Circadian rhythmicity of the activity of hydroxyindole-O-methyl transferase (HIOMT) in the formation of melatonin and 5-methoxytryptophol in the pineal, retina, and harderian gland of the golden hamster

Melatonin in the mammalian retina: Synthesis, mechanisms of action and neuroprotection

Melatonin is an important player in the regulation of many physiological functions within the body and in the retina. Melatonin synthesis in the retina primarily occurs during the night and its levels are low during the day. Retinal melatonin is primarily synthesized by the photoreceptors, but whether the synthesis occurs in the rods and/or cones is still unclear. Melatonin exerts its influence by binding to G protein-coupled receptors named melatonin receptor type 1 (MT1) and type 2 (MT2). MT1 and MT2 receptors activate a wide variety of signaling pathways and both receptors are present in the vertebrate photoreceptors where they may form MT1/MT2 heteromers (MT1/2h). Studies in rodents have shown that melatonin signaling plays an important role in the regulation of retinal dopamine levels, rod/cone coupling as well as the photopic and scotopic electroretinogram. In addition, melatonin may play an important role in protecting photoreceptors from oxidative stress and can protect photoreceptors from apoptosis. Critically, melatonin signaling is involved in the modulation of photoreceptor viability during aging and other studies have implicated melatonin in the pathogenesis of age-related macular degeneration. Hence melatonin may represent a useful tool in the fight to protect photoreceptors-and other retinal cells-against degeneration due to aging or diseases.

Is part-night lighting a suitable mitigation strategy to limit Artificial Light at Night effects on the biological rhythm at the behavioral and molecular scales of the oyster Crassostrea gigas?

Artificial Light at Night (ALAN) is a fast-spreading threat to organisms, especially in coastal environments, where night lighting is increasing due to constant anthropization. Considering that ALAN affects a large diversity of coastal organisms, finding efficient solutions to limit these effects is of great importance but poorly investigated. The potential benefit of one strategy, in particular, should be studied since its use is growing: part-night lighting (PNL), which consists in switching off the lights for a few hours during nighttime. The aim of this study is to investigate the positive potential of the PNL strategy on the daily rhythm of the oyster Crassostrea gigas, a key species of coastal areas of ecological and commercial interest. Oysters were exposed to a control condition and three different ALAN modalities. A realistic PNL condition is applied, recreating a strategy of city policy in a coastal city boarding an urbanized bay (Lanton, Arcachon Bay, France). The PNL modality consists in switching off ALAN direct sources (5 lx) for 4 h (23-3 h) during which oysters are in darkness. Then, a PNL + skyglow (PNL + S) modality reproduces the previous one mimicking a skyglow (0.1 lx), an indirect ALAN source, during the direct lighting switch off, to get as close as possible to realistic conditions. Finally, the third ALAN condition mimics full-night direct lighting (FNL). Results revealed that PNL reduces some adverse effects of FNL on the behavioral daily rhythm. But, counterintuitively, PNL + S appears more harmful than FNL for some parameters of the behavioral daily rhythm. PNL + S modality is also the only one that affect oysters' clock and melatonin synthesis gene expression, suggesting physiological consequences. Thus, in realistic conditions, the PNL mitigation strategy might not be beneficial in the presence of skyglow, seeing worse for a coastal organism such as the oysters.

Artificial light at night at environmental intensities disrupts daily rhythm of the oyster Crassostrea gigas

Artificial Light At Night (ALAN) masks the natural light cycles and thus can disturb the synchronization of organisms' biological rhythms with their environment. Although coastlines are highly exposed to this growing threat, studies concerning the impacts of ALAN on coastal organisms remain scarce. In this study, we investigated the ALAN exposure effects at environmentally realistic intensities (0.1, 1, 10, 25 lx) on the oyster Crassostrea gigas, a sessile bivalve subject to light pollution on shores. We focused on the effects on oyster's daily rhythm at behavioral and molecular levels. Our results showed that ALAN disrupts the oyster's daily rhythm by increasing valve activity and annihilating day / night differences of expression of circadian clock and clock-associated genes. ALAN effects occur starting from 0.1 lx, in the range of artificial skyglow illuminances. We concluded that realistic ALAN exposure affects oysters' biological rhythm, which could lead to severe physiological and ecological consequences.

Bivalve mollusc circadian clock genes can run at tidal frequency

Marine coastal habitats are complex cyclic environments as a result of sun and moon interactions. In contrast with the well-known circadian orchestration of the terrestrial animal rhythmicity (approx. 24 h), the mechanism responsible for the circatidal rhythm (approx. 12.4 h) remains largely elusive in marine organisms. We revealed in subtidal field conditions that the oyster Crassostrea gigas exhibits tidal rhythmicity of circadian clock genes and clock-associated genes. A free-running (FR) experiment showed an endogenous circatidal rhythm. In parallel, we showed in the field that oysters' valve behaviour exhibited a strong tidal rhythm combined with a daily rhythm. In the FR experiment, all behavioural rhythms were circatidal, and half of them were also circadian. Our results fuel the debate on endogenous circatidal mechanisms. In contrast with the current hypothesis on the existence of an independent tidal clock, we suggest that a single 'circadian/circatidal' clock in bivalves is sufficient to entrain behavioural patterns at tidal and daily frequencies.

