Synaptic organization of type 2 catecholamine amacrine cells in the rhesus monkey retina

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.

Type 2 wide-field amacrine cells in TH::GFP mice show a homogenous synapse distribution and contact small ganglion cells

In vertebrate retinas, wide-field amacrine cells represent a diverse class of interneurons, important for the extraction of selective features, like motion or objects, from the visual scene. Most types of wide-field amacrine cells lack dedicated output processes, whereas some types spatially segregate outputs from inputs. In the tyrosine hydroxylase (TH)::green fluorescent protein (GFP) mouse line, two types of GFP-expressing wide-field amacrine cells have been described: dopaminergic type 1 and γ-aminobutyric acid-ergic type 2 cells (TH2). TH2 cells possess short and long radial processes stratifying in the middle of the inner plexiform layer, where they collect excitatory and inhibitory inputs from bipolar cells and other amacrine cells, respectively. Although it was shown that these inputs lead to ON-OFF light responses, their spatial distribution along TH2 cell processes is unknown. Also, the postsynaptic targets of TH2 cells have not been identified so far. Here, we analysed the synapse distribution of these cells in TH::GFP mice and show that they form a weakly coupled network. Electrical synapses (made of connexin36) and chemical (excitatory and inhibitory) synapses are uniformly distributed along TH2 dendrites, independent of dendrite length or distance from soma. Moreover, we reveal that TH2 cells contact at least two types of small ganglion cells; one of them is the W3 cell, a ganglion cell sensitive to object motion. Contacts were often associated with markers of inhibitory synapses. Thus, TH2 wide-field amacrine cells likely provide postsynaptic inhibition to W3 ganglion cells and may contribute to object-motion detection in the mouse retina.

Inputs underlying the ON-OFF light responses of type 2 wide-field amacrine cells in TH::GFP mice

In the mammalian retina, two types of catecholaminergic amacrine cells have been described. Although dopaminergic type 1 cells are well characterized, the physiology of type 2 cells is, so far, unknown. To target type 2 cells specifically, we used a transgenic mouse line that expresses green fluorescent protein under the control of the tyrosine hydroxylase promoter. Type 2 cells are GABAergic and have an extensive dendritic arbor, which stratifies in the middle of the inner plexiform layer. Our data suggest that type 2 cells comprise two subpopulations with identical physiological properties: one has its somata located in the inner nuclear layer and the other in the ganglion cell layer. Immunostaining with bipolar cell markers suggested that type 2 cells receive excitatory inputs from type 3 OFF and type 5 ON bipolar cells. Consistently, patch-clamp recordings showed that type 2 cells are ON-OFF amacrine cells. Blocking excitatory inputs revealed that different rod and cone pathways are active under scotopic and mesopic light conditions. Blockade of inhibitory inputs led to membrane potential oscillations in type 2 cells, suggesting that GABAergic and glycinergic amacrine cells strongly influence type 2 cell signaling. Among the glycinergic amacrine cells, we identified the VGluT3-immunoreactive amacrine cell as a likely candidate. Collectively, light responses of type 2 cells were remarkably uniform over a wide range of light intensities. These properties point toward a general function of type 2 cells that is maintained under scotopic and mesopic conditions.

The retina in Parkinson's disease

As a more complete picture of the clinical phenotype of Parkinson's disease emerges, non-motor symptoms have become increasingly studied. Prominent among these non-motor phenomena are mood disturbance, cognitive decline and dementia, sleep disorders, hyposmia and autonomic failure. In addition, visual symptoms are common, ranging from complaints of dry eyes and reading difficulties, through to perceptual disturbances (feelings of presence and passage) and complex visual hallucinations. Such visual symptoms are a considerable cause of morbidity in Parkinson's disease and, with respect to visual hallucinations, are an important predictor of cognitive decline as well as institutional care and mortality. Evidence exists of visual dysfunction at several levels of the visual pathway in Parkinson's disease. This includes psychophysical, electrophysiological and morphological evidence of disruption of retinal structure and function, in addition to disorders of 'higher' (cortical) visual processing. In this review, we will draw together work from animal and human studies in an attempt to provide an insight into how Parkinson's disease affects the retina and how these changes might contribute to the visual symptoms experienced by patients.

