Modulation of endogenous dopamine release in the fish retina by light and prolonged darkness

Increased ocular dopamine levels in rabbits after blue light stimulation of the optic nerve head

The purpose was to quantify ocular dopamine in rabbits after stimulation of the optic nerve head with short-wavelength (blue) light to activate melanopsin expressed in the axons of intrinsically photosensitive retinal ganglion cells (ipRGCs). Dopamine levels in tears, aqueous humor, vitreous body, and retina (including choroid) were quantified after blue light stimulation of the optic nerve head of 15 rabbits with an optical fiber for 1 min, 10 min, or no stimulation (n = 5, each group). The left eye of all rabbits was operated on to introduce the optical fiber and stimulate the optic nerve, while the contralateral eye served as internal control. One minute of blue light stimulation significantly increased dopamine concentration in the vitreous body of the treated eyes compared to the contralateral ones (P = 0.015). Stimulation for 10 min significantly increased dopamine concentration in the vitreous body, as well as the aqueous humor (P < 0.05). Therefore, using an optical fiber approach to stimulate the optic nerve head with blue light significantly increased dopamine concentration in the aqueous humor and the vitreous body. This likely reflects an upregulation of retinal dopamine synthesis that could be attributed to ipRGC activation. However, the data provided in this study fell short of establishing a definitive link between dopamine release and ipRGC activation, mainly due to the lack of evidence supporting the expression of the melanopsin photopigment in the optic nerve.

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

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.

Sensory Integration: Cross-Modal Communication Between the Olfactory and Visual Systems in Zebrafish

Cross-modal sensory communication is an innate biological process that refers to the combination and/or interpretation of different types of sensory input in the brain. Often, this process conjugates with neural modulation, by which the neural signals that convey sensory information are adjusted, such as intensity, frequency, complexity, and/or novelty. Although the anatomic pathways involved in cross-modal sensory integration have been previously described, the course of development and the physiological roles of multisensory signaling integration in brain functions remain to be elucidated. In this article, I review some of the recent findings in sensory integration from research using the zebrafish models. In zebrafish, cross-modal sensory integration occurs between the olfactory and visual systems. It is mediated by the olfacto-retinal centrifugal (ORC) pathway, which originates from the terminalis nerve (TN) in the olfactory bulb and terminates in the neural retina. In the retina, the TNs synapse with the inner nuclear layer dopaminergic interplexiform cells (DA-IPCs). Through the ORC pathway, stimulation of the olfactory neurons alters the cellular activity of TNs and DA-IPCs, which in turn modulates retinal neural function and increases behavioral visual sensitivity.

Mapping physiological inputs from multiple photoreceptor systems to dopaminergic amacrine cells in the mouse retina

In the vertebrate retina, dopamine is synthesized and released by a specialized type of amacrine cell, the dopaminergic amacrine cell (DAC). DAC activity is stimulated by rods, cones, and melanopsin-expressing intrinsically photosensitive retinal ganglion cells upon illumination. However, the relative contributions of these three photoreceptor systems to the DAC light-induced response are unknown. Here we found that rods excite dark-adapted DACs across a wide range of stimulation intensities, primarily through connexin-36-dependent rod pathways. Similar rod-driven responses were observed in both ventral and dorsal DACs. We further found that in the dorsal retina, M-cones and melanopsin contribute to dark-adapted DAC responses with a similar threshold intensity. In the ventral retina, however, the threshold intensity for M-cone-driven responses was two log units greater than that observed in dorsal DACs, and melanopsin-driven responses were almost undetectable. We also examined the DAC response to prolonged adapting light and found such responses to be mediated by rods under dim lighting conditions, rods/M-cones/melanopsin under intermediate lighting conditions, and cones and melanopsin under bright lighting conditions. Our results elucidate the relative contributions of the three photoreceptor systems to DACs under different lighting conditions, furthering our understanding of the role these cells play in the visual system.

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.

Nitric oxide synthase inhibitors prevent the growth-inhibiting effects of quinpirole

PURPOSE

Both dopamine and nitric oxide (NO) have been implicated in the signal cascade mediating ocular growth inhibition. If both are part of the same pathway, which precedes the other? We tested the hypothesis that dopamine acts upstream of NO, by using two NOS inhibitors in combination with the dopamine agonist quinpirole, and measured the effects on ocular growth rate.

