Population Of Ganglion Cells Research Articles

Characterization of green fluorescent protein-expressing retinal cells in CD 44-transgenic mice

Sensory information in the retina is transferred from rod and cone photoreceptors to higher visual centers via numerous parallel circuits that sample the photoreceptor mosaic independently. Each circuit consists of a unique combination of ganglion cell, bipolar and amacrine cell types. The morphology and physiological responses of many amacrine cells have been characterized. However, the synaptic connections and retinal circuits in which they participate are only rarely understood. A major problem that has prevented fuller characterization of retinal circuitry is the need for specific cellular markers for the more than 50 inner retinal cell types. One potential strategy for labeling cells is to use transgenic expression of a reporter gene in a specific cell type. In a recent study of cluster of differentiation 44 (CD44)-enhanced green fluorescent protein (EGFP) transgenic mice, we observed that the green fluorescent protein (GFP) was expressed in a population of amacrine and ganglion cells in the inner nuclear layer (INL) and the GCL. To characterize the morphology of the GFP-labeled cells, whole mount preparations of the retina were used for targeted iontophoretic injections of Lucifer Yellow and Neurobiotin. Furthermore, immunocytochemistry was used to characterize the antigenic properties of the cells. We found that many GFP-expressing cells were GABAergic and also expressed calretinin. In addition to the somatic staining, there was a strong GFP +-band located about 50–60% depth in the inner plexiform layer (IPL). Double labeling with an antibody to choline acetyltransferase (ChAT) revealed that the GFP-band was located at strata 3 inner retina. The best-labeled GFP-expressing cell type in the INL was a wide-field amacrine cell that ramified in stratum 3. The GFP-expressing cells in the GCL resemble the type B1, or possibly A2 ganglion cells. The CD44-EGFP mice should provide a valuable resource for electrophysiological and connectivity studies of amacrine cells in the mouse retina.

Tissue resistivities determine the current flow in the cochlea

In individuals with severe to profound hearing loss, cochlear implants bypass normal inner ear function by applying electrical current directly into the cochlea, thereby stimulating cochlear nerve fibers. Stimulating discrete populations of spiral ganglion cells in cochlear implant users' ears is similar to the encoding of small acoustic frequency bands in a normal-hearing person's ear. Thus, spiral ganglion cells stimulated by an electrode convey the information contained by a small acoustic frequency band. Problems that refer to the current spread and subsequent nonselective stimulation of spiral ganglion cells in the cochlea are reviewed. Cochlear anatomy and tissue properties determine the current path in the cochlea. Current spreads largely via scala tympani and across turns. While most of the current leaves the cochlea via the modiolus, the facial canal and the round window constitute additional natural escape paths for the current from the cochlea. Moreover, degenerative processes change tissue resistivities and thus may affect current spread in the cochlea. Electrode design and coding strategies may result in more spatial stimulation of spiral ganglion cells, resulting in a better performance of the electrode-tissue interface.

Presbycusis: A Human Temporal Bone Study of Individuals With Downward Sloping Audiometric Patterns of Hearing Loss and Review of the Literature

The purpose of this retrospective case review was to identify patterns of cochlear element degeneration in individuals with presbycusis exhibiting downward sloping audiometric patterns of hearing loss and to correlate these findings with those reported in the literature to clarify conflicting concepts regarding the association between hearing loss and morphologic abnormalities. Archival human temporal bones from individuals with presbycusis were selected on the basis of strict audiometric criteria for downward-sloping audiometric thresholds. Twenty-one temporal bones that met these criteria were identified and compared with 10 temporal bones from individuals with normal hearing. The stria vascularis volumes, spiral ganglion cell populations, inner hair cells, and outer hair cells were quantitatively evaluated. The relationship between the severity of hearing loss and the degeneration of cochlear elements was analyzed using univariate linear regression models. Outer hair cell loss and ganglion cell loss was observed in all individuals with presbycusis. Inner hair cell loss was observed in 18 of the 21 individuals with presbycusis and stria vascularis loss was observed in 10 of the 21 individuals with presbycusis. The extent of degeneration of all four of the cochlear elements evaluated was highly associated with the severity of hearing loss based on audiometric thresholds at 8,000 Hz and the pure-tone average at 500, 1,000, and 2,000 Hz. The extent of ganglion cell degeneration was associated with the slope of the audiogram. Individuals with downward-sloping audiometric patterns of presbycusis exhibit degeneration of the stria vascularis, spiral ganglion cells, inner hair cells, and outer hair cells that is associated with the severity of hearing loss. This association has not been previously reported in studies that did not use quantitative methodologies for evaluating the cochlear elements and strict audiometric criteria for selecting cases.

