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R Weiler

Publications and source records attributed to R Weiler.

At least 19 recordsLinked to original sources
PubMed2001

Visual transmission deficits in mice with targeted disruption of the gap junction gene connexin36.

In the mammalian retina, rods feed into the cone pathway through electrotonic coupling, and recent histological data suggest the involvement of connexin36 (Cx36) in this pathway. We therefore generated Cx36 null mice and monitored the functional consequences of this deficiency on early visual transmission. The homozygous mutant mice had a normally developed retina and showed no changes in the cellular organization of the rod pathway. In contrast, the functional coupling between AII amacrine cells and bipolar cells was impaired. Recordings of electroretinograms revealed a significant decrease of the scotopic b-wave in mutant animals and an increased cone threshold that is compatible with a distorted, gap junctional transmission between AII amacrine cells and cone bipolar cells. Recordings of visual evoked potentials showed extended latency in mutant mice but unaffected ON and OFF components. Our results demonstrate that Cx36-containing gap junctions are essential for normal synaptic transmission within the rod pathway.

Animals↗
PubMed2001

Selective synaptic distribution of AMPA and kainate receptor subunits in the outer plexiform layer of the carp retina.

The subunit composition of ionotropic glutamate receptors (GluRs) is extremely diverse and responsible for the diversity of postsynaptic responses to the release of glutamate, which is the major excitatory neurotransmitter in the retina. To understand the functional consequences of this diversity, it is necessary to reveal the synaptic localization and subunit composition of GluRs. We have used immuno light and electron microscopy to localize AMPA and kainate (GluR1, GluR2/3, GluR4, GluR5-7) subunits in identified carp retinal neurons contributing to the outer plexiform layer. GluR1 could not be detected within the outer plexiform layer. Rod and cone horizontal cells all express only GluR2/3 at the tips of their invaginating dendrites. These receptors are also inserted into the membrane of spinules, light-dependent protrusions of the horizontal cell dendrites, flanking the synaptic ribbon of the cone synapse. Bipolar cells express GluR2/3, GluR4, and GluR5-7 at their terminal dendrites invaginating cone pedicles and rod spherules. Colocalization data suggest that each subunit is expressed by a distinct bipolar cell type. The majority of bipolar cells expressing these receptors seem to be of the functional OFF-type; however, in a few instances, GluR2/3 could also be detected on dendrites of bipolar cells that, based on their localization within the cone synaptic complex, appeared to be of the functional ON-type. The spatial arrangement of the different subunits within the cavity of the cone pedicle appeared not to be random: GluR2/3 was found predominantly at the apex of the cavity, GluR4 at its base and GluR5-7 dispersed between the two.

Animals↗
PubMed2001

Hemichannel-mediated inhibition in the outer retina.

An essential feature of the first synapse in the retina is a negative feedback pathway from horizontal cells to cones. Here we show that at this synapse, connexin26 forms hemichannels on horizontal cell dendrites near the glutamate release site of the cones. Blocking these hemichannels hyperpolarizes horizontal cells, modulates the Ca2+ channels of the cones, and abolishes all feedback-mediated responses. We propose a feedback mechanism in which the activity of the Ca2+ channels and the subsequent glutamate release of the cones are modulated by a current through these hemichannels. Because the current through the hemichannels depends on the polarization of the horizontal cells, their activity modulates the output of the cones.

Animals↗
PubMed2001

Expression of neuronal connexin36 in AII amacrine cells of the mammalian retina.

