Cholinergic, but not the rod pathway-related glycinergic (All), amacrine cells contain calretinin in the rat retina

Proteomic Analysis of Retinal Tissue in an S100B Autoimmune Glaucoma Model

Glaucoma is a neurodegenerative disease that leads to damage of retinal ganglion cells and the optic nerve. Patients display altered antibody profiles and increased antibody titer, e.g., against S100B. To identify the meaning of these antibodies, animals were immunized with S100B. Retinal ganglion cell loss, optic nerve degeneration, and increased glial cell activity were noted. Here, we aimed to gain more insights into the pathophysiology from a proteomic point of view. Hence, rats were immunized with S100B, while controls received sodium chloride. After 7 and 14 days, retinae were analyzed through mass spectrometry and immunohistology. Using data-independent acquisition-based mass spectrometry, we identified more than 1700 proteins on a high confidence level for both study groups, respectively. Of these 1700, 43 proteins were significantly altered in retinae after 7 days and 67 proteins revealed significant alterations at 14 days. For example, α2-macroglobulin was found significantly increased not only by mass spectrometry analysis, but also with immunohistological staining in S100B retinae at 7 and 14 days. All in all, the identified proteins are often associated with the immune system, such as heat shock protein 60. Once more, these data underline the important role of immunological factors in glaucoma pathogenesis.

An uncommon neuronal class conveys visual signals from rods and cones to retinal ganglion cells

Neurons in the central nervous system (CNS) are distinguished by the neurotransmitter types they release, their synaptic connections, morphology, and genetic profiles. To fully understand how the CNS works, it is critical to identify all neuronal classes and reveal their synaptic connections. The retina has been extensively used to study neuronal development and circuit formation. Here, we describe a previously unidentified interneuron in mammalian retina. This interneuron shares some morphological, physiological, and molecular features with retinal bipolar cells, such as receiving input from photoreceptors and relaying visual signals to retinal ganglion cells. It also shares some features with amacrine cells (ACs), particularly Aii-ACs, such as their neurite morphology in the inner plexiform layer, the expression of some AC-specific markers, and possibly the release of the inhibitory neurotransmitter glycine. Thus, we unveil an uncommon interneuron, which may play an atypical role in vision.

Regional Variation of Gap Junctional Connections in the Mammalian Inner Retina

The retinas of many species show regional specialisations that are evident in the differences in the processing of visual input from different parts of the visual field. Regional specialisation is thought to reflect an adaptation to the natural visual environment, optical constraints, and lifestyle of the species. Yet, little is known about regional differences in synaptic circuitry. Here, we were interested in the topographical distribution of connexin-36 (Cx36), the major constituent of electrical synapses in the retina. We compared the retinas of mice, rats, and cats to include species with different patterns of regional specialisations in the analysis. First, we used the density of Prox1-immunoreactive amacrine cells as a marker of any regional specialisation, with higher cell density signifying more central regions. Double-labelling experiments showed that Prox1 is expressed in AII amacrine cells in all three species. Interestingly, large Cx36 plaques were attached to about 8-10% of Prox1-positive amacrine cell somata, suggesting the strong electrical coupling of pairs or small clusters of cell bodies. When analysing the regional changes in the volumetric density of Cx36-immunoreactive plaques, we found a tight correlation with the density of Prox1-expressing amacrine cells in the ON, but not in the OFF sublamina in all three species. The results suggest that the relative contribution of electrical synapses to the ON- and OFF-pathways of the retina changes with retinal location, which may contribute to functional ON/OFF asymmetries across the visual field.

Age-Related Alterations of Proteins in Albino Wistar Rat Retina

Imbalance of homeostasis causes permanent changes in the body with time. The central nervous system is especially prone to these changes since it possesses limited regenerative capacity. In the retina, neurons are damaged during the aging process, and this eventually leads to deterioration of vision. In our 2-year-long study, we examined genetically closely related rat individuals to disclose the hidden retinal causes of age-associated visual dysfunction. Morphometric analysis showed significant reduction of the retina thickness with aging, particularly that of the inner plexiform layer. To reveal changes between the age groups, we used immunohistochemistry against vesicular glutamate transporter 1 protein for photoreceptor and bipolar cell terminals, Brn3a for ganglion cells, calbindin 28 kDa for horizontal cells, parvalbumin for AII amacrines, protein kinase Cα for rod bipolar cells, tyrosine hydroxylase for dopaminergic cells, glial fibrillary acidic protein for glial cells, and peanut-agglutinin labeling for cones. The most significant decrease was observed in the density of photoreceptor and the ganglion cells in the aging process. By using immunocytochemistry and western blot technique, we observed that calbindin and vesicular glutamate transporter 1 protein staining do not change much with aging; tyrosine hydroxylase, parvalbumin and calretinin showed the highest immunoreactivity during the midlife period. Most interestingly, the level of glial fibrillary acidic protein also changes similarly to the previously named markers. Our results provide further evidence that protein content is modified at least in some cell populations of the rat retina, and the number of retinal cells declined with aging. We conclude that senescence alone may cause structural and functional damage in the retinal tissue.

Melanopsin and calbindin immunoreactivity in the inner retina of humans and marmosets

In primate retina, the calcium-binding protein calbindin is expressed by a variety of neurons including cones, bipolar cells, and amacrine cells but it is not known which type(s) of cell express calbindin in the ganglion cell layer. The present study aimed to identify calbindin-positive cell type(s) in the amacrine and ganglion cell layer of human and marmoset retina using immunohistochemical markers for ganglion cells (RBPMS and melanopsin) and cholinergic amacrine (ChAT) cells. Intracellular injections following immunolabeling was used to reveal the morphology of calbindin-positive cells. In human retina, calbindin-labeled cells in the ganglion cell layer were identified as inner and outer stratifying melanopsin-expressing ganglion cells, and ON ChAT (starburst amacrine) cells. In marmoset, calbindin immunoreactivity in the ganglion cell layer was absent from ganglion cells but present in ON ChAT cells. In the inner nuclear layer of human retina, calbindin was found in melanopsin-expressing displaced ganglion cells and in at least two populations of amacrine cells including about a quarter of the OFF ChAT cells. In marmoset, a very low proportion of OFF ChAT cells was calbindin-positive. These results suggest that in both species there may be two types of OFF ChAT cells. Consistent with previous studies, the ratio of ON to OFF ChAT cells was about 70 to 30 in human and 30 to 70 in marmoset. Our results show that there are species-related differences between different primates with respect to the expression of calbindin.

