‘Starburst’ amacrine cells and cholinergic neurons: mirror-symmetric ON and OFF amacrine cells of rabbit retina

Golgi-impregnated 'starburst' amacrine cells share significant morphological features with cholinergic neurons in rabbit retina. They are mirror-symmetrical about the a/b (OFF/ON) sublaminar border of the inner plexiform layer. Type a starburst amacrines have cell bodies in the amacrine cell layer and dendrites in sublamina a, while type b cells have their cell bodies in the ganglion cell layer and dendrites in sublamina b of the inner plexiform layer (IPL). The two levels of narrow dendritic stratification are precisely those demonstrated by Masland and Mills for cholinergic amacrine cells. The morphological evidence indicates that the duality of ON and OFF pathways is served separately by type b (displaced) and type a starburst amacrine cells, respectively.     

Histochemical localization of cytochrome oxidase activity in the visual system of the tree shrew:normal patterns and the effect of retinal impulse blockage

The tree shrew Tupaia belangeri has three functional pathways (ON-center, OFF-center, and W-like cells) that arise in the retina and proceed through separate LGN laminae to separate cortical targets. To determine whether these pathways have consistent differences in activity, cytochrome oxidase (C.O.) patterns were examined in the retina, LGN, and striate cortex. In six normal tree shrews the outer and inner plexiform layers of the retina were highly reactive for C.O. A pale, vascularized cleft zone separated the a (OFF) and b (ON) inner plexiform sublaminae, which seemed about equally reactive for C.O. In the LGN, laminae 1 and 2 (ON-center cells) and laminae 4 and 5 (mostly OFF-center cells) were highly reactive for C.O. LGN lamina 3 and 6 are part of an W-like afferent pathway. Lamina 3 was distinctly paler than laminae 1, 2, 4, and 5 while lamina 6 was intermediate. In the striate cortex, layer IV was the most reactive layer. Sublayer IVb (predominantly an OFF region) was consistently more reactive than sublayer IVa (predominantly ON). The middle portion, layer IVm, was paler than either IVa or IVb. This paler region includes, but extends above and below, the cell-sparse "cleft" region. Thus, considering all three levels of the retinogeniculostriate pathway, the ON and OFF systems were equally active until they reached the striate cortex, where the OFF system appeared to be more active than the ON. The W-cell laminae in the LGN exhibited the lowest level of activity. The contribution of ganglion cell activity to these patterns was assessed by intravitreal administration of tetrodotoxin (TTX) blockade either monocularly (three animals) or binocularly (two animals). In the TTX-treated retinae, the inner plexiform a and b sublaminae were paler for C.O., although visible, and were still separated by the pale cleft. The ganglion cell layer was very pale in comparison to the normal. In the LGN, monocular TTX blockade reduced the C.O. reactivity in the ON and OFF laminae that received input from the treated eye but had little effect on the W-like cell laminae. The ipsilaterally innervated ON and OFF laminae were more affected than were the contralaterally innervated laminae. Binocular TTX treatment resulted in a decrease of C.O. activity in the binocular segment of the ON and OFF LGN laminae. In the striate cortex, the most marked changes following TTX treatment occurred in layer IV.(ABSTRACT TRUNCATED AT 400 WORDS)     

Asymmetry of on- and off-pathways of blue-sensitive cones of the retina of macaques

Macaque retinal ganglion cells whose receptive-field center recieves input from blue-sensitive cones show an overt asymmetry of the frequency of ON-center and OFF-center varieties, an asymmetry not present in ganglion cells whose center receives input from the other two cone types. A similar asymmetry of ON/OFF responses is found in the local electrotetinogram (d-wave) mediated by signals from blue-sensitive cones. ‘Blue-ON-center’ ganglion cells have larger receptive-field centers and shorter conduction latencies than other opponent-color varieties, suggesting an appreciable degree of receptor convergence and presumably large cell bodies. Intracellular stainings of these neurons with Procion Yellow show that they correspond to diffuse stratified (Parasol) ganglion cells whose flat-topped dendritic arborization stratifies in the sclerad half of the inner plexiform layer. In view of the known characteristics of macaque bipolar cells and of the ON/OFF asymmetry, it is proposed that these ganglion cells are postsynaptic to cone-specific flat bipolars possibly mediating sign-inverting synaptic contacts. The results also indicate a reversal, for the blue-cone pathway, of the ON/OFF lamination of the inner plexiform layer that has recently been described in other species.     

