Retinal ganglion cell morphology in the frog, Rana pipiens

The morphology of retinal ganglion cells in the frog, Rana pipiens, has been examined in retinal flatmounts following backfilling of axons with horseradish peroxidase (HRP). Size and shape of the cell body and of the dendritic arbor, the dendritic branching pattern, and the depth of dendritic arborization within the inner plexiform layer (IPL) were all used to classify these cells. All of the ganglion cells so visualized can be grouped into one of 7 distinct cell classes. Class 1 contains the largest ganglion cells, with a soma size of 323 +/- 5.3 microns2 and dendritic fields of 86,819 +/- 11,817 microns2; the dendrites branch within strata 1 and 2 of the IPL. The second largest cells are class 2, with somas of 245 +/- 19.7 microns2 and dendritic fields of 55,983 +/- 7,392 microns2; the dendrites also branch within strata 1 and 2 of the IPL. Class 3 cells are the next largest class with somas of 211 +/- 11.8 microns2 and dendritic fields of 18,186 +/- 1,394 microns2; there are three varieties of class 3 cells based on the depth of branching of the dendrites: some cells are bistratified, others are tristratified, while still other cells arborize diffusely within the IPL. Class 4 cells are intermediate in size, with somas of 113 +/- 7.4 microns2 and dendrites of 4800 +/- 759 microns2; the dendrites arborize within strata 4 and 5 of the IPL. Class 5 cells have not been quantitatively analyzed because they are heterogeneous in soma and dendritic size. However, class 5 cells all have cell bodies displaced in location into the inner nuclear layer and all have a unique dendritic specialization: they send from 1 to 3 processes into the outer plexiform layer. Class 6 cells are the second smallest cell class with somas of 68.1 +/- 5.13 microns2 and dendritic fields of 888 +/- 182 microns2; the dendrites arborize within strata 3, 4, and 5 of the IP. Class 7 contains the smallest ganglion cells with somas of 62.1 +/- 2.86 microns2 and dendritic fields of 831 +/- 74.2 microns2; the dendrites arborize within strata 3, 4, and 5 of the IPL. The frequency of each cell class is inversely proportional to the size of the dendritic field. Thus, class 7 cells are the most frequent; class 1 cells are the least frequent. Furthermore, each of these 7 classes of ganglion cells has representative cells located in the inner nuclear layer.(ABSTRACT TRUNCATED AT 400 WORDS)

Retina of the tadpole and frog: delayed dendritic development in a subpopulation of ganglion cells coincident with metamorphosis

In this study, the morphology of tadpole retinal ganglion cells was compared to that of frogs to determine if changes in dendritic structure occur during metamorphosis. Ganglion cells were analyzed in the tadpole and frog after backfilling with horseradish peroxidase. Representative ganglion cells are present in the tadpole retina, which directly correspond to each of the 7 cell classes found in the frog. However, cells in 3 of these classes (1, 3, and 7) exist in morphologically immature states in retinas from tadpole stages St. XIV-XIX. New dendritic branches appear and the dendritic arbors of these ganglion cells expand during metamorphosis. We propose that the increased dendritic arborization may be followed by new synaptic contacts onto these cells, which contributes to the emergence of new physiological receptive field properties in the frog.

Endocytosis in the inner segment of rod photoreceptors: analysis of Xenopus laevis retinas using horseradish peroxidase

To evaluate endocytosis in rod photoreceptor inner segments, isolated retinas from Xenopus laevis were maintained in vitro in the presence of the tracer, horseradish peroxidase (HRP). HRP-positive vesicles, 110 +/- 82 nm in diam. (mean +/- S.D.), appeared in the inner segment cytoplasm within 2 min of incubation. Within 8-15 min, 20-30% of the vesicles in the ellipsoid were HRP-positive, reaching approx. 50% by 30-60 min and maintaining this level through 4 hr of culture. In retinas incubated 30 min with HRP and then cultured for an additional period without tracer, the HRP-labeled vesicles were reduced in number by 50- and 75% in the 1- and 2-hr chase incubations, respectively. In retinas cultured for 8 min or longer, multivesicular bodies in the inner segments also contained HRP reaction product, suggesting that the HRP-labeled endocytotic vesicles enter multivesicular bodies. Retinas cultured with HRP for greater than or equal to 1 hr contained significantly more vesicles (per unit area of ellipsoid cytoplasm) as compared with unincubated retinas, retinas incubated from 1 hr in normal media or retinas incubated 30 min with HRP followed by normal chase medium for greater than or equal to 30 min. Taken together, these data indicate that the rod photoreceptor inner segment is capable of extensive endocytotic activity, a process which may function to recover components of the inner segment plasma membrane and the interphotoreceptor matrix.

