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

Mechanisms of emmetropization and what might go wrong in myopia

Studies in animal models and humans have shown that refractive state is optimized during postnatal development by a closed-loop negative feedback system that uses retinal image defocus as an error signal, a mechanism called emmetropization. The sensor to detect defocus and its sign resides in the retina itself. The retina and/or the retinal pigment epithelium (RPE) presumably releases biochemical messengers to change choroidal thickness and modulate the growth rates of the underlying sclera. A central question arises: if emmetropization operates as a closed-loop system, why does it not stop myopia development? Recent experiments in young human subjects have shown that (1) the emmetropic retina can perfectly distinguish between real positive defocus and simulated defocus, and trigger transient axial eye shortening or elongation, respectively. (2) Strikingly, the myopic retina has reduced ability to inhibit eye growth when positive defocus is imposed. (3) The bi-directional response of the emmetropic retina is elicited with low spatial frequency information below 8 cyc/deg, which makes it unlikely that optical higher-order aberrations play a role. (4) The retinal mechanism for the detection of the sign of defocus involves a comparison of defocus blur in the blue (S-cone) and red end of the spectrum (L + M-cones) but, again, the myopic retina is not responsive, at least not in short-term experiments. This suggests that it cannot fully trigger the inhibitory arm of the emmetropization feedback loop. As a result, with an open feedback loop, myopia development becomes "open-loop".

Endogenous Opioid Signaling in the Mouse Retina Modulates Pupillary Light Reflex

Opioid peptides and their receptors are expressed in the mammalian retina; however, little is known about how they might affect visual processing. The melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs), which mediate important non-image-forming visual processes such as the pupillary light reflex (PLR), express β-endorphin-preferring, µ-opioid receptors (MORs). The objective of the present study was to elucidate if opioids, endogenous or exogenous, modulate pupillary light reflex (PLR) via MORs expressed by ipRGCs. MOR-selective agonist [D-Ala², MePhe⁴, Gly-ol⁵]-enkephalin (DAMGO) or antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP) was administered via intravitreal injection. PLR was recorded in response to light stimuli of various intensities. DAMGO eliminated PLR evoked by light with intensities below melanopsin activation threshold but not that evoked by bright blue irradiance that activated melanopsin signaling, although in the latter case, DAMGO markedly slowed pupil constriction. CTAP or genetic ablation of MORs in ipRGCs slightly enhanced dim-light-evoked PLR but not that evoked by a bright blue stimulus. Our results suggest that endogenous opioid signaling in the retina contributes to the regulation of PLR. The slowing of bright light-evoked PLR by DAMGO is consistent with the observation that systemically applied opioids accumulate in the vitreous and that patients receiving chronic opioid treatment have slow PLR.

The acute effect of atropine eye drops on the human full-field electroretinogram

PURPOSE

Atropine eye drops are a common and effective treatment for slowing myopia progression, but the site and mode of action of atropine in controlling myopia are unclear. We investigated the early retinal sites of action of atropine by examining its effects on the human full-field electroretinogram (ffERG).

METHOD

Baseline ffERGs were recorded in both eyes of 24 healthy subjects (mean ± SD: 21.0 ± 2.3 years; spherical equivalent refraction, range: + 1.63 to - 0.75 D) using 6 standard ISCEV protocols, 30 min after bilateral pupil dilation with 1% Tropicamide. Atropine (1 drop, 0.1%) was then instilled into the non-dominant eye. 24 h later, ffERGs were again recorded in both eyes. Ratios (post-atropine: pre-atropine) of dark-adapted (DA) and light-adapted (LA) ffERGs were compared between atropine-treated and control eyes using multivariate repeated measures general linear models.

RESULTS

Atropine-treated eyes responded with 14% lower DA3.0 OP (oscillatory potential) amplitude (p = 0.003) and 4% delay in the DA10.0 a-wave peak time (p = 0.00099) compared with control eyes. Amplitudes and peak times were not different between atropine-treated and control eyes for DA0.01, LA3.0, and LA3.0 flicker ERGs. While atropine caused a small (1.26 mm², p = 0.03) extra increase in pupil area in the treated eye, atropine-induced changes in ffERG responses bore no relationship with changes in pupil area (R²= 2-5%, p > 0.05).

CONCLUSIONS

The observed changes in oscillatory potentials corroborate previous findings that atropine affects neural activity in the inner retina. However, observed changes to the a-wave suggest that atropine also affects activity in photoreceptors.

Genome-Wide Association Study in Asians Identifies Novel Loci for High Myopia and Highlights a Nervous System Role in Its Pathogenesis

PURPOSE

To identify novel susceptibility loci for high myopia.

DESIGN

Genome-wide association study (GWAS) followed by replication and meta-analysis.

PARTICIPANTS

A total of 14 096 samples from East and Southeast Asian populations (2549 patients with high myopia and 11 547 healthy controls).

METHODS

We performed a GWAS in 3269 Japanese individuals (1668 with high myopia and 1601 control participants), followed by replication analysis in a total of 10 827 additional samples (881 with high myopia and 9946 control participants) from Japan, Singapore, and Taiwan. To confirm the biological role of the identified loci in the pathogenesis of high myopia, we performed functional annotation and Gene Ontology (GO) analyses.

MAIN OUTCOME MEASURES

We evaluated the association of single nucleotide polymorphisms with high myopia and GO terms enriched among genes identified in the current study.

RESULTS

We identified 9 loci with genome-wide significance (P < 5.0 × 10^-8). Three loci were previously reported myopia-related loci (ZC3H11B on 1q41, GJD2 on 15q14, and RASGRF1 on 15q25.1), and the other 6 were novel (HIVEP3 on 1p34.2, NFASC/CNTN2 on 1q32.1, CNTN4/CNTN6 on 3p26.3, FRMD4B on 3p14.1, LINC02418 on 12q24.33, and AKAP13 on 15q25.3). The GO analysis revealed a significant role of the nervous system related to synaptic signaling, neuronal development, and Ras/Rho signaling in the pathogenesis of high myopia.

CONCLUSIONS

The current study identified 6 novel loci associated with high myopia and demonstrated an important role of the nervous system in the disease pathogenesis. Our findings give new insight into the genetic factors underlying myopia, including high myopia, by connecting previous findings and allowing for a clarified interpretation of the cause and pathophysiologic features of myopia at the molecular level.

Studies on retinal mechanisms possibly related to myopia inhibition by atropine in the chicken

PURPOSE

While low-dose atropine eye drops are currently widely used to inhibit myopia development in children, the underlying mechanisms are poorly understood. Therefore, we studied possible retinal mechanisms and receptors that are potentially involved in myopia inhibition by atropine.

