Immunocytochemical characterization of quisqualic acid- and N-methyl-D-aspartate-induced excitotoxicity in the retina of chicks

Transient expression of LIM-domain transcription factors is coincident with delayed maturation of photoreceptors in the chicken retina

In the retina of warm-blooded vertebrates, photoreceptors are specified many days before the onset of synaptogenesis and the expression of photopigments. The factors that regulate the maturation of photoreceptors in the developing retina remain unknown. We report here that photoreceptors transiently express LIM-domain transcription factors during the development of the chicken retina. We examined the differentiation of photoreceptors through the normal course of embryonic development and at the far periphery of the postnatal retina, where the differentiation of photoreceptors is slowed and persists across a spatial gradient. In the embryonic retina, we find visinin-positive photoreceptors that transiently express Islet2 and Lim3 starting at E8 and ending around E15, but persisting in far peripheral regions of the retina through the first 2 weeks of postnatal development. During early stages of photoreceptor maturation, there is coincident and transient expression of the LIM-domain factors with axonin1, a cell surface glycoprotein that is a member of the immunoglobulin superfamily. Coincident with the downregulation of Islet2 and Lim3, we find the upregulation of calbindin, red/green opsin, rhodopsin, and a synaptic marker in the outer plexiform layer (OPL; dystrophin). In the periphery of the postnatal retina, photoreceptors that express Islet2, Lim3, and axonin1 do not overlap with photoreceptors that express calbindin, red/green opsin, rhodopsin, and dystrophin. We propose that Islet2 and Lim3 may promote the expression of genes that are involved in the early stages of differentiation but may suppress the expression of genes that are required in the mature photoreceptors.

Bullwhip neurons in the retina regulate the size and shape of the eye

Bullwhip and mini-bullwhip cells are unconventional types of retinal neurons that utilize the neuropeptides glucagon, glucagon-like peptide 1 (GLP1) and substance P. These cells have been implicated in regulating the proliferation of neural progenitors in the circumferential marginal zone (CMZ) of the chicken retina. The purpose of this study was to investigate the roles of the bullwhip cells in regulating ocular size and shape. We found that intravitreal delivery of colchicine at postnatal day 7 destroys the vast majority (approximately 98%) of the bullwhip and mini-bullwhip cells and their peptidergic terminals that are concentrated in the CMZ near the equator of the eye. Interestingly, colchicine-treatment resulted in excessive ocular growth that involved the expansion of equatorial diameter, but not axial length. Intraocular injections of glucagon completely prevented the equatorial expansion that occurs with colchicine-treatment. In eyes with undamaged retinas, exogenous glucagon suppressed equatorial eye growth, whereas glucagon receptor antagonists caused excessive equatorial growth. Furthermore, visual stimuli that increase or decrease rates of ocular growth caused a down- or up-regulation, respectively, of the immediate early gene Egr1 in the bullwhip cells; indicating that the activity of the bullwhip cells is regulated by growth-guiding visual cues. We found that the glucagon receptor was expressed by cells in the fibrous and cartilaginous sclera in equatorial regions of the eye. Taken together, these findings suggest that glucagon peptide released from the terminals of the bullwhip and mini-bullwhip cells regulates the growth of the equatorial sclera in a vision-dependent manner. Although the bullwhip and mini-bullwhip cells are not abundant, less than 1000 cells per retina, their influence on the development of the eye is substantial and includes vision-guided ocular growth.

Muscarinic signaling influences the patterning and phenotype of cholinergic amacrine cells in the developing chick retina

Background

Many studies in the vertebrate retina have characterized the differentiation of amacrine cells as a homogenous class of neurons, but little is known about the genes and factors that regulate the development of distinct types of amacrine cells. Accordingly, the purpose of this study was to characterize the development of the cholinergic amacrine cells and identify factors that influence their development. Cholinergic amacrine cells in the embryonic chick retina were identified by using antibodies to choline acetyltransferase (ChAT).

Results

We found that as ChAT-immunoreactive cells differentiate they expressed the homeodomain transcription factors Pax6 and Islet1, and the cell-cycle inhibitor p27^kip1. As differentiation proceeds, type-II cholinergic cells, displaced to the ganglion cell layer, transiently expressed high levels of cellular retinoic acid binding protein (CRABP) and neurofilament, while type-I cells in the inner nuclear layer did not. Although there is a 1:1 ratio of type-I to type-II cells in vivo, in dissociated cell cultures the type-I cells (ChAT-positive and CRABP-negative) out-numbered the type-II cells (ChAT and CRABP-positive cells) by 2:1. The relative abundance of type-I to type-II cells was not influenced by Sonic Hedgehog (Shh), but was affected by compounds that act at muscarinic acetylcholine receptors. In addition, the abundance and mosaic patterning of type-II cholinergic amacrine cells is disrupted by interfering with muscarinic signaling.

Conclusion

We conclude that: (1) during development type-I and type-II cholinergic amacrine cells are not homotypic, (2) the phenotypic differences between these subtypes of cells is controlled by the local microenvironment, and (3) appropriate levels of muscarinic signaling between the cholinergic amacrine cells are required for proper mosaic patterning.

Development of bullwhip neurons in the embryonic chicken retina

We have recently described large, unipolar neurons (named bullwhip cells) that regulate the proliferation of progenitors in the circumferential marginal zone (CMZ) of the postnatal chicken retina (Fischer et al. [2005] J. Neurosci. 25:10157-10166; [2006] J. Comp. Neurol. 496:479-494). There are only about 240 bullwhip cells in the entire retina, and these cells are easily identified by their unique morphology and immunoreactivity for glucagon, glucagon-like peptide 1 (GLP1), and substance P. The purpose of this study was to elucidate the development of bullwhip cells in the embryonic chicken retina. By using bromodeoxyuridine birth dating, we found that the bullwhip cells are generated very early during retinal development, between E4 and E5. Glucagon peptide was first detected in bullwhip cells at about E10, whereas substance P was not detected in the bullwhip cells until E15. Although glucagon peptide is not present during early stages of retinal development, we detected mRNA for glucagon receptor beginning at E7 and mRNA for GLP1 receptor at E5 through E14. Morphological differentiation of the bullwhip cells begins at about E14 and is completed by E18. The bullwhip cells are greatly overproduced, and nearly 80% of these cells undergo apoptotic cell death during late stages of embryonic development. The bullwhip cells that survive are those that project an axon-like process directly toward the CMZ; the cells that project in an inappropriate direction fail to survive. We conclude that cells fated to become bullwhip neurons are generated long before they begin to differentiate and that their survival depends on the orientation of their primary neurite.

A muscarinic cholinergic antagonist and a dopamine agonist rapidly increase ZENK mRNA expression in the form-deprived chicken retina

Increases in the expression of the immediate early gene ZENK in the retina, measured by changes in the levels of mRNA and protein immunoreactivity, are amongst the most rapid responses so far measured to conditions that decrease the rate of eye growth in chickens. Our aim was to determine whether atropine, a muscarinic cholinergic antagonist, and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide, a dopamine agonist, which are known to block excessive eye growth, produce similar changes in ZENK expression. Form-deprivation resulted in significant down-regulation of the expression of retinal ZENK mRNA within 1 h of fitting the diffusers, whereas removal of the diffusers from the eyes of chickens that had developed form-deprivation myopia resulted in significant up-regulation of retinal ZENK expression within 1 h. When atropine (10 microL of 25 mM solution) and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (10 microL of a 10 mM solution) were injected intravitreally, just prior to fitting the diffusers, the down-regulation of retinal ZENK mRNA caused by form-deprivation was reversed. This resulted in levels of ZENK mRNA higher than in control or contralateral control eyes. The doses were chosen because they are known to block the excessive axial elongation induced by form-deprivation, without affecting the growth of control eyes. Neither agent had any effect on retinal ZENK expression within this time period when injected into control eyes. These results suggest that both muscarinic acetylcholine antagonists and dopamine agonists act early in the signal cascade controlling eye growth, possibly within the retina itself.

