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

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

The Nogo-66 Receptors NgR1 and NgR3 Are Required for Commissural Axon Pathfinding

Nogo-66 receptors (NgR1-3) are glycosylphosphatidyl inositol-linked proteins that belong to the leucine-rich repeat superfamily. Through binding to myelin-associated inhibitors, NgRs contribute to the inhibition of axonal regeneration after spinal cord injury. Their role in limiting synaptic plasticity and axonal outgrowth in the adult CNS has been described previously, but not much is known about their role during the development of the nervous system. Here, we show that NgR1 and NgR3 mRNAs are expressed during spinal cord development of the chicken embryo. In particular, they are expressed in the dI1 subpopulation of commissural neurons during the time when their axons navigate toward and across the floorplate, the ventral midline of the spinal cord. To assess a potential role of NgR1 and NgR3 in axon guidance, we downregulated them using in ovo RNAi and analyzed the trajectory of commissural axons by tracing them in open-book preparations of spinal cords. Our results show that loss of either NgR1 or NgR3 causes axons to stall in the midline area and to interfere with the rostral turn of postcrossing axons. In addition, we also show that NgR1, but not NgR3, requires neuronal PlexinA2 for the regulation of commissural axon guidance.SIGNIFICANCE STATEMENT Over the last decades, many studies have focused on the role of NgRs, particularly NgR1, in axonal regeneration in the injured adult CNS. Here, we show a physiological role of NgRs in guiding commissural axons during early development of the chicken spinal cord in vivo Both NgR1 and NgR3 are required for midline crossing and subsequent turning of postcrossing axons into the longitudinal axis of the spinal cord. NgR1, but not NgR3, forms a receptor complex with PlexinA2 during axon guidance. Overall, these findings provide a link between neural regenerative mechanisms and developmental processes.

The Role of Dopamine in Emmetropization Modulated by Wavelength and Temporal Frequency in Guinea Pigs

Purpose

Wavelength and temporal frequency have been found to influence refractive development. This study investigated whether retinal dopamine (DA) plays a role in these processes.

Methods

Guinea pigs were randomly divided into nine groups that received different lighting conditions for 4 weeks, as follows: white, green, or blue light at 0, 0.5, or 20.0 Hz. Refractions and axial lengths were measured using streak retinoscopy and A-scan ultrasound imaging. DA and its metabolites were measured by high-pressure liquid chromatography-electrochemical detection.

Results

At 0 Hz, green and blue light produced myopic and hyperopic shifts compared with that of white light. At 0.5 Hz, no significant changes were observed compared with those of green or blue light at 0 Hz, whereas white light at 0.5 Hz induced a myopic shift compared with white light at 0 or 20 Hz. At 20 Hz, green and blue light acted like white light. Among all levels of DA and its metabolites, only vitreous 3, 4-dihydroxyphenylacetic acid (DOPAC) levels and retinal DOPAC/DA ratios were dependent on wavelength, frequency, and their interaction. Specifically, retinal DOPAC/DA ratios were positively correlated with refractions in white and green light conditions. However, blue light (0, 0.5, and 20.0 Hz) produced hyperopic shifts but decreased vitreous DOPAC levels and retinal DOPAC/DA ratios.

Conclusions

The retinal DOPAC/DA ratio, indicating the metabolic efficiency of DA, is correlated with ocular growth. It may underlie myopic shifts from light exposure with a long wavelength and low temporal frequency. However, different biochemical pathways may contribute to the hyperopic shifts from short wavelength light.

