Circadian expression of tryptophan hydroxylase mRNA in the chicken 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.

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.

Circadian Regulation of Mitochondrial Dynamics in Retinal Photoreceptors

Energy expenditure and metabolism in the vertebrate retina are under circadian control, as we previously reported that the overall retinal ATP content and various signaling molecules related to metabolism display daily or circadian rhythms. Changes in the fission and fusion process of mitochondria, the major organelles producing ATP, in retinal photoreceptors are largely dependent on light exposure, but whether mitochondrial dynamics in photoreceptors and retinal neurons are under circadian control is not clear. Herein, we investigated the possible roles of circadian oscillators in regulating mitochondrial dynamics, mitophagy, and redox states in the chicken retina and mammalian photoreceptors. After entrainment to 12:12-h light-dark (LD) cycles for several days followed by free-running in constant darkness (DD), chicken embryonic retinas and cone-derived 661W cells were collected in either LD or DD at 6 different zeitgeber time (ZT) or circadian time (CT) points. The protein expression of mitochondrial dynamin-related protein 1 (DRP1), mitofusin 2 (MFN2), and PTEN-induced putative kinase 1 (PINK1) displayed daily rhythms, but only DRP1 was under circadian control in the chicken retinas and cultured 661W cells. In addition, cultured chicken retinal cells responded to acute oxidative stress differently from 661W cells. Using pMitoTimer as a mitochondrial redox indicator, we found that the mitochondrial redox states were more affected by light exposure than regulated by circadian oscillators. Thus, this study demonstrates that the influence of cyclic lights might outweigh the circadian regulation of complex mitochondrial dynamics in light-sensing retinal cells.

Riding the Rhythm of Melatonin Through Pregnancy to Deliver on Time

Pregnancy is influenced by the circadian ("circa" or approximately; diēm or day) system, which coordinates physiology and behavior with predictable daily changes in the environment such as light/dark cycles. For example, most species deliver around a particular time of day. In mammals, circadian rhythms are controlled by the master circadian pacemaker, the suprachiasmatic nucleus. One key way that the suprachiasmatic nucleus coordinates circadian rhythms throughout the body is by regulating production of the sleep-promoting hormone melatonin. Serum melatonin concentration, which peaks at night and is suppressed during the day, is one of the best biological indicators of circadian timing. Circadian misalignment causes maternal disturbances in the temporal organization of many physiological processes including melatonin synthesis, and these disturbances of the circadian system have been linked to an increased risk for pregnancy complications. Here, we review evidence that melatonin helps regulate the maternal and fetal circadian systems and the timing of birth. Finally, we discuss the potential for melatonin-based therapeutic strategies to alleviate poor pregnancy outcomes such as preeclampsia and preterm birth.

Serotonin in retina

The expression of serotonin (5-HT) in the retina was first reported in the sixties. The detection of vesicular monoamine transporter and serotonin receptors in several retinal cells confirm that 5-HT is playing a neuromodulatory role in this structure. Whereas signaling pathways activated by 5-HT receptor binding has been poorly investigated so far, numerous data demonstrated that 5-HT is involved in retinal physiology, retinal physiopathology and photoreceptor survival.

Updates on the biology of serotonin and tryptophan hydroxylase

PURPOSE OF REVIEW

To summarize the most recent findings relevant to the biology of serotonin (5-hydroxytryptamine; 5-HT) and the enzyme tryptophan hydroxylase (TPH) in human gastrointestinal disease.

RECENT FINDINGS

Serotonin is synthesized in the central nervous system (CNS) and the gastrointestinal tract where it is secreted from enteroendocrine cells. Its biosynthesis is regulated by two isoforms of the enzyme TPH of which TPH1 is localized predominantly in gastrointestinal enteroendocrine cells. Serotonin activates the peristaltic reflexes, regulates gastrointestinal motility, and has a role in intestinal inflammation. Inhibition of TPH with novel molecules represents a new pharmacological tool in the successful management of carcinoid syndrome in patients with gastrointestinal neuroendocrine tumors (GI-NETs). Certain 5-HT receptor subtype agonists and antagonists are useful in the treatment of functional gastrointestinal disorders.

SUMMARY

The gastrointestinal tract is the largest storage organ for serotonin where its biosynthesis is regulated by TPH1. It has several important functions in gastrointestinal motility, secretion, and inflammation. Furthermore, TPH represents a target for inhibitory pharmacological therapy of serotonin access states such as the carcinoid syndrome.

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.

