Immunocytochemical localization of GABA and glycine in amacrine and displaced amacrine cells of macaque monkey retina

Mammalian Retinal Cell Quantification

Normal retina and its cell layers are essential for processing visual stimuli, and loss of its integrity has been documented in many disease processes. The numbers and the axonal processes of retinal ganglion cells are reduced substantially in glaucoma, leading to vision loss and blindness. Similarly, selective loss of photoreceptors in age-related macular degeneration and hereditary retinal dystrophies also results in the compromise of visual acuity. Development of genetically modified mice has led to increased understanding of the pathogenesis of many retinal diseases. Similarly, in this digital era, usage of modalities to quantify the retinal cell loss has grown exponentially leading to a better understanding of the suitability of animal models to study human retinal diseases. These quantification modalities provide valuable quantifiable data in studying pathogenesis and disease progression. This review will discuss the immunohistochemical markers for various retinal cells, available automated tools to quantify retinal cells, and present an example of retinal ganglion cell quantification using HALO image analysis platform. Additionally, we briefly review retinal cell types and subtypes, salient features of retina in various laboratory animal species, and a few of the main disease processes that affect retinal cell numbers in humans.

Cell types and cell circuits in human and non-human primate retina

This review summarizes our current knowledge of primate including human retina focusing on bipolar, amacrine and ganglion cells and their connectivity. We have two main motivations in writing. Firstly, recent progress in non-invasive imaging methods to study retinal diseases mean that better understanding of the primate retina is becoming an important goal both for basic and for clinical sciences. Secondly, genetically modified mice are increasingly used as animal models for human retinal diseases. Thus, it is important to understand to which extent the retinas of primates and rodents are comparable. We first compare cell populations in primate and rodent retinas, with emphasis on how the fovea (despite its small size) dominates the neural landscape of primate retina. We next summarise what is known, and what is not known, about the postreceptoral neurone populations in primate retina. The inventories of bipolar and ganglion cells in primates are now nearing completion, comprising ~12 types of bipolar cell and at least 17 types of ganglion cell. Primate ganglion cells show clear differences in dendritic field size across the retina, and their morphology differs clearly from that of mouse retinal ganglion cells. Compared to bipolar and ganglion cells, amacrine cells show even higher morphological diversity: they could comprise over 40 types. Many amacrine types appear conserved between primates and mice, but functions of only a few types are understood in any primate or non-primate retina. Amacrine cells appear as the final frontier for retinal research in monkeys and mice alike.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

The RNA binding protein RBPMS is a selective marker of ganglion cells in the mammalian retina

There are few neurochemical markers that reliably identify retinal ganglion cells (RGCs), which are a heterogeneous population of cells that integrate and transmit the visual signal from the retina to the central visual nuclei. We have developed and characterized a new set of affinity-purified guinea pig and rabbit antibodies against RNA-binding protein with multiple splicing (RBPMS). On western blots these antibodies recognize a single band at 〜24 kDa, corresponding to RBPMS, and they strongly label RGC and displaced RGC (dRGC) somata in mouse, rat, guinea pig, rabbit, and monkey retina. RBPMS-immunoreactive cells and RGCs identified by other techniques have a similar range of somal diameters and areas. The density of RBPMS cells in mouse and rat retina is comparable to earlier semiquantitative estimates of RGCs. RBPMS is mainly expressed in medium and large DAPI-, DRAQ5-, NeuroTrace- and NeuN-stained cells in the ganglion cell layer (GCL), and RBPMS is not expressed in syntaxin (HPC-1)-immunoreactive cells in the inner nuclear layer (INL) and GCL, consistent with their identity as RGCs, and not displaced amacrine cells. In mouse and rat retina, most RBPMS cells are lost following optic nerve crush or transection at 3 weeks, and all Brn3a-, SMI-32-, and melanopsin-immunoreactive RGCs also express RBPMS immunoreactivity. RBPMS immunoreactivity is localized to cyan fluorescent protein (CFP)-fluorescent RGCs in the B6.Cg-Tg(Thy1-CFP)23Jrs/J mouse line. These findings show that antibodies against RBPMS are robust reagents that exclusively identify RGCs and dRGCs in multiple mammalian species, and they will be especially useful for quantification of RGCs.

Disinhibitory recruitment of NMDA receptor pathways in retina

Glutamate release at bipolar to ganglion cell synapses activates NMDA and AMPA/kainic acid (KA) ionotropic glutamate receptors. Their relative strength determines the output signals of the retina. We found that this balance is tightly regulated by presynaptic inhibition that preferentially suppresses NMDA receptor (NMDAR) activation. In transient ON-OFF neurons, block of GABA and glycine feedback enhanced total NMDAR charge by 35-fold in the ON response and 9-fold in the OFF compared with a 1.7-fold enhancement of AMPA/KA receptors. Blocking only glycine receptors enhanced the NMDAR excitatory postsynaptic current 10-fold in the ON and 2-fold in the OFF pathway. Blocking GABA(A) or GABA(C) receptors (GABA(C)Rs or GABA(A)Rs) produced small changes in total NMDAR charge. When both GABA(A)Rs and GABA(C)Rs were blocked, the total NMDAR charge increased ninefold in the ON and fivefold in the OFF pathway. This exposed a strong GABA(C)R feedback to bipolar cells that was suppressed by serial amacrine cell synapses mediated by GABA(A)Rs. The results indicate that NMDAR currents are large but latent, held in check by dual GABA and glycine presynaptic inhibition. One example of this controlled NMDAR activation is the cross talk between ON and OFF pathways. Blocking the ON pathway increased NMDAR relative strength in the OFF pathway. Stimulus prolongation similarly increased the NMDAR relative strength in the OFF response. This NMDAR enhancement was produced by a diminution in GABA and glycine feedback. Thus the retinal network recruits NMDAR pathways through presynaptic disinhibition.

Parvalbumin-immunoreactive amacrine cells of macaque retina

A number of authors have observed amacrine cells containing high levels of immunoreactive parvalbumin in primate retinas. The experiments described here were designed to identify these cells morphologically, to determine their neurotransmitter, to record their light responses, and to describe the other cells that they contact. Macaque retinas were fixed in paraformaldehyde and labeled with antibodies to parvalbumin and one or two other markers, and this double- and triple-labeled material was analyzed by confocal microscopy. In their morphology and dendritic stratification patterns, the parvalbumin-positive cells closely resembled the knotty type 2 amacrine cells described using the Golgi method in macaques. They contained immunoreactive glycine transporter, but not immunoreactive gamma-aminobutyric acid, and therefore, they use glycine as their neurotransmitter. Their spatial density was relatively high, roughly half that of AII amacrine cells. They contacted lobular dendrites of AII cells, and they are expected to be presynaptic to AII cells based on earlier ultrastructural studies. They also made extensive contacts with axon terminals of OFF midget bipolar cells whose polarity cannot be predicted with certainty. A macaque amacrine cell of the same morphological type depolarized at the onset of increments in light intensity, and it was well coupled to other amacrine cells. Previously, we described amacrine cells like these that contacted OFF parasol ganglion cells and OFF starburst amacrine cells. Taken together, these findings suggest that one function of these amacrine cells is to inhibit the transmission of signals from rods to OFF bipolar cells via AII amacrine cells. Another function may be inhibition of the OFF pathway following increments in light intensity.

