Dopamine modulates S-potential amplitude and dye-coupling between external horizontal cells in carp retina

Dendrites of rod dominant ON-bipolar cells are coupled by gap junctions in carp retina

Gap junctions are supposed to be the anatomical substrate for electrical coupling between neurons. In fish retina, bipolar cells are electrically coupled and their receptive field diameters are always larger than dendritic field size, however, gap junctions have not been described between dendrites of bipolar cells. In this paper, using immunostaining for protein kinase C, we show that every rod dominant ON-bipolar cell is connected with its neighboring dendrites by gap junctions forming a plexus in the outer plexiform layer. These dendritic processes provide the site of electrical coupling. We suggest that dendrites of bipolar cells could be involved in lateral pathways in retina.

Influence of dopamine on cyclic nucleotide enzymes in bovine retinal membrane fractions

Dopamine is a major neurotransmitter and neuromodulator in vertebrate retina. Although its pharmacological and physiological actions are well understood, the biochemical mechanisms of its signal transduction are less clear. Acting via D1 receptors, dopamine was shown to increase cyclic AMP levels in intact retina and to activate adenylate cyclase in retinal homogenates. The action via activation of D2 receptors is controversial: it was reported to decrease cyclic AMP levels in intact retina but inhibition of cyclase could not be demonstrated in retinal homogenates; also it was reported to activate rod outer segment cyclic GMP phosphodiesterase in vitro but did not decrease cyclic GMP levels in aspartate-treated retinas. We made an attempt to fractionate bovine retinal membranes and to investigate the effects of dopamine, via D1 and D2 receptors, on the synthesis and hydrolysis of cyclic AMP and cyclic GMP. Activation of cyclic AMP synthesis was noted in all fractions, but no effects were evident on cyclic nucleotide hydrolysis or cyclic GMP synthesis in any fraction. Also, D2 agonist did not inhibit cyclic AMP synthesis. These observations suggest that D2 receptors may not be directly coupled to cyclic nucleotide metabolizing enzymes in bovine retina.

New aspects of dopaminergic interplexiform cell organization in the goldfish retina

Dopaminergic interplexiform cells (DA-IPCs) in the goldfish retina have been reexamined by light and electron microscopic immunocytochemistry with antisera against dopamine (DA) or tyrosine hydroxylase (TH). Successful immunostaining with a specific anti-DA antiserum offers further direct support for DA-IPCs. Anti-DA immunocytochemistry in combination with [3H]-DA autoradiography shows 92% colocalization of the two markers, indicating that [3H]-DA autoradiography is a reliable technique for identification of DA-IPCs. Incubations with anti-TH antiserum show that immunoreactive DA-IPCs have a homogeneous distribution, with an average frequency of 71 +/- 8 cells/mm2 in retinas of 14-15 cm long goldfish. Their arrangement is distinctly nonrandom. Electron microscopy of TH-immunoreactive cell processes confirms that horizontal cell axons synapse onto DA-IPCs and adds the following junctional arrangements to the circuit diagram of the DA-IPC: 1) adjacent serial synapses between DA-IPCs, external horizontal cells, and putative glycinergic interplexiform cells, 2) junctional appositions between DA-IPCs and photoreceptor cells, 3) junctional appositions between neighbouring DA-IPCs, and 4) the "gap junctional complex," typically consisting of a DA-IPC process juxtaposed with a gap junction between horizontal cell axons. The gap junction is flanked by clusters of small, round vesicles and groups of electron-dense structures resembling intermediate filaments. These morphological results support the functional involvement of DA-IPCs in adaptive retinomotor movements and in horizontal cell gap junction modulation and/or dynamics. They also suggest particular interaction between the dopaminergic and the glycinergic IPC system in the outer plexiform layer of goldfish 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.

Localization of D2 dopamine receptors in vertebrate retinae with anti-peptide antibodies

Dopamine plays an important role in modulating various aspects of retinal signal processing. The morphology of dopaminergic neurons and its physiological effects are well characterized. Two classes of receptor molecules (D1 and D2) were shown pharmacologically to mediate specific actions, with differences between individual groups of vertebrates. In an attempt to better understand dopaminergic mechanisms at the cellular level, we used antisera against D2 receptors and investigated the localization of the dopamine D2 receptor in the retinae of rat, rabbit, cow, chick, turtle, frog, and two fish species with immunofluorescence techniques. Antisera were raised in rabbits to two oligopeptides predicted from rat D2 receptor cDNA; one specific for the splice-variant insertion in the third cytoplasmic loop and the other directed towards the extracellular amino terminal region shared by both short and long isoforms. Preadsorption with the synthetic peptide resulted in a significant reduction of label, indicating the presence of specific binding in all species except turtle and goldfish. The pattern of labelling produced by the two antisera was essentially identical; however, the staining obtained with antiserum to the extracellular motif was always more intense. Specific staining was present in photoreceptor inner and outer segments, and in the outer and inner plexiform layers of all species. In mammals and chick, strongly fluorescent perikarya were observed in the ganglion cell layer and at the proximal margin of the inner nuclear layer. Label may be present in the pigment epithelium but could not be established beyond doubt. This pattern of labelling is in accordance with previous observations on D2 receptor localization by means of radioactive ligand binding and in situ hybridization techniques. It suggests that retinal dopamine acts as a neuromodulator as well as a transmitter. In the distal retina, it may reach its targets via diffusion over considerable distances, even crossing the outer limiting membrane; in the inner and outer plexiform layers, conventional synaptic transmission seems to coexist with paracrine addressing of more distant targets, and D2 receptors are expressed by both amacrine and ganglion cells.