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.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Melatonin: an underappreciated player in retinal physiology and pathophysiology

In the vertebrate retina, melatonin is synthesized by the photoreceptors with high levels of melatonin at night and lower levels during the day. Melatonin exerts its influence by interacting with a family of G-protein-coupled receptors that are negatively coupled with adenylyl cyclase. Melatonin receptors belonging to the subtypes MT(1) and MT(2) have been identified in the mammalian retina. MT(1) and MT(2) receptors are found in all layers of the neural retina and in the retinal pigmented epithelium. Melatonin in the eye is believed to be involved in the modulation of many important retinal functions; it can modulate the electroretinogram (ERG), and administration of exogenous melatonin increases light-induced photoreceptor degeneration. Melatonin may also have protective effects on retinal pigment epithelial cells, photoreceptors and ganglion cells. A series of studies have implicated melatonin in the pathogenesis of age-related macular degeneration, and melatonin administration may represent a useful approach to prevent and treat glaucoma. Melatonin is used by millions of people around the world to retard aging, improve sleep performance, mitigate jet lag symptoms, and treat depression. Administration of exogenous melatonin at night may also be beneficial for ocular health, but additional investigation is needed to establish its potential.

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.

Serotonin released from amacrine neurons is scavenged and degraded in bipolar neurons in the retina

The neurotransmitter serotonin is synthesized in the retina by one type of amacrine neuron but accumulates in bipolar neurons in many vertebrates. The mechanisms, functions and purpose underlying serotonin accumulation in bipolar cells remain unknown. Here, we demonstrate that exogenous serotonin transiently accumulates in a distinct type of bipolar neuron. KCl-mediated depolarization causes the depletion of serotonin from amacrine neurons and, subsequently, serotonin is taken-up by bipolar neurons. The accumulation of endogenous and exogenous serotonin by bipolar neurons is blocked by selective reuptake inhibitors. Exogenous serotonin is specifically taken-up by bipolar neurons even when serotonin-synthesizing amacrine neurons are destroyed; excluding the possibility that serotonin diffuses through gap junctions from amacrine into bipolar neurons. Further, inhibition of monoamine oxidase A prevents the degradation of serotonin in bipolar neurons, suggesting that monoamine oxidase A is present in these neurons. However, the vesicular monoamine transporter 2 is present only in amacrine cells suggesting that serotonin is not transported into synaptic vesicles and reused as a transmitter in the bipolar neurons. We conclude that the serotonin-accumulating bipolar neurons perform glial functions in the retina by actively transporting and degrading serotonin that is synthesized in neighboring amacrine cells.

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.

Role of melatonin in the eye and ocular dysfunctions

Melatonin is a ubiquitous molecule and widely distributed in nature, with functional activity occurring in unicellular organisms, plants, fungi, and animals. Several studies have indicated that melatonin synthesis occurs in the retina of most vertebrates, including mammals. The retinal biosynthesis of melatonin and the mechanisms involved in the regulation of this process have been extensively studied. Circadian clocks located in the photoreceptors and retinal neurons regulate melatonin synthesis in the eye. Photoreceptors, dopaminergic amacrine neurons, and horizontal cells of the retina, corneal epithelium, stroma endothelium, and the sclera all have melatonin receptors, indicating a widespread ocular function for melatonin. In addition, melatonin is an effective antioxidant which scavenges free radicals and up-regulates several antioxidant enzymes. It also has a strong antiapoptotic signaling function, an effect that it exerts even during ischemia. Melatonin cytoprotective properties may have practical implications in the treatment of ocular diseases, like glaucoma and age-related macular degeneration.

Temporal coupling of cyclic AMP and Ca/calmodulin-stimulated adenylyl cyclase to the circadian clock in chick retinal photoreceptor cells

