Effects of calcium ions on L-type horizontal cells in the isolated turtle retina

Journal ArticleA technique by which the retina can be isolated from the turtle eye is described. Scanning electron microscopy revealed morphological variability between preparations and also between regions of the same one. Large areas were often totally free of any pigment epithelial cells, yet contained a high proportion of photoreceptors with complete outer segments. However, adjacent regions may contain photoreceptors without outer segments or with fragmented ones. The physiological properties of the horizontal cells also demonstrated large variability between different preparations. In all cases, lowering calcium concentration from 2 mM to 0.1-0.5 mM depolarized the horizontal cells and augmented the amplitude of the maximum photoresponses. However, these effects were accompanied by changes in the photoresponse kinetics and by a reduction in the horizontal cell sensitivity to light. Moreover, prolonged exposure to low calcium induced permanent damage to the retina as was indicated by the reduction in the response amplitude after superfusion with 2 mM calcium solution had been resumed. The toxic effects of low calcium were most apparent when superfusion with 0.1-1.0 microM calcium concentration was performed. These solutions induced complex time-dependent effects on the resting potential of horizontal cells and on the amplitude and kinetics of the photoresponses. We conclude from these observations that the normal concentration of extracellular calcium in the turtle retina is in the 2 mM range

Effects of background illumination on the photoresponses of red and green cones

Journal Article1. The photoresponses of light- and dark-adapted red and green cone photoreceptors were recorded intracellularly in the retina of the turtle, Pseduemys scripta elegans. Background illumination produced similar effects on both types of cones. 2. In response to the onset of a prolonged, steady background illumination the cone initially hyperpolarized to a peak which then sagged back to a steady-state polarization that was typically about one half the initial peak amplitude. This sag was observed for all backgrounds studied (dim as well as bright). 3. A resensitization was observed concomitantly with this sag; both the maximum increment and decrement responses grew in amplitude as light-adaptation proceeded. After about 2--3 min of background illumination, the amplitudes of these responses stabilized. 4. The dark-adapted cone produced graded responses to test pulses over a range of intensities spanning about 3.5 log units. The amplitudes of these responses were well fit by the relationship V = I.Vm/(I + sigma). 5. After 2--3 min of background illumination, 500 msec test pulses either brighter or dimmer than the background intensity were substituted for the background. The light-adapted intensity-response curves constructed from this data were similar to the dark-adapted curve but were shifted horizontally and slightly vertically, so that they still spanned about 3.5 log units of intensity. Thus, in the light-adapted cone, graded responses were elicited by a range of bright test pulses which would have produced saturated responses when delivered to the dark-adapted cone. 6. The 'off response' observed at the offset of the background became faster as the background intensity was increased. It also became faster with time following the onset of any particular background intensity. 7. It was concluded that cone sensitivity during any state of light-adaptation is determined by two mechanisms; response compression resulting from the instantaneous non-linearity between 'internal transmitter' concentration and membrane potential and a more active 'cellular adaptation' mechanism which is manifest as a shift in the intensity-response curve. In the steady-state condition of light-adaptation, most of the sensitivity changes are a result of the cellular adaptation mechanism. 8. Photopigment bleaching caused by the backgrounds, negative feed-back from horizontal cells and voltage dependent mechanisms in the cones could not account for this cellular adaptation. These effects of background illumination were interpreted in terms of the 'internal transmitter' hypothesis of phototransduction

Effects of GABA and related drugs on horizontal cells in the isolated turtle retina

