Avian melatonin synthesis: photic and circadian regulation of serotonin N-acetyltransferase mRNA in the chicken pineal gland and retina

One Health: Circadian Medicine Benefits Both Non-human Animals and Humans Alike

Circadian biology's impact on human physical health and its role in disease development and progression is widely recognized. The forefront of circadian rhythm research now focuses on translational applications to clinical medicine, aiming to enhance disease diagnosis, prognosis, and treatment responses. However, the field of circadian medicine has predominantly concentrated on human healthcare, neglecting its potential for transformative applications in veterinary medicine, thereby overlooking opportunities to improve non-human animal health and welfare. This review consists of three main sections. The first section focuses on the translational potential of circadian medicine into current industry practices of agricultural animals, with a particular emphasis on horses, broiler chickens, and laying hens. The second section delves into the potential applications of circadian medicine in small animal veterinary care, primarily focusing on our companion animals, namely dogs and cats. The final section explores emerging frontiers in circadian medicine, encompassing aquaculture, veterinary hospital care, and non-human animal welfare and concludes with the integration of One Health principles. In summary, circadian medicine represents a highly promising field of medicine that holds the potential to significantly enhance the clinical care and overall health of all animals, extending its impact beyond human healthcare.

Influence of Different Light Spectra on Melatonin Synthesis by the Pineal Gland and Influence on the Immune System in Chickens

It is well known that the pineal gland in birds influences behavioural and physiological functions, including those of the immune system. The purpose of this research is to examine the endocrine-immune correlations between melatonin and immune system activity. Through a description of the immune-pineal axis, we formulated the objective to determine and describe: the development of the pineal gland; how light influences secretory activity; and how melatonin influences the activity of primary and secondary lymphoid organs. The pineal gland has the ability to turn light information into an endocrine signal suitable for the immune system via the membrane receptors Mel1a, Mel1b, and Mel1c, as well as the nuclear receptors RORα, RORβ, and RORγ. We can state the following findings: green monochromatic light (560 nm) increased serum melatonin levels and promoted a stronger humoral and cellular immune response by proliferating B and T lymphocytes; the combination of green and blue monochromatic light (560-480 nm) ameliorated the inflammatory response and protected lymphoid organs from oxidative stress; and red monochromatic light (660 nm) maintained the inflammatory response and promoted the growth of pathogenic bacteria. Melatonin can be considered a potent antioxidant and immunomodulator and is a critical element in the coordination between external light stimulation and the body's internal response.

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

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

The effect of light regime and time of slaughter in broiler on broiler performance, liver antioxidant status, and expression of genes related to peptide absorption in the jejunum and melatonin synthesis in the brain

This study aimed to assess the effects of light regime and time of slaughter on primal cut and organ weights, peptide transporter 1 (PEPT1) gene expression in the jejunum, arylalkylamine N-acetyltransferase (AANAT) gene expression in the brain, and liver oxidant/antioxidant status in broilers aged 37 days. The experiment was conducted in a factorial completely randomized design, with two light regimes (intermittent light varying according to bird age and continuous light under an 18 h light/6 h dark photoperiod) and four times of slaughter (2:00, 8:00, 14:00 and 20:00 h). There was an interaction effect on PEPT1 and AANAT expression, lipid and protein oxidation and superoxide dismutase (SOD) activity. In both light regimes, PEPT1 expression responded cubically to slaughter time. In the continuous light group, PEPT1 expression was highest in birds slaughtered at 2:00 and 14:00 h, whereas, in the intermittent light treatment, expression was highest at 8:00 h. In the continuous light regime, AANAT expression had a cubic relationship with time of slaughter, with the greatest values recorded at 20:00 h. In the intermittent light regime, slaughter time showed a cubic effect on lipid oxidation, which was highest at 8:00 h. In the continuous light group, there was a cubic effect on nitrite concentration, lipid oxidation, protein oxidation, and SOD activity; nitrite levels, lipid oxidation, and protein oxidation were highest and SOD activity was lowest in birds slaughtered at 14:00 h. Time of slaughter influenced catalase activity, which responded cubically; catalase activity was lowest at 8:00 and 14:00 h. This study is the first to demonstrate that PEPT1 expression in the jejunum of broilers follows a diurnal rhythm and varies according to light regime. The results also suggest that mainly continuous lighting and slaughter at 14:00 h when the animals are possibly more active may be more stressful to broilers.

Effect of monochromatic light on the circadian clock of cultured chick retinal tissue

The central biological clock system of bird is formed by hypothalamus suprachiasmatic nucleus, pineal gland and retina thereby interacting with each other in a neuroendocrine loop. Previous results have confirmed that monochromatic light can influence the clock genes in the pineal gland, hypothalamus and retina of chicks in vivo. The present work was conducted to study whether the cultured retinal tissue of chick could maintain the circadian oscillation and whether the monochromatic light affect the expression level of cultured retinal circadian clock in vitro. Retinal tissues of 0-day-old chicks were cultured in vitro under 4 light treatments (white, red, green and blue lights) with light dark cycle 12:12 and constant dark. The tissues and culture medium were collected every each 4 h. Melanopsin, clock genes, cAanat, the positive-regulating clock proteins and melatonin were measured. The results showed that cOpn4-1, cOpn4-2, cBmal1, cCry1, cPer2, cPer3, cAanat and melatonin concentrations possessed a significant circadian rhythm in cultured chick retina tissues under different monochromatic lights; while, in constant dark, cBmal1, cCry1, cPer2, cPer3, cAanat and melatonin concentration possessed a significant circadian rhythm. Green light promoted the circadian expression level of cOpn4-1, cOpn4-2, cBmal1, cAanat and BMAL1 proteins and the circadian rhythm of melatonin secretion of retina by increasing the mesors and amplitudes. In addition, green light significantly increased the average expression levels of cClock, cBmal2 and CLOCK proteins which were expressed arrhythmically. Results suggested that the retina is a central oscillator with autonomous circadian rhythm. In isolated retina tissues, green light activated the expression of melanopsin and promoted the expression of positive-regulating clock genes, thereby up-regulating the expression of cAanat and resulting the increasing of the synthesis and secretion of melatonin.

Characterization of Arylalkylamine N-Acyltransferase from Tribolium castaneum: An Investigation into a Potential Next-Generation Insecticide Target

The growing issue of insecticide resistance has meant the identification of novel insecticide targets has never been more important. Arylalkylamine N-acyltransferases (AANATs) have been suggested as a potential new target. These promiscuous enzymes are involved in the N-acylation of biogenic amines to form N-acylamides. In insects, this process is a key step in melanism, hardening of the cuticle, removal of biogenic amines, and in the biosynthesis of fatty acid amides. The unique nature of each AANAT isoform characterized indicates each organism accommodates an assembly of discrete AANATs relatively exclusive to that organism. This implies a high potential for selectivity in insecticide design, while also maintaining polypharmacology. Presented here is a thorough kinetic and structural analysis of AANAT found in one of the most common secondary pests of all plant commodities in the world, Tribolium castaneum. The enzyme, named TcAANAT0, catalyzes the formation of short-chain N-acylarylalkylamines, with short-chain acyl-CoAs (C2-C10), benzoyl-CoA, and succinyl-CoA functioning in the role of acyl donor. Recombinant TcAANAT0 was expressed and purified from E. coli and was used to investigate the kinetic and chemical mechanism of catalysis. The kinetic mechanism is an ordered sequential mechanism with the acyl-CoA binding first. pH-rate profiles and site-directed mutagenesis studies identified amino acids critical to catalysis, providing insights about the chemical mechanism of TcAANAT0. A crystal structure was obtained for TcAANAT0 bound to acetyl-CoA, revealing valuable information about its active site. This combination of kinetic analysis and crystallography alongside mutagenesis and sequence analysis shines light on some approaches possible for targeting TcAANAT0 and other AANATs for novel insecticide design.