Heterogeneity of horizontal cells in the chicken retina

Despite numerous reports that different markers are expressed by horizontal cells in the avian retina, it remains unknown whether different types of horizontal cells can be defined by differences in their immunocytochemical profiles. The purpose of this study was to rectify this deficiency. We identified horizontal cells by indirect immunofluorescence with antibodies to calretinin, trkA, GABA, Prox1, AP2alpha, Pax6, islet1, and Lim1 + 2. We found two major groups of horizontal cells, those that express trkA and those that express calretinin. The trkA-immunoreactive (-IR) horizontal cells had small, round somata and robust, bulbous dendritic endings, whereas calretinin-IR horizontal cells had large, polygonal cell bodies and fine, diffuse dendritic endings, both contacting the calbindin-IR pedicles of double cones. Weak gamma-aminobutyric acid (GABA) immunoreactivity was observed only in a few of the trkA-IR horizontal cells, whereas the overlap of calretinin and GABA immunoreactivities was 100%. The majority of trkA-IR horizontal cells expressed islet1, and the majority of calretinin-IR horizontal cells expressed Lim1 + 2, AP2alpha, and Pax6. Islet1 immunoreactivity was observed in a small fraction of calretinin-IR/non-trkA-IR cells. In agreement with previous reports, we detected Prox1 immunoreactivity in all types of horizontal cells. These immunolabeling profiles suggest that there are four immunochemically distinct subtypes of horizontal cells in the postnatal chick retina, which may match the four types that have been observed in Golgi-impregnated pigeon and turtle retinas.

Patterning of the circumferential marginal zone of progenitors in the chicken retina

A circumferential marginal zone (CMZ) of retinal progenitors has been identified in most vertebrate classes, with the exception of mammals. Little is known about the formation of the CMZ during late stages of embryonic retinal histogenesis. Thus, the purpose of this study was to characterize the formation and patterning of the CMZ in the embryonic chicken retina. We identified progenitors by assaying for the expression of proliferating cell nuclear antigen (PCNA), N-cadherin and the nestin-related filament transitin, and newly generated cells by using BrdU-birthdating. We found that there is a gradual spatial restriction of progenitors into a discreet CMZ during late stages of embryonic development between E16 and hatching, at about E21. In addition, we found that retinal neurons remain immature for prolonged periods of time in far peripheral regions of the retina. Early markers of neuronal differentiation (such as HuC/D, calretinin and visinin) are expressed by neurons that are found directly adjacent to the CMZ. By contrast, genes (protein kinase C, calbindin, red/green opsin) that are expressed with a delay (7-10 days) after terminal mitosis in the central retina are not expressed until as many as 30 days after terminal mitosis in the far peripheral retina. We conclude that the neurons that are generated by late-stage CMZ progenitors differentiate much more slowly than neurons generated during early stages of retinal development. We propose that the microenvironment within the far peripheral retina at late stages of development permits the maintenance of a zone of progenitors and slows the differentiation of neurons.

Characterization of glucagon-expressing neurons in the chicken retina

We recently identified large glucagon-expressing neurons that densely ramify neurites in the peripheral edge of the retina and regulate the proliferation of progenitors in the circumferential marginal zone (CMZ) of the postnatal chicken eye (Fischer et al. [2005] J Neurosci 25:10157-10166). However, nothing is known about the transmitters and proteins that are expressed by the glucagon-expressing neurons in the avian retina. We used antibodies to cell-distinguishing markers to better characterize the different types of glucagon-expressing neurons. We found that the large glucagon-expressing neurons were immunoreactive for substance P, neurofilament, Pax6, AP2alpha, HuD, calretinin, trkB, and trkC. Colocalization of glucagon and substance P in the large glucagon-expressing neurons indicates that these cells are the "bullwhip cells" that have been briefly described by Ehrlich et al. ([1987] J Comp Neurol 266:220-233). Similar to the bullwhip cells, the conventional glucagon-expressing amacrine cells were immunoreactive for calretinin, HuD, Pax6, and AP2alpha. Unlike bullwhip cells, the conventional glucagon-expressing amacrine cells were immunoreactive for GABA. While glucagon-immunoreactive amacrine cells were negative for substance P in central regions of the retina, a subset of this type of amacrine cell was immunoreactive for substance P in far peripheral regions of the retina. An additional type of glucagon/substance P-expressing neuron, resembling the bullwhip cells, was found in far peripheral and dorsal regions of the retina. Based on morphology, distribution within the retina, and histological markers, we conclude that there may be four different types of glucagon-expressing neurons in the avian retina.

Effects of follistatin overexpression on cell differentiation in the chick embryo retina

Although activin is expressed in the embryonic central nervous system (CNS), its possible functions in the regulation of CNS neuronal differentiation remain largely unknown. We have investigated this question in the retina, a well-characterized CNS structure previously shown to respond to activin in vitro, and to express activin subunits and receptors in vivo. RCAS retroviruses were used to overexpress in the chick retina in ovo either follistatin (FS), an activin-binding protein and inhibitor, or alkaline phosphatase (AP), as control. FS-treated retinas appeared normal until ED 8, when they showed a reduction of the inner plexiform layer, accompanied by a marked decrease in the frequency of amacrine cells. The territory lacking amacrine cells showed downregulation of transcription factors necessary for amacrine cell differentiation, such as Pax6 and AP2alpha, accompanied by ectopic expression of transcription factors associated with the development of horizontal or bipolar neurons, such as Prox1, Chx10 and NeuroM. Increases in cell death were also observed in FS-treated retinas. Taken together with previous in vitro studies, our results suggest that activin is a powerful regulator of neuronal differentiation in the central nervous system.

Effects of muscarinic antagonists on ZENK expression in the chicken retina

Muscarinic antagonists, particularly atropine, can inhibit myopia development in several animal models and also in children. However, the biochemical basis of the inhibition of axial eye growth remains obscure, and there are doubts whether muscarinic receptors are involved at all. Experiments in chickens and monkeys have shown that the synthesis of the transcription factor ZENK, also named Egr-1, in retinal glucagon amacrine cells is strongly associated with inhibition of axial eye growth (assumed to create a STOP signal). We have tested whether the muscarinic antagonists atropine, pirenzepine, oxyphenonium, gallamine, MT-3, himbacine, and 4-DAMP can stimulate ZENK expression so that the drugs' inhibitory effect on myopia development could be explained by an enhanced STOP signal. Because it is known that intravitreal quisqualic acid (QA) eliminates most cholinergic neurons in the retina within 6 or 7 days, in a second set of experiments, we tested whether these antagonists could still stimulate ZENK production, 6 days after QA was applied. Muscarinic antagonists, injected intravitreally at various concentrations, affected ZENK synthesis in various and unpredictable ways. Pirenzepine, oxyphenonium, and MT-3 increased the proportion of glucagon cells that were ZENK-immunoreactive, whereas himbacine decreased that proportion, and gallamine and 4-DAMP had no significant effect. Atropine caused an upregulation of ZENK only if all positive amacrine and bipolar cells were counted and therefore appeared to affect primarily cells other than glucagon amacrines. The pattern of results remained unchanged after ablation of most cholinergic neurons by QA. Our results suggest that at least some muscarinic antagonists do not activate cells that synthesize ZENK when they inhibit axial eye growth. Therefore, in line with other studies they also cast doubt on the assumption that muscarinic transmission is crucial, and they suggest that muscarinic antagonists may inhibit myopia through extraretinal target sites or through non-cholinergic retinal actions.

