Rhythmic regulation of retinal melatonin: metabolic pathways, neurochemical mechanisms, and the ocular circadian clock

The role of the serotonergic system in atropine's anti-myopic effects

The muscarinic cholinergic antagonist atropine is the most widely used pharmacological treatment for the visual disorder myopia (short-sightedness), the leading cause of low-vision worldwide. This study sought to better define the mechanism by which atropine inhibits myopic growth. Although classified as a muscarinic-cholinergic antagonist, atropine has been found to bind and modulate the activity of several non-cholinergic systems (e.g., serotonin). Thus, this study investigated whether the serotonergic system could underly atropine's anti-myopic effects. Using a chick model of myopia, we report that atropine's growth-inhibitory effects can be attenuated by pharmacological stimulation of the serotonin system. This may suggest that atropine can slow the development of myopia through inhibiting serotonergic receptor activity. We also observed that pharmacological antagonism of serotonergic receptors inhibits the development of experimental myopia in a dose-dependent manner, further demonstrating that modulation of serotonergic receptor activity can alter ocular growth rates. Finally, we found that neither experimental myopia, nor atropine treatment, induced a significant change in retinal serotonergic output (i.e., synthesis, transport, release and catabolism). This may suggest that, although myopic growth can be inhibited through modulation of serotonergic receptor activity (by atropine or serotonergic antagonists), this does not require a change in serotonin levels. These findings regarding a serotonergic mechanism for atropine may have significant ramifications for the treatment of human myopia. This includes assessing the use of atropine in patients who are also undergoing treatment to upregulate serotonergic signaling (e.g., serotonergic anti-depressants).

Does exposure to moonlight affect day/night changes in melatonin and metabolic parameters in Amazonian fish?

Lunar cycle modulates the rhythmic activity patterns of many animals, including fish. The effect of the moonlight cycle on daily melatonin and metabolic parameters was evaluated in matrinxã (Brycon amazonicus) subjected to external natural lighting. Eighty juvenile were distributed in 4 tanks of 1m3 (20 fish/tank) and divided into two groups. One group was exposed to the full moon and the other group to the new moon for 30 days, which corresponds to the duration of the lunar period. At the end of the lunar phase, 6 fish from each group were anesthetized to collect blood, tissue and eye samples at midday and midnight. The comparison between the light and dark periods revealed a significant increase in plasma and ocular melatonin in the last period. However, there was no significant difference for plasma melatonin between moons. Ocular melatonin presented higher concentrations during the new moon. Glucose, total proteins, cortisol, liver glutathione and gill lipid peroxidation were higher in the full moon compared to in the new moon. Plasma triglyceride was higher during the night for the full moon, and the opposite was found for the new moon. Total cholesterol values were higher at night regardless the moon phase. Glutathione in the gills and lipid peroxidation in the liver showed no significant differences. These results highlight the importance of considering both the day and lunar cycles for melatonin and metabolic parameters in species of commercial interest and susceptible to stressful situations in rearing conditions.

Extrinsic and Intrinsic Factors Determine Expression Levels of Gap Junction-Forming Connexins in the Mammalian Retina

Gap junctions (GJs) are not static bridges; instead, GJs as well as the molecular building block connexin (Cx) proteins undergo major expression changes in the degenerating retinal tissue. Various progressive diseases, including retinitis pigmentosa, glaucoma, age-related retinal degeneration, etc., affect neurons of the retina and thus their neuronal connections endure irreversible changes as well. Although Cx expression changes might be the hallmarks of tissue deterioration, GJs are not static bridges and as such they undergo adaptive changes even in healthy tissue to respond to the ever-changing environment. It is, therefore, imperative to determine these latter adaptive changes in GJ functionality as well as in their morphology and Cx makeup to identify and distinguish them from alterations following tissue deterioration. In this review, we summarize GJ alterations that take place in healthy retinal tissue and occur on three different time scales: throughout the entire lifespan, during daily changes and as a result of quick changes of light adaptation.

Testis development in the Japanese eel is affected by photic signals through melatonin secretion

Objective

According to reported spawning characteristics of Japanese eel, Anguilla japonica, which exhibit spawning and migration patterns that are synchronized with lunar cycles and photoperiod, we hypothesized that a close association exists between specific photic signals (daylight, daylength, and moonlight) and endocrinological regulation. Given the photic control in melatonin secretion, this hypothesis was tested by investigating whether melatonin signals act as mediators relaying photic signals during testis development in the eel.

Methods

We examined changes in melatonin-secretion patterns using time-resolved fluorescence immunoassays in sexually immature and mature male Japanese eels under the condition of a new moon (NM) and a full moon (FM).

Results

The eye and plasma melatonin levels exhibited a nocturnal pattern under a 12-h light: dark cycle (12L12D) or under constant darkness (DD), but not with constant light (LL). Eye melatonin levels were similar under the 12L12D and short-day (9L15D) conditions. In the long-day condition (15L9D), secreted plasma melatonin levels were stable, whereas short-day melatonin secretion began when darkness commenced. Sexual maturation began at 8 weeks following intraperitoneal injection of human chorionic gonadotropin (hCG), and NM exposure led to significantly higher eye and plasma melatonin levels compared with those detected under FM exposure.

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

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

IMI - Report on Experimental Models of Emmetropization and Myopia

The results of many studies in a variety of species have significantly advanced our understanding of the role of visual experience and the mechanisms of postnatal eye growth, and the development of myopia. This paper surveys and reviews the major contributions that experimental studies using animal models have made to our thinking about emmetropization and development of myopia. These studies established important concepts informing our knowledge of the visual regulation of eye growth and refractive development and have transformed treatment strategies for myopia. Several major findings have come from studies of experimental animal models. These include the eye's ability to detect the sign of retinal defocus and undergo compensatory growth, the local retinal control of eye growth, regulatory changes in choroidal thickness, and the identification of components in the biochemistry of eye growth leading to the characterization of signal cascades regulating eye growth and refractive state. Several of these findings provided the proofs of concepts that form the scientific basis of new and effective clinical treatments for controlling myopia progression in humans. Experimental animal models continue to provide new insights into the cellular and molecular mechanisms of eye growth control, including the identification of potential new targets for drug development and future treatments needed to stem the increasing prevalence of myopia and the vision-threatening conditions associated with this disease.

