Potassium-Stimulated Release of Radiolabelled Taurine and Glycine from the Isolated Rat Retina

Abstract: The release of preloaded [³H]glycine and [³H]taurine in response to a depolarising stimulus (12.5-50 m M KCl) has been studied in the superfused rat retina. High external potassium concentration immediately increased the spontaneous efflux of [3H]glycine, the effect of 50 m M K⁺ apparently being abolished by omitting calcium from the superfusing medium. In contrast, although high potassium concentrations increased the spontaneous emux of [³H]taurine from the superfused rat retina, this release was not evident until the depolarising stimulus was removed from the superfusing medium. The magnitude of this "late" release of [³H]taurine was dependent on external K⁺ concentrations, and appeared immediately after cessation of the stimulus irrespective of whether it was applied for 4, 8, or 12 min. Potassium (50 m M )-induced release of taurine appeared partially calcium-dependent, being significantly reduced (p < 0.01) but not abolished by replacing calcium with 1 mM EDTA in the superfusate. High-affinity uptake systems for both [³H]glycine and [³H]taurine were demonstrated in the rat retina in vitro ( K _m values, 1.67 μ M and 2.97 μ M ; V_max values, 19.3 and 23.1 nmol/g wet weight tissue/h, respectively). The results are discussed with respect to the possible neuro-transmitter roles of both amino acids in the rat retina.     

THE UTILIZATION OF GLUTAMINE BY THE RETINA: AN AUTORADIOGRAPHIC AND METABOLIC STUDY

The cells able to accumulate exogenously applied [³H]glutamine in rat, cat, frog, pigeon and guinea pig retinas have been located by autoradiography, and the fate of the labelled glutamine, as regards its incorporation into aspartic, glutamic and γ-amino-butyric acids, followed for 60min. The results support the notion of glutamine as a precursor of transmitter amino acids in some neurones. In particular, it would appear to be a source of a relatively stable pool of GABA which may be located, with species variation, in amacrine or ganglion cells. In the pigeon retina a glutamate pool incorporates and retains a major percentage of the label, and perikarya in the middle of the inner nuclear layer of the tissue are predominantly labelled.     

FACTORS AFFECTING THE SPONTANEOUS RELEASE OF [3H] γ-AMINOBUTYRIC ACID FROM THE FROG RETINA IN VITRO

Abstract— The spontaneous efflux of [³H]GABA and its metabolites from the frog retina has been studied. The efflux of radioactivity was multiphasic in the presence or absence of amino-oxyacetic acid (AOAA), an inhibitor of GABA metabolism, and was not affected by light or dark adaptation.
Strong retention of radioactivity was evident in the presence of AOAA, about 90% of the label remaining in the tissue after 4 h superfusion. Under these conditions, increases in the rate of release of radioactivity were evoked by electrical stimulation, 40 m m -potassium. unlabelled GABA (5 m m ), ouabain (5 × 10⁻⁵ m ) and the absence of calcium. The amount of [³H]GABA released by electrical stimulation was not markedly calcium dependent, whereas the response to 40 m m -potassium was reduced by 96% in the absence of calcium.     

THE FORMATION OF GLUTAMATE, ASPARTATE AND GABA IN THE RAT RETINA; GLUCOSE AND GLUTAMINE AS PRECURSORS

Abstract— The distribution of the neuroactive amino acids taurine, GABA, glycine, glutamate and aspartate, together with glutamine, have been studied in the rat retina. Peak levels of taurine were found in photoreceptor cells and of GABA and glycine in a retinal fraction enriched in amacrine cells and, synaptic terminals. In vitro , GABA formation from [³H]glutamine and [¹⁴C]glucose was also most prominent in this fraction; at 500 μ m [³H]glutamine was the better precursor.
Observations on metabolism in the photoreceptor cell layer of the tissue suggest an active turnover of glutamate, aspartate and GABA, and show that glutamine may serve as an alternative substrate to glucose here, perhaps via the GABA bypath.     