GABAergic circuitry in the opossum retina: a GABA release induced by L-aspartate

Glutamate and gamma-amino butyric acid (GABA) are the major excitatory and inhibitory neurotransmitters, respectively, in the central nervous system (CNS), including the retina. Although in a number of studies the retinal source of GABA was identified, in several species, as horizontal, amacrine cells and cells in the ganglion cell layer, nothing was described for the opossum retina. Thus, the first goal of this study was to determine the pattern of GABAergic cell expression in the South America opossum retina by using an immunohistochemical approach for GABA and for its synthetic enzyme, glutamic acid decarboxylase (GAD). GABA and GAD immunoreactivity showed a similar cellular pattern by appearing in a few faint horizontal cells, topic and displaced amacrine cells. In an effort to extend the knowledge of the opossum retinal circuitry, the possible influence of glutamatergic inputs in GABAergic cells was also studied. Retinas were stimulated with different glutamatergic agonists and aspartate (Asp), and the GABA remaining in the tissue was detected by immunohistochemical procedures. The exposure of retinas to NMDA and kainate resulted the reduction of the number of GABA immunoreactive topic and displaced amacrine cells. The Asp treatment also resulted in reduction of the number of GABA immunoreactive amacrine cells but, in contrast, the displaced amacrine cells were not affected. Finally, the Asp effect was totally blocked by MK-801. This result suggests that Asp could be indeed a putative neurotransmitter in this non-placental animal by acting on an amacrine cell sub-population of GABA-positive NMDA-sensitive cells.

Specific immunolabeling of brain macrophages and microglial cells in the developing and mature chick central nervous system

The present study showed that the HIS-C7 monoclonal antibody, which recognizes the chick form of CD45, is a specific marker for macrophages/microglial cells in the developing and mature chick central nervous system (CNS). HIS-C7-positive cells were characterized according to their morphological features and chronotopographical distribution patterns within developing and adult CNS, similar to those of macrophages/microglial cells in the quail CNS and confirmed by their histochemical labeling with Ricinus communis agglutinin I, a lectin that recognizes chick microglial cells. Therefore, the HIS-C7 antibody is a valuable tool to identify brain macrophage and microglial cells in studies of the function, development, and pathology of the chick brain. CD45 expression differed between chick microglia (as revealed with HIS-C7 antibody) and mouse microglial cells (as revealed with an antibody against mouse form of CD45). Thus, a discontinuous label was seen on mouse microglial cells with the anti-mouse CD45 immunostaining, whereas the entire surface of chick microglial cells was labeled with the anti-chick CD45 staining. The functional relevance of these differences between species has yet to be determined.

Rhythms of glycerophospholipid synthesis in retinal inner nuclear layer cells

The present study demonstrates that the biosynthesis of phospholipids in the inner nuclear layer cells of the chicken retina displays daily rhythms under constant illumination conditions. The vertebrate retina contains circadian oscillators and photoreceptors (PRCs) that temporally regulate its own physiology and synchronize the whole organism to the daily environmental changes. We have previously reported that chicken photoreceptors and retinal ganglion cells (RGCs) present significant daily variations in their phospholipid biosynthesis under constant illumination conditions. Herein, we demonstrate that cell preparations highly enriched in inner nuclear layer cells also exhibit a circadian-regulated phospholipid labeling after the in vivo administration of [(32)P]phosphate or [(3)H]glycerol both in animals maintained under constant darkness or light for at least 48h. In constant darkness, there was a significant incorporation of both precursors into phospholipids with the highest levels of labeling around midday and dusk. In constant light, the labeling of (32)P-phospholipids was also significantly higher during the day and early night whereas the incorporation of [(3)H]glycerol into phospholipids, that indicates de novo biosynthesis, was greater during the day but probably reflecting a higher precursor availability at those phases. We also measured the in vitro activity of phosphatidate phosphohydrolase and diacylglycerol lipase in preparations obtained from the dark condition. The two enzymes exhibited the highest activity levels late in the day. When we assessed the in vitro incorporation of [(14)C]oleate into different lysophospholipids from samples collected at different phases in constant darkness, reaction catalyzed by lysophospholipid acyltransferases II, labeling showed a complex pattern of daily activity. Taken together, these results demonstrate that the biosynthesis of phospholipids in cells of the chicken retinal inner nuclear layer exhibits a daily rhythmicity under constant illumination conditions, which is controlled by a circadian clock.

Transitin, a nestin-related intermediate filament, is expressed by neural progenitors and can be induced in Müller glia in the chicken retina

The purpose of this study was to test whether transitin, the avian homologue of nestin, is expressed by retinal progenitors in the developing and postnatal chicken. Because nestin has been widely used as a cell-distinguishing marker of neural progenitors in the mammalian nervous system, we expected to find transitin expressed specifically by the neural progenitors of the retina. In early stages of development, transitin is expressed by neural progenitors in the retina and by cells in the developing ciliary body. During later stages of development, transitin expression persists in differentiating Müller glia but is down-regulated by these cells as maturation proceeds. In the postnatal chick, transitin expression is restricted to neural progenitors at the peripheral edge of the retina. We found that the expression of transitin in mature Müller glia was induced by intraocular injections of insulin and fibroblast growth factor-2 (FGF2) but not by ciliary neurotrophic factor. In response to insulin and FGF2, the expression of transitin was induced in the nonpigmented epithelium (NPE) of the ciliary body. In the postnatal retina, acute retinal damage transiently induces transitin expression in Müller glia. We propose that the expression of transitin by retinal Müller glia and NPE cells in the postnatal animal represents a state of de-differentiation and a step toward becoming neurogenic progenitor cells. Taken together, our findings indicate that transitin is expressed by neural progenitors in the embryonic and postnatal chicken retina. However, transitin is not exclusively expressed by neural progenitors and is also expressed by non-neurogenic cells.

BMP4 and CNTF are neuroprotective and suppress damage-induced proliferation of Müller glia in the retina

In response to acute damage, Müller glia in the chicken retina have been shown to be a source of proliferating progenitor-like cells. The secreted factors and signaling pathways that regulate this process remain unknown. The purpose of this study was to test whether secreted factors, which are known to promote glial differentiation during development, regulate the ability of Müller glia to proliferate and become retinal progenitors in response to acute damage in mature retina. We made intraocular injections of BMP4, BMP7, EGF, NGF, BDNF, or CNTF before or after a single, toxic dose of N-methyl-d-aspartate (NMDA) and assayed for proliferating progenitor-like cells within the retina. We found that injections of BMP4, BMP7, or CNTF, but not EGF, NGF, or BDNF, before NMDA treatment reduced the number of Müller glia that proliferated and gave rise to progenitor-like cells. CNTF and BMP4, but not NGF or BDNF, greatly reduced the number of cells destroyed by toxin treatment indicating that these factors protect retinal neurons from a severe excitotoxic insult. Injections of CNTF 5 days before NMDA treatment prevented neurotoxin-induced cell death and Müller glial proliferation, while injections of BMP4 had no protective effect. In addition, CNTF injected after NMDA treatment suppressed glial proliferation, while BMP4 did not. We conclude that BMP4 and CNTF, when applied before a toxic insult, act as neuroprotective agents and likely suppress the proliferative response of Müller glia to retinal damage by attenuating the retinal damage; protecting bipolar and amacrine neurons from NMDA-induced cell death. When applied after a toxic insult, CNTF suppressed glial proliferation independent of levels of retinal damage.

