Electron microscopy of glutamate decarboxylase (GAD) immunoreactivity in the inner plexiform layer of the rhesus monkey retina

Summary With indirect immunofluorescence, glutamate decarboxylase (GAD), the GABA synthesizing enzyme, was localized to cell bodies in the inner half of the inner nuclear layer and a few in the outer tier of the ganglion cell layer in the rhesus monkey retina. In the inner plexiform layer there were three strongly GAD-immunoreactive laminae separated by two less immunoreactive laminae. Electron microscopy demonstrated that the GAD was contained in amacrine cells and these GAD-immunoreactive amacrines were primarily pre- and postsynaptic to biopolar cell axon terminals. The GAD-containing processes possessed small synaptic vesicles and formed synapses that could be characterized as symmetrical. Large, dense-cored vesicles were often found in the cell bodies and synaptic processes of the GAD-immunoreactive amacrine cells. As the vast majority of the synaptic input and output of the GAD-containing amacrine cells was to and from bipolar cells and the strongest GAD-immunoreactivity correlated with the endings of bipolar cells that connect with a single cone, the functional effects of GABA in the primate retina are likely to be found in the responses of single cone pathways in the inner plexiform layer.     

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.     

Localization of GABA and glycine in goldfish retina by electron microscopic postembedding immunocytochemistry: improved visualization of synaptic structures with LR White resin

Summary A post-embedding, electron microscopic immunocytochemistry technique, modified from existing protocols, was used to examine the labelling patterns of GABA immunoreactivity and glycine immunoreactivity in goldfish retina. Retinae were fixed in mixed aldehyde solution, dehydrated in ethanol, staineden bloc with uranyl acetate and phosphotungstic acid and embedded in LR White resin. Substances were localized in thin sections by floating grids first on a drop of primary antiserum and then on a colloidal gold-IgG conjugate. Finally, grids were exposed to osmium vapour. The localization of GABA immunoreactivity matched that of [³H]-GABA uptake or glutamate decarboxylase immunoreactivity as described previously. In the outer retina, GABA immunoreactivity was found in the cell bodies and axon terminals of H1 horizontal cells and their dendrites opposite cone photoreceptor terminals. Selected amacrine cell bodies were labelled, as were many processes, both synaptic and non-synaptic, throughout the inner plexiform layer, including most amacrine cell processes contacting the synaptic terminals of type Mb bipolar cells. Numerous amacrine cells, their processes in the inner and outer plexiform layers, and photoreceptor terminals contained glycine immunoreactivity in a distribution similar to that shown by [³H]-glycine uptake. Despite the absence of osmium in the primary or secondary fixative, our protocol results in excellent visibility of synaptic structures and detectability of the colloidal gold immunolabel. Also, it does not cause extraction of the HRP/DAB reaction product and is therefore suitable for double-label analysis of neurons labelled with horseradish peroxidase.     

Regressive and reactive changes in the connectivity patterns of rod and cone pathways of P23H transgenic rat retina

We have used the P23H line 1 homozygous albino rat to study how progressive photoreceptor degeneration affects rod and cone relay pathways. We examined P23H retinas at different stages of degeneration by confocal microscopy of immunostained sections and electroretinogram (ERG) recordings. By 21 days of age in the P23H rat retina, there is already substantial loss of rods and reduction in rod bipolar dendrites along with reduction of metabotropic glutamate receptor 6 (mGluR6) and rod-associated bassoon staining. The cone pathway is relatively unaffected. By 150 days, when rods are absent from much of the retina, some rod bipolars remain and dendrites of rod and cone bipolar cells form synaptic complexes associated with cones and horizontal cell processes. These complexes include foci of mGluR6 and bassoon staining; they develop further by 270 days of age. Over the course of degeneration, beginning at 21 days, bipolar axon terminals atrophy and the inner retina undergoes further changes including a reduced and disorganized AII amacrine cell population and thinning of the inner plexiform layer. Electroretinogram (ERG) results at 23 days show reductions in a-wave amplitude, in rod and cone-associated b-waves (using a double flash paradigm) and in the amplitude of oscillatory potentials (OPs). By 38 days, rod scotopic a-wave responses and OPs are lost. B-wave amplitudes decline until 150 days, at which point they are purely cone-driven and remain stable up to 250 days. The results show that during the course of photoreceptor loss in the P23H rat, there are progressive degenerative changes, particularly in the rod relay pathway, and these are reflected in the changing ERG response patterns. Later reactive changes involving condensation of cone terminals and neurotransmitter receptors associated with rod and cone bipolar dendrites and with horizontal cell processes suggest that at this stage, there are likely to be complex changes in the relay of sensory information through the retina.     