Blockade of PARP activity attenuates poly(ADP‐ribosyl)ation but offers only partial neuroprotection against NMDA‐induced cell death in the rat retina

Recent reports have linked neuronal cell death by necrosis to poly(ADP-ribose) polymerase-1 (PARP-1) hyperactivation. It is believed that under stress, the activity of this enzyme is up-regulated, resulting in extensive poly(ADP-ribosyl)ation of nuclear proteins, using NAD(+) as its substrate, which, in turn, leads to the depletion of NAD(+). In efforts to restore the level of NAD(+), depletion of ATP occurs, resulting in the shutdown of ATP-dependent ionic pumps. This results in cell swelling and eventual loss of membrane selectivity, hallmarks of necrosis. Reports from in vitro and in vivo studies in the brain have shown that NMDA receptor activation stimulates PARP activity and that blockade of the enzyme provides substantial neuroprotection. The present study was undertaken to determine whether PARP activity is regulated by NMDA in the rat retina, and whether blockade of PARP activity provides protection against toxic effects of NMDA. Rat retinas exposed to intravitreal injections containing NMDA, with or without the PARP inhibitor N-(6-oxo-5, 6-dihydrophenanthridin-2-yl)-(N,-dimethylamino) acetamide hydrochloride (PJ-34), were assessed for changes in PARP-1 activity as evidenced by poly(ADP-ribosyl)ation (PAR), loss of membrane integrity, morphological indicators of apoptosis and necrosis, and ganglion cell loss. Results showed that: NMDA increased PAR formation in a concentration-dependent manner and caused a decline in retinal ATP levels; PJ-34 blockade attenuated the NMDA-induced formation of PAR and decline in ATP; NMDA induced the loss of membrane selectivity to ethidium bromide (EtBr) in inner retinal neurons, but loss of membrane selectivity was not prevented by blocking PARP activity; cells stained with EtBr, or reacted for TUNEL-labeling, displayed features characteristic of both apoptosis and necrosis. In the presence of PJ-34, greater numbers of cells exhibited apoptotic features; PJ-34 provided partial neuroprotection against NMDA-induced ganglion cell loss. These findings suggest that although blockade of PARP activity fully attenuates NMDA-induced PAR formation and loss of retinal ATP content, and improves the survival of select populations of ganglion cells, this approach does not provide full neuroprotection. In contrast, blockade of PARP activity promotes apoptotic-like cell death in the majority of cells undergoing cell death. Furthermore, these studies show that the loss of membrane selectivity is not dependent upon PAR formation or the resulting decline of ATP, and suggests that an alternative pathway, other than PARP activation, exists to mediate this event.

Electrical Resistivity Measurements in the Mammalian Cochlea After Neural Degeneration

In the present series of experiments, the effect of neural degeneration on the cochlear structure electrical resistivities was evaluated to test if it alters the current flow in the cochlea and if increased current levels are needed to stimulate the impaired cochlea. In cochlear implants, frequency information is encoded in part by stimulating discrete populations of spiral ganglion cells along the cochlea. However, electrical properties of the cochlear structures result in shunting of the current away from the auditory neurons. This consumes energy, makes cochlear implants less efficient, and drastically reduces battery life. Models of the electrically stimulated cochlea serve to make predictions on current paths using modified and improved cochlear implant electrodes. However, one of the model's shortcomings is that most of the values for tissue impedances are not direct measurements. They are derived from bulk impedance measurements, which are fitted to lumped-element models. The four-electrode reflection-coefficient technique was used to measure resistivities in the gerbil cochlea. In vivo and in vitro (the hemicochlea) models were used. Measurements were made in normal and in deafened animals. Cochlear damage was induced by neomycin injection into the animals' middle ears. Neural degeneration was allowed to occur over 2 months before performing the measurements in the deafened animals. The resistivity values in deafened animals were smaller than in the normal-hearing animals, thus altering the current flow within the cochlea. Resistivity changes and subsequent changes in current path should be considered in future designs of cochlear implants.

