Subset Of Bipolar Cells Research Articles

Dopaminergic Modulation of Signal Processing in a Subset of Retinal Bipolar Cells.

The retina and the olfactory bulb are the gateways to the visual and olfactory systems, respectively, similarly using neural networks to initiate sensory signal processing. Sensory receptors receive signals that are transmitted to neural networks before projecting to primary cortices. These networks filter sensory signals based on their unique features and adjust their sensitivities by gain control systems. Interestingly, dopamine modulates sensory signal transduction in both systems. In the retina, dopamine adjusts the retinal network for daylight conditions (“light adaptation”). In the olfactory system, dopamine mediates lateral inhibition between the glomeruli, resulting in odorant signal decorrelation and discrimination. While dopamine is essential for signal discrimination in the olfactory system, it is not understood whether dopamine has similar roles in visual signal processing in the retina. To elucidate dopaminergic effects on visual processing, we conducted patch-clamp recording from second-order retinal bipolar cells, which exhibit multiple types that can convey different temporal features of light. We recorded excitatory postsynaptic potentials (EPSPs) evoked by various frequencies of sinusoidal light in the absence and presence of a dopamine receptor 1 (D1R) agonist or antagonist. Application of a D1R agonist, SKF-38393, shifted the peak temporal responses toward higher frequencies in a subset of bipolar cells. In contrast, a D1R antagonist, SCH-23390, reversed the effects of SKF on these types of bipolar cells. To examine the mechanism of dopaminergic modulation, we recorded voltage-gated currents, hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and low-voltage activated (LVA) Ca2+ channels. SKF modulated HCN and LVA currents, suggesting that these channels are the target of D1R signaling to modulate visual signaling in these bipolar cells. Taken together, we found that dopamine modulates the temporal tuning of a subset of retinal bipolar cells. Consequently, we determined that dopamine plays a role in visual signal processing, which is similar to its role in signal decorrelation in the olfactory bulb.

Expression of LIM-homeodomain transcription factors in the developing and mature mouse retina

LIM-homeodomain (LIM-HD) transcription factors have been extensively studied for their role in the development of the central nervous system. Their function is key to several developmental events like cell proliferation, differentiation and subtype specification. However, their roles in retinal neurogenesis remain largely unknown. Here we report a detailed expression study of LIM-HD transcription factors LHX9 and LHX2, LHX3 and LHX4, and LHX6 in the developing and mature mouse retina using immunohistochemistry and in situ hybridization techniques. We show that LHX9 is expressed during the early stages of development in the retinal ganglion cell layer and the inner nuclear layer. We also show that LHX9 is expressed in a subset of amacrine cells in the adult retina. LHX2 is known to be expressed in retinal progenitor cells during development and in Müller glial cells and a subset of amacrine cells in the adult retina. We found that the LHX2 subset of amacrine cells is not cholinergic and that a very few of LHX2 amacrine cells express calretinin. LHX3 and LHX4 are expressed in a subset of bipolar cells in the adult retina. LHX6 is expressed in cells in the ganglion cell layer and the neuroblast layer starting at embryonic stage 13.5 (E13.5) and continues to be expressed in cells in the ganglion cell layer and inner nuclear layer, postnatally, suggesting its likely expression in amacrine cells or a subset thereof. Taken together, our comprehensive assay of expression patterns of LIM-HD transcription factors during mouse retinal development will help further studies elucidating their biological functions in the differentiation of retinal cell subtypes.

Dynamic cellular translocation of caldendrin is facilitated by the Ca2+‐myristoyl switch of recoverin

Caldendrin and recoverin are Ca(2+)-sensor proteins operating in neuronal systems. In a search for novel binding partners of recoverin, we employed an affinity column and identified caldendrin as a possible interaction partner. Caldendrin and recoverin co-localized in the retina in a subset of bipolar cells and in the pineal gland as revealed by immunofluorescence studies. The binding process was controlled by Ca(2+) as revealed by pull-down assays, and surface plasmon resonance studies. Importantly, caldendrin existed as a Ca(2+)-independent homodimer whereas a complex of recoverin and caldendrin formed with low to moderate affinity in the presence of Ca(2+). Co-transfection of COS-7 cells with plasmids harboring the gene for fluorescently labeled recoverin and caldendrin was used to study the cellular distribution by time-lapse fluorescence microscopy. Apparently, the increase of intracellular Ca(2+) facilitates the translocation of caldendrin to intracellular membranes, which is under control of complex formation with recoverin.

