Development of neuropeptide Y immunoreactive amacrine and ganglion cells in the pre- and postnatal cat retina

Seasonal Differences in Expression of Neuropeptide Y (NPY) in Visual Centers of Spotted Munia (Lonchura punctulata)

The visual perception of birds is an incredibly exciting subject of research. Birds have significantly higher visual acuity than most other animals, which helps them stay safe in flight and detect their prey. Understanding how the eyes send information to the brain for additional processing is crucial. The brain has sections (nuclei) that accept input from the retina. The key areas where information is processed are the hyperpallium apicale (HA), hippocampus (HP), optic tectum (TeO), nucleus rotundus (RoT), and the geniculatus lateralis ventralis (Glv); among these, the RoT is one of the most investigated nuclei for vision. This study looked at how the visual centers of non-photoperiodic songbirds (Spotted Munia) adapt in different life history stages by looking at NPY expression. We immunohistochemically quantified NPY expression in four different seasons, including pre-breeding (June), breeding (September), post-breeding (December), and regressed (March) in the brain of Spotted Munia. We evaluated changes in the expression levels of the peptide throughout the year, by determining the expression at four different periods throughout the year. Peptide expression levels were projected to fluctuate within photoperiod-induced seasons. It was discovered that the parts of the brain related to vision (RoT, HA, and HP) have a higher number of immunoreactive cells during their mating season, i.e., during the summer. The appearance of NPY, a non-photic marker, in brain areas linked with light perception, was fascinating. Indirectly, NPY aids avian reproduction in a variety of ways. These findings demonstrate the importance of these nuclei in the process of reproduction, as well as the involvement of NPY in the visual brain areas of Spotted Munia.

Emerging novel roles of neuropeptide Y in the retina: from neuromodulation to neuroprotection

Neuropeptide Y (NPY) and NPY receptors are widely expressed in the central nervous system, including the retina. Retinal cells, in particular neurons, astrocytes, and Müller, microglial and endothelial cells express this peptide and its receptors (Y1, Y2, Y4 and/or Y5). Several studies have shown that NPY is expressed in the retina of various mammalian and non-mammalian species. However, studies analyzing the distribution of NPY receptors in the retina are still scarce. Although the physiological roles of NPY in the retina have not been completely elucidated, its early expression strongly suggests that NPY may be involved in the development of retinal circuitry. NPY inhibits the increase in [Ca(2+)]i triggered by elevated KCl in retinal neurons, protects retinal neural cells against toxic insults and induces the proliferation of retinal progenitor cells. In this review, we will focus on the roles of NPY in the retina, specifically proliferation, neuromodulation and neuroprotection. Alterations in the NPY system in the retina might contribute to the pathogenesis of retinal degenerative diseases, such as diabetic retinopathy and glaucoma, and NPY and its receptors might be viewed as potentially novel therapeutic targets.

Neuropeptide Y receptors activation protects rat retinal neural cells against necrotic and apoptotic cell death induced by glutamate

It has been claimed that glutamate excitotoxicity might have a role in the pathogenesis of several retinal degenerative diseases, including glaucoma and diabetic retinopathy. Neuropeptide Y (NPY) has neuroprotective properties against excitotoxicity in the hippocampus, through the activation of Y1, Y2 and/or Y5 receptors. The principal objective of this study is to investigate the potential protective role of NPY against glutamate-induced toxicity in rat retinal cells (in vitro and in an animal model), unraveling the NPY receptors and intracellular mechanisms involved. Rat retinal neural cell cultures were prepared from newborn Wistar rats (P3-P5) and exposed to glutamate (500 μM) for 24 h. Necrotic cell death was evaluated by propidium iodide (PI) assay and apoptotic cell death using TUNEL and caspase-3 assays. The cell types present in culture were identified by immunocytochemistry. The involvement of NPY receptors was assessed using selective agonists and antagonists. Pre-treatment of cells with NPY (100 nM) inhibited both necrotic cell death (PI-positive cells) and apoptotic cell death (TUNEL-positive cells and caspase 3-positive cells) triggered by glutamate, with the neurons being the cells most strongly affected. The activation of NPY Y2, Y4 and Y5 receptors inhibited necrotic cell death, while apoptotic cell death was only prevented by the activation of NPY Y5 receptor. Moreover, NPY neuroprotective effect was mediated by the activation of PKA and p38K. In the animal model, NPY (2.35 nmol) was intravitreally injected 2 h before glutamate (500 nmol) injection into the vitreous. The protective role of NPY was assessed 24 h after glutamate (or saline) injection by TUNEL assay and Brn3a (marker of ganglion cells) immunohistochemistry. NPY inhibited the increase in the number of TUNEL-positive cells and the decrease in the number of Brn3a-positive cells induced by glutamate. In conclusion, NPY and NPY receptors can be considered potential targets to treat retinal degenerative diseases, such as glaucoma and diabetic retinopathy.

