Neurotransmitter properties of the newborn human retina

Human retinal tissue from a newborn was examined autoradiographically for the presence of high-affinity uptake and localization of the following putative neurotransmitters: dopamine, glycine, GABA, aspartate, and glutamate. In addition, the dopamine content of this newborn retina was measured by high pressure liquid chromatography. Our study reveals that specific uptake mechanisms for 3H-glycine, 3H-dopamine, and 3H-GABA are present at birth. However, the number and distribution of cells labeled with each of these 3H-transmitters are not identical to those observed in adult human retinas. Furthermore, the amount of endogenous dopamine in the newborn retina is approximately 1/20 the adult level. Photoreceptor-specific uptake of 3H-glutamate and 3H-aspartate are not observed. These findings indicate that, while some neurotransmitter-specific properties are present at birth, significant maturation of neurotransmitter systems occurs postnatally.

Simulation of the conditions of gyrate atrophy in vitro suppresses retinal protein synthesis

The effect of ornithine on retinal protein synthesis was evaluated using isolated baboon retinas maintained in vitro. When ornithine was present in the culture medium at a concentration equivalent to the plasma level of ornithine in gyrate atrophy patients (1 mM), the incorporation of 3H-leucine was reduced to 70 per cent of control levels. Progressively higher concentrations of ornithine resulted in a further suppression of protein synthesis. Soluble radioactivities after short incubation times from control and ornithine cultured retina were similar indicating no effect of ornithine on uptake of 3H-leucine. Grain counts in autoradiographs of control and ornithine-treated retinas revealed a suppression of incorporation in all retinal strata. These results suggest that elevated ornithine levels may suppress ongoing protein synthesis in the retina.

A dominantly inherited chorioretinal degeneration resembling sectoral retinitis pigmentosa

A light and electron microscopic study of an eye from a 79-year-old woman diagnosed as having sector retinitis pigmentosa is presented. Prominent bone spicule pigmentation was present bilaterally in the nasal quadrant. Retinal structure in the central fundus around the fovea and extending 2-3 mm peripherally was near normal with all photoreceptors and other retinal neurons present. Extensive degeneration of the retina occurred as one proceeded toward the peripheral regions from 3-5 mm from the fovea in all quadrants. Changes were evident within the entire choroid and were most severe in regions where retinal degeneration was most pronounced. It is likely that the extensive degenerative changes present in the retina were secondary effects that follow alterations in the choroidal blood supply in individuals affected with this disorder.

Retinal protein synthesis in relationship to environmental lighting

A series of in vivo and in vitro experiments using Xenopus laevis juvenile toads was conducted to probe the relationship between environmental lighting and protein synthesis in the retina. Autoradiographic and biochemical analyses indicated that measurable changes in protein synthesis did not occur during a normal diurnal cycle when animals were conditioned to 12 hr light followed by 12 hr darkness each day (LD). However, when retinas from animals maintained in continuous darkness (DD) for 3 days were incubated with 3H-leucine, there was a 40% reduction in the specific radioactivity of total retinal proteins compared with retinas from animals maintained in continuous light (LL) for 3 days or on the LD cycle. Retinas from DD animals injected with 3H-leucine showed a 48% reduction in protein synthesis compared with retinas of LL animals. In autoradiographs of retinas from in vivo or in vitro experiments, grain counts were 40% lower in the total retinas of the DD animals compared with retinas of LL animals. This reduction occurred throughout the entire retina and was not restricted to any specific cell type. There was also a 35% reduction in the rate of radioactive band displacement in the rod outer segments of DD animals, although the percent of 3H-leucine incorporated into opsin relative to total retinal protein was the same for both groups. We conclude from these studies that fluctuations in the rate of protein synthesis during the normal light-dark cycle are not detectable. However, major differences in protein synthesis are evident when animals are stressed with continuous darkness for several days. This effect is not restricted to any particular retinal layer but occurs throughout the entire retina. Moreover, prolonged darkness affects protein synthesis in extraocular tissues as well.U

