鲤属鱼L-型外水平细胞所接收的锥细胞信号

在保持正常血液循环的眼杯标本上,用细胞内微电极技术记录了 L-型外水平细胞(LHC)的反应,并且用颜色光适应方法分析了它所接收的锥细胞输入。结果表明,在白光明适应条件下,LHC 依然接收红敏和绿敏锥细胞的输入,但两者的相对贡献与暗视时有所不同。用蓝适应光可以基本上分离红敏锥细胞的信号;而在红色背景光下,绿敏锥细胞主导了反应的上升相和撤光后回跳相(off-rebound),但在反应平台处却是红敏锥细胞的信号作出主要贡献,表明不同的锥细胞信号显然有不同的潜伏期和时程。     

GABA transaminase in cyprinid fish retina: Localization and effects of inhibitors on temporal characteristics of S-potentials

Cellular localization and physiology of GABA-transaminase have been studied in the cyprinid fish retina. Immunohistochemical localization of the enzyme in the goldfish retina revealed a broad distribution pattern, including both outer and inner plexiform layers. Well known inhibitors of GABA-transaminase activity, such as gabaculine and γ-vinyl-GABA, caused an improvement of the temporal frequency transfer functions of luminosity type horizontal cells in the roach retina. In contrast, bicuculline had the opposite effect.Results suggest that GABA-transaminase has a physiological role in the retina consistent with the hypothesis that GABAergic (negative) feed-back from horizontal cells to cones regulates the temporal characteristics of light-evoked electrical activity in the outer plexiform layer.     

VA opsin,melanopsin, and an inherent light response within retinal interneurons

BACKGROUND: Although photoreception is best understood in rods and cones, it is increasingly clear that these are not the only photoreceptive cells of the vertebrate retina. While considerable attention has been paid to the role of melanopsin in the generation of intrinsic light sensitivity in the retinal ganglion cells of mammals, nothing is known about the photoreceptive capacity of the horizontal cells of the fish retina in which both VA opsin and melanopsin are expressed. As yet, there has been little more than speculation as to the physiological function of these opsins within local retinal circuit neurons. RESULTS: VA opsin and melanopsin have been isolated and localized within the well-characterized cyprinid retina of the roach (Rutilus rutilus). Parallel electrophysiological studies identified a novel subtype of horizontal cell (HC-RSD) characterized by a depolarizing response that fits an opsin photopigment with a lambda(max) of 477 nm. The HC-RSD cells mediate responses to light that are characterized by long integration times, well beyond those observed for rods and cones. Significantly, HC-RSD responses persist when the conventional photoreceptor inputs are saturated by background light. CONCLUSIONS: The syncytium of coupled horizontal cells has long been considered to provide a signal of overall retinal irradiance. Our data suggest that this light information is, at least in part, derived from a population of intrinsically photosensitive VA opsin and/or melanopsin horizontal cells.     

Cone connections of the horizontal cells of the rhesus monkey's retina

The presence in the rhesus monkey's retina of a second morphological type of horizontal cell (H2), described by Kolb et al. (1980), is confirmed. Both types of cell are here further described. Their cone connections are quantified and compared with those of mammals and other vertebrates. The dendrites and axons of the H2 type of cell contact only cones as do the dendrites of the H1 cell (originally described by Polyak (1941)) which has an axon contacting only rods. The dendrites of foveal H2 cells contact between 11 and 14 cones; those of H1 contact 7. The number of cones that each type of cell contacts increases with increasing distance from the fovea, so that, by 5-6 mm eccentricity, H2-type cells synapse with between 20 and 30 cones, and the H1 cells with 12-15. The qualitatively estimated coverage factors of each are 3 or 4; every cone synapses with more than one of both types. Neither type of horizontal cell makes chromatically specific connections that are anatomically recognizable, unlike the situation in some teleostean and turtle retinae. Individual horizontal cells, particularly those connected to foveal cones, may have different ratios of chromatic input. At equivalent eccentricities, up to about 6 mm from the fovea, the dendritic fields of H2 horizontal cells are about twice the size of H1 cells and contact about twice the number of cones. These relative differences are closely similar to those of the cat's horizontal cells and it is suggested that they are a basic feature of most placental mammals. The organization of foveal cone fibres within Henle's layer is described. The distribution of primate cone telodendria, gap junctions and synapses in the outer plexiform layer are briefly reviewed and compared with those of other vertebrate retinae.     