Stimulatory effect of taurine on Ca-uptake by disc membranes from photoreceptor cell outer segments

Effects of taurine and related compounds on Ca-uptake by the disc membranes prepared from dark-adapted frog retina were studied. Taurine stimulated ATP-dependent Ca-uptake and the turnover of 45Ca in the disc membranes without affecting basal activity, but it was not observed with the synaptic plasma membranes from rat brain. The stimulatory effect appears to be specific to taurine, since cysteine sulfinic acid, hypotaurine, isethionic acid, β-alanine and γ-aminobutyric acid (GABA) did not stimulate Ca-uptake. The maximal activation, observed at about 30 mM taurine, was about 3 fold, and the Km value for taurine was 10 mM. These results might suggest that taurine modifies translocation of Ca ion in the rod outer segment.     

High-affinity taurine uptake in developing retina

A specific system for taurine transport is present at the early stages of development in both chick and rat retinas. The results obtained with taurine analogs indicate a high degree of specificity of taurine uptake. Two transport systems were detected for the adult rat retina: a high-affinity (K _m 21 M) and a low-affinity transport system (K _m 312 M). On the other hand, in the adult chick retina, only a low-affinity transport system (K _m 580 M) could be detected. Nevertheless, embryo chick retina accumulated [³H]taurine by two different kinetic mechanisms withK _m s of 242 M and 21 M for the low- and high-affinity processes, respectively. Taurine uptake systems were absolutely Na⁺ dependent. The sodium-dependence curve for taurine uptake was sigmoid. These mechanisms appear not to be mediated by a Na⁺ cotransport system. In spite of the differences observed in taurine uptake in both species, in each of them it closely parallels the changes brought about by the morphological and functional maturation of the retina.     

UPTAKE, RELEASE AND HOMO- AND HETERO-EXCHANGE DIFFUSIONS OF INHIBITORY AMINO ACIDS IN GUINEA-PIG CEREBELLAR SLICES

Abstract— GABA, taurine and β-alanine are taken up by guinea-pig cerebellar slices by both the high-and low-affinity uptake processes, whereas glycine is taken up only by the low-affinity process. A considerable amount of labelled GABA loaded in the slice is released by unlabelled external GABA and a minute amount is released by external β-alanine, glycine and taurine. External glycine and β-alanine releases labelled glycine loaded in the slice. Labelled taurine loaded is effectively released by external taurine and β-alanine, while labelled β-alanine loaded is released only by external β-alanine.
It is suggested that hetero-exchanges which are one-directional in some cases also take place between the amino acids in addition to homo-exchanges. Therefore, high-affinity uptake processes observed with GABA and taurine could be the result of the homo-exchange diffusions, while that of β-alanine could be due to either the homo-exchange or the hetero-exchange diffusions or both.
K⁺'-evoked releases of GABA and to a lesser extent, taurine are partially dependent upon the presence of Ca⁺ in the superfusion media, whereas that of glycine and probably that of β-alanine, are not, K⁺ -evoked releases of labelled GABA and taurine are larger when loaded by their high-affinity uptake systems than by their low-affinity uptake processes. The reverse is the case with labelled glycine and β-alanine. These results do not rule out the possibility that taurine might act as a neurotransmitter in the cerebellum.     

EFFECTS OF GLUTAMATE AND OTHER AMINO ACIDS ON THE RETINA1

The application of unlabelled glutamate to the isolated chicken retina charged with [¹⁴C]glutamate caused an increase in the tissue transparency and a release of the label into the superfusion fluid. The processes causing the change in transparency were‘desensitized’by a prolonged application of unlabelled glutamate, whereas the release of the labelled amino acid was relatively unaffected. Mg²⁺ tended to depress the change in transparency caused by stimulation with unlabelled glutamate but had little effect on the release of labelled glutamate from the retina. The effect of a Ca²⁺-free superfusion fluid on the transparency and release of glutamate varied from retina to retina. Aspartate (in higher concentrations) elicited a change in transparency and release of the label in a manner similar to that of glutamate. Glutamine caused a change in transparency accompanied by a release of labelled glutamine and in some experiments the release of a small amount of labelled glutamate. Homocysteic acid elicited marked changes in transparency but no release of labelled glutamate. Pyroglutamate depressed both the change in transparency and the release of labelled glutamate caused by the unlabelled amino acid. Gamma-aminobutyric acid and glycine had no effect on the transparency of the tissue or on the release of amino acids. We have discussed the possibility that a release of glutamate from the intracellular compartment into the extracellular space is involved in the mechanism of spreading depression.     

