Quantitative assessment of localization and colocalization of glutamate, aspartate, glycine, and GABA immunoreactivity in the chick retina

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

The retinal basis of vision in chicken

The Avian retina is far less known than that of mammals such as mouse and macaque, and detailed study is overdue. The chicken (Gallus gallus) has potential as a model, in part because research can build on developmental studies of the eye and nervous system. One can expect differences between bird and mammal retinas simply because whereas most mammals have three types of visual photoreceptor birds normally have six. Spectral pathways and colour vision are of particular interest, because filtering by oil droplets narrows cone spectral sensitivities and birds are probably tetrachromatic. The number of receptor inputs is reflected in the retinal circuitry. The chicken probably has four types of horizontal cell, there are at least 11 types of bipolar cell, often with bi- or tri-stratified axon terminals, and there is a high density of ganglion cells, which make complex connections in the inner plexiform layer. In addition, there is likely to be retinal specialisation, for example chicken photoreceptors and ganglion cells have separate peaks of cell density in the central and dorsal retina, which probably serve different types of behaviour.

Expression of vesicular glutamate transporter 2 and 3 and glutamate receptor 1 and 2 mRNAs in the retina of adult laughing doves (Streptopelia senegalensis): An in situ hybridization study

The retina possesses few types of neurons so; it is considered an excellent model for understanding the neural mechanisms underlying basic neural information processing in the brain. Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system and retina. The present study was carried out to characterize the expression pattern of vesicular glutamate transporter 2 (Vglut2) and 3 (Vglut3) and glutamate receptor 1 (GluR1) and 2 (GluR2) mRNAs in the retina of adult laughing dove (Streptopelia senegalensis) by RT-PCR and in situ hybridization histochemistry. The cerebellum of adult laughing dove was used as a positive control in this study. Vglut2 mRNA was highly expressed only in the granular layer of the cerebellum while Vglut3 mRNA was weakly expressed only in the Purkinje cells layer. In the retina, Vglut2 mRNA was highly expressed in the ganglion cell layer and moderately expressed in the inner nuclear layer while Vglut3 mRNA was moderately expressed only in the inner nuclear layer. GluR1 mRNA was intensely expressed in the Purkinje cells layer while GluR2 mRNA signals were highly detectable in both granular and Purkinje cells layers of the cerebellum. In the retina, moderate expression of GluR1 and intense expression of GluR2 was found in both ganglion cell layer and the internal half of inner nuclear layer mostly amacrine cells. These results suggest that some retinal neuronal cells in the adult laughing dove are glutamatergic. Therefore, GluR1 and 2 are suggested as useful markers for glutamatergic retinal neuronal cells in the adult laughing doves.

Cholinergic nervous system and glaucoma: From basic science to clinical applications

The cholinergic system has a crucial role to play in visual function. Although cholinergic drugs have been a focus of attention as glaucoma medications for reducing eye pressure, little is known about the potential modality for neuronal survival and/or enhancement in visual impairments. Citicoline, a naturally occurring compound and FDA approved dietary supplement, is a nootropic agent that is recently demonstrated to be effective in ameliorating ischemic stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, cerebrovascular diseases, memory disorders and attention-deficit/hyperactivity disorder in both humans and animal models. The mechanisms of its action appear to be multifarious including (i) preservation of cardiolipin, sphingomyelin, and arachidonic acid contents of phosphatidylcholine and phosphatidylethanolamine, (ii) restoration of phosphatidylcholine, (iii) stimulation of glutathione synthesis, (iv) lowering glutamate concentrations and preventing glutamate excitotoxicity, (v) rescuing mitochondrial function thereby preventing oxidative damage and onset of neuronal apoptosis, (vi) synthesis of myelin leading to improvement in neuronal membrane integrity, (vii) improving acetylcholine synthesis and thereby reducing the effects of mental stress and (viii) preventing endothelial dysfunction. Such effects have vouched for citicoline as a neuroprotective, neurorestorative and neuroregenerative agent. Retinal ganglion cells are neurons with long myelinated axons which provide a strong rationale for citicoline use in visual pathway disorders. Since glaucoma is a form of neurodegeneration involving retinal ganglion cells, citicoline may help ameliorate glaucomatous damages in multiple facets. Additionally, trans-synaptic degeneration has been identified in humans and experimental models of glaucoma suggesting the cholinergic system as a new brain target for glaucoma management and therapy.