We have studied the expression pattern of neuronal connexin36 (Cx36) in the mouse and rat retina. In vertical sections of both retinas, a polyclonal antibody directed against Cx36 produced punctate labeling in the inner plexiform layer (IPL). Intense immunoreactivity was localized to the entire OFF sublamina of the IPL, and much weaker staining could be observed in the ON sublamina. Double-labeling experiments in the rat retina with antibodies directed against parvalbumin indicate that Cx36 is expressed on dendrites of AII amacrine cells. Cx36-like immunoreactivity in sublamina a of the IPL did not overlap with lobular appendages or cell bodies of AII amacrine cells. In a mouse retinal slice preparation, AII amacrine and ON cone bipolar cells were intracellularly injected with Neurobiotin and counterstained with antibody against Cx36. Punctate labeling appeared to be in register with dendritic arborization of AII amacrines and cone bipolar cells in the ON sublamina of the IPL. Whereas AII amacrine cells isolated from the rat retina clearly displayed Cx36-like immunoreactivity, isolated ON cone bipolar cells were negative for Cx36. Axon terminals of rod bipolar cells were decorated with Cx36-positive contacts but did not express Cx36 themselves. These results indicate that Cx36 is expressed by AII amacrine cells in homologous and heterologous gap junctions made with AII amacrines and cone bipolar cells, respectively. The heterologous gap junctions appear to be heterotypic, because ON cone bipolar cells do not express Cx36.

Animals↗
PubMed2000

Molecular and functional diversity of neural connexins in the retina.

Electrical synapses (gap junctions) in neuronal circuits have become a major focus in the study of network properties such as synchronization and oscillation (Galarreta and Hestrin, 1999; Gibson et al., 1999). Despite the recent progress made in unraveling the contribution of gap junctions to network behavior, little is known about the molecular composition of the junctional constituents. By cloning gap junction proteins [connexins (Cxs)] from zebrafish retina and through functional expression, we demonstrate that the retina possesses a high degree of connexin diversity, which may account for differential functional properties of electrical synapses. Three new Cxs, designated as zebrafish Cx27.5 (zfCx27.5), zfCx44.1, and zfCx55.5, and the carp ortholog of mammalian Cx43 were cloned. By in situ hybridization and in situ RT-PCR, we demonstrate that the four fish connexin mRNAs show differential localization in the retina. Transient functional expression in paired Xenopus oocytes and in the neuroblastoma N2A cell line indicate an extreme range of electrophysiological properties of these connexins in terms of voltage dependence and unitary conductance. For instance, the new zfCx44.1 exhibited high sensitivity to voltage-induced closure with currents decaying rapidly for transjunctional potentials >10 mV, whereas zfCx55.5 channels showed an opposite voltage dependence in response to voltage steps of either polarity. Moreover, although zfCx44.1 channels showed unitary conductance as high as any previously reported for junctional channels (nearly 300 pS), zfCx55. 5 and zfCx27.5 exhibited much lower unitary conductances (<60 pS).

Animals↗
PubMed2000

Expression patterns of connexin genes in mouse retina.

To analyze the molecular basis of gap junctional communication in mouse retina, we examined the expression pattern of the following 13 connexin (Cx) genes: Cx26, Cx30, Cx30.3, Cx31, Cx31.1, Cx32, Cx36, Cx37, Cx40, Cx43, Cx45, Cx46, and Cx50. By using reverse transcriptase-polymerase chain reactions with primer oligonucleotides to murine connexin genes, we detected mRNAs of Cx26, Cx31, Cx32, Cx36, Cx37, Cx40, Cx43, Cx45, and Cx50. Retinae from heterozygous mice with targeted replacement of most of the Cx45 open reading frame by a lacZ reporter gene showed Cx45 promoter activity in somata of the ganglion cell layer and the inner nuclear layer. Immunoblot and immunofluorescence analyses with antibodies generated to murine connexin epitopes revealed the presence of Cx36, Cx37, Cx43, and Cx45 proteins: The outer and inner plexiform layer were immunopositive for Cx36 and Cx45. Cx37 immunoreactivity was found in blood vessels of the inner retina. Cx43 immunolabeling was detected in the ganglion cell layer and nerve fiber layer where it was largely colocalized with immunostaining of glial fibrillary acidic protein suggesting that Cx43-positive cells could be of glial origin. No Cx26 protein was detected in retina by using Cx26 antibodies for immunoblot analyses or confocal microscopy. Furthermore, comparative immunofluorescence analyses of retinae from mice deficient for Cx31, Cx32, or Cx40 with retinae of wild-type mice revealed no specific immunostaining. Our results demonstrate regional specificity in expression of connexin genes in mouse retina and, thus, provide a basis for future assignments of functional defects in connexin-deficient mice to cells in different regions of the retina.