Expression of the homeodomain transcription factor Meis2 in the embryonic and postnatal retina

Members of the Meis subfamily of homeodomain-containing transcription factors play important roles during development and disease. Here we report that the Meis family protein Meis2 is expressed by a subpopulation of gamma-aminobutyric acid (GABA)ergic amacrine (AM) cells in the adult and embryonic retina of different vertebrate species. In mice, Meis2-expressing (Meis2+) AM cells are not cholinergic or dopaminergic, but some are immunoreactive for neuronal nitric oxide synthase (bNOS). About 50% of the mouse Meis2+ AM cell population expresses the calcium-binding protein calretinin, and some Meis2+ AM cells show characteristics of Type II CD-15+ cells. AM cell expression of Meis2 is lost in a conditional knockout mouse model for Pax6, indicating a dependency upon Pax6. Bromodeoxyuridine pulse labeling experiments and immunohistochemical staining for the neuronal marker NeuN in embryonic mouse retinae indicate that Meis2 is an early marker for newly postmitotic AM cells. In addition, taking advantage of the protracted retinal development in humans, we show that newly generated AM cells express Meis2 before adopting the GABAergic or glycinergic neurotransmitter phenotype. As development proceeds, some AM cells lose Meis2 expression concomitantly with the appearance of glycine, while other AM cells retain Meis2 expression after they express GABA. These data identify Meis2 as a suitable marker for the study of AM cell diversity and development in addition to providing evidence for the stepwise specification of the glycinergic and GABAergic neurotransmitter phenotypes during AM cell differentiation.

Expression of the gamma-aminobutyric acid (GABA) plasma membrane transporter-1 in monkey and human retina

PURPOSE

To determine the expression pattern of the predominant gamma-aminobutyric acid (GABA) plasma membrane transporter GAT-1 in Old World monkey (Macaca mulatta) and human retina.

METHODS

GAT-1 was localized in retinal sections by using immunohistochemical techniques with fluorescence and confocal microscopy. Double-labeling studies were performed with the GAT-1 antibody using antibodies to GABA, vasoactive intestinal polypeptide (VIP), tyrosine hydroxylase (TH), and the bipolar cell marker Mab115A10.

RESULTS

The pattern of GAT-1 immunostaining was similar in human and monkey retinas. Numerous small immunoreactive somata were in the inner nuclear layer (INL) and were present rarely in the inner plexiform layer (IPL) of all retinal regions. Medium GAT-1 somata were in the ganglion cell layer in the parafoveal and peripheral retinal regions. GAT-1 fibers were densely distributed throughout the IPL. Varicose processes, originating from both the IPL and somata in the INL, arborized in the outer plexiform layer (OPL), forming a sparse network in all retinal regions, except the fovea. Sparsely occurring GAT-1 processes were in the nerve fiber layer in parafoveal regions and near the optic nerve head but not in the optic nerve. In the INL, 99% of the GAT-1 somata contained GABA, and 66% of the GABA immunoreactive somata expressed GAT-1. GAT-1 immunoreactivity was in all VIP-containing cells, but it was absent in TH-immunoreactive amacrine cells and in Mab115A10 immunoreactive bipolar cells.

CONCLUSIONS

GAT-1 in primate retinas is expressed by amacrine and displaced amacrine cells. The predominant expression of GAT-1 in the inner retina is consistent with the idea that GABA transporters influence neurotransmission and thus participate in visual information processing in the retina.

Localization of glycine receptor alpha subunits on bipolar and amacrine cells in primate retina

The major inhibitory neurotransmitter glycine is used by about half of the amacrine cells in the retina. Amacrine cells provide synaptic output to bipolar, ganglion, and other amacrine cells. The present study investigated whether different bipolar and amacrine cell types in the primate retina differ with respect to the expression of glycine receptor (GlyR) subtypes. Antibodies specific for the alpha1, alpha2, and alpha3 subunits of the GlyR were combined with immunohistochemical markers for bipolar and amacrine cells and applied to vertical sections of macaque (Macaca fascicularis) and marmoset (Callithrix jacchus) retinae. For all subunits, punctate immunoreactivity was expressed in the inner plexiform layer. The GlyRalpha2 immunoreactive (IR) puncta occur at the highest density, followed by GlyR(alpha)3 and GlyR(alpha)1 IR puncta. Postembedding electron microscopy showed the postsynaptic location of all subunits. Double immunofluorescence demonstrated that the three alpha subunits are clustered at different postsynaptic sites. Two OFF cone bipolar cell types (flat midget and diffuse bipolar DB3), are predominantly associated with the alpha1 subunit. Two ON bipolar cell types, the DB6 and the rod bipolar cell, are predominantly associated with the alpha2 subunit. The glycinergic AII amacrine cell is presynaptic to the alpha1 subunit in the OFF-sublamina, and postsynaptic to the alpha2 subunit in the ON-sublamina. Another putative glycinergic cell, the vesicular glutamate transporter 3 cell, is predominantly presynaptic to the alpha2 subunit. The dopaminergic amacrine cell expresses the alpha3 subunit at a low density.

Morphological features of chick retinal ganglion cells

On average, in chicks, the total number of retinal ganglion cells is 4.9 x 10(6) and the cell density is 10400 cells/mm2. Two high-density areas, namely the central area (CA) and the dorsal area (DA), are located in the central and dorsal retinas, respectively, in post-hatching day 8 (P8) chicks (19000 cells/mm2 in the CA; 12800 cells/mm2 in the DA). Thirty percent of total cells in the ganglion cell layer are resistant to axotomy of the optic nerve. The distribution of the axotomy resistant cells shows two high-density areas in the central and dorsal retinas, corresponding to the CA (5800 cells/mm2) and DA (3200 cells/mm2). The number of presumptive ganglion cells in P8 chicks is estimated to be 4 x 10(6) (8600 cells/mm2 on average) and the density is 13500 and 10200 cells/mm2 in the CA and DA, respectively, and 4300 cell/mm2 in the temporal periphery (TP). The somal area of presumptive ganglion cells is small in the CA and DA (mean (+/- SD) 35.7 +/- 9.1 and 40.0 +/- 11.3 microm2, respectively) and their size increases towards the periphery (63.4 +/- 29.7 microm2 in the TP), accompanied by a decrease in cell density. Chick ganglion cells are classified according to dendritic field, somal size and branching density of the dendrites as follows: group Ic, Is, IIc, IIs, Ills, IVc. The density of branching points of dendrites is approximately 10-fold higher in the complex type (c) than in the simple type (s) in each group. The chick inner plexiform layer is divided into eight sublayers according to the dendritic strata of retinal ganglion cells and 26 stratification patterns are discriminated.

Morphological analysis of disabled-1-immunoreactive amacrine cells in the guinea pig retina

Disabled-1 (Dab1) is an adapter molecule in a signaling pathway, stimulated by reelin, that controls cell positioning in the developing brain. It localizes to selected neurons in the nervous system, including the retina, and Dab1-like immunoreactivity is present in AII amacrine cells in the mouse retina. This study was conducted to characterize Dab1-labeled cells in the guinea pig retina in detail using immunocytochemistry, quantitative analysis, and electron microscopy. Dab1 immunoreactivity is present in a class of amacrine cell bodies located in the inner nuclear layer adjacent to the inner plexiform layer (IPL). These cells give rise to processes that ramify the entire depth of the IPL. Double-labeling experiments demonstrated that these amacrine cells make contacts with the axon terminals of rod bipolar cells and that their processes make contacts with each other via connexin 36 in sublamina b of the IPL. In addition, all Dab1-labeled amacrine cells showed glycine transporter 1 immunoreactivity, indicating that they are glycinergic. The density of Dab1-labeled AII amacrine cells decreased from about 3,750 cells/mm(2) in the central retina to 1,725 cells/mm(2) in the peripheral retina. Dab1-labeled amacrine cells receive synaptic inputs from the axon terminals of rod bipolar cells in stratum 5 of the IPL. From these morphological features, Dab1-labeled cells of the guinea pig retina resemble the AII amacrine cells described in other mammalian species. Thus, the rod pathway of the guinea pig retina follows the general mammalian scheme and Dab1 antisera can be used to identify AII amacrine cells in the mammalian retina.