Localization of proteoglycan within the extracellular matrix sheath of cone photoreceptors

The interphotoreceptor matrix of the human retina was examined histochemically by staining with a cationic copper phthalocyanin dye, Cuprolinic Blue, in a critical electrolyte concentration method which allowed staining of sulfated polyanions. In the presence of Cuprolinic Blue, a dense punctate matrix component was visualized covering the cone from the outer limiting membrane of the retina to the tip of the cone outer segment and extending through the matrix to the apical surface of the pigment epithelium. The size and appearance of the stained material are consistent with its being proteoglycan and its distribution is consistent with its being a component of the cone extracellular matrix sheath. A fine reticular pattern was observed in the presence and absence of Cuprolinic Blue.

Localization of hydroxyindole-O-methyltransferase-like immunoreactivity in photoreceptors and cone bipolar cells in the human retina: a light and electron microscope study

The localization of the melatonin-synthesizing enzyme hydroxyindole-O-methyltransferase (HIOMT) was examined by light and electron microscopic immunocytochemistry in the human retina. HIOMT-like immunoreactivity was observed in the photoreceptor layers and the inner nuclear layer (INL). The immunoreactive cells in the INL were more numerous in the central retina than in the peripheral retina and sent processes to both the outer plexiform and inner plexiform layers. The HIOMT immunoreactivity in the inner plexiform layer (IPL) appeared as punctate terminals in the proximal and distal one-thirds of that layer. At the ultrastructural level, HIOMT-like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors and to a population of cone bipolar cells. HIOMT-immunoreactive bipolar cell dendrites were observed to make both invaginating and flat synaptic contacts with cone pedicles. No immunoreactive invaginating contacts in rod spherules were observed. HIOMT immunoreactivity was observed in the bipolar cell cytoplasm in the INL, and in the bipolar synaptic terminals in the IPL. These terminals contained synaptic ribbons, which formed synaptic contacts with unlabeled cells in the IPL. HIOMT radioenzymatic assays confirmed the presence of HIOMT in the human retina. Average HIOMT activity of eight donors was determined to be 15.0 pmol/mg protein/hour +/- 7.2 S.D. The ultrastructural localization of HIOMT observed in this study, combined with reports from other laboratories, suggests that the cytoplasm of the photoreceptors and a population of cone bipolar cells may be the sites of melatonin synthesis in the human retina.

Cholinergic neurons in the human retina

Choline acetyltransferase (ChAT)-like immunoreactivity in the human retina can be demonstrated using a polyclonal antiserum to ChAT isolated from chick brain. There is a population of ChAT-like immunoreactive cells along both margins of the inner plexiform layer (IPL). The labeled cells have a morphology and position characteristic of the cholinergic amacrine- and displaced amacrine cells demonstrated by other workers in the mammalian retina. Non-immune rabbit serum or pre-absorbed antiserum, used in place of the primary antiserum, verified the specificity of the method. Human retinas can also be labeled with the fluorescent dye 4',6-diamidino-2-phenylindole (DAPI), which has been reported to bind selectively to DNA in the nuclei of cholinergic cells. The fluorescent cells are similar in morphology, position, and distribution to the cells which show ChAT-like immunoreactivity. In addition, we have localized the presence of [3H]choline and [3H]choline metabolites in freeze-dried, vapor-fixed tissue using 'dry' autoradiographic techniques. Incubation in [3H]choline was followed by either stimulation or inhibition of calcium-dependent transmitter release during a 1-hr 'chase' period. Using tissue incubated in a chase designed to retain labeled neurotransmitters, silver grains were concentrated over a population of cell bodies at either margin of the IPL (i.e. in the same position as putative ChAT-immunoreactive cells and DAPI-labeled cells). In contrast, tissue incubated in a chase designed to release labeled acetylcholine was labeled uniformly throughout the neural retina, with a heavy band of label over the pigment epithelium. Taken together, the results presented here indicate that three independent markers for cholinergic cells (i.e. ChAT immunoreactivity, DAPI binding, and choline uptake) are present in a population of cells in the human retina. This suggests that acetylcholine may be a neurotransmitter synthesized by amacrine and displaced amacrine cells in the retina.