Hydroxyindole-O-methyltransferase (HIOMT) activity in the retina of melatonin-proficient mice

Numerous pieces of evidence support the expression by the mammalian retina of Hydroxyindole-O-methyltransferase (HIOMT, EC 2.1.1.4), the enzyme directly responsible for the biosynthesis of the pineal chronobiotic hormone melatonin (MLT). However, conflicting results obtained so far by enzyme-kinetic and immune-detection techniques still make HIOMT presence and relevance in the eye a matter of debate. This work aimed at evaluating unambiguously HIOMT activity in the mouse retina, a valuable model for studying the effects of MLT variations on ocular pathophysiology. Since laboratory mouse strains can bear genetic polymorphisms yielding defective enzymes of MLT biosynthesis, retinas and control pineal glands used in this study were obtained in a MLT-proficient crossing of A/J mice, the A/J/C57BL/10 strain. To improve the radiochemical reference assay, we tested different homogenization procedures coupled with HPLC detection. Concomitantly, we quantified MLT, and its precursor N-acetyl-serotonin (NAS) by HPLC coupled to electrochemical detection in retinas isolated from either light- or dark-adapted mice. Results showed that the standard radio-chemical assay was successful for pineal HIOMT only, whereas specific homogenization buffers and HPLC were required to detect retinal activity, presumably due to interfering methyl-transferases inhibited by NAS. Under present conditions, retinal HIOMT V_max accounted for by ≈ 40 fmol/h/mg protein, 2.6-hundreds-fold lower than the pineal counterpart, displaying equivalent K_Ms (≈10 μM). Moreover, NAS and MLT rapidly decreased in light-exposed isolated retinas, corroborating light-sensitive in-situ MLT formation. Conclusively, we measured mouse retinal HIOMT kinetics under basal conditions, a useful result to elucidate the regulatory patterns, the possible impact on eye health, and therapeutic approaches related to this enzyme.

Melatonin and Multiple Sclerosis: From Plausible Neuropharmacological Mechanisms of Action to Experimental and Clinical Evidence

Multiple sclerosis (MS) is a devastating chronic autoimmune demyelinating disease of the central nervous system (CNS), thought to affect more than 2.5 million people worldwide. Regulation of the sleep-wake cycle might influence disease activity and the frequency of relapses in patients. As melatonin (or sleep hormone) involves the regulation of circadian rhythms, much attention has been paid to the management of MS symptoms with melatonin. This review describes the pharmacological mechanisms underlying the neuroprotective effects of melatonin and recent clinical evidence from MS patients. Apparent risks and benefits of melatonin therapies are also discussed. Various in vivo and clinical data presented in this up-to-date review suggest that melatonin may possibly possess a protective role against the behavioral deficits and neuropathological characteristics of MS. Multiple mechanisms of the neuroprotective effects of melatonin such as mitochondrial protection and antioxidant, anti-inflammatory, and anti-apoptotic properties, as well as its anti-demyelinating function are also discussed. A large body of evidence shows that melatonin potently regulates the immune system, demyelination, free radical generation, and inflammatory responses in neural tissue, which are mediated by multiple signal transduction cascades. In the present article, we focus on different pathways that are targeted by melatonin to prevent the development and progression of MS.

Ocular non-P450 oxidative, reductive, hydrolytic, and conjugative drug metabolizing enzymes

Metabolism in the eye for any species, laboratory animals or human, is gaining rapid interest as pharmaceutical scientists aim to treat a wide range of so-called incurable ocular diseases. Over a period of decades, reports of metabolic activity toward various drugs and biochemical markers have emerged in select ocular tissues of animals and humans. Ocular cytochrome P450 (P450) enzymes and transporters have been recently reviewed. However, there is a dearth of collated information on non-P450 drug metabolizing enzymes in eyes of various preclinical species and humans in health and disease. In an effort to complement ocular P450s and transporters, which have been well reviewed in the literature, this review is aimed at presenting collective information on non-P450 oxidative, hydrolytic, and conjugative ocular drug metabolizing enzymes. Herein, we also present a list of xenobiotics or drugs that have been reported to be metabolized in the eye.

Neuroprotective action of N-acetyl serotonin in oxidative stress-induced apoptosis through the activation of both TrkB/CREB/BDNF pathway and Akt/Nrf2/Antioxidant enzyme in neuronal cells

