Neither the pituitary gland nor the sympathetic nervous system is responsible for eliciting the large drop in elevated rat pineal melatonin levels due to swimming

Melatonin Mitigates Mitochondrial Meltdown: Interactions with SIRT3

Melatonin exhibits extraordinary diversity in terms of its functions and distribution. When discovered, it was thought to be uniquely of pineal gland origin. Subsequently, melatonin synthesis was identified in a variety of organs and recently it was shown to be produced in the mitochondria. Since mitochondria exist in every cell, with a few exceptions, it means that every vertebrate, invertebrate, and plant cell produces melatonin. The mitochondrial synthesis of melatonin is not photoperiod-dependent, but it may be inducible under conditions of stress. Mitochondria-produced melatonin is not released into the systemic circulation, but rather is used primarily in its cell of origin. Melatonin's functions in the mitochondria are highly diverse, not unlike those of sirtuin 3 (SIRT3). SIRT3 is an NAD+-dependent deacetylase which regulates, among many functions, the redox state of the mitochondria. Recent data proves that melatonin and SIRT3 post-translationally collaborate in regulating free radical generation and removal from mitochondria. Since melatonin and SIRT3 have cohabitated in the mitochondria for many eons, we predict that these molecules interact in many other ways to control mitochondrial physiology. It is predicted that these mutual functions will be intensely investigated in the next decade and importantly, we assume that the findings will have significant applications for preventing/delaying some age-related diseases and aging itself.

Mitochondria: Central Organelles for Melatonin's Antioxidant and Anti-Aging Actions

Melatonin, along with its metabolites, have long been known to significantly reduce the oxidative stress burden of aging cells or cells exposed to toxins. Oxidative damage is a result of free radicals produced in cells, especially in mitochondria. When measured, melatonin, a potent antioxidant, was found to be in higher concentrations in mitochondria than in other organelles or subcellular locations. Recent evidence indicates that mitochondrial membranes possess transporters that aid in the rapid uptake of melatonin by these organelles against a gradient. Moreover, we predicted several years ago that, because of their origin from melatonin-producing bacteria, mitochondria likely also synthesize melatonin. Data accumulated within the last year supports this prediction. A high content of melatonin in mitochondria would be fortuitous, since these organelles produce an abundance of free radicals. Thus, melatonin is optimally positioned to scavenge the radicals and reduce the degree of oxidative damage. In light of the "free radical theory of aging", including all of its iterations, high melatonin levels in mitochondria would be expected to protect against age-related organismal decline. Also, there are many age-associated diseases that have, as a contributing factor, free radical damage. These multiple diseases may likely be deferred in their onset or progression if mitochondrial levels of melatonin can be maintained into advanced age.

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.

Reducing oxidative/nitrosative stress: a newly-discovered genre for melatonin

The discovery of melatonin and its derivatives as antioxidants has stimulated a very large number of studies which have, virtually uniformly, documented the ability of these molecules to detoxify harmful reactants and reduce molecular damage. These observations have clear clinical implications given that numerous age-related diseases in humans have an important free radical component. Moreover, a major theory to explain the processes of aging invokes radicals and their derivatives as causative agents. These conditions, coupled with the loss of melatonin as organisms age, suggest that some diseases and some aspects of aging may be aggravated by the diminished melatonin levels in advanced age. Another corollary of this is that the administration of melatonin, which has an uncommonly low toxicity profile, could theoretically defer the progression of some diseases and possibly forestall signs of aging. Certainly, research in the next decade will help to define the role of melatonin in age-related diseases and in determining successful aging. While increasing life span will not necessarily be a goal of these investigative efforts, improving health and the quality of life in the aged should be an aim of this research.

One molecule, many derivatives: a never-ending interaction of melatonin with reactive oxygen and nitrogen species?