Analyses of bipolar cell responses elicited by polarization of horizontal cells

Simultaneous intracellular recordings were made from a bipolar cell and a horizontal cell in the carp retina. The properties of the bipolar cell were studied while injecting current into the horizontal cell. Hyperpolarization of horizontal cells, irrespective of their type, elicited a hyperpolarizing response in on-center bipolar cells and a depolarizing response in off-center bipolar cells. Analyses of the ionic mechanisms of bipolar cell responses revealed that depolarization of horizontal cells simulated and hyperpolarization opposed the effect of central illumination. The effect of polarization was exerted in such a manner that each type of horizontal cells modified the transmission from those photoreceptors from which they receive main inputs. In on- center bipolar cells, for example, the L-type horizontal cells receiving inputs mainly from red cones modified the cone-bipolar transmission accompanied by a conductance change of K+ and/or Cl- channels, and the intermediate horizontal cells receiving inputs from rods modified the rod-bipolar transmission accompanied by a conductance change of Na+ channels. In off-center bipolar cells, the effect of polarization of any type of horizontal cells was mediated mainly by conductance changes of Na+ channels. Feedback mechanisms from horizontal cells to photoreceptors could explain these results reasonably well.     

Effects of long-term spectral deprivation on the morphological organization of the outer retina of the blue acara (Aequidens pulcher)

To investigate the developmental plasticity of colour vision, we reared fish with a trichromatic cone system (Aequidens pulcher) under three near-monochromatic lights, differentially stimulating each spectral cone type from the larval stage to the age of at least one year. Control conditions comprised white lights of two intensities. The treatments did not affect the visual pigments, hut led to significant changes in cone outer segment lengths. Furthermore, in the blue-reared group the density of single cones within the retina was reduced by two-thirds after 18 months of exposure, while no changes were observed in the other groups. The connectivity of cone horizontal cells with the single cones was influenced by the intensity and spectral composition of the rearing lights: H1 cells were more sensitive to the spectral component, whereas H2 cells responded to intensity cues. In the blue-light group the dynamics of horizontal cell synaptic spinule formation and degradation were severely compromised. These observations show that long-term spectral deprivation leads to significant morphological changes at the level of photoreceptors and horizontal cells. While the reactions of photoreceptors may be interpreted mostly in terms of compensation, the functional consequences of the changes observed on the horizontal cell level remain to be determined electrophysiologically.     

Synaptic transmission from horizontal cells to cones is impaired by loss of connexin hemichannels

In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.     

明适应条件下鲤属鱼L-型外水平细胞反应的给光-瞬变成分

在明适应条件下鲤属鱼 L-型外水平细胞的反应显示明显的给光-瞬变成分(on-transient)、它与刺激波长有关——对蓝、绿光的反应比对红光的反应有更明显的瞬变成分,其光谱特性提示它与绿敏锥细胞的输入信号有关。与已报道的其它动物 L 型水平细胞的给光-瞬变成分不同,它的出现在一定范围内与网膜受照射的面积无关。绿色(502nm)和红色(706nm)闪光同时照射所引起反应的给光-瞬变成分比各自单独刺激时要显著得多,提示它也与绿敏锥细胞和红敏锥细胞输入的相互作用有关。     

The nature of surround-induced depolarizing responses in goldfish cones

Cones in the vertebrate retina project to horizontal and bipolar cells and the horizontal cells feedback negatively to cones. This organization forms the basis for the center/surround organization of the bipolar cells, a fundamental step in the visual signal processing. Although the surround responses of bipolar cells have been recorded on many occasions, surprisingly, the underlying surround-induced responses in cones are not easily detected. In this paper, the nature of the surround-induced responses in cones is studied. Horizontal cells feed back to cones by shifting the activation function of the calcium current in cones to more negative potentials. This shift increases the calcium influx, which increases the neurotransmitter release of the cone. In this paper, we will show that under certain conditions, in addition to this increase of neurotransmitter release, a calcium-dependent chloride current will be activated, which polarizes the cone membrane potential. The question is, whether the modulation of the calcium current or the polarization of the cone membrane potential is the major determinant for feedback-mediated responses in second-order neurons. Depolarizing light responses of biphasic horizontal cells are generated by feedback from monophasic horizontal cells to cones. It was found that niflumic acid blocks the feedback-induced depolarizing responses in cones, while the shift of the calcium current activation function and the depolarizing biphasic horizontal cell responses remain intact. This shows that horizontal cells can feed back to cones, without inducing major changes in the cone membrane potential. This makes the feedback synapse from horizontal cells to cones a unique synapse. Polarization of the presynaptic (horizontal) cell leads to calcium influx in the postsynaptic cell (cone), but due to the combined activity of the calcium current and the calcium-dependent chloride current, the membrane potential of the postsynaptic cell will be hardly modulated, whereas the output of the postsynaptic cell will be strongly modulated. Since no polarization of the postsynaptic cell is needed for these feedback-mediated responses, this mechanism of synaptic transmission can modulate the neurotransmitter release in single synaptic terminals without affecting the membrane potential of the entire cell.     