N-acetyl serotonin (NAS) as a melatonin precursor has neuroprotective actions. Nonetheless, it is not clarified how NAS protects neuronal cells against oxidative stress. Recently, we have reported that N-palmitoyl serotonins possessed properties of antioxidants and neuroprotection. Based on those, we hypothesized that NAS, a N-acyl serotonin, may have similar actions in oxidative stress-induced neuronal cells, and examined the effects of NAS based on in vitro and in vivo tests. NAS dose-dependently inhibited oxidative stress-induced cell death in HT-22 cells. Moreover, NAS suppressed glutamate-induced apoptosis by suppressing expression of AIF, Bax, calpain, cytochrome c and cleaved caspase-3, whereas it enhanced expression of Bcl-2. Additionally, NAS improved phosphorylation of tropomyosin-related kinase receptor B (TrkB) and cAMP response element-binding protein (CREB) as well as expression of brain-derived neurotrophic factor (BDNF), whereas the inclusion of each inhibitor of JNK, p38 or Akt neutralized the neuroprotective effect of NAS, but not that of ERK. Meanwhile, NAS dose-dependently reduced the level of reactive oxygen species, and enhanced the level of glutathione in glutamate-treated HT-22 cells. Moreover, NAS significantly increased expression of heme oxygenase-1, NAD(P)H quinine oxidoreductase-1 and glutamate-cysteine ligase catalytic subunit as well as nuclear translocation of NF-E2-related factor-2. Separately, NAS at 30 mg/kg suppressed scopolamine-induced memory impairment and cell death in CA1 and CA3 regions in mice. In conclusion, NAS shows actions of antioxidant and anti-apoptosis by activating TrkB/CREB/BDNF pathway and expression of antioxidant enzymes in oxidative stress-induced neurotoxicity. Therefore, such effects of NAS may provide the information for the application of NAS against neurodegenerative diseases.

•

NAS protects apoptosis induced by oxidative stress in neuronal cells.

•

NAS exerts an antioxidant property in neuronal cells.

•

NAS improves activation of BDNF/TrkB/CREB pathway in neuronal cells.

•

NAS enhances activation of Akt/Nrf2/Antioxidant enzyme pathway in neuronal cells.

•

NAS recovers memory and neuronal cells in scopolamine-treated mice.

Pharmacometabolomics of response to sertraline and to placebo in major depressive disorder - possible role for methoxyindole pathway

Therapeutic response to selective serotonin (5-HT) reuptake inhibitors in Major Depressive Disorder (MDD) varies considerably among patients, and the onset of antidepressant therapeutic action is delayed until after 2 to 4 weeks of treatment. The objective of this study was to analyze changes within methoxyindole and kynurenine (KYN) branches of tryptophan pathway to determine whether differential regulation within these branches may contribute to mechanism of variation in response to treatment. Metabolomics approach was used to characterize early biochemical changes in tryptophan pathway and correlated biochemical changes with treatment outcome. Outpatients with MDD were randomly assigned to sertraline (n = 35) or placebo (n = 40) in a double-blind 4-week trial; response to treatment was measured using the 17-item Hamilton Rating Scale for Depression (HAMD₁₇). Targeted electrochemistry based metabolomic platform (LCECA) was used to profile serum samples from MDD patients. The response rate was slightly higher for sertraline than for placebo (21/35 [60%] vs. 20/40 [50%], respectively, χ²(1)  = 0.75, p = 0.39). Patients showing a good response to sertraline had higher pretreatment levels of 5-methoxytryptamine (5-MTPM), greater reduction in 5-MTPM levels after treatment, an increase in 5-Methoxytryptophol (5-MTPOL) and Melatonin (MEL) levels, and decreases in the (KYN)/MEL and 3-Hydroxykynurenine (3-OHKY)/MEL ratios post-treatment compared to pretreatment. These changes were not seen in the patients showing poor response to sertraline. In the placebo group, more favorable treatment outcome was associated with increases in 5-MTPOL and MEL levels and significant decreases in the KYN/MEL and 3-OHKY/MEL; changes in 5-MTPM levels were not associated with the 4-week response. These results suggest that recovery from a depressed state due to treatment with drug or with placebo could be associated with preferential utilization of serotonin for production of melatonin and 5-MTPOL.

Physiological and pharmacological properties of 5-methoxytryptophol

5-methoxytryptophol (5-ML) is a pineal indoleamine derived from serotonin shown to be biologically active in a number of species. This indolamine exhibits a circadian pattern synchronized with the day-night cycle with significant increases during daylight, already recognized in vertebrates. The multiplicity of physiological and endocrine functions of 5-ML is remarkable; it is involved in circadian rhythms, reproduction and sexual processes. Furthermore, a number of pharmacological benefits of 5-ML have been reported, including immunomodulatory, antitumor and antioxidative activities. However, the molecular mechanisms of these pharmacological effects remain unclear. The purpose of this article is to provide an overview on the general properties and physiological functions of 5-ML. An attempt has been made to fully document all studies performed using 5-ML. In addition, this article aims to gain insight into the current state of knowledge regarding pharmacological and therapeutic effects of this indoleamine.

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

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

Melatonin synthesis in the rat harderian gland: age- and time-related effects

The Harderian gland is considered as an extrapineal source of melatonin. In the pineal gland, melatonin is known to present a circadian rhythm with high concentration during nighttime in all species studied. We determined in Wistar male rats the effects of age and time of day on melatonin synthesis in the Harderian gland. We compared Harderian gland melatonin content and the hormone synthesizing enzymes, serotonin N-acetyltransferase and hydroxyindole-O-methyltransferase, in young (4 months) and old (22 months) animals at six circadian stages and found that melatonin synthesis in the Harderian gland was unaffected by age. We also studied the Wistar rat Harderian gland at ten different circadian stages and found that the Harderian gland did not exhibit a daily rhythm in its melatonin content. This study shows that, by contrast to the pineal gland, melatonin in Wistar rat Harderian gland does not exhibit daily variations and that aging does not affect the melatonin content of the 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.