Diversity of glycine receptors in the mouse retina: localization of the alpha3 subunit

Glycine receptors (GlyRs) and their role in retinal circuitry were analyzed immunocytochemically in wild-type and GlyR alpha3 subunit-deficient (Glra3(-/-)) mouse retinae. GlyRs are localized in the inner plexiform layer in brightly fluorescent puncta, which are likely to represent postsynaptically clustered GlyRs. Approximately one third of the clusters were found to contain the alpha1 subunit, and half possessed the alpha3 subunit. However, these two GlyR isoforms were localized at different glycinergic synapses. In the Glra3(-/-) mouse, alpha3 subunit clusters were completely eliminated, although the total number of GlyR clusters was only slightly reduced. This finding indicates that other GlyR subunits (such as alpha2 or alpha4) may have compensated for the loss of the alpha3 subunit. Characteristic expression patterns of the alpha1 and alpha3 subunits within the synaptic circuits of the retina were revealed by double labeling sections for GlyRs and markers that define specific retinal neurons. The alpha1 subunit mediates signal transfer in the rod pathway between AII amacrine cells and OFF-cone bipolar cells. In contrast, the alpha3 subunit appears to be predominantly involved with the cone pathways. Thus, expression of different GlyR alpha subunit genes correlates with anatomically defined connectivities.

Inner retinal neurons display differential responses to N-methyl-D-aspartate receptor activation

The N-methyl-D-aspartate (NMDA) responses of neurons from within the inner rabbit retina were mapped using a channel permeable cation, 1-amino-4-guanidobutane (agmatine, AGB). Serial sections were subsequently probed with immunoglobulins targeting AGB, glutamate, gamma-aminobutyric acid (GABA), and glycine to visualize the NMDA responses of neurochemical subpopulations of neurons. Most inner retinal subpopulations of neurons demonstrated an NMDA concentration-dependent increase in activation. This NMDA-induced activation displayed a distinct pattern, with the most sensitive class to least sensitive class ranking being GC > GABA cAC > GABA/Gly cAC > Gly cAC > GABA dAC (GC, ganglion cells; AC, amacrine cells; c, conventional; d, displaced; Gly, glycine). The variable NMDA response may reflect differences in NMDA receptor subunit disposition or differences in receptor density. In addition to the variable NMDA activation pattern, we found that virtually all ganglion cells (87%) showed NMDA-gated AGB entry, compared with only 58% of amacrine cells. We conclude that a large cohort of amacrine cells do not possess functional NMDA receptors. In addition to most ganglion cells being activated by NMDA, a large subpopulation displayed the highest sensitivity to NMDA application. The functional significance of this finding is that the ganglion cell population will be the first neuronal class to be susceptible to glutamate-induced neurotoxicity mediated through the NMDA receptor. The addition of betaxolol significantly reduced NMDA-mediated AGB entry into most neuronal groups (ganglion cells, GABA, and glycine amacrine cells), with the greatest effect being on ganglion cells. Betaxolol had no significant effect on NMDA-gated entry of AGB on the GABA/Gly amacrine cell population.

A precise retinotopic map of primate striate cortex generated from the representation of angioscotomas

Shadows cast by retinal blood vessels are represented in striate cortex of the squirrel monkey. Their pattern was exploited to generate a true retinotopic map of V1. For calibration, retinal landmarks were projected onto a tangent screen to measure their visual field location. Next, the retina was warped onto striate cortex, distorting it as necessary to match each retinal vessel to its cortical representation. Maps from four hemispheres of two normal adult squirrel monkeys were created and used to derive expressions for cortical magnification factor (M). A mean map was produced by averaging the individual maps. To address the controversial issue of whether the ratio of retinal ganglion cell (RGC) density to M is constant at all eccentricities, we stained a retinal whole mount from one of the two monkeys for Nissl substance. A ganglion cell density map was compiled by sampling the concentration of cells at 171 retinal points. Allowance was made for displaced amacrine cells and for the centripetal displacement of RGCs from central photoreceptors. After these corrections the V1 surface area and RGC density were compared at each eccentricity. The cortical representation of the macula was found to be amplified, even beyond the magnification expected from its high density of RGCs. For example, the central 4 degrees of visual field were allotted 27% of the surface area of V1 but were supplied by only 12% of RGCs. We conclude that, in monkey striate cortex, more tissue is allocated per ganglion cell for the analysis of information emanating from the macula as compared with the peripheral retina.

D-serine and serine racemase are present in the vertebrate retina and contribute to the physiological activation of NMDA receptors

d-serine has been proposed as an endogenous modulator of N-methyl-d-aspartate (NMDA) receptors in many brain regions, but its presence and function in the vertebrate retina have not been characterized. We have detected d-serine and its synthesizing enzyme, serine racemase, in the retinas of several vertebrate species, including salamanders, rats, and mice and have localized both constituents to Müller cells and astrocytes, the two major glial cell types in the retina. Physiological studies in rats and salamanders demonstrated that, in retinal ganglion cells, d-serine can enhance excitatory currents elicited by the application of NMDA, as well as the NMDA receptor component of light-evoked synaptic responses. Application of d-amino acid oxidase, which degrades d-serine, reduced the magnitude of NMDA receptor-mediated currents, raising the possibility that endogenous d-serine serves as a ligand for setting the sensitivity of NMDA receptors under physiological conditions. These observations raise exciting new questions about the role of glial cells in regulating the excitability of neurons through release of d-serine.

Light and electron microscopic analysis of aquaporin 1-like-immunoreactive amacrine cells in the rat retina

Aquaporin 1 (AQP1; also known as CHIP, a channel-forming integral membrane protein of 28 kDa) is the first protein to be shown to function as a water channel and has been recently shown to be present in the rat retina. We previously showed (Kim et al. [1998] Neurosci Lett 244:52-54) that AQP1-like immunoreactivity is present in a certain population of amacrine cells in the rat retina. This study was conducted to characterize these cells in more detail. With immunocytochemistry using specific antisera against AQP1, whole-mount preparations and 50-microm-thick vibratome sections were examined by light and electron microscopy. These cells were a class of amacrine cells, which had symmetric bistratified dendritic trees ramified in stratum 2 and in the border of strata 3 and 4 of the inner plexiform layer (IPL). Their dendritic field diameters ranged from 90 to 230 microm. Double labeling with antisera against AQP1 and gamma-aminobutyric acid or glycine demonstrated that these AQP1-like-immunoreactive amacrine cells were immunoreactive for glycine. Their most frequent synaptic input was from other amacrine cell processes in both sublaminae a and b of the IPL, followed by a few cone bipolar cells. Their primary targets were other amacrine cells and ganglion cells in both sublaminae a and b of the IPL. In addition, synaptic output onto bipolar cells was rarely observed in sublamina b of the IPL. Thus, the AQP1 antibody labels a class of glycinergic amacrine cells with small to medium-sized dendritic fields in the rat retina.