Immunocytochemical localization of cholinergic amacrine cells in the bat retina

The purpose of this study was to localize the cholinergic amacrine cells, one of the key elements of a functional retina, in the retina of a microbat, Rhinolophus ferrumequinum. The presence and localization of choline acetyltransferase-immunoreactive (ChAT-IR) cells in the microbat retina were investigated using immunocytochemistry, confocal microscopy, and quantitative analysis. These ChAT-IR cells were present in the ganglion cell layer (GCL) and inner part of the inner nuclear layer (INL), as previously reported in various animals. However, the bat retina also contained some ChAT-IR cells in the outer part of the INL. The dendrites of these cells extended into the outer plexiform layer, and those of the cells in the inner INL extended within the outer part of the inner plexiform layer (IPL). The dendrites of the ChAT-IR cells in the GCL extended into the middle of the IPL and some fibers ramified up to the outer IPL. The average densities of ChAT-IR cells in the GCL, inner INL, and outer INL were 259±31cells/mm², 469±48cells/mm², and 59±8cells/mm², respectively. The average total density of the ChAT-IR cells was 788±58cells/mm² (mean±S.D.; n=3; 2799±182 cells/retina). We also found that the cholinergic amacrine cells in the bat retina contained calbindin, one of the calcium-binding proteins, but not calretinin or parvalbumin. As the cholinergic amacrine cells play key roles in the direction selectivity and optokinetic eye reflex in the other mammalian retinas, the present study might provide better information of the cytoarchitecture of bat retina and the basic sources for further physiological studies.

Connexin36 Expression in the Mammalian Retina: A Multiple-Species Comparison

Much knowledge about interconnection of human retinal neurons is inferred from results on animal models. Likewise, there is a lack of information on human retinal electrical synapses/gap junctions (GJ). Connexin36 (Cx36) forms GJs in both the inner and outer plexiform layers (IPL and OPL) in most species including humans. However, a comparison of Cx36 GJ distribution in retinas of humans and popular animal models has not been presented. To this end a multiple-species comparison was performed in retinas of 12 mammals including humans to survey the Cx36 distribution. Areas of retinal specializations were avoided (e.g., fovea, visual streak, area centralis), thus observed Cx36 distribution differences were not attributed to these species-specific architecture of central retinal areas. Cx36 was expressed in both synaptic layers in all examined retinas. Cx36 plaques displayed an inhomogenous IPL distribution favoring the ON sublamina, however, this feature was more pronounced in the human, swine and guinea pig while it was less obvious in the rabbit, squirrel monkey, and ferret retinas. In contrast to the relative conservative Cx36 distribution in the IPL, the labels in the OPL varied considerably among mammals. In general, OPL plaques were rare and rather small in rod dominant carnivores and rodents, whereas the human and the cone rich guinea pig retinas displayed robust Cx36 labels. This survey presented that the human retina displayed two characteristic features, a pronounced ON dominance of Cx36 plaques in the IPL and prevalent Cx36 plaque conglomerates in the OPL. While many species showed either of these features, only the guinea pig retina shared both. The observed similarities and subtle differences in Cx36 plaque distribution across mammals do not correspond to evolutionary distances but may reflect accomodation to lifestyles of examined species.

Pre-treatment with vinpocetine protects against retinal ischemia

Vinpocetine has been shown to have beneficial effects for tissues of the central nervous system subjected to ischemia and other related metabolic insults. We recently showed vinpocetine promotes glucose availability, prevents unregulated cation channel permeability and regulates glial reactivity when present during retinal ischemia. Less is known however about the ability of vinpocetine to protect against future ischemic insults. This study explores the effect of vinpocetine when used as a pre-treatment in an ex vivo model for retinal ischemia using cation channel permeability of agmatine (AGB) combined with immunohistochemistry as a measure for cell functionality. We found that vinpocetine pre-treatment reduced cation channel permeability and apoptotic marker immunoreactivity in the GCL and increased parvalbumin immunoreactivity of inner retinal neurons in the inner nuclear layer following ischemic insult. Vinpocetine pre-treatment also reduced Müller cell reactivity following ischemic insults of up to 120 min compared to untreated controls. Many of vinpocetine's effects however were transient in nature suggesting the drug can protect retinal neurons against future ischemic damage but may have limited long-term applications.

Calcium buffer proteins are specific markers of human retinal neurons

Ca(2+)-buffer proteins (CaBPs) modulate the temporal and spatial characteristics of transient intracellular Ca(2+)-concentration changes in neurons in order to fine-tune the strength and duration of the output signal. CaBPs have been used as neurochemical markers to identify and trace neurons of several brain loci including the mammalian retina. The CaBP content of retinal neurons, however, varies between species and, thus, the results inferred from animal models cannot be utilised directly by clinical ophthalmologists. Moreover, the shortage of well-preserved human samples greatly impedes human retina studies at the cellular and network level. Our purpose has therefore been to examine the distribution of major CaBPs, including calretinin, calbindin-D28, parvalbumin and the recently discovered secretagogin in exceptionally well-preserved human retinal samples. Based on a combination of immunohistochemistry, Neurolucida tracing and Lucifer yellow injections, we have established a database in which the CaBP marker composition can be defined for morphologically identified cell types of the human retina. Hence, we describe the full CaBP make-up for a number of human retinal neurons, including HII horizontal cells, AII amacrine cells, type-1 tyrosine-hydroxylase-expressing amacrine cells and other lesser known neurons. We have also found a number of unidentified cells whose morphology remains to be characterised. We present several examples of the colocalisation of two or three CaBPs with slightly different subcellular distributions in the same cell strongly suggesting a compartment-specific division of labour of Ca(2+)-buffering by CaBPs. Our work thus provides a neurochemical framework for future ophthalmological studies and renders new information concerning the cellular and subcellular distribution of CaBPs for experimental neuroscience.

Prdm13 regulates subtype specification of retinal amacrine interneurons and modulates visual sensitivity

Amacrine interneurons, which are highly diversified in morphological, neurochemical, and physiological features, play crucial roles in visual information processing in the retina. However, the specification mechanisms and functions in vision for each amacrine subtype are not well understood. We found that the Prdm13 transcriptional regulator is specifically expressed in developing and mature amacrine cells in the mouse retina. Most Prdm13-positive amacrine cells are Calbindin- and Calretinin-positive GABAergic or glycinergic neurons. Absence of Prdm13 significantly reduces GABAergic and glycinergic amacrines, resulting in a specific defect of the S2/S3 border neurite bundle in the inner plexiform layer. Forced expression of Prdm13 distinctively induces GABAergic and glycinergic amacrine cells but not cholinergic amacrine cells, whereas Ptf1a, an upstream transcriptional regulator of Prdm13, induces all of these subtypes. Moreover, Prdm13-deficient mice showed abnormally elevated spatial, temporal, and contrast sensitivities in vision. Together, these results show that Prdm13 regulates development of a subset of amacrine cells, which newly defines an amacrine subtype to negatively modulate visual sensitivities. Our current study provides new insights into mechanisms of the diversification of amacrine cells and their function in vision.