cAMP signaling pathways play crucial roles in photoreceptor cells and other retinal cell types. Previous studies demonstrated a circadian rhythm of cAMP level in chick photoreceptor cell cultures that drives the rhythm of activity of the melatonin synthesizing enzyme arylalkylamine N-acetyltransferase and the rhythm of affinity of the cyclic nucleotide-gated channel for cGMP. Here, we report that the photoreceptor circadian clock generates a rhythm in Ca(2+)/calmodulin-stimulated adenylyl cyclase activity, which accounts for the temporal changes in the cAMP levels in the photoreceptors. The circadian rhythm of cAMP in photoreceptor cell cultures is abolished by treatment with the l-type Ca(2+) channel antagonist nitrendipine, while the Ca(2+) channel agonist, Bay K 8644, increased cAMP levels with continued circadian rhythmicity in constant darkness. These results indicate that the circadian rhythm of cAMP is dependent, in part, on Ca(2+) influx. Photoreceptor cell cultures exhibit a circadian rhythm in Ca(2+)/calmodulin-stimulated adenylyl cyclase enzyme activity with high levels at night and low levels during the day, correlating with the temporal changes of cAMP in these cells. Transcripts encoding two of the Ca(2+)/calmodulin-stimulated adenylyl cyclases, type 1 and type 8 (Adcy1 and Adcy8), displayed significant daily rhythms of mRNA expression under a light-dark cycle, but only the Adcy1 transcript rhythm persisted in constant darkness. Similar rhythms of Adcy1 mRNA level and Ca(2+)/calmodulin-stimulated adenylyl cyclase activity were observed in retinas of 2-week-old chickens. These results indicate that a circadian clock controls the expression of Adcy1 mRNA and Ca(2+)/calmodulin-stimulated adenylyl cyclase activity; and calcium influx into these cells gates the circadian rhythm of cAMP, a key component in the regulation of photoreceptor function.

Circadian clocks, clock networks, arylalkylamine N-acetyltransferase, and melatonin in the retina

Circadian clocks are self-sustaining genetically based molecular machines that impose approximately 24h rhythmicity on physiology and behavior that synchronize these functions with the solar day-night cycle. Circadian clocks in the vertebrate retina optimize retinal function by driving rhythms in gene expression, photoreceptor outer segment membrane turnover, and visual sensitivity. This review focuses on recent progress in understanding how clocks and light control arylalkylamine N-acetyltransferase (AANAT), which is thought to drive the daily rhythm in melatonin production in those retinas that synthesize the neurohormone; AANAT is also thought to detoxify arylalkylamines through N-acetylation. The review will cover evidence that cAMP is a major output of the circadian clock in photoreceptor cells; and recent advances indicating that clocks and clock networks occur in multiple cell types of the retina.

Transcriptional Profiling of Circadian Patterns of mRNA Expression in the Chick Retina

Previous transcriptome analyses have identified candidate molecular components of the avian pineal clock, and herein we employ high density cDNA microarrays of pineal gland transcripts to determine oscillating transcripts in the chick retina under daily and constant darkness conditions. Subsequent comparative transcriptome analysis of the pineal and retinal oscillators distinguished several transcriptional similarities between the two as well as significant differences. Rhythmic retinal transcripts were classified according to functional categories including phototransductive elements, transcription/translation factors, carrier proteins, cell signaling molecules, and stress response genes. Candidate retinal clock transcripts were also organized relative to time of day mRNA abundance, revealing groups accumulating peak mRNA levels across the circadian day but primarily reaching peak values at subjective dawn or subjective dusk. Comparison of the chick retina transcriptome to the pineal transcriptome under constant conditions yields an interesting group of conserved genes. This group includes putative clock elements cry1 and per3 in addition to several previously unidentified and uninvestigated genes exhibiting profiles of mRNA abundance that varied markedly under daily and constant conditions. In contrast, many transcripts were differentially regulated, including those believed to be involved in both melatonin biosynthesis and circadian clock mechanisms. Our results indicate an intimate transcriptional relationship between the avian pineal and retina in addition to providing previously uncharacterized molecular elements that we hypothesize to be involved in circadian rhythm generation.

Retinal ganglion cells are autonomous circadian oscillators synthesizing N-acetylserotonin during the day

Retinal ganglion cells send visual and circadian information to the brain regarding the environmental light-dark cycles. We investigated the capability of retinal ganglion cells of synthesizing melatonin, a highly reliable circadian marker that regulates retinal physiology, as well as the capacity of these cells to function as autonomous circadian oscillators. Chick retinal ganglion cells presented higher levels of melatonin assessed by radioimmunoassay during both the subjective day in constant darkness and the light phase of a light-dark cycle. Similar changes were observed in mRNA levels and activity of arylalkylamine N-acetyltransferase, a key enzyme in melatonin biosynthesis, with the highest levels of both parameters during the subjective day. These daily variations were preceded by the elevation of cyclic-AMP content, the second messenger involved in the regulation of melatonin biosynthesis. Moreover, cultures of immunopurified retinal ganglion cells at embryonic day 8 synchronized by medium exchange synthesized a [3H]melatonin-like indole from [3H]tryptophan. This [3H]indole was rapidly released to the culture medium and exhibited a daily variation, with levels peaking 8 h after synchronization, which declined a few hours later. Cultures of embryonic retinal ganglion cells also showed self-sustained daily rhythms in arylalkylamine N-acetyltransferase mRNA expression during at least three cycles with a period near 24 h. These rhythms were also observed after the application of glutamate. The results demonstrate that chick retinal ganglion cells may function as autonomous circadian oscillators synthesizing a melatonin-like indole during the day.