Light and myopia: from epidemiological studies to neurobiological mechanisms

Myopia is far beyond its inconvenience and represents a true, highly prevalent, sight-threatening ocular condition, especially in Asia. Without adequate interventions, the current epidemic of myopia is projected to affect 50% of the world population by 2050, becoming the leading cause of irreversible blindness. Although blurred vision, the predominant symptom of myopia, can be improved by contact lenses, glasses or refractive surgery, corrected myopia, particularly high myopia, still carries the risk of secondary blinding complications such as glaucoma, myopic maculopathy and retinal detachment, prompting the need for prevention. Epidemiological studies have reported an association between outdoor time and myopia prevention in children. The protective effect of time spent outdoors could be due to the unique characteristics (intensity, spectral distribution, temporal pattern, etc.) of sunlight that are lacking in artificial lighting. Concomitantly, studies in animal models have highlighted the efficacy of light and its components in delaying or even stopping the development of myopia and endeavoured to elucidate possible mechanisms involved in this process. In this narrative review, we (1) summarize the current knowledge concerning light modulation of ocular growth and refractive error development based on studies in human and animal models, (2) summarize potential neurobiological mechanisms involved in the effects of light on ocular growth and emmetropization and (3) highlight a potential pathway for the translational development of noninvasive light-therapy strategies for myopia prevention in children.

Visual conditions affecting eye growth alter diurnal levels of vitreous DOPAC

In chicks, the diurnal patterns of retinal dopamine synthesis and release are associated with refractive development. To assess the within-day patterns of dopamine release, we assayed vitreal levels of DOPAC (3,4-dihydroxyphenylacetic acid) using high performance liquid chromatography with electrochemical detection, at 4-h intervals over 24 h in eyes with experimental manipulations that change ocular growth rates. Chicks were reared under a 12 h light/12 h dark cycle; experiments began at 12 days of age. Output was assessed by modelling using the robust variance structure of Generalized Estimating Equations. Continuous spectacle lensdefocus or form deprivation: One group experienced non-restricted visual input to both eyes and served as untreated "normal" controls. Three experimental cohorts underwent monocular visual alterations known to alter eye growth and refraction: wearing a diffuser, a negative lens or a positive lens. After one full day of device-wear, chicks were euthanized at 4-h intervals over 24 h (8 birds per time/condition). Brief hyperopic defocus: Chicks wore negative lenses for only 2 daily hours either in the morning (starting at ZT 0; n = 16) or mid-day (starting at ZT 4; n = 8) for 3 days. Vitreal DOPAC was assayed. In chicks with bilateral non-restricted vision, or with continuous defocus or form-deprivation, there was a diurnal variation in vitreal DOPAC levels for all eyes (p < 0.001 for each). In normal controls, DOPAC was highest during the daytime, lowest at night, and equivalent for both eyes. In experimental groups, regardless of whether experiencing a growth stimulatory input (diffuser; negative lens) or growth inhibitory input (positive lens), DOPAC levels were reduced compared both to fellow eyes and to those of normal controls (p < 0.001 for each). These diurnal variations in vitreous DOPAC levels under different visual conditions indicate a complexity for dopaminergic mechanisms in refractive development that requires further study.

Protective effects of increased outdoor time against myopia: a review

Myopia has become a major cause for concern globally, particularly in East Asian countries. The increasing prevalence of myopia has been associated with a high socioeconomic burden owing to severe ocular complications that may occur with progressive myopia. There is an urgent need to identify effective and safe measures to address the growing number of people with myopia in the general population. Among the numerous strategies implemented to slow the progression of myopia, longer time spent outdoors has come to be recognized as a protective factor against this disorder. Although our understanding of the protective effects of outdoor time has increased in the past decade, considerably more research is needed to understand the mechanisms of action. Here, we summarize the main potential factors associated with the protective effects against myopia of increased outdoor time, namely, exposure to elevated levels and shorter wavelengths of light, and increased dopamine and vitamin D levels. In this review, we aimed to identify safe and effective therapeutic interventions to prevent myopia-related complications and vision loss.

Development of Experimental Myopia in Chicks in a Natural Environment

Purpose

The hypothesis that outdoor exposure might protect against myopia has generated much interest, although available data find only modest clinical efficacy. We tested the effect of outdoor rearing on form-deprivation myopia in chicks, a myopia model markedly inhibited by high-intensity indoor laboratory lighting.