Identification of AⅡ amacrine, displaced amacrine, and bistratified ganglion cell types in human retina with antibodies against calretinin

Antibodies against calretinin are markers for one type of rod pathway interneuron (AⅡ amacrine cell) in the retina of some but not all mammalian species. The AⅡ cells play a crucial role in night-time (scotopic) vision and have been proposed as a target for optogenetic restoration of vision in retinal disease. In the present study we aimed to characterize the AⅡ cells in human retina. Postmortem human donor eyes were obtained with ethical approval and processed for calretinin immunofluorescence. Calretinin-positive somas in the inner nuclear and the ganglion cell layer were filled with the lipophilic dye DiI. The large majority (over 80%) of calretinin-immunoreactive cells is located in the inner nuclear layer, is immunopositive for glycine transporter 1, and shows the typical morphology of AⅡ amacrine cells. In addition, a small proportion of calretinin-positive cells in the inner nuclear layer and in the ganglion cell layer is glutamic acid decarboxylase-positive and shows the morphology of widefield amacrine cells (stellate, semilunar, and thorny amacrine cells). About half of the calretinin cells in the ganglion cell layer are bistratified ganglion cells resembling the small bistratified (presumed blue-ON/yellow-OFF) and the G17 ganglion cell previously described in primates. We conclude that in human retina, antibodies against calretinin can be used to identify AⅡ amacrine cells in the inner nuclear layer as well as widefield amacrine and small bistratified ganglion cells in the ganglion cell layer.

Developmental changes in the expression level of connexin36 in the rat retina

Connexin36 (Cx36) is the major gap junction forming protein in the brain and the retina; thus, alterations in its expression indicate changes in the corresponding circuitry. Many structural changes occur in the early postnatal retina before functional neuronal circuits are finalized, including those that incorporate gap junctions. To reveal the time-lapse formation of inner retinal gap junctions, we examine the developing postnatal rat retina from birth (P0) to young adult age (P20) and follow the expression of Cx36 in the mRNA and protein levels. We found a continuous elevation in the expression of both the Cx36 transcript and protein between P0 and P20 and a somewhat delayed Cx36 plaque formation throughout the inner plexiform layer (IPL) starting at P10. By using tristratificated calretinin positive (CaR(+)) fibers in the IPL as a guide, we detected a clear preference of Cx36 plaques for the ON sublamina from the earliest time of detection. This distributional preference became more pronounced at P15 and P20 due to the emergence and widespread expression of large (>0.1 μm(2)) Cx36 plaques in the ON sublamina. Finally, we showed that parvalbumin-positive (PV(+)) AII amacrine cell dendrites colocalize with Cx36 plaques as early as P10 in strata 3 and 4, whereas colocalizations in stratum 5 became characteristic only around P20. We conclude that Cx36 expression in the rat IPL displays a characteristic succession of changes during retinogenesis reflecting the formation of the underlying electrical synaptic circuitry. In particular, AII cell gap junctions, first formed with ON cone bipolar cells and later with other AII amacrine cells, accounted for the observed Cx36 expressional changes.

Adenosine triphosphate-induced photoreceptor death and retinal remodeling in rats

Many common causes of blindness involve the death of retinal photoreceptors, followed by progressive inner retinal cell remodeling. For an inducible model of retinal degeneration to be useful, it must recapitulate these changes. Intravitreal administration of adenosine triphosphate (ATP) has recently been found to induce acute photoreceptor death. The aim of this study was to characterize the chronic effects of ATP on retinal integrity. Five-week-old, dark agouti rats were administered 50 mM ATP into the vitreous of one eye and saline into the other. Vision was assessed using the electroretinogram and optokinetic response and retinal morphology investigated via histology. ATP caused significant loss of visual function within 1 day and loss of 50% of the photoreceptors within 1 week. At 3 months, 80% of photoreceptor nuclei were lost, and total photoreceptor loss occurred by 6 months. The degeneration and remodeling were similar to those found in heritable retinal dystrophies and age-related macular degeneration and included inner retinal neuronal loss, migration, and formation of new synapses; Müller cell gliosis, migration, and scarring; blood vessel loss; and retinal pigment epithelium migration. In addition, extreme degeneration and remodeling events, such as neuronal and glial migration outside the neural retina and proliferative changes in glial cells, were observed. These extreme changes were also observed in the 2-year-old P23H rhodopsin transgenic rat model of retinitis pigmentosa. This ATP-induced model of retinal degeneration may provide a valuable tool for developing pharmaceutical therapies or for testing electronic implants aimed at restoring vision.

Starburst amacrine cells express parvalbumin but not calbindin and calretinin in rabbit retina

Calcium-binding proteins (CBPs) are important components in calcium-mediated cellular signal transduction. Among the many CBPs, at least three EF-hand CBPs, calbindin-D28K (CB), calretinin (CR), and parvalbumin (PV), have been extensively studied in the retina. In the present study, we investigated the expression patterns of these three CBPs in cholinergic starburst amacrine cells (SACs), which are the most important element for direction selectivity in the rabbit retina. Double-label immunocytochemical analysis of vibratome sections and single-cell injection after immunocytochemical analysis on whole mounts were carried out in rabbit retinas. We found that all SACs in the inner nuclear layer and the ganglion cell layer contained PV. However, none of the SACs in the inner nuclear layer or ganglion cell layer contained either CB or CR. These results suggest that PV, but not CR or CB, may act as a calcium-buffering protein in the SACs of the rabbit retina.

Inhibitor of apoptosis-stimulating protein of p53 (iASPP) is required for neuronal survival after axonal injury

The transcription factor p53 mediates the apoptosis of post-mitotic neurons exposed to a wide range of stress stimuli. The apoptotic activity of p53 is tightly regulated by the apoptosis-stimulating proteins of p53 (ASPP) family members: ASPP1, ASPP2 and iASPP. We previously showed that the pro-apoptotic members ASPP1 and ASPP2 contribute to p53-dependent death of retinal ganglion cells (RGCs). However, the role of the p53 inhibitor iASPP in the central nervous system (CNS) remains to be elucidated. To address this, we asked whether iASPP contributes to the survival of RGCs in an in vivo model of acute optic nerve damage. We demonstrate that iASPP is expressed by injured RGCs and that iASPP phosphorylation at serine residues, which increase iASPP affinity towards p53, is significantly reduced following axotomy. We show that short interference RNA (siRNA)-induced iASPP knockdown exacerbates RGC death, whereas adeno-associated virus (AAV)-mediated iASPP expression promotes RGC survival. Importantly, our data also demonstrate that increasing iASPP expression in RGCs downregulates p53 activity and blocks the expression of pro-apoptotic targets PUMA and Fas/CD95. This study demonstrates a novel role for iASPP in the survival of RGCs, and provides further evidence of the importance of the ASPP family in the regulation of neuronal loss after axonal injury.

Control of neuronal morphology by the atypical cadherin Fat3

Neurons receive signals through dendrites that vary widely in number and organization, ranging from one primary dendrite to multiple complex dendritic trees. For example, retinal amacrine cells (ACs) project primary dendrites into a discrete synaptic layer called the inner plexiform layer (IPL) and only rarely extend processes into other retinal layers. Here, we show that the atypical cadherin Fat3 ensures that ACs develop this unipolar morphology. AC precursors are initially multipolar but lose neurites as they migrate through the neuroblastic layer. In fat3 mutants, pruning is unreliable and ACs elaborate two dendritic trees: one in the IPL and a second projecting away from the IPL that stratifies to form an additional synaptic layer. Since complex nervous systems are characterized by the addition of layers, these results demonstrate that mutations in a single gene can cause fundamental changes in circuit organization that may drive nervous system evolution.

