Circadian rhythm of tryptophan hydroxylase activity in 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.

The effects of Levilactobacillus brevis on the physiological parameters and gut microbiota composition of rats subjected to desynchronosis

BACKGROUND

All living organisms have developed during evolution complex time-keeping biological clocks that allowed them to stay attuned to their environments. Circadian rhythms cycle on a near 24 h clock. These encompass a variety of changes in the body ranging from blood hormone levels to metabolism, to the gut microbiota composition and others. The gut microbiota, in return, influences the host stress response and the physiological changes associated with it, which makes it an important determinant of health. Lactobacilli are traditionally consumed for their prophylactic and therapeutic benefits against various diseases, namely, the inflammatory bowel syndrome, and even emerged recently as promising psychobiotics. However, the potential role of lactobacilli in the normalization of circadian rhythms has not been addressed.

RESULTS

Two-month-old male rats were randomly divided into three groups and housed under three different light/dark cycles for three months: natural light, constant light and constant darkness. The strain Levilactobacillus brevis 47f was administered to rats at a dose of 0.5 ml per rat for one month and The rats were observed for the following two months. As a result, we identified the biomarkers associated with intake of L. brevis 47f. Changing the light regime for three months depleted the reserves of the main buffer in the cell-reduced glutathione. Intake of L. brevis 47f for 30 days restored cellular reserves of reduced glutathione and promoted redox balance. Our results indicate that the levels of urinary catecholamines correlated with light/dark cycles and were influenced by intake of L. brevis 47f. The gut microbiota of rats was also influenced by these factors. L. brevis 47f intake was associated with an increase in the relative abundance of Faecalibacterium and Roseburia and a decrease in the relative abundance of Prevotella and Bacteroides.

CONCLUSIONS

The results of this study show that oral administration of L. brevis 47f, for one month, to rats housed under abnormal lightning conditions (constant light or constant darkness) normalized their physiological parameters and promoted the gut microbiome's balance.

The Role of Beta-Adrenergic Receptors in the Regulation of Circadian Intraocular Pressure Rhythm in Mice

PURPOSE

To investigate whether the elimination of β1- and β2-adrenergic receptors alters the diurnal intraocular pressure (IOP) rhythm in mice.

MATERIALS AND METHODS

β1-/β2-adrenergic receptor double-knockout and C57BL/6J mice were anesthetized intraperitoneally, with their IOPs measured via microneedle method. After entrainment to a 12-h light-dark (LD) cycle (light phase 6:00-18:00), IOPs were measured every 3 h from 9:00 to 24:00 (group 1, β1-/β2-adrenergic receptor double-knockout mice, n = 11; C57BL/6J, n = 15). The IOP measurements at 15:00 and 24:00 under a 12-h LD cycle and in the constant darkness (1 day and 8 days after exposure to darkness, respectively) were performed in another group of β1-/β2-adrenergic receptor double-knockout mice (group 2, n = 12). IOP variance throughout the day and mean IOP differences among time points were evaluated using a linear mixed model.

RESULTS

β1-/β2-adrenergic receptor double-knockout and C57BL/6J mice showed biphasic IOP curves, low during the light phase and high during the dark phase; the fluctuation was significant (P < 0.001). The peak IOP (18.7 ± 1.4 mmHg) occurred at 24:00 and the trough IOP (13.5 ± 1.5 mmHg) occurred at 15:00 in β1-/β2-adrenergic receptor double-knockout mice group. IOP curves of β1-/β2-adrenergic receptor double-knockout and C57BL/6J were nearly parallel, and the IOPs of β1-/β2-adrenergic receptor double-knockout mice were significantly higher than those of C57BL/6J mice (P < 0.001). Under constant dark (DD) conditions, IOP at 24:00 (18.1 ± 1.5 mmHg) was significantly higher than that at 15:00 (13.3 ± 1.2 mmHg) (P < 0.001). The transition from the LD cycle to DD environment produced no significant change in IOP (P = 0.728).

CONCLUSIONS

Elimination of both β1- and β2-adrenergic receptors did not disturb the biphasic diurnal IOP rhythm in mice.

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 rat with oxygen-induced retinopathy is myopic with low retinal dopamine

PURPOSE

Dopamine (DA) is a neurotransmitter implicated both in modulating neural retinal signals and in eye growth. Therefore, it may participate in the pathogenesis of the most common clinical sequelae of retinopathy of prematurity (ROP), visual dysfunction and myopia. Paradoxically, in ROP myopia the eye is usually small. The eye of the rat with oxygen-induced retinopathy (OIR) is characterized by retinal dysfunction and short axial length. There have been several investigations of the early maturation of DA in rat retina, but little at older ages, and not in the OIR rat. Therefore, DA, retinal function, and refractive state were investigated in the OIR rat.

METHODS

In one set of rats, the development of dopaminergic (DAergic) networks was evaluated in retinal cross-sections from rats aged 14 to 120 days using antibodies against tyrosine hydroxylase (TH, the rate-limiting enzyme in the biosynthesis of DA). In another set of rats, retinoscopy was used to evaluate spherical equivalent (SE), electoretinography (ERG) was used to evaluate retinal function, and high-pressure liquid chromatography (HPLC) was used to evaluate retinal contents of DA, its precursor levodopamine (DOPA), and its primary metabolite 3,4-dihydroxyphenylacetic acid (DOPAC).

