Dopaminergic regulation of cone retinomotor movement in isolated teleost retinas: II. Modulation by gamma-aminobutyric acid and serotonin

Aging related changes of retina and optic nerve of Uromastyx aegyptia and Falco tinnunculus

Aging is a biological phenomenon that involves gradual degradation of the structure and function of the retina and optic nerve. To our knowledge, little is known about the aging-related ocular cell loss in avian (Falco tinnunculus) and reptilian species (Uromastyx aegyptia). A selected 90 animals of pup, middle, and old age U. aegyptia (reptilian) and F. tinnunculus (avian) were used. The retinae and optic nerves were investigated by light and transmission electron microscopy (TEM) and assessments of neurotransmitters, antioxidant enzymes (catalase, superoxide dismustase and glutathione s transferase), caspase-3 and -7, malonadialdhyde, and DNA fragmentation. Light and TEM observations of the senile specimens revealed apparent deterioration of retinal cell layers, especially the pigmented epithelium and photoreceptor outer segments. Their inclusions of melanin were replaced by lipofuscins. Also, vacuolar degeneration and demyelination of the optic nerve axons were detected. Concomitantly, there was a marked increase of oxidative stress involved reduction of neurotransmitters and antioxidant enzymes and an increase of lipid peroxidation, caspase-3 and -7, subG0/G1 apoptosis, and P53. We conclude that aging showed an inverse relationship with the neurotransmitters and antioxidant enzymes and a linear relationship of caspases, malondialdhyde, DNA apoptosis, and P53 markers of cell death. These markers reflected the retinal cytological alterations and lipofuscin accumulation within inner segments.

Physiological endpoints for potential SSRI interactions in fish

Selective serotonin reuptake inhibitors (SSRIs) are among the pharmaceutical compounds frequently detected in sewage treatment plant effluents and surface waters, albeit at very low concentrations, and have therefore become a focus of interest as environmental pollutants. These neuroactive drugs are primarily used in the treatment of depression but have also found broader use as medication for other neurological dysfunctions, consequently resulting in a steady increase of prescriptions worldwide. SSRIs, via inhibition of the serotonin (5-hydroxytryptamine, 5-HT) reuptake mechanism, induce an increase in extracellular 5-HT concentration within the central nervous system of mammals. The phylogenetically ancient and highly conserved neurotransmitter and neurohormone 5-HT has been found in invertebrates and vertebrates, although its specific physiological role and mode of action is unknown for many species. Consequently, it is difficult to assess the impact of chronic SSRI exposure in the environment, especially in the aquatic ecosystem. In view of this, the current knowledge of the functions of 5-HT in fish physiology is reviewed and, via comparison to the physiological role and function of 5-HT in mammals, a characterization of the potential impact of chronic SSRI exposure on fish is provided. Moreover, the insight on the physiological function of 5-HT strongly suggests that the experimental approaches currently used are inadequate if not entirely improper for routine environmental risk assessment of pharmaceuticals (e.g., SSRIs), as relevant endpoints are not assessed or impossible to determine.

Adenosine stimulates cone photoreceptor myoid elongation via an adenosine A2-like receptor

In several parts of the nervous system, adenosine has been shown to function as an extracellular neuromodulator binding to surface receptors on target cells. This study examines the possible role of adenosine in mediating light and circadian regulation of retinomotor movements in teleost cone photoreceptors. Teleost cones elongate in the dark and contract in the light. In continuous darkness, the cones continue to elongate and contract at subjective dusk and dawn in response to circadian signals. We report here that exogenous adenosine triggers elongation (the dark/night movement) in isolated cone inner segment-cone outer segment preparations (CIS-COS) in vitro. Agonist/antagonist potency profiles indicate that adenosine's effect on cone movement is mediated by an A2-like adenosine receptor, which like other A2 receptors enhances adenylate cyclase activity. Although closest to that expected for A2 receptors, the antagonist potency profile for CIS-COS does not correspond exactly to any known A2 receptor subtype, suggesting that the cone receptor may be a novel A2 subtype. Our findings are consistent with previous reports that retinal adenosine levels are higher in the dark, and further suggest that adenosine could act as a neuromodulatory "dark signal" influencing photoreceptor metabolism and function in the fish retina.

Keeping an eye on retinal clocks

Circadian pacemakers that drive rhythmicity in retinal function are found in both invertebrates and vertebrates. They have been localized to photoreceptors in molluscs, amphibians, and mammals. Like other circadian pacemakers, they entrain to light, oscillate based on a negative feedback between transcription and translation of clock genes, and control a variety of physiological and behavioral rhythms that often includes rhythmic melatonin production. As a highly organized and accessible tissue, the retina is particularly well suited for the study of the input-output pathways and the mechanism for rhythm generation. Impressive advances can now be expected as researchers apply new molecular techniques toward looking into the eye's clock.