Synaptic distribution of ionotropic glutamate receptors in the inner plexiform layer of the primate retina

The distribution and synaptic clustering of glutamate receptors (GluRs) were studied in the inner plexiform layer (IPL) of the macaque monkey retina by using subunit specific antisera. A punctate immunofluorescence pattern was observed in the IPL for all subunits tested, and electron microscopy confirmed that the immunoreactive puncta represent clustering of receptors at sites postsynaptic to the bipolar cell ribbon synapses (dyads). Usually only one of the two postsynaptic processes at the dyads expressed a given subunit. Immunoreactive GluR2, GluR2/3, and GluR4 puncta were found at high density throughout the IPL and are probably expressed at every dyad. The GluR1 subunit was expressed at lower density. The N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR1C2' were restricted to synapses localized in two broad bands in the center of the IPL. They were often colocalized with GluR2/3 and GluR4 subunits. The orphan receptor subunits delta 1/2 predominated in three horizontal bands. The kainate receptor subunits GluR6/7 were clustered in large postsynaptic densities adjacent to bipolar cell axon terminals but lacking a synaptic ribbon on the presynaptic side. This might represent a conventional synapse made by a bipolar axon terminal. The results suggest that GluR2/3 and GluR4, together with NMDA receptors, are preferentially expressed on ganglion cell dendrites, whereas kainate receptors and the delta 1/2 subunits are mostly localized on amacrine cell processes.

Synaptic input to an ON parasol ganglion cell in the macaque retina: a serial section analysis

A labeled ON parasol ganglion cell from a macaque retina was analyzed in serial, ultrathin sections. It received 13% of its input from diffuse bipolar cells. These directed a large proportion of their output to amacrine cells but received a relatively small proportion of their amacrine cell input via feedback synapses. In these respects, they were similar to the DB3 bipolar cells that make synapses onto OFF parasol cells. Bipolar cell axons that contacted the ON parasol cell in stratum 4 of the inner plexiform layer always made synapses onto the dendrite, and therefore, the number of bipolar cell synapses onto these ganglion cells could be estimated reliably by light microscopy in the future. Amacrine cells provided the majority of inputs to the ON parasol cell. Only a few of the presynaptic amacrine cell processes received inputs from the same bipolar cells as the parasol cells, and most of the presynaptic amacrine cell processes did not receive any inputs at all within the series. These findings suggest that most of the inhibitory input to the ON parasol cell originates from other areas of the retina. Amacrine cells presynaptic to the parasol ganglion cell interacted very infrequently with other neurons in the circuit, and therefore, they would be expected to act independently, for the most part.

Synaptic currents generating the inhibitory surround of ganglion cells in the mammalian retina

The receptive field (RF) of retinal ganglion cells (RGCs) consists of an excitatory central region, the RF center, and an inhibitory peripheral region, the RF surround. It is still unknown in detail which inhibitory interneurons (horizontal or amacrine cells) and which inhibitory circuits (presynaptic or postsynaptic) generate the RF surround. To study surround inhibition, light-evoked whole-cell currents were recorded from RGCs of the isolated, intact rabbit retina. The RFs were stimulated with light or dark spots of increasing diameters and with annular light stimuli. Direct inhibitory currents could be isolated by voltage clamping ganglion cells close to the Na(+)/K(+) reversal potential. They mostly represent an input from GABAergic amacrine cells that contribute to the inhibitory surround of ganglion cells. This direct inhibitory input and its physiological function were also investigated by recording light-evoked action potentials of RGCs in the current-clamp mode and by changing the intracellular Cl(-) concentration. The excitatory input of the ganglion cells could be isolated by voltage clamping ganglion cells at the Cl(-) reversal potential. Large light spots and annular light stimuli caused a strong attenuation of the excitatory input. Both GABA(A) receptors and GABA(C) receptors contributed to this inhibition, and picrotoxinin was able to completely block it. Together, these results show that the RF surround of retinal ganglion cells is mediated by a combination of direct inhibitory synapses and presynaptic surround inhibition.

Effects of age on nerve fibers in the rhesus monkey optic nerve

During normal aging there is a reduction in white matter volume in the cerebral hemispheres and structural abnormalities in myelin in some parts of the central nervous system, but whether nerve fibers are lost with age and whether the myelin changes are ubiquitous is not known. Studying the optic nerve, which is a circumscribed bundle of nerve fibers, offers an opportunity to gain further insight into the effects of normal aging on white matter. The present study examined the optic nerves from young (4-10 years) and old (27-33 years) rhesus monkeys using light and electron microscopy. These nerves had been perfused transcardially to obtain optimal preservation of the tissue. Varying degrees of degeneration were encountered in all the optic nerves from the old monkeys. The changes included myelin abnormalities, similar to those reported in other parts of the central nervous system; the presence of degenerating axons and their sheaths; changes in neuroglial cells; and thickening of the trabeculae of connective tissue in the nerve. The total number of nerve fibers was reduced from an average of 1.6 x 10(6) in the young optic nerves to as few as 4 x 10(5) in one old monkey, and with one exception in all of the old optic nerves the packing density of nerve fibers was less than in any of the young optic nerves. The degenerative changes were most marked in those optic nerves that contained the fewest nerve fibers.

Spatial scale and cellular substrate of contrast adaptation by retinal ganglion cells

Human visual perception and many visual system neurons adapt to the luminance and contrast of the stimulus. Here we describe a form of contrast adaptation that occurs in the retina. This adaptation had a local scale smaller than the dendritic or receptive fields of single ganglion cells and was insensitive to pharmacological manipulation of amacrine cell function. These results implicate the bipolar cell pathway as a site of contrast adaptation. The time required for contrast adaptation varied with stimulus size, ranging from approximately 100 ms for the smallest stimuli, to seconds for stimuli the size of the receptive field. The differing scales and time courses of these effects suggest that multiple types of contrast adaptation are used in viewing natural scenes.

Fundamental GABAergic amacrine cell circuitries in the retina: nested feedback, concatenated inhibition, and axosomatic synapses

Presynaptic gamma-aminobutyrate-immunoreactive (GABA+) profiles were mapped in the cyprinid retina with overlay microscopy: a fusion of electron and optical imaging affording high-contrast ultrastructural and immunocytochemical visualization. GABA+ synapses, deriving primarily from amacrine cells (ACs), compose 92% of conventional synapses and 98% of the input to bipolar cells (BCs) in the inner plexiform layer. GABA+ AC synapses, the sign-inverting elements of signal processing, are deployed in micronetworks and distinctive synaptic source/target topologies. Nested feedback micronetworks are formed by three types of links (BC --> AC, reciprocal BC <-- AC, and AC --> AC synapses) arranged as nested BC<--> [AC --> AC] loops. Circuits using nested feedback can possess better temporal performance than those using simple reciprocal feedback loops. Concatenated GABA+ micronetworks of AC --> AC and AC --> AC --> AC chains are common and must be key elements for lateral spatial, temporal, and spectral signal processing. Concatenated inhibitions may represent exceptionally stable, low-gain, sign-conserving devices for receptive field construction. Some chain elements are GABA immunonegative (GABA-) and are, thus, likely glycinergic synapses. GABA+ synaptic baskets target the somas of certain GABA+ and GABA- cells, resembling cortical axosomatic synapses. Finally, all myelinated intraretinal profiles are GABA+, suggesting that some efferent systems are sources of GABAergic inhibition in the cyprinid retina and may comprise all axosomatic synapses. These micronetworks are likely the fundamental elements for receptive field shaping in the inner plexiform layer, although few receptive field models incorporate them as functional components. Conversely, simple feedback and feedforward synapses may often be chimeras: the result of an incomplete view of synaptic topology.