SUMOylation and deimination of proteins: two epigenetic modifications involved in Giardia encystation

SUMOylation, a posttranslational modification of proteins, has been recently described as vital in eukaryotic cells. In a previous work, we analyzed the role of SUMO protein and the genes encoding the putative enzymes of the SUMOylation pathway in the parasite Giardia lamblia. Although we observed several SUMOylated proteins, only the enzyme Arginine Deiminase (ADI) was confirmed as a SUMOylated substrate. ADI is involved in the survival of the parasite and, besides its role in ATP production, it also catalyzes the modification of arginine residues to citrulline in the cytoplasmic tail of surface proteins. During encystation, however, ADI translocates to the nuclei and downregulates the expression of the Cyst Wall Protein 2 (CWP2). In this work, we made site-specific mutation of the ADI SUMOylation site (Lys101) and observed that transgenic trophozoites did not translocate to the nuclei at the first steps of encystation but shuttled in the nuclei late during this process through classic nuclear localization signals. Inside the nuclei, ADI acts as a peptidyl arginine deiminase, being probably involved in the downregulation of CWPs expression and cyst wall formation. Our results strongly indicate that ADI plays a regulatory role during encystation in which posttranslational modifications of proteins are key players.

Functional and neurochemical development in the normal and degenerating mouse retina

The rd1 mouse is a well-established animal model for human retinitis pigmentosa (RP). We used electroretinography (ERG) to evaluate retinal function and postembedding immunocytochemistry to determine the changes in cellular amino acid expression in the normal (C57Bl6) and degenerating mouse retina (rd1), as a function of age during development and the onset of degeneration. In the normal mouse retina, photoreceptoral and post-photoreceptoral ERG responses improved simultaneously from eye-opening until adult levels were achieved at approximately postnatal day (P) 30. Maturation of amino acid neurochemistry preceded the development of retinal function in the normal retina. Amino acid levels increased immediately from birth and reached stable levels by eye-opening. In contrast, in the rd1 mouse, both rod and cone pathway function rapidly reduced from eye-opening and by P21 became undetectable. Interestingly, at P18 cone responses were still comparable between the normal and degenerating retina. Before eye opening, the pattern of amino acid immunoreactivity in the rd1 retina was similar to the normal retina. Alterations in neurochemistry were observed after the onset of rod photoreceptor cell death. The most obvious change was the reduction in neurotransmitter immunoreactivity within the synaptic layers and some cell classes of the rd1 retina. Reduction of glutamine and glutamate was observed in Müller cells before established gliosis markers. Overall, these results suggest the rapid maturation of neurochemistry by eye opening followed by functional maturation by P30 in the normal retina. The dystrophic retina displays similar neurochemistry to control retina before eye opening but a subsequent decline.

The giardial VPS35 retromer subunit is necessary for multimeric complex assembly and interaction with the vacuolar protein sorting receptor

The retromer is a pentameric protein complex that mediates the retrograde transport of acid hydrolase receptors between endosomes and the trans-Golgi network and is conserved across all eukaryotes. Unlike other eukaryotes, the endomembrane system of Giardia trophozoite is simple and is composed only of the endoplasmic reticulum and peripheral vesicles (PVs), which may represent an ancient organellar system converging compartments such as early and late endosomes and lysosomes. Sorting and trafficking of membrane proteins and soluble hydrolases from the endoplasmic reticulum to the PVs have been described as specific and conserved but whether the giardial retromer participates in receptor recycling remains elusive. Homologs of the retromer Vacuolar Protein Sorting (Vps35p, Vps26p, and Vps29p) have been identified in this parasite. Cloning the GlVPS35 subunit and antisera production enabled the localization of this protein in the PVs as well as in the cytosol. Tagged expression of the subunits was used to demonstrate their association with membranes, and immunofluorescence confocal laser scanning revealed high degrees of colabeling between the retromer subunits and also with the endoplasmic reticulum and PV compartment markers. Protein-protein interaction data revealed interaction between the subunits of GlVPS35 and the cytosolic domain of the hydrolase receptor GlVps. Altogether our data provide original information on the molecular interactions that mediate assembly of the cargo-selective retromer subcomplex and its involvement in the recycling of the acid hydrolase receptor in this parasite.

