Spectral plasticity of H1 horizontal cells in carp retina: independent modulation by dopamine and light-adaptation

Role of dopamine in distal retina

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

Proton feedback mediates the cascade of color-opponent signals onto H3 horizontal cells in goldfish retina

It has been postulated that horizontal cells (HCs) send feedback signals onto cones via a proton feedback mechanism, which generates the center-surround receptive field of bipolar cells, and color-opponent signals in many non-mammalian vertebrates. Here we used a strong pH buffer, HEPES, to reduce extracellular proton concentration changes and so determine whether protons mediate color-opponent signals in goldfish H3 (triphasic) HCs. Superfusion with 10mM HEPES-fortified saline elicited depolarization of H3 HCs' dark membrane potential and enhanced hyperpolarizing responses to blue stimuli, but suppressed both depolarization by yellow and orange and hyperpolarization by red stimuli. The response components suppressed by HEPES resembled the inverse of spectral responses of H2 (biphasic) HCs. These results are consistent with the Stell-Lightfoot cascade model, in which the HEPES-suppressed component of H3 HCs was calculated using light responses recorded experimentally in H1 (monophasic) and H2 HCs. Selective suppression of long- or long-+middle-wavelength cone signals by long-wavelength background enhanced the responses to short-wavelength stimuli. These results suggest that HEPES inhibited color opponent signals in H3 HCs, in which the source of opponent-color signals is primarily a feedback from H2 HCs and partly from H1 HCs onto short-wavelength cones, probably mediated by protons.

Interactions between dopamine and melatonin organize circadian rhythmicity in the retina of the green iguana

Circadian physiology in the vertebrate retina is regulated by several neurotransmitters. In the lateral eyes of the green iguana the circadian rhythm of melatonin content peaks during the night while the rhythm of dopamine peaks during the day. In the present work, the authors explore the interaction of these 2 neurotransmitters during the circadian cycle. They depleted retinal dopamine with intravitreal injections of 6-hydroxydopamine (6-OHDA) and measured ocular melatonin content in vivo throughout 1 circadian cycle. The circadian rhythm of ocular melatonin not only persisted but increased 10-fold in amplitude. This increase was substantially reduced by the intraocular administration of dopamine. 6-OHDA-treated retinas, unlike those from untreated animals, did not express a circadian rhythm of melatonin synthesis in vitro. To deplete retinal melatonin, the authors pinealectomized iguanas and blocked retinal melatonin synthesis by depleting serotonin with intraocular injections of 5,6-dihydroxytryptamine. In animals so treated, they found that the circadian rhythm of retinal dopamine content was abolished, the levels of dopamine were lowered, and the levels of dopamine metabolites were greatly increased. The data suggest that in iguanas, the amplitude of the circadian rhythm of melatonin synthesis in the eye is suppressed by dopamine while the rhythm of dopamine depends, at least in part, on the presence of melatonin.

Dopamine and nitric oxide control both flickering and steady-light-induced cone contraction and horizontal cell spinule formation in the teleost (carp) retina: serial interaction of dopamine and nitric oxide

Adaptation to ambient light, which is an important characteristic of the vertebrate visual system, involves cellular and subcellular (synaptic) plasticity of the retina. The present study investigated dopamine (DA) and nitric oxide (NO) as possible neurochemical modulators controlling cone photomechanical movements (PMMs) and horizontal cell (HC) spinules in relation to steady and flickering light adaptation in the carp retina. Haloperidol (HAL; a nonspecific DA receptor blocker) or cPTIO (a NO scavenger) largely inhibited the cone PMMs and HC spinule formation induced by either steady or flickering light. These results suggested that both DA and NO could be involved in the light-adaptation changes induced by either pattern of input and that DA and NO effects may not be completely independent. The possibility that NO and DA interact serially was evaluated pharmacologically by cross-antagonist application (i.e., DA + cPTIO or NO + HAL). When a NO donor was coapplied with HAL to dark-adapted eyecups, normal light-adaptive cone PMMs and HC spinules occurred. In contrast, when DA was applied in the presence of cPTIO, the dark-adapted state persisted. It was concluded 1) that DA and NO are both light-adaptive neurochemicals, released in the retina during either steady or flickering light; 2) that the effects of DA and NO on light-adaptive cone PMMs and HC spinules do not occur in parallel; and 3) that NO and DA act mainly in series, specifically as follows: Light --> DA --> NO --> Cone PMMs + HC spinules.

