Functional stability of retinal ganglion cells after degeneration-induced changes in synaptic input

Retinal ganglion cell (RGC) responses to different voltage stimulation parameters in rd1 mouse retina

Retinal prostheses are being developed to restore vision for the blind with retinal diseases such as retinitis pigmentosa (RP) or age-related macular degeneration (AMD). Since neural prostheses depend upon electrical stimulation to control neural activity, optimal stimulation parameters for successful encoding of visual information are one of the most important requirements to enable visual perception. Therefore, in this paper, we focused on RGC responses to different stimulation parameters in degenerated retina. For this purpose, we used in vitro preparation of rd1 mice retina on microelectrode arrays. When the neural network of rd1 mice retinas is stimulated with voltage-controlled pulses, RGCs in degenerated retina also respond to voltage amplitude or voltage duration modulation as well in wild-type RGCs. But the temporal pattern of RGCs response is very different; in wild-type RGCs, single peak within 100 ms appears while in RGCs in degenerated retina multiple peaks (approximately 4 peaks) with approximately 10 Hz rhythm within 400 ms appear. The threshold charge densities for activation of RGCs in rd1 mouse retinas were on average 70.50 approximately 99.87 microC/cm(2) in the experiment of voltage amplitude modulation and 120.5 approxiamtely 170.6 microC/cm(2) in the experiment of voltage duration modulation.

Developmental time course distinguishes changes in spontaneous and light-evoked retinal ganglion cell activity in rd1 and rd10 mice

In a subset of hereditary retinal diseases, early photoreceptor degeneration causes rapidly progressive blindness in children. To better understand how retinal development may interact with degenerative processes, we compared spontaneous and light-evoked activity among retinal ganglion cells in rd1 and rd10 mice, strains with closely related retinal disease. In each, a mutation in the Pde6b gene causes photoreceptor dysfunction and death, but in rd10 mice degeneration starts after a peak in developmental plasticity of retinal circuitry and thereafter progresses more slowly. In vitro multielectrode action potential recordings revealed that spontaneous waves of correlated ganglion cell activity comparable to those in wild-type mice were present in rd1 and rd10 retinas before eye opening [postnatal day (P) 7 to P8]. In both strains, spontaneous firing rates increased by P14-P15 and were many times higher by 4-6 wk of age. Among rd1 ganglion cells, all responses to light had disappeared by ~P28, yet in rd10 retinas vigorous ON and OFF responses were maintained well beyond this age and were not completely lost until after P60. This difference in developmental time course separates mechanisms underlying the hyperactivity from those that alter light-driven responses in rd10 retinas. Moreover, several broad physiological groups of cells remained identifiable according to response polarity and time course as late as P60. This raises hope that visual function might be preserved or restored despite ganglion cell hyperactivity seen in inherited retinal degenerations, particularly if treatment or manipulation of early developmental plasticity were to be timed appropriately.

Changes in physiological properties of rat ganglion cells during retinal degeneration

Retinitis pigmentosa (RP) is a leading cause of degenerative vision loss, yet its progressive effects on visual signals transmitted from the retina to the brain are not well understood. The transgenic P23H rat is a valuable model of human autosomal dominant RP, exhibiting extensive similarities to the human disease pathology, time course, and electrophysiology. In this study, we examined the physiological effects of degeneration in retinal ganglion cells (RGCs) of P23H rats aged between P37 and P752, and compared them with data from wild-type control animals. The strength and the size of visual receptive fields of RGCs decreased rapidly with age in P23H retinas. Light responses mediated by rod photoreceptors declined earlier (∼ P300) than cone-mediated light responses (∼ P600). Responses of ON and OFF RGCs diminished at a similar rate. However, OFF cells exhibited hyperactivity during degeneration, whereas ON cells showed a decrease in firing rate. The application of synaptic blockers abolished about half of the elevated firing in OFF RGCs, indicating that the remodeled circuitry was not the only source of degeneration-induced hyperactivity. These results advance our understanding of the functional changes associated with retinal degeneration.