Methods

Unilaterally goggled cohorts of White Leghorn chicks were maintained in a species-appropriate, outdoor rural setting during daylight hours to the extent permitted by weather. Control chicks were reared indoors with incandescent lighting. Besides ocular refraction and ultrasound, we determined dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) content in retina and vitreous and measured mRNA expression levels of selected clock and circadian rhythm-related genes in the retina/RPE.

Results

Myopia developed in the goggled eyes of all cohorts. Whereas outdoor rearing lessened myopia by 44% at 4 days, a protective effect was no longer evident at 11 days. Outdoor rearing had no consistent effect on retinal or vitreous content of dopamine or DOPAC. Conforming to prior data on form-deprivation myopia, retina and vitreous levels of DOPAC were reduced in goggled eyes. Compared with contralateral eyes, the retinal expression of clock and circadian rhythm-related genes was modestly altered in myopic eyes of chicks reared indoors or outdoors.

Conclusions

Outdoor rearing of chicks induces only a partial decrease of goggle-induced myopia that is not maintained, without evidence that retinal dopamine metabolism accounts for the partial myopia inhibition under these outdoor conditions. Although modest, alterations in retinal gene expression suggest that studying circadian signals might be informative for understanding refractive mechanisms.

Characterization of the relations between morphology and physiological status of the pineal gland in connection with the somatic development level in turkeys reared in Romania

This research started from the premises of the existence of some possible relationships between indole and pineal peptide hormones and the somatic development, with participation of hypothalamic-pituitary complex. Experimental factors, which were the subject of the present paper, influenced the dynamics of corporal mass and fodder consumption, leading to the occurrence of some important structural modifications at the level of pineal gland. The exposure of the individuals to continuous light (photic pinealectomy) produces increases in corporal mass, showing the involvement of the pineal gland in neuro-endocrine-metabolic reactions, which contributes to the maintenance of homeostatic balance, including somatic ones. Biological material was represented by a number of 50 individuals belonging to B.U.T. Big 6 hybrid, reared on soil, on a permanent litter, which could assure the expanding of knowledge area regarding the relation between rearing technology, modulation of some microclimate parameters and growing performances. Were also realised cytometric and hystometric muscular determinations.

Role of dopamine in distal retina

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

Bright light induces choroidal thickening in chickens

PURPOSE

Bright light is a potent inhibitor of myopia development in animal models. Because development of refractive errors has been linked to changes in choroidal thickness, we have studied in chickens whether bright light may exert its effects on myopia also through changes in choroidal thickness.

METHODS

Three-day-old chickens were exposed to "bright light" (15,000 lux; n = 14) from 10 AM to 4 PM but kept under "normal light" (500 lux) during the remaining time of the light phase for 5 days (total duration of light phase 8 AM to 6 PM). A control group (n = 14) was kept under normal light during the entire light phase. Choroidal thickness was measured in alert, hand-held animals with optical coherence tomography at 10 AM, 4 PM, and 8 PM every day.

RESULTS

Complete data sets were available for 12 chicks in bright light group and nine in normal light group. The striking inter-individual variability in choroidal thickness (coefficient of variance: 23%) made it necessary to normalize changes to the individual baseline thickness of the choroid. During the 6 hours of exposure to bright light, choroidal thickness decreased by -5.2 ± 4.0% (mean ± SEM). By contrast, in the group kept under normal light, choroidal thickness increased by +15.4 ± 4.7% (difference between both groups p = 0.003). After an additional 4 hours, choroidal thickness increased also in the "bright light group" by +17.8 ± 3.5%, while there was little further change (+0.6 ± 4.0%) in the "normal light group" (difference p = 0.004). Finally, the choroid was thicker in the "bright light group" (+7.6 ± 26.0%) than in the "normal light group" (day 5: -18.6 ± 26.9%; difference p = 0.036).

