Arylalkylamine (serotonin) N-acetyltransferase assay using high-performance liquid chromatography with fluorescence or electrochemical detection of N-acetyltryptamine

Changes in the Metabolic Profile of Melatonin Synthesis-Related Indoles during Post-Embryonic Development of the Turkey Pineal Organ

Research on age-dependent changes in pineal activity has been limited almost exclusively to melatonin (MLT). This study determined, for the first time, the alterations occurring in the metabolic profile of MLT synthesis-related indoles during the post-embryonic development period in birds. Turkeys reared under a 12 h light/dark cycle were euthanized at 2 h intervals for 24 h at the ages of 2, 7, 14, and 28 days and 10, 20, 30, and 45 weeks. The results showed prominent changes in the metabolic profile of indoles during development and could be distinguished in four stages. The first stage, from hatching to the age of 2 weeks, was characterized by a decrease in the 5-hydroxytryptophan concentration and an increase in the concentrations of serotonin (5-HT), MLT, 5-methoxyindoleacetic acid, and 5-methoxytryptamine (5-MTAM). During the second stage, around the age of 1 month, the concentrations of N-acetylserotonin (NAS) and MLT reached a maximum. The synthesis and degradation of 5-HT were also the highest. The third stage, around the age of 10 weeks, was characterized by decreased levels of 5-HT (approximately 50%) and 5-hydroxyindoleacetic acid and a high level of 5-MTAM. The last stage, covering the age of 20 to 45 weeks, was characterized by a large decrease in the synthesis, content, and degradation of 5-HT. Despite these changes, there were no prominent differences in the nocturnal levels of NAS and MLT between the third and fourth stages. The concentrations of all tryptophan derivatives showed daily fluctuations until the age of 45 weeks.

Metabolism of Melatonin Synthesis-Related Indoles in the Turkey Pineal Organ and Its Modification by Monochromatic Light

The metabolism of pineal indoles is closely related to alterations in the light and dark phases of a daily cycle. Recent research showed important interspecies differences in the pineal biochemistry, and a strong impact of monochromatic light on many physiological processes in birds. Therefore, the aims of study were to characterize the metabolism of melatonin-synthesis indoles in the pineal organ of the domestic turkey, and to determine the changes occurring in this metabolism under the influence of different wavelengths and intensities of light. For this purpose, 3-week-old turkeys were kept under 16 lx white light, or under blue, green, and red light with intensities of 16, 32, and 64 lx during the photophase, and after 7 d were sacrificed at 4 h intervals. The activities of melatonin-synthesizing enzymes and the contents of indoles were measured in the same pineal organ. The results revealed that the activities of tryptophan hydroxylase and arylalkylamine N-acetyltransferase, and the levels of all tryptophan derivatives had significant daily changes in birds kept under each light condition used. The profile of pineal indole metabolism in 4-week-old turkeys was characterized by high-amplitude rhythms in the activity of arylalkylamine N-acetyltransferase and the contents of N-acetylserotonin and melatonin, equal relative amounts of serotonin and 5-hydroxyindoleacetic acid, and higher content of melatonin than N-acetylserotonin. The monochromatic light significantly modified the pineal indole metabolism, and its effects were dependent on the color and intensity of light. Pronounced changes occurred in the level of serotonin synthesis and the daily rhythm course of melatonin synthesis.

A new arylalkylamine N-acetyltransferase in silkworm (Bombyx mori) affects integument pigmentation

Dopamine is a precursor for melanin synthesis. Arylalkylamine N-acetyltransferase (AANAT) is involved in the melatonin formation in insects because it could catalyze the transformation from dopamine to dopamine-N-acetyldopamine. In this study, we identified a new AANAT gene in the silkworm (Bombyx mori) and assessed its role in the silkworm. The cDNA of this gene encodes 233 amino acids that shares 57 % amino acid identity with the Bm-iAANAT protein. We thus refer to this gene as Bm-iAANAT2. To investigate the role of Bm-iAANAT2, we constructed a transgenic interference system using a 3xp3 promoter to suppress the expression of Bm-iAANAT2 in the silkworm. We observed that melanin deposition occurs in the head and integument in transgenic lines. To verify the melanism pattern, dopamine content and the enzyme activity of AANAT were determined by high-performance liquid chromatography (HPLC). We found that an increase in dopamine levels affects melanism patterns on the heads of transgenic B. mori. A reduction in the enzyme activity of AANAT leads to changes in dopamine levels. We analyzed the expression of the Bm-iAANAT2 genes by qPCR and found that the expression of Bm-iAANAT2 gene is significantly lower in transgenic lines. Our results lead us to conclude that Bm-iAANAT2 is a new arylalkylamine N-acetyltransferase gene in the silkworm and is involved in the metabolism of the dopamine to avoid the generation of melanin.

Photic regulation of arylalkylamine N-acetyltransferase binding to 14-3-3 proteins in retinal photoreceptor cells

14-3-3 proteins are a ubiquitous, highly conserved family of chaperone proteins involved in signal transduction, regulation of cell cycle, intracellular trafficking/targeting, cytoskeletal structure, and transcription. Although 14-3-3 proteins are among the most abundant proteins in the CNS, very little is known about their functional roles in the vertebrate retina. In the present study, we demonstrated that photoreceptors express 14-3-3 protein(s) and identified a 14-3-3 binding partner in photoreceptor cells, the melatonin-synthesizing enzyme arylalkylamine N-acetyltransferase (AANAT). Importantly, our data demonstrate that the binding of 14-3-3 to AANAT is regulated by light, with dramatic functional consequences. During the night in darkness, retinal AANAT is phosphorylated and forms a complex with 14-3-3 proteins with an apparent molecular weight of approximately 90 kDa. Phosphorylation of AANAT facilitates the binding of enzyme to 14-3-3 proteins. Within the complex, AANAT is catalytically activated and protected from dephosphorylation and degradation. Light disrupts the AANAT/14-3-3 complex, leading to catalytic inactivation, dephosphorylation, and proteolytic degradation of the enzyme. In the presence of the proteasome inhibitor, lactacystin, light results in the formation of a high molecular weight complex (>150 kDa), which may represent an intermediate in the AANAT degradation process. These findings provide new insight into the roles of 14-3-3 proteins in photoreceptor cells and to the mechanisms controlling melatonin synthesis in the vertebrate retina.