Animals↗
PubMed2000

Endogenous dopaminergic regulation of horizontal cell coupling in the mammalian retina.

Horizontal cells in an isolated wholemount preparation of the mouse retina were injected with Lucifer yellow and neurobiotin to characterize both the pattern of gap junctional connectivity and its regulation by dopamine. The injected horizontal cells had a uniform morphology of a round cell body, a compact dendritic tree, and an axon, which could sometimes be traced to an expansive terminal system. The dendro-dendritic gap junctions between neighboring cells mediated both weak Lucifer yellow dye coupling and strong neurobiotin tracer coupling. The extent of the tracer coupling was decreased by either exogenous dopamine (100 microM) or cyclic adenosine monophosphate (cAMP) analogs and was significantly increased by the D1 antagonist SCH 23390 (10 microM). These results provide the first evidence in the mammalian retina that the gap junctions between horizontal cells are endogenously regulated by dopamine, which acts through D1 receptors to increase the intracellular cAMP. It has been proposed that the gap junctional coupling between horizontal cells is mediated by connexin 32 (Cx32), but the pattern and dopaminergic regulation of horizontal cell coupling were unaffected in Cx32-knockout mice, ruling out the possible involvement of Cx32. Every tracer-coupled horizontal cell showed calbindin immunoreactivity, and vice versa, providing strong evidence that the horizontal cells in the mouse retina comprise a single cell type. Like the axonless horizontal cells in other mammalian retinas, the axon-bearing horizontal cells in the mouse retina are coupled by gap junctions that are permeable to Lucifer yellow and dopamine sensitive, suggesting that the mouse horizontal cells have hybrid properties to compensate for the absence of axonless horizontal cells.

Animals↗
PubMed2000

Multirecording of Ca(2+) signals from inner retinal neurons evoked by light stimulation of photoreceptors.

We simultaneously monitored changes of intracellular free Ca(2+) concentration ([Ca(2+)](i)) following different light stimuli from different inner retinal neurons of the turtle retina slice preparation. [Ca(2+)](i) increased with an increase of the light stimulus intensity. Some of the cells also showed color opponent Ca(2+) signals. 2-Amino-4-phosphonobutyric acid (APB) blocked in particular [Ca(2+)](i) increases and picrotoxin enhanced the observed [Ca(2+)](i) changes. These data support the idea that the observed [Ca(2+)](i) changes result from light stimulation and subsequent retinal processing. Similar Ca(2+) signals were observed when the release of Ca(2+) from internal stores was blocked with caffeine and thapsigargin. These results indicate that retinal Ca(2+) signals evoked by light stimulation depend to a large extent on voltage-dependent Ca(2+) influx and might therefore reflect signal processing.

Animals↗
PubMed2000

Gap junctions in the eye: evidence for heteromeric, heterotypic and mixed-homotypic interactions.

Some of the best evidence that different types of gap junction proteins (connexins) interact with each other in vivo has been found in the eye. This review focuses on three diverse ocular tissues that may contain heterotypic or heteromeric gap junction channels. Each of the tissues uses gap junctions in a superlative fashion: The crystalline lens has an exceptionally high density of gap junctions; the ciliary body expresses a surprising variety of connexins; the neural retina shows remarkable specificity in the patterns of intercellular coupling.

Animals↗
PubMed2000

Modulation of coupling between retinal horizontal cells by retinoic acid and endogenous dopamine.

The regulation of electrical coupling between retinal neurons appears to be an important component of the neuronal mechanism of light adaptation, which enables the retina to operate efficiently over a broad range of light intensities. The information about the ambient light conditions has to be transmitted to the neuronal network of the retina and previous evidence has indicated that dopamine is an important neurochemical signal. In addition, recent studies suggest that another important chemical signal is retinoic acid, which is a light-correlated byproduct of the phototransduction cycle. This review summarizes the latest findings about the effects of dopamine and retinoic acid on gap junctional coupling in the retinas of mouse, rabbit and fish.

Animals↗
PubMed2000

Light-adaptive effects of retinoic acid on receptive field properties of retinal horizontal cells.