Environmental Cycles, Melatonin, and Circadian Control of Stress Response in Fish

Fish have evolved a biological clock to cope with environmental cycles, so they display circadian rhythms in most physiological functions including stress response. Photoperiodic information is transduced by the pineal organ into a rhythmic secretion of melatonin, which is released into the blood circulation with high concentrations at night and low during the day. The melatonin rhythmic profile is under the control of circadian clocks in most fish (except salmonids), and it is considered as an important output of the circadian system, thus modulating most daily behavioral and physiological rhythms. Lighting conditions (intensity and spectrum) change in the underwater environment and affect fish embryo and larvae development: constant light/darkness or red lights can lead to increased malformations and mortality, whereas blue light usually results in best hatching rates and growth performance in marine fish. Many factors display daily rhythms along the hypothalamus-pituitary-interrenal (HPI) axis that controls stress response in fish, including corticotropin-releasing hormone (Crh) and its binding protein (Crhbp), proopiomelanocortin A and B (Pomca and Pomcb), and plasma cortisol, glucose, and lactate. Many of these circadian rhythms are under the control of endogenous molecular clocks, which consist of self-sustained transcriptional-translational feedback loops involving the cyclic expression of circadian clock genes (clock, bmal, per, and cry) which persists under constant light or darkness. Exposing fish to a stressor can result in altered rhythms of most stress indicators, such as cortisol, glucose, and lactate among others, as well as daily rhythms of most behavioral and physiological functions. In addition, crh and pomca expression profiles can be affected by other factors such as light spectrum, which strongly influence the expression profile of growth-related (igf1a, igf2a) genes. Additionally, the daily cycle of water temperature (warmer at day and cooler at night) is another factor that has to be considered. The response to any acute stressor is not only species dependent, but also depends on the time of the day when the stress occurs: nocturnal species show higher responses when stressed during day time, whereas diurnal fish respond stronger at night. Melatonin administration in fish has sedative effects with a reduction in locomotor activity and cortisol levels, as well as reduced liver glycogen and dopaminergic and serotonergic activities within the hypothalamus. In this paper, we are reviewing the role of environmental cycles and biological clocks on the entrainment of daily rhythms in the HPI axis and stress responses in fish.

Sleep-wake disturbances in the premotor and early stage of Parkinson's disease

PURPOSE OF REVIEW

Review of recent literature pertaining to frequency, associations, mechanisms, and overall significance of sleep--wake disturbances (SWD) in the premotor and early phase of Parkinson's disease.

RECENT FINDINGS

SWD are frequent in Parkinson's disease and their prevalence increases with disease progression. Recent studies confirm previous findings that SWD can appear as initial manifestation of Parkinson's disease even decades before motor signs appear and highlight their clinical associations in these early stages. More intriguingly, new evidence underpins their role as risk factors, predictors, or even as driving force for the neurodegenerative process. As our understanding of sleep--wake neurobiology increases, new hypotheses emerge concerning the pathophysiology of SWD in early Parkinson's disease stages involving dopaminergic and nondopaminergic mechanisms.

SUMMARY

SWD are predictors for the development of parkinsonian syndromes including Parkinson's disease. This may offer the opportunity of developing new preventive strategies and interventions at an early stage of this neurodegenerative disease.

Diurnal expression of proteins in the retina of the blind cone-rod homeobox (Crx-/- ) mouse and the 129/Sv mouse: a proteomic study

PURPOSE

The vertebrate retina contains a circadian clock participating in adaptations to day and night vision. This peripheral clock is independent of the master clock in the suprachiasmatic nucleus (SCN). The retinal clock is located in several cell types, including the photoreceptors. To investigate the role of the circadian clock of the photoreceptor cells in regulation of retinal protein rhythms, we analysed diurnal protein expression in the photoreceptor-deficient cone-rod homeobox knockout mouse (Crx^-/- ) and the 129/Sv mouse.

METHODS

2D gels were made from retinal homogenates of 129/Sv and Crx^-/- mice killed at midday and midnight. Stained gels were analysed by use of PDQuest 2D gel analysis software. After trypsin digestion of differential expressed spots, the proteins were identified by LC-MS/MS using a nano-liquid chromatograph connected to a Q-TOF Premier mass spectrometer. These data were used to search the SWISS-PROT database.

RESULTS

Both the retinae of the control and the Crx^-/- mice exhibited diurnal proteins rhythms. As expected, proteins involved in phototransduction were not detected in the Crx^-/- mouse; in this phenotype, however, proteins from spots showing diurnal rhythms were specifically identified as enzymes involved in glucose metabolism, Krebs cycle, and mitochondrial enzymes. Data are available via ProteomeXchange with identifier PXD005556.

CONCLUSION

We show diurnal protein rhythms in the retina of a mouse lacking the rods and cones. The diurnal protein rhythms in this genotype, lacking the circadian clock of the photoreceptors, might be caused by a circadian clock in other retinal cell types or a direct light input to the retina.

The 2004 Aschoff/Pittendrigh Lecture: Theory of the Origin of the Pineal Gland— A Tale of Conflict and Resolution

A theory is presented that explains the evolution of the pinealocyte from the common ancestral photoreceptor of both the pinealocyte and retinal photoreceptor. Central to the hypothesis is the previously unrecognized conflict between the two chemistries that define these cells—melatonin synthesis and retinoid recycling. At the core of the conflict is the formation of adducts composed of two molecules of retinaldehyde and one molecule of serotonin, analogous to formation in the retina of the toxic bis-retinyl ethanolamine (A2E). The hypothesis argues that early in chordate evolution, at a point before the genes required for melatonin synthesis were acquired, retinaldehyde—which is essential for photon capture—was depleted by reacting with naturally occurring arylalkylamines (tyramine, serotonin, tryptamine, phenylethylamine) and xenobiotic arylalkylamines. This generated toxic bis-retinyl arylalkylamines (A2AAs). The acquisition of arylalkylamine N-acetyltransferase (AANAT) prevented this by N-acetylating the arylalkylamines. HydroxyindoleOmethyltransferase enhanced detoxification in the primitive photoreceptor by increasing the lipid solubility of serotonin and bis-retinyl serotonin. After the serotonin. melatonin pathway was established, the next step leading toward the pinealocyte was the evolution of a daily rhythm in melatonin and the capacity to recognize it as a signal of darkness. The shift in melatonin from metabolic garbage to information developed a pressure to improve the reliability of the melatonin signal, which in turn led to higher levels of serotonin in the photodetector. This generated the conflict between serotonin and retinaldehyde, which was resolved by the cellular segregation of the two chemistries. The result, in primates, is a pineal gland that does not detect light and a retinal photodetector that does not make melatonin. High levels of AANAT in the latter tissue might serve the same function AANAT had when first acquired— prevention of A2AA formation.