Dopamine enhances both electrotonic coupling and chemical excitatory postsynaptic potentials at mixed synapses

The transmitter dopamine reduces electrotonic coupling between retinal horizontal cells and increases their sensitivity to glutamate. Since in other systems single afferents establish mixed electrotonic and chemical excitatory synapses with their targets, dopamine might be expected there to depress one component of excitation while enhancing the other. This hypothesis was tested by applying dopamine locally in the vicinity of the lateral dendrite of the goldfish Mauthner cell (M cell) and monitoring the composite electrotonic and chemical excitatory postsynaptic potentials and currents evoked by ipsilateral eighth nerve stimulation. Dopamine produces persistent enhancements of both components of the postsynaptic response while it also increases input conductance. All these dopamine actions are prevented by superfusing the brain with saline containing the dopamine D1 receptor antagonist SCH-23390. Postsynaptic injections of the cAMP-dependent protein kinase inhibitor (Walsh inhibitor, or PKI5-24) block the dopamine-induced changes in synaptic transmission, implicating a cAMP-dependent mechanism. Furthermore, there is a dopaminergic innervation of the M cell, as demonstrated immunohistochemically with antibodies against dopamine and the rate-limiting enzyme in its synthetic pathway, tyrosine hydroxylase. Varicose immunoreactive fibers lie in the vicinity of the distal part of the lateral dendrite between the large myelinated club endings that establish the mixed synapses. As determined with electron microscopy, the dopaminergic fibers contain small vesicles, and they do not have synaptic contacts with either the afferents or the M cell, remaining instead in the synaptic bed. Taken together, these results suggest that dopamine released at a distance from these terminals increases the gain of this primary sensory input to the M cell, most likely through a phosphorylation mechanism.

Activation of a D2 receptor increases electrical coupling between retinal horizontal cells by inhibiting dopamine release

In the fish retina, interplexiform cells release dopamine onto cone-driven horizontal cells. Dopamine decreases the electrical coupling between horizontal cells by activating adenylate cyclase through dopamine D1 receptors. Using intracellular recording, we have studied the effect of dopamine D2 receptor activation on horizontal cell electrical coupling in the intact goldfish retina. Superfusion of the D2 agonist LY171555 (quinpirole; 0.2-10 microM) increased horizontal cell coupling, as indicated by a decrease in responses to centered spots or slits of light. The length constant of the horizontal cell network increased an average of 31%. Although dopamine (0.5-20 microM) uncoupled horizontal cells, lower concentrations (e.g., 0.2 microM) initially uncoupled and then subsequently increased coupling beyond initial control levels. The coupling effect of LY171555 (10 microM) was blocked completely by prior application of the D1 agonist SKF 38393 at saturating (20 microM) or nonsaturating (2.5-5.0 microM) doses. Prior treatment of the retinas with 6-hydroxydopamine, which destroyed dopaminergic neurons, eliminated the coupling effect of LY171555 but not the uncoupling effect of SKF 38393. These results suggest that goldfish horizontal cells contain D1, but not D2, receptors and that dopamine activation of D2 autoreceptors on interplexiform cells inhibits dopamine release onto horizontal cells so that the electrical coupling between horizontal cells increases.

Dopamine decreases receptive field size of rod-driven horizontal cells in carp retina

Receptive field size of rod-driven horizontal cells (HCs) in the carp retina was measured by the spread of responses to the slit of light stimulus with changing the distance from the recording electrode and it was found to decay with a single exponential function. By perfusing 10 microM dopamine (DA) the length constant of rod-driven HCs was reduced to half and the response amplitude in the centre increased approximately two-fold, and the input resistance was markedly increased. This suggests that DA as a neuromodulator released from interplexiform cells could decouple the rod-driven HCs which had no direct synaptic contact with the interplexiform cells.

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.

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.

Glutamate antagonists that block hyperpolarizing bipolar cells increase the release of dopamine from turtle retina

Some neurochemical features of the neuronal circuitry regulating dopamine release were examined in the retina of the turtle, Pseudemys scripta elegans. Glutamate antagonists that block hyperpolarizing bipolar cells, such as 2,3 piperidine dicarboxylic acid (PDA), produced dose-dependent dopamine release. In contrast, the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), which blocks depolarizing bipolar cell responses with high specificity, had no effect on the release of dopamine. The gamma-aminobutyric acid (GABA) antagonist, bicuculline, also produced potent dose-dependent release of dopamine. The release of dopamine produced by PDA was blocked by exogenous GABA and muscimol, suggesting that the PDA-mediated release process was polysynaptic and involved a GABAergic synapse interposed between the bipolar and dopaminergic amacrine cells. The only other agents that produced dopamine release were chloride-free media and high extracellular K+; in particular, kainic acid and glutamate itself were ineffective. These results suggest that the primary neuronal chain mediating dopamine release in the turtle retina is: cone----hyperpolarizing bipolar cell----GABAergic amacrine cell----dopaminergic amacrine cell.

Contacts of dopaminergic interplexiform cells in the outer retina of the blue acara

Dopaminergic interplexiform cells in the retina of the blue acara were investigated using an antiserum against tyrosine hydroxylase and PAP visualization. In whole-mount preparations, we observed a homogeneous distribution of cell bodies throughout the retina without any indication of regional specialization. At the fine and ultrastructural level, we studied the morphology of labeled telodendria within the outer plexiform layer. Apart from contacts with horizontal cell perikarya and bipolar cell dendrites, we observed direct contacts, mostly in the form of close appositions, with cone pedicles and rod spherules. Quantitative evaluation and reconstruction of serial sections showed that all cone pedicles and most rod terminals were approached in this way. The dopaminergic pathway terminating on horizontal cells and photoreceptors is discussed with respect to the localization of dopamine receptors in the outer retina, and the control of adaptive changes such as retinomotor movements, spinule formation, and horizontal cell coupling.

