Dopaminergic Modulation of Transient Neurite Outgrowth from Horizontal Cells of the Fish Retina is not Mediated by cAMP

Synthesis and Evaluation of Two Novel All -trans-Retinoic Acid Conjugates: Biocompatible and Functional Tools for Retina Research

The vitamin A derivative all- trans-retinoic acid (ATRA) is an important biologically active metabolite that regulates a variety of essential biological processes in particular via gene-regulatory mechanisms. In the retina, ATRA is a light-dependent byproduct of the phototransduction cascade. Here, ATRA is not only needed for proper retinal development, but it also acts as a neuromodulator on horizontal cells, second-order inhibitory neurons in the outer retina, which reveal morphological and physiological changes when the retina is treated with ATRA. There is evidence that gene-regulatory mechanisms may only be partially involved in these neuromodulatory processes and the underlying nontranscriptional mechanisms are still elusive. This is, among other things, due to the lack of appropriately labeled ATRA, which would allow the tracking of ATRA in cells or a given tissue. To overcome this obstacle, we designed, synthesized, and evaluated two conjugates of ATRA, one conjugated with biotin (biotin-ATRA) and one conjugated with diaminoterephthalate fluorophore (DAT-ATRA), as molecular tools for different fields of application. The biocompatibility of both compounds was demonstrated via cell viability assays in cultured N2a-cells. N2a-cells exposed to the compounds showed no significant changes in the viability rate. The functionality of synthesized ATRA-conjugates was verified using retinal tissue derived from adult carp. The binding of ATRA-conjugates to distinct retinal cells was assessed in primary cultures of carp retina. Hereby, horizontal and Müller cells have been identified as specific target cells of the new ATRA compounds. Electron microscopy further confirmed that the new substances are still able to induce synaptic plasticity at horizontal cell dendrites resulting in formation of spine synapses, as it is shown for native ATRA. Taken together, the novel ATRA-conjugates represent biocompatible and functional molecular tools, which may further provide the possibility to track ATRA in neuronal cells and study its modulatory effects in different cell systems.

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.

Uptake of 3H-cAMP by retinal pigment epithelium isolated from bluegill sunfish (Lepomis macrochirus)

Background

In bluegill sunfish, the melanin-containing pigment granules of the retinal pigment epithelium undergo cyclic movements in response both to ambient lighting and circadian cues. Pigment granules aggregate into the cell body at night (in the dark), and disperse into apical processes during the day (in the light). Regulation of pigment granule aggregation in a number of fishes depends on modulating the intracellular levels of cyclic adenosine monophosphate.

Results

Here we show isolated RPE takes up cyclic adenosine monophosphate (cAMP) in a saturable manner, exogenously applied cAMP induces pigment granule aggregation in retinal pigment epithelium isolated from bluegill, and aggregation induced in this manner is inhibited by treatment with probenecid, an organic anion transport inhibitor.

Conclusion

Our results raise the possibility that cAMP functions as a messenger secreted from the neural retina to signal darkness to the RPE, which takes it up. It further suggests that organic anion transport systems are the route by which cAMP crosses RPE cell membranes since probenecid inhibits extracellular cAMP from causing pigment granule aggregation.

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.

Synaptic plasticity and functionality at the cone terminal of the developing zebrafish retina

Previous studies have analyzed photoreceptor development, some inner retina cell types, and specific neurotransmitters in the zebrafish retina. However, only minor attention has been paid to the morphology of the synaptic connection between photoreceptors and second order neurons even though it represents the transition from the light sensitive receptor to the neuronal network of the visual system. Here, we describe the appearance and differentiation of pre- and postsynaptic elements at cone synapses in the developing zebrafish retina together with the maturation of the directly connecting second order neurons and a dopaminergic third order feedback-neuron from the inner retina. Zebrafish larvae were examined at developmental stages from 2 to 7dpf (days postfertilization) and in the adult. Synaptic maturation at the photoreceptor terminals was examined with antibodies against synapse associated proteins. The appearance of synaptic plasticity at the so-called spinule-type synapses between cones and horizontal cells was assessed by electron microscopy, and the maturation of photoreceptor downstream connection was identified by immunocytochemistry for GluR4 (AMPA-type glutamate receptor subunit), protein kinase beta(1) (mixed rod-cone bipolar cells), and tyrosine hydroxylase (dopaminergic interplexiform cells). We found that developing zebrafish retinas possess first synaptic structures at the cone terminal as early as 3.5dpf. Morphological maturation of these synapses at 3.5-4dpf, together with the presence of synapse associated proteins at 2.5dpf and the maturation of second order neurons by 5dpf, indicate functional synaptic connectivity and plasticity between the cones and their second order neurons already at 5dpf. However, the mere number of spinules and ribbons at 7dpf still remains below the adult values, indicating that synaptic functionality of the zebrafish retina is not entirely completed at this stage of development.

