Characterization of the Canine Retinal Vasculature With Optical Coherence Tomography Angiography: Comparisons With Histology and Fluorescein Angiography

Purpose: To present a methodology for quantification of the canine retinal vasculature imaged by optical coherence tomography angiography (OCTA) and validate this approach by comparison with fluorescein angiography (FA) and confocal imaging of retinal wholemounts labelled by immunohistochemistry (IHC). Methods: Six normal adult dogs underwent retinal OCTA imaging in both eyes. The images extracted from the different microvascular plexuses at eight retinal locations spanning the central and mid-peripheral fundus were analyzed using the AngioTool software. FA was performed in one eye and was compared to the OCTA images. Six eyes from three dogs were processed by IHC to examine the retinal vasculature. Results: A total of four retinal plexuses were identified by OCTA in the canine retina, and their density and topographical pattern varied with eccentricity. OCTA offered improved resolution over FA with the advantage of allowing imaging of the individual plexuses. Detection by OCTA of small vessels within the deep capillary plexus was possible and approached the level of resolution achieved with ex vivo imaging of the retinal vasculature by confocal microscopy/IHC. The plexuses herein described are analogous to human retinal vasculature. Conclusion: OCTA can be used to image and quantify non-invasively the vascular retinal networks of the canine retina. We provide normative data in eight different retinal locations that can be imaged non-invasively with this technology. This could support analysis of retinal vascular changes associated with disease and following therapeutic intervention.

Identification of the Photoreceptor Transcriptional Co-Repressor SAMD11 as Novel Cause of Autosomal Recessive Retinitis Pigmentosa

Retinitis pigmentosa (RP), the most frequent form of inherited retinal dystrophy is characterized by progressive photoreceptor degeneration. Many genes have been implicated in RP development, but several others remain to be identified. Using a combination of homozygosity mapping, whole-exome and targeted next-generation sequencing, we found a novel homozygous nonsense mutation in SAMD11 in five individuals diagnosed with adult-onset RP from two unrelated consanguineous Spanish families. SAMD11 is ortholog to the mouse major retinal SAM domain (mr-s) protein that is implicated in CRX-mediated transcriptional regulation in the retina. Accordingly, protein-protein network analysis revealed a significant interaction of SAMD11 with CRX. Immunoblotting analysis confirmed strong expression of SAMD11 in human retina. Immunolocalization studies revealed SAMD11 was detected in the three nuclear layers of the human retina and interestingly differential expression between cone and rod photoreceptors was observed. Our study strongly implicates SAMD11 as novel cause of RP playing an important role in the pathogenesis of human degeneration of photoreceptors.

p75NTR and Its Ligand ProNGF Activate Paracrine Mechanisms Etiological to the Vascular, Inflammatory, and Neurodegenerative Pathologies of Diabetic Retinopathy

In many diseases, expression and ligand-dependent activity of the p75(NTR) receptor can promote pericyte and vascular dysfunction, inflammation, glial activation, and neurodegeneration. Diabetic retinopathy (DR) is characterized by all of these pathological events. However, the mechanisms by which p75(NTR) may be implicated at each stage of DR pathology remain poorly understood. Using a streptozotocin mouse model of diabetic retinopathy, we report that p75(NTR) is upregulated very early in glia and in pericytes to mediate ligand-dependent induction of inflammatory cytokines, disruption of the neuro-glia-vascular unit, promotion of blood-retina barrier breakdown, edema, and neuronal death. In a mouse model of oxygen-induced retinopathy, mimicking proliferative DR, p75(NTR)-dependent inflammation leads to ischemia and pathological angiogenesis through Semaphorin 3A. The acute use of antagonists of p75(NTR) or antagonists of the ligand proNGF suppresses each distinct phase of pathology, ameliorate disease, and prevent disease progression. Thus, our study documents novel disease mechanisms and validates druggable targets for diabetic retinopathy.

SIGNIFICANCE STATEMENT

Diabetic retinopathy (DR) affects an estimated 250 million people and has no effective treatment. Stages of progression comprise pericyte/vascular dysfunction, inflammation, glial activation, and neurodegeneration. The pathophysiology of each stage remains unclear. We postulated that the activity of p75NTR may be implicated. We show that p75NTR in glia and in pericytes mediate ligand-dependent induction of inflammatory cytokines, disruption of the neuro-glia-vascular unit, promotion of blood-retina barrier breakdown, edema, and neuronal death. p75NTR-promoted inflammation leads to ischemia and angiogenesis through Semaphorin 3A. Antagonists of p75NTR or antagonists of proNGF suppress each distinct phase of pathology, ameliorate disease, and prevent disease progression. Our study documents novel mechanisms in a pervasive disease and validates druggable targets for treatment.

Partial rescue of retinal function in chronically hypoglycemic mice

PURPOSE

Mice rendered hypoglycemic by a null mutation in the glucagon receptor gene Gcgr display late-onset retinal degeneration and loss of retinal sensitivity. Acute hyperglycemia induced by dextrose ingestion does not restore their retinal function, which is consistent with irreversible loss of vision. The goal of this study was to establish whether long-term administration of high dietary glucose rescues retinal function and circuit connectivity in aged Gcgr-/- mice.

