Evidence for a D2 dopamine receptor in frog retina that decreases cyclic AMP accumulation and serotonin N-acetyltransferase activity

Illuminating and Sniffing Out the Neuromodulatory Roles of Dopamine in the Retina and Olfactory Bulb

In the central nervous system, dopamine is well-known as the neuromodulator that is involved with regulating reward, addiction, motivation, and fine motor control. Yet, decades of findings are revealing another crucial function of dopamine: modulating sensory systems. Dopamine is endogenous to subsets of neurons in the retina and olfactory bulb (OB), where it sharpens sensory processing of visual and olfactory information. For example, dopamine modulation allows the neural circuity in the retina to transition from processing dim light to daylight and the neural circuity in the OB to regulate odor discrimination and detection. Dopamine accomplishes these tasks through numerous, complex mechanisms in both neural structures. In this review, we provide an overview of the established and emerging research on these mechanisms and describe similarities and differences in dopamine expression and modulation of synaptic transmission in the retinas and OBs of various vertebrate organisms. This includes discussion of dopamine neurons’ morphologies, potential identities, and biophysical properties along with their contributions to circadian rhythms and stimulus-driven synthesis, activation, and release of dopamine. As dysregulation of some of these mechanisms may occur in patients with Parkinson’s disease, these symptoms are also discussed. The exploration and comparison of these two separate dopamine populations shows just how remarkably similar the retina and OB are, even though they are functionally distinct. It also shows that the modulatory properties of dopamine neurons are just as important to vision and olfaction as they are to motor coordination and neuropsychiatric/neurodegenerative conditions, thus, we hope this review encourages further research to elucidate these mechanisms.

Circadian regulation in the retina: From molecules to network

The mammalian retina is the most unique tissue among those that display robust circadian/diurnal oscillations. The retina is not only a light sensing tissue that relays light information to the brain, it has its own circadian "system" independent from any influence from other circadian oscillators. While all retinal cells and retinal pigment epithelium (RPE) possess circadian oscillators, these oscillators integrate by means of neural synapses, electrical coupling (gap junctions), and released neurochemicals (such as dopamine, melatonin, adenosine, and ATP), so the whole retina functions as an integrated circadian system. Dysregulation of retinal clocks not only causes retinal or ocular diseases, it also impacts the circadian rhythm of the whole body, as the light information transmitted from the retina entrains the brain clock that governs the body circadian rhythms. In this review, how circadian oscillations in various retinal cells are integrated, and how retinal diseases affect daily rhythms.

Dopamine: A Modulator of Circadian Rhythms in the Central Nervous System

Circadian rhythms are daily rhythms that regulate many biological processes – from gene transcription to behavior – and a disruption of these rhythms can lead to a myriad of health risks. Circadian rhythms are entrained by light, and their 24-h oscillation is maintained by a core molecular feedback loop composed of canonical circadian (“clock”) genes and proteins. Different modulators help to maintain the proper rhythmicity of these genes and proteins, and one emerging modulator is dopamine. Dopamine has been shown to have circadian-like activities in the retina, olfactory bulb, striatum, midbrain, and hypothalamus, where it regulates, and is regulated by, clock genes in some of these areas. Thus, it is likely that dopamine is essential to mechanisms that maintain proper rhythmicity of these five brain areas. This review discusses studies that showcase different dopaminergic mechanisms that may be involved with the regulation of these brain areas’ circadian rhythms. Mechanisms include how dopamine and dopamine receptor activity directly and indirectly influence clock genes and proteins, how dopamine’s interactions with gap junctions influence daily neuronal excitability, and how dopamine’s release and effects are gated by low- and high-pass filters. Because the dopamine neurons described in this review also release the inhibitory neurotransmitter GABA which influences clock protein expression in the retina, we discuss articles that explore how GABA may contribute to the actions of dopamine neurons on circadian rhythms. Finally, to understand how the loss of function of dopamine neurons could influence circadian rhythms, we review studies linking the neurodegenerative disease Parkinson’s Disease to disruptions of circadian rhythms in these five brain areas. The purpose of this review is to summarize growing evidence that dopamine is involved in regulating circadian rhythms, either directly or indirectly, in the brain areas discussed here. An appreciation of the growing evidence of dopamine’s influence on circadian rhythms may lead to new treatments including pharmacological agents directed at alleviating the various symptoms of circadian rhythm disruption.

Cyclic AMP-dependent regulation of tyrosine hydroxylase mRNA and immunofluorescence levels in rat retinal precursor cells

Stimulation of tyrosine hydroxylase (TH) gene transcription by cyclic AMP (cAMP) has been clearly established in adrenal medula cells and neural-crest-derived cell lines but information on this mechanism is still lacking in dopaminergic neurons. Because they are easily amenable to in vitro experiments, dopaminergic amacrine cells of the retina might constitute a valuable model system to study this mechanism. We have used real-time reverse transcription with the polymerase chain reaction to quantify TH mRNA levels in the rat retina during post-natal development and in retinal precursor cells obtained from neonatal rats and cultured for 3 days in serum-free medium. Whereas the TH mRNA concentration remains consistantly low in control cultures, treatment with cAMP-increasing agents (forskolin, membrane depolarization, phosphodiesterase inhibitors) is sufficient to raise it to the level observed in adult retina (15-fold increase). Treatment of the cultured cells can be delayed by up to 2 days with identical results at the TH mRNA level, thus ruling out a survival-promoting effect of cAMP. TH immunofluorescence has confirmed cAMP-dependent regulation of TH expression at the protein level and indicates that the frequency of TH-positive cells in the cultures is similar to that observed in the adult retina. Selective phosphodiesterase inhibitors suggest that PDE4 is the major subtype involved in the dopaminergic amacrine cell response. Our data clearly establish the cAMP-dependent regulation of TH mRNA and immunofluorescence levels in retinal precursor cells. The possible role of this regulation mechanism in the developmental activation of TH gene expression is discussed.

Circadian regulation of ion channels and their functions

Ion channels are the gatekeepers to neuronal excitability. Retinal neurons of vertebrates and invertebrates, neurons of the suprachiasmatic nucleus (SCN) of vertebrates, and pinealocytes of non-mammalian vertebrates display daily rhythms in their activities. The interlocking transcription-translation feedback loops with specific post-translational modulations within individual cells form the molecular clock, the basic mechanism that maintains the autonomic approximately 24-h rhythm. The molecular clock regulates downstream output signaling pathways that further modulate activities of various ion channels. Ultimately, it is the circadian regulation of ion channel properties that govern excitability and behavior output of these neurons. In this review, we focus on the recent development of research in circadian neurobiology mainly from 1980 forward. We will emphasize the circadian regulation of various ion channels, including cGMP-gated cation channels, various voltage-gated calcium and potassium channels, Na(+)/K(+)-ATPase, and a long-opening cation channel. The cellular mechanisms underlying the circadian regulation of these ion channels and their functions in various tissues and organisms will also be discussed. Despite the magnitude of chronobiological studies in recent years, the circadian regulation of ion channels still remains largely unexplored. Through more investigation and understanding of the circadian regulation of ion channels, the future development of therapeutic strategies for the treatment of sleep disorders, cardiovascular diseases, and other illnesses linked to circadian misalignment will benefit.

