Retinomotor Movements Research Articles

The visual system of the Florida garfish, Lepisosteus platyrhincus (Ginglymodi). I. Retina.

The retina and choriocapillaris of the Florida garfish, Lepisosteus platyrhincus (Ginglymodi), was examined at the light and electron microscopic levels. The inner limiting membrane is covered by an extensive system of vitreal blood vessels emanating from the hyaloid artery, which enters the eye ventrally at the proximal end of the elongated optic nerve head. Two size classes of ganglion cell soma are segregated by optic axon fascicles and Müller cell endfeet, all of which lie at the level of the ganglion cell layer. A third class of 'displaced' ganglion cells lies at the border of the inner plexiform and inner nuclear layers amidst tightly packed amacrine, bipolar and Müller cell soma. Two layers of horizontal cells lie vitread of a synaptic zone consisting of a complex arrangement of horizontal and bipolar dendrites invaginating rod spherules and cone pedicles to form single and multiple (three to six) synaptic ribbon connections, respectively. Immediately vitread of the photoreceptor nuclei lie a population of 'displaced' bipolars. Three types of photoreceptors are characterised: unequal double cones, single cones (large and small) and rods. These show retinomotor movements where the rods elongate in the light and are masked by the pigment epithelium and contract in the dark as the pigment migrates sclerad. Ultrastructurally, 4 types of dark-staining (osmophilic) granules are described: (1) Small glycogen granules (0.033 microns) aggregated at the bases of the photoreceptor nuclei and larger similar granules (0.078 microns and termed paraboloids) vitread to the ellipsoid; (2) tapetal granules (0.32 microns) distributed throughout the dorsal four-fifths of the retinal pigment epithelium (RPE); (3) pigment granules (0.5-2.0 microns) in the RPE, concentrated in ventral retina; (4) granules or melanosomes (0.813 microns) of the choriocapillaris. The second class of granules constitute a tapetum lucidum eliciting a yellow eyeshine when viewed in the dark. Two other tapeta also exist, a guanine tapetum (irregular guanine crystals) and a tapetum fibrosum (stacks of collagen fibrils). Functional correlations are made, and the putative ancestral (primitive) condition of particular visual characters is established for the ray-finned fishes by out-group comparisons.

The Visual System of the Florida Garfish, <i>Lepisosteus platyrhincus</i>(Ginglymodi) (Part 2 of 2)

The retina and choriocapillaris of the Florida garfish, <i>Lepisosteus platyrhincus </i>(Ginglymodi), was examined at the light and electron microscopic levels. The inner limiting membrane is covered by an extensive system of vitreal blood vessels emanating from the hyaloid artery, which enters the eye ventrally at the proximal end of the elongated optic nerve head. Two size classes of ganglion cell soma are segregated by optic axon fascicles and M&#220;ller cell endfeet, all of which lie at the level of the ganglion cell layer. A third class of 'displaced' ganglion cells lies at the border of the inner plexiform and inner nuclear layers amidst tightly packed amacrine, bipolar and M&#220;ller cell soma. Two layers of horizontal cells lie vitread of a synaptic zone consisting of a complex arrangement of horizontal and bipolar dendrites invaginating rod spherules and cone pedicles to form single and multiple (three to six) synaptic ribbon connections, respectively. Immediately vitread of the photoreceptor nuclei lie a population of 'displaced' bipolars. Three types of photoreceptors are characterised: unequal double cones, single cones (large and small) and rods. These show retinomotor movements where the rods elongate in the light and are masked by the pigment epithelium and contract in the dark as the pigment migrates sclerad. Ultrastructurally, 4 types of dark-staining (osmophilic) granules are described: (1) Small glycogen granules (0.033 &#181;m) aggregated at the bases of the photoreceptor nuclei and larger similar granules (0.078 &#181;m and termed paraboloids) vitread to the ellipsoid; (2) tapetal granules (0.32 &#181;m) distributed throughout the dorsal four-fifths of the retinal pigment epithelium (RPE); (3) pigment granules (0.5–2.0 &#181;m) in the RPE, concentrated in ventral retina; (4) granules or melanosomes (0.813 &#181;m) of the choriocapillaris. The second class of granules constitute a tapetum lucidum eliciting a yellow eyeshine when viewed in the dark. Two other tapeta also exist, a guanine tapetum (irregular guanine crystals) and a tapetum fibrosum (stacks of collagen fibrils). Functional correlations are made, and the putative ancestral (primitive) condition of particular visual characters is established for the ray-finned fishes by out-group comparisons.