METHODS

Chicks wore -10 D lenses or diffusers (FD) for 4 days starting at age 13 days. Experimental eyes received daily 20 μL injections of the following: quinpirole-lens: n = 12, FD: n = 20; n-ω-propyl-L-arginine (NPA)-lens: n = 6, FD: n = 4; quinpirole + NPA-lens: n = 17, FD: n = 19; and quinpirole + L-NIO-lens: n = 12, FD: n = 12. Saline injections were done as controls. High-frequency ultrasonography was done at the start, and on day 5, prior to injections and 3 hours later. Refractions were measured on day 5.

RESULTS

As expected, quinpirole prevented the development of axial myopia in both paradigms. When quinpirole was combined with either NOS inhibitor, however, eyes became myopic compared to quinpirole (FD: NPA: -5.9 D vs. -3.4 D; L-NIO: -5.8 D vs. -3.4 D; lens: NPA: -3.5 D vs. -0.4 D; p < 0.05 for all; L-NIO was not significant). This was the result of a disinhibition of vitreous chamber growth versus quinpirole (FD: NPA: 401 vs. 275 μm/4 d; L-NIO: 440 vs. 275 μm/4 d; LENS: NPA: 407 vs. 253µm/4 d; L-NIO: 403 vs. 253 μm/4 d; p < 0.05). Only NPA prevented the quinpirole-induced choroidal thickening in lens-wearing eyes (0 vs. 31 μm/3 h; p < 0.05). Choroidal thickening was not inhibited by either drug in FD eyes.

CONCLUSIONS

Dopamine acts upstream of NO and the choroidal response in the signal cascade mediating ocular growth inhibition in both form deprivation and negative lens wear. That neither NOS inhibitor inhibits choroidal thickening in FD eyes suggests that the choroidal mechanisms differ in the two paradigms.

Dopamine D1 receptors regulate the light dependent development of retinal synaptic responses

Retinal synaptic connections and function are developmentally regulated. Retinal synaptic activity plays critical roles in the development of retinal synaptic circuitry. Dopamine receptors have been thought to play important roles in the activity-dependent synaptic plasticity in central nervous system. The primary goal of this study is to determine whether dopamine D1 receptor regulates the activity-dependent development of retinal light responsiveness. Accordingly, we recorded electroretinogram from wild type mice and mice with genetic deletion of D1 dopamine receptor (D1-/- mice) raised under cyclic light conditions and constant darkness. Our results demonstrated that D1-/- mice have reduced amplitudes of all three major components of electroretinogram in adulthood. When the relative strength of the responses is considered, the D1-/- mice have selective reduction of the amplitudes of a-wave and oscillatory potentials evoked by low-intermediate intensities of lights. During postnatal development, D1-/- mice have increased amplitude of b-wave at the time of eye-opening but reduced developmental increase of the amplitude of b-wave after eye opening. Light deprivation from birth significantly reduced the amplitudes of b-wave and oscillatory potentials, increased the outer retinal light response gain and altered the light response kinetics of both a- and b-waves of wild type mice. In D1-/- mice, the effect of dark rearing on the amplitude of oscillatory potentials was diminished and dark rearing induced effects on the response gain of outer retina and the kinetics of a-wave were reversed. These results demonstrated roles of dopamine D1 receptor in the activity-dependent functional development of mouse retina.

Inhibitory inputs tune the light response properties of dopaminergic amacrine cells in mouse retina

Dopamine (DA) is a neuromodulator that in the retina adjusts the circuitry for visual processing in dim and bright light conditions. It is synthesized and released from retinal interneurons called dopaminergic amacrine cells (DACs), whose basic physiology is not yet been fully characterized. To investigate their cellular and input properties as well as light responses, DACs were targeted for whole cell recording in isolated retina using two-photon fluorescence microscopy in a mouse line where the dopamine receptor 2 promoter drives green fluorescent protein (GFP) expression. Differences in membrane properties gave rise to cell-to-cell variation in the pattern of resting spontaneous spike activity ranging from silent to rhythmic to periodic burst discharge. All recorded DACs were light sensitive and generated responses that varied with intensity. The threshold response to light onset was a hyperpolarizing potential change initiated by rod photoreceptors that was blocked by strychnine, indicating a glycinergic amacrine input onto DACs at light onset. With increasing light intensity, the ON response acquired an excitatory component that grew to dominate the response to the strongest stimuli. Responses to bright light (photopic) stimuli also included an inhibitory OFF response mediated by GABAergic amacrine cells driven by the cone OFF pathway. DACs expressed GABA (GABA(A)α1 and GABA(A)α3) and glycine (α2) receptor clusters on soma, axon, and dendrites consistent with the light response being shaped by dual inhibitory inputs that may serve to tune spike discharge for optimal DA release.