Evidence for an endogenous clock in the retina of rainbow trout: II. Circadian rhythmicity of serotonin metabolism

The purpose of the present study was to investigate patterns of circadian rhythmicity in the retina of a salmonid fish, the rainbow trout (Oncorhynchus mykiss). Our data present the first demonstration of intraretinal variations of serotonergic substances under light/dark-conditions (LD) and during continuous darkness (DD). All substances examined (serotonin, N-acetyl serotonin, 5-hydroxytryptophol, 5-hydroxyindole acetic acid) were rhythmic in LD. Serotonin, N-acetyl-serotonin, and 5-hydroxyindole acetic acid showed a preservation of specific features of rhythmicity in DD indicating the involvement of an endogenous pacemaker in the regulation of serotonin metabolism in the rainbow trout eye.

Density ratio of dopaminergic versus serotonergic cells correlates with cone-to-rod ratio in teleost retinas

Dopaminergic and serotonergic cells were visualised immunohistochemically with antibodies against tyrosine hydroxylase and serotonin in retinal wholemounts of eight teleosts from different habitats and with different rod-to-cone ratios. The cell densities were calculated, and the density ratio of dopaminergic cells versus serotonergic cells was compared among these fish species. The density ratio was high (1.9-2.7) in three out of the four species of cichlid fish studied with cone densities roughly equalling rod densities, medium in roach (0.8) where rods dominate cone numbers, and low in deep-sea fish (0.2-0.4) with pure rod retinas. These observations confirm earlier findings on the species-specificity of the ratio of dopaminergic versus serotonergic cells in the retina and further demonstrate a close correlation between the rod-to-cone ratio, and the density ratio of dopaminergic versus serotonergic cells in the inner retina. The possible significance for the processing of photopic and scotopic information in the inner retina is discussed.

Similar effects of carbachol and dopamine on neurons in the distal retina of the tiger salamander

Though there is considerable evidence that dopamine is an important retinal neuromodulator that mediates many of the changes in the properties of retinal neurons that are normally seen during light adaptation, the mechanism by which dopamine release is controlled remains poorly understood. In this paper, we present evidence which indicates that dopamine release in the retina of the tiger salamander, Ambystoma tigrinum, is driven excitatorily by a cholinergic input. We compared the effects of applying carbachol to those of dopamine application on the responses of rods, horizontal cells, and bipolar cells recorded intracellularly from the isolated, perfused retina of the tiger salamander. Micromolar concentrations of dopamine reduced the amplitudes of rod responses throughout the rods' operating range. The ratio of amplitudes of the cone-driven to rod-driven components of the responses of both horizontal and bipolar cells was increased by activation of both D1 and D2 dopamine receptors. Dopamine acted to uncouple horizontal cells and also off-center bipolar cells, the mechanism in the case of horizontal cells depending only upon activation of D1 receptors. Carbachol, a specific cholinomimetic, applied in five- to ten-fold higher concentrations, produced effects that were essentially identical to those of dopamine. These effects of carbachol were blocked by application of specific dopamine blockers, however, indicating that they are mediated secondarily by dopamine. We propose that the dopamine-releasing amacrine cells in the salamander are under the control of cells, probably amacrine cells, which secrete acetylcholine as their transmitter.

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.

Changing the light intensity of the visual environment results in large differences in numbers of synapses and in photoreceptor size in the retina of the young adult rat

A quantitative light- and electron-microscopic study has been made of the retinae of rats which were exposed to different lighting conditions for between one and 15 weeks in young adulthood, having been reared in identical conditions during development. The width of the inner and outer segments of the photoreceptors and the width of the outer plexiform layer varied inversely with the light intensity under diurnal lighting conditions of 10 h light/14 h dark. Linear regression analysis showed that the widths were inversely related to the fourth root of the light intensity as measured in lux. Both central and peripheral areas of retina showed a similar change. No change was seen in the widths of the inner plexiform layer, or of the inner and outer nuclear cell layers. Nor was there a difference in the packing density or size of the nuclei in the nuclear cell layers. The number of ribbon synapses in the outer plexiform layer also varied inversely with the intensity of diurnal light. Linear regression analysis showed that the number of synapses was inversely correlated with the fourth root of the light intensity and was positively correlated with the width of the outer plexiform layer. The number of ribbon synapses was increased by up to two and a half times in constant darkness compared to diurnal light of 35 lux. The increase was present but not maximal after one week of exposure. The length of synaptic ribbons was unchanged. The nerve terminals forming such synapses were increased in size but not in number. After one week, there was little or no additional change in the retinal widths and number of synaptic ribbons with time. However, there was a progressive increase with time in nerve terminal size (two-fold in area) in constant darkness. There was some evidence of a slight decrease in nerve terminal number and increase in size of retinal nuclei with age. It is concluded that the adult retina responds to a different lighting environment by a relatively rapid change in the size of photoreceptor segments, by a progressive and large change in number of ribbon synapses and by a slower progressive and large change in the size of photoreceptor nerve terminals. The response is quantitatively determined by the strength of the stimulus but not in a linear fashion. These results are compared with the effects of environmental stimulation of other areas of the nervous system.