RESULTS

The normally rapid postnatal ramification of DAergic neurons was disrupted in OIR rats. Retinoscopy revealed that OIR rats were relatively myopic. In the same eyes, ERG confirmed retinal dysfunction in OIR. HPLC of those eyes' retinae confirmed low DA. Regression analysis indicated that DA metabolism (evaluated by the ratio of DOPAC to DA) was an important additional predictor of myopia beyond OIR.

CONCLUSIONS

The OIR rat is the first known animal model of myopia in which the eye is smaller than normal. Dopamine may modulate, or fail to modulate, neural activity in the OIR eye, and thus contribute to this peculiar myopia.

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 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.

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

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

Circadian rhythm and photic control of cAMP level in chick retinal cell cultures: a mechanism for coupling the circadian oscillator to the melatonin-synthesizing enzyme, arylalkylamine N-acetyltransferase, in photoreceptor cells

Arylalkylamine N-acetyltransferase (AANAT) is the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway. In chicken retina in vivo, AANAT is expressed in a circadian fashion, primarily in photoreceptor cells. AANAT activity is high at night in darkness, low during the daytime, and suppressed by light exposure at night. In the present study, we investigated the circadian and photic regulation of adenosine 3',5'-monophosphate (cAMP) in cultured retinal cells entrained to a daily light-dark (LD) cycle, as well as the role of Ca(2+) and cAMP in the regulation of AANAT activity. Similar to AANAT activity, cAMP levels fluctuate in a daily fashion, with high levels at night in darkness and low levels during the day in light. This daily fluctuation continued with reduced amplitude in constant (24 h/day) darkness (DD). These changes in cAMP appear to be causally related to control of AANAT activity. Adenylyl cyclase and protein kinase A inhibitors suppress the nocturnal increase of AANAT in DD, while 8Br-cAMP augments it. The nocturnal increase of AANAT activity also involves Ca(2+) influx, as it is inhibited by nitrendipine, an inhibitor of L-type voltage-gated channels, and augmented by Bay K 8644, a Ca(2+) channel agonist. The effect of Bay K 8644 was antagonized by the adenylyl cyclase inhibitor MDL 12330A, suggesting a link between Ca(2+) influx, cAMP formation, and AANAT activity in retinal cells. Light exposure at night, which rapidly suppresses AANAT activity, also suppressed cAMP levels. The effect of light on AANAT activity was reversed by Bay K 8644, 8Br-cAMP, and the proteasome inhibitor lactacystin. These results indicate a dynamic interplay of circadian oscillators and light in the regulation of cAMP levels and AANAT activity in photoreceptor cells.

14-3-3 proteins in pineal photoneuroendocrine transduction: how many roles?

Recent studies suggest that a common theme links the diverse elements of pineal photoneuroendocrine transduction--regulation via binding to 14-3-3 proteins. The elements include photoreception, neurotransmission, signal transduction and the synthesis of melatonin from tryptophan. We review general aspects of 14-3-3 proteins and their biological function as binding partners, and also focus on their roles in pineal photoneuroendocrine transduction.

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.

Photic regulation of melatonin in rat retina and the role of proteasomal proteolysis

Several investigations have shown that illumination at night reduces melatonin level in the mammalian pineal, but the effect of night illumination on the retina is not known. In this study retinas were cultured in a flow-through apparatus and then were exposed to light at ZT 18. Light exposure reduced melatonin levels to the daytime level within 30 min. The reduction of melatonin levels was due to a rapid decrease in the activity of the enzyme AA-NAT; AA-NAT mRNA levels were not affected by illumination. Pre-incubation with lactacystin (25 microM) prevented light-induced reduction of AA-NAT activity and melatonin levels. These results demonstrate that melatonin levels in the mammalian retina are affected by light exposure at night, via proteosomal proteolysis of AA-NAT.

Regulation of tryptophan hydroxylase activity in Xenopus laevis photoreceptor cells by cyclic AMP

The aim of this study was to investigate the role of cyclic AMP in the regulation of tryptophan hydroxylase activity localized in retinal photoreceptor cells of Xenopus laevis, where the enzyme plays a key role in circadian melatonin biosynthesis. In photoreceptor-enriched retinas that lack serotonergic neurons, tryptophan hydroxylase activity is markedly stimulated by treatments that increase intracellular levels of cyclic AMP or activate cyclic AMP-dependent protein kinase, including forskolin, phosphodiesterase inhibitors, and cyclic AMP analogues. In contrast, cyclic AMP has no effect on tryptophan hydroxylase mRNA abundance. Experiments using cycloheximide and actinomycin D demonstrate that cyclic AMP exerts its regulatory effect via posttranslational mechanisms mediated by cyclic AMP-dependent protein kinase. The effect of cyclic AMP is independent of the phase of the photoperiod, suggesting that the nucleotide is not a mediator of the circadian rhythm of tryptophan hydroxylase. Cyclic AMP accumulation is higher in darkness than in light, as is tryptophan hydroxylase activity. Furthermore, the stimulatory effect of forskolin and that of darkness are inhibited by H89, an inhibitor of cyclic AMP-dependent protein kinase. In conclusion, cyclic AMP may mediate the acute effects of light and darkness on tryptophan hydroxylase activity of retinal photoreceptor cells.