Dopaminergic agonists that result in ocular growth inhibition also elicit transient increases in choroidal thickness in chicks

The dopaminergic system has been implicated in ocular growth regulation in chicks and monkeys. In both, dopamine D2 agonists inhibit the development of myopia in response to form deprivation, and in chicks, to negative lenses as well. Because there is mounting evidence that the choroidal response to defocus plays a role in ocular growth regulation, we asked whether the effective agonists also elicit transient thickening of the choroid concomitant with the growth inhibition. Negative lenses mounted on velcro rings were worn on one eye starting at age 8-12 days. Intravitreal injections (20 μl; dose = 10 nmole) of the agonist (dissolved in saline) or saline, were given through the superior temporal sclera using a 30G needle. Eyes were injected daily at noon, for 4 days, and the lenses immediately replaced. Agonists used were apomorphine (non-specific; n = 17), quinpirole (D2; n = 10), SKF-38393 (D1; n = 9), and saline controls (n = 22). For the antagonists, the same protocol was used, but on each day, the lenses were removed for 2 h. Immediately prior to lens-removal, the antagonist was injected (20 μl; dose = 5 nmole). Antagonists used were methylergonovine (non-specific; n = 12), spiperone (D2; n = 20), SCH-23390 (D1; n = 6) and saline controls (n = 27). Comparisons to saline (continuous lens wear) controls were from the agonist experiment. Axial dimensions were measured using high frequency A-scan ultrasonography at the start of lens wear, and on day 4 prior to the injections, and then again 3 h later. Refractive errors were measured using a Hartinger's refractometer at the end of the experiment. Apomorphine and quinpirole inhibited the refractive response to the hyperopic defocus induced by the negative lenses (drug vs saline controls: -1.3 and 1.2 D vs -5.6 D; p < 0.005 for both). This effect was axial: both drugs prevented the excessive ocular elongation (change in axial length: 233 and 205 μm vs 417 μm; p < 0.01 for both). Both drugs were also associated with a transient thickening of the choroid over 3 h (41 and 32 μm vs -1 μm; p < 0.01; p = 0.059 respectively) that did not summate: choroids thinned significantly over the 4 day period in all lens-wearing eyes. Two daily hours of unrestricted vision during negative lens wear normally prevents the development of myopia. Spiperone and SCH-23390 inhibited the ameliorating effects of periods of vision on lens-induced refractive error (-2.9 and -2.8 D vs 0.6 D; p < 0.0001), however, the effects on neither axial length nor choroidal thickness were significant. These data support a role for both D1 and D2 receptors in the ocular growth responses.

ZO-1 and the spatial organization of gap junctions and glutamate receptors in the outer plexiform layer of the mammalian retina

Information processing in the retina starts at the first synaptic layer, where photoreceptors and second-order neurons exhibit a complex architecture of glutamatergic and electrical synapses. To investigate the composition of this highly organized synaptic network, we determined the spatial relationship of zonula occludens-1 (ZO-1) with different connexins (Cx) and glutamate receptor (GluR) subunits in the outer plexiform layer (OPL) of rabbit, mouse, and monkey retinas. ZO-1 is well known as an intracellular component of tight and adherens junctions, but also interacts with various connexins at gap junctions. We found ZO-1 closely associated with Cx50 on dendrites of A-type horizontal cells in rabbit, and with Cx57 at dendro-dendritic gap junctions of mouse horizontal cells. The spatial arrangement of ZO-1 at the giant gap-junctional plaques in rabbit was particularly striking. ZO-1 formed a clear margin around the large Cx50 plaques instead of being colocalized with the connexin staining. Our finding suggests the involvement of ZO-1 in the composition of tight or adherens junctions around gap-junctional plaques instead of interacting with connexins directly. Furthermore, gap junctions were found to be clustered in close proximity to GluRs at the level of desmosome-like junctions, where horizontal cell dendrites converge before invaginating the cone pedicle. Based on this distinct spatial organization of gap junctions and GluRs, it is tempting to speculate that glutamate released from the photoreceptors may play a role in modulating the conductance of electrical synapses in the OPL.