Development of cytochrome oxidase staining in the retina and lateral geniculate nucleus: a possible correlate of ON- and OFF-center channel maturation

A histochemical stain for cytochrome oxidase (CO) activity was used to examine the maturation of a neurochemical correlate of ON and OFF channels in the retina and dorsal lateral geniculate nucleus (LGN) of the tree shrew. In the adult tree shrew, the CO staining pattern can be used as a histochemical marker of segregated ON- and OFF-center channels in the retina, LGN, and striate cortex. Our previous studies have shown that the retina is immature and the LGN unlaminated at birth. In the present study, we show that the laminar development of CO reactivity emerges during the first postnatal week in the LGN, while the maturation of CO staining in the presumed ON and OFF sublaminae of the retinal inner plexiform layer develops slowly, well after the appearance of differential laminar CO staining in the LGN.     

A novel type of complex ganglion cell in rabbit retina

The 15-20 physiological types of retinal ganglion cells (RGCs) can be grouped according to whether they fire to increased illumination in the receptive-field center (ON cells), decreased illumination (OFF cells), or both (ON-OFF cells). The diversity of RGCs has been best described in the rabbit retina, which has three types of ON-OFF RGCs with complex receptive-field properties: the ON-OFF direction-selective ganglion cells (DSGCs), the local edge detectors, and the uniformity detectors. Here we describe a novel type of bistratified ON-OFF RGC that has not been described in either physiological or morphological studies of rabbit RGCs. These cells stratify in the ON and OFF sublaminae of the inner plexiform layer, branching at about 30% and 60% depth, between the ON and OFF arbors of the bistratified DSGCs. Similar to the ON-OFF DSGCs, these cells respond with transient firing to both bright and dark spots flashed in the receptive field but, unlike the DSGCs, they show no directional preference for moving stimuli. We have termed these cells "transient ON-OFF" RGCs. Area-response measurements show that both the ON and the OFF spike responses have an antagonistic receptive-field organization, but with different spatial extents. Voltage-clamp recordings reveal transient excitatory inputs at light ON and light OFF; this excitation is strongly suppressed by surround stimulation, which also elicits direct inhibitory inputs to the cells at light ON and light OFF. Thus the receptive-field organization is mediated both within the presynaptic circuitry and by direct feed-forward inhibition.     

Immunocytochemical localization of kainate-selective glutamate receptor subunits GluR5, GluR6, and GluR7 in the cat retina

Localizations of the kainate-selective glutamate receptor subunits GluR5, 6, and 7 were studied in the cat retina by light and electron microscopic immunocytochemistry. GluR5 immunoreactivity was observed in the cell bodies and dendrites of numerous cone bipolar cells and ganglion cells. The labeled cone bipolar cells make basal or flat contacts with cone pedicles in the outer plexiform layer, leading to their identification as OFF-center bipolar cells. Reaction product within the inner plexiform layer was observed in processes of ganglion cells at their sites of input from cone bipolar cells. Staining for GluR6 was localized to A- and B-type horizontal cells, numerous amacrine cells, and an occasional cone bipolar cell. The larger ganglion cells were also immunoreactive. As with other GluR molecules, labeling was usually confined to one of the two postsynaptic elements at a cone bipolar dyad contact. Immunoreactivity for GluR7 was very limited and was seen only in a few amacrine and displaced amacrine cells. Findings of this study are consistent with a major role for kainate receptors in mediating OFF pathways in the outer retina with participation in both OFF and ON pathways in the inner retina.     

The network-selective actions of quinoxalines on the neurocircuitry operations of the rabbit retina

We examined the contribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxalole-4-propionic acid (AMPA)/kainate (KA) receptors to the light-responses of rabbit retinal neurons. In the outer retina, bath application of the AMPA/KA receptor antagonists 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and 2,3,dihydroxy-6-nitro-7-sulfamoyl-benzo-f-quinoxaline (NBQX) blocked the light-responses of horizontal cells. Application of quinoxalines enhanced ON-bipolar cell light-responses, and was associated with a hyperpolarization of their resting potentials. In the inner retina, application of both AMPA/KA and NMDA antagonists to AII amacrine-like cells only partially blocked their light-responses. Their residual responses may reflect electrical coupling to neighboring ON-center cone bipolar cells, and can inhibit OFF-center ganglion cells. ON-sustained ganglion cells were highly sensitive to the quinoxalines, which reduced their light-evoked firing, while the firing of ON-transient cells remained as NMDA-mediated light-responses. Quinoxalines had differential effects on the firing rates of ON- and OFF-center ganglion cells: ON-cells were reduced, while OFF-cells were increased. In contrast, firing rates of ON-OFF ganglion cells were not excited by NBQX, and showed a recovered light-response mediated by NMDA receptors. The receptive field surround was lost in ganglion cells. For comparison, NMDA antagonists had only moderate effects on all ganglion cell light-responses. Our results indicate that NMDA and AMPA/KA receptors both contribute to ganglion cell light-responses. However, AMPA/KA receptors also significantly effect the light-response of neurons presynaptic to retinal ganglion cells, altering the observed pharmacology at the ganglion cell level.     