Role of the dopaminergic system in the development of myopia in children and adolescents

This review summarizes the experimental evidence that supports the role of dopamine in the regulation of ocular axial growth. The most important functions attributed to dopamine are light adaptation and regulation of the retinal circadian rhythm. An increase of the retinal levels of dopamine activates D1 and D2 dopaminergic receptors present throughout the retina, generating a signal that inhibits axial growth once the eye has reached emmetropization. Researchers induced form-deprivation myopia in animal models in order to assess the different changes of ocular axial growth. Other studies have shown that phenylethylamine is an endogenous precursor-neurotransmitter capable of modulating the activity of dopamine. Considering the role of the dopaminergic system in the development of myopia (in children and adolescents) and the fact that phenylethylamine improves the consequences of a dopamine deficit, it would be interesting to study the effect of phenylethylamine on the regulation of axial growth, which represents the genesis of myopia.

Diurnal variation of tight junction integrity associates inversely with matrix metalloproteinase expression in Xenopus laevis corneal epithelium: implications for circadian regulation of homeostatic surface cell desquamation

Background and Objectives

The corneal epithelium provides a protective barrier against pathogen entrance and abrasive forces, largely due to the intercellular junctional complexes between neighboring cells. After a prescribed duration at the corneal surface, tight junctions between squamous surface cells must be disrupted to enable them to desquamate as a component of the tissue homeostatic renewal. We hypothesize that matrix metalloproteinase (MMPs) are secreted by corneal epithelial cells and cleave intercellular junctional proteins extracellularly at the epithelial surface. The purpose of this study was to examine the expression of specific MMPs and tight junction proteins during both the light and dark phases of the circadian cycle, and to assess their temporal and spatial relationships in the Xenopus laevis corneal epithelium.

Methodology/Principal Findings

Expression of MMP-2, tissue inhibitor of MMP-2 (TIMP-2), membrane type 1-MMP (MT1-MMP) and the tight junction proteins occludin and claudin-4 were examined by confocal double-label immunohistochemistry on corneas obtained from Xenopus frogs at different circadian times. Occludin and claudin-4 expression was generally uniformly intact on the surface corneal epithelial cell lateral membranes during the daytime, but was frequently disrupted in small clusters of cells at night. Concomitantly, MMP-2 expression was often elevated in a mosaic pattern at nighttime and associated with clusters of desquamating surface cells. The MMP-2 binding partners, TIMP-2 and MT1-MMP were also localized to surface corneal epithelial cells during both the light and dark phases, with TIMP-2 tending to be elevated during the daytime.

Conclusions/Significance

MMP-2 protein expression is elevated in a mosaic pattern in surface corneal epithelial cells during the nighttime in Xenopus laevis, and may play a role in homeostatic surface cell desquamation by disrupting intercellular junctional proteins. The sequence of MMP secretion and activation, tight junction protein cleavage, and subsequent surface cell desquamation and renewal may be orchestrated by nocturnal circadian signals.

Effects of different colors of light on melatonin suppression and expression analysis of Aanat1 and melanopsin in the eye of a tropical damselfish

Ocular melatonin production exhibits a daily rhythm with a decrease during photophase and an increase during scotophase (nocturnal pattern) in teleost fish due to day-night changes in the activity of the rate-limiting melatonin synthesizing enzyme arylalkylamine N-acetyltransferase (AANAT). Acute light exposure during scotophase suppresses AANAT activity and melatonin production in the eyes, suggesting that external light signals are a principal regulator of ocular melatonin synthesis. To better understand the photic regulation of ocular melatonin synthesis in teleost fish, this study sought to characterize the effect of light on ocular melatonin synthesis in the sapphire devil Chrysiptera cyanea, which shows a nocturnal pattern and light-induced inhibition of ocular melatonin production during scotophase. Exposure to three different wavelengths of light (half-peak bandwidth=435-475 nm with a peak of 455 nm, 495-565 nm with a peak of 530 nm, and 607-647 nm with a peak of 627 nm for the blue, green, and red LEDs) for 2h during scotophase resulted in the blue wavelength significantly decreasing ocular melatonin content within 30 min after light exposure. This result clearly indicates that the effective range of visible light on ocular melatonin suppression is distributed within the wavelengths of blue light and that a blue light-sensitive opsin is involved in ocular melatonin suppression in the fish. A PCR-based cloning method revealed the expression of melanopsin, a putative blue light-sensitive nonvisual opsin, in the eyes. Furthermore, in situ hybridization using the sapphire devil Aanat1 and melanopsin RNA probes showed mRNA expressions of both genes in the inner nuclear and ganglion cell layer of the fish retina. These results suggest that melanopsin is a possible candidate photoreceptor involved in ocular melatonin suppression by an external light signal in the sapphire devil.

Moonlight affects mRNA abundance of arylalkylamine N-acetyltransferase in the retina of a lunar-synchronized spawner, the goldlined spinefoot

Melatonin synthesis in the pineal gland and retina shows a rhythmic fashion with high levels at night and is controlled by a rate-limiting enzyme, arylalkylamine N-acetyltransferase (AANAT). A previous study revealed that moonlight suppresses the plasma melatonin levels of the goldlined spinefoot (Siganus guttatus), which exhibits a lunar cycle in its reproductive activity and repeats gonadal development toward and spawning around the first quarter moon. Whether the retina of this species responds to moonlight is unknown. To clarify the photoperceptive ability of this species, we aimed to clone the full-length cDNA of Aanat1 (sgAanat1) from the retina and examine its transcriptional pattern under several daylight and moonlight regimes. The full-length sgAanat1 cDNA (1,038 bp) contained a reading frame encoding a protein of 225 amino acids, which was highly homologous to AANAT1 of other teleosts. Reverse transcription-polymerase chain reaction (PCR) analysis revealed that among the tissues tested, sgAanat1 fragments were expressed exclusively in the retina. Real-time quantitative PCR analysis revealed that sgAanat1 fluctuated with high abundance at night under light-dark cycle and at subjective night under constant darkness, but not under constant light. These results suggest that sgAanat1 is regulated by both the external light signal and internal clock system. The abundance of sgAanat1 in the retina was higher at the culmination time around new moon than full moon phase. Additionally, exposing fish to brightness around the full moon period suppressed sgAanat1 mRNA abundance. Thus, moonlight is perceived by fish and has an impact on melatonin fluctuation in the retina.

