D1 dopamine receptors in the rat retina: effect of dark adaptation and chronic blockade by SCH 23390

The development of and recovery from form-deprivation myopia in infant rhesus monkeys reared under reduced ambient lighting

Although reduced ambient lighting ("dim" light) can cause myopia in emmetropizing chicks, it does not necessarily lead to myopic changes in emmetropizing rhesus monkeys. Because myopia is rarely spontaneous, a question remained whether dim light would hasten the progression of visually induced myopia. To determine the effects of dim light on the development of and recovery from form-deprivation myopia (FDM), seven 3-week-old infant rhesus monkeys were reared under dim light (mean ± SD = 55 ± 9 lx) with monocular diffuser spectacles until ~154 days of age, then maintained in dim light with unrestricted vision until ~337 days of age to allow for recovery. Refractive errors, corneal powers, ocular axial dimensions and sub-foveal choroidal thicknesses were measured longitudinally and compared to those obtained from form-deprived monkeys reared under typical laboratory lighting (504 ± 168 lx). Five of the seven subjects developed FDMs that were similar to those observed among their normal-light-reared counterparts. The average degree of form-deprivation-induced myopic anisometropia did not differ significantly between dim-light subjects (-3.88 ± 3.26D) and normal-light subjects (-4.45 ± 3.75D). However, three of the five dim-light subjects that developed obvious FDM failed to exhibit any signs of recovery and the two monkeys that were isometropic at the end of the treatment period manifest abnormal refractive errors during the recovery period. All refractive changes were associated with alterations in vitreous chamber elongation rates. It appears that dim light is not a strong myopiagenic stimulus by itself, but it can impair the optical regulation of refractive development in primates.

Atypical pattern of rod electroretinogram modulation by recent light history: a possible biomarker of seasonal affective disorder

Our goal was to challenge both normal controls and patients with seasonal affective disorders (SAD) to various light histories and then measure their retinal response modulation using the electroretinogram (ERG) in both winter and summer. In winter and summer, 11 normal controls and 12 SAD patients were exposed to three different light conditions for 1 h (10,000, 100 and 5 lux) followed by an ERG. Groups showed similar ERG amplitudes in the 100 lux condition. Compared with the 100-lux condition, in controls, the ERG response was significantly increased in the 5-lux condition; in SAD, it was significantly decreased in the 10,000-lux condition. This pattern was present in both seasons. This is the first time a retinal response modulation anomaly has been observed in SAD patients in both the depressed and euthymic states. Retinal response modulation may represent an interesting biomarker of the disease for future research.

Dopamine receptor localization in the mammalian retina

After a short history of dopamine receptor discovery in the retina and a survey on dopamine receptor types and subtypes, the distribution of dopamine receptors in the retinal cells is described and correlated with their possible role in cell and retinal physiology. All the retinal cells probably bear dopamine receptors. For example, the recently discovered D1B receptor has a possible role in modulating phagocytosis by the pigment epithelium and a D4 receptor is likely to be involved in the inhibition of melatonin synthesis in photoreceptors. Dopamine uncouples horizontal and amacrine cell-gap junctions through D1-like receptors. Dopamine modulates the release of other transmitters by subpopulations of amacrine cells, including that of dopamine through a D2 autoreceptor. Ganglion cells express dopamine receptors, the role of which is still uncertain. Müller cells also are affected by dopamine. A puzzling action of dopamine is observed in the ciliary retina, in which D1- and D2-like receptors are likely to be involved in the cyclic regulation of intraocular pressure. Most of the dopaminergic actions appear to be extrasynaptic and the signaling pathways remain uncertain. Further studies are needed to better understand the multiple actions of dopamine in the retina, especially those that implicate rhythmic regulations.

Tracers for imaging melanin with positron emission tomography

The melanin binding properties of six radioligands were determined in vivo in the eyes of pigmented mice. Binding in the eyes of nonpigmented mice was used to assess nonmelanin binding characteristics. Of these radioligands, 3H-N-methylspiperone showed the best uptake and gave the best signal-to-noise ratio at all time points examined. Its binding appeared essentially irreversible. A PET study with 11C-N-methylspiperone was therefore carried out in a patient with a small ocular melanoma. Increased uptake of 11C-N-methylspiperone was observed in the melanoma. Our studies indicate that PET and radiolabeled NMSP might be used for imaging melanin and for the detection of pigmented melanoma. These results suggest that with a high resolution PET camera it may be feasible to image the melanin-containing cells (dopaminergic neurons) of the substantia nigra in the central nervous system, which could be of interest for the study of Parkinson's disease.