Temporal interactions during paired-electrode stimulation in two retinal prosthesis subjects

PURPOSE

Since 2002, six blind patients have undergone implantation of an epiretinal 4 × 4 electrode array designed to directly stimulate the remaining cells of the retina after severe photoreceptor degeneration due to retinitis pigmentosa. This study was conducted to investigate how the brightness of percepts is affected by pulse timing across electrodes in two of these patients.

METHODS

Subjects compared the perceived brightness of a standard stimulus (synchronous pulse trains presented across pairs of electrodes) to the perceived brightness of a test stimulus (pulse trains across the electrode pair phase shifted by 0.075, 0.375, 1.8, or 9 ms). The current amplitude necessary for each phase-shifted test stimulus to match the brightness of the standard was determined.

RESULTS

Depending on the electrode pair, interactions between electrodes were either facilitatory (the perceived brightness produced by stimulating the pair of electrodes was greater than that produced by stimulating either electrode alone) or suppressive (the perceived brightness produced by stimulating the pair of electrodes was less than that produced by stimulating either electrode alone). The amount of interaction between electrodes decreased as a function of increased separation both in time (the phase-shift between pulse trains) and space (center-to-center distance between the electrode pair).

CONCLUSIONS

For visual prostheses to represent visual scenes that are changing in both space and time requires the development of spatiotemporal models describing the effects of stimulation across multiple electrodes. During multielectrode stimulation, interactions between electrodes have a significant influence on subjective brightness that includes both facilitatory and suppressive effects, and these interactions can be described with a simple computational model. (ClinicalTrials.gov number, NCT00279500.).

Simulating electrical stimulation of degenerative retinal ganglion cells with bi-phasic pulse trains

The aim of this work was to investigate how retinal ganglion cells (RGCs) respond to repetitive electrical stimulation in degenerative retina. The response of modeled ON and OFF cells was examined to bi-phasic pulse train stimulation of varying frequencies. Previously developed models of RGCs were extended to include an experimentally observable balance of excitatory and inhibitory currents in degenerative retina. The phenomena of fading and dark phosphenes with retinal implants were investigated. A hypothesis for a mechanism contributing to these phenomena was formulated.

Retinal degeneration in two lines of transgenic S334ter rats

Aim of this study was to examine synaptic connectivity changes in the retina and the location and rate of apoptosis in transgenic S334ter line-3 and line-5 rats with photoreceptor degeneration. Heterozygous S334ter-line-3 and line-5 at P11-13, P30, P60, P90 and several control non-dystrophic rats (Long Evans and Sprague-Dawley) at P60, were studied anatomically by immunohistochemistry for various cell and synaptic markers, and by PNA and TUNEL label.- S334ter line-3 exhibited the fastest rate of degeneration with an early loss of photoreceptors, with 1-2 layers remaining at P30, and only cones left at P60. Line-5 had 4-5 layers left at P30, and very few rods left at P60-90. In both lines, horizontal cell processes (including dendrites and axon) were diminished at P11-13, showing gaps in the outer plexiform layer (OPL) at P60, and at P90, almost no terminal tips could be seen. Bipolar cells showed a retraction of their dendrites forming clusters along the OPL. Synaptic terminals of A-II amacrine cells in the IPL lost most of their parvalbumin-immunoreactivity. The apoptosis rate was different in both lines. Line-3 rats showed many photoreceptors affected at P11, occupying the innermost part of the outer nuclear layer. Line-5 showed a lower number of apoptotic cells within the same location at P13. In summary, the S334ter line-3 rat has a faster progression of degeneration than line-5. The horizontal and bipolar terminals are already affected at P11-P13 in both models. Apoptosis is related to the mutated rhodopsin transgene; the first photoreceptor cells affected are those close to the OPL.