Activation of Protein Kinase C Modulates Light Responses in Horizontal Cells of the Turtle Retina

The effect of phorbol esters on the light-evoked responses of horizontal cells were studied in the turtle eyecup preparation. Phorbol esters caused a reduction in receptive field size and a significant decrease in the amplitude of responses to annular and full-field illumination; however, they caused only minor changes in responses to small spots in the receptive field centre. The dark membrane potential was not affected. The results suggest that phorbol esters may affect both coupling resistance and membrane resistance in horizontal cells. The effects of phorbol esters were blocked by the protein kinase C inhibitor staurosporine, and inactive phorbol ester had no effect, making it very likely that the phorbol ester effects were mediated through activation of protein kinase C. The above effects of the phorbol esters were considerably reduced by the dopamine antagonists haloperidol and fluphenazine, suggesting that they were in part mediated by release of dopamine.

AMPA-preferring receptors with high Ca2+ permeability mediate dendritic plasticity of retinal horizontal cells

The synaptic complex formed by the cone photoreceptor pedicles and the dendrites of horizontal cells in the teleost retina undergoes structural changes during light adaptation. Numerous spinules are formed by the terminal dendrites, and they are subsequently retracted during dark adaptation. In a retina kept under continuous illumination, the retraction process can be initiated by analogues of the neurotransmitter glutamate acting at AMPA/kainate receptors. On the other hand, the retraction process depends on calcium influx and the subsequent activation of CaMkII. We show here that the retraction of spinules induced by AMPA or kainate is not impaired in the presence of cobalt, making an involvement of voltage-gated calcium channels unlikely. Using calcium imaging techniques with isolated horizontal cells, we demonstrate that AMPA and kainate, but not NMDA, increase [Ca2+]i in the presence of nicardipine, caffeine and thapsigargin. The increase of [Ca2+]i under these conditions depends on [Ca2+]o and on the agonist in a dose-dependent manner, suggesting that the increase of [Ca2+]i is largely due to calcium influx through the agonist-gated channel. Pharmacological studies were performed to determine whether AMPA- and/or kainate-preferring receptors mediate the calcium influx. The AMPA-preferring receptor antagonist LY303070 blocked glutamate- and kainate-evoked increases of [Ca2+]i in a concentration-dependent manner, indicating that kainate-preferring receptors contributed little or nothing to the observed [Ca2+]i increase. This was supported by experiments where cyclothiazide (which blocks the desensitization of AMPA receptors) and concanavalin A (which potentiates responses mediated by kainate receptors) were applied. In all cases, LY303070 blocked the agonist-evoked increase of [Ca2+]i. The presence of AMPA-preferring receptors with high Ca2+ permeability on horizontal cells was also supported by measuring agonist-induced currents using whole-cell recording techniques. Furthermore, LY303070 was able to impair the retraction of spinules during dark adaption in the in vivo situation.

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.