Acetylcholinesterase in the human retina

The distribution of cholinesterase activity in the human retina was evaluated using histochemical methods. The butyrylcholine esterase (BuChE) inhibitor tetraisopropylpyrophosphoramide was used to localize acetylcholinesterase (AChE) activity, the AChE inhibitor BW284c51 was used to localize BuChE activity, and eserine was used to inhibit all cholinesterase activity in control incubations. We have found specific AChE activity in the inner plexiform layer (IPL) and BuChE activity is apparently associated with blood vessels at the light microscopic level. At the electron microscopic level, AChE reaction product is seen both at apparently synaptic and manifestly non-synaptic sites. Reaction product is found adjacent to neural processes in the IPL; these processes have an ultrastructure characteristic of amacrine cells. Although reaction product is seen adjacent to bipolar, ganglion and Müller cells, it is only present where they come into contact with apparent amacrine cell profiles. Using longer incubations, reaction product is found in the perikarya of cells with the morphology and position of amacrine and displaced amacrine cells. There is also some evidence for AChE activity adjacent to lateral processes at a rare type of rod synapse in the outer plexiform layer. The observations reported here strengthen the evidence for cholinergic neurotransmission in the human retina.

Inhibition of oligosaccharide processing and membrane morphogenesis in retinal rod photoreceptor cells

Castanospermine (Cas), an inhibitor of alpha-glucosidase I, blocks "trimming" of the N-linked oligosaccharide Glc3Man9GlcNAc2, thus preventing normal glycoprotein maturation. With use of a dual-label protocol, Xenopus retinas incubated in the presence of Cas exhibited at least a 2.3-fold increase in the incorporation of [3H]mannose into total retina Cl3CCOOH-precipitable material, whereas incorporation of [14C]leucine was not significantly affected, relative to controls. Analysis of NaDodSO4/PAGE fluorograms of solubilized retinas and rod outer segment (ROS) membranes indicated a relatively selective effect of Cas on opsin (the rod visual pigment apoglycoprotein). The apparent molecular mass of opsin was increased by approximately 2500 in the presence of Cas; the incorporation of [3H]mannose into opsin was enhanced about 2.3-fold without a significant effect on [14C]leucine incorporation, relative to controls. Electron microscopic autoradiography of retinas incubated for 4 hr with [3H]mannose showed that the number of newly formed ROS discs in Cas-treated retinas was not significantly different from controls, but the silver grain density over those discs was about 2.6-fold greater than in controls. The morphology of the newly formed discs was comparable under both conditions. Thus, opsin bearing abnormally large oligosaccharides can be accommodated in the process of disc morphogenesis. These results suggest that the structural requirements for opsin's oligosaccharides, with regard to their potential role as determinants of disc morphogenesis, are not stringent. Furthermore, post-translational processing of N-linked oligosaccharides is not essential for the normal intracellular routing and cell surface expression of membrane glycoproteins.

Human retinas synthesize and release acetylcholine

Human retinas have the capacity to synthesize and release [3H]acetylcholine ([3H]ACh) after an incubation in [3H]choline ([3H]Ch). Synthesis of [3H]Ch by retinal homogenates was determined using either high-voltage paper electrophoresis (HVPE) or a two-step enzymatic/extraction assay for separating [3H]ACh from [3H]Ch. The enzymatic/extraction assay is shown to be accurate over a wide range of concentrations (10(-6)-10(-12) M). Homogenates of human retina synthesize [3H]ACh from [3H]Ch. We find an approximate Km of 50 microM and a Vmax of about 20 nmol/mg protein/h (at 37 degrees C) for the synthesis of labeled ACh by retinal homogenates. Human retinas also release [3H]ACh after a pulse of [3H]Ch. Release of labeled transmitter is stimulated by potassium depolarization. The potassium-stimulated release is partially blocked by magnesium or cobalt ions. Release data were analyzed by both the enzymatic/extraction assay and HVPE; the results are qualitatively identical in both cases. The data reported here provide additional evidence for cholinergic neurotransmission in the human retina.