Dopaminergic neurons in the human retina

The utilization of dopamine in the adult human retina was examined by using high-affinity uptake, localization, synthesis, and release as neurotransmitter-specific physiological probes. Autoradiographic and histochemical studies have shown that dopamine-accumulating and dopamine-containing cells of the human retina belong to a population of neurons whose somata are located in the proximal regional of the inner nuclear layer. Some of these are amacrine cells which are pre- and postsynaptic to other amacrine cells exclusively in the inner plexiform layer. However, evidence is presented which indicates the existence of interplexiform dopaminergic neurons which send processes to both plexiform layers of the retina. These neurons contain a high concentration of dopamine, take up 3H-dopamine by a hig-affinity mechanism, and release endogenous or accumulated dopamine by a Ca2+-dependent mechanism upon depolarization with high extracellular K+. An endogeneous level of about 20 pmoles dopamine per mg protein was measured in freshly isolated retina using high-pressure liquid chromatography with electrochemical detection. These results demonstrate that mechanisms for dopaminergic neurotransmission are present in the human retina.

Membrane addition to rod photoreceptor outer segments: light stimulates membrane assembly in the absence of increased membrane biosynthesis

The effect of light on the biosynthesis and the assembly of rod photoreceptor outer segment membranes was analyzed in vitro using retinas from Xenopus laevis. The number of open discs at the base of the outer segment was used as a morphologic index to evaluate relative differences in rates of membrane assembly. Assembly was stimulated in vitro by light, as evidenced by a greater number of open discs that accumulated in light-exposed retinas than that in retinas maintained under the same conditions but in darkness. Quantitative autoradiography indicated, however, that unlike membrane assembly, the incorporation of 3H-leucine or 3H-mannose into proteins by rods was nearly identical in light-stimulated and dark-maintained retinas. Likewise, the synthesis of opsin, a protein destined for the rod outer segment, was not affected by light. We conclude from these studies that the enhanced rate of outer segment membrane assembly that occurs during the early light phase of the diurnal cycle takes place in the absence of an increase in the rate of biosynthesis of opsin, the major outer segment membrane protein.

Regional effects of sodium aspartate and sodium glutamate on protein synthesis in the retina

Isolated retinas from Xenopus laevis tadpoles or juvenile frogs were incubated in Ringer's bicarbonate-glucose medium in which sodium chloride was replaced with equimolar amounts of either sodium aspartate (NaAsp) or sodium glutamate (NaGlu). At all concentrations tested (0.1 to 50 mM), both acidic amino acids caused a dramatic decrease in the incorporation of 3H-leucine into proteins in the inner retina. Higher concentrations of these acids (50 mM) caused an increase in the incorporation of the label by photoreceptors. Polyacrylamide gel disc electrophoresis of whole retinal proteins showed a selective increase in incorporation of the label into rhodopsin, a protein unique to rod photoreceptor cells. Quantitative autoradiographic and biochemical studies showed that NaAsp had no effect on the uptake of 3H-leucine by the retinal cells, suggesting that the effect is directly on protein synthesis. When retinas were removed from NaAsp treatment, protein synthetic capacity recovered. and the degree of recovery was related to the concentration of NaAsp in the preincubation medium. These studies demonstrate that acidic amino acids in concentration utilized in electrophysiologic studies to isolate the photoreceptor generated a-wave of the electroretinogram also have a profound effect on retinal protein synthesis. Whether these two actions are related remains to be determined.

Light stimulates the incorporation of inositol into phosphatidylinositol in the retina

The incorporation of [3H]inositol into phosphatidylinositol in retinas of Xenopus laevis tadpoles or young adults in short-term organ culture is stimulated by light, compared to retinas maintained under identical conditions in darkness. Over 95% of the label incorporated into lipid was in phosphatidylinositol, and none was incorporated into retinal proteins. The stimulation of incorporation was localized by autoradiography to the outer plexiform layer, a neurophil composed primarily of horizontal cell processes.