Essential roles of dopamine D4 receptors and the type 1 adenylyl cyclase in photic control of cyclic AMP in photoreceptor cells

Light and dopamine regulate many physiological functions in the vertebrate retina. Light exposure decreases cyclic AMP formation in photoreceptor cells. Dopamine D(4) receptor (D(4)R) activation promotes light adaptation and suppresses the light-sensitive pool of cyclic AMP in photoreceptor cells. The key signaling pathways involved in regulating cyclic AMP in photoreceptor cells have not been identified. In the present study, we show that the light- and D(4)R-signaling pathways converge on the type 1 Ca(2+)/calmodulin-stimulated adenylyl cyclase (AC1) to regulate cyclic AMP synthesis in photoreceptor cells. In addition, we present evidence that D(4)R activation tonically regulates the expression of AC1 in photoreceptors. In retinas of mice with targeted deletion of the gene (Adcy1) encoding AC1, cyclic AMP levels and Ca(2+)/calmodulin-stimulated adenylyl cyclase activity are markedly reduced, and cyclic AMP accumulation is unaffected by either light or D(4)R activation. Similarly, in mice with disruption of the gene (Drd4) encoding D(4)R, cyclic AMP levels in the dark-adapted retina are significantly lower compared to wild-type retina and are unresponsive to light. These changes in Drd4-/- mice were accompanied by significantly lower Adcy1 mRNA levels in photoreceptor cells and lower Ca(2+)/calmodulin-stimulated adenylyl cyclase activity in retinal membranes compared with wild-type controls. Reduced levels of Adcy1 mRNA were also observed in retinas of wild-type mice treated chronically with a D(4)R antagonist, L-745870. Thus, activation of D(4)R is required for normal expression of AC1 and for the regulation of its catalytic activity by light. These observations illustrate a novel mechanism for cross-talk between dopamine and photic signaling pathways regulating cyclic AMP in photoreceptor cells.

Inhibitory modulation of photoreceptor melatonin synthesis via a nitric oxide-mediated mechanism

Nitric oxide (NO) has been suggested to have many physiological functions in the vertebrate retina, including a role in light-adaptive processes. The aim of this study was to examine the influence of the NO-donor sodium nitroprusside (SNP) on the activity of arylalkylamine-N-acetyltransferase (AA-NAT; EC. 2.3.1.87), the activity of which responds to light and reflects the changes in retinal melatonin synthesis--a key feature of light-adaptive responses in photoreceptors. Incubation of dark-adapted and dark-maintained retinas with SNP lead to the NO-specific suppression of AA-NAT activity, with NO suppressing AA-NAT activity to a level similar to that seen in the presence of dopaminergic agonists or light. Increased levels of cGMP appeared to be causally involved in the suppression of AA-NAT activity by SNP, as non-hydrolysable analogues of cGMP and the cGMP-specific phosphodiesterase (PDE) inhibitor zaprinast also significantly suppressed AA-NAT activity, while an inhibitor of soluble guanylate cyclase blocked the effect of SNP. While this chain of events may not be part of the normal physiology of the retina, it could be important in pathological circumstances that are associated with marked increase in levels of cGMP, as is found to be the case in certain forms photoreceptor degeneration, which are produced by defects in cGMP phosphodiesterase activity.

Inhibitory interaction of cannabinoid CB1 receptor and dopamine D2 receptor agonists on voltage-gated currents of goldfish cones

Dopamine is a light-adaptive signal that desensitizes the retina, while cannabinoids reportedly increase photosensitivity. The presynaptic membrane of goldfish retinal cones has dopamine D2 receptors and cannabinoid CB1 receptors. This work focused on whether dopamine D2 receptor agonist quinpirole and cannabinoid CB1 receptor agonist WIN 55212-2 (WIN) interacted to modulate voltage-dependent membrane currents of cones. A conventional patch-clamp method was used to record depolarization evoked whole-cell outward currents (Iout) and an inward calcium current (ICa) from the inner segment of cones in goldfish retinal slices. WIN had biphasic actions: low concentrations (<1 μM) increased the currentsviaGs, while higher concentrations (>1 μM) decreased the currentsviaGi/Go. Neither dopamine nor the D2 agonist quinpirole (1–20 μM) had a significant effect on eitherIoutorICa. Quinpirole at 50 μM had a mild suppressive (∼20%) effect onIout. However, quinpirole (<10 μM) completely blocked the enhancement of both currents seen with 0.7 μM WIN. The effect of quinpirole was blocked by sulpiride and by pertussis toxin, indicating that quinpirole was actingviaa D2 receptor-Gi/o coupled mechanism. The suppressive action of 50 μM quinpirole (∼20%) was not additive with the suppressive effect of 3 μM WIN (∼40%). D2 agonistsviaGi/o oppose the action of low concentrations of CB1 agonists actingviaGs to modulate cone membrane currents, suggesting a role in shaping the cone light response and/or sensitivity to changes in ambient light conditions. The nonadditive effect of high concentrations of WIN and quinpirole suggests that both decrease membrane currentsviathe same transduction pathway, Gi/Go protein kinase A (PKA).

Regulation of the Circadian Oscillator in Xenopus Retinal Photoreceptors by Protein Kinases Sensitive to the Stress-activated Protein Kinase Inhibitor, SB 203580

Circadian rhythms are generated by transcriptional and translational feedback loops. Stress-activated protein kinases (SAPKs) are known to regulate transcription factors in response to a variety of extracellular stimuli. In the present study, we examined whether the SAPKs play a role in the circadian system in cultured Xenopus retinal photoreceptor layers. A 6-h pulse of SB 203580, an inhibitor of SAPKs, reset the circadian rhythm of melatonin in a phase-dependent manner similar to dark pulses. This phase-shifting effect was dose-dependent over the range of 1-100 microm. Treatment with SB 203580 also affected light-induced phase shifts, and light had no effect on the circadian oscillator in the presence of 100 microm SB 203580. In-gel kinase assays showed that SB 203580 selectively inhibited a small group of protein kinases in the photoreceptor cells. These SB 203580-sensitive kinases correspond to two isoforms of phosphorylated p38 MAPK and three isoforms of c-Jun N-terminal kinase (JNK). Further in vitro study demonstrated that SB 203580 also inhibited casein kinase Iepsilon (CKIepsilon), which has been shown to regulate circadian rhythms in several organisms. However, a pharmacological inhibition of CKI reset the circadian oscillator in a phase-dependent manner distinct from that of SB 203580. This argues against a primary role of CKI in the phase-shifting effects of SB 203580. These results suggest that SB 203580 affects the circadian system by inhibiting p38 MAPKs or JNKs and that these protein kinases are candidate cellular signals in the regulation of the circadian oscillator in the Xenopus retinal photoreceptors.