Immunohistochemical localization of dopamine D1 receptors in rat retina

We have localized the dopamine D1 receptor in rat retina using a subtype-specific monoclonal antibody. Immunolabelling can be detected in the inner and outer plexiform layers and in a number of cells in the inner nuclear layer. In the inner plexiform layer, labelled processes form four distinct horizontal bands and a series of patches. In order further to characterize the labelling pattern of the D1 receptor antibody, double-labelling experiments were performed with antibodies against population-specific neuronal markers in the retina. Antibodies against tyrosine hydroxylase, choline acetyltransferase, calretinin, calbindin, the glutamate transporter GLT-1, protein kinase C, recoverin and parvalbumin were co-applied with the D1 receptor antibody. With these cell markers we demonstrate that horizontal cells, at least three types of cone bipolar cells and a small number of amacrine cells are immunolabelled for the D1 receptor. In the inner plexiform layer, processes labelled by the D1 receptor antibody are co-stratified with processes labelled by the GLT-1 antibody. D1 receptor-labelled processes are not co-localized with the processes of amacrine cells and ganglion cells labelled by antibodies against tyrosine hydroxylase, choline acetyltransferase or calretinin. Our results indicate that dopamine D1 receptors are localized predominantly to horizontal cells and cone bipolar cells. Furthermore, the spatial disparity between dopaminergic processes and the site of the majority of D1 receptors supports the idea that in the retina dopamine acts as a neuromodulator that diffuses through extracellular space. The localization of D1 receptors to a number of identified cell types enables future physiological work to be directed towards specific synaptic circuits within the retina.

Dopaminergic transmission in the rat retina: evidence for volume transmission

The study was designed to determine whether dopaminergic neurotransmission in the retina can operate via volume transmission. In double immunolabelling experiments, a mismatch as well as a match was demonstrated in the rat retina between tyrosine hydroxylase (TH) and dopamine (DA) immunoreactive (ir) terminals and cell bodies and dopamine D2 receptor-like ir cell bodies and processes. The match regions were located in the inner nuclear and plexiform layers (D2 ir cell bodies plus processes). The mismatch regions were located in the ganglion cell layer, the outer plexiform layer, and the outer segment of the photoreceptor layer, where very few TH ir terminals can be found in relation to the D2 like ir processes. In similar experiments analyzing D1 receptor like ir processes versus TH ir nerve terminals, mainly a mismatch in their distribution could be demonstrated, with the D1 like ir processes present in the outer plexiform layer and the outer segment where a mismatch in D2 like receptors also exists. The demonstration of a mismatch between the localization of the TH terminal plexus and the dopamine D2 and D1 receptor subtypes in the outer plexiform layer, the outer segment and the ganglion cell layer (only D2 immunoreactivity (IR)) suggests that dopamine, mainly from the inner plexiform layer, may reach the D2 and D1 mismatch receptors via diffusion in the extracellular space. After injecting dopamine into the corpus vitreum, dopamine diffuses through the retina, and strong catecholamine (CA) fluorescence appears in the entire inner plexiform layer and the entire outer plexiform layer, representing the match and mismatch DA receptor areas, respectively. The DA is probably bound to D1 and D2 receptors in both plexiform layers, since the DA receptor antagonist chlorpromazine fully blocks the appearance of the DA fluorescence, while only a partial blockade is found after haloperidol treatment which mainly blocks D2 receptors. These results indicate that the amacrine and/or interplexiform DA cells, with sparse branches in the outer plexiform layer, can operate via volume transmission in the rat retina to influence the outer plexiform layer and the outer segment, as well as other layers of the rat retina such as the ganglion cell layer.

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.

Dopamine and melatonin interactions in the intact chicken eye. Electrooculographic and biochemical study

Electrophysiological and biochemical techniques were used to investigate the interactions between dopamine (DA) and melatonin (MEL) in the intact chicken eye. Endogenous DA depletion induced by intraocular administration of alpha-methyl-para-tyrosine (alpha-MPT), a selective tyrosine hydroxylase inhibitor, decreases the transepithelial potential (TEP) of the retinal pigment epithelium and reduces the light peak (LP) recorded by an indirect electro-oculographic (EOG) method. An intraocular injection of MEL also reduces the TEP but does not reduce the LP. Retinal MEL is increased after endogenous DA depletion and a tight inverse correlation between DA and MEL contents was found. The present data, together with other findings support the hypothesis (1) that in the intact chicken eye, DA and MEL play respectively a role of light and dark signals on the TEP, and (2) that a balance between these two neurohormones may be responsible for the regulation of RPE events which are dependent on light-dark conditions.

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.

Dopamine- and adenosine-3':5'-monophosphate (cAMP)-regulated phosphoprotein of Mr 32,000 (DARPP-32) in the retina of cat, monkey and human

The cellular localization of a dopamine- and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32) was investigated in cat, monkey and human retina by immunohistochemistry. In cat, DARPP-32-immunoreactive cell bodies identified as Müller cells were demonstrated in the inner nuclear layer (INL) with processes closely surrounding the cell soma of photoreceptors in the outer nuclear layer. Some DARPP-32-IR cells were also seen in the nerve fiber layer (NFL) sending processes to the inner plexiform layer. In monkey and human retina, DARPP-32-IR cell bodies were also demonstrated in the INL, with few cells located in the NFL.

