Vulnerability of Dopaminergic Amacrine Cells to Chronic Ischemia in a Mouse Model of Oxygen-Induced Retinopathy

Purpose

Retinal dopamine deficiency is a potential cause of myopia and visual deficits in retinopathy of prematurity (ROP). We investigated the cellular mechanisms responsible for lowered levels of retinal dopamine in an oxygen-induced retinopathy (OIR) mouse model of ROP.

Methods

Retinopathy was induced by exposing mice to 75% oxygen from postnatal day 7 (P7) to P12. Oxygen-induced retinopathy and age-matched control mice were euthanized at P12, P17, P25, or P42 to P50. Immunohistochemistry, electrophysiology, and biochemical approaches were used to determine the effect of OIR on the structure and function of dopaminergic amacrine cells (DACs).

Results

The total number of DACs was unchanged in OIR retinas at P12 despite significant capillary dropout in the central retina. However, a significant loss of DACs was observed in P17 OIR retinas (in which neovascularization was maximal), with the cell loss being more profound in the central (avascular) than in the peripheral (neovascular) regions. Cell loss was persistent in both regions at P25, at which time retinal neovascularization had regressed. At P42, the percentage of DACs lost (54%) was comparable to the percent decrease in total dopamine content (53%). Additionally, it was found that DACs recorded in OIR retinas at P42 to P50 had a complete dendritic field and exhibited relatively normal spontaneous and light-induced electrical activity.

Conclusions

The results suggest that remaining DACs are structurally and functionally intact and that loss of DACs is primarily responsible for the decreased levels of retinal dopamine observed after OIR.

Multiple cone pathways are involved in photic regulation of retinal dopamine

Dopamine is a key neurotransmitter in the retina and plays a central role in the light adaptive processes of the visual system. The sole source of retinal dopamine is dopaminergic amacrine cells (DACs). We and others have previously demonstrated that DACs are activated by rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) upon illumination. However, it is still not clear how each class of photosensitive cells generates light responses in DACs. We genetically isolated cone function in mice to specifically examine the cone-mediated responses of DACs and their neural pathways. In addition to the reported excitatory input to DACs from light-increment (ON) bipolar cells, we found that cones alternatively signal to DACs via a retrograde signalling pathway from ipRGCs. Cones also produce ON and light-decrement (OFF) inhibitory responses in DACs, which are mediated by other amacrine cells, likely driven by type 1 and type 2/3a OFF bipolar cells, respectively. Dye injections indicated that DACs had similar morphological profiles with or without ON/OFF inhibition. Our data demonstrate that cones utilize specific parallel excitatory and inhibitory circuits to modulate DAC activity and efficiently regulate dopamine release and the light-adaptive state of the retina.

Hyperactivity of ON-type retinal ganglion cells in streptozotocin-induced diabetic mice

Impairment of visual function has been detected in the early stage of diabetes but the underlying neural mechanisms involved are largely unknown. Morphological and functional alterations of retinal ganglion cells, the final output neurons of the vertebrate retina, are thought to be the major cause of visual defects in diabetes but direct evidence to support this notion is limited. In this study we investigated functional changes of retinal ganglion cells in a type 1-like diabetic mouse model. Our results demonstrated that the spontaneous spiking activity of ON-type retinal ganglion cells was increased in streptozotocin-diabetic mice after 3 to 4 months of diabetes. At this stage of diabetes, no apoptotic signals or cell loss were detected in the ganglion cell layer of the retina, suggesting that the functional alterations in ganglion cells occur prior to massive ganglion cell apoptosis. Furthermore, we found that the increased activity of ON-type ganglion cells was mainly a result of reduced inhibitory signaling to the cells in diabetes. This novel mechanism provides insight into how visual function is impaired in diabetic retinopathy.