Tyrosine hydroxylase expression in the Cebus monkey retina

Tyrosine hydroxylase (TH) expression was used as a marker to study the dopaminergic cells in the Cebus monkey retina. Two types of dopaminergic cells were identified by cell body size and location, level of arborization in the inner plexiform layer, and amount of immunolabeling. Type 1 cells displayed intense immunoreactivity and larger somata (12-24 microns) located in the inner nuclear layer or ganglion cell layer, whereas type 2 had smaller cell bodies (8-14 microns) found either in the inner plexiform layer or ganglion cell layer and were more faintly labeled. Interplexiform cells were characterized as type 1 dopaminergic cells. Immunoreactive axon-like processes were seen in the nerve fiber layer, and a net of fibers was visible in the foveal pit and in the extreme periphery of the retina. The population of TH+ cells was most numerous in the temporal superior quadrant and its density peaked at 1-2 mm from the fovea. Type 1 TH+ cells were more numerous than type 2 cells at any eccentricity. Along the horizontal meridian, type 1 cell density was slightly higher in temporal (29 cells/mm2) than in nasal (25 cells/mm2) retina, while type 2 cells had a homogeneous distribution (4.5 cells/mm2). Along the vertical meridian, type 1 cells reached lower peak density (average 17.7 cells/mm2) in the inferior retina (central 4 mm), compared to the superior portion (23.7 cells/mm2). Type 2 cell density varied from 4.5 cells/mm2 in the superior region to 9.4 cells/mm2 in the inferior region. The spatial density of the two cell types varied approximately inversely while the total density of TH+ cells was virtually constant across the retina. No correlation between dopaminergic cells and rod distribution was found. However, we suggest that dopaminergic cells could have a role in mesopic and/or photopic vision in this species, since TH+ fibers are present in cone-dominated regions like the foveola and extreme nasal periphery.

Immunocytochemical localization of dopamine D1 receptors in the retina of mammals

Dopamine is one of the major neurotransmitters in the retina. It is released from amacrine and interplexiform cells into both inner (IPL) and outer (OPL) plexiform layers. Several dopaminergic actions are known to occur through D1 receptors (D1R) but the precise location of these receptors has not been established. An antibody that recognizes the intracytoplasmic C-terminal of the rat D1R was used to detect D1R, immunohistochemically, in rats (Wistar and RCS), mouse, hamster, and macaque monkey retinas. The OPL was heavily stained in each species, consistent with the known actions of dopamine on horizontal cells. Three to five bands were observed in the IPL, depending on species. Three were in the a sublayer, the outermost of which was close to the amacrine cell layer, and may represent the massive dopamine input to the AII rod-amacrine cells. As observed in mice, where bipolar cells are D1-immunoreactive, the band located in sublayer 3 of the IPL may contain cone-bipolar cell terminals. A band of D1R-immunoreactivity in the b sublayer of the IPL contains ON-bipolar cell terminals and a second site of interaction between dopaminergic cells and the AII amacrine cells. This sublayer was absent from the RCS rat retina, suggesting a severe impairment of the rod-driven pathway following rod degeneration in these mutant rats. Cells in the ganglion cell layer exhibited relatively heavy staining, and may be ganglion cells or displaced amacrine cells. Some extrasynaptic localizations of D1R in the retina are suggested.

Dopaminergic transmission in the rat retina: evidence for volume transmission

The study was designed to determine whether dopaminergic neurotransmission in the retina can operate via volume transmission. In double immunolabelling experiments, a mismatch as well as a match was demonstrated in the rat retina between tyrosine hydroxylase (TH) and dopamine (DA) immunoreactive (ir) terminals and cell bodies and dopamine D2 receptor-like ir cell bodies and processes. The match regions were located in the inner nuclear and plexiform layers (D2 ir cell bodies plus processes). The mismatch regions were located in the ganglion cell layer, the outer plexiform layer, and the outer segment of the photoreceptor layer, where very few TH ir terminals can be found in relation to the D2 like ir processes. In similar experiments analyzing D1 receptor like ir processes versus TH ir nerve terminals, mainly a mismatch in their distribution could be demonstrated, with the D1 like ir processes present in the outer plexiform layer and the outer segment where a mismatch in D2 like receptors also exists. The demonstration of a mismatch between the localization of the TH terminal plexus and the dopamine D2 and D1 receptor subtypes in the outer plexiform layer, the outer segment and the ganglion cell layer (only D2 immunoreactivity (IR)) suggests that dopamine, mainly from the inner plexiform layer, may reach the D2 and D1 mismatch receptors via diffusion in the extracellular space. After injecting dopamine into the corpus vitreum, dopamine diffuses through the retina, and strong catecholamine (CA) fluorescence appears in the entire inner plexiform layer and the entire outer plexiform layer, representing the match and mismatch DA receptor areas, respectively. The DA is probably bound to D1 and D2 receptors in both plexiform layers, since the DA receptor antagonist chlorpromazine fully blocks the appearance of the DA fluorescence, while only a partial blockade is found after haloperidol treatment which mainly blocks D2 receptors. These results indicate that the amacrine and/or interplexiform DA cells, with sparse branches in the outer plexiform layer, can operate via volume transmission in the rat retina to influence the outer plexiform layer and the outer segment, as well as other layers of the rat retina such as the ganglion cell layer.