METHODS

A total of 250 μg atropine were intravitreally injected into one eye of 19 chickens, while the fellow eyes received saline and served as controls. After 1 h, 1.5 h, 2 h, 3 h, and 4 h, eyes were prepared for vitreal dopamine (DA) measurements, using high-pressure liquid chromatography with electrochemical detection. Twenty-four animals were kept either in bright light (8500 lx) or standard light (500 lx) after atropine injection for 1.5 h before DA was measured. In 10 chickens, the α_2A-adrenoreceptor (α_2A-ADR) agonists brimonidine and clonidine were intravitreally injected into one eye, the fellow eye served as control, and vitreal DA content was measured after 1.5 h. In 6 chickens, immunohistochemical analyses were performed 1.5 h after atropine injection.

RESULTS

Vitreal DA levels increased after a single intravitreal atropine injection, with a peak difference between both eyes after 1.97 h. DA was also enhanced in fellow eyes, suggesting a systemic action of intravitreally administered atropine. Bright light and atropine (which both inhibit myopia) had additive effects on DA release. Quantitative immunolabelling showed that atropine heavily stimulated retinal activity markers ZENK and c-Fos in cells of the inner nuclear layer. Since atropine was recently found to also bind to α_2A-ADRs at doses where it can inhibit myopia, their retinal localization was studied. In amacrine cells, α_2A-ADRs were colocalized with tyrosine hydroxylase (TH), glucagon, and nitric oxide synthase, peptides known to play a role in myopia development in chickens. Intravitreal atropine injection reduced the number of neurons that were double-labelled for TH and α_2A-ADR. α_2A-ADR agonists clonidine and brimonidine (which were also found by other authors to inhibit myopia) severely reduced vitreal DA content in both injected and fellow eyes, compared to eyes of untreated chicks.

CONCLUSIONS

Merging our results with published data, it can be concluded that both muscarinic and α_2A-adrenergic receptors are expressed on dopaminergic neurons and both atropine and α_2A-ADR antagonists stimulate DA release whereas α_2A-ADR agonists strongly suppress its release. Stimulation of DA by atropine was enhanced by bright light. Results are in line with the hypothesis that inhibition of deprivation myopia is correlated with DA stimulation, as long as no toxicity is involved.

Circadian Vision in Zebrafish: From Molecule to Cell and from Neural Network to Behavior

Most visual system functions, such as opsin gene expression, retinal neural transmission, light perception, and visual sensitivity, display robust day-night rhythms. The rhythms persist in constant lighting conditions, suggesting the involvement of endogenous circadian clocks. While the circadian pacemakers that control the rhythms of animal behaviors are mostly found in the forebrain and midbrain, self-sustained circadian oscillators are also present in the neural retina, where they play important roles in the regulation of circadian vision. This review highlights some of the correlative studies of the circadian control of visual system functions in zebrafish. Because zebrafish maintain a high evolutionary proximity to mammals, the findings from zebrafish research may provide insights for a better understanding of the mechanisms of circadian vision in other vertebrate species including humans.

μ-Opioid Receptor Activation Directly Modulates Intrinsically Photosensitive Retinal Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) encode light intensity and trigger reflexive responses to changes in environmental illumination. In addition to functioning as photoreceptors, ipRGCs are post-synaptic neurons in the inner retina, and there is increasing evidence that their output can be influenced by retinal neuromodulators. Here we show that opioids can modulate light-evoked ipRGC signaling, and we demonstrate that the M1, M2 and M3 types of ipRGCs are immunoreactive for μ-opioid receptors (MORs) in both mouse and rat. In the rat retina, application of the MOR-selective agonist DAMGO attenuated light-evoked firing ipRGCs in a dose-dependent manner (IC50 < 40 nM), and this effect was reversed or prevented by co-application of the MOR-selective antagonists CTOP or CTAP. Recordings from solitary ipRGCs, enzymatically dissociated from retinas obtained from melanopsin-driven fluorescent reporter mice, confirmed that DAMGO exerts its effect directly through MORs expressed by ipRGCs. Reduced ipRGC excitability occurred via modulation of voltage-gated potassium and calcium currents. These findings suggest a potential new role for endogenous opioids in the mammalian retina and identify a novel site of action-MORs on ipRGCs-through which opioids might exert effects on reflexive responses to environmental light.

Animal Studies and the Mechanism of Myopia-Protection by Light?

Epidemiological studies have demonstrated that spending time outdoors during your childhood is protective against the development of myopia. It has been hypothesized that this protective effect is associated with light-induced increases in retinal dopamine levels, a critical neuromodulator that has long been postulated to be involved in the regulation of ocular growth. This paper, along with the paper entitled "What do animal studies tell us about the mechanism of myopia-protection by light?" discusses the evidence provided by animal models for this hypothesis.

Intravitreally-administered dopamine D2-like (and D4), but not D1-like, receptor agonists reduce form-deprivation myopia in tree shrews

We examined the effect of intravitreal injections of D1-like and D2-like dopamine receptor agonists and antagonists and D4 receptor drugs on form-deprivation myopia (FDM) in tree shrews, mammals closely related to primates. In eleven groups (n = 7 per group), we measured the amount of FDM produced by monocular form deprivation (FD) over an 11-day treatment period. The untreated fellow eye served as a control. Animals also received daily 5 µL intravitreal injections in the FD eye. The reference group received 0.85% NaCl vehicle. Four groups received a higher, or lower, dose of a D1-like receptor agonist (SKF38393) or antagonist (SCH23390). Four groups received a higher, or lower, dose of a D2-like receptor agonist (quinpirole) or antagonist (spiperone). Two groups received the D4 receptor agonist (PD168077) or antagonist (PD168568). Refractions were measured daily; axial component dimensions were measured on day 1 (before treatment) and day 12. We found that in groups receiving the D1-like receptor agonist or antagonist, the development of FDM and altered ocular component dimensions did not differ from the NaCl group. Groups receiving the D2-like receptor agonist or antagonist at the higher dose developed significantly less FDM and had shorter vitreous chambers than the NaCl group. The D4 receptor agonist, but not the antagonist, was nearly as effective as the D2-like agonist in reducing FDM. Thus, using intravitreally-administered agents, we did not find evidence supporting a role for the D1-like receptor pathway in reducing FDM in tree shrews. The reduction of FDM by the dopamine D2-like agonist supported a role for the D2-like receptor pathway in the control of FDM. The reduction of FDM by the D4 receptor agonist, but not the D4 antagonist, suggests an important role for activation of the dopamine D4 receptor in the control of axial elongation and refractive development.

