Modification of nitric oxide synthase activity and neuronal response in rat striatum by melatonin and kynurenine derivatives

Melatonin: a ferroptosis inhibitor with potential therapeutic efficacy for the post-COVID-19 trajectory of accelerated brain aging and neurodegeneration

The unprecedented pandemic of COVID-19 swept millions of lives in a short period, yet its menace continues among its survivors in the form of post-COVID syndrome. An exponentially growing number of COVID-19 survivors suffer from cognitive impairment, with compelling evidence of a trajectory of accelerated aging and neurodegeneration. The novel and enigmatic nature of this yet-to-unfold pathology demands extensive research seeking answers for both the molecular underpinnings and potential therapeutic targets. Ferroptosis, an iron-dependent cell death, is a strongly proposed underlying mechanism in post-COVID-19 aging and neurodegeneration discourse. COVID-19 incites neuroinflammation, iron dysregulation, reactive oxygen species (ROS) accumulation, antioxidant system repression, renin-angiotensin system (RAS) disruption, and clock gene alteration. These events pave the way for ferroptosis, which shows its signature in COVID-19, premature aging, and neurodegenerative disorders. In the search for a treatment, melatonin shines as a promising ferroptosis inhibitor with its repeatedly reported safety and tolerability. According to various studies, melatonin has proven efficacy in attenuating the severity of certain COVID-19 manifestations, validating its reputation as an anti-viral compound. Melatonin has well-documented anti-aging properties and combating neurodegenerative-related pathologies. Melatonin can block the leading events of ferroptosis since it is an efficient anti-inflammatory, iron chelator, antioxidant, angiotensin II antagonist, and clock gene regulator. Therefore, we propose ferroptosis as the culprit behind the post-COVID-19 trajectory of aging and neurodegeneration and melatonin, a well-fitting ferroptosis inhibitor, as a potential treatment.

Physiological and pharmacological roles of melatonin in the pathophysiological components of cellular injury after ischemic stroke

Apart from its metabolic or physiological functions, melatonin has a potent cytoprotective activity in the physiological and pathological conditions. It is synthetized by the pineal gland and released into the blood circulation but particularly cerebrospinal fluid in a circadian manner. It can also easily diffuse through cellular membranes due its small size and lipophilic structure. Its cytoprotective activity has been linked to its potent free radical scavenger activity with the desirable characteristics of a clinically- reliable antioxidant. Melatonin detoxifies oxygen and nitrogen-based free radicals and oxidizing agents, including the highly toxic hydroxyl-and peroxynitrite radicals, initiating cellular damage. However, the cytoprotective activity of melatonin is complex and cannot be solely limited to its free radical scavenger activity. It regulates cellular signaling pathways through receptor– dependent and independent mechanisms. Most of these downstream molecules, such as PI3K/AKT pathway components, also contribute to the cytoprotective effects of melatonin. In this term, melatonin is a promising molecule for the treatment of neurodegenerative disorders, such as ischemic stroke, which melatonin reduces ischemic brain injury in animal models of ischemic stroke. It regulates also circadian rhythm proteins after ischemic stroke, playing roles in cellular survival. In this context, present article summarizes the possible role of melatonin in the pathophysiological events after ischemic stroke.

The relationship between epileptic seizure and melatonin in children

OBJECTIVE

Studies about the relationship between epileptic seizures (ESs) and melatonin are limited in children and have been performed in heterogeneous patient groups and with different methods. In this study, it was planned to investigate this relationship according to seizure and epilepsy characteristics.

MATERIAL AND METHODS

In 91 children with ES, serum melatonin levels were measured within half an hour following the seizure and on a seizure-free day. Seizures were categorized according to the diagnosis, semiology, etiology, duration, electroencephalography (EEG) findings, and response to treatment. Melatonin levels were compared between each group and control group. In addition, basal melatonin levels of 21 patients with electrical status epilepticus in sleep (ESES) were compared with a control group.

RESULTS

Basal melatonin levels were found to be lower in children with ESs and ESES group compared with the control group (p < 0.001, p < 0.001). Likewise, similar results were obtained in subgroups except for remote symptomatic etiology, severe EEG findings, and refractory epilepsy. No significant difference was observed between basal and postseizure levels of melatonin.

CONCLUSION

This is the first study to reveal the relationship between ESs and basal melatonin levels according to all the characteristics of seizure and epilepsy in the largest patient group. It also demonstrates the need for more detailed studies on the role of melatonin in the pathogenesis of both ESs and ESES, which may provide a basis for a future treatment.

Melatonin and the electron transport chain

Melatonin protects the electron transport chain (ETC) in multiple ways. It reduces levels of ·NO by downregulating inducible and inhibiting neuronal nitric oxide synthases (iNOS, nNOS), thereby preventing excessive levels of peroxynitrite. Both ·NO and peroxynitrite-derived free radicals, such as ·NO2, hydroxyl (·OH) and carbonate radicals (CO3·-) cause blockades or bottlenecks in the ETC, by ·NO binding to irons, protein nitrosation, nitration and oxidation, changes that lead to electron overflow or even backflow and, thus, increased formation of superoxide anions (O2·-). Melatonin improves the intramitochondrial antioxidative defense by enhancing reduced glutathione levels and inducing glutathione peroxidase and Mn-superoxide dismutase (Mn-SOD) in the matrix and Cu,Zn-SOD in the intermembrane space. An additional action concerns the inhibition of cardiolipin peroxidation. This oxidative change in the membrane does not only initiate apoptosis or mitophagy, as usually considered, but also seems to occur at low rate, e.g., in aging, and impairs the structural integrity of Complexes III and IV. Moreover, elevated levels of melatonin inhibit the opening of the mitochondrial permeability transition pore and shorten its duration. Additionally, high-affinity binding sites in mitochondria have been described. The assumption of direct binding to the amphipathic ramp of Complex I would require further substantiation. The mitochondrial presence of the melatonin receptor MT1 offers the possibility that melatonin acts via an inhibitory G protein, soluble adenylyl cyclase, decreased cAMP and lowered protein kinase A activity, a signaling pathway shown to reduce Complex I activity in the case of a mitochondrial cannabinoid receptor.

