Ординатура / Офтальмология / Английские материалы / The Retina and its Disorders_Besharse, Bok_2011

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inhibited by application of the A-protomer of pertussis toxin (the holotoxin moiety where the ribosyl-transferase activity resides), as predicted from the presence of a cysteine in the fourth position from the carboxy terminus of the Gao sequence. This is the hallmark for susceptibility to ADP-ribosylation.

Guanylate Cyclase

The downstream mechanisms of visual excitation call for a light-induced elevation of cGMP, which could conceivably arise by either of two schemes: (1) inhibition of a phosphodiesterase (PDE) over a background of constitutive guanylate cyclase (GC) activity (i.e., the mirror image of the cGMP cascade of rods and cones) and (2) light-dependent stimulation of a cyclase (i.e., parallel to the cAMP cascade of olfactory neurons). Pharmacological antagonists of PDE fail

to mimic the effects of light (i.e., they do not directly activate the photoconductance, although some of them augment the amplitude of the light response). By contrast, GC antagonists reversibly inhibit the photoresponse, suggesting that light may control cGMP production, rather than degradation (i.e., scheme (2)). Such a notion, unprecedented for visual cells, parallels the well-established role of adenylate cyclase in ciliary neurons of the olfactory epithelium, where cAMP is the internal messenger. The putative lightregulated GC, however, must not be one of the canonical soluble (sGC) or membrane (mGC) forms. First, the light response is impervious to manipulations of the nitric oxide (NO) pathway, which suggests exclusion of a soluble GC. Second, changes in intracellular Ca concentration do not alter the photocurrent, which indicates an essential divergence with respect to the regulatory mechanisms that operate in vertebrate membrane GCs. Most importantly, like in

all G-protein-coupled receptors, rhodopsin signals through a G-protein, and such a regulatory mechanism has not been reported by either class of GC. However, a new family of GCs has been uncovered in lower organisms (e.g., Plasmodium, Paramecium, and Dictyostelium), with the same topology as the G-protein-regulated adenylyl cyclase (class III) of olfactory neurons. In Dictyostelium, it has been reported that its activity is regulated by a heterotrimeric G-protein of the Go subtype. G protein dependency for GC activity has recently been documented in Leydig tumor cells and may thus not be confined to protozoa. In Pecten retinal lysates, polyclonal antibodies against the multitransmembrane domain GC of Paramecium label a distinct band, with an apparent molecular mass of 240 kDa, similar to that of the multitransmembrane domain GCs, and greatly exceeding that of either soluble (70–82 kDa) or other membrane-bound GCs (up to 140 kDa).

Light-Dependent Ion Channels

The molecular identity of the ion channels underlying the receptor potential of ciliary photoreceptors has yet to be established. Interestingly, in Western blots of Pecten retinal lysates, antibodies raised against CNG-2 (the a-subunit of the transduction channel of olfactory neurons) label a single band of the appropriate apparent molecular mass, 73 kDa; by contrast anti-CNG-1, the vertebrate retina form, and anti-CNG-3 (which is expressed in mammalian heart, kidney, and sperm) produce no signals. In wholeeye cryosections, the same antibodies selectively decorate the distal layer of the retina, where ciliary photoreceptors are found. By confocal fluorescence microscopy in dissociated cells, the target was localized in the ciliary appendages, presumed to be the light-transducing organelles. The results suggest the presence of an olfactory-like CNG channel in distal photoreceptors, with a subcellular distribution compatible with a role in visual excitation.

The light-dependent channels of distal photoreceptors are uniquely interesting because of their gating and ionselectivity properties. It has long been known that CNG channels are homologous to some voltage-gated K channels, and a common evolutionary origin has been proposed. Nonetheless, the two classes differ sharply in terms of ion permeation, which in CNG channels is characteristically cationic nonselective. The light-dependent channels of hyperpolarizing invertebrate photoreceptors are remarkably similar to both voltage-gated K channels and CNG channels: on the one hand, they are strongly selective for potassium and highly susceptible to blockage by certain K-channel antagonists like 4-aminopyridine. On the other hand, they are gated by cGMP and blocked by various antagonists of the light-sensitive conductance of rods, such as l-cis-diltiazem. As such, they seemingly constitute a missing link, bridging the gap between these two superfamilies of ion channels. This raises a question about

the origin of their gating mechanism. In Pecten, the light-dependent K conductance exhibits a pronounced outward rectification due to voltage-dependent occlusion of the permeation pathway by Ca2+ and Mg2+ ions, which bind at a site located about half-way through the membrane (electrical distance d 0.6 from the external surface). Blockage by Ca2+ and Mg2+ requires an open pore, and the channels can close with a divalent ion trapped inside. These observations suggest that the cGMP-controlled gate must reside near the extracellular side of the channel protein, in sharp contrast with the intracellularly located gate of its voltage-dependent K channel relatives.