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.

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.

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 μm), begin to contract before expected dawn, and with light onset contract to their fully contracted positions (5 μm) 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 μm). 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 D 2-receptor. Cone myoid contraction can be induced at midnight in LD or DD animals by intraocular injection of dopamine or the D 2-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 D 2-receptor agonist, or to elongate by intraocular injection of the dopamine D 2-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 D 2-antagonist sulpiride shortly before the time of expected light onset. In contrast, circadian cone myoid movements were unaffected by intraocular injection of the D 1-receptor agonist SCH23390, or the D 1-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. Our optic nerve section observations indicate that circadian cone movement does not require efferent control. Instead they suggest that circadian cone movement is regulated locally within the eye. Our results from intraocular injection of dopamine agonists and antagonists suggest that this local circadian regulation is mediated by endogenous dopamine, acting via D 2-receptors. We find no evidence for a role for melatonin in producing dark-adaptive retinomotor movements in fish retina.

Retinal photoreceptor fine structure in the velvet cichlid (Astronotus ocellatus).

The structure and arrangement of the retinal photoreceptors of the velvet cichlid fish (Astronotus ocellatus) have been studied by light and electron microscopy. Rods, single cones and double (twin) cones are present. In the light-adapted state, rods are very tall cells that reach deep into the retinal epithelial (RPE) layer. The long outer segment is composed of discs of uniform diameter displaying one or two incisures. The rod inner segment shows a distal ellipsoid of mitochondria, and then narrows dramatically in the myoid region. Rod nuclei are electron dense and located deep in the outer nuclear layer. Rod synaptic spherules are small and show two to three invaginated synaptic sites as well as superficial synapses. Single cones are similar to the individual members of a double cone and all display a short tapering outer segment, a large ellipsoid of mitochondria and a myoid rich in rough endoplasmic reticulum, polysomes, Golgi zones and autophagic vacuoles. Double cones have extensive subsurface cisternae along their entire contiguous surfaces. Cone nuclei are large and vesicular and located close to or through the external limiting membrane. The synaptic pedicles of cones are larger, more electron lucent, and display more invaginated (ribbon) synapses as well as conventional (superficial) synaptic sites than do the rod spherules. Rod photoreceptors certainly undergo retinomotor movements and it is probable that cones do as well. In the light-adapted state the cone photoreceptors are arranged in a repeating square mosaic pattern with one single cone surrounded by four double (twin) cones.

Retinal pigment epithelial fine structure in the velvet cichlid (Astronotus ocellatus)

The morphology of the retinal pigment epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) have been studied by light and electron microscopy in the velvet cichlid (Astronotus ocellatus). The RPE is composed of a single layer of large columnar cells. The basal (scleral) border of these cells is minimally infolded, whereas the apical (vitreal) surface displays numerous pigment-laden processes which in light-adaptation surround both rod and cone outer segments. Laterally the RPE cells are joined by a series of basally located tight junctions. Wandering phagocytes are a constant feature within this epithelial membrane. The RPE cells display a large, vesicular nucleus, numerous mitochondria, much smooth endoplasmic reticulum, polysomes, myeloid bodies, phagosomes and melanosomes. Rough endoplasmic reticulum is relatively scarce within these cells. Although only light-adapted specimens were examined, it is thought that the melanosomes are capable of extensive retinomotor movement. The endothelium of the choriocapillaris facing Bruch's membrane is typically very thin but shows few fenestrations. Bruch's membrane is typical of other teleost species in that it is composed of only three layers.