Melatonin as a Therapeutic Resource for Inflammatory Visual Diseases

BACKGROUND

Uveitis and optic neuritis are prevalent ocular inflammatory diseases, and highly damaging ocular conditions. Both diseases are currently treated with corticosteroids, but they do not have adequate efficacy and are often associated with severe side effects. Thus, uveitis and optic neuritis remain a challenging field to ophthalmologists and a significant public health concern.

OBJECTIVE

This review summarizes findings showing the benefits of a treatment with melatonin in experimental models of these inflammatory ocular diseases.

RESULTS

Oxidative and nitrosative damage, tumor necrosis factor, and prostaglandin production have been involved in the pathogeny of uveitis and optic neuritis. Melatonin is an efficient antioxidant and antinitridergic, and has the ability to reduce prostaglandin and tumor necrosis factor levels both in the retina and optic nerve. Moreover, melatonin not only prevents functional and structural consequences of experimental uveitis and optic neuritis, but it is also capable of suppressing the actively ongoing ocular inflammatory response.

CONCLUSIONS

Since melatonin protects ocular tissues against inflammation, it could be a potentially useful anti-inflammatory therapy in ophthalmology.

Evaluation of Plasma Melatonin Levels in Children With Afebrile and Febrile Seizures

BACKGROUND

Melatonin modulates central nervous system neuronal activity. We compared the melatonin levels of patients with febrile and afebrile seizures during and after seizure with those of healthy controls.

METHODS

We enrolled 59 individuals with afebrile and febrile seizures (mean age, 6.09 ± 4.46 years) and 28 age-, sex-, and weight-matched healthy children. Melatonin levels were measured near the time of a seizure (0 to 1 hour) and at 12 and 24 hours post-seizure, and control melatonin levels were measured from a single venous blood sample.

RESULTS

Plasma melatonin levels increased during seizures in the study group (P < 0.001). Post-seizure plasma melatonin levels were significantly lower in the study group than in the control group (P < 0.05). Plasma melatonin levels did not differ between patients with afebrile seizures who had and had not used antiepileptic drugs. Daytime (8 AM to 8 PM) and nighttime (8 PM to 8 AM) post-seizure melatonin levels were not significantly different.

CONCLUSIONS

Melatonin levels were lower in pediatric patients prone to seizures than in healthy children and increased during seizures. Further research is needed to test the role of melatonin in the pathophysiology and treatment of epilepsy.

Targeting different pathophysiological events after traumatic brain injury in mice: Role of melatonin and memantine

The tissue damage that emerges during traumatic brain injury (TBI) is a consequence of a variety of pathophysiological events, including free radical generation and over-activation of N-methyl-d-aspartate-type glutamate receptors (NMDAR). Considering the complex pathophysiology of TBI, we hypothesized that combination of neuroprotective compounds, targeting different events which appear during injury, may be a more promising approach for patients. In this context, both NMDAR antagonist memantine and free radical scavenger melatonin are safe in humans and promising agents for the treatment of TBI. Herein, we examined the effects of melatonin administered alone or in combination with memantine on the activation of signaling pathways, injury development and DNA fragmentation. Both compounds reduced brain injury moderately and the density of DNA fragmentation significantly. Notably, melatonin/memantine combination decreased brain injury and DNA fragmentation significantly, which was associated with reduced p38 and ERK-1/2 phosphorylation. As compared with melatonin and memantine groups, SAPK/JNK-1/2 phosphorylation was also reduced in melatonin/memantine combined animals. In addition, melatonin, memantine and their combination decreased iNOS activity significantly. Here, we provide evidence that melatonin/memantine combination protects brain from traumatic injury, which was associated with decreased DNA fragmentation, p38 phosphorylation and iNOS activity.

Development of urea and thiourea kynurenamine derivatives: synthesis, molecular modeling, and biological evaluation as nitric oxide synthase inhibitors

Herein we describe the synthesis of a new family of kynurenamine derivatives with a urea or thiourea moiety, together with their in vitro biological evaluation as inhibitors of both neuronal and inducible nitric oxide synthases (nNOS and iNOS, respectively), enzymes responsible for the biogenesis of NO. These compounds were synthesized from a 5-substituted-2-nitrophenyl vinyl ketone scaffold in a five-step procedure with moderate to high chemical yields. In general, the assayed compounds show greater inhibition of iNOS than of nNOS, with 1-[3-(2-amino-5-chlorophenyl)-3-oxopropyl]-3-ethylurea (compound 5 n) being the most potent iNOS inhibitor in the series and the most iNOS/nNOS-selective compound. In this regard, we performed molecular modeling studies to propose a binding mode for this family of compounds to both enzymes and, thereby, to elucidate the differential molecular features that could explain the observed selectivity between iNOS and nNOS.

Development of nitric oxide synthase inhibitors for neurodegeneration and neuropathic pain

Nitric oxide (NO) is an important signaling molecule in the human body, playing a crucial role in cell and neuronal communication, regulation of blood pressure, and in immune activation. However, overproduction of NO by the neuronal isoform of nitric oxide synthase (nNOS) is one of the fundamental causes underlying neurodegenerative disorders and neuropathic pain. Therefore, developing small molecules for selective inhibition of nNOS over related isoforms (eNOS and iNOS) is therapeutically desirable. The aims of this review focus on the regulation and dysregulation of NO signaling, the role of NO in neurodegeneration and pain, the structure and mechanism of nNOS, and the use of this information to design selective inhibitors of this enzyme. Structure-based drug design, the bioavailability and pharmacokinetics of these inhibitors, and extensive target validation through animal studies are addressed.