Functional integrity and modification of retinal dopaminergic neurons in the rd1 mutant mouse: roles of melanopsin and GABA

The progressive loss of rod and cone photoreceptors in human subjects with retinitis pigmentosa causes a gradual decline in vision and can result in blindness. Current treatment strategies for the disease rely on the integrity of inner retinal neurons, such as amacrine cells, that are postsynaptic to photoreceptors. Previous work has demonstrated that a specialized subclass of retinal amacrine cell that synthesizes and releases the key neurotransmitter dopamine remains morphologically intact during the disease; however, the pathophysiological function of these neurons remains poorly understood. Here we examined spontaneous and light-evoked spike activity of genetically labeled dopamine neurons from the retinas of retinal degeneration 1 (rd1) mice. Our results indicated that rd1 dopamine neurons remained functionally intact with preserved spontaneous spiking activity and light-evoked responses. The light responses were mediated exclusively by melanopsin phototransduction, not by surviving cones. Our data also suggested that dopamine neurons were altered during photoreceptor loss, as evidenced by less spontaneous bursting activity and increased light-evoked responses with age. Further evidence showed that these alterations were attributed to enhanced GABA/melanopsin signaling to dopamine neurons during disease progression. Taken together, our studies provide valuable information regarding the preservation and functional modification of the retinal dopamine neuronal system in rd1; this information should be considered when designing treatment strategies for retinitis pigmentosa.

Physiological and molecular characterization of connexin hemichannels in zebrafish retinal horizontal cells

Connexin channels mediate electrical synaptic transmission when assembled as cell-to-cell pores at gap junctions and can mediate transmembrane currents when expressed in plasma membranes as hemichannels. They are widely expressed in the vertebrate retina where in electrical synapses they are critical for transmission of visual signals. While the roles of connexins in electrical synapses are well-studied, the function and roles of connexin hemichannels in the nervous system are less well understood. Genetic deletion in zebrafish of connexin (Cx) 55.5 alters horizontal cell feedback to cones, spectral responses, and visual behavior. Here, we have characterized the properties of hemichannel currents in zebrafish retinal horizontal cells and examined the roles of two connexin isoforms, Cx55.5 and Cx52.6, that are coexpressed in these cells. We report that zebrafish horizontal cells express hemichannel currents that conduct inward current at physiological negative potentials and Ca(2+) levels. Manipulation of Cx55.5 and Cx52.6 gene expression in horizontal cells of adult zebrafish revealed that both Cx55.5 and Cx52.6 contribute to hemichannel currents; however, Cx55.5 expression is necessary for high-amplitude currents. Similarly, coexpression of Cx55.5 with Cx52.6 in oocytes increased hemichannel currents in a supra-additive manner. Taken together these results demonstrate that zebrafish horizontal cell hemichannel currents exhibit the functional characteristics necessary to contribute to synaptic feedback at the first visual synapse, that both Cx55.5 and Cx52.6 contribute to hemichannel currents, and that Cx55.5 may have an additional regulatory function enhancing the amplitude of hemichannel currents.

Celecoxib and acetylbritannilactone interact synergistically to suppress breast cancer cell growth via COX-2-dependent and -independent mechanisms

The use of celecoxib is associated with a significant decrease in breast cancer risk. However, the long-term use of high-dose celecoxib might be limited owing to cardiovascular side effects. In this study, we found that acetylbritannilactone (ABL), extract from a Chinese medicinal herb, could reduce celecoxib dose and potentiate the growth-inhibitory effect in breast cancer cells. ABL enhanced the apoptotic effect of celecoxib in COX-2-expressing cells, but had little effect in COX-2-negative cells. The apoptosis induced by the combination treatment disappeared when COX-2 was knocked down, whereas the lack of apoptotic effects in COX-2-negative cells was reversed after COX-2 transfection. However, the combination treatment induced a G(0)/G(1) phase arrest independent of whether or not the cells expressed COX-2. The G(0)/G(1) arrest was attributed to a decreased expression of cyclinD1, cyclinE, CDK2 and CDK6, especially the upregulation of p21. In addition, inhibition of Akt and p38 signaling pathways was required by the synergism, as the constitutively active Akt and p38 protected cells against apoptosis and cell cycle arrest induced by the combination treatment. In vivo, administration of celecoxib and ABL were more effective than the individual agents against xenograft tumor growth. Thus, our data suggested that the combinatorial approach of celecoxib and ABL might be helpful for breast cancer treatment.