Light levels, refractive development, and myopia--a speculative review

Recent epidemiological evidence in children indicates that time spent outdoors is protective against myopia. Studies in animal models (chick, macaque, tree shrew) have found that light levels (similar to being in the shade outdoors) that are mildly elevated compared to indoor levels, slow form-deprivation myopia and (in chick and tree shrew) lens-induced myopia. Normal chicks raised in low light levels (50 lux) with a circadian light on/off cycle often develop spontaneous myopia. We propose a model in which the ambient illuminance levels produce a continuum of effects on normal refractive development and the response to myopiagenic stimuli such that low light levels favor myopia development and elevated levels are protective. Among possible mechanisms, elevation of retinal dopamine activity seems the most likely. Inputs from intrinsically-photosensitive retinal ganglion cells (ipRGCs) at elevated light levels may be involved, providing additional activation of retinal dopaminergic pathways.

Pharmacology of myopia and potential role for intrinsic retinal circadian rhythms

Despite the high prevalence and public health impact of refractive errors, the mechanisms responsible for ametropias are poorly understood. Much evidence now supports the concept that the retina is central to the mechanism(s) regulating emmetropization and underlying refractive errors. Using a variety of pharmacologic methods and well-defined experimental eye growth models in laboratory animals, many retinal neurotransmitters and neuromodulators have been implicated in this process. Nonetheless, an accepted framework for understanding the molecular and/or cellular pathways that govern postnatal eye development is lacking. Here, we review two extensively studied signaling pathways whose general roles in refractive development are supported by both experimental and clinical data: acetylcholine signaling through muscarinic and/or nicotinic acetylcholine receptors and retinal dopamine pharmacology. The muscarinic acetylcholine receptor antagonist atropine was first studied as an anti-myopia drug some two centuries ago, and much subsequent work has continued to connect muscarinic receptors to eye growth regulation. Recent research implicates a potential role of nicotinic acetylcholine receptors; and the refractive effects in population surveys of passive exposure to cigarette smoke, of which nicotine is a constituent, support clinical relevance. Reviewed here, many puzzling results inhibit formulating a mechanistic framework that explains acetylcholine's role in refractive development. How cholinergic receptor mechanisms might be used to develop acceptable approaches to normalize refractive development remains a challenge. Retinal dopamine signaling not only has a putative role in refractive development, its upregulation by light comprises an important component of the retinal clock network and contributes to the regulation of retinal circadian physiology. During postnatal development, the ocular dimensions undergo circadian and/or diurnal fluctuations in magnitude; these rhythms shift in eyes developing experimental ametropia. Long-standing clinical ideas about myopia in particular have postulated a role for ambient lighting, although molecular or cellular mechanisms for these speculations have remained obscure. Experimental myopia induced by the wearing of a concave spectacle lens alters the retinal expression of a significant proportion of intrinsic circadian clock genes, as well as genes encoding a melatonin receptor and the photopigment melanopsin. Together this evidence suggests a hypothesis that the retinal clock and intrinsic retinal circadian rhythms may be fundamental to the mechanism(s) regulating refractive development, and that disruptions in circadian signals may produce refractive errors. Here we review the potential role of biological rhythms in refractive development. While much future research is needed, this hypothesis could unify many of the disparate clinical and laboratory observations addressing the pathogenesis of refractive errors.

Dopaminergic amacrine cells express opioid receptors in the mouse retina

The presence of opioid receptors has been confirmed by a variety of techniques in vertebrate retinas including those of mammals; however, in most reports, the location of these receptors has been limited to retinal regions rather than specific cell types. Concurrently, our knowledge of the physiological functions of opioid signaling in the retina is based on only a handful of studies. To date, the best-documented opioid effect is the modulation of retinal dopamine release, which has been shown in a variety of vertebrate species. Nonetheless, it is not known if opioids can affect dopaminergic amacrine cells (DACs) directly, via opioid receptors expressed by DACs. This study, using immunohistochemical methods, sought to determine whether (1) μ- and δ-opioid receptors (MORs and DORs, respectively) are present in the mouse retina, and if present, (2) are they expressed by DACs. We found that MOR and DOR immunolabeling were associated with multiple cell types in the inner retina, suggesting that opioids might influence visual information processing at multiple sites within the mammalian retinal circuitry. Specifically, colabeling studies with the DAC molecular marker anti-tyrosine hydroxylase antibody showed that both MOR and DOR immunolabeling localize to DACs. These findings predict that opioids can affect DACs in the mouse retina directly, via MOR and DOR signaling, and might modulate dopamine release as reported in other mammalian and nonmammalian retinas.

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.

Melatonin combats molecular terrorism at the mitochondrial level

The intracellular environmental is a hostile one. Free radicals and related oxygen and nitrogen-based oxidizing agents persistently pulverize and damage molecules in the vicinity of where they are formed. The mitochondria especially are subjected to frequent and abundant oxidative abuse. The carnage that is left in the wake of these oxygen and nitrogen-related reactants is referred to as oxidative damage or oxidative stress. When mitochondrial electron transport complex inhibitors are used, e.g., rotenone, 1-methyl-1-phenyl-1,2,3,6-tetrahydropyridine, 3-nitropropionic acid or cyanide, pandemonium breaks loose within mitochondria as electron leakage leads to the generation of massive amounts of free radicals and related toxicants. The resulting oxidative stress initiates a series of events that leads to cellular apoptosis. To alleviate mitochondrial destruction and the associated cellular implosion, the cell has at its disposal a variety of free radical scavengers and antioxidants. Among these are melatonin and its metabolites. While melatonin stimulates several antioxidative enzymes it, as well as its metabolites (cyclic 3-hydroxymelatonin, N¹-acetyl-N²-formyl-5-methoxykynuramine and N¹-acetyl-5-methoxykynuramine), likewise effectively neutralize free radicals. The resulting cascade of reactions greatly magnifies melatonin's efficacy in reducing oxidative stress and apoptosis even in the presence of mitochondrial electron transport inhibitors. The actions of melatonin at the mitochondrial level are a consequence of melatonin and/or any of its metabolites. Thus, the molecular terrorism meted out by reactive oxygen and nitrogen species is held in check by melatonin and its derivatives.