An electrophysiological test of the effect of the temporal pattern of light adaptation on teleost H1 type horizontal cell plasticity

The possible importance of the temporal pattern of photon delivery in light adaptation-induced physiological plasticity in the outer retina of carp was tested by intracellular recording. Steady and flicker (3 Hz) background adaptation was applied whilst recording chromatic voltage responses of H1 type horizontal cells (HCs) to 680 and 440 nm full-field test flashes (generating response amplitudes of V(r) and V(b), respectively). A third parameter V(b)/V(r) (B/R) was calculated as an indicator of the cells' short/long wavelength relative spectral contrast. Steady light adaptation increased V(r) and to a lesser extent V(b), and reduced B/R. Flicker adaptation also increased V(r) (by a similar amount), but, unlike steady adaptation, consistently decreased V(b). The reduction in B/R was statistically greater for flicker than for steady adaptation, although the former delivered half as many photons to the retina. These results suggest that the temporal pattern of light adaptation is indeed an important determinant of qualitative and quantitative aspects of plasticity induced in the outer retina, and complement earlier morphological findings. The effects are discussed in terms of dopamine and nitric oxide as underlying possible neurochemical control mechanisms.

Light-adaptive role of nitric oxide in the outer retina of lower vertebrates: a brief review

The role of nitric oxide (NO) as a novel neurochemical mechanism controlling light adaptation of the outer retina is discussed by considering mainly published results. The emphasis is on the retinae of fishes and amphibia, but some data from the mammalian (rabbit) retinae have also been included for completeness. In the fish retina, application of NO donors in the dark caused light-adaptive photomechanical movements of cones. The normal effect of light adaptation in inducing cone contractions was suppressed by pretreatment of retinae with an NO scavenger. NO donors modulated horizontal cell activity by uncoupling the cells' lateral gap junctional interconnections and enhancing negative feedback to cones, again consistent with a light-adaptive role of NO. Direct evidence for light adaptation-induced release of NO has been obtained in fish (carp) and rabbit retinae. The results strongly suggest that control of retinal light adaptation is, under multiple neurochemical control, with NO and dopamine having an interactive role.

GABA enhances short wavelength-sensitive cone input and reduces red cone input to carp L-type horizontal cells

Light responses of cone-driven horizontal cells were recorded intracellularly in the isolated superfused carp retina and the effects of gamma-aminobutyric acid (GABA) on signals from red-sensitive (R-) and short-wavelength-sensitive (S-) cones (green cones and/or blue cones) were studied. In the presence of a bright red (694 nm) background light, which substantially suppressed signal from R-cones, the responses of L-type horizontal cells (L-HCs) to 532-nm flashes, predominantly driven by the S-cone input, were potentiated by application of GABA. In contrast, the responses of these cells to 694-nm flashes driven by the R-cone input, were suppressed, when signal from S-cones was suppressed by a bright 532-nm background light. Both the effects could be reversed by co-application of bicuculline, suggesting the involvement of GABA(A) receptors. It was unlikely that the potentiation by GABA of the S-cone driven responses of the L-HCs was mediated by actions of GABA on the cone photoreceptors. The dual action of GABA persisted in the dopamine-depleted retina, indicating no involvement of the dopaminergic interplexiform cells. We speculate that this dual action may be partially due to differential modulation by GABA of different postsynaptic mechanisms respectively mediating signal transfer from R-cones and S-cones to L-HCs.

Stimulus pattern related plasticity of synapses between cones and horizontal cells in carp retina

Stimulus pattern related synaptic plasticity in the luminosity-type horizontal cell (LHC) of isolated carp retina was investigated. The major findings were: (1) repetitive red flashes progressively strengthened the synaptic connection between red-cone and LHC, whereas weakened that between green-cone and LHC; (2) repetitive green flashes remarkably depressed the LHC's red response, but caused little changes in the cell's green response. A competitive depression between different cone signals is suggested.

Effects of nitric oxide, light adaptation and APB on spectral characteristics of H1 horizontal cells in carp retina