Zinc modulation of hemi-gap-junction channel currents in retinal horizontal cells

Hemi-gap-junction (HGJ) channels of retinal horizontal cells (HCs) function as transmembrane ion channels that are modulated by voltage and calcium. As an endogenous retinal neuromodulator, zinc, which is coreleased with glutamate at photoreceptor synapses, plays an important role in shaping visual signals by acting on postsynaptic HCs in vivo. To understand more fully the regulation and function of HC HGJ channels, we examined the effect of Zn(2+) on HGJ channel currents in bass retinal HCs. Hemichannel currents elicited by depolarization in Ca(2+)-free medium and in 1 mM Ca(2+) medium were significantly inhibited by extracellular Zn(2+). The inhibition by Zn(2+) of hemichannel currents was dose dependent with a half-maximum inhibitory concentration of 37 microM. Compared with other divalent cations, Zn(2+) exhibited higher inhibitory potency, with the order being Zn(2+) > Cd(2+) approximately Co(2+) > Ca(2+) > Ba(2+) > Mg(2+). Zn(2+) and Ca(2+) were found to modulate HGJ channels independently in additivity experiments. Modification of histidine residues with N-bromosuccinimide suppressed the inhibitory action of Zn(2+), whereas modification of cysteine residues had no significant effect on Zn(2+) inhibition. Taken together, these results suggest that zinc acts on HGJ channels in a calcium-independent way and that histidine residues on the extracellular domain of HGJ channels mediate the inhibitory action of zinc.

Targeting retinal dopaminergic neurons in tyrosine hydroxylase-driven green fluorescent protein transgenic zebrafish

PURPOSE

Dopamine plays key roles in a variety of basic functions in the central nervous system. To study developmental and functional roles of dopaminergic cells in zebrafish, we have generated a transgenic line of zebrafish expressing green fluorescent protein (GFP) under the control of the tyrosine hydroxylase (th1) promoter.

METHODS

A 12 kb gene fragment that contains the th1 promoter was isolated and ligated to the MmGFP coding sequence, linearized, microinjected into 1-2 cell stage embryos and the founders crossed with wild-type fish to screen for transgenic lines. Tg(-12th:MmGFP) embryos were visualized under fluorescence microscopy for GFP expression during development. Confocal microscopy was used to visualize GFP-labeled cells in the living whole mount retina and immunostained vertical sections of adult zebrafish retina. Single-cell reverse transcription polymerase chain reaction (RT-PCR) was performed on individual GFP+ cells collected from dispersed retinal cell cultures for th1 and dopamine transporter (dat). Loose-patch recordings of spike activity of GFP+ neurons were made in isolated whole mount retinas.

RESULTS

th1 promoter-driven GFP exhibited robust expression in the brain and retina during zebrafish development. In juvenile and adult fish retinas, GFP was expressed in cells located in the inner nuclear layer. Immunocytochemistry with antibodies for GFP and TH showed that 29+/-2% of GFP-labeled cells also expressed TH. Two subpopulations of GFP-labeled cells were identified by fluorescent microscopy: bright GFP-expressing cells and dim GFP-expressing cells. Seminested single-cell RT-PCR showed that 71% of dim GFP-expressing cells expressed both th and dat mRNA. Loose-patch voltage-clamp recording from dim GFP-labeled cells in retinal whole mounts revealed that many of these dopaminergic neurons are spontaneously active in darkness.

CONCLUSIONS

Although this Tg(-12th:MmGFP) line is not a completely specific reporter for dopaminergic neurons, using relative GFP intensity we are able to enrich for the selection of retinal dopaminergic cells in vitro and in situ in molecular and electrophysiological experiments. This transgenic line provides a useful tool for studying retinal dopaminergic cells in the zebrafish.