ÈVIDENCE FOR THE COMPARTMENTATION OF GLUTAMATE METABOLISM IN ISOLATED RAT RETINA

—Glucose is a major precursor of glutamate and related amino acids in the retina of adult rats. ¹⁴C from labelled glucose appears to gain access to a large glutamate pool, and the resulting specific activity of glutamate labelled from glucose is always higher than that of glutamine or the other amino acids. Radioactive acetate appeared to label a small glutamate pool. The specific activity of glutamine labelled from acetate relative to that of glutamate was always greater than 1.0. Other precursors of the small glutamate pool were found to include glutamate, aspartate, GABA, serine, leucine and sodium bicarbonate. The level of radioactivity present in retinae incubated with [U-¹⁴C]glucose or [1-¹⁴C]sodium acetate was reduced in the presence of 10^?5m -ouabain. Under these conditions, the relative specific activity of glutamine labelled from [1-¹⁴C]sodium acetate was lowered, but it was raised when [U-¹⁴C]glucose was used as substrate. Ouabain also considerably reduced the synthesis of GABA from [1-¹⁴C]sodium acetate. In all cases ouabain caused a fall in the tissue levels of the amino acids. Aminooxyacetic acid (10^?4m ) almost completely abolished the labelling of GABA from both [U-¹⁴C]glucose and [1-¹⁴C]sodium acetate, while the RSA of glutamine labelled from the latter substrate was significantly increased. Aminooxyacetic acid raised the tissue concentration of glutamate, but caused a fall in the tissue concentrations of glutamine, aspartate and GABA. The results suggest that there are separate compartments for the metabolism of glutamate in retina and that these can be modified in different ways by different drugs.     

A COMPARISON OF THE AXONAL TRANSPORT OF TAURINE AND PROTEINS IN THE GOLDFISH VISUAL SYSTEM

Abstract— Radioactive cystathionine, a metabolic precursor of taurine, was injected into the right eye of goldfish. At various times after injection the retina and both optic tecta were extracted with trichloroacetic acid (TCA) and the amount and nature of the radioactivity was determined. Radioactive taurine and inorganic sulfate were present in the TCA-soluble extract of retina and radioactive taurine and a small amount of inorganic sulfate was found in the contralateral optic tectum. That taurine is migrating intraaxonally and is not diffusing in extraaxonal spaces is suggested from experiments in which the migration of taurine was compared with that of [¹⁴C]mannitol, used here as a marker of extracellular diffusion. In the time studied (up to 15 h) mannitol did not migrate to the tectum, whereas taurine was detectable in the tectum as early as 8 h after injection. Since intra-axonal diffusion of amino acids and other small molecules in this system has been ruled out, it is likely that taurine is being transported axonally. The axonal transport of taurine was found to be similar to the fast component of protein transport because: (1) their rates of transport are similar, (2) the transport of both is blocked by the protein synthesis inhibitor cycloheximide, (3) vinblastine, which disrupts neurotubules, appears to have similar effects on both protein and taurine transport, and (4) both rapidly transported proteins and taurine remain mostly intra-axonal once they have been transported to the tectum. Taurine and proteins differ in that rapidly transported proteins are primarily paniculate in nature and localized to a large extent in nerve endings, while taurine is primarily in a soluble fraction and is present in nerve endings only in trace amounts. We suggest that taurine may be loosely linked to a newly synthesized protein in the soma and is then transported along with that protein on a similar conveying mechanism in the axoplasm.     

Potassium modulation of taurine transport across the frog retinal pigment epithelium

Net taurine transport across the frog retinal pigment epithelium-choroid was measured as a function of extracellular potassium concentration, [K+]o. The net rate of retina-to-choroid transport increased monotonically as [K+]o increased from 0.2 mM to 2 mM on the apical (neural retinal) side of the tissue. No further increase was observed when [k+]o was elevated to 5 mM. The [K+]o changes that modulate taurine transport approximate the light-induced [K+]o changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium. The taurine-potassium interaction was studied by using rubidium as a substitute for potassium and measuring active rubidium transport as a function of extracellular taurine concentration. An increase in apical taurine concentration, from 0.2 mM to 2 mM, produced a threefold increase in active rubidium transport, retina to choroid. Net taurine transport can also be altered by relatively large, 55 mM, changes in [Na+]o. Apical ouabain, 10(-4) M, inhibited active taurine, rubidium, and potassium transport; in the case of taurine, this inhibition is most likely due to a decrease in the sodium electrochemical gradient. In sum, these results suggest that the apical membrane contains a taurine, sodium co-transport mechanism whose rate is modulated, indirectly, through the sodium pump. This pump has previously been shown to be electrogenic and located on the apical membrane, and its rate is modulated, indirectly, by the taurine co-transport mechanism.     