Circadian regulation in the retina: From molecules to network

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

Roles of Direct Photoreception and the Internal Circadian Oscillator in the Regulation of Melatonin Secretion in the Pineal Organ of the Domestic Turkey: A Novel In Vitro Clock and Calendar Model

The regulation of melatonin secretion in the avian pineal organ is highly complex and shows prominent interspecies differences. The aim of this study was to determine the roles of direct photoreception and the internal oscillator in the regulation of melatonin secretion in the pineal organ of the domestic turkey. The pineal organs were collected from 12-, 13- and 14-week-old female turkeys reared under a 12 L:12 D cycle with the photophase from 07.00 to 19.00, and were incubated in superfusion culture for 3-6 days. The cultures were subjected to different light conditions including 12 L:12 D cycles with photophases between 07.00 and 19.00, 13.00 and 01.00 or 01.00 and 13.00, a reversed cycle 12 D:12 L, cycles with long (16 L:8 D) and short (8 L:16 D) photophases, and continuous darkness or illumination. The pineal organs were also exposed to light pulses of variable duration during incubation in darkness or to periods of darkness during the photophase. The secretion of melatonin was determined by direct radioimmunoassay. The turkey pineal organs secreted melatonin in a well-entrained diurnal rhythm with a very high amplitude. Direct photoreception as an independently acting mechanism was able to ensure quick and precise adaptation of the melatonin secretion rhythm to changes in light-dark conditions. The pineal organs secreted melatonin in circadian rhythms during incubation in continuous darkness or illumination. The endogenous oscillator of turkey pinealocytes was able to acquire and store information about the light-dark cycle and then to generate the circadian rhythm of melatonin secretion in continuous darkness according to the stored data. The obtained data suggest that the turkey pineal gland is highly autonomous in the generation and regulation of the melatonin secretion rhythm. They also demonstrate that the turkey pineal organ in superfusion culture is a valuable model for chronobiological studies, providing a highly precise clock and calendar. This system has several features which make it an attractive alternative to other avian pineal glands for circadian studies.

Temporal expression of genes coding for aryl-alkamine-N-acetyltransferase and melatonin receptors in circadian clock tissues: Circadian rhythm dependent role of melatonin in seasonal responses

We investigated at the transcriptional level the role of daily rhythm in melatonin secretion in seasonal responses in the migratory blackheaded bunting (Emberiza melanocephala), which when exposed to short (SP) and long (LP) photoperiods exhibits distinct seasonal life-history states (LHSs). We reproduced the seasonal LHS by subjecting buntings to SP (8 h light: 16 h darkness, 8 L:16D), which maintained the nonmigratory/ nonbreeding phenotype, and to LP (16 L:8D), which induced the premigratory/ prebreeding, migratory/ breeding and nonmigratory/ postbreeding phenotypes. Plasma melatonin measured at 4 h intervals showed loss of the daily rhythm in the LP-induced premigratory/ prebreeding and migratory/ breeding LHSs. Subsequently, mRNA expression of genes coding for the aryl-alkamine-N-acetyltransferase (AANAT; the rate-liming enzyme of melatonin biosynthesis) and for the receptors for melatonin (Mel1A, Mel1B and Mel1C) was examined in the retina, pineal and hypothalamus; the interacting independent circadian clocks comprising the songbird circadian timing system. Except AANAT that was not amplified in the hypothalamus, we found significant alterations in both, the level and persistence of 24 h rhythm in mRNA expression of all genes, albeit with photoperiod and seasonal differences between three circadian clock tissues. Particularly, 24 h mRNA expression pattern of all genes, except retinal Mel1A, lacked a significant daily rhythm in the LP-induced migratory/ breeding LHS. These results underscore the overall importance of the circadian rhythm in the role of melatonin in photoperiodically-controlled seasonal responses in migratory songbirds.

Molecular Evolution of Tryptophan Hydroxylases in Vertebrates: A Comparative Genomic Survey

Serotonin is a neurotransmitter involved in various physiological processes in the central and peripheral nervous systems. Serotonin is also a precursor for melatonin biosynthesis, which mainly occurs in the pineal gland of vertebrates. Tryptophan hydroxylase (TPH) acts as the rate-limiting enzyme in serotonin biosynthesis and is the initial enzyme involved in the synthesis of melatonin. Recently, two enzymes—TPH1 and TPH2—were reported to form the TPH family in vertebrates and to play divergent roles in serotonergic systems. Here, we examined the evolution of the TPH family from 70 vertebrate genomes. Based on the sequence similarity, we extracted 184 predicted tph homologs in the examined vertebrates. A phylogenetic tree, constructed on the basis of these protein sequences, indicated that tph genes could be divided into two main clades (tph1 and tph2), and that the two clades were further split into two subgroups of tetrapods and Actinopterygii. In tetrapods, and some basal non-teleost ray-finned fishes, only two tph isotypes exist. Notably, tph1 in most teleosts that had undergone the teleost-specific genome duplication could be further divided into tph1a and tph1b. Moreover, protein sequence comparisons indicated that TPH protein changes among vertebrates were concentrated at the NH₂-terminal. The tertiary structures of TPH1 and TPH2 revealed obvious differences in the structural elements. Five positively selected sites were characterized in TPH2 compared with TPH1; these sites may reflect the functional divergence in enzyme activity and substrate specificity. In summary, our current work provides novel insights into the evolution of tph genes in vertebrates from a comprehensive genomic perspective.

Effect of Monochromatic Light on Circadian Rhythm of Clock Genes in Chick Pinealocytes

The avian circadian system is a complex of mutually coupled pacemakers residing in pineal gland, retina and suprachiasmatic nucleus. In this study, the self-regulation mechanism of pineal circadian rhythm was investigated by culturing chick primary pinealocytes exposed to red light (RL), green light (GL), blue light (BL), white light (WL) and constant darkness (DD), respectively. All illuminations were set up with a photoperiod of 12 light: 12 dark. The 24-h expression profiles of seven core clock genes (cBmal1/2, cClock, cCry1/2 and cPer2/3), cAanat and melatonin showed significant circadian oscillation in all groups, except for the loss of cCry1 rhythm in BL. Compared to WL, GL increased the amplitudes and mesors of positive elements (cClock and cBmal1/2) and reduced those of negative elements (cCry1/2 and cPer2/3), in contrast to RL. The temporal patterns of cAanatmRNA and melatonin secretion have always been consistent with the positive genes. Besides, GL advanced the acrophases of the positive elements, cAanat and melatonin, but RL and BL showed the opposite effect. Thereby, GL could promote the secretion of melatonin by enhancing the expressions of positive clock genes and repressing the expressions of negative clock genes.