Melatonin: a well-documented antioxidant with conditional pro-oxidant actions

Melatonin (N-acetyl-5-methoxytryptamine), an indoleamine produced in many organs including the pineal gland, was initially characterized as a hormone primarily involved in circadian regulation of physiological and neuroendocrine function. Subsequent studies found that melatonin and its metabolic derivatives possess strong free radical scavenging properties. These metabolites are potent antioxidants against both ROS (reactive oxygen species) and RNS (reactive nitrogen species). The mechanisms by which melatonin and its metabolites protect against free radicals and oxidative stress include direct scavenging of radicals and radical products, induction of the expression of antioxidant enzymes, reduction of the activation of pro-oxidant enzymes, and maintenance of mitochondrial homeostasis. In both in vitro and in vivo studies, melatonin has been shown to reduce oxidative damage to lipids, proteins and DNA under a very wide set of conditions where toxic derivatives of oxygen are known to be produced. Although the vast majority of studies proved the antioxidant capacity of melatonin and its derivatives, a few studies using cultured cells found that melatonin promoted the generation of ROS at pharmacological concentrations (μm to mm range) in several tumor and nontumor cells; thus, melatonin functioned as a conditional pro-oxidant. Mechanistically, melatonin may stimulate ROS production through its interaction with calmodulin. Also, melatonin may interact with mitochondrial complex III or mitochondrial transition pore to promote ROS production. Whether melatonin functions as a pro-oxidant under in vivo conditions is not well documented; thus, whether the reported in vitro pro-oxidant actions come into play in live organisms remains to be established.

Melatonin and the theories of aging: a critical appraisal of melatonin's role in antiaging mechanisms

The classic theories of aging such as the free radical theory, including its mitochondria-related versions, have largely focused on a few specific processes of senescence. Meanwhile, numerous interconnections have become apparent between age-dependent changes previously thought to proceed more or less independently. Increased damage by free radicals is not only linked to impairments of mitochondrial function, but also to inflammaging as it occurs during immune remodeling and by release of proinflammatory cytokines from mitotically arrested, DNA-damaged cells that exhibit the senescence-associated secretory phenotype (SASP). Among other effects, SASP can cause mutations in stem cells that reduce the capacity for tissue regeneration or, in worst case, lead to cancer stem cells. Oxidative stress has also been shown to promote telomere attrition. Moreover, damage by free radicals is connected to impaired circadian rhythmicity. Another nexus exists between cellular oscillators and metabolic sensing, in particular to the aging-suppressor SIRT1, which acts as an accessory clock protein. Melatonin, being a highly pleiotropic regulator molecule, interacts directly or indirectly with all the processes mentioned. These influences are critically reviewed, with emphasis on data from aged organisms and senescence-accelerated animals. The sometimes-controversial findings obtained either in a nongerontological context or in comparisons of tumor with nontumor cells are discussed in light of evidence obtained in senescent organisms. Although, in mammals, lifetime extension by melatonin has been rarely documented in a fully conclusive way, a support of healthy aging has been observed in rodents and is highly likely in humans.

Management of the aging risk factor for Parkinson's disease

The aging risk factor for Parkinson's disease is described in terms of specific disease markers including mitochondrial and gene dysfunctions relevant to energy metabolism. This review details evidence for the ability of nutritional agents to manage these aging risk factors. The combination of alpha lipoic acid, acetyl-l-carnitine, coenzyme Q10, and melatonin supports energy metabolism via carbohydrate and fatty acid utilization, assists electron transport and adenosine triphosphate synthesis, counters oxidative and nitrosative stress, and raises defenses against protein misfolding, inflammatory stimuli, iron, and other endogenous or xenobiotic toxins. These effects are supported by gene expression via the antioxidant response element (ARE; Keap/Nrf2 pathway), and by peroxisome proliferator-activated receptor gamma co-activator 1 alpha (PGC-1 alpha), a transcription coactivator, which regulates gene expression for energy metabolism and mitochondrial biogenesis, and maintains the structural integrity of mitochondria. The effectiveness and synergies of the combination against disease risks are discussed in relation to gene action, dopamine cell loss, and the accumulation and spread of pathology via misfolded alpha-synuclein. In addition there are potential synergies to support a neurorestorative role via glial derived neurotrophic factor expression.

Neuroprotective effect of melatonin: a novel therapy against perinatal hypoxia-ischemia

One of the most common causes of mortality and morbidity in children is perinatal hypoxia-ischemia (HI). In spite of the advances in neonatology, its incidence is not diminishing, generating a pediatric population that will require an extended amount of chronic care throughout their lifetime. For this reason, new and more effective neuroprotective strategies are urgently required, in order to minimize as much as possible the neurological consequences of this encephalopathy. In this sense, interest has grown in the neuroprotective possibilities of melatonin, as this hormone may help to maintain cell survival through the modulation of a wide range of physiological functions. Although some of the mechanisms by which melatonin is neuroprotective after neonatal asphyxia remain a subject of investigation, this review tries to summarize some of the most recent advances related with its use as a therapeutic drug against perinatal hypoxic-ischemic brain injury, supporting the high interest in this indoleamine as a future feasible strategy for cerebral asphyctic events.

Kynurenines in Parkinson's disease: therapeutic perspectives

Parkinson's disease (PD) is a chronic progressive neurodegenerative disorder the pathomechanism of which is not yet fully known. With regard to the molecular mechanism of development of the disease, oxidative stress/mitochondrial impairment, glutamate excitotoxicity and neuroinflammation are certainly involved. Alterations in the kynurenine pathway, the main pathway of the tryptophan metabolism, can contribute to the complex pathomechanism. There are several possibilities for therapeutic intervention involving targeting of this altered metabolic route. The development of synthetic molecules that would shift the altered balance towards the achievement of neuroprotective effects would be of great promise for future clinical studies on PD.