Expression and localization of ionotropic glutamate receptor subunits in the goldfish retina--an in situ hybridization and immunocytochemical study

The expression and distribution of AMPA, kainate and NMDA glutamate receptor subunits was studied in the goldfish retina. For the immunocytochemical localization of the AMPA receptor antisera against GluR2, GluR2/3 and GluR4 were used, and for in situ hybridization rat specific probes for GluR1 and GluR2 and goldfish specific probes for GluR3 and GluR4 were used. The localization of the low affinity kainate receptor and NMDA receptor was studied using antisera against GluR5-7 and NR1. All AMPA receptor subtypes were demonstrated to be present in the goldfish retina both by immunocytochemistry and in situ hybridization. In situ hybridization revealed expression of all AMPA receptors subunit at the inner border of the INL. Only GluR3 was also strongly expressed in the outer border of the INL. Some of the ganglion cells displayed a strong signal for GluR1, GluR3 and GluR4. GluR1-immunoreactivity was present in subsets of bipolar, amacrine, and ganglion cells. GluR2 and GluR2/3-immunoreactivity was mainly localized in the outer plexiform layer. GluR2 and GluR2/3-immunoreactivity are associated with the photoreceptor synaptic terminals. GluR4-immunoreactivity is present on Müller cells in the inner retina and on dendrites of bipolar cells in the OPL, whereas GluR5-7-immunoreactivity was prominently present on horizontal cell axon terminals. Finally, NR1-immunoreactivity was confined to amacrine cells, the inner plexiform layer and ganglion cells. This study shows that there is a strong heterogeneity of glutamate receptor subunit expression in the various layers of the retina. Of the AMPA receptor subunits GluR3 seems to be expressed the most widely in all layers with strong glutamatergic synaptic interactions whereas all the other subunits seem to have a more restricted expressed pattern.     

The inner plexiform layer in the retina of the cat: electron microscopic observations

Summary Neural connections of cells ramifying in the inner plexiform layer of the cat retina have been studied by serial section electron microscopy. Flat cone bipolars and invaginating cone bipolars segregate their axon terminals to different sublaminae of the IPL (sublaminaa and sublaminab, respectively) where they relate to different subtypes of the same class of ganglion cell (a andb types respectively).Rod bipolar axon terminals end solely in sublaminab and synapse with amacrine cells (AI and AII). AI provides reciprocal synapses to clusters of rod bipolar axon terminals. The AII amacrine provides rod input toa type ganglion cells by means of chemical synapses and tob type ganglion cells through gap junctions with invaginating cone bipolar terminals.Amacrine cells exist which interconnect rod and cone bipolars, but some amacrines appear to be related specifically to neurons branching in particular sublaminae. Both large- and small-bodied ganglion cells have amacrine-dominated input while the medium-bodied ganglion cells with small dendritic trees have cone bipolar-dominated input.     

Synaptic connections of the interplexiform cell in the retina of the cat

Summary Electron microscopy of Golgi-impregnated material and of well fixed, ultrathin serial sections has revealed the synaptic connections of interplexiform cells in cat retina. In the inner plexiform layer these cells are postsynaptic to amacrine cells and probably presynaptic to both bipolars and amacrines. In the outer plexiform layer they are presynaptic to rod and cone bipolar cells and also pre- and postsynaptic to other interplexiform cell dendrites. The interplexiform cell in cat retina appears to be concerned with feeding back information from the inner plexiform layer to the dendrites of bipolar cells in the outer plexiform layer.     

