Neuronal and glial cell types revealed by NADPH-diaphorase histochemistry in the retina of a teleost fish, the grass goby ( Zosterisessor ophiocephalus , Perciformes, Gobiidae)

The grass goby is a mud-burrowing fish with a rich retinal vasculature appropriate to its hypoxic habitat. NADPH-diaphorase histochemistry was performed on retinal sections and wholemounts to reveal cells that contain nitric oxide synthase and so may be presumed to synthesise nitric oxide, a gaseous intercellular messenger with many roles including vasodilation. Structures that were consistently stained by this method included cone ellipsoids, horizontal cells, Müller cells and their processes, large displaced ganglion cells in the inner nuclear layer (identified by their axons), large interstitial ganglion cells in the inner plexiform layer, and capillary endothelial cells. In wholemounts, horizontal cells were seen to form a regular pattern, contacting each other at their dendritic terminals. Some cells in the ganglion cell layer were weakly stained, but stained bipolar and amacrine cells were not seen. The diaphorase-positive large ganglion cells all formed large, sparsely branched dendritic trees, arborizing near the scleral border of the inner plexiform layer. The displaced and interstitial cells seemed to belong to distinct morphological types, the interstitial cells having smaller somata and trees. Analysis of their spatial distributions in one representative retina confirmed this: the displaced cells formed a highly regular mosaic with a mean spacing (nearest-neighbour distance) of 303 μm, whereas the interstitial cells formed a separate mosaic, almost as regular but with a smaller mean spacing of 193 μm, rising to 217 μm in a sample that excluded the area retinae temporalis. Spatial correlogram analysis showed that these two mosaics were spatially independent. Nitric oxide probably has many roles in the retina. The presence of its synthetic enzyme in Müller cells, which communicate with retinal blood vessels, is consistent with a role in the control of retinal blood flow. Its function in large, mosaic-forming retinal ganglion cells is unknown. Accepted: 29 April 1999     

The distribution of displaced ganglion cells in the retina of the pigeon

Displaced ganglion cells in the pigeon's retina, at the inner margin of the inner nuclear layer, were labelled by retrograde axonal transport of horseradish peroxidase (HRP). Large HRP injections were made in order to fill all the retinal projection sites in the thalamus and midbrain. The distribution of labelled cells was studied in retinal whole mounts incubated with tetramethyl benzidine (TMB) substrate for HRP. A maximum of 5,300 HRP labelled displaced ganglion cells was found. They were concentrated in a band of retina centred on the horizontal meridian, with high density areas (of about 110 cells/mm2) near the area centralis and in the mid-temporal retina. This is a different distribution to that of ganglion and inner nuclear layer cells; these are concentrated in the area centralis and red field. The orientation of retinal maps was checked by ophthalmoscopic measurements of the angle of the pecten to the horizontal in alert pigeons; this was found to be approximately 70 degrees. The array of displaced ganglion cells, studied by nearest neighbour distributions, was irregular and nearly random, which is consistent with a system of low spatial acuity. In the central retina only the cell bodies and not the dendrites of small displaced ganglion cells (7.5 microns diameter) were labelled; towards the periphery large displaced ganglion cells (16 microns diameter) with 2-5 radially arranged primary dendrites were found.     

Displaced retinal ganglion cells in normal frogs and those with regenerated optic nerves