Histamine Enhances Voltage-Gated Potassium Currents of ON Bipolar Cells in Macaque Retina

The goal was to understand the functions of retinopetal axons containing histamine. In prior work, type 3 histamine receptors (HR3) have been localized to the tips of ON bipolar cell dendrites in macaque retinas. Voltage-gated potassium channels have also been localized to bipolar cell dendrites, and the hypothesis tested in the present study was that these are modulated by histamine. Whole-cell recordings of potassium currents were made from bipolar cells in slice preparations of macaque retina. In voltage-clamp mode, the cells were held at -60 mV and stepped to values from -60 to 80 mV. Recordings of the membrane potential were also made in current-clamp mode. Histamine, the HR3 agonist (R) alpha-methylhistamine (RAMH), tetraethyl ammonium (TEA), and 4-aminopyridine (4-AP) were applied in the superfusate. Histamine produced a dose-dependent increase in potassium currents in a subset of bipolar cells. At 5 microM, histamine increased the currents by 15% or more in the ON bipolar cells but not in the OFF bipolar cells. RAMH at 5 microM increased the amplitude of the potassium currents in the ON bipolar cells. In 10 mM TEA, potassium currents were reduced in all the bipolar cells, and there was no effect of histamine. Histamine hyperpolarized the resting membrane potential of the ON bipolar cells by 5 mV. By enhancing potassium currents in the ON bipolar cells, histamine is expected to reduce the amplitude of the light responses and limit their duration. The hyperpolarization of the resting membrane potential would also reduce neurotransmitter release at their output synapses.

In vivodevelopment of retinal ON-bipolar cell axonal terminals visualized innyx::MYFPtransgenic zebrafish

Axonal differentiation of retinal bipolar cells has largely been studied by comparing the morphology of these interneurons in fixed tissue at different ages. To better understand how bipolar axonal terminals develop in vivo, we imaged fluorescently labeled cells in the zebrafish retina using time-lapse confocal and two photon microscopy. Using the upstream regulatory sequences from the nyx gene that encodes nyctalopin, we constructed a transgenic fish in which a subset of retinal bipolar cells express membrane targeted yellow fluorescent protein (MYFP). Axonal terminals of these YFP-labeled bipolar cells laminated primarily in the inner half of the inner plexiform layer, suggesting that they are likely to be ON-bipolar cells. Transient expression of MYFP in isolated bipolar cells indicates that two or more subsets of bipolar cells, with one or two terminal boutons, are labeled. Live imaging of YFP-expressing bipolar cells in the nyx::MYFP transgenic fish at different ages showed that initially, filopodial-like structures extend and retract from their primary axonal process throughout the inner plexiform layer (IPL). Over time, filopodial exploration becomes concentrated at discrete foci prior to the establishment of large terminal boutons, characteristic of the mature form. This sequence of axonal differentiation suggests that synaptic targeting by bipolar cell axons may involve an early process of trial and error, rather than a process of directed outgrowth and contact. Our observations represent the first in vivo visualization of axonal development of bipolar cells in a vertebrate retina.

Axons and dendrites originate from neuroepithelial-like processes of retinal bipolar cells

The cellular mechanisms underlying axogenesis and dendritogenesis are not completely understood. The axons and dendrites of retinal bipolar cells, which contact their synaptic partners within specific laminae in the inner and outer retina, provide a good system for exploring these issues. Using transgenic mice expressing enhanced green fluorescent protein (GFP) in a subset of bipolar cells, we determined that axonal and dendritic arbors of these interneurons develop directly from apical and basal processes attached to the outer and inner limiting membranes, respectively. Selective stabilization of processes contributed to stratification of axonal and dendritic arbors within the appropriate synaptic layer. This unusual mode of axogenesis and dendritogenesis from neuroepithelial-like processes may act to preserve neighbor-neighbor relationships in synaptic wiring between the outer and inner retina.

Synaptic connections of starburst amacrine cells and localization of acetylcholine receptors in primate retinas

Starburst amacrine cells in the macaque retina were studied by electron microscopic immunohistochemistry. We found that these amacrine cells make a type of synapse not described previously; they are presynaptic to axon terminals of bipolar cells. We also confirmed that starburst amacrine cells are presynaptic to ganglion cell dendrites and amacrine cell processes. In order to determine the functions of these synapses, we localized acetylcholine receptors using a monoclonal antibody (mAb210) that recognizes human alpha3- and alpha5-containing nicotinic receptors and also antisera against the five known subtypes of muscarinic receptors. The majority of the mAb210-immunoreactive perikarya were amacrine cells and ganglion cells, but a subpopulation of bipolar cells was also labeled. A subset of bipolar cells and a subset of horizontal cells were labeled with antibodies to M3 muscarinic receptors. A subset of amacrine cells, including those that contain cholecystokinin, were labeled with antibodies to M2 receptors. Taken together, these results suggest that acetylcholine can modulate the activity of retinal ganglion cells by multiple pathways.