Somatostatin-immunoreactive amacrine cells of chicken retina: retinal mosaic, ultrastructural features, and light-driven variations in peptide metabolism

Somatostatin-like immunoreactive amacrine cells of the chicken retina have been characterized by immunohistochemistry at the light and electron microscope levels. The cell bodies were set back from the junction of the inner nuclear and inner plexiform layers, and prominent fibre plexuses were found in sublaminas 1 and 3-5 of the inner plexiform layer. The cells were distributed across the retinal surface with a centroperipheral gradient of cell density. Locally, the cells were organized in a non-random mosaic. Ultrastructurally, immunohistochemical reaction product was found throughout the cytoplasm of the cell bodies, particularly associated with membranous structures, including the cytoplasmic surfaces of the Golgi apparatus, and within large dense-core vesicles. In dendritic varicosities in the inner plexiform layer, reaction product was associated with the external surfaces of small, clear synaptic vesicles. The synaptic relationships of the somatostatin-immunoreactive terminals in sublamina 1 were distinct from those in sublaminas 3-5. Those in sublamina 1 received input predominantly, possibly exclusively, from bipolar cells. Feedback synapses onto bipolar terminals or to the other amacrine cell process at a synaptic dyad were observed. In sublaminas 3-5, input came predominantly, possibly exclusively, from other, non-immunoreactive amacrine cells, and output was primarily onto other amacrine cells. No synaptic contacts with ganglion cells or with other somatostatin-immunoreactive amacrine cells were identified. Changes in levels of somatostatin-like immunoreactivity in retinas of chicks kept on 12:12 light:dark cycles were detected by radioimmunoassay, and by light and electron microscopic immunohistochemistry. Levels of retinal somatostatin-like immunoreactivity increased in the light and decreased in the dark. The changes appear to be light-driven rather than circadian, since with prolonged exposure to light or dark, the levels of somatostatin-like immunoreactivity continued to increase or decrease until plateaus were reached. The light-driven change in levels of somatostatin-like immunoreactivity may be related to the predominance of bipolar input to the immunoreactive processes in sublamina 1 of the inner plexiform layer. The reduction in peptide levels in the dark may indicate greater release of somatostatin-like immunoreactivity from the amacrine cells in the dark, resulting in an inability of peptide synthesis to keep pace with breakdown. In the light, release of somatostatin-like immunoreactivity may be lower, leading to a net synthesis of peptide.     

Histology of the ferret retina

The retina of the adult ferret, Mustelo furo, was studied with light and transmission electron microscopy to provide an anatomical basis for use of the ferret as a model for retinal research. The pigment epithelium is a simple cuboidal layer of cells characterized by a zone of basal folds, apical microvilli, and pigment granules at various stages of maturation. The distinction between rod and cone photoreceptor cells is based on their location, morphology, heterochromatin pattern and the electron density of their inner segments. The round, light-staining cone cell nuclei occupy the layer of perikarya along the apical border of the outer nuclear layer. The remainder of the outer nuclear layer consists of oblong, deeply-stained rod cell nuclei. Ribbon type synaptic complexes involving photoreceptor cell axons, horizontal cell processes, and bipolar cell dendrites characterize the outer plexiform layer. The inner nuclear layer is comprised of horizontal, bipolar, and amacrine cell perikarya as well as the perikarya of the Müller cells. The light-staining horizontal cell nuclei are prominent along the apical border of the inner nuclear layer. The light-staining amacrine cell nuclei form a more or less continuous layer along the basal border of the inner nuclear layer. Both conventional and ribbon-type synapses characterize the inner plexiform layer. The ganglion cells form a single cell layer. The optic fiber layer contains bundles of axons surrounded by Müller cell processes. Small blood vessels and capillaries are present in the basal portion of the retina throughout the region extending from the internal limiting membrane to the outer plexiform layer. The adult one-year-old retina is compared with the retina at the time of eye opening.     