Inhibitory effect of somatostatin-14 on L-type voltage-gated calcium channels in cultured cone photoreceptors requires intracellular calcium

The inhibitory effects of somatostatin have been well documented for many physiological processes. The action of somatostatin is through G-protein-coupled receptor-mediated second-messenger signaling, which in turn affects other downstream targets including ion channels. In the retina, somatostatin is released from a specific class of amacrine cells. Here we report that there was a circadian phase-dependent effect of somatostatin-14 (SS14) on the L-type voltage-gated calcium channels (L-VGCCs) in cultured chicken cone photoreceptors, and our study reveals that this process is dependent on intracellular calcium stores. Application of 500 nM SS14 for 2 h caused a decrease in L-VGCC currents only during the subjective night but not the subjective day. We then explored the cellular mechanisms underlying the circadian phase-dependent effect of SS14. The inhibitory effect of SS14 on L-VGCCs was mediated through the pertussis-toxin-sensitive G-protein-dependent somatostatin receptor 2 (sst2). Activation of sst2 by SS14 further activated downstream signaling involving phospholipase C and intracellular calcium stores. Mobilization of intracellular Ca2+ was required for somatostatin induced inhibition of photoreceptor L-VGCCs, suggesting that somatostatin plays an important role in the modulation of photoreceptor physiology.

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.

An autonomous circadian clock in the inner mouse retina regulated by dopamine and GABA

The influence of the mammalian retinal circadian clock on retinal physiology and function is widely recognized, yet the cellular elements and neural regulation of retinal circadian pacemaking remain unclear due to the challenge of long-term culture of adult mammalian retina and the lack of an ideal experimental measure of the retinal circadian clock. In the current study, we developed a protocol for long-term culture of intact mouse retinas, which allows retinal circadian rhythms to be monitored in real time as luminescence rhythms from a PERIOD2::LUCIFERASE (PER2::LUC) clock gene reporter. With this in vitro assay, we studied the characteristics and location within the retina of circadian PER2::LUC rhythms, the influence of major retinal neurotransmitters, and the resetting of the retinal circadian clock by light. Retinal PER2::LUC rhythms were routinely measured from whole-mount retinal explants for 10 d and for up to 30 d. Imaging of vertical retinal slices demonstrated that the rhythmic luminescence signals were concentrated in the inner nuclear layer. Interruption of cell communication via the major neurotransmitter systems of photoreceptors and ganglion cells (melatonin and glutamate) and the inner nuclear layer (dopamine, acetylcholine, GABA, glycine, and glutamate) did not disrupt generation of retinal circadian PER2::LUC rhythms, nor did interruption of intercellular communication through sodium-dependent action potentials or connexin 36 (cx36)-containing gap junctions, indicating that PER2::LUC rhythms generation in the inner nuclear layer is likely cell autonomous. However, dopamine, acting through D1 receptors, and GABA, acting through membrane hyperpolarization and casein kinase, set the phase and amplitude of retinal PER2::LUC rhythms, respectively. Light pulses reset the phase of the in vitro retinal oscillator and dopamine D1 receptor antagonists attenuated these phase shifts. Thus, dopamine and GABA act at the molecular level of PER proteins to play key roles in the organization of the retinal circadian clock.

The circadian clock in the mammalian retina regulates many retinal functions, and its output modulates the central circadian clock in the brain. Details about the cellular location and neural regulation of the mammalian retinal circadian clock remain unclear, however, largely due to the difficulty of maintaining long-term culture of adult mammalian retina and the lack of an ideal experimental measure of the retinal clock. We have circumvented these limitations by developing a protocol for long-term culture of intact mouse retinas to monitor circadian rhythms of clock gene expression in real time. Using this protocol, we have localized expression of molecular retinal circadian rhythms to the inner nuclear layer. We find molecular retinal rhythms generation is independent of many forms of signaling from photoreceptors and ganglion cells, or major forms of neural communication within the inner nuclear layer, and have characterized light-induced resetting of the retinal clock. Retinal dopamine and GABA, although not necessary for the generation of molecular retinal rhythms, were revealed to regulate the phase and amplitude of retinal molecular rhythms, respectively, with dopamine participating in light-induced resetting. Our data indicate that dopamine and GABA play prominent roles in the organization of the retinal circadian clock.

Long-term culture of mouse retinas reveals a circadian clock in the inner retina that can be reset by light and is regulated by the neurotransmitters dopamine and GABA.

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.

Interactions between dopamine and melatonin organize circadian rhythmicity in the retina of the green iguana

Circadian physiology in the vertebrate retina is regulated by several neurotransmitters. In the lateral eyes of the green iguana the circadian rhythm of melatonin content peaks during the night while the rhythm of dopamine peaks during the day. In the present work, the authors explore the interaction of these 2 neurotransmitters during the circadian cycle. They depleted retinal dopamine with intravitreal injections of 6-hydroxydopamine (6-OHDA) and measured ocular melatonin content in vivo throughout 1 circadian cycle. The circadian rhythm of ocular melatonin not only persisted but increased 10-fold in amplitude. This increase was substantially reduced by the intraocular administration of dopamine. 6-OHDA-treated retinas, unlike those from untreated animals, did not express a circadian rhythm of melatonin synthesis in vitro. To deplete retinal melatonin, the authors pinealectomized iguanas and blocked retinal melatonin synthesis by depleting serotonin with intraocular injections of 5,6-dihydroxytryptamine. In animals so treated, they found that the circadian rhythm of retinal dopamine content was abolished, the levels of dopamine were lowered, and the levels of dopamine metabolites were greatly increased. The data suggest that in iguanas, the amplitude of the circadian rhythm of melatonin synthesis in the eye is suppressed by dopamine while the rhythm of dopamine depends, at least in part, on the presence of melatonin.

Season of birth, natural light, and myopia

PURPOSE

To investigate the possible roles of season of birth and perinatal duration of daylight hours (photoperiod) in the development of myopia.

DESIGN

Retrospective, population-based, epidemiological study.

PARTICIPANTS

A total of 276 911 adolescents (157 663 male, 119 248 female) 16 to 22 years old. All were Israeli-born conscripts to the Israeli Defense Forces who were examined during the 5-year period 2000 through 2004.

METHODS

Noncycloplegic refraction was determined by autorefractometer and validated by qualified optometrists. Myopia, defined on the basis of right eye spherical equivalence, was classified as mild (-0.75 to -2.99 diopters [D]), moderate (-3.0 to -5.99 D), or severe (-6.0 D or worse). The photoperiod was recorded from astronomical tables and classified into 4 categories. Using multivariate logistic regression models, we calculated odds ratios (ORs) for several risk factors of myopia including season of birth.

MAIN OUTCOME MEASURE

The OR for photoperiod categories as risk factors for myopia.