Serotonin metabolism in rat skin: characterization by liquid chromatography-mass spectrometry

We have recently uncovered the full expression of novel cutaneous serotoninergic and melatoninergic systems in the human and hamster skin. In this work, we have characterized serotonin metabolism in the rat skin using liquid chromatography-mass spectrometry and found that serotonin undergoes acetylation in the presence of acetyl coenzyme A. Inhibition of serotonin acetylation with Cole bisubstrate inhibitor shows that rat skin expresses both arylalkylamine and arylamine N-acetyltransferase activities. The serotonin degradation product-5-hydroxyindole acetic acid is also detected and pargyline (monoaminooxidase inhibitor) suppresses almost completely 5-hydroxyindole acetic acid accumulation. Together with previous data, the present study clearly demonstrates that biotransformation of serotonin in mammalian skin follows two alternate pathways. In the first pathway, serotonin is acetylated by arylalkylamine and arylamine N-acetyltransferases to generate the precursor of melatonin. Alternately, serotonin may undergo oxidative deamination by monoaminooxidase followed by enzymatic degradation by aldehyde dehydrogenase into 5-hydroxyindole acetic acid, which is presumably devoid of biological activity. Thus, the current methodological development of a liquid chromatography-mass spectrometry-based assay allows rapid resolution of the cutaneous metabolism of serotonin.

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

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

Characterization of the serotoninergic system in the C57BL/6 mouse skin

We showed expression of the tryptophan hydroxylase gene and of tryptophan hydroxylase protein immunoreactivity in mouse skin and skin cells. Extracts from skin and melanocyte samples acetylated serotonin to N-acetylserotonin and tryptamine to N-acetyltryptamine. A different enzyme from arylalkylamine N-acetyltransferase mediated this reaction, as this gene was defective in the C57BL6 mouse, coding predominantly for a protein without enzymatic activity. Serotonin (but not tryptamine) acetylation varied according to hair cycle phase and anatomic location. Serotonin was also metabolized to 5-hydroxytryptophol and 5-hydroxyindole acetic acid, probably through stepwise transformation catalyzed by monoamine oxidase, aldehyde dehydrogenase and aldehyde reductase. Activity of the melatonin-forming enzyme hydroxyindole-O-methyltransferase was notably below detectable levels in all samples of mouse corporal skin, although it was detectable at low levels in the ears and in Cloudman melanoma (derived from the DBA/2 J mouse strain). In conclusion, mouse skin has the molecular and biochemical apparatus necessary to produce and metabolize serotonin and N-acetylserotonin, and its activity is determined by topography, physiological status of the skin, cell type and mouse strain.

Melatonin synthesis in retina: circadian regulation of arylalkylamine N-acetyltransferase activity in cultured photoreceptor cells of embryonic chicken retina

The key regulatory enzyme in melatonin synthesis is arylalkylamine N-acetyltransferase (AANAT). In vivo, AANAT activity in chicken retinal photoreceptor cells exhibits a circadian rhythm that peaks at night. The purpose of the present study was to investigate the temporal development of light/dark and circadian oscillations of AANAT activity in cultured retinal cells prepared from 6- and 8-day-old chicken embryos (E6, E8, respectively). Photoreceptor cells prepared from E6 retinas and incubated under a 14-h light/10-h dark (LD) cycle of illumination for 5-7 days displayed prominent daily fluctuations in AANAT activity on days 5 and 6 in vitro. However, when E6 cells, incubated for 5 days under LD, were transferred to continuous (24 h/day) darkness (DD) on day 6, no daily pattern of activity was observed. This result indicates that AANAT fluctuations were light-driven and not circadian at this stage. In contrast, cells prepared from E8 embryos and incubated under conditions identical to those for E6 cells displayed prominent rhythms of AANAT activity in both LD and DD, indicative of circadian control. To determine if circadian control of AANAT activity would develop in E6 cells incubated for a longer period of time to allow maturation, cells were incubated for 8 days in LD followed by 2 days in DD. AANAT activity in these cells was rhythmic in both LD and DD. In cells incubated in this manner, a 2-h light pulse in the middle of the subjective night suppressed AANAT activity, indicating that the enzyme activity in the cultured cells is acutely suppressed by light, as it is in vivo. These results indicate that the ability to express circadian regulation of AANAT activity is an intrinsic property of retinal cells that can develop in vitro. Development of light-dark regulation of AANAT activity appears to precede the circadian clock-control of enzyme activity.

