Sensitive assay for serotonin N-acetyltransferase activity in rat pineal

Melatonin Synthesis Enzymes Activity: Radiometric Assays for AANAT, ASMT, and TPH

Melatonin is synthesized and secreted by the pineal gland in mammals. Its synthesis is triggered at night by norepinephrine released in the interstices of the gland. This nocturnal production is dependent on the transcription, translation, and/or activation of the enzymes arylalkylamine-N-acetyltransferase (AANAT), acetylserotonin O-methyltransferase (ASMT), and tryptophan hydroxylase (TPH). In this chapter, the methodology for the analysis of AANAT, ASMT, and TPH activities by radiometric assays will be presented. Several papers were published by our group utilizing these methodologies, evaluating the enzymes modulation by voltage-gated calcium channels, angiotensin II, insulin, anhydroecgonine methyl ester (AEME, crack-cocaine product), ethanol, monosodium glutamate (MSG), signaling pathways such as NFkB, and pathophysiological conditions such as diabetes.

The Effect of Melatonin on Locomotor Behavior and Muscle Physiology in the Sea Cucumber Apostichopus japonicus

Melatonin is a highly conserved hormone in evolutionary history. It occurs in numerous organisms and plays a role in the endocrine and immune systems. Locomotor behavior is a basic behavior in animals and is an important indicator of circadian rhythms, which are coordinated by the nervous and endocrine systems. To date, the effect of melatonin on locomotor behavior has been studied in vertebrates, including syrian hamsters, sparrows, rats, zebrafish, goldfish, and flatworms. However, there have been few studies of the effects of melatonin on locomotor behavior in marine invertebrates. The goals of present study were to show the existence of melatonin in the sea cucumber Apostichopus japonicus and to evaluate its effect on locomotor activity. In addition, muscle tissues from control and melatonin-treated sea cucumbers were tested using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) to determine the changes of metabolic activity in muscle. Melatonin was present in the coelomic fluid of A. japonicus at a concentration of ∼135.0 ng/L. The total distance traveled and number steps taken over 9 h after melatonin administration decreased with increasing concentration of the melatonin dose. Mean and maximum velocity of movement and stride length and stride frequency also decreased, but their differences were not statistically significant. Overall, these results suggest that melatonin administration had a sedative effect on A. japonicus. The levels of 22 different metabolites were altered in the muscle tissues of melatonin-treated sea cucumbers. Serotonin, 9-cis retinoic acid, all-trans retinoic acid, flavin mononucleotide in muscles were downregulated after melatonin administration. Moreover, a high free fatty acid (FFA) concentration and a decrease in the adenosine 5′-triphosphate (ATP) concentration in the muscle tissues of the melatonin-treated group were detected as well. These results suggest that the sedative effect of melatonin involves some other metabolic pathways, and the reduced locomotor modulator—serotonin, inhibited fatty acid oxidation and disturbed oxidative phosphorylation are potential physiological mechanisms that result in the inhibitory effect of melatonin on locomotion in sea cucumbers.

Chronic treatment with dexamethasone alters clock gene expression and melatonin synthesis in rat pineal gland at night

BACKGROUND

Melatonin is a neuroendocrine hormone that regulates many functions involving energy metabolism and behavior in mammals throughout the light/dark cycle. It is considered an output signal of the central circadian clock, located in the suprachiasmatic nucleus of the hypothalamus. Melatonin synthesis can be influenced by other hormones, such as insulin and glucocorticoids in pathological conditions or during stress. Furthermore, glucocorticoids appear to modulate circadian clock genes in peripheral tissues and are associated with the onset of metabolic diseases. In the pineal gland, the modulation of melatonin synthesis by clock genes has already been demonstrated. However, few studies have shown the effects of glucocorticoids on clock genes expression in the pineal gland.

RESULTS

We verified that rats treated with dexamethasone (2 mg/kg body weight, intraperitoneal) for 10 consecutive days, showed hyperglycemia and pronounced hyperinsulinemia during the dark phase. Insulin sensitivity, glucose tolerance, melatonin synthesis, and enzymatic activity of arylalkylamine N-acetyltransferase, the key enzyme of melatonin synthesis, were reduced. Furthermore, we observed an increase in the expression of Bmal1, Per1, Per2, Cry1, and Cry2 in pineal glands of rats treated with dexamethasone.

CONCLUSION

These results show that chronic treatment with dexamethasone can modulate both melatonin synthesis and circadian clock expression during the dark phase.

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

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

Developmental and light-entrained expression of melatonin and its relationship to the circadian clock in the sea anemone Nematostella vectensis

Background

The primary hormone of the vertebrate pineal gland, melatonin, has been identified broadly throughout the eukaryotes. While the role for melatonin in cyclic behavior via interactions with the circadian clock has only been reported in vertebrates, comparative research has shown that the transcription-translation loops of the animal circadian clock likely date to the cnidarian-bilaterian ancestor, leaving open significant questions about the evolutionary origin of melatonin signaling in circadian behavior by interacting with the molecular clock.

Results

Expression of melatonin in adult anemones showed peak expression at the end of light period (zeitgeber time (ZT) = 12) when cultured under diel conditions, coinciding with expression of genes and enzyme activity for members of the melatonin synthesis pathway (tryptophan hydroxylase and hydroxyindol-O-methyltransferase), which also showed rhythmic expression. During embryogenesis and juvenile stages, melatonin showed cyclic oscillations in concentration, peaking in midday. Spatial (in situ hybridization) and quantitative (real-time PCR) transcription of clock genes during development of N. vectensis showed these ‘clock’ genes are expressed early in the development, prior to rhythmic oscillations, suggesting functions independent of a function in the circadian clock. Finally, time-course studies revealed that animals transferred from diel conditions to constant darkness lose circadian expression for most of the clock genes within 4 days, which can be reset by melatonin supplementation.

Conclusions

Our results support an ancient role for melatonin in the circadian behavior of animals by showing cyclic expression of this hormone under diel conditions, light-dependent oscillations in genes in the melatonin synthesis pathway, and the function of melatonin in initiating expression of circadian clock genes in the cnidarian N. vectensis. The differences in expression melatonin and the circadian clock gene network in the adult stage when compared with developmental stages of N. vectensis suggests new research directions to characterize stage-specific mechanisms of circadian clock function in animals.

