LED-Induced Microglial Activation and Rise in Caspase3 Suggest a Reorganization in the Retina.

Vision is our primary sense as the human eye is the gateway for more than 65% of information reaching the human brain. Today's increased exposure to different wavelengths and intensities of light from light emitting diode (LED) sources could induce retinal degeneration and accompanying neuronal cell death. Damage induced by chronic phototoxic reactions occurring in the retina accumulates over years and it has been suggested as being responsible for the etiology of many debilitating ocular conditions. In this work, we examined how LED stimulation affects vision by monitoring changes in the expression of death and survival factors as well as microglial activation in LED-induced damage (LID) of the retinal tissue. We found an LED-exposure-induced increase in the mRNA levels of major apoptosis-related markers BAX, Bcl-2, and Caspase-3 and accompanying widespread microglial and Caspase-3 activation. Everyday LED light exposure was accounted for in all the described changes in the retinal tissue of mice in this study, indicating that overuse of non-filtered direct LED light can have detrimental effects on the human retina as well.

Regional Variation of Gap Junctional Connections in the Mammalian Inner Retina.

The retinas of many species show regional specialisations that are evident in the differences in the processing of visual input from different parts of the visual field. Regional specialisation is thought to reflect an adaptation to the natural visual environment, optical constraints, and lifestyle of the species. Yet, little is known about regional differences in synaptic circuitry. Here, we were interested in the topographical distribution of connexin-36 (Cx36), the major constituent of electrical synapses in the retina. We compared the retinas of mice, rats, and cats to include species with different patterns of regional specialisations in the analysis. First, we used the density of Prox1-immunoreactive amacrine cells as a marker of any regional specialisation, with higher cell density signifying more central regions. Double-labelling experiments showed that Prox1 is expressed in AII amacrine cells in all three species. Interestingly, large Cx36 plaques were attached to about 8-10% of Prox1-positive amacrine cell somata, suggesting the strong electrical coupling of pairs or small clusters of cell bodies. When analysing the regional changes in the volumetric density of Cx36-immunoreactive plaques, we found a tight correlation with the density of Prox1-expressing amacrine cells in the ON, but not in the OFF sublamina in all three species. The results suggest that the relative contribution of electrical synapses to the ON- and OFF-pathways of the retina changes with retinal location, which may contribute to functional ON/OFF asymmetries across the visual field.

Unmasking inhibition prolongs neuronal function in retinal degeneration mouse model.

All neurodegenerative diseases involve a relatively long period of timeframe from the onset of the disease to complete loss of functions. Extending this timeframe, even at a reduced level of function, would improve the quality of life of patients with these devastating diseases. The retina, as the part of the central nervous system and a frequent site of many distressing neurodegenerative disease, provides an ideal model to investigate the feasibility of extending the functional timeframe through pharmacologic intervention. Retinitis Pigmentosa (RP) is a group of blinding diseases. Although the rate of progression and degree of visual loss varies, there is usually a prolonged time before patients totally lose their photoreceptors and vision. It is believed that inhibitory mechanisms are still intact and may become relatively strong after the gradual loss of photoreceptors in RP patients. Therefore, it is possible that light-evoked responses of retinal ganglion cells and visual information processes in retinal circuits could be "unmasked" by blocking these inhibitory mechanisms restoring some level of visual function. Our results indicate that if the inhibition in the inner retina was unmasked in the retina of the rd10 mouse (the well-characterized RP mimicking, clinically relevant mouse model), the light-evoked responses of many retinal ganglion cells can be induced and restore their normal light sensitivity. GABA A receptor plays a major role in this masking inhibition. ERG b-wave and behavioral tests of spatial vision partly recovered after the application of PTX. Hence, removing retinal inhibition unmasks signalling mediated by surviving cones, thereby restoring some degree of visual function. These results may offer a novel strategy to restore the visual function with the surviving cones in RP patients and other gradual and progressive neurodegenerative diseases.

Characterization of connexin36 gap junctions in the human outer retina.