Melatonin is a highly conserved molecule. Its presence can be traced back to ancient photosynthetic prokaryotes. A primitive and primary function of melatonin is that it acts as a receptor-independent free radical scavenger and a broad-spectrum antioxidant. The receptor-dependent functions of melatonin were subsequently acquired during evolution. In the current review, we focus on melatonin metabolism which includes the synthetic rate-limiting enzymes, synthetic sites, potential regulatory mechanisms, bioavailability in humans, mechanisms of breakdown and functions of its metabolites. Recent evidence indicates that the original melatonin metabolite may be N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) rather than its commonly measured urinary excretory product 6-hydroxymelatonin sulfate. Numerous pathways for AFMK formation have been identified both in vitro and in vivo. These include enzymatic and pseudo-enzymatic pathways, interactions with reactive oxygen species (ROS)/reactive nitrogen species (RNS) and with ultraviolet irradiation. AFMK is present in mammals including humans, and is the only detectable melatonin metabolite in unicellular organisms and metazoans. 6-hydroxymelatonin sulfate has not been observed in these low evolutionary-ranked organisms. This implies that AFMK evolved earlier in evolution than 6-hydroxymelatonin sulfate as a melatonin metabolite. Via the AFMK pathway, a single melatonin molecule is reported to scavenge up to 10 ROS/RNS. That the free radical scavenging capacity of melatonin extends to its secondary, tertiary and quaternary metabolites is now documented. It appears that melatonin's interaction with ROS/RNS is a prolonged process that involves many of its derivatives. The process by which melatonin and its metabolites successively scavenge ROS/RNS is referred as the free radical scavenging cascade. This cascade reaction is a novel property of melatonin and explains how it differs from other conventional antioxidants. This cascade reaction makes melatonin highly effective, even at low concentrations, in protecting organisms from oxidative stress. In accordance with its protective function, substantial amounts of melatonin are found in tissues and organs which are frequently exposed to the hostile environmental insults such as the gut and skin or organs which have high oxygen consumption such as the brain. In addition, melatonin production may be upregulated by low intensity stressors such as dietary restriction in rats and exercise in humans. Intensive oxidative stress results in a rapid drop of circulating melatonin levels. This melatonin decline is not related to its reduced synthesis but to its rapid consumption, i.e. circulating melatonin is rapidly metabolized by interaction with ROS/RNS induced by stress. Rapid melatonin consumption during elevated stress may serve as a protective mechanism of organisms in which melatonin is used as a first-line defensive molecule against oxidative damage. The oxidative status of organisms modifies melatonin metabolism. It has been reported that the higher the oxidative state, the more AFMK is produced. The ratio of AFMK and another melatonin metabolite, cyclic 3-hydroxymelatonin, may serve as an indicator of the level of oxidative stress in organisms.

The antidepressant-like effect of the melatonin receptor ligand luzindole in mice during forced swimming requires expression of MT2 but not MT1 melatonin receptors

We previously reported an antidepressant-like effect in C3H/HeN mice during the forced swimming test (FST) following treatment with the MT1/MT2 melatonin receptor ligand, luzindole. This study investigated the role melatonin receptors (MT1 and/or MT2) may play in the effect of luzindole in the FST using C3H/HeN mice with a genetic deletion of either MT1 (MT1KO) or MT2 (MT2KO) melatonin receptors. In the light phase (ZT 9-11), luzindole (30 mg/kg, i.p.) significantly decreased immobility during swimming in both wild type (WT) (135.6 +/- 25.3 s, n = 7) and MT(1)KO (132.6 +/- 13.3 s, n = 8) as compared with vehicle-treated mice (WT: 207.1 +/- 6.0 s, n = 7; MT1KO: 209.5 +/- 6.2 s, n = 8) (P < 0.001). In the dark phase (ZT 20-22), luzindole also decreased time of immobility in both WT (89.5 +/- 13.9 s, n = 8) and MT1KO (66.5 +/- 6.4 s, n = 8) mice as compared with the vehicle treated (WT: 193.8 +/- 3.5, n = 6; MT1KO: 176.6 +/- 6.2 s, n = 8) (P < 0.001). Genetic disruption of the MT1 gene did not alter the diurnal rhythm of serum melatonin in MT1KO mice (ZT 9-11: 1.3 +/- 0.6 pg/mL, n = 7; ZT 20-22: 10.3 +/- 1.1 pg/mL, n = 8) as compared with WT (ZT 9-11: 1.4 +/- 0.7 pg/mL; ZT 20-22: 10.6 pg/mL). Swimming did not alter the serum melatonin diurnal rhythm in WT and MT1KO mice. Decreases in immobility of WT and MT1KO mice by luzindole treatment were not affected by gender or age (3 months versus 8 months). In contrast, luzindole did not decrease immobility during the FST in MT2KO mice. We conclude that the antidepressant-like effect of luzindole may be mediated through blockade of MT2 rather than MT1 melatonin receptors.