Voltammetric and flow amperometric methods for the determination of melatonin in pharmaceuticals

Melatonin can be sensitively detected in pharmaceuticals by two different and simple electrochemical methods: cyclic voltammetry (CV) and amperometric detection in a flow injection analysis system (FIA-ED). An adequate pre-treatment of the carbon paste electrode in the first case and the employ of a high flow rate in the second one were the key for obtaining a very good reproducibility (R.S.D. values of 1.5 (n=10) and 1.3% (n=20), respectively). Low limits of detection were achieved and with the coupling of a flow system a linear dynamic range of three orders of magnitude (from 10(-8) to 10(-5) M) was obtained. Both methods were applied to the determination of melatonin in pharmaceuticals. In order to best validate these methodologies a fluorescent procedure was developed to contrast the results. As no interferences from the matrix were found the employ of a separation technique is not necessary. In this way the procedure is fastened and simplified. Moreover, the low price, ease of handling, possibility of automation and high sample throughput are important advantages that convert the flow methodology in an attractive alternative for quality control of pharmaceuticals.

An intramolecular disulfide bridge as a catalytic switch for serotonin N-acetyltransferase

Serotonin N-acetyltransferase (EC. 2.3.1.87) (AA-NAT) is a melatonin rhythm-generating enzyme in pineal glands. To establish a melatonin rhythm, AA-NAT activity is precisely regulated through several signaling pathways. Here we show novel regulation of AA-NAT activity, in which an intramolecular disulfide bond may function as a switch for the catalysis. Recombinant AA-NAT activity was irreversibly inhibited by N-ethylmaleimide (NEM) in an acetyl-CoA-protected manner. Oxidized glutathione or dissolved oxygen reversibly inhibited AA-NAT in an acetyl-CoA-protected manner. To identify the cysteine residues responsible for the inhibition, AA-NAT was first oxidized with dissolved oxygen, treated with NEM, reduced with dithiothreitol, and then labeled with [(14)C]NEM. Cys(61) and Cys(177) were specifically labeled in an acetyl-CoA-protected manner. The AA-NAT with the Cys(61) to Ala and Cys(177) to Ala double substitutions (C61A/C177A-AA-NAT) was fully active but did not exhibit sensitivity to either oxidation or NEM, whereas the AA-NATs with only the single substitutions retained about 40% of these sensitivities. An intramolecular disulfide bond between Cys(61) and Cys(177) formed upon oxidation and cleaved upon reduction was identified. Furthermore, C61A/C177A-AA-NAT expressed in COS7 cells was relatively insensitive to H(2)O(2)-evoked oxidative stress, whereas wild-type AA-NAT was strongly inhibited under the same conditions. These results indicate that the formation and cleavage of the disulfide bond between Cys(61) and Cys(177) produce the active and inactive states of AA-NAT. It is possible that intracellular redox conditions regulate AA-NAT activity through switching via an intramolecular disulfide bridge.

Serotoninergic system in hamster skin

We have cloned the tryptophan hydroxylase cDNA from hamster pituitary and demonstrated its expression in the skin, melanotic and amelanotic melanomas, spleen, heart, and the eye. We further demonstrated that skin, melanomas, spleen, pituitary, and eye but not heart expressed arylalkylamine N-acetyltransferase mRNA. The cutaneous expression of the arylalkylamine N-acetyltransferase gene was accompanied by enzymatic activity for the conversion of serotonin and tryptamine to N-acetylserotonin and N-acetyltryptamine, respectively. There was marked regional variation in the serotonin N-acetyltransferase activity, which was higher in ear skin than in corpus skin, and was lower in melanomas than in normal skin. Serotonin N-acetyltransferase activity was significantly inhibited by Cole bisubstrate at low concentration (</= 1 micro m); this evidence in conjunction with arylalkylamine N-acetyltransferase mRNA expression implies an involvement of arylalkylamine N-acetyltransferase in serotonin metabolism in the skin. We also documented both the in vitro transformation of serotonin to N-acetylserotonin using liquid chromatography/mass spectrometry and the generation/storage of N-acetylserotonin in cultured melanoma cells. Thus, we have uncovered a cutaneous pathway displaying capabilities for serotonin biosynthesis and/or its metabolism to N-acetylserotonin in rodent skin. As serotonin has powerful vasodilator, immunomodulator, and growth factor actions, this pathway could be involved in skin physiology and/or pathology.

Serotoninergic and melatoninergic systems are fully expressed in human skin

We investigated the cutaneous expression of genes and enzymes responsible for the multistep conversion of tryptophan to serotonin and further to melatonin. Samples tested were human skin, normal and pathologic (basal cell carcinoma and melanoma), cultured normal epidermal and follicular melanocytes, melanoma cell lines, normal neonatal and adult epidermal and follicular keratinocytes, squamous cell carcinoma cells, and fibroblasts from dermis and follicular papilla. The majority of the samples showed simultaneous expression of the genes for tryptophan hydroxylase, arylalkylamine N-acetyltransferase (AANAT), and hydroxyindole-O-methyltransferase (HIOMT). The products of AANAT activity were identified by RP-HPLC with fluorimetric detection in human skin and in cultured normal and malignant melanocytes and immortalized keratinocytes; HIOMT activity was detected in human skin, keratinocytes, and melanoma cells. N-acetylserotonin (NAS) was detected by RP-HPLC in human skin extracts. NAS identity was confirmed further by LC/MS in keratinocytes. In conclusion, we provide evidence that the human skin expresses intrinsic serotonin and melatonin biosynthetic pathways.

Conversion of L-tryptophan to serotonin and melatonin in human melanoma cells

We showed in human melanoma cells tryptophan hydroxylase (TPH) and hydroxyindole methyltransferase genes expression with the sequential enzymatic activities of TPH, serotonin (Ser) N-acetyltransferase and hydroxyindole methyltransferase. The presence of the products Ser, 5OH-tryptophan, N-acetylserotonin, melatonin (Mel), 5-methoxytryptamine and 5-methoxytryptophol was documented by liquid chromatography-mass spectrometry. Thus, human melanoma cells can synthesize and metabolize Ser and Mel.