Receptive fields of retinal bipolar cells are mediated by heterogeneous synaptic circuitry

Center-surround antagonistic receptive field (CSARF) organization is the basic synaptic circuit that serves as elementary building blocks for spatial information processing in the visual system. Cells with such receptive fields converge into higher-order visual neurons to form more complex receptive fields. Retinal bipolar cells (BCs) are the first neurons along the visual pathway that exhibit CSARF organization. BCs have been classified according to their response polarities and rod/cone inputs, and they project signals to target cells at different sublaminae of the inner plexiform layer. On the other hand, CSARFs of various types of BCs have been assumed be organized the same way. Here we examined center and surround responses of over 250 salamander BCs, and demonstrated that different types of BCs exhibit different patterns of dye coupling, receptive field center size, surround response strength, and conductance changes associated with center and surround responses. We show that BC receptive field center sizes varied with the degree of BC-BC coupling, and that surround responses of different BCs are mediated by different combinations of five lateral synaptic pathways mediated by the horizontal cells and amacrine cells. The finding of heterogeneous receptive field circuitry fundamentally challenges the common assumption that CSARFs of different subtypes of visual neurons are mediated by the same synaptic pathways. BCs carrying different visual signals use different synaptic circuits to process spatial information, allowing shape and contrast computation be differentially modulated by various lighting and adaptation conditions.

Hyperpolarization-activated cation current is involved in modulation of the excitability of rat retinal ganglion cells by dopamine

Modulation of membrane properties and excitability of retinal ganglion cells (RGCs) by dopamine was investigated in rat retinal slices, using whole cell patch clamp techniques. Application of dopamine (10 microM) caused a small depolarization of the membrane potential, a reduction of the input resistance and a decrease in the number of current-evoked action potentials of RGCs, and these effects were blocked by a D1 antagonist (SCH23390, 10 microM), but not by a D2 antagonist (sulpiride, 10 microM). SKF38393 (10 microM), a D1 agonist, but not quinpirole (10 microM), a D2 agonist, mimicked the effects of dopamine on RGCs. Like dopamine, 8-Br-cAMP, a membrane-permeable analog of cAMP, produced similar changes in the membrane properties and the excitability of RGCs. All these results suggest that these effects of dopamine are likely mediated by D1 receptors. Pre-application of KT5720, an inhibitor of protein kinase A (PKA), blocked the dopamine effects, indicating that the effects were PKA-dependent. Possible involvement of hyperpolarization-activated cation currents (I(h)) in the dopamine effects was tested. Inward currents were induced by voltage steps, with an activation threshold of around -70 mV, in the presence of TTX, Cd(2+), TEA-Cl and 4-AP. These currents, with a reversal potential of -33.2 mV, displayed inward rectification and were blocked by ZD7288, a specific I(h) channel blocker. These results are indicative of the presence of I(h) in rat RGCs. Dopamine increased the amplitude of I(h) and shifted the activation curve of I(h) to a range of more positive potentials. SKF38393 and 8-Br-cAMP increased the amplitude of I(h), which was blocked by KT5720. The dopamine effects were abolished when the preparations were pre-incubated by ZD7288. These data strongly suggest that the dopamine effects on rat RGCs may be, at least in part, mediated by modulation of I(h) through the cAMP- and PKA-dependent pathway.

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.

Mitogen-associated protein kinase- and protein kinase A-dependent regulation of rhodopsin promoter expression in zebrafish rod photoreceptor cells

Mitogen-associated protein kinase (MAPK)- and protein kinase A (PKA)-dependent signal transductions play important roles in the regulation of gene expression. Both MAPK and PKA pathways can be activated by light exposure. In this study, we investigated the effect of light on MAPK and PKA signal transduction and their roles in the regulation of rhodopsin promoter expression by using transgenic zebrafish [Tg(rhod::GFP)]. The Tg(rhod::GFP) fish express short half-life GFP that is under the transcriptional control of the zebrafish rhodopsin promoter and can therefore be used for in vivo studies of rhodopsin gene transcription in live cells. Blue light plays a role in the regulation of rhodopsin promoter expression via an MAPK-mediated signal transduction cascade. Blue light excites cryptochromes (CRY), which activate the downstream PKC-dependent MAPK signal pathway. White light, on the other hand, regulates rhodopsin promoter expression via a G-protein-coupled cAMP-dependent PKA pathway. White light promotes dopamine release in the retina, which activates dopamine receptors and the downstream PKA pathway. Blocking MAPK signaling diminishes the blue light-induced increases in rhodopsin promoter expression, but this treatment has no effect on white light-mediated rhodopsin promoter expression. Conversely, blocking the PKA pathway diminishes the white light-induced rhodopsin promoter expression but does not affect rhodopsin promoter expression regulated by blue light. Together, the data suggest that MAPK and PKA regulate rhodopsin transcription through parallel signal transduction pathways.