METHODS

Gcgr-/- mice were administered a carbohydrate-rich diet starting at 12 months of age. After 1 month of treatment, retinal function and structure were evaluated using electroretinographic (ERG) recordings and immunohistochemistry.

RESULTS

Treatment with a carbohydrate-rich diet raised blood glucose levels and improved retinal function in Gcgr-/- mice. Blood glucose increased from moderate hypoglycemia to euglycemic levels, whereas ERG b-wave sensitivity improved approximately 10-fold. Because the b-wave reflects the electrical activity of second-order cells, we examined for changes in rod-to-bipolar cell synapses. Gcgr-/- retinas have 20% fewer synaptic pairings than Gcgr+/- retinas. Remarkably, most of the lost synapses were located farthest from the bipolar cell body, near the distal boundary of the outer plexiform layer (OPL), suggesting that apical synapses are most vulnerable to chronic hypoglycemia. Although treatment with the carbohydrate-rich diet restored retinal function, it did not restore these synaptic contacts.

CONCLUSIONS

Prolonged exposure to diet-induced euglycemia improves retinal function but does not reestablish synaptic contacts lost by chronic hypoglycemia. These results suggest that retinal neurons have a homeostatic mechanism that integrates energetic status over prolonged periods of time and allows them to recover functionality despite synaptic loss.

Retinal degeneration in two lines of transgenic S334ter rats

Aim of this study was to examine synaptic connectivity changes in the retina and the location and rate of apoptosis in transgenic S334ter line-3 and line-5 rats with photoreceptor degeneration. Heterozygous S334ter-line-3 and line-5 at P11-13, P30, P60, P90 and several control non-dystrophic rats (Long Evans and Sprague-Dawley) at P60, were studied anatomically by immunohistochemistry for various cell and synaptic markers, and by PNA and TUNEL label.- S334ter line-3 exhibited the fastest rate of degeneration with an early loss of photoreceptors, with 1-2 layers remaining at P30, and only cones left at P60. Line-5 had 4-5 layers left at P30, and very few rods left at P60-90. In both lines, horizontal cell processes (including dendrites and axon) were diminished at P11-13, showing gaps in the outer plexiform layer (OPL) at P60, and at P90, almost no terminal tips could be seen. Bipolar cells showed a retraction of their dendrites forming clusters along the OPL. Synaptic terminals of A-II amacrine cells in the IPL lost most of their parvalbumin-immunoreactivity. The apoptosis rate was different in both lines. Line-3 rats showed many photoreceptors affected at P11, occupying the innermost part of the outer nuclear layer. Line-5 showed a lower number of apoptotic cells within the same location at P13. In summary, the S334ter line-3 rat has a faster progression of degeneration than line-5. The horizontal and bipolar terminals are already affected at P11-P13 in both models. Apoptosis is related to the mutated rhodopsin transgene; the first photoreceptor cells affected are those close to the OPL.

Intraretinal processing following photoreceptor rescue by non-retinal cells

Royal College of Surgeon (RCS) rats undergo retinal degeneration due to the inability of retinal pigment epithelial (RPE) cells to phagocytose shed outer segments. We explored the effect of introducing Schwann cells to the subretinal space of RCS rats (before the onset of retinal degeneration), by relying on electroretinogram (ERG) recordings and correlative retinal morphology. Scotopic ERGs recorded from cell-injected eyes showed preserved amplitudes of mixed a-wave b-wave, rod b-waves, and cone b-waves over controls (sham-injected eyes); photopic b-wave amplitudes and critical flicker fusion were also improved. Normal retinal morphology was found in areas of retinas that had received cell injections. Since Schwann cells have no phagocytic properties, their therapeutic effect is best explained through a paracrine mechanism (secretion of factors that ensure photoreceptor survival).

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.

A novel isoform of acetylcholinesterase exacerbates photoreceptors death after photic stress

PURPOSE

To study the involvement of stress-induced acetylcholinesterase (AChE) expression in light-induced retinal damage in albino rats.

METHODS

Adult albino rats were exposed for 24 hours to bright, damaging light. AChE expression was monitored by in situ hybridization, by histochemistry for AChE activity, and by immunocytochemistry. An orphan antisense agent (Monarsen; Ester Neurosciences, Ltd., Herzlia Pituach, Israel) was administered intraperitoneally to minimize light-induced AChE expression. The electroretinogram (ERG) was recorded to assess retinal function.

RESULTS

Twenty-four-hour exposure to bright light caused severe reduction in the ERG responses and augmented expression of mRNA for the "read-through" variant of AChE (AChE-R) in photoreceptor inner segments (IS), bipolar cells, and ganglion cells. AChE activity increased in IS. The expressed AChE protein was a novel variant, characterized by an extended N terminus (N-AChE). Systemic administration of the orphan antisense agent, Monarsen, reduced the photic induction of mRNA for AChE-R, and of the N-AChE protein. Rats exposed to bright, damaging light and treated daily with Monarsen exhibited larger ERG responses, relatively thicker outer nuclear layer (ONL), and more ONL nuclei than did rats exposed to the same damaging light but treated daily with saline.

CONCLUSIONS

The findings indicate that the photic-induced novel variant of AChE (N-AChE-R) may be causally involved with retinal light damage and suggest the use of RNA targeting for limiting such damage.