The emergence, localization, and maturation of neurotransmitter systems during development of the retina in Xenopus laevis: II. Glycine

The high-affinity uptake and release of glycine was studied in retinas of Xenopus laevis. In the toad and tadpole retina, 3H-glycine was accumulated by a population of cells located predominantly in the inner nuclear layer. When retinas preloaded with 3H-glycine were subjected to high K+-concentrations, these retinas released large amounts of 3H-glycine by a Ca++-dependent mechanism. The appearance and maturation of these putative glycinergic properties was followed during retinal development. Our results indicate that the high-affinity uptake of glycine first appears around stage 33/34 whereas K+-stimulated glycine release cannot be detected until stage 42.

The emergence, localization, and maturation of neurotransmitter systems during development of the retina in Xenopus laevis. III. Dopamine

The uptake, synthesis, and release of dopamine was studied in retinas of Xenopus laevis. In the tadpole and adult retina, 3H-dopamine is accumulated by cells located in the inner nuclear layer. Retinas preloaded with 3H-dopamine release this compound in response to high K+ concentrations in the medium. This release is probably Ca++-dependent as it is inhibited by Co++ in the medium. Adult retinas are also capable of synthesizing 3H-dopamine from 3H-tyrosine. The appearance and maturation of these dopaminergic properties were followed during retinal development. Our data indicate that synthesis of dopamine can first be detected as early as stage 35/36 whereas uptake of dopamine first occurs at stage 43. K+-stimulated release of preloaded 3H-dopamine from putative dopaminergic neurons is, however, not evident until stage 46. These results show that similar to the development of GABA-ergic and glycinergic properties, the uptake, synthesis, and release mechanisms for dopamine emerge at different stages during retinal differentiation in Xenopus Laevis.

Sensitivity of photoreceptors to elevated levels of cGMP in the human retina

When isolated human retinas were cultured in the presence of a phosphodiesterase inhibitor or dibutyryl cyclic guanosine monophosphate (dbcGMP), degenerative changes occurred which were proportional to the concentration of drug used and the period of exposure. Low concentrations of either drug did not alter retinal morphology as compared to controls. Higher concentrations provoked vesiculation of rod inner segments and rounding up of cones. Numerous pyknotic nuclei were noted in the outer nuclear layer of those preparations. Combining IBMX and dbcGMP in the same medium destroyed virtually every rod in the specimen within 8 hr of incubation. Under those conditions, cones remained structurally intact although somewhat rounded. In all treatments, cells of the innerretinal layers maintained normal morphology. Our results suggest that elevated levels of cGMP in the human retina can alter certain metabolic processes in photoreceptors, which leads to degenerative changes and cell death uniquely in rod photoreceptors.

Specification of retinotectal connexions during development of the toad Xenopus laevis

The specification of central connexions of retinal ganglion cells was studied in Xenopus laevis. In one series of experiments, the right eye primordium was rotated 180 degrees at embryonic stages 24-32. In the other series, the left eye was transplanted into the right orbit, and vice versa, with either 0 degrees or 180 degrees rotation. After metamorphosis the visual projections from the operated eye to the contralateral optic tectum were mapped electrophysiologically and compared with the normal retinotectal map. In all cases the visual projection map was rotated through the same angle as was indicated by the position of the choroidal fissure. The left eye exchanged into the right orbit retained its original axes and projected to the contralateral tectum. These results suggest that retinal ganglion cell connexions are specified before stage 24.

Specification of retinotectal connexions during development of the toad Xenopus laevis

The specification of central connexions of retinal ganglion cells was studied in Xenopus laevis. In one series of experiments, the right eye primordium was rotated 180° at embryonic stages 24–32. In the other series, the left eye was transplanted into the right orbit, and vice versa, with either 0° or 180° rotation. After metamorphosis the visual projections from the operated eye to the contralateral optic tectum were mapped electrophysiologically and compared with the normal retinotectal map. In all cases the visual projection map was rotated through the same angle as was indicated by the position of the choroidal fissure. The left eye exchanged into the right orbit retained its original axes and projected to the contralateral tectum. These results suggest that retinal ganglion cell connexions are specified before stage 24.