Protective effects of synthetic kynurenines on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism in mice

Mitochondrial complex I inhibition is thought to underlie the neurodegenerative process in Parkinson's disease (PD). Moreover, an overproduction of nitric oxide due to both cytosolic (iNOS) and mitochondrial (i-mtNOS) inducible nitric oxide synthases causes free radicals generation and oxidative/nitrosative stress, contributing to mitochondrial dysfunction and neuronal cell death. Looking for active molecules against mitochondrial dysfunction and inflammatory response in PD, we show here the effects of four synthetic kynurenines in the MPTP model of PD in mice. After MPTP administration, mitochondria from substantia nigra and, in a lesser extent, from striatum showed a significant increase in i-mtNOS activity, nitric oxide production, oxidative stress, and complex I inhibition. The four kynurenines assayed counteracted the effects of MPTP, reducing iNOS/i-mtNOS activity, and restoring the activity of the complex I. Consequently, the cytosolic and mitochondrial oxidative/nitrosative stress returned to control values. The results suggest that the kynurenines here reported represent a family of synthetic compounds with neuroprotective properties against PD, and that they can serve as templates for the design of new drugs able to target the mitochondria.

Melatonin as a therapeutic tool in ophthalmology: implications for glaucoma and uveitis

Several lines of evidence support the view that increased free radical generation and altered nitric oxide (NO) metabolism play a role in the pathogenesis of highly prevalent ocular diseases, such as glaucoma and uveitis. Data are discussed indicating that melatonin, being an efficient antioxidant that displays antinitridergic properties, has a promising role in the treatment of these ocular dysfunctions. Melatonin synthesis occurs in the eye of most species, and melatonin receptors are localized in different ocular structures. In view of the fact that melatonin lacks significant adverse collateral effects even at high doses, the application of melatonin could potentially protect ocular tissues by effectively scavenging free radicals and excessive amounts of NO generated in the glaucomatous or uveitic eye.

Light deprivation improves melatonin related suppression of hippocampal plasticity

In early postnatal life, sensory inputs deeply influence development as well as function of the brain. Plasticity of synaptic transmission including its experimentally induced form, long-term potentiation (LTP), is affected by sensory deprivation in neocortex. This study is devoted to assess if dark rearing and a dark phase synthesized hormone melatonin influence LTP in the hippocampus, an area of brain involved in learning and memory. In vivo experiments were carried out on two groups of 45-days-old male Wistar rats kept in standard 12-h light/dark condition [light reared (LR) tested during the light phase] or in complete darkness [dark reared (DR)] since birth to testing. Each group, in turn, was divided to two, vehicle- and melatonin-treated, groups. Stimulating the Schaffer collaterals of CA3 area of hippocampus extracellular postsynaptic potentials (EPSPs) were recorded in the CA1 area. Having the stable baseline responses to the test pulses, the hippocampus was perfused by either vehicle or 2 microg melatonin and EPSPs were recorded for 30 min. Then, for induction of LTP, the tetanus was applied to the Schaffer collaterals and the field potentials were pooled for 120-min post-tetanus. The light deprivation resulted in a significant augmentation in the amplitude of baseline responses. Also, we observed a melatonin-induced increase in amplitude of the baseline recordings in either LR or DR animals. Tetanic stimulation elicited LTP of EPSPs in both LR and DR groups, robustly in the former where it lasted for about 90 min. Generally, melatonin inhibited the production of LTP in the two groups especially in the LR animals leading to a noticeable depression. We concluded that higher level of neuronal activity in the DR rats gives rise to a lower level of LTP. Weaker effect of melatonin on blocking the potentiation of post-tetanus EPSPs in the DR rats may be the result of a desensitization of melatonin receptors due to chronically increased levels of this hormone in the visually deprived rats.

Melatonin: a novel neuroprotectant for the treatment of glaucoma

Glaucoma is a leading cause of blindness. Although ocular hypertension is the most important risk factor, several concomitant factors such as elevation of glutamate and decrease in gamma-aminobutyric acid (GABA) levels, disorganized NO metabolism, and oxidative damage could significantly contribute to the neurodegeneration. The aim of this report was to analyze the effect of melatonin on retinal glutamate clearance, GABA concentrations, NO synthesis, and retinal redox status, as well as on functional and histological alterations provoked by chronic ocular hypertension induced by intracameral injections of hyaluronic acid (HA) in the rat eye. In normal retinas, melatonin increased glutamate uptake, glutamine synthase activity, GABA turnover rate, glutamic acid decarboxylase activity, superoxide dismutase activity, and reduced glutathione (GSH) levels, whereas it decreased NOS activity, L-arginine uptake, and lipid peroxidation. To assess the effect of melatonin on glaucomatous neuropathy, weekly injections of HA were performed in the eye anterior chamber. A pellet of melatonin was implanted subcutaneously 24 hr before the first injection or after six weekly injections of HA. Melatonin, which did not affect intraocular pressure (IOP), prevented and reversed the effect of ocular hypertension on retinal function (assessed by electroretinography) and diminished the vulnerability of retinal ganglion cells to the deleterious effects of ocular hypertension. These results indicate that melatonin could be a promissory resource in the management of glaucoma.

Neuroprotection by radical avoidance: search for suitable agents

Neurodegeneration is frequently associated with damage by free radicals. However, increases in reactive oxygen and nitrogen species, which may ultimately lead to neuronal cell death, do not necessarily reflect its primary cause, but can be a consequence of otherwise induced cellular dysfunction. Detrimental processes which promote free radical formation are initiated, e.g., by disturbances in calcium homeostasis, mitochondrial malfunction, and an age-related decline in the circadian oscillator system. Free radicals generated at high rates under pathophysiological conditions are insufficiently detoxified by scavengers. Interventions at the primary causes of dysfunction, which avoid secondary rises in radical formation, may be more efficient. The aim of such approaches should be to prevent calcium overload, to reduce mitochondrial electron dissipation, to support electron transport capacity, and to avoid circadian perturbations. L-theanine and several amphiphilic nitrones are capable of counteracting excitotoxicity and/or mitochondrial radical formation. Resveratrol seems to promote mitochondrial biogenesis. Mitochondrial effects of leptin include attenuation of electron leakage. Melatonin combines all the requirements mentioned, additionally regulates anti- and pro-oxidant enzymes and is, with few exceptions, very well tolerated. In this review, the perspectives, problems and limits of drugs are compared which may be suitable for reducing the formation of free radicals.