Light Adaptation

Because the primary function of ciliary cells is to produce an OFF discharge upon the dimming of a continuous light, the photoresponse is bound to have a prominent sustained component. Nonetheless, during prolonged light stimulation the photocurrent decays to a plateau, and background illumination or conditioning flashes produce all the classical manifestations of light adaptation: shift in the sensitivity curve, compression of the response amplitude range, and acceleration of response kinetics. However, the underlying modulatory mechanisms operate in an unusual way: the lack of a detectable Ca permeability of the light-activated channels and of a functional IP3 signaling pathway implies that light stimulation is not coupled to either influx or internal release of calcium. In fact, unlike all other known photoreceptors, fluorescent Ca indicators report no discernible light-induced changes in cytosolic calcium. As a consequence, this ion would not be in a position to play a significant role in light adaptation. Not surprisingly, direct manipulations of intracellular Ca, either buffering it with the rapid Ca chelator BAPTA, or, conversely, elevating it to mM levels fails to significantly change basal light sensitivity or to alter adaptation. The Ca-indepen- dent signaling pathway responsible for light adaptation appears to implicate cGMP, the same messenger that governs visual excitation: application of cGMP analogs not only activates the photoconductance, but, on a slower timescale, also depresses the light response to an extent that far exceeds what one would expect from the decreased pool of available channels (i.e., a simple competition for a common effector mechanism). This excess reduction of the photoresponse amplitude is accompanied by a shift in the sensitivity curve and acceleration of response kinetics, the hallmark signs of light adaptation (Figure 11). Tests with pharmacological antagonists indicate that the changes in sensitivity during light adaptation mediated by cGMP may be in part controlled by a cGMP-dependent protein kinase.

In summary, ciliary photoreceptors found in the retina of several bivalve mollusks diverge sharply from classical (microvillar) invertebrate photoreceptors, and partake

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instead of several morphological and structural features of vertebrate rods and cones; nonetheless, they utilize a fundamentally different cascade both for light transduction and for light adaptation, warranting their inclusion in a novel separate class of light-transducing cells. The parallelism with the odor-transduction cascade of olfactory neurons suggests a common lineage with an ancestral chemoreceptor cell.

See also: Genetic Dissection of Invertebrate Phototransduction; Phototransduction: Inactivation in Rods; Phototransduction in Limulus Photoreceptors; Phototransduction: Phototransduction in Cones; Phototransduction: Phototransduction in Rods.

Further Reading

Barber, V. C., Evans, E. M., and Land, M. F. (1967). The fine structure of the eye of the mollusk Pecten maximus. Zeitschrift fu¨r Zellforschung und Mikroskopische Anatomie 76: 295–312.

Cornwall, M. C. and Gorman, A. L. F. (1979). Contribution of calcium and potassium permeability changes to the off response of scallop hyperpolarizing photoreceptors. Journal of Physiology 291: 207–232.

Gorman, A. L. F. and McReynolds, J. S. (1969). Hyperpolarizing and depolarizing receptor potentials in the scallop eye. Science

165: 309–310.

Gomez, M. and Nasi, E. (1994). The light-sensitive conductance of hyperpolarizing invertebrate photoreceptors: A patch-clamp study.

Journal of General Physiology 103: 939–956.

Gomez, M. and Nasi, E. (1995). Activation of light-dependent potassium channels in ciliary invertebrate photoreceptors involves cGMP but not the IP3/Ca cascade. Neuron 15: 607–618.

Gomez, M. and Nasi, E. (1998). Membrane current induced by protein kinase C activators in rhabdomeric photoreceptors: implications for visual excitation. Journal of Neuroscience 18: 5253–5263.

Gomez, M. and Nasi, E. (2000). Light transduction in invertebrate hyperpolarizing photoreceptors: Involvement of a Go-regulated guanylate cyclase. Journal of Neuroscience 20: 5254–5263.

Gomez, M. and Nasi, E. (2005). A direct signalling role for PIP2 in the visual excitation process of microvillar receptors. Journal of Biological Chemistry 280: 16784–16789.

Gomez, M. and Nasi, E. (2005). Calcium-independent, cGMP-mediated light adaptation in ciliary photoreceptors. Journal of Neuroscience 25: 2042–2049.

Kojima, D., Terakita, A., Ishikawa, T., et al. (1997). A novel Go-mediated phototransduction cascade in scallop visual cells. Journal of Biological Chemistry 272: 22979–22982.