Effects of dark-rearing on the retinal dopaminergic system in the neonatal and postnatal guinea pig

To appraise the extent of light responsivity of the retinal dopaminergic system in the early postnatal period, dopamine content (DA) of retinas of cycling-light-reared and dark-reared guinea pigs was determined. DA storage postnatally was suppressed in dark-reared animals. Their retinas responded to a brief light stimulus by rapidly increasing DOPA synthesis and DA utilization, but not to the extent of the response of cycling-light-reared animals. Dopaminergic retinal neurones depend on light stimulus for the complete development of some of their neurochemical characteristics.

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.

Multiple fluorescent ligands for dopamine receptors. II. Visualization in neural tissues

Selective dopamine receptor ligands, (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4, 5-tetrahydro-3-methyl-[1H]-3-benzazepin-7-ol, the 4'-amino derivative of the high affinity D1 receptor antagonist SCH 23390, the high affinity D2 receptor antagonist N-(p-aminophenethyl)-spiperone or NAPS, and the D2 selective agonist, 2-(N-phenethyl-N-propyl)-amino-5-hydroxytetralin or PPHT were chemically coupled to the fluorescent compounds, Bodipy, Cascade blue, coumarin, fluorescein, rhodamine, or Texas red. The utility of the 6 fluorescent moieties linked to the 3 dopamine receptor binding ligands for anatomical study of regional and cellular distribution patterns of the two dopaminergic receptor subtypes has been assessed in frozen sections of the rat striatum and compared to our previous report using the rhodamine-labeled antagonists. The regional staining for the two dopaminergic receptor binding sites supports previous work using in vitro receptor autoradiographic analyses; the D1 receptor binding was more robust than that of D2 receptors in the caudate nucleus. The cellular element which most frequently expressed striatal D1 binding sites had a medium-diameter cell body. Medium-sized cells also exhibited fluorescence for the D2 binding site, as did a much larger diameter element; potentially the cholinergic interneuron of the caudate nucleus. The pharmacological specificity for each of the different D1 fluorescent antagonist ligands in the tissues was determined by competition with 100-fold excess of unlabeled SCH 23390 (non-specific binding), spiroperidol (binding selectivity), the stereoactive paired isomers of butaclamol, and the serotonin 5-HT2 receptor antagonist ketanserin. The same criteria were used to assess the different D2 fluorescent agonist and antagonist ligand derivatives. The anatomical efficacy of these novel ligands was determined using selective dichroic filters to stimulate the fluorescent moieties in the optimal excitation wavelength, and the amount of fluorescent dopamine receptor binding was photographically measured and contrasted for each of the newly synthesized fluoroprobes. Using the most pharmacologically specific and anatomically efficient of these novel fluoroprobes, we determined the localization pattern of the D1 and D2 dopamine receptor binding sites in tissues reported to exhibit both subtypes of the receptor. The cellular distribution of the dopamine receptor binding sites was determined concurrently using fluoroprobes in the forebrain, mesencephalon, pituitary, retina, and superior cervical ganglion of the rodent, and bovine adrenal medullary chromaffin cells were examined using the rhodamine-labeled antagonists.

Effect of in vivo modulation of membrane docosahexaenoic acid levels on the dopamine-dependent adenylate cyclase activity in the rat retina

We have studied the effect of a dietary deprivation of n-3 fatty acids on the activity of the dopamine (DA)-dependent adenylate cyclase in the rat retina. Experiments were conducted in 6-month-old rats raised on semipurified diets containing either safflower oil (n-3 deficient diet) or soybean oil (control diet). The levels of docosahexaenoic acid [22:6 (n-3)] in retinal phospholipids were significantly decreased in n-3 deficient rats (35-42% of control levels). This was compensated by a rise in 22:5 (n-6), the total content of polyunsaturated fatty acids (PUFA) remaining approximately constant. Adenylate cyclase activity was measured in retinal membrane preparations from dark-adapted or light-exposed rats. The enzyme activity was stimulated by DA and SKF 38393 in a light-dependent fashion. The activation was lower in rats exposed to light than in dark-adapted animals, suggesting a down-regulation of the D1 DA receptors by light. The activation by guanine nucleotides and forskolin was also decreased in light-exposed rats. There was no significant effect of the dietary regimen on the various adenylate cyclase activities and their response to light. Furthermore, the guanine nucleotide- and DA-dependent adenylate cyclase activities of retinal membranes were found to be relatively resistant to changes in membrane fluidity induced in vitro by benzyl alcohol. The results indicate that in the absence of changes in total PUFA content, a decreased ratio of n-3 to n-6 fatty acids in membrane phospholipids does not significantly affect the properties of adenylate cyclase in the rat retina.