CONCLUSIONS

Bright light stimulates choroidal thickening in chickens, although the response is smaller than with experimentally imposed myopic defocus, and it occurs with some time delay. It nevertheless suggests that choroidal thickening is also involved in myopia inhibition by bright light.

An updated view on the role of dopamine in myopia

A large body of data is available to support the hypothesis that dopamine (DA) is one of the retinal neurotransmitters involved in the signaling cascade that controls eye growth by vision. Initially, reduced retinal DA levels were observed in eyes deprived of sharp vision by either diffusers ("deprivation myopia", DM) or negative lenses ("lens induced myopia", LIM). Simulating high retinal DA levels by intravitreal application of a DA agonist can suppress the development of both DM and LIM. Also more recent studies using knock-out mouse models of DA receptors support the idea of an association between decreased DA levels and DM. There seem to be differences in the magnitude of the effects of DA on DM and LIM, with larger changes in DM but the degrees of image degradation by both treatments need to be matched to support this conclusion. Although a number of studies have shown that the inhibitory effects of dopamine agonists on DM and LIM are mediated through stimulation of the D2-receptor, there is also recent evidence that the balance of D2- and D1-receptor activation is important. Inhibition of D2-receptors can also slow the development of spontaneous myopia in albino guinea pigs. Retinal DA content displays a distinct endogenous diurnal, and partially circadian rhythm. In addition, retinal DA is regulated by a number of visual stimuli like retinal illuminance, spatial frequency content of the image, temporal contrast and, in chicks, by the light input from the pineal organ. A close interaction was found between muscarinergic and dopaminergic systems, and between nitric oxide and dopaminergic pathways, and there is evidence for crosstalk between the different pathways, perhaps multiple binding of the ligands to different receptors. It was shown that DA agonists interact with the immediate early signaling molecule ZENK which triggers the first steps in eye growth regulation. However, since long treatment periods were often needed to induce significant changes in retinal dopamine synthesis and release, the role of dopamine in the early steps is unclear. The wide spatial distribution of dopaminergic amacrine cells in the retina and the observation that changes in dopamine levels can be locally induced by local retinal deprivation is in line with the assumption that dopaminergic mechanisms control both central and peripheral eye growth. The protective effect of outdoor activity on myopia development in children seems to be partly mediated by the stimulatory effect of light on retinal dopamine production and release. However, the dose-response function linking light exposure to dopamine and to the suppression of myopia is not known and requires further studies.

Ambient illuminance, retinal dopamine release and refractive development in chicks

Form deprivation and low illuminance of ambient light are known to induce myopia in chicks. Low concentrations of retinal dopamine, a light-driven neurohormone, was previously shown to be associated with form deprivation myopia. In the present study we examined the dependence of retinal dopamine release in chicks on illuminance during light-dark cycles and in continuous light, and the role of retinal dopamine release in illuminance dependent refractive development. Newly hatched chicks (n = 166) were divided into two experimental groups, a dopamine (n = 88) and a refraction group (n = 78). Both groups were further divided into six illumination groups for exposure of chicks to illuminances of 50, 500 or 10,000 lux of incandescent illumination (referred to throughout as low, medium, and high illuminance, respectively), either under a light-dark cycle with lights on between 7 AM and 7 PM or under continuous illumination. For the dopamine experiment, chicks were euthanized and vitreous was extracted on day 14 post-hatching at 7, 8 AM and 1 PM. Vitreal dihydroxyphenylacetic acid (DOPAC) and dopamine concentrations were quantified by high-performance liquid chromatography coupled to electrochemical detection. For the refraction experiment, chicks underwent refraction, keratometry and A-scan ultrasonography on days 30, 60 and 90 post-hatching, and each of those measurements was correlated with vitreal DOPAC concentration measured at 1 PM (representing the index of retinal dopamine release). The results showed that under light-dark cycles, vitreal DOPAC concentration was strongly correlated with log illuminance, and was significantly correlated with the developing refraction, corneal radius of curvature, and axial length values. On day 90, low vitreal DOPAC concentrations were associated with myopia (-2.41 ± 1.23 D), flat cornea, deep anterior and vitreous chambers, and thin lens. Under continuous light, vitreal DOPAC concentrations measured at 1 PM in the low, medium, and high illuminance groups did not differ from the concentrations measured at 8 AM. On day 90, low DOPAC concentrations were associated with emmetropia (+0.63 ± 3.61), steep cornea, and shallow vitreous chamber. We concluded that ambient light over a log illuminance range of 1.69-4 is linearly related to vitreal DOPAC concentration. Under both light-dark cycles and continuous light, the intensity of ambient light regulates the release of retinal dopamine. Refractive development is associated with illuminance dependent dopamine release.