Nasi, E. (1991). Two light-dependent conductances in the membrane

of Lima photoreceptor cells. Journal of General Physiology 97: 55–72. Nasi, E. and Gomez, M. (1992). Light-activated ion channels in

solitary photoreceptors from the eye of the scallop Pecten irradians.

Journal of General Physiology 99: 747–769.

Nasi, E. and Gomez, M. (1999). Divalent cation interactions with lightdependent K channels: Kinetics of voltage-dependent block and requirement for an open pore. Journal of General Physiology

114: 653–671.

Piccoli, G., Gomez, M., and Nasi, E. (2002). Role of protein kinase C in light adaptation of microvillar photoreceptors. Journal of Physiology 543: 481–494.

Glossary

Coverage factor – The product of mean dendritic field size and cell density. It gives a measure of the number of neurons of a particular type whose receptive fields overlap a particular point on the retina. Because of their great varieties in shapes and frequencies, amacrine cells of different types can have coverage factors anywhere between 1 and 500. Retinal mosaic – The ordinate spatial pattern formed over the retinal surface by cells of a certain type, arranged regularly. Each retinal cell type has a unique mosaic that can be described formally in mathematical terms. Starburst amacrine cells tile the retinal surface very regularly, while dopaminergic amacrines have highly irregular mosaics.

Near the end of the nineteenth century, Santiago Ramon y Cajal named amacrine cells those retinal neurons whose cell bodies occupy the innermost tier of the inner nuclear layer (INL) and that ramify at various depths in the inner plexiform layer (IPL). Literally, their name means cells without an axon. From Cajal’s initial observations and following early studies based on Golgi-impregnated neurons, the notion emerged that amacrine cells come in all shapes, sizes, and stratification patterns (Figure 1). In time, many morphological types were described thanks to the development of various techniques. These included intracellular recordings followed by dye injections, immunocytochemical staining, assorted anatomical tracing methods and, more recently, transgenic expression of fluorescent molecules. At present, it is accepted that the retina of mammals contains more than 26 varieties of amacrine cells; their catalog can be considered substantially complete. Classification schemes with a tendency to emphasize subtle differences among presumptive categories separate amacrine cells in as many as 40 different types. Surely enough, amacrine cells represent the most diverse cell types in the retina, as they comprise more varieties than bipolar and ganglion cells.

In the IPL, amacrine cells receive their excitatory input from bipolar cells and provide inhibitory output onto the dendrites of both other amacrine and ganglion cells. They also establish inhibitory synapses onto the axonal arborizations of bipolar cells. Functionally, amacrine cells are capable of modulating the activity of ganglion cells by direct inhibition, or by inhibiting the

activity of bipolar cells that carry excitatory inputs to the inner retina. Many amacrine cells are also coupled by means of gap junctions.

Coupling can be with amacrine cells of the same (homologous) or different (heterologous) type. In addition, amacrine cell processes can be coupled to ganglion cell dendrites. Although many amacrine cells, such as the AIIs, generate graded signals in response to light, it has been shown that certain amacrine cells are capable of producing true action potentials that initiate locally and then propagate into every dendrite, exciting the entire cell. Thus, each amacrine cell can mediate both local as well as long-range lateral inhibition, regulating the spatial and temporal pattern of synaptic outputs from its dendrites.

Despite their name, certain wide-field amacrine cells have long, axon-like processes which probably function as true axons as they represent output fibers of the cell. These particular amacrines have been described as polyaxonal and their soma are often found in an interstitial position, that is, within the IPL. However, their long processes remain confined within the retina and do not contribute to the optic nerve as do the axons of ganglion cells.

Traditionally, amacrine cells have been divided into the broad categories of narrow-field (30–150 mm), small-field (150–300 mm), medium-field (300–500 mm), and widefield (>500 mm) cells, based on the size of their dendritic field diameters. It is worth pointing out that the dendritic spread of the cells is correlated quite strongly with their degree of stratification. Hence, wide-field cells are highly stratified, medium-field cells are less so, and almost all narrow-field cells span radially across two or more levels of the IPL (Figure 2).