The absence of the clathrin-dependent endocytosis in rod bipolar cells of the FVB/N mouse retina

The high rate of exocytosis at the ribbon synapses is balanced by following compensatory endocytosis. Unlike conventional synaptic terminals where clathrin-mediated endocytosis (CME) is a predominant mechanism for membrane retrieval, CME is thought to be only a minor mechanism of endocytosis at the retinal ribbon synapses, but CME is present there and it works. We examined the clathrin expression in the FVB/N rd1 mouse, which is an animal model of retinitis pigmentosa. The broadly distributed pattern of clathrin immunoreactivity in the inner plexiform layer was similar in both the control and FVB/N mouse retinas, but the immunoreactive punta within the rod bipolar axon terminals located in the proximal IPL were decreased in number and reduced in size at postnatal days 14 and they came to disappear at postnatal days 21. This preferential decrease of the clathrin expression at ribbon synapses in the rod bipolar cell axon terminals of the FVB/N mouse retina demonstrates another plastic change after photoreceptor degeneration and this suggests that clathrin may be important for normal synaptic function at the rod bipolar ribbon synapses in the mammalian retina.     

Functional and structural modifications during retinal degeneration in the rd10 mouse

Mouse models of retinal degeneration are useful tools to study therapeutic approaches for patients affected by hereditary retinal dystrophies. We have studied degeneration in the rd10 mice both by immunocytochemistry and TUNEL-labeling of retinal cells, and through electrophysiological recordings. The cell degeneration in the retina of rd10 mice produced appreciable morphological changes in rod and cone cells by P20. Retinal cell death is clearly observed in the central retina and it peaked at P25 when there were 800 TUNEL-positive cells per mm(2). In the central retina, only one row of photoreceptors remained in the outer nuclear layer by P40 and there was a remarkable deterioration of bipolar cell dendrites postsynaptic to photoreceptors. The axon terminals of bipolar cells also underwent atrophy and the inner retina was subject to further changes, including a reduction and disorganization of AII amacrine cell population. Glutamate sensitivity was tested in rod bipolar cells with the single cell patch-clamp technique in slice preparations, although at P60 no significant differences were observed with age-matched controls. Thus, we conclude that rod and cone degeneration in the rd10 mouse model is followed by deterioration of their postsynaptic cells and the cells in the inner retina. However, the functional preservation of receptors for photoreceptor transmission in bipolar cells may open new therapeutic possibilities.     

Localization and possible function of P2Y(4) receptors in the rodent retina

Extracellular ATP acts as a neurotransmitter in the retina, via the activation of ionotropic P2X receptors and metabotropic P2Y receptors. The expression of various P2X and P2Y receptor subtypes has been demonstrated in the retina, but the localization of P2Y receptors and their role in retinal signaling remains ill defined. In this study, we were interested in determining the localization of the P2Y(4) receptor subtype in the rat retina, and using the electroretinogram (ERG) to assess whether activation of these receptors modulated visual transmission. Using light and electron microscopy, we demonstrated that P2Y(4) receptors were expressed pre-synaptically in rod bipolar cells and in processes postsynaptic to cone bipolar cells. Furthermore, we show that the expression of P2Y(4) receptors on rod bipolar cell axon terminals is reduced following dark adaptation, suggesting receptor expression may be dependent on retinal activity. Finally, using the electroretinogram, we show that intravitreal injection of uridine triphosphate, a P2Y receptor agonist, decreases the amplitude of the rod PII, supporting a role for P2Y receptors in altering inner retinal function. Taken together, these results suggest a role for P2Y(4) receptors in the modulation of inner retinal signaling.     

Quantitative analysis of GABA-immunoreactive synapses in the inner plexiform layer of theBufo marinus retina: identification of direct output to ganglion cells and contacts with dopaminergic amacrine cells

Summary We have recently reported that about 50% of amacrine cells and some of the bipolar and ganglion cells are GABA-immunoreactive in the retina ofBufo marinus. Synapses formed by these elements in the inner plexiform layer were studied. GABA-immunoreactive amacrine cell processes were found most frequently in synaptic contact with non-immunoreactive amacrine cells. Double-label experiments showed that some of these non-GABA-immunoreactive elements contain tyrosine hydroxylase immunoreactivity. Another source of input to the GABA-immunoreactive amacrine cells were the bipolar cells; some of which were GABA-immunoreactive. GABA-immunoreactive amacrine cells synapsed also onto bipolar cell terminals, and ganglion cell dendrites that were identified by the retrograde transport of horseradish peroxidase from the optic nerve. Synapses between GABA-immunoreactive amacrine cells and bipolar and ganglion cells were non-uniformly distributed in the inner plexiform layer. Synaptic contacts with bipolar cells were more frequent in the OFF-sublamina, and those with ganglion cell dendrites in the ON-sublamina. These results demonstrate that GABA-immunoreactive amacrine cells (1) preferentially synapse with OFF-responding bipolar and ON-centre ganglion cells in the through-pathway, (2) synapse with tyrosine hydroxylase-immunoreactive amacrine cells in both the OFF- and ON-sublaminae, and (3) synapse directly with GABA-immunoreactive ganglion cells. The synapses between GABA-immunoreactive amacrine and GABA-immunoreactive ganglion cells may inhibit the centrally projecting inhibitory ganglion cells, causing disinhibition in the visual centres.On leave from Department of Zoology, Attila József University, Szeged, Hungary.     