Neuromodulation of pigment movement in the RPE of normal and 6-OHDA-lesioned goldfish retinas

The role of dopamine as the endogenous signal-initiating light-dependent changes in the distribution of pigment granules in goldfish retinal pigment epithelium was investigated. In normal retinas, light adaptation resulted in the dispersion of pigment granules. This effect of light was mimicked by the intraocular injection of dopamine or serotonin, which is thought to increase endogenous dopamine release, into dark-adapted eyes. The effect of light, dopamine, or serotonin on dark-adapted retinas was blocked by the dopamine receptor antagonists haloperidol and sulpiride. However, lesioning the endogenous source of retinal dopamine, by prior intraocular injection of 6-hydroxydopamine (6-OHDA), did not block the dispersion of pigment granules in light-adapted retinas. No significant differences in pigment dispersion were noted between unlesioned and lesioned light- or dark-adapted retinas. However, the effect of light on pigment dispersion was no longer blocked by haloperidol or sulpiride in 6-OHDA lesioned animals. Dopamine and serotonin mimicked the effect of light when injected into lesioned dark-adapted eyes, but their effects were also not blocked by haloperidol or sulpiride. These results suggest that dopamine, acting on D2 receptors, is sufficient to induce pigment migration in unlesioned animals. In 6-OHDA-lesioned animals, however, pigment migration is mediated by a receptor mechanism other than D2.

Dark-adaptive cone elongation in the blue acara retina is triggered by green-sensitive cones

In a dichromatic teleost species, we determined the intensity of light of various wavelengths required to prevent cone elongation by exposing fish at the time of their normal "dusk" phase to monochromatic light (479, 623, and 660 nm) at eight to ten different intensities for 75 min. The positions of single and double cones were measured in tangential sections and expressed as cone indices. At all wavelengths, the spectral responses of both cone types were virtually identical. Furthermore, the sensitivity of the blocking effect was highest at shorter wavelengths. When comparing the relative quantal sensitivities of myoid elongation for the two cone types to the spectral sensitivities of the three types of Aequidens pulcher photoreceptor, we found the closest match between the action spectrum and the absorption spectrum of the green-sensitive single cones. This may indicate that this cone type is capable of reacting directly to decreasing levels of illumination. On the other hand, the identical sensitivity of both cone types argues for an indirect control mechanism of dark-adaptive cone elongation, possibly via a neural pathway involving the inner retinal layers, complementary to the neural control of light adaptation. Green-sensitive single cones are well suited to trigger this response, since (1) their sensitivity is inferior to that of double cones; (2) waters inhabited by the blue acara transmit best at long wavelengths; and (3) at dusk, long-wavelength radiation dominates over other parts of the spectrum. Therefore, green-sensitive cone threshold will be reached first at dusk.

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.

Photoreceptor plasticity in the albino rat retina following unilateral optic nerve section

Rhodopsin content of the retina increased when an albino rat was moved to a lower intensity cyclic light, and decreased when moved to a higher intensity. Unilateral optic nerve section was employed to test if intact optic nerves are necessary to maintain regulation of rhodopsin when albino rats are switched between two intensities, 3 lx and 600 lx (12 hr/12 hr, cyclic light). The ability to regulate rhodopsin content was altered but not lost in the eye which had the optic nerve transected. This alteration was in the direction of incomplete up-regulation when the intensity change was from high to low, and was in the direction of excessive down-regulation when the intensity change was from low to high. This indicates that efferent fibers from the brain to the retina may be involved in the regulation of rhodopsin content. Using histological techniques, cell number and rod outer segment (ROS) length were measured: surgery alone had no effect on ROS length or outer nuclear layer thickness and in all cases ROS lengths were inversely related to habitat illuminance.

