Persistence of circadian rhythms of melatonin and N-acetylserotonin in the serum of rats after pinealectomy

The mammalian gastro-intestinal tract is a NOT a major extra-pineal source of melatonin

In 1992, a paper reported that the melatonin content of the rat duodenum was 24 000 ± 2000 pg/g tissue (range: 4000-100 000 pg/g) while the pineal melatonin content was 580 000 ± 36 000 pg/g. The data has been used for the last 30 years to infer that the gut produces 400 hundred times more melatonin than the pineal gland and that it is the source of plasma melatonin during the daytime. No-one has ever challenged the statement. In this review, evidence is summarised from the literature that pinealectomy eliminates melatonin from the circulation and that studies to the contrary have relied upon poorly validated immunoassays that overstate the levels. Similarly studies that have reported increases in plasma melatonin following tryptophan administration failed to account for cross reactivity of tryptophan and its metabolites in immunoassays. The most extraordinary observation from the literature is that in those studies that have measured melatonin in the gut since 1992, the tissue content is vastly lower than the original report, even when the methodology used could be overestimating the melatonin content due to cross reactivity. Using the more contemporary results we can calculate that rather than a 400:1 ratio of duodenum: pineal melatonin, a ratio of 0.05-0.19: 1 is likely. The gut is not a major extra-pineal source of melatonin; indeed it may well not produce any.

A surgical modification in the technique of rat pinealectomy

BACKGROUND

Several experimental intents require pineal gland removal. The main challenge of the pinealectomy surgical procedure is the hemorrhage due to the transverse sinus torn. The study aimed to modify the rat pinealectomy surgical procedure to reduce the risk of bleeding and the mortality rate.

METHODS

Adult male rats experienced pinealectomy surgery. A mini-drill was used to remove a small skull area in the junction of the lambda and sagittal sutures. The pineal gland was removed using a curved-head hook. Animals experienced intensive post-surgical care. Locomotion, cerebellar motor function, working memory, and anxiety were evaluated 2 weeks after pinealectomy by the open field, rotarod, Y maze, and the elevated plus maze, respectively.

RESULTS

Surgical modification reduced the bleeding risk and animal mortality rate. No significant alteration was found in locomotion and working memory. However, the pinealectomy was anxiogenic and decreased entry to the open arm. The cerebellar motor performance did not change in the rotarod test. Hematoxylin-Eosin staining of removed tissue confirmed the histology of the pineal gland.

CONCLUSION

Advantages of this technique were removing a small skull area, modifying the hook insertion point to prevent damaging the brain veins, reducing the bleeding risk and the mortality rate. Surgery modification was associated with a decreased final number of animals used. Regardless of the melatonin shortage, pinealectomy affects different organs, which should be considered in the research study design.

Pineal Calcification, Melatonin Production, Aging, Associated Health Consequences and Rejuvenation of the Pineal Gland

The pineal gland is a unique organ that synthesizes melatonin as the signaling molecule of natural photoperiodic environment and as a potent neuronal protective antioxidant. An intact and functional pineal gland is necessary for preserving optimal human health. Unfortunately, this gland has the highest calcification rate among all organs and tissues of the human body. Pineal calcification jeopardizes melatonin's synthetic capacity and is associated with a variety of neuronal diseases. In the current review, we summarized the potential mechanisms of how this process may occur under pathological conditions or during aging. We hypothesized that pineal calcification is an active process and resembles in some respects of bone formation. The mesenchymal stem cells and melatonin participate in this process. Finally, we suggest that preservation of pineal health can be achieved by retarding its premature calcification or even rejuvenating the calcified gland.

Melatonin as a Potent and Inducible Endogenous Antioxidant: Synthesis and Metabolism

Melatonin is a tryptophan-derived molecule with pleiotropic activities. It is present in almost all or all organisms. Its synthetic pathway depends on the species in which it is measured. For example, the tryptophan to melatonin pathway differs in plants and animals. It is speculated that the melatonin synthetic machinery in eukaryotes was inherited from bacteria as a result of endosymbiosis. However, melatonin's synthetic mechanisms in microorganisms are currently unknown. Melatonin metabolism is highly complex with these enzymatic processes having evolved from cytochrome C. In addition to its enzymatic degradation, melatonin is metabolized via pseudoenzymatic and free radical interactive processes. The metabolic products of these processes overlap and it is often difficult to determine which process is dominant. However, under oxidative stress, the free radical interactive pathway may be featured over the others. Because of the complexity of the melatonin degradative processes, it is expected that additional novel melatonin metabolites will be identified in future investigations. The original and primary function of melatonin in early life forms such as in unicellular organisms was as a free radical scavenger and antioxidant. During evolution, melatonin was selected as a signaling molecule to transduce the environmental photoperiodic information into an endocrine message in multicellular organisms and for other purposes as well. As an antioxidant, melatonin exhibits several unique features which differ from the classic antioxidants. These include its cascade reaction with free radicals and its capacity to be induced under moderate oxidative stress. These features make melatonin a potent endogenously-occurring antioxidant that protects organisms from catastrophic oxidative stress.

Melatonin, light and chronobiological disorders

Human plasma melatonin concentrations can be measured accurately and sensitively by gas chromatography-negative chemical ionization mass spectrometry. With this assay, we have shown that: in rats and in humans, plasma melatonin is exclusively derived from the pineal gland; propranolol and clonidine reduce melatonin levels in human; some blind people appear to have free-running melatonin secretory circadian rhythms; bright light can acutely suppress human melatonin production according to a linear fluence-response relationship; manic-depressive patients appear to be supersensitive to light, even when they are well; melatonin levels are greater in manic patients than in depressed patients; in experiments to test the clock-gate model and the hypothesized phase-response curve, two different effects of light appear to present in humans: an acute suppressant effect (mainly in the evening during long photoperiods) and an entrainment effect (particularly during the morning but also in the evening). When blood is sampled for measuring melatonin levels as a marker for circadian phase position, bright light should be avoided after 5 p.m. (the dim light melatonin onset). Bright-light exposure in the morning appears to advance circadian rhythms, whereas bright-light exposure in the evening appears to delay them. Once a patient has been 'phase typed' (phase-advanced vs. phase-delayed), predictions can be made about whether morning or evening light would be more effective in treating the sleep or mood disorder.