Circadian rhythms and endocrine functions in adult insects

Many behavioral and physiological processes in adult insects are influenced by both the endocrine and circadian systems, suggesting that these two key physiological systems interact. We reviewed the literature and found that experiments explicitly testing these interactions in adult insects have only been conducted for a few species. There is a shortage of measurements of hormone titers throughout the day under constant conditions even for the juvenile hormones (JHs) and ecdysteroids, the best studied insect hormones. Nevertheless, the available measurements of hormone titers coupled with indirect evidence for circadian modulation of hormone biosynthesis rate, and the expression of genes encoding proteins involved in hormone biosynthesis, binding or degradation are consistent with the hypothesis that the circulating levels of many insect hormones are influenced by the circadian system. Whole genome microarray studies suggest that the modulation of farnesol oxidase levels is important for the circadian regulation of JH biosynthesis in honey bees, mosquitoes, and fruit flies. Several studies have begun to address the functional significance of circadian oscillations in endocrine signaling. The best understood system is the circadian regulation of Pheromone Biosynthesis Activating Neuropeptide (PBAN) titers which is important for the temporal organization of sexual behavior in female moths. The evidence that the circadian and endocrine systems interact has important implications for studies of insect physiology and behavior. Additional studies on diverse species and physiological processes are needed for identifying basic principles underlying the interactions between the circadian and endocrine systems in insects.

Melanopsin, photosensitive ganglion cells, and seasonal affective disorder

In two recent reports, melanopsin gene variations were associated with seasonal affective disorder (SAD), and in changes in the timing of sleep and activity in healthy individuals. New studies have deepened our understanding of the retinohypothalamic tract, which translates environmental light received by the retina into neural signals sent to a set of nonvisual nuclei in the brain that are responsible for functions other than sight including circadian, neuroendocrine and neurobehavioral regulation. Because this pathway mediates seasonal changes in physiology, behavior, and mood, individual variations in the pathway may explain why approximately 1-2% of the North American population develops mood disorders with a seasonal pattern (i.e., Major Depressive and Bipolar Disorders with a seasonal pattern, also known as seasonal affective disorder/SAD). Components of depression including mood changes, sleep patterns, appetite, and cognitive performance can be affected by the biological and behavioral responses to light. Specifically, variations in the gene sequence for the retinal photopigment, melanopsin, may be responsible for significant increased risk for mood disorders with a seasonal pattern, and may do so by leading to changes in activity and sleep timing in winter. The retinal sensitivity of SAD is hypothesized to be decreased compared to controls, and that further decrements in winter light levels may combine to trigger depression in winter. Here we outline steps for new research to address the possible role of melanopsin in seasonal affective disorder including chromatic pupillometry designed to measure the sensitivity of melanopsin containing retinal ganglion cells.

Melatonin receptors are anatomically organized to modulate transmission specifically to cone pathways in the retina of Xenopus laevis

Melatonin receptors have been identified in several retinal cell types, including photoreceptors, horizontal cells, amacrine cells, and ganglion cells. Recent reports suggest that melatonin potentiates signaling from rods to inner retinal neurons. However, the organization of the melatonin receptors mediating this action in the outer plexiform layer (OPL) is not clear. To assess melatonin receptor localization in the OPL, double-label confocal immunohistochemistry for Mel1a or Mel1b melatonin receptors was performed in combination with markers for cone photoreceptors (calbindin, XAP-1) and ON bipolar cells (guanine nucleotide binding protein alpha, Goα) on the retina of Xenopus laevis. Both Mel1a and Mel1b receptors were specifically associated with processes contacting the pedicles of cones, but localized to processes from different sets of second-order neurons. Mel1a receptors localized to the large axonal processes of horizontal cells, while Mel1b receptors localized to the dendrites of OFF bipolar cells. Both receptors also localized to third-order amacrine and ganglion cells and their processes in the inner plexiform layer. This study indicates that Mel1a and Mel1b melatonin receptors are expressed specifically in the Xenopus OPL to modulate transmission from cones to horizontal cells and OFF bipolar cells, respectively; they are second-order neurons that predominantly contact ribbon synapses and display OFF responses to light. When combined with results from recent physiological studies, the current results suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons across species, although the precise mechanisms by which melatonin enhances this transmission are likely to vary in a species-dependent manner.

Electroretinographic assessment in major depressed patients receiving duloxetine: might differences between responders and non-responders indicate a differential biological background?

INTRODUCTION

Despite intense research efforts, still too little is known about the biological determinants of depression, thus soliciting diverse study approaches. Among others, the electroretinography (ERG) has been proposed even as a putative proxy (retinal) measurement of central dopaminergic activity for Major Depressive Disorder (MDD) both in drug-naïve patients and subjects receiving antidepressant treatments. Nonetheless, current evidences are merely preliminary, essentially considering just older classes of antidepressants, thus requiring confirmation studies even with newer agents as duloxetine.

METHOD

Twenty MDD subjects and 20 matched controls received duloxetine 60 mg/day for 12 weeks, being monitored both by standard ERG recording and by administration of the Hamilton scales for Depression and Anxiety and the Young Mania Rating Scale at baseline and week 12 (end of the study).

RESULTS

ERG mean rod b-wave amplitude significantly reduced from baseline to week 12 in those depressed subjects achieving final response (p=.024), decreasing from the highest rank values to the ones, substantially unmodified, seen among non-responders and controls.

LIMITATIONS

Small sample size and lack of multiple assessments.

CONCLUSIONS

At least some MDD patients responding to duloxetine might exhibit a peculiar ERG pattern, hypothetically indicating a specific biological background. If confirmed by larger-sampled studies, these results might shed further light in the understanding of the biological determinants of different subtypes of depression, ideally showing alternative patterns of response upon different treatment interventions.

Dopamine D₄ receptor activation controls circadian timing of the adenylyl cyclase 1/cyclic AMP signaling system in mouse retina

In the mammalian retina, dopamine binding to the dopamine D₄ receptor (D₄R) affects a light-sensitive pool of cyclic AMP by negatively coupling to the type 1 adenylyl cyclase (AC1). AC1 is the primary enzyme controlling cyclic AMP production in dark-adapted photoreceptors. A previous study demonstrated that expression of the gene encoding AC1, Adcy1, is downregulated in mice lacking Drd4, the gene encoding the D₄R. The present investigation provides evidence that D₄R activation entrains the circadian rhythm of Adcy1 mRNA expression. Diurnal and circadian rhythms of Drd4 and Adcy1 mRNA levels were observed in wild-type mouse retina. Also, rhythms in the Ca²⁺-stimulated AC activity and cyclic AMP levels were observed. However, these rhythmic activities were damped or undetectable in mice lacking the D₄R. Pharmacologically activating the D₄R 4 h before its normal stimulation at light onset in the morning advances the phase of the Adcy1 mRNA expression pattern. These data demonstrate that stimulating the D₄R is essential in maintaining the normal rhythmic production of AC1 from transcript to enzyme activity. Thus, dopamine/D₄R signaling is a novel zeitgeber that entrains the rhythm of Adcy1 expression and, consequently, modulates the rhythmic synthesis of cyclic AMP in mouse retina.

