Rhythm Of Melatonin Synthesis Research Articles

КЛИНИЧЕСКОЕ ЗНАЧЕНИЕ СУТОЧНОГО МОНИТОРИРОВАНИЯ АРТЕРИАЛЬНОГО ДАВЛЕНИЯ И ЦИРКАДНОГО СИНТЕЗА МЕЛАТОНИНА В ПРОГНОЗИРОВАНИИ СИНДРОМА ОБСТРУКТИВНОГО АПНОЭ СНА У ПАЦИЕНТОВ С АРТЕРИАЛЬНОЙ ГИПЕРТЕНЗИЕЙ I-II СТЕПЕНИ

Objective. To study the role of 24-hour ambulatory blood pressure monitoring (ABPM) and circadian melatonin (MT) synthesis in predicting obstructive sleep apnea syndrome (OSAS) in patients with grade 1-2 arterial hypertension (AH) and to reveal the relationship between the study findings. Material and methods. 67 patients aged 50.0 (41.0; 56.0) having grade 1-2 AH were examined and divided into 2 groups in regard to the complex of clinical and instrumental research methods: Group 1 – 25 patients having AH and Group – 45 patients having AH associated with OSAS. 24-hour ABPM, anthropometric measurements, and respiratory monitoring were performed. MT levels were evaluated by the content of its metabolite 6-sulfatoxymelatonin (6-COMT) in the urine. Results It has been found that according to ABPM diastolic blood pressure variability during the day was ≥12 mm Hg and had high sensitivity (74.1%) and specificity (91.7%) for predicting the OSAS in patients with hypertension receiving standard antihypertensive therapy. The characteristic feature of the daily MT production in patients with AH in combination with OSAS is an increase in the level of 6-SOMT in daily urine (p=0.031) and its daily portion (p=0.037), correlating with the severity of OSAS (r=0.43; p=0.00065 and r=0.40; p=0.00079, respectively) and independent of the grade of AH (p>0.05). Conclusion. The obtained results allow us to recommend the use of ABPM as a screening, low-cost study to predict OSAS in patients with grade 1-2 AH. The identified changes in the daily rhythm of MT synthesis weakly correlate with the severity of OSAS and the degree of hypertension.

The ISL LIM-homeobox 2 transcription factor is negatively regulated by circadian adrenergic signaling to repress the expression of Aanat in pinealocytes of the rat pineal gland.

Melatonin is synthesized in the pineal gland during nighttime in response to nocturnal increase in the activity of the enzyme aralkylamine N-acetyltransferase (AANAT), the transcription of which is modulated by several homeodomain transcription factors. Recent work suggests that the homeodomain transcription factor ISL LIM homeobox 2 (ISL2) is expressed in the pineal gland, but its role is currently unknown. With the purpose of identifying the mechanisms that control pineal expression of Isl2 and the possible function of Isl2 in circadian pineal biology, we report that Isl2 is specifically expressed in the pinealocytes of the rat pineal gland. Its expression exhibits a 24 h rhythm with high transcript and protein levels during the day and a trough in the second half of the night. This rhythm persists in darkness, and lesion studies reveal that it requires intact function of the suprachiasmatic nuclei, suggesting intrinsic circadian regulation. In vivo and in vitro experiments show that pineal Isl2 expression is repressed by adrenergic signaling acting via cyclic AMP; further, Isl2 is negatively regulated by the nocturnal transcription factor cone-rod homeobox. During development, pineal Isl2 expression is detectable from embryonic day 19, preceding Aanat by several days. In vitro knockdown of Isl2 is accompanied by an increase in Aanat transcript levels suggesting that ISL2 represses its daytime expression. Thus, rhythmic expression of ISL2 in pinealocytes is under the control of the suprachiasmatic nucleus acting via adrenergic signaling in the gland to repress nocturnal expression, while ISL2 itself negatively regulates daytime pineal expression of Aanat and thereby suggestively enhances the circadian rhythm in melatonin synthesis.