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.

Functional properties, developmental regulation, and chromosomal localization of murine connexin36, a gap-junctional protein expressed preferentially in retina and brain

Retinal neurons of virtually every type are coupled by gap-junctional channels whose pharmacological and gating properties have been studied extensively. We have begun to identify the molecular composition and functional properties of the connexins that form these 'electrical synapses,' and have cloned several that constitute a new subclass (gamma) of the connexin family expressed predominantly in retina and brain. In this paper, we present a series of experiments characterizing connexin36 (Cx36), a member of the gamma subclass that was cloned from a mouse retinal cDNA library. Cx36 has been localized to mouse chromosome 2, in a region syntenic to human chromosome 5, and immunocytochemistry showed strong labeling in the ganglion cell and inner nuclear layers of the mouse retina. Comparison of the developmental time course of Cx36 expression in mouse retina with the genesis of the various classes of retinal cells suggests that the expression of Cx36 occurs primarily after cellular differentiation is complete. Because photic stimulation can affect the gap-junctional coupling between retinal neurons, we determined whether lighting conditions might influence the steady state levels of Cx36 transcript in the mouse retina. Steady-state levels of Cx36 transcript were significantly higher in animals reared under typical cyclic-light conditions; exposure either to constant darkness or to continuous illumination reduced the steady-state level of mRNA approximately 40%. Injection of Cx36 cRNA into pairs of Xenopus oocytes induced intercellular conductances that were relatively insensitive to transjunctional voltage, a property shared with other members of the gamma subclass of connexins. Like skate Cx35, mouse Cx36 was unable to form heterotypic gap-junctional channels when paired with two other rodent connexins. In addition, mouse Cx36 failed to form voltage-activated hemichannels, whereas both skate and perch Cx35 displayed quinine-sensitive hemichannel activity. The conservation of intercellular channel gating contrasts with the failure of Cx36 to make hemichannels, suggesting that the voltage-gating mechanisms of hemichannels may be distinct from those of intact intercellular channels.

Identification of specific histidine residues and the carboxyl terminus are essential for serotonin N-acetyltransferase enzymatic activity

Melatonin is synthesized in pinealocytes of the pineal gland and in photoreceptors of the retina. Synthesis rate from serotonin to melatonin is controlled by the rapid and dramatic enzymatic increase in darkness of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT, EC 2.3.1.87) and hydroxyindole-O-methyltransferase (HIOMT, EC 2.1.1.4). The primary structure of these critical indoleamine enzymes is now known and the regulation of the enzyme catalysis can be examined. As a first step, the conserved cysteine (C) and histidine (H) residues were targeted for site-directed mutagenesis as potential amino acid residues involved in the N-acetylation reaction of AA-NAT. Our studies concluded that among 6 histidine (H) to alanine (A) mutations, three residues (H110A, H118A, H120A) within the AA-NAT protein showed little or no enzymatic activity, whereas the others (H28A, H70A, H125A) retained enzymatic activity, compared to the unaltered AA-NAT protein. Cysteine to alanine mutations, C37A and C177A, had no significant effect on the AA-NAT enzymatic activity; however, C61A had a four-fold increase in K(m) for acetyl CoA and an altered sensitivity to the thiol modification chemical, N-ethylmaleimide (NEM), implying that C61 may participate in the acetyl CoA binding. Further studies examined the AA-NAT enzyme regulation of the highly conserved carboxyl terminus. When 12 terminal amino acid residues were deleted systematically from the carboxyl terminus of the 205 amino acid residue AA-NAT protein, enzyme activity was retained. However, further residue deletion resulted in enzyme activity plummeting, implicating that the essential information either for the correct structural folding into an active enzyme form or for enzyme stability is in the 193 residues. To test the relative importance of the AA-NAT carboxyl terminal region, a single leucine (L) was altered to alanine (A) or proline (P). Both mutants, either L193A or L193P, had a marked decrease in AA-NAT enzymatic activity and a decrease in thermal stability, suggesting the leucine, in addition to the cysteine and histidine residues, is involved in either enzyme catalysis or stability. In light of the recently reported three-dimensional structure of AA-NAT (17,18), the site-directed mutagenesis data demonstrate experimentally the importance of essential amino acid residues for acetyl CoA binding and AA-NAT activation.

Diurnal rhythms of tryptophan hydroxylase activity in Xenopus laevis retina: opposing phases in photoreceptors and inner retinal neurons

Tryptophan hydroxylase (TPH) is the first enzyme in the biosynthetic pathways of melatonin in photoreceptor cells and of serotonin in amacrine cells. To assess the regulation of TPH activity in photoreceptor cells, we pretreated retinas with kainic acid. The neurotoxin selectively killed inner retinal neurons while sparing photoreceptors. TPH activity in both control and kainate-treated retinas undergoes a day-night rhythm. The rhythms in both preparations fit sinusoidal functions. However, the rhythm in intact retinas peaks at midday while that in kainate-lesioned retinas does so at midnight. The daily rhythm of tryptophan hydroxylase activity in photoreceptors parallels that of melatonin release. Comparing the mean level of activity in rhythms of intact and lesioned retinas, we calculate that the TPH activity in photoreceptors represents 24% of the total activity. Therefore, the TPH activity measured in intact retinas reflects mainly the enzymatic activity in serotonergic neurons, masking that from photoreceptors. In contrast, the levels and diurnal variation of TPH mRNA did not differ in intact and kainate-lesioned retinas indicating that measurements of TPH mRNA content reflect primarily that in photoreceptor cells. Thus, TPH mRNA levels and enzyme activity are differentially regulated in amacrine neurons and photoreceptor cells. This differential regulation markedly impacts the patterns of daily rhythms observed in the intact 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.