Alteration of melatonin secretion in patients with type 2 diabetes and proliferative diabetic retinopathy

Background:

The purpose of this study was to evaluate the dynamics of plasma melatonin secretion in patients with type 2 diabetes mellitus and diabetic retinopathy.

Methods:

Plasma melatonin levels were measured by high-performance liquid chromatography in 56 patients. Patients were divided into a diabetic group (30 patients) and a nondiabetic group (26 patients). The diabetic group was divided further into a proliferative diabetic retinopathy (PDR) group (n = 14) and a nonproliferative diabetic retinopathy (NPDR) group (n = 16). Plasma melatonin levels obtained at midnight and 3 am were compared between the groups.

Results:

Nighttime melatonin levels were significantly lower in the diabetic group than in the nondiabetic group (P < 0.03) and lower in the PDR group than in the nondiabetic and NPDR groups (P < 0.01 and P < 0.03, respectively), but no significant difference was found between the nondiabetic and NPDR groups. The daytime melatonin level did not significantly differ between the nondiabetic and diabetic groups or between the nondiabetic, NPDR, and PDR groups.

Conclusion:

The nighttime melatonin level is altered in patients with diabetes and PDR but not in diabetic patients without PDR. Although patients with PDR may have various dysfunctions that affect melatonin secretion more severely, advanced dysfunction of retinal light perception may cause altered melatonin secretion. Alteration of melatonin secretion may accelerate further occurrence of complications in diabetic patients.

Effects of 530 nm green light on refractive status, melatonin, MT1 receptor, and melanopsin in the guinea pig

PURPOSE

To investigate (i) the effect of monochromatic light on inhibiting induction of light-induced melatonin and (ii) the roles of melanopsin and MT1 receptor in light-induced myopia in the guinea pig.

METHODS

Forty-eight guinea pigs were randomly distributed into three treatment groups: white-light (control), green-light (530 nm), and blue-light (480 nm) groups. Levels of pineal gland melatonin were measured twice daily--10:00 a.m. and 10:00 p.m.--10 days after initial light treatment. Thirty additional guinea pigs were also assigned to these groups and treated similarly. For these latter animals, refractive status, ocular length, and vitreous depth were measured before and after light treatment. Eight weeks after light treatment, retinal and sceral levels of melanopsin, melatonin receptor type (MT) 1, and mRNA protein were determined by Western blotting, real-time polymerase chain reaction (RT-PCR), and immunohistochemistry.

RESULTS

The level of pineal gland melatonin in the green-light group was significantly higher than that in the blue-light group. Biometric measurements showed that guinea pigs in the green-light group had a somewhat myopic refractive status. Expressions of retinal melanopsin mRNA and protein were significantly higher in the blue-light group and lower in the green-light group when compared to controls. Conversely, expressions of MT1 receptor mRNA and protein in retina and sclera were significantly higher in the green-light group and lower in the blue-light group when compared to controls.

CONCLUSIONS

Green light appears to suppress induction of melatonin production. In addition, 530 nm of green light is involved in the development of myopia. In the guinea pig, MT1 receptor and melanopsin appear to play roles in the development of myopia induced by 530 nm of light.

Current knowledge on the melatonin system in teleost fish

Melatonin is a much conserved feature in vertebrates that plays a central role in the entrainment of daily and annual physiological rhythms. Investigations aiming at understanding how melatonin mediates the effects of photoperiod on crucial functions and behaviors have been very active in the last decades, particularly in mammals. In fish a clear-cut picture is still missing. Here we review the available data on (i) the sites of melatonin production in fish, (ii) the mechanisms that control its daily and annual rhythms of production and (iii) the characterization of its different receptor subtypes, their location and regulation. The in vivo and in vitro data on melatonin effects on crucial neuroendocrine regulations, including reproduction, growth, feeding and behavioral responses, are also reviewed. Finally we discuss how manipulation of the photic cues impact on fish circannual clock and annual cycle of reproduction, and how this can be used for aquaculture purposes.

Current knowledge on the photoneuroendocrine regulation of reproduction in temperate fish species

Seasonality is an important adaptive trait in temperate fish species as it entrains or regulates most physiological events such as reproductive cycle, growth profile, locomotor activity and key life-stage transitions. Photoperiod is undoubtedly one of the most predictable environmental signals that can be used by most living organisms including fishes in temperate areas. This said, however, understanding of how such a simple signal can dictate the time of gonadal recruitment and spawning, for example, is a complex task. Over the past few decades, many scientists attempted to unravel the roots of photoperiodic signalling in teleosts by investigating the role of melatonin in reproduction, but without great success. In fact, the hormone melatonin is recognized as the biological time-keeping hormone in fishes mainly due to the fact that it reflects the seasonal variation in daylength across the whole animal kingdom rather than the existence of direct evidences of its role in the entrainment of reproduction in fishes. Recently, however, some new studies clearly suggested that melatonin interacts with the reproductive cascade at a number of key steps such as through the dopaminergic system in the brain or the synchronization of the final oocyte maturation in the gonad. Interestingly, in the past few years, additional pathways have become apparent in the search for a fish photoneuroendocrine system including the clock-gene network and kisspeptin signalling and although research on these topics are still in their infancy, it is moving at great pace. This review thus aims to bring together the current knowledge on the photic control of reproduction mainly focusing on seasonal temperate fish species and shape the current working hypotheses supported by recent findings obtained in teleosts or based on knowledge gathered in mammalian and avian species. Four of the main potential regulatory systems (light perception, melatonin, clock genes and kisspeptin) in fish reproduction are reviewed.

Evolution of The Vertebrate Pineal Gland: The Aanat Hypothesis

The defining feature of the pineal gland is the capacity to function as a melatonin factory that operates on a approximately 24 h schedule, reflecting the unique synthetic capacities of the pinealocyte. Melatonin synthesis is typically elevated at night and serves to provide the organism with a signal of nighttime. Melatonin levels can be viewed as hands of the clock. Issues relating to the evolutionary events leading up to the immergence of this system have not received significant attention. When did melatonin synthesis appear in the evolutionary line leading to vertebrates? When did a distinct pineal gland first appear? What were the forces driving this evolutionary trend? As more knowledge has grown about the pinealocyte and the relationship it has to retinal photoreceptors, it has become possible to generate a plausible hypothesis to explain how the pineal gland and the melatonin rhythm evolved. At the heart of the hypothesis is the melatonin rhythm enzyme arylalkylamine N-acetyltransferase (AANAT). The advances supporting the hypothesis will be reviewed here and expanded beyond the original foundation; the hypothesis and its implications will be addressed.