Synaptic organization and ionic basis of on and off channels in mudpuppy retina. I. Intracellular analysis of chloride-sensitive electrogenic properties of receptors, horizontal cells, bipolar cells, and amacrine cells

Intracellular recordings from receptors, horizontal cells, bipolars, and amacrines have been carried out in the perfused mudpuppy eyecup. The introduction of a chloride-free (c-f) medium results in initial transient potential changes in many cells followed by a slow loss of light-evoked activity of the depolarizing bipolar, the horizontal cell, and the on depolarization of amacrine cells. The hyperpolarizing bipolar remains responsive to light stimulation in a c-f medium, but the antagonistic surround mechanism is abolished. These effects are reversible after returning to a normal ionic medium. The results of this study provide insight into the retinal connections which underlie ganglion cell receptive field organization. It is concluded that the depolarizing bipolar is excitatory to on ganglion cells and is also the pathway for on-excitation of on-off cells. The hyperpolarizing bipolar mediates the off discharge of off and on-off cells. Amacrine cells receive input from both depolarizing and hyperpolarizing bipolar cells. These findings raise the possibility that transmembrane movements of chloride ions are critical for the light responsiveness of horizontal and depolarizing bipolar cell activity.     

Orientation tuning of motion-sensitive neurons shaped by vertical-horizontal network interactions

We measured the orientation tuning of two neurons of the fly lobula plate (H1 and H2 cells) sensitive to horizontal image motion. Our results show that H1 and H2 cells are sensitive to vertical motion, too. Their response depended on the position of the vertically moving stimuli within their receptive field. Stimulation within the frontal receptive field produced an asymmetric response: upward motion left the H1/H2 spike frequency nearly unaltered while downward motion increased the spike frequency to about 40% of their maximum responses to horizontal motion. In the lateral parts of their receptive fields, no such asymmetry in the responses to vertical image motion was found. Since downward motion is known to be the preferred direction of neurons of the vertical system in the lobula plate, we analyzed possible interactions between vertical system cells and H1 and H2 cells. Depolarizing current injection into the most frontal vertical system cell (VS1) led to an increased spike frequency, hyperpolarizing current injection to a decreased spike frequency in both H1 and H2 cells. Apart from VS1, no other vertical system cell (VS2-8) had any detectable influence on either H1 or H2 cells. The connectivity of VS1 and H1/H2 is also shown to influence the response properties of both centrifugal horizontal cells in the contralateral lobula plate, which are known to be postsynaptic to the H1 and H2 cells. The vCH cell receives additional input from the contralateral VS2-3 cells via the spiking interneuron V1.     

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.     

Functional anatomy of the photoreceptor and second-order cell mosaics in the retina of Xenopus laevis

Mosaics of photoreceptors, and horizontal and bipolar cells of the Xenopus laevis retina were studied in whole-mount preparations applying lectin-cytochemical, immunocytochemical and intracellular labeling techniques. The combined density of all photoreceptor types was about 13700/mm2, of which rods represented 53%. Of the cones, the large long-wavelength-sensitive (86% of all cones) and the miniature ultraviolet-wavelength-sensitive (4%) ones could be labeled with peanut agglutinin, whereas the large short-wavelength-sensitive (10%) cones remained unlabeled. There were no significant regional differences in photoreceptor distribution. Bipolar cells were selectively labeled with antibodies against calretinin. Their density was between 4000 and 6000 cells/cm2, with slightly elevated numbers in the superior nasal quadrant. Two types of horizontal cell were injected intracellularly. The luminosity-type cells were more frequent (approximately 1000 cells/mm2) than the chromaticity cells (approximately 450 cells/mm2). The dendritic field size of the latter cell type was threefold bigger than that of the luminosity cells. The coverage factors were estimated to be 3.3 for the luminosity cells and 5.2 for the chromaticity cells. The luminosity cells contacted all photoreceptor types, whereas chromatic horizontal cells received their inputs from the short-wavelength-sensitive cones and from some, but not all, rods. Luminosity cells encounter about 50-60 potential synaptic partners within their dendritic fields, whereas chromatic horizontal cells only about 20. Chromatic horizontal cells form multiple synaptic contacts with the short-wavelength-sensitive cones. The results indicate that the overall photoreceptor to bipolar and bipolar to ganglion cell convergence in Xenopus retina is similar to that in the central retinal specialized regions of mammals, predicting comparable spatial resolutions.     