The effect of L-tryptophan on hemorrhagic shock induced bacterial translocation1

Hemorrhagic shock causes mucosal damage in intestine and it results in translocation of bacteria to distant organs. In this study, effects of various doses of L-Tryptophan on the prevention of bacterial translocation in hemorrhagic shock induced rabbits were investigated. This study was carried out on six groups, each was consisting of 10 rabbits. While any procedure was conducted on the rabbits in group 1 (as a control group), 1 x 10(10)Escherichia coli isolate were administered rabbits in the other groups by gavage. In groups 3, 4, 5, and 6, hemorrhagic shock was induced. After induction of hemorrhagic shock, 10, 50, and 200 mg/kg L-Tryptophan were intragastrically administered to animals in groups 4, 5, and 6, respectively. Blood and terminal ileum samples were taken to detect bacterial translocation by polymerase chain reaction and mucosal damage by histopathological examination at 24 h after hemorrhagic shock. The occurrence of bacterial translocation increased as well when intestinal bacterial intensity was increased (P < 0.05). The most intensive bacterial translocation was formed in group 3 as a result of the additive effect of hemorrhagic shock to bacterial augmentation. It was observed that bacterial translocation was significantly reduced in groups 5 and 6 that are 50 and 200 mg/kg L-Tryptophan were administered (P < 0.01). Histopathological changes on mucosa and submucosa support these results. As a result, we concluded that augmentation of intestinal bacterial intensity induces bacterial translocation, the addition of hemorrhagic shock to bacterial augmentation makes more excessive translocation and mucosal changes have effective roles in these events. L-Tryptophan decreased the intestinal mucosal damage and bacterial translocation induced by hemorrhagic shock, in a dose-dependent manner.

Cerebral arteriolar structure and function in pinealectomized rats

We examined cerebral arteriolar structure and autoregulation of cerebral blood flow (CBF) in control (n = 8), sham-operated (n = 8), pinealectomized (n = 10), and pinealectomized plus melatonin-treated (0.51 +/- 0.01 mg x kg(-1) x day(-1) in drinking water, n = 9) young Wistar rats. The lower limit of CBF autoregulation (LLCBF) was determined by measurement of CBF (in arbitrary units, laser Doppler) during stepwise hypotensive hemorrhage; the arteriolar internal diameter (ID; in microm, cranial window) was also measured. Measurements of ID were repeated during a second stepwise hypotension after smooth muscle cell deactivation (67 mmol/l EDTA). The cross-sectional area (CSA) was measured by histometry. CSA and EDTA-induced vasodilatation decreased after pinealectomy (517 +/- 21 vs. 819 +/- 40 microm(2) in sham and 829 +/- 55 microm(2) in control, P < 0.05, and 81 +/- 4 vs. 102 +/- 5 microm in sham and 104 +/- 4 microm in control, P < 0.05, respectively) and were restored by melatonin (924 +/- 39 microm(2) and 102 +/- 5 microm, respectively). These results suggest that melatonin deprival makes the arteriolar wall thinner and stiffer. However, these changes had little effect on LLCBF. In conclusion, pinealectomy of young rats induces atrophy and decreases distensibility of the cerebral arteriolar wall; these effects are prevented by melatonin. They do not modify LLCBF.

The effect of short intermittent light exposures on the melatonin circadian rhythm and NMU-induced breast cancer in female F344/N rats

We investigated the effects of altered endogenous nighttime melatonin concentrations on mammary tumor production in an N-nitroso-N-methylurea (NMU)-induced breast cancer model in female Fischer 344 (F344)/N rats. Experiments were designed 1) to evaluate whether short-duration intermittent exposures to light at night would affect the nocturnal rise of melatonin, resulting in a decrease in nighttime serum melatonin concentrations, 2) to evaluate whether any suppression of nighttime serum melatonin concentrations could be maintained for a period of weeks, and 3) to determine the effects of suppressed serum melatonin concentrations on the incidence and progression of NMU-induced breast cancer. In vivo studies were used to assess serum melatonin concentrations after 1 day and 2 and 10 weeks of nightly administration of short-duration intermittent light exposure at night and incidence of NMU-induced tumors. Five 1-minute exposures to incandescent light every 2 hours after the start of the dark phase of the light: dark cycle decreased the magnitude of the nocturnal rise of serum melatonin concentrations in rats by approximately 65%. After 2 weeks of nightly intermittent light exposures, an average decrease of the peak nighttime serum melatonin concentrations of approximately 35% occurred. The amelioration continued and, at 10 weeks, peak nighttime serum melatonin concentrations were still decreased, by approximately 25%. Because peak endogenous nighttime serum melatonin values could be moderately suppressed for at least 10 weeks, a 26-week NMU mammary tumor study was conducted. Serum melatonin concentrations and incidence, multiplicity, and weight of NMU-induced mammary tumors were assessed. A group of pinealectomized (Px) animals was also included in the tumor study. No effect on the development of mammary tumors in an NMU-induced tumor model in rats occurred when endogenous nighttime serum melatonin concentrations were moderately suppressed by short-duration intermittent light exposures at night. At necropsy, there were no alterations in mammary tumor incidence (28/40 NMU controls, 28/40 NMU + light, 31/40 NMU + Px), multiplicity (2.18 tumors/tumor-bearing NMU control, 1.89 NMU + light, 2.39 NMU + Px), or average tumor weight (1.20 g NMU control, 1.19 g NMU + light, 0.74 g NMU + Px). Tumor burden had no effect on the serum melatonin cycle. At 26 weeks, however, animals exposed to intermittent light at night exhibited approximately 3-fold higher serum melatonin concentrations as compared with controls. Additionally, rats that had been pinealectomized at 4 weeks of age had serum melatonin concentrations that were markedly higher than the expected baseline concentrations for pinealectomized rats (<15 pg/ml), suggesting the reestablishment of a melatonin cycle. This finding was unexpected and suggests that melatonin can be produced by an organ or tissue other than the pineal gland.