Morphometry and distribution of displaced dopaminergic cells in rat retina

The majority of dopaminergic (DA) cells, labeled by tyrosine hydroxylase (TH) immunohistochemistry, are located in the amacrine cell layer (i.e., the innermost sublayer of the inner nuclear layer) in the rat retina. We describe a small population of DA cells, observed in retinal wholemounts, that are displaced to either the inner plexiform layer (DAIcs) or the ganglion cell layer (DAGcs). Contrary to some other species, such cells are few in number in the rat retina. Their systematic study was made in young and adult retinas by retinal mapping, camera lucida drawing, and computer-aided three-dimensional reconstruction. Located predominantly in the superior temporal quadrant, they are observed as soon as the second postnatal day. Most of the morphometric parameters studied were not significantly different between the two types of displaced DA cells, despite the characteristic appearance of interstitial cells. Two hypotheses are proposed for the origin of their displacement: either it is accidental or programmed. Our results favor the former possibility.

Synaptic organization of dopaminergic amacrine cells in the larval tiger salamander retina

The ultrastructural features and synaptic interactions of tyrosine hydroxylase-like-immuno-reactive amacrine cells in the larval tiger salamander retina were examined using routine immunoelectron microscopy. The somas of tyrosine hydroxylase-like-immunoreactive amacrine cells were immunostained evenly throughout their cytoplasm. Their nuclei were generally unstained and possessed indented nuclear membranes. The processes of tyrosine hydroxylase-like-immunoreactive amacrine cells were homogeneously stained with the exception of their mitochondria, whose morphology was often disrupted by the staining procedure. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were characterized by an occasional dense-cored vesicle(s), in addition to a generally homogeneous population of small, round, agranular synaptic vesicles. They formed conventional synaptic junctions that were characterized by symmetrical synaptic membrane densities. A total of 168 synapses were observed that involved tyrosine hydroxylase-like-immunoreactive amacrine cell processes. A large percentage (79.8%) of these synaptic arrangements were found in sublayer 1 of the inner plexiform layer, while substantially lower percentages were observed in sublayers 3 (9.5%) and 5 (10.7%). They served as pre- and postsynaptic elements 63.1 and 36.9% of the time, respectively. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were presynaptic to amacrine cell processes (36.9% of total synaptic involvement) and processes that lack synaptic vesicles and whose origin remains uncertain (26.2%). They received synaptic input primarily from amacrine cell processes (31.0%). Tyrosine hydroxylase-like-immunoreactive amacrine cell processes also received a few ribbon synapses from bipolar cells (5.9%). Each of these synaptic relationships were observed in each of sublayers 1, 3 and 5 of the inner plexiform layer, with the majority of each arrangement being found in sublayer 1.

Localization of GABA, glycine, glutamate and tyrosine hydroxylase in the human retina

A light microscope study using postembedding immunocytochemistry techniques to demonstrate the common neurotransmitter candidates gamma-aminobutyric acid (GABA), glycine, glutamate, and tyrosine hydroxylase for dopamine has been done on human retina. By using an antiserum to GABA, we found GABA-immunoreactivity (GABA-IR) to be primarily in amacrine cells lying in the inner nuclear layer (INL) or displaced to the ganglion cell layer (GCL). A few stained cells in the INL, which are probably interplexiform cells, were observed to project thin processes towards the outer plexiform layer (OPL). There were heavily stained bands of immunoreactivity in strata 1, 3 and 5 of the inner plexiform layer (IPL). An occasional ganglion cell was also GABA-IR. By using an antiserum to glycine, stained cells were observed at all levels of the INL. Most of these were amacrines, but a few bipolar cells were also glycine-IR. Displaced amacrine cells and large-bodied cells, which are probably ganglion cells, stained in the GCL. The bipolar cells that stained appeared to include both diffuse and midget varieties. The AII amacrine cell of the rod pathway was clearly stained in our material but at a lower intensity than two other amacrine cell types tentatively identified as A8 and A3 or A4. Again, there was stratified staining in the IPL, with strata 2 and 4 being most immunoreactive. An antiserum to glutamate revealed that most of the neurons of the vertical pathways in the human retina were glutamate-IR. Rod and cone photoreceptor synaptic endings labeled as did the majority of bipolar and ganglion cells. The rod photoreceptor stained more heavily than the cone photoreceptor in our material. While both midget and diffuse cone bipolar cell types were clearly glutamate-IR, rod bipolars were not noticeably stained. The most strongly staining glutamate-IR processes of the IPL lay in the outer half, in sublamina a. The antiserum to tyrosine hydroxylase (TOH) revealed two different amacrine cell types. Strongly immunoreactive cells (TOH1) had their cell bodies in the INL and their dendrites ramified in a dense plexus in stratum 1 of the IPL. Fine processes arising from their cell bodies or from the stratum 1 plexus passed through the INL to reach the OPL but did not produce long-ranging ramifications therein. The less immunoreactive amacrines (TOH2) lay in the INL, the center of the IPL or the GCL and emitted thick dendrites that were monostratified in stratum 3 of the IPL.