Functional heterogeneity of retinal dopaminergic neurons underlying their multiple roles in vision

Dopaminergic neurons play key roles in the CNS, mediating basic mechanisms of vision, movement, motivation, and mood. The most accessible dopaminergic neurons of the vertebrate CNS are the dopaminergic amacrine cells of the retina. Here, we have characterized the intrinsic neural activity, synaptic input, and light responses of retinal dopaminergic neurons in situ, using targeted electrophysiological recordings of fluorescent neurons in TH::RFP (tyrosine hydroxylase gene promoter::red fluorescent protein) transgenic mice. Dopaminergic amacrine cells exhibit two classes of intrinsic bursting in the dark, shaped by inhibitory synaptic inputs, and two classes of light responses, ON-transient and ON-sustained, as well as light-independent activity, tuned to mediate specific dopaminergic functions in vision. The functional heterogeneity revealed in dopaminergic amacrine cells provides a cellular basis for the multiple roles of dopaminergic amacrine neurons in vision and is likely a general property of dopaminergic neurons throughout the CNS.

Gene profiling after knocking-down expression of nucleostemin in Hela cells using oligonucleotide DNA microarray

Nucleostemin (NS) is preferentially and exclusively expressed in the stem cells and cancer cells, but not in differentiated adult tissues and cells. NS is likely to take part in controlling the proliferation and differentiation switch in stem cells and progenitor cells. Its deregulation in cancer also contributes to the elevated proliferation and undifferentiation of cancer cells. However, the mechanisms by which NS helps to maintain both cancer and stem cells in undifferentiated state remain unclear. In this study, we carried out gene profilings using oligonucleotide DNA microarray after knocking down the expression of NS in Hela cells. Of the 21,329 genes, 200 genes were found differentially expressed in NS silenced Hela cells with > 2 fold ratio (either > 2 or < 0.5). Category analysis indicated these differential genes were mainly related with cancer pathogenesis, cell death, cell growth and proliferation. NS related gene pathway analysis suggested NS was mostly involved in the networks of cell cycle and differentiation controls. p53 may not be the only partner of NS in its regulated pathways. c-Myc may directly or indirectly interact with it to control the proliferation and differentiation switch in cancer cells. Our study provides a general view of the NS-target genes, and indicates the possible pathways in which NS plays its role in proliferation control.

Modulation of A-type potassium currents in retinal horizontal cells by extracellular calcium and zinc

Extracellular Ca2+ and Zn2+ influence many aspects of retinal function. Here, we examined the effect of external Ca2+ and Zn2+ on potassium channels of retinal horizontal cells. When extracellular Ca2+ was lowered from 3 mM to 0.3 mM, horizontal cell transient outward currents elicited by voltage steps from resting membrane potential (−70 mV) were decreased by approximately 50%, whereas the sustained currents remained unchanged. This effect was due to a hyperpolarizing shift in the steady-state inactivation curve of A-type K+ currents when extracellular Ca2+ concentration was lowered. The mean half inactivation potential of the steady-state inactivation curves was hyperpolarized from −56.3 ± 4.7 mV in 3 mM Ca2+ to −76.4 ± 3.9 mV in 0.3 mM Ca2+. Neither the state-steady activation curve nor the kinetics of inactivation was significantly changed in low extracellular Ca2+. The addition of 30 μM Zn2+ restored peak outward currents in 0.3 mM Ca2+. The half inactivation voltages were depolarized from −70 ± 2.8 mV in 0.3 mM Ca2+ to −56 ± 2.6 mV in 0.3 mM Ca2+ plus 30 μM Zn2+. Taken together, the results indicate that external Ca2+ and Zn2+ maintain the activity of A-type potassium channels in retinal horizontal cells by influencing the voltage dependence of steady-state inactivation.