Synaptic transmission at retinal ribbon synapses

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

Identification of retinal neurons in a regressive rodent eye (the naked mole-rat)

The retina consists of many parallel circuits designed to maximize the gathering of important information from the environment. Each of these circuits is comprised of a number of different cell types combined in modules that tile the retina. To a subterranean animal, vision is of relatively less importance. Knowledge of how circuits and their elements are altered in response to the subterranean environment is useful both in understanding processes of regressive evolution and in retinal processing itself. We examined common cell types in the retina of the naked mole-rat,Heterocephalus glaberwith immunocytochemical markers and retrograde staining of ganglion cells from optic nerve injections. The stains used show that the naked mole-rat eye has retained multiple ganglion cell types, 1–2 types of horizontal cell, rod bipolar and multiple types of cone bipolar cells, and several types of common amacrine cells. However, no labeling was found with antibodies to the dopamine-synthesizing enzyme, tyrosine hydroxylase. Although most of the well-characterized mammalian cell types are present in the regressive mole-rat eye, their structural organization is considerably less regular than in more sighted mammals. We found less precision of depth of stratification in the inner plexiform layer and also less precision in their lateral coverage of the retina. The results suggest that image formation is not very important in these animals, but that circuits beyond those required for circadian entrainment remain in place.

Localization of dopamine D1-receptor to A-type horizontal cells in the rabbit retina by single cell RT-PCR

Dopamine is an important neurotransmitter regulating light-dark adaptation in the retina. The effect of dopamine is widespread and dependent on the localization of its receptors. We performed single cell RT-PCR on A-type horizontal cells visually selected from dissociated rabbit retinal neurons. Dopamine D1-receptor mRNA was positively identified. Subsequent sequencing of the fragment showed 82% homology with rat D1 receptor and 87% homology with human D1 receptor. This study supported previous observations that dopamine regulated A-type horizontal cell coupling via D1 receptors at the level of transcription.

Characterization of retinoic acid neuromodulation in the carp retina

Visual sensation in vertebrates starts with the isomerization of 11-cis retinaldehyde into all-trans retinaldehyde. Aldehyde dehydrogenases, present in the pigment epithelium and some retinal cells, convert all-trans retinaldehyde into all-trans retinoic acid (at-RA). Evidence in the retina and the hippocampus has accumulated, showing that at-RA, besides being a morphogenetic factor, also acts as a neuromodulator. In mature retina, at-RA affects visual processing by acting on gap junctional conductances and the synaptic transfer between photoreceptors and horizontal cells. We present evidence supporting a neuromodulatory role of at-RA in the carp retina. High performance liquid chromatography (HPLC) measurements and an RA bioassay indicate a light dependency of at-RA formation, which can explain the observed effects of at-RA on spinule formation at horizontal cell dendrites in this retina. Furthermore, inhibiting endogenous metabolism and catabolism of at-RA affects formation and persistence of spinules in a way, supporting a direct involvement of at-RA in this light-dependent mechanism of synaptic plasticity. The action of at-RA, however, seems independent of the dopaminergic system, known for its light-signaling role in the retina, because at-RA effects on spinule formation persisted in retina depleted of dopaminergic neurons or in the presence of haloperidol. Together, these data indicate that at-RA acts effectively as a direct neuromodulator in carp retina, transmitting information about ambient light conditions to the neuronal retina.

Olfactory input increases visual sensitivity in zebrafish: a possible function for the terminal nerve and dopaminergic interplexiform cells

Centrifugal innervation of the neural retina has been documented in many species. In zebrafish Danio rerio, the only so-far described centrifugal pathway originates from terminal nerve (TN) cell bodies that are located in the olfactory bulb. Most of the TN axons terminate in the forebrain and midbrain, but some project via the optic nerve to the neural retina, where they synapse onto dopaminergic interplexiform cells (DA-IPCs). While the anatomical pathway between the olfactory and visual organs has been described, it is unknown if and how olfactory signals influence visual system functions. We demonstrate here that olfactory input is involved in the modulation of visual sensitivity in zebrafish. As determined by a behavioral assay and by electroretinographic (ERG) recording, zebrafish visual sensitivity was increased upon presentation of amino acids as olfactory stimuli. This effect, however, was observed only in the early morning hours when zebrafish are least sensitive to light. The effect of olfactory input on vision was eliminated after lesion of the olfactory bulbs or after the destruction of DA-IPCs. Intraocular injections of a dopamine D(2) but not a D(1) receptor antagonist blocked the effect of olfactory input on visual sensitivity. Although we cannot exclude the involvement of other anatomical pathways, our data suggest that the TN and DA-IPCs are the prime candidates for olfactory modulation of visual sensitivity.