Robust circadian and circannual rhythms of expression of clock genes and clock controlled aanat2 gene in the photoreceptive pineal organ of catfish, Clarias gariepinus under natural conditions

ABSTRACT Diurnal changes in daylength and temperature influence circadian rhythm of melatonin synthesis in the fish pineal organ. All core clock genes and clock-controlled aanat2 gene are expressed in the pineal organ of teleosts. However, there is paucity of information on the circadian rhythm of expression of clock genes and any clock-controlled gene in the pineal organ of any tropical/subtropical fish species under natural photoperiod and temperature during different phases of the annual breeding cycle and/or seasons. Therefore, we studied circadian rhythms of expression of clock genes (clock, bmal1, per1, per2, per3, cry1 and cry2) and aanat2 in the pineal organ of the catfish, Clarias gariepinus maintained under natural environmental conditions during different phases of its annual breeding cycle. All clock genes and aanat2 gene invariably exhibited prominent circadian rhythm of expression in the fish pineal organ irrespective of seasons/breeding phases. Further, acrophase, mesor and amplitude of the circadian rhythm of expression of the clock genes and aanat2 gene responded differently to different seasons. Our findings suggest that photoperiod-, temperature- and season-independent circadian rhythm of expression of clock genes and aanat2 gene in the pineal organ enable it to act as a potential master clock in the fish under natural conditions.

CHRONONEONATOLOGY: BIORHYTHMIC ENTITY OF A NEWBORN

Human biorhythms display the adaptation of living organisms to the external environment. These are mechanisms that are so well-regulated by nature that they are often called ‘biological clock’.There are definite functions in the body of the newborn that have a daily rhythm (with a period of 2 to 25 hours). Such a rhythmic setup depends on the maturity of the newborn: the circadian rhythm of melatonin synthesis in humans arises from the first days after birth, and its formation ends up 9-12 weeks and in premature babies for 2-3 weeks later. The development of the ‘biological clock’ of the newborn is significantly influenced by the conditions of the environment. Melatonin provides recovery, stabilization and synchronization of chronorhythms of different frequencies, including daily periodism.At this rate, the pleiotropic effects of melatonin on the control of biological rhythms during this period are a matter of particular attention. Our reference is devoted to it.During early postnatal adaptation, it is reasonable to determine the biorhythmologic features of the main physiological indicators of life: hourly blood pressure, heart rate and respiratory movements, body temperature, cortisol, adrenaline, norepinephrine, 6-sulfatoxymelatonin (melatonin metabolite) in fractional urine in the dynamics of the early neonatal period.The tasks of chrononeonatology are the study of the peculiarities of the formation of the circadian setup of physiological functions and the level of the main adaptive hormones in the dynamics of the early neonatal period.In the prenatal period, melatonin penetrates the fetus from the pregnant woman through the placenta, and after birth it enters the baby's body with the mother's milk. It is believed that periodic signals emanating from the mother's pineal gland (PG) cells synchronize the fetal chronorhythms. In the early stages of embryogenesis, these signals have a nervous and humoral genesis, and after birth, only humoral influences.The concentration of melatonin in the blood of a pregnant woman reaches a maximum level to the 32nd week of pregnancy and is restored by the 2nd day after childbirth.Fetal growth restriction is due to a significant decrease in the secretion of melatonin during the first 3 months of life of newborns. It is important that in the preterm infants, the relative deficit of melatonin lasts from 2-4 to 7-8 months.Maternal influence on development does not end with delivery, but continues in the neonatal period. Breast milk contains more than 60 biologically active substances (STH, lactogenic hormone, IGF-1, insulin, etc., in particular melatonin), whose level significantly exceeds their concentration in the peripheral blood of the mother.Early breastfeeding, the rooming-in in the postpartum period, an arbitrary feeding regime contribute to the earlier formation of biorhythms of physiological indicators of life and the favorable course of adaptive processes in newborns.So, summing up, we note that the child's condition is associated with an adequate biorhythmic daily activity. Metabolism lability in the newborn requires keeping in mind the structure of biorhythms in the postnatal period to prevent the development of morbid conditions.

Melatonin modulates daytime-dependent synaptic plasticity and learning efficiency.

Mechanisms of hippocampus-related memory formation are time-of-day-dependent. While the circadian system and clock genes are related to timing of hippocampal mnemonic processes (acquisition, consolidation, and retrieval of long-term memory [LTM]) and long-term potentiation (LTP), little is known about temporal gating mechanisms. Here, the role of the neurohormone melatonin as a circadian time cue for hippocampal signaling and memory formation was investigated in C3H/He wildtype (WT) and melatonin receptor-knockout ( ) mice. Immunohistochemical and immunoblot analyses revealed the presence of melatonin receptors on mouse hippocampal neurons. Temporal patterns of time-of-day-dependent clock gene protein levels were profoundly altered in mice compared to WT animals. On the behavioral level, WT mice displayed better spatial learning efficiency during daytime as compared to nighttime. In contrast, high error scores were observed in mice during both, daytime and nighttime acquisition. Day-night difference in LTP, as observed in WT mice, was absent in mice and in WT animals, in which the sympathetic innervation of the pineal gland was surgically removed to erase rhythmic melatonin synthesis. In addition, treatment of melatonin-deficient C57BL/6 mice with melatonin at nighttime significantly improved their working memory performance at daytime. These results illustrate that melatonin shapes time-of-day-dependent learning efficiency in parallel to consolidating expression patterns of clock genes in the mouse hippocampus. Our data suggest that melatonin imprints a time cue on mouse hippocampal signaling and gene expression to foster better learning during daytime.