The mere size of their dendritic field, however, cannot account for the great variability in the morphology of these interneurons. Multiple classification schemes coexist: some amacrine cells have traditionally maintained numbers from old, partial nomenclatures (i.e., AII amacrines and A17 amacrines), while others are indicated with names reminiscent of their shapes, such as starburst cell, fountain amacrine, flag amacrine, or spider cell. As for bipolar and ganglion cells, a more functional, and therefore relevant criterion of identification involves knowing the stratification level of the cells. It is well known that the IPL can be subdivided into five equally thick strata or sublayers to which amacrine, bipolar, and ganglion cell processes can be assigned. The level of stratification of a given neuron in the IPL is predictive of the polarity ofthe cell response to light stimuli, so that neurons confined in the outermost two

layers of the IPL, the so-called sublamina a, are excited when the light goes off, while neurons ramified in the innermost three tiers of the IPL, the sublamina b, are excited at the light onset. This strong correlation between anatomy and function makes it convenient to classify neuronal types primarily according to the strata of the IPL in which their processes are confined. Altogether, the combination of dendritic field size, pattern, and depth of

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Figure 1 Small-, medium-, and wide-field amacrine cells from the human retina. Golgi silver impregnation. Modified from Poljak, S. (1941). The Retina.

stratification within the IPL are sufficient criteria to separate cells into unique types. Supplementary information includes dendritic caliber, size and distribution of varicosities, and the course of dendrites.

The fact that branching of individual amacrine cells takes place at various levels in the IPL implicates that different amacrine cells contact different types of bipolar and ganglion cells and end up having different functional properties. Indeed, amacrine cells are major players in the retina’s processing of visual information. They constitute at least 40% of all neurons in the INL of mammalian retinas and contribute to 64–87% of all synapses in the IPL according to the species. While it was once generally believed that primate retinas were more bipolar dominated as opposed to lower-mammal (i.e., rabbits) retinas, considered more amacrine dominated, quantitative morphology demonstrated that these cells represent some 40% of the INL cells in rabbits, mice, and primates.

Neurotransmitter immunocytochemistry (based on the localization of amino acids or rate-limiting enzymes) has shown that amacrine cells either contain gammaaminobutyric acid (GABA) or glycine, two inhibitory amino acids used as neurotransmitters ubiquitously in the central nervous system. Remarkably, GABAergic amacrine cells are morphologically wide-field cells, while glycinergic amacrines are smalland medium-field neurons. Glycinergic amacrines are slightly more numerous, at least in the rabbit retina, in which they account for some 56% of all the amacrines. This fits well with the percentage of small-field amacrines (55%) revealed by the technique

of random photofilling. Glycinergic amacrines, with their narrow fields and little overlapping dendrites, mediate radial inhibition in the retina. These cells are meant for vertical transmission rather then for modulation of light signals.

GABAergic amacrines, instead, are more designated for later inhibition, a function traditionally ascribed to amacrine cells in general. In GABAergic amacrines, the fast neurotransmitter GABA usually coexists with another neurotransmitter or neuromodulator; this is typically a peptide, such as somatostatin, substance P, vasoactive intestinal peptide (VIP), and so on. Because they contain high levels of one amino acid, amacrine cells can be identified through their typical amino acidic signature in appropriately stained histological sections of the retina. Noticeably, cholinergic (starburst) amacrine cells are stained with methods revealing both GABA and acetylcholine and are known to corelease both transmitters.

Each particular point of the retina is covered by the dendritic trees of cells of the same type, overlapping to different extents. The parameter indicating the degree of overlapping is known as the coverage factor. This is obtained essentially by histological measurements and is defined as the product of cell density and area of the dendritic tree. A coverage factor of 10 means that a retinal point is covered by the dendritic trees of 10 cells of a given type. Amacrine cells have highly variable coverage factors. In general, small-field amacrine cells have lower coverage factors than wide-field amacrine cells. The well-known indoleamine-accumulating cells (IACs), which comprise two varieties of wide-field amacrines, narrowly stratified in the deepest stratum of the IPL, have coverage factors of 500–900. This means that in each point of the retina, a stack of processes from overlapping IAC cells are regularly piled in sublamina 5 of the IPL. Low-power electron microscopy shows bundles of IAC cells fasciculating in sublamina 5, running parallel to each other and interrupted by the large varicosities of rod bipolar axonal endings, also located at the border with ganglion cell bodies.

Individual types of retinal neuron exhibit regular spacing, such that cells of a certain type maintain a minimum distance from other neurons of the same type. Each cell is surrounded by an exclusion zone from which other cells of the same type are barred. On the contrary, spacing of neurons of different types is completely casual. Amacrine cells adhere to this rule. Because of the availability of cell-specific markers, the mosaics of certain amacrine cells have been described in detail. Starburst amacrine cells can be stained by antibodies against choline acetyl trasferase (ChAT), the ratelimiting enzyme in acetylcholine synthesis; their mosaic is highly characteristic. Similarly, AII amacrine cells can be labeled with antibodies against the protein disabled 1, or, in certain mammals including humans, by parvalbumin antibodies. Dopaminergic amacrines (historically, the first to be described, using enzyme histochemistry) can be

revealed by antibodies against tyrosine hydroxylase. Each of these amacrine cells forms over the retinal surface a peculiar, well-recognizable mosaic that can be described formally in rigorous, mathematical terms. Whatever the mechanism leading to mosaic formation during retinal development might be, the regularity of a mosaic is a powerful means to assess that a given cell has been properly identified and correctly assigned to a type. Mutations in various genes controlling retinal development can alter mosaic formation and lead to retinal structural abnormalities.

