Role of melatonin in retina

Increased risk of Parkinson's disease amongst patients with age-related macular degeneration and visual disability: A nationwide cohort study

BACKGROUND AND PURPOSE

The association between Parkinson's disease (PD) and age-related macular degeneration (AMD) has been shown in previous reports. However, the association between the severity of AMD and PD development is unknown. The aim was to evaluate the association of AMD with/without visual disability (VD) with the risk of PD occurrence using the National Health Insurance data in South Korea.

METHODS

A total of 4,205,520 individuals, 50 years or older and without a previous diagnosis of PD, participated in the Korean National Health Screening Program in 2009. AMD was verified using diagnostic codes, and participants with VD were defined as those with loss of vision or visual field defect as certified by the Korean Government. The participants were followed up until 31 December 2019, and incident cases of PD were identified using registered diagnostic codes. The hazard ratio was calculated for groups (control and AMD with/without VD) using multivariable adjusted Cox regression analysis.

RESULTS

In total, 37,507 participants (0.89%) were diagnosed with PD. Amongst individuals with AMD, the risk of PD development was higher in individuals with VD (adjusted hazard ratio [aHR] 1.35, 95% confidence interval [CI] 1.09-1.67) than in those without (aHR 1.22, 95% CI 1.15-1.30) compared with controls. Additionally, an increased risk of PD was observed in individuals with AMD compared with controls, regardless of the presence of VD (aHR 1.23, 95% CI 1.16-1.31).

CONCLUSIONS

Visual disability in AMD was associated with the development of PD. This suggests that neurodegeneration in PD and AMD may have common pathways.

Impact of endogenous melatonin on rhythmic behaviors, reproduction, and survival revealed in melatonin-proficient C57BL/6J congenic mice

The hormone melatonin is synthesized from serotonin by two enzymatic reactions (AANAT and ASMT/HIOMT) in the pineal gland following a circadian rhythm with low levels during the day and high levels at night. The robust nightly peak of melatonin secretion is an output signal of the circadian clock to the whole organism. However, so far the regulatory roles of endogenous melatonin in mammalian biological rhythms and physiology processes are poorly understood. Here, we establish congenic mouse lines (>N10 generations) that are proficient or deficient in melatonin synthesis (AH+/+ or AH-/- mice, respectively) on the C57BL/6J genetic background by crossing melatonin-proficient MSM/Ms with C57BL/6J. AH+/+ mice displayed robust nightly peak of melatonin secretion and had significantly higher levels of pineal and plasma melatonin vs AH-/- mice. Using this mice model, we investigated the role of endogenous melatonin in regulating multiple biological rhythms, physiological processes, and rhythmic behaviors. In the melatonin-proficient (AH+/+) mice, the rate of re-entrainment of wheel-running activity was accelerated following a 6-hour phase advance of dark onset when comparted with AH-/- mice, suggesting a role of endogenous melatonin in facilitating clock adjustment. Further in the AH+/+ mice, there was a significant decrease in body weight, gonadal weight and reproductive performance, and a significant increase in daily torpor (a hypothermic and hypometabolic state lasting only hours during adverse conditions). Endogenous melatonin, however, had no effect in the modulation of the diurnal rhythm of 2-[¹²⁵ I]-iodomelatonin receptor expression in the SCN, free-running wheel behavior in constant darkness, life span, spontaneous homecage behaviors, and various types of social-emotional behaviors. The findings also shed light on the role of endogenous melatonin in mice domestication and provide new insights into melatonin's action in reducing energy expenditure during a food shortage. In summary, the congenic mice model generated in this study offers a significant advantage toward understanding of the role of endogenous melatonin in regulating melatonin receptor-mediated rhythm behaviors and physiological functions.

Ophthalmological features of Parkinson disease

Background

The aim of this study was to determine the type and frequency of ophthalmologic changes occurring in patients with Parkinson disease (PD).

Material/Methods

One hundred consecutive patients (196 eyes) with idiopathic PD and a control group consisting of 100 healthy patients (196 eyes) matched for age and sex underwent a complete ophthalmological examination of both eyes, including assessment of patient medical history, dry eye questionare, and visual hallucinations questionnaire, distance and near best corrected visual acuity (DBCVA, NBCVA), color vision, distance photopic contrast sensitivity, near point of convergence, slit lamp examination of the eye anterior segment, tear film osmolarity and breakup time, aqueous tear production, and intraocular pressure, as well as fundus examination and evaluation of the perimacular retinal thickness (RT) and peripapillary retinal nerve fiber layer (RNFL) thickness.