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.

Sensitivity of human peripheral blood mononuclear leukocytes to visible light

Overnight light exposure of cultured human peripheral blood mononuclear leukocytes [PBML], significantly increased basal [3H]thymidine incorporation and upon stimulation with phytohemagglutinin [PHA]. Melatonin (10(-9) to 10(-5) M) enhanced the light-induced increase of [3H]thymidine incorporation, while serotonin (10(-9) to 10(-7) M) stimulated [3H]thymidine incorporation in the dark. The wavelengths responsible of this effect were restricted to the blue-green zone of the spectrum. The stimulatory effect of visible light on PHA-induced DNA replication had a circannual rhythm, being maximal during winter. In winter, white light also reduced melatonin and serotonin binding to PBML membranes and switched the PBML indole metabolism towards serotonin and 5-hydroxy-indole-acetic acid [HIAA] synthesis, with a concomitant decrease 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.

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.

Postnatal development of the harderian gland in the Syrian golden hamster (Mesocricetus auratus): a light and electron microscopic study

The main objective of the present investigation was to study the morphological and chronological aspects of the development of the Harderian gland in the Syrian golden hamster. Tissues were obtained from male and female hamsters at days 1,3,5,7,10,12,15,17,20,27,37,46, and 90 after birth and processed for light and transmission electron microscopy. The present observations indicate that a well-defined temporal sequence in microscopic and ultrastructural modification is recognizable in the development of the hamster Harderian gland. Four stages of development were proposed. Between days 1-5 (first stage), the gland shows characteristics of an immature structure. The glandular cells contain many free ribosomes, few and small organelles, and large irregular-shape nuclei. Between days 7-17 (second stage), there is a marked increase of organelles involved in synthesis and secretion. The gland begins the secretion of lipids and porphyrins, but no morphological differences between male and female glands are observed. Between days 20-36 (third stage), the morphological differences between the two sexes appear and progressively develop. In 45-day-old hamsters, the Harderian gland possesses the structural characteristics of adult glands, and further developmental changes are essentially quantitative in nature (fourth stage). At all stages of development, the population of secretory cells has a uniform appearance. The morphological results are discussed as well as the possible relationship of this temporal sequence with hormonal changes.

Melatonin induced increase in gamma-interferon production by murine splenocytes

Previously, we demonstrated that production of gamma-interferon (gamma-IFN) by the mouse splenocytes isolated at night was higher than from those isolated in the morning. In this paper we show that melatonin increased gamma-IFN production by murine splenocytes. Moreover, this stimulating effect was significantly higher (10 times) in the cells isolated at night than in those isolated in the morning (2 times).

Serotonin content and melatonin production in the pineal gland of the male Djungarian hamster (Phodopus sungorus)

Diurnal variations in serum melatonin levels and pineal concentrations of serotonin (5-HT), N-acetylserotonin (NAS), 5-hydroxyindole acetic acid (5-HIAA), and melatonin were estimated in adult male Djungarian hamsters kept under long-day (LD 16:8) or short-day (LD 8:16) photoperiods. The nocturnal increase in melatonin production was accompanied by a marked drop in pineal serotonin concentrations. Serotonin levels, however, decreased approximately 4 hr before pineal melatonin increased. Correlations of the mean values for pineal serotonin and pineal melatonin indicated significant correlations at both LD 16:8 (r = -0.92, P less than 0.001) and LD 8:16 (r = -0.85, P less than 0.001). The mean levels of pineal serotonin and serum melatonin were correlated as well (LD 16:8, r = -0.91, P less than 0.001; LD 8:16, r = -0.81, P less than 0.01). The levels of pineal serotonin declined at approximately the same time as serum melatonin levels increased. These data suggest that the drop in pineal serotonin is primarily a consequence of melatonin production (as reflected by increasing serum concentrations). Consequently, pineal concentrations of melatonin may not be the best estimate of actual melatonin production, but a measure of how much melatonin is accumulated within the pineal due to high synthesis rates while the release of the hormone from the gland is limited.

Day and nighttime concentrations of 5-methoxytryptophol and melatonin in the retina and pineal gland from different classes of vertebrates

5-Methoxytryptophol (ML) and melatonin concentrations were determined by radioimmunoassay in the retinae and pineal glands of frogs (Rana esculenta), chickens (Gallus domesticus), quails (Coturnix coturnix), and European hamsters (Cricetus cricetus), killed in the afternoon (1500-1700 hr) and around midnight (2400-0200 hr). ML was only detectable in significant amounts in the frog retina. In the pineal gland ML levels showed the opposite profile to retinal ML, with measurable levels in the chicken, quail, and hamster and undetectable levels in the frog (less than 2.0 pg/gland). A significant day/night variation was observed in the bird pineal only, with higher levels occurring during the day. In contrast to ML, the interspecies variation in melatonin content was similar in the retina and pineal, with the highest concentrations occurring in the chicken greater than quail much greater than frog greater than hamster. Significantly higher night levels of melatonin were observed in the retina of the chicken and quail and in the pineal of the chicken, quail, and frog. The ratio of ML:melatonin concentrations in the retina and pineal also showed interspecies variations. In the birds the amount of ML was minimal in comparison with the amount of melatonin (1-4% retina; 0.3-12% pineal). The highest ML:melatonin occurred in the retina of the frog and in the pineal of the European hamster.