Effects of serotonergic agonists and antagonists on ganglion cells in the goldfish retina

Extracellular recordings were made from the isolated goldfish retina during superfusion with various serotonergic agonists and antagonists to determine the effects of these drugs on the maintained activity and response properties of the ganglion cells. Superfusion of the retina with serotonin (25-500 microM) increased the maintained activity of OFF-center ganglion cells and decreased the maintained activity of ON-center ganglion cells. In addition, serotonin also attenuated the excitatory responses to annular stimuli, suggesting a decrease in the strength of surround input to the ganglion cells. The effects of serotonin on OFF-center ganglion cells were mimicked by the nonselective 5-HT1 agonist 5-MeOT and the 5-HT1A receptor agonist 8-OH-DPAT, while only 5-MeOT mimicked the action of serotonin on ON-center ganglion cells. The effects of exogenously applied serotonin on the ganglion cells could be blocked by the mixed 5-HT1/5-HT2 receptor antagonist methysergide but not by the 5-HT2 receptor antagonist mianserin or the dopamine receptor antagonist haloperidol. These results support previous anatomical and biochemical evidence that serotonin functions in a neurotransmitter or neuromodulatory role in the teleost retina and suggest that serotonin may be involved in modulating the maintained activity and surround input to the ganglion cells. The results also indicate that two different types of receptors may mediate the actions of serotonin in the ON and OFF pathways, respectively.

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)

The "ON"-bipolar agonist, L-2-amino-4-phosphonobutyrate, blocks light-evoked cone contraction in xenopus eye cups

Rhythmic photoreceptor metabolism in relationship to light-dark cycles is now thought to be regulated through a retinal feed-back mechanism with dopamine serving as a principal signal initiating light-evoked events. In order to test the hypothesis that depolarizing "ON"-bipolar neurons participate in the retinal signalling pathway, we determined the effects of L-2-amino-4-phosphonobutyrate (L-APB) on light-evoked cone contraction in eye cups from Xenopus laevis. L-APB blocked the response stereospecifically when applied over a broad concentration range. The high specificity of L-APB in retina suggests that sign-inverting bipolar neurons which depolarize in light are in the signalling pathway. One possibility is that this pathway conveys signals that regulate dopamine release.

Rhythmic regulation of retinal melatonin: metabolic pathways, neurochemical mechanisms, and the ocular circadian clock

1. Current knowledge of the mechanisms of circadian and photic regulation of retinal melatonin in vertebrates is reviewed, with a focus on recent progress and unanswered questions. 2. Retinal melatonin synthesis is elevated at night, as a result of acute suppression by light and rhythmic regulation by a circadian oscillator, or clock, which has been localized to the eye in some species. 3. The development of suitable in vitro retinal preparations, particularly the eyecup from the African clawed frog, Xenopus laevis, has enabled identification of neural, cellular, and molecular mechanisms of retinal melatonin regulation. 4. Recent findings indicate that retinal melatonin levels can be regulated at multiple points in indoleamine metabolic pathways, including synthesis and availability of the precursor serotonin, activity of the enzyme serotonin N-acetyltransferase, and a novel pathway for degradation of melatonin within the retina. 5. Retinal dopamine appears to act through D2 receptors as a signal for light in this system, both in the acute suppression of melatonin synthesis and in the entrainment of the ocular circadian oscillator. 6. A recently developed in vitro system that enables high-resolution measurement of retinal circadian rhythmicity for mechanistic analysis of the circadian oscillator is described, along with preliminary results that suggest its potential for elucidating general circadian mechanisms. 7. A model describing hypothesized interactions among circadian, neurochemical, and cellular mechanisms in regulation of retinal melatonin is presented.

Effects of circadian phase on cone retinomotor movements in the Midas cichlid

In retinas of the Midas cichlid (Cichlasoma citrinellum), cone myoid lengths change in response to both light and circadian signals. Cone myoids are short in day (or in the light), and long at night (or in darkness). During the normal light/dark cycle, cone myoids elongate to 58 microns after light offset at dusk, and remain long until just before dawn, at which time they begin to contract before the time of expected light onset and eventually (after light onset) assume daytime lengths of 5 microns. In continuous light, cone myoids remain fully contracted regardless of the time in the cycle. In continuous darkness, cone myoids undergo circadian movements; they elongate fully at night and contract partly during expected day to 23 microns (65% of the contraction seen at dawn in a normal light/dark cycle). To clarify further the regulation of these retinomotor movements, we have investigated the effects of circadian phase on cone movements induced by light or dark onset. Circadian phase had no significant effect on either the initial rate or the final extent of light-induced cone myoid contraction: at mid-dark cones began to contract immediately after light onset and contracted fully at 3.0 +/- 0.5 microns min-1; at mid-night cones contracted immediately and fully at 4.0 +/- 0.2 microns min-1. At all of the relatively high intensities of light tested (30-5000 lx) cones contracted at similar rates and to similar final extents.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of unilateral optic nerve section on retinal light damage in rats

The effect of unilateral optic nerve section on the susceptibility of rat photoreceptors to damage by constant light was studied. Optic nerves of albino rats were cut intracranially by a ventral approach, so as not to interfere with structures in the orbit of the eye, with the brain or with the blood supply to the retina. Two separate experiments were performed on unilaterally optic nerve sectioned rats from two different sources. One group was purchased from a commercial supplier and the other group was born and raised in 3 lux cyclic light in our laboratory. For 1-4 weeks after surgery they were exposed to either 1000 lux for 24 hr or 80 lux for 48 hr. Light damage was quantified by measuring the outer nuclear layer area remaining in histological sections through the vertical meridian of the retina. It was found that retinas with optic nerves cut suffered substantially less damage from light than did those with intact optic nerves. Sham operated rats suffered the same amount of damage as did the optic nerve intact retinas of rats with one nerve cut. The extent of protection was greatest in the region previously shown to be most susceptible to damage. The protection afforded by optic nerve section could not be explained on the basis of behavior or rhodopsin photochemistry. The possible role of heat-shock proteins and a neuromodulator is discussed.