The glutamatergic neurons in the spinal cord of the sea lamprey: an in situ hybridization and immunohistochemical study

Glutamate is the main excitatory neurotransmitter involved in spinal cord circuits in vertebrates, but in most groups the distribution of glutamatergic spinal neurons is still unknown. Lampreys have been extensively used as a model to investigate the neuronal circuits underlying locomotion. Glutamatergic circuits have been characterized on the basis of the excitatory responses elicited in postsynaptic neurons. However, the presence of glutamatergic neurochemical markers in spinal neurons has not been investigated. In this study, we report for the first time the expression of a vesicular glutamate transporter (VGLUT) in the spinal cord of the sea lamprey. We also study the distribution of glutamate in perikarya and fibers. The largest glutamatergic neurons found were the dorsal cells and caudal giant cells. Two additional VGLUT-positive gray matter populations, one dorsomedial consisting of small cells and another one lateral consisting of small and large cells were observed. Some cerebrospinal fluid-contacting cells also expressed VGLUT. In the white matter, some edge cells and some cells associated with giant axons (Müller and Mauthner axons) and the dorsolateral funiculus expressed VGLUT. Large lateral cells and the cells associated with reticulospinal axons are in a key position to receive descending inputs involved in the control of locomotion. We also compared the distribution of glutamate immunoreactivity with that of γ-aminobutyric acid (GABA) and glycine. Colocalization of glutamate and GABA or glycine was observed in some small spinal cells. These results confirm the glutamatergic nature of various neuronal populations, and reveal new small-celled glutamatergic populations, predicting that some glutamatergic neurons would exert complex actions on postsynaptic neurons.

Vacuolar protein sorting receptor in Giardia lamblia

In Giardia, lysosome-like peripheral vacuoles (PVs) need to specifically coordinate their endosomal and lysosomal functions to be able to successfully perform endocytosis, protein degradation and protein delivery, but how cargo, ligands and molecular components generate specific routes to the PVs remains poorly understood. Recently, we found that delivering membrane Cathepsin C and the soluble acid phosphatase (AcPh) to the PVs is adaptin (AP1)-dependent. However, the receptor that links AcPh and AP1 was never described. We have studied protein-binding to AcPh by using H6-tagged AcPh, and found that a membrane protein interacted with AcPh. This protein, named GlVps (for Giardia lamblia Vacuolar protein sorting), mainly localized to the ER-nuclear envelope and in some PVs, probably functioning as the sorting receptor for AcPh. The tyrosine-binding motif found in the C-terminal cytoplasmic tail domain of GlVps was essential for its exit from the endoplasmic reticulum and transport to the vacuoles, with this motif being necessary for the interaction with the medium subunit of AP1. Thus, the mechanism by which soluble proteins, such as AcPh, reach the peripheral vacuoles in Giardia appears to be very similar to the mechanism of lysosomal protein-sorting in more evolved eukaryotic cells.

Neurochemical phenotype and birthdating of specific cell populations in the chick retina

The chick embryo is one of the most traditional models in developing neuroscience and its visual system has been one of the most exhaustively studied. The retina has been used as a model for studying the development of the nervous system. Here, we describe the morphological features that characterize each stage of the retina development and studies of the neurogenesis period of some specific neurochemical subpopulations of retinal cells by using a combination of immunohistochemistry and autoradiography of tritiated-thymidine. It could be concluded that the proliferation period of dopaminergic, GABAergic, cholinoceptive and GABAceptive cells does not follow a common rule of the neurogenesis. In addition, some specific neurochemical cell groups can have a restrict proliferation period when compared to the total cell population.