Developmental regulation of calcium-dependent feedback in Xenopus rods

The kinetics of activation and inactivation in the phototransduction pathway of developing Xenopus rods were studied. The gain of the activation steps in transduction (amplification) increased and photoresponses became more rapid as the rods matured from the larval to the adult stage. The time to peak was significantly shorter in adults (1.3 s) than tadpoles (2 s). Moreover, adult rods recovered twice as fast from saturating flashes than did larval rods without changes of the dominant time constant (2.5 s). Guanylate cyclase (GC) activity, determined using IBMX steps, increased in adult rods from ∼1.1 s⁻¹ to 3.7 s⁻¹ 5 s after a saturating flash delivering 6,000 photoisomerizations. In larval rods, it increased from 1.8 s⁻¹ to 4.0 s⁻¹ 9 s after an equivalent flash. However, the ratio of amplification to the measured dark phosphodiesterase activity was constant. Guanylate cyclase–activating protein (GCAP1) levels and normalized Na⁺/Ca²⁺, K⁺ exchanger currents were increased in adults compared with tadpoles. Together, these results are consistent with the acceleration of the recovery phase in adult rods via developmental regulation of calcium homeostasis. Despite these large changes, the single photon response amplitude was ∼0.6 pA throughout development. Reduction of calcium feedback with BAPTA increased adult single photon response amplitudes threefold and reduced its cutoff frequency to that observed with tadpole rods. Linear mathematical modeling suggests that calcium-dependent feedback can account for the observed differences in the power spectra of larval and adult rods. We conclude that larval Xenopus maximize sensitivity at the expense of slower response kinetics while adults maximize response kinetics at the expense of sensitivity.

The neurons of the ground squirrel retina as revealed by immunostains for calcium binding proteins and neurotransmitters

Ground squirrel retinas were immunostained with antibodies against calcium binding proteins (CBPs) and classical neurotransmitters in order to describe neuronal phenotypes in a diurnal mammalian retina and to then compare these neurons with those of more commonly studied nocturnal retinas like cats' and rabbits'. Double immunostained tissue was examined by confocal microscopy using antibodies against the following: rhodopsin and the CBPs, calbindin, calretinin, parvalbumin, calmodulin and recoverin (CB, CR, PV, CM, RV), glycine, GABA, choline acetyltransferase (CHAT) and tyrosine hydroxylase (TOH). In ground squirrel retina, the traditional cholinergic mirror symmetric amacrine cells colocalize CHAT with PV and GABA and faintly with glycine. A second cholinergic amacrine cell type colocalizes glycine alone. CR is found in at least 3 different amacrine cell types. The CR-immunoreactive (IR) cell population is a mixture of glycinergic and GABAergic types. The dopamine cell type IR to tyrosine hydroxylase has the typical morphology of a wide field cell with dendrites in S1 but the "rings" seen in cat or rabbit retina are not as numerous. TOH-IR amacrine cells send large club-shaped processes to the outer plexiform layer. CB and CR are in bipolar cells, A- and B-type horizontal cells and several amacrine cell types. Anti-rhodopsin labels the low density rod photoreceptor population in this species. Anti-recoverin labels cones and some bipolar cells while PKC is found in several different bipolar cell types. One ganglion cell with dendritic branching in S3 is strongly CR-IR. We find no evidence for an AII amacrine cell in the ground squirrel, with either anti-CR or anti-PV. An amacrine cell with similarity to the DAP1-3 cell of rabbit is CR-IR and glycine-IR. We discuss this labeling pattern in relationship to other mammalian species. The differences in staining patterns and phenotypes revealed suggest a functional diversity in the populations of amacrine cells according to whether the retinas are rod or cone dominated.

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.

Identification and light-dependent translocation of a cone-specific antigen, cone arrestin, recognized by monoclonal antibody 7G6

PURPOSE

To elucidate the antigen recognized by monoclonal antibody (mAb) 7G6, a widely used cone-specific marker.

METHODS

7G6 immunocytochemistry was performed on sections of human, primate, and bovine retina. The antigen was immunoprecipitated from human retinal lysates and purified with protein G. Edman degradation and liquid chromatography of tryptic peptides combined with tandem mass spectrometry (LC-MS/MS) identified the antigen.

RESULTS

Sequencing of peptides derived from the immunoprecipitated 7G6 antigen identified it as cone arrestin. The identity was confirmed by Western blot analysis with recombinant human cone arrestin and competition with the antibody in immunocytochemistry. Subcellular localization of cone arrestin in dark-adapted and bleached bovine retinas showed that cone arrestin accumulated in cone outer segments of light-adapted retina but was more concentrated in the inner segments of dark-adapted retina. By expression of truncated human cone arrestin mutants systematically deleting areas divergent from bovine and primate cone arrestins, the epitope of 7G6 was identified as a divergent loop exposed at the surface within the N-domain of cone arrestin.

CONCLUSIONS

Several independent methods established that the 7G6 antigen is cone arrestin. The 7G6 epitope is contained in a divergent loop, the sequence of which is conserved in bovine and primates but not other vertebrate species consistent with the specificity of the antibody. The light-dependent translocation of cone arrestin suggests a role in light-dark adaptation of cones. Because of the location of its gene on the X-chromosome, cone arrestin is a candidate gene for X-linked cone dystrophies.