Presbycusic Neuritic Degeneration Within the Osseous Spiral Lamina

To describe a neglected anatomic variant occurring with presbycusis. Retrospective temporal bone histopathology study. Quantitative analysis of peripheral hair cells, neurites, neurons, and the stria vascularis in temporal bones from individuals who had presbycusis. Fifty-three patients aged 65 years or older and with a down-sloping audiogram and clinical diagnosis of presbycusis were reviewed. Nine cases had normal hair and ganglion cell populations but reduced peripheral processes (neuritic presbycusis). These were compared with five normal-hearing controls on measurements of anterior middle and basal turn fiber bundle diameter and the ratio of basal to middle diameters. Thresholds at 4 and 8 kHz were significantly poorer in the neuritic presbycusis group than in the control group (p<or=0.004 and 0.05, respectively), as was speech discrimination score (p<or=0.028). The ratio of basal to middle turn diameters was significantly smaller in the neuritic presbycusis group (p<or=0.003). This effect was quite marked in that there was no overlap in ratios between the groups, with the maximum neuritic presbycusis group ratio smaller than the minimum control group ratio. There was a moderate negative correlation between ratio and threshold at 4 kHz (sigma=-0.49, p<or=0.075). Loss of peripheral neurites in the anterior basal cochlear segment is found in conjunction with presbycusis in temporal bones that have no other morphologic abnormalities. These cases can be identified by a gradual down-sloping audiogram in contrast to sensory (hair cell) presbycusis, which is characterized by a precipitous high tone loss.

The number, morphology, and distribution of retinal ganglion cells and optic axons in the Australian lungfishNeoceratodus forsteri(Krefft 1870)

Australian lungfish Neoceratodus forsteri may be the closest living relative to the first tetrapods and yet little is known about their retinal ganglion cells. This study reveals that lungfish possess a heterogeneous population of ganglion cells distributed in a horizontal streak across the retinal meridian, which is formed early in development and maintained through to adult stages. The number and complement of both ganglion cells and a population of putative amacrine cells within the ganglion cell layer are examined using retrograde labelling from the optic nerve and transmission electron-microscopic analysis of axons within the optic nerve. At least four types of retinal ganglion cells are present and lie predominantly within a thin ganglion cell layer, although two subpopulations are identified, one within the inner plexiform and the other within the inner nuclear layer. A subpopulation of retinal ganglion cells comprising up to 7% of the total population are significantly larger (>400 microm2) and are characterized as giant or alpha-like cells. Up to 44% of cells within the retinal ganglion cell layer represent a population of presumed amacrine cells. The optic nerve is heavily fasciculated and the proportion of myelinated axons increases with body length from 17% in subadults to 74% in adults. Spatial resolving power, based on ganglion cell spacing, is low (1.6-1.9 cycles deg(-1), n = 2) and does not significantly increase with growth. This represents the first detailed study of retinal ganglion cells in sarcopterygian fish, and reveals that, despite variation amongst animal groups, trends in ganglion cell density distribution and characteristics of cell types were defined early in vertebrate evolution.

Discrete stimulus estimation from neural responses in the turtle retina

In this paper, we investigate the decoding of flashed, full-field visual stimuli while recording from a population of retinal ganglion cells. We present a direct statistical method for determining the likelihood that a response was evoked by a particular stimulus, and use this method to estimate stimuli based on microelectrode array recordings in the turtle retina. This method uses the well-known time-varying Poisson model of neural firing, along with extensions to accommodate neural refractory periods. Unlike other approaches commonly used for Poisson processes, the specific formulation presented here is bin free and requires few user-specified parameters. Statistical dependency issues and the effects of stationarity on the estimation method are also discussed.

Morphological properties of mouse retinal ganglion cells

The mouse retina offers an increasingly valuable model for vision research given the possibilities for genetic manipulation. Here we assess how the structural properties of mouse retinal ganglion cells relate to the stratification pattern of the dendrites of these neurons within the inner plexiform layer. For this purpose, we used 14 morphological measures to classify mouse retinal ganglion cells parametrically into different clusters. Retinal ganglion cells were labeled in one of three ways: Lucifer Yellow injection, ‘DiOlistics’ or transgenic expression of yellow fluorescent protein. The resulting analysis of 182 cells revealed 10 clusters of monostratified cells, with dendrites confined to either On or Off sublaminae of the inner plexiform layer, and four clusters of bistratified cells, dendrites spanning the On and Off sublaminae. We also sought to establish how these parametrically identified retinal ganglion cell clusters relate to cell types identified previously on the basis of immunocytochemical staining and the expression of yellow fluorescent protein. Cells labeled with an antibody against melanopsin were found to be located within a single cluster, while those labeled with the SMI-32 antibody were in four different clusters. Yellow fluorescent protein expressing cells were distributed within 13 of the 14 clusters identified here, which demonstrates that yellow fluorescent protein expression is a useful method for labeling virtually the entire population of mouse retinal ganglion cells. Collectively, these findings provide a valuable baseline for future studies dealing with the effects of genetic mutations on the morphological development of these neurons.