The effects of theβ2-agonist drug clenbuterol on taurine levels in heart and other tissues in the rat

Summary The administration of a single subcutaneous dose of clenbuterol to rats altered the level of taurine in certain tissues. Taurine levels in cardiac tissue were significantly decreased 3 h after the administration of 250g/kg of clenbuterol and remained significantly depressed at 12h post-dose only returning to control values by 24h. The level of taurine in the liver increased 3 h after clenbuterol administration but was lower than the control value at 24 h post dose. Lung taurine levels were significantly lower than the control value at 12 hr post dose and remained depressed until 24h post dose. Clenbuterol caused a significant increase in taurine levels in serum and muscle at 3 and 6 hr postdosing respectively but not at other time points. Serum creatine kinase (CK), activity was slightly but significantly raised at the 12 and 24 h time point.The effects of clenbuterol on tissue taurine content were not dose-dependent over the range studied (63–500g/kg). However taurine levels in the lung were significantly reduced at all doses and in the heart were significantly lower in the treated groups at all except the lowest dose, 12h post dosing. Liver taurine levels were significantly increased at the highest dose of 500g/kg.The reduction of taurine concentrations in the heart, caused by clenbuterol, is of concern as taurine has been shown to have protective properties in many tissues especially the heart.     

Modulation of taurine uptake in the goldfish retina and axonal transport to the tectum¶Effect of crushing the optic nerve or axotomy

Summary. Although there are a great number of studies concerning the uptake of taurine in several tissues, the regulation of taurine transport has not been studied in the retina after lesioning the optic nerve. In the present study, isolated retinal cells of the goldfish retina were used either immediatly after cell suspension or in culture. The high-affinity transport system of [³H]taurine in these cells was sodium-, temperature- and energy-dependent, and was inhibited by hypotaurine and β-alanine, but not by γ-aminobutyric acid. There was a decrease in the maximal velocity (V_max) without modifications in the substrate affinity (K_m) after optic axotomy. These changes were mantained for up to 15 days after the lesion. The results might be the summation of mechanisms for providing extracellular taurine to be taken up by other retinal cells or eye structures, or regulation by the substrate taurine, which increases after lesioning the optic nerve. The in vivo accumulation of [³H]taurine in the retina after intraocular injection of [³H]taurine was affected by crushing the optic nerve or by axotomy. A progressive retinal decrease in taurine transport was observed after crushing the optic nerve, starting at 7 hours after surgery on the nerve. The uptake of [³H]taurine by the tectum was compensated in the animals that were subjected to crushing of the optic nerve, since the concentration of [³H]taurine was only different from the control value 24 hours after the lesion, indicating an efficient transport by the remaining axons. On the contrary, the low levels of [³H]taurine in the tectum after axotomy might be an index of the non-axonal origin of taurine in the tectum. Axonal transport was illustrated by the differential presence of [³H]taurine in the intact or crushed optic nerve. The uptake of [³H]taurine into retinal cells in culture in the absence or in the presence of taurine might indicate the existence of an adaptive regulation of taurine transport in this tissue, however taurine transport probably differentially occurs in specific populations of retinal cells. The use of a purified preparation of cells might be useful for future studies on the modulation of taurine transport by taurine in the retina and its role during regeneration. Received June 11, 1999/Accepted August 31, 1999     

Taurine and its Trophic Effects in the Retina Lucimey Lima

The sulphur amino acid taurine possesses variable functions during development and regeneration of the central nervous system. The retina synthesize and uptake taurine, which is the amino acid present in the highest concentration in this tissue. Deficiency of taurine alters the structure and the function of the cerebral and cerebelar cortex, as well as the retina. Taurine increases outgrowth of postcrush goldfish retina in culture, partially by elevating calcium influx, and also by the modulation of protein phosphorylation. Its concentration increases in the retina after the lesion of the optic nerve, and the intraocular injection of it, between the crush and the explantation, stimulates the outgrowth of neurites. Taken together, although there are a great number of unresolved questions on the mechanisms of action of this amino acid as atrophic substance, the results support the role of taurine during regeneration of the optic nerve.     