A transgenic mouse line for molecular genetic analysis of excitatory glutamatergic neurons

Excitatory glutamatergic neurons are part of most of the neuronal circuits in the mammalian nervous system. We have used BAC-technology to generate a BAC-Vglut2::Cre mouse line where Cre expression is driven by the vesicular glutamate transporter 2 (Vglut2) promotor. This BAC-Vglut2::Cre mouse line showed specific expression of Cre in Vglut2 positive cells in the spinal cord with no ectopic expression in GABAergic or glycinergic neurons. This mouse line also showed specific Cre expression in Vglut2 positive structures in the brain such as thalamus, hypothalamus, superior colliculi, inferior colliculi and deep cerebellar nuclei together with nuclei in the midbrain and hindbrain. Cre-mediated recombination was restricted to Cre expressing cells in the spinal cord and brain and occurred as early as E 12.5. Known Vglut2 positive neurons showed normal electrophysiological properties in the BAC-Vglut2::Cre transgenic mice. Altogether, this BAC-Vglut2::Cre mouse line provides a valuable tool for molecular genetic analysis of excitatory neuronal populations throughout the mouse nervous system.

Distribution of vesicular glutamate transporter 2 and glutamate receptor 1 and 2 mRNA in the pigeon retina

Glutamate is an excitatory neurotransmitter in the central and peripheral nervous systems of the vertebrate. The previous studies show the presence of mRNAs of AMPA-type glutamate receptors, GluR1 and GluR2, in the optic tectum of the pigeon, suggesting glutamatergic input from the retina. The present study examined localization of vesicular glutamate transporter 2 (VGLUT2) and GluR1 and GluR2 to confirm source of glutamatergic neurons in the pigeon retina by in situ hybridization histochemistry. VGLUT2 mRNA expressed in the inner nuclear layer and ganglion cells, while GluR1 and GluR2 mRNAs were observed in the inner nuclear layer, ganglion cells, and superficial layers of the optic tectum. The results suggest that photoreceptor cells, bipolar cells and ganglion cells are glutamatergic in the avian retina as in mammals.

Immunoreactivity of glutamate in mouse retina inner segment of photoreceptors with in vivo cryotechnique

The purpose of this study was to clarify a previously controversial issue concerning glutamate (Glu) immunoreactivity (IR) in the inner segment (IS) of photoreceptors by using in vivo cryotechnique (IVCT) followed by freeze substitution (FS), which enabled us to analyze the cells and tissues reflecting living states. Eyeballs from anesthetized mice were directly frozen using IVCT. The frozen tissues were processed for FS fixation in acetone containing chemical fixatives, and embedded in paraffin. Deparaffinized sections were immunostained with an anti-Glu antibody. The strongest Glu-IR was obtained in the specimens prepared by FS with paraformaldehyde or a low concentration of glutaraldehyde, whereas no Glu-IR was obtained without the chemical fixatives. The Glu was immunolocalized in the IS, outer and inner plexiform and ganglion cell layers. Thus, the immunolocalization of Glu in the IS was clearly demonstrated using IVCT.

Axon-bearing and axon-less horizontal cell subtypes are generated consecutively during chick retinal development from progenitors that are sensitive to follistatin

BACKGROUND

Horizontal cells are retinal interneurons that modulate the output from photoreceptors. A rich literature on the morphological classification and functional properties of HCs in different animals exists, however, the understanding of the events underlying their development is still limited. In most vertebrates including chicken, two main horizontal cell (HC) subtypes are identified based on the presence or absence of an axon.

RESULTS

In this work we have molecularly characterized three HC subtypes based on Lim1, Isl1, GABA and TrkA, a classification that is consistent with three chick HC subtypes previously defined by morphology. The axon-bearing and axon-less HC subpopulations molecularly defined by Lim1 and Isl1, are born consecutively on embryonic day (E) 3-4 and E4-5, respectively, and exhibit temporally distinguishable periods of migration. Their relative numbers are not adjusted by apoptosis. A sharp decrease of high endogenous levels of the activin-inhibitor follistatin at E3 coincides with the appearance of the Lim1 positive cells. Extending the follistatin exposure of the HC retinal progenitor cells by injection of follistatin at E3 increased the number of both Lim1- and Isl1 positive HCs when analysed at E9.

CONCLUSION

The results imply that the axon-bearing and axon-less HC subgroups are defined early and are generated consecutively from a retinal progenitor cell population that is sensitive to the inhibitory action of follistatin. The results are consistent with a model wherein added follistatin causes HC-generating progenitors to proliferate beyond the normal period of HC generation, thus producing extra HCs of both types that migrate to the HC layer.