Retinal dopamine D1 and D2 receptors: characterization by binding or pharmacological studies and physiological functions

1. In the retinal inner nuclear layer of the majority of species, a dopaminergic neuronal network has been visualized in either amacrine cells or the so-called interplexiform cells. 2. Binding studies of retinal dopamine receptors have revealed the existence of both D1- as well D2-subtypes. The D1-subtype was characterized by labeled SCH 23390 (Kd ranging from 0.175 to 1.6 nM and Bmax from 16 to 482 fmol/mg protein) and the D2-subtype by labelled spiroperidol (Kd ranging from 0.087 to 1.35 nM and Bmax from 12 to 1500 fmol/mg protein) and more selectively by iodosulpiride (Kd 0.6 nM and Bmax 82 fmol/mg protein) or methylspiperone (Kd 0.14 nM and Bmax 223 fmol/mg protein). 3. Retinal dopamine receptors have been also shown to be positively coupled with adenylate cyclase activity in most species, arguing for the existence of D1-subtype, whereas in some others (lower vertebrates and rats), a negative coupling (D2-subtype) has been also detected in peculiar pharmacological conditions implying various combinations of dopamine or a D2-agonist with a D1-antagonist or a D2-antagonist in the absence or presence of forskolin. 4. A subpopulation of autoreceptors of D2-subtype (probably not coupled to adenylate cyclase) also seems to be involved in the modulation of retinal dopamine synthesis and/or release. 5. Light/darkness conditions can affect the sensitivity of retinal dopamine D1 and/or D2-receptors, as studied in binding or pharmacological experiments (cAMP levels, dopamine synthesis, metabolism and release). 6. Visual function(s) of retinal dopamine receptors were connected with the regulation of electrical activity and communication (through gap junctions) between horizontal cells mediated by D1 and D2 receptor stimulation. Movements of photoreceptor cells and migration of melanin granules in retinal pigment epithelial cells as well as synthesis of melatonin in photoreceptors were on the other hand mediated by the stimulation of D2-receptors. 7. Other physiological functions of dopamine D1-receptors respectively in rabbit and in embryonic avian retina would imply the modulation of acetylcholine release and the inhibition of neuronal growth cones.

Serotonin-like immunoreactivity in the retina of the clawed frog Xenopus laevis

Using an antiserum directed against serotonin, we have studied the morphology and distribution of serotonin-containing and serotonin-accumulating neurons in the retina of Xenopus laevis. Endogenous serotonin-like immunoreactivity was found in two classes of amacrine cell, one class of bipolar cell and a few centrifugal fibres. Kainic acid-induced depletion of serotonin, under various conditions, enabled us to determine the distribution of stained bipolars, amacrine cells and centrifugal fibres within the meshwork of serotonin-like immunoreactivity-labelled processes. Kainic acid-induced release of serotonin by bipolar cells is calcium dependent. Stimulation of release by kainic acid as well as the fact that all serotonin-like immunoreactive bipolar cells ramify in sublayer 1 of the inner plexiform layer suggest that serotonergic bipolars are OFF centre cells. Release of serotonin from amacrine cells is largely calcium independent. Serotonin-containing amacrines send primary dendrites into layer 1 of the inner plexiform layer; short off-shoots from the primary dendrites descend into sublayers 3-5 in which they ramify into a fine network. Serotonergic amacrines have an uneven distribution in the Xenopus retina. Their highest density occurs in the posterolateral quadrant, whereas large portions of the anteromedial quadrant lack serotonin-like immunoreactivity altogether. The uneven distribution of serotonin-containing elements in the Xenopus retina with its peak falling onto the retinal area which generates binocular vision, suggests its involvement in binocular perception.