Multidisciplinary studies on the currently well-known amacrine cell types have led to the notion that, as for other retinal neurons, diversity of shape means diversity of function. This concept is supported by a number of considerations: first, cells whose morphological features are very different have different membrane properties associated with the size of the dendritic tree, the caliber of individual dendrites, and the number and position of input and output synapses with respect to the cell soma. In addition, amacrine processes that occupy different sublayers of the IPL communicate with different sets of bipolar and ganglion cells. Since connectivity shapes function, the functional properties of amacrine cells with different patterns of connections must be different. Also, the area of the dendritic field (which varies greatly among various cells types) is strictly related to the retinal sampling capability (or area of visual space) of a given cell. A different coverage factor also reflects different sampling rates of cells.

The following sections describe two types of amacrine cells whose morphological and functional properties have been studied in detail and appear extremely different. They can be considered as paradigmatic of amacrine- cell-variegated structural and functional aspects.

AII Amacrine Cells

These small-field amacrines were first described in the cat retina by Kolb and co-workers in 1978 and can be considered as hallmark components of the retina of mammals. They bear a distinctive morphology as their dendrites are organized in two different layers: an ovoidal cell body gives rise to a thick, primary dendrite producing two orders of branches – a series of long, thin processes mostly restricted in sublamina 5 of the IPL, near the cell bodies of ganglion cells; and a second set of globose varicosities, called lobular appendages, clustered in a bushy and narrow ramification, spanning vertically through sublaminae 1–2 of the IPL (Figure 3). Electron microscopy has demonstrated that AII amacrine cells are placed in a pivotal position along the rod pathway. Scotopic signals transmitted from rods to rod bipolar cells (which belong to the category of ON-center, depolarizing neurons) are conveyed through sign-conserving, glutamatergic synapses to the vitreal

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Figure 3 An individual AII amacrine cell from the mouse retina labeled after delivering the fluorescent probe DiI with a gene gun. Note the typical bistratified morphology, with lobular appendages (arrowheads) confined in the outermost part of the inner plexiform layer (IPL), the sublamina a. Long and thin processes of the same amacrine reach the innermost portion of the IPL, the sublamina b.

dendrites of AII amacrine cells in sublamina 5 of the IPL. At this point, a dichotomy is generated in the rod pathway, as the signal is literally split into two channels by means of the connections of AII amacrines. These cells establish sign-conserving gap junctions with the axonal arbors of cone bipolar cells located in sublamina ON of the IPL, and glycinergic, sign-inverting synapses with axonal endings of cone bipolar cells terminating in sublamina OFF of the IPL. Finally, rod-initiated signals are transferred to the ganglion cell through sign-conserving chemical synapses established by axonal endings of cone bipolar cells in the ON and OFF halves of the IPL. Therefore, the rod pathway ultimately makes use of the axonal endings of cone bipolar cells to gain access to ganglion cells. The fiveneuron chain in which the main rod pathway is organized is usually called piggyback arrangement, to indicate the fact that rod-generated signals use a common route, represented by cone bipolars, to exit the retina. AII amacrines are key elements of the piggyback pathway: because of their peculiar morphology and connectivity, they occupy a strategic position when illumination conditions switch from the scotopic range, in which rod photoreceptors are active, to the photopic range, in which cones become functional. AII amacrines, therefore, have to be informed about adaptation in the inner retina. This information is provided, among others, by dopaminergic amacrine cells, which play a relevant role in adaptation processes in the neural retina, and which provide a rich innervation of the primary dendrites of AII amacrine cells in sublamina 1 of the IPL.

Because of their synaptic circuitry, cells such as the AII do not conform to the general concept of amacrine cells as laterally placed, modulatory elements of the retinal connectivity, deputed to lateral transmission of electric

Figure 4 Example of three adjacent starburst amacrines stained by intracellular injection with Lucifer yellow. Reproduced from Vaney, D.I. (1999). Neuronal coupling in the central nervous system: Lessons from the retina. Novartis Foundation Symposium 219: 113–125.

signals. Within the rod pathway, AII amacrines occupy a vertical position and rod-generated signals cannot enter ganglion cells without this strategically placed neuronal type. Hence, in very dim light, the AII amacrine is an obligatory connection in the retina’s through-pathway. Generally speaking, small-field amacrine cells introduce little more lateral conduction than bipolar cells do; besides the AII, the function of other small-field amacrines is yet to be clarified.