Quantitative assessment of localization and colocalization of glutamate, aspartate, glycine, and GABA immunoreactivity in the chick retina

We examined the posthatch chick retina for the frequency of occurrence of localization and colocalization of four amino acid transmitter candidates: glutamate (Glu), aspartate (Asp), gamma aminobutyric acid (GABA), and glycine (Gly) using postembedding methods. We support previous studies of Glu, Asp, GABA, and Gly localization in the direct and indirect functional pathways of the chick retina and extend these studies with new qualitative and quantitative observations. We found that photoreceptors show distinct cellular immunoreactivity for both Glu (Glu+) and Asp+, but not for Gly (Gly-) or GABA. Moreover, there is compartmentalization of Glu and Asp staining within the photoreceptors. All horizontal cells react strongly with Asp and Glu, about three-fourths are GABA+ and three-fourths of these are Gly+. Bipolar cells are uniformly Glu+, heterogeneously Asp+, occasionally Gly+, but GABA-. A majority of amacrine cells stain heterogeneously with all antibodies: 90% are Gly+, slightly more than half colocalize Glu, GABA, and Gly. Furthermore, amacrine cells in the outer two or three rows of cells are more likely to be stained by Gly than Glu, Asp, or GABA. Confirming previous studies, ganglion cells were mostly immunoreactive for Glu and Asp with fewer reactive for GABA and Gly. Strong and distinctly cellular immunoreactivity was found in both central and peripheral retina. Our findings show: 1) there is extensive colocalization of Glu, Asp, GABA, and Gly among most retinal neurons, including some cells that contain all four; 2) cells of the direct functional pathway tend to be labeled by Glu and Asp generally to the exclusion of GABA and Gly, while those of the indirect pathway tend to be labeled by GABA+ and/or Gly+ in addition to Glu+ and Asp+; 3) different cell body layers have distinct patterns of colocalization; and 4) there is no qualitative difference in staining patterns between peripheral and central retina.

A quantitative analysis of cells in the ganglion cell layer of the chick retina

This study investigated the organization of cells in the ganglion cell layer (GCL) using Nissl staining, retrograde cell degeneration with axotomy of the optic nerve, and retrograde cell labeling by injections of horseradish peroxidase (HRP) into the optic nerve of chicks (posthatching day 1 and 8, P-1 and P-8). The total number of cells in the GCL was 6.1 x 10(6) (P-1) and 4.9 x 10(6) (P-8), and the cell density was 14,300 cells/mm2 (P-1) and 10,400 cells/ mm2 (P-8) on average. Two high-density areas, the central area (CA) and the dorsal area (DA), were observed in the central and dorsal retinas in both P-1 (22,000 cells/mm2 in CA, 19,000 cells/mm2 in DA) and P-8 chicks (19,000 cells/mm2 in CA, 12,800 cells/mm2 in DA). The cell densities in the temporal periphery (TP) and the nasal (NP) peripheral retinas were 7,800 cells/mm2 and 12,500 cells/mm2, respectively, in P-1 and 5,000 cells/ mm2 and 8,000 cells/mm2, respectively, in P-8 chicks. The cell density in the temporal periphery was 35% (P-8) lower than in the nasal periphery in both P-1 and P-8 chicks. Thirty percent (1.9 x 10(6) cells in P-1) of the total cells in the GCL were resistant to axotomy of the optic nerve. The distribution of the axotomy-resistant cells showed two high-density areas in the central and dorsal retinas, corresponding to the CA (5,800 cells/mm2) and the DA (3,200 cells/mm2). These cells also exhibited a center-peripheral increase (2,200 cells/mm2 in the TP) in P-1 chicks, but the high-density area was not found in the dorsal retina of P-8 chicks. From these data and the HRP study, the number of presumptive ganglion cells in P-8 chicks was estimated to be 4 x 10(6) (8,600 cells/mm2 on average), and the density in each area was 13,500 (CA), 10,200 (DA), and 4,300 (TP) cells/mm2. The peripheral/ center ratios of the density of ganglion cells were significantly different along the nasotemporal and dorsoventral axes. The density of ganglion cells decreased more rapidly toward the temporal periphery (TP/CA ratio: 0.47 in P-1 and 0.32 in P-8) than toward the nasal periphery (NP/CA ratio: 0.67 in P-1 and 0.52 in P-8). In contrast, there was no significant difference in the peripheral/center ratios between the dorsal retina (DP/CA ratio: 0.6 in P-1 and 0.56 in P-8) and ventral retina (VP/CA ratio: 0.58 in P-1 and 0.51 in P-8). A small peak in the density of the presumptive ganglion cells was detected in the dorsal retina of both P-1 chicks (10,800 cells/mm2) and P-8 chicks (10,200 cells/mm2). The HRP-labeled cells were small in the CA (M +/- SD: 35.7 +/- 9.1 microm2) and DA (40.0 +/- 11.3 microm2), and their sizes increased toward the periphery (63.4 +/- 29.7 microm2 in the TP) accompanied by a decrease in the cell density. However, the axotomy-resistant cells did not significantly increase in size toward the peripheral retina (12.2 +/- 2.2 microm2 in the CA, 15.2 +/- 3.2 microm2 in the DA, 15.1 +/- 3.8 microm2 in the TP). The characteristic distribution of ganglion cells could be related to visual behavior based upon the specialization of avian visual fields.

Alterations in neurochemistry during retinal degeneration

Retinitis pigmentosa refers to a family of hereditary retinal degenerations that lead to photoreceptor death and vision loss. The underlying cause(s) are not known. In recent years there has been accumulating evidence of neurochemical changes during degeneration. In particular, the amino acids glutamate, GABA, and glycine show alterations in labelling intensity in subsets of neurons. Furthermore, there are differences in the labelling of the precursors, glutamine and aspartate, prior to, during, and following loss of photoreceptors, suggesting that the metabolic pathways involved in neurotransmitter formation and degradation may be abnormal. In addition, there is an elevation in glutamine and arginine content within Müller cells prior to the onset of photoreceptor death. Investigations evaluating Müller cell function indicate that formation and degradation of glutamate, in particular, is abnormal in the degenerating retina from an early age. These studies suggest that even though the primary genetic defect of the RCS rat is within the retinal pigment epithelium, Müller cells develop abnormally, and may contribute to the observed photoreceptor loss.

Synaptic input to small bistratified (blue-ON) ganglion cells in the retina of a new world monkey, the marmoset Callithrix jacchus

Small bistratified (blue-ON) ganglion cells in the primate retina are involved in processing short wavelength sensitive cone signals. These ganglion cells stratify in both the ON- and OFF-sublamina of the inner plexiform layer. We investigated the origin of synaptic input to the small bistratified ganglion cell in the retina of a New World primate, the marmoset Callithrix jacchus. Two small bistratified cells from peripheral retina were intracellularly filled with Lucifer Yellow, subsequently photoconverted and processed for electron microscopy. Serial ultrathin sections were cut through portions of each cell, and these were analysed in the electron microscope. The majority of synaptic input (about 84%) to both the inner and outer tier of dendrites was from amacrine cells. Both dendritic tiers also received bipolar cell input. These findings are consistent with predictions from physiological studies that synaptic input to the inner and outer tier of small bistratified cells should be excitatory. However, the tiny fraction of total input supplied from bipolar cells to the outer tier is not consistent with the strong excitatory OFF response in cells of this pathway.