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. As regards the retinal part of the visual system specifically, it would follow that the dopaminergic pathways are likely to be involved in a variety of functions including sensitivity control, chromaticity, spatial resolution/acuity and temporal signalling.

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 μm 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 μm. 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 μm (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 μm min −1; at mid-night cones contracted immediately and fully at 4·0±0·2 μm 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. In contrast, circadian phase did affect dark-induced cone myoid elongation: both the delay before initiation and the final extent of cone elongation were strongly influenced by circadian phase. At dusk, elongation began immediately after dark onset and plateaued at 58 μm, while at dawn, elongation was delayed by 50 min and plateaued at 30 μm. At mid-day, elongation began 30 min after dark onset and plateaued at 25 μm. Our observations indicate that circadian phase has significant effects on the time of initiation and the final extent of dark-induced cone elongation, but has little or no effect on light-induced cone contraction at light intensities between 30 and 5000 lx.

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.

Dopamine Induces Light‐Adaptive Retinomotor Movements in Bullfrog Cones via D2 Receptors and in Retinal Pigment Epithelium via D1 Receptors

In the eyes of lower vertebrates, retinal photoreceptors and melanin pigment granules of the retinal pigment epithelium (RPE) exhibit characteristic retinomotor movements in response to changes in ambient illumination and to signals from an endogenous circadian clock. We previously reported that 3,4-dihydroxyphenylethylamine (dopamine) mimicked the effect of light on these movements in photo-receptors and RPE cells of green sunfish, Lepomis cyanellus, by interacting with D2 dopaminergic receptors. Here, we report that dopamine also mimics the effect of light on cone and RPE retinomotor movements in bullfrogs, Rana catesbeiana, i.e., dopamine induces cone contraction and RPE pigment dispersion. Dopamine induced cone contraction in isolated dark-adapted bullfrog retinas incubated in constant darkness in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). This effect of dopamine was inhibited by a D2 but not a D1 antagonist and mimicked by a D2 but not a D1 agonist. These results suggest that induction of cone contraction by dopamine is mediated by D2 dopaminergic receptors and that cone adenylate cyclase activity is inhibited. Thus, dopamine acts via the same type of receptor in both bullfrog and green sunfish retinas to induce cone contraction. In contrast, dopamine influences RPE retinomotor movement via different receptors in fish and bullfrog. Dopamine induced light-adaptive pigment dispersion in isolated dark-adapted bullfrog RPE-eyecups incubated in constant darkness in normal Ringer's solution. Because the retina was not present, these experiments demonstrate a direct effect of dopamine on bullfrog RPE. This effect of dopamine on bullfrog RPE was inhibited by a D1 but not a D2 antagonist and mimicked by a D1 but not a D2 agonist. Furthermore, agents that increase the concentration of intracellular cyclic AMP also induced pigment dispersion in dark-adapted bullfrog RPE-eyecups incubated in the dark. These results suggest that dopamine induces pigment dispersion in bullfrog RPE via D1 dopaminergic receptors. Thus, dopamine acts via different receptors on bullfrog (D1) versus green sunfish (D2) RPE to induce pigment dispersion. In addition, inhibitor studies indicate that pigment dispersion is actin dependent in teleost but not in bullfrog RPE. Dopamine-induced pigment dispersion was inhibited by cytochalasin D in isolated RPE sheets of green sunfish but not in RPE-eyecups of bullfrogs. Together, these observations indicate that dopamine mimics the effect of light on cone and RPE retinomotor movements in both fish and bullfrogs. However, in the RPE, different receptors mediate the effect of dopamine, and different cytoskeletal mechanisms are used to affect pigment transport.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structural and volumetric changes of lamprey photoreceptor cells during light and dark periods