Long-term retinal function and structure rescue using capsid mutant AAV8 vector in the rd10 mouse, a model of recessive retinitis pigmentosa

The retinal degeneration 10 (rd10) mouse is a well-characterized model of autosomal recessive retinitis pigmentosa (RP), which carries a spontaneous mutation in the β subunit of rod cGMP-phosphodiesterase (PDEβ). Rd10 mouse exhibits photoreceptor dysfunction and rapid rod photoreceptor degeneration followed by cone degeneration and remodeling of the inner retina. Here, we evaluate whether gene replacement using the fast-acting tyrosine-capsid mutant AAV8 (Y733F) can provide long-term therapy in this model. AAV8 (Y733F)-smCBA-PDEβ was subretinally delivered to postnatal day 14 (P14) rd10 mice in one eye only. Six months after injection, spectral domain optical coherence tomography (SD-OCT), electroretinogram (ERG), optomotor behavior tests, and immunohistochemistry showed that AAV8 (Y733F)-mediated PDEβ expression restored retinal function and visual behavior and preserved retinal structure in treated rd10 eyes for at least 6 months. This is the first demonstration of long-term phenotypic rescue by gene therapy in an animal model of PDEβ-RP. It is also the first example of tyrosine-capsid mutant AAV8 (Y733F)-mediated correction of a retinal phenotype. These results lay the groundwork for the development of PDEβ-RP gene therapy trial and suggest that tyrosine-capsid mutant AAV vectors may be effective for treating other rapidly degenerating models of retinal degeneration.

Viability of the inner retina in a novel mouse model of retinitis pigmentosa

Retinal prostheses aim to restore vision to patients who are blind from photoreceptor diseases such as Retinitis Pigmentosa (RP). All implants target the neural cells in the inner retina, the retinal ganglion cells (RGCs). Our research focuses on further understanding the disease process of RP during mid to late stages when total loss of photoreceptors has occurred and significant remodeling of inner retinal neurons has taken place. We have used a novel transgenic mouse, Rd1-FTL, to observe different degenerative stages of RP. Notably, in the aged retina we have evidence that there was gross inner retinal remodeling as well as glial dysfunction that occurred in confined regions in the central retina that worsened overtime. Consequently, the timing of implantation and location of the prosthesis both need to account for the state of the retina at different stages in the disease process.

Plasticity of TRPM1 expression and localization in the wild type and degenerating mouse retina

The light response in retinal ON bipolar cells is associated with disinhibition of current flow through cation channels recently identified as type 1 members of the melastatin transient receptor potential (TRPM) family. We determined the developmental expression of Trpm1 in the wild type C57BL/6, DBA/2J, DBA2J-Gpnmb mouse retinas and in Pde6brd1 retinas characterized by degeneration of rod photoreceptors. Trpm1 mRNA in wild type retinas was low at birth but exhibited progressive increases in abundance up to early adulthood at postnatal day 21 (P21). Retinal Trpm1 mRNA content did not decrease following loss of photoreceptors. At P21, TRPM1-immunopositive perikarya migrated into the outer nuclear layer. The TRPM1 protein was trafficked to discrete postsynaptic puncta in wild type retinas whereas in adult Pde6brd1 mouse retinas, TRPM1 translocated to bipolar perikarya and bar-like structures in the distal inner nuclear layer. These findings show that expression and localization of the TRPM1 in the mouse retina is plastic, modulated by use-dependence and availability of sustained excitatory input.

Selective activation of neuronal targets with sinusoidal electric stimulation

Electric stimulation of the CNS is being evaluated as a treatment modality for a variety of neurological, psychiatric, and sensory disorders. Despite considerable success in some applications, existing stimulation techniques offer little control over which cell types or neuronal substructures are activated by stimulation. The ability to more precisely control neuronal activation would likely improve the clinical outcomes associated with these applications. Here, we show that specific frequencies of sinusoidal stimulation can be used to preferentially activate certain retinal cell types: photoreceptors are activated at 5 Hz, bipolar cells at 25 Hz, and ganglion cells at 100 Hz. In addition, low-frequency stimulation (≤25 Hz) did not activate passing axons but still elicited robust synaptically mediated responses in ganglion cells; therefore, elicited neural activity is confined to within a focal region around the stimulating electrode. Our results suggest that sinusoidal stimulation provides significantly improved control over elicited neural activity relative to conventional pulsatile stimulation.