Effects of nitric oxide on the horizontal cell network and dopamine release in the carp retina

In the teleost retina the intercellular messenger nitric oxide can be synthesized by several cell types including cone photoreceptors and H1 horizontal cells, indicating a modulatory role within the outer plexiform layer, the first stage of the visual information processing. Therefore, the aim of this study was to elucidate the effects of nitric oxide on the physiology of cone horizontal cells in the intact retina. The nitric oxide donor sodium nitroprusside (0.5-2.5 mM) enhanced the light responsiveness of cone horizontal cells and reduced the degree of electrical coupling in the network. Furthermore, the spread of intracellularly injected Lucifer Yellow was restricted. The effects on light responsiveness and electrical coupling were qualitatively mimicked by 8-bromo-cGMP (0.5 mM) and could not be achieved by ferrocyanide (1 mM), the byproduct of nitric oxide liberation from nitroprusside. The effects of NO on the responsiveness of horizontal cells may be due to an action on green- and red-sensitive cones. Nitroprusside (0.1 mM) diminished the K(+)-stimulated release of endogenous dopamine by 50%, whereas the basal dopamine release was not affected, indicating that the effects on electrotonic horizontal cell coupling were not elicited by an NO-induced release of dopamine. With respect to the morphologic plasticity of the cone-horizontal cell synapse the inhibitor of endogenous nitric oxide synthesis L-nitroarginine (0.1 mM) had no influence on the formation or retraction of spinules. These results show that NO affects the responsiveness and coupling of the horizontal cell network in a dopamine-independent way.

Dopaminergic control of light-adaptive synaptic plasticity and role in goldfish visual behavior

Dopamine has been implicated in processes of retinal light and dark adaptation. In goldfish retina, horizontal cell dendrites elaborate neurite processes (spinules) into cone terminals, in a light- and dopamine-dependent manner. However, the functions of retinal dopamine and the horizontal cell spinules in visual behavior are unknown. These issues were addressed in behavioral, electroretinographic, and anatomical studies of normal fish and those with unilateral depletion of retinal dopamine induced by intraocular (i.o.) injections with 6-hydroxydopamine (6-OHDA). Dopamine interplexiform cells (DA-IPC) disappear within 2 weeks after 6-OHDA injection; cell bodies appear at the marginal zone within 6 weeks at which time neurites slowly reinnervate the retina with a sparse plexus over the next 12 months. We found that dopamine depletion increased light sensitivity at photopic but not scotopic backgrounds by 2.5 log units, an effect mimicked by i.o. injections of dopamine D1 and D2 antagonists. The ERG b-wave increment thresholds were the same for control and dopamine depleted eyes, indicating a normal transition from rod to cone systems in the ON pathway. Light-dependent spinule formation was reduced by about 60% in dopamine-depleted retinas, but returned to normal by 3 months and 9 months after injection in the entire retina, even areas not directly innervated with DA-IPC processes. Spinule formation in vivo was inhibited 50% with i.o. injection of SCH 23390 in control retinas as well as throughout 3 month 6-OHDA injected retinas, including DA-IPC free areas. This latter result indicates a volume effect of dopamine, diffusing laterally through the retina over several millimeters, in regulating spinules. We conclude that DA-IPCs regulate sensitivity to background at photopic levels not via the ON pathway, but perhaps the OFF pathway. Goldfish display both increased sensitivity to light and a normal Purkinje shift in the ERG b-wave whether or not horizontal cell spinules are present, indicating that dopamine control of photopic vision in fish is not mediated through light-induced spinule formation of horizontal cell dendrites.

Ca(2+)-dependency of spinule plasticity at dendrites of retinal horizontal cells and its possible implication for the functional role of spinules

Calcium is involved in many aspects of synaptic plasticity and we have analyzed its involvement in spinule dynamics at retinal horizontal cell dendrites. We show here that in particular the retraction of spinules is a Ca(2+)-dependent process. Inhibiting calmodulin or CaMKII, blocked the retraction that was also impaired in low calcium Ringer. Changes of the cytosolic Ca(2+)-concentration through depletion of internal Ca(2+)-stores were without effect. This suggested that Ca(2+)-influx during dark adaption and subsequent activation of CaMKII is an important step for spinule retraction. Voltage dependent Ca(2+)-channels were not responsible for the Ca(2+)-influx, rather Ca2+ leaking through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-gated channels. This suggested a close local link between AMPA/kainate receptors and CaMKII indicating a possible postsynaptic function of spinules. The distribution of bound, omega-shaped vesicles within the cone pedicles and its dependence on artificial depolarization further supported the idea of a postsynaptic function of spinules.