Summary We have analysed the number and spatial distribution of displaced retinal ganglion cells in the frog Litoria (Hyla) moorei. A series of normal animals was compared with one in which the optic nerve was crushed and allowed to regenerate. Ganglion cells were labelled with horseradish peroxidase (HRP) applied to the optic nerve, and retinae were examined as sections or whole mounts. We analysed separately ganglion cells with somata displaced to the inner nuclear (Dogiel cells, DGCs) and to the inner plexiform layer (IPLGCs). These findings were related to data for the orthotopic ganglion cells (OGCs). The mean number of DGCs in the normal series was 2,550 (±281) and fell to 1,630 (±321) after regeneration, representing a mean loss of 36%. This reduction was not significantly different from the mean loss of 43% from the OGC population in which mean values fell from 474,700 (±47,136) to 268,700 (±54,395). In both the normal and the regenerate series, DGCs were estimated to represent means of only 0.6% of the OGC population. Densities of DGCs were highest in the nasoventral and temporo-dorsal peripheries; densities of both DGCs and OGCs were lower after optic nerve regeneration. We conclude that the factors which affect ganglion cell death during optic nerve regeneration, do so to similar extents amongst the DGC and the OGC populations. The IPLGCs were very rare in normal animals with a mean of 420 (±95). However, their numbers increased after regeneration to a mean of 3,350 (±690), estimated to be 1.2% of the OGC population. These cells normally favoured peripheral retina but became pan-retinal after regeneration. The primary dendrites of the majority of IPLGCs were oriented in the same direction as those of OGCs. We conclude that most IPLGCs were OGCs which had relocated their somata to the inner plexiform layer.     

Neuronal and glial cell types revealed by NADPH-diaphorase histochemistry in the retina of a teleost fish, the grass goby (Zosterisessor ophiocephalus, Perciformes, Gobiidae)

The grass goby is a mud-burrowing fish with a rich retinal vasculature appropriate to its hypoxic habitat. NADPH-diaphorase histochemistry was performed on retinal sections and wholemounts to reveal cells that contain nitric oxide synthase and so may be presumed to synthesise nitric oxide, a gaseous intercellular messenger with many roles including vasodilation. Structures that were consistently stained by this method included cone ellipsoids, horizontal cells, Müller cells and their processes, large displaced ganglion cells in the inner nuclear layer (identified by their axons), large interstitial ganglion cells in the inner plexiform layer, and capillary endothelial cells. In wholemounts, horizontal cells were seen to form a regular pattern, contacting each other at their dendritic terminals. Some cells in the ganglion cell layer were weakly stained, but stained bipolar and amacrine cells were not seen. The diaphorase-positive large ganglion cells all formed large, sparsely branched dendritic trees, arborizing near the scleral border of the inner plexiform layer. The displaced and interstitial cells seemed to belong to distinct morphological types, the interstitial cells having smaller somata and trees. Analysis of their spatial distributions in one representative retina confirmed this: the displaced cells formed a highly regular mosaic with a mean spacing (nearest-neighbour distance) of 303 µm, whereas the interstitial cells formed a separate mosaic, almost as regular but with a smaller mean spacing of 193 µm, rising to 217 µm in a sample that excluded the area retinae temporalis. Spatial correlogram analysis showed that these two mosaics were spatially independent. Nitric oxide probably has many roles in the retina. The presence of its synthetic enzyme in Müller cells, which communicate with retinal blood vessels, is consistent with a role in the control of retinal blood flow. Its function in large, mosaic-forming retinal ganglion cells is unknown.     

Displaced ganglion cells in the chick retina

New morphological and cytological data on the displaced ganglion cells (DGCs) in the chick retina are presented. Analysis of the topographic distribution, cellular number, dendritic field, perikaryon size and ultrastructural characteristics are included. The DGCs were found predominantly in the peripheral retina. The sizes of the DGCs, 18-42 microns, observed either by Normarsky's interferential contrast or by silver impregnation techniques, spanned the size range of the other retinal neurons. The results support the hypothesis that DGCs, in the chick retina, may constitute a specific morphofunctional system, and therefore they might not be considered as neurons that fail to attain the normal location of ganglion cells during the developmental process of migration.     

Functional lamination in the ganglion cell layer of the macaque's retina

Close to the fovea of the primate retina the ganglion cell layer is at its maximal thickness and several layers of cells deep. In whole-mount preparations in which the ganglion cells had been retrogradely labelled to reveal the dendritic trees we have studied the distribution of the different ganglion cell types across the depth of the ganglion cell layer. The ganglion cells which project to the parvocellular layers (P ganglion cells) are found more vitread than those which project to the magnocellular layers (M ganglion cells). The cells which project to the midbrain lie in the outer part of the ganglion cell layer among the M cells and adjacent to the inner plexiform layer. Within the P and M classes of ganglion cell the On-centre cells lie more vitread than the Off-centre cells. These results are discussed with relation to the proportions of different cell types sampled with intraocular recordings from ganglion cells and the possible significance for the development of different types of ganglion cell.     