Neuropeptide Y stimulates retinal neural cell proliferation--involvement of nitric oxide

Neuropeptide Y (NPY) is a 36 amino acid peptide widely present in the CNS, including the retina. Previous studies have demonstrated that NPY promotes cell proliferation of rat post-natal hippocampal and olfactory epithelium precursor cells. The aim of this work was to investigate the role of NPY on cell proliferation of rat retinal neural cells. For this purpose, primary retinal cell cultures expressing NPY, and NPY Y(1), Y(2), Y(4) and Y(5) receptors [Alvaro et al., (2007) Neurochem. Int., 50, 757] were used. NPY (10-1000 nM) stimulated cell proliferation through the activation of NPY Y(1), Y(2) and Y(5) receptors. NPY also increased the number of proliferating neuronal progenitor cells (BrdU(+)/nestin(+) cells). The intracellular mechanisms coupled to NPY receptors activation that mediate the increase in cell proliferation were also investigated. The stimulatory effect of NPY on cell proliferation was reduced by L-nitroarginine-methyl-esther (L-NAME; 500 microM), a nitric oxide synthase inhibitor, 1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one (ODQ; 20 microM), a soluble guanylyl cyclase inhibitor or U0126 (1 microM), an inhibitor of the extracellular signal-regulated kinase 1/2 (ERK 1/2). In conclusion, NPY stimulates retinal neural cell proliferation, and this effect is mediated through nitric oxide-cyclic GMP and ERK 1/2 pathways.

Neuropeptide Y inhibits [Ca2+]i changes in rat retinal neurons through NPY Y1, Y4, and Y5 receptors

Neuropeptide Y (NPY) and NPY receptors are widely distributed in the CNS, including the retina, but the role of NPY in the retina is largely unknown. The aim of this study was to investigate whether NPY modulates intracellular calcium concentration ([Ca(2+)](i)) changes in retinal neurons and identify the NPY receptors involved. As NPY decreased the [Ca(2+)](i) amplitudes evoked by 30 mM KCl in only 50% of neurons analyzed, we divided them in two populations: NPY-non-responsive neurons (Delta2/Delta1 > or = 0.80) and NPY-responsive neurons (Delta2/Delta1 < 0.80), being the Delta2/Delta1 the ratio between the amplitude of [Ca(2+)](i) increase evoked by the second (Delta2) and the first (Delta1) stimuli of KCl. The NPY Y(1)/Y(5), Y(4), and Y(5) receptor agonists (100 nM), but not the Y(2) receptor agonist (300 nM), inhibited the [Ca(2+)](i) increase induced by KCl. In addition, the inhibitory effect of NPY on evoked-[Ca(2+)](i) changes was reduced in the presence of the Y(1) or the Y(5) receptor antagonists. In conclusion, NPY inhibits KCl-evoked [Ca(2+)](i) increase in retinal neurons through the activation of NPY Y(1), Y(4), and Y(5) receptors. This effect may be viewed as a potential neuroprotective mechanism of NPY against retinal neurodegeneration.