Differential effect of aging on 3H-SCH 23390 binding sites in the retina and in distinct areas of the rat brain

The effects of age on the binding parameters of the selective D-1 dopamine (DA) receptor antagonist 3H-SCH 23390 were studied in membrane preparations from rat striatum, substantia nigra, olfactory tubercle, prefrontal cortex and retina. When compared with 3-month-old animals, there was a significant decrease in the density of 3H-SCH 23390 binding sites in the striatum (-25%), substantia nigra (-24%), and olfactory tubercle (-23%), but not in the prefrontal cortex of senescent (23-month-old) rats. The affinity of 3H-SCH 23390 for D-1 DA receptors did not change with age in any of the brain areas analyzed. In contrast, the density of 3H-SCH 23390 binding sites was higher (+53%) in the retina of aged rats that in 3-month-old controls. Confirming previous studies, we observed that light deprivation induced a significant increment in the density of 3H-SCH 23390 binding sites in the retina of adult rats (+31%) but not in the retina of aged animals. The ability of light exposure to activate DAergic neurons in the rat retina was not altered by normal aging. In fact, a similar increase in the concentration of DOPAC was observed in the retina of light-adapted adult and senescent rats when compared to their respective dark-adapted controls (+94% and +95%, respectively). The results indicate that aging has a differential effect on D-1 DA receptors in the retina and different areas of the rat brain. Finally, the age-related increment in the density of retinal D-1 DA receptors does not appear to depend on presynaptic mechanisms, since DA metabolism is increased by light to the same extent in young and aged rats.

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.

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.

Failure of dark adaptation to upregulate D-1 dopamine receptors in retina of senescent rats

The effect of aging on the binding parameters of 3H-SCH 23390, the most selective ligand of D-1 DA receptors, was studied in membrane preparations from the rat retina. DA-stimulated adenylate cyclase activity was also measured in order to better characterize the changes in retinal D-1 DA receptors induced by aging. The binding studies revealed that the density of 3H-SCH 23390 was increased (34 and 73%) in the retina of 14- and 26-month-old rats, when compared to young adult animals, respectively. In contrast, aging failed to alter the sensitivity of the adenylate cyclase to the action of DA. In fact, DA (10(-6) M to 10(-4) M) elicited a similar enhancement in cyclic AMP formation in retinal homogenates of both adult and senescent rats. Since dark adaptation increases the density of D-1 DA receptors in the retina of adult rats we studied the effect of light deprivation on 3H-SCH 23390 binding and DA-sensitive adenylate cyclase activity in the retina of senescent rats. As previously shown (25) light deprivation increased 3H-SCH 23390 binding and enhanced DA-sensitive adenylate cyclase activity in the retina of young adult rats. On the contrary, dark adaptation failed to increase 3H-SCH 23390 binding and to enhance DA-sensitive adenylate cyclase activity in the retina of senescent rats. Taken together these results indicate that D-1 DA receptors in the retina of aged rats have biochemical and functional properties different from those found in the retina of adult animals; these changes may result in an altered response to the physiological stimuli elicited by environmental lighting.

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)

Involvement of D1 and D2 Dopamine Receptors in the Control of Horizontal Cell Electrical Coupling in the Turtle Retina

We studied the actions of D1 and D2 dopamine agonists and antagonists on the coupling of horizontal cell axons in the turtle retina by a combination of pharmacological and electrophysiological methods. Both D1 and D2 receptors were identified in membrane fractions by radioligand binding using [3H]-SCH 23390 and [3H]-spiperone, respectively. The KD of both receptor classes were identical (0.21 nM) but D1 receptor density exceeded that of D2 receptors by more than four-fold. D1 agonists increased the activity of adenylate cyclase in a dose-dependent manner, whereas D2 agonists were without significant effect by themselves, nor did D2 antagonists block the D1-mediated increase in adenylate cyclase activity. Intracellular recordings and Lucifer Yellow dye injections were used to characterize the modifications of the receptive field profile of horizontal cell axons (H1AT) exposed to different pharmacological agents. Dopamine or D1 agonists (0.05 - 10 microM) induced a marked constriction of the H1AT receptive field, whereas D2 agonists elicited a small expansion of the receptive field. However, in the presence of a D1 antagonist, as well as IBMX to inhibit phosphodiesterase, D2 agonists (10 - 70 microM) induced a marked increase in the receptive field profile. These results indicate that both D1 and D2 dopamine receptors play a role in shaping the receptive field profile of the horizontal cell axon terminal in the turtle retina.