Dopamine in the Turkey retina-an impact of environmental light, circadian clock, and melatonin

Substantial evidence suggests that dopamine and melatonin are mutually inhibitory factors that act in the retina as chemical analogs of day and night. Here, we show an impact of environmental light, biological clock, and melatonin on retinal levels of dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the turkey. In turkeys held under different light (L) to dark (D) cycles (16L:8D, 12L:12D, 8L:16D), retinal levels of dopamine and DOPAC fluctuated with daily rhythms. High levels of dopamine and DOPAC were observed during light hours and low during dark hours. Under the three photoperiodic regimes, rhythms of dopamine and DOPAC were out of phase with daily oscillation in retinal melatonin content. In constant darkness, dopamine and DOPAC levels oscillated in circadian rhythms. Light deprivation resulted, however, in a significant decline in amplitudes of both rhythms. Injections of melatonin (0.1-1 mumol/eye) during daytime significantly reduced retinal levels of DOPAC. This suppressive effect of melatonin was more pronounced in the dark-adapted than light-exposed turkeys. Quinpirole (a D(2)/D(4)-dopamine receptor agonist; 0.1-10 nmol/eye) injected to dark-adapted turkeys significantly decreased retinal melatonin. Our results indicate that in the turkey retina: (1) environmental light is the major factor regulating dopamine synthesis and metabolism; (2) dopaminergic neurones are controlled, in part, by intrinsic circadian clock; and (3) dopamine and melatonin are components of the mutually inhibitory loop.

Localization and regulation of dopamine receptor D4 expression in the adult and developing rat retina

Levels of dopamine and melatonin exhibit diurnal rhythms in the rat retina. Dopamine is high during daytime adapting the retina to light, whereas melatonin is high during nighttime participating in the adaptation of the retina to low light intensities. Dopamine inhibits the synthesis of melatonin in the photoreceptors via Drd4 receptors located on the cell membrane of these cells. In this study, we show by semiquantitative in situ hybridization a prominent day/night variation in Drd4 expression in the retina of the Sprague-Dawley rat with a peak during the nighttime. Drd4 expression is seen in all retinal layers but the nocturnal increase is confined to the photoreceptors. Retinal Drd4 expression is not affected by removal of the sympathetic input to the eye, but triiodothyronine treatment induces Drd4 expression in the photoreceptors. In a developmental series, we show that the expression of Drd4 is restricted to postnatal stages with a peak at postnatal day 12. The high Drd4 expression in the rat retinal photoreceptors during the night supports physiological and pharmacologic evidence that the Drd4 receptor is involved in the dopaminergic inhibition of melatonin synthesis upon light stimulation. The sharp increase of Drd4 expression at a specific postnatal time suggests that dopamine is involved in retinal development.