Besides its role in ocular development, retinoic acid (RA), which is a light-correlated byproduct of the phototransduction cycle, was recently shown to affect light-driven synaptic plasticity in the outer plexiform layer of the adult fish retina. Tuning by ambient light conditions of the retinal network properties is very prominent in outer plexiform layer circuits, and we therefore examined whether RA could affect cone horizontal cell physiology similar to ambient light. Performing intracellular recordings and dye injections in the dark-adapted inverted eyecup preparation of the carp, we found that RA reduced the receptive fields of horizontal cell somata and impaired gap junctional communication. This action was not observed among coupled axon terminals of horizontal cells and appeared to be stereospecific because it could only be attributed to all-trans and 13-cis RA but not to the 9-cis isomer and photoisomerized all-trans RA. Modulation of receptive field size occurred independently of the dopaminergic system. Furthermore, RA affected the light responsiveness of cone horizontal cells. Compared to the dark-adapted condition, responsiveness to intense light stimulation was enhanced but decreased when low intensities were used. Moreover, following RA treatment H2-type horizontal cells of dark-adapted retinae which do not give rise to colour-opponent light properties became colour-opponent and performed depolarizing responses to long-wavelength stimulation. In all these cases RA perfectly matched the effects of light adaptation, supporting the notion that RA acts as an endogenous neuromodulator.

Acyclic Monoterpenes↗
PubMed1999

AMPA-preferring receptors with high Ca2+ permeability mediate dendritic plasticity of retinal horizontal cells.

The synaptic complex formed by the cone photoreceptor pedicles and the dendrites of horizontal cells in the teleost retina undergoes structural changes during light adaptation. Numerous spinules are formed by the terminal dendrites, and they are subsequently retracted during dark adaptation. In a retina kept under continuous illumination, the retraction process can be initiated by analogues of the neurotransmitter glutamate acting at AMPA/kainate receptors. On the other hand, the retraction process depends on calcium influx and the subsequent activation of CaMkII. We show here that the retraction of spinules induced by AMPA or kainate is not impaired in the presence of cobalt, making an involvement of voltage-gated calcium channels unlikely. Using calcium imaging techniques with isolated horizontal cells, we demonstrate that AMPA and kainate, but not NMDA, increase [Ca2+]i in the presence of nicardipine, caffeine and thapsigargin. The increase of [Ca2+]i under these conditions depends on [Ca2+]o and on the agonist in a dose-dependent manner, suggesting that the increase of [Ca2+]i is largely due to calcium influx through the agonist-gated channel. Pharmacological studies were performed to determine whether AMPA- and/or kainate-preferring receptors mediate the calcium influx. The AMPA-preferring receptor antagonist LY303070 blocked glutamate- and kainate-evoked increases of [Ca2+]i in a concentration-dependent manner, indicating that kainate-preferring receptors contributed little or nothing to the observed [Ca2+]i increase. This was supported by experiments where cyclothiazide (which blocks the desensitization of AMPA receptors) and concanavalin A (which potentiates responses mediated by kainate receptors) were applied. In all cases, LY303070 blocked the agonist-evoked increase of [Ca2+]i. The presence of AMPA-preferring receptors with high Ca2+ permeability on horizontal cells was also supported by measuring agonist-induced currents using whole-cell recording techniques. Furthermore, LY303070 was able to impair the retraction of spinules during dark adaption in the in vivo situation.

Animals↗
PubMed1999

Retinoic acid modulates gap junctional permeability between horizontal cells of the mammalian retina.