Effects of D1- and D2-dopamine receptor activation on melatonin synthesis in bovine pinealocytes

Previous studies have identified and characterized D1- and D2-dopamine receptors in bovine pineal glands. The data indicate that the density of D1-dopamine receptors (974 fmol/mg protein) far exceed that of D2-dopamine receptors (37 fmol/mg protein). The objective of this study was to identify the mRNAs for both D1- and D2-dopamine receptors and to elucidate the status of dopamine and its possible involvement in the pineal function, particularly on melatonin synthesis. The expression of these dopamine receptor subtypes were determined by using a reverse transcriptase-polymerase chain reaction technique with specific pairs of primers to amplify D1- and D2-dopamine receptor mRNAs. Amplification of RNAs from bovine striatum (positive control) and bovine pineal gland resulted in products of the predicted lengths of 231 bp for D1- and 333 bp for D2-dopamine receptors. The results indicate that both D1- and D2-dopamine receptor mRNAs are present in the bovine pineal gland. The role of dopamine receptors was investigated by studying the effects of selective D1- and D2-dopamine agonists and antagonists on the N-acetyltransferase (NAT) activity of cultured bovine pinealocytes. The data showed that SKF-38393, a selective D1-agonist, enhanced NAT activity, and increased melatonin level, and the stimulatory effect was blocked by SCH-23390, a D1-selective antagonist, whereas quinpirole, a selective D2-agonist, inhibited NAT basal activity and decreased the melatonin basal level. Furthermore the inhibitory effect was blocked by D2-selective antagonists, spiperone, haloperidol, and domperidone. The present results indicate that the pineal dopamine receptors have a distinct effect on pineal function. The precise mechanism whereby activation of dopamine receptors altered the NAT activity and melatonin level needs to be further delineated.

Circadian phase-dependent modulation of cGMP-gated channels of cone photoreceptors by dopamine and D2 agonist

The affinity of cGMP-gated ion channels (CNGCs) for cGMP in chick retinal cone photoreceptors is under circadian control. Here we report that dopamine (DA) and D2 receptor agonists evoke phase-dependent shifts in the affinity of CNGCs for activating ligand. Inside-out patch recordings from cultured chick cones were performed at circadian time (CT) 4-7 and CT 16-19 on the second day of constant darkness. Exposing intact cells to DA or the D2 agonist quinpirole for 2 hr before patch excision caused a significant increase in the K(D) for cGMP during the night (CT 16-19) but had no effect during the day (CT 4-7). DA or quinpirole treatment had no effect on the Hill slope or the average number of channels per patch. The effect of DA was blocked by the D2 antagonist eticlopride and was not mimicked by D1 agonists or blocked by D1 antagonists. By contrast, a brief (15 min) exposure to DA or quinpirole caused a decrease in K(D) during the subjective day and had no effect during the subjective night. Thus, the effect of D2 agonists depends on both the duration of agonist exposure and the time of day. Application of DA or quinpirole evoked a transient activation of the MAP kinase Erk (extracellular signal-related kinase) during the day but caused a sustained inhibition during the night. Conversely, D2 agonists caused activation of Ca2+/calmodulin-dependent protein kinase II during the night and inhibited this enzyme during the day. A circadian oscillator in cones appears to regulate the nature of the transduction cascade used by D2 receptors.

Photic regulation of heme oxygenase activity in the golden hamster retina: involvement of dopamine

The photic regulation of heme oxygenase (HO) activity was examined in the golden hamster retina. This enzymatic activity was significantly higher at midday than at midnight. When the hamsters were placed under constant darkness for 48 h and killed at subjective day or at subjective night, the differences in HO activity disappeared. Western blot analysis showed no differences in HO levels among these time points. Dopamine significantly increased this activity in retinas excised at noon or at midnight, with a higher sensitivity at night. The effect of dopamine was reversed by SCH 23390 but not by spiperone and clozapine and it was not reproduced by quinpirole. In vitro, the increase in HO activity found in retinas incubated under light for 1 h was significantly reduced by SCH 23390. Two cAMP analogs increased HO activity and their effect, as well as the effect of dopamine was blocked by H-89, a protein kinase A (PKA) inhibitor. Tin protoporphyrin IX, an HO inhibitor, significantly decreased cGMP accumulation with maximal effects during the day. Low concentrations of bilirubin decreased retinal thiobarbituric acid substances levels (an index of lipid peroxidation) in basal conditions and after exposing retinal cells to H2O2. These results suggest that hamster retinal HO activity is regulated by the photic stimulus, probably through a dopamine/cAMP/PKA dependent pathway.

Rhythmic changes in metabolism of dopamine in the chick retina: the importance of light versus biological clock

Rhythmic changes in dopamine (DA) content and metabolism were studied in retinas of chicks that were adapted to three different lighting conditions: 12-h light : 12-h dark (LD), constant darkness (DD) and continuous light (LL). Retinas of chicks kept under LD conditions exhibited light-dark-dependent variations in the steady-state level of DA and the two metabolites of DA, i.e. 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA). Concentrations of DA, DOPAC and HVA were high in light hours and low in dark hours of the LD illumination cycle. In retinas of chicks kept under DD, the content of DA, DOPAC and HVA oscillated in a rhythmic manner for 2 days, with higher values during the subjective light phase than during the subjective dark phase. The amplitudes of the observed oscillations markedly and progressively declined compared with the amplitudes recorded under the LD cycle. In retinas of chicks kept under LL conditions, levels of DA, DOPAC and HVA were similar to those found during the light phase of the LD cycle. Changes in the retinal contents of DA and HVA did not exhibit pronounced daily oscillations, while on the first day of LL the retinal concentrations of DOPAC were significantly higher during the subjective light phase than during the subjective dark phase. Acute exposure of chicks to light during the dark phase of the LD cycle markedly increased DA and DOPAC content in the retina. In contrast, light deprivation during the day decreased the retinal concentrations of DA and DOPAC. It is suggested that of the two regulatory factors controlling the level and metabolism of DA in the retina of chick, i.e. light and biological clock, environmental lighting conditions seem to be of major importance, with light conveying a stimulatory signal for the retinal dopaminergic cells.