A congenic line of the C57BL/6J mouse strain that is proficient in melatonin synthesis.

The C57BL/6J (B6) is the most common inbred mouse strain used in biomedical research in the United States. Yet, this strain is notoriously known for being deficient in the biosynthesis of melatonin, an important effector of circadian clocks in the brain and in the retina. Melatonin deficiency in this strain results from nonfunctional alleles of the genes coding 2 key enzymes of the melatonin synthesis pathway: arylalkylamine-N-acetyltransferase (Aanat) and N-acetylserotonin-O-methyltransferase (Asmt). By introducing functional alleles of the Aanat and Asmt genes from the melatonin-proficient CBA/CaJ (CBA) mouse strain to B6, we have generated a B6 congenic line that has acquired the capacity of rhythmic melatonin synthesis. In addition, the melatonin-dependent rhythm of dopamine release in the retina is restored in the B6 congenic line. Finally, we have partially characterized the Aanat and Asmt genes of the CBA strain and have identified multiple differences between CBA and B6 alleles, including single nucleotide polymorphism and deletion/insertion of DNA segments of various sizes. As an improved model organism with functional components of the melatonin synthesis pathway and melatonin-dependent circadian regulations, the new line will be useful to researchers studying melatonin physiological functions in a variety of fields including, but not limited to, circadian biology and neuroscience. In particular, the congenic line will be useful to speed up introduction of melatonin production capacity into genetically modified mouse lines of interest such as knockout lines, many of which are on B6 or mixed B6 backgrounds. The melatonin-proficient B6 congenic line will be widely distributed.

Heterogeneous ribonucleoprotein R regulates arylalkylamine N-acetyltransferase synthesis via internal ribosomal entry site-mediated translation in a circadian manner.

Rhythmic arylalkylamine N-acetyltransferase (AANAT) synthesis is a prominent circadian-controlled response that occurs in most mammals. AANAT is the core enzyme in melatonin production; because melatonin participates in many physiological processes, the regulation of AANAT is an important research topic. In this study, we focused on the role of heterogeneous ribonucleoprotein R (hnRNP R) in the translation of AANAT. A novel RNA-binding protein hnRNP R widely interacted with the 5' untranslated region (UTR) of AANAT mRNA and contributed to translation through an internal ribosomal entry site (IRES). Fine-tuning of AANAT protein synthesis occurred in response to knockdown and overexpression of hnRNP R. Nocturnal elevation of AANAT protein was dependent on the rhythmic changes of hnRNP R, whose levels are elevated in the pineal gland during nighttime. Increases in hnRNP R additionally improved AANAT production in rat pinealocytes under norepinephrine (NE) treatment. These results suggest that cap-independent translation of AANAT mRNA plays a role in the rhythmic synthesis of melatonin through the recruitment of translational machinery to hnRNP R-bound AANAT mRNA.

Comparative study of pineal clock gene and AANAT2 expression in relation to melatonin synthesis in Atlantic salmon (Salmo salar) and European seabass (Dicentrarchus labrax)