A new look at calretinin-immunoreactive amacrine cell types in the monkey retina

We have examined amacrine cells that are calretinin-immunoreactive (-IR) in the macaque monkey retina with the aim of classifying them into morphological and functional subtypes. There are calretinin-IR cells in the fovea and throughout the retina. Their highest density is reached at 1.0 mm from the foveal pit (10500 cells/mm(2)) and falls to 2600/mm(2) by 10 mm of eccentricity. Nearest-neighbor statistics for the calretinin-IR cell body distribution indicate a nonregular pattern, with a regularity index of 1.4-1.6. There is an increase or "bump" of cell density 3.5-4.0 mm from the foveal pit, corresponding to the rod photoreceptor density peak. Based on morphological differences, there appear to be three types of amacrine cell that are calretinin-IR. To determine the types, we doubly immunolabeled retinas, from fovea to periphery, for calretinin-IR in combination with other calcium binding proteins and inhibitory amino acid neurotransmitters. Labeling with parvalbumin and calretinin antibodies indicated that 70% of the amacrine cells were solely calretinin-IR, and 30% contained parvalbumin-IR as well. In the same way, 70% of the calretinin-IR amacrine cells colocalized calbindin, but 30% were only calretinin-IR. Among the calretinin/calbindin-colocalized cells, there were small-field and wide-field types. Double labeling with antibodies to calretinin and gamma-aminobutyric acid (GABA) and to calretinin and glycine revealed the majority to be glycine-IR, but some were GABA-IR. The glycine-IR population consists mainly of AII amacrine cell types, but clearly another non-AII type is involved. The non-AII glycine-IR population resembles a small- to medium-field diffuse type. The calretinin-IR wide-field type is GABAergic and corresponds to an A19 type. The central, rod-free, fovea contains the calretinin-IR, non-AII glycine-IR type and the calretinin-IR, GABAergic type only. To learn more concerning the circuitry of the calretinin/glycine-IR, non-AII amacrine cell type in isolation from AII amacrine cells, we concentrated on the rod-free fovea, where AII amacrine cells are absent. We performed a serial section electron microscopy (EM) study on four calretinin-IR cells. They were involved with cone pathway circuitry. They got input from ON and OFF midget bipolar cells, reciprocated synapses to these bipolar cells, and provided synapses to ON-center ganglion cells. Thus we have obtained new information on a cone pathway amacrine cell of the central monkey fovea that is involved in the midget system.

Membrane properties of an unusual intrinsically oscillating, wide-field teleost retinal amacrine cell

In the retina, amacrine cells modulate the transfer of information from bipolar to ganglion cells. The nature of the modulation depends on the synaptic input and the membrane properties of the cells. In the retina of white bass, we identified a class of bistratified, wide-field amacrine cell characterized by immunopositive labelling for GABA and calmodulin. In isolation, the cells presented resting membrane potentials averaging -69 mV although some cells settled at more depolarized values (-30 mV). Injection of depolarizing current pulses induced oscillatory membrane responses. When elicited from depolarized cells, the oscillations were short-lived (< 40 ms). For the most part, the oscillatory potentials of hyperpolarized cells remained unattenuated throughout the depolarizing pulse. The frequency of the oscillations increased logarithmically with mean membrane potential, ranging from 74 to 140 Hz. Cells exhibiting depolarized membrane potentials oscillated at twice that rate. When the membrane potential of these cells was hyperpolarized to -70 mV, the oscillations became unattenuated and slowed. We found the cells expressed voltage-gated sodium, potassium and calcium currents and calcium-dependent potassium currents. We demonstrate that the oscillatory potentials arose as a result of the interplay between calcium and potassium currents. The cells responded to local application of GABA and glycine, both of which modulate the oscillatory potentials. Glutamate and its analogues depolarized the cell and induced oscillatory potentials. Our results indicate that oscillatory responses of a type of wide-field amacrine cell are an intrinsic feature of the cell and not due to circuit properties.

Localization of neurotransmitters and calcium binding proteins to neurons of salamander and mudpuppy retinas

We wished to identify the different types of retinal neurons on the basis of their content of neuroactive substances in both larval tiger salamander and mudpuppy retinas, favored species for electrophysiological investigation. Sections and wholemounts of retinas were labeled by immunocytochemical methods to demonstrate three calcium binding protein species and the common neurotransmitters, glycine, GABA and acetylcholine. Double immunostained sections and single labeled wholemount retinas were examined by confocal microscopy. Immunostaining patterns appeared to be the same in salamander and mudpuppy. Double and single cones, horizontal cells, some amacrine cells and ganglion cells were strongly calbindin-immunoreactive (IR). Calbindin-IR horizontal cells colocalized GABA. Many bipolar cells, horizontal cells, some amacrine cells and ganglion cells were strongly calretinin-IR. One type of horizontal cell and an infrequently occurring amacrine cell were parvalbumin-IR. Acetylcholine as visualized by ChAT-immunoreactivity was seen in a mirror-symmetric pair of amacrine cells that colocalized GABA and glycine. Glycine and GABA colocalized with calretinin, calbindin and occasionally with parvalbumin in amacrine cells.