Circadian rhythmicity in dopamine content of mammalian retina: role of the photoreceptors

Dopamine, the predominant retinal catecholamine, is a neurotransmitter and neuromodulator known to regulate light-adaptive retinal processes. Because dopamine influences several rhythmic events in the retina it is also a candidate for a retinal circadian signal. Using high performance liquid chromatography (HPLC), we have tested whether dopamine and its breakdown products are rhythmic in Royal College of Surgeons (RCS) rats with normal and dystrophic retinas. In both normal and mutant animals entrained to a 12-h light/12-h dark cycle, we found robust daily rhythms of dopamine and its two major metabolites. To address circadian rhythmicity of dopamine content, rats were entrained to light/dark cycles and released into constant darkness, using the circadian rhythm of wheel-running activity as a marker of each individual's circadian phase. Circadian rhythms of dopamine and metabolite content persisted in both wild type and retinally degenerate animals held for two weeks in constant darkness. Our results demonstrate for the first time clear circadian rhythms of dopamine content and turnover in a free-running mammal, and suggest that rods and cones are not required for dopamine rhythmicity.

Dysfunctional light-evoked regulation of cAMP in photoreceptors and abnormal retinal adaptation in mice lacking dopamine D4 receptors

Dopamine is a retinal neuromodulator that has been implicated in many aspects of retinal physiology. Photoreceptor cells express dopamine D4 receptors that regulate cAMP metabolism. To assess the effects of dopamine on photoreceptor physiology, we examined the morphology, electrophysiology, and regulation of cAMP metabolism in mice with targeted disruption of the dopamine D4 receptor gene. Photoreceptor morphology and outer segment disc shedding after light onset were normal in D4 knock-out (D4KO) mice. Quinpirole, a dopamine D2/D3/D4 receptor agonist, decreased cAMP synthesis in retinas of wild-type (WT) mice but not in retinas of D4KO mice. In WT retinas, the photoreceptors of which were functionally isolated by incubation in the presence of exogenous glutamate, light also suppressed cAMP synthesis. Despite the similar inhibition of cAMP synthesis, the effect of light is directly on the photoreceptors and independent of dopamine modulation, because it was unaffected by application of the D4 receptor antagonist l-745,870. Nevertheless, compared with WT retinas, basal cAMP formation was reduced in the photoreceptors of D4KO retinas, and light had no additional inhibitory effect. The results suggest that dopamine, via D4 receptors, normally modulates the cascade that couples light responses to adenylyl cyclase activity in photoreceptor cells, and the absence of this modulation results in dysfunction of the cascade. Dark-adapted electroretinogram (ERG) responses were normal in D4KO mice. However, ERG b-wave responses were greatly suppressed during both light adaptation and early stages of dark adaptation. Thus, the absence of D4 receptors affects adaptation, altering transmission of light responses from photoreceptors to inner retinal neurons. These findings indicate that dopamine D4 receptors normally play a major role in regulating photoreceptor cAMP metabolism and adaptive retinal responses to changing environmental illumination.

Regulation of cAMP by light and dopamine receptors is dysfunctional in photoreceptors of dystrophic retinal degeneration slow(rds) mice

cAMP levels in dark and light were studied in dystrophic retinal degeneration slow(rds) mice, which carry a mutation in the rds/peripherin gene. cAMP levels were measured in vivo, in freshly isolated retinas, and in vitro in the presence of glutamate, which confines light modulation to photoreceptors. Experiments were conducted on young animals, when significant numbers of viable photoreceptor cells are present. In vivo levels of cAMP are higher in illuminated rds/rds retinas than levels measured in normal BALB/c retinas. Light-evoked down-regulation of cAMP levels was observed in vitro in normal photoreceptors. These measurements were made in the presence of the cyclic nucleotide phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine; therefore, they reflect an inhibition of cAMP formation. In contrast, light had no effect on cAMP formation in photoreceptors of mutant mice, measured under identical conditions. Thus, elevated levels of cAMP in rds/rds retinas in vivo result from abnormalities in cAMP synthesis in the mutant photoreceptor cells. In addition to regulation by light, cAMP formation in photoreceptor cells is regulated by dopamine, acting through dopamine D4 receptors. A dopamine D2/D4 receptor agonist, quinpirole, reduced cAMP levels in dark-adapted normal retinas in vitro, but not in rds/rds retinas. Our data indicate that alterations in a signal transduction pathway that leads to inhibition of adenylyl cyclase might underlie the abnormalities in cAMP levels in mutant rds/rds retinas. Heterozygous rds/+ photoreceptors demonstrated a normal pattern of light-evoked and quinpirole-mediated down-regulation of cAMP. Thus, partial expression of the normal phenotype is sufficient to render normal characteristics of cAMP regulation to the photoreceptors of the heterozygous mouse. The data obtained in the present study might be relevant to the understanding of photoreceptor pathology of patients with peripherin/rds mutations.

Day/night variations of dopamine ocular content during Xenopus laevis ontogeny

Concentration of dopamine (DA) and its metabolite, 3,4-dihydroxyphenylacetic acid is quantified by high-pressure liquid chromatography with a coulometric detection system in the eye of Xenopus laevis through ontogeny and in adults at two times during photocycle (midday and midnight). Ocular dopaminergic activity remains low during pre- and prometamorphosis and significantly rises in postmetamorphic froglets. This increase is more pronounced at midnight than at midday. The dualism of DA content versus DA release in Xenopus ocular tissue is studied in an eyecup culture system. On a 24-h cycle of DA release from adult Xenopus eyecups the highest DA release by eyecups is produced during daytime, and significantly decreases in darkness. From these results it can be concluded that in spite of the early development of the retinal dopaminergic system in the ontogeny of Xenopus, the final maturation must occur during the metamorphic climax. Endogenous DA release is significantly inhibited by light offset, which explains the higher ocular DA content found at midnight as compared to midday in postmetamorphic froglets and adults.

Symphony of rhythms in the Xenopus laevis retina

The photoreceptor layer in the retina of Xenopus laevis harbors a circadian clock. Many molecular components known to drive the molecular clock in other organisms have been identified in Xenopus, such as XClock, Xper2, and Xcrys, demonstrating phylogenetic conservation. This model system displays a wide array of rhythms, including melatonin release, ERG rhythms, and retinomotor movements, suggesting that the ocular clock is important for proper retinal function. A flow-through culture system allows measurements of retinal rhythms such as melatonin release in vitro over time from a single eyecup. This system is suited for pharmacological perturbations of the clock, and has led to important observations regarding the circadian control of melatonin release, the roles of light and dopamine as entraining agents, and the circadian mechanisms regulating retinomotor movements. The development of a transgenic technique in Xenopus allows precise and reliable molecular perturbations. Since it is possible to follow rhythms in eyecups obtained from adults or tadpoles, the combination of the flow-through culture system and the transgenic technique leads to the fast generation of transgenic tadpoles to monitor the effects of molecular perturbations on the clock.