Morphology of bipolar cells labeled by DAPI in the rabbit retina

The morphology, distribution, and coverage of certain cone bipolar cell types were investigated in rabbit retina. Brief in vitro incubation of isolated rabbit retina in the fluorescent dye 4,6-diamino-2-phenylindole labeled only a few cell types in the inner nuclear layer. Intracellular injection of Lucifer Yellow into these types showed them to be horizontal cells and cone bipolar cells. All stained bipolar cells ramified in sublamina a of the inner plexiform layer (IPL) and formed three classes. Two types ranged from 20 to 60 microns in diameter in both plexiform layers; the other large bipolar cell was 40-70 microns in diameter in the outer plexiform layer (OPL) and up to 150 microns in diameter in the IPL. The brightest type was narrowly stratified in the outer portion of sublamina a. Its density increased from about 500 cells/mm2 in the periphery to about 2,500 cells/mm2 in the visual streak. Staining of neighboring cells of this type showed that processes in the IPL rarely crossed, but often converged at a common site so as to impart a "honeycomb" appearance to a single sublayer of retina. The other small bipolar cell was similar in density and coverage, but stratified diffusely throughout sublamina a. The large bipolar cell stratified narrowly in the distal portion of sublamina a and was more sparsely distributed. Whether determined by staining adjacent cells or by density vs. area calculations, coverage in the OPL approached 1 for each type, as did coverage in the IPL for the two types with narrow fields.

Effects of background illuminations on the receptive field size of horizontal cells in the turtle retina are mediated by dopamine

Intracellular recordings from luminosity-type horizontal cells of the turtle retina were used to analyze the effects of steady and flickering background illumination on the size of their receptive fields. Both types of background illumination reduce the size of the receptive field to about the same extent. The reduction seems largely due an increase in the coupling resistance between horizontal cells. The effects of both types of background illumination are sensitive to the dopamine antagonist fluphenazine. This suggests that steady and flickering illuminations stimulate the release of endogenous dopamine.

Responses of cat horizontal cells to sinusoidal gratings

The spatiotemporal properties of cat horizontal (H-) cells were studied by recording the intracellular responses in the optically intact, in vivo, eye to sinusoidal gratings at a photopic mean illumination level. In order to investigate the linearity of spatial summation a "null test" was performed in which the responses to contrast reversal gratings were measured at different positions of the grating relative to the receptive field. Spatial and temporal transfer functions were measured using drifting sinusoidal gratings of variable spatial and temporal frequencies. The amplitudes of cat H-cell responses to contrast reversal gratings modulated with a square wave time-course showed a sinusoidal dependence on spatial phase. When zero crossings of the grating were lined up with the receptive field center, as defined by the maximum of the measured line weighting function, contrast reversal produced no response modulation. This result did not depend on the spatial frequency of the grating or the temporal frequency of contrast modulation over substantial ranges. The response waveform was found not to depend on the spatial phase of the grating. The spatial transfer function of cat H-cells has low-pass characteristics with a cut-off frequency in the range of about 0.4-1.5 c/deg. The shape of the spatial transfer function was roughly the same for temporal frequencies ranging from 3 to 10 Hz. The temporal transfer function exhibited band-pass characteristics with a maximum response amplitude at 3-6 Hz. The amplitude fall-off for low and high temporal frequencies was independent of the spatial frequency of the grating. The results obtained with sine gratings were found not to agree with the receptive field profiles measured with narrow slits flashed at different positions in the receptive field.

Modulation of connexon densities in gap junctions of horizontal cell perikarya and axon terminals in fish retina: effects of light/dark cycles, interruption of the optic nerve and application of dopamine

In the fish retina, connexon densities of gap junctions in the outer horizontal cells are modulated in response to different light or dark adaptation times and wavelengths. We have examined whether the connexon density is a suitable parameter of gap junction coupling under in situ conditions. Short-term light adaptation evoked low connexon densities, regardless of whether white or red light was used. Short-term dark adaptation evoked high connexon densities; this was more pronounced in the axon terminal than in perikaryal gap junctions. Under a 12 h red light/12 h dark cycle, a significant difference in connexon densities between the light and the dark period could be established in the gap junctions of the perikarya and axon terminals. Under a white light/dark cycle, only the gap junctions of axon terminals showed a significant difference. Crushing of the optic nerve resulted in an increase in connexon densities; this was more pronounced in axon terminals than in perikarya. Dopamine injected into the right eye of white-light-adapted animals had no effect. However, dopamine prevented the effect of optic-nerve crushing on connexon density. The reaction of axon-terminal gap junctions to different conditions thus resembles that of perikaryal gap junctions, but is more intense. Axon terminals are therefore thought to play an important role in the adaptation process.

Effects of dopaminergic and noradrenergic mechanisms on the neuronal activity of the isolated pineal organ of the trout, Oncorhynchus mykiss

The effects of exogenous applied catecholamines on the neuronal activity of ganglion cells of the luminance type (achromatic cells) were investigated in the photosensitive pineal organ of the trout, Oncorhynchus mykiss. Extracellular recordings were performed on neurons of the superfused isolated pineal organ. Addition of dopamine to the superfusion medium increased the spontaneous activity of more than 60% of the achromatic neurons (n = 25). The D1-dopamine antagonist SCH-23390 and D2-dopamine antagonist spiperone reversed the dopamine-induced stimulation of ganglion cells and inhibited their maintained activity, which suggests that dopamine acts via both D1- and D2-receptors. Norepinephrine, the beta-adrenergic agonist isoproterenol, and DOPA enhanced the spontaneous activity of most of the ganglion cells, whereas the beta-antagonist propranolol depressed the discharge rate and reversed the action of isoproterenol. This suggests that catecholamines might play a modulatory role in the regulation of the neural activity of pineal luminance neurons.