Morphological and neurochemical diversity of neuronal nitric oxide synthase-positive amacrine cells in the turtle retina

The histochemistry of reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) and immunoreactivity of neuronal nitric oxide synthase (nNOS-IR) can be demonstrated in various cell types of the vertebrate retina. In this study, we have focused on characterizing the different NADPH-d-positive amacrine cell types in turtle retina. Cryostat sections were examined by confocal laser scanning microscopy for double immunofluorescence with antibodies against nNOS and either GABA or glycine, or by combining histochemistry with immunocytochemistry to obtain triple labeling with NADPH-d, GABA, and glycine. Forty-eight percent of the NADPH-d-labeled amacrine cells colocalized GABA, 52% glycine. Here we show that two morphologically different types of amacrine cell are nNOS/glycine-IR and three types are nNOS/GABA-IR. Antibodies against calretinin, parvalbumin, somatostatin, tyrosine hydroxylase, and choline acetyltransferase did not colocalize with nNOS-IR or NADPH-d-labeled amacrine cells, but 15% of the NOS-labeled amacrine cells showed immunoreactivity against calbindin. Only GABA has been seen to colocalize with NADPH-d in amacrine cells in previous reports in other species. The finding here of glycine colocalizing with NO-containing cells is novel. We suggest that NO, apart from its well known function in gap junction regulation, can also modulate the release of both GABA and glycine in the turtle retina.

Choline acetyltransferase is found in terminals of horizontal cells that label with GABA, nitric oxide synthase and calcium binding proteins in the turtle retina

In this study, we discriminated the various types of horizontal cell in the turtle retina on their content of neuroactive substances. Double label immunocytochemistry was performed on sectioned and wholemount retina using antisera to neural- and endothelial-nitric oxide synthase (nNOS, and eNOS), calretinin (CR), calbindin (CB), gamma-aminobutyric acid (GABA) and choline acetyltransferase (ChAT). H1 cells and their axon terminals label with CR, CB and GABA. Only H1 axon terminals label with eNOS. H2 cells contain CB, CR, nNOS and GABA maybe in their dendrites. H3 cells label only with nNOS. The localization of nNOS in the H2 and H3 cells is a novel finding. None of these antibodies labels H4 cells. The photoreceptor subtypes have been differentiated by different intensity of labeling with CB. The accessory member of the double cone is less intensely labeled with CB than the principal member and rods and blue cones do not appear to label at all. ChAT-IR is located in terminal boutons of H1 and H2 horizontal cells and H1 axon terminals and these boutons contact rods and all spectral types of cones. Clearly, GABA is present in H1 horizontal cells and may be used in neurotransmission between horizontal cells and possibly for feedback pathways to photoreceptors. The evidence of nNOS immunoreactivity in H2 and H3 horizontal cells, combined with available physiological evidence, suggests that NO may be involved in electrical coupling and/or modulation of synaptic input to these types of cells. Furthermore, our results raise the possibility that cholinergic synaptic transmission may occur from horizontal cell processes to photoreceptors in the outer plexiform layer of the turtle retina.

Endothelial nitric oxide synthase (eNOS) is localized to Müller cells in all vertebrate retinas

The distribution of endothelial nitric oxide synthase immunoreactivity (eNOS-IR) was investigated in the retinas of all phylogenetic vertebrate classes by using a monoclonal eNOS antibody. Confocal light microscopy showed immunoreactive labeling in Müller cells of fish, frog, salamander, turtle, chicken, rat, ground squirrel, and monkey retina. In vascularized retinas (rat, monkey), astrocytes and some blood vessels were also stained. Furthermore, eNOS-IR was localized to axon terminals of turtle and fish horizontal cells. These observations are the first to show the presence of eNOS-IR in Muller glia and horizontal cell structures of the vertebrate retina.

The localization of guanylyl cyclase-activating proteins in the mammalian retina

PURPOSE

To explore the distribution of guanylyl cylase-activating proteins 1 and 2 (GCAP1 and GCAP2) in the mammalian retina.

METHODS

Cryostat and vibratome vertical sections and wholemount retinas from mouse, rat, cat, bovine, monkey, and human eyes were prepared for immunocytochemistry and viewing by light and confocal microscopy.

RESULTS

In all mammalian retinas investigated, intense GCAP1 immunoreactivity (GCAP1-IR) was seen in cone photoreceptor inner and outer segments, cell bodies, and synaptic regions. Intensity of the GCAP1-IR was strong in inner segments of rods in all species but weaker in outer segments-particularly so in primates and cats. GCAP2 immunoreactivity (GCAP2-IR) was weak in bovine, mouse, and rat cones but was intense in human and monkey cones. In all species except primates, GCAP2 staining was intense in rod inner and outer segments. In primates GCAP2-IR was intense in the rod inner segment but faint in the rod outer segment. A striking difference from the GCAP1 pattern of immunoreactivity was seen with GCAP2 antibodies as far as the inner retina was concerned. GCAP2-IR was evident in certain populations of bipolar, amacrine, and ganglion cells in all species.