Formation and dissolution of spinules and changes in nematosome size require optic nerve integrity in black bass (Micropterus salmoides) retina

Teleost retinas adapted to light show numerous spinules invaginated in the cone pedicles and small nematosomes in the distal horizontal cells. Darkness induces the dissolution of spinules and the presence of large and numerous nematosomes. The aim of this work is to study the influence of optic nerve integrity on spinule formation/dissolution and changes in nematosome size during light or dark adaptation of black bass (Micropterus salmoides) retinas. Eyes from fish, dark- or light-adapted, were removed and the eyecups placed in oxygenated Ringer's solution and immediately exposed to light or dark, respectively, for 1 h. The number of spinules per pedicle and the nematosome diameter were measured on electron micrographs. Isolation of eyecups in the dark, impaired both spinule formation and nematosome size reduction when they were superfused in light. In the same way, isolation of eyecups in the light, impaired both spinule dissolution and nematosome size increase when they were superfused in dark. No significant differences in spinule number and nematosome size, following dopamine superfusion, were found in comparison to retinas superfused with Ringer's solution only. Our results suggest: (1) optic nerve integrity is necessary to yield spinule formation/disruption and changes in nematosome size during light or dark adaptation. (2) dopamine does not appear to be the primary agent responsible for spinule formation.

Volume transmission of dopamine may modulate light-adaptive plasticity of horizontal cell dendrites in the recovery phase following dopamine depletion in goldfish retina

We investigated the recovery of light-adaptive spinule formation following dopamine depletion with intraocular injection of 6-hydroxydopamine (6-OHDA) and subsequent neogeneration of dopamine interplexiform cells (DA-IPC) at the marginal zone. DA-IPCs were gone by 2 weeks postinjection and appeared at the marginal zone by 6 weeks postinjection, at which time DA-IPC neurites grew toward the central retina, reaching within 0.5 mm of the central retina by 1 year. Retinas from day time, light-adapted fish at 2 weeks, 4 weeks, 3 months, and 1 year postinjection with 6-OHDA were processed for pre-embedding tyrosine hydroxylase immunoreactivity (TOH-IR) and compared to sham-injected and control retinas at the electron-microscopical (EM) level. Only 6-OHDA fish that tilted markedly toward the injected eye were used for these experiments. The tilt mimics the dorsal light reaction, indicating a 2-2.5 log unit increase in the photopic sensitivity of the 6-OHDA eye. Spinule formation was reduced by about 60% in the 2- and 4-week 6-OHDA retinas, but returned to control levels throughout the entire retina of 3-month and 1 year 6-OHDA retinas even though the central region of these retinas contained no detectable TOH-IR. Intraocular injection with 10 microM SCH 23390 (a D1 antagonist) reduced light-adaptive spinule formation by 50% both in control eyes as well as those eyes that were 3 months post 6-OHDA injected. The full return of spinule formation with only partial reinnervation of the retina with DA-IPC processes and their subsequent inhibition by SCH 23390 indicates that dopamine diffused large distances within the retina to regulate this synaptic plasticity (i.e. displayed volume transmission). Also, since all 6-OHDA injected fish displayed an increased photopic sensitivity in the injected eye when sacrificed, we suggest that horizontal cell spinules are not required for photopic luminosity coding in the outer retina.

Evidence for calcium/calmodulin dependence of spinule retraction in retinal horizontal cells