Short photoreceptor cells of the lamprey retina exhibited a 30% increase in the width of the myoid process and a 20% increase in that of the axonal process during a 12-h light period, compared to the measurements obtained during a 12-h dark period. An increase in the amount of cytoplasm, dilation of ER cisterns, and swelling of the nucleus appeared to cause the enlargement of the myoid parts. Accumulation of synaptic vesicles occurred concurrently with a thickening of the axonal process. These morphological changes presumably represent a phase of the diurnal cycle and current synaptic activity of the short cell. By contrast, the long photorecpetor cell showed neither measurable changes nor any indication of “retinomotor movement”.

Retinomotor movements in the frog retinal pigment epithelium: Dependence of pigment migration on Na + and Ca 2+

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 O 2. 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 Ca 2+, (b) high external Na +, and (c) drugs expected to increase the cytoplasmic levels of either Na +, or Ca 2+, like ouabain, caffeine and the ionophore A23187. However, the inhibition caused by low Ca 2+ did not occur if Na + was also reduced in the incubation medium. On the other hand, an increase in the concentration of external Ca 2+ or the addition of Co 2+ 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 Ca 2+ dependence of RPE movements, and are compatible with current views on ionic mechanisms in analogous systems of intracellular transport.

Ontogenesis of Diurnal Rhythms of cAMP Concentration, Outer Segment Disc Shedding and Retinomotor Movements in the Eye of the Brown Trout, &lt;i&gt;Salmo trutta&lt;/i&gt;

Histological analyses of the retina of Salmo trutta post-embryos exhibit rhythmic disc shedding at 6 weeks, followed by cyclic retinomotor movements at 7 weeks. Diurnal variations in cAMP concentration, as established by a radioreceptor assay, develop simultaneously with retinomotor movements and are fully expressed at 9.5 weeks. In continuous darkness, neither retinomotor movements nor intra-ocular peaks of cAMP occur, indicating that the endogenous oscillator for retinomotor movements and cAMP levels is not yet developed.

Rods trigger light adaptive retinomotor movements in all spectral cone types of a teleost fish

Action spectra for light induced cone contraction are described for the two spectral cone types of a dichromatic (red/green) cichlid species ( Aequidens pulcher). Criterion response thresholds (50% of maximal response amplitude) were determined for seven wavelengths. After correcting for absorption by cornea and lens, the resulting action spectra were compared with the absorption spectra of the rod and cone visual pigments. We find (i) that the action spectra of red and green sensitive cones are almost identical and (ii) matched most closely the absorption spectrum of the rod visual pigment. We therefore conclude that light adaptive cone contraction is triggered by light absorption in rods, which in the dark adapted state are located next to the external limiting membrane and are therefore in an optimal position to capture the incident light during early dawn. Possible mechanisms of signal transfer from rods to cones are discussed.

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.

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.