The photoreceptive teleost pineal is considered to be essential to the generation, synchronisation and maintenance of biological rhythms, primarily via melatonin release. The role of internal (circadian clock) and external (light) signals controlling melatonin production in the fish pineal differs between species, yet the reasons underpinning this remain largely unknown. Whilst in salmonids, pineal melatonin is apparently regulated directly by light, in all other studied teleosts, rhythmic melatonin production persists endogenously under the regulation of clock gene expression. To better understand the role of clocks in teleost pineals, this study aimed to characterise the expression of selected clock genes in vitro under different photoperiodic conditions in comparison to in vivo in both Atlantic salmon (Salmo salar) and in European seabass (Dicentrarchus labrax) (in vitro 12L:12D), a species known to display endogenous rhythmic melatonin synthesis. Results revealed no rhythmic clock gene (Clock, Period 1 &2) expression in Atlantic salmon or European seabass (Clock and Period 1) pineal in vitro. However rhythmic expression of Cryptochrome 2 and Period 1 in the Atlantic salmon pineal was observed in vivo, which infers extra-pineal regulation of clocks in this species. No rhythmic arylalkylamine N-acetyltransferase 2 (Aanat2) expression was observed in the Atlantic salmon yet in the European seabass, circadian Aanat2 expression was observed. Subsequent in silico analysis of available Aanat2 genomic sequences reveals that Atlantic salmon Aanat2 promoter sequences do not contain similar regulatory architecture as present in European seabass, and previously described in other teleosts which alludes to a loss in functional connection in the pathway.

Dynamics in enzymatic protein complexes offer a novel principle for the regulation of melatonin synthesis in the human pineal gland

Time of day is communicated to the body through rhythmic cues, including pineal gland melatonin synthesis, which is restricted to nighttime. Whereas in most rodents transcriptional regulation of the arylalkylamine N-acetyltransferase (Aanat) gene is essential for rhythmic melatonin synthesis, investigations into nonrodent mammalian species have shown post-transcriptional regulation to be of central importance, with molecular mechanisms still elusive. Therefore, human pineal tissues, taken from routine autopsies were allocated to four time-of-death groups (night/dawn/day/dusk) and analyzed for daytime-dependent changes in phosphorylated AANAT (p31T-AANAT) and in acetyl-serotonin-methyltransferase (ASMT) expression and activity. Protein content, intracellular localization, and colocalization of p31T-AANAT and ASMT were assessed, using immunoblotting, immunofluorescence, and immunoprecipitation techniques. Fresh sheep pineal gland preparations were used for comparative purposes. The amount of p31T-AANAT and ASMT proteins as well as their intracellular localization showed no diurnal variation in autoptic human and fresh sheep pineal glands. Moreover, in human and sheep pineal extracts, AANAT could not be dephosphorylated, which was at variance to data derived from rat pineal extracts. P31T-AANAT and ASMT were often found to colocalize in cellular rod-like structures that were also partly immunoreactive for the pinealocyte process-specific marker S-antigen (arrestin) in both, human and sheep pinealocytes. Protein-protein interaction studies with p31T-AANAT, ASMT, and S-antigen demonstrated a direct association and formation of robust complexes, involving also 14-3-3. This work provides evidence for a regulation principle for AANAT activity in the human pineal gland, which may not be based on a p31T-AANAT phosphorylation/dephosphorylation switch, as described for other mammalian species.

Evolution of AANAT: expansion of the gene family in the cephalochordate amphioxus

BackgroundThe arylalkylamine N-acetyltransferase (AANAT) family is divided into structurally distinct vertebrate and non-vertebrate groups. Expression of vertebrate AANATs is limited primarily to the pineal gland and retina, where it plays a role in controlling the circadian rhythm in melatonin synthesis. Based on the role melatonin plays in biological timing, AANAT has been given the moniker "the Timezyme". Non-vertebrate AANATs, which occur in fungi and protists, are thought to play a role in detoxification and are not known to be associated with a specific tissue.ResultsWe have found that the amphioxus genome contains seven AANATs, all having non-vertebrate type features. This and the absence of AANATs from the genomes of Hemichordates and Urochordates support the view that a major transition in the evolution of the AANATs may have occurred at the onset of vertebrate evolution. Analysis of the expression pattern of the two most structurally divergent AANATs in Branchiostoma lanceolatum (bl) revealed that they are expressed early in development and also in the adult at low levels throughout the body, possibly associated with the neural tube. Expression is clearly not exclusively associated with the proposed analogs of the pineal gland and retina. blAANAT activity is influenced by environmental lighting, but light/dark differences do not persist under constant light or constant dark conditions, indicating they are not circadian in nature. bfAANATα and bfAANATδ' have unusually alkaline (> 9.0) optimal pH, more than two pH units higher than that of vertebrate AANATs.ConclusionsThe substrate selectivity profiles of bfAANATα and δ' are relatively broad, including alkylamines, arylalkylamines and diamines, in contrast to vertebrate forms, which selectively acetylate serotonin and other arylalkylamines. Based on these features, it appears that amphioxus AANATs could play several roles, including detoxification and biogenic amine inactivation. The presence of seven AANATs in amphioxus genome supports the view that arylalkylamine and polyamine acetylation is important to the biology of this organism and that these genes evolved in response to specific pressures related to requirements for amine acetylation.