Melatonin synthesis impairment as a new deleterious outcome of diabetes-derived hyperglycemia

Melatonin is a neurohormone that works as a nighttime signal for circadian integrity and health maintenance. It is crucial for energy metabolism regulation, and the diabetes effects on its synthesis are unresolved. Using diverse techniques that included pineal microdialysis and ultrahigh-performance liquid chromatography, the present data show a clear acute and sustained melatonin synthesis reduction in diabetic rats as a result of pineal metabolism impairment that is unrelated to cell death. Hyperglycemia is the main cause of several diabetic complications, and its consequences in terms of melatonin production were assessed. Here, we show that local high glucose (HG) concentration is acutely detrimental to pineal melatonin synthesis in rats both in vivo and in vitro. The clinically depressive action of high blood glucose concentration in melatonin levels was also observed in type 1 diabetes patients who presented a negative correlation between hyperglycemia and 6-sulfatoxymelatonin excretion. Additionally, high-mean-glycemia type 1 diabetes patients presented lower 6-sulfatoxymelatonin levels when compared to control subjects. Although further studies are needed to fully clarify the mechanisms, the present results provide evidence that high circulating glucose levels interfere with pineal melatonin production. Given the essential role played by melatonin as a powerful antioxidant and in the control of energy homeostasis, sleep and biological rhythms and knowing that optimal glycemic control is usually an issue for patients with diabetes, melatonin supplementation may be considered as an additional tool to the current treatment.

Melatonin production in the sea star Echinaster brasiliensis (Echinodermata)

The primary hormone of the vertebrate pineal gland, melatonin, has been identified broadly throughout the tree of life, in animals, plants, and fungi, supporting a deep evolutionary origin for this signaling molecule. However, some key groups have not been studied. Echinoderms, deuterostome animals, are one of these groups. Herein we study the presence of melatonin and enzymes of its pathway in the sea star Echinaster brasiliensis. We demonstrate that E. brasiliensis produces endogenous melatonin, in the gonads, under a circadian pattern with a nocturnal peak of production. We also show that the enzymes arylalkylamine N-acetyltransferase (AANAT) and tryptophan hydroxylase (TPH) are present and are probably regulating the melatonin production.

Leptin modulates norepinephrine-mediated melatonin synthesis in cultured rat pineal gland

Pineal melatonin synthesis can be modulated by many peptides, including insulin. Because melatonin appears to alter leptin synthesis, in this work we aimed to investigate whether leptin would have a role on norepinephrine- (NE-)mediated melatonin synthesis in cultured rat pineal glands. According to our data, cultured rat pineal glands express leptin receptor isoform b (Ob-Rb). Pineal expression of Ob-Rb mRNA was also observed in vivo. Administration of leptin (1 nM) associated with NE ( 1 µM) reduced melatonin content as well as arylalkylamine-N-acetyl transferase (AANAT) activity and expression in cultured pineal glands. Leptin treatment per se induced the expression of STAT3 in cultured pineal glands, but STAT3 does not participate in the leptin modulation of NE-mediated pineal melatonin synthesis. In addition, the expression of inducible cAMP early repressor (ICER) was further induced by leptin challenge when associated with NE. In conclusion, leptin inhibition of pineal melatonin synthesis appears to be mediated by a reduction in AANAT activity and expression as well as by increased expression of Icer mRNA. Peptidergic signaling within the pineal gland appears to be one of the most important signals which modulates melatonin synthesis; leptin, as a member of this system, is not an exception.

Limited recovery of pineal function after regeneration of preganglionic sympathetic axons: evidence for loss of ganglionic synaptic specificity

The cervical sympathetic trunks (CSTs) contain axons of preganglionic neurons that innervate the superior cervical ganglia (SCGs). Because regeneration of CST fibers can be extensive and can reestablish certain specific patterns of SCG connections, restoration of end organ function would be expected. This expectation was examined with respect to the pineal gland, an organ innervated by the two SCGs. The activity of pineal serotonin N-acetyltransferase (NAT) exhibits a large circadian rhythm that is dependent on the sympathetic input of the gland, with high activity at night. Thirty-six hours after the CSTs were crushed bilaterally, nocturnal NAT was decreased by 99%. Three months later, enzyme activity had recovered only to 15% of control values, a recovery dependent on regeneration of CST fibers. Nevertheless, a small day/night rhythm was present in lesioned animals. Neither the density of the adrenergic innervation of the gland nor the ability of an adrenergic agonist to stimulate NAT activity was reduced in rats with regenerated CSTs. In addition, stimulation of the regenerated CST at a variety of frequencies was at least as effective in increasing NAT activity as seen with control nerves. These data suggest that the failure of pineal function to recover is not attributable to a quantitative deficit in the extent of reinnervation or synaptic efficacy. Rather, we suggest that there is some loss of specificity in the synaptic connections made in the SCG during reinnervation, resulting in a loss of the central neuronal information necessary for directing a normal NAT rhythm and thus normal pineal function.

Characterization of arylalkylamine N-acetyltransferase from silkmoth (Antheraea pernyi) and pesticidal drug design based on the baculovirus-expressed enzyme

Arylalkylamine N-acetyltransferase (AANAT; EC 2.3.1.87) catalyzes the N-acetylation of arylalkylamines. A cDNA encoding AANAT (ApAANAT) was cloned from Antheraea pernyi by PCR. The cDNA of 1966 bp encodes a 261 amino acid protein. The amino acid sequence was found to have a high homology with Bombyx mori AANAT (BmNAT) but had very low homology with vertebrate AANATs. Amino acid sequence analysis revealed that four insect AANATs cloned from three species including ApAANAT formed a distinct cluster from the vertebrate group. A recombinant ApAANAT protein was expressed in Sf9 cells using a baculovirus expression system, having AANAT activity. The transformed cell extract acetylated tryptamine, serotonin, dopamine, tyramine, octopamine and norepinephrine. The AANAT activity was inhibited at over 0.03 mM tryptamine. Although insect AANATs have been considered as a target of insecticide, this type of insecticide has never been developed. Screening a chemical library of Otsuka Chemical Co., Ltd., we found a novel compound and its derivatives that inhibited the AANAT activity of ApAANAT. This may facilitate investigation of the monoamine metabolic pathway in insects and the development of new types of insecticides and inhibitors of AANATs.