Horizontal cells of the carp retina alter their synaptic connections with cones during dark and light adaptation. At light onset, dendrites of horizontal cells, which are positioned laterally at the ribbon synapse, form "spinules," little processes with membrane densities. Spinules are retracted again during dark adaptation. Spinule retraction is also elicited upon glutamate application to the retina. In the present study, we address the question whether calcium/calmodulin-dependent pathways are involved in dark- and glutamate-evoked spinule retraction. Light-adapted retinas were isolated and subsequently dark adapted during incubation in media of different calcium concentrations. Spinule retraction was clearly blocked in low-calcium solutions (5 microM and 50 nM CaCl2). Incubation in medium containing cobalt chloride (2 mM) had the same effect. Both treatments blocked the glutamate-induced spinule retraction as well. These results indicate that spinule retraction is induced by a calcium influx into horizontal cells. To investigate whether calmodulin, the primary calcium receptor in eukaryotic cells, is present at the site of spinule formation, light- and dark-adapted retinas, embedded in LR White resin, were labelled with an antibody against calmodulin and gold-conjugated secondary antibodies. Horizontal cell dendrites at the ribbon synapse revealed strong calmodulin immunoreactivity, which was more than twice as high in light- as in dark-adapted retinas. The incubation of isolated retinas with the calmodulin antagonists W5 and W13 inhibited spinule retraction. In summary, these results suggest that spinule retraction may be regulated by calcium influx into horizontal cells and subsequent calcium/calmodulin-dependent pathways.

Depletion of retinal dopamine does not affect the ERG b-wave increment threshold function in goldfish in vivo

Increment threshold functions of the electroretinogram (ERG) b-wave were obtained from goldfish using an in vivo preparation to study intraretinal mechanisms underlying the increase in perceived brightness induced by depletion of retinal dopamine by 6-hydroxydopamine (6-OHDA). Goldfish received unilateral intraocular injections of 6-OHDA plus pargyline on successive days. Depletion of retinal dopamine was confirmed by the absence of tyrosine-hydroxylase immunoreactivity at 2 to 3 weeks postinjection as compared to sham-injected eyes from the same fish. There was no difference among normal, sham-injected or 6-OHDA-injected eyes with regard to ERG waveform, intensity-response functions or increment threshold functions. Dopamine-depleted eyes showed a Purkinje shift, that is, a transition from rod-to-cone dominated vision with increasing levels of adaptation. We conclude (1) dopamine-depleted eyes are capable of photopic vision; and (2) the ERG b-wave is not diagnostic for luminosity coding at photopic backgrounds. We also predict that (1) dopamine is not required for the transition from scotopic to photopic vision in goldfish; (2) the ERG b-wave in goldfish is influenced by chromatic interactions; (3) horizontal cell spinules, though correlated with photopic mechanisms in the fish retina, are not necessary for the transition from scotopic to photopic vision; and (4) the OFF pathway, not the ON pathway, is involved in the action of dopamine on luminosity coding in the retina.

Circadian regulation of teleost retinal cone movements in vitro

In the retinas of many species of lower vertebrates, retinal photoreceptors and pigment epithelium pigment granules undergo daily movements in response to both diurnal, and in the case of teleost cone photoreceptors, endogenous circadian signals. Typically, these cone movements take place at dawn and at dusk when teleosts are maintained on a cyclic light (LD) regime, and at expected dawn and expected dusk when animals are maintained in continuous darkness (DD). Because these movements are so strictly controlled, they provide an overt indicator of the stage of the underlying clock mechanism. In this study we report that both light-induced and circadian-driven cone myoid movements in the Midas cichlid (Cichlasoma citrinellum), occur normally in vitro. Many of the features of retinomotor movements found in vivo also occur in our culture conditions, including responses to light and circadian stimuli and dopamine. Circadian induced predawn contraction and maintenance of expected day position in response to circadian modulation, are also normal. Our studies suggest that circadian regulation of cone myoid movement in vitro is mediated locally by dopamine, acting via a D2 receptor. Cone myoid contraction can be induced at midnight and expected mid-day by dark culture with dopamine or the D2 receptor agonist LY171555. Further, circadian induced predawn contraction can be increased with either dopamine or LY171555, or may be reversed with the dopamine D2 antagonist, sulpiride. Sulpiride will also induce cone myoid elongation in retinal cultures at expected mid-day, but will not induce cone myoid elongation at dusk. In contrast, circadian cone myoid movements in vitro were unaffected by the D1 receptor agonist SCH23390, or the D1 receptor antagonist SKF38393. Our short-term culture experiments indicate that circadian regulation of immediate cone myoid movement does not require humoral control but is regulated locally within the retina. The inclusion of dopamine, or dopamine receptor agonists and antagonists in our cultures, has indicated that retinal circadian regulation may be mediated by endogenously produced dopamine, which acts via a D2 mechanism.

