Regional and diurnal studies of melatonin and melatonin binding sites in the duck gastro-intestinal tract

Melatonin as an Ameliorative Agent Against Cadmium- and Lead-Induced Toxicity in Fish: an Overview

Diverse anthropogenic activities and lack of knowledge on its consequences have promoted serious heavy metal contaminations in different aquatic systems throughout the globe. The non-biodegradable nature of most of these toxic heavy metals has increased the concern on their possible bioaccumulation in aquatic organisms as well as in other vertebrates. Among these aquatic species, fish are most sensitive to such contaminated water that not only decreases their chance of survivability in the nature but also increases the probability of biomagnifications of these heavy metals in higher order food chain. After entering the fish body, heavy metals induce detrimental changes in different vital organs by impairing multiple physiological and biochemical pathways that are essential for the species. Such alterations may include tissue damage, induction of oxidative stress, immune-suppression, endocrine disorders, uncontrolled cell proliferation, DNA damage, and even apoptosis. Although uncountable reports have explored the toxic effects of different heavy metals in diverse fish species, but surprisingly, only a few attempts have been made to ameliorate such toxic effects. Since, oxidative stress seems to be the underlying common factor in such heavy metal-induced toxicity, therefore, a potent and endogenous antioxidant with no side effect may be an appropriate therapeutic solution. Apart from summarizing the toxic effects of two important toxicants, i.e., cadmium and lead in fish, the novelty of the present treatise lies in its arguments in favor of using melatonin, an endogenous free radical scavenger and indirect antioxidant, in ameliorating the toxic effects of heavy metals in any fish species.

Influence of Different Light Spectra on Melatonin Synthesis by the Pineal Gland and Influence on the Immune System in Chickens

It is well known that the pineal gland in birds influences behavioural and physiological functions, including those of the immune system. The purpose of this research is to examine the endocrine-immune correlations between melatonin and immune system activity. Through a description of the immune-pineal axis, we formulated the objective to determine and describe: the development of the pineal gland; how light influences secretory activity; and how melatonin influences the activity of primary and secondary lymphoid organs. The pineal gland has the ability to turn light information into an endocrine signal suitable for the immune system via the membrane receptors Mel1a, Mel1b, and Mel1c, as well as the nuclear receptors RORα, RORβ, and RORγ. We can state the following findings: green monochromatic light (560 nm) increased serum melatonin levels and promoted a stronger humoral and cellular immune response by proliferating B and T lymphocytes; the combination of green and blue monochromatic light (560-480 nm) ameliorated the inflammatory response and protected lymphoid organs from oxidative stress; and red monochromatic light (660 nm) maintained the inflammatory response and promoted the growth of pathogenic bacteria. Melatonin can be considered a potent antioxidant and immunomodulator and is a critical element in the coordination between external light stimulation and the body's internal response.

Gut melatonin: A potent candidate in the diversified journey of melatonin research

After being discovered from the bovine pineal gland by Aaron Lerner and co-workers in the year 1958, various distinguished researchers have reported melatonin (5-methoxy-N-acetyl-tryptamine) from several extra-pineal sources, including the gastrointestinal tract (GIT). In the year 1974, Raikhlin and Kvetnoy first detected this molecule in the gastrointestinal tissue. Later, within the last 45 years, many renowned investigators found that the GIT is a rich source of melatonin, in addition to the pineal gland. In the carp gut, the estimation of Arylalkylamine-N-acetyltransferase (AANAT) mRNA/protein levels, which is the rate-determining enzyme for melatonin biosynthesis in the pineal gland, confirmed the endogenous synthesis of melatonin. The remarkable feature of the pineal gland melatonin is its rhythmic synthesis with a peak at dark-phase and lowest at light-phase in synchronization with seasonal environmental light-dark (LD) cycle. Recent studies on carp demonstrated that the melatonin concentrations and the AANAT protein intensities in different gut segments underwent significant daily fluctuations. However, compared to the melatonin rhythm in the pineal gland, the melatonin profiles in gut tissue displayed daily rhythm in parallel with the feeding cycle of the carp, irrespective of LD conditions of the environment. Notably, in carp, the temporal pattern of the gut melatoninergic system found to vary with the environmental non-photic signal(s), such as food entrainment factors (viz. availability of food, timing of food supply, number(s) of feed per day, quality of food) those act as the most dependable synchronizer(s) in daily rhythm characteristics of gut melatonin and AANAT. Thereby in this review, it appears meaningful to highlight the existing data on the mode of synthesis of melatonin in cells of the digestive tract, and most importantly, the regulation of its synthesis. Finally, in comparison with the dynamic actions of melatonin derived from the pineal gland, this review will lead to underline the role of gut-derived melatonin in a variety of physiological functions.