Day/night variation of tryptophan hydroxylase and serotonin N-acetyltransferase mRNA levels in the ovine pineal gland and retina

In mammals, the photoperiodic information, received by the retina, is transmitted to the pineal gland. In both organs, melatonin is produced and functions as a neurohormone giving temporal information to the organism. A four-step enzymatic pathway, involving in particular the tryptophan hydroxylase (TPOH), the rate-limiting enzyme in serotonin synthesis, and the serotonin N-acetyltransferase (NAT) that converts serotonin to N-acetylserotonin, allows the synthesis of melatonin. Many studies on melatonin synthesis modulation have focused on the enzyme NAT, but the regulation of TPOH is less well understood. We report here a quantitative study, using a reverse transcription polymerase chain reaction (RT-PCR) analysis, of the nycthemeral expression of TPOH and NAT mRNAs in the ovine retina and pineal gland. In both organs, we show a nocturnal increase in mRNA levels of the two enzymes. suggesting a role of transcriptional mechanisms in the regulation of melatonin synthesis. However, the amplitude of the observed increase in TPOH and NAT mRNAs expression can not entirely explain the 7-fold nocturnal increase in the plasma melatonin level. Our results suggest that, in the sheep, post-transcriptional mechanisms might also be involved in the day/night modulation of melatonin production.

CCAAT displacement protein (CDP/cut) binds a negative regulatory element in the human tryptophan hydroxylase gene

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin, a neurotransmitter that has been implicated in many psychiatric illnesses. The mechanism of transcriptional regulation of the human TPH gene is largely unknown. We have identified a negative regulatory element located between nucleotides -310 and -220 in the human TPH (hTPH) gene. Electromobility shift analyses performed with the -310/-220 hTPH probe and nuclear extract from P815-HTR (a TPH-expressing cell line) revealed two slow migrating protein-DNA complexes, designated I and II. CCAAT displacement protein (CDP/Cut) is involved in complex I formation as shown in electromobility shift analysis, using consensus oligonucleotide competitor and antibody. Mutations in the CDP/Cut binding site not only disrupted the CDP-DNA complex but also disrupted the second complex, suggesting that the core binding sequences of the two proteins are overlapping. The functional importance of these protein-DNA interactions was assessed by transiently transfecting wild-type and mutant pTPH/luciferase reporter constructs into P815-HTR cells. Mutations in the core CDP/Cut site resulted in an approximately fourfold increase in relative luciferase activities. Because CDP/Cut has been shown to repress transcription of many target genes, we speculate that disruption of the CDP/Cut binding was responsible, at least in part, for the activation of hTPH gene.

Elevation of melatonin in chicken retina by 5-hydroxytryptophan: differential light/dark responses

Melatonin is synthesized in the chicken retina under the influence of a circadian clock, which also regulates the expression of tryptophan hydroxylase (TPH) and serotonin N-acetyltransferase (AA-NAT). In order to examine the role of substrate supply in the rhythmic synthesis of melatonin in chicken retina, tryptophan and 5-hydroxytryptophan were administered day and night in light or darkness. When administered systemically at night in darkness, 5-hydroxytryptophan, but not tryptophan, dramatically stimulates melatonin levels in the chick retina in a dose-dependent manner. Intraocular administration of 5-hydroxytryptophan also increases melatonin levels locally, indicating a retinal site of action of the serotonin precursor. The effect of 5-hydroxytryptophan is much greater at night, when TPH and AA-NAT activities are high, than during the day, when the enzyme activities are low. Similarly, unexpected light exposure at night, which inactivates AA-NAT, significantly reduces the ability of 5-hydroxytryptophan to increase retinal melatonin levels. The results suggest that TPH, but not AA-NAT or other enzymes in the melatonin biosynthetic pathway, is saturated with substrate in situ. The rate of melatonin production appears to be a function of the concentration of serotonin, which is regulated by TPH, and by the level of activity of AA-NAT.

Circadian expression of tryptophan hydroxylase mRNA in the chicken retina

Many aspects of retinal physiology are controlled by a circadian clock located within the eye. This clock controls the rhythmic synthesis of melatonin, which results in elevated levels during the night and low levels during the day. The rate-limiting enzyme in melatonin biosynthesis in retina appears to be tryptophan hydroxylase (TPH)[G.M. Cahill and J.C. Besharse, Circadian regulation of melatonin in the retina of Xenopus laevis: Limitation by serotonin availability, J. Neurochem. 54 (1990) 716-719]. In this report, we found that TPH mRNA is strongly expressed in the photoreceptor layer and the vitread portion of the inner nuclear layer; the message is also expressed, but to a lesser extent, in the ganglion cell layer. The abundance of retinal TPH mRNA exhibits a circadian rhythm which persists in constant light or constant darkness. The phase of the rhythm can be reversed by reversing the light:dark cycle. In parallel experiments we found a similar pattern of expression in the chicken pineal gland. However, whereas a pulse of light at midnight suppressed retinal TPH mRNA by 25%, it did not alter pineal TPH mRNA, suggesting that there are tissue-specific differences in photic regulation of TPH mRNA. In retinas treated with kainic acid to destroy serotonin-containing amacrine and bipolar cells, a high amplitude rhythm of TPH mRNA was observed indicating that melatonin-synthesizing photoreceptors are the primary source of the rhythmic message. These observations provide the first evidence that chick retinal TPH mRNA is under control of a circadian clock.