Evidence for melatonin synthesis in rodent Harderian gland: a dynamic in vitro study

Melatonin content and release from Harderian glands (HGs) has been measured by an in vitro perifusion technique in three rodent species: Wistar rat, Syrian hamster, and Siberian hamster. Melatonin immunoreactive concentrations in HGs of animals killed at 10.00 hr were 0.31 +/- 0.031 pg/mg gland in male Wistar rat, 0.54 +/- 0.026 pg/mg gland in male Siberian hamster, 0.17 +/- 0.070 and 0.20 +/- 0.059 pg/mg gland in male and female Syrian hamster, respectively. In all species examined, isolated HGs perifused for 9-15 hr released melatonin but did not stabilize their melatonin release rate. No sex-related difference could be noted in the HG melatonin release rate. The total amount of melatonin released over a 15 hr long perifusion was about 0.075 +/- 0.004 ng/15 h/mg gland and 0.063 +/- 0.010 ng/15 hr/mg gland in male and female Wistar rat, respectively; 0.155 +/- 0.019 ng/15 hr/mg gland and 0.141 +/- 0.006 ng/15 hr/mg gland in male and female Siberian hamster, respectively; 0.035 +/- 0.003 ng/15 hr/mg gland and 0.045 +/- 0.004 ng/15 hr/mg gland in male and female Syrian hamster, respectively. This amount, which is higher than the tissue levels, demonstrates the de novo melatonin synthesis. This is confirmed by the fact that infusion of the indoleamine precursor, tryptophan (TRP), stimulated melatonin secretion from HGs. The melatonin release is increased by 2.5-fold in male and female Wistar rat, 1.5-fold in male and female Siberian hamster, and 2.0- and 3.0-fold in male and female Syrian hamster, respectively. Treatment with a TRP hydroxylase inhibitor, para-chlorophenylalanine, reduced basal melatonin release and inhibited the TRP-induced melatonin stimulation. Kinetics and amounts of melatonin released were not affected by pinealectomy, ruling out a possible plasmatic origin of the HG melatonin. Isoproterenol, a beta-adrenergic agonist, and dibutyryl cyclic AMP, a cyclic AMP analogue, failed to stimulate HG melatonin secretion. In conclusion, these results confirm the presence of melatonin in the HGs and demonstrate that melatonin is synthesized in and released from isolated rodent HGs.

The localization of the site of arylalkylamine N-acetyltransferase circadian expression in the photoreceptor cells of mammalian retina

To investigate the molecular mechanism of the melatonin rhythm in the mammalian retina, we examined the temporal mRNA expression pattern of arylalkylamine (serotonin) N-acetyltransferase (AA-NAT), the rate-limiting enzyme in melatonin synthesis in the rat retina. Rat AA-NAT mRNA was detected exclusively in the retinal photoreceptors in the outer nuclear layer--low during the day and increased more than threefold at night. The nocturnal AA-NAT expression in rat retina was also confirmed by RNase protection and the AA-NAT enzymatic activity. This is the first report to localize the site of AA-NAT mRNA circadian expression in mammalian photoreceptor cells.

Vitamin E consumption induced by oxidative stress in red blood cells is enhanced by melatonin and reduced by N-acetylserotonin

The effect of melatonin and its precursor N-acetylserotonin was studied in a model of lipid peroxidation induced in human red blood cells by incubation with cumene hydroperoxide (CHP) and H2O2. The oxidative stress was expressed as vitamin E consumption in the presence of melatonin or N-acetylserotonin (concentration ranging from 0.3 to 400 microM): incubation with melatonin not only lacked any protective effect but it induced a dose-dependent extra vitamin E consumption with both CHP and H2O2. On the contrary, N-acetylserotonin showed a strong antioxidant effect at concentrations between 100 and 400 microM. The hydrogen-donating capacity of melatonin and N-acetylserotonin was also evaluated from the decay of the ESR signal of galvinoxyl radical used as hydrogen abstractor. Lack of hydrogen-donating capacity was observed with melatonin, whereas N-acetylserotonin showed a significant hydrogendonating capacity although inferior to vitamin E, thus suggesting that N-acetylserotonin acts by the classical antioxidant mechanism of hydrogen donation. The measurement of the oxidation potential and the specific molecular structure suggest that the vitamin E consumption effect observed with melatonin could be due to the interactions of its radical cation or derivatives on vitamin E.

The effects of melatonin on vasopressin secretion in vivo: interactions with acetylcholine and prostaglandins

The pineal hormone melatonin influences the neurohypophysial hormone release from the isolated hypothalamus in vitro through the effect on the cholinergic pathways as well as the biosynthesis of prostaglandins. The aim of the present study was, therefore, to investigate the effects of melatonin (0.5, 1, or 5 ng) administered in vivo on the vasopressin and oxytocin release as well as to examine whether similar interactions between melatonin and acetylcholine or prostaglandins occur in vivo. In the initial study on the effect of melatonin male Sprague-Dawley rats were implanted under anaesthesia with an arterial and venous cannula. Melatonin in a dose of 0.5 ng injected intravenously had no effect on plasma vasopressin concentration. The higher dose of 1 ng caused a significant decrease in vasopressin release 10 min after injection, whereas 5 ng melatonin caused an increase in plasma hormone concentrations, the difference being significant 20 min after injection. No significant effects of melatonin on the oxytocin release was found. In the second study in which an I.C.V. cannula was additionally implanted, the cholinergic muscarinic receptor antagonist atropine (10 microg) injected I.C.V. abolished the melatonin-induced effects on plasma vasopressin level. On the other hand, a cyclo-oxygenase inhibitor ibuprofen (75 microg) injected I.C.V. blocked the vasopressin release induced by 5 ng melatonin and reversed the inhibitory effect of 1 ng melatonin. These results demonstrate that melatonin affects the neurosecretory function of the hypothalamo-neurohypophysial complex in vivo possibly via mechanisms involving cholinergic transmission and/or prostaglandin biosynthesis.

Cell biology of the harderian gland

The harderian gland is an orbital gland of the majority of land vertebrates. It is the only orbital gland in anuran amphibians since the lacrimal gland develops later during phylogenesis in some reptilian species. Perhaps because it is not found in man, little interest was paid to this gland until about four decades ago. In recent years, however, the scientific community has shown new interest in analyzing the ontogenetic and morphofunctional aspects of the harderian gland, particularly in rodents, which are the preferred experimental model for physiologists and pathologists. One of the main characteristics of the gland is the extreme variety not only in its morphology, but also in its biochemical properties. This most likely reflects the versatility of functions related to different adaptations of the species considered. The complexity of the harderian gland is further shown in its control by many exogenous and endogenous factors, which vary from species to species. The information gained so far points to the following functions for the gland: (1) lubrication of the eye and nictitating membrane, (2) a site of immune response, particularly in birds, (3) a source of pheromones, (4) a source of saliva in some chelonians, (5) osmoregulation in some reptiles, (6) photoreception in rodents, (7) thermoregulation in some rodents, and (8) a source of growth factors.