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

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

Meeting report: the role of environmental lighting and circadian disruption in cancer and other diseases

Light, including artificial light, has a range of effects on human physiology and behavior and can therefore alter human physiology when inappropriately timed. One example of potential light-induced disruption is the effect of light on circadian organization, including the production of several hormone rhythms. Changes in light-dark exposure (e.g., by nonday occupation or transmeridian travel) shift the timing of the circadian system such that internal rhythms can become desynchronized from both the external environment and internally with each other, impairing our ability to sleep and wake at the appropriate times and compromising physiologic and metabolic processes. Light can also have direct acute effects on neuroendocrine systems, for example, in suppressing melatonin synthesis or elevating cortisol production that may have untoward long-term consequences. For these reasons, the National Institute of Environmental Health Sciences convened a workshop of a diverse group of scientists to consider how best to conduct research on possible connections between lighting and health. According to the participants in the workshop, there are three broad areas of research effort that need to be addressed. First are the basic biophysical and molecular genetic mechanisms for phototransduction for circadian, neuroendocrine, and neurobehavioral regulation. Second are the possible physiologic consequences of disrupting these circadian regulatory processes such as on hormone production, particularly melatonin, and normal and neoplastic tissue growth dynamics. Third are effects of light-induced physiologic disruption on disease occurrence and prognosis, and how prevention and treatment could be improved by application of this knowledge.

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.

Light, dark, and melatonin: emerging evidence for the importance of melatonin in ocular physiology

Melatonin is a hormone, which is mainly produced by the pineal gland, a vestigial eye. Rather than the rods and cones, it is a newly discovered subgroup of photosensitive retinal ganglion cells, which is responsible for mediating the light-dark cycles, thus regulating melatonin's secretion. One of the correlates of the circadian rhythm of melatonin release is the habitual sleep pattern. Patients with circadian rhythm sleep disorders, including some blind patients with no light-induced suppression of melatonin, benefit from melatonin treatment. Melatonin is synthesized in the retina, lens, ciliary body as well as other parts of the body. In this review, we discuss the physiological roles of melatonin in the eye, as well as the potential therapeutic avenues currently under study.

Evolution of arylalkylamine N-acetyltransferase: emergence and divergence

The melatonin rhythm-generating enzyme, arylalkylamine N-acetyltransferase (AANAT) is known to have recognizable ancient homologs in bacteria and fungi, but not in other eukaryotes. Analysis of new cDNA and genomic sequences has identified several additional homologs in other groupings. First, an AANAT homolog has been found in the genome of the cephalochordate amphioxus, representing the oldest homolog in chordates. Second, two AANAT homologs have been identified in unicellular green algae. The homologs in amphioxus, unicellular green algae, fungi and bacteria are similarly primitive in that they lack sequences found in vertebrate AANATs that are involved in regulation and that facilitate binding and catalysis. In addition, all these sequences are intronless. These features are consistent with horizontal transfer of the AANAT ancestor from bacteria to green algae, fungi and chordates. Lastly, a third AANAT gene has been found in teleost fish, suggesting that AANAT genes serve multiple functions in addition to melatonin synthesis.

Circadian rhythms of ocular melatonin in the wrasse Halichoeres tenuispinnis, a labrid teleost

Using in vivo and in vitro methods we studied the regulation of ocular melatonin rhythms in the wrasse Halichoeres tenuispinnis, by either light or the circadian clock. Rhythmic changes in ocular melatonin levels under light-dark (LD) cycles were persistent under constant darkness (DD), and had a circadian periodicity of approximately 24h. However, ocular melatonin levels remained low under constant light conditions. When wrasse were exposed to a single 6-h light pulse at three different circadian phases under DD, phase-dependent phase shifts in the circadian rhythms of ocular melatonin were observed. When eyecups were prepared during mid-light periods or at the onset of darkness, and incubated in vitro in either light or dark periods, both time and light conditions affected melatonin release. These results indicate that the melatonin rhythms in the wrasse eye are driven by an ocular circadian clock that is entrained to LD cycles via local photoreceptors.

Time course and development of light adaptation processes in the outer zebrafish retina

Retinomotor movements are morphological changes in the outer retina in response to changing light conditions. They can be separated into two components: Migration of pigment granules within the microvilli of the retinal pigment epithelium (RPE) and positional changes in photoreceptor cells. These positional changes optimize exposure of the cone and rod photoreceptors to light. The aim of this study was to analyze both the time course of retinomotor movements in the adult zebrafish and the maturation of these processes in the developing fish. We show that retinomotor movements are used as a dark/light adaptation mechanism in zebrafish. In adult zebrafish, melanin granules of the RPE migrate with constant speed and reach the fully light adapted (LA) state approximately after 1 h. In contrast, about two thirds of double cone outer segment movements are finished in 5 min, and are fully completed in 10 to 20 min. During development there are three crucial stages leading to mature retinomotor movements in response to light: at 5 dpf (days post fertilization) the migration of pigment granules begins, at 20 dpf the pigment granules condense in the apical part of the RPE microvilli, and at 28 dpf, concomitant with the functional maturation of rods, the double cones contract as in adult retinas.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Melatonin regulates circadian electroretinogram rhythms in a dose- and time-dependent fashion

The circadian rhythm of the chick electroretinogram (ERG) is regulated by the indoleamine hormone melatonin. To determine if the concentration of melatonin or the time at which it was administered would have differential effects on ERG parameters, we conducted experiments analyzing the effects of melatonin at different times of the day. Circadian rhythms of a- and b-wave implicit times and amplitudes were observed in both light:dark (LD) and in continuous darkness (DD). Intramuscular melatonin administration of 1 mg/kg and 100 ng/kg decreased a- and b-wave amplitudes and increased a- and b-wave implicit times. This effect was significantly greater than that observed for 1 ng/kg melatonin, which had little to no effect over the saline controls. The effect of 1 mg/kg and 100 ng/kg melatonin on a- and b-wave amplitude in LD and on b-wave amplitude in DD was greater during the night (ZT/CT 17) than during the day (ZT/CT 5). The fold change in b-wave implicit time over that of controls was greater during the day (ZT/CT 5) than during the night (ZT/CT 17). These data indicate that melatonin may play a role in regulating a day and night functional shift in the retina, and that it does so via regulation of a retinal clock.