Results

In the eyes of PD patients DBCVA, NBCVA, contrast sensitivity, and color discrimination were significantly reduced. We also detected increased frequency of convergence insufficiency, seborrhoic blepharitis, meibomian gland disease (MGD), dry eye syndrome, nuclear and posterior subcapsular cataract, and glaucoma (p<0.05). However, intraocular pressure (IOP) was significantly lower in the PD group compared to controls. The frequency of visual hallucinations, age-related macular degeneration (ARMD), and other ophthalmological diseases, as well as RT and RNFL thickness, did not significantly differ between investigated groups.

Conclusions

Clinicians need to be aware of the association between PD and ophthalmological changes. Restoration of good-quality vision has a great impact on PD patients’ quality of life, reduction of costs of treatment and care, and rehabilitation.

Parkinson's disease, lights and melanocytes: looking beyond the retina

Critical analysis of recent research suggesting that light pollution causes Parkinson's disease (PD) reveals that such a hypothesis is unsustainable in the context of therapeutic use of light in treating various neuropsychiatric conditions. Reinterpretation of their findings suggests that retinal damage caused by prolonged light exposure may have contributed to the observed enhancement of experimental PD. To test this hypothesis further, forty-two Sprague Dawley rats received microinjections of 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-2, 4, 6-tetrahydropyridine (MPTP), paraquat or rotenone into the vitreal mass in doses so minute that the effects could not be attributed to diffusion into brain. Significant changes in five motor parameters consistent with symptoms of experimental PD were observed. These findings support the interpretation that the retina is involved in the control of motor function and in the aetiology of PD.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, classification, and pharmacology of G protein-coupled melatonin receptors

The hormone melatonin (5-methoxy-N-acetyltryptamine) is synthesized primarily in the pineal gland and retina, and in several peripheral tissues and organs. In the circulation, the concentration of melatonin follows a circadian rhythm, with high levels at night providing timing cues to target tissues endowed with melatonin receptors. Melatonin receptors receive and translate melatonin's message to influence daily and seasonal rhythms of physiology and behavior. The melatonin message is translated through activation of two G protein-coupled receptors, MT(1) and MT(2), that are potential therapeutic targets in disorders ranging from insomnia and circadian sleep disorders to depression, cardiovascular diseases, and cancer. This review summarizes the steps taken since melatonin's discovery by Aaron Lerner in 1958 to functionally characterize, clone, and localize receptors in mammalian tissues. The pharmacological and molecular properties of the receptors are described as well as current efforts to discover and develop ligands for treatment of a number of illnesses, including sleep disorders, depression, and cancer.

Parkinson's disease as a neuroendocrine disorder of circadian function: dopamine-melatonin imbalance and the visual system in the genesis and progression of the degenerative process

For more than 50 years, Parkinson's disease (PD) has been conceptualized as a product of nigro-striatal dopamine (NSD) system degeneration. In spite of a growing body of evidence depicting the mammalian brain as an interrelated complexity of circuitous systems, dopamine (DA) deficiency of the NSD is still regarded as the main problem, with DA replacement being the purpose of therapeutic intervention. For at least 191 years circadian involvement in various aspects of PD, including depression and insomnia, has been recognized as an integral part of the symptom matrix of PD and yet attempts to elucidate the involvement of this system is uncharted territory. The present review attempts a major reorganization of mammalian brain into a coordinated complex involving the NSD and the retinal hypothalamic tract (RHT) as the primary systems involved in the retino-diencephalic/mesencephalic-pineal (RDMP) axis. Secondary systems including the lateral hypothalamus (LH), the area postraema (AP) and the subthalamic nucleus (STN) also form an integral part of this system as they have been shown to be either intimately related to the primary systems of the RDMP axis or have been shown to be significantly involved in the expression and treatment of PD. A large volume of evidence suggests that the RDMP axis is activated during the course of PD and during therapeutic intervention. Four types of neurotoxicity associated with melatonin are identified and the susceptibility of various parts of the RDMP axis to undergo neuropathological change, the tendency for melatonin to induce PD-like behavioural toxicity, and the relationship of this to PD symptomotology are described. This includes adverse effects of melatonin on motor function, hypotension, the adjuvant use of benzodiazepines, depression, insomnia, body weight regulation and various biochemical effects of melatonin administration: all problems currently facing the proposal to introduce melatonin as an adjuvant. It is suggested further that traditional DA replacement may well work by exerting its effect upon the circadian system, rather than simply replacing deficient DA. Activation of the circadian function by antagonizing melatonin with bright light not only has therapeutic value in treating the primary symptoms of PD but it shares a common mechanism with L-dopa in reducing the occurrence of seborrheic dermatitis. Concepts at the centre of understanding pineal function in PD, including pineal calcification, melatonin deficiency, symptomatic versus protective features of melatonin and antioxidative effects, are explained in a counterintuitive context. Intriguing propositions including the role of the retina in the aetiology of PD and that the nigra functions as a retina in this disorder are presented with the intention to provide a new understanding of the underlying compromised function in PD and to provide new treatment strategies. For the first time, abundant evidence is presented describing PD as an endocrine disorder of melatonin hyperplasia. The role of circadian interventive therapies and internal desynchrony in the aetiology and progression of PD provides a new direction for understanding the underlying physiology of a disease which is currently in a state of impasse and provides new hope for those who suffer from its debilitating effects.

