Synaptic organization of regenerated retina in the goldfish

In the adult goldfish, any manipulation that significantly depletes retinal neurons stimulates neurogenesis and the regeneration of nearly normal retina. We sought to determine the extent to which the regenerated neurons formed normal synaptic connections. We used qualitative and quantitative electron microscopy to compare the organization of the synaptic layers in regenerated and normal retinas. In eight eyes, a small patch of retina was surgically excised, stimulating regeneration of new retina in its place. Animals were killed 16–20 weeks after surgery. Qualitative comparisons of the synaptic architecture of photoreceptor terminals in the outer plexiform layer and quantitative comparisons of the synaptic organization in the inner plexiform layer were made between the patch of regenerated retina and an adjacent intact site. In the regenerated outer plexiform layer, cone pedicles and rod spherules were not arranged as regularly as normal, but they formed normal-appearing synaptic contacts. In the regenerated inner plexiform layer, with one exception, the quantitative descriptors of the synaptic organization in the normal and regenerate were n ot significantly different: The planimetric and numerical densities of the synapses, number of synapses/inner retinal neuron, and, with the exception of the bipolar terminals in the inner plexiform layer, and synapse depth profiles were similar. These data suggest that (1) relatively normal synaptic connections are recreated during regeneration, (2) the cellular mechanisms that guide synaptogenesis during development act during retinal regeneration, and (3) the physiological response properties of regenerated neurons should be comparable to that found in the normal retina. © 1994 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50060/1/903430410_ftp.pd

Morphology and distribution of synapses onto a type of large field ganglion cell in the retina of the goldfish

The morphology and dendritic distribution of terminals that synapse onto a type of large-field ganglion cell in the retina of the goldfish are described. Electron microscopy was combined with retrograde labelling of cells with horseradish peroxidase (HRP). Synapses from both amacrine (four types) and bipolar cells contacted the dendrites (all orders) of these cells. In contrast to a recent report describing the synaptic organization of large-field ganglion cells in the catfish (Sakai et al., '86), the synapses were relatively evenly distributed throughout the dendritic arbor, not clustered at discrete sites, and no presynaptic specializations were seen in the dendrites of the ganglion cells.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50044/1/902830203_ftp.pd

Intrinsiccone adaptation modulates feedback efficiency from horizontal cells to cones

Processing of visual stimuli by the retina changes strongly during light/dark adaptation. These changes are due to both local photoreceptor-based processes and to changes in the retinal network. The feedback pathway from horizontal cells to cones is known to be one of the pathways that is modulated strongly during adaptation. Although this phenomenon is well described, the mechanism for this change is poorly characterized. The aim of this paper is to describe the mechanism for the increase in efficiency of the feedback synapse from horizontal cells to cones. We show that a train of flashes can increase the feedback response from the horizontal cells, as measured in the cones, up to threefold. This process has a time constant of ∼3 s and can be attributed to processes intrinsic to the cones. It does not require dopamine, is not the result of changes in the kinetics of the cone light response and is not due to changes in horizontal cells themselves. During a flash train, cones adapt to the mean light intensity, resulting in a slight (4 mV) depolarization of the cones. The time constant of this depolarization is ∼3 s. We will show that at this depolarized membrane potential, a light-induced change of the cone membrane potential induces a larger change in the calcium current than in the unadapted condition. Furthermore, we will show that negative feedback from horizontal cells to cones can modulate the calcium current more efficiently at this depolarized cone membrane potential. The change in horizontal cell response properties during the train of flashes can be fully attributed to these changes in the synaptic efficiency. Since feedback has major consequences for the dynamic, spatial, and spectral processing, the described mechanism might be very important to optimize the retina for ambient light conditions

Neuroprotective effects of the cannabinoid agonist HU210 on retinal degeneration

Cannabinoids have been demonstrated to exert neuroprotective effects on different types of neuronal insults. Here we have addressed the therapeutic potential of the synthetic cannabinoid HU210 on photoreceptor degeneration, synaptic connectivity and functional activity of the retina in the transgenic P23H rat, an animal model for autosomal dominant retinitis pigmentosa (RP). In P23H rats administered with HU210 (100 μg/kg, i.p.) from P24 to P90, ERG recordings showed an amelioration of vision loss, as compared to vehicle-administered animals. Under scotopic conditions, the maximum a-wave amplitudes recorded at P60 and P90 were higher in HU210-treated animals, as compared to the values obtained in untreated animals. The scotopic b-waves were significantly higher in treated animals than in untreated rats at P30, P60 and P90. This attenuation of visual deterioration correlated with a delay in photoreceptor degeneration and the preservation of retinal cytoarchitecture. HU210-treated animals had 40% more photoreceptors than untreated animals. Presynaptic and postsynaptic elements, as well as the synaptic contacts between photoreceptors and bipolar or horizontal cells, were also preserved in HU210-treated P23H rats. These results indicate that HU210 preserves cone and rod structure and function, together with their contacts with postsynaptic neurons, in P23H rats. These data suggest that cannabinoids are potentially useful to delay retinal degeneration in RP patients.This research was supported by grants from the Spanish Ministry of Economy and Competitiveness (BFU2012-36845-FEDER), Instituto de Salud Carlos III (RETICS RD12/0034/0010), Universidad de Alicante (UA2010-48536273), and the Organización Nacional de Ciegos Españoles (ONCE)