High-capacity screening of arylalkylamine N-acetyltransferase inhibitors using a high-performance liquid chromatography system

Systematic screening is a natural development of any pharmacological program. Most enzyme inhibitor screens use indirect or "aspecific" methods, such as colorimetric or fluorimetric ones. These screening methods cause quite a few false-positive and false-negative hits. In order to limit these as much as possible, we developed a methodology using a high-performance liquid chromatography (HPLC) system for the medium throughput screening of serotonin N-acetyltransferase inhibitors. The core of this screening system is (1) the dramatic shortening of the analytical time down to 100 s per run by using a high-performance analytical column (Turbo), and (2) the use of absorption as opposed to radioactivity for detection of the product of the reaction (N-acetylserotonin). This system permits the analysis of about 1,000 compounds per day to be performed with a single HPLC system. This enzymatic system was taken as an example, because the methodology can be extended to many other enzymes, particularly transferases, phosphatases, and kinases.

Substrate specificity and inhibition studies of human serotonin N-acetyltransferase

Arylalkylamine N-acetyltransferase (AANAT) catalyzes the reaction of serotonin with acetyl-CoA to form N-acetylserotonin and plays a major role in the regulation of the melatonin circadian rhythm in vertebrates. In the present study, the human cloned enzyme has been expressed in bacteria, purified, cleaved, and characterized. The specificity of the human enzyme toward substrates (natural as well as synthetic arylethylamines) and cosubstrates (essentially acyl homologs of acetyl-CoA) has been investigated. Peptide combinatorial libraries of tri-, tetra-, and pentapeptides with various amino acid compositions were also screened as potential sources of inhibitors. We report the findings of several peptides with low micromolar inhibitory potency. For activity measurement as well as for specificity studies, an original and rapid method of analysis was developed. The assay was based on the separation and detection of N-[(3)H]acetylarylethylamine formed from various arylethylamines and tritiated acetyl-CoA, by means of high performance liquid chromatography with radiochemical detection. The assay proved to be robust and flexible, could accommodate the use of numerous synthetic substrates, and was successfully used throughout this study. We also screened a large number of pharmacological bioamines among which only one, tranylcypromine, behaved as a substrate. The synthesis and survey of simple arylethylamines also showed that AANAT has a large recognition pattern, including compounds as different as phenyl-, naphthyl-, benzothienyl-, or benzofuranyl-ethylamine derivatives. An extensive enzymatic study allowed us to pinpoint the amino acid residue of the pentapeptide inhibitor, S 34461, which interacts with the cosubstrate-binding site area, in agreement with an in silico study based on the available coordinates of the hAANAT crystal.

Hydroxyindole-O-methyltransferase is another target for L-glutamate-evoked inhibition of melatonin synthesis in rat pinealocytes

Rat pinealocytes use L-glutamate as a modulator for melatonin synthesis. Upon binding of L-glutamate to the class II metabotropic glutamate receptor, norepinephrine (NE)-dependent formation of cAMP was inhibited, resulting in decreased serotonin-N-acetyltransferase (NAT) activity and melatonin output. Although L-glutamate at 1 mM caused 90% inhibition of melatonin synthesis, about 30% of the NAT activity remained, suggesting the presence of another target for L-glutamate. In this study, we found that L-glutamate also inhibits hydroxyindole-O-methyltransferase (HIOMT). The inhibition is reversible and dose-dependent: the maximal inhibition was obtained with more than 0.4 mM L-glutamate. Contrary to L-glutamate-evoked inhibition of NAT, agonists for class II metabotropic receptors such as (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) had no effect on HIOMT. Neither (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG), an specific antagonist for class II mGluRs, nor dibutyryl cAMP restored the L-glutamate-evoked inhibition of HIOMT. Northern blot analyses revealed that L-glutamate significantly inhibits the expression of mRNA of NAT, but not that of HIOMT. These results indicated that HIOMT is an another target for L-glutamate due to its inhibition of melatonin synthesis, and the signaling pathway toward the inhibition is distinct from that of NAT.

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.

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

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

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

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

Acetylcholine triggers L-glutamate exocytosis via nicotinic receptors and inhibits melatonin synthesis in rat pinealocytes

Rat pinealocytes, melatonin-secreting endocrine cells, contain peripheral glutaminergic systems. L-Glutamate is a negative regulator of melatonin synthesis through a metabotropic receptor-mediated inhibitory cAMP cascade. Previously, we reported that depolarization of pinealocytes by externally added KCl and activation of L-type Ca2+ channels resulted in secretion of L-glutamate by microvesicle exocytosis. What is unknown is how and what kinds of stimuli trigger glutamate exocytosis under physiological conditions. Here, we report that the nicotinic acetylcholine receptor can trigger glutamate exocytosis from cultured rat pinealocytes. Moreover, acetylcholine or nicotine inhibited norepinephrine-dependent serotonin N-acetyltransferase activity, which results in decreased melatonin synthesis. These activities were blocked by (2S,3S, 4S)-2-methyl-2-(carboxycyclopropyl)glycine, an antagonist of the metabotropic glutamate receptor. These results suggest that cholinergic stimulation initiates the glutaminergic signaling cascade in pineal glands and that parasympathetic neurons innervating the gland exert negative control over melatonin synthesis by way of the glutaminergic systems.