Substance P-immunoreactive neurons in the retina of two lizards.

The dendritic morphology and retinal distribution of substance P(SP)-immunoreactive neurons was determined in two Australian lizard species Pogona vitticeps and Varanus gouldii, by using immunohistochemistry on retinal wholemounts and sectioned materials. In both species, two classes of SP-immunoreactive neurons were described in the inner nuclear layer (INL) and classified as amacrine cells (types A and B). Type A amacrine cells had large somata and wide-field, bistratified dendrites branching in sublaminas 1 and 5 of the inner plexiform layer (IPL). Their morphology and retinal distribution differed between the two species. Type B amacrine cells in both species had small somata and small-field dendritic branching. A population of SP-immunoreactive neurons with classical ganglion cell morphology were identified in the ganglion cell layer (GCL). Immunostained ganglion cells occurred in larger numbers of Varanus gouldii than in Pogona vitticeps. In both species type B SP cells were the most numerous and were estimated to be about 60,000-70,000. They were distributed non-uniformly with a high density band across the horizontal meridian of the retina, from where the density decreased towards the dorsal and ventral retinal margins. In both species type A amacrine cells occurred in small numbers distributed sparsely in the peripheral retina. The faint immunostaining of SP-immunoreactive neurons in the GCL, did not allow us to reliably determine their numbers and retinal distribution. The functional significance of SP-immunoreactive amacrine and ganglion cells in the lizard retina remains to be determined.     

Distinct patterns of distribution among NADPH-diaphorase neurones of the guinea pig retina

We have examined the morphology and distribution of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-diaphorase) cells in the retina of the guinea pig. Two morphologically distinct classes of labelled cells were detected, one with larger, darkly labelled somata commonly located in the inner nuclear layer (INL: NDa cells) and the other with smaller, lightly labelled somata in the ganglion cell layer (GCL: NDb cells). The somata of NDb cells did not vary in diameter with eccentricity, whereas those of the NDa cells were smallest in the visual streak. The number of NDa cells was approximately 3,500, with a mean density of 26/mm2 and NDb cells numbered approximately 4,400, with a mean density of 33 mm2. NDa cells were distributed relatively uniformly across the retina, whereas NDb cells concentrated in the visual streak and were restricted to the superior half of the retina. In these features of morphology and distribution. NADPH-diaphorase neurones of the guinea pig retina are distinct from those observed in other species. It remains to be elucidated whether the diversity in the morphology and distribution of NADPH-diaphorase neurones between species reflects a diversity in their function.     

Somatostatinergic neurones of the developing human and cat retinae

We have examined somatostatin-immunoreactive (S-IR) neurones in developing retinae of the human and cat. At 14 and 16 weeks' gestation (G14 and G16) in the human, S-IR cells were only found close to the putative fovea centralis, but by 18 weeks' gestation (G18), they were located in all retinal regions. By adulthood, the majority of S-IR cells were restricted to inferior retina. In the developing cat retina, two classes of S-IR cells were recognized. S1-IR cells were similar in morphology and distribution to adult cells: they had small round somata which were only found in inferior retina and gave rise to beaded processes which traversed the inner plexiform layer (IPL) and nerve fibre layer (NFL). S2-IR cells had larger somata located in the ganglion cell layer (GCL) and the label was compartmentalized within their cytoplasm. Most S2-IR cells had lost immunoreactivity by P (postnatal day) 25 and may have been alpha-ganglion cells transiently expressing somatostatin in association with their retention of plasticity into postnatal life.     