NPY in rat retina is present in neurons, in endothelial cells and also in microglial and Müller cells

NPY is present in the retina of different species but its role is not elucidated yet. In this work, using different rat retina in vitro models (whole retina, retinal cells in culture, microglial cell cultures, rat Müller cell line and retina endothelial cell line), we demonstrated that NPY staining is present in the retina in different cell types: neurons, macroglial, microglial and endothelial cells. Retinal cells in culture express NPY Y(1), Y(2), Y(4) and Y(5) receptors. Retina endothelial cells express all NPY receptors except NPY Y(5) receptor. Moreover, NPY is released from retinal cells in culture upon depolarization. In this study we showed for the first time that NPY is present in rat retina microglial cells and also in rat Müller cells. These in vitro models may open new perspectives to study the physiology and the potential pathophysiological role of NPY in the retina.

Y2 receptor expression and inhibition of voltage-dependent Ca2+ influx into rod bipolar cell terminals

Neuropeptide Y (NPY) is a potent inhibitory neuropeptide expressed by amacrine cells in the rat retina. NPY modulates the release of multiple neurotransmitters in mammalian retina, yet the mechanisms mediating this regulation are not well defined. To further understand the action of NPY in the retina, Y receptor coupling to voltage-dependent Ca(2+) channels was investigated using Ca(2+) imaging with fura-2 AM to measure [Ca(2+)](i) increases in rod bipolar cell terminals. Y receptor expression was studied in rat retinal tissue with reverse transcription-polymerase chain reaction (RT-PCR). NPY inhibited the depolarization-evoked Ca(2+) influx into rod bipolar cell axon terminals and caused a dose-dependent reduction and an average maximal inhibition of 72% at 1 microM, which was reversed upon washout. K(+)-evoked Ca(2+) increases were also inhibited by the selective Y2 receptor agonists, C2-NPY and NPY(13-36), at concentrations of 1 microM, but not by the selective Y1 receptor agonist, [Leu(31)Pro(34)]NPY, selective Y4 receptor agonist, rPP, or the selective Y5 receptor agonist, [d-Trp32]-NPY. Y receptor expression was determined using RT-PCR for all known Y receptor subtypes. Y2 receptor mRNA, as well as Y1, Y4, and Y5 receptor mRNAs, are present in the rat retina. Like the rod bipolar cell, other studies in central neurons have shown that the Y2 receptor is expressed predominantly as a presynaptic receptor and that it modulates transmitter release. Together, these findings suggest that NPY activates presynaptic Y2 receptors to inhibit voltage-dependent Ca(2+) influx into rod bipolar cell terminals, and establishes one mechanism by which NPY may reduce l-glutamate release from the rod bipolar cell synapse.

Neurokinin 1 receptor expression and substance p physiological actions are developmentally regulated in the rabbit retina

We investigated the expression of the substance P (SP) receptor (the neurokinin 1 receptor, NK1 receptor) and SP functional effects in developing rabbit retinas. NK1 receptors in adult retinas were in a population of cone bipolar cells and in dopaminergic amacrine cells, as previously described. In contrast, at birth and at postnatal day (PND) 6, NK1 receptors were exclusively expressed by cholinergic amacrine and displaced amacrine cells. NK1 receptor expression in cholinergic cells was still observed at PND10 (eye opening), while at PND21 it was confined to cholinergic cells of the inner nuclear layer. Starting at PND10, NK1 receptors were also in bipolar cells and in dopaminergic amacrine cells. A fully mature NK1 receptor expression pattern was observed at PND35. Dopamine release was assessed in isolated retinas in the presence of SP, the NK1 receptor agonist GR73632 or the NK1 receptor antagonist GR82334. At PND35, extracellular dopamine was significantly increased by 10 microM SP or 0.01-100 microM GR73632, and it was decreased by 0.01-10 microM GR82334. No effects were detected in developing retinas up to PND21. Ca2+ imaging experiments were performed in single cholinergic cells identified by their "starburst" morphology in perinatal retinas. Intracellular Ca2+ levels were significantly increased by 1 microM SP or GR73632. This effect was reversibly inhibited by 1 microM GR82334. These data demonstrate that both NK1 receptor expression and SP physiological actions are developmentally regulated in the retina. SP neurotransmission in the immature retina may subserve developmental events, and SP is likely to represent an important developmental factor for the maturation of retinal neurons and circuitries.