A Comparative Genomic Survey Provides Novel Insights into Molecular Evolution of l-Aromatic Amino Acid Decarboxylase in Vertebrates

Melatonin is a pleiotropic molecule with various important physiological roles in vertebrates. l-aromatic amino acid decarboxylase (AAAD) is the second enzyme for melatonin synthesis. By far, a clear-cut gene function of AAAD in the biosynthesis of melatonin has been unclear in vertebrates. Here, we provide novel insights into the evolution of AAAD based on 77 vertebrate genomes. According to our genome-wide alignments, we extracted a total of 151 aaad nucleotide sequences. A phylogenetic tree was constructed on the basis of these sequences and corresponding protein alignments, indicating that tetrapods and diploid bony fish genomes contained one aaad gene and a new aaad-like gene, which formed a novel AAAD family. However, in tetraploid teleosts, there were two copies of the aaad gene due to whole genome duplication. A subsequent synteny analysis investigated 81 aaad sequences and revealed their collinearity and systematic evolution. Interestingly, we discovered that platypus (Ornithorhynchus anatinus), Atlantic cod (Guadus morhua), Mexican tetra (Astyanax mexicanus), and a Sinocyclocheilus cavefish (S. anshuiensis) have long evolutionary branches in the phylogenetic topology. We also performed pseudogene identification and selection pressure analysis; however, the results revealed a deletion of 37 amino acids in Atlantic cod and premature stop codons in the cave-restricted S. anshuiensis and A. mexicanus, suggesting weakening or disappearing rhythms in these cavefishes. Selective pressure analysis of aaad between platypus and other tetrapods showed that rates of nonsynonymous (Ka) and synonymous (Ks) substitutions were higher when comparing the platypus to other representative tetrapods, indicating that, in this semiaquatic mammal, the aaad gene experienced selection during the process of evolution. In summary, our current work provides novel insights into aaad genes in vertebrates from a genome-wide view.

Seeing the light to change colour: An evolutionary perspective on the role of melanopsin in neuroendocrine circuits regulating light-mediated skin pigmentation

Melanopsin photopigments, Opn4x and Opn4m, were evolutionary selected to "see the light" in systems that regulate skin colour change. In this review, we analyse the roles of melanopsins, and how critical evolutionary developments, including the requirement for thermoregulation and ultraviolet protection, the emergence of a background adaptation mechanism in land-dwelling amphibian ancestors and the loss of a photosensitive pineal gland in mammals, may have helped sculpt the mechanisms that regulate light-controlled skin pigmentation. These mechanisms include melanopsin in skin pigment cells directly inducing skin darkening for thermoregulation/ultraviolet protection; melanopsin-expressing eye cells controlling neuroendocrine circuits to mediate background adaptation in amphibians in response to surface-reflected light; and pineal gland secretion of melatonin phased to environmental illuminance to regulate circadian and seasonal variation in skin colour, a process initiated by melanopsin-expressing eye cells in mammals, and by as yet unknown non-visual opsins in the pineal gland of non-mammals.

Daily Rhythm in Plasma N-acetyltryptamine

Normal physiology undergoes 24-h changes in function that include daily rhythms in circulating hormones, most notably melatonin and cortical steroids. This study focused on N-acetyltryptamine, a little-studied melatonin receptor mixed agonist-antagonist and the likely evolutionary precursor of melatonin. The central issue addressed was whether N-acetyltryptamine is physiologically present in the circulation. N-acetyltryptamine was detected by LC-MS/MS in daytime plasma of 3 different mammals in subnanomolar levels (mean ± SEM: rat, 0.29 ± 0.05 nM, n = 5; rhesus macaque, 0.54 ± 0.24 nM, n = 4; human, 0.03 ± 0.01 nM, n = 32). Analysis of 24-h blood collections from rhesus macaques revealed a nocturnal increase in plasma N-acetyltryptamine (p < 0.001), which varied from 2- to 15-fold over daytime levels among the 4 animals studied. Related RNA sequencing studies indicated that the transcript encoding the tryptamine acetylating enzyme arylalkylamine N-acetyltransferase (AANAT) is expressed at similar levels in the rhesus pineal gland and retina, thereby indicating that either tissue could contribute to circulating N-acetyltryptamine. The evidence that N-acetyltryptamine is a physiological component of mammalian blood and exhibits a daily rhythm, together with known effects as a melatonin receptor mixed agonist-antagonist, shifts the status of N-acetyltryptamine from pharmacological tool to candidate for a physiological role. This provides a new opportunity to extend our understanding of 24-h biology.

Retinal Circadian Clocks and Control of Retinal Physiology

Retinas of all classes of vertebrates contain endogenous circadian clocks that control many aspects of retinal physiology, including retinal sensitivity to light, neurohormone synthesis, and cellular events such as rod disk shedding, intracellular signaling pathways, and gene expression. The vertebrate retina is an example of a “peripheral” oscillator that is particularly amenable to study because this tissue is well characterized, the relationships between the various cell types are extensively studied, and many local clock-controlled rhythms are known. Although the existence of a photoreceptor clock is well established in several species, emerging data are consistent with multiple or dual oscillators within the retina that interact to control local physiology. Aprominent example is the antiphasic regulation of melaton in and dopamine in photoreceptors and inner retina, respectively. This review focuses on the similarities and differences in the molecular mechanisms of the retinal versus the SCN oscillators, as well as on the expression of core components of the circadian clockwork in retina. Finally, the interactions between the retinal clock(s) and the master clock in the SCN are examined.

Degradation of Serotonin N-Acetyltransferase, a Circadian Regulator, by the N-end Rule Pathway

Serotonin N-acetyltransferase (AANAT) converts serotonin to N-acetylserotonin (NAS), a distinct biological regulator and the immediate precursor of melatonin, a circulating hormone that influences circadian processes, including sleep. N-terminal sequences of AANAT enzymes vary among vertebrates. Mechanisms that regulate the levels of AANAT are incompletely understood. Previous findings were consistent with the possibility that AANAT may be controlled through its degradation by the N-end rule pathway. By expressing the rat and human AANATs and their mutants not only in mammalian cells but also in the yeast Saccharomyces cerevisiae, and by taking advantage of yeast genetics, we show here that two "complementary" forms of rat AANAT are targeted for degradation by two "complementary" branches of the N-end rule pathway. Specifically, the N(α)-terminally acetylated (Nt-acetylated) Ac-AANAT is destroyed through the recognition of its Nt-acetylated N-terminal Met residue by the Ac/N-end rule pathway, whereas the non-Nt-acetylated AANAT is targeted by the Arg/N-end rule pathway, which recognizes the unacetylated N-terminal Met-Leu sequence of rat AANAT. We also show, by constructing lysine-to-arginine mutants of rat AANAT, that its degradation is mediated by polyubiquitylation of its Lys residue(s). Human AANAT, whose N-terminal sequence differs from that of rodent AANATs, is longer-lived than its rat counterpart and appears to be refractory to degradation by the N-end rule pathway. Together, these and related results indicate both a major involvement of the N-end rule pathway in the control of rodent AANATs and substantial differences in the regulation of rodent and human AANATs that stem from differences in their N-terminal sequences.

Growth responses of broiler chickens to different periods of artificial light

This study aimed to establish response curves between broiler chicken growth parameters and artificial light periods, as opposed to optimizing a lighting regimen for broiler production. Medium-growing broiler chickens were illuminated for periods of 12, 14, 16, 18, 20, 22, or 24 h each day. The BW of the broilers were significantly influenced by light periods ( < 0.05). Moreover, BW responded to light periods in a linear fashion, suggesting that long light periods result in greater BW. In addition, a linear relationship was found between feed intake and light periods. However, the relationship between shank length and light period was quadratic. When the light period was too short (12 h) or too long (24 h), the light stimulus did not enhance shank growth in the broiler chickens ( < 0.05). In addition, a quadratic relationship between the quantity of abdominal adipose tissue and light period suggested that the quantity of abdominal adipose decreases when the period of the light stimulus was too short or too long ( < 0.05). Moreover, a broken-stick analysis suggested that the triiodothyronine (T3) concentration in the blood was minimally affected beyond 18 h ( = 0.267), although a quadratic relationship was found between the period (from 18 to 24 h) and T3 concentrations in the blood. The response curves established in the present study will be valuable for designing future lighting regimes for medium-growing broiler strains.