Ethanol consumption and pineal melatonin daily profile in rats

It is well known that melatonin participates in the regulation of many important physiological functions such as sleep-wakefulness cycle, motor coordination and neural plasticity, and cognition. However, as there are contradictory results regarding the melatonin production diurnal profile under alcohol consumption, the aim of this paper was to study the phenomenology and mechanisms of the putative modifications on the daily profile of melatonin production in rats submitted to chronic alcohol intake. The present results show that rats receiving 10% ethanol in drinking water for 35 days display an altered daily profile of melatonin production, with a phase delay and a reduction in the nocturnal peak. This can be partially explained by a loss of the daily rhythm and the 25% reduction in tryptophan hydroxylase activity and, mainly, by a phase delay in arylalkylamine N-acetyltransferase gene expression and a 70% reduction in its peak activity. Upstream in the melatonin synthesis pathway, the results showed that noradrenergic signaling is impaired as well, with a decrease in β1 and α1 adrenergic receptors' mRNA contents and in vitro sustained loss of noradrenergic-stimulated melatonin production by glands from alcohol-treated rats. Together, these results confirm the alterations in the daily melatonin profile of alcoholic rats and suggest the possible mechanisms for the observed melatonin synthesis modification.

Characterization of arylalkylamine N-acetyltransferase (AANAT) activities and action spectrum for suppression in the band-legged cricket, Dianemobius nigrofasciatus (Orthoptera: Gryllidae)

Arylalkylamine N-acetyltransferase (AANAT), constituting a large family of enzymes, catalyzes the transacetylation from acetyl-CoA to monoamine substrates, although homology among species is not very high. AANAT in vertebrates is photosensitive and mediates circadian regulation. Here, we analyzed AANAT of the cricket, Dianemobius nigrofasciatus. The central nervous system contained AANAT activity. The optimum pHs were 6.0 (a minor peak) and 10.5 (a major peak) with crude enzyme solution. We analyzed the kinetics at pH 10.5 using the sample containing collective AANAT activities, which we term AANAT. Lineweaver-Burk plot and secondary plot yielded a K(m) for tryptamine as substrate of 0.42 microM, and a V(max) of 9.39 nmol/mg protein/min. The apparent K(m) for acetyl-CoA was 59.9 microM and the V(max) was 8.14 nmol/mg protein/min. AANAT of D. nigrofasciatus was light-sensitive. The activity was higher at night-time than at day-time as in vertebrates. To investigate most effective wavelengths on AANAT activity, a series of monochromatic lights was applied (350, 400, 450, 500, 550, 600 and 650 nm). AANAT showed the highest sensitivity to around 450 nm and 550 nm. 450 nm light was more effective than 550 nm light. Therefore, the most effective light affecting AANAT activity is blue light, which corresponds to the absorption spectrum of blue wave (BW)-opsin.

Action spectrum for the suppression of arylalkylamine N-acetyltransferase activity in the two-spotted spider mite Tetranychus urticae

An action spectrum was obtained for the suppression of arylalkylamine N-acetyltransferase (NAT) activity in the two-spotted spider mite Tetranychus urticae by irradiating the mite with monochromatic lights of various wavelengths using the Okazaki Large Spectrograph at the National Institute for Basic Biology, Okazaki, Japan. Fluence-response curves were obtained for wavelengths between 300 and 650 nm by irradiating the mite for 4 h day(-1). The samples were frozen after the third exposure. A negative correlation between the logarithmic fluence rate and NAT activity was detected in the range of 0.01-1 micromol m(-2) s(-1) for wavelengths between 300 and 500 nm and in the range of 0.1-10 micromol m(-2) s(-1) for wavelengths between 550 and 650 nm. The constructed action spectrum indicated that the photoreceptors mediating the circadian and/or photoperiodic systems might be UV-A- and blue-type photoreceptors with absorption peaks at 350 and 450 nm.

UV radiation elevates arylalkylamine N-acetyltransferase activity and melatonin content in the two-spotted spider mite, Tetranychus urticae

Ultraviolet (UV) radiation produces reactive oxygen species (ROS) in mammals, where melatonin plays the role of a ROS scavenger. The melatonin synthetic enzyme arylalkylamine N-acetyltransferase (NAT) is a significant element in a possible ROS removal system. Changes in NAT activity and melatonin content were determined in the two-spotted spider mite Tetranychus urticae by irradiating it with monochromatic light using the Okazaki Large Spectrograph at the National Institute for Basic Biology, Okazaki, Japan. The NAT activity and melatonin content were suppressed by blue light (450nm). No effects of red light (650nm) on the NAT activity and melatonin content were observed. UV radiation had intensity-dependent dual effects on the NAT activity and melatonin content. In the UV-B (300nm) treatment, the NAT activity and melatonin content were suppressed at the intensity below 1micromolm(-2)s(-1) but elevated when the intensity was as high as 10micromolm(-2)s(-1). In the UV-A (350nm) treatment, the melatonin content was elevated when the intensity was as high as 10micromolm(-2)s(-1), though the NAT activity and melatonin content were suppressed at the intensity below 10 and 1micromolm(-2)s(-1), respectively. Elevation of the NAT activity and melatonin content by high intensity UV irradiation may indicate that the UV signals initiate melatonin synthesis for ROS removal in mites.