Validation of an Ex Vivo Permeation Method for the Intestinal Permeability of Different BCS Drugs and Its Correlation with Caco-2 In Vitro Experiments

The absorption study of drugs through different biological membranes constitutes an essential step in the development of new pharmaceutical dosage forms. Concerning orally administered forms, methods based on monolayer cell culture of Caco-2 (Caucasian colon adenocarcinoma) have been developed to emulate intestinal mucosa in permeability studies. Although it is widely accepted, it has disadvantages, such as high costs or high technical complexity, and limitations related to the simplified structure of the monolayer or the class of molecules that can be permeated according to the transport mechanisms. The aim of this work was to develop a new ex vivo methodology which allows the evaluation of the intestinal apparent permeability coefficient (P_app) while using fewer resources and to assess the correlation with Caco-2. To this end, pig (Sus scrofa) duodenum segments were mounted in Franz diffusion cells and used to permeate four different drugs: ketorolac tromethamine (Kt), melatonin (Mel), hydrochlorothiazide (Htz), and furosemide (Fur). No statistically significant differences (p > 0.05) were observed corelating P_app values from Franz diffusion cells and Caco-2 cell experiments for Kt, Htz, and Fur. However, there were statistically significant differences (p < 0.05) correlating P_app values and Mel. The difference is explained by the role of Mel in the duodenal epithelial paracellular permeability reduction. Ex vivo permeation may be an equivalent method to Caco-2 for drugs that do not produce intestinal membrane phenomena that could affect absorption.

Melatonin Receptor Mel1b- and Mel1c-mediated Green Light Induced the Secretion of Growth Hormone in Anterior Pituitary of Chicks

Previous studies have found that melatonin was related to the growth and development in avian. Therefore, the newly hatched broilers were exposed to colors of light to establish a model of pinealectomy and explored the mechanism of pineal melatonin promoting growth hormone (GH) secretion. The results showed that green light (GL) promoted the levels of GH, pituitary-specific transcription factor-1 (Pit-1) genes and proteins in pituitary. Moreover, the mRNA and protein levels of melatonin receptor subtypes Mel1b and Mel1c in the pituitary in GL were higher than other monochromatic light groups. After pinealectomy, the expression of Pit-1, GH, Mel1b and Mel1c in pituitary decreased. In vitro, exogenous melatonin promoted the level of Pit-1 mRNA and the secretion of GH in anterior pituitary cells. However, when melatonin was added with exogenous selective Mel1b antagonist (4-phenyl-2-propionamideotetralin) and selective Mel1c antagonist (prazosin), the level of Pit-1 mRNA and the GH secretion decreased. When selective Mel1b and Mel1c antagonists were added simultaneously, the decrease in Pit-1 mRNA and GH secretion was more significant. These results indicated that pineal melatonin promotes the expression of Pit-1 under GL by binding to melatonin receptor subtypes Mel1b and Mel1c in the pituitary, thereby increasing GH secretion and promoting the growth.