Nuclear lamins are differentially expressed in retinal neurons of the adult rat retina

Lamins are type V intermediate filament proteins that support nuclear membranes. They are divided into A-type lamins, which include lamin A and C, and B-type lamins, which include lamin B1 and B2. In the rat brain, lamin A and C are expressed in relatively equal amounts, while the expressions of lamin B1 and B2 vary depending on the cell type. Lamins play important roles in normal morphogenesis and function. In the nervous system, their abnormal expression causes several neurodegenerative diseases such as peripheral neuropathy, leukodystrophy and lissencephaly. The retina belongs to the central nervous system (CNS) and has widely been used as a source of CNS neurons. We investigated the expression patterns of lamin subtypes in the adult rat retina by immunohistochemistry and found that the staining patterns differed when compared with the brain. All retinal neurons expressed lamin B1 and B2 in relatively equal amounts. In addition, horizontal cells and a subpopulation of retinal ganglion cells expressed lamin A and C, while photoreceptor cells expressed neither lamin A nor C, and all other retinal neurons expressed lamin C only. This differential expression pattern of lamins in retinal neurons suggests that they may be involved in cellular differentiation and expression of cell-specific genes in individual retinal neurons.

MATH5 controls the acquisition of multiple retinal cell fates

Math5-null mutation results in the loss of retinal ganglion cells (RGCs) and in a concurrent increase of amacrine and cone cells. However, it remains unclear whether there is a cell fate switch of Math5-lineage cells in the absence of Math5 and whether MATH5 cell-autonomously regulates the differentiation of the above retinal neurons. Here, we performed a lineage analysis of Math5-expressing cells in developing mouse retinas using a conditional GFP reporter (Z/EG) activated by a Math5-Cre knock-in allele. We show that during normal retinogenesis, Math5-lineage cells mostly develop into RGCs, horizontal cells, cone photoreceptors, rod photoreceptors, and amacrine cells. Interestingly, amacrine cells of Math5-lineage cells are predominately of GABAergic, cholinergic, and A2 subtypes, indicating that Math5 plays a role in amacrine subtype specification. In the absence of Math5, more Math5-lineage cells undergo cell fate conversion from RGCs to the above retinal cell subtypes, and occasionally to cone-bipolar cells and Müller cells. This change in cell fate choices is accompanied by an up-regulation of NEUROD1, RXRγ and BHLHB5, the transcription factors essential for the differentiation of retinal cells other than RGCs. Additionally, loss of Math5 causes the failure of early progenitors to exit cell cycle and leads to a significant increase of Math5-lineage cells remaining in cell cycle. Collectively, these data suggest that Math5 regulates the generation of multiple retinal cell types via different mechanisms during retinogenesis.

Localization of the calcium-binding protein secretagogin in cone bipolar cells of the mammalian retina

Secretagogin, a recently cloned member of the EF-hand family of calcium binding proteins, was localized in the mouse, rat, and rabbit retina using immunofluorescence immunohistochemistry. Secretagogin is expressed in subpopulations of ON and OFF cone bipolar cells; however, no immunoreactivity was observed in rod bipolar cells in any of these species. Using subtype-specific markers and mice expressing green fluorescent protein (GFP) within specific cell classes, we found that secretagogin is expressed in Types 2, 3, 4, 5, 6 and possibly Type 8 cone bipolar cells in the mouse retina. The expression pattern in the rat retina differs slightly with expression in cone bipolar cell Types 2, 5, 6, 7, and 8. Evaluation of secretagogin in the developing mouse retina revealed expression as early as postnatal day 6, with OFF cone bipolar cells showing secretagogin expression prior to the ON cone bipolar cells. Secretagogin is a useful marker of cone bipolar cells for studying alterations in bipolar cell morphology during development and degeneration. Further work will be necessary to elucidate the functional role of this protein in bipolar cells.

Immunotoxin-induced ablation of melanopsin retinal ganglion cells in a non-murine mammalian model

In mammals, non-image-forming visual functions, including circadian photoentrainment and the pupillary light reflex, are thought to be mediated by the combination of rods, cones, and the melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs). Although several genetic models have been developed to clarify the individual roles of the rod, cone, and ipRGC systems in mediating non-image visual function, assessing the in vivo role(s) of the ipRGCs has been complicated by the possibility of ontogenetic issues in these genetically modified animal models. In the present study, we describe the development and validation of an immunotoxin that specifically targets the ipRGC population in the mature mammalian retina. This ipRGC immunotoxin, consisting of saporin conjugated to a melanopsin polyclonal antibody, was evaluated with respect to its effectiveness and specificity in depleting the ipRGC population in the fully developed rat retina. The results showed that the ipRGC toxin rapidly and permanently depleted approximately 70% of the ipRGC population, without inducing appreciable changes in the cell number or morphology of any of the non-melanopsin-containing retinal cell populations investigated. These findings suggest that the newly developed ipRGC immunotoxin provides a potent method for achieving relatively rapid, permanent, and selective depletion of the ipRGC population in a non-murine model system. The development of this ipRGC-ablation method is the next step in elucidating the role of ipRGCs in mediating non-visual and circadian light-resetting responses in a wide range of non-murine mammalian models.

Evaluation of the protective effects of PACAP with cell-specific markers in ischemia-induced retinal degeneration

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurotrophic and neuroprotective peptide that has been shown to exert protective effects in different neuronal injuries, such as traumatic brain injury, models of neurodegenerative diseases and cerebral ischemia. We have provided evidence that PACAP is neuroprotective in several models of retinal degeneration in vivo. In our previous studies we showed that PACAP treatment significantly ameliorated the damaging effects of permanent bilateral common carotid artery occlusion (BCCAO). In the present study cell-type-specific markers were used in the same models in order to further specify the protective effects of PACAP. In rats BCCAO led to severe degeneration of all retinal layers that was attenuated by PACAP (100 pmol) administered unilaterally immediately following BCCAO into the vitreous body of one eye. Retinas were processed for immunohistochemistry after 3 weeks. Immunolabeling was executed for vesicular glutamate transporter 1 (VGLUT 1), vesicular gamma-aminobutyric acid transporter (VGAT), protein kinase Calpha (PKCalpha), glial fibrillary acidic protein (GFAP) and calcium-binding proteins, such as calbindin, calretinin, parvalbumin. In BCCAO retinas, intensity of immunopositivity for all antisera was dramatically decreased, except in the case of GFAP. In PACAP-treated retinas, immunostaining was similar to that of the control animals. In summary, our study presented immunohistochemical identification of cell types sensitive to chronic retinal hypoperfusion and the protective effects of PACAP. This analysis revealed that the retinoprotective effects of PACAP are not phenotype-specific, but it rather influences general cytoprotective pathways irrespective of the neuronal subtypes in the retina subjected to chronic hypoperfusion.