Cellular origin of spontaneous ganglion cell spike activity in animal models of retinitis pigmentosa

Here we review evidence that loss of photoreceptors due to degenerative retinal disease causes an increase in the rate of spontaneous ganglion spike discharge. Information about persistent spike activity is important since it is expected to add noise to the communication between the eye and the brain and thus impact the design and effective use of retinal prosthetics for restoring visual function in patients blinded by disease. Patch-clamp recordings from identified types of ON and OFF retinal ganglion cells in the adult (36–210 d old) rd1 mouse show that the ongoing oscillatory spike activity in both cell types is driven by strong rhythmic synaptic input from presynaptic neurons that is blocked by CNQX. The recurrent synaptic activity may arise in a negative feedback loop between a bipolar cell and an amacrine cell that exhibits resonant behavior and oscillations in membrane potential when the normal balance between excitation and inhibition is disrupted by the absence of photoreceptor input.

Visual function in mice with photoreceptor degeneration and transgenic expression of channelrhodopsin 2 in ganglion cells

The progression of rod and cone degeneration in retinally degenerate (rd) mice ultimately results in a complete loss of photoreceptors and blindness. The inner retinal neurons survive and several recent studies using genetically targeted, light activated channels have made these neurons intrinsically light sensitive. We crossbred a transgenic mouse line expressing channelrhodopsin2 (ChR2) under the control of the Thy1 promoter with the Pde6b(rd1) mouse, a model for retinal degeneration (rd1/rd1). Approximately 30-40% of the ganglion cells of the offspring expressed ChR2. Extracellular recordings from ChR2-expressing ganglion cells in degenerated retinas revealed their intrinsic light sensitivity which was approximately 7 log U less sensitive than the scotopic threshold and approximately 2 log U less sensitive than photopic responses of normal mice. All ChR2-expressing ganglion cells were excited at light ON. The visual performance of rd1/rd1 mice and ChR2 rd1/rd1 mice was compared. Behavioral tests showed that both mouse strains had a pupil light reflex and they were able to discriminate light fields from dark fields in the visual water task. Cortical activity maps were recorded with optical imaging. The ChR2rd1/rd1 mice did not show a better visual performance than rd1/rd1 mice. In both strains the residual vision was correlated with the density of cones surviving in the peripheral retina. The expression of ChR2 under the control of the Thy1 promoter in retinal ganglion cells does not rescue vision.

Dendritic calcium signaling in ON and OFF mouse retinal ganglion cells

Retinal ganglion cells (RGCs) are the output cells of the retina; they convert synaptic input into spike output that carries visual information to the brain. Synaptic inputs are received, integrated and communicated to the spike initiation zone of the axon by dendrites whose properties are poorly understood. Here simultaneous patch-clamp recording and 2-photon Ca(2+) imaging are used to study voltage- and light-evoked Ca(2+) signals in the dendrites of identified types of mouse RGCs from parallel ON and OFF pathways, which encode the onset and offset of light, respectively. The results show pathway-specific differences in voltage-dependent Ca(2+) signaling. While both ON and OFF cells express high-voltage-activated (HVA) Ca(2+) channels, only OFF RGCs also express low-voltage-activated (LVA) Ca(2+) channels. LVA Ca(2+) channels in OFF cells are deinactivated by hyperpolarization from the resting potential and give rise to rebound excited Ca(2+) spikes at the termination of a step of either hyperpolarizing current or light. This suggests that the differential expression of voltage-gated Ca(2+) channels in ON and OFF RGC dendrites contributes to differences in the way the two cell types process visual stimuli.