When melatonin gets on your nerves: its beneficial actions in experimental models of stroke

This article summarizes the evidence that endogenously produced and exogenously administered melatonin reduces the degree of tissue damage and limits the biobehavioral deficits associated with experimental models of ischemia/reperfusion injury in the brain (i.e., stroke). Melatonin's efficacy in curtailing neural damage under conditions of transitory interruption of the blood supply to the brain has been documented in models of both focal and global ischemia. In these studies many indices have been shown to be improved as a consequence of melatonin treatment. For example, when given at the time of ischemia or reperfusion onset, melatonin reduces neurophysiological deficits, infarct volume, the degree of neural edema, lipid peroxidation, protein carbonyls, DNA damage, neuron and glial loss, and death of the animals. Melatonin's protective actions against these adverse changes are believed to stem from its direct free radical scavenging and indirect antioxidant activities, possibly from its ability to limit free radical generation at the mitochondrial level and because of yet-undefined functions. Considering its high efficacy in overcoming much of the damage associated with ischemia/reperfusion injury, not only in the brain but in other organs as well, its use in clinical trials for the purpose of improving stroke outcome should be seriously considered.

Decreased nocturnal melatonin levels during acute myocardial infarction

Acute myocardial infarction is accompanied by an increase in cellular oxidative stress in the pericardial coverings of the heart. Melatonin is a highly potent and efficient radical scavenger. Little research has been carried out concerning the relationship between this antioxidant and acute myocardial infarction in humans. In this work, serum levels of melatonin and parameters of oxidative stress, such as glutathione peroxidase and lipid peroxidation levels were examined in light/dark periods in patients with acute myocardial infarction. Twenty-five patients diagnosed with acute myocardial infarction were studied and 25 patients with no evidence of coronary artery disease served as controls. Venous blood samples were obtained from the patients and control subjects to determine melatonin, glutathione peroxidase and lipid peroxidation; the samples were collected at 10:00 hr (light period) and 03:00 hr (dark period) in the first 24 hr after admission to the coronary care unit. Our results demonstrate the existence of differences between changes in melatonin levels in control subjects and acute myocardial infarction patients, revealing a reduced nocturnal elevation in the acute myocardial infarction group. Glutathione peroxidase levels were lower after acute myocardial infarction and did not show diurnal variations. In the control group, lipid peroxidation levels presented a light/dark pattern but in the acute myocardial infarction group diurnal variations of this parameter were lost. Our data show that acute myocardial infarction is associated with a nocturnal serum melatonin deficit as well as increased oxidative stress, suggesting that melatonin is, at least in part, depleted during the dark phase to reduce the free radicals formed in acute myocardial infarction.

Hormone levels of world class cyclists during the Tour of Spain stage race

OBJECTIVES

To evaluate the hormonal response to strenuous endurance exercise performed by elite athletes.

METHODS

Nine professional cyclists (mean (SD) age 28 (1) years; mean (SD) VO(2)MAX 75.3 (2.3) ml/kg/min) who participated in a three week tour race (Vuelta a España 1999) were selected as subjects. Morning urinary levels of 6-sulphatoxymelatonin (aMT6s) and morning serum levels of testosterone, follicle stimulating (FSH), luteinising hormone (LH), and cortisol were measured in each subject at t(0) (before the competition), t(1) (end of first week), t(2) (end of second week), and t(3) (end of third week). Urine samples of aMT6s were also evaluated in the evening at t(0), t(1), t(2), and t(3).

RESULTS

Mean urinary aMT6s levels had increased significantly (p<0.01) during the day after each stage (1091 (33) v 683 (68) ng/ml at t(1); 955 (19) v 473 (53) ng/ml at t(2); 647 (61) v 337 (47) ng/ml at t(3)). Both morning and evening aMT6s levels decreased significantly during the study. A similar pattern was observed for morning serum levels of cortisol and testosterone.

CONCLUSIONS

The results suggest that the basal activity of the pineal gland, adrenal glands, and testis may be decreased after consecutive days of intense, long term exercise.