Rhythmic regulation of retinal melatonin: metabolic pathways, neurochemical mechanisms, and the ocular circadian clock

1. Current knowledge of the mechanisms of circadian and photic regulation of retinal melatonin in vertebrates is reviewed, with a focus on recent progress and unanswered questions. 2. Retinal melatonin synthesis is elevated at night, as a result of acute suppression by light and rhythmic regulation by a circadian oscillator, or clock, which has been localized to the eye in some species. 3. The development of suitable in vitro retinal preparations, particularly the eyecup from the African clawed frog, Xenopus laevis, has enabled identification of neural, cellular, and molecular mechanisms of retinal melatonin regulation. 4. Recent findings indicate that retinal melatonin levels can be regulated at multiple points in indoleamine metabolic pathways, including synthesis and availability of the precursor serotonin, activity of the enzyme serotonin N-acetyltransferase, and a novel pathway for degradation of melatonin within the retina. 5. Retinal dopamine appears to act through D2 receptors as a signal for light in this system, both in the acute suppression of melatonin synthesis and in the entrainment of the ocular circadian oscillator. 6. A recently developed in vitro system that enables high-resolution measurement of retinal circadian rhythmicity for mechanistic analysis of the circadian oscillator is described, along with preliminary results that suggest its potential for elucidating general circadian mechanisms. 7. A model describing hypothesized interactions among circadian, neurochemical, and cellular mechanisms in regulation of retinal melatonin is presented.

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.

Immunoreactive somatostatin diurnal rhythms in rat pineal, retina and harderian gland: effects of sex, season, continuous darkness and estrous cycle

Diurnal profiles of the content of immunoreactive somatostatin (IRS) in the male and female rat pineal, Harderian gland and retina have been studied. Supplementary experiments have been performed to elucidate a possible effect of infradian cycles, namely estrous and seasonal cycles, and continuous dark on IRS concentration. Results demonstrate that the IRS content in the rat pineal gland, Harderian gland and retina is submitted to diurnal variations, but not under all studied conditions. A sexual dimorphism exists between male and female animals: male rats showed higher IRS content in pineal (103.8 +/- 4.7 vs 32.3 +/- 1.8 pg IRS/gland), but lower in retina (1,362.1 +/- 82.7 vs 2,176 +/- 102.2 +/- pg IRS/mg of protein) and Harderian gland (10.3 +/- 0.8 vs 30.6 +/- 3.5 pg IRS/mg of protein). Additionally, seasonal differences appeared: in male and female animals pineal IRS content was lower in spring than in November. This decrease also appeared in female retina IRS concentration. Estrous cycle did not seem to change IRS content in the three studied tissues. Finally, pineal IRS rhythm persisted after continuous dark for a week. These results demonstrate, in the rat, sexual differences in the IRS content of the various tissues studied and suggest a physiological role for somatostatin, possibly related to seasonal adaptation.

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.

5-Methoxytryptophol injections in the Syrian hamster: plasma and pineal concentrations

Plasma and pineal 5-methoxytryptophol (ML) levels were determined by radioimmunoassay (RIA) following subcutaneous (s.c.) injection of ML (1, 25 and 100 micrograms) to hamsters, Mesocricetus auratus. A dose-dependent increase in plasma ML occurred, with the duration of the supra-physiological levels being related to the dose. However, only the 100 micrograms dose significantly increased pineal ML levels above control values. Plasma profiles after 1 or 25 micrograms ML fitted a one-compartment model with half-lives of 18.6 and 25.2 min, respectively. Following 100 micrograms ML a biphasic decay curve was evident, the half-lives of the 2 phases being 6.6 min and 2.95 h.

Melatonin and porphyrin in the harderian glands of the Syrian hamster: circadian patterns and response to autumnal conditions

1. Adult male Syrian hamsters were killed at nine intervals during a 24 hr period in the autumn, after 2 months either indoors in controlled conditions or in natural outdoor conditions. 2. Harderian glands were taken for determination of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) activities and melatonin and porphyrin concentrations. 3. Mean 24 hr Harderian NAT and melatonin values were lower outside than inside. 4. Twenty-four hour melatonin rhythms were detected with similar daytime (afternoon) acrophases in both environmental conditions. 5. An NAT rhythm was seen only in animals kept inside, with a circadian maximum in the late dark phase. 6. Mean 24 hr HIOMT activity was slightly higher outdoors than indoors, and 24 hr rhythms were not detected in either condition. 7. Mean porphyrin concentrations were higher outdoors, with 24 hr rhythms detected in both conditions and a significantly earlier nocturnal circadian maximum outdoors.