RESULTS

Overall prevalences of mild, moderate, and severe myopia were 18.8%, 8.7%, and 2.4%, respectively. There were seasonal variations in moderate and severe myopia according to birth month, with prevalence highest for June/July births and lowest for December/January. On multivariate logistic regression, the ORs of photoperiod categories for moderate and severe myopia were highly significant and demonstrated a dose-response pattern. Odds ratios for severe myopia were highest for the shortest versus the longest photoperiods (1.24; 95% confidence interval, 1.15-1.33; P<0.001). Mild myopia was not associated with season of birth or perinatal light exposure. Other risk factors were gender (1.14 for female), education level (1.32 for age above 12), and father's origin (1.31 for Eastern vs. Israeli origin).

CONCLUSION

Myopia in this population is associated with birth during summer months. The exact associating mechanism is not known but might be related to exposure to natural light during the early perinatal period.

Dopaminergic agents affect the ability of brief periods of normal vision to prevent form-deprivation myopia

Placing a translucent diffuser over the eye of a chick causes the eye to grow excessively, resulting in form-deprivation myopia. For chickens kept on a 12:12 h light/dark cycle, removing the diffuser for 3 h during the light period protects against the excessive growth, but if the bird is kept in the dark for this 3-h period, the protective effect is abolished. Injecting dopamine agonists into the eye during this 3-h dark period restores the protective effect, which can be blocked by dopamine antagonists injected just prior to diffuser removal in the light. These responses are mediated by D2 receptors, suggesting that the protective effect of normal vision against form-deprivation is mediated through the stimulation of dopamine release and activation of D2-dopamine receptors.

Expression of aquaporin-9 immunoreactivity by catecholaminergic amacrine cells in the rat retina

Aquaporins are involved in the maintenance of ionic and osmotic balance in the central nervous system and in the eye. Whereas the expression of aquaporin-9 immunoreactivity in the brain has been described for catecholaminergic neurons and glial cells, the expression of aquaporin-9 in the neural retina is unclear. We examined the distribution of aquaporin-1 and -9 immunoreactivities in retinas of the rat. Aquaporin-9 immunoreactivity was expressed exclusively by tyrosine hydroxylase (TH) positive amacrine cells, while aquaporin-1 immunoreactivity was expressed by photoreceptor cells and by TH negative amacrine cells. The staining pattern of aquaporin-9 did not change up to 4 weeks after pressure-induced transient retinal ischemia. It is concluded that catecholaminergic, putatively dopaminergic, amacrine cells of the retina express aquaporin-9.

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.

Glutamate-evoked alterations of glial and neuronal cell morphology in the guinea pig retina

Neuronal activity is accompanied by transmembranous ion fluxes that cause cell volume changes. In whole mounts of the guinea pig retina, application of glutamate resulted in fast swelling of neuronal cell bodies in the ganglion cell layer (GCL) and the inner nuclear layer (INL) (by approximately 40%) and a concomitant decrease of the thickness of glial cell processes in the inner plexiform layer (IPL) (by approximately 40%) that was accompanied by an elongation of the glial cells, by a thickening of the whole retinal tissue, and by a shrinkage of the extracellular space (by approximately 18%). The half-maximal effect of glutamate was observed at approximately 250 mum, after approximately 4 min. The swelling was caused predominantly by AMPA-kainate receptor-mediated influx of Na+ into retinal neurons. Similar but transient morphological alterations were induced by high K+ and dopamine, which caused release of endogenous glutamate and subsequent activation of AMPA-kainate receptors. Apparently, retinal glutamatergic transmission is accompanied by neuronal cell swelling that causes compensatory morphological alterations of glial cells. The effect of dopamine was elicitable only during light adaptation but not in the dark, and glutamate and high K+ induced strong ereffects in the dark than in the light. This suggests that not only the endogenous release of dopamine but also the responsiveness of glutamatergic neurons to dopamine is regulated by light-dark adaptation. Similar morphological alterations (neuronal swelling and decreased glial process thickness) were observed in whole mounts isolated immediately after experimental retinal ischemia, suggesting an involvement of AMPA-kainate receptor activation in putative neurotoxic cell swelling in the postischemic retina.

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.

Emmetropisation under continuous but non-constant light in chicks

It has been suggested that ambient lighting at night influences eye growth and might play a causal role in human myopia. To test this hypothesis, we reared newly hatched chicks under 12 hr light-dark or light-dim cycles with a light phase intensity of 1500 microW/cm(2) and variable dim phase intensities between 0.01 and 500 microW/cm(2). Other chicks were reared under constant light conditions with intensities between 1 and 1500 microW/cm(2). After three weeks, the chicks were examined by refractometry, ultrasound and caliper measurements of enucleated eyes. To relate ocular parameters with a retinal neurotransmitter likely involved in eye growth control, retinal and vitreal levels of dopamine and its principal metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were measured by high performance liquid chromatography with electrochemical detection in the light, dark and dim phases. Diurnal fluctuations in axial length and choroidal thickness also were measured twice daily by partial coherence interferometry (PCI) in chicks under light-dark and the two brightest light-dim conditions. The eyes of chicks reared under most light-dim conditions had refractions and ocular dimensions comparable to those reared under light-dark conditions. At dim phase light intensities of 10 microW/cm(2) and above, the day-night changes in retinal dopamine metabolism were not observed. The daily fluctuations of axial length and choroidal thickness were altered with rearing under the two brightest dim light intensities, compared to the light-dark condition. Rearing under constant light with intensities ranging between 1 and 1500 microW/cm(2) produced a shallow anterior chamber and other eye alterations previously described for constant light rearing even though rearing under continuous light that fluctuated between these same intensities generally permitted normal eye growth. Thus, continuous but fluctuating light exerts different developmental effects on the eye than constant non-fluctuating light. Light-dim rearing may be more relevant to daily human light exposures than other laboratory lighting conditions and may provide an opportunity to study developmental interactions of visual quality (e.g. blur, defocus, etc.) and features of the light-dark cycle under conditions that perturb daily rhythms in dopamine metabolism and ocular dimensions. Such studies also could provide mechanistic insights into whether and how daily rhythms in retinal dopamine metabolism, axial length or choroidal thickness modulate refractive development.