Morphological alterations in retinal neurons in the S334ter-line3 transgenic rat

The S334ter-line-3 rat is a transgenic model of retinal degeneration developed to express a rhodopsin mutation similar to that found in human retinitis pigmentosa (RP) patients. Previous studies have focused on physiological changes in retinal cells and higher centers of the visual system with this model of retinal degeneration. However, little is known about the morphological changes in retinal cells during the development of the S334ter-line-3 rat. In order to understand and aid vision-rescue strategies, our aim has been to describe the retinal degeneration pattern in this model. We focus on changes in the morphologies of horizontal, bipolar, and amacrine cells in developing S334ter-line-3 rat retinas. Degeneration of photoreceptors begins in the central retina and progresses toward the periphery. In retinas at post-natal day 15 (P15), horizontal and rod bipolar cells show normal morphology. However, at P21, horizontal and rod bipolar cells exhibit abnormal processes at the outer plexiform layer, whereas the outer nuclear layer is significantly thinner. A glial reaction occurs concomitantly. In contrast, modifications in cone-bipolar and amacrine cells are much slower and do not occur until P90 and P180, respectively. The density of horizontal and rod-bipolar cells significantly drops after P60. Overall, the S334ter-line-3 model exhibits the hallmarks of cellular remodeling caused by photoreceptor degeneration. Its moderately fast time course makes the S334ter-line-3 a good model for studying vision-rescue strategies.

Late histological and functional changes in the P23H rat retina after photoreceptor loss

Several strategies have been proposed to restore useful vision following photoreceptor degeneration. However, a very few studies have investigated late anatomical changes and functional state of residual retinal neurons after complete photoreceptor loss. We investigated the progressive degeneration of retinal ganglion cells (RGCs) in P23H rats. The RGC multielectrode array recordings indicated lower firing rates, disappearance of broad-scale, and maintenance of short-scale pairwise correlations. Up to 11% of RGCs displayed repetitive and often correlated spike discharges, reminiscent of developmental rhythmic activity, which could be reversibly suppressed by blockade of the AMPA/kainite glutamate receptors. RGCs in P23H rats remain sensitive to local electrical stimulation, generating short-latency responses as in the normal retina. These results provide evidence that, despite the demonstrated RGC degeneration, remaining active RGCs maintain their basic physiological and network properties with some emerging functional changes such as the spontaneous rhythmic activity in late stages of the degenerative disease.

Electrically-evoked Neural Activities of rd1 Mice Retinal Ganglion Cells by Repetitive Pulse Stimulation

For successful visual perception by visual prosthesis using electrical stimulation, it is essential to develop an effective stimulation strategy based on understanding of retinal ganglion cell (RGC) responses to electrical stimulation. We studied RGC responses to repetitive electrical stimulation pulses to develop a stimulation strategy using stimulation pulse frequency modulation. Retinal patches of photoreceptor-degenerated retinas from rd1 mice were attached to a planar multi-electrode array (MEA) and RGC spike trains responding to electrical stimulation pulse trains with various pulse frequencies were observed. RGC responses were strongly dependent on inter-pulse interval when it was varied from 500 to 10 ms. Although the evoked spikes were suppressed with increasing pulse rate, the number of evoked spikes were >60% of the maximal responses when the inter-pulse intervals exceeded 100 ms. Based on this, we investigated the modulation of evoked RGC firing rates while increasing the pulse frequency from 1 to 10 pulses per second (or Hz) to deduce the optimal pulse frequency range for modulation of RGC response strength. RGC response strength monotonically and linearly increased within the stimulation frequency of 1~9 Hz. The results suggest that the evoked neural activities of RGCs in degenerated retina can be reliably controlled by pulse frequency modulation, and may be used as a stimulation strategy for visual neural prosthesis.

Spatiotemporal interactions in retinal prosthesis subjects

PURPOSE

Vision loss due to retinitis pigmentosa affects an estimated 15 million people worldwide. Through collaboration between Second Sight Medical Products, Inc., and the Doheny Eye Institute, six blind human subjects underwent implantation with epiretinal 4 x 4 electrode arrays designed to directly stimulate the remaining cells of the retina, with the goal of restoring functional vision by applying spatiotemporal patterns of stimulation. To better understand spatiotemporal interactions between electrodes during synchronous and asynchronous stimulation, the authors investigated how percepts changed as a function of pulse timing across the electrodes.