Acetylcholine receptors in the human retina

Evidence for a population of acetylcholine (ACh) receptors in the human retina is presented. The authors have used the irreversible ligand 3H-propylbenzilylcholine mustard (3H-PrBCM) to label muscarinic receptors. 3H- or 125I-alpha-bungarotoxin (alpha-BTx) was used to label putative nicotinic receptors. Muscarinic receptors are apparently present in the inner plexiform layer of the retina. Autoradiographic grain densities are reduced in the presence of saturating concentrations of atropine, quinuclidinyl benzilate or scopolamine; this indicates that 3H-PrBCM binding is specific for a population of muscarinic receptors in the human retina. Binding sites for radiolabeled alpha-BTx are found predominantly in the inner plexiform layer of the retina. Grain densities are reduced in the presence of d-tubocurarine, indicating that alpha-BTx may bind to a pharmacologically relevant nicotinic ACh receptor. The work reported here is consistent with earlier data on cholinergic neurons in the retina of other mammalian species, as well as with previous work on the psychophysical effects of cholinergic agonists on human vision. This study provides evidence for cholinergic neurotransmission in the human retina.

Endocytosis and degradation of interstitial retinol-binding protein: differential capabilities of cells that border the interphotoreceptor matrix

Between the pigment epithelium and the outer limiting membrane of the retina is an extracellular compartment filled with the interphotoreceptor matrix (IPM). A prominent component of the IPM is a glycoprotein known as interstitial retinol-binding protein (IRBP). Using in vitro techniques, we compared the ability of the cells that border this compartment to internalize colloidal gold (CG) coated with either IRBP or ovalbumin, a glycoprotein not found in the IPM. Neither IRBP-CG nor ovalbumin-CG was internalized by the Muller's cells. Both rod and cone photoreceptors take up IRBP-CG, which is observed in small vesicles and multivesicular bodies. Neither photoreceptor type takes up ovalbumin-CG. Acid phosphatase cytochemistry indicates that acid phosphatase reaction product in the multivesicular bodies co-localizes with IRBP-CG, which suggests that this molecule is degraded by rod and cone photoreceptors and is not recycled. The pigment epithelium internalizes IRBP-CG and ovalbumin-CG, both of which remain in small cytoplasmic vesicles near the apical plasma membrane. There is no indication that vesicles that contain either IRBP-CG or ovalbumin-CG fuse with the lysosomal system in the pigment epithelial cells during the incubation.

Effect of light on the metabolism of lipids in the rat retina

The effect of light on the in vitro incorporation of a variety of radioactive precursors into glycerolipids was tested in isolated retinas of albino rats. There was an increase in the incorporation of [2-3H]myo-inositol, 32Pi, [2-3H]glycerol, and [methyl-3H]choline into retinal phospholipids in light compared to that in darkness. [2-3H]myo-Inositol was incorporated primarily into phosphatidylinositol. 32Pi was incorporated primarily into the phosphoinositides, although there were significant increases in the specific activities of all retinal phospholipids in light compared to those in darkness. Likewise, [2-3H]glycerol incorporation into all retinal phospholipids and diglycerides was greater in light than in the dark. There was no effect of light on the incorporation of [2-3H]ethanolamine into phosphatidylethanolamine or of [3-3H]serine into phosphatidylserine, although these phospholipids were labeled to a greater extent in light with [2-3H]glycerol. There was no effect of light on the incorporation of [3H]palmitic acid into diglycerides and phospholipids, with the exception of phosphatidylinositol. Light also had no effect on the uptake of [2-3H]glycerol, [2-3H]inositol, or [methyl-3H]choline into the retina. We conclude from these studies that light stimulates the phosphoinositide effect in the rat retina. Although some of the results are consistent with a stimulation of de novo synthesis of all lipid classes, our studies with [3H]palmitate, [2-3H]ethanolamine, and [3-3H]serine do not support this conclusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane morphogenesis in retinal rod outer segments: inhibition by tunicamycin