Tryptophan hydroxylase is modulated by L-type calcium channels in the rat pineal gland

Calcium is an important second messenger in the rat pineal gland, as well as cAMP. They both contribute to melatonin synthesis mediated by the three main enzymes of the melatonin synthesis pathway: tryptophan hydroxylase, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase. The cytosolic calcium is elevated in pinealocytes following alpha(1)-adrenergic stimulation, through IP(3)-and membrane calcium channels activation. Nifedipine, an L-type calcium channel blocker, reduces melatonin synthesis in rat pineal glands in vitro. With the purpose of investigating the mechanisms involved in melatonin synthesis regulation by the L-type calcium channel, we studied the effects of nifedipine on noradrenergic stimulated cultured rat pineal glands. Tryptophan hydroxylase, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase activities were quantified by radiometric assays and 5-hydroxytryptophan, serotonin, N-acetylserotonin and melatonin contents were quantified by HPLC with electrochemical detection. The data showed that calcium influx blockaded by nifedipine caused a decrease in tryptophan hydroxylase activity, but did not change either arylalkylamine N-acetyltransferase or hydroxyindole-O-methyltransferase activities. Moreover, there was a reduction of 5-hydroxytryptophan, serotonin, N-acetylserotonin and melatonin intracellular content, as well as a reduction of serotonin and melatonin secretion. Thus, it seems that the calcium influx through L-type high voltage-activated calcium channels is essential for the full activation of tryptophan hydroxylase leading to melatonin synthesis in the pineal gland.

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

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

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

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

Effects of the blockade of high voltage-activated calcium channels on in vitro pineal melatonin synthesis

The presence of high voltage-activated calcium channels in the rat pineal gland is well known. However, their role in pineal metabolism is not completely understood and is even controversial. Better to understand this matter, we investigated the effects of L-, N- or P/Q-type calcium channel blockers (nifedipine, omega-conotoxin GVIA, omega-agatoxin IVA, respectively) on melatonin content and arylalkylamine-N-acetyltransferase activity of denervated rat pineal glands kept for 48 h in culture and stimulated with norepinephrine. Melatonin was measured by high performance liquid chromatography with electrochemical detection and arylalkylamine-N-acetyltransferase activity was quantified by radiometric assay. Pre-incubation with any of these high voltage-activated calcium channel blockers reduced the melatonin production induced by norepinephrine although arylalkylamine-N-acetyltransferase activity was reduced only by the N-type calcium channel antagonist, omega-conotoxin GVIA. The results indicate that calcium influx through L-, N- or P/Q-type of high voltage-activated calcium channels is necessary for the full expression of the metabolic process leading to melatonin synthesis in the rat pineal glands. However, the mechanisms involved in this process are different for the L- or P/Q- and N-type calcium channels.

Diabetic Goto Kakizaki rats as well as type 2 diabetic patients show a decreased diurnal serum melatonin level and an increased pancreatic melatonin-receptor status

There are functional inter-relationships between the beta cells of the endocrine pancreas and the pineal gland, where the synchronizing circadian molecule melatonin originates. The aim of this study was to elucidate a putative interaction between insulin and melatonin in diabetic patients and a diabetic rat model. We analyzed glucose, insulin, and melatonin levels of type 2 patients, as well as type 2 diabetic Goto Kakizaki (GK) rats by radioimmunoassay. Expression of pancreatic melatonin and pineal insulin receptors, as well as arylalkylamine-N-acetyltransferase (AANAT), was determined by real-time reverse transcriptase polymerase chain reaction (RT-PCR). The AANAT enzyme activity was measured in pineal homogenates. Diabetic patients showed a decrease in melatonin levels, while in the pancreas of GK rats an upregulation of the melatonin-receptor mRNA was determined. The pancreatic islets of GK rats showed expression of the mRNA for the pancreatic melatonin (MT1) receptor, which had previously been identified in rats and insulinoma (INS1) cells. Besides their presence in animal cells, the MT1-receptor transcript was also detected in human pancreas by RT-PCR. Whereas the rat pancreatic mRNA expression of the MT1-receptor was significantly increased, the activity of the pineal AANAT enzyme was reduced. The latter observation was in accordance with plasma melatonin levels. The insulin-receptor mRNA of the pineal gland was found to be reduced in GK rats. Our observations suggest a functional inter-relationship between melatonin and insulin, and may indicate a reduction of melatonin in the genesis of diabetes.

Rat pineal arylalkylamine-N-acetyltransferase: cyclic AMP inducibility of its gene depends on prior entrained photoperiod

The nocturnal biosynthesis of melatonin in the rat pineal depends on strongly enhanced expression of the enzyme N-acetyltransferase [arylalkylamine N-acetyltransferase (AA-NAT); EC 2.3.1.87]. AA-NAT transcription is stimulated during darkness by adrenergic inputs to the pineal from the suprachiasmatic nucleus (SCN). Nocturnal activation of the AA-NAT promotor following stimulation of pinealocyte adrenoceptors involves cAMP-dependent stimulation of protein kinase A (PKA). The nocturnal rise in AA-NAT depends on the lighting conditions. As compared with light/dark (LD) 12:12, the delay between dark onset and the nocturnal rise in AA-NAT is shortened under long photoperiods and prolonged under short photoperiods. Here, we report that the rapidity of nocturnal AA-NAT induction depends on cAMP inducibility of the gene. Accordingly, cAMP produces a strong AA-NAT response in pineals obtained from rats housed under long photoperiods and a weak AA-NAT response under short photoperiods. Changes in AA-NAT inducibility are fully developed not earlier than after seven cycles. This observation suggests that long-term changes in the photoperiod are necessary to achieve full adjustment of cAMP inducibility of the gene. A direct relationship was found between cAMP-dependent AA-NAT inducibility and the pineal protein kinase A (PKA) activity. As compared to LD 12:12, PKA activity was increased under LD 20:4 and attenuated under LD 4:20. On the basis of the present findings, we suggest that the photoperiod determines the effectiveness of nocturnal AA-NAT induction by long-term modulation of the intrapineal pathway that transmits the cAMP signal to the AA-NAT gene.