Photoreceptors of a cyprinid fish,the roach: morphological and spectral characteristics

Summary Structure-function studies were undertaken on cones in the retina of the cyprinid fish, the roach,Rutilus rutilus, in order to provide a basis for analysis of colour information by post-receptoral neurones. Measurements were made with two main aims: (i) To determine by microspectrophotometry the absorbance spectra of the photopigments present within the retina, and correlate these with the morphological types of photoreceptor; (ii) To characterize the morphologies of the photoreceptors at both light and electron microscopical levels and determine their relative abundance in the retina. In addition to red-, green-, and blue-sensitive cones, an ultra-violet-sensitive photoreceptor has been found in a sub-population of miniature short single cones. Possible relevance of this finding to vertebrate vision is discussed.     

Lateral feedback from monophasic horizontal cells to cones in carp retina. II. A quantitative model

About half of the monophasic horizontal cells in carp retina receive input from both red- and green-sensitive cones. Since the horizontal cells feed back to cones, the color and feedback pathways result in wavelength- and intensity-dependent changes of the dynamics and of the receptive field amplitude profile of the horizontal cell responses. In this paper we present a quantitative model that describes adequately the color and spatial coding and the dynamics of the responses from monophasic horizontal cells in carp. Lateral feedback plays a distinct role in this model.     

Receptor contacts of horizontal cells in the retina of the domestic cat.

The terminal aggregations of A- and B-type horizontal cells, stained by the Golgi-Colonnier method, have been analysed. The pattern of the aggregations is regular and is shown to be in register with the cone mosaic. Both tyes of horizontal cell are in contact with at least 80% of the cones above their dendritic fields. Therefore, the different horizontal cell classes cannot be selective for a special kind of cone but must have at least 60% of the cone input in common. Each A-type horizontal cell makes contacts with between 120 and 170 cones, and each B-type horizontal cell with 60-90 cones. An individual A-type horizontal cell occupies an average of 20% of the lateral elements of the triads in a cone pedicle, but an individual B-type cell fills only some 13%. Each and every cone is connected with several of both types of horizontal cell. An estimation of the number of rods converging onto a single axon terminal system showed that it could be as many as 3000.     

Fish cell lines: Characterization by isozyme analysis

Summary The electrophoretic mobilities of isozymes from 16 enzyme systems were determined for nine fish cell lines. The relative migration of the malate dehydrogenase and 6-phosphoglucose dehydrogenase isozymes could be used together to distinguish between seven of the fish cell lines. Two cyprinid cell lines could not be distinguished from each other but were readily separated from the six noncyprinid lines and the one other line of cyprinid origin.     

Glutamate-induced K+ flux in the photoreceptor layer of isolated retina of cyprinid fish: possible consequences for second-order neurones

The effect of L-glutamate on extracellular K+ activity (aK0) in the micro-environment of the isolated retina of a cyprinid fish was investigated using double-barrelled K+-sensitive micro-electrodes. Glutamate was pulsed onto the retina from an atomizer, and aK0's recorded at different retinal depths. Glutamate induced a concentration-dependent rise in aK0 which was maximal in the first 100 microns of the retinal surface. The aK0 change was transient, with a time to peak of 20-30 s, and complete reversal within 4 min. However, if the retina was pre-treated with dinitrophenol or ouabain, the maximum rise in aK0 was greater than normal, and the effect was sustained. Application of equimolar glutamate transiently depolarized the membrane potentials of horizontal cells. The effects of glutamate on aK0 and horizontal cell membrane potential had some common characteristics. It is concluded that glutamate may have non-synaptic effects on post-receptoral retinal neurones, and possible mechanisms of this are considered.     

Feedback connections and operation of the outer plexiform layer of the retina.

The primary feedback control apparatus in the outer retina is the sign-inverting feedback synapse between horizontal cells and cones. In many lower vertebrates horizontal cells release GABA in darkness, which opens Cl- channels in cones. Input-output relations of the feedback synapse reveal that the synaptic gain is light-dependent with the highest negative gain near the dark horizontal cell potential. The horizontal cell-cone feedback synapse improves the reliability of the photoreceptor output synapses. It also modulates the dynamic range and mediates color opponency and surround responses in second-order retinal neurons.     

A model of anuran retina relating interneurons to ganglion cell responses

A model is presented which accounts for many characteristic response properties used to classify anuran ganglion cell types while being consistent with data concerning interneurons. In the model color is ignored and input stimuli are assumed to be only black and white at high contrast. We show that accurate ganglion cell responses are obtained even with simplified receptors and horizontal cells: Receptors are modeled as responding with a step change, while horizontal cells respond only to global changes in intensity brought about by full field illumination changes. A hyperpolarizing and depolarizing bipolar cell are generated y subtracting local receptor and horizontal potentials. Two transient amacrine cells (On and Off) are generated using a high-pass filter like mechanism with a thresholded output which responds to positive going changes in the corresponding bipolar cell potentials. The model shows how a selective combination of bipolar and amacrine channels can account for many of the response properties used to classify the anuran ganglion cell types (class-0 through 4) and makes several experimental predictions.