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.

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

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

Temporal-spatial characterization of chicken clock genes: circadian expression in retina, pineal gland, and peripheral tissues

The molecular core of the vertebrate circadian clock is a set of clock genes, whose products interact to control circadian changes in physiology. These clock genes are expressed in all tissues known to possess an endogenous self-sustaining clock, and many are also found in peripheral tissues. In the present study, the expression patterns of two clock genes, cBmal1 and cMOP4, were examined in the chicken, a useful model for analysis of the avian circadian system. In two tissues which contain endogenous clocks--the pineal gland and retina--circadian fluctuations of both cBmal1 and cMOP4 mRNAs were observed to be synchronous; highest levels occurred at Zeitgeber time 12. Expression of these genes is also rhythmic in several peripheral tissues; however, the phases of these rhythms differ from those in the pineal gland and retina: in the liver the peaks of cMOP4 and cBmal1 mRNAs are delayed 4-8 h and in the heart they are advanced by 4 h, relative to those in the pineal gland and retina. These results provide the first temporal characterization of cBmal1 and cMOP4 mRNAs in avian tissues: their presence in avian peripheral tissues indicates they may influence temporal features of daily rhythms in biochemical, physiological, and behavioral functions at these sites.

Purification and immunohistochemical analysis of calcium-binding proteins expressed in the chick pineal gland

The pineal gland is a site of melatonin production, of which intracellular calcium ions (Ca2+) are likely involved in various aspects. To investigate the identity of molecules responsible for the Ca2+-dependent processes in the pineal cells, we prepared a cellular extract from 2000 chick pineal glands and isolated a series of Ca2+-binding proteins by taking advantage of their Ca2+-dependent hydrophobic interaction with phenyl-Sepharose beads. The proteins identified by micro-sequencing analysis included calmodulin, neurocalcin, sorcin, annexin II and annexin V. Immunohistochemical analysis of the chick pineal sections revealed that both calmodulin and sorcin are expressed in the follicular and parafollicular pinealocytes. On the other hand, neurocalcin was expressed in a few neuron-like cells located predominantly in the parafollicular layer of the pineal follicle. These results suggest that calmodulin and sorcin may contribute to cellular functions in the chick pinealocytes.

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.

Circadian phototransduction and the regulation of biological rhythms

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Involvement of NF-Y and Sp1 in basal and cAMP-stimulated transcriptional activation of the tryptophan hydroxylase (TPH ) gene in the pineal gland

The expression of the tryptophan hydroxylase (TPH) gene, encoding the rate-limiting enzyme of serotonin biosynthesis, is tightly regulated both at the transcriptional and at the post-transcriptional levels. In the pineal gland, transcription of the gene is activated in response to an intracellular circadian increase of the cAMP concentration. We have previously shown that transcription of a 2.1-kb fragment of the human TPH promoter is induced by cAMP, although it lacks the canonical cAMP responsive element, CRE. The minimal promoter (-73/+29) has only weak transcriptional activity but is responsive to cAMP. It contains an inverted CCAAT box, which was demonstrated to be involved in this response. Here, we have extended our investigation to the functional features of the inverted CCAAT box in the -252/+29 TPH promoter, which has a higher basal activity. We show that an additional cis -acting sequence, the adjacent GC-rich region, cooperates with the inverted CCAAT box for the full activation of basal transcription, and that both elements are essential for the full cAMP response. We also show that in pinealocytes, NF-Y and Sp1 transactivators bind the inverted CCAAT box and GC-rich-region, respectively. These factors participate in a novel pathway for the cAMP-mediated response of the TPH promoter, which is independent of the canonical CRE-mediated response.

Circadian 5-HT production regulated by adrenergic signaling

Using on-line microdialysis, we have characterized in vivo dynamics of pineal 5-hydroxytryptamine (5-HT; serotonin) release. Daily pineal 5-HT output is triphasic: (i) 5-HT levels are constant and high during the day; (ii) early in the night, there is a novel sharp rise in 5-HT synthesis and release, which precedes the nocturnal rise in melatonin synthesis; and (iii) late in the night, levels are low. This triphasic 5-HT production persists in constant darkness and is influenced strongly by intrusion of light at night. We demonstrate that both diurnal 5-HT synthesis and 5-HT release are activated by sympathetic innervation from the superior cervical ganglion and show that these processes are controlled by distinct receptors. The increase in 5-HT synthesis is controlled by beta-adrenergic receptors, whereas the increase in 5-HT release is mediated by alpha-adrenergic signaling. On the other hand, the marked decrease in 5-HT content and release late at night is a passive process, influenced by the extent of melatonin synthesis. In the absence of melatonin synthesis, the late-night decline in 5-HT release is prevented, reaching levels roughly twice as high as that of the day value. In summary, our results demonstrate that 5-HT levels display marked circadian rhythms that depend on adrenergic signaling.