Dopamine mediates circadian clock regulation of rod and cone input to fish retinal horizontal cells

A circadian (24-hour) clock regulates the light responses of fish cone horizontal cells, second order neurones in the retina that receive synaptic contact from cones and not from rods. Due to the action of the clock, cone horizontal cells are driven by cones in the day, but primarily driven by rods at night. We show here that dopamine, a retinal neurotransmitter, acts as a clock signal for the day by increasing cone input and decreasing rod input to cone horizontal cells. The amount of endogenous dopamine released from in vitro retinae was greater during the subjective day than the subjective night. Application of dopamine or quinpirole, a dopamine D(2)-like agonist, during the subjective night increased cone input and eliminated rod input to the cells, a state usually observed during the subjective day. In contrast, application of spiperone, a D(2)-like antagonist, or forskolin, an activator of adenylyl cyclase, during the subjective day reduced cone input and increased rod input. SCH23390, a D(1) antagonist, had no effect. Application of R(p)-cAMPS, an inhibitor of cAMP-dependent protein kinase, or octanol, an alcohol that uncouples gap junctions, during the night increased cone input and decreased rod input. Because D(2)-like receptors are on photoreceptor cells, but not horizontal cells, the results suggest that the clock-induced increase in dopamine release during the day activates D(2)-like receptors on photoreceptor cells. The resultant decrease in intracellular cyclic AMP and protein kinase A activation then mediates the increase in cone input and decrease in rod input.

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.

Day/night variations of dopamine ocular content during Xenopus laevis ontogeny

Concentration of dopamine (DA) and its metabolite, 3,4-dihydroxyphenylacetic acid is quantified by high-pressure liquid chromatography with a coulometric detection system in the eye of Xenopus laevis through ontogeny and in adults at two times during photocycle (midday and midnight). Ocular dopaminergic activity remains low during pre- and prometamorphosis and significantly rises in postmetamorphic froglets. This increase is more pronounced at midnight than at midday. The dualism of DA content versus DA release in Xenopus ocular tissue is studied in an eyecup culture system. On a 24-h cycle of DA release from adult Xenopus eyecups the highest DA release by eyecups is produced during daytime, and significantly decreases in darkness. From these results it can be concluded that in spite of the early development of the retinal dopaminergic system in the ontogeny of Xenopus, the final maturation must occur during the metamorphic climax. Endogenous DA release is significantly inhibited by light offset, which explains the higher ocular DA content found at midnight as compared to midday in postmetamorphic froglets and adults.

Long-term light history modulates the light response kinetics of luminosity (L)-type horizontal cells in the roach retina

We have examined the effects of prolonged periods of darkness on the responses of luminosity-type horizontal cells (L-HCs) in the freshwater cyprinid, Rutilus rutilus. Two groups of retinae were compared, those recorded after 10 min dark adaptation (SA) and those recorded after 3 h dark adaptation (LA). The results suggest that long-term light history does not modify the general responsiveness of the L-HCs in this species. However, there are apparent changes in the receptive field of the cells and modifications to the kinetics of the light-evoked response. The kinetics changes involve both a delay in the onset of light response and a selective effect on the hyperpolarizing light-ON response. Thus the mean time constant (tau) for the SA cells was 32.4+/-2.39 ms (n=62), whilst that for the LA cells was 53.4+/-3.03 ms (n=61). These effects occur in the absence of changes in the relative spectral sensitivity or threshold sensitivity of the HCs. The results suggest that in some vertebrate retinae, prolonged darkness (light-history) may regulate long-term plasticity in the kinetics of the cone-HC pathway.

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.

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.

Effects of dopamine depletion on visual sensitivity of zebrafish

The visual sensitivity of zebrafish in which the retinal dopaminergic interplexiform cells (DA-IPCs) were destroyed by 6-hydroxydopamine was measured behaviorally. During the first 6-8 min of dark adaptation, visual thresholds of DA-IPC-depleted animals were similar to those of control animals. Thereafter, their visual thresholds were elevated so that by 14-18 min of dark adaptation, they were 2-3 log units above those of control animals. In DA-IPC-depleted animals, the electroretinogram was normal in terms of light sensitivity and waveform, but the light threshold for eliciting a ganglion cell discharge was raised by 1.8 log units as compared with control animals. No obvious rod system function was detected in DA-IPC-depleted animals as measured behaviorally. Partial rescue of the behavioral visual sensitivity loss in DA-IPC-depleted animals occurred when dopamine or a long-acting dopamine agonist (2-amino-6, 7-dihydroxy-1, 2, 3, 4-tetrahydronaphthalene hydrobromide) were injected intraocularly. Our data suggest that the principal visual defect shown by DA-IPC-depleted animals is attributable to effects occurring in the inner retina, mainly on rod signals. We also show that dopamine is involved in mediating the effect of the circadian clock on visual sensitivity.