Extrapineal melatonin: sources, regulation, and potential functions

Endogenous melatonin is synthesized from tryptophan via 5-hydroxytryptamine. It is considered an indoleamine from a biochemical point of view because the melatonin molecule contains a substituted indolic ring with an amino group. The circadian production of melatonin by the pineal gland explains its chronobiotic influence on organismal activity, including the endocrine and non-endocrine rhythms. Other functions of melatonin, including its antioxidant and anti-inflammatory properties, its genomic effects, and its capacity to modulate mitochondrial homeostasis, are linked to the redox status of cells and tissues. With the aid of specific melatonin antibodies, the presence of melatonin has been detected in multiple extrapineal tissues including the brain, retina, lens, cochlea, Harderian gland, airway epithelium, skin, gastrointestinal tract, liver, kidney, thyroid, pancreas, thymus, spleen, immune system cells, carotid body, reproductive tract, and endothelial cells. In most of these tissues, the melatonin-synthesizing enzymes have been identified. Melatonin is present in essentially all biological fluids including cerebrospinal fluid, saliva, bile, synovial fluid, amniotic fluid, and breast milk. In several of these fluids, melatonin concentrations exceed those in the blood. The importance of the continual availability of melatonin at the cellular level is important for its physiological regulation of cell homeostasis, and may be relevant to its therapeutic applications. Because of this, it is essential to compile information related to its peripheral production and regulation of this ubiquitously acting indoleamine. Thus, this review emphasizes the presence of melatonin in extrapineal organs, tissues, and fluids of mammals including humans.

Characterization of signaling pathways coupled to melatonin receptors in gastrointestinal smooth muscle

Melatonin, a close derivative of serotonin, is involved in physiological regulation of circadian rhythms. In the gastrointestinal (GI) system, melatonin exhibits endocrine, paracrine and autocrine actions and is implicated in the regulation of GI motility. However, it is not known whether melatonin can also act directly on GI smooth muscle cells. The aim of the present study was to determine the expression of melatonin receptors in smooth muscle and identify their signaling pathways. MT1, but not MT2 receptors are expressed in freshly dispersed and cultured gastric smooth muscle cells. Melatonin selectively activated Gq and stimulated phosphoinositide (PI) hydrolysis in freshly dispersed and cultured muscle cells. PI hydrolysis was blocked by the expression of Gq, but not Gi minigene in cultured muscle cells. Melatonin also caused rapid increase in cytosolic Ca(2+) as determined by epifluorescence microscopy in fura-2 loaded single smooth muscle cells, and induced rapid contraction. Melatonin-induced PI hydrolysis and contraction were blocked by a non-selective MT1/MT2 antagonist luzindole (1 μM), but not by a selective MT2 antagonist 4P-PDOT (100 nM), and by the PLC inhibitor U73122. MT2 selective agonist IIK7 (100 nM) had no effect on PI hydrolysis and contraction. We conclude that rabbit gastric smooth muscle cells express melatonin MT1 receptors coupled to Gq. Activation of these receptors causes stimulation of PI hydrolysis and increase in cytosolic Ca(2+), and elicits muscle contraction.

Gender-related invasion differences associated with mRNA expression levels of melatonin membrane receptors in colorectal cancer

Melatonin inhibits growth and invasive capacity of colon cancer cells in vitro through its membrane (MT1 and MT2) and/or nuclear receptors (RORα). Previous studies showed that this indoleamine is present in both the normal and colon cancer at similar levels. Therefore, we analyzed MT1, MT2, and RORα expression in tumor samples versus normal mucosa (NM) from patients suffering from colorectal cancer (CRC). Given the existence of sex differences in the incidence and pathology of CRC and the involvement of steroid receptors in the oncostatic actions of melatonin in some types of cancer, we also analyzed the expression of androgen (AR) and estrogen receptor (ER) α and ERβ. Finally, we conducted some experiments in colon cancer cell lines to corroborate the experiments carried out in human tumors. We found a decreased expression of MT1, MT2, AR, ERα, and ERβ in tumor samples versus NM, but no changes in RORα expression in the whole cohort of patients. Classifying tumors by stage and gender, MT1, MT2, AR, ERα, and ERβ expression decreased in both early stage and advanced tumors, but only in male patients. On the other hand, MT1 and MT2 expression correlated positively with AR, ERα, and ERβ expression in male patients and with ERα or ERβ in female patients. In vitro, the invasive capacity was higher in cells with the least expression of MT1, MT2, and AR, and nonselective MT1/MT2 agonists inhibited cell growth and invasion. These results could indicate a possible interaction of these pathways.