D-aspartate modulates melatonin synthesis in rat pinealocytes

It has been known that pinealocytes contain the highest level of D-aspartate among various neuroendocrine cells in the rat. Here, we report that exogenous D-aspartate strongly inhibited norepinephrine-dependent melatonin synthesis in the rat pineal gland, the concentration required for 50% inhibition being 75 microM. This inhibition was due at least partly to decreased norepinephrine-dependent serotonin N-acetyltransferase activity. Upon incubation, D-aspartate was gradually released from pinealocytes and accumulated in the incubation medium as determined by high-performance liquid chromatography on a Pirkle-type chiral column. These results suggest that D-aspartate acts as a negative regulator for melatonin synthesis in the pineal gland.

Metabotropic glutamate receptors negatively regulate melatonin synthesis in rat pinealocytes

Rat pinealocytes receive noradrenergic innervation that stimulates melatonin synthesis in a cAMP-mediated manner. In addition to melatonin, we showed previously that pinealocytes secrete L-glutamate through an exocytic mechanism. The released glutamate inhibits norepinephrine (NE)-dependent melatonin synthesis. Consistent with this observation, specific agonists of class II metabotropic glutamate receptors (mGluRs), including 1-(1S,3R)-aminocyclopentane-1,3-dicarboxylic acid (tACPD), inhibited NE-dependent melatonin synthesis, whereas agonists for other types of glutamate receptors did not. Furthermore, reverse transcription-PCR, Northern blotting, and immunohistochemistry analyses indicated expression of class II mGluR3 in pinealocytes. Inhibitory guanine nucleotide-binding protein (Gi) was also detected in pinealocytes. L-Glutamate or agonists of class II receptors decreased NE- or forskolin-dependent increase of cAMP and serotonin-N-acetyltransferase activities to similar extents. These effects were blocked by pertussis toxin or dibutyryl cAMP. These results indicate that the inhibitory cAMP cascade is involved in the glutamate-evoked inhibition of melatonin synthesis. We propose that the glutaminergic system negatively regulates NE-dependent melatonin synthesis in rat pinealocytes.

Kinetic analysis of the catalytic mechanism of serotonin N-acetyltransferase (EC 2.3.1.87)

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87) is the penultimate enzyme in melatonin biosynthesis. This enzyme is of special biological interest because large changes in its activity drive the large night/day rhythm in circulating melatonin in vertebrates. In this study the kinetic mechanism of AANAT action was studied using bacterially expressed glutathione S-transferase (GST)-AANAT fusion protein. The enzymologic behavior of GST-AANAT and cleaved AANAT was essentially identical. Two-substrate kinetic analysis generated an intersecting line pattern characteristic of a ternary complex mechanism. The dead end inhibitor analog desulfo-CoA was competitive versus acetyl-CoA and noncompetitive versus tryptamine. Tryptophol was not an alternative substrate but was a dead end competitive inhibitor versus tryptamine and an uncompetitive inhibitor versus acetyl-CoA, indicative of an ordered binding mechanism requiring binding of acetyl-CoA first. N-Acetyltryptamine, a reaction product, was a noncompetitive inhibitor versus tryptamine and uncompetitive with respect to acetyl-CoA. Taken together these results support an ordered BiBi ternary complex (sequential) kinetic mechanism for AANAT and provide a framework for inhibitor design.

Melatonin and serotonin N-acetyltransferase activity in developing eggs of the cricket Gryllus bimaculatus

Melatonin (N-acetyl-5-methoxytryptamine) and serotonin N-acetyltransferase (NAT), a key regulatory enzyme in melatonin synthesis, are present in the adults and larvae of several insect species, as well as in vertebrates. To determine when melatonin and NAT first appear in insects ontogenetically, melatonin levels and NAT-like activity were measured in developing eggs of the cricket Gryllus bimaculatus. When the eggs were incubated under a 12-h light/12-h dark (LD) cycle at 24-26 degrees C, melatonin was detected in the egg extracts at all of the developmental stages examined. NAT-like activity was first found in the eggs 3 days after oviposition. From 5 to 11 days after oviposition, both NAT-like activity and melatonin levels showed significant day/night changes with the high levels occurring during the dark period of the LD cycle. By contrast, significant day/night changes were not detected in eggs just before hatching. To determine more detailed temporal changes, NAT-like activity was assayed in eggs 6 to 7 days after oviposition at 2- or 4-h intervals over a 48-h period. The activity in the eggs clearly exhibited a diurnal rhythm, peaking in the dark period of the LD cycle, and the rhythm persisted in constant darkness. These results suggest that the cricket egg (probably the embryo) synthesizes melatonin, and that its melatonin synthesis may fluctuate with a circadian rhythm. In addition, the results of the present study strongly suggest that a circadian clock controlling NAT activity functions in the cricket at the embryonic stage.

Detection of melatonin and serotonin N-acetyltransferase and hydroxyindole-O-methyltransferase activities in rat ovary

Melatonin (N-acetyl-5-methoxytryptamine) and the activities of two melatonin-synthesizing enzymes, serotonin N-acetyltransferase (acetyl coenzyme A: arylalkylamine N-acetyltransferase EC 2.3.1.87; NAT) and hydroxyindole-O-methyltransferase (S-adenosyl-L-methionine: N-acetylserotonin-O-methyltransferase EC 2.1.1.4; HIOMT), were assayed in extracts of ovaries obtained from virgin Wistar-derived rats (7-9 week-old) during the light period of a 12 h light/12 h dark cycle. Melatonin was detected in the rat ovary using reverse-phase high-performance liquid chromatography (HPLC) coupled with fluorometric detection and radioimmunoassay (RIA). In addition, NAT and HIOMT activities were found in rat ovary. The apparent Michaelis constants (Km) for the substrates of NAT and HIOMT in the rat ovary were similar to those reported for the pineal gland and retina. These data suggest that the rat ovary, like the pineal gland and the retina, may synthesize melatonin from serotonin by the sequential action of NAT and HIOMT.