The phototransduction cascade in the isolated chick pineal gland revisited

It is well established that the isolated chick pineal gland is directly light sensitive and that melatonin synthesis of the gland can be inhibited by exposing the gland to light during scotophase. Since not all the steps of the phototransduction cascade have been clarified to the same extent as in the retina, we have treated isolated chick pineal glands with 90 min of light during scotophase and with drugs that affect key-components of vertebrate phototransduction, i.e., cyclic guanosine monophosphate (cGMP) phosphodiesterase 6 (PDE6), cGMP levels and cGMP-gated calcium channels. The endpoint measured was the activity of the rate-limiting enzyme of melatonin synthesis, arylalkylamine N-acetyltransferase (AA-NAT), which is inhibited by light. The effects on AA-NAT activity of light were negated by addition of dipyridamol and zaprinast, either of which inhibits the light-induced activation of PDE6. The effect of light was also counteracted by the nitric oxide donor sodium nitroprusside and C-type natriuretic peptide, both of which increase cGMP levels, and by the calcium channel agonist Bay K 8644, which prevents the cGMP-decrease-induced closure of cGMP-gated calcium channels. Inhibition of nitric oxide synthase (NOS) by N(G)-nitro-l-arginine did not influence the inhibitory effect of light, suggesting that the NOS pathway does not play a role. Since the light effect on AA-NAT activity involves both cGMP and cyclic adenosine monophosphate (cAMP) hydrolysis, we have also studied whether the cGMP-inhibited cAMP phosphodiesterase 3 (PDE3) is involved. As the specific PDE3 inhibitor cilostamide is without effect, we assume that the light-induced decrease of cAMP levels does not involve PDE3. These results taken together strongly suggest that the investigated steps of the phototransduction cascade in the isolated chick pineal gland are basically similar to those in the retina.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Multiple forms of arylalkylamine N-acetyltransferase (NAT) from cockroach female colleterial glands and activity changes during oocyte maturation

Arylalkylamine N-acetyltransferase (NAT) from the female colleterial glands of Periplaneta americana showed activity peaks at pH 6.0 and 9.5 and the pH profile changed during oogenesis. The left gland contained higher activity than the right gland but the right gland also contained recognizable activity. The patterns in activity change depended on the substrate used, tryptamine (TN) or serotonin (5-HT). When TN was used as the substrate, the alkaline peak was higher than the acidic peak. In contrast, when 5-HT was used, the acidic peak was much higher than the alkaline peak. This suggests that at least two NATs are present in this species that are specific to pH and substrate species. Of the four combinations of the two pH ranges and two substrate indolamines, the enzyme activity that showed a similar change to the oocyte maturation was obtained in the combination of pH 6.0 and TN. TN was actually detected in the colleterial glands by fluorescent measurements according to Hess and Uderfriend [J. Pharmacol. Exp., 127 (1959) 175-177]. It peaked on the 6th day of emergence, which corresponded to the first rise of oocyte length and yolk accumulation, whereas a small peak appeared in the phase of the second rise. TN, or more likely N-acetyl TN, may therefore be involved in the regulation of oocyte maturation which could be a novel mechanism in oocyte maturation.

Pineal melatonin synthesis and release are not altered throughout the estrous cycle in female rats

Melatonin times reproduction with seasons in many photoperiodic mammalian species. Whether sexual hormones reflect on melatonin synthesis is still debated. The aim of this work was to study, using a large panel of technical approaches, whether the daily profile of pineal melatonin synthesis and release varies with the estrous cycle in the female rat. The mRNA levels and enzyme activities of the melatonin synthesizing enzymes, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase were similar at the four stages of the rat estrous cycle. The endogenous release of melatonin, followed by transpineal microdialysis during six consecutive days in cycling female rats, displayed no significant variation during this interval. Taken together, the present results demonstrate that there is no regular fluctuation in the pineal metabolism leading to melatonin synthesis and release throughout the estrous cycle in female rats.

The induction of low nocturnal secretion of melatonin caused by reverse feeding rhythms depends on availability of macronutrient diets

The present study investigated the effect of single chow diet and two-way dietary choice between a protein-rich and a carbohydrate-rich diet ingested during free access or daytime access (08.00-16.00 h) on the rhythms of ingestion and on nocturnal levels of serum melatonin and pineal N-acetyltransferase activity in rats. Animals with free access to the single or the choice diets displayed a circadian rhythmicity of total food intake with peaks at the beginning, the middle and the end of the dark phase. Rats fed the dietary choice displayed a preference for carbohydrate at the beginning, and a preference for protein at the middle and the end of the dark phase. Under daytime feeding access, both dietary conditions resulted in only one peak of total intake at the beginning of the access period. Rats fed the choice diets on the daytime feeding schedule initially preferred carbohydrate to protein but this preference disappeared subsequently. Nocturnal peaks of serum melatonin and pineal NAT activity were lower with daytime access to dietary choice compared to ad libitum access. The results suggest that in a two-way selection of macronutrients, feeding cycle can directly act on the rhythms expressed within the pineal gland.

Characterization and purification of polymorphic arylalkylamine N-acetyltransferase from the American cockroach, Periplaneta americana

We separated two forms of arylalkylamine N-acetyltransferase (AANAT) from various organs of the American cockroach, Periplaneta americana. Both forms of the enzyme had an equivalent molecular mass of 28 kDa. One form isolated from the testicular accessory glands had high enzyme activity at acidic pHs. The isoelectric point was 5-6 and the substrate specificity was wider than the other type. The other isolated form from female midguts had a higher level of enzyme activity at basic pHs. These findings suggested that P. americana contains polymorphic AANAT, as is the case in Drosophila melanogaster. These forms differed not only in pH specificity, and substrate specificity but in chromatographic behavior and kinetic properties. Most of the organs we examined contained a mixture of the two forms since two types of AANAT activity were separated in different chromatographic fractions when two pH conditions were used for activity measurement.

Food access schedule and diet composition alter rhythmicity of serum melatonin and pineal NAT activity

This study investigated the effect of dietary composition and food access schedule on the rhythmicity of serum melatonin and pineal N-acetyltransferase (NAT) activity. Wistar rats maintained on a 12:12 h light-dark cycle were assigned to two dietary groups: a group fed rat chow and a group fed a choice between a protein-rich and a carbohydrate-rich diet. Each dietary group was further divided based on feeding schedule, with food available between 0800 and 1600 h or ad lib access to food. Regardless of dietary condition, total food and carbohydrate intake of rats having free access to food was higher than under the restricted food access schedule. Protein intake of rats fed the dietary choice was lower with the restricted access than in the free access. In rats fed the dietary choice, melatonin levels and NAT activity were significantly decreased with restricted access compared to free access. Such results were not found in rats offered restricted chow. This study suggests that the rhythms of melatonin secretion and NAT activity can be altered by dietary composition.