Natural melatonin fluctuation and its minimally invasive simulation in the zebra finch

Melatonin is a key hormone in the regulation of circadian rhythms of vertebrates, including songbirds. Understanding diurnal melatonin fluctuations and being able to reverse or simulate natural melatonin levels are critical to investigating the influence of melatonin on various behaviors such as singing in birds. Here we give a detailed overview of natural fluctuations in plasma melatonin concentration throughout the night in the zebra finch. As shown in previous studies, we confirm that “lights off” initiates melatonin production at night in a natural situation. Notably, we find that melatonin levels return to daytime levels as early as two hours prior to the end of the dark-phase in some individuals and 30 min before “lights on” in all animals, suggesting that the presence of light in the morning is not essential for cessation of melatonin production in zebra finches. Thus, the duration of melatonin production seems not to be specified by the length of night and might therefore be less likely to directly couple circadian and annual rhythms. Additionally, we show that natural melatonin levels can be successfully simulated through a combination of light-treatment (daytime levels during subjective night) and the application of melatonin containing skin-cream (nighttime levels during subjective day). Moreover, natural levels and their fluctuation in the transition from day to night can be imitated, enabling the decoupling of the effects of melatonin, for example on neuronal activity, from sleep and circadian rhythmicity. Taken together, our high-resolution profile of natural melatonin levels and manipulation techniques open up new possibilities to answer various melatonin related questions in songbirds.

The Retina and Other Light-sensitive Ocular Clocks

Ocular clocks, first identified in the retina, are also found in the retinal pigment epithelium (RPE), cornea, and ciliary body. The retina is a complex tissue of many cell types and considerable effort has gone into determining which cell types exhibit clock properties. Current data suggest that photoreceptors as well as inner retinal neurons exhibit clock properties with photoreceptors dominating in nonmammalian vertebrates and inner retinal neurons dominating in mice. However, these differences may in part reflect the choice of circadian output, and it is likely that clock properties are widely dispersed among many retinal cell types. The phase of the retinal clock can be set directly by light. In nonmammalian vertebrates, direct light sensitivity is commonplace among body clocks, but in mice only the retina and cornea retain direct light-dependent phase regulation. This distinguishes the retina and possibly other ocular clocks from peripheral oscillators whose phase depends on the pace-making properties of the hypothalamic central brain clock, the suprachiasmatic nuclei (SCN). However, in mice, retinal circadian oscillations dampen quickly in isolation due to weak coupling of its individual cell-autonomous oscillators, and there is no evidence that retinal clocks are directly controlled through input from other oscillators. Retinal circadian regulation in both mammals and nonmammalian vertebrates uses melatonin and dopamine as dark- and light-adaptive neuromodulators, respectively, and light can regulate circadian phase indirectly through dopamine signaling. The melatonin/dopamine system appears to have evolved among nonmammalian vertebrates and retained with modification in mammals. Circadian clocks in the eye are critical for optimum visual function where they play a role fine tuning visual sensitivity, and their disruption can affect diseases such as glaucoma or retinal degeneration syndromes.

Clock-Controlled Regulation of the Acute Effects of Norepinephrine on Chick Pineal Melatonin Rhythms

The chicken pineal gland synthesizes and releases melatonin rhythmically in light/dark (LD) cycles, with high melatonin levels during the dark phase, and in constant darkness (DD) for several cycles before it gradually damps to arrhythmicity in DD. Daily administration of norepinephrine (NE) in vivo and in vitro prevents the damping and restores the melatonin rhythm. To investigate the role of the circadian clock on melatonin rhythm damping and of its restoration by NE, the effects of NE administration at different phases of the melatonin cycle revealed a robust rhythm in NE sensitivity in which NE efficacy in increasing melatonin amplitude peaked in late subjective night and early subjective day, suggesting a clock underlying NE sensitivity. However, NE itself had no effect on circadian phase or period of the melatonin rhythms. Transcriptional analyses indicated that even though the rhythm of melatonin output damped to arrhythmicity, messenger RNA (mRNA) encoding clock genes gper2, gper3, gBmal1, gclock, gcry1, and gcry2; enzymes associated with melatonin biosynthesis; and enzymes involved in cyclic nucleotide signaling remained robustly rhythmic. Of these, only gADCY1 (adenylate cyclase 1) and gPDE4D (cAMP-specific 3',5'-cyclic phosphodiesterase 4D) were affected by NE administration at the mRNA levels, and only ADCY1 was affected at the protein level. The data strongly suggest that damping of the melatonin rhythm in the chick pineal gland occurs at the posttranscriptional level and that a major role of the clock is to regulate pinealocytes' sensitivity to neuronal input from the brain.

Structures and functions of insect arylalkylamine N-acetyltransferase (iaaNAT); a key enzyme for physiological and behavioral switch in arthropods

The evolution of N-acetyltransfeases (NATs) seems complex. Vertebrate arylalkylamine N-acetyltransferase (aaNAT) has been extensively studied since it leads to the synthesis of melatonin, a multifunctional neurohormone prevalent in photoreceptor cells, and is known as a chemical token of the night. Melatonin also serves as a scavenger for reactive oxygen species. This is also true with invertebrates. NAT therefore has distinct functional implications in circadian function, as timezymes (aaNAT), and also xenobiotic reactions (arylamine NAT or simply NAT). NATs belong to a broader enzyme group, the GCN5-related N-acetyltransferase superfamily. Due to low sequence homology and a seemingly fast rate of structural differentiation, the nomenclature for NATs can be confusing. The advent of bioinformatics, however, has helped to classify this group of enzymes; vertebrates have two distinct subgroups, the timezyme type and the xenobiotic type, which has a wider substrate range including imidazolamine, pharmacological drugs, environmental toxicants and even histone. Insect aaNAT (iaaNAT) form their own clade in the phylogeny, distinct from vertebrate aaNATs. Arthropods are unique, since the phylum has exoskeleton in which quinones derived from N-acetylated monoamines function in coupling chitin and arthropodins. Monoamine oxidase (MAO) activity is limited in insects, but NAT-mediated degradation prevails. However, unexpectedly iaaNAT occurs not only among arthropods but also among basal deuterostomia, and is therefore more apomorphic. Our analyses illustrate that iaaNATs has unique physiological roles but at the same time it plays a role in a timezyme function, at least in photoperiodism. Photoperiodism has been considered as a function of circadian system but the detailed molecular mechanism is not well understood. We propose a molecular hypothesis for photoperiodism in Antheraea pernyi based on the transcription regulation of NAT interlocked by the circadian system. Therefore, the enzyme plays both unique and universal roles in insects. The unique role of iaaNATs in physiological regulation urges the targeting of this system for integrated pest management (IPM). We indeed showed a successful example of chemical compound screening with reconstituted enzyme and further attempts seem promising.