Alpha-Gustducin expression in the vomeronasal organ of the mouse.

The expression of alpha-gustducin, a G protein alpha subunit involved in bitter and sweet taste transduction, was investigated in chemosensory tissues of adult mice. By immunohistochemistry, alpha gustducin was absent in the olfactory neuroepithelium. Instead, alpha gustducin was expressed in a subset of bipolar cells in the proliferative zone of the vomeronasal neuroepithelium as well as in taste buds. Northern blot analysis confirmed the presence of alpha gustducin in isolated vomeronasal organs. Moreover, immunohisto- chemistry revealed the expression of alpha gustducin in scattered cells of the nasal respiratory epithelium. These results show for the first time that alpha gustducin is expressed in chemosensory tissue outside the alimentary tract, suggesting that common transduction mechanisms could be shared by apparently unrelated chemosensory tissues.

Development of serotoninergic chick retinal neurons

Numerous neurotransmitters have been studied in detail in the developing retina. Almost all known neurotransmitters and neuromodulators were demonstrated in vertebrate retinas using formaldehyde-induced fluorescence, uptake autoradiography or immunohistochemistry procedures. Serotoninergic (5HT) amacrine neurons were described in the inner nuclear layer (INL) of the retina with their dendrites spreading within the inner plexiform layer (IPL). The present work describes the morphological pattern of development of serotoninergic amacrine neurons with a stratified dendritic branching pattern in the chick retina from embryonic day 12 to postnatal day 7. Serotoninergic-bipolar neurons are also described. 5HT-amacrine neurons have round or pear-shaped somata and primary dendritic trees oriented toward the IPL that runs through the INL, showing several varicosities. Secondary dendrites then go through the INL, without any collateral branch. At the outer and inner margin of the IPL the primary and secondary dendrites originate an outer and an inner serotoninergic network, respectively. When the primary dendritic tree reaches the IPL it deflects laterally in sublayer 1—the outer serotoninergic network. Tertiary branches then arise from the secondary dendrite and deflect in the innermost sublayer of the IPL— the inner serotoninergic network. The final pattern of branching of 5HT amacrine cells was present at embryonic day 14 and was completely developed at hatching. Serotoninergic (5HT) bipolar neurons were also present in the INL at hatching. They are weakly immunoreactive and are probably a subset of bipolar cells that accumulate serotonin from the intersynaptic cleft and are not ‘‘true’’ 5HT neurons.

Serotonergic and serotonin-accumulating neurons in the goldfish retina

Autoradiography of goldfish retinas incubated in micromolar levels of 3H-serotonin displayed 3 kinds of labeled somas in the inner nuclear layer: S1 amacrine cells with heavy labeling, large somas, and a sparse distribution (approximately 93/mm2); S2 amacrine cells with moderate labeling, smaller somas, and a denser distribution (approximately 500/mm2); and a subset of bipolar cells with light labeling, small somas, and a very dense distribution (approximately 4000/mm2). Serotonin-like immunoreactivity was observed only in S1 amacrine cells and their synaptic terminals. Radiolabeled terminals in the inner plexiform layer formed 4 strata that were differentially assigned to the 3 cell types. S1 amacrine cells arborized in sublayers 1 and 5, received inputs from type a1 bipolar cells and amacrine cells, and made synapses on other amacrine cells, type a1 bipolar cells and unidentified processes. Thus, S1 amacrine cells seem to receive significant input from "off-center" pathways. S2 amacrine cells arborized in sublayer 3 and made synapses onto amacrine cells. Labeled bipolar cell terminals were exclusively located in sublayer 2 and were identified as type a2 mixed rod-cone bipolar cells. We conclude that the S1 amacrine cell is truly serotonergic and that radiolabeling of S2 amacrine cells and type a2 bipolar cells is due to cross-specificity for another carrier or processes unrelated to their neurochemical identities. These observations partially reconcile many previous observations on the types, numbers, and synaptologies of teleost retinal neurons identified by different markers for indoleaminergic transmission.