Serine/threonine phosphatase inhibitors decrease adrenergic arylalkylamine n-acetyltransferase induction in the rat pineal gland

Adrenergic regulation of the pineal enzyme serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AA-NAT); EC 2.3.1.87] accounts for the circadian rhythm in melatonin formation. In the present study, the role of protein phosphatases in the adrenergic regulation of rat pineal AA-NAT was investigated using specific inhibitors. In cultured pineals, the serine/threonine phosphatase type 1 and type 2A inhibitors okadaic acid and calyculin A significantly decreased adrenergically or cAMP-induced AA-NAT activity, whereas the serine/threonine phosphatase type 2B inhibitor cypermethrin and tyrosine phosphatase inhibitor dephostatin were ineffective. Reverse transcriptase-polymerase chain reaction (RT-PCR) data indicate that okadaic acid exerts its effect on cAMP-dependent AA-NAT induction by downregulating the amount of AA-NAT transcript. The 'third' messengers, inducible cAMP early repressor (ICER) and Fos-related antigene-2 (Fra-2), are believed to play a negative role in pineal AA-NAT transcription. Okadaic acid increased the cAMP responsiveness of neither ICER mRNA nor Fra-2 mRNA. Therefore, the regulatory role of pineal serine/threonine phosphatases in adrenergically stimulated AA-NAT expression probably does not depend on ICER or Fra-2.

Sulfhydryl G proteins and phospholipase A(2)-associated G proteins are involved in adrenergic signal transduction in the rat pineal gland

The rat pineal gland with its circadian noradrenaline-regulated melatonin rhythm is an excellent model for studying adrenergic signal transduction with respect to cAMP and cGMP formation. The stimulatory G(s) proteins play a well-established role in this process. In contrast, the potential roles of the inhibitory G(i) proteins, the functionally unclear other G(o) proteins, and a number of G protein subtypes are not known. The present study examines the effects on beta(1)- and beta(1)-plus-alpha(1)-stimulated cAMP and cGMP formation of a number of G protein modulators in rat pinealocyte suspension cultures. The effects of the nitric oxide donor sodium nitroprusside on cGMP were also examined. The results showed that drugs that activate G proteins of the G(i)/G(o) family, i.e., pertussis toxin, mastoparan, and compound 48/80, had no effect on unstimulated, isoproterenol (beta(1))-stimulated, or combined isoproterenol/phenylephrine (beta(1)-plus()-alpha(1))-stimulated cAMP and cGMP accumulation. However, in this experimental paradigm, the inhibitors of sulfhydryl G proteins (N-ethylmaleimide) and those of phospholipase A2-related G proteins (isotetrandrine) exerted a clear inhibitory effect. Sodium-nitroprusside-stimulated cGMP accumulation was also inhibited. These results confirm a previous report that members of the G(i)/G(o) family, which are present in the rat pineal gland, do not play a major role in adrenergic signal transduction. The new finding that sulfhydryl G proteins and phospholipase A2-associated G proteins exert a clear stimulatory effect on adrenergic signal transduction suggests that they are subtypes of G(s) proteins.

Influence of photoperiod on N-acetyltransferase activity and melatonin in the fiddler crab Uca pugilator

Melatonin and N-acetyltransferase (NAT) activity were measured in the eyestalks of fiddler crabs acclimated to various photoperiods: constant light, a L:D 12:12 h photoperiod, or constant dark. Following acclimation, eyestalks were collected every 3 h over a 24-h period; they were assayed for melatonin with a radioimmunoassay and for NAT activity with a radioenzymatic assay. In constant light, melatonin levels increased at 1300 h, from 142 to 431 pg x mg(-1) eyestalk; NAT activity increased concurrently, from 97 to 203 pmol x h(-1) x mg(-1) eyestalk, and both remained elevated until 0400 h. In the L:D 12:12 h photoperiod, melatonin levels increased at 1300 h from 28 to 230 pg x mg(-1) eyestalk, and though NAT activity increased significantly, from 80 to 122 pmol x h(-1) x mg(-1) eyestalk, an even greater increase occurred at 0400 h, when melatonin levels were low. In constant dark, melatonin levels increased at 1600 h, from 22 to 196 pg x mg(-1) eyestalk, with a concurrent increase in NAT activity from 93 to 140 pmol x mg(-1) x h(-1) eyestalk. However, the second peak in melatonin (111 pg x mg(-1)), occurring at 0400 h, was out of phase with the second peak of NAT activity (113 pmol x mg(-1) x h(-1) eyestalk) which occurred at 0700 h. NAT may be a rate-limiting step in melatonin synthesis in fiddler crabs under some conditions (constant light and the 1300 h peak in constant dark); however, NAT activity correlates poorly with melatonin levels in a L:D 12:12 h photoperiod and in constant dark relative to the 0400 h melatonin peak.

Effects of restricted food access on circadian fluctuation of serotonin N-acetyltransferase activities in hereditary microphthalmic rats

The characteristics in diurnal fluctuation of serotonin N-acetyltransferase activity were examined in normal and microphthalmic mutant rats of the Donryu strain under ad lib or restricted feeding conditions. Under a 12:12-h light:dark (12-h LD) cycle with free access to food, normal-sighted rats exhibited typical nocturnal increases in the activity of pineal serotonin N-acetyltransferase, being more than 50-fold higher in the dark period than that in the light period, but hereditary blind rats showed nonperiodic change in the pineal enzyme activity in the average, suggesting that the rhythms in individuals have become free-running, asynchronous. When the subjective night or subjective day of the mutants was discerned by active or inactive in the locomotor activity, the pineal enzyme activities in the mutants increased at the subjective night but depressed at the subjective daytime. When food access was restricted only for 6 h in the light period of the LD cycle, normal rats still showed the nocturnal increases in the pineal enzyme activity, but hereditary blind rats manifested a blunt peak in the activity of the pineal enzyme at eating time in the light period. The results suggest that microphthalmic mutant rats maintain the ability to shift and to synchronize their circadian phases induced by restricted access to food, even if they completely lack their optic nerve and visual input to the circadian clock.