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)

Ultrastructure of electrical synapses: review

This article reviews studies providing information on the ultrastructure of electrical synapses. Although the review focuses on electron-microscopic investigations, its aim is to examine how the structure of an electrical synapse relates to its function. It begins by presenting a historical overview of the early studies which were responsible for the recognition of electrical synapses. The structure of gap junctions which are the morphological correlates of electrical synapses is illustrated and the ultrastructure and function of the two types of electrical synapse, rectifying and non-rectifying, described. Recent papers investigating the ultrastructure of electrical and mixed electrical-chemical synapses in invertebrates and vertebrates are reviewed. For earlier references, the reader is directed to previous reviews on the subject. Much new information, however, on the structure and formation of electrical synapses has been obtained from work on cultured neurons and from electron-microscopic, immunocytochemical, conformational and molecular studies. This article reviews those studies and in light of their findings, re-examines the relationships of the structure of electrical synapses with their function.

Effects of 2-amino-4-phosphonobutyric acid on cells in the distal layers of the tiger salamander's retina

1. We studied the effects of 2-amino-4-phosphonobutyric acid (APB) on the response properties of rods, horizontal cells and bipolar cells in the isolated, perfused retina of the tiger salamander, Ambystoma tigrinum. A concentration of 100 microM was found to be sufficient to elicit maximal effects. 2. Rods hyperpolarized slightly upon exposure to 100 microM-APB and their response amplitudes were slightly reduced. The amplitude of the cone-generated component of the rod's response to 700 nm light was not significantly affected by APB. 3. Horizontal cells hyperpolarized by 2-5 mV upon exposure to 100 microM-APB. The rod-driven component of the horizontal cell response increased in amplitude while the cone-driven component decreased in amplitude. APB thus causes an increase in voltage gain between rods and horizontal cells and a decrease in cone/horizontal cell gain. These findings can be explained in terms of an APB-induced reduction in transmitter release from the cones. 4. APB at a concentration of 100 microM caused an increase in the length constant of the horizontal cell syncytium. Our analysis shows this to be due primarily to a 50% reduction in the coupling impedance between the cells of the syncytium. 5. The effects of APB on off-centre bipolar cells were qualitatively similar to those on horizontal cells. APB increased the amplitudes of rod-driven responses and reduced those of cone-driven responses. The length constants, both of the receptive field centre and of the surround, were increased and the strength of the surround relative to the centre was reduced by about 20%. 6. APB abolished the depolarizing light responses of the receptive field centres of on-centre bipolar cells. A hyperpolarizing response remained whose spatial properties were similar to those of the receptive field surround. We believe this response to reflect a direct (feedforward) input to on-centre bipolar cells from horizontal cells.

Effects of light stimuli on the release of dopamine from interplexiform cells in the white perch retina

Interplexiform cells are centrifugal neurons in the retina carrying information from the inner to the outer plexiform layers. In teleost fish, interplexiform cells appear to release dopamine in the outer plexiform layer after prolonged darkness that modulates the receptive-field size and light responsiveness of horizontal cells (Mangel & Dowling, 1985; Yang et al., 1988a, b). It has been proposed that interplexiform cells may also release dopamine upon steady illumination because horizontal cells' receptive fields shrink in the light (Shigematsu & Yamada, 1988). Here, we report the shrinkage of the receptive fields of horizontal cells seen in the presence of background illumination is not blocked by dopamine antagonists, indicating that dopamine does not underlie the receptive-field size changes observed during steady illumination. Flickering light, however, does appear to stimulate the release of dopamine from the interplexiform cells, resulting in a marked reduction of horizontal cell receptive-field size. Taken together, experiments on horizontal cells indicate that dopamine is released from interplexiform cells in the teleost retina after prolonged darkness and during flickering light, but that dopamine release from interplexiform cells during steady retinal illumination is minimal.

The organization of dopaminergic neurons in vertebrate retinas

A survey of the shapes of dopaminergic (DA) neurons in the retinas of representative vertebrates reveals that they are divisible into three groups. In teleosts and Cebus monkey, DA cells are interplexiform (IPC) neurons with an ascending process that ramifies to create an extensive arbor in the outer plexiform layer (OPL). All other vertebrates studied, including several primate species, have either DA amacrine cells or IPCs with an ascending process that either does not branch within the OPL or does so to a very limited degree. DA neurons of non-teleosts exhibit a dense plexus of fine caliber fibers which extends in the distal most sublamina of the inner plexiform layer (IPL). Teleosts lack this plexus. In all vertebrates, DA cells are distributed more or less evenly and at a low density (10-60 cells/mm2) over the retinal surface. Dendritic fields of adjacent DA neurons overlap. Most of the membrane area of the DA cell is contained within the plexus of fine fibers, which we postulate to be the major source of dopamine release. Thus, dopamine release can be modeled as occurring uniformly from a thin sheet located either in the OPL (teleosts) or in the distal IPL (most other vertebrates) or both (Cebus monkey). Assuming that net lateral spread of dopamine is zero, the fall of dopamine concentration with distance at right angles to the sheet (i.e. in the scleral-vitreal axis) will be exponential. The factors that influence the rate of fall-diffusion in extracellular space, uptake, and transport--are not yet quantified for dopamine, hence the dopamine concentration around its target cells cannot yet be assessed. This point is important in relation to the thresholds for activation of D1 and D2 dopamine receptors that are found on a variety of retinal cells.

The rod circuit in the rabbit retina

Mammalian retinae have a well-defined neuronal pathway that serves rod vision. In rabbit retina, the different populations of interneurons in the rod pathway can be selectively labeled, either separately or in combination. The rod bipolar cells show protein kinase C immunoreactivity; the rod (AII) amacrine cells can be distinguished in nuclear-yellow labeled retina; the rod reciprocal (S1 & S2) amacrine cells accumulate serotonin; and the dopaminergic amacrine cells show tyrosine-hydroxylase immunoreactivity. Furthermore, intracellular dye injection of the microscopically identified interneurons enables whole-population and single-cell studies to be combined in the same tissue. Using this approach, we have been able to analyze systematically the neuronal architecture of the rod circuit across the rabbit retina and compare its organization with that of the rod circuit in central cat retina. In rabbit retina, the rod interneurons are not organized in a uniform neuronal module that is simply scaled up from central to peripheral retina. Moreover, peripheral fields in superior and inferior retina that have equivalent densities of each neuronal type show markedly different rod bipolar to AII amacrine convergence ratios, with the result that many more rod photoreceptors converge on an AII amacrine cell in superior retina. In rabbit retina, much of the convergence in the rod circuit occurs in the outer retina whereas, in central cat retina, it is more evenly distributed between the inner and outer retina.