Neural regeneration in the chick retina

In warm-blooded vertebrates, possibilities for retinal regeneration have recently become reality with the discovery of neural stem cells in the mature eye. A number of different cellular sources of neural stem cells have been identified. These sources include stem cells at the retinal margin, pigmented cells in the ciliary body and iris, non-pigmented cells in the ciliary body and Müller glia within the retina. This review focuses on recent reports of neural stem cells and regeneration in the postnatal chicken retina. In the chicken eye sources of neurogensis and regeneration include: (1) retinal stem cells at the peripheral edge of the retina; (2) Müller glia in central regions of the retina; (3) non-pigmented epithelial cells in the posterior portion of the ciliary body; and (4) possibly pigmented cells in the pars plana of the ciliary body. This review discusses the similarities between the retinal progenitor cells in the postnatal eye and those found in the embryo. In addition, I discuss combinations of growth factors, (insulin, IGF-I, EGF and FGF2) that are capable of stimulating the proliferation and production of neurons from neural progenitors, non-neural epithelial cells, and postmitotic support cells in the avian eye. In summary, the mechanisms that regulate the proliferation and differentiation of cells with neurogenic potential are beginning to be understood and the postnatal chicken eye has proven to be a useful model system to study retinal regeneration.

Potential for neural regeneration after neurotoxic injury in the adult mammalian retina

It has long been believed that the retina of mature mammals is incapable of regeneration. In this study, using the N-methyl-D-aspartate neurotoxicity model of adult rat retina, we observed that some Müller glial cells were stimulated to proliferate in response to a toxic injury and produce bipolar cells and rod photoreceptors. Although these newly produced neurons were limited in number, retinoic acid treatment promoted the number of regenerated bipolar cells. Moreover, misexpression of basic helix-loop-helix and homeobox genes promoted the induction of amacrine, horizontal, and rod photoreceptor specific phenotypes. These findings demonstrated that retinal neurons regenerated even in adult mammalian retina after toxic injury. Furthermore, we could partially control the fate of the regenerated neurons with extrinsic factors or intrinsic genes. The Müller glial cells constitute a potential source for the regeneration of adult mammalian retina and can be a target for drug delivery and gene therapy in retinal degenerative diseases.

Localization and regulation of glucagon receptors in the chick eye and preproglucagon and glucagon receptor expression in the mouse eye

Myopia is a condition in which the eye is too long for the focal length of cornea and lens. Analysis of the messengers that are released by the retina to control axial eye growth in the animal model of the chicken revealed that glucagon-immunoreactive amacrine cells are involved in the retinal image processing that controls the growth of the sclera. It was found that the amount of retinal glucagon mRNA increased during treatment with positive lenses and pharmacological studies supported the idea that glucagon may act as a stop signal for eye growth. Glucagon exerts its regulatory effects by binding to a single type of glucagon receptor. In this study, we have sequenced the chicken glucagon receptor and compared its DNA and amino acid sequence with the human and mouse homologues. After sequencing about 80% of the receptor, we found a homology between 79.4 and 75.6% on cDNA level. At the protein level, about 73% of the amino acids were identical. Moreover, the cellular localization and regulation of the glucagon receptor in the chick retina was studied. In situ hybridization studies showed that many cells in the ganglion cell layer and inner nuclear layer, and some cells in the outer nuclear layer, express the receptor mRNA. Injection of the glucagon agonist Lys17,18,Glu21-glucagon induced a down-regulation of glucagon receptor mRNA content. Since the mouse would be an attractive mammalian model to study the biochemical and genetic basis of myopia, and because recent studies have demonstrated that form deprivation myopia can be induced, the expression of preproglucagon and glucagon receptor genes were also studied in the mouse retina and were found to be expressed.

Cellular positioning and dendritic field size of cholinergic amacrine cells are impervious to early ablation of neighboring cells in the mouse retina

We have examined the role of neighbor relationships between cholinergic amacrine cells upon their positioning and dendritic field size by producing partial ablations of this population of cells during early development. We first determined the effectiveness of L-glutamate as an excitotoxin for ablating cholinergic amacrine cells in the developing mouse retina. Subcutaneous injections (4 mg/g) made on P-3 and thereafter were found to produce a near-complete elimination, while injections at P-2 were ineffective. Lower doses on P-3 produced only partial reductions, and were subsequently used to examine the effect of partial ablation upon mosaic organization and dendritic growth of the remaining cells. Four different Voronoi-based measures of mosaic geometry were examined in L-glutamate-treated and normal (saline-treated) retinas. Partial depletions of around 40% produced cholinergic mosaics that, when scaled for density, approximated the mosaic geometry of the normal retina. Separate comparisons simulating a 40% random deletion of the normal retina produced mosaics that were no different from those experimentally depleted retinas. Consequently, no evidence was found for positional regulation in the absence of normal neighbor relationships. Single cells in the ganglion cell layer were intracellularly filled with Lucifer Yellow to examine the morphology and dendritic field extent following partial ablation of the cholinergic amacrine cells. No discernable effect was found on their starburst morphology, and total dendritic field area, number of primary dendrites, and branch frequency were not significantly different. Cholinergic amacrine cells normally increase their dendritic field area after P-3 in excess of retinal expansion; despite this, the present results show that this growth is not controlled by the density of neighboring processes.

Retinoic acid signals the direction of ocular elongation in the guinea pig eye

A growing eye becomes myopic after form deprivation (FD) or compensates for the power and sign of imposed spectacle lenses. A possible mediator of the underlying growth changes is all-trans retinoic acid (RA). Eye elongation and refractive error (RE) was manipulated by raising guinea pigs with FD, or a spectacle lens worn on one eye. We found retinal-RA increased in myopic eyes with accelerated elongation and was lower in eyes with inhibited elongation. RA levels in the choroid/sclera combined mirrored these directional changes. Feeding RA (25 mg/kg) repeatedly to guinea pigs, also resulted in rapid eye elongation (up to 5 times normal), and yet the RE was not effected. In conclusion, RA may act as a signal for the direction of ocular growth.

Effects of Quisqualic Acid on Retinal ZENK Expression Induced by Imposed Defocus in the Chick Eye

PURPOSE

Expression of the transcription factor ZENK in glucagon amacrine cells of the chicken retina is enhanced after treatment with positive spectacle lenses and reduced after treatment with negative lenses. ZENK may, therefore, have an important role in emmetropization. To learn more about its regulation, we have studied its expression after retinal intoxication with quisqualic acid (QA, a glutamatergic excitotoxin).