Effects of cycloheximide and aminophylline on 5-methoxytryptophol and melatonin contents in the chick pineal gland

The chick pineal gland rhythmically synthesizes two 5-methoxyindoles, melatonin and 5-methoxytryptophol. These rhythms are circadian in nature and have opposite phases. The aim of this study was to determine the effects of cycloheximide, a protein synthesis inhibitor, and aminophylline, an inhibitor of phosphodiesterase, on 5-methoxytryptophol content in the chick pineal gland and to compare this with the drugs' action on pineal melatonin production. Inhibition of melatonin biosynthesis by cycloheximide (1 mg/kg, i.p. ), revealed by a marked reduction in the nighttime activity of serotonin N-acetyltransferase (AA-NAT; a key regulatory enzyme in melatonin synthesis) and melatonin concentrations, was accompanied by a significant increase in 5-methoxytryptophol content. In contrast, administration of aminophylline (100 mg/kg, i.p.) to light-exposed chicks significantly increased pineal AA-NAT activity and melatonin levels and decreased 5-methoxytryptophol concentrations. It is concluded that in the chick the production of pineal 5-methoxytryptophol and melatonin is inversely correlated.

A differential role of CREB phosphorylation in cAMP-inducible gene expression in the rat pineal

In the rat pineal gland cAMP mediates nocturnal induction of the enzyme arylalkylamine N-acetyltransferase (AA-NAT) as well as of transcription factors such as inducible cAMP early repressor (ICER), Fos-related antigen-2 (Fra-2) and JunB. Cyclic AMP stimulates the phosphorylation of the DNA binding protein cAMP response element binding protein (CREB). While cAMP-induced CREB phosphorylation appears to be a prerequisite for AA-NAT and ICER gene expression, it is not known whether CREB phosphorylation accounts for the full cAMP response of the two genes. Furthermore, the significance of CREB phosphorylation in cAMP-activated Fra-2 and JunB transcription is unknown. In the present in vitro study we used the serine/threonine protein phosphatase inhibitor okadaic acid (OA) to phosphorylate CREB without altering intrapineal cAMP concentration. It was observed that OA (10(-7) M) was less effective than dibutyryl cAMP (dbcAMP; 10(-3) M) in inducing AA-NAT mRNA and ICER mRNA, respectively. On the basis of this finding, it is concluded that CREB phosphorylation alone is apparently not sufficient for the full cAMP response of the two genes. By contrast, OA and dbcAMP equally stimulated the accumulation of the mRNAs of Fra-2 and JunB. Therefore cAMP may induce Fra-2 and JunB transcripts via CREB phosphorylation. Our observations suggest that CREB phosphorylation plays a critical role in diversification of cAMP-dependent gene induction in the rat pineal.

HIOMT drives the photoperiodic changes in the amplitude of the melatonin peak of the Siberian hamster

In the pineal, melatonin (Mel) is synthesized from serotonin by arylalkylamine-N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT). Although it is clear that AA-NAT drives the daily rhythm in Mel synthesis, the mechanisms involved in the photoperiodic changes of the amplitude of the Mel peak, as observed in the Siberian hamster, remain to be determined. We investigated the characteristics of AA-NAT and HIOMT in Siberian hamsters kept either under a short (SP) or a long photoperiod (LP). The amplitude of the nocturnal peak of Mel was about two times higher under SP than under LP, whereas AA-NAT activity was about two times smaller under SP. In contrast, a twofold increase of HIOMT activity was observed under SP compared with LP. No change in the affinity of the enzymes for their substrates was observed between the two photoperiods. Our data strongly suggest that the photoperiodic variations in the amplitude of the nocturnal peak of Mel are driven by HIOMT, thereby promoting an important physiological role for this enzyme in the seasonal regulation of Mel production.

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.

Rapid and simple measurement of serotonin N-acetyltransferase activity by liquid biphasic diffusion assay

We report here a rapid, simple, and accurate method to assay for serotonin N-acetyltransferase (NAT) activity. This assay relies on the selective diffusion of radiolabeled acetyltryptamine into a water-immiscible scintillation fluid. Unlike organic solvent extraction, thin-layer chromatography, or high performance liquid chromatography, the separation of acetyltryptamine from acetyl CoA and tryptamine is not required in the method. Moreover, the limit of sensitivity is less than 4 pmol of N-acetyltryptamine formed per sample. Enhancement of NAT activity upon beta-adrenergic receptor stimulation in the rat pineal gland was clearly detected with this method. In addition, the NAT activity measurements obtained with this method agreed quantitatively in the pineal gland and other brain tissues with the conventional organic solvent extraction method. The results suggest that this liquid biphasic diffusion assay is applicable to the detection of NAT activity in tissues and cells.

Identification of highly elevated levels of melatonin in bone marrow: its origin and significance

Bone marrow is an important tissue in generation of immunocompetent and peripheral blood cells. The progenitors of hematopoietic cells in bone marrow exhibit continuous proliferation and differentiation and they are highly vulnerable to acute or chronic oxidative stress. In this investigation, highly elevated levels of the antioxidant melatonin were identified in rat bone marrow using immunocytochemistry, radioimmunoassay, high performance liquid chromatography with electrochemical detection and mass spectrometry. Night-time melatonin concentrations (expressed as pg melatonin/mg protein) in the bone marrow of rats were roughly two orders of magnitude higher than those in peripheral blood. Measurement of the activities of the two enzymes (N-acetyltransferase (NAT) and hydroxyindole-O-methoxyltransferase (HIOMT)) which synthesize melatonin from serotonin showed that bone marrow cells have measurable NAT activity, but they have very low levels of HIOMT activity (at the one time they were measured). From these studies we could not definitively determine whether melatonin was produced in bone marrow cells or elsewhere. To investigate the potential pineal origin of bone marrow melatonin, long-term (8-month) pinealectomized rats were used to ascertain if the pineal gland is the primary source of this antioxidant. The bone marrow of pinealectomized rats, however, still exhibited high levels of melatonin. These results indicate that a major portion of the bone marrow's melatonin is of extrapineal origin. Immunocytochemistry clearly showed a positive melatonin reaction intracellularly in bone marrow cells. A melatonin concentrating mechanism in these cells is suggested by these findings and this may involve a specific melatonin binding protein. Since melatonin is an endogenous free radical scavenger and an immune-enhancing agent, the high levels of melatonin in bone marrow cells may provide on-site protection to reduce oxidative damage to these highly vulnerable hematopoietic cells and may enhance the immune capacity of cells such as lymphocytes.