Mass spectrometric identification and quantification of 5-methoxytryptophol in quail retina

The occurrence of 5-methoxytryptophol (5-MTL) in the quail retina was investigated by capillary column gas chromatography/mass spectrometry/selected ion monitoring using a deuterated internal standard. Based on ion intensity ratios in the mass spectra of pentafluoropropionyl and heptafluorobutyryl derivatives of 5-MTL and deuterated 5-MTL, 5-MTL was unequivocally identified in the quail retina. Similar to the circadian rhythm of retinal melatonin, retinal 5-MTL also exhibited a diurnal variation with high levels at mid-dark. However, no significant correlation between the diurnal levels of 5-MTL and melatonin was observed in the quail retina at mid-light or mid-dark.

Cyclic AMP-dependent melatonin production in Y79 human retinoblastoma cells

Melatonin is rhythmically synthesized in some vertebrate retinas and has been implicated in the regulation of key rhythmic events in the photoreceptor-pigment epithelial complex. In human retina, melatonin is present; however, no information exists on the cellular regulation of this hormone. We report here that the established human retinoblastoma cell line Y79 synthesizes and releases melatonin. Treatments that elevate cyclic AMP (cAMP) levels (forskolin, 8-Br-cAMP, and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine) all stimulate melatonin release from static cultures of Y79 cells. Other 8-bromo nucleotide analogues (cyclic GMP, ATP, and AMP) are not effective. These results suggest that Y79 human retinoblastoma cells require a cAMP-dependent mechanism for melatonin biosynthesis similar to that described previously in other vertebrates. This is the first demonstration of melatonin release from a cultured human cell line. These results support the idea that human retinal cells share homologies with pineal cells, as suggested by the condition trilateral retinoblastoma.

The influence of light of different wavelengths on the methylating capacity of the pineal gland of male golden hamsters in relation to reproduction

In the present experiments the influence of light of different wavelengths on pineal indole metabolism in relation to reproduction was studied. Therefore, during autumn and winter male golden hamsters were kept under natural conditions but for the sunlight which was filtered exposing the hamsters to either normal (control), red or blue light. During the gradually shortening photoperiod at the start of the experiments under normal light conditions, a marked decrease of FSH and LH plasma content as well as testicular weight was found, indicating the onset of gonadal atrophy. During this period a high synthesis of 5-methoxytryptophan (MW) and 5-methoxytryptamine (MT) was determined. The synthesis of other 5-methoxyindoles (MI) was low, while O-acetyl-5-methoxytryptophol (aML) synthesis even markedly decreased. Red and blue light did not cause significant changes in MI synthesis. As long as MT synthesis is high (under blue light), there is no increase in FSH content and testes weight is still decreasing. This influence of blue light confirms the putative antigonadotropic properties of MT. The increase of FSH content at week 9 was the first indication that recrudescence had started. At week 19, this recrudescence was also manifested in the increasing testes weight. The synthesis of melatonin (aMT), 5-methoxytryptophol (ML), 5-methoxyindole-3-acetic acid (MA) and aML increased whereas the production of MT decreased. Blue light exposure caused a significantly higher increase of synthesis of ML, MA, aML and, not-significantly, of aMT, whereas red light caused a significantly lower synthesis of MA. It was concluded that MT, a putative antigonadotropic, and aML, a putative counter-antigonadotropic, are probably important pineal compounds that transduce the photoperiodic messages, which cause either gonadal atrophy or recrudescence. The effect of blue light on indole metabolism and the reproductive cycle was more clear than that of red light. From the present results of blue light on indole metabolism, it was suggested that blue light delayed gonadal atrophy and stimulated gonadal growth, compared to red light. An opposite effect of red light was less obvious.

Histochemical detection of monoamine oxidase and alcohol dehydrogenase activities in the Syrian hamster Harderian glands: existence of a sexual dimorphism

Monoamine oxidase (MAO) and alcohol dehydrogenase (AD) activities were studied histochemically in the Syrian hamster Harderian gland using tryptamine as substrate and Nitroblue Tetrazolium as the final electron acceptor. No dark: light-related changes were observed. Male type I secretory cells showed an intense MAO reaction. Female type I cells exhibited a moderate MAO activity. Both male and female glands showed a moderate/intense AD-positive reaction. Male type II cells were lacking MAO and AD activities. MAO activity found in the hamster Harderian glands corresponded mainly to MAO type A since treatment with chlorgyline (0.01, 0.1 and 0.5 mM) totally inhibited it. The possible role of these two enzymes in Harderian gland indolalkylamine metabolism is discussed.

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.

Serotonin and melatonin synthesis in peripheral blood mononuclear cells: stimulation by interferon-gamma as part of an immunomodulatory pathway

Serotonin and melatonin inhibit phytohemagglutinin- (PHA) induced interferon-gamma (IFN-gamma) production by lymphocytes. In this paper, it is shown that IFN-gamma-increased tryptophan uptake by lymphocytes and macrophages led to an enhanced production of serotonin. When IFN-gamma and serotonin were added together to a lymphocyte culture, N-acetyl serotonin and melatonin production was increased, whereas the path to 5-hydroxy-indoleacetic acid remained unchanged. Therefore, the stimulated IFN-gamma production of serotonin and melatonin by lymphocytes and macrophages and the inhibition of IFN-gamma synthesis by these indoleamines suggest a hypothesis for an immunoregulatory circuit.