N-acetylserotonin: circadian activation of the BDNF receptor and neuroprotection in the retina and brain

TrkB is the cognate receptor for brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family involved in neuronal survival, neurogenesis and synaptic plasticity. BDNF has been shown to protect photoreceptors from light-induced retinal degeneration (LIRD) and to improve ganglion cell survival following optic nerve damage. However, the utility of BDNF as a retinal neuroprotectant is limited by its short half-life, inability to cross the blood-brain and blood-retinal barriers, and activation of the proapoptotic p75 neurotrophin receptor. N-Acetylserotonin (NAS) is a naturally occurring chemical intermediate in the melatonin biosynthetic pathway in the pineal gland and retina. Its synthesis occurs in a circadian fashion with high levels at night and is suppressed by light exposure. Until recently, NAS was thought to function primarily as a melatonin precursor with little or no biological function of its own. We have now shown that TrkB activation in the retina and hippocampus is circadian in C3H/f(+/+) mice, which synthesize NAS, but not in C57BL/6 mice, which have a mutation in the gene encoding the enzyme that converts serotonin to NAS. In addition, treatment of mice exogenous NAS, but not with serotonin or melatonin, activates TrkB during the daytime in a BDNF-independent manner. NAS appears to have neuroprotective properties and its administration reduces caspase 3 activation in the brain in response to kainic acid, a neurotoxic glutamate analog. We have developed structural analogs of NAS that activate TrkB. One of these derivatives, N- [2-(-indol-3-yl)ethyl]-2-oxopiperideine-3-carboximide (HIOC), selectively activates TrkB with greater potency than NAS and has a significantly 5-hydroxy-1Hlonger biological half-life than NAS after systemic administration. HIOC administration results in long-lasting activation of TrkB and downstream signaling kinases. The compound can pass the blood-brain and blood-retinal barriers when administered systemically and reduces kainic acid-induced neuronal cell death in a TrkB-dependent manner. Systemic administration of HIOC mitigates LIRD, assessed electrophysiologically and morphometrically. Hence, NAS may function as an endogenous circadian neurotrophin-like compound and HIOC is a good lead compound for further drug development for treatment of retinal degenerative diseases.

Avian circadian organization: a chorus of clocks

In birds, biological clock function pervades all aspects of biology, controlling daily changes in sleep: wake, visual function, song, migratory patterns and orientation, as well as seasonal patterns of reproduction, song and migration. The molecular bases for circadian clocks are highly conserved, and it is likely the avian molecular mechanisms are similar to those expressed in mammals, including humans. The central pacemakers in the avian pineal gland, retinae and SCN dynamically interact to maintain stable phase relationships and then influence downstream rhythms through entrainment of peripheral oscillators in the brain controlling behavior and peripheral tissues. Birds represent an excellent model for the role played by biological clocks in human neurobiology; unlike most rodent models, they are diurnal, they exhibit cognitively complex social interactions, and their circadian clocks are more sensitive to the hormone melatonin than are those of nocturnal rodents.

Pineal oscillator functioning in the chicken--effect of photoperiod and melatonin

The avian pineal gland, apart from the hypothalamic master clock (suprachiasmatic nuclei, SCN) and retina, functions as an independent circadian oscillator, receiving external photic cues that it translates into the rhythmical synthesis of melatonin, a biochemical signal of darkness. Functional similarity to the mammalian SCN makes the avian pineal gland a convenient model for studies on biological clock mechanisms in general. Pineal melatonin is produced not only in a light-dependent manner but also remains under the control of the endogenous oscillator, while the possible involvement of melatonin in maintaining cyclic expression of the avian clock genes remains to be elucidated. The aim of the present study was to characterize the diurnal profiles of main clock genes transcription in the pineal glands of chickens exposed to continuous light (LL) and supplemented with exogenous melatonin. We hypothesized that rearing chickens from the day of hatch under LL conditions would evoke a functional pinealectomy, influencing, in turn, pineal clock function. To verify this hypothesis, we examined the diurnal transcriptional profiles of selected clock genes as well as the essential parameters of pineal gland function: transcription of the genes encoding arylalkylamine N-acetyltransferase (Aanat), a key enzyme in melatonin biosynthesis, and the melatonin receptor (Mel1c), along with the blood melatonin level. Chickens hatched in summer or winter were maintained under LD 16:8 and 8:16, corresponding to the respective photoperiods, as the seasonal control groups. Another set of chickens was kept in parallel under LL conditions and some were supplemented with melatonin to check the ability of exogenous hormone to antagonize the effects evoked by continuous light. Twelve-day-old chickens were sacrificed every 3 h over a 24-h period and the mRNAs of selected clock genes, Bmal1, Cry1, Per3, E4bp4, together with those of Aanat and Mel1c, were quantified in the isolated pineal glands. Our results indicate that the profiles of clock gene transcription are not dependent on the duration of the light phase, while LL conditions decrease the amplitude of diurnal changes, but do not abolish them entirely. Melatonin supplied in drinking water to the birds kept in LL seems to desynchronize transcription of the majority of clock genes in the summer, while in the winter, it restores the pattern, but not the diurnal rhythmicity. Rhythmic expression of Bmal1 appears to provide a direct link between the circadian clock and the melatonin output pathway, while the availability of cyclic melatonin is clearly involved in the canonical transcription pattern of Per3 in the chicken pineal gland. Regardless of the experimental conditions, a negative correlation was identified between the transcription of genes involved in melatonin biosynthesis (Aanat) and melatonin signal perception (Mel1c receptor).

Negative impact of melatonin ingestion on the photopic electroretinogram of dogs

Melatonin follows a circadian rhythm entrained by the light/dark cycle and plays a role in promoting light sensitivity at night. It has been suggested that melatonin and dopamine reciprocal inhibition may contribute to the switch between day and night vision. The purpose of this study was to investigate the impact of a high dose of melatonin administration on the photopic and scotopic electroretinogram (ERG) of dogs in the daytime, when it is not thought to be present. Photopic and scotopic ERG luminance response functions were obtained from 7 anaesthetized beagle dogs (3 males and 4 females), once without melatonin (control) and once after oral administration of melatonin (90 mg/dog). Vmax (maximal b-wave amplitude achieved) and logK (retinal sensitivity) were calculated from the derived luminance response function. Photopic flicker ERG was also recorded. In photopic condition, a-wave amplitude (control: -126.90 μV; with melatonin: -49.64 μV; p<0.001) and Vmax (control: 252.50 μV; with melatonin: 115.40 μV; p<0.001) were decreased. A significant reduction of the photopic flicker ERG amplitude was observed after melatonin ingestion. In scotopic condition, an overall difference was reported before and after melatonin ingestion for the a- and b-wave amplitude, but no change was significant for Vmax. Melatonin ingestion at a high dose during the day decreases the photopic amplitude of a- and b-wave, but has no impact on implicit time. This negative impact of melatonin on photopic system may serve to promote night vision.

Visual photoreceptor subtypes in the chicken retina: melatonin-synthesizing activity and in vitro differentiation

The chicken retina contains five visual photoreceptor subtypes, based on the specific opsin gene they express. In addition to the central role they play in vision, some or all of these photoreceptors translate photoperiodic information into a day-night rhythm of melatonin production. This indolic hormone plays an important role in the photoperiodic regulation of retinal physiology. Previous studies have stopped short of establishing whether melatonin synthesis takes place in all the photoreceptor spectral subtypes. Another issue that has been left unsettled by previous studies is when during development are retinal precursor cells committed to a specific photoreceptor subtype and to a melatoninergic phenotype? To address the first question, in situ hybridization of the five opsins was combined with immunofluorescent detection of the melatonin-synthesizing enzyme hydroxyindole O-methyltransferase (HIOMT, EC.2.1.1.4). Confocal microscopy clearly indicated that all photoreceptor spectral subtypes are involved in melatonin synthesis. To tackle the second question, retinal precursor cells were dissociated between embryonic day 6 (E6) and E13 and cultured in serum-free medium for 4 days to examine their ability to autonomously activate the expression of opsins and HIOMT. Real-time PCR on cultured precursors indicated that red-, green- and violet-sensitive cones are committed at E6, rods at E10 and blue-sensitive cones at E12. HIOMT gene expression was programmed at E6, probably reflecting the differentiation of early cones. The present study provides a better characterization of photoreceptor subtypes in the chicken retina and describes a combination of serum-free culture and real-time PCR that should facilitate further developmental studies.