Neuronal and glial cell expression of angiotensin II type 1 (AT1) and type 2 (AT2) receptors in the rat retina

The bio-active peptide, angiotensin II (Ang II), has been suggested to exert a neuromodulatory effect on inner retinal neurons. In this study, we examined the distribution of angiotensin receptors (ATRs) in the developing and mature rat retina and optic nerve using immunofluorescence immunocytochemistry. Double-labeling experiments were performed with established markers to identify different retinal cell populations. In adult retinae, ATRs were observed on neurons involved in "ON" pathways of neurotransmission. Angiotensin II type 1 receptors (AT(1)Rs) were expressed by a sub-population of "ON" cone bipolar cells that also labeled for G alpha(0) and islet-1. Extra-neuronal expression of AT(1)Rs was evident on retinal astrocytes, Müller cells and blood vessels. Immunoreactivity for the angiotensin II type 2 receptor (AT(2)R) was observed on conventional and displaced GABAergic amacrine cells. Co-localization studies showed that AT(2)R-expressing amacrine cells constituted at least two separate sub-populations. Cell counts revealed that all wide-field amacrine cells expressing protein kinase C-alpha were also AT(2)R-positive; a further subset of amacrine cells expressing AT(2)Rs and stratifying in sublamina "b" of the inner plexiform layer (IPL) was identified. Developmental expression of AT(1)Rs was dynamic, involving multiple inner neuronal classes. At postnatal day 8 (P8), AT(1)R immunoreactivity was observed on putative ganglion cells. The characteristic bipolar cell labeling observed in adults was not evident until P13. In contrast, AT(2)Rs were detected as early as P2 and localized specifically to amacrine cells from this age onward. These data provide further evidence for the potential role of angiotensin II in the modulation of retinal neurons and glia. The differential pattern of expression of these receptors across these cell types is similar to that observed in the brain and suggests that a similar functional role for Ang II may also exist within the retina.

Subsets of retinal neurons and glia express P2Y1 receptors

Recent evidence suggests that extracellular ATP modulates retinal processing and could play a role in modulating glial cells during retinal diseases. Here, we evaluated the localization of P2Y(1) receptors in the rat retina using indirect immunofluorescence immunocytochemistry. We observed labeling within defined populations of inner retinal neurons and Müller cell processes and end feet. Double labeling of P2Y(1) receptor with choline acetyltransferase revealed extensive colocalization indicating the expression of this receptor by cholinergic amacrine cells. Ganglion cell labeling for P2Y(1) receptors was also observed. Having established the normal pattern of immunolabeling within the retina, we next examined whether immunolabeling was altered by retinal disease. P2Y(1) receptor immunolabeling of Müller cells was of greater intensity following light-induced retinal degeneration, suggesting that Müller cell gliosis is accompanied by changes in P2Y(1) receptor expression. Overall, these data provide further evidence for a role of extracellular ATP in retinal signaling within subsets of retinal neurons as well as glia.

Developmentally regulated and evolutionarily conserved expression of SLITRK1 in brain circuits implicated in Tourette syndrome

Tourette syndrome (TS) is an inherited developmental neuropsychiatric disorder characterized by vocal and motor tics. Multiple lines of neurophysiological evidence implicate dysfunction in the corticostriatal-thalamocortical circuits in the etiology of TS. We recently identified rare sequence variants in the Slit and Trk-like family member 1 (SLITRK1) gene associated with TS. SLITRK1, a single-pass transmembrane protein, displays similarities to the SLIT family of secreted ligands, which have roles in axonal repulsion and dendritic patterning, but its function and developmental expression remain largely unknown. Here we provide evidence that SLITRK1 has a developmentally regulated expression pattern in projection neurons of the corticostriatal-thalamocortical circuits. SLITRK1 is further enriched in the somatodendritic compartment and cytoplasmic vesicles of cortical pyramidal neurons in mouse, monkey, and human brain, observations suggestive of an evolutionarily conserved function in mammals. SLITRK1 is transiently expressed in the striosomal/patch compartment of the mammalian striatum and moreover is associated with the direct output pathway; adult striatal expression is confined to cholinergic interneurons. These analyses demonstrate that the expression of SLITRK1 is dynamic and specifically associated with the circuits most commonly implicated in TS and related disorders, suggesting that SLITRK1 contributes to the development of corticostriatal-thalamocortical circuits.

Retinal anatomy and visual performance in a diurnal cone-rich laboratory rodent, the Nile grass rat (Arvicanthis niloticus)

Unlike laboratory rats and mice, muridae of the Arvicanthis family (A. ansorgei and A. niloticus) are adapted to functioning best in daylight. To date, they have been used as experimental models mainly in studies of circadian rhythms. However, recent work aimed at optimizing photoreceptor-directed gene delivery vectors (Khani et al. [2007] Invest Ophthalmol Vis Sci 48:3954-3961) suggests their potential usefulness for studying retinal pathologies and therapies. In the present study we analyzed the retinal anatomy and visual performance of the Nile grass rat (A. niloticus) using immunohistofluorescence and the optokinetic response (OKR). We found that approximately 35-40% of photoreceptors are cones; that many neural features of the inner retina are similar to those in other diurnal mammals; and that spatial acuity, measured by the OKR, is more than two times that of the usual laboratory rodents. These observations are consistent with the known diurnal habits of this animal, and further support its pertinence as a complementary model for studies of structure, function, and pathology in cone-rich mammalian retinae.

Expression of the homeodomain transcription factor Meis2 in the embryonic and postnatal retina

Members of the Meis subfamily of homeodomain-containing transcription factors play important roles during development and disease. Here we report that the Meis family protein Meis2 is expressed by a subpopulation of gamma-aminobutyric acid (GABA)ergic amacrine (AM) cells in the adult and embryonic retina of different vertebrate species. In mice, Meis2-expressing (Meis2+) AM cells are not cholinergic or dopaminergic, but some are immunoreactive for neuronal nitric oxide synthase (bNOS). About 50% of the mouse Meis2+ AM cell population expresses the calcium-binding protein calretinin, and some Meis2+ AM cells show characteristics of Type II CD-15+ cells. AM cell expression of Meis2 is lost in a conditional knockout mouse model for Pax6, indicating a dependency upon Pax6. Bromodeoxyuridine pulse labeling experiments and immunohistochemical staining for the neuronal marker NeuN in embryonic mouse retinae indicate that Meis2 is an early marker for newly postmitotic AM cells. In addition, taking advantage of the protracted retinal development in humans, we show that newly generated AM cells express Meis2 before adopting the GABAergic or glycinergic neurotransmitter phenotype. As development proceeds, some AM cells lose Meis2 expression concomitantly with the appearance of glycine, while other AM cells retain Meis2 expression after they express GABA. These data identify Meis2 as a suitable marker for the study of AM cell diversity and development in addition to providing evidence for the stepwise specification of the glycinergic and GABAergic neurotransmitter phenotypes during AM cell differentiation.