Hypothalamic and neurohypophysial vasopressin and oxytocin in melatonin-treated pinealectomized male rats

The effect of melatonin on hypothalamic and neurohypophysial vasopressin and oxytocin was investigated in normal and pinealectomized rats. Pinealectomy was followed by a decrease of both vasopressin and oxytocin content in the hypothalamus and neurohypophysis. In unpinealectomized rats, melatonin decreased vasopressin and oxytocin storage in the hypothalamo-neurohypophysial system. Following pineal removal, melatonin did not augment the pinealectomy-induced decrease of vasopressin and oxytocin in the neurohypophysis; the hypothalamic storage of both neurohormones was even higher when compared with vehicle-treated animals.

N-acetyltransferase activity, hydroxyindole-O-methyltransferase activity, and melatonin levels in the Harderian glands of the female Syrian hamster: changes during the light:dark cycle and the effect of 6-parachlorophenylalanine administration

The activities of NAT and HIOMT and the melatonin content of the Harderian glands of female Syrian hamsters were studied. When hamsters were kept under a light:dark cycle of 14:10 (lights on at 06.00 h), NAT activity exhibited a sharp, short term rise at one hour after lights on. Simultaneously, the activity of HIOMT, which forms melatonin, exhibited a rapid decline. Melatonin levels, like HIOMT activity, also showed a precipitous drop at one hour after light onset. After the respective changes, both NAT and HIOMT activity reverted back to night time levels. Melatonin levels remained depressed for several hours but by 1400 h (8 hours after lights on), nighttime melatonin values were re-established. Treatment of female hamsters with PCPA, a trytophan hydroxylase inhibitor, led to depressed levels of Harderian melatonin without affecting the activities of either NAT or HIOMT.

Localization of hydroxyindole-O-methyltransferase-like immunoreactivity in photoreceptors and cone bipolar cells in the human retina: a light and electron microscope study

The localization of the melatonin-synthesizing enzyme hydroxyindole-O-methyltransferase (HIOMT) was examined by light and electron microscopic immunocytochemistry in the human retina. HIOMT-like immunoreactivity was observed in the photoreceptor layers and the inner nuclear layer (INL). The immunoreactive cells in the INL were more numerous in the central retina than in the peripheral retina and sent processes to both the outer plexiform and inner plexiform layers. The HIOMT immunoreactivity in the inner plexiform layer (IPL) appeared as punctate terminals in the proximal and distal one-thirds of that layer. At the ultrastructural level, HIOMT-like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors and to a population of cone bipolar cells. HIOMT-immunoreactive bipolar cell dendrites were observed to make both invaginating and flat synaptic contacts with cone pedicles. No immunoreactive invaginating contacts in rod spherules were observed. HIOMT immunoreactivity was observed in the bipolar cell cytoplasm in the INL, and in the bipolar synaptic terminals in the IPL. These terminals contained synaptic ribbons, which formed synaptic contacts with unlabeled cells in the IPL. HIOMT radioenzymatic assays confirmed the presence of HIOMT in the human retina. Average HIOMT activity of eight donors was determined to be 15.0 pmol/mg protein/hour +/- 7.2 S.D. The ultrastructural localization of HIOMT observed in this study, combined with reports from other laboratories, suggests that the cytoplasm of the photoreceptors and a population of cone bipolar cells may be the sites of melatonin synthesis in the human retina.

Decreased nocturnal plasma melatonin peak in patients with a functional alteration of the retina in relation with uveitis

A functional alteration of the retina is present in patients suffering from uveitis. Because of the relation between the pineal gland and the retina we documented possible modifications of melatonin secretion in patients with uveitis. Plasma melatonin was assayed in 19 patients and 16 age-matched controls. Blood samples were drawn at the known high and low points of the circadian rhythm of the hormone, i.e. 02.00 and 11.00 h, respectively. We found that the nocturnal peak of plasma melatonin was greatly decreased (45%) in patients with uveitis. These data cannot be related to the impairment of retinal melatonin synthesis alone. The possibility exists that the decline of the nocturnal peak of melatonin we have reported is due to a pineal inflammation in patients with uveitis, as observed in the experimental autoimmune uveitis induced in rats by retinal S-antigen.

Pineal indoles: significance and measurement

Despite intensive investigation, particularly over the past fifteen years, many aspects of pineal function with respect to mammalian physiology remain obscure. Much of this work is reviewed and particular attention focussed on indole metabolism within the pineal gland. Emphasis is placed on the development of new analytical techniques with special reference to high performance liquid chromatography coupled with electrochemical detection. The growth in knowledge regarding pineal indole synthesis which can be attributed to the use of this technique is discussed. The possibility that pineal indoles other than melatonin may function as hormones or neuromodulators is considered. A functional role for 5-hydroxytryptophol as a neuromodulator, possibly associated with diffuse neuroendocrine function (amine precursor, uptake and decarboxylation, APUD) is suggested.

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.