Melatonin synthesis in the greenfrog retina in culture: II. Dopaminergic and adrenergic control

Serotonin N-acetyltransferase (NAT) activity and melatonin show a daily rhythm with high levels at night. Although the rhythmic properties of NAT and melatonin are similar in pineal gland and retina, great differences in the light perception and transmission mechanisms exist. We have analyzed the effects of adrenergic and dopaminergic agents on greenfrog (Rana perezi) eyecup culture, in order to identify the receptors involved in the regulation of retinal melatonin synthesis. A D2-like receptor is directly involved in the regulation of NAT activity and melatonin release in R. perezi retina. Quinpirole mimics the effect of light, reducing the darkness-stimulated NAT activity and melatonin release, while sulpiride antagonized these actions. Neither D1-agonist (SKF 38393) nor D1-antagonist (SCH 23390) had effect on NAT activity. However, a significant inhibition of darkness-evoked melatonin release was produced by SKF 38393 after 6 hours of culture. The beta- and antagonist1-agonists showed a clear inhibition. However, a direct effect of beta, alpha1 and D1-agonists on photoreceptors is unproven, being more probable that the adrenergic actions imply a non-photoreceptor retinal cell. In conclusion, eyecup culture of Rana perezi revealed a dopaminergic control of melatonin synthesis and a possible modulation of dopaminergic tone by adrenergic receptors. Melatonin release is a more sensitive parameter than NAT activity to the action of neuroactive agents, suggesting that melatonin synthesis can be regulated by more than one enzymatic step in Rana perezi.

Dopamine mediates circadian rhythms of rod-cone dominance in the Japanese quail retina

A circadian clock modulates the functional organization of the Japanese quail retina. Under conditions of constant darkness, rods dominate electroretinogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod-cone dominance. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, also exhibits a circadian rhythm in the retina with approximately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activity of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rhythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amplitude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selective antagonist for D1 dopamine receptors has no effect on retinal sensitivity or rod-cone dominance. Reducing retinal dopamine partially abolishes rhythms in sensitivity and yields a rod-dominated retina regardless of the time of day. These results suggest that dopamine, under the control of a circadian oscillator, has a key role in modulating sensitivity and rod-cone dominance in the Japanese quail retina.

A role for cyclic AMP in entrainment of the circadian oscillator in Xenopus retinal photoreceptors by dopamine but not by light

The circadian oscillator in Xenopus retinal photoreceptor layers can be reset in similar ways by light and agonists of D2-like dopamine receptors. Treatments that increase cyclic AMP levels act on this oscillator in an opposite fashion, mimicking darkness in the induction of phase shifts. Light and dopamine have each been reported to inhibit adenylate cyclase in photoreceptors. Together, these data suggest that the transduction pathways for entrainment by dopamine and/or light include suppression of cyclic AMP or a cyclic AMP-sensitive step. In these studies, we examined this hypothesis by measuring the effects of treatment with a cyclic AMP analogue on the phase shifts induced in photoreceptor melatonin rhythms by light or a D2 receptor agonist (quinpirole). When photoreceptor layers were treated simultaneously with 8-(4-chlorophenylthio)cyclic AMP (8-CPT-cAMP) and quinpirole at any of three different phases of the circadian cycle, the resulting phase shifts of the melatonin rhythm were always the same as those caused by 8-CPT-cAMP alone. This indicates that there is a cyclic AMP-sensitive step in the dopamine entrainment pathway. In contrast, light pulses did reset the oscillator in the presence of elevated cyclic AMP. This suggests a separate cyclic AMP-insensitive transduction pathway for entrainment by light. Quinpirole reduced basal levels of cyclic AMP in photoreceptors, but light did not. These data suggest that cyclic AMP plays a role in the entrainment pathway activated by dopamine but not in the entrainment pathway activated by light.

Modulation of rhythmic melatonin synthesis in Xenopus retinal photoreceptors by cyclic AMP

Cyclic AMP regulates melatonin synthesis in vertebrate photoreceptor cells. In the present study, we investigated whether the circadian rhythm of melatonin synthesis in Xenopus retinal photoreceptor layers is driven by rhythmic changes in cyclic AMP. When the photoreceptor layers were continuously treated with 8-(4-chlorophenylthio)-cyclic AMP (8-CPT-cAMP) at a saturating concentration (1 mM), melatonin release was increased at all times of the day, but robust melatonin rhythms were maintained for 2 days in constant darkness (DD). We also measured cyclic AMP efflux and melatonin release simultaneously from photoreceptor layers that were continuously treated with forskolin and/or 3-isobutyl-1-methylxanthine (IBMX) in light-dark (LD) and DD. Circadian rhythmicity was observed in melatonin release, but not in cyclic AMP efflux, suggesting that changes of melatonin levels are not always caused by the changes of the cyclic AMP levels. In addition, the simultaneous treatment of forskolin and IBMX appeared to saturate sensitivity of melatonin synthesis to cyclic AMP, but this treatment did not abolish melatonin rhythms. These results suggest that circadian rhythms of melatonin can be driven without rhythmic changes of cyclic AMP, and that cyclic AMP regulates melatonin in parallel with the output pathways from the circadian oscillator.

Endogenous activation of dopamine D2 receptors regulates dopamine release in the fish retina

In the fish retina, horizontal cell electrical coupling and light responsiveness is regulated by activation of dopamine D1 receptors that are located on the horizontal cells themselves. The effects of dopamine and dopamine D2 receptor agonists and antagonists on cone horizontal cell light responses were studied in in vitro superfused goldfish retinas. Horizontal cell light responses and electrical coupling were assessed by monitoring responses to full-field stimuli and to small, centered (0.4 mm diam) spots of light, respectively. Dopamine (0.2-10 microM) application uncoupled horizontal cells and decreased their responses to full-field stimuli. Application of the D2 antagonist eticlopride (10-50 microM) produced similar effects, whereas quinpirole (0.1-10 microM), a D2 agonist, had the opposite effects. The uncoupling effect of eticlopride was blocked by prior application of SCH23390 (10 microM), a D1 receptor antagonist, and was eliminated after destruction of dopaminergic neurons by prior treatment of the retinas with 6-hydroxydopamine. The effects of these D2 drugs were observed following flickering light stimulation, but were not observed following sustained light stimulation. Application of the D2 antagonists sulpiride (0.5-20 microM) and spiperone (0.25-10 microM) uncoupled horizontal cells when the total concentration of divalent cations (Mg2+ and Ca2+) in the Ringer solution was 1.1 mM. However, when the concentration of divalent cations was 0.2 mM, spiperone had no effect on the horizontal cells and sulpiride increased coupling. In contrast, eticlopride uncoupled the cells and decreased their light responsiveness irrespective of the concentration of divalent cations. The effects of quinpirole also depended on the concentration of divalent cations; its coupling effect was reduced when the divalent cation concentration was increased from 0.2 to 1.0 mM. The results suggest that activation of D2 receptors in the fish retina by endogenous dopamine decreases dopamine release and is greater after flickering compared with sustained light stimulation. These D2 receptors thus function as presynaptic autoreceptors that inhibit dopamine release from dopaminergic cells. In addition, the results also indicate that the effectiveness of some D2 drugs at these receptors is dependent on the concentration of divalent cations.