Inconsistent suppression of nocturnal pineal melatonin synthesis and serum melatonin levels in rats exposed to pulsed DC magnetic fields

The purpose of these experiments was to determine whether the exposure of rats at night to pulsed DC magnetic fields (MF) would influence the nocturnal production and secretion of melatonin, as indicated by pineal N-acetyltransferase (NAT) activity (the rate limiting enzyme in melatonin production) and pineal and serum melatonin levels. By using a computer-driven exposure system, 15 experiments were conducted. MF exposure onset was always during the night, with the duration of exposure varying from 15 to 120 min. A variety of field strengths, ranging from 50 to 500 microT (0.5 to 5.0 G) were used with the bulk of the studies being conducted using a 100 microT (1.0 G) field. During the interval of DC MF exposure, the field was turned on and off at 1-s intervals with a rise/fall time constant of 5 ms. Because the studies were performed during the night, all procedures were carried out under weak red light (intensity of <5 microW/cm2). At the conclusion of each study, a blood sample and the pineal gland were collected for analysis of serum melatonin titers and pineal NAT and melatonin levels. The outcome of individual studies varied. Of the 23 cases in which pineal NAT activity, pineal melatonin, and serum melatonin levels were measured, the following results were obtained; in 5 cases (21.7%) pineal NAT activity was depressed, in 2 cases (8.7%) studies pineal melatonin levels were lowered, and in 10 cases (43.5%) serum melatonin concentrations were reduced. Never was there a measured rise in any of the end points that were considered in this study. The magnitudes of the reductions were not correlated with field strength (i.e., no dose-response relationships were apparent), and likewise the reductions could not be correlated with the season of the year (experiments conducted at 12-month intervals under identical exposure conditions yielded different results). Duration of exposure also seemed not to be a factor in the degree of melatonin suppression. The inconsistency of the results does not permit the conclusion that pineal melatonin production or release are routinely influenced by pulsed DC MF exposure. In the current series of studies, a suppression of serum melatonin sometimes occurred in the absence of any apparent change in the synthesis of this indoleamine within the pineal gland (no alteration in either pineal NAT activity or pineal melatonin levels). Because melatonin is a direct free radical scavenger, the drop in serum melatonin could theoretically be explained by an increased uptake of melatonin by tissues that were experiencing augmented levels of free radicals as a consequence of MF exposure. This hypothetical possibly requires additional experimental documentation.

Varying responses to the rat forced-swim test under diurnal and nocturnal conditions

The paradox that experiments in behavioural pharmacology employing nocturnal rodent species are carried out almost exclusively in the resting phase of the animals' circadian cycle has remained largely unexamined and unquestioned. This is despite the fact that all major physiological systems in the body are intrinsically aligned with its natural circadian rhythm. The forced-swim test (FST) is a rodent model that is used extensively as a screening test for antidepressant activity. The objectives of the present study were to examine the behaviour of rats in the FST under diurnal and nocturnal conditions and, in addition, to profile the response of neurochemical, neuroendocrine, and cellular indices of stress at time points up to 120 min following exposure to the FST. The time spent in escape-oriented activity was significantly less when animals were tested in the dark phase. The profile of serum corticosterone and adrenal ascorbic acid concentrations indicates that the animals were less stressed by the test situation during the active (i.e., dark) phase of their circadian cycle. Similarly, increases in blood enzymatic markers of stress-induced cellular damage were less marked following FST exposure in the nocturnal period. Characteristic stress-induced increases in 5-HT turnover in the frontal cortex and amygdala observed in the diurnal phase were reversed in the nocturnal period. In conclusion, circadian differences in behaviour in the FST may be related to parallel alterations in the ability of animals to adapt to exposure to stress.

Modification of the beta- and alpha2-adrenergic sensitivity of rat submandibular glands by environmental stimuli and stress

In man, the rate of resting salivary secretion can be influenced by environmental stimuli related to light dark cycles or by noxious stimuli (stressors) of psychological origin. The sympathetic branch of the autonomic nervous system and the adrenal medulla play an important part in homeostatic responses. Previous observations have shown that chronic exposure of rats to constant light promotes degranulation of parotid acini and desensitization of submandibular beta-adrenergic receptors. Now the submandibular secretory response elicited by beta- and alpha2-adrenergic agonists was studied in rats chronically exposed to environmental conditions that modified the activities of sympathetic efferents to the pineal, salivary and adrenal glands. Adult male rats were exposed to constant light (LL) or constant darkness (DD) for 20 days, or to stress (2 h daily immobilization) for 14 days. Control animals were kept under the usual lighting conditions and without immobilization. Dose response curves to isoproterenol (i.v), before and after administration (i.v.) of a dose (20 microg/kg) of clonidine were obtained. Beta-adrenergic desensitization was observed in all the experimental groups, while alpha2-adrenergic desensitization was only observed in the stress and LL groups. The results suggest that circulating catecholamines could mediate light and stress effects on submandibular beta-adrenergic secretory responses. Extrasynaptic alpha2-adrenoceptors might modulate the submandibular secretory response when predictable environmental stimuli (daily light phase) or unpredictable stressors raise the concentrations of circulating catecholamines.