Circadian organization of the mammalian retina

The mammalian retina contains an endogenous circadian pacemaker that broadly regulates retinal physiology and function, yet the cellular origin and organization of the mammalian retinal circadian clock remains unclear. Circadian clock neurons generate daily rhythms via cell-autonomous autoregulatory clock gene networks, and, thus, to localize circadian clock neurons within the mammalian retina, we have studied the cell type-specific expression of six core circadian clock genes in individual, identified mouse retinal neurons, as well as characterized the clock gene expression rhythms in photoreceptor degenerate rd mouse retinas. Individual photoreceptors, horizontal, bipolar, dopaminergic (DA) amacrines, catecholaminergic (CA) amacrines, and ganglion neurons were identified either by morphology or by a tyrosine hydroxylase (TH) promoter-driven red fluorescent protein (RFP) fluorescent reporter. Cells were collected, and their transcriptomes were subjected to multiplex single-cell RT-PCR for the core clock genes Period (Per) 1 and 2, Cryptochrome (Cry) 1 and 2, Clock, and Bmal1. Individual horizontal, bipolar, DA, CA, and ganglion neurons, but not photoreceptors, were found to coordinately express all six core clock genes, with the lowest proportion of putative clock cells in photoreceptors (0%) and the highest proportion in DA neurons (30%). In addition, clock gene rhythms were found to persist for >25 days in isolated, cultured rd mouse retinas in which photoreceptors had degenerated. Our results indicate that multiple types of retinal neurons are potential circadian clock neurons that express key elements of the circadian autoregulatory gene network and that the inner nuclear and ganglion cell layers of the mammalian retina contain functionally autonomous circadian clocks.

Circadian rhythm of Period1 clock gene expression in NOS amacrine cells of the mouse retina

The vertebrate retina contains self-sustained circadian clocks that broadly influence retinal physiology. In the present study, we have examined the relationship of nitric oxide, GABAergic and glycinergic inner retinal neurons with expression of a reporter for the circadian clock gene Period1 (Per1). Using Per1 : :GFP transgenic mice, we found that 72% of brain nitric oxide synthase (bNOS) expressing amacrine cells (NOS amacrine cells) sampled during the daytime were also immunoreactive for Per1-driven GFP. The number of bright GFP(+) NOS(+) cells was greater at Zeitgeber time (ZT) 10 than at 22, and this pattern persisted in retinas from animals which were placed in constant darkness [Circadian time (CT) 10 vs. 22]. Intensities of GFP-IR for individual NOS amacrine cells were analyzed at ZT4, 10, 16 and 22, with the peak value occurring at ZT10. Similar results were obtained from retinas sampled at CT4, 10, 16 and 22 in constant darkness, indicating that an endogenous circadian clock drives the transcription of the Per1 clock gene within NOS amacrine cells. The predominance of Per1 : :GFP(+) amacrine cells (82%), was immunoreactive to glutamate decarboxylase 65, but no Per1 : :GFP(+) amacrine cells colabeled with a glycine transporter 1 antibody. The results demonstrate circadian rhythms in Per1 promoter activation in nitric oxide (NO) and GABA secreting amacrine cells, and suggest that NO and GABA could be controlled by circadian clock mechanisms in the mammalian retina.

Characterization of genetically labeled catecholamine neurons in the mouse retina

Mouse neurons were labeled transgenically with red fluorescent protein (RFP) driven by the tyrosine hydroxylase (TH) promoter and observed in living retinas and brain slices. Two types of retinal amacrine cells expressed TH::RFP. One type had large cell bodies, processes that ramified in S1 of the inner plaxiform layer (IPL) and were TH immunoreactive, identifying them as dopaminergic neurons. A second type had smaller somas, ramified in S3 and lacked TH. Dopaminergic cells had large dendritic fields and exceptionally long axon-like processes, whereas type 2 cells were more compact. Neither cell type exhibited tracer coupling. Thus, murine retinal dopaminergic neurons exhibit functional anatomy similar to their primate counterparts and TH::RFP mice are useful for in situ characterization of catecholaminergic neurons.