Melatonin in edible plants identified by radioimmunoassay and by high performance liquid chromatography-mass spectrometry

Melatonin, the chief hormone of the pineal gland in vertebrates, is widely distributed in the animal kingdom. Among many functions, melatonin synchronizes circadian and circannual rhythms, stimulates immune function, may increase life span, inhibits growth of cancer cells in vitro and cancer progression and promotion in vivo, and was recently shown to be a potent hydroxyl radical scavenger and antioxidant. Hydroxyl radicals are highly toxic by-products of oxygen metabolism that damage cellular DNA and other macromolecules. Herein we report that melatonin, in varying concentrations, is also found in a variety of plants. Melatonin concentrations, measured in nine different plants by radioimmunoassay, ranged from 0 to 862 pg melatonin/mg protein. The presence of melatonin was verified by gas chromatography/mass spectrometry. Our findings suggest that the consumption of plant materials that contain high levels of melatonin could alter blood melatonin levels of the indole as well as provide protection of macromolecules against oxidative damage.

Superior cervical ganglionectomy suppresses circadian corticotropic rhythms in male rats in the short term (5 days) and long term (10 days)

Superior cervical ganglionectomy (SCGx) has drastic effects on numerous hormonal circadian rhythms and particularly on pineal melatonin secretion. We investigated the hormonal consequences of ablation of the superior cervical ganglion on the corticotropic circadian rhythms in the male rat. Plasma were obtained by sampling blood every 4 h, using a chronic carotid cannula. Adreno-corticotropin hormone (ACTH) was assayed by radioimmunoassay (RIA) and corticosterone (B) by radiocompetition. Urinary 6-sulphatoxymelatonin (aMT6s), considered as an index of the pineal gland activity, was assayed by specific RIA: a decrease in the aMT6s concentration after ganglionectomy was taken as proof of adequate surgical operation. Control animals showed classical circadian rhythms for ACTH and B with basal values during the light phase and circadian peaks around the light/dark interface. Five and ten days after ganglionectomy, the circadian rhythms of ACTH and B were suppressed. In addition, the mean ACTH concentrations increased significantly 10 days after ganglionectomy compared to those in sham-operated rats and 5 days post-operation group. The mean plasma corticosterone levels were similar in those three groups of animals. This is the first study demonstrating the suppressive effect of superior cervical ganglionectomy on the circadian corticotropic hormonal cycle.

Duodenum is not a consistent source of melatonin in rats

Markedly increased melatonin levels in plasma have been observed in response to tryptophan administration. This post-tryptophan melatonin increase has been attributed to the duodenum. Because extra-pineal sources of melatonin may be important in interpreting the meaning of altered melatonin production observed in patient populations, this work was undertaken to confirm whether melatonin is produced in the duodenum and to know whether the duodenum need be considered when investigating the circadian control of melatonin production. We measured melatonin in rat duodenum by HPLC both under basal conditions and following tryptophan load. No melatonin was observed in duodenum under conditions of 2.5 ng/g measurement limits. Neither was there any evidence found for the melatonin precursor N-acetylserotonin. Treatment with N-acetylserotonin resulted in increased melatonin content in the pineal gland, but no evidence for melatonin in the duodenum. In vitro incubation of duodenum tissue with 5-hydroxytryptophan, 5-methoxytryptophan, or N-acetylserotonin revealed no detectable melatonin synthesis, and incubation with melatonin revealed no detectable melatonin degradation. The lack of confirmation of melatonin content and the lack of either synthetic or degradative enzyme activity in duodenum tissue suggest that melatonin production from duodenum need not be considered in human or animal studies of melatonin production.

The significance of the metabolism of the neurohormone melatonin: antioxidative protection and formation of bioactive substances

Recent findings suggest that the ability of melatonin to enter all body tissues and to be metabolized, enzymatically or nonenzymatically, in any of them results in a spectrum of effects, which exceed substantially those transduced by membrane receptors. These actions comprise the formation of various bioactive compounds such as N-acetylserotonin, 5-methoxytryptamine, N,N-dimethyl-5-methoxytryptamine, 5-methoxytryptophol, cyclic 2-hydroxymelatonin, pinoline, and 5-methoxylated kynuramines. Apart from enzymatic metabolism, nonenzymatic reactions with free radicals, in particular the superoxide anion and the hydroxyl radical, represent a new and significant aspect of melatonin's biological role. Melatonin represents the most potent physiological scavenger of hydroxyl radicals found to date, and recent findings suggest an essential role of this indoleamine for protection from hydroxyl radical-induced carcinogenesis and neurodegeneration.

The contribution of extrapineal sites of melatonin synthesis to circulating melatonin levels in higher vertebrates

While the production of melatonin in higher vertebrates occurs in other organs and tissues besides the pineal, the contribution of extrapineal sites of melatonin synthesis such as the retina, the Harderian glands and the gut to circulating melatonin levels is still a matter of debate. The amount of melatonin found in the gastrointestinal tract is much higher than in any other organ including the pineal and the gut appears to make a significant contribution to circulating melatonin at least under certain conditions. The gut has been identified to be the major source of the elevated plasma concentrations of melatonin seen after tryptophan administration and of the changes of circulating melatonin level induced by the feeding regimen. Whereas the circadian and circannual fluctuations of the concentration of melatonin in the blood seem to be triggered by changes of the photoenvironment and its effect of pineal melatonin formation, basal daytime melatonin levels and the extent of their elevation at nighttime appear to be additionally controlled by nutritional factors, such as the amount and the composition of ingested food and therefore availability of tryptophan as a rate-limiting precursor of melatonin formation by the enterochromaffin cells of the gastrointestinal tract.

Immunohistochemical demonstration of melatonin in the female mink harderian gland

In the Harderian gland of the female mink, either intact or killed after a bilateral ablation of the cervical superior ganglion, almost all of the cells of the alveoli were immunolabeled with anti-melatonin antiserum. Animals were killed during the day or during the night. The immunolabelling was observed only in the cytoplasm, while the nucleus remained unstained. Using successive dilutions of the antiserum on serial sections of the Harderian gland to qualitate the melatonin content, a circadian rhythm of melatonin immunoreactivity was observed. The intensity of immunofluorescence labelling was higher in intact animals killed during the day than in those killed during the night. These results could be explained by the inhibitory or stimulatory influence of pineal melatonin released during the night on melatonin synthesis or release in the Harderian gland, respectively. In the Harderian gland of ganglionectomized animals, the intensity of melatonin immunofluorescence was lower than in intact animals killed during the day. It is concluded that the Harderian gland might be involved in the perception of the day/night cycle and that melatonin synthesis/secretion was likely controlled by the cervical superior ganglion in this organ.