A review on animal models for screening potential anti-stress agents

Stress is a state of threatened homeostasis that produces different physiological as well as pathological changes depending on severity, type and duration of stress. The animal models are pivotal for understanding the pathophysiology of stress-induced behavioral alterations and development of effective therapy for its optimal management. A battery of models has been developed to simulate the clinical pain conditions with diverse etiology. An ideal animal model should be able to reproduce each of the aspects of stress response and should be able to mimic the natural progression of the disease. The present review describes the different types of acute and chronic stress models including immersion in cold water with no escape, cold environment isolation, immobilization/restraint-induced stress, cold-water restraint stress, electric foot shock-induced stress, forced swimming-induced stress, food-deprived activity stress, neonatal isolation-induced stress, predatory stress, day-night light change-induced stress, noise-induced stress, model of post-traumatic stress disorder and chronic unpredictable stress models.

Melatonin synthesis in retina: cAMP-dependent transcriptional regulation of chicken arylalkylamine N-acetyltransferase by a CRE-like sequence and a TTATT repeat motif in the proximal promoter

Arylalkylamine N-acetyltransferase (AANAT) is the key regulatory enzyme controlling the daily rhythm of melatonin biosynthesis. In chicken retinal photoreceptor cells, Aanat transcription and AANAT activity are regulated in part by cAMP-dependent mechanisms. The purpose of this study was to identify regulatory elements within the chicken Aanat promoter responsible for cAMP-dependent induction. Photoreceptor-enriched retinal cell cultures were transfected with a luciferase reporter construct containing up to 4 kb of 5'-flanking region and the first exon of Aanat. Forskolin treatment stimulated luciferase activity driven by the ∼4 kb promoter construct and by all 5'-deletion constructs except the smallest, Aanat (-217 to +120)luc. Maximal basal and forskolin-stimulated expression levels were generated by the Aanat (-484 to +120)luc construct. This construct lacks a canonical cyclic AMP-response element (CRE), but contains two other potentially important elements in its sequence: an eight times TTATT repeat (TTATT₈) and a CRE-like sequence. Electrophoretic mobility shift assays, luciferase reporter assays, chromatin immunoprecipitation, and siRNA experiments provide evidence that these elements bind c-Fos, JunD, and CREB to enhance basal and forskolin-stimulated Aanat transcription. We propose that the CRE-like sequence and TTATT₈ elements in the 484 bp proximal promoter interact to mediate cAMP-dependent transcriptional regulation of Aanat.

Circadian melatonin production develops faster in birds than in mammals

The development of circadian rhythmicity of melatonin biosynthesis in the pineal gland starts during embryonic period in birds while it is delayed to the postnatal life in mammals. Daily rhythms of melatonin in isolated pinealocytes and in intact pineal glands under in vivo conditions were demonstrated during the last third of embryonic development in chick embryos, with higher levels during the dark (D) than during the light (L) phase. In addition to the LD cycle, rhythmic temperature changes with the amplitude of 4.5°C can entrain rhythmic melatonin biosynthesis in chick embryos, with higher concentrations found during the low-temperature phase (33.0 vs 37.5°C). Molecular clockwork starts to operate during the embryonic life in birds in line with the early development of melatonin rhythmicity. Expression of per2 and cry genes is rhythmic at least at day 16 and 18, respectively, and the circadian system operates in a mature-like manner soon after hatching. Rhythmic oscillations are detected earlier in the central oscillator (the pineal gland) than in the peripheral structures, reflecting the synchronization of individual cells which is necessary for detection of the rhythm. The early development of the circadian system in birds reflects an absence of rhythmic maternal melatonin which in mammals synchronizes physiological processes of offspring. Developmental consequences of modified development of circadian system for its stability later in development are not known and should be studied.

CLOCK and NPAS2 have overlapping roles in the circadian oscillation of arylalkylamine N-acetyltransferase mRNA in chicken cone photoreceptors

Circadian clocks in vertebrates are thought to be composed of transcriptional-translational feedback loops involving a highly conversed set of 'clock genes' namely, period (Per1-3) and cryptochrome (Cry1-2), which encode negative transcriptional regulators; and Bmal1, Clock, and Npas2, which encode positive regulators. Aanat, which encodes arylalkylamine N-acetyltransferase (AANAT), the key regulatory enzyme that drives the circadian rhythm of melatonin synthesis, contains a circadian E-box element (CACGTG) in its proximal promoter that is potentially capable of binding CLOCK : BMAL1 and NPAS2 : BMAL1 heterodimers. The present study was conducted to investigate whether CLOCK and/or NPAS2 regulates Aanat expression in photoreceptor cells. Npas2 and Clock are both expressed in photoreceptor cells in vivo and in vitro. To assess the roles of CLOCK and NPAS2 in Aanat expression, gene-specific micro RNA vectors were used to knock down expression of these clock genes in photoreceptor-enriched cell cultures. The knockdown of CLOCK protein significantly reduced the circadian expression of Npas2, Per2, and Aanat transcripts but had no effect on the circadian rhythm of Bmal1 transcript level. The knockdown of NPAS2 significantly damped the circadian rhythm of Aanat mRNAs but had no effect on circadian expression of any of clock genes examined, except Npas2 itself. Chromatin immunoprecipitation studies indicated that both CLOCK and NPAS2 bound to the Aanat promoter in situ. Thus, CLOCK and NPAS2 have overlapping roles in the clock output pathway that regulates the rhythmic expression of Aanat in photoreceptors. However, CLOCK plays the predominant role in the chicken photoreceptor circadian clockwork mechanism, including the regulation of NPAS2 expression.

Physiology and pharmacology of melatonin in relation to biological rhythms

Melatonin is an evolutionarily conserved molecule that serves a time-keeping function in various species. In vertebrates, melatonin is produced predominantly by the pineal gland with a marked circadian rhythm that is governed by the central circadian pacemaker (biological clock) in the suprachiasmatic nuclei of the hypothalamus. High levels of melatonin are normally found at night, and low levels are seen during daylight hours. As a consequence, melatonin has been called the "darkness hormone". This review surveys the current state of knowledge regarding the regulation of melatonin synthesis, receptor expression, and function. In particular, it addresses the physiological, pathological, and therapeutic aspects of melatonin in humans, with an emphasis on biological rhythms.

Impact of oral melatonin on the electroretinogram cone response

BACKGROUND

In the eye, melatonin plays a role in promoting light sensitivity at night and modulating many aspects of circadian retinal physiology. It is also an inhibitor of retinal dopamine, which is a promoter of day vision through the cone system. Consequently, it is possible that oral melatonin (an inhibitor of retinal dopamine) taken to alleviate circadian disorders may affect cone functioning. Our aim was to assess the impact of melatonin on the cone response of the human retina using electroretinography (ERG).