Dopamine and its agonists reduce a light-sensitive pool of cyclic AMP in mouse photoreceptors

The exposure to bright light of dark-adapted (DKA) mouse retinas incubated in the dark (DI) in IBMX-containing medium causes a marked loss of cyclic AMP. This light response also occurs if the medium contains 10 mM aspartate or cobaltous ion, agents believed to confine the effects of light to photoreceptors. Thus, the action of light in the presence of either of these agents defines a light-sensitive pool of cyclic AMP in photoreceptors. This pool could also be reduced or eliminated in DKA-DI retinas by nanomolar to micromolar levels of dopamine (if the medium contained SCH23390, a potent antagonist of D1 receptors), thus indicating an agonistic action of dopamine at D2 receptors. The D2 agonists LY171555 (EC50 10 nM) or (+)-3-PPP also reduced the cyclic AMP level in the dark. Of the D2 antagonists tested, the butyrophenone spiperone (in the presence of the 5HT-2 blocker ketanserin) countered the action of the D2 agonists but substituted benzamides were ineffective. Consistently, the D2 agonists had no effect on cyclic AMP levels of mutant retinas lacking photoreceptors (rd/rd), but reduced cyclic AMP in DKA-DI glutamate-modified retinas which exhibit a major loss of inner retinal neurons without apparent loss of photoreceptors. The D1 antagonist SCH23390 only reduced cyclic AMP levels of DKA-DI retinas when cyclic AMP levels had been elevated by adding dopamine to the incubation medium.

Darkness stimulates rapid synthesis and release of melatonin in rat retina

The presence of melatonin in retina has been widely reported for over two decades although studies of its functional importance within the retina have only recently been emphasized. We have analyzed the biochemical characteristics of melatonin synthesis and release, focusing on rapid changes in response to light/dark conditions. Our major findings are consistent with the following conclusions: (1) melatonin synthesis is stimulated within minutes after exposure to darkness, and may reflect an increase in N-acetyl transferase activity; (2) melatonin is not stored, but rather it diffuses freely throughout the retina immediately after it is synthesized; and (3) the dark-induced increase in retinal melatonin release is a synthesis-coupled response and does not involve separate secretion mechanisms. The characteristics of melatonin synthesis and release described herein would be consistent with the proposed role of melatonin as a local paracrine effector of dark-adaptive responses in retina.

Light-induced dopamine release from teleost retinas acts as a light-adaptive signal to the retinal pigment epithelium

In the retinal pigment epithelium (RPE) of lower vertebrates, melanin pigment granules migrate in and out of the cells' long apical projections in response to changes in light condition. When the RPE is in its normal association with the retina, light onset induces pigment granules to disperse into the apical projections; dark onset induces pigment granules to aggregate into the cell bodies. However, when the RPE is separated from the retina, pigment granule movement in the isolated RPE is insensitive to light onset. It thus seems likely that a signal from the retina communicates light onset to the RPE to initiate pigment dispersion. We have examined the nature of this retina-to-RPE signal in green sunfish, Lepomis cyanellus. In isolated retinas with adherent RPE, light-induced pigment dispersion in the RPE is blocked by treatments known to block Ca2+-dependent transmitter release in the retina. In addition, the medium obtained from incubating previously dark-adapted retinas in the light induces light-adaptive pigment dispersion when added to isolated RPE. In contrast, the medium obtained from incubating dark-adapted retinas in constant darkness does not affect pigment distribution when added to isolated RPE. These results are consistent with the idea that RPE pigment dispersion is triggered by a substance that diffuses from the retina at light onset. The capacity of the conditioned medium from light-incubated retinas to induce pigment dispersion in isolated RPE is inhibited by a D2 dopamine antagonist, but not by D1 or alpha-adrenergic antagonists. Light-induced pigment dispersion in whole RPE-retinas is also blocked by a D2 dopamine antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinomotor movements in the frog retinal pigment epithelium: dependence of pigment migration on Na+ and Ca2+

The ionic dependence of the screening-pigment migrations in the frog retinal epithelium (RPE) was quantitatively studied with eyecups incubated in media of different compositions. Typical migrations in response to light and darkness, equivalent to those observed in the intact animal, were fully accomplished and maintained for up to 6 hr by the isolated organ bathed in Ringer solution rich with O2. Pigment migration in either direction was completed under the appropriate illumination conditions at any time during the day, indicating that circadian influences, if present in the intact animal, can be overridden in the isolated organ by light or darkness alone. Pigment aggregation toward the dark-adapted position was inhibited by: (a) low external Ca2+, (b) high external Na+, and (c) drugs expected to increase the cytoplasmic levels of either Na+, or Ca2+, like ouabain, caffeine and the ionophore A23187. However, the inhibition caused by low Ca2+ did not occur if Na+ was also reduced in the incubation medium. On the other hand, an increase in the concentration of external Ca2+ or the addition of Co2+ to the normal Ringer facilitated pigment aggregation in the dark. Pigment dispersion to the light-adapted position was unaffected by any of the above conditions. This is the first report of full and stable pigment responses in the RPE of vertebrate eyes incubated under simple physiological conditions. The results seem to conciliate a discrepancy of previous reports on the Ca2+ dependence of RPE movements, and are compatible with current views on ionic mechanisms in analogous systems of intracellular transport.