The marker for nitric oxide synthase, NADPH-diaphorase, co-localizes with GABA in horizontal cells and cells of the inner retina in the carp retina

NADPH-diaphorase histochemistry selectively stained discrete populations of neurons contributing to the inner plexiform layer of the carp retina. In addition H1 horizontal cells contributing to the outer plexiform layer were labeled. In these cells as in a subpopulation of the labeled cells in the inner retina, NADPH-activity co-localized with GABA immunoreactivity. NADPH-activity is a direct marker for NO synthesis and it is concluded that NO might be an important mediator of light-dependent adaptational processes in the outer retina.

Microanatomy of the dopaminergic system in the rainbow trout retina

We have investigated the morphology of dopaminergic interplexiform cells as well as the distribution of two classes of dopamine receptors in the retina of the rainbow trout. Interplexiform cells were visualized using an antiserum against tyrosine hydroxylase and PAP immunocytochemistry. In whole amounts, these cells have a density of between 91 and 182 cells per mm2 with highest values in the lower temporal quadrant. Their cell bodies lie at the inner margin of the inner nuclear layer with only 12-17 cells per retina displaced to the ganglion cell layer. There are three levels of stratification in the inner plexiform layer, one at the distal and proximal borders respectively, and one in the middle. They arise mostly from a radially oriented, stout primary dendrite. Tangential processes are about 1 micron in diameter and show a number of varicosities. The density of processes is greatest in sublayer 5, but no major difference in the general organization is apparent between the three sublayers. In the outer retina, there are two levels of dense ramification confined to the layer of horizontal cells. Light and electron microscopic analysis shows synaptic input to horizontal cells, but not to photoreceptors. The distribution of D1 receptors was assessed by studying the binding pattern of a specific, fluorescent-labelled antagonist, SCH 23390, in unfixed frozen sections. We found displaceable binding in the inner and outer plexiform layers and in the region of horizontal cell perikarya. We used an anti-peptide antibody directed to an extracellular domain of the rat D2 receptor and a fluorescent secondary antiserum to study the localization of D2 receptors. In addition to marked label in both plexiform layers, the outer, and especially the inner segments of rods and cones show specific immunoreactivity. In addition, there is distinct label at the level of the horizontal cell bodies; in the inner retina, specific fluorescence is found in somata of some amacrine cells. The significance of the connectivity pattern and the distribution of the two receptor types is discussed with respect to the role of dopamine in controlling adaptational processes in the outer retina, such as retinomotor movements and changes in horizontal cell morphology and physiology.

In vitro phosphorylation in isolated horizontal cells of the fish retina: effects of the state of light adaptation

Horizontal cells, which are second-order neurons of the vertebrate retina, exhibit synaptic plasticity governed by light and dark adaptation. We have investigated the alterations in the protein phosphorylation patterns of isolated carp (Cyprinus carpio) horizontal cells in relation to their state of light adaptation by using an in vitro phosphorylation assay and compared the resulting data with protein synthesis patterns of the whole retina. Phosphoproteins and [35S]methionine-labelled proteins were analysed by one- and two-dimensional gel electrophoresis followed by autoradiography. The state of light adaptation significantly affected the in vitro phosphorylation of horizontal cell proteins with molecular weights of 68, 56/58, 47, 28 and 15 kDa, but had no effect on the protein synthesis of retinal proteins. In the light the most prominent increase of 32P incorporation was observed in the 47 kDa protein. The biochemical properties of this protein closely resembled those of the growth-associated GAP-48, found in the fish retina. In addition, the phosphorylation of horizontal cell homogenates in the presence of protein kinase activators such as cyclic AMP, calcium, calmodulin and phospholipids revealed that horizontal cells of the fish retina contain cyclic AMP-, calcium/calmodulin- and calcium/phospholipid-dependent protein kinase activity resulting in the phosphorylation of several horizontal cell proteins, including the phosphoproteins which were affected by the state of light adaptation.