Transcripts encoding two melatonin synthesis enzymes in the teleost pineal organ: circadian regulation in pike and zebrafish, but not in trout

In this report the photosensitive teleost pineal organ was studied in three teleosts, in which melatonin production is known to exhibit a daily rhythm with higher levels at night; in pike and zebrafish this increase is driven by a pineal clock, whereas in trout it occurs exclusively in response to darkness. Here we investigated the regulation of messenger RNA (mRNA) encoding serotonin N-acetyltransferase (AA-NAT), the penultimate enzyme in melatonin synthesis, which is thought to be primarily responsible for changes in melatonin production. AA-NAT mRNA was found in the pineal organ of all three species and in the zebrafish retina. A rhythm in AA-NAT mRNA occurs in vivo in the pike pineal organ in a light/dark (L/D) lighting environment, in constant lighting (L/L), or in constant darkness (D/D) and in vitro in the zebrafish pineal organ in L/D and L/L, indicating that these transcripts are regulated by a circadian clock. In contrast, trout pineal AA-NAT mRNA levels are stable in vivo and in vitro in L/D, L/L, and D/D. Analysis of mRNA encoding the first enzyme in melatonin synthesis, tryptophan hydroxylase, reveals that the in vivo abundance of this transcript changes on a circadian basis in pike, but not in trout. A parsimonious hypothesis to explain the absence of circadian rhythms in both AA-NAT and tryptophan hydroxylase mRNAs in the trout pineal is that one circadian system regulates the expression of both genes and that this system has been disrupted by a single mutation in this species.

Ocular melatonin rhythms in the goldfish, Carassius auratus

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

Molecular cloning of chick pineal tryptophan hydroxylase and circadian oscillation of its mRNA levels

We have previously shown that the level of [35S]methionine incorporation into tryptophan hydroxylase (TPH) shows a circadian rhythm in cultured chick pineal cells. The TPH protein oscillation persists in constant darkness, peaks in the early night and can be phase-shifted by light, in parallel to the effect of these treatments on melatonin synthesis. We have cloned and sequenced a full-length cDNA for chick pineal TPH. Levels of TPH mRNA show a robust diurnal oscillation both in vivo and in vitro. The rhythm in TPH mRNA also persists in constant darkness, suggesting that TPH mRNA synthesis and/or turnover is regulated by an endogenous circadian clock in cultured chick pineal cells. The circadian oscillation of TPH constitutes the first described circadian rhythm of a chick pineal gene at the mRNA level.

Tryptophan hydroxylase mRNA levels are regulated by the circadian clock, temperature, and cAMP in chick pineal cells

Chick pineal cells contain a circadian oscillator that derives rhythmic synthesis and secretion of melatonin even in dispersed cell culture. Here, we demonstrate that the mRNA encoding tryptophan hydroxylase (TPH), the first enzyme in the melatonin synthetic pathway, is expressed rhythmically under the control of the circadian clock. TPH message levels doubled between early day and early night, under both cyclic lightning and constant lightning conditions. The amplitude of the TPH mRNA rhythm was increased to 4-fold by culturing the cells at 43.3 degrees C for 48 h instead of 36.7 degrees C. Addition of forskolin to the cultures in early day produced a modest increase (50%) in TPH message levels but had no effect at other times. Because TPH mRNA are regulated by the endogenous pineal circadian clock, this provides a valuable system in which the molecular mechanism of clock control of gene expression.

Quantitative two-dimensional gel electrophoretic analysis of clock-controlled proteins in cultured chick pineal cells: circadian regulation of tryptophan hydroxylase

The progression of the circadian oscillator through its cycle and the circadian rhythm of melatonin production in dissociated chick pineal cultures both require daily de novo protein synthesis during defined circadian phases. To identify specific proteins involved in these two processes, we have performed a quantitative two-dimensional polyacrylamide gel electrophoretic screen of proteins that are synthesized at different times of the day in chick pineal cell cultures. Out of approximately 700 proteins analyzed, we have identified several proteins whose levels of 35S incorporation oscillate in a light/dark cycle. One protein of 56 kDa, pI 6 (p56) undergoes a diurnal oscillation that parallels the melatonin rhythm, reaching a peak early in the night and falling to minimal levels during the day. A second protein of 22 kDa, pI 4.5 (p22) also expresses a diurnal rhythm in 35S incorporation; however, it peaks at the end of the night. The oscillations of both proteins persist, with a reduced amplitude, in constant darkness. Furthermore, the phases of the p56 and p22 rhythms are regulated by the light/dark cycle. Both p56 and p22 appear to be under direct control of the chick pineal circadian oscillator, and therefore can be described as "clock-controlled proteins." We have identified p56 as tryptophan hydroxylase by microsequencing and western blotting. Chick pineal tryptophan hydroxylase also expresses a 24-h oscillation in abundance both in vitro and in vivo. The rhythm in tryptophan hydroxylase expression represents a newly discovered level of regulation of the melatonin synthesis pathway by the circadian clock in chick pineal cells.