Islet-1 controls the differentiation of retinal bipolar and cholinergic amacrine cells

Whereas the mammalian retina possesses a repertoire of factors known to establish general retinal cell types, these factors alone cannot explain the vast diversity of neuronal subtypes. In other CNS regions, the differentiation of diverse neuronal pools is governed by coordinately acting LIM-homeodomain proteins including the Islet-class factor Islet-1 (Isl1). We report that deletion of Isl1 profoundly disrupts retinal function as assessed by electroretinograms and vision as assessed by optomotor behavior. These deficits are coupled with marked reductions in mature ON- and OFF-bipolar (>76%), cholinergic amacrine (93%), and ganglion (71%) cells. Mosaic deletion of Isl1 permitted a chimeric analysis of "wild-type" cells in a predominantly Isl1-null environment, demonstrating a cell-autonomous role for Isl1 in rod bipolar and cholinergic amacrine development. Furthermore, the effects on bipolar cell development appear to be dissociable from the preceding retinal ganglion cell loss, because Pou4f2-null mice are devoid of similar defects in bipolar cell marker expression. Expression of the ON- and OFF-bipolar cell differentiation factors Bhlhb4 and Vsx1, respectively, requires the presence of Isl1, whereas the early bipolar cell marker Prox1 initially did not. Thus, Isl1 is required for engaging bipolar differentiation pathways but not for general bipolar cell specification. Spatiotemporal expression analysis of additional LIM-homeobox genes identifies a LIM-homeobox gene network during bipolar cell development that includes Lhx3 and Lhx4. We conclude that Isl1 has an indispensable role in retinal neuron differentiation within restricted cell populations and this function may reflect a broader role for other LIM-homeobox genes in retinal development, and perhaps in establishing neuronal subtypes.

Expression of somatostatin receptor subtype 5 in rat retinal amacrine cells

Somatostatin (SRIF), as a neuroactive peptide in the CNS, exerts its actions via five subtypes of specific receptors (ssts). In this work, the localization of sst(5) was studied immunocytochemically in rat retinal amacrine cells (ACs). Labeling for sst(5) was diffusely distributed throughout the full thickness of the inner plexiform layer (IPL) and formed two distinct fluorescence bands in the distal part of the IPL. Double labeling experiments showed that sst(5) was expressed in GABAergic ACs. It was further shown that labeling for sst(5) was observed in both dopaminergic and cholinergic ACs, stained by tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT), respectively. The immunostaining appeared mainly on the cell membranes and somatodendritic compartments of these ACs. For the cholinergic ACs, weak sst(5)-immunoreactivity was also observed in the processes terminating in the IPL. In contrast, no sst(5)-immunoreactivity was found in glycinergic AII ACs, stained by parvalbumin (PV). Furthermore, labeling for SRIF was co-localized with sst(5) in both dopaminergic and cholinergic ACs. These results suggest that sst(5) may serve as an autoreceptor or conventional receptor in retinal ACs.

P2X2 receptors on ganglion and amacrine cells in cone pathways of the rat retina

Extracellular ATP is known to mediate fast, excitatory neurotransmission through activation of ionotropic P2X receptors. In this study, the localization of the P2X(2) receptor (P2X(2)R) subunit was studied in rat retina by using immunofluorescence immunohistochemistry and preembedding immunoelectron microscopy. The P2X(2)R was observed in large ganglion cells as well as in a subset of amacrine cells. Double labeling revealed that 96% of all P2X(2)R-immunoreactive amacrine cells showed gamma-aminobutyric acid (GABA) immunoreactivity. Subsets of P2X(2)R-immunoreactive amacrine cells expressed nitric oxide synthase and substance P; however, no colocalization was observed with choline acetyltransferase, vasoactive intestinal peptide, or tyrosine hydroxylase. Nearest-neighbor analysis confirmed that P2X(2)Rs were expressed by a heterogeneous population of amacrine cells. The synaptic connectivity of P2X(2)R amacrine cells was also investigated. It was interesting that P2X(2)R-immunoreactive amacrine cell dendrites stratified in the sublaminae of the inner plexiform layer occupied by cone, but not rod bipolar cell axon terminals. Immunoelectron microscopy revealed that P2X(2)-immunoreactive amacrine cell processes were associated with cone bipolar cell axon terminals as well as other conventional synapses in the inner plexiform layer. Taken together, these data provide further evidence for the involvement of extracellular ATP in neuronal signaling in the retina, particularly within cone pathways.

Degree of damage compensation by various PACAP treatments in monosodium glutamate-induced retinal degeneration

Pituitary adenylate cyclase activating polypeptide (PACAP) has been shown to be neuroprotective in retinal ischemia and monosodium L-glutamate (MSG)-induced retinal degeneration. Here we describe how different MSG treatments (1x and 3x application) cause retinal damage and finally lead to the destruction of the entire inner retina and how PACAP attenuates this effect. Newborn rats from both sexes were injected subcutaneously with 2 mg/g bodyweight MSG on postnatal days 1, 5 and 9. The left eye was left intact while we injected 5 microl PACAP38 solution (100 pmol) into the vitreous of the right eye with a Hamilton syringe at the time of (i) the first, (ii) the first two or (iii) all three MSG injections. Histological analysis has shown that the above described MSG treatment caused the entire inner plexiform layer (IPL) to degenerate, and the inner nuclear (INL) and ganglion cell layers (GCL) seemed fused. One time PACAP38 treatment at the first MSG application did not change the degenerative capacity of MSG. However, if animals received PACAP38 into the vitreous of the eye at the first 2 or all 3 times, a substantial protective effect could be observed. The IPL remained well discernible, the INL retained 2-3 cell rows and the number of cells in the GCL was substantially higher than in the MSG-treated retinas, and was not significantly different from that observed in the control tissue. We conclude that (i) 2 or 3 times PACAP treatment attenuates retinal degeneration; (ii) one PACAP treatment does not provide protection against repeated excitotoxic insults, and (iii) repeated application of PACAP under these experimental conditions may lead to a primed state in which further neurotoxic insults are ineffective.

Runx1 expression defines a subpopulation of displaced amacrine cells in the developing mouse retina

AML1/Runx1 (Runx1) is a mammalian transcription factor that plays critical roles in regulating the differentiation of a number of different cell types. In the present study, we have utilized mice expressing beta-galactosidase (beta-gal) under the control of the Runx1 promoter to characterize the spatiotemporal expression pattern of Runx1 during retinogenesis. Expression of beta-gal was first detected at embryonic day 13.5 in post-mitotic cells located in the inner retina and overlapped with expression of the early amacrine and ganglion cell marker protein Islet1. During subsequent developmental stages, the number of beta-gal-positive cells increased in a central-to-peripheral gradient until late embryogenesis but then decreased in the early post-natal retina. beta-gal-positive cells were located primarily in the ganglion cell layer by late embryonic/early post-natal stages and were identified as a subpopulation of displaced amacrine cells by the continued expression of Islet1, as well as Pax6, and the coexpression of the amacrine cell subtype-specific markers choline acetyltransferase, calretinin and the 65-kDa isoform of glutamic acid decarboxylase. These findings identify Runx1 as a novel marker for a restricted amacrine cell subtype and suggest a role for this gene in regulating the post-mitotic development of these cells.