Isolated Xenopus laevis retinas were incubated with 3H-labeled mannose or leucine in the presence or absence of tunicamycin (TM), a selective inhibitor of dolichyl phosphate-dependent protein glycosylation. At a TM concentration of 20 micrograms/ml, the incorporation of [3H]mannose and [3H]leucine into retinal macromolecules was inhibited by approximately 66 and 12-16%, respectively, relative to controls. Cellular uptake of the radiolabeled substrates was not inhibited at this TM concentration. Polyacrylamide gel electrophoresis revealed that TM had little effect on the incorporation of [3H]leucine into the proteins of whole retinas and that labeling of proteins (especially opsin) in isolated rod outer segment (ROS) membranes was negligible. The incorporation of [3H]mannose into proteins of whole retinas and ROS membranes was nearly abolished in the presence of TM. Autoradiograms of control retinas incubated with either [3H]mannose or [3H]leucine exhibited a discrete concentration of silver grains over ROS basal disc membranes. In TM-treated retinas, the extracellular space between rod inner and outer segments was dilated and filled with numerous heterogeneously size vesicles, which were labeled with [3H]leucine but not with [3H]mannose. ROS disc membranes per se were not labeled in the TM-treated retinas. Quantitative light microscopic autoradiography of retinas pulse-labeled with [3H]leucine showed no differences in labeling of rod cellular compartments in the presence or absence of TM as a function of increasing chase time. These results demonstrate that TM can block retinal protein glycosylation and normal disc membrane assembly under conditions where synthesis and intracellular transport of rod cell proteins (e.g., opsin) are not inhibited.

Synthesis and secretion of interstitial retinol-binding protein by the human retina

Interstitial retinol-binding protein (IRBP) is a soluble glycoprotein present between the retina and pigmented epithelium, which may function to shuttle vitamin A derivatives between these tissues. While previous studies have shown that the retina is solely responsible for IRBP synthesis, the specific retinal cell(s) in which this occurs has not been established. Since the carbohydrate moiety of IRBP contains fucose, the authors have analyzed the sites of incorporation of 3H-fucose in the human retina in vitro, using autoradiography. Following a 30-min pulse incubation, all retinal layers exhibited incorporation of label; however, the rod photoreceptor inner segments contained one- to two-fold more radioactivity than was present in any other retinal compartment. In autoradiographs of retinas recovered following a 4 hr chase incubation, all retinal layers retained similar levels of radioactivity with the exception of the rod photoreceptors, cone photoreceptors and cells in the inner nuclear layer, which lost 75, 11, and 14 percent, respectively of the radioactivity present immediately following the 30-min pulse. Proteins present in the chase incubation medium were analyzed by polyacrylamide gel electrophoresis and fluorography. The principal labeled component in the chase medium was identified as IRBP by immunoprecipitation with antibovine-IRBP immunoglobulins. Thus, the major loss of 3H-fucose radioactivity from rod photoreceptors coupled with the appearance of 3H-labeled IRBP in the incubation media suggests that the rod photoreceptors are primarily responsible for the synthesis and secretion of IRBP.

Inhibitors of metalloendoprotease activity prevent K+-stimulated neurotransmitter release from the retina of Xenopus laevis

Metalloendoprotease activity was identified in retinal homogenates using a synthetic fluorogenic metalloendoprotease substrate and specific metalloendoprotease inhibitors. The requirement of metalloendoprotease activity in neurotransmitter release was examined during the depolarization-induced release of [3H]glycine from the retina of Xenopus laevis. Neurons with high affinity uptake and calcium-dependent, K+-stimulated release of glycine have been described previously in this retina. When isolated retinas preloaded with [3H]glycine are depolarized by 22 mM K+, the usual efflux of [3H]glycine is completely abolished by the metalloendoprotease inhibitor, 1,10-phenanthroline (100 micrograms/ml). The inhibition of [3H]glycine release by 1,10-phenanthroline is dose dependent; furthermore, 1,10-phenanthroline blocks release by chelating metal and not Ca2+, since addition of equimolar calcium does not alter the inhibition. The metalloendoprotease inhibitor, carbobenzoxy (CBZ)-L-phenylalanine, also prevents release, whereas the amino acid, L-phenylalanine, has no effect. Synthetic carbobenzoxy dipeptide amides which are metalloendoprotease substrates (e.g., CBZ-Gly-Leu-amide, CBZ-Ser-Leu-amide, and CBZ-Gly-Phe-amide) also prevent [3H]glycine release in a dose-dependent and reversible manner. The synthetic dipeptide CBZ-Gly-Gly-amide, however, is not a metalloendoprotease substrate and has no effect on release. The ability of synthetic dipeptides to inhibit neurotransmitter release is amino acid specific, dose dependent, reversible, and matches their ability to interact with characterized metalloendoproteases. Depolarization-stimulated transmitter release may therefore require the activity of a metalloendoprotease.