Hydroxyindole-O-methyltransferase mRNA expression in a subpopulation of photoreceptors in the chicken retina

Hydroxyindole O-methyltransferase (HIOMT, EC 2.1.1.4) catalyzes the final step in the synthesis of melatonin in the pineal gland and retina. HIOMT mRNA was localized by in situ hybridization in the chicken retina to some, but clearly not all, photoreceptors, while in the pineal gland, most pinealocytes displayed a positive hybridization signal. The in situ hybridization localization was confirmed by immunocytochemistry, using an antibody directed against a synthetic chicken HIOMT peptide. Western blot analysis demonstrated an immunoreactive protein of about 40 kilodaltons in the pineal, but the HIOMT protein was below detectable levels in the retina. However, the HIOMT-peptide antibody did identify a modestly immunoreactive subpopulation of retinal photoreceptors. These observations suggest that, in the chicken, melatonin biosynthetic activity is located mainly in a subpopulation of retinal photoreceptors and in most pinealocytes.

Melatonin and deprivation myopia in chickens

Chicken eyes elongate and become myopic if they are covered with translucent diffusors which degrade the retinal image ('deprivation myopia'). Since it has been shown that dopamine D2/D4 receptors (which mediate inhibition of melatonin synthesis) are also implicated in deprivation myopia, we have studied the role of melatonin in the visual control of eye growth. We have found that (1) diurnal melatonin rhythms and melatonin content in the retina are unchanged during deprivation myopia development despite the breakdown of both diurnal growth rhythms of the eye and diurnal rhythms in retinal dopamine metabolism, (2) diurnal melatonin rhythms and melatonin content in the retina remain unchanged after application of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) and presumably also after 6-hydroxydopamine (6-OHDA) application which both have a suppressive effect on deprivation myopia and (3) deprivation myopia was slightly reduced in both eyes after unilateral intravitreal injection of melatonin, despite that deprivation myopia is based on a mechanism intrinsic to the eye. We conclude that melatonin is not involved in the retinal signaling pathway translating visual experience to deprivation myopia.

Stimulation of dopaminergic amacrine cells by stroboscopic illumination or fibroblast growth factor (bFGF, FGF-2) injections: possible roles in prevention of form-deprivation myopia in the chick

Form-deprivation myopia (FDM) in the chick is a popular model for studying the postnatal regulation of ocular growth. Using this model, we have shown previously that dopamine and FGF-2 can counteract the effects of form-deprivation, thereby producing emmetropia. In the present study, we tested the hypothesis that the emmetropizing effects of flickering light and intraocular injections of FGF-2 in the chick are mediated by the activity of dopaminergic retinal amacrine cells. We have assessed the rate of dopamine synthesis in the retina by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA). We found that form-deprivation reduces the rate of dopamine synthesis in the light-adapted retina, and that the normal rate of dopamine synthesis in the light can be restored by stroboscopic illumination at frequencies around 10 Hz. By labeling cells immunocytochemically we have shown that the synthesis of c-fos, a putative transcriptional regulator of the tyrosine hydroxylase gene, is induced in dopaminergic amacrine cells by stroboscopic illumination at around 10 Hz. These observations are consistent with a critical role for dopaminergic amacrine cells in the regulation of ocular growth by intermittent illumination. We have found also that intraocular injections of FGF-2 cause emmetropization without altering levels of expression of c-fos, amounts of tyrosine hydroxylase, or rates of dopamine synthesis with respect to vehicle-injected controls. We conclude that FGF acts either in parallel to or downstream from the dopaminergic amacrine cells, rather than through them. We observed that intravitreal injection per se induces high levels of c-fos expression in both form-deprived and non-deprived retinas, and causes partial emmetropization in form-deprived eyes, while inhibiting dopamine synthesis in non-deprived retinas. It is likely, therefore, that injection stimulates the production and/or release of unknown factors whose diverse effects on ocular growth and dopamine metabolism are mediated by complex pathways. Taken together, our results are consistent with the view that the retinal circuitry that controls postnatal ocular growth in the chick involves multiple messengers and pathways.

Tryptophan hydroxylase is expressed by photoreceptors in Xenopus laevis retina

Serotonin has important roles, both as a neurotransmitter and as a precursor for melatonin synthesis. In the vertebrate retina, the role and the localization of serotonin have been controversial. Studies examining serotonin immunoreactivity and uptake of radiolabeled serotonin have localized serotonin to inner retinal neurons, particularly populations of amacrine cells, and have proposed that these cells are the sites of serotonin synthesis. However, other reports identify other cells, such as bipolars and photoreceptors, as serotonergic neurons. Tryptophan hydroxylase (TPH), the rate-limiting enzyme in the serotonin synthetic pathway, was recently cloned from Xenopus laevis retina, providing a specific probe for localization of serotonin synthesis. Here we demonstrate that the majority of retinal mRNA encoding TPH is present in photoreceptor cells in Xenopus laevis retina. These cells also contain TPH enzyme activity. Therefore, in addition to being the site of melatonin synthesis, the photoreceptor cells also synthesize serotonin, providing a supply of the substrate needed for the production of melatonin.