Recurrent headache as the main symptom of acquired cerebral toxoplasmosis in nonhuman immunodeficiency virus-infected subjects with no lymphadenopathy: the parasite may be responsible for the neurogenic inflammation postulated as a cause of different types of headaches

Headache and/or migraine, a common problem in pediatrics and internal medicine, affect about 5% to 10% children and adolescents, and nearly 30% of middle-aged women. Headache is also one of the most common clinical manifestations of acquired Toxoplasma gondii infection of the central nervous system (CNS) in immunosuppressed subjects. We present 11 apparently nonhuman immunodeficiency virus-infected children aged 7 to 17 years (8 girls, 3 boys) and 1 adult woman with recurrent severe headaches in whom latent chronic CNS T. gondii infection not manifested by enlarged peripheral lymph nodes typical for toxoplasmosis, was found. In 7 patients, the mean serum IgG Toxoplasma antibodies concentration was 189 +/- 85 (SD) IU/mL (range 89 to 300 IU/mL), and in 5 other subjects, the indirect fluorescent antibody test titer ranged from 1:40 to 1:5120 IU/mL (n= <1:10 IU/mL). Some of the patients suffered also from atopic dermatitis (AD) and were exposed to cat and/or other pet allergens, associated with an increased IL-4 and decreased IFN-gamma production. These cytokine irregularities caused limited control of cerebral toxoplasmosis probably because IL-4 down-regulated both the production of IFN-gamma and its activity, and stimulated production of a low NO-producing population of monocytes, which allowed cysts rupture, increased parasite multiplication and finally reactivation of T. gondii infection. The immune studies performed in 4 subjects showed a decreased percentage of T lymphocytes, increased total number of lymphocytes B and serum IgM concentration, and impaired phagocytosis. In addition, few of them had also urinary tract diseases known to produce IL-6 that can mediate immunosuppressive functions, involving induction of the anti-inflammatory cytokine IL-10. These disturbances probably resulted from the host protective immune reactions associated with the chronic latent CNS T. gondii infection/inflammation. This is consistent with significantly lower enzyme indoleamine 2,3-dioxygenase (IDO) activity reported in atopic than in nonatopic individuals, and an important role that IDO and tryptophan degradation pathways plays in both, the host resistance to T. gondii infection and its reactivation. Analysis of literature information on the subjects with different types of headaches caused by foods, medications, and other substances, may suggest that their clinical symptoms and changes in laboratory data result at least in part from interference of these factors with dietary tryptophan biotransformation pathways. Several of these agents caused headache attacks through enhancing NO production via the conversion of arginine to citrulline and NO by the inducible nitric oxide synthase enzyme, which results in the high-output pathway of NO synthesis. This increased production of NO is, however, quickly down-regulated by NO itself because this biomolecule can directly inactivate NOS, may inhibit Ia expression on IFN-gamma-activated macrophages, which would limit antigen-presenting capability, and block T-cell proliferation, thus decreasing the antitoxoplasmatic activity. Moreover, NO inhibits IDO activity, thereby suppressing kynurenine formation, and at least one member of the kynurenine pathway, 3-hydroxyanthranilic acid, has been shown to inhibit NOS enzyme activity, the expression of NOS mRNA, and activation of the inflammatory transcription factor, nuclear factor-kB. In addition, the anti-inflammatory cytokines IL-4 and IL-10, TGF-beta, and a cytokine known as macrophage deactivating factor, have been shown to directly modulate NO production, sometimes expressing synergistic activity. On the other hand, IL-4 and TGF-beta can suppress IDO activity in some cells, for example human monocytes and fibroblasts, which is consistent with metabolic pathways controlled by IDO being a significant contributor to the proinflammatory system. Also, it seems that idiopathic intracranial hypertension, pseudotumor cerebri, and aseptic meningitis, induced by various factors, may result from their interference with IDO and inducible nitric oxide synthase activities, endogenous NO level, and cytokine irregularities which finally affect former T. gondii status 2mo in the brain. All these biochemical disturbances caused by the CNS T. gondii infection/inflammation may also be responsible for the relationship found between neurologic symptoms, such as headache, vertigo, and syncope observed in apparently immunocompetent children and adolescents, and physical and psychiatric symptoms in adulthood. We therefore believe that tests for T. gondii should be performed obligatorily in apparently immunocompetent patients with different types of headaches, even if they have no enlarged peripheral lymph nodes. This may help to avoid overlooking this treatable cause of the CNS disease, markedly reduce costs of hospitalization, diagnosis and treatment, and eventually prevent developing serious neurologic and psychiatric disorders.