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

In the accompanying paper we reported that 3,4-dihydroxyphenylethylamine (dopamine) induced light-adaptive retinomotor movements in teleost photoreceptors and that this effect was mediated by D2 dopamine receptors located on the photoreceptors themselves. In this study, we investigated the effects on cone retinomotor movement of three agents that have been reported by others to modulate retinal dopamine release: gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT, serotonin), and melatonin. We report here that the GABA antagonists bicuculline and picrotoxin induced light-adaptive cone contraction in dark-adapted green sunfish retinas cultured in constant darkness; thus they mimic the effect of light or exogenously applied dopamine. Since their effects were blocked by either the D2 dopamine antagonist sulpiride or by Co2+, it seems likely that these agents act by enhancing retinal dopamine release. The GABA agonist muscimol produced effects opposite to those of GABA antagonists. Muscimol inhibited light-induced cone contraction in previously dark-adapted retinas and induced dark-adaptive cone elongation in light-adapted retinas. These results suggest that in green sunfish retinas, as has been reported for other retinas, GABA inhibits dopamine release. 5-HT induced light-adaptive cone contraction in dark-adapted retinas; thus 5-HT also mimics the effect of light or exogenously applied dopamine. The effect of 5-HT was blocked by sulpiride, Co2+, or the 5-HT antagonist mianserin. These results suggest that 5-HT induces cone contraction by stimulating dopamine release. Melatonin neither inhibited dopamine-induced cone contraction in retinas cultured in the dark nor induced cone elongation in retinas cultured in the light. Our results suggest that both GABA and 5-HT (but not melatonin) affect cone retinomotor movements in green sunfish by modulating dopamine release: GABA by inhibiting and 5-HT by stimulating dopamine release. We report in the companion paper that dopamine induced contraction in isolated cone fragments. Together these observations strongly suggest that dopamine serves as the final extracellular messenger directly inducing light-adaptive cone retinomotor movement, and that GABA and 5-HT affect these movements by modulating dopamine release.

Circadian regulation of retinomotor movements. I. Interaction of melatonin and dopamine in the control of cone length.

In lower vertebrates, cone retinomotor movements occur in response to changes in lighting conditions and to an endogenous circadian clock. In the light, cone myoids contract, while in the dark, they elongate. In order to test the hypothesis that melatonin and dopamine may be involved in the regulation of cone movement, we have used an in vitro eyecup preparation from Xenopus laevis that sustains light- and dark-adaptive cone retinomotor movement. Melatonin mimics darkness by causing cone elongation. Dark- and melatonin-induced cone elongation are blocked by dopamine. Dopamine also stimulates cone contraction in dark-adapted eyecups. The effect of dopamine appears to be mediated specifically by a dopamine receptor, possibly of the D2 type. The dopamine agonist apomorphine and the putative D2 agonist LY171555 induced cone contraction. In contrast, the putative D1 agonist SKF38393-A and specific alpha 1-, alpha 2-, and beta-adrenergic receptor agonists were without effect. Furthermore, the dopamine antagonist spiroperidol not only blocked light-induced cone contraction, but also stimulated cone elongation in the light. These results suggest that dopamine is part of the light signal for cone contraction, and that its suppression is part of the dark signal for cone elongation. Melatonin may affect cone movement indirectly through its influence on the dopaminergic system.

Dopamine inhibits forskolin- and 3-isobutyl-1-methylxanthine-induced dark-adaptive retinomotor movements in isolated teleost retinas.

We have been investigating the mechanisms of diurnal and circadian regulation of teleost retinomotor movements. In the retinas of lower vertebrates, photoreceptors and melanin pigment granules of the retinal pigment epithelium (RPE) undergo movements at dawn and dusk. These movements continue to occur at subjective dawn and dusk in animals maintained in constant darkness. Cone myoids contract at dawn and elongate at dusk; RPE pigment disperses into the epithelial cells' long apical processes at dawn and aggregates into the cell bodies at dusk. We report here that forskolin, an adenylate cyclase activator, and 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor, each induces dark-adaptive cone and RPE retinomotor movements in isolated light-adapted green sunfish retinas cultured in constant light. Forskolin induces a 22-fold elevation in retinal cyclic AMP content. Forskolin- and IBMX-induced movements are inhibited approximately 65% and 95%, respectively, by 3,4-dihydroxyphenylethylamine (dopamine). However, dopamine does not inhibit dark-adaptive movements induced by dibutyryl cyclic AMP. Epinephrine is much less effective than dopamine in inhibiting forskolin-induced movements, while phenylephrine and clonidine are totally ineffective. These results are consistent with our previous findings that treatments that increase intracellular cyclic AMP content promote dark-adaptive retinomotor movement. They further suggest that dopamine inhibits adenylate cyclase activity in photoreceptors and RPE cells and thereby favors light-adaptive retinomotor movements.