Photoreceptor-specific degeneration caused by tunicamycin

The antibiotic tunicamycin inhibits the biosynthesis of N-acetylglucosaminylpyrophosphoryl polyisoprenol, a key intermediate in the formation of the asparagine-linked oligosaccharides of glycoproteins. The effects of tunicamycin have been studied in various biological systems, primarily with the aim of elucidating the role of the carbohydrate moieties in the cellular function of glycoproteins. Rhodopsin, the visual pigment of retinal rod photoreceptor cells, is a membrane glycoprotein which consists of a single polypeptide chain (opsin) to which a chromophoric prosthetic group (II-cis-retinaldehyde) and two asparagine-linked oligosaccharide chains are covalently attached. The glycosylation of opsin can be blocked with tunicamycin in vitro in conditions where polypeptide synthesis is only slightly decreased. We have reported that tunicamycin can disrupt the normal assembly of rod outer segment membranes in vitro without significantly inhibiting the biosynthesis or intracellular transport of opsin. Here we report that intraocular injection of tunicamycin produces a photoreceptor-specific degeneration characterized by progressive shortening of rod outer segment, decreased membrane assembly, and eventual photoreceptor cell death.

Glycinergic neurons in the human retina

Neurotransmitter-specific properties of glycinergic neurons in the human retina were studied using 11 pairs of eyes from donors ranging from 2 1/2 to 54 years in age. A mean endogenous level of 10.3 nmoles glycine per mg protein was measured by amino acid analysis in retinas isolated within 1 hour postmortem. When retinas were incubated with 3H-glycine (2 microM) and processed for autoradiography, label was found associated with neurons whose somata reside within the inner nuclear layer. Some heavily labeled neurons located at the vitread border of the inner nuclear layer were identified as amacrine cells based on ultrastructural verification of the conventional synaptic contacts made by their processes in distal regions of the inner plexiform layer. In proximal regions of the inner plexiform layer, dendrites of glycine-accumulating amacrine cells were postsynaptic to both ribbon and conventional synaptic contacts, suggesting input from bipolar and other, nonglycinergic amacrine cells. Their density (30 +/- 11 S.D. cells/mm linear retinal expanse) tended to be greater toward the central fundus. A second population of lightly labeled, probable bipolar cells was present in the middle of the inner nuclear layer; the density of this second set of glycine-accumulating cells approximated that of the heavily labeled population from the fovea, centrally, to the ora serrata, peripherally. Release of either accumulated or endogenous glycine was elicited by K+-depolarization in a Ca2+-dependent manner. Tissue fragments exposed for 6 minutes to normal medium, 40 mM K+-substituted medium, or K+-substituted medium with Co2+, release endogenous glycine into each bathing solution in average amounts of 0.6, 2.6, and 0.7 nmoles per mg protein, respectively. Together these data strongly implicate glycine as a neurotransmitter in the human retina.