Retinal neuromodulation: the role of dopamine

Dopamine exerts multiple effects on retinal horizontal cells. Dopamine, via cyclic AMP and protein kinase A, reduces the light responsiveness of horizontal cells and the electrical coupling between the cells. The gating kinetics of both gap-junctional and glutamate channels are altered as a result of phosphorylation by protein kinase A. Dopamine also causes a reversible retraction of neurites of horizontal cells maintained in culture. Diacylglycerol analogues as well as phorbol esters mimic this effect of dopamine, but not cyclic AMP analogues or Forskolin. The results suggest that dopamine causes neurite retraction by the activation of protein kinase C via diacylglycerol.

Synaptic organization of type 2 catecholamine amacrine cells in the rhesus monkey retina

Two types of amacrine cell immunoreactive for tyrosine hydroxylase, the rate-limiting enzyme in the catecholamine synthetic pathway, are present in the retina of the rhesus monkey, Macaca mulatta. The well-known dopaminergic, or type 1 catecholamine amacrine cells have relatively large cell bodies almost exclusively in the inner nuclear layer with processes that densely arborize in the outermost stratum of the inner plexiform layer and fine, radially-oriented fibres in the inner nuclear layer. Type 2 catecholamine amacrine cells, in contrast, have smaller cell bodies in the inner nuclear layer, the inner plexiform layer and the ganglion cell layer, and have sparsely-branching processes ramifying in the centre of the inner plexiform layer. Although type 2 catecholamine cells are more numerous than type 1 catecholamine amacrines, type 2 cells contain less than one-third the amount of tyrosine hydrolase as the type 1 cells. Electron microscopy of retinal tissue immunoreacted for tyrosine hydrolase by the peroxidase-antiperoxidase method revealed synaptic input from amacrine cells at conventional synapses, and bipolar cells at ribbon synapses onto the type 2 catecholamine amacrine cells. Curiously, although the synaptic input is comparatively easily found, the output synapses, or synapses of the type 2 catecholamine amacrine cells onto other neuronal elements, are rarely found. Some synapses of the type 2 catecholamine cells onto non-immunoreactive amacrine cells have been identified, however. This unusual pattern of synaptic organization, with many identifiable input synapses but few morphologically characterizable output synapses, suggests a paracrine function for the dopamine released by the type 2 catecholamine amacrine cells in the primate retina.

An interplexiform cell in the goldfish retina: light-evoked response pattern and intracellular staining with horseradish peroxidase

The light-evoked response pattern and morphology of one interplexiform cell were studied in the goldfish retina by intracellular recording and staining. The membrane potential of the cell spontaneously oscillated in the dark. In response to a brief light stimulus, the membrane potential initially gave a slow transient depolarization. During maintained light, the oscillations showed a tendency to be suppressed; the response of the cell to the offset of the stimulus was not so prominent. The perikaryon of the interplexiform cell was positioned at the proximal boundary of the inner nuclear layer. The cell had two broad layers of dendrites; one was diffuse in the inner plexiform layer, the other was more sparse in the outer plexiform layer. The morphological and electrophysiological characteristics of the cell are discussed in relation to dopaminergic interplexiform cells and the light-evoked release pattern of dopamine in the teleost retina.

Dopaminergic interplexiform cells and centrifugal fibres in the Xenopus retina

Putative dopaminergic neurons in the Xenopus retina were identified using an immunoreaction against tyrosine hydroxylase. A single class of cell was stained whose perikaryon (12-15 microns in diameter) was located at the border of the inner nuclear and inner plexiform layers. About 2% of the stained cell bodies were located in the ganglion cell layer, but the distribution of the processes of displaced cells had the same geometry as for the majority of stained cells. Tyrosine hydroxylase-like immunoreactive perikarya gave rise to one to four stout processes that descended to the most proximal level of the inner plexiform layer, within which they branched repeatedly to generate a diffuse network of fine processes. Secondary branches ascended to the most distal sublayer of the inner plexiform layer where they ramified into fine processes that joined other fibres arising horizontally from the cell body and confined to the distal inner plexiform layer throughout their course. The diameter of the dendritic arbor of stained cells was in the range of 350-600 microns. The dense network of fine fibres within the distal inner plexiform layer was arrayed in rings that surrounded other amacrine cells; using an antiserum against glycine we found that at least some of these were glycinergic neurons. Most tyrosine hydroxylase-positive neurons emitted one or two fine ascending processes that arose from the perikaryon, traversed the inner plexiform layer and arborized within the outer plexiform layer. Additionally, fine varicose fibres arising from the sublayer 1 of the inner plexiform layer and running to the outer retina were observed. Thus, based on light microscopic criteria, dopaminergic neurons in the Xenopus retina appeared to be interplexiform cells. A few tyrosine hydroxylase-immunoreactive fibres were observed in the optic nerve, some of which entered the inner retina where they ramified, thus indicating that they were centrifugal axons. In addition, a small number of stout smooth processes were observed to traverse the entire inner nuclear layer and course laterally at the level of the photoreceptor bases. Whether this second class of ascending process arises from the tyrosine hydroxylase-like immunoreactive efferents remains to be determined. The total number of dopaminergic neurons per retina was 750-800, equivalent to an average density of 30 cells mm-2. The dendritic fields of adjacent cells strongly overlapped, with an estimated coverage factor of 4.8.