NF-Y activates mouse tryptophan hydroxylase transcription

Tryptophan hydroxylase catalyses the rate-limiting step in the biosynthesis of serotonin, a neurotransmitter which has been implicated in the etiologies of clinically important psychiatric illnesses. Tryptophan hydroxylase is expressed in a tissue-specific manner, but little is known about its transcriptional regulation. By analysing transcriptional activities of a set 5'-deletion constructs of promoter-reporter plasmids in P815-HTR mastocytoma cells, we found that transcription was activated by sequences between nucleotides -343 and -21. DNase I footprint analysis, using nuclear protein extracts from P815-HTR cells, revealed a protein-DNA interaction between nucleotides -77 and -46. A double stranded oligonucleotide, representing this binding site, specifically bound nuclear protein in a gel shift assay. Methylation interference analysis of this complex revealed that nuclear protein interacted with an inverted GGCCAAT element, which is a high-affinity binding motif for the transcription factor NF-Y (also known as CP1 or CBF). An NF-Y specific antibody abolished protein binding in a gel shift assay. Mutagenesis of specific base pairs abolished protein binding in vitro, and mutagenesis of the same base pairs in a reporter gene construct resulted in a 65% decrease in transcriptional activity. Our results suggest that the transcription factor NF-Y binds to a GGCCAAT motif in the tph proximal promoter and activates transcription.

Regulation of tryptophan hydroxylase expression by a retinal circadian oscillator in vitro

Many aspects of retinal physiology are controlled by a circadian clock including at least two steps in the melatonin synthetic pathway: the activity of the enzyme, N-acetyltransferase (NAT), and mRNA levels of the rate-limiting enzyme trytophan hydroxylase (TPH). Light and dopamine (through D2-like dopamine receptors) can phase shift the clock, and can also acutely inhibit NAT activity, resulting in supressed melatonin synthesis. In this paper, we show that eyecups cultured in constant darkness maintain a clock-controlled rhythm in TPH mRNA, with low levels in early day, rising to a peak in early night. Both eyecups and isolated retinas, cultured in light during the day, also exhibit a similar increase in TPH mRNA levels, indicating that this expression is not acutely inhibited by light. Treatment with light or quinpirole (D2 dopamine receptor agonist) in early night, at a time and dose that acutely inhibits NAT activity, does not change levels of TPH mRNA. Addition of eticlopride (D2 dopamine receptor antagonist) during the day, also has no effect on the normal daytime increase in TPH message levels. Therefore, TPH mRNA level is controlled by a circadian clock located within the eye, but acute effects of light or dopamine are not detected.

Dopamine and serotonin turnover rate in the retina of rabbit, rat, goldfish, and Eugerres plumieri: light effects in goldfish and rat

The concentration of dopamine, and its metabolites 3,4-dihydroxyphenylacetic and homovanillic acids, as well as serotonin and its metabolite 5-hydroxyindoleacetic acid, were determined in the retina of two teleosts, C. auratus (goldfish) and E. plumieri (mojarra), and two mammals, R. norvegicus (rat) and O. cuniculus (rabbit). The turnover rate of these monoamines were investigated in the four species by the calculation of the ratio monoamine/metabolite as an indirect index, and in goldfish and rat by the inhibition of the synthesis with alpha-methyl-p-tyrosine or p-chlorophenylalanine, by the increase in dopamine or serotonin by the corresponding precursors, 3,4-dihydroxyphenylalanine or 5-hydroxytryptophan, and by inhibition of monoaminooxidase with pargyline. The modulation by light and dark stimulation was studied in the goldfish and the rat. Differences in the concentration and turnover rate were observed among the species. Serotonin concentration was higher in the teleosts. The administration of inhibitors of dopamine and serotonin synthesis differentially decreased the levels of the monoamines in the retina of goldfish and rat. The rate of formation of dopamine and serotonin by the corresponding precursors was much higher in the goldfish than in the rat. Pargyline administration decreased 3,4-dihydroxyphenylacetic and 5-hydroxyindoleacetic acids at different rates and time dependency in the retina of goldfish and rat. Dopamine and serotonin concentration did not exhibit high modifications by the inhibitor, suggesting the function of regulatory mechanisms or additional effect of pargyline at other sites different from monoaminooxidase.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of endogenous dopamine release in amphibian retina by melatonin: the role of GABA

In the retina of the African clawed frog (Xenopus laevis), endogenous dopamine release increases in light and decreases in darkness. Exogenous melatonin and several chemical analogs of melatonin suppressed light-evoked dopamine release from frog retina in a concentration-dependent manner. The rank order of potency for inhibition of light-evoked dopamine release was melatonin >> 5-methoxytryptamine > or = N-acetylserotonin > 5-methoxytryptophol >>> serotonin. Melatonin did not suppress dopamine release below levels seen in darkness. The putative melatonin receptor antagonist luzindole inhibited the effect of melatonin. Luzindole enhanced dopamine release in darkness but had little effect in light. These data suggest a role for endogenous melatonin in dark-induced suppression of retinal dopamine. Picrotoxin and bicuculline, GABA-A receptor antagonists, blocked melatonin-induced suppression of dopamine release. In the presence of melatonin, bicuculline was significantly less potent in stimulating dopamine release. These results suggest that melatonin enhances GABAergic inhibition of light-evoked dopamine release. This mechanism may underlie the light/dark difference in dopamine release in vertebrate retina.