Inositol incorporation into phosphoinositides in retinal horizontal cells of Xenopus laevis: enhancement by acetylcholine, inhibition by glycine

The absorption of light by photoreceptor cells leads to an increased incorporation of [2-3H]inositol into phosphoinositides of horizontal cells in the retina of Xenopus laevis in vitro. We have identified several retinal neurotransmitters that are involved in regulating this response. Incubation with glycine, the neurotransmitter of an interplexiform cell that has direct synaptic input onto horizontal cells, abolishes the light effect. This inhibition is reversed by preincubation with strychnine. Acetylcholine added to the culture medium enhances the incorporation of [2-3H]inositol into phosphoinositides in horizontal cells when retinas are incubated in the dark. This effect is inhibited by preincubation with atropine. However, atropine alone does not inhibit the light-enhanced incorporation of [2-3H]inositol into phosphoinositides in the retina. gamma-Aminobutyric acid, the neurotransmitter of retinal horizontal cells in X. laevis, as well as dopamine and norepinephrine, have no effect on the incorporation of [2-3H]inositol into phosphoinositides. These studies demonstrate that the light-enhanced incorporation of [2-3H]inositol into phosphoinositides of retinal horizontal cells is regulated by specific neurotransmitters, and that there are probably several synaptic inputs into horizontal cells which control this process.

Glycoprotein synthesis in the human retina: localization of the lipid intermediate pathway

Retinal tissue from human eye donors was incubated with [3H]-labeled mannose, glucosamine, fucose, and leucine in the presence and absence of tunicamycin, a selective inhibitor of dolichol-dependent glycoprotein biosynthesis. The incorporation of labeled substrates was examined by quantitative light microscopic autoradiography and biochemical methods. [3H]-Mannose and [3H]-fucose were predominantly incorporated into the photoreceptor layer, while [3H]-glucosamine and [3H]-leucine labeled the entire retinal expanse. Tunicamycin caused a marked and selective reduction in the incorporation of [3H]-mannose and [3H]-glucosamine in the photoreceptor layer (especially the inner segments) without affecting the relative distribution of labeled products derived from [3H]-fucose or [3H]-leucine. All three [3H]-labeled sugars were incorporated preferentially into rod inner segments, relative to cones. Dual-label experiments with [3H]-sugars and [14C]-leucine revealed that tunicamycin selectively inhibited the incorporation of [3H]-mannose, but not the other substrates, into total retinal TCA-precipitable material. This inhibition was not due to decreased mannose uptake by retinal cells. Detergent-solubilized retinas were analyzed by polyacrylamide gel electrophoresis and fluorography. The results indicated that each labeled substrate was incorporated into a variety of glyoproteins, including a component having the electrophoretic mobility of opsin. These results suggest that the lipid intermediate pathway of glycoprotein synthesis is localized preferentially to the photoreceptor layer of the retina, and may subserve the biosynthesis of rod cell glycoproteins (e.g. opsin).

Autoradiographic identification of muscarinic receptors in human iris smooth muscle

We have used the specific, irreversible muscarinic ligand [3H]-propylbenzilylcholine mustard to localize putative muscarinic cholinergic receptors in the smooth muscle tissue of the human iris. Analysis of autoradiograms from labeled irides reveals high grain densities over the iris sphincter muscle, consistent with the well-known pharmacology of this muscle. In addition, a smaller but significant population of muscarinic binding sites was seen in the iris dilator muscle as well. Grain densities in both muscles are substantially reduced in control tissue treated with relatively high concentrations of the muscarinic antagonists quinuclidinyl benzilate (QNB) and atropine. This is, to our knowledge, the first report of autoradiographic localization of putative muscarinic receptors in the human iris.

Differential sensitivity of protein synthesis in human retina to a phosphodiesterase inhibitor and cyclic nucleotides

When human retinas are cultured in the presence of various phosphodiesterase (PDE) inhibitors or cyclic nucleotide analogues, rod photoreceptors undergo degenerative changes followed by cell death within 8 hours of incubation, whereas cone photoreceptors and other retinal cells are affected minimally. In the present study we found that the PDE inhibitor, isobutylmethylxanthine (IBMX), as well as the dibutyryl analogues of cGMP and cAMP, inhibited protein synthesis in short-term cultures of human retinas under conditions where uptake of exogenously supplied 3H-leucine was not diminished (relative to controls). The results of an autoradiographic analysis suggested that inhibition of protein synthesis by these drugs occurred to a greater extent in rod photoreceptors than in cones or other retinal cells, and that this phenomenon happened prior to the onset of any morphological changes. When retinas incubated in IBMX for 4 hours were returned to control medium for an additional incubation, rods recovered their ability to synthesize proteins and cell viability was maintained. The results of polyacrylamide gel electrophoresis indicated that IBMX caused a general inhibition of retinal protein synthesis rather than affecting the synthesis of specific retinal proteins. These observations are discussed with regard to possible mechanisms underlying rod-specific photoreceptor degeneration.