METHODS

Lenses of either +7 or -7 D power were placed in front of the eyes of young chickens 6 days after intravitreal QA injections. By this time, QA had caused severe damage to the retina. After 2 hours of lens wearing, changes in ZENK immunoreactivity were measured by means of double staining. In another experiment, lenses were worn for 4 days to study the residual function of emmetropization.

RESULTS

QA injections caused a massive loss of cells in the inner nuclear layer and the ganglion cell layer but left the numbers of glucagon cells unchanged. Four of six QA-injected eyes became more myopic in response to wearing positive lenses, and all eyes with negative lenses also became myopic. QA caused a general reduction in ZENK expression, and there was no clear evidence that ZENK expression was still controlled by the sign of imposed defocus.

CONCLUSIONS

After severe destruction of the inner retina by QA, retinal image processing appeared to be reduced to blur detection with no sign, causing myopia with both types of lenses. QA must remove synaptic input to the glucagon cells, which is necessary to transmit the information on the sign of imposed defocus.

NeuroD induces the expression of visinin and calretinin by proliferating cells derived from toxin-damaged chicken retina

Müller glia have been shown to be a potential source of neural regeneration in the avian retina. In response to acute damage Müller glia de-differentiate, proliferate, express transcription factors found in embryonic retinal progenitors, and some of the progeny differentiate into neurons and glia (Fischer and Reh [2001a] Nat. Neurosci. 4:247-252). However, most of the cells produced by proliferating Müller cells appear to remain undifferentiated. The purpose of this study was to test whether the neurogenic gene NeuroD can promote the differentiation of proliferating cells derived from the postnatal chick retina. We used recombinant avian retroviruses to transfect green fluorescent protein (GFP) or NeuroD. The majority of cells transfected with GFP remained undifferentiated, with a few cells differentiating into calretinin-immunoreactive neurons. Many cells transfected with the NeuroD-virus expressed calretinin, neurofilament, or visinin, while most cells remained undifferentiated. The number of calretinin-expressing cells that were generated was increased approximately 20-fold with forced expression of NeuroD. In addition, we found that cells transfected with NeuroD never expressed glutamine synthetase, a marker of mature Müller glia, suggesting that NeuroD suppresses glial differentiation. We conclude that NeuroD stimulates cells from the toxin-damaged chicken retina to acquire some neuronal phenotypes. We propose that most of these cells were derived from Müller glia.

A non-mitochondrial carboxylase, related to glutamate action is synthesized in the retina of the chick embryo

SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot of chick retinal proteins at 21 days after intravitreal injection with 5 or 10 microM/eye Glu showed decreases in 37, 42, 53 and 57 kDa proteins and increases in 35, 72 and >94 kDa proteins. These proteins were carboxylases except for the 35 and 37 kDa proteins. With > or =15 microM/eye Glu, non-specific loss of retinal proteins was observed. In embryonic retinas, the 42 kDa protein was seen a few days before hatching, with biotin incorporation on days 3-6 after hatching. Immunohistochemistry indicated that this protein was a component of both the inner nuclear layer and the photoreceptor. Immunocytochemistry located it to the cell surface.

Growth factors induce neurogenesis in the ciliary body

The ciliary body of the eye is a nonneural tissue that is derived from the anterior rim of the optic cup, an extension of the neural tube. This tissue normally does not contain neurons and functions to produce components of the aqueous humor. We found that intraocular injections of insulin, EGF, or FGF2 stimulate NPE cells to proliferate and differentiate into neurons. These growth factors had region-specific effects along the radial axis of the ciliary body, with insulin and EGF stimulating proliferation of NPE cells close to the retina, while FGF2 stimulated the proliferation of NPE cells further toward the lens. Similar region-specific effects were observed for accumulations of neurons in the NPE in response to injections of different growth factors. The neurons derived from NPE cells express neurofilament, beta3 tubulin, RA4, calretinin, Islet1, or Hu, and a few produced long axonal projections, several millimeters in length that extend across the ciliary body. Our results suggest that the ciliary body has the capacity to generate retinal neurons, but normally neurogenesis is actively inhibited.

Potential of Müller glia to become neurogenic retinal progenitor cells

The possibility of neural regeneration has gained credence with the identification of neural stem cells seeded within different regions of the adult central nervous system (CNS). Recently, this possibility has received an additional boost from reports that glia, the support cells of the CNS, might provide a source of neural regeneration. We review some of our findings that Müller glia in the chicken retina are a source of proliferating progenitors that can generate neurons. These Müller cells are fully differentiated glial cells that serve functions ascribed to this cell type. In response to damage or exogenous growth factors, Müller glia dedifferentiate, proliferate, express combinations of transcription factors normally expressed by embryonic retinal progenitors, and produce new neurons and glia. In light of these data, the potential of Müller glia as a source of neural regeneration in the retina of nonavian species, namely humans, seems an avenue of investigation that warrants serious consideration.

Choline acetyltransferase is expressed by non-starburst amacrine cells in the ground squirrel retina

We have used immunostaining techniques to reveal a new type of amacrine cell that is immunoreactive for choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, in the Ground Squirrel (Spermophilus beecheyi) retina. Cryostat sections and double immunostained wholemount preparations were examined by confocal microscopy. This new ChAT type III cell is distinct in morphology and neurotransmitter content from the well know 'starburst' amacrine cells (types I and II) that are so well represented in the ground squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. The type III cell colocalizes glycine with the acetylcholine and does not appear to be GABAergic or exhibit calcium-binding proteins like the well-known starburst type. As well, type III cells do not occur as a mirror-symmetric pair with normally placed and displaced varieties. The type III cell is probably a small field amacrine type branching broadly in upper sublamina b of the inner plexiform layer, and is most likely A6 of the Ground Squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. Type III cells are ideally placed in the architecture of the Ground Squirrel retina to influence ON directionally selective ganglion cell types.

Protective effect of nilvadipine against glutamate neurotoxicity in purified retinal ganglion cells

We determined the effect of nilvadipine, a dihydropyridine-type calcium channel blocker, in preventing glutamate neurotoxicity in purified retinal ganglion cells (RGCs). RGCs were purified from dissociated rat retinal cells (postnatal days 6-8), using a modified two-step panning method, and cultured in serum-free medium containing neurotrophic factors and forskolin. RGC survival after exposure to glutamate (25 microM) with nilvadipine or other calcium channel blockers was measured by calcein-acetoxymethyl ester staining after 3 days in culture. Changes in the level of intracellular Ca(2+) ([Ca(2+)](i)) were measured with fura-2 fluorescence. Induction of apoptosis was evaluated using the TDT-dUTP terminal nick-end labeling technique. The neurotoxic effects of low doses of glutamate were blocked by a specific alpha-amino-3-dihydro-5-methylisoxazole-4-propionate-kainate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (20 microM). Simultaneous application of nilvadipine (1-100 nM) with glutamate protected against glutamate neurotoxicity in a dose-dependent manner. Calcium-imaging experiments showed that the glutamate-evoked [Ca(2+)](i) increase was significantly blocked by nilvadipine (P<0.001), but not nifedipine and diltiazem, in about 50% of RGCs. In addition, the application of nilvadipine significantly reduced glutamate-induced apoptosis (P<0.001). These findings suggest that nilvadipine may partly inhibit glutamate-induced apoptotic cell death by blocking calcium influx via voltage-dependent calcium channels in purified RGCs.