Effects of dopamine antagonists on receptive fields of brisk cells and directionally selective cells in the rabbit retina

The effects of dopamine antagonists on the extracellularly recorded activity of ON- and OFF-center brisk ganglion cells and ON-OFF directionally selective ganglion cells in the rabbit retina were investigated. Haloperidol, fluphenazine, and cis-flupenthixol, infused in the arterial system supplying the eye, produced similar effects. In general, these drugs reduced the antagonistic surround responses of brisk ganglion cells, reduced the sustained excitation of the center response of ON-center brisk-sustained cells, reduced the leading edge response of ON-OFF directionally selective cells to moving light stimuli along with any sustained excitation to stationary light stimuli, and affected the spontaneous activity of the cells. These drug effects appear to be due to a blockade of D-1 (adenylate cyclase-linked) receptors and not to D-2 receptors. Neither S-sulpiride nor metoclopramide, two specific D-2 antagonists, had much effect. The findings are the first to describe the functional effects of dopamine antagonists on single cells in the mammalian retina and on ganglion cell activity in the vertebrate retina.     

Microglial cell responses in the rabbit retina following transection of the optic nerve

The response of retinal microglial cells, which accompanies retrograde degeneration of ganglion cell axons and perikarya (induced by transection of the optic nerve), was studied in whole-mounted rabbit retinae labeled enzyme-histochemically for nucleoside diphosphatase (NDPase), which is a microglial cell marker. A few days after transection, the number of microglial cells/mm2, as well as their staining intensity, began to increase in the inner plexiform layer. The mosaic-like distribution of the star-shaped microglial cells present in the inner plexiform layer of a normal rabbit retina was preserved during ganglion cell degeneration. As in the normal retina, processes of individual cells never overlapped with those of neighboring cells in the inner plexiform layer because individual cells in the "degenerating" retina acquired shorter processes, i.e., the cells occupied a smaller territory compared to the normal retina. In the nerve fiber layer the number and staining intensity of NDPase-labeled microglial cell processes (most of which are aligned in parallel with degenerating ganglion cell axons) transiently increased and returned to normal values by 5 months post-transection. Microglial cells that are not detectably NDPase labeled in the outer plexiform layer of a normal rabbit retina acquire intense staining a few days after the nerve is cut. The functional significance of the increased NDPase activity in the plasma membrane of microglial cells during degeneration remains to be determined.     

Transient responses of rabbit retinal ganglion cells to photic and electrical stimuli.

The relationships between the center and the surround of the receptive field of the rabbit retinal ganglion cell were investigated. This was done by coupling localized light spots and electrical activation of the retina and by analyzing the time of the excitatory and inhibitory periods. The responsiveness to the electrical transretinal pulse revealed a) that ON stimulation in OFF-center cells and OFF stimulation in ON-center cells, elicited a primary period of inhibition with a short latency; b) the long latency response of surround stimulation was not preceded by an inhibitory period unless the center was simultaneously stimulated in the same direction; c) a transient response to a stationary spot of light is followed by a period of inhibition. These results are discussed in relation to the known cellular retinal networks.     

Starburst amacrine cells in cat retina are associated with bistratified, presumed directionally selective, ganglion cells

Starburst amacrine cells of cat retina are similar in form, though more delicate and less profusely branched, when compared to the starburst/cholinergic amacrine cells of rabbit retina, as identified in Golgi preparations. In both species, type a cells branch in the middle of sublamina a of the inner plexiform layer (IPL), but type b (displaced) starburst amacrine cells of cat branch near the a/b sublaminar border (stratum 3) of the IPL, not in the middle of sublamina b (stratum 4), as do those of rabbit. Nevertheless, in each species, this starburst substratum in sublamina b coincides with the sublamina b-level branching of a bistratified ganglion cell, which in rabbit retina shows directionally selective responses. It is proposed that starburst amacrine cells of cat retina are cholinergic and, as in rabbit retina, make selective connections with on-off directionally selective ganglion cells.     