Restricted feeding restores rhythmicity in the pineal gland of arrhythmic suprachiasmatic‐lesioned rats

In mammals, the rhythmic synthesis of melatonin by the pineal gland is tightly controlled by the master clock located in the suprachiasmatic nuclei (SCN). In behaviourally arrhythmic SCN-lesioned rats, we investigated the effects of daily restricted feeding (RF) on pineal melatonin synthesis. RF restored not only a rhythmic transcription of the rate-limiting enzyme for melatonin biosynthesis [arylalkylamine-N-acetyltransferase (AANAT)] and a rhythmic expression of c-FOS but also a rhythmic synthesis of melatonin in the pineal gland. In control rats without functional SCN and fed ad libitum, a daily immobilization stress did not restore any rhythmicity in the pineal gland. Interestingly, a combination of RF and daily stress prior to the time of food access did not markedly impair AaNat mRNA and c-FOS rhythmicity but did abolish the restoration of rhythmic pineal melatonin. These data indicate that the synchronizing effects of RF on the pineal rhythmicity are not due to, and cannot be mimicked by, high levels of circulating glucocorticoids. In keeping with the multi-oscillatory nature of the circadian system, the rhythmicity of pineal melatonin in mammals, until now an exclusive output of the SCN, can also be controlled by daily feeding cues when the SCN clock is lacking. Thus, the present study demonstrates that daily RF in SCN-lesioned rats provides, probably via sympathetic fibres, synchronizing stimuli strong enough to drive rhythmicity in the pineal gland.

Photic and circadian regulation of melatonin production in the Mozambique tilapia Oreochromis mossambicus

Diverse circadian systems related to phylogeny and ecological adaptive strategies are proposed in teleosts. Recently, retinal photoreception was reported to be important for the circadian pacemaking activities of the Nile tilapia Oreochromis niloticus. We aimed to confirm the photic and circadian responsiveness of its close relative—the Mozambique tilapia O. mossambicus. Melatonin production in cannulated or ophthalmectomized fish and its secretion from cultured pineal glands were examined under several light regimes. Melatonin production in the cannulated tilapias was measured at 3-h intervals; it fluctuated daily, with a nocturnal increase and a diurnal decrease. Exposing the cannulated fish to several light intensities (1500–0.1 lx) and to natural light (0.1 and 0.3 lx) suppressed melatonin levels within 30 min. Static pineal gland culture under light–dark and reverse light–dark cycles revealed that melatonin synthesis increased during the dark periods. Rhythmic melatonin synthesis disappeared on pineal gland culture under constant dark and light conditions. After ophthalmectomy, plasma melatonin levels did not vary with light–dark cycles. These results suggest that (1) Mozambique tilapias possess strong photic responsiveness, (2) their pineal glands are sensitive to light but lack circadian pacemaker activity, and (3) they require lateral eyes for rhythmic melatonin secretion from the pineal gland.

Localization of a circadian clock in mammalian photoreceptors

Several studies have demonstrated that the mammalian retina contains an autonomous circadian clock. Dopaminergic and other inner retinal neurons express many of the clock genes, whereas some of these genes seem to be absent from the photoreceptors. This observation has led to the suggestion that in mammalian retina the circadian pacemaker driving retinal rhythms is located in the inner nuclear layer. However, other evidence points to the photoreceptor layer as the site of the mammalian retinal clock. The goal of the present study was to demonstrate the presence of a functional circadian clock in photoreceptors. First, using laser capture microdissection and reverse transcriptase-polymerase chain reaction, we investigated which of the clock genes are expressed in rat photoreceptors. We then prepared photoreceptor layer cultures from the retina to test whether these isolated cultures were viable and could drive circadian rhythms. Our data indicated that Per1, Per3, Cry1, Cry2, Clock, Bmal1, Rev-erb alpha, and Rora RNAs were present in the photoreceptors, whereas we were unable to amplify mRNA for Per2 and Npas2. Photoreceptor layers obtained from Period1-luciferase rats expressed a robust circadian rhythm in bioluminescence and melatonin synthesis. These results demonstrate that mammalian photoreceptors contain the circadian pacemaker driving rhythmic melatonin synthesis.