Quantitative estimations of foveal and extra-foveal retinal circuitry in humans

For an understanding of the basis for psychophysical measurement of visual resolution, quantitative morphological studies of retinal neuronal architecture are needed. Here we report on cell densities and retinal ganglion cell:cone ratio (RGC:C) from the foveal border to the peripheral retina (34 degrees eccentricity). Quantitative estimates of RGC and C densities were made using a modified disector method in three vertically sectioned human retinae and were adjusted for RGC displacement. In agreement with our previous data on humans, we found an RGC:C ratio close to 3 at 2-3 degrees eccentricity. Outside the foveal border, the ratio declined to 1.0 at 7.5 degrees eccentricity and to 0.5 at eccentricities larger than 19 degrees. Center-to-center separation of C and RGC in addition to center-to-center separation of estimated 'receptive fields' was calculated at corresponding locations along the superior and inferior hemimeridians. The center-to-center separation of estimated 'receptive fields' was found to be more closely related to resolution thresholds from the fovea to 19 degrees eccentricity than was the separation of RGC and C. On the basis of these quantitative estimates, models for neural circuitry involved in central and peripheral spatial vision can be discussed.

Gap junctions with amacrine cells provide a feedback pathway for ganglion cells within the retina

In primates, one type of retinal ganglion cell, the parasol cell, makes gap junctions with amacrine cells, the inhibitory, local circuit neurons. To study the effects of these gap junctions, we developed a linear, mathematical model of the retinal circuitry providing input to parasol cells. Electrophysiological studies have indicated that gap junctions do not enlarge the receptive field centres of parasol cells, but our results suggest that they make other contributions to their light responses. According to our model, the coupled amacrine cells enhance the responses of parasol cells to luminance contrast by disinhibition. We also show how a mixed chemical and electrical synapse between two sets of amacrine cells presynaptic to the parasol cells might make the responses of parasol cells more transient and, therefore, more sensitive to motion. Finally, we show how coupling via amacrine cells can synchronize the firing of parasol cells. An action potential in a model parasol cell can excite neighbouring parasol cells, but only when the coupled amacrine cells also fire action potentials. Passive conduction was ineffective due to low-pass temporal filtering. Inhibition from the axons of the coupled amacrine cells also produced oscillations that might synchronize the firing of more distant ganglion cells.

Glycine and GABA receptors in the mammalian retina

Molecular cloning has introduced an unexpected diversity of neurotransmitter receptors. In this study we review the types, the localization and possible synaptic function of the inhibitory neurotransmitter receptors in the mammalian retina. Glycine receptors (GlyRs) and their localization in the mammalian retina were analyzed immunocytochemically. Specific antibodies against the alpha 1 subunit of the GlyR (mAb2b) and against all subunits of the GlyR (mAb4a) were used. Both antibodies produced a punctate immunofluorescence, which was shown by electron microscopy to represent clustering of GlyRs at synaptic sites. Synapses expressing the alpha 1 subunit of the GlyR were found on ganglion cell dendrites and on bipolar cell axons. GlyRs were also investigated in the oscillator mutant mouse. The complete loss of the alpha 1 subunit was compensated for by an apparent upregulation of the other subunits of the GlyR. GABAA receptors (GABAARs) and their retinal distribution were studied with specific antibodies that recognize the alpha 1, alpha 2, alpha 3, beta 1, beta 2, beta 3, gamma 2 and delta subunits. Most antibodies produced a punctate immunofluorescence in the inner plexiform layer (IPL) which was shown by electron microscopy to represent synaptic clustering of GABAARs. The density of puncta varied across the IPL and different subunits were found in characteristic strata. This stratification pattern was analyzed with respect to the ramification of cholinergic amacrine cells. Using intracellular injection with Lucifer yellow followed by immunofluorescence, we found that GABAARs composed of different subunits were expressed by the same ganglion cell, however, they were clustered at different synaptic sites. The distribution of GABAC receptors was studied in the mouse and in the rabbit retina using an antiserum that recognizes the rho 1, rho 2 and rho 3 subunits. GABAC receptors were found to be clustered at postsynaptic sites. Most, if not all of the synapses were found on rod and cone bipolar axon terminals. In conclusion we find a great diversity of glycine and GABA receptors in the mammalian retina, which might match the plethora of morphological types of amacrine cells. This may also point to subtle differences in synaptic function still to be elucidated.

Glutamine immunoreactivity in Müller cells of monkey eyes with experimental glaucoma

The action of glutamate in retina is largely terminated through rapid uptake by Müller cells and subsequent conversion primarily to glutamine. Glutamine, transferred from Müller cells to neurons, serves as a precursor for the formation of glutamate in neurons completing the glutamate-glutamine cycle. In a monkey model of high-tension glaucoma, we have examined glutamine immunoreactivity in the Müller cell as well as the number of Müller cells to determine whether the activity of these cells in the glutamate-glutamine cycle is affected, particularly since high vitreal glutamate has been reported in glaucoma. Unilateral glaucoma was induced in three monkeys by argon laser application to the trabecular meshwork. LR White sections of retina from the temporal mid-periphery (about 23 degrees) and the parafovea (central 3 degrees) were immunolabeled for glutamine using immunogold and silver intensification. The percentage difference in labeling intensity (darkness) in the glaucomatous retina was determined relative to the labeling found in the control retina by image analysis. Ganglion cell density was estimated from radial sections in the parafovea and from retinal whole mounts in the mid-periphery. The number of Müller cells was estimated from vibratome sections immunolabeled by vimentin antibodies in the temporal mid-periphery (about 30 degrees). Glutamine immunoreactivity was localized predominately in ganglion cells and Müller cells. However, the intensity of glutamine immunolabeling was greater in Müller cells of glaucomatous eyes than in control eyes. This increase in glutamine immunolabeling was 25-32% in the temporal mid-periphery and 27-48% in the parafovea. Müller cell number in the glaucomatous eye was similar to that of the control in the temporal mid-periphery. The data in this study indicate that the increase in glutamine in Müller cells is not a consequence of their loss and that Müller cell function in the glutamate-glutamine cycle continues in glaucomatous eyes. These findings are consistent with a previous report that extracellular/vitreal glutamate concentration is elevated in high-tension glaucoma.

Presynaptic and postsynaptic localization of GABA(B) receptors in neurons of the rat retina

The recently cloned GABA(B) receptors were localized in rat retina using specific antisera. Immunolabelling was detected in the inner and outer plexiform layers (IPL, OPL), and in a number of cells in the inner nuclear layer and the ganglion cell layer. Double-labelling experiments for GABA (gamma-aminobutyric acid) and GABA(B) receptors, respectively, demonstrated a co-localization in horizontal cells and amacrine cells. Electron microscopy showed that GABA(B) receptors of the OPL were localized presynaptically in horizontal cell processes invaginating into photoreceptor terminals. In the IPL, GABA(B) receptors were present presynaptically in amacrine cells, as well as postsynaptically in amacrine and ganglion cells. The postnatal development of GABA(B) receptors was also studied, and immunoreactivity was observed well before morphological and synaptic differentiation of retinal neurons. The present results suggest a presynaptic (autoreceptor) as well as postsynaptic role for GABA(B) receptors. In addition, the extrasynaptic localization of GABA(B) receptors could indicate a paracrine function of GABA in the retina.