Cone bipolar cells in the retina of the microbat Carollia perspicillata

We studied the retinal cone bipolar cells of Carollia perspicillata, a microchiropteran bat of the phyllostomid family. Microchiroptera are strongly nocturnal, with small eyes and rod‐dominated retinae. However, they also possess a significant cone population (2–4%) comprising two spectral types, which are hence the basis for daylight and color vision. We used antibodies against the calcium‐binding protein recoverin and the carbohydrate epitope 15 (CD15) as reliable markers for certain cone bipolar cells. Dye injections of recoverin‐ or CD15‐prelabeled cone bipolar cells in vertical slices revealed the morphology of the axon terminal system of individual bipolar cells. Seven distinct cone bipolar cell types were identified. They differed in the morphology and stratification level of their axon terminal system in the inner plexiform layer and in immunoreactivity for recoverin and/or CD15. Additional immunocytochemical markers were used to assess the functional ON/OFF subdivision of the inner plexiform layer. In line with the extended thickness of the ON sublayer of the inner plexiform layer in the microbat retina, more ON than OFF cone bipolar cell types were found, namely, four versus three. Most likely, in the bats' predominantly dark environment, ON signals have greater importance for contrast perception. We conclude that the microbat retina conforms to the general mammalian blueprint, in which light signals of intensities above rod sensitivity are detected by cones and transmitted to various types of ON and OFF cone bipolar cells. J. Comp. Neurol. 523:963–981, 2015. © 2014 Wiley Periodicals, Inc.     

A novel type of interplexiform amacrine cell in the mouse retina

Mammalian retinas comprise an enormous variety of amacrine cells with distinct properties and functions. The present paper describes a new interplexiform amacrine cell type in the mouse retina. A transgenic mouse mutant was used that expressed the gene for the enhanced green fluorescent protein (EGFP) instead of the coding DNA of connexin45 in several retinal cell classes, among which a single amacrine cell population was most prominently labelled. Staining for EGFP and different marker proteins showed that these amacrine cells are interplexiform: they stratify in stratum S4/5 of the inner plexiform layer and send processes to the outer plexiform layer. These cells were termed IPA-S4/5 cells. They belong to the group of medium-field amacrine cells and are coupled homologously and heterologously to other amacrine cells by connexin45. Immunostaining revealed that IPA-S4/5 cells are GABAergic and express GAT-1, a plasma-membrane-bound GABA transporter possibly involved in non-vesicular GABA release. To characterize the light responses of IPA-S4/5 cells, patch-clamp recordings in retinal slices were made. Consistent with their stratification in the ON sublamina of the inner plexiform layer, cells depolarized in response to light ON stimuli and transiently hyperpolarized in response to light OFF. Responses of cells to green (578 nm) and blue (400 nm) light suggest that they receive input from cone bipolar cells contacting both M- and S-cones, possibly with reduced S-cone input. A new type of interplexiform ON amacrine cell is described, which is strongly coupled and uses GABA but not dopamine as its neurotransmitter.     

Is taurine a neurotransmitter in rabbit retina?

Rabbit retina was used as a model to study the possible role of taurine in the retina. The taurine-synthesizing enzyme, cysteine sulfinic acid decarboxylase (CSAD), is localized immunohistochemically using specific antibodies against CSAD. The CSAD-immunoreactivity appears to be most prominent in the inner nuclear layer (INL) and ganglion cell layer (GCL). The inner plexiform layer (IPL), the outer nuclear layer and outer plexiform layer are sporadically stained. The CSAD-positive neurons include some amacrine cells and probably the bipolar cells in the INL and some large and small ganglion cells in the GCL. Autoradiographic studies reveal that the uptake of [3H]taurine is most prominent in the INL. The IPL and GCL, as well as the Müller cells, also show a moderate degree of [3H]taurine accumulation. In conclusion, we have demonstrated the presence of the taurine-synthesizing enzyme and uptake systems in rabbit retina. Based on the above evidence, we propose that taurine may be used by some neurons, presumably amacrine cells, as a transmitter in the rabbit retina.     