Modulation of metabolic and clock gene mRNA rhythms by pineal and retinal circadian oscillators

Avian circadian organization involves interactions between three neural pacemakers: the suprachiasmatic nuclei (SCN), pineal, and retina. Each of these structures is linked within a neuroendocrine loop to influence downstream processes and peripheral oscillations. However, the contribution of each structure to drive or synchronize peripheral oscillators or circadian outputs in avian species is largely unknown. To explore these interactions in the chick, we measured 2-deoxy[(14)C]-glucose (2DG) uptake and mRNA expression of the chick clock genes bmal1, cry1, and per3 in three brain areas and in two peripheral organs in chicks that underwent pinealectomy, enucleation, or sham surgery. We found that 2DG uptake rhythms damp under constant darkness in intact animals, while clock gene mRNA levels continue to cycle, demonstrating that metabolic rhythms are not directly driven by clock gene transcription. Moreover, 2DG rhythms are not phase-locked to rhythms of clock gene mRNA. However, pinealectomy and enucleation had similar disruptive effects on both metabolic and clock gene rhythms, suggesting that both of these oscillators act similarly to reinforce molecular and physiological rhythms in the chicken. Finally, we show that the relative phasing of at least one clock gene, cry1, varies between central and peripheral oscillators in a tissue specific manner. These data point to a complex, differential orchestration of central and peripheral oscillators in the chick, and, importantly, indicate a disconnect between canonical clock gene regulation and circadian control of metabolism.

Effects of exogenous melatonin and tryptophan on fecal shedding of E. coli O157:H7 in cattle

Fecal prevalence of Escherichia coli O157 in ruminants is highest in the summer decreasing to very low levels in the winter. We hypothesize that this seasonal variation is a result of physiological responses within the host animal to changing day-length. To determine the effects of melatonin (MEL) on fecal shedding of E. coli O157:H7 in cattle, eight crossbred beef steers identified as shedding E. coli O157:H7, were allotted to treatment: control or MEL (0.5 mg/kg body weight (BW); 1x) administered orally daily for 7 days. After a 5-day period of no treatment, a second MEL dose (5.0 mg/kg BW; 10x) was administered daily for 4 days. Fecal samples were collected daily for qualification of E. coli O157:H7. No differences (P > 0.10) were observed in the percentage of E. coli O157:H7 positive fecal samples in steers receiving the 1x MEL dose, however the 10x dose decreased (P = 0.05) the percentage of fecal samples E. coli O157:H7 positive. Serum MEL concentrations were higher in the 1x, but not 10x, treated animals compared to control animals. Although it is difficult to explain, this may be a result of decreasing day-length increasing serum melatonin concentrations that may have masked any treatment effect on serum melatonin. In a second similar experiment, a second group of cattle (heifers and steers) were administered tryptophan (TRP) over a 17-day experimental period (5 g/head/day for 10 days followed by 10 g/head/day for 7 days). Tryptophan had no effect (P > 0.20) on the percentage of fecal samples positive for E. coli O157. Serum TRP (P < 0.05), but not MEL (P > 0.20), concentrations were elevated in TRP-treated animals. The decrease in the number of positive fecal samples observed in the first experiment, may be related to gastrointestinal MEL, affected by the 10x, but not 1x MEL dose.

Effects of melatonin administration on intestinal adaptive response after massive bowel resection in rats

This study evaluates whether melatonin can improve the structure of the small intestine and enhance adaptation in an experimental model of short bowel syndrome. Thirty Sprague-Dawley rats were divided randomly into three experimental groups of 10 animals each. In one group, only laparotomy was performed and these rats served as the sham-control group (G1). The remaining 20 rats underwent 90% small bowel resection (SBR) and formed the two experimental groups: the SBR/untreated group (G2), and the SBR/melatonin-treated group (G3). Rats in the SBR/untreated group received no therapeutic treatment. Rats in the SBR/melatonin-treated group received melatonin intraperitoneally for 3 weeks. The animals were weighed daily. All rats underwent relaparotomy on day 21 of the experiment. Remnant small bowel was excised and evaluated for villus height, total mucosal thickness, and crypt cell mitosis. After the 90% SBR, all animals suffered from diarrhea and weight loss between the first and the sixth postoperative days. The body weight of the SBR/melatonin group showed significant increases at the beginning of postoperative day 10 and day 21 in comparison to that of the SBR/untreated group. The rats treated with melatonin had significantly greater villus height and crypt cell mitosis compared to the sham-control group and the SBR/untreated group. In addition, the mucosal thickness was significantly increased in the SBR/melatonin-treated group compared to the SBR/untreated rats. These observations suggest that melatonin treatment increases villus height, total mucosal thickness, and crypt cell mitosis after massive SBR and it may exert a considerable effect on the mucosal adaptive response in short bowel syndrome in rats.