Pinealectomy decelerates the circadian food intake rhythm of cervically sympathectomized rabbits

Long-term records of the free-running food intake rhythms were obtained from 26 young adult blinded rabbits that were subjected in the course of recording to bilateral superior cervical ganglionectomy (sympathectomy). This always resulted in acceleration of the rhythm (mean delta tau = -0.35 h). Some 4 months afterwards, 12 rabbits were pinealectomized and after another 4 months sham pinealectomized also. In the other 14 animals, these operations were performed in the reverse order. It appeared that suctioning away part of the ventricular walls during sham pinealectomy was followed by a slight reduction in tau (mean = -0.07 h) that could be largely attributed to the spontaneous gradual reduction of tau that occurred throughout all experiments. On the other hand, total pinealectomy in these already sympathectomized blinded rabbits always resulted in a substantial deceleration of the rhythms (mean delta tau = +0.23 h). These observations on blinded rabbits suggest that a sympathetically denervated pineal gland releases appreciable amounts of melatonin or of another hormone with a melatonin-like accelerating effect on the circadian pacemaker (SCN).

Feedback in the rabbit's central circadian system, revealed by the changes in its free-running food intake pattern induced by blinding, cervical sympathectomy, pinealectomy, and melatonin administration

Long-term records of the number of food approaches per 30 min were obtained from 16 rabbits in constant light and after blinding. Optic nerve sectioning usually resulted in a shortening of the free-running period (tau; mean reduction = -0.26 hr) of the oscillator governing the rabbit's food intake pattern. Subsequent resection of both superior cervical ganglia always reduced this tau (mean = -0.41 hr), and it could be further shortened (mean = -0.24 hr) by chronic administration of melatonin. These observations led to experiments in which the food intake of 15 blinded rabbits was recorded before and after pinealectomy. This always resulted in a lengthening of tau (mean = +0.27 hr), which was never seen after subtotal or sham pinealectomy in 10 other blinded animals (mean = -0.05 hr). Sympathectomy also reduced tau (mean = -0.50 hr) in the pinealectomized rabbits, whereas it induced a smaller reduction (mean = -0.29 hr) after sham pinealectomy. These results, together with previous observations on the effect of sympathectomy on the retinal "dark" discharge and on the influence of the latter upon the food intake pattern, indicate that the cervical sympathetic nerves may form part of three feedback loops for the rabbit's central circadian system. These have separate postganglionic pathways; there are also differences in the circadian phasing of their activities and in their effects on tau.

Diurnal rhythms of immunoreactive melatonin in the aqueous humor and serum of male pigmented rabbits

Using specific radioimmunoassays, melatonin was quantified in the aqueous humor and serum of male pigmented rabbits adapted to 12L:12D with lights on at 06.00 h. Melatonin concentrations in the aqueous humor at 08.00 and 16.00 h were similar (mean: 5.23 pg/ml) and significantly lower than those at 22.00 and 01.00 h (mean: 22.06 pg/ml). A parallel rhythm was also demonstrated in the serum with higher melatonin concentrations (daytime mean: 75.26 pg/ml; nighttime mean: 168.94 pg/ml). We propose that the aqueous melatonin rhythm is associated with the rhythmic change in aqueous flow.

Ultrastructure of the blood vessels in the Harderian gland of the hamster (Mesocricetus auratus): existence of sinusoids

The Harderian gland blood supply of female and male hamsters was studied using light and electron microscopy. A profuse vascularization surrounding secretory acini was observed. Among the blood vessels, the existence of large and irregular sinusoidal capillaries was apparent. These sinusoids appeared in close association to the basal aspect of the secretory cells. Typical, small, fenestrated capillaries were also observed within the connective tissue. The existence of this particular vascularization together with other morphological features of the secretory cell basal pole suggest a possible endocrine function of these orbital glands.

Hydroxyindole-O-methyltransferase in rat retinal bipolar cells: persistence following photoreceptor destruction

The presence of hydroxyindole-O-methyltransferase (HIOMT) activity and localization of HIOMT immunoreactivity was examined in albino rat retinas following photoreceptor destruction. Male Sprague-Dawley rats were exposed to high intensity fluorescent light for 4 consecutive days, then placed on a 14:10 h light:dark cycle for two weeks to allow for phagocytic removal of damaged cells from the retina. Histologic examination revealed almost complete destruction and removal of all photoreceptors. The damaged retinas exhibited an increase in HIOMT activity relative to controls, when expressed as activity per mg of protein. HIOMT activity in the pineal glands was not affected. When control and light damaged retinas were examined for HIOMT localization by immunocytochemistry, the control retinas displayed intense HIOMT immunoreactivity in all photoreceptors, and a somewhat lighter labeling in a population of bipolar cells, whereas the light damaged retinas (lacking photoreceptors) showed intense HIOMT immunoreactivity in bipolar cells. These results suggest that the increase in HIOMT activity following photoreceptor destruction is due to increased synthesis of this enzyme in a population of bipolar cells. These HIOMT-immunoreactive bipolar cells may perhaps respond in a compensatory manner to changing levels of melatonin in the retina.

Darkness stimulates rapid synthesis and release of melatonin in rat retina

The presence of melatonin in retina has been widely reported for over two decades although studies of its functional importance within the retina have only recently been emphasized. We have analyzed the biochemical characteristics of melatonin synthesis and release, focusing on rapid changes in response to light/dark conditions. Our major findings are consistent with the following conclusions: (1) melatonin synthesis is stimulated within minutes after exposure to darkness, and may reflect an increase in N-acetyl transferase activity; (2) melatonin is not stored, but rather it diffuses freely throughout the retina immediately after it is synthesized; and (3) the dark-induced increase in retinal melatonin release is a synthesis-coupled response and does not involve separate secretion mechanisms. The characteristics of melatonin synthesis and release described herein would be consistent with the proposed role of melatonin as a local paracrine effector of dark-adaptive responses in retina.

N-acetyltransferase activity and indole contents of the male Syrian hamster Harderian gland: changes during the light:dark cycle

The activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) and the indole contents of the Harderian glands of male Syrian hamsters were studied throughout a 24-h period. NAT activity exhibited a sharp rise 1 h after lights on, decreasing to basal levels 1 h later. Neither a HIOMT activity nor a melatonin concentration rhythm was detected throughout the 24 h. The 5-hydroxytryptamine (serotonin) concentration was highest during the dark phase reaching a peak at 0300 h; with light onset serotonin levels exhibited a rapid short-term drop. The 5-hydroxytryptophol concentration was highest during the mid- to late photophase; the lowest values to this constituent were measured late in the dark phase and at 1 h after lights on. The 5-hydroxyindole acetic acid concentration of the Harderian glands was rather stable throughout the 24-h period but levels did show a short-lived drop 1 h after light onset. Only a few animals contained detectable amounts of N-acetyl-5-hydroxytryptamine (N-acetylserotonin) in their Harderian glands. In agreement with previous work on the Harderian glands of female Syrian hamsters, the present results in males suggest that light onset is associated with marked changes in Harderian indoleamine metabolism.