A caspase-3-dependent pathway is predominantly activated by the excitotoxin pregnenolone sulfate and requires early and late cytochrome c release and cell-specific caspase-2 activation in the retinal cell death

This study investigates the implication of mitochondria- and caspase-dependent pathways in the death of retinal neurones exposed to the neurosteroid pregnenolone sulfate (PS) shown to evoke apoptosis and contribute to amplification and propagation of excitotoxicity. After a brief PS challenge of intact retinas, caspase-3 and caspase-2 activation and cytochrome c release occur early and independent of changes in the oxidative state measured by superoxide dismutase activity. The temporal and spatial relationship of these events suggests that a caspase-3-dependent pathway is activated in response to cytochrome c release and requires caspase-2 activation and a late cytochrome c release in specific cellular subsets of retinal layers. The protection by caspase inhibitors indicates a predominant role of the pathway in PS-induced retinal apoptosis, although a limited use of caspase inhibitors is upheld on a conceivable shift from apoptosis toward necrosis. Conversely, 3alpha-hydroxy-5beta-pregnan-20-one sulfate and 17beta-oestradiol provide complete prevention of PS-induced retinal death.

Exogenous growth factors stimulate the regeneration of ganglion cells in the chicken retina

Recent reports have found that the posthatch chicken retina has the capacity for neuronal regeneration. The purpose of this study was to test whether the types of cells destroyed by neurotoxic lesions influence the types of cells that are regenerated, and whether exogenous growth factors stimulate neural regeneration in the chicken retina. N-methyl-D-aspartate (NMDA) was used to destroy amacrine and bipolar cells; kainate was used to destroy bipolar, amacrine, and ganglion cells; colchicine was used to selectively destroy ganglion cells. Following toxin-induced damage, bromo-deoxyuridine was used to label proliferating cells. In some animals, growth factors were injected into the vitreous chamber of the eye. We found that the proliferation of cells within the retina was stimulated by toxin-induced cell loss, and by insulin and FGF2. After either kainate- or colchicine-induced retinal damage, some of the newly generated cells expressed markers and had the morphology of ganglion cells. The combination of insulin and FGF2 stimulated the regeneration of ganglion cells in kainate- and colchicine-treated retinas. We conclude that exogenous growth factors can be used to stimulate neural regeneration in the retina. We propose that the type of neuron destroyed in the retina may allow or promote the regeneration of that neuronal type.

Simultaneous expression of c-Jun and p53 in retinal ganglion cells of adult rat retinal slice cultures

PURPOSE

To determine whether the apoptosis-associated transcription factor c-Jun and the regulator protein p53 are expressed together during retinal ganglion cell (RGC) death in slice cultures of adult rat retina, and whether c-Jun expression or p53 expression is altered by glutamate.

METHODS

Newborn rat RGCs were retrogradely labeled by Di-I microinjections into the superior colliculus. Retinas were isolated 2 to 4 months later, cut into 200 microm-thick slices. These slices were cultured with 0-300 microM glutamate in the presence or absence of MK801. Survival was assessed using Sytox green and ethidium homodimer. Cultures also were immunostained for Thy-1, c-Jun and/or p53. The linear density of stained RGCs was determined by confocal laser microscopy.

RESULTS

RGCs in freshly-isolated adult retina slices did not express either c-Jun or p53. By 12 hours in vitro, 12.0 +/- 3.1 cells/mm of RGCs expressed c-Jun and 18.5 +/- 3.5 cells/mm of RGCs expressed p53 in control cultures. Exposure to glutamate increased both c-Jun and p53 positive RGCs in dose-dependent manner, and decreased survival in the RGC layer. Following double staining, up to 58% of cells in the RGC layer simultaneously expressed both c-Jun and p53. Time-course analysis showed that peak c-Jun expression preceded peak p53 expression in control and glutamate-treated cultures.

CONCLUSIONS

The simultaneous expression of both c-Jun and p53 in the cultured adult rat slices raises the possibility that each may contribute to the mechanism of RGC death. This is supported by the increased p53 and c-Jun inductions in the presence of glutamate.

Pirenzepine affects scleral metabolic changes in myopia through a non-toxic mechanism

Whilst the precise mechanism regulating ocular growth is unknown, it has been shown that various pharmacological agents, including the muscarinic receptor antagonists, atropine and pirenzepine, are effective at preventing the development of myopia. A recent study, which demonstrated that muscarinic antagonists reduce the synthesis of glycosaminoglycans and DNA in chick sclera in vitro, led to the suggestion that such drugs may act directly on the sclera, possibly through a toxic mechanism. Accepted markers of scleral metabolism and cell viability were used in conjunction with a non-invasive, physiological method of ocular growth regulation to determine whether the selective muscarinic antagonist pirenzepine inhibits the development of myopia via toxicity to the sclera. Chicks were monocularly deprived (MD) of pattern vision and given daily intravitreal injections of either pirenzepine (700 microg) or saline vehicle into the deprived eye over 5 days. Unoccluded animals also received intravitreal injections of either pirenzepine or saline into one eye (n=6, all groups). The contralateral eye of all animals was left untreated for comparison. Optical and ocular biometric measures were collected on the final experimental day. Following in vivo delivery of [(35)S] labelled sulphate, levels of sulphate incorporation into scleral glycosaminoglycans were measured in proteinase K digests following selective precipitation with alcian blue dye. The DNA content was also assessed through luminescence spectrometry after binding to Hoechst 33258 dye. To allow comparison with an accepted non-invasive, physiological method of ocular growth regulation, myopia was prevented in additional groups of MD animals by allowing 3hr of unoccluded vision each day, over 5 days, before levels of sulphate incorporation were measured. Scleral DNA content, a marker of cell viability, was not significantly altered between treated and control eyes in any injected group. Relative levels of sulphate incorporation (% difference between treated and contralateral control eyes) were significantly lower in the cartilaginous sclera of pirenzepine-MD animals, compared to saline-MD controls (+35.9 +/- 10.1% vs +121.2 +/- 28.6%, P<0.05), after 2hr of incorporation. However, after 6hr incorporation, there was no significant difference in sulphate incorporation in the cartilaginous sclera between the two groups (+87.2 +/- 33.1% vs +111.0 +/- 14.4%, P=0.53). No significant change was found in the levels of glycosaminoglycan synthesis in the fibrous sclera of any pirenzepine treated group, when compared to the appropriate saline control. Relative patterns of sulphate incorporation, between treatment and control groups, were essentially identical at both time points examined, regardless of whether myopia was prevented through pirenzepine injection or periods of unoccluded vision. The present study shows that, at a dose of pirenzepine sufficient to prevent experimentally-induced axial myopia, glycosaminoglycan synthesis in the cartilaginous sclera was significantly reduced for a transient period following the injection. These pirenzepine-induced reductions in glycosaminoglycan synthesis were not caused by direct drug toxicity to scleral cells as these changes were reversible and no significant reduction in DNA content was observed in pirenzepine treated eyes. Similar patterns of scleral glycosaminoglycan synthesis changes were found following the provision of brief periods of unoccluded vision further demonstrating that pirenzepine is effective in myopia prevention via a non-toxic mechanism. Consequently, the prevention of myopia development in chicks, with either pirenzepine or brief periods of unoccluded vision, is associated with the transient modulation of scleral glycosaminoglycan synthesis in the cartilaginous sclera.