The circadian rhythm of melatonin modulates the severity of caerulein-induced pancreatitis in the rat

Melatonin, an antioxidant, protects the pancreas against acute inflammation but, although this indole is released mainly at night, no study has been undertaken to determine circadian changes of plasma melatonin levels and the severity of acute pancreatitis. The aims of this study were: (a) to compare the severity of caerulein-induced pancreatitis (CIP) produced in the rat during the day and at the night, and (b) to assess the changes of plasma melatonin level and the activity of an antioxidative enzyme; superoxide dismutase (SOD), in the pancreas subjected to CIP during the day time and at night without or with administration of exogenous melatonin or its precursor; l-tryptophan. Rats were kept in 12 hr light/dark cycle. CIP was induced by subcutaneous infusion of caerulein (5 microg/kg/hr for 5 hr). Melatonin (5 or 25 mg/kg) or l-tryptophan (50 or 250 mg/kg) was given intraperitoneally 30 min prior to the start of CIP. CIP induced during the day time was confirmed by histological examination and manifested by pancreatic edema, and rises of amylase and lipase plasma activities (by 400 and 500%, respectively), whereas pancreatic SOD, pancreatic blood flow (PBF) and oxygen consumption by pancreatic tissue (VO(2)) were decreased by 70, 40 and 45%, respectively, as compared with the appropriate controls. All morphological and biochemical parameters of CIP induced at night were significantly less severe, compared with those recorded during the light phase. Plasma melatonin immunoreactivity was significantly higher during the night, than during the day, especially following administration of melatonin or its precursor, which reversed all manifestations of CIP. In conclusion, a circadian rhythm modulates the severity of CIP with a decrease of pancreatitis severity during the night compared with that at the day time and this may be due to the increased plasma level of melatonin and higher activity of SOD in the pancreas.

Pineal melatonin secretion, but not ocular melatonin secretion, is sufficient to maintain normal immune responses in Japanese quail (Coturnix coturnix japonica)

Reports that plasma melatonin is an important immune regulator in avian species have been rather sparse and contradictory. Also, the primary source of immune-modulating melatonin has yet to be determined in birds. In Japanese quail (Coturnix coturnix japonica), the pineal gland and eyes contribute roughly two thirds and one third of the melatonin found in the blood, respectively. Two experiments were conducted to evaluate melatonin as an immune modulator in Japanese quail and to determine the primary source of immune-modulating melatonin in this species. Experiment 1 was designed to evaluate the involvement of the pineal gland and the eyes in immunocompetence. Each of three groups of quail was assigned a surgical treatment and the cellular and humoral immune responses were determined 8 weeks following surgery. The surgical treatments were pinealectomy (Px), sham pinealectomy (SH-Px), and ocular enucleation (eye removal (Ex)). Experiment 2 utilized exogenous melatonin as a replacement to reconstitute immune responses in surgically immunocompromised birds. In this experiment, 50.0 microg/ml of melatonin, or diluent only, was provided to Px and SH-Px birds in the drinking water ad libitum. The cellular and humoral immune responses were determined after 8 weeks of melatonin treatment. In both experiments, a cutaneous basophil hypersensitivity reaction to phytohemagglutinin was measured to evaluate the cellular immune response. To evaluate the humoral immune response, primary antibody titers were determined 7 days postintravenous injection with a Chukar red blood cell suspension. Flow cytometric analysis of peripheral blood lymphocytes was performed to determine the relative percentage of CD4(+) and CD8(+) T- and B-lymphocytes in all treatments of Experiment 2. In Experiment 1, both the SH-Px and Ex surgical treatments produced similar cellular and humoral immune responses, and these responses were significantly greater than those in Px-treated birds. Pinealectomy significantly reduced the cellular and humoral immune responses from SH-Px by 25.8% and 41.3%, respectively. In Experiment 2, Px again resulted in depressed cellular and humoral immune responses. In addition, Px significantly reduced CD8(+) T-lymphocyte numbers compared to SH-Px, while B-lymphocytes remained unchanged. Melatonin administration to Px birds increased the cellular (32.9%) and humoral (30.6%) immune responses to the level of control (SH-Px) birds, although this reconstitution was not due to increased CD8(+) T- or B-lymphocytes. From these data, it was clear that removal of the pineal gland, but not the eyes, reduced cellular and humoral immune responses, which were reconstituted to normal levels by exogenous melatonin. These data suggest that immunodepression is only observed in birds with two thirds of the plasma melatonin removed by pinealectomy. Removal of one third of the plasma melatonin (by ocular enucleation) is not sufficient to reduce cellular and humoral responses in the Japanese quail.