Rat retinal microglial cells under normal conditions, after optic nerve section, and after optic nerve section and intravitreal injection of trophic factors or macrophage inhibitory factor

Retinal microglial cells may have a role in both degeneration and neuroprotection of retinal ganglion cells (RGC) after optic nerve (ON) section. We have used NDPase enzymohistochemistry to label adult rat retinal microglial cells and have studied these cells under normal conditions, after left ON section, and after left ON section and eye puncture or intravitreal injection of different substances: vehicle, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin 3 (NT3), or macrophage inhibitory factor (MIF). Resident microglial cells are present in four layers in the adult rat retina: the nerve fiber layer (NFL), ganglion cell layer (GCL), inner plexiform layer (IPL), and outer plexiform layer (OPL). Left ON section induces microglial activation in the ipsilateral and contralateral retina as manifested by stronger staining intensity in both retinas and increased microglial cell densities in the NFL, IPL, and GCL of the ipsilateral retina. Left ON section followed by left eye puncture or intravitreal injection increases microglial cell density in both retinas and induces changes in the microglial cells of the ipsilateral retina that vary depending on the substance injected: BDNF injections delay microglial activation, possibly through retinal ganglion cell neuroprotection, whereas NT3 partially inhibits microglial activation in the NFL; MIF injections have no clear effects on microglial activation. In conclusion, retinal microglial cells become activated after an ON section and react more intensely when the eye is also punctured or injected, and this response may be altered by using neurotrophic factors, although the effects of MIF are less clear.     

Biocytin wide-field bipolar cells in rabbit retina selectively contact blue cones

The biocytin wide-field bipolar cell in rabbit retina has a broad axonal arbor in layer 5 of the inner plexiform layer and a wide dendritic arbor that does not contact all cones in its dendritic field. The purpose of our study was to identify the types of cones that this cell contacts. We identified the bipolar cells by selective uptake of biocytin, labeled the cones with peanut agglutinin, and then used antibodies against blue cone opsin and red-green cone opsin to identify the individual cone types. The biocytin-labeled cells selectively contacted cones whose outer segments stained for blue cone opsin and avoided cones that did not. We conclude that the biocytin wide-field bipolar cell is an ON blue cone bipolar cell in the rabbit retina and is homologous to the blue cone bipolar cells that have been previously described in primate, mouse, and ground squirrel retinas.     

Midget ganglion cells of the parafovea of the human retina: a study by electron microscopy and serial section reconstructions.

In this study we used serial section electron microscopy and three-dimensional reconstructions to examine four midget ganglion cells of the human retina. The four cells were located in the parafoveal retina 2.5 mm or 8 degrees from the foveal center. Both type a (with dendritic trees in distal inner plexiform layer) and type b (with dendritic trees in proximal inner plexiform layer) midget ganglion cells have been studied. These cells have dendritic trees of 7-9 microns diameter, and their complete dendritic trees in the neuropil of the inner plexiform layer can be analyzed, as well as the bipolar cell axon terminals having synaptic input, by a study of 100-150 serial ultrathin sections. Type a midget ganglion cells appear to be in a one-to-one relationship with flat midget bipolar cell axon terminals ending in distal inner plexiform layer. Type b midget ganglion cells are in a one-to-one synaptic relationship with invaginating midget bipolar cell axon terminals in proximal inner plexiform layer. The midget bipolar cells primarily involved with the midget ganglion cells do not contact other ganglion cell dendrites. In other words, midget bipolar cells appear to be in exclusive contact with single midget ganglion cells in the human retina. The midget ganglion cells receive most of their input from their associated midget bipolar cells in the form of ribbon synapses at dyads or monads (55-81 ribbons total), although ribbonless synapses are seen occasionally. In all four midget ganglion cells reconstructed, one or two other bipolar cell axon terminals, presumed to be from wide-field bipolar types, provide 1-3 ribbon synapses each. The number of amacrine synapses upon a midget ganglion cell's dendritic tree is approximately equal to the number of bipolar ribbon inputs (43%-56% bipolar ribbons: 44%-57% amacrine synapses). We assume from our knowledge of response characteristics of ganglion cells in other mammalian retinas (Nelson et al., '78: J. Neurophysiol. 41:427-483), that the type a midget ganglion cell and its exclusive connectivity with a flat midget bipolar cell forms a single cone connected OFF-center pathway, whereas the type b midget ganglion cell with its exclusive connectivity to an invaginating midget bipolar cell forms a single cone connected ON-center pathway, through the retina to the brain.     