Induction and growth of mammary tumors after superior cervical ganglionectomy in sighted and blinded-anosmic rats

Female rats were subjected to superior cervical ganglionectomy (Gx), blinding and anosmia (BAs) or combined procedures (BAsGx). Onset and growth of dimethylbenz(a)anthracene (DBMA)-induced mammary tumors was studied in these animals and compared to tumorigenesis in intact control rats. Carcinostatic effects were present in all surgically altered animals, as evidenced by a trend toward reduced tumor incidence, reduced final tumor mass, and a significant reduction in mean number of tumors in Gx and BAsGx rats, and increased regression of tumors in BAs rats compared to intact group. Reduced tumorigenesis was paralleled by a trend toward either an increase (BAs) or a decrease (Gx and BAsGx) in the activity of pineal hydroxyindole-O-methyltransferase (HIOMT) compared to intact group. In addition, BAs and BAsGx animals showed a significant reduction in body weight. These results suggest that Gx reduces mammary tumorigenesis in both sighted and BAs rats. They further confirm the findings of others on reduced mammary tumorigenesis in BAs rats. Possible involvement of multiple carcinostatic mechanisms in different animal models is discussed.

The effect of melatonin administration and short exposures to cold on body temperature of the blind subterranean mole rat (Rodentia, Spalax ehrenberghi, Nehring)

Improvement of the cold resistance capacity of the mole rat (Spalax ehrenbergi, Nehring), a blind subterranean rodent, was achieved following subcutaneous melatonin administration. Melatonin-treated mole rats exhibited reduction both in initial body temperature (Tbo) and in the decrease of body temperature (delta Tb) after 6 hours of exposure to cold. The tested mole rats were acclimated to and kept during the experiment, under long (16L/8D) photoperiod schedule while the effects of melatonin administration on body temperature simulated the effects of short photoperiod conditions. These results suggest that in mole rats, melatonin biochemically mediates photoperiodic information into body temperature physiological response. As a byproduct of the original experiment, it was found that frequently repeated short (6 hours) exposure to cold (Ta = 5 +/- 1 degrees C) can in itself initiate an improvement of cold resistance capacity similar to the effect of melatonin administration. This supports the idea that mole rats may improve their cold resistance in response to either photoperiodic or climatic changes.

The effect of different photoperiods on the methylating capacity of the pineal gland of adult, male golden hamsters, with special reference to 5-methoxyindoles

Testes weight, plasma FSH and LH concentration and pineal methylating capacity were compared in hamsters housed under either long (LD14:10) or short (LD8:16) photoperiods. Hamsters housed for 14 weeks under short photoperiod showed gonadal atrophy, which was complete after 6 weeks. Also plasma FSH and LH concentration showed a marked decline after transfer to short photoperiod. However, after 14 weeks the concentration of FSH and LH as well as testes weight increased again. Under both photoperiods day/night rhythms in plasma FSH and LH concentration were measured. Under both light regimes the concentrations did not show significant differences. Under long as well as short photoperiods in the pineal gland of animals no significant differences were found in the daily synthesis of various MI tested. Only the synthesis of ML was significantly higher in the pineal of hamsters housed under short photoperiod. The function of this higher synthesis of ML remains unknown. Although the maxima of the rhythm for the various MI found under different LD regimes did not differ in magnitude or duration, their location in respect to the onset of darkness was different. It is suggested that this specific location is of more physiological importance than the quantity or duration of synthesis, concentration or release of MI. At the moment the day/night rhythms were determined there were indications that recrudescence of the testes had already started. It is suggested that this recrudescence is responsible for the fact that no differences in the synthesis of MI were found comparing the influence of both photoperiods. After 14 weeks of exposure to short photoperiod, aML synthesis was, in contrast to the synthesis of the other MI, (not significantly) higher under LD8:16. Moreover, opposite results for aMT and aML synthesis during darkness were found. It is suggested that the ratio of synthesis of these compounds is of physiological significance.

Effects of melatonin implants on structures and behaviors of the house finch (Carpodacus mexicanus) eye

This study aimed to determine the extraretinal effects of melatonin upon the eyes of an avian species, the House Finch (Carpodacus mexicanus). Twelve birds (full-grown, second-year males) each received a Silastic tubing intraperitoneal implant, six containing melatonin (average release = 24 micrograms/d/bird; = M birds) and six being empty (= C birds). Microscopic study of pupillary and palpebral behaviors during the final week demonstrated lesser pupillary diameters and interpalpebral distances in M birds under all test conditions. These effects could have diminished mean light levels reaching parts of the retina. Characteristics of the relative miosis and ptosis of M birds resemble signs in some CNS disorders, such as altered inhibition of the Edinger-Westphal nucleus, and especially lesions in, or lowered activity of, higher sympathetic centers (a subtype of Horner's syndrome). Weights of eyes and their parts were the same in M and C birds, contrasting with previously reported results from male Golden Hamsters, possibly due to species differences and/or preexperimental attainment of full growth in the finches. Effects of melatonin on pupillary and palpebral behaviors, demonstrated here for the first time, foster caveats for simplistic experimental designs and interpretations with melatonin when sensory-neural-behavioral interactions are affected. Quantitative changes in pupillary and palpebral behaviors may, nevertheless, provide a window for monitoring central actions of melatonin in living test subjects in chronic studies.