A retinal dark-light switch: a review of the evidence

We propose that there exists within the avian, and perhaps more generally in the vertebrate retina, a two-state nonadapting flip-flop circuit, based on reciprocal inhibitory interactions between the photoreceptors, releasing melatonin, the dopaminergic amacrine cells, and amacrine cells which contain enkephalin-, neurotensin-, and somatostatin-like immunoreactivity (the ENSLI amacrine cells). This circuit consists of two loops, one based on the photoreceptors and dopaminergic amacrine cells, and the other on the dopaminergic and ENSLI amacrine cells. In the dark, the photoreceptors and ENSLI amacrine cells are active, with the dopaminergic amacrine cells inactive. In the light, the dopaminergic amacrine cells are active, with the photoreceptors and ENSLI amacrine cells inactive. The transition from dark to light state occurs over a narrow (< 1 log unit) range of low light intensities, and we postulate that this transition is driven by a graded, adapting pathway from photoreceptors, releasing glutamate, to ON-bipolar cells to dopaminergic amacrine cells. The properties of this pathway suggest that, once released from the reciprocal inhibitory controls of the dark state, dopamine release will show graded, adapting characteristics. Thus, we postulate that retinal function will be divided into two phases: a dopamine-independent phase at low light intensities, and a dopamine-dependent phase at higher light intensities. Dopamine-dependent functions may show two-state properties, or two-state properties on which are superimposed graded, adapting characteristics. Functions dependent upon melatonin, the enkephalins, neurotensin, and somatostatin may tend to show simpler two-state properties. We propose that the dark-light switch may have a role in a range of light-adaptive phenomena, in signalling night-day transitions to the suprachiasmatic nucleus and the pineal, and in the control of eye growth during development.

Clozapine and other neuroleptic drugs antagonize the light-evoked suppression of melatonin biosynthesis in chick retina: involvement of the D4-like dopamine receptor

The subtype of dopamine receptor mediating the suppressive effect of light on melatonin biosynthesis in chick retina was characterized pharmacologically. Acute exposure of animals to light during the dark phase of the light-dark cycle dramatically decreased melatonin levels and activity of serotonin N-acetyltransferase (NAT; a key regulatory enzyme in melatonin biosynthetic pathway). Various antagonists of dopamine receptors were tested for their ability to block this action of light on the retinal melatonin formation. Intraocular (i. oc.) pretreatment of chicks with neuroleptic drugs--blockers of the D2-family of dopamine receptors, i.e., clotiapine, clozapine (an atypical neuroleptic with high affinity for a D4-subtype dopamine receptor), haloperidol, spiroperidol, sulpiride, and YM-09151-2, significantly antagonized the light-evoked suppression of the nighttime NAT activity of the chick retina in a dose-dependent manner. In contrast, remoxipride (a D2-selective dopamine antagonist), raclopride and (+)-UH-232 (D2/D3-dopamine receptor antagonists), as well as SCH 23390, a blocker of the D1-family of dopamine receptors, were ineffective. Clozapine, haloperidol, spiroperidol and sulpiride also potently antagonized the suppressive action of light on melatonin content of the chick retina. It is suggested that the dopamine receptor mediating the inhibitory effect of light stimulation on the nighttime melatonin biosynthesis in the retina of chick represents a D4-like subtype.

Tryptophan hydroxylase expression is regulated by a circadian clock in Xenopus laevis retina

A circadian clock has been localized to the photoreceptor layer in the Xenopus laevis retina. This clock controls the rhythmic synthesis of melatonin, which results in elevated levels during the night and low levels during the day. The rate-limiting enzyme in melatonin synthesis in Xenopus laevis retina is tryptophan hydroxylase. A cDNA clone coding for Xenopus tryptophan hydroxylase was isolated, characterized, and used as a probe for analysis of tryptophan hydroxylase mRNA expression. Northern blot analyses of total retinal RNA show that the tryptophan hydroxylase message levels are low in the day and higher at night. The expression of tryptophan hydroxylase mRNA is under circadian control because rhythmic changes are also seen in constant darkness, with elevated levels during the subjective night. Nuclear run-on analysis during the first subjective day in constant darkness revealed that transcription initiation is low early in the day and increases throughout the day. Our observations suggest that the circadian clock modulates tryptophan hydroxylase gene expression. An understanding of how the circadian clock controls tryptophan hydroxylase expression may lead to a clearer understanding of the melatonin biosynthetic pathway, and possibly the clock itself.

Dopamine receptor regulating serotonin N-acetyltransferase activity in chick retina represents a D4-like subtype: pharmacological characterization

The dopamine (DA) receptor regulating serotonin N-acetyltransferase (NAT) activity in chick retina was characterized pharmacologically. Intraocular (i.o.) administration of DA significantly decreased the nighttime NAT activity of chick retina. The effect of DA was antagonized by blockers of the D2 family of DA receptors, spiroperidol, YM-09151-2 and clozapine, and it was not affected by SCH23390, a selective antagonist of D1 DA receptor. Several agonists of D2 family of DA receptors given i.o. suppressed the nighttime NAT activity of the chick retina with quinpirole (D3/D4 receptor selective) and bromocriptine (D2/D3 receptor selective) being the most and the least potent drugs, respectively. The rank-order potency of antagonists of D2 family of DA receptors to block the inhibitory effect of quinpirole on the enzyme activity (with clozapine being relatively very potent, and (+)-butaclamol, raclopride and remoxipride-ineffective) match the characteristics of the D4 DA receptor. Moreover, although sulpiride effectively prevented the quinpirole-induced decline in the nighttime NAT activity of the chick retina, there was no marked stereoselectivity in its action. It is suggested that DA receptor regulating NAT activity in chick retina represents a D4-like subtype.

Does D4 dopamine receptor mediate the inhibitory effect of light on melatonin biosynthesis in chick retina?