As rod photoreceptors are numerous, AII amacrine cells are also abundant in the retina. Indeed, they are the largest population of amacrine cells accounted for, reaching 11–13% of all the amacrines in the retina of rabbits and mice. The remaining amacrine cell types come at a lower frequency, each reaching 3% maximum of the total population.

Starburst Amacrines

Starburst amacrine cells, so defined thanks to the shape of their circular and rich dendritic tree, are the second, most numerous amacrine cells in mammals, accounting for about 3% of the whole amacrine population. They are a recurrent finding of vertebrate retinas, from dogfish to primates. These neurons have a characteristic, radially symmetric morphology, with higher-order dendritic branches emerging from dichotomous ramification of thicker fibers. In the rabbit retina, their dendritic field size is approximately 400 mm and therefore they belong to the category of wide-field cells (Figure 4). It has been shown that starburst amacrines overlap extensively and their dendrites occupy a large percentage of the volume of the IPL. In the rabbit, calculations show that each millimeter of IPL is covered by 6 m of dendrites from starburst amacrine cells, so that the whole retina contains some 2 km of processes from this neuronal type.

Starburst amacrines occur in two, mirror-symmetric populations, composed of almost equal numbers of cells. In the first group, cell bodies reside in the innermost tier of the INL and dendrites form a narrow plexus in sublamina 2 of the IPL.

These are OFF-starburst cells. In the second population, cell bodies are located in the ganglion cell layer instead and therefore contribute to the heterogeneous group of displaced amacrine cells. Displaced starbursts can be easily identified in the ganglion cell layer even with simple nuclear staining because of the smaller nuclear size and visible regularity of their mosaic. Their dendrites form a second plexus in sublamina 4 of the IPL and are therefore ON-starbursts. Because of their early formation during retinal development and precise positions in the ON and OFF halves of the IPL, the two tiers of starburst dendrites (also known as cholinergic bands) are landmarks often used as references to define the stratification levels of other neurons in the IPL.

In the first days of postnatal development, when retinal neurons are being generated and assembled in regular tiers, waves of light-independent electrical activity traverse the inner retinal surface intermittently. Retinal waves can be recorded from the ganglion cell layer in the form of correlated activity of these neurons. Waves are thought to play an instructive role in the formation of topographic maps of projection neurons in the central-most visual areas. Pharmacological experiments indicate that developmental waves require the presence of starburst amacrines, releasing both GABA and acetylcholine, the latter acting on nicotinic receptors. The action of both neurotransmitters is excitatory, since GABA has an excitatory role during development. During retinal maturation, starburst cells communicate directly with each other, so that electrical activity generated at one retinal location can propagate to distant areas. However, during the subsequent developmental stages, excitation between starbursts becomes almost undetectable while GABAergic synapses among starburst cells switch from being excitatory to inhibitory.

Starburst cells participate in transretinal waves during early development but their role is totally different in the adult retina. The exact cofasciculation of starburst processes and the dendrites of a highly distinctive ganglion cell type, the ON-OFF directional selective (DS) ganglion cell, which senses stimuli moving in one direction, led to the hypothesis that, in the adult retina, starburst amacrines could provide a major source of synaptic input to this category of neurons. A wealth of data generated the notion that acetylcholine and starburst amacrines in particular, do contribute to the functional properties of directional selectivity. A current view of this complex problem that has fascinated physiologists for decades is that starburst amacrines are capable of releasing the neurotransmitter in a directional fashion, contributing to the tuning of DS ganglion cells for the preferred direction of motion.

Only for a few other types of amacrine cells has a clear role been elucidated. Among them, dopaminergic amacrines, also known as interplexiform cells, have a crucial function in neural adaptation and circadian rhythms; A17 amacrine cells, a long-time known type of wide-field amacrine, are typical local interneurons, providing feedback inhibition to the axonal ending of rod bipolar cells. For most of the amacrine cell types, however, the role is simply inferred on the basis of their costratification with other retinal neurons. Obviously, the fact that our inventory of amacrine cells is presumably complete represents only the first stage in the comprehension of the role of these interneurons. The next step toward a true understanding of retinal architecture will be to learn synaptic and functional interactions of the amacrine cell with individual types of bipolar and ganglion cells, constituting parallel pathways across the retina – a challenge for future years.