Retinal ganglion cells: dying to survive

This review examines the maturation of the retinal ganglion cell (RGC) population within the nascent retina. Apoptosis, a form of programmed cell death prevalent throughout the developing central nervous system (CNS), is evident in the growth of RGCs within the ganglion cell layer. These cells provide an accessible and illuminating platform to elucidate the apoptotic pathways present in the developing CNS and the role this form of cell death plays in RGC growth. This article focuses on the seminal stages of RGC development and the role played by neurotrophic factors and apoptosis in this process.

Roller Culture of Free-Floating Retinal Slices: A New System of Organotypic Cultures of Adult Rat Retina

No experimental system exists to date for the in vitro study of retinal ganglion cell populations in a three-dimensional organotypic tissue environment. Here, we describe such a novel method for roller cultivation of adult retinas. Retinas of adult (1–3 months old) rats were cut into rectangular slices of approximately 1 mm². Free-floating slices were cultured on a horizontal rotating roller drum (50–60 rpm) in a dry incubator at 36.5°C. During the first days of cultivation, primary flat retinal slices changed their configuration and transformed into ball-shaped tissue spheres (retinal bodies). Histological and immunocytochemical studies showed that the outer wall of the retinal bodies was formed by cell and fibre layers typical of mature retina with photoreceptors located on the outside. Initially, retinal bodies contained an inner cavity which later was completely obliterated and filled with glial cells, sprouting nerve fibres, and vascular structures. This culture system was further developed into a robust model of glutamate-induced neurotoxicity. Using a novel culture method of adult rat retina, preservation of the three-dimensional organotypic retinal cytoarchitecture was achieved, including survival of neurons in the ganglion cell layer and sprouting of nerve fibres of the axotomized retinal ganglion cells. This novel culture model promises to facilitate studies of retinal physiology and pathology.

Functional Organization of Ganglion Cells in the Salamander Retina

Recently, we reported a novel technique for recording all of the ganglion cells in a retinal patch and showed that their receptive fields cover visual space roughly 60 times over in the tiger salamander. Here, we carry this analysis further and divide the population of ganglion cells into functional classes using quantitative clustering algorithms that combine several response characteristics. Using only the receptive field to classify ganglion cells revealed six cell types, in agreement with anatomical studies. Adding other response measures served to blur the distinctions between these cell types rather than resolve further classes. Only the biphasic off type had receptive fields that tiled the retina. Even when we attempted to split these classes more finely, ganglion cells with almost identical functional properties were found to have strongly overlapping spatial receptive fields. A territorial spatial organization, where ganglion cell receptive fields tend to avoid those of other cells of the same type, was only found for the biphasic off cell. We further studied the functional segregation of the ganglion cell population by computing the amount of visual information shared between pairs of cells under natural movie stimulation. This analysis revealed an extensive mixing of visual information among cells of different functional type. Together, our results indicate that the salamander retina uses a population code in which every point in visual space is represented by multiple neurons with subtly different visual sensitivities.

A bHLH transcriptional network regulating the specification of retinal ganglion cells

In the developing retina, the production of ganglion cells is dependent on the proneural proteins NGN2 and ATH5, whose activities define stages along the pathway converting progenitors into newborn neurons. Crossregulatory interactions between NGN2, ATH5 and HES1 maintain the uncommitted status of ATH5-expressing cells during progenitor patterning, and later on regulate the transition from competence to cell fate commitment. Prior to exiting the cell cycle, a subset of progenitors is selected from the pool of ATH5-expressing cells to go through a crucial step in the acquisition of a definitive retinal ganglion cell fate. The selected cells are those in which the upregulation of NGN2, the downregulation of HES1 and the autostimulation of ATH5 are coordinated with the progression of progenitors through the last cell cycle. This coordinated pattern initiates the transcription of ganglion cell-specific traits and determines the size of the ganglion cell population.