CONCLUSIONS

GCAP1 and GCAP2, which are involved in Ca2+-dependent stimulation and inhibition of photoreceptor guanylyl cyclase, can be detected in mammalian photoreceptor inner and outer segments, consistent with their physiological function. The occurrence of both GCAPs in the synaptic region of the photoreceptors indicates participation of these proteins in pathways other than regulation of phototransduction. The occurrence of GCAP2 in inner retinal neurons is indicative of second-messenger chemical transduction, possibly in metabotropic glutamate, gamma-aminobutyric acid (GABA) receptor, and nitric oxide-activated neural circuits.

Circuitry and role of substance P-immunoreactive neurons in the primate retina

In this paper, we extend our previous light microscopic (LM) study of substance P (SP)-containing amacrine and ganglion cell types of the human retina (Cuenca et al. [1995] J. Comp. Neurol. 356:491-504) to an electron microscopic (EM) and confocal-imaging study in order to reveal synaptic circuitry and putative input and output neurons. SP-immunoreactive (-IR) amacrine cells in primate retina are typically wide-field cells with large cell bodies occurring in normal or displaced positions relative to the inner plexiform layer (IPL). Their main dendrites bear many spines and are monostratified in stratum 3 (S3) of the IPL. Axon-like processes arise from dendrites close to the cell body and run for hundreds of microns at the same level as the dendrites, thus forming a relatively dense plexus in S3 of the IPL. SP-IR axon processes also climb to S1 to surround some amacrine cell bodies, and others pass into the outer plexiform layer (OPL). Still other axons run down to the ganglion cell layer, where they encircle SP-IR ganglion cells and pass on to end in the nerve fiber layer. The SP-IR ganglion cell types have large cell bodies (20-22 microm diameter) and dendrites that costratify in S3 among the SP-IR amacrine cell processes. Double immunostaining and study by confocal microscopy reveals that SP-IR amacrine cells in the monkey colocalize gamma-aminobutyric acid (GABA). Their main plexus of dendrites in S3 of the IPL is skirted on the S2/S3 border by cone bipolar axons that stain for calbindin but intermingles primarily with glycinergic bipolar cell types of S3 and S3-S4. Strongly GABA-IR/weakly glycine-IR amacrine cell bodies, in addition to the SP-IR large-bodied ganglion cell type, are targets of encircling SP-IR axon processes. EM study of the human SP-IR amacrine cell indicates that input synapses to its dendrites are from bipolar cell axons of the S2/S3 border, S3, and the S3/S4 border of the IPL neuropil (33% of the synaptic input) and from amacrine cell processes (67% of the synaptic input). The input amacrine cells are of at least two distinct types based on cytological criteria. Synaptic output from the SP-IR amacrine cell dendrites is to bipolar cell axons as reciprocal synapses (31%), to amacrine cells (40%), and to ganglion cell profiles, primarily in S3 (29%) of the IPL. The SP-IR axons synapse upon SP-IR ganglion cell bodies and axons, upon normally placed and displaced amacrine cell bodies, and upon bipolar cell dendrites in the OPL. In addition, they appear to synapse among themselves. We shall discuss a wiring diagram and the possible role of SP-IR amacrine cells in the primate retina.

Formation and dissolution of spinules and changes in nematosome size require optic nerve integrity in black bass (Micropterus salmoides) retina

Teleost retinas adapted to light show numerous spinules invaginated in the cone pedicles and small nematosomes in the distal horizontal cells. Darkness induces the dissolution of spinules and the presence of large and numerous nematosomes. The aim of this work is to study the influence of optic nerve integrity on spinule formation/dissolution and changes in nematosome size during light or dark adaptation of black bass (Micropterus salmoides) retinas. Eyes from fish, dark- or light-adapted, were removed and the eyecups placed in oxygenated Ringer's solution and immediately exposed to light or dark, respectively, for 1 h. The number of spinules per pedicle and the nematosome diameter were measured on electron micrographs. Isolation of eyecups in the dark, impaired both spinule formation and nematosome size reduction when they were superfused in light. In the same way, isolation of eyecups in the light, impaired both spinule dissolution and nematosome size increase when they were superfused in dark. No significant differences in spinule number and nematosome size, following dopamine superfusion, were found in comparison to retinas superfused with Ringer's solution only. Our results suggest: (1) optic nerve integrity is necessary to yield spinule formation/disruption and changes in nematosome size during light or dark adaptation. (2) dopamine does not appear to be the primary agent responsible for spinule formation.

Substance P: a neurotransmitter of amacrine and ganglion cells in the vertebrate retina

A short history and summary of the occurrence of substance P in the vertebrate body is presented. Substance P is now generally accepted to be a neurotransmitter and can be visualized by immunocytochemistry to occur in various nerve cells in the CNS. In the retina, substance P-immunoreactivity (SP-IR) occurs in amacrine cell populations in all the species so far studied. In some vertebrates retinas SP is also apparent in one or more ganglion cell types. Anatomical investigations have revealed the morphology and connectivity of SP-IR amacrine cells: they branch in several strata of the inner plexiform layer receiving input from bipolar and amacrine cells and making synapses upon bipolar and ganglion cells. Most commonly SP-IR amacrines emit axon-like process that pass to both the outer plexiform layer and the ganglion cell and nerve fiber layers. These processes often end upon the retinal vasculature. SP-IR ganglion cells have been described in turtle, rabbit and human retinas. In turtle, intracellular dye injection has revealed the morphology of one type of SP-IR ganglion cell as being a large-field monostratified cell with a branches in the outer stratum of the inner plexiform layer. It may correspond to a "Dogiel cell" type. Intracellular investigation of SP-IR amacrine cells in turtle reveal their physiological responses to be ON-OFF in nature with some color-coding characteristics. In general SP acts as an excitatory neurotransmitter raising the spontaneous activity level of ganglion cell responses. The SP-IR ganglion cell is an OFF-center unit in the turtle retina and may be driven in the center of its receptive field by luminosity bipolar cells and in its surround by amacrine cells with color-opponent properties.