Metabolic studies on retinal tissue from a donor with a dominantly inherited chorioretinal degeneration resembling sectoral retinitis pigmentosa

Protein synthesis, glycosylation, RNA synthesis, and neurotransmitter uptake were monitored using biochemical and autoradiographic techniques following in vitro labeling of retinal tissue from a 79-year-old female with sectoral retinitis pigmentosa (RP). Comparisons were made between degenerate and non-degenerate regions of the RP retina, and normal retinal tissues from an age- and postmortem-matched donor. Autoradiographs of non-degenerate retina from the RP eye following 3H-uridine incubation revealed virtually identical silver grain density over nuclei in all retinal strata as compared to normal control retinas. In contrast, a photoreceptor-specific reduction in silver grain density in the non-degenerate RP retina was noted following 3H-leucine incubation. In the normal retina, rod photoreceptor labeling with 3H-mannose was always greater than cone photoreceptor labeling. This pattern of incorporation was reversed in the non-degenerate region of the RP retina where rod photoreceptor labeling was less pronounced than that observed for cone photoreceptors. In non-degenerate regions of the RP retina, a marked accumulation of 3H-GABA by the Müller's cells was observed. Few cells exhibited selective uptake of 3H-muscimol, a GABA analog, indicating that few GABAergic neurons remained in the degenerate retina. 3H-dopamine-accumulating cell terminals were observed in the usual positions in the non-degenerate RP retina. In the degenerate region of the RP retina, heavy and diffuse uptake of 3H-GABA and 3H-muscimol, respectively, into broad cellular processes were noted, whereas 3H-dopamine was accumulated by only a few punctate terminals.

Two populations of rod photoreceptors in the retina of Xenopus laevis identified with 3H-fucose autoradiography

The retina of Xenopus laevis contains two populations of rod photoreceptors that differ in their utilization of L-fucose. Following intraperitoneal injections with 3H-fucose, all rod and cone photoreceptors incorporate the label. One day after labeling, much of the radioactivity is associated with the photoreceptor outer segments. In rods, a band of radioactivity is initially located at the base of the outer segment. As the time interval between injection and recovery is extended, the band of radioactivity is progressively displaced toward the outer segment tip. When the autoradiography exposure times are reduced so that cone and most rod outer segments no longer appear labeled, a minor population of rod photoreceptors can be distinguished which remains heavily labeled. The outer segment of the principal rod is 29.8 +/- 0.6 micrometers long and 6.4 +/- 0.6 micrometers in diameter, whereas the outer segment of the minor rod is 19.7 +/- 3.4 micrometers long and 4.5 +/- 0.6 micrometers in diameter, the latter accounting for approximately 2-3% of the total rod photoreceptor population. The rate of 3H-band displacement is 2-fold greater in the minor rod outer segment than the renewal rate of the outer segment in the principal rod. The similarity in relative cell density and size of the minor rod suggests that this photoreceptor corresponds to the blue-sensitive rod of Xenopus recently described by Witkovsky et al. (1981a, b), Vision Res. 21, 863-867, 875-883).

Phosphoinositide metabolism in the retina: localization to horizontal cells and regulation by light and divalent cations

Isolated retinas from Xenopus laevis incorporated greater amounts of [3H]inositol and 32Pi into phosphoinositides when incubated in light than did control retinas incubated in the dark. Inositol was primarily incorporated into phosphatidylinositol (83-86%), while phosphate labeled the polyphosphoinositides (72-79%). The incorporation of radioactive glycerol, serine, choline, or ethanolamine into retinal lipids was unaffected by light. Following incubation with [3H]inositol, the cell type involved in the light response was identified by light and electron microscope autoradiography to be the horizontal cell. These results are consistent with a classic phosphatidylinositol effect in the retina. An interesting feature of this response is that the stimulus (light) is received in the photoreceptor cell and the effect is manifest in the horizontal cell.