Endogenous dopaminergic regulation of horizontal cell coupling in the mammalian retina

Horizontal cells in an isolated wholemount preparation of the mouse retina were injected with Lucifer yellow and neurobiotin to characterize both the pattern of gap junctional connectivity and its regulation by dopamine. The injected horizontal cells had a uniform morphology of a round cell body, a compact dendritic tree, and an axon, which could sometimes be traced to an expansive terminal system. The dendro-dendritic gap junctions between neighboring cells mediated both weak Lucifer yellow dye coupling and strong neurobiotin tracer coupling. The extent of the tracer coupling was decreased by either exogenous dopamine (100 microM) or cyclic adenosine monophosphate (cAMP) analogs and was significantly increased by the D1 antagonist SCH 23390 (10 microM). These results provide the first evidence in the mammalian retina that the gap junctions between horizontal cells are endogenously regulated by dopamine, which acts through D1 receptors to increase the intracellular cAMP. It has been proposed that the gap junctional coupling between horizontal cells is mediated by connexin 32 (Cx32), but the pattern and dopaminergic regulation of horizontal cell coupling were unaffected in Cx32-knockout mice, ruling out the possible involvement of Cx32. Every tracer-coupled horizontal cell showed calbindin immunoreactivity, and vice versa, providing strong evidence that the horizontal cells in the mouse retina comprise a single cell type. Like the axonless horizontal cells in other mammalian retinas, the axon-bearing horizontal cells in the mouse retina are coupled by gap junctions that are permeable to Lucifer yellow and dopamine sensitive, suggesting that the mouse horizontal cells have hybrid properties to compensate for the absence of axonless horizontal cells.

Light-adaptive effects of retinoic acid on receptive field properties of retinal horizontal cells

Besides its role in ocular development, retinoic acid (RA), which is a light-correlated byproduct of the phototransduction cycle, was recently shown to affect light-driven synaptic plasticity in the outer plexiform layer of the adult fish retina. Tuning by ambient light conditions of the retinal network properties is very prominent in outer plexiform layer circuits, and we therefore examined whether RA could affect cone horizontal cell physiology similar to ambient light. Performing intracellular recordings and dye injections in the dark-adapted inverted eyecup preparation of the carp, we found that RA reduced the receptive fields of horizontal cell somata and impaired gap junctional communication. This action was not observed among coupled axon terminals of horizontal cells and appeared to be stereospecific because it could only be attributed to all-trans and 13-cis RA but not to the 9-cis isomer and photoisomerized all-trans RA. Modulation of receptive field size occurred independently of the dopaminergic system. Furthermore, RA affected the light responsiveness of cone horizontal cells. Compared to the dark-adapted condition, responsiveness to intense light stimulation was enhanced but decreased when low intensities were used. Moreover, following RA treatment H2-type horizontal cells of dark-adapted retinae which do not give rise to colour-opponent light properties became colour-opponent and performed depolarizing responses to long-wavelength stimulation. In all these cases RA perfectly matched the effects of light adaptation, supporting the notion that RA acts as an endogenous neuromodulator.

Retinoic acid modulates gap junctional permeability between horizontal cells of the mammalian retina

In the retina, all-trans retinoic acid (at-RA) could function as a light signal because its production increases with the level of illumination. Given the well-established effects of retinoic acid on cell coupling in other tissues, it is possible that the changing levels of at-RA modulate the gap junctional permeability between retinal neurons. This study examines the effects of retinoic acid on horizontal cell coupling, which is known to be modulated by the ambient light level. Single horizontal cells were injected under visual control with either Neurobiotin (mouse retina) or Lucifer yellow (rabbit retina) and the extent of tracer coupling or dye coupling was used to monitor the gap junctional permeability. In the mouse retina, the injection of Neurobiotin revealed a network of approximately 150-250 tracer-coupled horizontal cells. The tracer coupling was completely abolished by incubating the retina in 150 microM at-RA for 35 min. In the rabbit retina, the injection of Lucifer yellow into A-type horizontal cells revealed networks of approximately 15-30 dye-coupled horizontal cells. Incubation in 150 microM at-RA reduced the dye coupling within 12 min and complete uncoupling was achieved after 35 min. The uncoupling effects of at-RA in the mouse and rabbit retinas were concentration- and time-dependent and they were reversible after washout. The coupling was not affected by either the 9-cis form of retinoic acid or by at-RA that had been isomerized by intensive light. The uncoupling effect of at-RA persisted following treatment with a D1 receptor antagonist and thus was dopamine-independent. This study has established that at-RA is able to modulate the gap junctional permeability between horizontal cells in the mammalian retina, where its light-dependent release has already been demonstrated.