Discovering Light Effects on the Brain

Back to table of contents Previous article Next article Images in NeuroscienceFull AccessDiscovering Light Effects on the BrainSolomon H. Snyder M.D.Jimo Borjigin PH.D.,Paolo Sassone-Corsi, PH.D.,Solomon H. Snyder M.D.Search for more papers by this authorJimo Borjigin PH.D.Search for more papers by this author,Paolo Sassone-Corsi, PH.D.Search for more papers by this author,Published Online:1 May 2006AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail Circadian rhythms in the pineal gland were initially noted by Wilbur Quay in 1963. Axelrod and his colleagues showed it to be a true circadian rhythm, maintained during constant dark periods, but dramatically modulated by light. As seen in the diagram above, the neuronal pathway regulating melatonin synthesis in the rodent pineal gland connects 1) the retina to the suprachiasmatic nucleus, 2) the suprachiasmatic nucleus, through the intermediolateral cell column, to the superior cervical ganglion, and 3) the superior cervical ganglion to the pineal gland. Serotonin levels vary diurnally with the light/dark cycle: high levels in daylight, varying inversely with melatonin, which shows high levels in darkness. The biochemical link between serotonin and melatonin includes two enzymes that convert serotonin to melatonin (above): the first acetylating serotonin to form N -acetyl serotonin (5-HT N -acetyltransferase), and the other methylating N -acetyl serotonin to melatonin (hydroxyindole- O -methyltransferase). The molecular mechanisms of this process have been characterized as the following: nighttime release of noradrenaline in the mammalian pineal gland (which activates β-adrenergic receptors to stimulate adenylate cyclase activity and elevate cAMP) up-regulates the activity of 5-HT N -acetyltransferase and subsequently increases melatonin levels in a circadian fashion. The molecular signal is mediated by the expression of an inducible cAMP early repressor (ICER) factor that participates in an autoregulatory negative feedback process for melatonin synthesis at 5-HT N -acetyltransferase. An additional fundamental property of nighttime melatonin synthesis by the pineal gland is its sensitivity to the length of night, conveying not only the day/night transition to the animal but also seasonal information. The mechanism of this involves ICER factor, the levels of which reflect not only light and dark but also the duration of the darkness. Adaptation of an organism to a changing environment, mediated in part by these processes, is an essential feature of physiological regulation. Conversion of the day/night rhythm into hormonal oscillations governs human and animal metabolism. Melatonin is synthesized rhythmically in the pineal gland in response to signals originating from the biological clock in the suprachiasmatic nucleus. The molecular mechanisms involved in rhythmic synthesis of melatonin have been largely described and involve the rhythmically expressed ICER factor, which creates an autoregulatory loop that controls the amplitude of 5-HT N -acetyltransferase oscillations. Thus, the transcription factor (ICER) modulates the oscillatory levels of its target hormone (melatonin). The effect of altering the light/dark cycle, especially during seasons of long darkness with the goal of altering mood, is thought to be mediated to some degree by these molecular mechanisms. Baltimore, Md.Strasbourg, FranceAddress reprint requests to Dr. Tamminga, UT Southwestern Medical Center, Department of Psychiatry, 5323 Harry Hines Blvd., #NC5.914, Dallas, TX 75390-9070; [email protected] (e-mail). FiguresReferencesCited byDetailsCited ByExpert Opinion on Therapeutic Targets, Vol. 19, No. 6Clinical Psychology Review, Vol. 31, No. 2 Volume 163Issue 5 May, 2006Pages 771-771THE AMERICAN JOURNAL OF PSYCHIATRY May 2006 Volume 163 Number 5 Metrics PDF download History Published online 1 May 2006 Published in print 1 May 2006