Dopamine and plasticity of horizontal cell function in the teleost retina: regulation of a spectral mechanism through D1-receptors

The negative feed-back interaction between horizontal cells (HCs) and cones in the cyprinid fish retina is thought to be mediated by horizontal cell spinules. These are "plastic" structures, largely absent from the dark-adapted retina and formed anew during light adaptation. We have previously shown that horizontal cell feed-back is similarly enhanced by light adaptation. The role of the interplexiform cell transmitter dopamine in both processes has been studied in the roach retina. Application of dopamine to dark-adapted retinae induced spinule formation in a dose-dependent way. The effect of dopamine was mimicked by dibutyryl-cAMP and suppressed selectively by D1 receptor antagonists. The effect of light in inducing spinule formation was lost in retinae depleted of endogenous dopamine. However, application of exogenous dopamine to these retinae triggered normal spinule formation. For all pharmacological treatments used, there was a strong correlation between spinule number and degree of feed-back activity in biphasic horizontal cells. Thus, when the spinule content of the cone pedicles was high, biphasic horizontal cell responses exhibited strong depolarizing components and vice versa. It is concluded that light-evoked formation of spinules in HC dendrites involves the action of dopamine upon D1 receptors. Spinules, in turn, are likely to be presynaptic terminals mediating the dynamic negative feed-back effect of horizontal cells upon cones.

Synaptic inputs to turtle horizontal cells analyzed after blocking of gap junctions by intracellular injection of cyclic nucleotides

Intracellular injection of cAMP or cGMP into turtle horizontal cells significantly increased the input resistances, and the cells could thus be easily polarized by current injection, suggesting that the cyclic nucleotides blocked gap junctions between cells. Then, synaptic inputs onto triphasic chromaticity-type cells were analyzed. Hyperpolarizing and depolarizing light responses were all reduced with depolarizing current, and their polarities were reversed by further depolarization. Their reversal potentials coincided at around 0 mV. This level was the same as observed in luminosity-type and biphasic chromaticity-type cells, suggesting that the ionic mechanisms of synaptic transmission are common among horizontal cell types.

Voltage clamp study of electrophysiologically-identified horizontal cells in carp retina

Passive membrane properties and electromotive force of light modulated currents of L-, R/G-type and rod-driven horizontal cells were studied by voltage-clamp using double-barrelled micro-electrodes whilst perfusing with 5 microM dopamine to uncouple the gap junctions. Input impedances of horizontal cells in darkness were 31 +/- 1.4 M omega (mean +/- SE, n = 63); the resting potentials were -37 +/- 1.3 mV. Current-voltage relationships had regions of both inward and outward rectification and a region of negative resistance was commonly observed. Reversal potentials of light modulated currents were estimated on average to be -7 +/- 4 mV (n = 14), which is consistent with the involvement of K+ and Na+ and/or Ca2+ gradients. Importantly in R/G cells both depolarizing and hyperpolarizing components of the response had essentially the same reversal potential.

Is dopamine involved in the generation of the light peak in the intact chicken eye?

The implication for dopamine (DA) in the modulation of the standing potential (SP) and the light peak (LP) was tested in intact chickens using an indirect EOG method. After an intravenous or intravitreal injection of DA, a transient, dose-dependent increase in the SP was observed. The LP, recorded after an intravenous injection, was preserved. But after an intravitreal injection, the LP was strongly reduced or even abolished depending on the dose of DA, whereas the photoreceptor response was unchanged. The data supports the hypothesis that the light peak, which is generated by a neural retina-pigment epithelium interaction, could be triggered by dopamine released at light onset from the inner retinal layers.

The relationship between light, dopamine release and horizontal cell coupling in the mudpuppy retina

1. The effect of different experimental conditions on electrical coupling between horizontal cells in the mudpuppy retina was studied by comparing the changes in responses to illumination of the central and peripheral portions of the receptive field, using centred spot and annulus stimuli. An increase in the amplitude of the response to a centred spot stimulus and a decrease in the amplitude of the response to a concentric annulus indicated a decrease in coupling, and vice versa. 2. Dopamine (10-250 microM) caused a decrease in coupling between horizontal cells. The uncoupling effect of dopamine was much greater in dark-adapted than in light-adapted retinas. The effect of the D1-receptor agonist SKF38393 was similar to that of dopamine. The effect of the D2-receptor agonist LY171555 on coupling was opposite to that of dopamine; this was attributed to a reduction in endogenous dopamine release. 3. The D1 antagonist SCH23390 (15 microM) caused an increase in coupling between horizontal cells. This effect was much greater in light-adapted than in dark-adapted retinas. 4. The glutamate analogue 2-amino-4-phosphonobutyrate (APB), which hyperpolarizes on-centre bipolar cells and blocks their responses to light, caused an increase in coupling between horizontal cells. This effect of APB was greater in light-adapted retinas than in dark-adapted retinas. The effect of APB on coupling could be reversed by the addition of dopamine, but the effect of dopamine on coupling could not be reversed by the addition of APB. These results suggest that APB increases horizontal cell coupling by causing a decrease in dopamine release. 5. In dark-adapted retinas, 2.5 min exposure to an adapting light caused a decrease in coupling between horizontal cells; the uncoupling effect of the adapting light was blocked in the presence of either SCH23390 or APB. 6. The results suggest that coupling between horizontal cells in the mudpuppy retina is decreased by dopamine acting at D1 receptors, that the release of dopamine affecting horizontal cells is greater under light-adapted conditions, and that the pathway by which exposure to light increases this dopamine release is mainly via on-centre bipolar cells.

Horizontal cell axon terminals in growing goldfish

In the retina of teleost fish, cone horizontal cell axons penetrate the inner nuclear layer, where they enlarge into fusiform terminal swellings. The present study shows that horizontal cell axon terminals enlarge disproportionately during postembryonic growth of the retina in juvenile and adult goldfish: the relative volume of axon terminals increases almost 20-fold, while the volume of the entire retina increases only about fourfold during a 2-3-yr period. The enlarging axon terminals fill in the gaps created as the numerical density of nuclei in the inner nuclear layer falls. Horizontal cell axon terminals are thought to participate in cone-dominated visual pathways, although their precise role is unclear. The results of this study suggest that a comparison of horizontal cell function in small and large fish might help to resolve this issue.