L-aspartate-evoked inhibition of melatonin production in rat pineal glands

Our previous studies in rat indicated that pinealocytes secrete L-glutamate through microvesicle-mediated exocytosis to regulate negatively melatonin production. Recently, we further found that pinealocytes secrete L-aspartate through microvesicle-mediated exocytosis. In the present study, we investigated the role of L-aspartate in the melatonin production in isolated rat pineal glands. It was found that L-aspartate inhibits norepinephrine-stimulated melatonin production as well as serotonin N-acetyltransferase activity reversibly and dose-dependently, the concentrations required for 50% inhibition being 150 and 175 microM, respectively. L-Asparagine and oxaloacetate, metabolites of L-aspartate, had no effect on the melatonin production. These results suggest that pinealocytes use L-aspartate, as well as L-glutamate, as a negative regulator for melatonin production.

Parasympathetic inhibition of pineal indole metabolism by prejunctional modulation of noradrenaline release

The role of the parasympathetic nervous system in rat pineal indole metabolism was investigated by transpineal in vivo microdialysis. On-line coupling to a high performance liquid chromatography system with fluorescence detection (HPLC-FD) allowed simultaneous analysis of three major indolic compounds from the pineal, i.e. serotonin, N-acetylserotonin and melatonin. Infusion of the muscarinic receptor agonists, carbachol and oxotremorine, during the dark period resulted in a marked decrease of melatonin release. This effect was suggested to be mediated by a decrease in N-acetyltransferase activity, since a similar decrease was seen in N-acetylserotonin release, while serotonin levels increased simultaneously. Nicotine did show a very slight effect on the three indoles under these circumstances. Neostigmine failed to influence pineal indole metabolism, indicating that the endogenous tonus of acetylcholine release is either absent or extremely low in the middle of the dark period. The involvement of sympathetic innervation in the muscarinic effects was investigated by measurement of noradrenaline release from the pineal by sensitive off-line HPLC-FD analysis of noradrenaline in the dialysates. Carbachol markedly decreased the noradrenaline input during the infusion. Noradrenaline release returned to baseline values immediately after infusion with carbachol. These data suggest that the in vivo inhibitory effect of muscarinic receptor agonists on pineal melatonin production is mediated by presynaptic muscarinic receptors, located on the sympathetic nerve endings. This prejunctional inhibition of noradrenaline release causes a reduced induction of N-acetyltransferase activity, resulting in decreased melatonin release.

Iron decreases the nuclear but not the cytosolic content of the neurohormone melatonin in several tissues in chicks

This paper describes the influence of iron on both nuclear and cytosolic melatonin contents in several tissues of chicks. The neurohormone melatonin was estimated by means of radioimmunoassay. Iron, administered as FeCl3, decreased the nuclear melatonin level in a variety of tissues, including brain, heart, lung, kidney, and erythrocytes (nucleated cells in chicks) but was not seen in either the liver or gut. All variations related with iron were seen in the nuclear fraction, while only in the pineal gland did the melatonin content of the cytosol change as a result of iron treatment. We also observed a day-night rhythm in the nuclear melatonin: high nuclear levels of melatonin at night and low levels during the light period. This is the first report of nuclear localization of melatonin in any avian cell.

Melatonin and arylalkylamine N-acetyltransferase activity in the silkworm, Bombyx mori

Melatonin (N-acetyl-5-methoxytryptamine) was identified in the head and hemolymph of the silkworm, Bombyx mori, using reversed-phase high-performance liquid chromatography coupled with fluorometric detection and radioimmunoassay. In addition, evidence of arylakylamine (serotonin) N-acetyltransferase (NAT) a key enzyme controlling the synthesis of melatonin in vertebrates, was found in the head of the silkworm. Melatonin levels in the head and hemolymph and the NAT activity in the head were significantly higher during the dark period than during the light period of a 12-h light/12-h dark cycle. The day-night changes persisted in constant darkness but were suppressed by constant light. The results suggest that the synthesis and release of melatonin in the silkworm head occur as a circadian rhythm that is entrained by environmental light/dark cycles, as it is in the pineal gland of vertebrates. Melatonin in the silkworm head may function as a neurochemical mediator of photoperiodic control of developmental events such as molting, eclosion and diapause.

Melatonin biosynthesis in photoreceptor-enriched chick retinal cell cultures: role of cyclic AMP in the K(+)-evoked, Ca(2+)-dependent induction of serotonin N-acetyltransferase activity

The roles of cyclic AMP and calcium in the regulation of serotonin N-acetyltransferase (NAT) activity were studied in low density monolayer cultures of chick retinal photoreceptors and neurons. Photoreceptor-enriched retinal cell cultures were prepared from embryonic day 6 retinas and cultured for 6 days. NAT activity in these cultures could be induced by treatment with cyclic AMP protagonists, 8Br-cyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine (IBMX), or by treatment with depolarizing concentrations of extracellular K+. The stimulatory effect of K+, which involves Ca2+ influx through dihydropyridine-sensitive channels, was mediated at least in part by cyclic AMP, as indicated by the following observations. Depolarizing concentrations of K+ stimulated the formation of cyclic AMP, and the stimulatory effects of K+ on both cyclic AMP formation and on NAT activity were synergistically potentiated by the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). MDL 12,330A, a putative adenylate cyclase inhibitor, inhibited K(+)-evoked cyclic AMP accumulation and induction of NAT activity over the identical concentration range. In contrast, MDL 12,300A failed to inhibit the induction of NAT elicited by 8Br-cyclic AMP. H-89, an inhibitor of cyclic AMP-dependent protein kinase, antagonized the induction of NAT activity by either forskolin or K+ with equal potency for both stimuli. These results suggest that cyclic AMP plays an essential role in the induction of NAT activity that occurs as a consequence of membrane depolarization. Cyclic AMP and Ca2+ may also interact at a step distal to adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of enzyme activity by high-performance liquid chromatography

The application of high-performance liquid chromatography (HPLC) in the study of enzymatic reactions is reviewed. The rationale for using HPLC is given and whether the components of the reaction mixture should be derivatized prior to or after HPLC. An alphabetical list of enzymes assayed by HPLC is given. Substrate and product are included as well the derivatization reagent, detection method and biological matrix. Specific examples of these assays in a complex biological matrix viz. faeces are given. Future prospects are the detection of new enzymes using synthetic substrates and implementation of mass spectrometry to elucidate enzyme specificities.