See also: Information Processing: Amacrine Cells; Morphology of Interneurons: Interplexiform Cells.

Further Reading

Baccus, S. A. (2007). Timing and computation in inner retinal circuitry.

Annual Review of Physiology 69: 271–290.

Demb, J. B. (2007). Cellular mechanisms for direction selectivity in the retina. Neuron 55: 179–186.

Galli-Resta, L. (2002). Putting neurons in the right places: Local interactions in the genesis of retinal architecture. Trends in Neuroscience 25: 638–643.

Jeon, C. J., Strettoi, E., and Masland, R. H. (1998). The major cell populations of the mouse retina. Journal of Neuroscience 18: 8936–8946.

MacNeil, M. A., Heussy, J. K., Dacheux, R. F., Raviola, E., and Masland, R. H. (1999). The shapes and numbers of amacrine cells: Matching of photofilled with Golgi-stained cells in the rabbit retina and comparison with other mammalian species. Journal of Comparative Neurology 413: 305–326.

MacNeil, M. A. and Masland, R. H. (1998). Extreme diversity among amacrine cells: Implications for function. Neuron 20: 971–982.

Marc, R. E., Murry, R. F., Fisher, S. K., et al. (1998). Amino acid signatures in the normal cat retina. Investigative Ophthalmology and Visual Science 39: 1685–1693.

Masland, R. H. (2004). Neuronal cell types. Current Biology 14: R497–R500.

Masland, R. H. (2005). The many roles of starburst amacrine cells.

Trends in Neuroscience 28: 395–396.

Masland, R. H. and Raviola, E. (2000). Confronting complexity: Strategies for understanding the microcircuitry of the retina. Annual Review of Neurosciences 23: 249–284.

O’Malley, D. M., Sandell, J. H., and Masland, R. H. (1992). Co-release of acetylcholine and GABA by the starburst amacrine cells. Journal of Neuroscience 12: 1394–1408.

Sharpe, L. T. and Stockman, A. (1999). Rod pathways: The importance of seeing nothing. Trends in Neuroscience 22: 497–504.

Strettoi, E. and Masland, R. H. (1995). The organization of the inner nuclear layer of the rabbit retina. Journal of Neuroscience 15: 875–888.

Taylor, W. R. and Vaney, D. I. (2003). New directions in retinal research.

Trends in Neuroscience 26: 379–385.

Torborg, C. L. and Feller, M. B. (2005). Spontaneous patterned retinal activity and the refinement of retinal projections. Progress in Neurobiology 76: 213–235.

Vaney, D. I. (1999). Neuronal coupling in the central nervous system: Lessons from the retina. Novartis Foundation Symposium 219: 113–125.

Glossary

Clomeleon – A ratiometric genetically encoded indicator comprising a fusion of cyan and yellow fluorescent protein that allows noninvasive chloride measurements in living tissue.

Dyad – Synaptic arrangement of the bipolar cell output synapses within the inner plexiform layer of the retina; the presynaptic ribbon of a dyad is surrounded by vesicles and two postsynaptic elements.

Genuine S-cones – In ventral mouse retina, the great majority of cones express both mediumwavelength (M) and short-wavelength (S) opsin. Blue cone bipolar cells contact only those cones, which express S-opsin only. These are the genuine S-cones.

Landolt club – Process that extends distally from many bipolar cells in cold-blooded vertebrates from the outer plexiform layer to the outer limiting membrane. The function of the Landolt club is not known in any retina.

Midget bipolar cell – The term midget refers to the small spread of their dendritic and axonal arbors. The one-to-one relationship between midget bipolar cells and cones on the dendritic end, and between midget bipolar cells and ganglion cells on the axonal end, is a distinctive property of midget bipolar cells around the fovea.

Type – Members of each cell type show very similar properties, that is, release the same transmitter, make the same connections to other cell types, and generally have the same morphology.

Introduction

The retina contains a large diversity of individual cell types, each carrying out a specific set of functions. They are mainly defined by their morphological appearance. Bipolar cells differ in their dendritic branching pattern, the number of photoreceptors contacted, and the shape and stratification level of their axons in the inner plexiform layer. The cells transfer the light signals from the photoreceptors to amacrine and ganglion cells. They can be subdivided, according to their light responses, into ON

and OFF bipolar cells. This functional dichotomy is the result of the expression of different glutamate receptors at the synapses between photoreceptors and bipolar cell dendrites. OFF bipolar cells make flat or basal contacts with cone pedicles, ON bipolar cells make invaginating contacts. The axon terminals of OFF cone bipolar cells terminate in the outer half of the IPL and synapse with the dendrites of OFF ganglion cells, whereas those of ON bipolar cells terminate in the inner half of the IPL and contact the dendrites of ON ganglion cells.