Mosaic properties of midget and parasol ganglion cells in the marmoset retina

We measured mosaic properties of midget and parasol ganglion cells in the retina of a New World monkey, the common marmoset Callithrix jacchus . We addressed the functional specialization of these populations for color and spatial vision, by comparing the mosaic of ganglion cells in dichromatic ("red-green color blind") and trichromatic marmosets. Ganglion cells were labelled by photolytic amplification of retrograde marker ("photofilling") following injections into the lateral geniculate nucleus, or by intracellular injection in an in vitro retinal preparation. The dendritic-field size, shape, and overlap of neighboring cells were measured. We show that in marmosets, both midget and parasol cells exhibit a radial bias, so that the long axis of the dendritic field points towards the fovea. The radial bias is similar for parasol cells and midget cells, despite the fact that midget cell dendritic fields are more elongated than are those of parasol cells. The dendritic fields of midget ganglion cells from the same (ON or OFF) response-type array show very little overlap, consistent with the low coverage of the midget mosaic in humans. No large differences in radial bias, or overlap, were seen on comparing retinae from dichromatic and trichromatic animals. These data suggest that radial bias in ganglion cell populations is a consistent feature of the primate retina. Furthermore, they suggest that the mosaic properties of the midget cell population are associated with high spatial resolution rather than being specifically associated with trichromatic color vision.

Redundancy in the Population Code of the Retina

We have explored the manner in which the population of retinal ganglion cells collectively represent the visual world. Ganglion cells in the salamander were recorded simultaneously with a multielectrode array during stimulation with both artificial and natural visual stimuli, and the mutual information that single cells and pairs of cells conveyed about the stimulus was estimated. We found significant redundancy between cells spaced as far as 500 mum apart. When we used standard methods for defining functional types, only ON-type and OFF-type cells emerged as truly independent information channels. Although the average redundancy between nearby cell pairs was moderate, each ganglion cell shared information with many neighbors, so that visual information was represented approximately 10-fold within the ganglion cell population. This high degree of retinal redundancy suggests that design principles beyond coding efficiency may be important at the population level.

Effect of Cochlear Implantation on Residual Spiral Ganglion Cell Count as Determined by Comparison with the Contralateral Nonimplanted Inner Ear in Humans

It is generally assumed that at least a minimal number of spiral ganglion cells is essential for successful speech perception with a cochlear implant. Although the insertion of a multichannel cochlear implant frequently results in loss of residual hearing in the implanted ear, this outcome does not imply that significant damage to residual populations of spiral ganglion cells has occurred. The purpose of the current study was to compare spiral ganglion cell counts in implanted and nonimplanted cochleas in 11 patients for whom both temporal bones were available and in whom a multichannel cochlear implant had been placed unilaterally. The temporal bones were processed for light microscopy by standard techniques. The cochleas were reconstructed by 2-dimensional methods. Spiral ganglion cell counts of the implanted and nonimplanted sides were compared by a paired t-test (2-tailed). The mean spiral ganglion cell counts for implanted and nonimplanted ears were not statistically different in the most basal three segments of the cochlea. However, the mean spiral ganglion cell count in segment 4 (apical segment) and the mean total spiral ganglion cell count were lower in the implanted cochleas than in the nonimplanted cochleas (p < .01). The results of this study suggest a modest decrease in the total spiral ganglion cell count in the implanted ears as compared to the nonimplanted ears, principally in the apical segment. Possible interpretations of this finding are discussed.

Is Word Recognition Correlated With the Number of Surviving Spiral Ganglion Cells and Electrode Insertion Depth in Human Subjects With Cochlear Implants?

Speech perception scores using cochlear implants have ranged widely in all published series. The underlying determinants of success in word recognition are incompletely defined. Although it has been assumed that residual spiral ganglion cell population in the deaf ear may play a critical role, published data from temporal bone specimens from patients have not supported this hypothesis. The depth of insertion of a multichannel cochlear implant has also been suggested as a clinical variable that may be correlated with word recognition. In the current study these correlations were evaluated in 15 human subjects. Retrospective review of temporal bone histopathology. Temporal bones were fixed and prepared for histological study by standard techniques. Specimens were then serially sectioned and reconstructed by two-dimensional methods. The spiral ganglion cells were counted, and the depth of insertion of the cochlear implant as measured from the round window was determined. Correlation analyses were then performed between the NU6 word scores and spiral ganglion cell counts and the depth of insertion. The segmental and total spiral ganglion cell counts were not significantly correlated (P > .50) with NU6 word scores for the 15 subjects. Statistically significant correlations were not achieved by separate analysis of implant types. Similarly, no significant correlation between the depth of insertion of the electrode array and postoperative NU6 word score was identified for the group. Although it is unlikely that the number of residual spiral ganglion cell counts is irrelevant to the determination of word recognition following cochlear implantation, there are, clearly, other clinical variables not yet identified that play an important role in determining success with cochlear implantation.