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

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

High-performance liquid chromatography-based assays of enzyme activities

Interest in using HPLC to assay enzymatic reactions continues to grow as evidenced by the more than 100 papers published during the early 1990s. HPLC can be used for any enzymatic assay that requires separation of substrates and products before quantifying the extent of the reaction. The popularity of HPLC-based assays is due to several reasons: (1) HPLC offers unsurpassed precision, specificity, sensitivity, and reproducibility. (2) Powerful microcomputers and user-friendly software automate the running of samples and collection and processing of data. (3) Current columns, especially C18 packings, separate a very wide variety of samples, and (4) A variety of on-line detectors provide a means to detect virtually any compound. This review surveys recent papers on the development of HPLC-based assays for enzymes that degrade or otherwise modify macromolecules. Methods for assaying enzymes involved in metabolic pathways are also reviewed. Work by the authors in developing HPLC-based assays for mitochondrial enzymes that use GTP/GDP and other nucleotides that cannot be or are not easily assayed by enzyme-coupled assays is discussed. These enzymes include nucleoside diphosphate kinase, succinate thiokinase, and GTP-AMP phosphotransferase. The assays are suitable for determining the submitochondrial compartmentation of enzyme activities. Finally, current and anticipated trends in HPLC technology, including new column packings and the trend toward smaller columns that give faster separations, are reviewed in relation to enzyme assays.

Effect of iron and estrogen on melatonin secretion by the chicken pineal gland

Estrogen effects on pineal secretion of melatonin are controversial. Some feel that estrogen inhibits melatonin output in vitro but not in vivo. Melatonin levels vary with the age in chickens where circulating estrogen levels also vary. Laying hens have minimal melatonin levels and maximal serum iron concentrations. Thus, we reasoned that iron released by estrogen may inhibit melatonin secretion from the chick pineal gland. The present study shows that perifusion of estrogen-treated chick pineal glands with several concentrations of iron greatly inhibited melatonin secretion.

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

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

Melatonin increases serotonin N-acetyltransferase activity and decreases dopamine synthesis in light-exposed chick retina: in vivo evidence supporting melatonin-dopamine interaction in retina

The administration of melatonin, either peripherally (0.01-10 mg/kg) or intraocularly (0.001-10 mumol/eye), to light-exposed chicks dose-dependently increased serotonin N-acetyltransferase (NAT) activity in retina but not in pineal gland. The effect of melatonin was slightly but significantly reduced by luzindole (2-benzyl-N-acetyltryptamine), and not affected by two other purported melatonin antagonists, N-acetyltryptamine and N-(2,4-dinitrophenyl)-5-methoxytryptamine (ML-23). The elevation of the enzyme activity induced by melatonin was substantially stronger than that evoked by 5-hydroxytryptamine, N-acetyl-5-hydroxytryptamine, or 5-methoxytryptamine. The melatonin-evoked rise in the retinal NAT activity was counteracted by two dopamine D2 receptor agonists, quinpirole and apomorphine, and prevented by the dopamine D2 receptor blocker spiroperidol, and by an inhibitor of dopamine synthesis, alpha-methyl-p-tyrosine. Melatonin (0.1-10 mg/kg i.p.) dose-dependently decreased the levels of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), as well as the DOPAC/dopamine ratio, in chick retina but not in forebrain. The results obtained (1) indicate that melatonin in vivo potently inhibits dopamine synthesis selectively in retina, and (2) suggest that the increase in retinal NAT activity evoked by melatonin in light-exposed chicks is an indirect action of the compound, and results from the disinhibition of the NAT induction process from the dopaminergic (inhibitory) signal. The results provide in vivo evidence supporting the idea (derived on the basis of in vitro findings) that a mutually antagonistic interaction between melatonin and dopamine operates in retinas of living animals.

Determination of kinetic properties of serotonin-N-acetyltransferase in bovine pineal gland using HPLC with fluorimetric detection

The determination of serotonin-N-acetyltransferase (NAT) activity in the bovine pineal gland and other rat tissues was based upon the separation and detection of N-acetyltryptamine formed from tryptamine and acetyl CoA by means of high performance liquid chromatography with fluorimetric detection. In the bovine pineal the enzyme exhibited a Km value of 31.45 +/- 4.98 microM and a Vmax value of 30.90 +/- 1.18 pmol N-acetyltryptamine/min/mg protein for tryptamine, and a Km value of 28.72 +/- 7.50 microM and a Vmax value of 25.90 +/- 1.50 pmol N-acetyltryptamine/min/mg protein for acetyl CoA. The present method is simple, allows the determination of NAT activity from a variety of enzyme sources, has application to pharmacological studies of NAT regulation in tissue cultures, and provides an alternative to current radioenzymatic assays.