Bipolar Cell Types of the Mammalian

Retina

Bipolar cells of the mammalian retina can be subdivided into many different morphological types (Figure 1). There are at least nine types of cone bipolar cells and one type of rod bipolar cell. Most mammalian retinas are rod dominated. Therefore, rod bipolar cells form the numerically superior part of the bipolar cell population. Their dendrites make invaginating contacts with rod spherules (Figure 3(b)) and their axons terminate in the innermost part of the IPL (Figure 1: rod bipolar cell, RB). The number of rods converging on a single rod bipolar cell varies greatly between the species, and, within a species, with retinal eccentricity. In the peripheral human retina, each rod bipolar cell contacts 40–50 rods. Near to the fovea, the dendritic trees become smaller and 15–20 rods are connected. Regardless to retinal eccentricity, convergence in the rod pathway is usually higher than in the cone pathway.

Several types of cone bipolar cells have been recognized in different mammalian species (rabbit: 13, human: 10, cat: 8–10, rat: 9–11, ground squirrel: 6–8). The diagram in (Figure 1) compares the bipolar cells of the mouse retina with those of the peripheral macaque monkey retina. The nine putative cone bipolar cell types (labeled 1–9) and the rod bipolar cell of the mouse retina are arranged according to the stratification level of their axon terminals in the IPL. The cells were drawn from vertical sections following intracellular injections. Selective markers, which stain the whole population, are now available for most of the bipolar cell types of the mouse retina (see below). Types 1–4 are OFF cone bipolar cells; types 5–9 are ON cone bipolar cells. The cells contact on average between five and eight neighboring cone pedicles with one exception: type 9 has a wide dendritic tree that appears to be cone selective and it will be shown later that it contacts S-cones.

The mouse retina is considered to be rod dominated because only 3% of their photoreceptors are cones. However, the perspective changes if one examines the absolute number of cones; the cone density is about 13 000 cones/ mm2, similar to peripheral cat, rabbit, and macaque monkey retinas. Consequently, the types and retinal distributions of cone bipolar cells are closely similar between mammalian species. The bipolar cell types of the monkey retina (Figure 1(b)) were determined initially from Golgistained whole mounts. There is a striking similarity between mouse and monkey bipolar cells with respect to the shapes and stratification levels of their axons. However, there is also a clear difference; midget bipolar cells (flat midget bipolar cell (FMB); invaginating midget bipolar cell (IMB)) are only found in the monkey retina. FMB and IMB cells have dendritic trees, which contact a single cone. Bipolar cells contacting several neighboring cone pedicles were named diffuse bipolar cells (DB1–DB6).

Midget Bipolar Cells of the Primate Retina

Primates have trichromatic color vision based on three spectral types of cones: long-wavelength (red or L-), middle-wavelength (green or M-), and short-wavelength (blue or S-) sensitive cones. Midget bipolar cells receive inputs either from red or green cones. Thus, in terms of their input and the polarity of their response, there are four types of midget bipolar cells: red ON, red OFF, green ON, and green OFF. The term midget refers to the small spread of their dendritic and axonal arbors (Figure 1b:

FMB, IMB). In the central retina, a midget bipolar cell receives direct input from just one cone (Figure 2(a)). The bipolar cell axon terminal, which contacts just one ganglion cell, is correspondingly small. This one-to-one relationship between midget bipolar cells and cones on the dendritic end, and between midget bipolar cells and ganglion cells on the axonal end, is a distinctive property of midget bipolar cells around the fovea. Four to five millimeters beyond the fovea, in the near periphery, the midget bipolar cells become two and three headed connecting to two and three cones, respectively (Figure 2(b)).

Reconstructions of Golgi-impregnated midget bipolar cells of the primate retina by serial electron microscopy (EM) revealed a clear dichotomy of their dendritic contacts at the cone pedicle base: IMB cells made exclusively invaginating contacts, whereas FMB cells made only flat contacts (Figure 3(a)). In central retina, they are all in the vicinity of the ribbons (triad associated, TA), and at eccentricities beyond 3– 4 mm, approximately 20% are nontriad associated (NTA). Individual IMB bipolar cells make up to 25 contacts with a cone pedicle, and an FMB cell makes approximately 2–3.5 times that number of basal synapses.

Blue Cone Bipolar Cells

Placental mammals other than primates have only two types of cones: M-cones, in which the visual pigment has an absorption maximum of >500 nm and S-cones with an absorption maximum at <500 nm. They are, therefore, dichromats. In an evolutionary comparison of color