Absence of circadian clock regulation of horizontal cell gap junctional coupling reveals two dopamine systems in the goldfish retina

In fish and other vertebrate retinas, although dopamine release is regulated by both light and an endogenous circadian (24-hour) clock, light increases dopamine release to a greater extent than the clock. The clock increases dopamine release during the subjective day so that D2-like receptors are activated. It is not known, however, whether the retinal clock also activates D1 receptors, which display a much lower sensitivity to dopamine in intact tissue. Because activation of the D1 receptors on fish cone horizontal (H1) cells uncouples the gap junctions between the cells, we studied whether the clock regulates the extent of biocytin tracer coupling in the goldfish retina. Tracer coupling between H1 cells was extensive under dark-adapted conditions (low scotopic range) and similar in the subjective day, subjective night, day, and night. An average of approximately 180 cells were coupled in each dark-adapted condition. However, bright light stimulation or application of the D1 agonist SKF38393 (10 microM) dramatically reduced H1 cell coupling. The D2 agonist quinpirole (1 microM) or application of the D1 antagonist SCH23390 (10 microM) and/or the D2 antagonist spiperone (10 microM) had no effect on H1 cell coupling in dark-adapted retinas. These observations demonstrate that H1 cell gap junctional coupling and thus D1 receptor activity are not affected by endogenous dopamine under dark-adapted conditions. The results suggest that two different dopamine systems are present in the goldfish retina. One system is controlled by an endogenous clock that activates low threshold D2-like receptors in the day, whereas the second system is controlled by light and involves activation of higher threshold D1 receptors.

Calcium channels at the photoreceptor synapse

Presynaptic Ca2+ channels mediate early stages of visual information processing in photoreceptors by facilitating the release of neurotransmitter and by receiving modulatory input that alters transmission. Two types of L-type Ca2+ channels, composed of alpha1F and alpha1D subunits and having similar biophysical andpharmacological properties, appear to form the principle voltage-dependent Ca2+ influx pathways in rods and cones, respectively. The role played by these channels in neurotransmitter release at these graded potential, non-spiking synapses, has been well described. The channels mediate sustained glutamate release in darkness where the cells rest at potentials near -40 mV, and signal increases in light intensity as the cells hyperpolarize negative to this value. Synaptic modulation and integration mediated by these channels has not yet been as fully described but appears to involve GABA, nitric oxide (NO), glutamate, and dopamine. Ca2+ permeable cyclic nucleotide gated (CNG) channels appear to have supporting roles at the photoreceptor output synapse and may transduce NO signals from other cells by either directly permitting Ca2+ influx or by providing depolarizing influences that gate voltage dependent Ca2+ channels.

Dopamine mediates circadian clock regulation of rod and cone input to fish retinal horizontal cells

A circadian (24-hour) clock regulates the light responses of fish cone horizontal cells, second order neurones in the retina that receive synaptic contact from cones and not from rods. Due to the action of the clock, cone horizontal cells are driven by cones in the day, but primarily driven by rods at night. We show here that dopamine, a retinal neurotransmitter, acts as a clock signal for the day by increasing cone input and decreasing rod input to cone horizontal cells. The amount of endogenous dopamine released from in vitro retinae was greater during the subjective day than the subjective night. Application of dopamine or quinpirole, a dopamine D(2)-like agonist, during the subjective night increased cone input and eliminated rod input to the cells, a state usually observed during the subjective day. In contrast, application of spiperone, a D(2)-like antagonist, or forskolin, an activator of adenylyl cyclase, during the subjective day reduced cone input and increased rod input. SCH23390, a D(1) antagonist, had no effect. Application of R(p)-cAMPS, an inhibitor of cAMP-dependent protein kinase, or octanol, an alcohol that uncouples gap junctions, during the night increased cone input and decreased rod input. Because D(2)-like receptors are on photoreceptor cells, but not horizontal cells, the results suggest that the clock-induced increase in dopamine release during the day activates D(2)-like receptors on photoreceptor cells. The resultant decrease in intracellular cyclic AMP and protein kinase A activation then mediates the increase in cone input and decrease in rod input.

Somatostatin (SRIF) and SRIF receptors in the mouse retina

In the retina, somatostatin (SRIF) acts as a neuromodulator by interacting with specific SRIF subtype (sst) receptors. The aim of this study was to detect mRNAs for sst(1-5) receptors by semiquantitative RT-PCR and to determine the cellular localization of either SRIF or individual SRIF receptor immunoreactivities. Size, density and absolute number of immunolabeled somata were measured using computer-assisted image analysis. With RT-PCR we found that all five sst receptor mRNAs were expressed, with highest levels of sst(2) and sst(4) receptors. SRIF immunolabeling was localized to sparse-occurring amacrine cells in the inner nuclear layer (INL) and to displaced amacrine cells in the ganglion cell layer (GCL). sst(2A) receptors were localized to protein kinase- (PKC) immunoreactive (IR) rod bipolar cells, calbindin- (CaBP-) IR horizontal cells, tyrosine hydroxylase- (TH-) IR amacrine cells and glycinergic amacrine cells. None of the sst(2A)-IR amacrine cells were found to express parvalbumin (PV) immunoreactivity. sst(4) receptor immunolabeling was localized to CaBP-IR and CaBP-non-IR cells in the GCL that originated long process bundles in the GC axon layer. These cells were not observed after optic nerve transection and they were therefore interpreted as ganglion cells. Quantitative analysis showed that all of the PKC-IR rod bipolar cells, CaBP-IR horizontal cells, and TH-IR amacrine cells and 5% of the glycinergic amacrine cells expressed sst(2A) receptors. In addition, 4-6% of the putative ganglion cells expressed sst(4) receptors. The localization of SRIF to sparse-occurring retinal neurons, together with the widespread expression of sst(2A) and sst(4) receptors suggests that SRIF acts at multiple levels of retinal circuitry. These results provide a database for investigations of the functional retinal networks in mice with genetic alterations of somatostatinergic transmission.

Darkness and near work: myopia and its progression in third-year law students

PURPOSE

To evaluate myopia prevalence, myopia progression, and various potential myopia risk factors in third-year law students.

DESIGN

Cross-sectional study and survey.

PARTICIPANTS

One hundred seventy-nine third-year law students at the University of Pennsylvania.

METHODS

We administered a questionnaire to assess the prevalence of myopia, myopia progression, and risk factors, including near work, family history, and daily light/dark exposure. We conducted a screening eye examination to ascertain myopia status. Myopia was defined as the mean spherical equivalent of the two eyes of </=-0.5 diopters; myopia progression was defined by the self-reported need for a stronger eyeglass prescription during law school.

MAIN OUTCOME MEASURES

(1) prevalence of myopia, (2) progression of myopia.