A possible role of the pineal gland in acute immobilization-related suppression of naloxone-induced LH release in ovariectomized estrogen-primed rats

It has been recently reported that acute immobilization stress almost completely suppresses the luteinizing hormone (LH) release induced by naloxone, a mu-opioid antagonist, in ovariectomized estrogen-primed rats. The present study examined the possible involvement of the pineal gland in the acute immobilization-related suppression of the naloxone-induced LH release. An intraventricular (ICV) injection of 15 microg naloxone produced an abrupt increase in circulating LH concentrations in non-stressed rats. The naloxone-induced LH release was completely eliminated when tested 60 min after the end of a 30 min session of acute immobilization. The same stress conditions did not affect LH-releasing hormone (LHRH)-induced LH release, suggesting that the stress-related suppression of the naloxone-induced LH release was a suprapituitary event. In chronically-pinealectomized rats, but not in sham-pinealectomized rats, naloxone injected 60 min after the end of the stress session evoked a significant increase in serum LH concentrations. However, naloxone injected ICV during the acute immobilization did not elicit LH release in either pinealectomized or sham-operated rats. Under non-stressed conditions, the LH secretory response to naloxone was similar in pinealectomized and sham-operated animals. The same stress (30 min immobilization) significantly increased pineal melatonin content as well as plasma melatonin concentrations in rats bearing intact pineal glands, indicating that stress actually affected the pineal function. These results provide evidence for a role of the pineal in the suppression of the LH response to naloxone after stress, but not during stress.

Melatonin effects on behavior: possible mediation by the central GABAergic system

The best described function of the pineal hormone melatonin is to regulate seasonal reproduction, with its daily production and secretion varying throughout the seasons or the photoperiod. Additionally, a number of behavioral effects of the hormone have been found. This review describes the effects of melatonin in rodent behavior. We focus on: (a) inhibitory effects (sedation, hypnotic activity, pain perception threshold elevation, anti-convulsive activity, anti-anxiety effects); and (b) direct effects on circadian rhythmicity (entrainment, resynchronization, alleviation of jet-lag symptoms, phase-shifting). Most of these effects are clearly time-dependent, with a peak of melatonin activity during the night. One of the possible mechanisms of action for melatonin in the brain is the interaction with the GABAergic system, as suggested by neurochemical and behavioral data. Finally, some pineal hormone effects might be candidates as putative therapies for several human disorders.

The melatonin rhythm: both a clock and a calendar

The paper briefly reviews the data which shows that the circadian production and secretion of melatonin by the pineal gland can impart both daily, i.e., clock, and seasonal, i.e., calendar, information to the organism. The paper summarizes the 3 patterns of nocturnal melatonin production that have been described. Clearly, regardless of the pattern of nocturnal melatonin production a particular species normally displays, the duration of nightime elevated melatonin is proportional to the duration of the night length. Since daylength under natural conditions changes daily the melatonin rhythm, which adjusts to the photoperiod sends time of year information to the organism. The melatonin receptors which subserve the clock message sent by the pineal gland in the form of a melatonin cycle may reside in the biological clock itself, namely, the suprachiasmatic nuclei (SCN). The melatonin receptors that mediate seasonal changes in reproductive physiology are presumably those that are located on the pars tuberalis cells of the anterior pituitary gland. Besides these receptors which likely mediate clock and calendar information, melatonin receptors have been described in other organs. Interestingly, the distribution of melatonin receptors is highly species-specific. Whereas the clock and calendar information that the melatonin cycle imparts to the organism relies on cell membrane receptors, a fact that is of some interest considering the high lipophilicity of melatonin, recent studies indicate that other functions of melatonin may require no receptor whatsoever.