Somatostatin peptides produce multiple effects on gating properties of native cone photoreceptor cGMP-gated channels that depend on circadian phase and previous illumination

A subpopulation of avian amacrine cells expresses somatostatin-14 (SS14) and somatostatin-28 (SS28), which provide a potential efferent limb for light-dependent regulation of photoreceptors. Here, we demonstrate that SS14 and SS28 modulate cone photoreceptor cGMP-gated channels (CNGCs) through multiple mechanisms. In chicken cones cultured in constant darkness for 2 d after previous entrainment to light-dark (LD) cycles or in cells maintained in LD, application of 100 nm SS14 or 100 nm SS28 for either 15 min or 2 h caused a decrease in the sensitivity of CNGCs to cGMP during the night, at circadian time 16 (CT16)-CT20 or zeitgeber time 16 (ZT16)-ZT20. SS14 had no effect during the day (CT4-CT8 or ZT4-ZT8). These effects persist in cells pretreated with pertussis toxin (PTX) and, like dopamine, may work to reinforce long-term circadian fluctuations in CNGCs driven by oscillators within the photoreceptors themselves. In contrast, a 15 min exposure to SS28 caused a seemingly paradoxical increase in the sensitivity of CNGCs to cGMP during the early day (ZT4-ZT6), but only in cones maintained in LD. This effect of SS28 desensitizes rapidly, is blocked by pretreatment with PTX, and is selectively mimicked by the cyclohexapeptide agonist MK-678. This transient response also requires activation of phospholipase C and protein kinase C. The transient response to SS28 may play a role in photoreceptor adaptation to rapid changes in ambient illumination. These data also show that photoreceptor responses to at least some peptide neurotransmitters depend on the previous history of light exposure.

Constant light rearing disrupts compensation to imposed- but not induced-hyperopia and facilitates compensation to imposed myopia in chicks

PURPOSE

While rearing chicks in constant light (CL) inhibits anterior segment growth, these conditions also induce excessive enlargement of the vitreous chamber. The mechanisms underlying these effects are poorly understood although it has been speculated that the enlarged vitreous chambers are a product of emmetropization, a compensatory response to the altered anterior segments. We examined the ability of eyes to compensate to defocusing lenses in CL as a direct test of their ability to emmetropize. We also studied recovery responses, i.e. from lens-induced changes in CL as well as CL-induced changes alone or combined with lens-induced changes in eyes returned to normal diurnal lighting (NL).

METHODS

Hatchling White-Leghorn chicks were reared in either CL or NL (control) lighting conditions (n=36) for 2 weeks, with lenses of either +10 or -10D power fitted to one eye of all chicks at the beginning of the second week. The lenses were removed at the end of the same week, at which time some CL chicks (n=14) were shifted to NL, the rest of the chicks remaining in their respective original lighting conditions. Retinoscopy, IR photo-keratometry and high-frequency A-scan ultrasonography were used to track refractions, corneal radii of curvature and ocular axial dimensions, respectively; data were collected on experimental days 0, 7, 9, 14 and 21.

RESULTS

Under CL, eyes showed near normal, albeit slightly exaggerated responses to +10D lenses while the response to -10D lenses was disrupted. With +10D lenses, lens-wearing eyes became more hyperopic (RE), and had shorter vitreous chambers (VC) and optical axial lengths (OL) relative to their fellows by the end of the lens period [RE: +10.5+/-1.5D, CL, +8.25+/-2.5D, NL; VC: -0.363+/-0.129mm, CL; -0.306+/-0.110mm, NL; OL: -0.493+/-0.115mm, CL, -0.379+/-0.106mm, NL (mean interocular difference+/-SD)]. With -10D lenses, the NL group showed a myopic shift in RE and increased elongation of both VC depth and OL (RE: -10.75+/-2.0D; VC depth: 0.554+/-0.097mm; OL: 0.746+/-0.166mm), while the CL group showed a small hyperopic shift in RE (+4.0+/-6.0D). Nonetheless, CL eyes were able to recover from lens-induced hyperopia, whether they were left in CL or returned to NL. One week of exposure to NL was sufficient to reverse the effects of 2 weeks of CL on anterior and vitreous chamber dimensions.