Substance P-immunoreactive neurons in the human retina

Substance P (SP) is a neuropeptide that acts as a neurotransmitter or a neuromodulator in the retina. The aim of this study was to identify the type(s) and the distribution of the SP-immunoreactive (SP-IR) cells in the human retina. We have used an antiserum to SP to immunostain neurons in postmortem human retinae. Immunostained retinae were processed with the avidin-biotin complex (ABC) to visualize the cells either whole mounted in glycerol or embedded in plastic. Some retinae were also sectioned at 20 microns in order to obtain radial views of stained cells. SP-IR amacrine cells stain intensely and appear to be of a single type in the human retina. They are large-field cells with large cell bodies (16 microns diameter) lying in normal or displaced positions on either side of the inner plexiform layer (IPL). Their sturdy, spiny, and appendage-bearing dendrites stratify in stratum 3 (S3) of the IPL, where many overlapping, fine dendrites intermingle to form a plexus of stained processes. Either cell bodies or primary dendrites emit an "axon-like" process that, typically, divides into two long, fine processes, which run in opposite directions for hundreds of micrometers in S5 and S3 before disappearing as distinct entities in the stained plexus in S3. Long, fine dendrites also pass from the dendritic plexus to run in S5 and down to the nerve fiber layer to end as large varicosities at blood vessel walls. In addition, fine processes are emitted from the dendritic plexus that runs in S1, and some pass up to the outer plexiform layer (OPL) to run therein for short distances. The SP-IR amacrine cell has many similarities to the thorny, type 2 amacrine cells described from Golgi studies. In addition to the SP-IR amacrine cells, a presumed ganglion cell type is faintly immunoreactive. Its 20-22 microns cell body gives rise to a radiate, sparsely branched, wide-spreading dendritic tree running in S3. Its dendrites and cell body become enveloped by the more intensely SP-IR processes and boutons from the SP-IR amacrine cell type. The SP-IR ganglion cell type most resembles G21 from a Golgi study.

Two types of mitochondria are evidenced by protein kinase C immunoreactivity in the Müller cells of the carp retina

The localization of protein kinase C (PKC) was studied immunocytochemically in the Müller cells of the carp retina. Electron microscope immunocytochemistry (using a monoclonal antibody to the alpha and beta isoenzymes of PKC) showed PKC-immunoreactivity mainly inside some mitochondria, especially along the mitochondrial cristae whereas other mitochondria in the same Müller cells showed no staining. Despite a detailed analysis we did not find any significant morphological difference between labeled and unlabeled mitochondria. These results demonstrate, for the first time, the presence of PKC immunoreactivity inside mitochondria and suggest that individual mitochondria may differ in signal transduction pathway.

Visual experience during postnatal development determines the size of optic nerve axons

The size of any axon is generally related to an internal programme (probably common for all neurones) that determines the number and organization of its cytoskeletal components. Here we show that axons from visually deprived rats are smaller than controls, although the number of microtubules and neurofilaments remains unchanged. Moreover the distance between both microtubules and neurofilaments and the amounts of 200 kDa, 160 kDa and 68 kDa neurofilament proteins are also diminished in the deprived axons. We suggest that cytoskeletal organization and axonal calibre are not only determined by intrinsic (genomic) factors, but that environmental stimulation is important for normal growth of nerve cells.

Dendrites of rod dominant ON-bipolar cells are coupled by gap junctions in carp retina

Gap junctions are supposed to be the anatomical substrate for electrical coupling between neurons. In fish retina, bipolar cells are electrically coupled and their receptive field diameters are always larger than dendritic field size, however, gap junctions have not been described between dendrites of bipolar cells. In this paper, using immunostaining for protein kinase C, we show that every rod dominant ON-bipolar cell is connected with its neighboring dendrites by gap junctions forming a plexus in the outer plexiform layer. These dendritic processes provide the site of electrical coupling. We suggest that dendrites of bipolar cells could be involved in lateral pathways in retina.

A compiled BASIC program for analysis of spatial point patterns: application to retinal studies

The pattern of distribution of a population of cells is of considerable interest to biologists and neurobiologists. However, the labor involved in collecting and analyzing the data often requires a significant amount of time. This paper presents a compiled BASIC program written using the Microsoft QuickBasic compiler for Apple Macintosh to facilitate such studies. The program allows collection and analysis of data that can be introduced either with the aid of a digitizing tablet of directly imported as x,y coordinates from different sources as, for example, word processors or image analysis software. Subsequently the program provides a quick, easy and interactive way of access to statistical, mathematical and graphical techniques used in the analysis of spatial point patterns. These techniques include several measures of dispersion (quadrat count, nearest neighbor and a 2-dimensional point autocorrelogram analysis) and arrangement. Although the program has been tested on spatial organization of retinal cells, it can be used to study the distribution of other cells in the nervous system and for different projects, as for example the distribution of microtubules and neurofilaments inside the axons. This software is available from the authors.