Release pattern of endogenous dopamine in teleost retinae during light adaptation and pharmacological stimulation

The release of endogenous dopamine from teleost retinae was studied using high-performance-liquid-chromatography and electrochemical detection. Dopamine was measured in superfusates of isolated retinae after stimulation with flickering light as well as in the presence of GABA, L-glutamate, kainate or taurine. The effect of the receptor antagonists bicuculline, picrotoxin and kynurenic acid was also tested in retinae kept in the dark. We report a low level, basal release of dopamine in the dark (20 pg x 10 min-1/retina), which is transiently increased by stimulation with flickering light. This light evoked release of dopamine is inhibited by GABA and L-glutamate, whilst antagonists of these retinal transmitters stimulate release in the dark.

Mesencephalic innervation of the turtle retina by a single serotonin-containing neuron

Co-localization of retrograde transported Nuclear yellow and serotonin immunoreactivity revealed the existence of a central serotonergic projection to the retina in the turtle Pseudemys scripta elegans. This serotonergic retinopetal system consists of only one fiber originating in the contralateral caudal mesencephalon. In the retina, the fiber arborizes exclusively in the temporal hemisphere, covering about one third of the total retinal surface. Interestingly this projecting area lies in the part of the retina, to which binocular perception can be attributed.

Circadian regulation of retinomotor movements: II. The role of GABA in the regulation of cone position

Cone photoreceptor movements in lower vertebrates are regulated by the interaction of the light-dark cycle and an endogenous circadian clock. We have suggested that melatonin and dopamine interact to regulate dark- and light-adaptive movements, respectively, and that melatonin affects cones indirectly by inhibiting dopamine release. In fact, any factor modulating dopaminergic neurons in the retina may have effects on either cone elongation or contraction. We have utilized an in vitro eyecup preparation from the African clawed frog, Xenopus laevis, to evaluate a possible role of the neurotransmitter GABA, which is thought to tonically suppress dopamine release. GABA agonists mimic the effects of darkness and induce cone elongation; the effects of GABA agonists are blocked by dopamine. Muscimol-induced cone elongation occurs at low light intensity but is inhibited by bright light in eyecups prepared from cyclic-light-maintained animals. Although neither melatonin nor muscimol stimulates cone elongation in bright light, simultaneous application of both drugs induces elongation. The GABA antagonist picrotoxin induces cone contraction which is blocked by the dopamine receptor antagonist spiroperidol, which suggests that GABA may affect cone movement in Xenopus by regulating dopamine neurons. Consistent with this, picrotoxin-induced cone contraction is Ca+2 dependent and is blocked by high Mg+2 or the Ca+2 antagonist nifedipine. Pharmacological analysis suggests that the effects of GABA may result from its action at more than one receptor subtype. Our results support the hypothesis that dopamine is part of the light signal for cone contraction and that its suppression is part of the signal for cone elongation.

Synaptic organization of dopaminergic interplexiform cells in the goldfish retina

The synaptic organization of dopaminergic interplexiform cells (DA-IPC) in the goldfish retina was studied by a combined double-label electron-microscopical (EM) immunocytochemical/autoradiographical study. DA-IPCs were labeled with antisera against tyrosine hydroxylase. The possibility of synaptic contact with GABAergic amacrine cells in the proximal inner plexiform layer (IPL) was studied by using 3H-GABA uptake. Most synaptic input and output from DA-IPC processes involved amacrine cell processes. In addition, synaptic interactions were observed between DA-IPC processes and bipolar cell terminals, other DA-IPC processes, very small dendrites in the IPL, ganglion cell and optic fiber layers (OFL), and cell bodies in the ganglion cell layer (GCL). Input and output synapses with GABAergic amacrine processes also were observed. Two-thirds of the DA-IPC boutons in the proximal IPL were involved in "junctional appositions," that is, the junctions appeared to be specialized but they were different than classical chemical synapses. The synaptic organization of DA-IPCs in the goldfish IPL appears to be far more complex than previously thought. Although earlier studies have attempted to explain the action of dopamine in terms of interaction only with amacrine cells, the present study shows that effects involving bipolar cells, other DA-IPCs, unidentified processes and cell bodies in the GCL and OFL must be considered as well.