The emergence, localization and maturation of neurotransmitter systems during development of the retina in Xenopus laevis. I. Gamma aminobutyric acid

The high-affinity uptake, biosynthesis and release of GABA have been studied in the retina of Xenopus laevis. In the mature retina, [3H]-GABA is accumulated predominantly by horizontal cells. A second population of cells located in the inner nuclear layer (possibly a type of amacrine cell) also showed a specific GABA uptake. In addition, this retina contains significant activities of L-glutamic acid decarboxylase and also releases [3H]-GABA in response to increasing K+ concentrations in the medium. We have followed the appearance and maturation of these GABA-ergic properties during embryonic development of this retina. Our results indicate that these properties emerge in a precise temporal pattern during retinal differentiation: the specific neuronal uptake of GABA precedes GABA synthesis which is followed by K+-stimulated GABA release.

Turnover of mouse photoreceptor outer segments in constant light and darkness

Pigmented mice differ from frogs in that lighting regime has only a small effect on rod outer segment turnover. During 8 days in constant light or darkness, disc addition measured as total displacement of a radioactive band in rod outer segments of mice which has received injections of tritiated amino acids was never modified by more than 7% compared to cyclic light controls. Disc shedding continued in each lighting regime, and as judged from outer segment dimensions, disc shedding approximately balanced disc addition.

Membrane addition to photoreceptor outer segments: progressive reduction in the stimulatory effect of light with increased temperature

Previous studies have indicated that temperature and light regimens affect the rates of membrane addition to rod outer segments in amphibia. However, it is not known how membrane renewal responds to simultaneous alterations of these two environmental factors. In the present study, the rates of membrane addition have been assessed in adult Rana pipiens maintained in three lighting conditions (constant light, constant darkness, and cyclic light) at three different temperatures by following the displacement of a radioactive band in autoradiographs after 3H-leucine injection. At 16 degrees, 21 degrees, and 26 degrees C, constant light increases the rate of membrane addition in red rods above the rates which occur in darkness by 39%, 14%, and 9%, respectively, whereas in the green rods, the rates of increase were 41%, 22%, and 8%. This progressive reduction in the stimulatory effect of light suggest that at the higher temperatures, rod outer segment renewal is proceeding at near maximal rates and can no longer be further accelerated by light stimulation.

Photoreceptor outer segment development: light and dark regulate the rate of membrane addition and loss

The rate of membrane addition during outer segment development and the onset of membrane loss through shedding was evaluated for Xenopus laevis embryos reared at 23 degrees C in constant light (LL), constant darkness (DD), and cyclic light (12 hr L/12 hr D; LD). Embryos were placed under these lighting conditions 1 day following fertilization at open neural plate stages and were sampled each day for 9 days. Rod and cone outer segments were present in each treatment group on day 3 of development. At the end of 10 days of development, rod outer segment volumes were 708 +/- 195, 380 +/- 40, and 297 +/- 33 micrometer3 (mean +/- S.D.) for LL, LD, and DD treatment groups, respectively, whereas mean cone outer segment volumes were 57 +/- 8, 48 +/- 8, and 35 +/- 9 micrometer3 for these same treatment groups. The loss of outer segment material through shedding was assessed by monitoring the phagosome content of the pigment epithelium. Phagosomes were present in the pigment epithelium in DD embryos as early as 6 days of development, were present at day 5 only after the onset of the light cycle in LD embryos, and were rarely observed in LL embryos. The rate of outer segment membrane elaboration in rods (determined with 3H-leucine autoradiography) was virtually identical in LL and LD (100 and 98 micrometer3/day) but was greatly reduced in DD (70 micrometer3/day). These findings indicate that the relatively rapid rate of rod outer segment volume accumulation in LL embryos is due to a reduction in membrane loss through shedding and not to a higher rate of membrane addition as compared to animals reared in LD. In contrast, the reduced rate of rod outer segment development in DD embryos is the result of both a slower rate of membrane elaboration accompanied by the early onset and enhanced rate of membrane loss through shedding in darkness.