Influence of pinealectomy on plasma and extrapineal melatonin rhythms in young chickens (Gallus domesticus)

A specific radioimmunoassay was validated for the quantitative measurement of melatonin (MT) levels in plasma and homogenates of the pineal gland, Harderian gland, or retinae of young chickens. Single-comb White Leghorn (SCWL) cockerels were raised under a 12L:12D light/dark cycle for two experiments. In Experiment 1, 12 birds were bled and immediately killed for their pineal glands at 4-hr intervals during a single light/dark cycle at 25 days of age (25 da) for simultaneous determination of changes in MT levels in the plasma and pineal gland. Plasma MT levels were low during photophase (100 pg/ml) and reached a distinct peak (390 pg/ml) at mid-scotophase. A parallel MT rhythm was found in pineal homogenates where the average MT content during scotophase (7.4 ng/gland) was 10 times higher than the average MT content of pineal glands obtained during photophase. In Experiment 2, SCWL cockerels were either pinealectomized or sham-operated (PN) at 8 to 10 da. At 25 da, six birds from each surgical treatment group, including unoperated controls (C), were bled at 4-hr intervals, corresponding to those in Experiment 1, during a single light/dark cycle. Immediately after being bled, each bird was killed and the eyes and Harderian glands were removed for measurement of their MT contents. Pinealectomy completely abolished the plasma MT rhythm which in intact chicks (PN and C) reached a sharp peak (298 pg/ml) at mid-scotophase. Although not affected by surgical treatment, retinal MT levels showed a higher amplitude rhythm with a prominent peak (4 ng/retina) at mid-scotophase that was 15 times higher than the average retinal MT content during photophase. A modest nocturnal MT rhythm was found in the Harderian gland where the average MT level for all surgical treatment groups during scotophase (89 pg/100 mg wet wt) was only 51% higher than that observed for photophase. These data indicate that the plasma MT rhythm in chickens is derived solely from MT secreted into blood by the pineal gland and that the extrapineal MT produced rhythmically in both the retina and Harderian gland does not contribute to the plasma MT rhythm.

Melatonin reduces the production and secretion of prolactin and growth hormone from rat pituitary cells in culture

A culture of clonal tumour cells from rat pituitary gland that secrete both prolactin and growth hormone were used to investigate whether the pineal hormone melatonin can act directly on the pituitary gland to control prolactin production. Melatonin inhibition of prolactin and growth hormone production was significant but mild. Concentrations of between 10(-8) M and 10(-6) M reduced both prolactin and growth hormone production and prolactin secretion by 10-50%. 17 beta-oestradiol (OE) and thyrotrophin-releasing hormone (TRH) stimulated prolactin production but had no significant effect on growth hormone production. Melatonin reduced the effects of both of these compounds. Both TRH and vasoactive intestinal peptide (VIP) stimulated secretion of prolactin, and TRH also of growth hormone. Melatonin reduced these effects significantly. TRH and VIP increased cAMP production two- and 12-fold, respectively. Melatonin had no effect on basal or stimulated cAMP production. The melatonin-induced changes in prolactin and growth hormone production and secretion seen here do not approach the magnitude of the fluctuations seen in plasma in vivo. It is concluded that while melatonin does have a direct effect on the lactotroph in the regulation of prolactin production, its main physiological target must be elsewhere.

Melatonin: parallels in pineal gland and retina

It is apparent that several relationships exist between the pineal gland and retina. The similarities in development and morphology have been obvious for many years. A recent resurgence of interest in this field has led to a further understanding of many functional similarities between these two organs. A notable feature of the pineal gland and retina is their common ability to synthesize the indolamine hormone, melatonin. Many investigators suspect that the cyclic rhythm of retinal melatonin synthesis may be related to other cyclic events which normally occur in the retina.

A fluctuation in plasma melatonin level in the Weddell seal during constant natural light

Blood was collected over a 24-h period from free-living adult Weddell seals that had been exposed to natural constant daylight for some 12 weeks. The plasma melatonin concentration was low throughout the trial (range of means: 30.8-70.4 pg/ml) with the exception of a 3-h period centered on 1700 h local time, when the plasma level rose to a mean of 131.8 pg/ml. The pineal gland of the Weddell seal is thus capable of transient bouts of activity during constant and bright daylight, a situation usually inhibitory to the function of the gland. It is suggested that circadian fluctuations in the intensity of constant light entrain variation in the pineal gland functioning of this seal during summer.

Effects of time, photoperiod, and pinealectomy on ocular and plasma melatonin concentrations in the chick

Pinealectomized and intact chicks raised under cyclic (14L:10D) and noncyclic (OL;24L) conditions of environmental lighting were sampled 60 hr and 6 weeks after pinealectomy to evaluate the effects of pineal ablation on plasma and ocular melatonin concentrations. Little or no immunoreactive melatonin was present in plasma 60 hr after pinealectomy, while appreciable amounts (38-70% of control values) were measurable 6 weeks later. We also found photoperiod to be an important variable in determining the relative amounts of melatonin present in the eyes and the pineal gland. At night or in continuous darkness, the eyes contain proportionately larger amounts of melatonin; by day, or in continuous light, they contain less. Finally, it was determined that pinealectomy significantly increases ocular melatonin concentrations (+ 62-80%) in chicks raised in the 14L:10D and 24L environments. These results show that compensatory mechanisms exist for maintaining circulating levels of melatonin in the absence of the pineal, and that they require some time to develop. Furthermore, there appears to be a dynamic interrelationship between the eyes and the pineal gland in terms of melatonin production. This is evidenced by the differential effects of environmental lighting on melatonin concentrations in these tissues and by the increased ocular concentrations of melatonin in pinealectomized animals.

Melatonin and N-acetylserotonin stress responses: effects of type of stimulation and housing conditions

The effects of housing condition and type of stimulation on serum melatonin and N-acetylserotonin (NAS) were investigated. Male rats were housed under a 12/12-hour light-dark cycle, with ad libitum food and water, either individually or in groups of four. At the start of the light phase, separate groups were sacrificed at rest or subjected for 3 minutes to the stimulation of cold water, noise, novel environment, or ether vapour and then decapitated at 0, 5, 15, 30 or 60 minutes after the end of stimulation. Melatonin was measured by a modified radioimmunoassay and NAS by a specific radioimmunoassay. Melatonin levels responded to stimulation with an increase, while NAS levels responded with a decrease. Housing condition had no effect on hormone response. However, the pattern of response for each of the two hormones differed greatly among the stimuli. For melatonin, cold water was the most potent stimulus, followed by noise, novel environment, and ether. NAS responded most to ether, fleetingly to cold, and in a bimodal manner to noise. The data are interpreted as suggesting that separate mechanisms regulate serum melatonin and serum NAS is response to environmental stimulation and that under appropriate control conditions melatonin from the pineal is very responsive to environmental stimuli, in a manner similar to that of pituitary hormones.