RESULTS

Seventy-nine percent of the class participated (n = 179, two were excluded for amblyopia leaving 177 students). Fifty-eight percent were male, 75% were Caucasian, and the mean age was 27 years. Seventy-nine percent reported parental myopia. The mean amount of near work was 7.4 hours/day; mean sleep was 7.9 hours/day; mean darkness was 5.3 hours/day. Sixty-six percent of the students were myopic. Of 96 participants myopic before law school, myopia increased in 83 (86%) during law school. Among 75 students not myopic at the beginning of law school, 14 (19%) became myopic. The onset of myopia could not be determined for 6 patients. There were trends for higher myopia prevalence among those with a parental myopia history (P = 0.14) and for increased myopia progression among those reporting more daily near work (P = 0.18). Students with </=5.6 hours of daily darkness were more likely to report myopia progression than those with >5.6 hours of darkness per day (95% vs. 80%, P = 0.07). To account for possible confounding effects of risk factors with myopia progression, logistic regression with categorization of the continuous exposure variables (hours of near work, sleep, and darkness) above or below median values weakened the near work association (odds ratio 1.8, 95% confidence interval 0.5-6.7, P = 0.35) but continued to identify darkness association with daily hours of darkness (odds ratio 4.8, 95% confidence interval 1.0 >/= 23.3, P < 0.05). Among the 77 students with myopia onset before college, those with </=5.6 hours of daily darkness were more likely to progress than those with more hours of daily darkness (97% vs. 76%, P = 0.01).

CONCLUSIONS

This study confirms high rates of myopia prevalence and myopia progression among law students. The strongest association, especially in those with myopia onset before college, was a relation of myopia progression during law school with less daily exposure to darkness, a potential risk factor previously identified in childhood myopia. The role of exposure to darkness in refractive development warrants additional study.

Ionotropic glutamate receptors expressed in human retinoblastoma Y79 cells

Mammalian retinal photoreceptors and pinealocytes have common characteristic that they secrete melatonin and L-glutamate as chemical transmitters. Although pinealocytes express glutamate receptors and receive glutamate signals, whether or not photoreceptors express glutamate receptors is unknown. Here, we investigated the expression of the glutamate receptors in cultured Y79 clonal human retinoblastoma cells, as model systems of photoreceptors. Reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that GluR1, GluR5, GluR7, EAA2, NR1, NR2A and NR2D mRNAs were present in the cultured cells. Northern analysis confirmed the presence of GluR7, EAA2, NR1, NR2A and NR2D mRNAs, while other mRNAs were under the detection limit. Addition of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainate increases intracellular (Ca(2+)) in Fura-2 loaded cells, which is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. N-methyl-D-aspartate also increases intracellular (Ca(2+)). These results demonstrated the presence of functional ionotropic receptors in Y79 cells.

Diurnal illumination patterns affect the development of the chick eye

Exposure to continuous illumination disrupts normal ocular development in young chicks, causing severe corneal flattening, shallow anterior chambers and progressive hyperopia ('constant light (CL) effects'). We have studied the minimum requirements of a diurnal light cycle to prevent CL effects. (1) Seven groups of 10 chicks were reared under a 0 (constant light, CL), or 1, 2, 3,4, 6, or 12/12 h (normal) light-dark cycles. It was found that CL effects were prevented if the dark period was 4 h or longer. Below 4 h, the effects were dose-dependent and inversely correlated with the amplitude of the Fourier component of illumination at 1 cycle per day (CPD). (2) Three groups of 20 chicks were exposed to 4 h of darkness distributed differently over 24 h to vary the amplitude of the Fourier component at 1 CPD. It was found that complete suppression of the CL effects required that the 4 h of darkness were given in one block and at the same time each day. Our results show that normal ocular development in the chick requires a minimum of 4 h darkness per day, provided at the same time of the day without interruption, and suggest that the light-dark cycle interacts with a linear or weakly nonlinear oscillating system.

Modulation of constant light effects on the eye by ciliary ganglionectomy and optic nerve section

Our previous studies have shown that an environment of constant light (CL) can lead to development of high degree of hyperopia in newborn chicks by inducing severe corneal flattening, and compensatory growth of the vitreous chamber. We wish to know whether the abnormal eye growth and progressive hyperopia under CL conditions is accomplished by a mechanism that uses the visual processing pathways of the central nervous system (CNS) or by a mechanism located in the eye. Thirty white leghorn chicks (Cornell K-strain) were raised under 12 h light/12 h dark (12L/12D) for either optic nerve section (ONS) or ciliary ganglion section (CGS). Another 30 chicks were raised under CL for ONS or CGS. Refractive states and corneal curvatures were measured by infrared (IR) photoretinoscopy and IR keratometry, respectively. The axial lengths of the ocular components were measured by A-scan ultrasonography. Both ONS and CGS surgery produced dilated pupils and accommodative paralysis. Four weeks after surgery, CGS eyes exhibited a hyperopic defocus, flatter cornea, and shorter vitreous chamber depth under both CL and normal conditions, whereas ONS eyes showed a smaller radius of corneal curvature and shallow vitreous chamber only in the normal light cycle group. CGS eyes of CL chicks showed significantly deeper vitreous chambers than did fellow control eyes. Our results indicate that optic nerve section does not seem to influence CL effects. Thus, local mechanisms may play a major role in the ocular development of chicks. The ciliary nerve is necessary for the normal corneal and anterior chamber growth, and prevents CL effects. The progressively increasing vitreous chamber depth under CL may be influenced by both local and central mechanisms.

The role of photoreceptors in the control of refractive state

The current state of research into experimentally induced refractive errors is reviewed. The area is analysed in three components-the transduction of defocus or deprivation, the vector for transmitting the error message from the retina to the outer tunics of the eye, and the identity of the effector for causing growth modulation in the sclera. Anatomical, pharmacological, electrophysiological and optical factors are considered in terms of which elements of the retina are necessary to support a refractive response to deprivation or defocus. Two of the current models are discussed-one emphasizing the role of the choroid in effecting ocular and refractive change, while the second model approaches the problem from the aspect of scleral changes that are associated with growth adaptation without emphasis on the error detection mechanism. A third model is proposed in which the error signal for deprivation or defocus is detected in the outer retina and where error is translated through separate signals for stimulus brightening and darkening into a net signal for fluid flow across and under the active control of the retinal pigment epithelium with the fluid communication between the vitreous chamber and the choroidal lymphatics. The directions of research both fundamental and clinical which are needed to create pharmaceutical or environmental solutions to refractive control are discussed.