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.

Chickens' Cry2: molecular analysis of an avian cryptochrome in retinal and pineal photoreceptors

We have identified and characterized an ortholog of the putative mammalian clock gene cryptochrome 2 (Cry2) in the chicken, Gallus domesticus. Northern blot analysis of gCry2 mRNA indicates widespread distribution in central nervous and peripheral tissues, with very high expression in pineal and retina. In situ hybridization of chick brain and retina reveals expression in photoreceptors and in visual and circadian system structures. Expression is rhythmic; mRNA levels predominate in late subjective night. The present data suggests that gCry2 is a candidate avian clock gene and/or photopigment and set the stage for functional studies of gCry2.

Characterization of the chicken serotonin N-acetyltransferase gene. Activation via clock gene heterodimer/E box interaction

The abundance of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) mRNA in the chicken pineal gland exhibits a circadian rhythm, which is translated into a circadian rhythm in melatonin production. Here we have started to elucidate the molecular basis of the circadian rhythm in chicken AANAT (cAANAT). The 5'-flanking region of the cAANAT gene was isolated and found to contain an E box DNA element that confers strong luciferase reporter activity. In transfection experiments using chicken pineal cells, an E box mutation dramatically decreased reporter activity. Northern blot analysis indicated that several putative clock genes (bmal1, Clock, and MOP4) are co-expressed in the chicken pineal gland. bmal1 mRNA is expressed in a rhythmic manner in the chicken pineal gland, with peak levels at early subjective night, coincident with the increase in cAANAT expression. Co-transfection experiments in COS cells demonstrated that chicken BMAL1/CLOCK and human BMAL1/MOP4 heterodimers bound the AANAT E box element and enhanced transcription. These observations suggest that binding of clock gene heterodimers to the cAANAT E box is a critical element in the expression of the cAANAT gene in vitro.

The effects of melatonin and L-DOPA on the diurnal rhythms of free amino acids content in the rat retina

The effects of melatonin and dopamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA) intraperitoneal administration on the rhythms of free amino acids content in the retina of rats were studied. The authors found that the levels of those amino acids, which are protein constituents but not neurotransmitters in the rat retina, change diurnally with maximum at 3-6 h after light onset. Diurnal changes of Ala, Arg, Asn, Ile, Met, Ser, Trp, and Val content persisted in the retina of rats maintained at constant darkness. This fact confirms the true circadian nature of these rhythms. Constant lighting abolished diurnal changes of the content of all amino acids with the exception of Trp. Daytime but not nighttime administration of melatonin decreased the levels of Ala, Asn, Gln, Ile, Met, and Ser down to nocturnal values. Diurnal changes of amino acids content vanished in melatonin-injected rats. The effect of melatonin administration disappeared when the protein synthesis was inhibited by cycloheximide. The effect of intraperitoneal administration of L-DOPA on the levels of free amino acids was opposite the effect of melatonin administration. L-DOPA increased nocturnal levels of Gly, Thr, Trp, and Val but had no effect on the daytime amino acids content. As in the case of melatonin administration, significant diurnal changes of amino acid levels disappeared in L-DOPA-injected rats. The authors hypothesize that melatonin and dopamine can serve as zeitgebers-antagonists of amino acids content rhythms in the rat retina.

Dopamine mediates circadian rhythms of rod-cone dominance in the Japanese quail retina

A circadian clock modulates the functional organization of the Japanese quail retina. Under conditions of constant darkness, rods dominate electroretinogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod-cone dominance. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, also exhibits a circadian rhythm in the retina with approximately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activity of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rhythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amplitude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selective antagonist for D1 dopamine receptors has no effect on retinal sensitivity or rod-cone dominance. Reducing retinal dopamine partially abolishes rhythms in sensitivity and yields a rod-dominated retina regardless of the time of day. These results suggest that dopamine, under the control of a circadian oscillator, has a key role in modulating sensitivity and rod-cone dominance in the Japanese quail retina.