Endogenous dopamine and cyclic events in the fish retina, II: Correlation of retinomotor movement, spinule formation, and connexon density of gap junctions with dopamine activity during light/dark cycles

In the fish retina, retinomotor movement, spinule formation, and alteration of connexon density within gap junctions occur in response to changes in ambient light conditions. All of these morphological parameters can also be influenced by the application of dopamine. This study examines whether the morphological alterations of these structures are correlated with the activity of endogenous dopamine during an entrained 12-h light/12-h dark cycle and after 1-h sort-term adaptation periods. The two measured parameters of retinomotor movement, cone inner segment length and pigment dispersion, were well-correlated with endogenous cyclic dopamine activity. However, retinomotor movement was initiated already at the end of the entrained dark period, before the onset of light and before the onset of dopamine turnover. Furthermore, a 1-h dark-adaptation period in the middle of the light phase reduced dopamine activity but did not affect retinomotor movement. At the switch from light to dark and after a 1-h light period at midnight retinomotor movement correlated exactly with dopamine turnover and illumination conditions. The formation of spinules was correlated with dopaminergic activity during all phases of the light/dark cycle and during short-term adaptation periods. Spinules were expressed in the light when dopamine activity was high and they were retracted when dopamine activity was reduced during darkness. Connexon density of horizontal cell gap junctions showed a weaker correlation with the endogenous dopamine turnover. In this case, a high activity of endogenous dopamine was paralleled by a high density of connexons. Our results suggest that endogenous dopamine is involved in the cyclic regulation of the observed morphological alterations and that dopamine is part of the light signal for these mechanisms.

Slow light and dark adaptation of horizontal cells in the Xenopus retina: a role for endogenous dopamine

A role for endogenous dopamine in the control of rod and cone contributions to a second-order retinal neuron, the horizontal cell (HC) was studied in the Xenopus retina. Relative rod and cone contributions were estimated from HC responses to scotopically balanced 491- and 650-nm flashes. In eyecups prepared in light then placed in darkness, cone input to the HC slowed and diminished on a time scale of hours. The decline in cone input was balanced by a slow growth of rod input to the HC. Administration of D-amphetamine, a dopamine releasing agent, restored the light-adapted waveform. The kinetics of slow light adaptation were examined by recording HC responses from eyecups that had been dark-adapted previously for 11-14 h. When test flashes fell on a dark field, cone input to the HC grew for 2-4 h, reached a plateau, and later declined. If, however, flashes were superimposed on a weak background field, cone input to the HC continued to increase monotonically at about 10%/h. This increase was abolished by superfusion with a nonspecific dopamine receptor blocker, cis-flupenthixol (50 microM), resulting in the complete suppression of cone-to-horizontal cell synaptic transfer and the enhancement of rod-to-horizontal cell communication. Subcutaneous injection of reserpine, a drug that depletes dopamine stores (2 mg/kg on 1-4 successive days), or intraocular injection of the dopamine neurotoxin, 6-hydroxydopamine (10-30 micrograms) slowed and reduced the amplitude of cone input to the HC, even in completely light-adapted eyes.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrooculographic and electroretinographic study in the chicken after dopamine and haloperidol

The implication of dopamine in the modulation of the standing potential of the eye was tested in the chicken through an indirect electrooculographic method and direct current electroretinogram (ERG) recording after haloperidol, a mixed D1-D2 antagonist. The standing potential of the eye was reduced within 15 min after intravitreal injection of the antagonist (150 micrograms). This effect is rapidly reversed by an application of dopamine. The fast oscillation was preserved but the light peak was either strongly reduced or abolished. The dark trough showed an apparently normal time course. The intensity-voltage function was studied for the various ERG components. After haloperidol the b-wave and the c-wave were strongly reduced, whereas the a-wave was little affected. Together with previous data obtained with intraocular injections of dopamine, our data suggest the involvement of dopamine in the modulation of the standing potential. They also support the hypothesis that the light peak, which is generated by a photoreceptor-pigment epithelium interaction, is influenced by dopamine or by a related substance. The modulatory effect could also be due to a balance between several neurotransmitter systems.

Close tip-to-tip contacts between dendrites of transient amacrine cells in carp retina

In isolated retinas of the carp (Cyprinus carpio) placed receptor side up in a plastic chamber, a subclass of amacrine cells, generating a fast ON-OFF transient response to spot and annular light stimuli, were intracellularly recorded and injected with a fluorescent dye, Lucifer yellow (LY). After brief fixation of the same preparations in aldehyde solution, the retinas were wholemounted vitreous side up in a tissue chamber. Under a fluorescence microscope, one LY-injected cell and several dye-coupled cells were seen; these cells belonged to type Fnd, having a fusiform soma, narrow dendritic field and bistratified dentrites in the inner plexiform layer (IPL). To reveal the interconnections between dendrites, one of such dye-coupled cells was further injected with LY. Close tip-to-tip contacts were predominantly found between dendrites of neighboring type Fnd cells in sublaminae a and b of the IPL, respectively.

Interplexiform cells of the goldfish retina

Dopaminergic and glycinergic interplexiform cells (IPCs) in the goldfish retina were impregnated by using two new Golgi protocols. The two cell types have markedly different morphological characteristics: Dopaminergic IPCs have primary dendrites that descend into and stratify in the inner plexiform layer, where they give rise to processes that project to the outer plexiform layer. Conversely, glycinergic IPCs have primary dendrites that ascend to the outer plexiform layer and from this dendritic arbor, many processes then project into the inner plexiform layer. The apparent coverage of dopaminergic IPCs is almost four times that of glycinergic IPCs. Even so, the coverage of each glycinergic IPC in the outer plexiform layer allows it to provide an accurate copy of the S-space to the inner plexiform layer. Considering the known GABAergic and glycinergic synaptologies in the inner plexiform layer, the glycinergic IPC must form a major element in the retinal circuitry of the goldfish.