Dietary Taurine Supplementation Prevents Glial Alterations in Retina of Diabetic Rats

The preventive effect of dietary taurine supplementation on glial alterations in retina of streptozotocin-induced diabetic rats was examined in this study. Blood glucose content, content of taurine, glutamate and <gamma>-amino butyric acid (GABA) and expression of glial fibrillary acid protein (GFAP), vascular endothelial growth factor (VEGF), glutamate transporter (GLAST), glutamine synthetase (GS) and glutamate decarboxylase (GAD) in retina were determined in diabetic rats fed without or with 5% taurine in a controlled trial lasting 12 weeks, with normal rats fed without or with 5% taurine served as controls. Dietary taurine supplementation could not lower glucose concentration in blood (P > 0.05), but caused an elevation of taurine content and a decline in levels of glutamate and GABA in retina of diabetic rats (P < 0.05). The content of GABA in normal control group was not altered by taurine supplementation. With supplementation of taurine in diet, lower expression of GFAP and VEGF while higher expression of GLAST, GS and GAD in retina of diabetic rats were determinated by RT-PCR, Western-blotting and immunofluorescence (P < 0.05). GFAP, VEGF, GLAST, GS and GAD expressions in normal controls were not altered by taurine treatment. This may have prospective implications of using taurine to treat complications in diabetic retinopathy.     

Neuron-specific enolase-like immunoreactivity in the vertebrate retina: selective labelling of Müller cells in Anura

Summary Neuron-specific enolase (NSE) immunocytochemistry was carried out in retinae of goldfish, axolotl, clawed frog, cane toad, lizard, chick, guinea-pig, rabbit, rat, cat and human. With the exception of Anura, strong immunoreactivity was seen in the large ganglion, amacrine cells and horizontal cells of the retina in all of the other species. Photoreceptors were found to be labelled in the rat and human retina and only one cone type in rabbit. Photoreceptor pedicles and ellipsoids were stained in the goldfish and the somata and inner segments of some photoreceptors in axolotl. In the axolotl retina, besides neurons, Müller cells (MCs) were also immunolabelled. In the retina of the cane toad and the clawed frog MCs were the only stained elements. Similarly in other parts of the central nervous system of the cane toad, glial elements of the optic tectum and spinal cord were immunoreactive. In contrast, in the peripheral nervous system, neurons of the 1st sympathetic ganglion and the 2nd dorsal root ganglion were labelled. In double-labelling experiments, glial fibrillary acidic protein and NSE showed colocalisation both in the glial elements of the optic tectum and spinal cord and in MCs of the retina of the cane toad.On leave of absence from Department of Zoology, Attila József University, Szeged, Hungary     

INCORPORATION OF 3H-LABELLED LEUCINE INTO THE PROTEIN FRACTION IN THE RETINA OF THE HONEYBEE DRONE

Abstract— Protein synthesis in the retina of the honey-bee drone was studied by incubating head slices in labelled leucine and measuring the TCA insoluble radioactivity. It was found that the protein-bound radioactivity in illuminated retinas was half of that in dark-adapted ones. This ratio was not affected by pre-treatment with puromycin. It was therefore concluded that, in the drone, the main influence of illumination is to increase the rate of breakdown of proteins.
Relatively high concentrations of labelled protein were found in dark-adapted retinas when the retinula cells were hyperpolarized by bathing the preparation in a sodiumfree medium; low concentrations were found when retinula cells were depolarized by increasing the extracellular potassium concentration. These findings suggest that protein metabolism of the retina is influenced by the membrane potential of retinula cells.     

Taurine activation of a bicarbonate-dependent,ATP-supported calcium uptake in frog rod outer segments

Isolated frog rod outer segments (ROS) with a leaky plasma membrane showed a bicarbonate-dependent, ATP-activated⁴⁵Ca accumulation. This calcium uptake requires magnesium and is specific for ATP; Other nucleotides, ITP, GTP, UTP and the non-hydrolysable analogue of ATP --methylene ATP did not substitute for ATP.⁴⁵Ca accumulation was inhibited by mersalyl, ethylmaleimide, ruthenium red, oligomycin and dicyclohexylcarbodiimide and was unaffected by ouabain. Addition of taurine to the incubation medium enhanced⁴⁵Ca uptake in a concentration-dependent manner; increases of more than 100% being produced by 25 mM taurine. The taurine-induced stimulation of⁴⁵Ca uptake was also sensitive to the tested inhibitors. The effect of taurine was only exerted on the bicarbonate-dependent, ATP-activated⁴⁵Ca uptake. Calcium accumulation observed in the absence of ATP or in a tris-buffered medium was unaffected by taurine. Other amino acids, glycine, GABA, -alanine, glutamic acid and the taurine analogue guanidinoethyl-sulfonate did not stimulate⁴⁵Ca uptake. These results suggest that taurine is affecting a Mg-ATPase activity responsible for calcium accumulation in frog ROS.     