Axon types classified by morphometric and multivariate analysis in the rat optic nerve

Calibers of the rat optic nerve axons distribute unimodally and it is difficult to distinguish groups among them. However, these fibers arose from 3 types of ganglion cells and showed 3 conduction velocities. Performing a cluster analysis over several ultrastructural parameters we found 3 main groups of fibers. These groups are present in a very similar proportion to the ganglion cells groups described in the rat retina.

A useful programme in BASIC for axonal morphometry with introduction of new cytoskeletal parameters

Interest in the structure of axons and quantification of their components has been growing over the last years. However, the existing literature contains few reports of available computer programmes to facilitate such studies. This paper presents a fully comprehensive BASIC programme for the morphometric analysis of electron micrographs of cross-sectional nerve fibres. From drawings of fibre and axonal contours and dots of the microtubules and neurofilaments, the programme calculates the following parameters: area, diameter and form factor of the fibres and axons, number and density of microtubules and neurofilaments, proportion between microtubules and neurofilaments (R-proportion), myelin thickness and the diameter of the axon relative to its sheath (g-ratio). The programme also introduces three new parameters to analyse the degree of uniformity of microtubule and neurofilament distribution: distances between microtubules and between neurofilaments, equilateral index and cytoskeletal intermingling index. The programme is written in Microsoft BASIC Interpreter for Apple Macintosh (Microsoft Corporation) but can be used on other computers. Although the programme has been tested on adult rat optic nerve fibres, it can be used for different projects concerning axonal morphometry.

Postembedding immunocytochemistry for GABA and glycine reveals the synaptic relationships of the dopaminergic amacrine cell of the cat retina

Postembedding electron microscope immunocytochemistry of glycine and GABA conjugated to colloidal gold has been applied to pre-embedded cat retina stained with the antibody against tyrosine hydroxylase (Toh+). Toh+ stained cells are the equivalent of A18 amacrine cells of Golgi descriptions (Kolb et al., '81). The dendrites of Toh+ cells synapse upon several different types of glycine-positive amacrine cell bodies. We suggest that these are the A8, A3/A4, and AII amacrine cell varieties by analogous immunocytochemical staining intensity, to glycine autoradiographic labeling intensity (Pourcho and Goebel, '85). The greatest number of synapses from Toh+ dendrites are directed at the least glycine-positive amacrine, which is the AII cell by all morphological criteria. A few glycine-positive profiles are also presynapatic to the Toh+ stained cell body itself. Toh+ profiles are also presynaptic to GABA-positive amacrine cell bodies. The commonest amacrine synapsed upon is very heavily labeled with GABA immunocytochemistry. We consider it to be the A17 amacrine cell, which is known to label strongly by [3H] muscimol autoradiography (Pourcho and Goebel, '83). The cell body of the Toh+ amacrine cell also receives many synapses, which appear to be GABA-positive, and Toh+ profiles running in stratum 1 of the inner plexiform layer (IPL) are both pre- and postsynaptic to GABA-positive amacrine cell profiles. In addition, the cell body and primary dendrites of the Toh+ cell receive input from a bipolar type and GABA- or glycine-negative profiles. GABA-positive profiles, belonging to the interplexiform cell (IPC), are synapsed upon by Toh+ profiles that run in the outer plexiform layer (OPL).(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of immunoreactivity to protein kinase C in the turtle retina

Immunocytochemical staining procedures using the HRP-complexed antibody to protein kinase C (PKC) have been carried out on the turtle retina. Wholemounts and frozen sections of retina have been studied by light microscopy to evaluate PKC immunoreactivity after stimulation of the retina with light and neurotransmitters known to be active in the vertebrate retina. The most dramatically stained sites are cone synaptic pedicles and bipolar cells under all conditions. Ganglion cells stain weakly under certain conditions. Applying the antibody to a 'control' retina under dark adapted conditions results in uniform background staining of both hyperpolarizing and depolarizing bipolar pathways, while stimulating the retina with K+ under dim light conditions results in discretely stained bipolar cells and a prominent band of staining in stratum 4 of the inner plexiform layer. Stronger stimulation of bipolar cells with their terminals contributing to strata 3 and 4 and the continuous dominant band in stratum 4 can be elicited with incubation of the retina in neurotransmitter agonists, GABA and dopamine. Incubation with dopamine, in particular, brings out the putative dopaminergic amacrine cell. The only condition in which a strong band in stratum 2 can be demonstrated is under stimulation with a flashing bar of spot of light. Thus K+ and neurotransmitter stimulation elicit PKC staining in neurons contributing to the ON or depolarizing sublamina of the IPL, while intermittent flashing light stimulus is required to elicit PKC staining in the OFF or hyperpolarizing sublamina of the IPL.