Journal ArticleThe role of GABA in the outer plexiform layer of the turtle retina has been examined by intracellular recordings from L- and C-type horizontal cells in the isolated retina preparation. GABA (1-5 mM) slightly depolarized the L-type horizontal cells, reduced the amplitude of their photoresponses, and slowed down the rate of hyperpolarization during the ON component of the photoresponse. These effects could not be replicated by either muscimol or baclofen. When synaptic transmission from the photoreceptors had been blocked by either kynurenic acid or cobalt ions, GABA depolarized L-type horizontal cells and augmented the remaining photoresponses. Neither muscimol nor baclofen exerted any effect on L-type horizontal cells under these conditions. Nipecotic acid, a competitive inhibitor of the GABA-uptake system, induced effects on turtle L-type horizontal cells which were similar to those exerted by GABA. Thus, the complex GABA effect on turtle L-type horizontal cells seems to represent the summation of at least two actions; an indirect one mediated by the red cones via GABAa-type receptors and a direct one which probably reflects the activation of an electrogenic GABA-uptake system. GABA (1-5 mM) induced a transient depolarization in C-type horizontal cells but eliminated color opponency in only three cells out of seven studied. This observation is inconsistent with the notion that the only neural mechanism responsible for the chromatic properties of C-type horizontal cells in the turtle retina is a GABAergic negative feedback from the L-type horizontal cells onto the green ones

Neural organization of the retina of the turtle Mauremys caspica: a light microscope and Golgi study

Journal ArticleThe organization of the retina of the turtle species Mauremys caspica, found in fresh water ponds of Israel, has been examined by light microscopical techniques including examination of fresh wholemount retina, one micron blue-stained vertical sections and Golgi-stained material. The anatomical findings on Mauremys retina have been compared with those of the Pseudemys retina (Kolb, 1982) which is more commonly used for electrophysiological and neurochemical studies in the USA. The photoreceptors of Mauremys are similar in type and oil droplet content to Pseudemys photoreceptors except for the double cone in Mauremys. This cone type appears more abundant than in Pseudemys and the principal member contains a yellow oil droplet instead of an orange oil droplet. Golgi staining reveals that the cell types that have been seen in Pseudemys are found in Mauremys with identical morphology. In addition, two amacrine cell types that were not before described for Pseudemys have been added to the classification. One of these is the tristratified dopaminergic amacrine cell described in immunocytochemical studies (Witkovsky et al., 1984; Nguyen-Legros et al., 1985; Kolb et al., 1987). We have used these anatomical studies on Pseudemys and Mauremys retina to form a catalogue of neural types for the turtle retina in general. We conclude with an attempt to combine findings from anatomy, electrophysiology, and neurochemistry to form an overview of the organization of this reptilian retina

NADPH diaphorase activity in the rat retina during the early stages of experimental diabetes

Nitric oxide (NO) plays an important physiological role in inter-cellular communication, but when produced in excess it can become toxic. Our goal was to evaluate possible involvement of NO in the development of retinopathy in diabetic rats.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47392/1/417_2003_Article_740.pd

Visual evoked cortical potential can be used to differentiate between uncorrected refractive error and macular disorders

The visual evoked cortical potential (VECP) is widely used to verify complaints of reduced visual performance and to identify the site of the disorder. In this study, we investigated the correlation between reduced visual acuity and VECP in volunteers with normal corrected visual acuity and in patients suffering from inherited macular degeneration or from age related macular degeneration (ARMD). Flash evoked VECP was not affected by the visual acuity in the cases of refractive error and in ARMD patients but was reduced in amplitude and delayed in implicit time in the patients suffering from inherited macular degeneration. The VECP elicited by pattern reversal checkerboard (PVECP) was not affected by the quality of the visual image in volunteers with uncorrected refractive error when checks of 60′ or larger were used but were considerably reduced in size and prolonged in implicit time for checks smaller than 15′. In both groups of patients suffering from macular dysfunction, pattern reversal VECP was very subnormal and was characterized by prolonged implicit time compared to values expected from their visual acuity. These findings indicate that the PVECP does not directly correlate with visual acuity but rather with foveal function. Therefore, we suggest that recordings of PVECP can be used to differentiate between refractive error and macular disorders as causing reduction in visual acuity when other clinical signs are missing or not available.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42621/1/10633_2004_Article_336241.pd