In the retina, all-trans retinoic acid (at-RA) could function as a light signal because its production increases with the level of illumination. Given the well-established effects of retinoic acid on cell coupling in other tissues, it is possible that the changing levels of at-RA modulate the gap junctional permeability between retinal neurons. This study examines the effects of retinoic acid on horizontal cell coupling, which is known to be modulated by the ambient light level. Single horizontal cells were injected under visual control with either Neurobiotin (mouse retina) or Lucifer yellow (rabbit retina) and the extent of tracer coupling or dye coupling was used to monitor the gap junctional permeability. In the mouse retina, the injection of Neurobiotin revealed a network of approximately 150-250 tracer-coupled horizontal cells. The tracer coupling was completely abolished by incubating the retina in 150 microM at-RA for 35 min. In the rabbit retina, the injection of Lucifer yellow into A-type horizontal cells revealed networks of approximately 15-30 dye-coupled horizontal cells. Incubation in 150 microM at-RA reduced the dye coupling within 12 min and complete uncoupling was achieved after 35 min. The uncoupling effects of at-RA in the mouse and rabbit retinas were concentration- and time-dependent and they were reversible after washout. The coupling was not affected by either the 9-cis form of retinoic acid or by at-RA that had been isomerized by intensive light. The uncoupling effect of at-RA persisted following treatment with a D1 receptor antagonist and thus was dopamine-independent. This study has established that at-RA is able to modulate the gap junctional permeability between horizontal cells in the mammalian retina, where its light-dependent release has already been demonstrated.

Animals↗
PubMed1998

Distribution of connexin43 immunoreactivity in the retinas of different vertebrates.

The distribution of Connexin43 (Cx43) was examined by immunoblotting and immunofluorescence microscopy in the retinas of five different vertebrates by using a C-terminal specific peptide antibody. The specificity of the antibody was proved on immunoblots, in which it showed cross reactivity with a 43-kDa protein in rat heart homogenates as well as in homogenates of rabbit, rat, chicken, turtle, and fish (carp and zebrafish) retinas. Immunofluorescence histochemistry with retinal cryosections revealed the presence of Cx43 in the retinal pigment epithelium cells of all tested species and in blood vessels of vascularized retinas (fish and rat). Cx43 immunoreactivity was further localized in the stria medullaris of rabbit retina, in the nerve fiber layer of rat retina, most likely in astrocytes, and in the area of the outer limiting membrane of the fish retina, most likely representing Cx43 in Müller glia cells. A punctate Cx43-immunoreactive pattern consistent with gap junctions was also detected in the outer plexiform layer of carp and zebrafish retinas, and a specific amacrine cell type, which ramified in two layers of the inner plexiform layer, was labeled in the zebrafish retina. The present results are in accordance with previous findings showing the abundance of Cx43 in astrocytes, endothelium, and epithelial cells. However, the presence of Cx43 immunoreactivity in a specific population of amacrine cells of the zebrafish retina might indicate that a Cx43-like protein is also expressed in neurons.

Animals↗
PubMed1998

Retinoic acid has light-adaptive effects on horizontal cells in the retina.

Ambient light conditions affect the morphology of synaptic elements within the cone pedicle and modulate the spatial properties of the horizontal cell receptive field. We describe here that the effects of retinoic acid on these properties are similar to those of light adaptation. Intraorbital injection of retinoic acid into eyes of dark-adapted carp that subsequently were kept in complete darkness results in the formation of numerous spinules at the terminal dendrites of horizontal cells, a typical feature of light-adapted retinae. The formation of these spinules during light adaptation is impaired in the presence of citral, a competitive inhibitor of the dehydrogenase responsible for the generation of retinoic acid in vivo. Intracellularly recorded responses of horizontal cells from dark-adapted eyecup preparations superfused with retinoic acid reveal typical light-adapted spatial properties. Retinoic acid thus appears to act as a light-signaling modulator. Its activity appears not to be at the transcriptional level because its action was not blocked by actinomycin.

Adaptation, Ocular↗
PubMed1998

The modulation of intercellular coupling in the retina.

Gap-junction coupling between retinal neurons of the same (homologous) or different (heterologous) type can be modulated dynamically by neurotransmitters and by the level of ambient illumination. The homologous coupling of both horizontal cells and rod (AII) amacrine cells is reduced by dopamine, but there appear to be cell and species differences in how the ambient illumination interacts with the dopaminergic system to modulate the coupling. Both the homologous coupling of horizontal cells and the heterologous coupling between AII amacrine cells and cone bipolar cells are reduced by nitric oxide, but it is not known if changes in illumination modulate coupling through an endogenous nitrergic mechanism. The heterologous coupling between rod and cone photoreceptors is modulated by illumination, but the endogenous mechanism has not been identified.

Animals↗