Cellular location and circadian rhythm of expression of the biological clock gene Period 1 in the mouse retina

The cellular location and rhythmic expression of Period 1 (Per1) circadian clock gene were examined in the retina of a Per1::GFP transgenic mouse. Mouse Per1 (mPer1) RNA was localized to inner nuclear and ganglion cell layers but was absent in the outer nuclear (photoreceptor) layer. Green fluorescent protein (GFP), which was shown to colocalize with PER1 protein, was found in a few subtypes of amacrine neuron, including those containing tyrosine hydroxylase, calbindin, and calretinin, but not in cholinergic amacrine cells. A small subset of ganglion cells also contained GFP immunoreactivity (GFP-IR), but the melanopsin-containing subtype, which projects to the suprachiasmatic nuclei (SCN), lacked GFP-IR. Although the intensity of GFP-IR varied among the populations of amacrine cells at each time point that was examined, both diurnal and circadian rhythms were found for the fraction of neurons showing strong GFP-IR, with peak expression between Zeitgeber/circadian (ZT/CT) times 10 and 14. In SCNs that were examined in the same mice used for the retinal measures, the peak in GFP-IR also occurred at approximately ZT/CT 10. Our results are the first to demonstrate a circadian rhythm of a biological clock component in identified neurons of a mammalian retina.

Suppression by zinc of AMPA receptor-mediated synaptic transmission in the retina

Zinc is strikingly co-localized with glutamate-containing vesicles in the synaptic terminals of retinal photoreceptors, and it is thought to be co-released with glutamate onto postsynaptic neurons such as horizontal cells and bipolar cells. Here we examined exogenous zinc modulation of glutamate receptors on cultured retinal horizontal cells using patch-clamp recording and endogenous zinc effect on intact horizontal cells using intracellular recording techniques. Application of 3, 30, and 300 microM zinc reduced the whole cell peak current of response to 200 microM glutamate by 2, 30, and 56%, respectively. Zinc suppression of glutamate response persisted in the presence of 10 microM cyclothiazide (CTZ). Glutamate responses of outside-out patches were completely abolished by 30 microM 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), and the receptor desensitization was blocked by 30 microM CTZ, indicating that receptor target for the zinc action on horizontal cells is alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid (AMPA) receptors. Zinc decreased the amplitude of outside-out patch peak current without an effect on either its 10-90% rise time or the rate of receptor desensitization. Dose-response curves for glutamate show that zinc reduced the maximal current evoked by glutamate and increased EC(50) from 50 +/- 3 to 70 +/- 6 microM without changing the Hill coefficient. Chelation of endogenous zinc with 1 mM Ca-EDTA depolarized horizontal cells in the intact retina by 3 mV, consistent with relief of the partial glutamate receptor inhibition by zinc. Overall, the results describe a unimodal form of zinc modulation of AMPA-type glutamate receptor responses not previously described in native neuronal preparations and a novel role for endogenous zinc in modulating neurotransmission.

Gating of retinal horizontal cell hemi gap junction channels by voltage, Ca2+, and retinoic acid

PURPOSE

Hemi gap junction (HGJ) channels, precursors of gap junctional channels, are functionally expressed in retinal horizontal cells where they may play roles in osmoeregulation and ephaptic regulation of synaptic feedback to photoreceptors. In this study we examined mechanisms of gating of these channels by transmembrane voltage, Ca2+ and retinoic acid (RA).

METHODS

Experiments were performed on cultured bass horizontal cells using the conventional whole cell patch clamp configuration.