Phenotype of V2-derived interneurons and their relationship to the axon guidance molecule EphA4 in the developing mouse spinal cord

The ventral spinal cord consists of interneuron groups arising from distinct, genetically defined, progenitor domains along the dorsoventral axis. Many of these interneuron groups settle in the ventral spinal cord which, in mammals, contains the central pattern generator for locomotion. In order to better understand the locomotor networks, we have used different transgenic mice for anatomical characterization of one of these interneuron groups, called V2 interneurons. Neurons in this group are either V2a interneurons marked by the postmitotic expression of the transcription factor Chx10, or V2b interneurons which express the transcription factors Gata2 and Gata3. We found that all V2a and most V2b interneurons were ipsilaterally projecting in embryos as well as in newborns. V2a interneurons were for the most part glutamatergic while V2b interneurons were mainly GABAergic or glycinergic. Furthermore, we demonstrated that a large proportion of V2 interneurons expressed the axon guidance molecule EphA4, a molecule previously shown to be important for correct organization of locomotor networks. We also showed that V2 interneurons and motor neurons alone did not account for all EphA4-expressing neurons in the spinal cord. Together, these findings enable a better interpretation of neural networks underlying locomotion, and open up the search for as yet unknown components of the mammalian central pattern generator.

Neuronal and glial cell changes are determined by retinal vascularization in retinopathy of prematurity

We have characterized the vascular, neuronal, and glial changes in oxygen-induced retinopathy, a model of retinopathy of prematurity (ROP). Newborn Sprague-Dawley rats were exposed to either 80% +/- 2% oxygen to postnatal day P11 and then room air until P18 (ROP) or room air for the entire duration (controls). Retinal structure was examined under the light microscope and following postembedding immunocytochemistry in central, midperipheral, and peripheral regions. Müller cells were evaluated immunocytochemically with glial fibrillary acidic protein. The extent of vascularization was established histologically. ROP caused significant thinning of the inner cellular and plexiform layers, which became more pronounced in the peripheral inner nuclear layer of ROP animals (11.3% loss vs. 25.4% loss). Amacrine cell amino acid levels were particularly vulnerable in the peripheral retina; bipolar cells showed similar but less prominent changes. Müller cells had elevated glutamine levels and were most gliotic in the periphery. The vasculature extended to peripheral retinal regions at P18 in controls but not in ROP rats. The most striking pattern of change was evident in the midperipheral "transition zone" of ROP animals. Areas close to blood vessels showed neurochemical properties that were similar to those of the central retina, indicating a local protective effect of the inner retinal blood supply. We find that ROP produces complex vascular, neural, and glial changes that relate to the proximity of inner retinal blood vessels.

Transporter mediated GABA release in the retina: Role of excitatory amino acids and dopamine

In general, the release of neurotransmitters in the central nervous system is accomplished by a calcium-dependent process which constitutes a common feature of exocytosis, a conserved mechanism for transmitter release in all species. However, neurotransmitters can also be released by the reversal of their transporters. In the retina, a large portion of GABA is released by this mechanism, which is under the control of neuroactive agents, such as excitatory amino acids and dopamine. In this review, we will focus on the transporter mediated GABA release and the role played by excitatory amino acids and dopamine in this process. First, we will discuss the works that used radiolabeled GABA to study the outflow of the neurotransmitter and then the works that took into consideration the endogenous pool of GABA and the topography of GABAergic circuits influenced by excitatory amino acids and dopamine.

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.

Darkness induced neuroplastic changes in the serotoninergic system of the chick retina

Sensory experience is critical for the formation of neuronal circuits and it is well known that neuronal activity plays a crucial role in the formation and maintenance of synapses. In the vertebrate retina, exposure to different environmental conditions results in structural, physiological, neurochemical and pharmacological changes. Serotoninergic (5HT) amacrine cells of the chicken retina are bistratified interneurons whose primary dendrites descend through the inner nuclear layer (INL) to branch in the inner plexiform layer (IPL) forming two plexi, an outer network, localized to sublamina 1, and an inner network, localized to sublamina 4 and 5 of the IPL. Their development is temporally correlated with the establishment of synapses in the retina and with the emergence of the typical adult electroretinogram. It is unknown, however, which role these cells play in processing visual information and whether visual deprivation modifies their phenotype. Here, we show that, in the chicken, red-light rearing from hatching to postnatal day 12 significantly alters the stratification pattern of 5HT amacrine cells, inhibiting their age-dependent pruning measured with morphometric and densitometric procedures; as well as increasing serotonin immunoreactivity measured as relative optical density. This change in dendritic arborization, accompanied by an increase in serotonin concentration in dark adapted conditions, may decrease visual threshold, thus increasing visual sensitivity.