Synaptic contacts between an identified type of ON cone bipolar cell and ganglion cells in the mouse retina

We surveyed the potential contacts between an identified type of bipolar cell and retinal ganglion cells in the mouse. By crossing two existing mouse strains (line 357 and line GFP-M), we created a double transgenic strain in which GFP is expressed by all members of a single type of ON cone bipolar cell and a sparse, mixed population of retinal ganglion cells. The GFP-expressing bipolar cells appear to be those termed CB4a of Pignatelli & Strettoi [(2004) J. Comp. Neurol., 476, 254-266] and type 7 of Ghosh et al. [(2004) J. Comp. Neurol., 469, 70-82 and J. Comp. Neurol., 476, 202-203]. The labelled ganglion cells include examples of most or all types of ganglion cells present in the mouse. By studying the juxtaposition of their processes in three dimensions, we could learn which ganglion cell types are potential synaptic targets of the line 357 bipolar cell. Of 12 ganglion cell types observed, 10 types could be definitively ruled out as major synaptic targets of the line 357 bipolar cells. One type of monostratified ganglion cell and one bistratified cell tightly cofasciculate with axon terminals of the line 357 bipolar cells. Double labelling for kinesin II demonstrates colocalization of bipolar cell ribbons at the sites of contact between these two types of ganglion cell and the line 357 bipolar cells.

Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN

Human vision starts with the activation of rod photoreceptors in dim light and short (S)-, medium (M)-, and long (L)- wavelength-sensitive cone photoreceptors in daylight. Recently a parallel, non-rod, non-cone photoreceptive pathway, arising from a population of retinal ganglion cells, was discovered in nocturnal rodents. These ganglion cells express the putative photopigment melanopsin and by signalling gross changes in light intensity serve the subconscious, 'non-image-forming' functions of circadian photoentrainment and pupil constriction. Here we show an anatomically distinct population of 'giant', melanopsin-expressing ganglion cells in the primate retina that, in addition to being intrinsically photosensitive, are strongly activated by rods and cones, and display a rare, S-Off, (L + M)-On type of colour-opponent receptive field. The intrinsic, rod and (L + M) cone-derived light responses combine in these giant cells to signal irradiance over the full dynamic range of human vision. In accordance with cone-based colour opponency, the giant cells project to the lateral geniculate nucleus, the thalamic relay to primary visual cortex. Thus, in the diurnal trichromatic primate, 'non-image-forming' and conventional 'image-forming' retinal pathways are merged, and the melanopsin-based signal might contribute to conscious visual perception.

Deletion of Connexin45 in Mouse Retinal Neurons Disrupts the Rod/Cone Signaling Pathway between AII Amacrine and ON Cone Bipolar Cells and Leads to Impaired Visual Transmission

Connexin45 (Cx45) is known to be expressed in the retina, but its functional analysis was problematic because general deletion of Cx45 coding DNA resulted in cardiovascular defects and embryonic lethality at embryonic day 10.5. We generated mice with neuron-directed deletion of Cx45 and concomitant activation of the enhanced green fluorescent protein (EGFP). EGFP labeling was observed in bipolar, amacrine, and ganglion cell populations. Intracellular microinjection of fluorescent dyes in EGFP-labeled somata combined with immunohistological markers revealed Cx45 expression in both ON and OFF cone bipolar cells. The scotopic electroretinogram of mutant mice revealed a normal a-wave but a 40% reduction in the b-wave amplitude, similar to that found in Cx36-deficient animals, suggesting a possible defect in the rod pathway of visual transmission. Indeed, neurotransmitter coupling between AII amacrine cells and Cx45-expressing cone bipolar cells was disrupted in Cx45-deficient mice. These data suggest that both Cx45 and Cx36 participate in the formation of functional heterotypic electrical synapses between these two types of retinal neurons that make up the major rod pathway.