Visual deficits related to dopamine deficiency in experimental animals and Parkinson's disease patients

In patients affected by Parkinson's disease, and in the monkey model of this disease, visual defects have been shown using psychophysical and electrophysiological measures of spatial and temporal contrast sensitivity. These studies imply an essential role for dopamine in primate vision. There is electrophysiological and neurochemical evidence to suggest that at least part of the problem is impaired retinal processing caused by systemic dopaminergic deficiency. Some of the deficits that have been demonstrated, consistent with physiological studies, suggest that center-surround interaction of neurons may suffer as a consequence of dopaminergic deficiency. The role of the regulation of retinal dopamine (D1 and D2) receptors in primate vision and of the balance of these receptors in presynaptic dopaminergic deficiency is not yet determined. Using sinusoidal grating stimuli in cognitively loaded tasks may increase understanding of the behavioral consequences of visual deficits seen in dopamine deficiency syndromes.

Desensitization of the dopaminergic system in bovine retina following incubation with high potassium

The effect of potassium depolarization on dopamine D1 receptor activity in bovine retina was investigated. Preincubation of bovine retinas in buffer containing high KCl (56 mM) as compared to a low KCl control buffer resulted in a significant decrease in dopamine-stimulated adenylate cyclase activity with no change in basal or GTP-stimulated adenylate cyclase activity. The apparent Vmax for dopamine was decreased from 102 +/- 15 pmol/min/mg protein in retinas preincubated in high KCl to 71 +/- 11 pmol/min/mg protein in control retinas (n = 5). The apparent Ka for dopamine stimulation of the enzyme did not change. The potassium-induced desensitization could be blocked by preincubation with the dopamine antagonist cis-flupenthixol suggesting that the desensitization was caused by the release of dopamine. The rapid desensitization was not accompanied by a change in D1 receptor density as assessed by binding of [3H]SCH23390 nor in agonist binding as assessed by competition of the selective D1 agonist, SKF38393, for [3H]SCH23390 binding. The potassium-induced desensitization was mimicked by preincubation of retinas in control medium containing isobutylmethylxanthine or dibutyryl cyclic AMP. Incubation of retinas in 56 mM KCl also led to a decrease in activation of adenylate cyclase by vasoactive intestinal polypeptide. These results strongly suggest that potassium depolarization leads to a very rapid heterologous desensitization of adenylate cyclase in bovine retinas.

GABA and tyrosine hydroxylase immunocytochemistry reveal different patterns of colocalization in retinal neurons of various vertebrates

Colocalization of GABA- and tyrosine hydroxylase-like immunoreactivity was studied in the retinae of various vertebrate species in order to ascertain whether the presumed coexistence of GABA and dopamine, reported earlier for mammals (Kosaka et al.: Exp. Brain Res. 66:191-210, '87: Wässle and Chun: J. Neurosci. 8:3383-3394,'88) is a common phenomenon. GABA-immunopositive cells constituted a separate population from tyrosine hydroxylase-positive cells in fish and amphibians, whilst in higher--i.e., amniote--vertebrates, such as reptiles, birds, and mammals, all dopaminergic cells contained GABA-like immunoreactivity. No clear correlation was found between the type of dopaminergic cell (amacrine/interplexiform) and the presence or absence of colocalization.

ATP directly affects junctional conductance between paired ventricular myocytes isolated from guinea pig heart

Effects of ATP on junctional conductance (gj) were investigated in paired ventricular myocytes isolated from guinea pig hearts. One cell of the pair was voltage-clamped with a single-patch pipette, and gj was measured after the perforation of the nonjunctional membrane of the partner cell. The current-voltage relation of gj was linear between -30 and +30 mV. The control gj at 5.0 mM ATP in 88 pairs of cells ranged from 100 to 1,055 nS (average, 268 nS). ATP within the range from 0.1 to 5.0 mM increased gj in a dose-dependent manner. The Hill coefficient was 2.6, and the half-maximum effective concentration of ATP was 0.68 mM. Adenylylimidodiphosphate (2 mM) caused a transient increase in gj in the presence of 0.5 mM ATP, but forskolin (30 microM), cyclic AMP (50 microM), catalytic subunit of cyclic AMP-dependent protein kinase (1 microM), and ADP (10 mM) had no significant effect on gj. The temperature coefficient of gj in the presence of 5.0 mM ATP was 1.29. These findings suggest that gj in paired ventricular myocytes is directly regulated by ATP probably through a specific ligand-receptor interaction between ATP and gap junctional channel protein.

Retinal neurotransmitter interaction as reflected in horizontal cell spatial behaviour

The effects of 5-hydroxytryptamine (5-HT) and its precursors 5-hydroxytryptophan (5-HTP) and L-tryptophan (L-Tryp) on the spatial properties of horizontal cells were studied in the isolated and perfused retina of the teleost Eugerres plumieri. All three compounds produce a contraction of the receptive field, evaluated by the ratio of responses evoked by local and distant light stimuli. This is the result of cell uncoupling, revealed by the hindrance to diffusion of intracellularly injected Lucifer yellow. Similar effects are produced by dopamine (DA) and the effectiveness is DA much greater than 5-HT greater than 5-HTP greater than L-Tryp. All these effects are blocked by Haloperidol. HPLC studies of endogenous DA release reveal that it occurs when isolated retinas are incubated with 50 mM potassium, 10 microM 5-HT or 5-HTP, but is not found with up to 1 mM L-Tryp. The results indicate that indolaminergic cells induce the release of DA from interplexiform cells, which in turn uncouples horizontal cells in the fish retina.