Cell populations of the ganglion cell layer: displaced amacrine and matching amacrine cells in the pigeon retina

Summary The proportion and size distribution of ganglion and non-ganglion cells in the ganglion cell layer of different areas of the pigeon retina was examined in whole-mounts of the retina by retrograde axonal transport of horseradish peroxidase (HRP) from large brain injections. A maximum of 98% of cells were labelled in the red field and a maximum of 77% in the peripheral yellow field. Unlabelled cell bodies were 30% smaller than labelled ganglion cells and had a mean diameter of 6.2 m and a size range of 4 to 9 m. The morphology of cells in the ganglion cell layer was examined by Golgi staining of retinal whole-mounts. Small glia, displaced amacrine and ganglion cells were found. Displaced amacrine cell bodies were about 30% smaller than ganglion cells and their size distribution was similar to the unlabelled cells in HRP preparations. Displaced amacrine cells had small rounded cell bodies (mean diameter 6.2 m) increasing in size with eccentricity, and a unistratified dendritic tree of fine, nearly radial, varicose dendrites in sublamina 4 of the inner plexiform layer. They had elliptical dendritic fields (mean diameter 66 m) aligned parallel to the retina's horizontal meridian. A population of amacrine cells was found with somas at the inner margin of the inner nuclear layer and soma and dendritic morphology matching those of displaced amacrines. These amacrine cells had unistratified dendritic trees at the junction of sublaminae 1 and 2 of the inner plexiform layer. Pigeon displaced amacrine cells and their matching amacrines are similar to starburst cells of the rabbit retina. They may participate in on and off pathways to ganglion cells and their lamination suggests that they are cholinergic.     

An immunocytochemical marker for hamster retinal ganglion cells

Summary We examined the specificity and developmental time course of the labelling of retinal ganglion cells in Syrian hamsters by a monoclonal antibody AB5. In adult hamsters, AB5 selectively labelled somata in the ganglion cell layer, dendrites in the inner plexiform layer and axons in the nerve fibre layer. When retinal ganglion cells were retrogradely labelled with Dil prior to AB5 immunocytochemistry, all of the retrogradely labelled retinal ganglion cells in the ganglion cell layer were AB5 immunoreactive, indicating that AB5 labels all classes of ganglion cell in that layer. In retinae depleted of retinal ganglion cells by neonatal optic nerve transections, AB5 did not label any somata or processes, indicating that AB5 specifically labels retinal ganglion cells. During development, AB5 labelling first appeared as a weak staining of cell bodies in the ganglion cell layer on postnatal day 12 (P12; PO=first 24 h following birth) and acquired the staining pattern seen in the adult by postnatal day 14. From the onset of AB5 immunoreactivity, AB5-labelled somata of varying sizes were present across the entire retinal surface. Although AB5 labelled retinal ganglion cell axons in the nerve fibre layer of the retina it did not label the optic nerve or retinal ganglion cell axons in the brain at any age examined. AB5 labelling was also found to be compatible with bromodeoxyuridine immunocytochemistry and, therefore, useful for determining the time of generation of hamster retinal ganglion cells.     

Morphology and retinal distribution of tyrosine hydroxylase-like immunoreactive amacrine cells in the retina of developingXenopus laevis

Summary The development of neurons immunoreactive to tyrosine hydroxylase (TH-IR) in the retina ofXenopus laevis was investigated from stage 53 tadpoles to adult, by using an antibody against tyrosine hydroxylase. At all developmental stages, most of the immunoreactive somata were located in the inner nuclear layer, and a few in the ganglion cell layer. Immunoreactive processes arborised in the scleral and vitreal sublaminae of the inner plexiform layer, indicating that these cells were bistratified amacrine cells. However, occasionally a few immunoreactive processes were observed projecting to the outer plexiform layer, suggesting the presence of THIR interplexiform cells. The number of immunoreactive amacrine cells in the inner nuclear layer per retina increased from 204 at stage 53 tadpole to 735 in adult, while the number of immunoreactive amacrine cells in the ganglion cell layer did not change significantly over the same period. Retinal area increased from 1.95 mm² at stage 53 to 23.40 mm² in the adult, and correspondingly cell density in the inner nuclear layer decreased from 104/mm² to 31/mm². At all stages there was an increasing density towards the ciliary margin, but this gradient decreased with age. The soma size of immunoreactive amacrine cells increased with age, and was consistently larger in the central than in the peripheral retina. Dendritic field size was estimated to increase 13-fold, from stage 53 to adult. This study shows that tyrosine hydroxylase-like immunoreactive amacrine cells are generated continuously throughout life, that after metamorphosis the retina grows more by stretching than by cell generation at the ciliary margin, and that the increase of dendritic field size is proportional to the increase in retinal surface area.On leave from Department of Anatomy, Zhanjiang Medical College, Guangdong, People's Republic of China     