Spinule-type neurite outgrowth from horizontal cells during light adaptation in the carp retina: an actin-dependent process

Dendrites of horizontal cells in the carp retina which invaginate the cone pedicles form numerous spinules during light adaptation. We have analyzed the contribution of cytoskeletal elements to this process. Isolated horizontal cells and frozen sections were screened with phalloidin for the existence of F-actin. F-actin was present in all types of horizontal cells and particularly enriched in the distal parts of the dendrites. Electron microscopical analysis demonstrated that interruption of the F-actin polymerization with cytochalasin B inhibited the formation of spinules during light adaptation. The persistence of spinules was also affected. Cytochalasin B also prevented the light-independent, phorbol ester-induced formation of spinules. Cytochalasin B only affected the morphology of the lateral, spinule-forming dendrites of cone horizontal cells within the cone pedicles, leaving the central, non spinule-forming dendrites of cone horizontal cells and the processes of rod horizontal cells within rod spherules unaffected. Whereas cytochalasin B prevented the protrusion of spinules, the spinule-associated membrane densities were only slightly affected. The two main characteristics of spinules, protrusion and membrane densities are therefore independently regulated processes.

cAMP-mediated second messenger mechanisms are involved in spinule formation in teleost cone horizontal cells

A number of light adaptive changes of teleost horizontal cells are mediated by dopamine D1 receptors coupled positively with the cAMP second messenger system. Spinules, finger-like extensions from horizontal cell dendrites directed towards the cone pedicle cytoplasm, are formed in response to a stimulation of D1 receptors. We studied the second messenger mechanism associated with this process using isolated dark-adapted cyprinid retinae. Increasing intracellular cAMP concentrations by adding a membrane permeable analogue, or by stimulating the adenylate cyclase and simultaneously blocking the degradation of cAMP, resulted in a significant increase of spinule numbers in spite of the absence of light. In contradistinction to using isolated retinae for pharmacological experiments, injection of drugs into the vitreous had inconsistent or negative results.

The circadian component of spinule dynamics in teleost retinal horizontal cells is dependent on the dopaminergic system

During the light phase of a light/dark cycle, dendrites of teleost cone horizontal cells display numerous finger-like projections, called spinules, which are formed at dawn and degraded at dusk, and are thought to be involved in chromatic feedback processes. We have studied the oscillations of these spinules during a normal light/dark cycle and during 48 h of constant darkness in two groups of strongly rhythmic, diurnal fish, Aequidens pulcher. In one group the retinal dopaminergic system had been destroyed by the application of 6-OHDA, while in the other (control) group, the dopaminergic system was intact. In control fish, oscillations of spinule numbers were observed under both normal and constant dark conditions, indicating the presence of a robust circadian rhythm. However, spinule dynamics were severely affected by the absence of retinal dopamine. During the normal light phase, the number of spinules in 6-OHDA injected retinae was strongly reduced, and throughout continual darkness, spinule formation was almost completely suppressed. These results indicate that dopamine is essential for both light-evoked and circadian spinule formation; furthermore, we conclude that there is no circadian oscillator within horizontal cells controlling the formation of spinules.