Melatonin synthesis in chicken retina: effect of kainic acid-induced lesions on the diurnal rhythm and D2-dopamine receptor-mediated regulation of serotonin N-acetyltransferase activity

The effect of kainic acid (KA)-induced lesions of retinal neurons on regulation of serotonin N-acetyltransferase (NAT) activity in chicken retina was investigated. Although NAT activity was higher in KA-lesioned retinas than in controls, the pattern of diurnal variation of enzyme activity throughout 36 h of constant darkness was similar for both tissues. Quinpirole, a selective D2-dopamine receptor agonist, inhibited the nocturnal increase of NAT activity in both control and KA-treated retinas. Quinpirole was significantly more potent in KA-treated retinas than in controls; the ED50 value for quinpirole was 3 times lower in KA-treated retinas than in control tissues. KA treatment markedly reduced retinal levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). We conclude that: (1) NAT activity in retina is localized primarily to KA-insensitive cells, presumably photoreceptors; (2) KA-sensitive inner retinal neurons are not essential to the maintenance of the circadian rhythm of NAT activity; and (3) KA-induced lesions of retinal cells result in supersensitivity of D2-dopamine receptors regulating NAT activity in a mechanism that involves adaptive changes following a decline in retinal dopamine neurotransmission.

Circadian rhythm of tryptophan hydroxylase activity in chicken retina

1. Retinal tryptophan hydroxylase activity in chickens (1-4 weeks old and embryos) was estimated by determination of levels of 5-hydroxytryptophan (5HTP) in retinas at defined intervals after inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine (NSD1015). 2. The relationship of tryptophan hydroxylase activity to photoperiod was explored. In chickens maintained on a 12-hr light: 12-hr dark cycle, a diurnal cycle in tryptophan hydroxylase activity was observed. Activity during middark phase was 4.4 times that seen in midlight phase. Cyclic changes in tryptophan hydroxylase activity persisted in constant darkness with a period of approximately 1 day, indicating regulation of the enzyme by a circadian oscillator. The phase of the tryptophan hydroxylase rhythm was found to be determined by the phase of the light/dark cycle. The relationship of the tryptophan hydroxylase rhythm to the light/dark cycle mirrors previously described rhythms of melatonin synthesis and serotonin N-acetyltransferase (NAT) activity in the retina. 3. Light exposure for 1 hr during dark phase suppressed NAT activity by 82%, while tryptophan hydroxylase activity was suppressed by only 30%. 4. Based on the differential responses of retinal NAT activity and tryptophan hydroxylase activity to acute light exposure during dark phase, it was predicted that exposure to light during dark phase would divert serotonin in the retina from melatonin biosynthesis to oxidation by MAO. In support of this, levels of 5-hydroxyindole acetic acid (5HIAA) in retina were found to be elevated approximately two-fold in chickens exposed to 30 min of light during dark phase. In pargyline-treated chickens, 2 hr of light exposure during dark phase was found to increase retinal serotonin levels by 64% over pargyline-treated controls. 5. Cyclic changes in tryptophan hydroxylase activity and NAT activity persisted for 2-3 days in constant light. Tryptophan hydroxylase activity at mid-night gradually decreased on successive days in constant light; on the first day of constant light, tryptophan hydroxylase activity at mid-night was 70% of activity seen during middark phase of the normal light/dark cycle and decreased further on subsequent days. In contrast, on each of 3 days of constant light, NAT activity at mid-night was approximately 15% of normal middark phase activity. 6. Cycloheximide completely inhibited the nocturnal increase in tryptophan hydroxylase activity when given immediately before light offset. The nocturnal increase in NAT activity was inhibited in a similar fashion. 7. Like the development of the NAT rhythm, cyclic changes of tryptophan hydroxylase activity in the retinas of chickens began on or immediately before the day of hatching. hatching.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic AMP-dependent induction of serotonin N-acetyltransferase activity in photoreceptor-enriched chick retinal cell cultures: characterization and inhibition by dopamine

The activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme in the melatonin biosynthetic pathway, was examined in low-density monolayer cultures of chick embryo retinal cells prepared with three levels of photoreceptor enrichment. In cultures prepared from embryonic day 8 retinas (E8), photoreceptors represented approximately 30% of the total cell population, whereas in those prepared from embryonic day 6 retinas (E6), approximately 70% of the cells were photoreceptors. In E8 retinas treated with kainic acid to destroy neurons (E8K), the relative content of photoreceptors was increased to approximately 50%. NAT activity was detectable in the cultures under all conditions studied, and was markedly increased by drugs that increase intracellular cyclic AMP levels and cyclic AMP-dependent protein kinase activity: 8-bromocyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine (IBMX). Consistent with the hypothesis that NAT is localized in photoreceptors, the effects of the stimulatory treatments were significantly greater in E6 and E8K cultures than in E8 cultures. The stimulation of NAT activity in E6 cultures was inhibited by actinomycin D and cycloheximide, suggesting the involvement of RNA and protein synthesis. Dopamine inhibited the induction of NAT activity by forskolin and IBMX, but not that elicited by 8-bromocyclic AMP. The dopamine-mediated suppression of activity was significantly inhibited by pertussis toxin and by spiperone and sulpiride, both D2-dopamine receptor antagonists, but not by SCH 23390, a D1-dopamine receptor blocker, or antagonists of alpha-adrenergic, beta-adrenergic, or serotonergic receptors. Because the inhibitory effect of dopamine on E6 and E8K cultures was at least as great as that on E8 cultures, the results suggest that dopamine acts on D2-like receptors on photoreceptors. The receptors appear to be coupled to adenylate cyclase through an inhibitory GTP-binding protein and to mediate inhibition of cyclic AMP synthesis and consequent induction of NAT activity.