Melatonin neutralizes neurotoxicity induced by quinolinic acid in brain tissue culture

Quinolinic acid is a well-known excitotoxin that induces oxidative stress and damage. In the present study, oxidative damage to biomolecules was followed by measuring lipid peroxidation and protein carbonyl formation in rat brain tissue culture over a period of 24 hr of exposure to this prooxidant agent at a concentration of 0.5 mm. Quinolinic acid enhanced lipid peroxidation in an early stage of tissue culture, and protein carbonyl at a later stage. These data confirm and extend previous studies demonstrating that quinolinic acid can induce significant oxidative damage. Melatonin, an antioxidant and neuroprotective agent with multiple actions as a radical scavenger and signaling molecule, completely prevented these prooxidant actions of quinolinic acid at a concentration of 1 mm. Morphological lesions and neurotoxicity induced by quinolinic acid were evaluated by light microscopy. Quinolinic acid produced extensive apoptosis/necrosis which was significantly attenuated by melatonin. Cotreatment with melatonin exerted a profound protective effect antagonizing the neurotoxicity induced by quinolinic acid. Glutathione reductase and catalase activities were increased by quinolinic acid and these effects were antagonized by melatonin. Furthermore, melatonin induced superoxide dismutase activity. Quinolinic acid and melatonin acted independently and by different mechanisms in modulating antioxidant enzyme activities. Our findings using quinolinic acid and melatonin clearly demonstrate that such changes should always be seen in the context of oxidative neurotoxicity and antioxidant neuroprotection.

Melatonin decreases calcium levels in retinotectal axons of Xenopus laevis by indirect activation of group III metabotropic glutamate receptors

Melatonin is a neuromodulator that binds to receptors in the retinotectal laminae of the amphibian optic tectum. The effect of melatonin on calcium dynamics in Xenopus retinotectal axons was investigated by imaging retinotectal axons labeled with the fluorescent indicator Fluo-4. Melatonin exerted an inhibitory influence on depolarization-evoked calcium increases, and the melatonin receptor antagonist 4-P-PDOT blocked this effect. Blockade of group III metabotropic receptors (mGluRs) counteracted the effect of melatonin on retinotectal axons. Application of the group II/group III mGluR antagonist MSPG or the group III-selective antagonist MSOP abolished the effect of melatonin. Conversely, this effect was not significantly affected by the group I mGluR antagonist LY367385 nor by EGLU or LY341495 at concentrations that specifically inhibit group II mGluRs. Furthermore, a higher concentration of LY341495 that affects group III mGluRs inhibited the effect of melatonin. The data therefore support the hypothesis that, in retinotectal axons, melatonin reduces cAMP levels, thereby relieving PKA-induced inhibition of group III mGluRs; the newly activated mGluRs in turn inhibit voltage-sensitive calcium channels, leading to a decrease in Ca2+ concentrations. The role of GABA(C) receptors in retinotectal responses was also evaluated. GABA(C) receptor antagonists did not block the effects of melatonin but instead were additive. Moreover, while other studies have shown that in Xenopus tectal cells, GABA(C) receptors mediate inhibition, in retinotectal axons, the opposite appears to occur since depolarization-evoked calcium rises in retinotectal axons were inhibited by GABA(C) receptor blockade. This result suggests that activation of GABA(C) receptors produces an increase in the synaptic excitability of retinotectal axon terminals.

The actions of a charged melatonin receptor ligand, TMEPI, and an irreversible MT2 receptor agonist, BMNEP, on mouse hippocampal evoked potentials in vitro

We have previously determined that melatonin modulates hippocampal synaptic transmission in a biphasic way: an initial depression was followed by a recovery/amplification phase. Here we describe the influence of two novel melatonin receptor ligands, BMNEP (N-bromoacetyl-2-iodo-5-methoxytryptamine) and TMPEI (N-[2-(2-Trimethylammoniumethyleneoxy-7-methoxy)ethyl]propionamide iodide), on the population spike (PS) and excitatory postsynaptic potentials (EPSP) recorded from mouse hippocampal slices. BMNEP, which specifically alkylates and constitutively activates the MT2 melatonin receptor, mimicked the first phase of melatonin's action by irreversibly depressing both the PS and EPSP. TMPEI, a charged ligand of plasma membrane melatonin receptors, amplified those potentials in a manner similar to the effect of melatonin observed during the second, recovery phase. Melatonin had no influence on the potentials amplified by the action of TMPEI. Our results suggest that the biphasic, receptor-dependent action of melatonin and its analogs modulates the efficiency of the hippocampal glutamergic synapse and is most likely mediated through two different, sequentially occurring mechanisms.

Melatonin mitigates mitochondrial malfunction

Melatonin, or N-acetyl-5-methoxytryptamine, is a compound derived from tryptophan that is found in all organisms from unicells to vertebrates. This indoleamine may act as a protective agent in disease conditions such as Parkinson's, Alzheimer's, aging, sepsis and other disorders including ischemia/reperfusion. In addition, melatonin has been proposed as a drug for the treatment of cancer. These disorders have in common a dysfunction of the apoptotic program. Thus, while defects which reduce apoptotic processes can exaggerate cancer, neurodegenerative disorders and ischemic conditions are made worse by enhanced apoptosis. The mechanism by which melatonin controls cell death is not entirely known. Recently, mitochondria, which are implicated in the intrinsic pathway of apoptosis, have been identified as a target for melatonin actions. It is known that melatonin scavenges oxygen and nitrogen-based reactants generated in mitochondria. This limits the loss of the intramitochondrial glutathione and lowers mitochondrial protein damage, improving electron transport chain (ETC) activity and reducing mtDNA damage. Melatonin also increases the activity of the complex I and complex IV of the ETC, thereby improving mitochondrial respiration and increasing ATP synthesis under normal and stressful conditions. These effects reflect the ability of melatonin to reduce the harmful reduction in the mitochondrial membrane potential that may trigger mitochondrial transition pore (MTP) opening and the apoptotic cascade. In addition, a reported direct action of melatonin in the control of currents through the MTP opens a new perspective in the understanding of the regulation of apoptotic cell death by the indoleamine.