Generation of retinal cells in the wallaby, Setonix brachyurus (quokka)

We have examined the generation of retinal cells in the wallaby, Setonix brachyurus (quokka). Animals received a single injection of tritiated thymidine between postnatal days 1-85 and retinae were examined at postnatal day 100. Retinae were sectioned, processed for autoradiography and stained with Cresyl Violet. Ganglion cells were labelled by injection of horseradish peroxidase into the optic tracts and primary visual centres. Other cells were classified according to their morphology and location. Retinal cell generation takes place in two phases. During the first phase, which concludes by postnatal day 30, cells destined to lie in all three cellular layers of the retina are produced. In the second phase, which starts by postnatal day 50, cell generation is almost entirely restricted to the inner and outer nuclear layers. Cells produced in the first phase are orthotopic and displaced ganglion cells, displaced and orthotopic amacrine cells, horizontal cells and cones. Glia in the ganglion cell layer, orthotopic amacrine cells, bipolar and horizontal cells. Muller glia, and rods are generated in the second phase. Cells became heavily labelled with tritiated thymidine in the central retina before postnatal day 7, over the entire retina (panretinal) by postnatal day 7 and from postnatal day 18, only in the periphery. The second phase of cell generation is initiated at P50, in a region extending from the optic nerve head to mid-temporal retina. Subsequently, cells are generated in annuli, centred on mid-temporal retina, which are seen at progressively more peripheral locations. Therefore, cell addition to the inner and outer nuclear layers continues for longer in peripheral than in mid-temporal retina. We suggest that such later differential cell addition to the inner and outer nuclear layers contributes to an asymmetric increase in retinal area. This non-uniform growth presumably results in more expansion of the ganglion cell layer peripherally than in mid-temporal retina and may play a role in establishing density gradients of ganglion cells.     

Development of NADPH-diaphorase cells in the rat's retina

This study has examined the development of cells in the rat retina which contain nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase. NADPH-diaphorase cells were first detected at postnatal day (P) 3, in somata located in the inner part of the cytoblast layer (CBL). At this age, NADPH-diaphorase reactivity was also seen in weakly labelled fibers in the presumptive outer plexiform layer (OPL). By P5, the somata of most labelled cells were in the inner part of the inner nuclear layer (INL), and by P11, their processes had spread extensively within the inner plexiform layer (IPL). By P25, there was a striking change in the pattern of NADPH-diaphorase reactivity. First, cells had lost reactivity from their large and extensive dendrites and second, there was a distinct reduction in the diameters of labelled somata. Thus, NADPH-diaphorase reactivity was most prominent during the period of synaptogenesis in the IPL. Labelled cells at P3 numbered 120 and were largely found at the superior margin of the retina. By P11, their total number had increased to the adult value of about 3400 and their density was highest in peripheral retina. With further development, the differential expansion of the retina appeared to lower the peripheral densities, resulting in an approximately uniform distribution by adulthood.     