A role for endogenous dopamine in circadian regulation of retinal cone movement

Cone movements in the retina of the Midas cichlid (Cichlasoma citrinellum) take place in response both to light and endogenous circadian signals. In the normal light/dark cycle (LD) cone myoids are long at night (50-55 microns), begin to contract before expected dawn, and with light onset contract to their fully contracted positions (5 microns) which are retained throughout the day. In continuous darkness (DD) cone myoids are fully elongate at night, but undergo pre-dawn contractions to partially contracted positions which they retain throughout expected day (20-25 microns). To investigate the mechanisms by which circadian signals modulate cone myoid movements in teleost retinas, we have tested the effects on circadian cone movements of optic nerve section, intraocular injection of dopamine agonists or antagonists, and intraocular injection of melatonin. We report here that both light-induced and circadian-driven cone myoid movements can occur in the absence of efferent input from higher centres: both are retained with full amplitude after optic nerve section in vivo. Intraocular injection studies suggest that circadian regulation of cone myoid movement is mediated locally within the eye by dopamine acting via a dopaminergic D2-receptor. Cone myoid contraction can be induced at midnight in LD or DD animals by intraocular injection of dopamine or the D2-receptor agonist LY171555. The partially contracted cones of DD animals at expected mid-day can be induced to fully contract by intraocular injection of dopamine or the D2-receptor agonist, or to elongate by intraocular injection of the dopamine D2-antagonist sulpiride. Furthermore, the pre-dawn cone myoid contraction observed in both LD and DD animals in response to circadian signals can be completely blocked in DD animals by intraocular injection of the D2-antagonist sulpiride shortly before the time of expected light onset. In contrast, circadian cone myoid movements were unaffected by intraocular injection of the D1-receptor agonist SCH23390, or the D1-receptor antagonist SKF38393. In addition, we report that intraocularly injected melatonin had no effect on cone position when injected in the light at mid-day, in darkness at midnight or in darkness just before expected light onset at dawn. However, both melatonin and iodomelatonin induced cone myoid contraction (the light-adaptive movement) when injected in darkness at expected mid-day in DD animals. This paradoxical result is not consistent with observations from other species in which melatonin induces dark-adaptive photoreceptor responses.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of dopamine depletion on light-evoked and circadian retinomotor movements in the teleost retina

The retinae of lower vertebrates undergo a number of structural changes during light adaptation, including the photomechanical contraction of cone myoids and the dispersion of melanin granules within the epithelial pigment. Since the application of dopamine to dark-adapted retinae is known to produce morphological changes that are characteristic of light adaptation, dopamine is accepted as a casual mechanism for such retinomotor movements. However, we report here that in the teleost fish, Aequidens pulcher, the intraocular injection of 6-hydroxydopamine (6-OHDA), a substance known to destroy dopaminergic retinal cells, has no effect on the triggering of light-adaptive retinomotor movements of the cones and epithelial pigment and only slightly depresses the final level of light adaptation reached. Furthermore, the retina continues to show circadian retinomotor changes even after 48 h in continual darkness that are similar in both control and 6-OHDA injected fish. Biochemical assay and microscopic examination showed that 6-OHDA had destroyed dopaminergic retinal cells. We conclude, therefore, that although a dopaminergic mechanism is probably involved in the control of light-induced retinomotor movements, it cannot be the only control mechanism, nor can it be the cause of circadian retinomotor migrations. Interestingly, 6-OHDA injected eyes never reached full retinomotor dark adaptation, suggesting that dopamine has a role to play in the retina's response to darkness.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein kinase C mediates transient spinule-type neurite outgrowth in the retina during light adaptation

Light and dark adaptation of the teleost retina is accompanied by a remarkable morphological rearrangement of the synaptic connections between photoreceptors and second-order neurons: during light adaptation, numerous new neurites, the so-called spinules, arise from the terminal dendrites of horizontal cells invaginating the cone pedicle, and during dark adaptation, these spinules are retracted. The formation of these spinules is paralleled by the appearance of color opponency in horizontal and ganglion cells, which led to the suggestion that these spinules are the site of the inhibitory synapses in the negative feedback loop between cones and horizontal cells. The formation of the spinules in the light and their disappearance in darkness have a time course of minutes and are modulated by the neurotransmitters dopamine and glutamate, respectively. Neurotransmitters can modulate neuronal processing through a variety of second messengers that activate protein kinases, resulting most commonly in protein phosphorylation. Herein we report that activation of protein kinase C by phorbol esters promotes the formation of new horizontal-cell spinules in animals kept in the dark. Partial inhibition of protein kinase C activation with sphingosines prevents the formation of new spinules during light adaptation but does not affect established spinules. The spinule-forming effect of phorbol esters is not mediated by dopaminergic neurons, since the effect is also seen in retinas depleted of dopaminergic neurons. Phorbol esters also initiate the formation of spinules in synaptically isolated horizontal cells, demonstrating that they have a direct action on these cells. In addition, isolated horizontal cells have substrate proteins that are phosphorylated in a protein kinase C-dependent manner.