Unusual responses of nocturnal pineal melatonin synthesis and secretion to swimming: attempts to define mechanisms

The effect of swimming at night on rat pineal melatonin synthesis was compared with that of light exposure at night. Rats were forced to swim at 0030 hr (lights out at 2000 hr) and sacrificed by decapitation 15 and 30 min later, immediately after swimming. Other groups of animals were exposed to white light (650 muW/cm2) for 15 and 30 min at same time. Swimming caused a rapid and highly significant drop in the melatonin content in the pineal gland; however, the activity of N-acetyltransferase (NAT), the supposed rate limiting enzyme in the melatonin production, was not changed. Despite the drop in pineal melatonin levels, serum concentrations of the indole remained elevated in the rats that swam. In contrast, melatonin levels in the pineal and serum of light exposed rats fell precipitously, accompanied by a significant suppression of NAT activity. Since we anticipated that the strenuous exercise associated with swimming may induce release of artrial natriuretic peptide (ANP) from the heart, which in turn could cause the release of pineal melatonin, in a second study we injected physiological saline intravenously to stretch the cardiac muscle and release ANP. Three milliliters of normal saline was injected during the day into the jugular vein of anesthetized rats that were pretreated with isoproterenol to stimulate pineal melatonin production. Animals were killed 15 min after the saline injection, and pineal NAT activity and pineal melatonin levels were measured. The saline injections caused no alteration in the elevated levels of either NAT or melatonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Some perturbations that disturb the circadian melatonin rhythm

The circadian melatonin rhythm is highly reproducible and generally not easily altered. The few perturbations that are capable of significantly changing either the amplitude or the pattern of the 24-h melatonin rhythm are summarized herein. Aging alters cyclic melatonin production by decreasing the amplitude of the nocturnal melatonin peak in all species in which it has been studied. The best known acute suppressor of nocturnal melatonin is light exposure. The brightness of light required to acutely depress pineal melatonin production is species dependent; of the visible wavelengths, those in the blue range (approximately 500-520 nm) seem most effective in suppressing melatonin production. Nonvisible, nonionizing radiation in the extremely low frequency range (e.g., 60 Hz) seems also capable of altering pineal melatonin synthesis. Hormones have relatively little influence on the circadian production of melatonin, although either adrenalectomy or hypophysectomy does attenuate the amplitude of the melatonin cycle. Exercise at the time of high melatonin production rapidly depresses pineal concentrations of the indole without influencing its synthesis; the mechanism of this suppression remains unknown.

Psychosocial stress affects pineal function in the tree shrew (Tupaia belangeri)

Using a recently developed commercially available radioimmunoassay the concentration of the principal melatonin metabolite 6-sulfatoxymelatonin (aMT6s) in the morning urine of male tree shrews was determined. Chronic social confrontation elicited a drastic increase of aMT6s excretion in subordinate tree shrews, whereas there was a tendency to reduced excretion of the melatonin metabolite in dominant animals. These results substantiate the function of the pineal gland in transforming stimuli from the social environment to endocrine information and, therefore, are indicative for the relevant role the gland may play in the physiological reactions to chronic psychosocial stress.

Physical exercise and brain monoamines: a review

A review of the literature on the relationship of exercise to mental health strongly suggests that the two are closely linked. Thus, physical activity has been reported to reduce depression symptoms and anxiety, and to improve coping with stress. Logically, many investigators have then tried to determine the physiological mechanisms that are responsible for this mood-elevating effect of exercise. Among the current hypotheses, those regarding the endorphin and the monoamine systems have received a great deal of attention. On that basis, the respective effects of physical exercise on brain dopamine, noradrenaline and serotonin transmission are reviewed herein. In addition, suggestions are advanced for future research in this field.

Pineal sensitivity to nighttime swimming stress changes during the active season in Richardson's ground squirrels (Spermophilus richardsonii)

Melatonin synthesis in the pineal gland, which is primarily regulated by the environmental lighting regime, can also be influenced by other factors that elicit modifications in sympathetic tone. The objectives of this study were to determine if forced swimming alters the normal pattern of melatonin production in the pineal gland of the Richardson's ground squirrel (Spermophilus richardsonii). In early June, the squirrels were forced to swim for 10 min during the photophase or during the scotophase. In mid-July squirrels swam only during the scotophase. Animals were sacrificed 15, 30, or 60 min after the onset of swimming. Activities of pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were assessed by radioenzyme assay, and pineal melatonin content was measured by radioimmunoassay. Daytime swimming elicited no major changes in enzyme activity or pineal melatonin. In June, swimming at night prevented the normal rises in NAT activity and pineal melatonin seen in nonswimming controls. In contrast, the pineals of squirrels that were tested 6 weeks later in mid-July did not appear to be as sensitive to nighttime swimming, as there were only minor differences in both NAT activity and melatonin content compared to controls. These results demonstrate that forced nighttime swimming, unlike several other aversive stimuli, can evoke changes in the normal pattern of pineal melatonin production in this species. Furthermore, the pineal's response to such stimuli may not be stable over the course of the active season.