RESULTS

HGJ currents in isolated bass horizontal cells, revealed by perfusion with Ca2+ free media, were opened by positive holding potentials and inhibited by negative holding potentials. These currents were also inhibited by external application of either Ca2+ or RA. Using a rapid perfusion system, the latency of 2 mM Ca2+ to begin channel closure was unmeasurably brief, whereas the latency for 30 microM RA action was 177+/-9 ms (mean+/-standard error of the mean). The total inhibition of HGJ channel currents by coapplication of 0.3 microM RA and 100 microM Ca2+ was less than the sum of inhibition by RA alone and Ca2+ alone suggesting that the actions of RA and Ca2+ were not independent. In the presence of 0.3 microM RA, the half maximal concentration for Ca2+ inhibition was increased from a control value of 192 microM to 375 microM without affecting maximal inhibition. Similarly, the half maximal concentration for RA inhibition was increased from a control value of 0.44 microM to 1.1 microM without affecting maximal inhibition in the presence of 100 microM Ca2+.

CONCLUSIONS

These results suggest that horizontal cell HGJ channels are closed by the normal negative resting potentials of these cells. Extracellular Ca2+ and the retinal neuromodulator RA also act to close HGJ channels through mechanisms or sites which are not independent.

Direct gating by retinoic acid of retinal electrical synapses

Retinoic acid (RA), a signaling molecule derived from vitamin A, controls growth and differentiation of a variety of cell types through regulation of gene transcription. In the vertebrate retina, RA also regulates gap junction-mediated physiological coupling of retinal neurons through a nontranscriptional mechanism. Here we report that RA rapidly and specifically modulates synaptic transmission at electrical synapses of cultured retinal horizontal cells through an external RAR(beta)(/gamma)-like binding site, the action of which is independent of second messenger cascades. External application of all-trans retinoic acid (at-RA) reversibly reduced the amplitude of gap junctional conductance in a dose-dependent manner, but failed to affect non-gap-junctional channels, including glutamate receptors. In contrast, internal dialysis with at-RA was ineffective, indicating an external site of action. Selective RAR(beta)(/gamma) ligands, but not an RAR(alpha)-selective agonist, mimicked the action of at-RA, suggesting that gating of gap junctional channels is mediated through an RAR(beta)(/gamma)-like binding site. At-RA did not act on gap junctional conductance by lowering [pH](i) or by increasing [Ca(2+)](i). A G protein inhibitor and protein kinase inhibitors did not block at-RA uncoupling effects indicating no second messenger systems were involved. Direct action of at-RA on gap junction channels was further supported by its equivalent action on whole-cell hemi-gap-junctional currents and on cell-free excised patch hemichannel currents. At-RA significantly reduced single-channel open probability but did not change unitary conductance. Overall, the results indicate that RA modulates horizontal cell electrical synapses by activation of novel nonnuclear RAR(beta)(/gamma)-like sites either directly on, or intimately associated with, gap junction channels.

Characterization of T cell lines derived from glatiramer-acetate-treated multiple sclerosis patients

We analyzed the effects of glatiramer acetate (GA) therapy on in vitro proliferative responses and cytokine production by lymphocytes derived from multiple sclerosis patients receiving this therapy. We confirmed that lymphocytes derived from GA naïve patients show a high frequency of response when initially exposed to GA in vitro; this frequency decreased following GA therapy. The frequency of lymphocytes responding to whole MBP stimulation did not change with GA therapy. GA- and MBP-specific T cell lines generated from these patients by repeated cycles of in vitro stimulation did not cross react. Some (23%) whole MBP-reactive T cell lines did cross react with MBP peptide 83-99. The mean levels of interferon (IFN) gamma secretion and the mean ratio of IFN-gamma/IL-5 were lower for GA-reactive cell lines, derived from patients both prior to and during GA therapy, compared to MBP-reactive T cell lines. The proportion of IFN-gamma(+) cells in unfractionated lymphocyte preparations derived from the GA-treated patients did not differ from that found for healthy controls. Our findings indicate that GA-reactive T cell lines derived from GA-treated MS patients continue to show a relative Th2 cytokine bias consistent with a bystander suppressor function. GA treatment is not associated with a cytokine phenotype shift in the total T cell or MBP-reactive T cell populations.