Mechanisms of N-methyl-D-aspartate receptor inhibition by melatonin in the rat striatum

N-methyl-D-aspartate (NMDA) receptor activation comprises multiple regulatory sites controlling Ca2+ influx into the cell. NMDA-induced increases in intracellular [Ca(+2)] lead to nitric oxide (NO) production through activation of neuronal NO synthase (nNOS). Melatonin inhibits either glutamate or NMDA-induced excitation, but the mechanism of this inhibition is unknown. In the present study, the mechanism of melatonin action in the rat striatum was studied using extracellular single unit recording of NMDA-dependent neuronal activity with micro-iontophoresis. Melatonin inhibited neuronal excitation produced by either NMDA or L-arginine. The effects of both NMDA and L-arginine were blocked by nitro-L-arginine methyl ester, suggesting that nNOS participates in responses to NMDA. However, excitation of NMDA-sensitive neurones induced by the NO donor sodium nitroprusside was only slightly modified by melatonin. Melatonin iontophoresis also counteracted excitation induced by tris(2-carboxyethyl)phosphine hydrochloride, showing that the redox site of the NMDA receptor may be a target for melatonin action. The lack of effects of the membrane melatonin receptor ligands luzindole, 4-phenyl-2-propionamidotetralin and 5-methoxycarbonylamino-N-acetyltryptamine, and the nuclear melatonin ligand, CGP 52608, a thiazolidine dione, excluded the participation of known membrane and nuclear receptors for melatonin. The data suggest that inhibition of NMDA-dependent excitation by melatonin involves both nNOS inhibition and redox site modulation.

Circadian rhythms, oxidative stress, and antioxidative defense mechanisms

Endogenous circadian and exogenously driven daily rhythms of antioxidative enzyme activities and of low molecular weight antioxidants (LMWAs) are described in various phylogenetically distant organisms. Substantial amplitudes are detected in several cases, suggesting the significance of rhythmicity in avoiding excessive oxidative stress. Mammalian and/or avian glutathione peroxidase and, as a consequence, glutathione reductase activities follow the rhythm of melatonin. Another hint for an involvement of melatonin in the control of redox processes is seen in its high-affinity binding to cytosolic quinone reductase 2, previously believed to be a melatonin receptor. Although antioxidative protection by pharmacological doses of melatonin is repeatedly reported, explanations of these findings are still insufficient and their physiological and chronobiological relevance is not yet settled. Recent data indicate a role of melatonin in the avoidance of mitochondrial radical formation, a function which may prevail over direct scavenging. Rhythmic changes in oxidative damage of protein and lipid molecules are also reported. Enhanced oxidative protein modification accompanied by a marked increase in the circadian amplitude of this parameter is detected in the Drosophila mutant rosy, which is deficient in the LMWA urate. Preliminary evidence for the significance of circadian rhythmicity in diminishing oxidative stress comes from clock mutants. In Drosophila, moderately enhanced protein damage is described for the arrhythmic and melatonin null mutant per0, but even more elevated, periodic damage is found in the short-period mutant per(s), synchronized to LD 12:12. Remarkably large increases in oxidative protein damage, along with impairment of tissue integrity and--obviously insufficient--compensatory elevations in protective enzymes are observed in a particularly vulnerable organ, the Harderian gland, of another short-period mutant tau, in the Syrian hamster. Mice deficient in the per2 gene homolog are reported to be cancer-prone, a finding which might also relate to oxidative stress. In the dinoflagellate Lingulodinium polyedrum [Gonyaulax polyedra], various treatments that cause oxidative stress result in strong suppressions of melatonin and its metabolite 5-methoxytryptamine (5-MT) and to secondary effects on overt rhythmicity. The glow maximum, depending on the presence of elevated 5-MT at the end of subjective night, decreases in a dose-dependent manner already under moderate, non-lethal oxidative stress, but is restored by replenishing melatonin. Therefore, a general effect of oxidative stress may consist in declines of easily oxidizable signaling molecules such as melatonin, and this can have consequences on the circadian intraorganismal organization and expression of overt rhythms. Recent findings on a redox-sensitive input into the core oscillator via modulation of NPAS2/BMAL1 or CLK/BMAL1 heterodimer binding to DNA indicate a direct influence of cellular redox balance, including oxidative stress, on the circadian clock.

Melatonin and mitochondrial function

Melatonin is a natural occurring compound with well-known antioxidant properties. In the last decade a new effect of melatonin on mitochondrial homeostasis has been discovered and, although the exact molecular mechanism for this effect remains unknown, it may explain, at least in part, the protective properties found for the indoleamine in degenerative conditions such as aging as well as Parkinson's disease, Alzheimer's disease, epilepsy, sepsis and other injuries such as ischemia-reperfusion. A common feature in these diseases is the existence of mitochondrial damage due to oxidative stress, which may lead to a decrease in the activities of mitochondrial complexes and ATP production, and, as a consequence, a further increase in free radical generation. A vicious cycle thus results under these conditions of oxidative stress with the final consequence being cell death by necrosis or apoptosis. Melatonin is able of directly scavenging a variety of toxic oxygen and nitrogen-based reactants, stimulates antioxidative enzymes, increases the efficiency of the electron transport chain thereby limiting electron leakage and free radical generation, and promotes ATP synthesis. Via these actions, melatonin preserves the integrity of the mitochondria and helps to maintain cell functions and survival.

Effects of some synthetic kynurenines on brain amino acids and nitric oxide after pentylenetetrazole administration to rats

We have previously proven that some synthetic kynurenines behave as antagonists of the N-methyl-d-aspartate receptor inhibiting neuronal subtype of nitric oxide synthase activity. We now investigate the anticonvulsant activity of four of these kynurenines in pentylenetetrazole (PTZ)-treated rats. The rats were treated with each kynurenine (10-160 mg/kg, s.c.) 30 min before PTZ administration (100 mg/kg, s.c.). Then, latency, duration and intensity of the first seizure and the percent animal survival were noted. PTZ-induced death was counteracted by high doses of kynurenines. Latency of the first seizure was significantly increased and its intensity reduced at the same doses, whereas the duration of the first seizure significantly decreased with doses of 20 mg/kg in most of the kynurenines tested. Three hours after PTZ administration, the surviving animals were sacrificed and the levels of brain amino acids and nitrite were measured. PTZ administration increased glutamate, glutamine, serine and taurine levels in different brain areas. High doses of kynurenines generally counteracted the effects of PTZ on excitatory amino acids, but they also reduced inhibitory aminoacids. However, the most consistent effect of kynurenines was the dose-dependent reduction of brain nitrite levels induced by PTZ. These results reveal a new family of anticonvulsant drugs that affect mainly to nitric oxide production in the brain.