Immunocytochemical development of the guinea pig retina

The aim of the present study was to establish the neurochemical profile of amacrine and horizontal cells during ontogeny in the guinea pig, a precocial species where significant retinal development occurs prenatally as opposed to altricial species where development largely occurs postnatally. The expression of neurochemical markers of horizontal cells and specific amacrine cell populations was investigated from 20 days of gestation (dg, term approximately 67 dg) to adulthood. Amacrine cell populations were identified immunohistochemically using antibodies to gamma-amino-butyric acid, cholineacetyltransferase, calbindin, calretinin, neuronal nitric oxide synthetase and tyrosine hydroxylase; horizontal cells were labelled with calbindin. All markers were present at 30 dg and had attained their mature (adult) laminar distribution and expression by 60 dg. Horizontal cells appeared in their final location at 30 dg with amacrine cell populations appearing in their final locations by 45 dg. Thus, in the guinea pig retina, the amacrine and horizontal cell populations investigated in this study are fully mature prior to birth.

Differential effects of ischemia/reperfusion on amacrine cell subtype-specific transcript levels in the rat retina

Transient retinal ischemia induces loss of retinal ganglion cells, supporting the hypothesis that ischemic conditions contribute to the induction and progression of glaucoma. However, after 60 min of ischemia, also amacrine cells are lost from the inner nuclear layer. The main goal was to determine the relative vulnerability of various amacrine subpopulations by measuring the levels of transcripts that are known to be specifically expressed by different amacrine subpopulations. A 60-min ischemic period was administered to the rat eye by raising the intraocular pressure, followed by a reperfusion period lasting between 2 h and 4 weeks. Total RNA was isolated from the whole retina and expression levels were assessed by real-time quantitative polymerase chain reaction (qPCR). Retinal ischemia/reperfusion has differential effects on the levels of the various transcripts. Three main patterns of changes were identified. (i) A gradual decrease of transcript level without recovery was observed for parvalbumin; this transcript is expressed by the glycinergic AII cells. (ii) A gradual reduction to different levels at 72 h of reperfusion followed by a partial or complete recovery (glycine transporter 1, glutamate decarboxylase, calretinin, and several other transcripts). The glycinergic amacrine cell markers recovered to 65-75% of the control level, while the main GABAergic markers had completely recovered at 4 weeks. (iii) No significant changes of transcript levels were found for markers of several smaller GABAergic subpopulations [including substance P (Tac1), somatostatin, and others]. Expression levels of photoreceptor-, horizontal cell-, and bipolar cell-specific transcripts were not altered. These patterns were confirmed by a cluster analysis of the data. Based on gene expression levels, it may be concluded that amacrine cells are vulnerable to ischemic insults and that the glycinergic amacrine cells are relatively more sensitive to ischemia than the GABAergic population. In particular, the extensive loss of the parvalbumin-containing AII amacrine cells, which serve in the rod pathway, may have functional implications for vision under scotopic conditions. In the accompanying paper [F. Dijk and W. Kamphuis, An immunocytochemical study on specific amacrine subpopulations in the rat retina after ischemia, Brain Res. (2004).], the results are evaluated at the protein level by immunostaining for a selection of the amacrine cell markers.

An immunocytochemical study on specific amacrine cell subpopulations in the rat retina after ischemia

Transient retinal ischemia leads to the loss of neurons in the inner retina. In an accompanying paper [F. Dijk, S. Van Leeuwen, W. Kamphuis, Differential effects of ischemia/reperfusion on amacrine cell subtype-specific transcript levels in the rat retina, Brain Res., 1026 (2004) 194-204] we present the results of a study on the effects of experimentally induced retinal ischemia on transcript levels of genes expressed by distinct subpopulations of amacrine cells. In response to 60-min ischemia, three different patterns of changes in transcript levels were found, indicating a differential vulnerability of amacrine subtypes: (i) a gradual decrease of transcript level without recovery (parvalbumin; PV); (ii) a gradual decrease, with varying rates and degrees, followed by partial recovery after 72 h of reperfusion (choline acetyltransferase (ChAT), calretinin (CR) and glycine transporter (Glyt1)); (iii) no significant changes (substance P (SP)). In order to verify whether the degree of cell loss can be predicted from the quantified alterations in gene expression level, immunocytochemical stainings were carried out. A 60-min ischemic period was administered to the rat eye by raising the intraocular pressure, followed by a reperfusion period lasting between 2 h and 4 weeks. Cryosections were immunostained for Glyt1, PV, ChAT, CR, and SP. Double-labelling with apoptosis marker TUNEL was used to demonstrate cell type-specific apoptosis. Following ischemia, the numbers of detected PV-, Glyt1, ChAT-, and CR-immunopositive somata showed a substantial, but differential, reduction at 1-4 weeks after ischemia. The total amount of immunoreactivity present in the inner plexiform layer (IPL) also decreased. The extent of alterations derived from immunocytochemical staining was greater than was anticipated from the decrease of transcript levels. Only for SP, no significant decrease in number of cells or in the intensity of immunoreactivity in IPL was observed, which is in agreement with the absence of significant changes in transcript levels. In conclusion, retinal ischemia/reperfusion differentially affects amacrine cell populations. Although both protein and mRNA levels are reduced, transcript levels are less attenuated. Caution must be applied in the use of real-time quantitative PCR (qPCR) screening as a tool to assess the cellular pattern of neurodegeneration in the retina.

DARPP-32-like immunoreactivity in AII amacrine cells of rat retina

Previous studies demonstrated that the dopamine- and adenosine 3',5'-monophosphate-regulated phosphatase inhibitor known as "DARPP-32" is present in rat, cat, monkey, and human retinas. We have followed up these studies by asking what specific cell subtypes contain DARPP-32. Using a polyclonal antibody directed against a peptide sequence of human DARPP-32, we immunostained adult rat retinas that were either transretinally sectioned or flat mounted and found DARPP-32-like immunoreactivity in some cells of the amacrine cell layer across the entire retinal surface. We report here, based on the shape and spatial distribution of these cells, their staining by an anti-parvalbumin antibody, and their juxtaposition with processes containing tyrosine hydroxylase, that DARPP-32-like immunoreactivity is present in AII amacrine cells of rat retina. These results suggest that the response of AII amacrine cells to dopamine is not mediated as simply as previously supposed.