Ectopic photoreceptors and cone bipolar cells in the developing and mature retina

An antibody against recoverin, the calcium-binding protein, labels photoreceptors, cone bipolar cells, and a subpopulation of cells in the ganglion cell layer. In the present study, we sought to establish the origin and identity of the cells expressing recoverin in the ganglion cell layer of the rat retina. By double labeling with rhodopsin, we demonstrate that early in development some of the recoverin-positive cells in the ganglion cell layer are photoreceptors. During the first postnatal week, these rhodopsin-positive cells are eliminated from the ganglion cell layer, but such neurons remain in the inner nuclear layer well into the first postnatal month. Another contingent of recoverin-positive cells, with morphological features equivalent to those of bipolar cells, is present in the postnatal retina, and approximately 50% of these neurons survive to maturity. The incidence of such cells in the ganglion cell layer was not affected by early transection of the optic nerve, a manipulation that causes rapid loss of retinal ganglion cells. These recoverin-positive cells were not double-labeled by cell-specific markers expressed by photoreceptors, rod bipolar cells, or horizontal and amacrine cells. Based on their staining with recoverin and salient morphological features, these ectopic profiles in the ganglion cell layer are most likely cone bipolar cells. Collectively, the results provide evidence for photoreceptors in the ganglion cell and inner nuclear layers of the developing retina, and a more permanent subpopulation of cone bipolar cells displaced to the ganglion cell layer.

Localization of neuropeptide Y1 receptor immunoreactivity in the rat retina and the synaptic connectivity of Y1 immunoreactive cells

Neuropeptide Y (NPY), an inhibitory neuropeptide expressed by a moderately dense population of wide-field amacrine cells in the rat retina, acts through multiple (Y1-y6) G-protein-coupled receptors. This study determined the cellular localization of Y1 receptors and the synaptic connectivity of Y1 processes in the inner plexiform layer (IPL) of the rat retina. Specific Y1 immunoreactivity was localized to horizontal cell bodies in the distal inner nuclear layer and their processes in the outer plexiform layer. Immunoreactivity was also prominent in cell processes located in strata 2 and 4, and puncta in strata 4 and 5 of the IPL. Double-label immunohistochemical experiments with calbindin, a horizontal cell marker, confirmed Y1 immunostaining in all horizontal cells. Double-label immunohistochemical experiments, using antibodies to choline acetyltransferase and vesicular acetylcholine transporter to label cholinergic amacrine cell processes, demonstrated that Y1 immunoreactivity in strata 2 and 4 of the IPL was localized to cholinergic amacrine cell processes. Electron microscopic studies of the inner retina showed that Y1-immunostained amacrine cell processes and puncta received synaptic inputs from unlabeled amacrine cell processes (65.2%) and bipolar cell axon terminals (34.8%). Y1-immunoreactive amacrine cell processes most frequently formed synaptic outputs onto unlabeled amacrine cell processes (34.0%) and ganglion cell dendrites (54.1%). NPY immunoreactivity in the rat retina is distributed primarily to strata 1 and 5 of the IPL, and the present findings, thus, suggest that NPY acts in a paracrine manner on Y1 receptors to influence both horizontal and amacrine cells.