Differences in the retinal GABA system among control, spastic mutant and retinal degeneration mutant mice

Immunocytochemical methods were used to compare the GABA system in control mice and two mutant strains: spastic which has reduced glycine receptors and retinal degeneration mutant in which the photoreceptors degenerate and reportedly have increased GABA and GAD levels. We found that the spastic mutant retina had reduced GABA-immunoreactivity (IR) in the proximal retina, reduced staining for GAD-1440 in the OPL, and reduced GABAA receptor staining in the OPL, compared to control. The retinal degeneration mutant retinas had enhanced GABA-IR throughout the retina, particularly in Müller cells, bipolar cells and IPL, and enhancement of GABAA receptor staining in the OPL, compared to control. The distributions of GABA-IR, GAD-1440-IR and GABAA receptor-IR in retinas of spastic mutant mice that also expressed the retinal degeneration phenotype resembled those found in retinas of mice that expressed only the retinal degeneration phenotype rather than those that expressed only the spastic mutation. No differences were observed among the conditions for GAD-65, GAD-67 or GABA-T. Our results with the spastic and retinal degeneration mutant mice demonstrate that attenuation in the glycinergic system and photoreceptor degeneration, respectively, is accompanied by alterations in different aspects of the GABA system, giving impetus for caution in the interpretation of experiments involving genetic manipulation of complex phenotypes.

GABAergic and glycinergic IPSCs in ganglion cells of rat retinal slices

GABAergic and glycinergic IPSCs were studied in identified retinal ganglion cells (RGCs) of light-adapted rat retinal slices, using whole-cell recording techniques. GABAergic IPSCs were blocked specifically by SR95531 (3 microM) and bicuculline (3 microM) and glycinergic IPSCs by strychnine (0.3 microM). From 37 RGCs studied, 25 showed exclusively GABAergic IPSCs, 6 presented only glycinergic IPSCs, and 6 showed both. This distribution may result from differences in amacrine cells input rather than from receptor heterogeneity, because both GABA and glycine elicited Cl--selective currents in all RGCs tested. TTX markedly reduced GABAergic IPSCs frequency, whereas glycinergic IPSCs were unaffected. Ca2+-free media, with or without high Mg2+, blocked TTX-resistant GABAergic and glycinergic IPSCs. These results suggest that GABAergic IPSCs in RGCs can be elicited either by Na+-dependent action potentials or by local Ca2+ influx in medium or large dendritic field GABAergic amacrine cells, whereas glycinergic IPSCs are generated by action potential-independent Ca2+ influx in narrow field glycinergic amacrine cells. Both types of IPSCs had fast rise times and biexponential decays, but glycinergic IPSC decay was significantly slower than that of GABAergic IPSCs. An elementary conductance of 54 pS for the glycine-gated channels was estimated from single-channel events, clearly detected in the falling phase of glycinergic IPSCs, and from responses to exogenous glycine.

Expression of GABA in the fetal, postnatal, and adult human retinas: an immunohistochemical study

The expression of GABA in the human fetal (12-25 weeks of gestation), postnatal (five-month-old), and adult (35-year-old) retinas was investigated by immunohistochemistry. GABA expression was seen as early as 12 weeks in the undifferentiated cells of the inner neuroblast zone; a few optic nerve fiber layer axons were clearly labeled, suggesting that some of the stained cell bodies were prospective ganglion cells, others could be displaced amacrine cells. From 16-17 to 24-25 weeks, intense labeling was found in the amacrine, displaced amacrine, and some ganglion cells. During this time period, horizontal cells (identified by calbindin immunohistochemistry), undergoing migration (periphery) and differentiation (center), expressed GABA prominently. In the postnatal retina, some horizontal cells were moderately labeled, but very weakly in a few cells, in the adult. The Müller cells developed immunoreactivity first weakly at 12 weeks and then moderately from 16-17 weeks onward. The staining was also evident in the postnatal and adult retinas, showing labeled processes of these glial cells. Virtually no axons in the adult optic nerve and nerve fiber layer were stained; the staining was restricted to a few, large ganglion cells and displaced amacrine cells: Some amacrines were also labeled. The possibility that GABA might play a role in horizontal cell differentiation and maturation is highlighted. Other evidences suggest that GABA might play a role in metabolism during retinal development.

The DAPI-3 amacrine cells of the rabbit retina

In the rabbit retina, the nuclear dye, 4,6,diamidino-2-phenylindole (DAPI), selectively labels a third type of amacrine cell, in addition to the previously characterized type a and type b cholinergic amacrine cells. In this study, these "DAPI-3" amacrine cells have been characterized with respect to their somatic distribution, dendritic morphology, and neurotransmitter content by combining intracellular injection of biotinylated tracers with wholemount immunocytochemistry. There are about 100,000 DAPI-3 amacrine cells in total, accounting for 2% of all amacrine cells in the rabbit retina, and their cell density ranges from about 130 cells/mm2 in far-peripheral retina to 770 cells/mm2 in the visual streak. The thin varicose dendrites of the DAPI-3 amacrine cells form a convoluted dendritic tree that is symmetrically bistratified in S1/S2 and S4 of the inner plexiform layer. Tracer coupling shows that the DAPI-3 amacrine cells have a fivefold dendritic-field overlap in each sublamina, with the gaps in the arborization of each cell being occupied by dendrites from neighboring cells. The DAPI-3 amacrine cells consistently show the strongest glycine immunoreactivity in the rabbit retina and they also accumulate exogenous [3H]-glycine to a high level. By contrast, the AII amacrine cells, which are the best characterized glycinergic cells in the retina, are amongst the most weakly labelled of the glycine-immunopositive amacrine cells. The DAPI-3 amacrine cells costratify narrowly with the cholinergic amacrine cells and the On-Off direction-selective ganglion cells, suggesting that they may play an important role in movement detection.

Amino acid signatures in the primate retina

Pattern recognition of amino acid signals partitions virtually all of the macaque retina into 16 separable biochemical theme classes, some further divisible by additional criteria. The photoreceptor-->bipolar cell-->ganglion cell pathway is composed of six separable theme classes, each possessing a characteristic glutamate signature. Neuronal aspartate and glutamine levels are always positively correlated with glutamate signals, implying that they largely represent glutamate precursor pools. Amacrine cells may be parsed into four glycine-dominated (including one glycine/GABA immunoreactive population) and four GABA-dominated populations. Horizontal cells in central retina possess a distinctive GABA signature, although their GABA content is constitutively lower than that of amacrine cells and shows both regional and sample variability. Finally, a taurine-glutamine signature defines Müller's cells. We thus have established the fundamental biochemical signatures of the primate retina along with multiple metabolic subtypes for each neurochemical class and demonstrated that virtually all neuronal space can be accounted for by cells bearing characteristic glutamate, GABA, or glycine signatures.