Dark- and light-induced changes in coupling between horizontal cells in mammalian retina

Retinal horizontal cells exhibit large receptive fields derived from their extensive electrical coupling by means of gap junctions. The conductance of these gap junctions seems to be regulated by dopamine acting through a cAMP-mediated cascade. There is now abundant evidence that extracellular dopamine levels vary with changes in ambient light intensity, suggesting that changes in the dark/light adaptational state of the retina can modulate coupling between horizontal cells. We studied this question in the mammalian retina by determining the effects of ambient light levels, in the form of changing background light intensity, on the coupling profiles of A- and B-type horizontal cells in the rabbit. Changes in coupling were assessed by measurements of the space constants of the syncytium formed by horizontal cells and the intercellular spread of the biotinylated tracer Neurobiotin. Our results indicate that dark-adapted horizontal cells show relatively weak coupling. However, presentation of background lights as dim as one-quarter log unit above rod threshold resulted in increases in both the averaged extent of tracer coupling and space constants of A- and B-type horizontal cells. Coupling expanded further as background light intensities were increased by 1-1.5 log units, after which additional light adaptation brought about an uncoupling of cells. Coupling reached its minimum at light intensities about 3 log units above rod threshold, after which, with further light adaptation, it stabilized at levels close to those seen in dark-adapted retinas. Our results indicate that electrical coupling between mammalian horizontal cells is modulated dramatically by changes in the adaptational state of the retina: coupling is maximized under dim ambient light conditions and diminishes as the retina is dark or light adapted from this level.

Retinoic acid has light-adaptive effects on horizontal cells in the retina

Ambient light conditions affect the morphology of synaptic elements within the cone pedicle and modulate the spatial properties of the horizontal cell receptive field. We describe here that the effects of retinoic acid on these properties are similar to those of light adaptation. Intraorbital injection of retinoic acid into eyes of dark-adapted carp that subsequently were kept in complete darkness results in the formation of numerous spinules at the terminal dendrites of horizontal cells, a typical feature of light-adapted retinae. The formation of these spinules during light adaptation is impaired in the presence of citral, a competitive inhibitor of the dehydrogenase responsible for the generation of retinoic acid in vivo. Intracellularly recorded responses of horizontal cells from dark-adapted eyecup preparations superfused with retinoic acid reveal typical light-adapted spatial properties. Retinoic acid thus appears to act as a light-signaling modulator. Its activity appears not to be at the transcriptional level because its action was not blocked by actinomycin.

Endogenous activation of dopamine D2 receptors regulates dopamine release in the fish retina

In the fish retina, horizontal cell electrical coupling and light responsiveness is regulated by activation of dopamine D1 receptors that are located on the horizontal cells themselves. The effects of dopamine and dopamine D2 receptor agonists and antagonists on cone horizontal cell light responses were studied in in vitro superfused goldfish retinas. Horizontal cell light responses and electrical coupling were assessed by monitoring responses to full-field stimuli and to small, centered (0.4 mm diam) spots of light, respectively. Dopamine (0.2-10 microM) application uncoupled horizontal cells and decreased their responses to full-field stimuli. Application of the D2 antagonist eticlopride (10-50 microM) produced similar effects, whereas quinpirole (0.1-10 microM), a D2 agonist, had the opposite effects. The uncoupling effect of eticlopride was blocked by prior application of SCH23390 (10 microM), a D1 receptor antagonist, and was eliminated after destruction of dopaminergic neurons by prior treatment of the retinas with 6-hydroxydopamine. The effects of these D2 drugs were observed following flickering light stimulation, but were not observed following sustained light stimulation. Application of the D2 antagonists sulpiride (0.5-20 microM) and spiperone (0.25-10 microM) uncoupled horizontal cells when the total concentration of divalent cations (Mg2+ and Ca2+) in the Ringer solution was 1.1 mM. However, when the concentration of divalent cations was 0.2 mM, spiperone had no effect on the horizontal cells and sulpiride increased coupling. In contrast, eticlopride uncoupled the cells and decreased their light responsiveness irrespective of the concentration of divalent cations. The effects of quinpirole also depended on the concentration of divalent cations; its coupling effect was reduced when the divalent cation concentration was increased from 0.2 to 1.0 mM. The results suggest that activation of D2 receptors in the fish retina by endogenous dopamine decreases dopamine release and is greater after flickering compared with sustained light stimulation. These D2 receptors thus function as presynaptic autoreceptors that inhibit dopamine release from dopaminergic cells. In addition, the results also indicate that the effectiveness of some D2 drugs at these receptors is dependent on the concentration of divalent cations.