METHODS

Twelve healthy participants aged between 18 to 52 years old were submitted to a placebo-controlled, double-blind, crossover, and counterbalanced-order design. The subjects were tested on 2 sessions beginning first with a baseline ERG, followed by the administration of the placebo or melatonin condition and then, 30 min later, a second ERG to test the effect.

RESULTS

Following oral melatonin administration, a significant decrease of about 8% of the cone maximal response was observed (mean 6.9 muV +/- SEM 2.0; P = 0.0065) along with a prolonged b-wave implicit time of 0.4 ms +/- 0.1, 50 minutes after ingestion.

CONCLUSION

Oral melatonin appears to reach the eye through the circulation. When it is administered at a time of day when it is not usually present, melatonin appears to reduce input to retinal cones. We believe that the impact of melatonin on retinal function should be taken into consideration when used without supervision in chronic self-medication for sleep or circadian disorder treatment.

Rhythmic expression of microRNA-26a regulates the L-type voltage-gated calcium channel alpha1C subunit in chicken cone photoreceptors

MicroRNAs (miRNAs) modulate gene expression by degrading or inhibiting translation of messenger RNAs (mRNAs). Here, we demonstrated that chicken microRNA-26a (gga-mir-26a) is a key posttranscriptional regulator of photoreceptor L-type voltage-gated calcium channel alpha1C subunit (L-VGCCalpha1C) expression, and its own expression has a diurnal rhythm, thereby explaining the rhythmic nature of L-VGCCalpha1Cs. Circadian oscillators in retinal photoreceptors provide a mechanism that allows photoreceptors to anticipate daily illumination changes. In photoreceptors, L-VGCC activities are under circadian control, which are higher at night and lower during the day. Interestingly, the mRNA level of VGCCalpha1D oscillates, but those for VGCCalpha1C do not. However, the protein expression of both VGCCalpha1C and alpha1D are higher at night in cone photoreceptors. The underlying mechanism regulating L-VGCCalpha1C protein expression was not clear until now. In vitro targeting reporter assays verified that gga-mir-26a specifically targeted the L-VGCCalpha1C 3'-untranslated region, and gga-mir-26a expression in the retina peaked during the day. After transfection with gga-mir-26a, L-VGCCalpha1C protein expression and L-VGCC current density decreased. Therefore, the rhythmic expression of gga-mir-26a regulated the protein expression of the L-VGCCalpha1C subunit. Additionally, both CLOCK (circadian locomoter output cycles kaput) and CREB (cAMP-response element-binding protein-1) activated gga-mir-26a expression in vitro. This result implies that gga-mir-26a might be a downstream target of circadian oscillators. Our work has uncovered new functional roles for miRNAs in the regulation of circadian rhythms in cone photoreceptors. Circadian regulated miRNAs could serve as the link between the core oscillator and output signaling that further govern biological functions.

Circadian regulation of ion channels and their functions

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

Pineal melatonin acts as a circadian zeitgeber and growth factor in chick astrocytes

Melatonin is rhythmically synthesized and released by the avian pineal gland and retina during the night, targeting an array of tissues and affecting a variety of physiological and behavioral processes. Among these targets, astrocytes express two melatonin receptor subtypes in vitro, the Mel(1A) and Mel(1C) receptors, which play a role in regulating metabolic activity and calcium homeostasis in these cells. Molecular characterization of chick astrocytes has revealed the expression of orthologs of the mammalian clock genes including clock, cry1, cry2, per2, and per3. To test the hypothesis that pineal melatonin entrains molecular clockworks in downstream cells, we asked whether coculturing astrocytes with pinealocytes or administration of exogenous melatonin cycles would entrain metabolic rhythms of 2-deoxy [14C]-glucose (2DG] uptake and/or clock gene expression in cultured astrocytes. Rhythmic secretion of melatonin from light-entrained pinealocytes in coculture as well as cyclic administration of exogenous melatonin entrained rhythms of 2DG uptake and expression of Gallus per2 (gper2) and/or gper3, but not of gcry1 mRNA. Surprisingly, melatonin also caused a dose-dependent increase in mitotic activity of astrocytes, both in coculture and when administered exogenously. The observation that melatonin stimulates mitotic activity in diencephalic astrocytes suggests a trophic role of the hormone in brain development. The data suggest a dual role for melatonin in avian astrocytes: synchronization of rhythmic processes in these cells and regulation of growth and differentiation. These two processes may or may not be mutually exclusive.

Night/day changes in pineal expression of >600 genes: central role of adrenergic/cAMP signaling

The pineal gland plays an essential role in vertebrate chronobiology by converting time into a hormonal signal, melatonin, which is always elevated at night. Here we have analyzed the rodent pineal transcriptome using Affymetrix GeneChip(R) technology to obtain a more complete description of pineal cell biology. The effort revealed that 604 genes (1,268 probe sets) with Entrez Gene identifiers are differentially expressed greater than 2-fold between midnight and mid-day (false discovery rate <0.20). Expression is greater at night in approximately 70%. These findings were supported by the results of radiochemical in situ hybridization histology and quantitative real time-PCR studies. We also found that the regulatory mechanism controlling the night/day changes in the expression of most genes involves norepinephrine-cyclic AMP signaling. Comparison of the pineal gene expression profile with that in other tissues identified 334 genes (496 probe sets) that are expressed greater than 8-fold higher in the pineal gland relative to other tissues. Of these genes, 17% are expressed at similar levels in the retina, consistent with a common evolutionary origin of these tissues. Functional categorization of the highly expressed and/or night/day differentially expressed genes identified clusters that are markers of specialized functions, including the immune/inflammation response, melatonin synthesis, photodetection, thyroid hormone signaling, and diverse aspects of cellular signaling and cell biology. These studies produce a paradigm shift in our understanding of the 24-h dynamics of the pineal gland from one focused on melatonin synthesis to one including many cellular processes.

The circadian clock system in the mammalian retina

Daily rhythms are a ubiquitous feature of living systems. Generally, these rhythms are not just passive consequences of cyclic fluctuations in the environment, but instead originate within the organism. In mammals, including humans, the master pacemaker controlling 24-hour rhythms is localized in the suprachiasmatic nuclei of the hypothalamus. This circadian clock is responsible for the temporal organization of a wide variety of functions, ranging from sleep and food intake, to physiological measures such as body temperature, heart rate and hormone release. The retinal circadian clock was the first extra-SCN circadian oscillator to be discovered in mammals and several studies have now demonstrated that many of the physiological, cellular and molecular rhythms that are present within the retina are under the control of a retinal circadian clock, or more likely a network of hierarchically organized circadian clocks that are present within this tissue. BioEssays 30:624-633, 2008. (c) 2008 Wiley Periodicals, Inc.

Ontogeny of melatonin, Per2 and E4bp4 light responsiveness in the chicken embryonic pineal gland

The chicken pineal gland possesses the capacity to generate circadian oscillations, is able to synchronize to external light:dark cycles and can generate an hormonal output--melatonin. We examined the light responses of the chicken pineal gland and its effects on melatonin and Per2, Bmal1 and E4bp4 expression in 19-day old embryos and hatchlings during the dark phase, subjective light phase and in constant darkness. Expression of Per2 and E4bp4 were rhythmic under light:dark conditions, but the rhythms of E4bp4 and Bmal1 mRNA did not persist in constant darkness in 19-day old embryos. Per2 mRNA expression persisted in constant darkness, but with a reduced amplitude. Per2 expression was inducible by light only during the subjective day. Melatonin release was inhibited by light only at end of the dark phase and during the subjective light phase in embryos. Our data demonstrate that the embryonic avian pineal pacemaker is light sensitive and can generate rhythmic output, however the effects of light were diminished in chick embryos in compared to hatchlings.

