaptation, Ocular↗
PubMed1990

Dopamine modulates the kinetics of ion channels gated by excitatory amino acids in retinal horizontal cells.

Upon exposure to dopamine, cultured teleost retinal horizontal cells become more responsive to the putative photoreceptor neurotransmitter L-glutamate and to its analog kainate. We have recorded unitary and whole-cell currents to determine the mechanism by which dopamine enhances ion channels activated by these agents. In single-channel recordings from cell-attached patches with agonist in the patch pipette, the frequency of 5- to 10-pS unitary events, but not their amplitude, increased by as much as 150% after application of dopamine to the rest of the cell. The duration of channel openings also increased somewhat, by 20-30%. In whole-cell experiments, agonists with and without dopamine were applied to voltage-clamped horizontal cells by slow superfusion. Analysis of whole-cell current variance as a function of mean current indicated that dopamine increased the probability of channel opening for a give agonist concentration without changing the amount of current passed by an individual channel. For kainate, noise analysis additionally demonstrated that dopamine did not alter the number of functional channels. Dopamine also increased a slow spectral component of whole-cell currents elicited by kainate or glutamate, suggesting a change in the open-time kinetics of the channels. This effect was more pronounced for currents induced by glutamate than for those induced by kainate. We conclude that dopamine potentiates the activity of horizontal cell glutamate receptors by altering the kinetics of the ion channel to favor the open state.

Analysis of Variance↗
PubMed1990

Dopamine induces neurite retraction in retinal horizontal cells via diacylglycerol and protein kinase C.

Dopamine causes a significant retraction of neurites of bull-head catfish horizontal cells maintained in culture. The effects of dopamine are blocked by haloperidol and SCH 23390, a D1 antagonist, but not by sulpiride, a D2 antagonist. The dopamine-induced morphological changes were mimicked by SKF 38393, a D1 agonist, but not by quinpirole, a D2 agonist. Kainate also caused process retraction, but other neuroactive substances tested including glutamate, 5-hydroxytryptamine, N-methyl-D-aspartate, gamma-aminobutyric acid, and glycine caused only minor changes in neurite length. Cyclic AMP analogues do not induce neurite retraction in horizontal cells, indicating that this effect of dopamine is not mediated by cyclic AMP. However, a protein kinase C activator (phorbol 12-myristate 13-acetate) and synthetic diacylglycerol analogs (1-oleoyl-2-acetyl-sn-glycerol and dioctanoglycerol) caused marked neurite retraction. Their effects, as well as the dopamine-induced changes, were blocked by staurosporine, a potent protein kinase antagonist. The results suggest that dopamine causes neurite retraction by the activation of protein kinase C via diacylglycerol.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗
PubMed1990

Structural and functional properties of two types of horizontal cell in the skate retina.

Two morphologically distinct types of horizontal cell have been identified in the all-rod skate retina by light- and electron-microscopy as well as after isolation by enzymatic dissociation. The external horizontal cell is more distally positioned in the retina and has a much larger cell body than does the internal horizontal cell. However, both external and internal horizontal cells extend processes to the photoreceptor terminals where they end as lateral elements adjacent to the synaptic ribbons within the terminal invaginations. Whole-cell voltage-clamp studies on isolated cells similar in appearance to those seen in situ showed that both types displayed five separate voltage-sensitive conductances: a TTX-sensitive sodium conductance, a calcium current, and three potassium-mediated conductances (an anomalous rectifier, a transient outward current resembling an A current, and a delayed rectifier). There was, however, a striking difference between external and internal horizontal cells in the magnitude of the current carried by the anomalous rectifier. Even after compensating for differences in the surface areas of the two cell types, the sustained inward current elicited by hyperpolarizing voltage steps was a significantly greater component of the current profile of external horizontal cells. A difference between external and internal horizontal cells was seen also in the magnitudes of their TEA-sensitive currents; larger currents were usually obtained in recordings from internal horizontal cells. However, the currents through these K+ channels were quite small, the TEA block was often judged to be incomplete, and except for depolarizing potentials greater than or equal to +20 mV (i.e., outside the normal operating range of horizontal cells), this current did not provide a reliable indicator of cell type. The fact that two classes of horizontal cell can be distinguished by their electrophysiological responses, as well as by their morphological appearance and spatial distribution in the retina, suggests that they may play different roles in the processing of visual information within the retina.

Animals↗
PubMed1990

A monoclonal antibody specific to Müller cells and selective synaptic sites in the retina.

We have produced a monoclonal antibody which stains the Müller cells in the region of the photoreceptors, nerve terminals surrounding the horizontal cells, and nerve terminals in the inner plexiform layer in carp, goldfish, and white perch retinas. Electron microscopy showed that the staining in the outer and inner retina was confined to Müller cells and presynaptic terminals, respectively. In the teleost brain, the antibody stained only the optic tectum and efferent fibers from the stratum album centrale. Biochemical characterization by immunoblotting showed that this monoclonal antibody recognizes an approximately 70-kD molecule in both whole retina and brain homogenates, suggesting that the antibody recognizes an identical molecule. No staining was noted in the spleen or the liver. This monoclonal antibody appears to be specific to a molecule common to the Müller cells and presynaptic terminals in the teleost retina, and although it is present in other parts of the central nervous system, it is confined to the visual pathway.