Ionotropic glutamate receptors during the development of the chick retina

Glutamate is the main neurotransmitter of photoreceptors, bipolar cells, and ganglion cells of the vertebrate retina. Three main classes of ionotropic glutamate receptors comprising different subunits can be distinguished: AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionate), KA (kainate), and NMDA (N-methyl-D-aspartate). This study was undertaken to characterize the AMPA (GluR1, GluR2/3, and GluR4), KA (GluR5/6/7), and NMDA (NR1) ionotropic glutamate receptor subunits and to determine their distribution during the development of the chick retina by Western blotting and immunohistochemistry. Western blotting analysis at 1 day after hatching indicated that the antibodies against GluR1, 2/3, 4, and 5/6/7 and NR1 recognized specifically a single band of 100-110 kDa. In turn, immunohistochemistry at P1 showed that all subunits were expressed in cells of the inner nuclear and ganglion cell layers of the chick retina, mostly amacrine and ganglion cells, and their processes in the inner plexiform layer. In addition, stained processes in the outer plexiform layer were observed with the antibodies against GluR2/3, GluR4, and GluR5/6/7. Although all subunits appeared around E5-E6 in the prospective ganglion cell layer, and later in the prospective inner nuclear layer, the distribution of cells containing these glutamate receptor subunits revealed distinct ontogenetic patterns. This multiplicity of glutamate receptors may contribute to different processes that occur in the chick retina during development.

Comparative study of glutamate mediated gamma-aminobutyric acid release from nitric oxide synthase and tyrosine hydroxylase immunoreactive cells of the Cebus apella retina

The effects of excitatory amino acids (EAAs) upon transporter-mediated gamma-aminobutyric acid (GABA) release were investigated in cells containing tyrosine hydroxylase (TH) or nitric oxide synthase (NOS) in retina of the primate Cebus apella. Retinas were treated in vitro with 50 microM Kainate (KA) or 5 mM L-Glutamate (L-Glu), for 30 min at 37 degrees C, in an Mg2+-free Locke's solution with or without Ca2+. The effects of EAAs were measured immunocytochemically by determining the GABA content in TH or NOS-immunoreactive cells in the inner retina, after stimulation. L-Glu and KA induced a Ca2+-independent GABA release from most GABA-immunoreactive cells of the inner retina. Double label experiments indicated that this release occurs in NOS+/GABA+ cells, but not in TH+/GABA+ cells suggesting that these cell subpopulations may be differentiated in some functional aspects.

Müller glia are a potential source of neural regeneration in the postnatal chicken retina

The retina of warm-blooded vertebrates is believed to be incapable of neural regeneration. Here we provide evidence that the retina of postnatal chickens has the potential to generate new neurons. In response to acute damage, numerous Müller glia re-entered the cell cycle, and shortly thereafter, expressed CASH-1, Pax6 and Chx10, transcription factors expressed by embryonic retinal progenitors. These progenitor-like cells transiently expressed neurofilament. Newly formed cells became distributed throughout the inner and outer nuclear layers of the retina, and remained for at least three weeks after damage. Some of these newly formed cells differentiated into retinal neurons, a few formed Müller glia, and most remained undifferentiated, with continued expression of Pax6 and Chx10. These cells continued to proliferate when grown in culture, with some differentiating into retinal neurons or Müller glia. We propose that, in response to damage, Müller glia in the retina are a potential source of neural regeneration.

Pregnenolone sulfate, a naturally occurring excitotoxin involved in delayed retinal cell death

The present study was designed to investigate the neurosteroid pregnenolone sulfate (PS), known for its ability to modulate NMDA receptors and interfere with acute excitotoxicity, in delayed retinal cell death. Three hours after exposure of the isolated and intact retina to a 30-min PS pulse, DNA fragmentation as assessed by genomic DNA gel electrophoresis and a modified in situ terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method appeared concurrently with an increase in superoxide dismutase (SOD) activity and thiobarbituric acid-reactive substances (TBARS) levels. At 7 h, the increased amount of DNA laddering was accompanied by a higher number of TUNEL-positive cells in the inner nuclear and ganglion cell layers. Necrotic signs were characterized by DNA smear migration, lactate dehydrogenase (LDH) release, and damage mainly in the inner nuclear layer. PS-induced delayed cell death was markedly reduced by the NMDA receptor antagonists 4-(3-phosphonopropyl)-2-piperazinecarboxylic acid and 3alpha-hydroxy-5beta-pregnan-20-one sulfate but completely blocked after concomitant addition of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. Steroids with antioxidant properties (progesterone, dehydroepiandrosterone and its sulfate ester, and 17beta-estradiol) differently prevented PS-induced delayed cell death. Cycloheximide treatment protected against DNA fragmentation and LDH release but failed to prevent the rise in SOD activity and TBARS level. We conclude that a brief PS pulse causes delayed cell death in a slowly evolving apoptotic fashion characterized by a cycloheximide-sensitive death program downstream of reactive oxygen species generation and lipid peroxidation, turning into secondary necrosis in a retinal cell subset.

Identification of a proliferating marginal zone of retinal progenitors in postnatal chickens

In warm-blooded vertebrates it is generally accepted that after early stages of development new neurons are not added to the retina. Contrary to this belief, we show here that hatched chickens have a zone of proliferating cells at the peripheral margin of the retina, similar to that of fish and amphibians. We found that cells at the peripheral edge of the retina incorporated the thymidine analog BrdU and expressed the cell cycle regulator proliferating cell nuclear antigen (PCNA). Furthermore, cells in the ciliary epithelium and retinal margin coexpressed the homeodomain transcription factors Pax6 and Chx-10, similar to multipotent progenitors of embryonic retina. Expression of PCNA, Pax6, and Chx-10 in cells at the retinal margin was maintained in adult birds. Double-labeling studies showed that BrdU-labeled cells that were integrated into the retina expressed proteins found only in differentiated neurons. Increased rates of ocular growth, induced by visual deprivation, resulted in increased numbers of BrdU-labeled cells at the retinal margin. Unlike the progenitors in the retinal marginal zone of fish and amphibians, the progenitors of the chick retina do not increase their rate of proliferation in response to acute damage. Furthermore, insulin, insulin-like growth factor-I, and epidermal growth factor increased proliferation of progenitors at the retinal margin, while basic fibroblast growth factor had no effect. These results indicate that the avian retina has a marginal growth zone containing proliferating cells that share similarities with multipotent embryonic retinal progenitors and the retinal stem cells of cold-blooded vertebrates.

Visually induced changes in components of the retinoic acid system in fundal layers of the chick

Eye growth is visually regulated via messengers that are released from the retina. The retina involves a yet unknown algorithm to analyse the projected image so that the appropriate growth rates for the back of the eye are ensured. One biochemical candidate that could act as a growth controller, is retinoic acid (RA). Previous work (Seko, Shimokawa and Tokoro, 1996; Mertz et al., 1999) has shown that retinal and choroidal RA levels are indeed predictably changed by visual conditions that cause myopia or hyperopia, respectively. We have studied in which fundal tissues aldehyde dehydrogenase-2 (AHD2) and retinaldehyde dehydrogenase-2 (RALDH2), enzymes involved in RA synthesis, are expressed and at which levels the effects of vision on RA levels may be controlled. Using Northern blot analysis, we have found that the retinal mRNA level of the AHD2 is up-regulated after 3 days of treatment with negative lenses (negative lenses place the image behind the retina). The abundance of the retinal mRNA of a RA receptor, RAR-beta, was up-regulated already after 6 hr of treatment with positive lenses (positive lenses place the image in front of the retina). The up-regulation persisted for at least 1 week. Finally, we have studied the effects of an inhibitor of RA synthesis, disulfiram, on the visual control of eye growth. We found inhibition of myopia as induced by frosted goggles ('deprivation myopia') but no significant inhibitory effects on refractive errors induced by +7D or -7D lenses. Our results are in line with the hypothesis that RA may play a role in the visual control of eye growth. The RA system differs from a number of other candidates (dopamine, cholinergic agents, opiates) in that it distinguishes between positive and negative defocus, similar to the immediate early gene ZENK (Stell et al., 1999). The exact time kinetics of the changes have still to be worked out since it is possible that the changes in RA relate to already occurring changes in growth rather than to initial steps of the signaling cascade.