Ocular Melatonin Rhythms in Teleost Fish

Melatonin (N-acetyl-5-methoxytryptamine) is synthesized in the pineal organ and the retina of vertebrates. In some teleost species, ocular melatonin levels can exhibit a circadian periodicity with elevated levels during the dark phase under light-dark (LD) cycles and this periodicity can persist even under constant dark (DD) cycles. However, reversed melatonin profiles and an absence of circadian ocular melatonin rhythms have also been reported. In this study, we investigated the daily rhythms of ocular melatonin in 32 teleost species under LD cycles. The melatonin profiles could be classified into three types: (1) normal profiles, with higher melatonin levels during the dark phase than the light phase; (2) reversed profiles, with higher levels during the light phase than the dark phase; (3) no significant differences in melatonin levels. We also studied whether ocular melatonin exhibits circadian rhythms under DD in selected species. Our results showed that ocular melatonin exhibited circadian rhythms in some but not all of the species examined. These results indicate that ocular melatonin rhythms in teleost fish exhibit species-specific variations as a result of the changes in the regulatory mechanisms during the course of evolution.

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.

Daily and Circadian Variations of the Pineal and Ocular Melatonin Contents and their Contributions to the Circulating Melatonin in the Japanese Newt, Cynops pyrrhogaster

Daily and circadian variations of melatonin contents in the diencephalic region containing the pineal organ, the lateral eyes, and plasma were studied in a urodele amphibian, the Japanese newt (Cynops pyrrhogaster), to investigate the possible roles of melatonin in the circadian system. Melatonin levels in the pineal region and the lateral eyes exhibited daily variations with higher levels during the dark phase than during the light phase under a light-dark cycle of 12 h light and 12 h darkness (LD12:12). These rhythms persisted even under constant darkness but the phase of the rhythm was different from each other. Melatonin levels in the plasma also exhibited significant day-night changes with higher values at mid-dark than at mid-light under LD 12:12. The day-night changes in plasma melatonin levels were abolished in the pinealectomized (Px), ophthalmectomized (Ex), and Px+Ex newts but not in the sham-operated newts. These results indicate that in the Japanese newts, melatonin production in the pineal organ and the lateral eyes were regulated by both environmental light-dark cycles and endogenous circadian clocks, probably located in the pineal organ and the retina, respectively, and that both the pineal organ and the lateral eyes are required to maintain the daily variations of circulating melatonin levels.

Effects of photic environment on ocular melatonin contents in a labrid teleost, the wrasse Halichoeres tenuispinnis

The wrasse Halichoeres tenuispinnis is a labrid teleost that exhibits robust circadian rhythms in locomotor activity under constant light (LL). This fish buries itself in the bottom sand during the subjective-night, thereby suggesting that behaviorally it adjusts its circadian clock to avoid photoreception. In this study, we determined ocular melatonin contents of the wrasse under various photic environments and used ocular melatonin to indicate photoreception. Under light-dark (LD) cycles, ocular melatonin contents of the wrasse exhibited a daily rhythm, with higher levels during the dark phase than those during the light phase. The duration of nocturnal melatonin elevation was longer under LD 9:15 than under LD 15:9. Acute exposure to 2-h light during the dark phase resulted in a significant decrease in ocular melatonin at mid-dark in an intensity-dependent manner. However, acute exposure to different intensities of light for 2h during the light phase had only a small effect on ocular melatonin contents at mid-day. Under LL, ocular melatonin contents in the wrasse reared with bottom sand present exhibited circadian rhythms and were significantly higher than those with transluscent acryl pellets on the bottom. These results indicate that the ocular melatonin rhythm in the wrasse is driven both by the photic environment and by a circadian clock, and that the wrasse that buries itself in the bottom sand can perceive low intensity of light.