The dopamine (DA) receptor mediating the inhibitory effect of light on melatonin formation in the chick retina was characterized pharmacologically. Nighttime serotonin N-acetyltransferase (NAT) activity was significantly decreased by either light exposure or by intraocular (i.oc.) administration of DA and quinpirole (a predominant D3/D4 DA receptor agonist). Several D2-like DA receptor antagonists, i.e. clozapine, haloperidol, spiroperidol, sulpiride, (+)-UH-232 and YM-09151-2, given i.oc. to light-adapted chicks markedly elevated retinal NAT activity. In contrast, raclopride (a D2/D3 DA receptor antagonist) and remoxipride (a D2-selective DA receptor antagonist) were ineffective. Spiroperidol, clozapine, haloperidol and sulpiride significantly increased melatonin content in the light-exposed retina, but had no effect on the activity of hydroxyindole-O-methyltransferase. None of D2-like DA receptor blockers tested modified the nighttime NAT activity in the chick retina. Our results indicate that the light-evoked inhibition of the nocturnal increase in melatonin biosynthesis in chick retina may involve stimulation of the D4 subtype DA receptor.

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

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

Physiological roles of dopamine and neuropeptides in the retina

The retina is a highly complex nervous tissue that converts light into patterns of electrical action potentials in order to process visual information. To carry out its function as a transducer and processor of visual information, the retina must be structurally and biochemically organized to send a coherent signal to the visual areas of the brain. In recent years, a number of biologically active substances have been demonstrated to be located within neurons in the retina. Most of them are thought to be involved in the modulation of the signal and its transmission to the brain through the optic nerve. The present paper attempts to summarize the immunocytochemical distribution and physiology of some neuronally localized substances in the mammalian retina, namely dopamine and neuropeptides.

Regulation of melatonin and dopamine biosynthesis in chick retina: the role of GABA

Melatonin biosynthesis in chick retina occurs as a circadian rhythm. Biosynthesis of the neurohormone is highest at night in darkness, and is suppressed by light. The role of gamma-aminobutyric acid (GABA) in the nocturnal regulation of melatonin synthesis was examined. Systemic or intravitreal administration of muscimol, a GABA-A receptor agonist, to light-exposed chicks at the beginning of the dark phase of the light/dark cycle increased retinal melatonin levels and the activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme of the melatonin biosynthetic pathway. Baclofen, a GABA-B receptor agonist, also increased NAT activity of light-exposed retinas, but muscimol was approximately 40-fold more potent than baclofen. Effects of both muscimol and baclofen on NAT activity were inhibited by GABA-A antagonists, bicuculline and picrotoxin, and the effect of baclofen was unaffected by the GABA-B selective antagonist, CGP 35348. Thus, activation of GABA-A receptors appears to be associated with increased melatonin biosynthesis. The GABA-uptake inhibitor, nipecotic acid, and the GABA-transaminase inhibitor, aminooxyacetic acid, also increased NAT activity of light-exposed retinas. The high levels of NAT activity associated with exposure to darkness were unaffected by either muscimol or baclofen, but picrotoxin and bicuculline significantly inhibited retinal NAT activity in darkness. The rate of dopamine synthesis, estimated from in situ tyrosine hydroxylase activity, was higher in light-exposed retinas than in darkness. Muscimol inhibited dopamine synthesis in light, and picrotoxin stimulated dopamine synthesis in darkness. The stimulation of melatonin synthesis by muscimol in light-exposed retinas appears to be related to inhibition of retinal dopamine neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Radioimmunoligand characterization and immunohistochemical localization of dopamine D2 receptors on rods in the rat retina

The retinal neurotransmitter dopamine (DA), elaborated from intrinsic dopaminergic neurons as amacrine and interplexiform cells, is known to modulate several complex functions mediated by D1 and D2 receptors in the vertebrate retina. In this paper, we characterized and localized DA receptors of the D2 family on rod outer segments (ROS) of the rat retina by a radioimmunoligand binding assay and by immunohistochemistry. Anti-anti-DA conjugated antibodies (or anti-idiotypic antibodies Ab2) were used as ligand; BSA-glutaraldehyde-conjugated spiperone, eticlopride (D2 antagonists) and DA were used as displacers. The linear Scatchard transformation indicated that data were best fit to the one-site model. By using the peroxidase-antiperoxidase technique, an intense labeling was located on rods. These results supported the paracrine action of DA on the photoreceptor cell.

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

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

D2-like dopamine receptors in amphibian retina: localization with fluorescent ligands

Dopamine induces several light adaptive changes in amphibian retina via receptors with D2-like pharmacology, but the identity of the primary target cells has not been determined. Using a fluorescent probe consisting of a selective D2 antagonist, N-(p-aminophenethyl)-spiperone (NAPS), derivatized with the fluorophore Bodipy (NAPS-Bodipy), we identified the distribution of dopamine binding sites in the retina of two amphibians, post-metamorphic Xenopus laevis and larval Ambystoma tigrinum. Specific labeling was defined as staining that was displaced by D2 selective ligands (eticlopride or sulpiride), but insensitive to D1 selective drugs (SCH 23390), adrenergic catecholamines (epinephrine or norepinephrine), or serotoninergic analogues (ketanserin). Both rod and cone cells showed specific dopamine D2-like binding sites arranged in clustered arrays on discrete membrane domains of the inner segment. Labeling of photoreceptor outer segments was continuous and was not displaced by competition with D2 selective ligands; this labeling was considered nonspecific. In addition, in both species, clustered binding of the D2-probe was found on Müller cells and on a subset of inner retinal cells with the morphology of amacrine/interplexiform cells. Our data provide direct evidence for D2 receptors on both rods and cones, and suggest that the receptors may be clustered into patches within a discrete cellular domain, the inner segment.

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

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

Melatonin increases serotonin N-acetyltransferase activity and decreases dopamine synthesis in light-exposed chick retina: in vivo evidence supporting melatonin-dopamine interaction in retina

The administration of melatonin, either peripherally (0.01-10 mg/kg) or intraocularly (0.001-10 mumol/eye), to light-exposed chicks dose-dependently increased serotonin N-acetyltransferase (NAT) activity in retina but not in pineal gland. The effect of melatonin was slightly but significantly reduced by luzindole (2-benzyl-N-acetyltryptamine), and not affected by two other purported melatonin antagonists, N-acetyltryptamine and N-(2,4-dinitrophenyl)-5-methoxytryptamine (ML-23). The elevation of the enzyme activity induced by melatonin was substantially stronger than that evoked by 5-hydroxytryptamine, N-acetyl-5-hydroxytryptamine, or 5-methoxytryptamine. The melatonin-evoked rise in the retinal NAT activity was counteracted by two dopamine D2 receptor agonists, quinpirole and apomorphine, and prevented by the dopamine D2 receptor blocker spiroperidol, and by an inhibitor of dopamine synthesis, alpha-methyl-p-tyrosine. Melatonin (0.1-10 mg/kg i.p.) dose-dependently decreased the levels of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), as well as the DOPAC/dopamine ratio, in chick retina but not in forebrain. The results obtained (1) indicate that melatonin in vivo potently inhibits dopamine synthesis selectively in retina, and (2) suggest that the increase in retinal NAT activity evoked by melatonin in light-exposed chicks is an indirect action of the compound, and results from the disinhibition of the NAT induction process from the dopaminergic (inhibitory) signal. The results provide in vivo evidence supporting the idea (derived on the basis of in vitro findings) that a mutually antagonistic interaction between melatonin and dopamine operates in retinas of living animals.