Light- and focus-dependent expression of the transcription factor ZENK in the chick retina

Ocular growth and refraction are regulated by visual processing in the retina. We identified candidate regulatory neurons by immunocytochemistry for immediate-early gene products, ZENK (zif268, Egr-1) and Fos, after appropriate visual stimulation. ZENK synthesis was enhanced by conditions that suppress ocular elongation (plus defocus, termination of form deprivation) and suppressed by conditions that enhance ocular elongation (minus defocus, form deprivation), particularly in glucagon-containing amacrine cells. Fos synthesis was enhanced by termination of visual deprivation, but not by defocus and not in glucagon-containing amacrine cells. We conclude that glucagon-containing amacrine cells respond differentially to the sign of defocus and may mediate lens-induced changes in ocular growth and refraction.

Long-term changes in retinal contrast sensitivity in chicks from frosted occluders and drugs: relations to myopia?

Experiments in animal models have shown that the retinal analyzes the image to identify the position of the plane of focus and fine-tunes the growth of the underlying sclera. It is fundamental to the understanding of the development of refractive errors to know which image features are processed. Since the position of the image plane fluctuates continuously with accommodative status and viewing distance, a meaningful control of refractive development can only occur by an averaging procedure with a long time constant. As a candidate for a retinal signal for enhanced eye growth and myopia we propose the level of contrast adaptation which varies with the average amount of defocus. Using a behavioural paradigm, we have found in chickens (1) that contrast adaptation (CA, here referred to as an increase in contrast sensitivity) occurs at low spatial frequencies (0.2 cyc/deg) already after 1.5 h of wearing frosted goggles which cause deprivation myopia, (2) that CA also occurs with negative lenses (-7.4D) and positive lenses (+6.9D) after 1.5 h, at least if accommodation is paralyzed and, (3) that CA occurs at a retinal level or has, at least, a retinal component. Furthermore, we have studied the effects of atropine and reserpine, which both suppress myopia development, on CA. Quisqualate, which causes retinal degeneration but leaves emmetropization functional, was also tested. We found that both atropine and reserpine increase contrast sensitivity to a level where no further CA could be induced by frosted goggles. Quisqualate increased only the variability of refractive development and of contrast sensitivity. Taken together, CA occurring during extended periods of defocus is a possible candidate for a retinal error signal for myopia development. However, the situation is complicated by the fact that there must be a second image processing mode generating a powerful inhibitory growth signal if the image is in front of the retina, even with poor images (Diether, S., & Schaeffel, F. (1999).

Nitric oxide synthase-containing cells in the retina, pigmented epithelium, choroid, and sclera of the chick eye

Nitric oxide is a nonconventional neurotransmitter that is produced as needed by the enzyme nitric oxide synthase (NOS). NOS has been detected in numerous neural structures, including distinct populations of retinal neurons in a variety of vertebrate species. The purpose of this study was to identify NOS-containing cells in the retina and extraretinal ocular tissues of hatched chicks. NOS was detected in frozen sections by using nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and antisera to neuronal NOS. In the retina, NADPH-diaphorase and NOS immunolabelling were present in four subtypes of amacrine cells, some ganglion cells, efferent fibers, efferent target cells, and neuronal processes in both plexiform layers, whereas diaphorase alone was detected in photoreceptor ellipsoids and Müller cells. In addition, NADPH-diaphorase and immunoreactive NOS were detected in axon bundles and innervation to vascular smooth muscle in the choroid, whereas stromal and endothelial cells in the choroid, scleral chondrocytes, and the retinal pigmented epithelium contained only NADPH-diaphorase. The excitotoxin quisqualate destroyed all but one subtype of NOS-immunoreactive amacrine cell and caused increased NADPH-diaphorase activity in Müller cells. We conclude that nitric oxide is produced by many different cells in the chick eye, including retinal amacrine and ganglion cells, Müller cells, retinal pigmented epithelium, and cells in the choroid, and likely has a broad range of visual and regulatory functions.

Colchicine causes excessive ocular growth and myopia in chicks

Colchicine has been reported to destroy ganglion cells (GCs) in the retina of hatchling chicks. We tested whether colchicine influences normal ocular growth and form-deprivation myopia, and whether it affects cells other than GCs. Colchicine greatly increased axial length, equatorial diameter, eye weight, and myopic refractive error, while reducing corneal curvature. Colchicine caused DNA fragmentation in many GCs and some amacrine cells and photoreceptors, ultimately leading to the destruction of most GCs and particular sub-sets of amacrine cells. Colchicine-induced ocular growth may result from the destruction of amacrine cells that normally suppress ocular growth, and corneal flattening may result from the destruction of GCs whose central pathway normally plays a role in shaping the cornea.

Cholinergic amacrine cells are not required for the progression and atropine-mediated suppression of form-deprivation myopia

Muscarinic cholinergic pathways have been implicated in the visual control of ocular growth. However, the source(s) of acetylcholine and the tissue(s) which regulate ocular growth via muscarinic acetylcholine receptors (mAChRs) remain unknown. We sought to determine whether retinal sources of acetylcholine and mAChRs contribute to visually guided ocular growth in the chick. Cholinergic amacrine cells were ablated by intraocular injections of either ethylcholine mustard aziridinium ion (ECMA; a selective cholinotoxin) or quisqualic acid (QA; an excitotoxin that destroys many amacrine cells, including those that release acetylcholine). Disruption of cholinergic pathways was assessed immunocytochemically with antibodies to the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT) and three different isoforms of mAChR, and by biochemical assay for ChAT activity. ECMA (25 nmol) destroyed two of the four subtypes of cholinergic amacrine cells and attenuated retinal ChAT activity, but left retinal mAChR-immunoreactivity intact. QA (200 nmol) destroyed the majority of all four subtypes of cholinergic amacrine cells, and ablated most mAChR-immunoreactivity and ChAT activity in the retina. ECMA and QA had no apparent effect on mAChRs or cholinergic fibres in the choroid, only marginally reduced choroidal ChAT activity, and had little effect on ChAT activity in the anterior segment. Toxin-treated eyes remained emmetropic and responded to form-deprivation by growing excessively and becoming myopic. Furthermore, daily intravitreal injection of 40 microg atropine for 6 days into form-deprived toxin-treated eyes completely prevented ocular elongation and myopia. We conclude that neither cholinergic amacrine cells nor mAChRs in the retina are required for visual regulation of ocular growth, and that atropine may exert its growth-suppressing influence by acting upon extraretinal mAChRs, possibly in the choroid, retinal pigmented epithelium, or sclera.