Discovering Light Effects on the Brain

Circadian rhythms in the pineal gland were initially noted by Wilbur Quay in 1963. Axelrod and his colleagues showed it to be a true circadian rhythm, maintained during constant dark periods, but dramatically modulated by light. As seen in the diagram above, the neuronal pathway regulating melatonin synthesis in the rodent pineal gland connects 1) the retina to the suprachiasmatic nucleus, 2) the suprachiasmatic nucleus, through the intermediolateral cell column, to the superior cervical ganglion, and 3) the superior cervical ganglion to the pineal gland. Serotonin levels vary diurnally with the light/dark cycle: high levels in daylight, varying inversely with melatonin, which shows high levels in darkness. The biochemical link between serotonin and melatonin includes two enzymes that convert serotonin to melatonin (above): the first acetylating serotonin to form N-acetyl serotonin (5-HT N-acetyltransferase), and the other methylating N-acetyl serotonin to melatonin (hydroxyindole-O-methyltransferase). The molecular mechanisms of this process have been characterized as the following: nighttime release of noradrenaline in the mammalian pineal gland (which activates β-adrenergic receptors to stimulate adenylate cyclase activity and elevate cAMP) up-regulates the activity of 5-HT N-acetyltransferase and subsequently increases melatonin levels in a circadian fashion. The molecular signal is mediated by the expression of an inducible cAMP early repressor (ICER) factor that participates in an autoregulatory negative feedback process for melatonin synthesis at 5-HT Nacetyltransferase. An additional fundamental property of nighttime melatonin synthesis by the pineal gland is its sensitivity to the length of night, conveying not only the day/night transition to the animal but also seasonal information. The mechanism of this involves ICER factor, the levels of which reflect not only light and dark but also the duration of the darkness. Adaptation of an organism to a changing environment, mediated in part by these processes, is an essential feature of physiological regulation. Conversion of the day/night rhythm into hormonal oscillations governs human and animal metabolism. Melatonin is synthesized rhythmically in the pineal gland in response to signals originating from the biological clock in the suprachiasmatic nucleus. The molecular mechanisms involved in rhythmic synthesis of melatonin have been largely described and involve the rhythmically expressed ICER factor, which creates an autoregulatory loop that controls the amplitude of 5-HT N-acetyltransferase oscillations. Thus, the transcription factor (ICER) modulates the oscillatory levels of its target hormone (melatonin). The effect of altering the light/dark cycle, especially during seasons of long darkness with the goal of altering mood, is thought to be mediated to some degree by these molecular mechanisms.

Characterization of Human Melatonin Synthesis Using Autoptic Pineal Tissue

The mammalian pineal gland synthesizes rhythmically the hormone melatonin, which provides the body with a signal coding the duration of the night period. The ultimate enzymatic step in melatonin synthesis is achieved by the hydroxyindole O-methyltransferase (HIOMT); the rate-limiting enzyme is, however, the arylalkylamine N-acetyltransferase (AA-NAT). In contrast to the central importance of a transcriptional regulation of the Aa-nat gene for rodent melatonin synthesis, mechanisms in the human pineal gland are elusive. Therefore, pineal tissue, taken from regular autopsies (n = 69; postmortem intervals ranging from 9 to 147 h) was analyzed simultaneously for Aa-nat and Hiomt mRNA levels by PCR, AA-NAT activity using (14)C-acetyl-coenzyme A, HIOMT activity using S-adenosyl-l-[(14)C]-methionine, and melatonin content using an ELISA. Results were allocated to asserted time-of-death groups (day, 1,000 to 1,630 h; dusk, 1,630 to 2,200 h; night, 2,200 to 0730 h; dawn, 0730 to 1,000 h). RNA degradation rates of genes of interest ran in parallel, and, therefore, data normalization could be established, regardless of postmortem delay in tissue sampling. Aa-nat and Hiomt mRNA and HIOMT activity showed no diurnal rhythm. In contrast, a significant rhythm was found for the correlation between time of death and both AA-NAT activity and melatonin content, with elevated values during dusk and night. Presented data demonstrate that postmortem brain tissue can be used to detect the remnant of premortem adaptive changes in neuronal activity. In particular, our results give strong experimental support for the idea that transcriptional mechanisms are not dominant for the generation of rhythmic melatonin synthesis in the human pineal gland.

Time keeping by the quail’s eye: Circadian regulation of melatonin production

Previous studies have shown that eye removal disrupts the circadian body temperature and activity rhythms of Japanese quail supporting the hypothesis that the eyes act as pacemakers within the quail circadian system. Furthermore, the putative ocular pacemakers are coupled to the rest of the circadian system via neural and hormonal outputs. Although the neural pathway has yet to be identified, experiments suggest that the daily rhythm of ocular melatonin synthesis and release is the hormonal output. We sought to strengthen the hypothesis that the eyes are the loci of circadian pacemakers, and that melatonin output is involved, by examining melatonin secretion in cultured quail retinas. Using an in vitro flow-through system we demonstrated that (1) isolated retinal tissue could exhibit a rhythm of melatonin release, (2) the rhythm of melatonin synthesis is directly entrainable by 24-h light–dark cycles, and (3) supplementation of the culture medium with serotonin is necessary for robust, rhythmic production of melatonin in constant darkness. These results show definitively that the eyes are the loci of a biological clock and, in light of previous studies showing the disruptive effects of blinding on the circadian system, strengthen the hypothesis that the ocular clock is a circadian pacemaker that can affect the rest of the circadian system via the cyclic synthesis and release of melatonin. The quail retina is proving to be a valuable in vitro model for investigating properties of circadian pacemakers.