Isolation and characteristics of autoreactive T cells specific to aggrecan G1 domain from rheumatoid arthritis patients

Our previous work showed that the cartilage proteoglycan aggrecan could induce an erosive polyarthritis and spondylitis in BALB/c mice and the G1 globular domain of the aggrecan (G1) contained the arthritogenic region. To elucidate whether autoreactive T cells to G1 are expressed in rheumatoid arthritis patients, we analyzed the frequency of human G1-specific T cells in the peripheral blood of five rheumatoid arthritis patients and tried to establish G1-reactive T cell lines from these rheumatoid arthritis patients. The results showed that the G1-specific T cells in PBL were detectable at the range of 4.97 +/- 0.5 x 10(-6) in peripheral blood lymphocytes. We have also generated 15 G1-specific T lymphocyte lines from these patients with a standard split-well method. All these cells expressed fine specificity to human recombinant G1, but not to unrelated antigen. All the 15 lines expressed a pan-T cell marker and 13 of them selectively used the alphabeta T cell receptor. Two of them used gammadelta T cell receptor. The 13 of these T cell lines was CD4 positive. One line expressed CD8. One line expressed both CD4 and CD8. Moreover, 14 out of 15 lines expressed the Th-1 cytokine profile, characterized by interferon-gamma positivity and IL-4 negativity. No Th-2 type cell line was generated. These data provide strong evidence in favor of the presence of autoreactive T cells in the rheumatoid arthritis patients. What is the mechanism(s) that these autoreactive T cells attack self-target and whether these G1-specific, Th-1 type T cell lines can induce arthritis in immune deficiency mice are currently under investigation.

Electrical coupling of retinal horizontal cells mediated by distinct voltage-independent junctions

Electrical coupling between H2 horizontal cell pairs isolated from the hybrid bass retina was studied using dual whole-cell, voltage-clamp technique. Voltage-dependent inactivation of junctional currents in response to steps in transjunctional voltage (Vj) over a range of +/-100 mV was characterized for 89 cell pairs. Approximately one-quarter of the pairs exhibited strongly voltage-dependent junctions (>50% reduction in junctional current at +/-100 mV), another quarter of the pairs exhibited voltage-independent junctional current (<5% reduction at +/-100 mV), and the remainder of the pairs exhibited intermediate values for voltage inactivation. We focused on further characterizing the Vj-independent junctions of horizontal cells, which have not been described previously in detail. When Lucifer Yellow dye was included in one recording pipette, pairs exhibiting Vj-independent coupling showed no (9/12), or limited (3/12), passage of dye. Vj-independent coupling was markedly less sensitive to the modulators SNP (100-300 microM, -9% reduction in coupling) and dopamine (100-300 microM, -6%) than were Vj-dependent junctions (-45% and -44%). However, simultaneous application of both SNP and dopamine significantly reduced Vj-independent coupling (-56%). Both Vj-independent and Vj-dependent junctions were blocked by DMSO (1-2%), but Vj-independent junctions were not blocked by heptanol. Single-channel junctional conductances of Vj-independent junctions range from 112-180 pS, versus 50-60 pS for Vj-dependent junctions. The results reveal that Vj-independent coupling in a subpopulation of horizontal cells from the hybrid bass retina is mediated by cellular junctions with physiological and pharmacological characteristics distinct from those previously described in fish horizontal cells.

GABA modulates color-opponent bipolar cells in carp retina

In the present work, the effects of gamma-aminobutyric acid (GABA) were examined while recording intracellularly from color-opponent bipolar cells in the isolated, superfused carp retina. Bath-applied GABA differentially suppressed light responses of R/G cells, which depolarized and hyperpolarized, respectively, in response to red and green flashes, and G/R cells, which showed the opposite responses. In either R/G or G/R cells, the response driven by input from red cones was invariably suppressed to a greater extent. Such effects of GABA were completely blocked by picrotoxin, but only partially by bicuculline, suggesting that the GABAC receptor, in addition to the GABAA receptor, may also be involved. It is postulated that GABA modulates the color-opponent bipolar cells either through activating GABA receptors on these cells directly or those on cone terminals indirectly.