CONCLUSION

CL impairs emmetropization. Specifically, it disrupts compensation to lens-imposed hyperopia but not imposed myopia. However, CL eyes are able to recover from lens-induced hyperopia, suggesting that the mechanisms underlying the compensatory responses to defocusing lenses are different from those involved in recovery responses. The ocular growth effects of CL on young eyes are reversible under NL.

Signaling mediated by the dopamine D2 receptor potentiates circadian regulation by CLOCK:BMAL1

Environmental cues modulate a variety of intracellular pathways whose signaling is integrated by the molecular mechanism that constitutes the circadian clock. Although the essential gears of the circadian machinery have been elucidated, very little is known about the signaling systems regulating it. Here, we report that signaling mediated by the dopamine D2 receptor (D2R) enhances the transcriptional capacity of the CLOCK:BMAL1 complex. This effect involves the mitogen-activated protein kinase transduction cascade and is associated with a D2R-induced increase in the recruiting and phosphorylation of the transcriptional coactivator cAMP-responsive element-binding protein (CREB) binding protein. Importantly, CLOCK:BMAL1-dependent activation and light-inducibility of mPer1 gene transcription is drastically dampened in retinas of D2R-null mice. Because dopamine is the major catecholamine in the retina, central for the neural adaptation to light, our findings establish a physiological link among photic input, dopamine signaling, and the molecular clock machinery.

A possible new role for fish retinal serotonin-N-acetyltransferase-1 (AANAT1): Dopamine metabolism

Serotonin-N-acetyltransferase (arylalkylamine-N-acetyltransferase, AANAT) is the key enzyme in the generation of melatonin rhythms in the pineal gland and retinal photoreceptors. Rhythmic AANAT activity drives rhythmic melatonin production in these tissues. Two AANATs, AANAT1 and AANAT2, are present in teleost fish species. Different spatial expression patterns, enzyme kinetics and substrate preferences suggest that they may have different functions. Enzyme activity assays revealed that recombinant seabream and zebrafish AANAT1s, but not AANAT2s, acetylate dopamine with kinetic characteristics that are similar to those for tryptamine acetylation. High performance liquid chromatography analysis of seabream retinal extracts indicated the presence of N-acetyldopamine. Time-of-day analysis of retinal AANAT activity and concentration of melatonin, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and N-acetyldopamine revealed a daily pattern of retinal melatonin and N-acetyldopamine production that are correlated with retinal AANAT1 activity. In situ hybridization analysis of seabream retinal sections indicated that tyrosine hydroxylase is expressed in the inner nuclear layer (INL) and that AANAT1 is expressed in the outer nuclear layer (ONL) and INL. Together, these observations point to the possibility that dopamine is acetylated by retinal AANAT1 in the INL. Such novel activity of AANAT1 may reflect an important function in the circadian physiology of the retina.

Diurnal patterns of dopamine release in chicken retina

The retinal dopaminergic system appears to play a major role in the regulation of global retinal processes related to light adaptation. Although most reports agree that dopamine release is stimulated by light, some retinal functions that are mediated by dopamine exhibit circadian patterns of activity, suggesting that dopamine release may be controlled by a circadian oscillator as well as by light. Using the accumulation of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC) in the vitreous as a measure of dopamine release rates, we have investigated the balance between circadian- and light control over dopamine release. In chickens held under diurnal light:dark conditions, vitreal levels of DOPAC showed daily oscillations with the steady-state levels increasing nine-fold during the light phase. Kinetic analysis of this data indicates that apparent dopamine release rates increased almost four-fold at the onset of light and then remained continuously elevated throughout the 12h light phase. In constant darkness, vitreal levels of DOPAC displayed circadian oscillations, with an almost two-fold increase in dopamine release rates coinciding with subjective dawn/early morning. This circadian rise in vitreal DOPAC could be blocked by intravitreal administration of melatonin (10 nmol), as predicted by the model of the dark-light switch where a circadian fall in melatonin would relieve dopamine release of inhibition and thus be responsible for the slight circadian increase in dopamine release. The increase in vitreal DOPAC in response to light, however, was only partially suppressed by melatonin. The activity of the dopaminergic amacrine cell in the chicken retina thus appears to be dominated by light-activated input.