Synapses of cone horizontal cell axons in goldfish retina

The axon terminals of cone horizontal cells in the goldfish retina form typical chemical synaptic contacts in the middle of the inner nuclear layer. Approximately 60% of the identified postsynaptic elements were perikarya, axons and dendrites of bipolar cells. The other identified postsynaptic elements were perikarya and processes of interplexiform cells. We propose that the horizontal cell axon terminal contribute to the antagonistic surround responses of the bipolar cells and that they modulate inputs to the outer plexiform layer conveyed by interplexiform cells. Output synapses from horizontal cell axons to unidentified neuronal processes as well as occasional input synapses to the axons from interplexiform cell processes and unidentified perikarya were also observed in the same region of the inner nuclear layer.     

Synaptic inputs to physiologically identified retinal X-cells in the cat.

The cat retina contains a number of different classes of ganglion cells, each of which has a unique set of receptive field properties. The mechanisms that underlie the functional differences among classes, however, are not well understood. All of the afferent input to retinal ganglion cells are from bipolar and amacrine cell terminals in the inner plexiform layer, suggesting that the physiological differences among cat retinal ganglion cells might be due to differences in the proportion of input that they receive from these cell types. In this study, we have combined in vivo intracellular recording and labeling with subsequent ultrastructural analysis to determine directly the patterns of synaptic input to physiologically identified X-cells in the cat retina. Our primary aim in these analyses was to determine whether retinal X-cells receive a characteristic pattern of bipolar and amacrine cell input, and further, whether the functional properties of this cell type can be related to identifiable patterns of synaptic input in the inner plexiform layer. We reconstructed the entire dendritic arbor of an OFF-center X-cell and greater than 75% of the dendritic tree of an ON-center X-cell and found that 1) both ON- and OFF-center X-cells are contacted with approximately the same frequency by bipolar and amacrine cell terminals, 2) each of these input types is distributed widely over their dendritic fields, and 3) there is no significant difference in the pattern of distribution of bipolar and amacrine cell synapses onto the dendrites of either cell type. Comparisons of the inputs to the ON- and the OFF-center cell, however, did reveal differences in the complexity of the synaptic arrangements found in association with the two neurons; a number of complex synaptic arrangements, including serial amacrine cell synapses, were found exclusively in association with the dendrites of the OFF-center X-cell. Most models of retinal X-cell receptive fields, because their visual responses share a number of features with those of bipolar cells, have attributed X-cell receptive field properties to their bipolar cell inputs. The data presented here, the first obtained from analyzing the inputs to the entire dendritic arbors of retinal X-cells, demonstrate that these retinal ganglion cells receive nearly one-half of their input from amacrine cells. These results clearly indicate that further data concerning the functional consequences of amacrine cell input are needed to understand more fully visual processing in the X-cell pathway.     

Localization and developmental expression patterns of the neuronal K-Cl cotransporter (KCC2) in the rat retina.

The processing of signals by integrative neurons in the retina and CNS relies strongly on inhibitory synaptic inputs, principally from GABAergic and glycinergic neurons that serve primarily to hyperpolarize postsynaptic neurons. Recent evidence indicates that the neuron-specific K-Cl cotransporter 2 (KCC2) is the major chloride extrusion system permitting hyperpolarizing inhibitory responses. It has been hypothesized that depolarizing GABA responses observed in immature neurons are converted to hyperpolarizing responses in large part by the expression of KCC2 during the second week of postnatal development. The cell-specific localization and developmental expression of KCC2 protein have been examined in relatively few neural tissues and have never been studied in retina, of which much is known physiologically and morphologically about inhibitory synaptic circuits. We examined the localization of KCC2 in adult rat retina with immunohistochemical techniques and determined the time course of its postnatal expression. KCC2 expression was localized in horizontal cells, bipolar cells, amacrine cells, and, most likely, ganglion cells, all of which are known to express GABA receptor subtypes. Developmentally, KCC2 expression in the retina increased gradually from postnatal day 1 (P1) until P14 in the inner retina, whereas expression was delayed in the outer plexiform layer until P7 but reached its adult level by P14. These data support the hypothesis that the function of KCC2 is intimately involved in GABAergic synaptic processing. Furthermore, the delayed temporal expression of KCC2 in the outer plexiform layer indicates that GABAergic function may be differentially regulated in retina during postnatal development and that GABA may produce depolarizing responses in the outer plexiform layer at times when it generates hyperpolarizing responses in the inner plexiform layer.