Clustering of gephyrin at GABAergic but not glutamatergic synapses in cultured rat hippocampal neurons

The molecular mechanisms underlying the establishment of a postsynaptic receptor mosaic on CNS neurons are poorly understood. One protein thought to be involved is gephyrin, a peripheral membrane protein that binds to the inhibitory glycine receptor and functions in clustering this receptor at synapses in cultured rat spinal cord neurons. We investigated the possible association of gephyrin with synapses in cultured rat hippocampal neurons, where glutamate and GABA but not glycine are the principal transmitters. Gephyrin immunoreactivity was detected in axons as well as dendrites, changing from a predominantly axonal to a more dendritic distribution with time in culture. Gephyrin staining was not distributed uniformly, but always took the form of clusters. Small clusters of gephyrin (0.2 microns 2), present throughout development, were distributed widely and not restricted to synaptic sites. Larger clusters of gephyrin (0.4-10.0 microns 2, sometimes composed of groups of small clusters), which developed in older cells, were localized to a subset of contacts between axons and dendrites. These large clusters were not present at glutamatergic synapses (marked by immunostaining for GluR1), but were closely associated with GABAergic synapses (marked by immunostaining for GABA and glutamic acid decarboxylase). These results, together with previous findings, suggest that gephyrin may function to anchor GABA and glycine receptors, but not glutamate receptors, at postsynaptic sites on central neurons. They also raise the possibility that gephyrin has additional functions, independent of its role at synapses.

Topography of ganglion cells and photoreceptors in the retina of a New World monkey: the marmoset Callithrix jacchus

We studied the anatomical substrates of spatial vision in a New World monkey, the marmoset Callithrix jacchus. This species has good visual acuity and a foveal specialization which is qualitatively similar to that of humans and other Old World primates. We measured the spatial density of retinal ganglion cells and photoreceptors, and calculated the relative numbers of these cell populations. We find that ganglion cells outnumber photoreceptors by between 2.4:1 and 4.2:1 in the fovea. The peak sampling density of ganglion cells is close to 550,000 cells/mm2. This value falls by almost 1000-fold between the fovea and peripheral retina; a value which approaches recent estimates of the centroperipheral ganglion cell gradient for human and macaque monkey retina and primary visual cortex. The marmoset shows a sex-linked polymorphism of color vision: all male and some female marmosets are dichromats. Six of the retinas used in the present study came from animals whose chromatic phenotype was identified in electrophysiological experiments and confirmed by polymerase chain reaction (PCR) amplification of cone opsin encoding genes. One animal was a trichromat and the others were dichromats. Antibodies against short wavelength-sensitive (SWS) cones labeled close to 8% of all cones near the fovea of one dichromat animal, consistent with electrophysiological evidence that the SWS system is present in all marmosets. The topography and spatial density of cone photoreceptors and ganglion cells was similar to that reported for macaque retina, and we found no obvious difference between dichromatic and trichromatic marmoset retinas. These results reinforce the view that the main determinate of primate foveal topography is the requirement for maximal spatial resolution.

Glycine receptors in the rod pathway of the macaque monkey retina

The distribution of glycinergic synapses in macaque monkey retina was investigated. The monoclonal antibody (mAb2b) against the alpha 1 subunit of the glycine receptor produced a punctate immunoreactivity that was localized to synapses. In central retina about 70% of the alpha 1 subunit-containing synapses were located in strata 1 and 2 of the inner plexiform layer, about 30% were located in strata 3 and 4, and immunoreactivity was absent in stratum 5. Electron microscopy showed that the majority of the synapses in strata 1 and 2 were on cone bipolar axons. The presynaptic profile always belonged to an amacrine cell. Presynaptic and postsynaptic profiles were further characterized using double-label immunofluorescence with cell-type specific antibodies against calcium-binding proteins. An antiserum against calretinin was used to label AII amacrine cells and an antiserum against recoverin was used to label flat midget bipolar cells. In the outer part of the IPL, 75% of the alpha 1-immunoreactive puncta were colocalized with calretinin-immunoreactive AII processes and 61% of the alpha 1-immunoreactive puncta were colocalized with recoverin-positive midget bipolar axons. These results suggest that the alpha 1 subunit of the glycine receptor is present at the chemical synapse made by AII amacrine cells with flat midget bipolar cells, thus providing a pathway for rod signals to reach midget ganglion cells.

The rod pathway of the macaque monkey retina: identification of AII-amacrine cells with antibodies against calretinin

AII-amacrine cells were characterized from Golgi-stained sections and wholemounts of the macaque monkey retina. Similar to other mammalian retinae, they are narrow-field, bistratified amacrine cells with lobular appendages in the outer half of the inner plexiform layer (IPL) and a bushy, smoother dendritic tree in the inner half. AII cells of the monkey retina were stained immunocytochemically with antibodies against the calcium-binding protein calretinin. Their retinal mosaic was elaborated, and their density distribution across the retina was measured. Convergence within the rod pathway was calculated. Electron microscopy of calretinin-immunolabelled sections was used to study the synaptic connections of the AII cells. They receive a major input from rod bipolar cells, and their output is largely onto cone bipolar cells. Thus, the rod pathway of the primate retina follows the general mammalian scheme as it is known from the cat, the rabbit, and the rat retina. The spatial sampling properties of macaque AII-amacrine cells are discussed and related to human scotopic visual acuity.

Colocalization of gephyrin and GABAA-receptor subunits in the rat retina

Gephyrin is a protein that copurifies with the glycine receptor (GlyR) and is required for the clustering of GlyRs at postsynaptic sites. Previously, it was thought that antibody mAb 7a, directed against gephyrin, was a specific marker for GlyR. However, there is evidence that gephyrin can also be found at nonglycinergic synapses. Here, immunocytochemistry was applied to show this directly for the rat retina. Both gephyrin and different subunits of the gamma-aminobutyric acid (GABA)A receptor were localized to discrete puncta in the inner plexiform layer, and these puncta were shown by electron microscopy to represent synaptic sites. Double immunocytochemistry revealed that GABAA receptors and GlyRs are not colocalized. However, gephyrin and different subunits of GABAA receptors were found to occur at the same synapses. The amount of colocalization varied with the GABAA receptor subunit composition and was most extensive for the alpha 2 subunit, less for the alpha 3 subunit, and minimal for the alpha 1 subunit. The gephyrin present at GABAergic synapses of the retina might also be involved with clustering of receptors at the postsynaptic sites. Hence, localization of gephyrin can no longer be considered as a unique marker of glycinergic synapses.

Specific cell types in cat retina express different forms of glutamic acid decarboxylase

We studied the expression of glutamate decarboxylase (GAD), GAD65 and GAD67, in cat retina by immunocytochemistry. About 10% of GABAergic amacrine cells express only GAD65 and 30% express only GAD67. Roughly 60% contain both forms of the enzyme, but GAD67 is present only at low levels in the majority of these double-labeled amacrine cells. The staining pattern in the inner plexiform layer (IPL) for the two GAD forms was also different. GAD65 was restricted to strata 1-4, and GAD67 was apparent throughout the IPL but was strongest in strata 1 and 5. This indicates that somas, as well as their processes, are differentially stained for the two forms of GAD. Cell types expressing only GAD65 include interplexiform cells, one type of cone bipolar cell, and at least one type of serotonin-accumulating amacrine cell. Cell types expressing only GAD67 include amacrine cells synthesizing dopamine, amacrine cells synthesizing nitric oxide (NO), and amacrine cells accumulating serotonin. Cholinergic amacrine cells express a low level of both GAD forms. Our findings in the retina are consistent with previous observations in the brain that GAD65 expression is greater in terminals than in somas. In addition, in retina most neurons expressing GAD67 also contain a second neurotransmitter as well as GABA, and they tend to be larger than neurons expressing GAD65. We propose that large cells have a greater demand for GABA than small cells, and thus require the constant, relatively unmodulated level of GABA that is provided by GAD67.