The Nature of Surround-Induced Depolarizing Responses in Goldfish Cones

Cones in the vertebrate retina project to horizontal and bipolar cells and the horizontal cells feedback negatively to cones. This organization forms the basis for the center/surround organization of the bipolar cells, a fundamental step in the visual signal processing. Although the surround responses of bipolar cells have been recorded on many occasions, surprisingly, the underlying surround-induced responses in cones are not easily detected. In this paper, the nature of the surround-induced responses in cones is studied. Horizontal cells feed back to cones by shifting the activation function of the calcium current in cones to more negative potentials. This shift increases the calcium influx, which increases the neurotransmitter release of the cone. In this paper, we will show that under certain conditions, in addition to this increase of neurotransmitter release, a calcium-dependent chloride current will be activated, which polarizes the cone membrane potential. The question is, whether the modulation of the calcium current or the polarization of the cone membrane potential is the major determinant for feedback-mediated responses in second-order neurons. Depolarizing light responses of biphasic horizontal cells are generated by feedback from monophasic horizontal cells to cones. It was found that niflumic acid blocks the feedback-induced depolarizing responses in cones, while the shift of the calcium current activation function and the depolarizing biphasic horizontal cell responses remain intact. This shows that horizontal cells can feed back to cones, without inducing major changes in the cone membrane potential. This makes the feedback synapse from horizontal cells to cones a unique synapse. Polarization of the presynaptic (horizontal) cell leads to calcium influx in the postsynaptic cell (cone), but due to the combined activity of the calcium current and the calcium-dependent chloride current, the membrane potential of the postsynaptic cell will be hardly modulated, whereas the output of the postsynaptic cell will be strongly modulated. Since no polarization of the postsynaptic cell is needed for these feedback-mediated responses, this mechanism of synaptic transmission can modulate the neurotransmitter release in single synaptic terminals without affecting the membrane potential of the entire cell

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

Conditional gene expression and lineage tracing of tuba1a expressing cells during zebrafish development and retina regeneration

The tuba1a gene encodes a neural-specific Α-tubulin isoform whose expression is restricted to the developing and regenerating nervous system. By using zebrafish as a model system for studying CNS regeneration, we recently showed that retinal injury induces tuba1a gene expression in MÜller glia that reentered the cell cycle. However, because of the transient nature of tuba1a gene expression during development and regeneration, it was not possible to trace the lineage of the tuba1a -expressing cells with a reporter directly under the control of the tuba1a promoter. To overcome this limitation, we generated tuba1a:CreER T2 and Β- actin2:loxP-mCherrry-loxP-GFP double transgenic fish that allowed us to label tuba1a -expressing cells conditionally and permanently via ligand-induced recombination. During development, recombination revealed transient tuba1a expression in not only neural progenitors but also cells that contribute to skeletal muscle, heart, and intestine. In the adult, recombination revealed tuba1a expression in brain, olfactory neurons, and sensory cells of the lateral line, but not in the retina. After retinal injury, recombination showed tuba1a expression in MÜller glia that had reentered the cell cycle, and lineage tracing indicated that these cells are responsible for regenerating retinal neurons and glia. These results suggest that tuba1a -expressing progenitors contribute to multiple cell lineages during development and that tuba1a -expressing MÜller glia are retinal progenitors in the adult. J. Comp. Neurol. 518:4196–4212, 2010. © 2010 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77972/1/22448_ftp.pd

Immunolocalisation of P2Y receptors in the rat eye

Nucleotides present an important role in ocular physiology which has been demonstrated by recent works that indicate their involvement in many ocular processes. P2Y are important among P2 receptors since they can control tear production, corneal wound healing, aqueous humour dynamics and retinal physiology. Commercial antibodies have allowed us to investigate the distribution of P2Y receptors in the cornea, anterior and posterior chamber of the eye and retina. The P2Y1 receptor was present mainly in cornea, ciliary processes, and trabecular meshwork. The P2Y2 receptors were present in cornea, ciliary processes and retinal pigmented epithelium. P2Y4 was present in cornea, ciliary processes, photoreceptors, outer plexiform layer and ganglion cell layer. The P2Y6 presented almost an identical distribution as the P2Y4 receptor. The P2Y11 was also detectable in the retinal pigmented epithelium. The detailed distribution of the receptors clearly supports the recent findings indicating the relevant role of nucleotides in the ocular function