Animals↗
PubMed1989

Localization of cyclic adenosine monophosphate in the teleost retina: effects of dopamine and prolonged darkness.

Localization of cyclic adenosine monophosphate (cAMP) in the white perch retina was carried out with immunohistochemical and autoradiographic methods. Following exposure to dopamine or prolonged darkness, cAMP staining was observed by immunohistochemistry in the distal part of the inner nuclear layer, i.e. in the horizontal cells. After exposure to dopamine, increased levels of cAMP were also observed by autoradiography in many horizontal cells. Finally, increased levels of cAMP staining were observed immunohistochemically following incubation with dopamine in all types of cone-related horizontal cells that had been isolated and maintained in culture.

Animals↗
PubMed1989

Junctions form between catfish horizontal cells in culture.

Cone horizontal cells from the catfish retina extend out processes after a few days in culture that sometimes contact adjacent cone horizontal cells. Two types of specialized junctions were observed by electron microscopy along the newly formed contact areas. One junctional type consisted of prominent electron-dense material along and just under the plasma membrane of one or both of the contacting elements. Sometimes vesicle clusters were associated with these junctions. The other type of junction showed some electron-dense material along the membranes of both processes and patchy areas of close membrane apposition resembling gap junctions. In about half of the cases tested, electrical coupling was detected between cone horizontal cells that had made contact in culture. In no case was the coupling as tight as is typically found between horizontal cells that had formed gap junctions in vivo.

Animals↗
PubMed1989

Responses of isolated white perch horizontal cells to changes in the concentration of photoreceptor transmitter agonists.

Current and voltage responses elicited by increasing or decreasing the concentration of L-glutamate or its analog kainate around isolated cone horizontal cells were measured with patch pipettes using the whole cell recording configuration. Application of these photoreceptor transmitter agonists induced inward currents in voltage-clamp experiments (for negative holding potentials) and depolarizing responses in current-clamp experiments. Continuous exposure to either drug produced inward currents which were maintained for as long as superfusion with the drugs continued. Reducing the concentration of the agonists by pressure ejection of pulses of drug-free Ringer's solution onto the cells completely turned off the drug-induced currents. Under current-clamp conditions, pulses of control Ringer's elicited hyperpolarizing responses of large amplitude (40-80 mV). The data demonstrate the ability to simulate in vitro the horizontal cell's photoresponses and thus support the use of cultured cells as a model system for studying horizontal cell physiology and pharmacology.

Animals↗
PubMed1989

Enhancement of kainate-gated currents in retinal horizontal cells by cyclic AMP-dependent protein kinase.

Dopamine, acting via cyclic adenosine 3':5'-monophosphate (cAMP), has been shown to enhance a kainate-gated ionic conductance in white perch retinal horizontal cells in vitro. To determine whether this effect involves stimulation of a protein kinase, kainate-gated currents were observed in cultured horizontal cells that were dialyzed with the catalytic subunit of cAMP-dependent protein kinase. Intracellular application of catalytic subunit or cAMP, but not heat-inactivated catalytic subunit, caused significant enhancement of the kainate-evoked currents. These results suggest that kainate-gated channels in horizontal cells may be modified by a phosphorylation event.

Animals↗
PubMed1989

D-aspartate potentiates the effects of both L-aspartate and L-glutamate on carp horizontal cells.

L-Aspartate, L-glutamate and D-aspartate, alone or in combination, were applied by superfusion or by atomization to the isolated carp retina while recording from cone horizontal cells. Each of these agents when applied alone depolarized horizontal cells and reduced the size of their light responses, an action which mimics the effect of the endogenous photoreceptor transmitter. Application of D-aspartate in conjunction with either of the L-amino acids potentiated the effects the L-amino acids so that the threshold concentration was reduced by about five-fold, compared to when the L-amino acids were applied alone. The potentiating effect of D-aspartate occurred with all types of cone horizontal cells--both L- and C-types. Furthermore, the potentiating effect of D-aspartate occurred not only in the dark but also in the presence of bright light background or Co2+ ions, conditions during which the release of photoreceptor transmitter is reduced or blocked. D-Aspartate also potentiated the depolarizing effects of the acidic amino acid cysteine sulfinate. The potentiating effect of D-aspartate can be attributed to its action as an amino acid uptake blocker in the outer retina. Thus, these findings, in themselves, cannot eliminate L-aspartate, L-glutamate or cysteine sulfinate as candidates for the carp cone transmitter. However, other evidence, previously reported, strongly suggests that L-glutamate and not L-aspartate is the cone transmitter.

Animals↗