The expression of L-type voltage-gated calcium channels in retinal photoreceptors is under circadian control

Photoreceptors are non-spiking neurons, and their synapses mediate the continuous release of neurotransmitters under the control of L-type voltage-gated calcium channels (VGCCs). Photoreceptors express endogenous circadian oscillators that play important roles in regulating photoreceptor physiology and function. Here, we report that the L-type VGCCs in chick cone photoreceptors are under circadian control. The L-type VGCC currents are greater when measured during the subjective night than during the subjective day. Using antibodies against the VGCCalpha1C and VGCCalpha1D subunits, we found that the immunofluorescence intensities of both VGCCalpha1C and VGCCalpha1D in photoreceptors are higher during the subjective night. However, the mRNA levels of VGCCalpha1D, but not VGCCalpha1C, are rhythmic. Nocturnal increases in L-type VGCCs are blocked by manumycin A, PD98059, and KN93, which suggest that the circadian output pathway includes Ras, Erk, and calcium-calmodulin dependent kinase II. In summary, four independent lines of evidence show that the L-VGCCs in cone photoreceptors are under circadian control.

Cloning and characterization of insect arylalkylamine N-acetyltransferase from Bombyx mori

Arylalkylamine N-acetyltransferase (AANAT) catalyzes N-acetylation of arylarkylamines. A cDNA of Bombyx mori insect AANAT (Bm-iAANAT) was found by searching an expressed-sequence tag (EST) database of B. mori (SilkBase). The cDNA encoded a 261 amino acid protein. The mRNA of Bm-iAANAT was expressed in eggs, larvae, adults and various tissues. Recombinant Bm-iAANAT protein was expressed in Sf9 cells by a baculovirus expression system. The AANAT activity of Bm-iAANAT was inhibited by high concentrations (over 0.01 mM) of tryptamine used as a substrate. The Bm-iAANAT acetylated tryptamine, serotonin, dopamine, octopamine, tyramine and norepinephrine. This is the first report of a cloned AANAT that acetylated norepinephrine. These results suggest that Bm-iAANAT is a novel member of insect AANAT family with unique kinetic properties and a broad substrate range.

Melatonin and its synthesizing enzymes (arylalkylamine N-acetyltransferase-like and hydroxyindole-O-methyltransferase) in avian eggs and early embryos

The presence of melatonin and the enzymes (transcripts and activities) involved in its synthesis, i.e. arylalkylamine N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT), was investigated in the eggs and early embryos of Japanese quail at Hamburger-Hamilton stages 1-10. Melatonin was present in the egg yolk (approximately 70 pg/g) and albumen (approximately 20 pg/g). The average content of melatonin was approximately 416 pg/egg. AA-NAT and HIOMT transcripts were present in the oocytes, blastoderms, and ovarian follicles. AA-NAT-like and HIOMT activities were detected in quail egg yolk. The activity of AA-NAT in yolk was comparable with that found in the pineal gland when calculated per milligram of yolk or pineal gland, but was significantly lower when re-calculated per milligram of protein in the yolk or pineal gland. AA-NAT-like activity was also identified in the ovarian follicles. Low HIOMT activity was detected in yolk, but not in the ovarian follicle. Both enzymes were essentially absent from early embryos although some residual activities, probably of yolk origin, were present in the stage 1 embryo. Melatonin and all the constituents needed for its synthesis (serotonin, AA-NAT and HIOMT activities) are contained within the avian yolk and could be used by the embryo from the very beginning of its development. The role of extrapineal melatonin in early avian development may be in protecting the embryo from the action of free radicals formed during intensive embryonic metabolism and/or it may participate (together with serotonin) in a 'diffuse neuroendocrine system' acting at early developmental stages, before differentiation of the nervous system.

Tyrosine phosphorylation of cGMP-gated ion channels is under circadian control in chick retina photoreceptors

PURPOSE

To investigate the role of tyrosine phosphorylation in circadian regulation of cGMP-gated cation channels (CNGCs) of chicken cone photoreceptors.

METHODS

Chick retinas were studied on the second day of constant darkness (DD) after several days of entrainment to 12:12 hr light-dark (LD) cycles in vitro. Inside-out patch recordings were made during the subjective day and subjective night to quantify circadian changes in the sensitivity of CNGCs to activation by cGMP after treatment with various tyrosine kinase and tyrosine phosphatase inhibitors. Immunoprecipitation and immunoblot analysis were also used to examine tyrosine phosphorylation of CNGCs and closely associated proteins after separation by conventional and two-dimensional SDS-PAGE.

RESULTS

Treatment with tyrosine kinase inhibitors caused a significant decrease in K(1/2) for cGMP activation of CNGCs in patches excised from cones during the subjective day, but had no effect on K(1/2) during the subjective night. Conversely, treatment with a tyrosine phosphatase inhibitor caused a significant increase in the K(1/2) of CNGCs in patches excised during the subjective night but had no effect on channel K(1/2) during the subjective day. Broad spectrum serine-threonine phosphatase inhibitors had no effect. An 85-kDa tyrosine polypeptide that coimmunoprecipitated with CNGC alpha-subunits was detectable at higher levels during the subjective day than during the subjective night. CNGC alpha-subunits were not tyrosine phosphorylated as a function of the time of day.

CONCLUSIONS

Circadian control of cone CNGCs appears to entail elevated daytime tyrosine phosphorylation of an approximately 85-kDa auxiliary protein or another subunit of the CNGCs.

Transcriptional regulation of arylalkylamine-N-acetyltransferase-2 gene in the pineal gland of the gilthead seabream

Pineal serotonin-N-acetyltransferase (arylalkylamine-N-acetyltransferase; AANAT) is considered the key enzyme in the generation of circulating melatonin rhythms; the rate of melatonin production is determined by AANAT activity. In all the examined species, AANAT activity is regulated at the post-translational level and, to a variable degree, also at the transcriptional level. Here, the transcriptional regulation of pineal aanat (aanat2) of the gilthead seabream (Sparus aurata) was investigated. Real-time polymerase chain reaction quantification of aanat2 mRNA levels in the pineal gland collected throughout the 24-h cycle revealed a rhythmic expression pattern. In cultured pineal glands, the amplitude was reduced, but the daily rhythmic expression pattern was maintained under constant illumination, indicating a circadian clock-controlled regulation of seabream aanat2. DNA constructs were prepared in which green fluorescent protein was driven by the aanat2 promoters of seabream and Northern pike. In vivo transient expression analyses in zebrafish embryos indicated that these promoters contain the necessary elements to drive enhanced expression in the pineal gland. In the light-entrainable clock-containing PAC-2 zebrafish cell line, a stably transfected seabream aanat2 promoter-luciferase DNA construct exhibited a clock-controlled circadian rhythm of luciferase activity, characteristic for an E-box-driven expression. In NIH-3T3 cells, the seabream aanat2 promoter was activated by a synergistic action of BMAL/CLOCK and orthodenticle homeobox 5 (OTX5). Promoter sequence analyses revealed the presence of the photoreceptor conserved element and an extended E-box (i.e. the binding sites for BMAL/CLOCK and OTX5 that have been previously associated with pineal-specific and rhythmic gene expression). These results suggest that seabream aanat2 is a clock-controlled gene that is regulated by conserved mechanisms.