METHODS

Pulse trains (20, 40, 80, and 160 Hz) were presented on groups of electrodes with 800, 1600, or 2400 microm center-to-center separation. Stimulation was either synchronous (pulses were presented simultaneously across electrodes) or asynchronous (pulses were phase shifted). Using a same-different discrimination task, the authors were able to evaluate how the perceptual quality of the stimuli changed as a function of phase shifts across multiple electrodes.

RESULTS

Even after controlling for electric field interactions, subjects could discriminate between spatiotemporal pulse train patterns based on differences of phase across electrodes as small as 3 ms. These findings suggest that the quality of the percept is affected not only by electric field interactions but also by spatiotemporal interactions at the neural level.

CONCLUSIONS

During multielectrode stimulation, interactions between electrodes have a significant influence on the quality of the percept. Understanding how these spatiotemporal interactions at the neural level influence percepts during multielectrode stimulation is fundamental to the successful design of a retinal prosthesis.

Loss of responses to visual but not electrical stimulation in ganglion cells of rats with severe photoreceptor degeneration

Retinal implants are intended to help patients with degenerative conditions by electrically stimulating surviving cells to produce artificial vision. However, little is known about how individual retinal ganglion cells respond to direct electrical stimulation in degenerating retina. Here we used a transgenic rat model to characterize ganglion cell responses to light and electrical stimulation during photoreceptor degeneration. Retinas from pigmented P23H-1 rats were compared with wild-type retinas between ages P37 and P752. During degeneration, retinal thickness declined by 50%, largely as a consequence of photoreceptor loss. Spontaneous electrical activity in retinal ganglion cells initially increased two- to threefold, but returned to nearly normal levels around P600. A profound decrease in the number of light-responsive ganglion cells was observed during degeneration, culminating in retinas without detectable light responses by P550. Ganglion cells from transgenic and wild-type animals were targeted for focal electrical stimulation using multielectrode arrays with electrode diameters of approximately 10 microns. Ganglion cells were stimulated directly and the success rate of stimulation in both groups was 60-70% at all ages. Surprisingly, thresholds (approximately 0.05 mC/cm(2)) and latencies (approximately 0.25 ms) in P23H rat ganglion cells were comparable to those in wild-type ganglion cells at all ages and showed no change over time. Thus ganglion cells in P23H rats respond normally to direct electrical stimulation despite severe photoreceptor degeneration and complete loss of light responses. These findings suggest that high-resolution epiretinal prosthetic devices may be effective in treating vision loss resulting from photoreceptor degeneration.

Activation of ganglion cells in wild-type and rd1 mouse retinas with monophasic and biphasic current pulses

We and other research groups are designing an electronic retinal prosthesis to provide vision for patients who are blind due to photoreceptor degeneration. In this study, we examined the effect of stimulus waveform on the amount of current needed to activate retinal ganglion cells (RGCs) when the retinal neural network is stimulated. Isolated retinas of wild-type and rd1 mice were stimulated with cathodal and anodal monophasic current pulses of 1 ms duration and symmetric biphasic current pulses (1 ms per phase) delivered through an electrode that was located subretinally. For both wild-type and rd1 mouse retinas, cathodal current pulses were least effective in activating most RGCs. The median threshold current for a cathodal current pulse was 2.0-4.4 fold higher than the median threshold current for either an anodal or a biphasic current pulse. In wild-type mouse retinas, the median threshold current for activating RGCs with anodal current pulses was 23% lower than that with biphasic current pulses. In rd1 mouse retinas, the median threshold currents for anodal and biphasic current pulses were about the same. However, the variance in thresholds of rd1 RGCs for biphasic pulse stimulation was much smaller than for anodal pulse stimulation. Thus, a symmetric biphasic current pulse may be the best stimulus for activating the greatest number of RGCs in retinas devoid of photoreceptors.