Effect of continuous melatonin infusions on steady-state plasma melatonin levels, metabolic fate and tissue retention in rats under near physiological conditions

The fate and disposition of melatonin released into the circulation is still poorly understood, and almost all current knowledge is derived from measurements made after one single, often very large dose of labeled melatonin. In continuous infusion experiments in freely moving rats, 500 ng melatonin/mL hr had to be infused in order to elevate the circulating hormone from low daytime levels to the 10-fold higher nocturnal steady state concentrations. To study the fate and tissue accumulation of the infused melatonin, tritiated melatonin was added to the infusion solution, and the retention of [3H]-melatonin and chloroform-insoluble [3H]-melatonin-metabolites were measured in almost all body tissues and their subcellular compartments immediately at the end of the infusion period and six hours later. A considerable amount of the infused melatonin was found in the gut and in all tissues, some melatonin was covalently attached to proteins.

Tissue retention and subcellular distribution of continuously infused melatonin in rats under near physiological conditions

The fate and disposition of the melatonin released into the circulation is still poorly understood, and almost all current knowledge is derived from measurements made after a single and often a very large dose of labelled melatonin. In continuous infusion experiments in freely moving rats, we have recently demonstrated that considerable amounts of melatonin must be endogenously released in order to achieve and maintain approximately a 10-fold elevation of the low daytime plasma levels of this hormone. We have now applied this infusion paradigm to study the fate and tissue accumulation of [3H]-melatonin continuously infused under near physiological conditions into the jugular vein for a period of 2 hr. The retention of [3H]-melatonin and chloroform-insoluble [3H]-melatonin-metabolites was measured in almost all body tissues and their subcellular compartments immediately at the end of the infusion period and 6 hr later. At the end of the 2 hr infusion period, about 45% of the administered melatonin was recovered as water-soluble metabolites in the urine and about 20% in the small intestine. Some accumulation of [3H]-melatonin-derived water-soluble radioactivity was also noticed in the liver, colon, adrenals, and pituitary, as well as in the feces. The subcellular distribution of this radioactivity differed between tissues. During the period of 6 hr after the termination of infusion, a considerable amount of melatonin-derived radioactivity was found to become increasingly attached to the proteous interlayer of chloroform extracts of tissues and subcellular fractions, from where it could only be liberated by protease treatment.

Effect of continuous melatonin infusions on steady-state plasma melatonin levels in rats under near physiological conditions

It was the aim of this study to measure the actual amount of melatonin required for elevating the circulating hormone from low daytime levels to the 10-fold higher nocturnal steady-state concentrations in rats. For this purpose, escalating doses of melatonin were continuously infused into the right jugular vein and blood samples were repeatedly drawn from the left jugular vein for a period of 2 hr in freely moving catheterized rats. In order to achieve an about 10-fold elevation of the plasma melatonin concentration, 500 ng melatonin/hr had to be infused, i.e., about 300 times the normal nocturnal melatonin content of the pineal. Infusions of up to 61 ng melatonin/hr (equivalent to the melatonin content of 40 pineals at darkness) failed to cause a significant rise of the low daytime steady-state concentrations in the blood. If the dose of 500 ng melatonin/h was infused at night, a less-pronounced rise of the blood levels was observed, as compared to that caused by the infusion of the same dose during daytime. No differences were found in the rate of metabolism between daytime and nighttime. The results of this study indicate 1) that the low basal concentrations of melatonin in the blood are not affected by an increased melatonin supply up to a certain critical threshold, 2) that the rat pineal gland would have to release all its melatonin content almost every 10 sec in order to sustain the elevated steady-state level of melatonin in the circulation during the dark period, and 3) that significant day/night differences exist in the disposition of circulating melatonin if administered in near physiological amounts and under near physiological conditions.