大鼠,金黄地鼠和家兔视网膜内一氧化氮合酶分布的比较

用ＮＡＤＰＨ黄递酶组织化学染色法观察了正常成年大鼠、金黄地鼠和家兔视网膜内一氧化氮合酶的分布，并比较了３种不同动物的区别。结果显示，在视网膜内ＮＯＳ阳性神经元主要为分布于内核层的无长突细胞、节细胞层的移位无长突细胞和少数节细胞，不同种类动物的视网膜内，ＮＯＳ阳性细胞的配布、密度和细胞形态均有差异。大鼠视网膜内ＮＯＳ阳性细胞多尾于内核层无长突细胞和节细胞层移位无长细胞，偶见于视网膜节细胞。金黄地鼠视     

Dendritic morphology and retinal distribution of tyrosine hydroxylase-like immunoreactive amacrine cells in Bufo marinus

Summary Tyrosine hydroxylase-like immunoreactive (TH-IR) amacrine cells (ACs) in the retina of metamorphosing and adult Bufo marinus were visualized, and their retinal distribution established, using immunohistochemistry on retinal wholemount and sectioned material. The somata of TH-IR ACs were located in the innermost part of the inner nuclear layer (INL). Their dendrites branched predominantly in the scieral sublamina of the inner plexiform layer (IPL), with sparse branching also in the vitreal sublamina. In the retinae of metamorphosing animals 592 ± 113 (mean ± S.D.) immunoreactive cells and in adult 5,670 ± 528 cells were found. Usually 1, 2 or 3 stem dendrites arose from the somata of TH-IR cells which branched 2 or 3 times. In the adult retinae the dendritic field sizes of immunoreactive cells were in the range of 0.059 ± 0.012 mm², which resulted in a considerable dendritic overlap across the retina. TH-IR cells were unevenly distributed over the retina, with 72 cells/mm² in the central temporal retina, 45–50 cells/mm² along the naso-temporal axis of the retina and 25 cells/mm² in the dorsal and ventral peripheral retina. The average density was 36 ± 6 cells/mm². A considerable number of TH-IR cells (range 52–133, n=4) were displaced into the ganglion cell layer (GCL) of the retina. The mean soma sizes of immunoreactive cells were significantly higher in the low density (95 ± 13 m²) than in the high cell density areas (86 ± 12 m²). There was also a slight but significant increase of the dendritic field sizes of these cells towards the low cell density areas of the retina. These observations show that the retinal distribution of TH-IR ACs parallels the non-uniform distribution of neurons of the INL demonstrated recently in Bufo marinus (Zhu et al. 1990). The class of TH-IR ACs appears to correspond to a subgroup of morphologically distinct dopaminergic ACs found in a number of other vertebrate species.On leave from Department of Anatomy, Zhanjiang Medical College, Guangdong, People's Republic of China     

A specific projection of retinal displaced ganglion cells to the nucleus of the basal optic root in the chicken.

The displaced ganglion cells of Dogiel are a class of retinal ganglion cells whose perikarya are located along the inner margin of the inner nuclear layer. Found in all vertebrate classes, they are particularly conspicuous in avians. Recently, Karten, Fite &; Brecha (1977) found that these cells in the pigeon gave rise to a seemingly exclusive projection to the contralateral nucleus of the basal optic root, the major component of the avian accessory optic system. In the present work, the projections of displaced ganglion cells were investigated in hatchling and adult chickens. The cells were found to project to the nucleus of the basal optic root but not to the tectum. Labeled displaced ganglion cells following injections of horseradish peroxidase into the nucleus of the basal optic root were 15 × 20μm in size in both hatchlings and adults. Labeled cells tended to have a higher concentration in the peripheral than in the central retina. Cells were widely but irregularly spaced, with adjacent cells seldom closer than 100 μm. Up to 7700 displaced ganglion cells were labeled in the adult chicken.These results, together with those of Kartenet al. (1977), suggest that in birds, displaced ganglion cells may constitute a unique class of retinal ganglion cells that project exclusively to the nucleus of the basal optic root. In light of the projections of the nucleus of the basal optic root to the oculomotor complex and vestibulocerebellum, the displaced ganglion cells may be an initial link in a visual pathway involved in the control of oculomotor reflexes.     