The spectral characteristics of cone-driven horizontal cells of H1 subtype (H1 HCs) receiving main synaptic input from red-sensitive cones were studied in light- and dark-adapted retinae of carp. The spectral sensitivity profile of H1 HCs in dark-adapted retinae was practically the same as the absorption spectrum of red-sensitive cones. Light-adaptation decreased the sensitivity preferentially in the short-wavelength (blue/green) region, resulting in a relative enhancement of the 617 nm peak. Application of nitric oxide (NO) donors, sodium nitroprusside (SNP) and S-nitrosoglutathione (SNOG or GSNO), or dopamine to dark-adapted retinae decreased the sensitivity preferentially in blue/green region, an effect similar to that of light-adaptation. Application of haemoglobin (Hb, an NO scavenger) or 2-amino-4-phosphonobutyrate (APB, a metabotropic glutamate receptor agonist), to light-adapted retinae increased the sensitivity preferentially in the blue/green region, an effect similar to dark-adaptation. The photoresponses of H1 HCs were univariant in dark-adapted retinae as well as Hb-treated light-adapted retinae. In light-adapted retinae with normal Ringer, however, the univariance did not hold. These results suggested that the photoresponses of H1 HCs to short-wavelength stimuli contain a depolarising (sign-reversing) component, which can be activated by light-adaptation or application of NO and dopamine, and inactivated by dark-adaptation or deprivation of NO or application of APB.

Modulation of chromatic difference in receptive field size of H1 horizontal cells in carp retina: dopamine- and APB-sensitive mechanisms

Chromatic aspects of receptive field size in the H1 horizontal cell syncytium of the carp retina were investigated using spectral photostimuli (blue or red) presented in the form of either a pair of a small spot and annulus, or a narrow moving slit. In the light-adapted retina, the receptive field for the blue stimulus was found to be significantly smaller than that for the red, i.e. there was a chromatic difference in the receptive field size. During the course of dark adaptation, the overall receptive field size increased, but the chromatic difference decreased. Immediately after adaptation to bright light, the receptive field sizes were reduced significantly, but the chromatic difference increased, mainly due to a greater reduction in the receptive field for the blue stimulus. Application of dopamine (5 microM) to a dark-adapted retina gradually decreased the receptive field size for both colours, but the chromatic difference became larger, again due to a greater reduction in the receptive field size for the blue stimulus. 2-Amino-4-phosphonobutyrate (APB) applied to light-adapted retinae at a working concentration of 1 mM, greatly expanded the receptive field size and suppressed the chromatic difference due to the effect being greater for the receptive field for the blue stimulus. The effect of APB was slow and cumulative. On the other hand, intracellular injection of cGMP or dibutyryl-cGMP increased the chromatic difference in the receptive field size. It is suggested (i) that the chromatic difference in the receptive field size could be due to a cGMP-coupled, conductance-decreasing receptor mechanism activated by APB; and (ii) that the mechanism is associated with short-wavelength sensitive cone input to the H1 cells and operates in the light-adapted state of the retina.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Quantitative analysis of triphasic (H3) horizontal cell-cone connectivity in the cyprinid fish (roach) retina

Although horizontal cells encode chromatic information by means of a variety of spectrally opponent light-evoked response patterns, their synaptic connections with the different spectral classes of cone are not completely understood. In the cyprinid fish retina, where a hierarchical set of interactions between horizontal cells and cone types has been proposed, a particular type of horizontal cell generates light-evoked triphasic (red-hyperpolarizing/green depolarizing/blue-hyperpolarizing) responses. In the present study, we have studied the cone connectivity of these cells by intracellular recording and staining in the roach retina. The horizontal cells were first identified electrophysiologically using spectral stimuli, and then stained intracellularly with horseradish peroxidase. Light microscopy revealed that the cells had consistent H3-like morphologies. At an ultrastructural level, these horizontal cells were deduced to contact selectively blue-sensitive cones. Within the cone pedicles, the majority (approximately 80%) of the contacts were "central" to synaptic ribbons. Some 50% of the "lateral" processes were large and engulfed cone cytoplasm. Spinules were present within the contacted pedicles but not upon the dendrites of the stained horizontal cells, although previous work had suggested that horseradish peroxidase would not interfere with spinule dynamics. The results are discussed in terms of existing modes of horizontal cell-cone connectivity in cyprinid fish retinae.

Effects of different patterns of light adaptation on cellular and synaptic plasticity in teleost retina: comparison of flickering and steady lights

The importance of the pattern of light stimulus in inducing light-adaptive morphological (cellular and sub-cellular) changes in the outer retina of the carp was assessed. Thus, the effects of steady and flickering backgrounds (of the same intensity) on cone photomechanical movements (PMMs) and horizontal cell (HC) spinules were compared by quantitative measurements. For both phenomena, flickering stimuli had a significantly greater effect than steady, although the number of photons delivered by the former was one-half. The results demonstrate clearly that the pattern of light stimulus is indeed an important parameter in determining the extent of adaptation in the outer retina. The effects are discussed in relation to possible neurochemical bases of light-adaptive neural control mechanisms.