Light-dependent regulation of the transcriptional activity of the mammalian gene for arrestin

The purpose of this investigation was to determine if the cyclic variations in the amount of arrestin mRNA that occur in the mouse retina during the light/dark cycle are controlled by light, a circadian clock, or both. In this study, an experimental paradigm was designed and used to distinguish among these molecular mechanisms. The data show that the initiation and the maintenance of an increased rate of syntheses of arrestin mRNA requires light and is not dependent on the time of day. Therefore the changes in transcriptional activity of the mouse arrestin gene are controlled by light, independent of circadian clock. The mechanism for this light-dependent regulation of gene transcription in the retina is distinct from the light-dependent increases in mRNA concentrations of early response genes which occur in the suprachiasmatic nucleus and a comparative analysis of such changes in these different regions of the CNS may lead to the identification of the molecular relationship between light- and circadian-dependent gene regulation.

Tryptophan hydroxylase expression is regulated by a circadian clock in Xenopus laevis retina

A circadian clock has been localized to the photoreceptor layer in the Xenopus laevis retina. This clock controls the rhythmic synthesis of melatonin, which results in elevated levels during the night and low levels during the day. The rate-limiting enzyme in melatonin synthesis in Xenopus laevis retina is tryptophan hydroxylase. A cDNA clone coding for Xenopus tryptophan hydroxylase was isolated, characterized, and used as a probe for analysis of tryptophan hydroxylase mRNA expression. Northern blot analyses of total retinal RNA show that the tryptophan hydroxylase message levels are low in the day and higher at night. The expression of tryptophan hydroxylase mRNA is under circadian control because rhythmic changes are also seen in constant darkness, with elevated levels during the subjective night. Nuclear run-on analysis during the first subjective day in constant darkness revealed that transcription initiation is low early in the day and increases throughout the day. Our observations suggest that the circadian clock modulates tryptophan hydroxylase gene expression. An understanding of how the circadian clock controls tryptophan hydroxylase expression may lead to a clearer understanding of the melatonin biosynthetic pathway, and possibly the clock itself.

Regulation of melatonin and dopamine biosynthesis in chick retina: the role of GABA

Melatonin biosynthesis in chick retina occurs as a circadian rhythm. Biosynthesis of the neurohormone is highest at night in darkness, and is suppressed by light. The role of gamma-aminobutyric acid (GABA) in the nocturnal regulation of melatonin synthesis was examined. Systemic or intravitreal administration of muscimol, a GABA-A receptor agonist, to light-exposed chicks at the beginning of the dark phase of the light/dark cycle increased retinal melatonin levels and the activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme of the melatonin biosynthetic pathway. Baclofen, a GABA-B receptor agonist, also increased NAT activity of light-exposed retinas, but muscimol was approximately 40-fold more potent than baclofen. Effects of both muscimol and baclofen on NAT activity were inhibited by GABA-A antagonists, bicuculline and picrotoxin, and the effect of baclofen was unaffected by the GABA-B selective antagonist, CGP 35348. Thus, activation of GABA-A receptors appears to be associated with increased melatonin biosynthesis. The GABA-uptake inhibitor, nipecotic acid, and the GABA-transaminase inhibitor, aminooxyacetic acid, also increased NAT activity of light-exposed retinas. The high levels of NAT activity associated with exposure to darkness were unaffected by either muscimol or baclofen, but picrotoxin and bicuculline significantly inhibited retinal NAT activity in darkness. The rate of dopamine synthesis, estimated from in situ tyrosine hydroxylase activity, was higher in light-exposed retinas than in darkness. Muscimol inhibited dopamine synthesis in light, and picrotoxin stimulated dopamine synthesis in darkness. The stimulation of melatonin synthesis by muscimol in light-exposed retinas appears to be related to inhibition of retinal dopamine neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Chick retina and pineal gland differentially respond to constant light and darkness: in vivo studies on serotonin N-acetyltransferase (NAT) activity and melatonin content

Oscillations in serotonin N-acetyltransferase (NAT) activity and melatonin content were investigated in retina and pineal gland of chicks kept for 5 days in constant darkness (DD) or continuous light (LL). Under DD the rhythm of the pineal melatonin biosynthesis resembled that found under diurnal illumination (LD), whereas in the retina DD resulted in significant elevations of NAT activity and melatonin level during subjective light. A low-amplitude rhythm of retinal NAT activity continued under LL with a period close to 24 h. In the pineal gland, light exposure suppressed the level of NAT activity and melatonin content (an effect being substantially weaker than that observed in retina), delayed the first peak of NAT activity by 3 h, and prolonged the rhythm's period to about 26 h. Our data suggest the existence of some differences in the activity of circadian oscillators that regulate the melatonin generating system in the retina and pineal gland of chick.