Diurnal and circadian rhythms in melatonin synthesis in the turkey pineal gland and retina

The pineal gland and retina of the turkey rhythmically produce melatonin. In birds kept under a daily light–dark (LD) illumination cycle melatonin concentrations in the pineal gland and retina were low during the light phase and high during the dark phase. A similar melatonin rhythm with high night-time values was also observed in the plasma. The pineal and retinal melatonin rhythms mirror oscillations in the activity of serotonin N-acetyltransferase (AANAT; the penultimate enzyme in the melatonin biosynthetic pathway). In contrast, in both the pineal gland and retina the activity of the enzyme hydroxyindole- O-methyltransferase (HIOMT) did not exhibit significant changes throughout the 24-h period. Acute exposure of turkeys to light at night dramatically decreased melatonin levels in the pineal gland, retina and plasma. The rhythms in AANAT activity and melatonin concentrations in the turkey pineal gland and retina were circadian in nature as they persisted under conditions of constant darkness (DD). Under DD, however, the amplitudes of AANAT and melatonin rhythms were significantly lower (by 50–80%) than those found under the LD cycle. The findings indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The rapid dampening of the rhythms under DD suggests that of these two regulatory factors, environmental light may be the primary stimulus in the maintenance of the high amplitude melatonin rhythms in the turkey.

The Rhythm and Blues of Gene Expression in the Rodent Pineal Gland

In all vertebrates, melatonin is rhythmically synthesized in the pineal gland and functions as a hormonal message, encoding for the duration of night. In rodents, the nocturnal rise and fall of the arylalkylamine N-ace-tyltransferase (AA-NAT) activity controls the rhythmic synthesis of melatonin. This rhythm is centered around the transcriptional regulation of the AA-NAT by two norepinephrine-inducible transcription factors, the activator CREB (Ca2+/cAMP-response element binding protein) and the inhibitor ICER (inducible cAMP early repressor). CREB is activated by phosphorylation, which is one of the fastest responses in pinealocytes upon adrenergic stimulation, occurring within minutes. ICER in turn accumulates only after several hours, a time gap resulting from the required de novo protein synthesis upon adrenergic stimulation. However, these molecular components of neuroendocrine signaling in the rodent pineal gland are supplemented by the impact of a variety of neurotransmitters and neuromodulators, and by translational and post-translational mechanisms. By molecular crosstalk, those different inputs on pinealocytes seem to fine-tune the shape of the melatonin signal, by interacting at various levels with the NE/cAMP/pCREB/ICER pathway. In addition, these alternate signaling routes may be important in acute "emergency" situations. Together, concerted signaling events in the rodent pineal gland help to generate a stable and reliable hormonal message of darkness for the body, that, however, can be altered rapidly upon sudden and unexpected "error" signals.

Glutamatergic clock output stimulates melatonin synthesis at night.

The rhythm of melatonin synthesis in the rat pineal gland is under the control of the biological clock, which is located in the suprachiasmatic nucleus of the hypothalamus (SCN). Previous studies demonstrated a daytime inhibitory influence of the SCN on melatonin synthesis, by using gamma-aminobutyric acid input to the paraventricular nucleus of the hypothalamus (PVN). Nevertheless, a recent lesion study suggested the presence of a stimulatory clock output in the control of the melatonin rhythm as well. In order to further investigate this output in acute in vivo conditions, we first measured the release of melatonin in the pineal gland before, during and after a temporary shutdown of either SCN or PVN neuronal activity, using multiple microdialysis. For both targets, SCN and PVN, the application of tetrodotoxin by reverse dialysis in the middle of the night decreased melatonin levels. Due to recent evidence of the existence of glutamatergic clock output, we then studied the effect on melatonin release of glutamate antagonist application within the PVN in the middle of the night. Blockade of the glutamatergic input to the PVN significantly decreased melatonin release. These results demonstrate that (i) neuronal activity of both PVN and SCN is necessary to stimulate melatonin synthesis during the dark period and (ii) glutamatergic signalling within the PVN plays an important role in melatonin synthesis.