Melatonin synthesis in chicken retina: effect of kainic acid-induced lesions on the diurnal rhythm and D2-dopamine receptor-mediated regulation of serotonin N-acetyltransferase activity

The effect of kainic acid (KA)-induced lesions of retinal neurons on regulation of serotonin N-acetyltransferase (NAT) activity in chicken retina was investigated. Although NAT activity was higher in KA-lesioned retinas than in controls, the pattern of diurnal variation of enzyme activity throughout 36 h of constant darkness was similar for both tissues. Quinpirole, a selective D2-dopamine receptor agonist, inhibited the nocturnal increase of NAT activity in both control and KA-treated retinas. Quinpirole was significantly more potent in KA-treated retinas than in controls; the ED50 value for quinpirole was 3 times lower in KA-treated retinas than in control tissues. KA treatment markedly reduced retinal levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). We conclude that: (1) NAT activity in retina is localized primarily to KA-insensitive cells, presumably photoreceptors; (2) KA-sensitive inner retinal neurons are not essential to the maintenance of the circadian rhythm of NAT activity; and (3) KA-induced lesions of retinal cells result in supersensitivity of D2-dopamine receptors regulating NAT activity in a mechanism that involves adaptive changes following a decline in retinal dopamine neurotransmission.

Paracrine control of photomembrane removal

Photomembrane turnover in vertebrate photoreceptors is regulated by light. Rod outer segments (ROS) shed membrane filled tips at light onset, during the coexistence of two light modulated processes: a dark priming factor and a light induction event. Transduction of these two signals is not direct but appears to involve the neural retina and diffusible paracrine molecules. I propose a model wherein three paracrines control this ROS tip shedding. Melatonin, a lipid soluble dark priming molecule, is synthesized in the dark by all photoreceptor cells, diffusing freely and separating the ROS disk membranes. A second paracrine, dopamine is released from the inner retina whenever light is absorbed by the 502 nm-cones, inhibiting melatonin synthesis. Third, a proposed trophic paracrine, "rostrophin", is released in the dark from internal horizontal cells, and stabilizes the photomembrane. Shedding occurs as rostrophin decreases in the presence melatonin; briefly at light onset or continuously in red or dim white light.

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.

Serotonin N-acetyltransferase activity in chicken retina: in vivo effects of phosphodiesterase inhibitors, forskolin, and drugs affecting dopamine receptors

A role of D2-dopaminergic neurotransmission in the regulation of melatonin biosynthesis in retina was studied in vivo in chickens. The nighttime rise in serotonin N-acetyltransferase (NAT)--the penultimate and key regulatory melatonin-synthesizing enzyme--was potently inhibited by both acute light exposure and agonists of dopamine D2-receptor (quinpirole, bromocriptine, and apomorphine). Spiroperidol, a selective dopamine D2-receptor blocker, increased the enzyme activity in light-exposed chickens, but had no effect in animals kept in darkness. Inhibitors of cyclic nucleotide phosphodiesterase, aminophylline, and 3-isobutyl-1-methylxanthine given peripherally, along with a direct adenylate cyclase activator forskolin injected directly into the eye, mimicked the action of darkness, and markedly enhanced the retinal NAT activity when administered to animals maintained in an illuminated environment. Dopamine D2-receptor agonists had no effect on aminophylline-stimulated enzyme activity, whereas spiroperidol enhanced it. Forskolin-driven NAT activity was suppressed by quinpirole. Spiroperidol and aminophylline given alone at different times of day under light conditions stimulated NAT activity, and their effects were mainly additive when given in combination. SCH 23390, a selective D1-dopamine receptor antagonist, did not affect the rise in NAT activity of chicken retina produced by either darkness or by aminophylline. The results provide further evidence that dopamine, acting via D2-receptors, mediates the inhibitory effects of light on the cyclic AMP-dependent dark-evoked induction of NAT activity in chicken retina.

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.

D1 dopamine receptors in neurite regions of embryonic and differentiated retina are highly coupled to adenylate cyclase in the embryonic but not in the mature tissue

[3H]SCH 23390 binds stereospecifically and with high affinity to D1 dopaminergic receptors in the developing chick retina. Autoradiographic experiments revealed that in retinas from 3-day-old chicken and embryos with 12, 14 and 16 days of development, specific labeling of [3H]SCH 23390 was mainly observed over the plexiform layers of the tissue, showing that dopaminergic D1 receptors are localized in retina cell neurites since the initial stages of neurite formation. The total number of [3H]SCH 23390 binding sites increased 5-fold during the differentiation of the retina, while the dopamine-dependent cyclic adenosine monophosphate (AMP) accumulation was significantly decreased. Consequently, the ratio between dopamine-dependent cyclic AMP accumulation and [3H]SCH 23390 binding sites decreased 10-fold as retina differentiated, indicating that a significant portion of D1 receptors in retinas from adult chicken are not effectively coupled to adenylate cyclase molecules.

Bidirectional regulation of cAMP generating system by dopamine-D1 and D2-receptors in the rat retina

Accumulation or inhibition of cAMP formation in response to dopamine or dopamine related drugs in the absence or in the presence of forskolin and/or IBMX was investigated in isolated rat retina. While the existence of D1-receptors (positively coupled with adenylate cyclase) was confirmed, D2-receptors (negatively coupled to adenylate cyclase) were also revealed by using a selective D1-antagonist (SCH 23390), a D2-agonist (LY 171555) or two D2-antagonists (S-sulpiride, spiroperidol). These results indicate that rat retina may be used for the study of both types of dopamine-receptors.

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.

Autoradiographic localization of D1 and D2 dopamine binding sites in the human retina

The localization of dopamine binding sites was studied by in vitro autoradiography in the normal human retina using [125I]SCH 23982 for D1 receptor labelling and [125I]iodosulpride for D2 receptors. Results demonstrated that both types of binding sites were present in human retina. Binding of [125I]SCH 23982 to D1 dopamine receptor was blocked by 1 microM SKF 38393, SCH 23390 (D1 specific compounds) whereas bromocriptine and domperidone (D2 specific compounds) were inactive at the same concentration. On the contrary, binding of [125I]iodosulpride to D2 dopamine receptor was inhibited only by D2 drugs. Precise cellular distribution was given by microautoradiographic techniques and showed that binding sites were exclusively localized to the plexiform layers.