Extracellular histones activate autophagy and apoptosis via mTOR signaling in human endothelial cells

Circulating histones have been proposed as targets for therapy in sepsis and hyperinflammatory symptoms. However, the proposed strategies have failed in clinical trials. Although different mechanisms for histone-related cytotoxicity are being explored, those mediated by circulating histones are not fully understood. Extracellular histones induce endothelial cell death, thereby contributing to the pathogenesis of complex diseases such as sepsis and septic shock. Therefore, the comprehension of cellular responses triggered by histones is capital to design effective therapeutic strategies. Here we report how extracellular histones induce autophagy and apoptosis in a dose-dependent manner in cultured human endothelial cells. In addition, we describe how histones regulate these pathways via Sestrin2/AMPK/ULK1-mTOR and AKT/mTOR. Furthermore, we evaluate the effect of Toll-like receptors in mediating autophagy and apoptosis demonstrating how TLR inhibitors do not prevent apoptosis and/or autophagy induced by histones. Our results confirm that histones and autophagic pathways can be considered as novel targets to design therapeutic strategies in endothelial damage

In Vivo Kinetics Of Rhodopsin In Rcs(+ Royal College Of Surgeons) And Normal Rats.

PhDAnatomy & physiologyAnimalsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/189598/2/7708011.pd

Interpretation of Length Constants in Retinal Horizontal Cell Layers

Introduction Horizontal Cells (HC's) of the vertebrate retina are second order neurons receiving excitatory synaptic input from photoreceptors (PR). Through gap junctions they form homological electrical syncytia, sampling and distributing signals over large areas. Through a feedback mechanism onto PRs and feedforward synapses onto bipolar cells they are thought to play an important role in spatial and color organization of retinal receptive fields and in adaptation processes. Figure 1: A H1 network in the turtle retina stained with neuro-biotin. The bar represents 50 ¯m. Effects of background light or drugs (e.g. dopamine) on the coupling strength or the receptive field size of HC's were quantified by determining directly or indirectly a length constant (whatever it is) from the membrane potential. Because of the interpretation of 2 as the ratio of the membrane resistance<F52

A novel isoform of acetylcholinesterase exacerbates photoreceptors death after photic stress

PURPOSE: To study the involvement of stress-induced acetylcholinesterase (AChE) expression in light-induced retinal damage in albino rats. METHODS: Adult albino rats were exposed for 24 hours to bright, damaging light. AChE expression was monitored by in situ hybridization, by histochemistry for AChE activity, and by immunocytochemistry. An orphan antisense agent (Monarsen; Ester Neurosciences, Ltd., Herzlia Pituach, Israel) was administered intraperitoneally to minimize light-induced AChE expression. The electroretinogram (ERG) was recorded to assess retinal function. RESULTS: Twenty-four-hour exposure to bright light caused severe reduction in the ERG responses and augmented expression of mRNA for the "read-through" variant of AChE (AChE-R) in photoreceptor inner segments (IS), bipolar cells, and ganglion cells. AChE activity increased in IS. The expressed AChE protein was a novel variant, characterized by an extended N terminus (N-AChE). Systemic administration of the orphan antisense agent, Monarsen, reduced the photic induction of mRNA for AChE-R, and of the N-AChE protein. Rats exposed to bright, damaging light and treated daily with Monarsen exhibited larger ERG responses, relatively thicker outer nuclear layer (ONL), and more ONL nuclei than did rats exposed to the same damaging light but treated daily with saline. CONCLUSIONS: The findings indicate that the photic-induced novel variant of AChE (N-AChE-R) may be causally involved with retinal light damage and suggest the use of RNA targeting for limiting such damage.Supported in part by the Chief Scientist, Israel Ministry of Health, the Selma Mitrani Age Related Macular Degeneration Research Fund (IP), and the European Alternative Splicing Network of Excellence Grant EURASNET LSH-2004-1.1.5-3 (HS)