Transient synaptic ribbons in the mammalian retina at unusual sites

In the vertebrate retina the presence of synaptic ribbons (SRs) is well documented in two sites only, viz., in photoreceptor axon terminals in the outer plexiform layer and in bipolar cell axons in the inner plexiform layer. The present paper reports the presence of non-photoreceptor SRs in the outer plexiform layer of cattle and mouse, where they were seen in small numbers in thin cell processes near cone pedicles of light-adapted animals. They were never seen near rod spherules. Quantitative data obtained in mice killed at different time-points revealed that the SRs under consideration increased in number during day time and were absent during the dark phase. Moreover, under high light intensity of 10000 lux they were more frequent in number compared to 100-lux-exposed animals. It is concluded that the cell processes revealing the temporary presence of SRs are processes of flat bipolar cells which may provide a feedback to cones during the light phase.     

Immunocytochemical staining with antibodies against protein kinase C and its isozymes in the turtle retina

Summary An LM immunocytochemical study has investigated the patterns of staining in turtle retina with monoclonal antibodies to the , and isozymes of protein kinase C. The protein kinase C- antibody reveals cells in the ganglion cell layer, occasional amacrine cells and faint banding in strata 2 and 4 of the inner plexiform layer. The protein kinase C- antibody stains primarily amacrine cells that have dendrites running in strata 2, in 4 close to the 3/4 border and on the 4/5 border of the inner plexiform layer. Protein kinase C- immunoreactivity is seen in a population of bipolar cells. The latter are characterized by stained axon terminals in strata 3 and 4 of the inner plexiform layer. A type of amacrine cell, different from those seen with the other antibodies, is also immunoreactive to protein kinase C-. EM immunocytochemistry (using a polyclonal antibody) reveals protein kinase C immunoreactivity in photoreceptor cells, bipolar cells, amacrine cells and ganglion cells. In photoreceptors protein kinase C immunoreactivity occurs as patchy staining associated with vesicles and the plasmalemma in pedicles and telodendria. Some varieties of bipolar cell display protein kinase C reaction product throughout the entire cell. Their dendrites contact photoreceptor pedicles at wide-cleft basal junctions and ribbon and non-ribbon related narrow cleft junctions. A few lateral elements per cone or rod pedicle are always protein kinase C-immunoreactive. Amacrine and ganglion cells typically show small clumps of protein kinase C immunoreactivity around vesicles and close to the postsynaptic membranes. Synaptic boutons of some varieties of amacrine cell stain more uniformly. Protein kinase C-immunoreactive bipolar cells are most commonly presynaptic in stratum 4 of the inner plexiform layer, while protein kinase C-immunoreactive amacrine cells are both pre- and postsynaptic throughout strata 1, 2, 3 and 4. Stratum 5 appears to be almost devoid of protein kinase C-immunoreactive neural profiles.     