Retinal connexins (Cx) form gap junctions (GJ) in key circuits that transmit average or synchronize signals. Expression of Cx36, -45, -50 and -57 have been described in many species but there is still a disconcerting paucity of information regarding the Cx makeup of human retinal GJs. We used well-preserved human postmortem samples to characterize Cx36 GJ constituent circuits of the outer plexiform layer (OPL). Based on their location, morphometric characteristics and co-localizations with outer retinal neuronal markers, we distinguished four populations of Cx36 plaques in the human OPL. Three of these were comprised of loosely scattered Cx36 plaques; the distalmost population 1 formed cone-to-rod GJs, population 2 in the mid-OPL formed cone-to-cone GJs, whereas the proximalmost population 4 likely connected bipolar cell dendrites. The fourth population (population 3) of Cx36 plaques conglomerated beneath cone pedicles and connected dendritic tips of bipolar cells that shared a common presynaptic cone. Overall, we show that the human outer retina displays a diverse cohort of Cx36 GJ that follows the general mammalian scheme and display a great functional diversity.

Development-related splicing regulates pituitary adenylate cyclase-activating polypeptide (PACAP) receptors in the retina.

PURPOSE: The ubiquitous pituitary adenylate cyclase-activating peptide (PACAP) has a disparate array of functions in development (e.g., proliferation and apoptosis). Among three types of PACAP receptor (VPAC1, VPAC2, and PAC1), PAC1 is subject to alternative splicing that generates isoforms. Although the literature documenting the presence of PACAP receptors in the central nervous system is vast, their expression during development has not been established yet. Here, we performed quantitative analyses on the expression of PACAP receptors during the postnatal development of the rat retina. METHODS: Retinas were harvested from postnatal days 0 to 20 (P0-P20). Using a comprehensive primer system, expression changes were followed employing quantitative real-time PCR. Changes at the protein level were detected by immunoblotting using anti-VPAC1, -VPAC2, and -PAC1 receptor antibodies. RESULTS: The expression of VPAC1 showed increases at P10 and P15. Peaks in VPAC2 expression were observed at P5 and P15. Using splicing variant-specific primers for PAC1 receptor, splicing regulation of Null, Hip, Hop1, and Hiphop1 variants was revealed in correlation with postnatal development. Transcript levels of the Null and Hip variants showed a decline, while Hop1 became the major PACAP receptor by P20. Hiphop1 transcript levels did not display remarkable changes except for a transient increase at P10. Immunoblotting confirmed the presence and expression level changes of the receptors. CONCLUSIONS: We conclude that both VPAC1 and VPAC2 could have roles at all stages of retinal development, that PACAP acts through a specific set of PAC1 isoforms, and that Hip and Hop1 are predominantly involved in the postnatal development of rat retina.

Light increases the gap junctional coupling of retinal ganglion cells.

We examined the effect of light adaptation on the gap junctional coupling of α-ganglion cells (α-GCs) in rabbit and mouse retinas. We assayed changes in coupling by measuring parameters of tracer coupling following injection of α-GCs with Neurobiotin and the concerted spike activity of α-GC neighbours under dark- and light-adapted conditions. We found that light adaptation using mesopic or photopic background lights resulted in a dramatic increase in the labelling intensity, number, and spatial extent of ganglion and amacrine cells coupled to OFF α-GCs when compared to levels seen under dark adaptation. While this augmentation of coupling by light did not produce an increase in the concerted spontaneous activity of OFF α-GC neighbours, it did significantly increase correlated light-evoked spiking. This was seen as an increase in the number of correlated spikes for α-GC neighbours and an extension of correlations to second-tier neighbours that was not seen under dark-adapted conditions. Pharmacological studies in the rabbit retina indicated that dopamine mediates the observed changes in coupling by differentially activating D1 and D2 receptors under different adaptation states. In this scheme, activation of dopamine D1 receptors following light exposure triggers cAMP-mediated intracellular pathways resulting in an increase in gap junctional conductance. Overall, our results indicate that as we move from night to day there is an enhanced electrical coupling between α-GCs, thereby increasing the concerted activity believed to strengthen the capacity and efficiency of information flow across the optic nerve.