Melatonin rhythms in the eyes, pineal bodies, and blood of Japanese quail (Coturnix coturnix japonica)

Melatonin levels in the eyes, pineal bodies, and blood of Japanese quail exposed to 12L:12D show robust daily rhythms with high levels occurring in the night and low levels occurring during the day. Since melatonin is synthesized in both the eyes and pineal bodies of birds, the relative contribution of these structures to the blood melatonin levels was determined. A rhythm of blood melatonin persisted in 12L:12D in birds blinded by complete orbital enucleation and in pinealectomized birds but the nighttime levels were reduced by 33 and 54%, respectively, as compared to melatonin levels in control quail. Only a small melatonin rhythm (13% of control levels) was detected in the blood of pinealectomized, blinded quail. This "residual" rhythm could indicate either the contribution of extrapineal, extraocular sources of melatonin or melatonin secretion from remnants (if any) of pineal body tissue remaining after pinealectomy. Blinding did not obviously affect pineal melatonin levels nor did pinealectomy affect ocular melatonin levels. It was concluded that (1) daily rhythms in melatonin content occur in the pineal bodies, the eyes, and the blood of quail; (2) the blood rhythm is the result of melatonin secretion from both the pineal body and the eyes; (3) extraretinal photoreceptors can mediate entrainment of the pineal melatonin rhythm; and (4) obvious compensatory changes in melatonin levels do not occur in the eye following pinealectomy or in the pineal body following blinding.

Pineal ablation in varying photoperiods and the incidence of 9,10-dimethyl-1,2-benzanthracene induced mammary cancer in rats

Our earlier observation of increased incidence of 9,10-dimethyl-1,2-benzanthracene (DMBA) induced mammary carcinoma in young, virgin 'functionally' pinealectomized Holtzman rats poses the question whether or not a comparable incidence would occur in surgically pinealectomized rats reared in varying photoperiods (e.g. light/dark (LD) 24/0 or LD 10/14 schedules). Results show that functionally or surgically pinealectomized rats in LD 24/0 schedule have comparable mammary tumor incidence (95% and 83%, respectively) and latency period of tumor appearance (60 +/- 3.1 and 69.2 +/- 6.6 days, respectively). However, when surgically pinealectomized rats were kept in short photoperiods (LD 10/14), a significant difference was observed in both tumor incidence (60.9%) and latency period (91.8 +/- 11.0 days). Our data suggest that the susceptibility of the mammary gland to carcinogenic insult may be modulated by the concentration of the pineal hormone, melatonin, in the CNS.

Studies on the kinetics of melatonin and N-acetylserotonin in the rat at mid-light and mid-dark

The distribution and disappearance of melatonin and N-acetylserotonin after intravenous injection were studied at mid-light and mid-dark. Male rats were adapted under a photoperiod of 12 h light and 12 h dark cycle for a minimum of 1 week. After the intravenous injection of melatonin (10 micrograms), N-acetylserotonin (10 micrograms), or 3H-melatonin (76 ng), blood samples were collected at 1, 3, 5, 10, 20, 30, 45, 60, 75, 90, and 120 min. The concentrations of indoles in the plasma samples were determined by radioimmunoassay, while the radioactivity of 3H-melatonin was determined by scintillation counting. The distribution of melatonin and N-acetylserotonin followed a two-compartment model, showing an initial distribution phase followed by an elimination phase. The biological half-life of melatonin at mid-light and mid-dark were similar but the volume of distribution (V beta and Vc) and metabolic clearance rate (C1s) were greater at mid-dark than at mid-light. The estimated melatonin secretory rate at mid-dark was fivefold greater than that at mid-light. When 3H-melatonin was used, the half-life was significantly longer. A diurnal difference in the distribution was also evident in the N-acetylserotonin studies. The volume of distribution (V beta and Vc) was greater at mid-dark while the C1s was not significantly faster at mid-dark compared with that at mid-light. However, the estimated N-acetylserotonin secretory rate at mid-dark was eightfold greater than at mid-light.

Regional concentrations of melatonin in the rat brain in the light and dark period

The levels of melatonin in five brain regions, whole brain, pineal and serum samples were studied in rats adapted under a photoperiod of 12 h light and 12 h dark. It was found that the melatonin levels for all the tissues obtained in the dark period were significantly higher than those obtained in the light period. Regional study of melatonin levels in the brain in the light and dark period demonstrated a high level in the hypothalamus, intermediate levels in the mid-brain, cerebellum and pons-medulla and low level in the telencephalon. Our findings indicate that melatonin in the brain is unevenly distributed and that there are diurnal rhythms of melatonin in all the five brain regions studied.

Diurnal variations of melatonin and N-acetylserotonin in the tissues of quails (Coturnix sp.), pigeons (Columba livia), and chickens (Gallus domesticus)

The levels of melatonin and N-acetylserotonin in the pineal, serum, retina, and brain of quails (Coturnix sp.), pigeons (Columba livia), and chickens (Gallus domesticus) were studied. The birds were kept under a photoperiod of 12 hr light and 12 hr dark for a minimum of 5 days before they were decapitated at midlight or middark under a 25 W red light. Melatonin and N-acetylserotonin in the pineal, serum, retina, and brain samples were extracted and determined by radioimmunoassay. It was found that the levels of melatonin in all the tissues collected in the dark period were significantly higher (P less than 0.05) than the melatonin levels of the tissues obtained in the light period. Similarly, N-acetylserotonin levels in the pineal and retinal of birds collected in the dark period were significantly higher (P less than 0.05) than those collected in the light period. But the levels of N-acetylserotonin in the serum and brain samples collected in the daytime were not significantly different from those collected in the nighttime. The results suggest that in quails, pigeons, and chickens, diurnal rhythms of melatonin are present in the pineal, serum, retina, and brain, but diurnal rhythms of N-acetylserotonin may only be present in the pineal and retina.