Retinal ganglion cells survive and maintain normal dendritic morphology in a mouse model of inherited photoreceptor degeneration

Retinitis pigmentosa (RP), a family of inherited disorders characterized by progressive photoreceptor death, is a leading cause of blindness with no available cure. Despite the genetic heterogeneity underlying the disease, recent data on animal models show that the degeneration of photoreceptors triggers stereotyped remodeling among their postsynaptic partners. In particular, bipolar and horizontal cells might undergo dendritic atrophy and secondary death. The aim of this study was to investigate whether or not concomitant changes also occur in retinal ganglion cells (RGCs), the only retinal projection neurons to the brain and the proposed substrate for various therapeutic approaches for RP. We assessed the retention of morphology, overall architecture, and survival of RGCs in a mouse model of RP at various stages of the disease. To study the morphology of single RGCs, we generated a new mouse line by crossing Thy1-GFP-M mice (Feng et al., 2000), which express GFP (green fluorescent protein) in a small number of heterogeneous RGCs types, and rd10 mutants, a model of autosomal recessive RP, which exhibit a typical rod-cone degeneration (Chang et al., 2002). We show remarkable preservation of RGC structure, survival, and projections to higher visual centers in the time span from 3 to 9 months of life, well beyond the death of photoreceptors. Thus, unlike second-order neurons, RGCs appear as a considerably stable population of cells, potentially constituting a favorable substrate for restoring vision in RP individuals by means of electronic prostheses or direct expression of photosensitive proteins.

Early remodeling in an inducible animal model of retinal degeneration

Photoreceptor degeneration is followed by significant morphological changes in the second-order retinal neurons in humans and in several genetic animal models. However, it is not clear whether similar changes occur when photoreceptor degeneration is induced nongenetically, raising the question whether these changes are a general effect of deafferentation independent of the cause of degeneration. We addressed this by inducing selective photoreceptor degeneration with N-methyl-N-nitrosourea (MNU) and studying its effects on inner retinal neurons in a mouse for up to 3 months, using immunocytochemistry and iontophoretic labeling. To develop objective measures of photoreceptor degeneration and of retinal remodeling, we measured several retinal proteins using immunoblot analysis, and quantified gross visual ability of the animal in a visual cliff test. The MNU-induced progressive degeneration of rods and cones was associated with declining levels of postsynaptic density 95 protein in the retina, and with deteriorating visual performance of the animal. Müller glial cells showed enhanced reactivity for glial fibrillary acidic protein as demonstrated by immunocytochemistry, which also reflected in increased levels of the protein as demonstrated by immunoblotting. Horizontal cells and rod bipolar cells progressively lost their dendritic processes, which correlated with a slight decline in the levels of calbindin and protein kinase C alpha respectively. Horizontal cell axons, immunoreactive for nonphosphorylated neurofilaments, showed sprouting into the inner nuclear layer. Ganglion cells and their synaptic inputs, probed by immunolocalizing beta-III-tubulin, neurofilaments, bassoon and synaptophysin, appeared to be unaffected. These results demonstrate that MNU-induced photoreceptor degeneration leads to retinal remodeling similar to that observed in genetic models, suggesting that the remodeling does not depend on the etiopathology that underlies photoreceptor degeneration.

Eyecup scope--optical recordings of light stimulus-evoked fluorescence signals in the retina

Dendritic signals play an essential role in processing visual information in the retina. To study them in neurites too small for electrical recording, we developed an instrument that combines a multi-photon (MP) microscope with a through-the-objective high-resolution visual stimulator. An upright microscope was designed that uses the objective lens for both MP imaging and delivery of visual stimuli to functionally intact retinal explants or eyecup preparations. The stimulator consists of a miniature liquid-crystal-on-silicon display coupled into the optical path of an infrared-excitation laser-scanning microscope. A pair of custom-made dichroic filters allows light from the excitation laser and three spectral bands ('colors') from the stimulator to reach the retina, leaving two intermediate bands for fluorescence imaging. Special optics allow displacement of the stimulator focus relative to the imaging focus. Spatially resolved changes in calcium-indicator fluorescence in response to visual stimuli were recorded in dendrites of different types of mammalian retinal neurons.