Autoradiographic localization of 2[125I]iodomelatonin binding sites in the gastrointestinal tract of mammals including humans and birds

In-vitro autoradiography was utilized to compare the distribution of 2[125I]iodomelatonin binding sites or putative melatonin receptors in the gastrointestinal tracts of humans, guinea pigs, mice, rats, hamsters, rabbits, ducks, chickens, pigeons, and quail. In humans, binding was detected in the mucosa of the colon, caecum, appendix, and on their blood vessels but not in the ileum. In the other mammals, significant binding was only demonstrated in the mucosa of the rabbit rectum, mouse colon, mouse rectum, and guinea pig ileum. The distribution of 2[125I]iodomelatonin binding in the avian gut varied with species. In the esophagus, binding was present in the lamina propria and blood vessels of all four birds. However, only the lamina propria of the chicken and quail proventriculus and ventriculus showed positive binding. For the duodenum and ileum, binding was very strong in the duck lamina propria, weak in the chicken lamina propria, and absent in the quail. In contrast, the pigeon muscle layer was weakly positive. The most striking species difference was found in the caecum where the duck lamina propria showed very strong binding, while the chicken lamina propria was only weakly positive. Conversely, the caecal muscle layer was strongly positive in chicken and quail but negative in duck and pigeon. In the rectum, a similar but less intense pattern of distribution was observed. The tremendous diversity in the distribution of 2[125I]iodomelatonin binding sites in the gastrointestinal tract is in accord with the hypothesis that melatonin may serve different functions in the gut of different species.

Autoradiographic distribution and physiological regulation of 2-[125I]iodomelatonin binding in rat epididymis

Autoradiographic study was conducted to localize 2-[125I]iodomelatonin binding in the rat epididymis. In the peripubertal (6 weeks old), postpubertal (8 weeks old) and adult (3 months old) rats, intense specific 2-[125I]iodomelatonin labelling of the corpus epididymidis was observed. The intensity of 2-[125I]iodomelatonin binding in the distal epididymal segment was significantly decreased in orchidectomized rats but the effect could be reversed with testosterone replacement. The intensity of 2-[125I]iodomelatonin binding in the distal rat epididymal segment did not show any diurnal rhythmicity when mid-light period and mid-dark period levels were compared, and was unaffected by constant lighting. Our data suggest androgen-dependent expression of 2-[125I]iodomelatonin binding sites, independent of light-induced changes in circulating melatonin, in the rat corpus epididymidis. A novel role of melatonin and its receptor in the regulation of the functions of rat corpus epididymidis is strongly implicated.

N-acetyltransferase activity in the quail (Coturnix coturnix jap) duodenum

The activity and kinetics of N-acetyltransferase (NAT) in the quail duodenum were studied by radioenzymatic assay. NAT activity was highest when incubated under 37 degrees C, at pH 5.8 for 15 sec. Of the four substrates tested (tryptamine, 5,6-dihydroxytryptamine, serotonin, and N-acetylserotonin at concentrations of 0.08-4 mM), only tryptamine showed the substrate saturation phenomenon when incubated with the duodenal enzyme and acetyl-14C-coenzyme A. The saturation concentration of tryptamine was about 4 mM. Using the double reciprocal plot and regression equation, the Michaelis constant (Km) and maximal rate (Vmax) of NAT activity were found to be 0.204 mM and 0.917 nmol.mg protein-1.min-1, respectively. Diurnal study demonstrated higher NAT activity at middark (3.7 nmol.mg protein-1.min-1) and lower activity at midlight (2.4 nmol.mg protein-1.min-1), suggesting a circadian rhythm of the enzyme activities in the quail duodenum.