Biochemical characterization of recombinant serotonin N-acetyltransferase

Pineal and retinal melatonin synthesis is controlled by the enzymatic activity of arylalkylamine N-acetyltransferase (AA-NAT, EC 2.3.1.87), which is regulated by light/dark signals and circadian factors. This enzyme converts serotonin to N-acetylserotonin by the transfer of an acetyl group from acetyl coenzyme A. Endogenous AA-NAT instability during routine purification has made enzyme characterization difficult, but now a stable recombinant protein for AA-NAT has been synthesized to investigate the intrinsic biochemical properties of AA-NAT from a rat pineal cDNA encoding a 205 amino acid, 23 kilodalton protein, by using a glutathione-S-transferase (GST) fusion protein system. Recombinant GST-AA-NAT showed substrate specificity for arylalkylamines and stability at 4 degrees C; however, the enzyme activity was reduced by 40% upon preincubation at 37 degrees C for 2 hr. GST-AA-NAT is preferentially phosphorylated by either cyclic AMP- or cyclic GMP-dependent kinases in vitro, but no detrimental effect was observed on AA-NAT enzymatic activity. Among the metal cations tested in this study, Ca2+, Mg2+, Mn2+, Fe2+, and Co2 showed little or no inhibitory potency, while either 1 mM Zn2+ or 0.1 mM Cu2+ nearly abolished the enzymatic activity. GST-AA-NAT enzyme activity is also inhibited by reagents that are known biochemically to modify thiol groups (N-ethylmaleimide, NEM) and histidine residues (p-chloromercuribenzoate, NBS and diethyl pyrocarbonate, DEPC), suggesting the presence of essential cysteine and histidine moieties. Moreover, preincubation of acetyl CoA completely protects the recombinant AA-NAT from inactivation by NEM and DEPC, indicating that specific cysteine and histidine residues may be at the acetylation site. The conclusion is that the biochemical properties of rat recombinant AA-NAT is similar to the endogenous pineal and retinal AA-NAT with respect to the sensitivity to temperature, metal cations, as well as the thiol modification reagents. These data also suggest that the phosphorylation status of the AA-NAT does not affect enzymatic activity directly, and histidine residues are potentially important residues required for high catalytic activity.

Age-related differences in serum melatonin and pineal NAT activity and in the response of rat pineal to a 50-Hz magnetic field

In a previous study we have shown that exposure to a 50-Hz sinusoidal magnetic field decreased serum melatonin concentration and pineal enzyme activities in young rats (9 weeks). In the present study we looked for the effect of a magnetic field of 100 microT on serum melatonin and pineal NAT activity in aged rats and compared them to young rats. We hypothesized that aging may change sensitivity of rats to a magnetic field. Two groups of Wistar male rats [aged rats (23 months) and young rats (9 weeks)] were exposed to 50-Hz magnetic fields of 100 microT for one week (18h/day). The animals were kept under a standard 12:12 light: dark cycle with a temperature of 25 degrees C and a relative humidity of 45 to 50%. Control (sham-exposed) animals were kept in a similar environment but without exposure to a magnetic field. The animals were sacrificed under red dim light. Serum melatonin concentration and pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) activities were studied. Our results showed that sinusoidal magnetic fields altered the production of melatonin (28% decrease; P <0.05) through an inhibition of pineal NAT activity (52% decrease; P <0.05) in the young rats whereas no effect was observed in aged ones. On the other hand, when comparing data from control animals between young and aged rats, we observed that serum melatonin level and NAT activity, but not HIOMT activity, decreased in aged rats (decrease by about 38% and 36% respectively). Our data strongly suggest that old rats are insensitive to the magnetic field.

The expression of the melatonin synthesis enzyme: arylalkylamine N-acetyltransferase in the suprachiasmatic nucleus of rat brain

The hormone melatonin, secreted primarily from the pineal gland, plays an important physiological role in synchronizing biological rhythms and neuroendocrine. Presently, we find the expression of the serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT) mRNA, the rate-limiting enzyme in the conversion of serotonin to melatonin, in the rat suprachiasmatic nucleus (SCN) which contains the biological circadian clock in mammals. AA-NAT mRNA content in rat SCN did not show a significant circadian rhythm. However, AA-NAT enzyme activity was lowest at midday and highest at early night, and the rhythm persisted under constant dark conditions. These results indicate that the rat SCN is capable of synthesizing melatonin and suggest that melatonin synthesis in the SCN may be regulated by the circadian clock at the post transcriptional level.

Arginine vasotocin activates phosphoinositide signal transduction system and potentiates N-acetyltransferase activity in the rat pineal gland

The pineal gland is innervated by pinealopetal peptidergic fibers originating in the hypothalamic nuclei which release arginine vasopressin (AVP) and arginine vasotocin (AVT) from their endings. Since the mechanism of AVT action on the pineal signal transduction and melatonin synthesis has not been determined so far, we examined the effect of AVT on the phosphoinositide signalling system and the N-acetyltransferase (NAT) activity in the rat pineal gland. The effect of AVP 4-9 fragment and AVP analogue desmopressin was also tested. The phosphoinositide signalling system was studied by measuring 32P labelling of phosphatidylinositol (PI), phosphatidylinositol phosphate (PIP) and phosphatidylinositol bisphosphate (PIP2) which reflects PI cycle activation. AVT (10(-5) and 10(-4) M) induced a significant increase in 32P labelling of PI, PIP and PIP2. The AVT mediated activation of the PI signal cascade was supressed by the vasopressin V1 receptor antagonist. The desmopressin and AVP 4-9 fragment were without the effect on PI signalling. To assess the AVT role in the melatonin synthesis we studied the daily pattern of the pineal NAT activity in rats treated by AVT (10 microg/100 g b.w). AVT application in the dark period of the day significantly increased nocturnal NAT activity. It can be summarized that AVT activates PI signalling system and potentiates NAT activity in the rat pineal gland.