Developmental switch in excitability, Ca(2+) and K(+) currents of retinal ganglion cells and their dendritic structure

In the retina of teleost fish, continuous neuronal development occurs at the margin, in the peripheral growth zone (PGZ). We prepared tissue slices from the retina of rainbow trout that include the PGZ and that comprise a time line of retinal development, in which cells at progressive stages of differentiation are present side by side. We studied the changes in dendritic structure and voltage-dependent Ca(2+), Na(+), and K(+) currents that occur as ganglion cells mature. The youngest ganglion cells form a distinct bulge. Cells in the bulge have spare and short dendritic trees. Only half express Ca(2+) currents and then only high-voltage-activated currents with slow inactivation (HVAslow). Bulge cells are rarely electrically excitable. They express a mixture of rapidly inactivating and noninactivating K(+) currents (IKA and IKdr). The ganglion cells next organize into a transition zone, consisting of a layered structure two to three nuclei thick, before forming the single layered structure characteristic of the mature retina. In the transition zone, the dendritic arbor is elaborately branched and extends over multiple laminae in the inner plexiform layer, without apparent stratification. The arbor of the mature cells is stratified, and the span of the dendritic arbor is well over five times the cell body's diameter. The electrical properties of cells in the transition and mature zones differ significantly from those in the bulge cells. Correlated with the more elaborate dendritic structures are the expression of both rapidly inactivating HVA (HVAfast) and of low-voltage-activated (LVA) Ca(2+) currents and of a high density of Na(+) currents that renders the cells electrically excitable. The older ganglion cells also express a slowly activating K(+) current (IKsa).     

The topography of magnocellular projecting ganglion cells (M-ganglion cells) in the primate retina.

The projection from the retina to the dorsal lateral geniculate nucleus in the primate arises from two morphologically distinct types of ganglion cells. The P-ganglion cells project to the parvocellular layers, the M-ganglion cells to the magnocellular layers. We have developed a neurofibrillar stain which stains the M-ganglion cell population with a high degree of selectivity allowing us to map their distribution across the retina. As with other ganglion cell types the M-ganglion cell density peaks close to the fovea and declines towards the periphery. At 1 mm from the fovea the proportion of M-ganglion cells ranges from 6 to 10% and then increases to about 8-10% over much of the retina except along the nasal horizontal meridian. Along the nasal horizontal meridian the percentage increases from 10% at 7 mm eccentricity to 20% or more at higher eccentricities. The increased percentage of M-ganglion cells in the nasal quadrant of the retina correlates with the relatively smaller dendritic trees of M-ganglion cells in this region.     

Expression of the neurokinin 1 receptor in the rabbit retina

Substance P is the preferred ligand for the neurokinin 1 (NK1) receptor. In vertebrate retinas, substance P is expressed by amacrine, interplexiform and ganglion cells. Substance P influences the activity of amacrine and ganglion cells and it is reported to evoke dopamine release. We investigated NK1 receptor expression in the rabbit retina using affinity-purified NK1 receptor antibodies. NK1 receptors were expressed by two distinct populations of retinal neurons. One is a population of ON-type bipolar cells characterized by axonal arborizations that ramified in the inner plexiform layer near the ganglion cell layer. Double-label studies showed that NK1 receptor-expressing bipolar cells were distinct from rod bipolar cells and from other immunocytochemically identified types of cone bipolar cells. Their density was about 2250 cells/mm2 in the visual streak and 1115 cells/mm2 in ventral mid-periphery. They were distributed in a non-random pattern. In the outer plexiform layer, the dendrites of these bipolar cells converged into heavily immunostained clusters having a punctate appearance. The density of these clusters in mid-peripheral ventral regions (about 13000 clusters/mm2) was similar to the reported cone density [Famiglietti and Sharpe (1995) Vis. Neurosci. 12, 1151-1175], suggesting these dendrites contact all cone photoreceptors. The second NK1 receptor expressing cell population corresponds to the tyrosine hydroxylase-containing amacrine cell population. NK1 receptor immunostaining was localized to the cell body and processes, but not to the processes that form the 'rings' that are known to encircle somata of AII amacrine cells. These findings show that NK1 receptor immunoreactivity is localized to a population of ON-type cone bipolar cells and to dopaminergic amacrine cells, suggesting that substance P acting on NK1 receptors influences multiple retinal circuits in the rabbit retina.