The synaptic organization of the dopaminergic amacrine cell in the cat retina

Summary The dopaminergic amacrine cells of the cat retina have been stained by immunocytochemistry using an antibody to tyrosine hydroxylase (Toh). The complete population of Toh+cells has been studied by light microscopy of retinal wholemounts to evaluate morphological details of dendritic structure and branching patterns. Selected Toh+amacrine cells have been studied by serial-section electron microscopy to analyse synaptic input and output relationships. The majority of Toh+amacrine cells occur in the amacrine cell layer of the retina and have their dendrites ramifying and forming the characteristic rings in stratum 1 of the inner plexiform layer. A minority of Toh+cells have cell bodies displaced to the ganglion cell layer but their dendrites also stratify in stratum 1. All Toh+cells have some dendritic branches running in stratum 2 as well as in stratum 1, and frequently they have long axon-like processes (500–1000 m long) dipping down to run in stratum 5 before passing up to rejoin the major dendritic arbors in stratum 1. In addition Toh+stained processes follow blood vessels in the inner plexiform layer and in the ganglion cell layer. A population of Toh+cells found in the inferior retina appears to give rise to stained processes that pass to the outer plexiform layer and therein to run for as far as one millimeter.Electron microscopy reveals that Toh+amacrine cells are postsynaptic to amacrine cells and a few bipolar cell terminals in stratum 1 of the inner plexiform layer and are primarily presynaptic to All amacrine cell bodies and lobular appendages, and to another type of amacrine cell body and amacrine dendrites hypothesized to be the A17 amacrine cell. The Toh+dendrites in stratum 2 are presynaptic to All lobular appendages primarily. Stained axon-like processes running in stratum 5 prove to be presynaptic to All amacrine dendrites as they approach the rod bipolar axon terminals and they may also be presynaptic to the rod bipolar terminal itself. The Toh+stained dendrites that have been followed in the outer plexiform layer run along the top of the B-type horizontal cell somata and may have small synapses upon them. The only clear synapses seen in the outer plexiform layer are from the Toh+profiles upon vesicle filled amacrine-like profiles that are in turn presynaptic to bipolar cell dendrites in the outer plexiform layer. We presume the cells postsynaptic to the Toh+dendrites in the outer plexiform layer are interplexiform cells. Finally the Toh+profiles that course along blood vessel walls and in the ganglion cell layer appear to end either against the basal lamina of the blood vessel or at intercellular channels of vesicle-laden Muller cell end-feet.     

Glycinergic synaptic transmission to bullfrog retinal bipolar cells is input-specific

Glycinergic inhibitory postsynaptic currents (IPSCs) focally elicited at the dendrites and axon terminals were recorded from bipolar cells in the bullfrog retinal slice, using the whole-cell clamp technique. IPSCs driven by input from interplexiform cells at bipolar cell dendrites (ipc-IPSCs) had a much slower decay time constant (25.2 +/- 7.8 ms) than IPSCs driven by input from amacrine cells at bipolar cell axon terminals (ac-IPSCs) (14.7 +/- 5.5 ms). Furthermore, peak-scaled non-stationary noise analysis revealed that the weighted mean single-channel conductance of the glycine receptors underlying bipolar cell dendritic ipc-IPSCs (20.8 +/- 6.6 pS) was significantly larger than that of those underlying bipolar cell axon terminal ac-IPSCs (12.9 +/- 2.9 pS). These results demonstrate that glycinergic synaptic transmission with different properties at bipolar cell dendrites and axon terminals differentially mediates intraretinal centrofugal signal transfer from the inner retina to the outer retina provided by interplexiform cells and lateral inhibition offered by amacrine cells in the inner retina.     

Characterization of histamine projections and their potential cellular targets in the mouse retina

The vertebrate retina receives histaminergic input from the brain via retinopetal axons that originate from perikarya in the posterior hypothalamus. In the nervous system, histamine acts on three G-protein-coupled receptors, histamine receptor (HR) 1, HR2 and HR3. In order to look for potential cellular targets of histamine in the mouse retina, we have examined the retina for the expression of histamine and the presence of these three receptors. Consistent with studies of retina from other vertebrates, histamine was only found in retinopetal axons, which coursed extensively through the ganglion cell and inner plexiform layers. mRNA for all three receptors was expressed in the mouse retina, and immunohistochemical studies further localized HR1 and HR2. HR1 immunoreactivity was observed on dopaminergic amacrine cells, calretinin-positive ganglion cells and axon bundles in the ganglion cell layer. Furthermore, a distinct group of processes in the inner plexiform layer was labeled, which most likely represents the processes of cholinergic amacrine cells. HR2 immunoreactivity was observed on the processes and cell bodies of the primary glial cells of the mammalian retina, the Müller cells. This distribution of histamine and its receptors is consistent with a brain-derived source of histamine acting on diverse populations of cells in the retina, including both neurons and glia.