Circadian rhythms in the green sunfish retina

We investigated the occurrence of circadian rhythms in retinomotor movements and retinal sensitivity in the green sunfish, Lepomis cyanellus. When green sunfish were kept in constant darkness, cone photoreceptors exhibited circadian retinomotor movements; rod photoreceptors and retinal pigment epithelium (RPE) pigment granules did not. Cones elongated during subjective night and contracted during subjective day. These results corroborate those of Burnside and Ackland (1984. Investigative Ophthalmology and Visual Science. 25:539-545). Electroretinograms (ERGs) recorded in constant darkness in response to dim flashes (lambda = 640 nm) exhibited a greater amplitude during subjective night than during subjective day. The nighttime increase in the ERG amplitude corresponded to a 3-10-fold increase in retinal sensitivity. The rhythmic changes in the ERG amplitude continued in constant darkness with a period of approximately 24 h, which indicates that the rhythm is generated by a circadian oscillator. The spectral sensitivity of the ERG recorded in constant darkness suggests that cones contribute to retinal responses during both day and night. Thus, the elongation of cone myoids during the night does not abolish the response of the cones. To examine the role of retinal efferents in generating retinal circadian rhythms, we cut the optic nerve. This procedure did not abolish the rhythms of retinomotor movement or of the ERG amplitude, but it did reduce the magnitude of the nighttime phases of both rhythms. Our results suggest that more than one endogenous oscillator regulates the retinal circadian rhythms in green sunfish. Circadian signals controlling the rhythms may be either generated within the eye or transferred to the eye via a humoral pathway.

Melatonin and rhythmic photoreceptor metabolism: melatonin-induced cone elongation is blocked at high light intensity

We have proposed a model for circadian regulation of cone position in Xenopus laevis that involves interaction of melatonin and dopamine as signals for darkness and light respectively. One problem, however, is that the effects of melatonin have not been detected in eye cups prepared from animals maintained on a cyclic light schedule. Since melatonin's effect would be expected to occur in low light intensity at night, we have investigated the relationship among melatonin, light intensity, and cone length. We report that melatonin mimics the effects of darkness and stimulates cone elongation in eye cups from cyclic light animals incubated at low but not at high light intensities.

Serotonin containing neurons in the retina of the teleost Eugerres plumieri

The concentration and localization of serotonin was determined in the retina of the teleost Eugerres plumieri by using high performance liquid chromatography (HPLC) and immunohistochemical techniques. Serotonin and dopamine were measured simultaneously, their concentrations in the retina being 77 +/- 8 and 516 +/- 23 ng/mg tissue respectively. Treatment of the animals with pargyline significantly increased the levels of dopamine and serotonin. When retinas were treated with the neurotoxin 5,6-dihydroxytryptamine, the level of serotonin was reduced by more than 90% while the dopamine content only diminished by 20% when compared to controls. By using immunohistochemistry with a monoclonal anti-serotonin antibody it was possible to localize this amine in cell bodies of a population of amacrine cells with processes extending mainly into a thin layer of the most external lamina of the inner plexiform layer. Very few ramifications were seen projecting to the internal lamina of this layer. When visualized in flat mount preparations, dense arborization of fluorescent processes was observed. This is the first direct evidence that serotonin is apparently present in amacrine cells of the retina of E. plumieri with a distribution of the serotonergic terminals similar to goldfish but somewhat different when compared to other species.

Dopaminergic regulation of cone retinomotor movement in isolated teleost retinas: I. Induction of cone contraction is mediated by D2 receptors

In the retinas of lower vertebrates, retinal photoreceptors and melanin pigment granules of the retinal pigment epithelium (RPE) undergo characteristic movements in response to changes in light intensity and to signals from an endogenous circadian clock. To identify agents responsible for mediating light and/or circadian regulation of these retinomotor movements, we investigated the effects of hormones and neurotransmitters on cone, rod, and RPE movements in the green sunfish, Lepomis cyanellus. We report here that 3,4-dihydroxyphenylethylamine (dopamine) mimics the effect of light by inducing light-adaptive retinomotor movements in all three cell types. In isolated dark-cultured retinas, dopamine induced light-adaptive cone contraction with a half-maximal effect at 10(-8) M. This effect of dopamine was inhibited by antagonists with a potency order characteristic of D2 receptor mediation. The dopamine uptake blocker benztropine also induced light-adaptive cone contraction in isolated dark-cultured retinas, suggesting that there is continuous dopamine release in the dark but that concomitant uptake normally prevents activation of cone contraction. That dopamine plays a role in light regulation of cone movement is further suggested by the observation that light-induced cone contraction was partially inhibited by sulpiride, a selective D2 dopamine antagonist, or by Co2+, a blocker of synaptic transmission. Sulpiride also promoted dark-adaptive cone elongation in isolated light-adapted retinas, suggesting that continuous dopamine action is required in the light to maintain the light-adapted cone position. Dopamine can act directly on D2 receptors located on rod and cone inner/outer segments: dopamine induced light-adaptive retinomotor movements in isolated distal fragments of dark-adapted photoreceptors cultured in the dark. Together our results indicate that dopamine induces light-adaptive retinomotor movements in cones, rods, and RPE cells by activating D2 receptors. We suggest that, in vivo, dopamine plays a role in both light and circadian regulation of retinomotor movements.