Neuron-specific enolase-like immunoreactivity in the vertebrate retina: selective labelling of Müller cells in Anura.

Neuron-specific enolase (NSE) immunocytochemistry was carried out in retinae of goldfish, axolotl, clawed frog, cane toad, lizard, chick, guinea-pig, rabbit, rat, cat and human. With the exception of Anura, strong immunoreactivity was seen in the large ganglion, amacrine cells and horizontal cells of the retina in all of the other species. Photoreceptors were found to be labelled in the rat and human retina and only one cone type in rabbit. Photoreceptor pedicles and ellipsoids were stained in the goldfish and the somata and inner segments of some photoreceptors in axolotl. In the axolotl retina, besides neurons, Müller cells (MCs) were also immunolabelled. In the retina of the cane toad and the clawed frog MCs were the only stained elements. Similarly in other parts of the central nervous system of the cane toad, glial elements of the optic tectum and spinal cord were immunoreactive. In contrast, in the peripheral nervous system, neurons of the 1st sympathetic ganglion and the 2nd dorsal root ganglion were labelled. In double-labelling experiments, glial fibrillary acidic protein and NSE showed colocalisation both in the glial elements of the optic tectum and spinal cord and in MCs of the retina of the cane toad.     

Taurine and Zinc Modulate Outgrowth from Goldfish Retinal Explants

Taurine and zinc, highly concentrated in the retina, possess similar properties in this structure, such as neuro-protection, membrane stabilization, influencing regeneration, and modulating development, maybe by acting in parallel or as interacting agents. We previously demonstrated that there are some correlations between taurine and zinc levels in hippocampus, dentate gyrus and retina of the developing rat. In the present study we evaluate the possible effects of taurine and zinc on outgrowth from goldfish retinal explants. The optic nerve was crushed 10 days before plating and culturing retinal explants in Leibovitz medium with 10% fetal calf serum and gentamicin. Neurites were measured with SigmaScanPro after 5 days in culture. Taurine (HPLC) and zinc (ICP) concentrations were determined in the retina between 1 and 180 days after crushing the optic nerve. Zinc sulfate (0.01–100 μM), N,N, N′,N′-tetrakis (pyridylmethyl) ethylenediamine (TPEN, 0.1–5 nM) and diethylenetriamine penta-acetic acid (DTPA, 10–300 μM), intracellular and extracellular zinc chelators, respectively, were added to the medium. TPEN was also injected intraocular (0.1 nM). Combinations of them were added with taurine (1–16 mM). Taurine concentrations were elevated in the retina 72 h after the crush, but were normalized by 180 days, those of zinc increased at 24 h, preceding the increase of taurine. The axonal transport of [³H]taurine from the optic tectum to the retina was not affected in fish with or without crush of the optic nerve at early periods after the injection, indicating an increase of it post-lesion. Zinc sulfate produced a bell-shaped concentration dependency on in vitro outgrowth, with stimulation at 0.05 μM, and inhibition at higher levels, also increased the effect of 4 mM taurine at 0.02 μM, but diminished it at higher concentrations in the medium. TPEN decreased outgrowth at 1 nM, but not at 0.5 nM, although the simultaneous presence of 4 mM taurine and 0.5 nM TPEN decreased outgrowth respecting the stimulation by taurine alone. The intraocular administration of TPEN decreased outgrowth in vitro, an effect counteracted by the addition of 4 mM taurine to the culture medium. DTPA decreased outgrowth from 10 μM in the medium. The present results indicate that an optimal zinc concentration is necessary for outgrowth of goldfish retinal explants and that, in zinc deficient retina, taurine could stimulate outgrowth. In addition, the observations of variations in tissue concentrations and of the effects of intraocular administration of TPEN indicate that these effects could occur in vivo. Special issue dedicated to Dr. Simo S. Oja