Melatonin differentially modulates the expression and function of the hMT1 and hMT2 melatonin receptors upon prolonged withdrawal

Melatonin is synthesized and released following a circadian rhythm and reaches its highest blood levels during the night. It relays signals of darkness to target tissues involved in regulating circadian and seasonal rhythms. Here, we report the expression of human melatonin receptors type 1 and 2 (hMT(1) and hMT(2), respectively) in Chinese hamster ovary (CHO) cells following exposure to melatonin treatments mimicking the amplitude (400 pM) and duration (8 hr) of the nightly melatonin peak and upon withdrawal. Exposure of CHO-MT(1) cells to melatonin (400 pM) for 0.5, 1, 2, 4, and 8 hr significantly increased specific 2-[125I]iodomelatonin (500 pM) binding to hMT(1) melatonin receptors upon 16-hr withdrawal. However, the same treatment did not affect the expression of hMT(2) melatonin receptors. The increase in specific 2-[125I]iodomelatonin (500 pM) binding (162+/-29%, N=3, P<0.05) 16 hr after melatonin withdrawal was parallel to increases in hMT(1) melatonin receptor mRNA (231+/-33%, N=4, P<0.05). This effect was due to an increase in the total number of hMT(1) receptors [B(max) 833+/-97 fmol/mg protein (N=3), control; 1449+/-41 fmol/mg protein (N=3), treated], with no change in binding affinity. The melatonin-mediated increase in MT(1) melatonin receptor expression upon withdrawal was not mediated through either a direct effect of the hormone in the promoter's vector or in the rate of mRNA degradation. In conclusion, melatonin differentially regulates the expression of its own receptors, which may have important implications in the transduction of dark signals in vivo.

Daily morphological variations in the viscacha (Lagostomus maximus maximus) retina. Probable local modulatory action of melatonin

Given that the local melatonin levels exhibit rhythmic daily changes in the retina of the viscacha, we considered it important to study the likely daily variations in morphology and specific 2-[(125)I]-iodomelatonin binding in retinas from this rodent and to correlate these putative changes with local indole levels. Adult animals of both sexes were captured in their habitat and were kept under a natural photoperiod. For light and electron microscopic studies the viscachas were sacrificed by decapitation at 08:00, 16:00, and 24:00 hr. A computer-assisted image analysis system was used to measure the thickness of the complete retina, the photoreceptor layer, the rod outer and inner segments, and the outer nuclear layer. The daily variation in 2-[(125)I]-iodomelatonin binding sites was followed during a 24-hr light-dark cycle, the animals being sacrificed at six time points. The parameters studied showed significant variations throughout the 24-hr period. Maximal specific binding, lysosomal content in the pigment epithelium, and photoreceptor layer outer segment thicknesses were observed at 24:00 hr. Close contact between photoreceptor membranes and microvilli of the pigment epithelium was observed at 08:00 and 16:00 hr. Moreover, the minimal outer segment thickness at 16:00 hr was accompanied by a scarcity of dense bodies, such as lysosomes, a maximum dispersion of melanin pigment granules, and a minimum density of radioligand binding sites. Therefore, in the retina of the viscacha, we suggest that the interaction between melatonin and specific sites could be one of the factors or causes that participate in the regulation of the daily morphological changes observed in viscacha.

The primary visual pathway in humans is regulated according to long-term light exposure through the action of a nonclassical photopigment

BACKGROUND

The mammalian eye shows marked adaptations to time of day. Some of these modifications are not acute responses to short-term light exposure but rely upon assessments of the photic environment made over several hours. In the past, all attempts at a mechanistic understanding have assumed that these adaptations originate with light detection by one or other of the classical photoreceptor cells (rods or cones). However, previous work has demonstrated that the mammalian eye contains non-rod, non-cone photoreceptors. This study aimed to determine whether such photoreceptors contribute to retinal adaptation.

RESULTS

In the human retina, second-order processing of signals originating in cones takes significantly longer at night than during the day. Long-term light exposure at night is capable of reversing this effect. Here, we employed the cone ERG as a tool to examine the properties of the irradiance measurement pathway driving this reversal. Our findings indicate that this pathway (1) integrates irradiance measures over time periods ranging from at least 15 to 120 min; (2) responds to relatively bright light, having a dynamic range almost entirely outside the sensitivity of rods; (3) acts on the cone pathway primarily through a local retinal mechanism; and (4) detects light via an opsin:vitamin A photopigment (lambda(max) approximately 483 nm).

CONCLUSIONS

A photopigment with a spectral sensitivity profile quite different from those of the classical rod and cone opsins but matching the standard profile of an opsin:vitamin A-based pigment drives adaptations of the human primary cone visual pathway according to time of day.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

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.