A unique role for Kv3 voltage-gated potassium channels in starburst amacrine cell signaling in mouse retina

Direction-selective retinal ganglion cells show an increased activity evoked by light stimuli moving in the preferred direction. This selectivity is governed by direction-selective inhibition from starburst amacrine cells occurring during stimulus movement in the opposite or null direction. To understand the intrinsic membrane properties of starburst cells responsible for direction-selective GABA release, we performed whole-cell recordings from starburst cells in mouse retina. Voltage-clamp recordings revealed prominent voltage-dependent K(+) currents. The currents were mostly blocked by 1 mm TEA, activated rapidly at voltages more positive than -20 mV, and deactivated quickly, properties reminiscent of the currents carried by the Kv3 subfamily of K+ channels. Immunoblots confirmed the presence of Kv3.1 and Kv3.2 proteins in retina and immunohistochemistry revealed their expression in starburst cell somata and dendrites. The Kv3-like current in starburst cells was absent in Kv3.1-Kv3.2 knock-out mice. Current-clamp recordings showed that the fast activation of the Kv3 channels provides a voltage-dependent shunt that limits depolarization of the soma to potentials more positive than -20 mV. This provides a mechanism likely to contribute to the electrical isolation of individual starburst cell dendrites, a property thought essential for direction selectivity. This function of Kv3 channels differs from that in other neurons where they facilitate high-frequency repetitive firing. Moreover, we found a gradient in the intensity of Kv3.1b immunolabeling favoring proximal regions of starburst cells. We hypothesize that this Kv3 channel gradient contributes to the preference for centrifugal signal flow in dendrites underlying direction-selective GABA release from starburst amacrine cells

Loss of calretinin immunoreactive fibers in subcortical visual recipient structures of the RCS dystrophic rat

The retinae of dystrophic Royal College of Surgeons (RCS) rats exhibit progressive photoreceptor degeneration accompanied by pathology of ganglion cells. To date, little work has examined the consequences of retinal degeneration for central visual structures in dystrophic rats. Here, we use immunohistochemistry for calretinin (CR) to label retinal afferents in the superior colliculus (SC), lateral geniculate nucleus, and olivary pretectal nucleus of RCS rats aged between 2 and 26 months of age. Early indications of fiber loss in the medial dystrophic SC were apparent between 9 and 13 months. Quantitative methods reveal a significant reduction in the level of CR immunoreactivity in visual layers of the medial dystrophic SC at 13 months (P < 0.02). In dystrophic animals aged 19-26 months the loss of CR fibers in SC was dramatic, with well-defined patches of fiber degeneration predominating in medial aspects of the structure. This fiber degeneration in SC was accompanied by increased detection of cells immunoreactive for CR. In several animals, regions of fiber loss were also found to contain strongly parvalbumin-immunoreactive cells. Loss of CR fibers was also observed in the lateral geniculate nucleus and olivary pretectal nucleus. Patterns of fiber loss in the dystrophic SC compliment reports of ganglion cell degeneration in these animals and the response of collicular neurons to degeneration is discussed in terms of plasticity of the dystrophic visual system and properties of calcium binding proteins.

Developmental changes in calbindin-D28k and calretinin expression in the mouse suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus, the primary circadian pacemaker in mammals, and the retinohypothalamic tract, the retinal afferent fibres to the suprachiasmatic nucleus, both mature during early postnatal life. The establishment of circadian rhythms is thought to depend on input from the retina, but the mechanism remains unknown. Here we examined developmental changes in the expression of the Ca2+-binding proteins calbindin-D28k and calretinin in the mouse hypothalamus. Robust calbindin-D28k immunoreactivity was observed in the dorsomedial suprachiasmatic nucleus and the supraoptic nucleus in neonatal mice (postnatal day 3). The calbindin-D28k immunoreactivity decreased significantly in the suprachiasmatic nucleus but not in the supraoptic nucleus during postnatal days 9-15, when retinohypothalamic tract projections to the suprachiasmatic nucleus are completed. Calretinin immunoreactivity was low in the neonatal suprachiasmatic nucleus and increased with development in the ventrolateral suprachiasmatic nucleus, in parallel with the developmental reduction of calbindin-D28k immunoreactivity observed in the dorsomedial suprachiasmatic nucleus. Developmentally stable calretinin immunoreactivity was also observed in retinohypothalamic tract fibres. Organotypic slice cultures of the suprachiasmatic nucleus were prepared from postnatal day 3 mice to examine the effect of the absence of retinohypothalamic tract inputs on developmental changes in calbindin-D28k and calretinin expression. After 12 days in vitro, the cultured suprachiasmatic nucleus slices exhibited dense calbindin-D28k immunoreactivity similar to neonatal mice, and calretinin immunoreactivity in the ventrolateral suprachiasmatic nucleus similar to young adult mice. These results demonstrate a developmental reduction in calbindin-D28k expression that paralleled retinohypothalamic tract formation and a developmental increase in calretinin expression that is independent of retinohypothalamic tract connections to suprachiasmatic nucleus neurons.

A key role of starburst amacrine cells in originating retinal directional selectivity and optokinetic eye movement

The directional selectivity of retinal ganglion cell responses represents a primitive pattern recognition that operates within a retinal neural circuit. The cellular origin and mechanism of directional selectivity were investigated by selectively eliminating retinal starburst amacrine cells, using immunotoxin-mediated cell targeting techniques. Starburst cell ablation in the adult retina abolished not only directional selectivity of ganglion cell responses but also an optokinetic eye reflex derived by stimulus movement. Starburst cells therefore serve as the key element that discriminates the direction of stimulus movement through integrative synaptic transmission and play a pivotal role in information processing that stabilizes image motion.

Calretinin co-localizes with the NMDA receptor subunit NR1 in cholinergic amacrine cells of the rat retina

Immunohistochemistry was used to verify whether choline-acetiltransferase colocalizes with calcium-binding proteins and NMDA receptor subunit NR1 in the rat retina. Whereas calbindin and parvalbumin were not observed in cholinergic amacrine cells, calretinin and NR1 were very frequently colocalized with ChAT. Calretinin/NR1-positive cells were also shown, suggesting that calretinin in cholinergic cells of the rat may be related to the buffering of excess intracellular calcium generated by activation of NMDA receptors.

Calretinin is present in serotonin- and gamma-aminobutyric acid-positive amacrine cell populations in the retina of Xenopus laevis

In the frog retina most bipolar cells, sparsely distributed amacrine cells and some ganglion cells contain calretinin (CaR). Double-label immunocytochemistry shows that in the Xenopus retina many calretinin positive amacrine cells are also gamma-aminobutyric acid (GABA)-immunoreactive (IR), none colocalizes glycine or dopamine but some contain serotonin (SER). The CaR-IR cells represent 8-9% of all GABA-IR amacrine cells. Only 4.6% of the CaR-positive cells contain SER. The SER-positive cells are present in two sizes in the anuran retina: the large cells never contain CaR but some of the small cells do. To further characterize the CaR+/SER+ amacrine cell population, immunolabeling for tryptophan hydroxylase (TrOH), the rate-limiting enzyme for SER synthesis, was performed. The results showed that large TrOH-containing cells are always negative for CaR. However, the small SER-synthesizing amacrine cells are invariably CaR-positive. Thus the anuran retina contains three neurochemically distinct SER-positive amacrine cell types, one of which (the small SER-synthesizing cell type) is also CaR-IR.