Binding characteristics and daily rhythms of melatonin receptors are distinct in the retina and the brain areas of the European sea bass retina (Dicentrarchus labrax)

Melatonin is synthesized, with a circadian rhythm, in the pineal organ of vertebrates, high levels being produced during the scotophase and low levels during the photophase. The retina also produces melatonin, although in the case of the European sea bass, its secretion pattern appears to be inverted. In the study described here, radioreceptor assay techniques were used to characterize the melatonin binding sites, their regional distribution and their daily variations. Brain and retina membrane preparations were used in all the binding assays and 2-[125I]iodomelatonin ([125I]Mel) as radioligand at 25 degrees C. The specific binding of [125I]Mel was seen to be saturable, reversible, specific and of high affinity. In all the tissues assayed, the power of the ligands to inhibit [125I]Mel binding decreased in the following order: melatonin>>4-P-PDOT>luzindole> or =N-acetylserotonin, which points to the presence of Mel1-like receptors. The inhibition curves of 4-P-PDOT suggested the presence of two different binding sites in the brain areas, but only one type of site of low affinity in the neural retina. No daily variations in [125I]Mel binding capacity (Bmax) or affinity (Kd) were detected in the brain areas, while a clear rhythm in Kd melatonin receptor affinity and Bmax binding capacity was observed in the retina. Kd and Bmax retinal rhythms were out of phase with the lowest Kd and the highest Bmax occurring at scotophase. This result suggests that retinal melatonin is a paracrine factor able to control receptor desensitization during photophase when ocular melatonin is higher in this species.

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.

Localization of Mel1b melatonin receptor-like immunoreactivity in ocular tissues of Xenopus laevis

The circadian signaling molecule, melatonin, is produced by pinealocytes and retinal photoreceptors. In the retina, melatonin is thought to diffuse into the inner retina to act as a paracrine signal of darkness by binding to specific receptors in retinal neurons. The retinal cell locations of the Mel1a and Mel1c melatonin receptor types have been reported, but the localization of the Mel1b receptor, which is the most highly expressed melatonin receptor type in the retina, is unknown. To determine the cellular distribution of Mel1b melatonin receptor protein in the Xenopus laevis retina and other ocular tissues, polyclonal antibodies were raised against a peptide fragment of the X. laevis Mel1b receptor. Western blot analysis of several ocular tissues revealed the presence of one or more immunoreactive bands in the sclera, cornea, lens, retinal pigment epithelium (RPE)/choroid, and neural retina. In the neural retina, the major immunoreactive bands displayed electrophoretic mobilities corresponding to approximately 35, 42, 45, and 80 Kd. Sections of X. laevis eyes were analyzed by immunocytochemistry and confocal microscopy, in combination with antibodies against the Mel1a melatonin receptor, a rod photoreceptor-specific protein, opsin, and two amacrine cell-specific markers, tyrosine hydroxylase (TOH; dopaminergic cells) and glutamic acid decarboxylase (GAD; GABA-ergic cells). Mel1b immunoreactivity was localized to the apical membranes of RPE cells, and punctate Mel1b immunoreactivity was observed in both rod and cone photoreceptor inner segments. Presumptive horizontal cells that ramify in the outer plexiform layer (OPL) were immunoreactive for Mel1b, and were exclusive of the Mel1a immunoreactivity present in the OPL. Neither TOH nor GAD co-localized with the Mel1b immunoreactivity that was present in the inner plexiform layer (IPL), suggesting that Mel1b is not expressed in dopaminergic or GABA-ergic amacrine cells. Mel1b immunoreactivity was observed in ganglion cells of the retina, a population of cells covering the outer surface of the outer fibrous layer of the sclera, and in lens fibers located in the outer regions of the lens. These results suggest that melatonin may influence retinal function by binding to receptors on RPE and photoreceptor cells, and by acting on neurons of the inner retina that do not use dopamine or GABA as a neurotransmitter. Furthermore, melatonin may bind to receptors on cells located in the sclera and lens, perhaps to modify the growth or function of these ocular tissues.

Diurnal rhythms of tryptophan hydroxylase 1 and 2 mRNA expression in the rat retina

Tryptophan hydroxylase is the first of four enzymes in the melatonin biosynthetic pathway. Recent studies have shown that there are two genes, Tph1 and Tph2, that encode tryptophan hydroxylase in mammals. In this study, we investigated which of the two genes is expressed in the rat retina. To that end, we measured Tph1 (classical Tph) and Tph2 mRNA levels using real-time quantitative RT-PCR in the retina. Our data demonstrate that Tph1 mRNA is the prevalent form expressed in the retina; Tph2 mRNA is also present but the level is very low. We also measured Tph1 expression levels in the outer nuclear layer, inner nuclear layer, and ganglion cell layer by combining laser capture microdissection and real-time RT-PCR. Tph1 mRNA is more abundant in the photoreceptors of the outer nuclear layer than in the inner nuclear layer or ganglion cell layer. Tph1 and Tph2 transcripts showed robust diurnal rhythms of abundance, with highest levels at night. Our results support the hypothesis that Tph1 is involved in melatonin synthesis in retinal photoreceptor cells.

Melatonin binding sites in the brain of European sea bass (Dicentrarchus labrax)

Characteristics, day-night changes, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) modulation, and localization of melatonin binding sites in the brain of a marine teleost, European sea bass Dicentrarchus labrax, were studied by radioreceptor assay using 2-[(125)I]iodomelatonin as a radioligand. The specific binding to the sea bass brain membranes was rapid, stable, saturable and reversible. The radioligand binds to a single class of receptor site with the affinity (Kd) of 9.3 +/-0.6 pM and total binding capacity (Bmax) of 39.08 +/-0.86 fmol/mg protein (mean+/-SEM, n=4) at mid-light under light-dark (LD) cycles of 12:12. Day-night changes were observed neither in the Kd nor in the Bmax under LD 12:12. Treatment with GTPgammaS significantly increased the Kd and decreased the Bmax both at mid-light and mid-dark. The binding sites were highly specific for 2-phenylmelatonin, 2-iodomelatonin, melatonin, and 6-chloromelatonin. Distribution of melatonin binding sites in the sea bass brain was uneven: The Bmax was determined to be highest in mesencephalic optic tectum-tegmentum and hypothalamus, intermediate in telencephalon, cerebellum-vestibulolateral lobe and medulla oblongata-spinal cord, and lowest in olfactory bulbs with the Kd in the low picomolar range. These results indicate that melatonin released from the pineal organ and/or retina plays neuromodulatory roles in the sea bass brain via G protein-coupled melatonin receptors.