Two arylalkylamine N-acetyltransferase genes mediate melatonin synthesis in fish

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87) is the first enzyme in the conversion of serotonin to melatonin. Large changes in AANAT activity play an important role in the daily rhythms in melatonin production. Although a single AANAT gene has been found in mammals and the chicken, we have now identified two AANAT genes in fish. These genes are designated AANAT-1 and AANAT-2; all known AANATs belong to the AANAT-1 subfamily. Pike AANAT-1 is nearly exclusively expressed in the retina and AANAT-2 in the pineal gland. The abundance of each mRNA changes on a circadian basis, with retinal AANAT-1 mRNA peaking in late afternoon and pineal AANAT-2 mRNA peaking 6 h later. The pike AANAT-1 and AANAT-2 enzymes (66% identical amino acids) exhibit marked differences in their affinity for serotonin, relative affinity for indoleethylamines versus phenylethylamines and temperature-activity relationships. Two AANAT genes also exist in another fish, the trout. The evolution of two AANATs may represent a strategy to optimally meet tissue-related requirements for synthesis of melatonin: pineal melatonin serves an endocrine role and retinal melatonin plays a paracrine role.

Enkephalin, neurotensin and somatostatin increase cAMP levels in the chicken retina

PURPOSE

Enkephalin, neurotensin and somatostatin are released at high rates in the dark and at low rates in the light n the chicken retina. The present study examines the effects of these peptide transmitters on retinal cAMP METHODS: Chicken retinas were incubated in vitro with various drugs for 10min. Cyclic AMP was extracted with acidified ethanol and retinal levels of cAMP were measured using a radioassay kit.

RESULTS/CONCLUSIONS

These peptides increased cAMP levels in the chicken retina in vitro, which is surprising as their receptors are generally thought to be negatively coupled to adenylate cyclase. The paradoxical increase in retinal cAMP may be due to unique types of peptide receptors that are positively coupled to adenylate cyclase. A more plausible explanation is that these peptides act indirectly and change the rate of release of another transmitter, whose receptor is coupled to adenylate cyclase.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Development of the enkephalin-, neurotensin- and somatostatin-like (ENSLI) amacrine cells in the chicken retina

The development of the enkephalin-, neurotensin- and somatostatin-like immunoreactive (ENSLI) amacrine cells in the chicken retina has been investigated by radioimmunoassay (RIA) and immunocytochemistry (ICC). By RIA, enkephalin-like immunoreactivity (ENK-LI) was detected at embryonic day (E) 5 at only very low levels, which gradually increased until E17. From E18 to E21, there was a relatively rapid increase in ENK-LI levels, and just after hatching, there was a very steep rise. By ICC, the cell bodies of the ENSLI amacrine cells were first detected in the inner nuclear layer on E18, with no immunostaining in the inner plexiform layer (IPL). On E21, more cells were detected and processes in the IPL were visible, but detailed arborisations were not clear. On postnatal day (P) 1, the ENSLI amacrine cells showed a morphology similar to that in mature retina in both the density of cell bodies and the ramification pattern of processes. Antibodies to neurotensin and somatostatin revealed a similar developmental pattern. Thus, the three peptides appear to follow a similar developmental pattern in the ENSLI amacrine cells, suggesting that the three peptides respond similarly to developmental stimuli, just as they are released in parallel in response to physiological stimulation from mature ENSLI amacrine cells. After hatching, higher levels of ENK-LI were detected by RIA and more ENSLI amacrine cell bodies and processes were detected by ICC in animals kept in the light than in those kept in the dark. In retinas kept in the light for 12 h, it was found that immunoreactive processes in the IPL formed strongly stained patches, but this was not observed in retinas kept in the dark for 12 h.

Dopaminergic behaviour in chicken retina and the effect of form deprivation

PURPOSE

Dopamine (DA) is considered to be a neurotransmitter involved in light-adaptive responses in the retina and has been implicated in the control of the eye growth induced by form deprivation. Vitreal DOPAC was shown to be a good indicator of retinal dopaminergic activity.

METHODS/RESULTS

Dopaminergic activity was highest during the light; with vitreal DOPAC levels rising within 3 h of light exposure. Form deprivation attenuated dopaminergic activity, as the rise in vitreal DOPAC levels on light exposure was reduced in form-deprived eyes, compared with control eyes.

CONCLUSION

The lack of sustained activation of DA release may explain the role of DA in the control of eye-growth.

A fundamental step-transition in retinal function at low light intensities

There appears to be a fundamental step-transition in retinal function at low light intensities, close to the scotopic-mesopic transition. This step-transition is observed for elements of the retinal dark-light switch, which has been described in the chicken retina. Over the same range of light intensities, there is a step-transition in photoreceptor retinomotor movements and in the coupling of horizontal and All amacrine cells, which suggests a switch in retinal circuitry from rod-processing to cone-processing regimes. A similar step-transition in pineal function suggests that the retinal step-transition signals to the central circadian systems. Finally, this step-transition may also inhibit eye growth, and thus be responsible for the reported diurnal rhythm in eye growth. Disturbances to this step-transition may be the initial cause of disordered eye growth in the form-deprivation myopia paradigm.

A light-driven rhythm in neurotensin-like immunoreactivity in the chicken retina

PURPOSE

To determine if the pattern of release of neurotensin from the enkephalin-, neurotensin- and somatostatin-like immunoreactive amacrine cells in response to light and dark is the same as that of the enkephalins and somatostatin.

METHODS/RESULTS

Both the enkephalins and somatostatin are released at high rates in the dark and at lower rates in the light, and these rate changes are reflected in increasing intracellular levels of the peptides in vivo in the light and decreasing levels in the dark. The levels of neurotensin-like immunoreactivity show a similar diurnal light-driven and non-circadian rhythm in vivo.

CONCLUSION

This implies that the actual release rates of neurotensin follow the same patterns as those demonstrated in vitro for the enkephalins and somatostatin.

Localization of D1 dopamine receptors in the chicken retina

PURPOSE

The localization of dopamine D1 receptors (DIR) in the chicken retina was examined using an anti-human DIR monoclonal antibody and PAP techniques.

RESULTS

A clear band of staining was seen in the outer plexiform layer, as well as cellular staining in the outer-most part of the inner nuclear layer, probably in a subset of horizontal cells. Many different amacrine cell bodies were labelled in the inner one-third of the inner nuclear layer. There was also extensive staining in the inner plexiform layer, which showed some striation. Occasional labelled ganglion cells were also detected.

CONCLUSION

Localization of D1-dopamine receptors has been shown in the chicken retina.

Deprivation of form vision suppresses diurnal cycling of retinal levels of leu-enkephalin

Deprivation of form vision by the fitting of translucent occluders suppressed the diurnal cycling of enkephalinergic amacrine cells (the ENSLI amacrine cells), in the chicken. Daily periods of normal vision or enforcing temporal contrast using strobe lighting appeared to restore normal functioning of the ENSLI cells. These results suggest that the ENSLI cells are involved in retinal circuits that assess the quality of the visual image and control eye growth.