Local differences in GABA release induced by excitatory amino acids during retina development: selective activation of NMDA receptors by aspartate in the inner retina

Glutamate and GABA are the major excitatory and inhibitory neurotransmitters in the CNS. In the retina, it has been shown that glutamate and aspartate and their agonists kainate and NMDA promote the release of GABA. In the chick retina, at embryonic day 14 (E14), glutamate and kainate were able to induce the release of GABA from amacrine and horizontal cells as detected by GABA-immunoreactivity. NMDA also induced GABA release restricted to amacrine cell population and its projections to the inner plexiform layer (E14 and E18). Although aspartate reduced GABA immunoreactivity, specifically in amacrine cells of E18 retinas, it was not efficient to promote GABA release from retinas at E14. As observed in differentiated retinas, dopamine inhibited the GABA release promoted by NMDA and aspartate but not by kainate. Our data show that different retinal sites respond to distinct EAAs via different receptor systems.

Demonstration of cholinergic ganglion cells in rat retina: expression of an alternative splice variant of choline acetyltransferase

Acetylcholine acts as a neurotransmitter in the retina. Although previous physiological studies have indicated that some retinal ganglion cells may be cholinergic, several immunohistochemical studies using antibodies to choline acetyltransferase (ChAT) have stained only amacrine cells but not ganglion cells. Recently, we identified a splice variant of ChAT mRNA, lacking exons 6-9, in rat peripheral nervous system. The encoded protein was designated as ChAT of a peripheral type (pChAT), against which an antiserum was raised. In the present study, we examined expression of pChAT in rat retina, both at the protein level by immunohistochemistry using the antiserum and at the mRNA level by RT-PCR. Immunohistochemistry revealed that although no positive neurons were found in untreated intact retinas, many neurons became immunoreactive for pChAT after intravitreal injection of colchicine. Damage of the optic nerve was also effective in disclosing positive cells. Such positive neurons were shown to be ganglion cells by double labeling with a retrograde tracer that had been injected into the contralateral superior colliculus. Western blot analysis and RT-PCR revealed a corresponding band to the pChAT protein and to the amplified pChAT gene fragment, respectively, in retinal samples. In addition, ChAT activity was definitely detected in retinofugal fibers of the optic nerve. These results indicate the presence of cholinergic ganglion cells in rat retina.

Quantitative organization of neurotransmitters in the deep cerebellar nuclei of the Lurcher mutant

The Lurcher mutant mouse is characterized by a primary selective loss of Purkinje cells, leading to the near total apoptotic death of these neurons. In contrast to the subsequent massive secondary degeneration of the granule cells and the inferior olivary neurons, only mild degeneration occurs in the deep cerebellar nuclei (DCN). However, it is not known to what extent the different populations of DCN neurons-glutamatergic principal projection neurons, gamma-aminobutyric acid (GABA)-ergic inferior olivary projection neurons, and glycinergic neurons-are affected in their neurotransmitter composition. To answer this question we studied the neurotransmitter contents (glutamate, GABA, and glycine) of DCN neurons and the size of synaptic boutons immunohistochemically on serial semithin sections in both Lurcher and wild-type mice. Applying the physical dissector counting method, our results confirmed the mild degeneration (a reduction by 20%) of large glutamatergic neurons and a more pronounced degeneration of GABAergic (by 42%) and glycinergic neurons (by 45%). On the other hand, an analysis of neurons colabeled for both GABA and glycine, revealed that this specific colabeling increased in the Lurcher mutant (by 40%). In addition, both the GABA-immunolabeled (IL) (by 56%) and the glycine-IL (by 45%) synaptic boutons showed an increase in diameter in the mutant. The density of these boutons showed a decrease of 30% each. In summary, the increase in the number of neurons colabeled for GABA and glycine, together with the increase in the size of the inhibitory synaptic boutons, could help in providing the minimum inhibition needed to maintain a residual "cerebellar" functionality in the Lurcher DCN.