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 receptor expression in the cornea and sclera

The scornea and sclera have been shown to exhibit circadian rhythms in cellular proliferation, wound healing and extracellular matrix synthesis. The distribution of melatonin Mel1a and Mel1c receptors was examined in the cornea and sclera of the Xenopus laevis eye in order to determine whether melatonin may potentially influence the growth and/or development of these ocular tissues. Sections of adult X. laevis eyes were analyzed by immunocytochemistry and confocal microscopy, using antibodies prepared against specific peptide sequences of the Xenopus Mel1a and Mel1c receptor proteins. Antibodies were pre- incubated with their appropriate antigenic peptides to control for non-specific labelling. Analysis of the distribution of Mel1a and Mel1c receptor immunoreactivity in the Xenopus eye revealed that both the Mel1a and Mel1c receptors were located in the outer fibrous layer (OFL) of the sclera, with Mel1c labelling being the most prominent. Similarly, Mel1a and Mel1c (Mel1c mostly) were also located in cells of the inner fibrous layer (IFL) with Mel1c being most abundant. The chondrocytes of the cartilaginous layer also appeared to express Mel1a, Mel1c, or both receptors. Both Mel1a and Mel1c receptor immunoreactivity were observed in the corneal epithelium and endothelium. Whereas the Mel1a antibody labelled the entire corneal epithelial layer, the Mel1c antibody labelled only the most superficial layer of epithelial cells. Cell processes of fibroblasts of the corneal stroma were immunoreactive for either Mel1a or Mel1c receptors. The identification of Mel1a and Mel1c receptors in restricted distributions in the cornea and sclera suggests that melatonin may play a role in the cellular physiology of these ocular tissues.

Induction of blindness by formoguanamine hydrochloride in adult male roseringed parakeets (Psittacula krameri)

Formoguanamine (2,4-diamino-s-triazine) was known to be an effective chemical agent in inducing blindness in poultry chicks, but not in adult birds. The present study was undertaken to demonstrate the influences, if any, of this chemical on the visual performance and retinal histology in an adult sub-tropical wild bird the roseringed parakeet (Psittacula krameri). Formoguanamine (FG) hydrochloride was subcutaneously injected into adult parakeets at the dosage of 25 mg (dissolved in 0.75 ml physiological saline)/100 g body weight/day, for two consecutive days while the control birds were injected only with the placebo. The effects were studied after 10, 20, and 30 days of the last treatment of FG. Within 24 h of the treatment of FG, about 90% of the total birds exhibited lack of visual responses to any light stimulus and even absence of pupillary light reactions. The remaining birds became totally blind on the day following the last injection of FG and remained so till the end of investigation. At the microscopic level, conspicuous degenerative changes were noted in the outer pigmented epithelium and the photoreceptive layer of rods and cones in the retinas of FG treated birds. A significant reduction in the thickness of the outer nuclear layer was also found in the retinas of FG treated parakeets, compared to that in the control birds. However, the inner cell layers of the retina in the control and FG administered parakeets were almost identical. It deserves special mention that the effects of FG, noted after 30 days of last treatment, were not very different from those noted just after 10 days of treatment. Collectively, the results of the present investigation demonstrate that FG can be used as a potent pharmacological agent for inducing irreversible blindness through selective damage in retinal tissue even in the adult wild bird, thereby making FG treatment an alternative euthanasic device to a cumbersome, stressful, surgical method of enucleation of the ocular system for laboratory studies.

Interaction of the retina with suprachiasmatic pacemakers in the control of circadian behavior

The suprachiasmatic nucleus (SCN) is the central circadian pacemaker governing the circadian rhythm of locomotor activity in mammals. The mammalian retina also contains circadian oscillators, but their roles are unknown. To test whether the retina influences circadian rhythms of locomotor behavior, the authors compared the activity of bilaterally enucleated hamsters with the activity of intact controls held in constant darkness (DD). Enucleated hamsters showed a broader range of free-running periods (tau) than did intact hamsters held for the same length of time in DD. This effect was independent of the age at enucleation (on postnatal days 1, 7, or 28). The average tau of intact animals kept in DD from days 7 or 28 was significantly longer than that of intact animals kept in DD from day 1 or any of the enucleated groups. This indicates that early exposure to light-dark cycles lengthens the tau and that the eye is required to maintain this effect even in DD. These data suggest that hypothalamic circadian pacemakers may interact continuously with the retina to determine the tau of locomotor activity. Enucleation caused a large decrease in glial fibrillary acidic protein in the SCN but has no (or slight) effects on calbindin, neuropeptide Y, vasopressin, or vasoactive intestinal polypeptide, which suggests that enucleation does not produce major damage to the SCN, an interpretation that is supported by the fact that enucleated animals retain robust circadian rhythmicity. The presence of an intact retina appears to contribute to system-level circadian organization in mammals perhaps as a consequence of interaction between its circadian oscillators and those in the SCN.

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.

In vivo disruption of Xenopus CLOCK in the retinal photoreceptor cells abolishes circadian melatonin rhythmicity without affecting its production levels

Xenopus laevis retinas, like retinas from all vertebrate classes, have endogenous circadian clocks that control many aspects of normal retinal physiology occurring in cells throughout all layers of the retina. The localization of the clock(s) that controls these various rhythms remains unclear. One of the best studied rhythmic events is the nocturnal release of melatonin. Photoreceptor layers can synthesize rhythmic melatonin when these cells are in isolation. However, within the intact retina, melatonin is controlled in a complex way, indicating that signals from many parts of the retina may contribute to the production of melatonin rhythmicity. To test this hypothesis, we generated transgenic tadpoles that express different levels of a dominant negative Xenopus CLOCK specifically in the retinal photoreceptors. Eyes from these tadpoles continued to produce melatonin at normal levels, but with greatly disrupted rhythmicity, the severity of which correlated with the transgene expression level. These results demonstrate that although many things contribute to melatonin production in vivo, the circadian clock localized in the retinal photoreceptors is necessary for its rhythmicity. Furthermore, these data show that the control of the level of melatonin synthesis is separable from the control of its rhythmicity and may be controlled by different molecular machinery. This type of specific "molecular lesion" allows perturbation of the clock in intact tissues and is valuable for dissection of clock control of tissue-level processes in this and other complex systems.