Effects of blinding and pinealectomy on regional brain monoamine concentrations

The effects of blinding with or without pineal ablation on brain monoamine levels were studied in male rats. Brain dopamine (DA), norepinephrine (NE), epinephrine (E), and serotonin (5-HT) were measured by radioenzymatic assays. Four weeks following pinealectomy, E levels were significantly enhanced in the frontal cortex. Chronic blinding decreased striatal DA levels and increased striatal 5-HT levels in both sham-operated and pinealectomized (Px) animals. In a second experiment Px animals were sacrificed 1 or 7 d after pinealectomy in order to examine the short-term effects of pinealectomy. There were no differences between controls and Px animals in their cortical levels of DA, NE, and E and their hippocampal and hypothalamic 5-HT levels. However, the E concentrations measured 1 d after surgery were significantly greater than after 7 d. The implications of these findings with regard to the reported role of the pineal and melatonin in brain homeostasis and endocrine regulation are discussed.

Chronic lithium treatment and twenty-four hour rhythm of serum prolactin, growth hormone and melatonin in rats

This study was undertaken to extend the observations that lithium chlordie has the ability to phase shift the 24-hour pattern of a large number of physiological parameters relative to the environmental lighting cycle. Adult male Wistar rats were housed individually in a temperature and sound controlled environment under a lighting schedule of 12 hours dark/12 hours light. Six point 24-hour maps were generated by taking measurements every four hours throughout 24 hours on separate groups of rats maintained for six weeks on ad lib water and one of three diets: 1) normal lab chow, 2) lab chow supplemented with 50 mM/kg of lithium chloride, 3) lab chow supplemented with 50 mM/kg of sodium chloride to control for the effects of a possible salt load. Variables measured included plasma and red blood cell lithium and sodium concentrations, serum prolactin, growth hormone and melatonin. Plasma lithium levels were 0.7 - 1.0 mEq/L. Plasma, but not red blood cell lithium levels demonstrated a 24-hour rhythm with higher levels following the normal feeding period. Serum and red blood cell sodium levels did not differ among the diets and showed no 24-hour variation. The sodium diet had no significant effects on any of the variables measured. The 24-hour pattern of prolactin was not significantly affected by the diets, while growth hormone and melatonin levels differed in animals on the lithium diet. During the light hours when growth hormone levels are normally falling and melatonin levels are barely detectable, there were no differences on these measurements among the diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of melatonin in the retinae of guinea pigs: effect of environmental lighting

We studied the effect of environmental lighting on the level of melatonin in the retinae of guinea pigs. Male pigmented guinea pigs were adapted to 12-hour light:12-hour dark (with lights on at 0600 hours) for 2 weeks. In experiment 1, two groups of animals were decapitated at 1600 and 0400 hours. Melatonin was extracted from retinae and quantified by radioimmunoassay. A diurnal rhythm of retinal melatonin level per pair of retinae was demonstrated. In experiment 2, one group was subjected to an extended dark period (lights remained off after 0600 hours) while the control group remained on the normal photoperiod on the day of sacrifice. Both groups were killed at 1200 hours. The retinal melatonin level in the group under extended dark period was significantly higher (P less than 0.05) than that of the control. In experiment 3, one group was exposed to an extended light period (lights remained on after 1800 hours) and the control group remained on the normal lighting schedule. Both groups were killed at 2400 hours. The retinal melatonin level in the group exposed to an extended light period was significantly lower (P less than 0.05) than that of the control. Thus, darkness induces an increase while light causes a decrease in retinal melatonin level.

Stimulatory effects of melatonin on ependymal epithelium of choroid plexuses in golden hamsters

Ependymal epithelial cells from the choroid plexuses (CPs) separately of lateral (I + II), IIIrd and IVth ventricles of male Golden Hamsters were studied by electron microscopy and morphometry. The 16 hamsters were distributed between three injections groups: vehicle only, 25 micrograms and 2500 micrograms melatonin (M) by subcutaneous injection daily at L11 to L11.75 in a LD 14:10 daily photoperiod. After 28 consecutive daily injections, animals were killed and the CPs were dissected, fixed and prepared for electronmicroscopy. Thirteen measures of the CP ependymal cells were made, by planimetry, morphometry or direct counting or linear measurement on the EM prints. Effects of melatonin occurred only on the cells from the lateral ventricles. Here M at high dosage caused cell swelling (averaging 50% increase in area), and other cellular changes were graded in relation to M dosage. These were increased (to 26%) in mitochondrial area per cell, and increased (to 50%) in length of apical microvilli. Since in other work the latter form a major locus of ouabain-sensitive Na+, K+-ATPase, it is suggested that M may possibly have a stimulatory effects on transport and related CSF secretory activities by these cells.

The level of N-acetylserotonin and melatonin in the brain of male rats: diurnal variations and effects of pinealectomy

The levels of N-acetylserotonin and melatonin (n-acetyl-5-methoxytryptamine) in the rat brain were studied. Male rats were housed under a photoperiod of 12L:12D. After 2 weeks, the rats were sacrificed in light or in dark at 4-hour intervals round the clock. Brain melatonin and N-acetylserotonin were extracted by chloroform at greater than pH 10 and ethyl acetate at less than pH 3 respectively and quantified by radioimmunoassay. There was a circadian rhythm of brain melatonin (F less than 0.05) with a nadir at 1600 h and a zenith at 0400 h. Brain N-acetylserotonin, unlike brain melatonin, demonstrated no diurnal variation. Effects of pinealectomy on brain N-acetylserotonin and melatonin were also studied. Male rats were housed as noted. After 2 weeks, they were pinealectomized or shamoperated. The animals were killed in light or in dark 1 week after operation. The levels of brain melatonin at 2400 h and 0400 h were significantly lowered (P less than 0.05) following pineal removal. Pinealectomy, however, has no observable effect on the level of brain N-acetylserotonin. Our findings suggest that 1) there is a diurnal rhythm of brain melatonin, 2) the elevation of brain melatonin in the dark period is dependent on the pineal, 3) brain N-acetylserotonin has no diurnal variation, and 4) brain N-acetylserotonin is not affected by pinealectomy.

A diurnal rhythm of N-acetylserotonin in the retina of rats

Diurnal variation of N-acetylserotonin in the retina was studied in male rats housed under a photoperiod of 12 h light and 12 h darkness (lights on at 06.00 h) for two weeks. N-acetylserotonin was extracted from the retina by ethyl acetate at pH less than or equal to 3 and was determined by radioimmunoassay. There is a diurnal rhythm of N-acetylserotonin in the retina with high at night and low in the daytime. It is suggested that N-acetylserotonin in the retina may contribute partly to the levels of circulating N-acetylserotonin and may play a role in the regulation of photomechanical changes in the eye of rats.