Unusual responses to light and darkness of ocular melatonin in European sea bass

Regulation by light and darkness of melatonin rhythms in the plasma and eye of the European sea bass (Dicentrarchus labrax) was studied. During light-dark cycles, plasma and ocular melatonin exhibited day-night changes with higher levels at mid-dark and at mid-light, respectively. Circulating melatonin levels were low in constant light but high in constant darkness (DD); ocular melatonin levels showed the reverse pattern. Plasma melatonin exhibited circadian rhythm for 1 cycle but the rhythm was no longer apparent on day 2. There was no circadian rhythm in ocular melatonin. Acute light exposure in DD decreased plasma melatonin but increased ocular melatonin. These results suggest that circulating melatonin may be used as a signal for darkness but ocular melatonin is used as a signal for the light phase.

Ocular melatonin rhythms in the goldfish, Carassius auratus

Ocular melatonin rhythms in the goldfish were studied and compared to those in the pineal organ and plasma. Under light:dark (LD) of 12 h light:12 h dark, melatonin contents in the eye as well as the pineal organ and plasma exhibited clear day-night changes with higher levels at mid-dark than at mid-light. However, melatonin contents in the eye at mid-light and mid-dark were approximately 100 and 9 times greater than those in the pineal organ, respectively. Day-night changes of ocular melatonin persisted after pinealectomy, which abolished those in plasma melatonin under LD 12:12. Ocular melatonin contents in the pinealectomized fish at mid-light were significantly higher than those in the sham-operated control. Under constant darkness (DD), circadian melatonin rhythms were observed in the eye but damped on the 3rd day, whereas plasma melatonin rhythms generated by the pineal organ persisted for at least 3 days. Under constant light, ocular melatonin contents exhibited a significant fluctuation with a smaller amplitude than that under DD, whereas plasma melatonin remained at low levels. These results indicate the involvement of LD cycles, a circadian clock, and the pineal organ in the regulation of ocular melatonin rhythms in the goldfish.

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

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

Electrophysiological signs of retinal dopamine deficiency in recently diagnosed Parkinson's disease and a follow up study

Electrophysiological studies, including electrooculogram (EOG), and simultaneously recorded flash and pattern evoked electroretinograms (FERG and PERG) and visually evoked potentials (FVEP and PVEP) were made in 1988 on 10 newly diagnosed untreated Parkinson's patients at Stage 1 of the Hoehn and Yahr scale. Follow up studies were made on five out of the 10 patients when their disease had progressed to Stage 2 during 1993. The earliest and only sign of abnormality detected in the Stage 1 of Parkinson's patients in 1988 was a delay in the time to reach the peak light rise in the EOG. When the disease had progressed to Stage 2, not only a delay in the time to reach the peak light rise but also a reduction in the amplitude of the peak light rise in the EOG, together with changes in PERG, FERG and PVEPs were demonstrable. These changes observed in PERG, FERG and PVEPs were generally consistent with those reported by previous studies. It is suggested that the reason for the susceptibility of pigment epithelial function to dopamine deficiency in Parkinson's disease may be due to the pigment epithelium being at the extremity of the diffusion pathway from dopamine release sites at the inner plexiform layer.

Light and the immature visual system

Preterm neonates are cared for in an environment which is continuously and brightly lit, and is very different to that experienced at any other time of life. The amount of light reaching the neonatal eye is governed by two groups of factors. Physical factors include intensity, spectral characteristics and duration of light exposure. Physiological factors are: Frequency of eyelid opening, transmission of light through the closed eyelid, pupil reactivity and area, transmission by the ocular tissues and retinal surface area. Retinal irradiance declines with increasing postmenstrual age. Mechanisms by which light may affect the neonatal eye and clinical conditions in which light have been implicated are considered.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

The visual input stage of the mammalian circadian pacemaking system: I. Is there a clock in the mammalian eye?

Threads of evidence from recent experimentation in retinal morphology, neurochemistry, electrophysiology, and visual perception point toward rhythmic ocular processes that may be integral components of circadian entrainment in mammals. Components of retinal cell biology (rod outer-segment disk shedding, inner-segment degradation, melatonin and dopamine synthesis, electrophysiological responses) show self-sustaining circadian oscillations whose phase can be controlled by light-dark cycles. A complete phase response curve in visual sensitivity can be generated from light-pulse-induced phase shifting. Following lesions of the suprachiasmatic nuclei, circadian rhythms of visual detectability and rod outer-segment disk shedding persist, even though behavioral activity becomes arrhythmic. We discuss the converging evidence for an ocular circadian timing system in terms of interactions between rhythmic retinal processes and the central suprachiasmatic pacemaker, and propose that retinal phase shifts to light provide a critical input signal.