Nogo-A and nogo receptor expression in demyelinating lesions of multiple sclerosis

A myelin-associated neurite outgrowth inhibitor, Nogo-A, plays a key role in inhibition of axonal regeneration following injury and ischemia in the central nervous system (CNS). Because axonal injury is a pathologic hallmark of multiple sclerosis (MS), we have investigated the expression of Nogo-A and its receptor NgR in four MS and 12 non-MS control brains by immunohistochemistry. Nogo-A expression was markedly upregulated in surviving oligodendrocytes at the edge of chronic active demyelinating lesions of MS and ischemic lesions of acute and old cerebral infarction, whereas NgR expression was greatly enhanced in reactive astrocytes and microglia/macrophages in these lesions when compared with their expression in the brains of neurologically normal controls. Nogo-A and NgR were also identified in a subpopulation of neurons. In contrast, Nogo-A was undetectable in reactive astrocytes and microglia/macrophages and NgR was not expressed on oligodendrocytes in any cases examined. Western blot analysis and double labeling immunocytochemistry identified the constitutive expression of NgR in cultured human astrocytes. These results suggest that Nogo-A expressed on oligodendrocytes might interact with NgR presented by reactive astrocytes and microglia/macrophages in active demyelinating lesions of MS, although biologic effects caused by Nogo-A/NgR interaction among glial cells remain unknown.

A circadian clock in the fish retina regulates dopamine release via activation of melatonin receptors

Although many biochemical, morphological and physiological processes in the vertebrate retina are controlled by a circadian (24 h) clock, the location of the clock and how the clock alters retinal function are unclear. For instance, several observations have suggested that dopamine, a retinal neuromodulator, may play an important role in retinal rhythmicity but the link between dopamine and a clock located within or outside the retina remains to be established. We found that endogenous dopamine release from isolated goldfish retinae cultured in continuous darkness for 56 h clearly exhibited a circadian rhythm with high values during the subjective day. The continuous presence of melatonin (1 nM) in the culture medium abolished the circadian rhythm of dopamine release and kept values constantly low and equal to the night-time values. The selective melatonin antagonist luzindole (1 microM) also abolished the dopamine rhythm but the values were high and equal to the daytime values. Melatonin application during the late subjective day introduced rod input and reduced cone input to fish cone horizontal cells, a state usually observed during the subjective night. In contrast, luzindole application during the subjective night decreased rod input and increased cone input. Prior application of dopamine or spiperone, a selective dopamine D(2)-like antagonist, blocked the above effects of melatonin and luzindole, respectively. These findings indicate that a circadian clock in the vertebrate retina regulates dopamine release by the activation of melatonin receptors and that endogenous melatonin modulates rod and cone pathways through dopamine-mediated D(2)-like receptor activation.

The relationship between melatonin and dopamine rhythms in the duck retina

In the retina of duck, levels of dopamine (DA) and its main metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), fluctuate throughout the day, with high values during the light phase. The rhythmic changes in DA content and metabolism are out of phase with the daily oscillations in melatonin (MEL) and serotonin N-acetyltransferase (AA-NAT; the penultimate and key regulatory enzyme in MEL biosynthesis) activity. Acute exposure of ducks to light at night potently increased levels of DA and DOPAC, and decreased AA-NAT activity and MEL content in the retina. Intraocular administration of MEL to light-adapted ducks produced a significant decline in retinal DA and DOPAC concentrations. On the other hand, quinpirole, a D(2)/D(4)-DA receptor agonist, administered intraocularly, markedly suppressed the night-time retinal AA-NAT activity and MEL. These findings provide, for the first time, evidence for an inverse relationship between the DA system and MEL in the duck retina.