Neuropeptide tyrosine-like immunoreactive system in the brain, olfactory organ and retina of the zebrafish, Danio rerio, during development

The anatomical distribution of neuropeptide tyrosine (NPY)-like immunoreactivity was investigated in the brain, olfactory organ and retina of the zebrafish, Danio rerio, during development and in juvenile specimens, by using the indirect immunofluorescence and the peroxidase-antiperoxidase methods. In 60 h post fertilization (hpf) embryos, NPY-like immunoreactive cell bodies appeared in the hypothalamus, within the posterior periventricular nucleus. Few positive nerve fibers were found in the hypothalamus and in the tegmentum of the mesencephalon. In 72 hpf embryos, a new group of NPY-like immunoreactive cells was found in the olfactory pit. At day 4 of development, NPY-like immunoreactive cell bodies were detected between the olfactory pit and the olfactory organ. In the hypothalamus the location of positive cell bodies was similar to that reported in the previous developmental stages. A few positive nerve fibers appeared in the tegmentum of the rhombencephalon. At days 7 and 15 of development, the distribution of NPY-like immunoreactivity was very similar to that reported at day 4. However, at day 15, NPY-like immunoreactivity appeared for the first time in amacrine cells of the retina and in nerve fibers of the tectum of the mesencephalon. In 1-month/3-month-old animals, additional groups of NPY-like immunoreactive cell bodies appeared in the glomerular layer of the olfactory bulbs, the terminal nerve, the lateral nucleus of the ventral telencephalic area, the entopeduncular nucleus and in the medial region of the reticular formation of the rhombencephalon. These results show that NPY-like immunoreactive structures appear early during ontogeny of zebrafish. The distribution of the immunoreactive system increases during the ontogeny, the juvenile stages, and reaches the complete development in mature animals. The location of NPY-like immunoreactivity indicates that, during development, NPY could be involved in several neuromodulatory functions, including the processing of visual and olfactory information. In 1-month/3-month-old animals, NPY-like immunoreactive nerve fibers are present in the pituitary, suggesting that, from these stages onward, NPY may influence the secretion of pituitary hormones.

Distribution and synaptic connectivity of neuropeptide Y-immunoreactive amacrine cells in the rat retina

Neuropeptide Y (NPY) is a potent bioactive peptide that is widely expressed in the nervous system, including the retina. Here we show that specific NPY immunoreactivity was localized to amacrine and displaced amacrine cells in the rat retina. Immunoreactive cells had a regular distribution across the retina and an overall cell density of 280 cells/mm(2) in the inner nuclear layer (INL) and 90 cells/mm(2) in the ganglion cell layer (GCL). In the INL, most immunoreactive cells were characterized by small cell bodies and fine processes that appeared to ramify primarily in stratum 1 of the inner plexiform layer (IPL). A few cells in the INL also ramified in stratum 3 of the IPL. In the GCL, small to medium immunoreactive cells appeared to ramify primarily in stratum 5 of the IPL. A few immunoreactive processes, originating from somata in the INL and processes in the IPL, ramified in the OPL. NPY-immunoreactive cells contained GABA immunoreactivity, and some amacrine cells also contained tyrosine hydroxylase immunoreactivity. NPY-immunostained processes were most frequently presynaptic to nonimmunostained amacrine and ganglion cell processes and postsynaptic to nonimmunostained amacrine cell processes and cone bipolar cell axonal terminals. These findings indicate that NPY immunoreactivity is present in two populations of amacrine cells, one located in the INL and the other in the GCL, and that these cells mainly form synaptic contacts with other amacrine cells. These observations suggest that NPY-immunoreactive cells participate in multiple circuits mediating visual information processing in the inner retina.

Depletion of cholinergic amacrine cells by a novel immunotoxin does not perturb the formation of segregated on and off cone bipolar cell projections

Cone bipolar cells are the first retinal neurons that respond in a differential manner to light onset and offset. In the mature retina, the terminal arbors of On and Off cone bipolar cells terminate in different sublaminas of the inner plexiform layer (IPL) where they form synapses with the dendrites of On and Off retinal ganglion cells and with the stratified processes of cholinergic amacrine cells. Here we first show that cholinergic processes within the On and Off sublaminas of the IPL are present early in development, being evident in the rat on the day of birth, approximately 10 d before the formation of segregated cone bipolar cell axons. This temporal sequence, as well as our previous finding that the segregation of On and Off cone bipolar cell inputs occurs in the absence of retinal ganglion cells, suggested that cholinergic amacrine cells could provide a scaffold for the subsequent in-growth of bipolar cell axons. To test this hypothesis directly, a new cholinergic cell immunotoxin was constructed by conjugating saporin, the ribosome-inactivating protein toxin, to an antibody against the vesicular acetylcholine transporter. A single intraocular injection of the immunotoxin caused a rapid, complete, and selective loss of cholinergic amacrine cells from the developing rat retina. On and Off cone bipolar cells were visualized using an antibody against recoverin, the calcium-binding protein that labels the soma and processes of these interneurons. After complete depletion of cholinergic amacrine cells, cone bipolar cell axon terminals still formed their two characteristic strata within the IPL. These findings demonstrate that the presence of cholinergic amacrine cells is not required for the segregation of recoverin-positive On and Off cone bipolar cell projections.

Transient expression of somatostatin mRNA in developing ganglion cell layers of rat retina

Somatostatin (SOM) mRNA in developing ganglion cell layer (GCL) detected by in situ hybridization histochemistry and SOM peptide in developing optic chiasma and optic tract detected by immunocytochemistry were monitored to explore whether ganglion cells expressing SOM project to the visual center. Most of these cells in the developing GCL expressed SOM transiently from embryonic day 13 (E13) to E21. The cells expressing SOM mRNA initially followed a central-to-peripheral pattern of development. The cells expressing SOM mRNA in the retinas of fetuses became detectable at E13. From E14 to E17 the number of cells expressing SOM mRNA increased rapidly. At E17 most of the cells in the developing GCL expressed SOM mRNA. From E18 to postnatal days the positive cells became sparse except at the postnatal day 0 (PND0) the positive cells decreased dramatically in comparison with that at the E21. At PND15, the positive cells only can be found in the inner neuroblastic layer and in the ganglion cell layer. At PND20 the distribution pattern and the number of the positive cells were essentially the same as that in adult rat. SOM immunoreactivity was detectable at E16 in the developing optic chiasma and optic tract; the majority of the fibers in these area were SOM positive. From E16 to E18 the density of the immunostaining increased rapidly, whereas from E19 to E21 the density decreased. At PND0 no positive fibers were seen. The transient presence of SOM in most of the ganglion cells in the developing ganglion cell layer has prompted us to study the role of SOM in generation and differentiation of the retinal ganglion cells, and formation of the retina-visual center projections.

Retinal mosaics: new insights into an old concept

It has been known since the middle of the 19th century that different neuronal types are distributed across the retinal surface in non-random arrays: indeed, these arrays, called 'mosaics', have long been considered to be a fundamental feature of retinal organization. However, until recently, little was known about how such mosaics are established during development. In the hope of stimulating further research, this article reviews the current status of three very different approaches to this intriguing general problem. The first postulates arrays of molecular markers, which are produced by specific cell types shortly after their final mitotic divisions and could be influential in the differentiation of other cell types. The second invokes a tangential dispersion of differentiating cells to generate spatial order, either while these cells are still migrating or soon after they reach their laminar destinations. The third involves the elimination of wrongly positioned cells through the process of naturally occurring cell death.

Mosaics of islet-1-expressing amacrine cells assembled by short-range cellular interactions

The nervous system has a modular architecture with neurons of the same type commonly organized in nonrandom arrays or mosaics. Modularity is essential to parallel processing of sensory information and has provided a key element for brain evolution, but we still know very little of the way neuronal mosaics form during development. Here we have identified the immature elements of two retinal mosaics, the choline acetyltransferase (ChAT) amacrine cells, by their early expression of the homeodomain protein Islet-1, and we show that spatial ordering is an intrinsic property of the two Islet-1 mosaics, dynamically maintained while new elements are inserted into the mosaics. Migrating Islet-1 cells do not show this spatial ordering, indicating that they must move tangentially as they enter the mosaic, under the action of local mechanisms. Clonal territory analysis in X-inactivation transgenic mice confirms the lateral displacement of ChAT amacrine cells away from their clonal columns of origin, and mathematical models show how short-range cellular interactions can guide the assemblage of these mosaics via a simple biological rule.