Changes in brain amino acids and nitric oxide after melatonin administration in rats with pentylenetetrazole-induced seizures

We examined the effect of melatonin on brain levels of amino acids and nitric oxide (NO) after pentylenetetrazole (PTZ)-induced seizures in rats. Animals were treated with melatonin (10-160 mg/kg, i.p.) 30 min before PTZ administration (100 mg/kg, s.c.), and were killed 3 hr later. At the dose of 80 mg/kg, melatonin significantly increased the latency (5.7-12.7 min) and decreased the duration (31.2-18.4 s) of the first seizure, reducing PTZ induced mortality from 87.5 to 25%. After kill, brains were removed and neurotransmitters and nitrite levels measured in prefrontal cortex (PF), parieto-temporal cortex (PF), striatum (ST), hippocampus (HP) and brain stem (BS) by high performance liquid chromatography. PTZ treatment increased glutamine levels in all brain areas studied, without changes in glutamate, gamma-amino butyric acid (GABA) and glycine. Aspartate and taurine increased in PF and PT and in HS and PT, respectively. Melatonin administration displayed a dose-dependent effect. At doses of 10-40 mg/kg, melatonin counteracted the PTZ-induced glutamine increase and reduced both glutamate and aspartate levels in the studied areas, with minor changes in GABA and glycine content. At doses of 80 and 160 mg/kg, the levels of glutamine, and glutamate, and to a lesser extent aspartate increased, whereas serine levels did not change. These two doses of melatonin also increased taurine, GABA and glycine in most brain areas studied. Treatment with melatonin (40-160 mg/kg) significantly decreased nitrite content in PT cortex, ST and BS areas of epileptic rats, without changes in the other brain regions. The results suggest that the anticonvulsant property of melatonin involves a modulation of both brain amino acids and NO production.

Oxidation of melatonin by carbonate radicals and chemiluminescence emitted during pyrrole ring cleavage

Oxidation of melatonin was followed by measuring chemiluminescence emitted during pyrrole ring cleavage, a process leading to the main oxidation product of this indoleamine, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK). Radical reactions of melatonin were studied in two variants of a moderately alkaline (pH 8) H2O2 system, one of which contained hemin as a catalyst. In both systems, light emission from melatonin oxidation lasted several hours. Time courses and turnover rates depended on the presence or absence of hemin; the catalyst enhanced light emission many-fold. In the two reaction systems, the presence of hydrogen carbonate (HCO)(3)(-) enhanced chemiluminescence by more than 10-fold, indicating scavenging of carbonate radicals. In the presence of 10% dimethylsulfoxide (DMSO) or 1 m mannitol, HCO(3)(-)-dependent as well as independent light emissions were only partially inhibited. With regard to the stimulatory effect of HCO(3)(-), this implies a formation of carbonate radicals (CO)(3)(-) independent of hydroxyl (OH) radicals, presumably involving superoxide anions abundantly present in the system. Tiron, a scavenger of superoxide anions, strongly and almost instantaneously inhibited chemiluminescence, in accordance to the requirement of this reactive oxygen species for AFMK formation and its involvement in -radical formation. Melatonin's capability of scavenging CO(3)(-) may contribute to its protective potency.

Interferon-gamma independent oxidation of melatonin by macrophages

Mononuclear phagocytes appear to synthesize kynurenine-like products from the oxidation of biologically active indole compounds including melatonin, catalyzed by interferon (IFN)-gamma-inducible enzyme indoleamine 2,3-dioxygenase (IDO). Concanavalin A (Con A) is a plant lectin that induces interferon-gamma (IFN-gamma) production by T cells. In this study we investigated whether Con A-primed peritoneal macrophages are able to oxidize melatonin to N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK). The AFMK production was accompanied by chemiluminescence. It was found that Con A-primed but not resident macrophages produce AFMK. Surprisingly, Con A-primed macrophages from IFN-gamma-deficient mice were as effective as macrophages from IFN-gamma-sufficient mice in oxidizing melatonin. Moreover, addition of an inhibitor of IDO (1-methyltryptophan) did not affect melatonin oxidation. Con A-primed but not resident macrophages have a significant content of myeloperoxidase (MPO) and inhibition of MPO by azide completely blocked chemiluminescence and AFMK production. Thus, our findings provide evidence that melatonin oxidation by macrophages may occur through a mechanism dependent of MPO and independent of IFN-gamma and IDO activity.

Factors regulating the influence of melatonin on hippocampal evoked potentials: comparative studies on different strains of mice

Factors regulating the influence of melatonin on the hippocampal glutamergic system in mouse hippocampal slices were evaluated. The sensitivity of hippocampal pyramidal neurons to melatonin (Sigma) was highest at 2 h following slice preparation and then declined with time. This pattern of sensitivity to melatonin correlated well with a reduced binding of melatonin to its receptors. The slices obtained from older animals remained sensitive to melatonin through the entire incubation period. Most of the experiments evaluating the influence of melatonin on hippocampal evoked potentials were performed within 2 h following slice preparation. The effect of melatonin was biphasic: an initial depression of the potential was followed by a recovery/amplification phase. The recovery phase was not a result of melatonin decomposition. The effect of melatonin was similar in three different strains of mice tested: CD-1, C57J/B6, and Swiss Webster. While the melatonin from another vendor (Regis) gave similar results, it was effective at much lower concentrations. In slices obtained from CD-1 light-deprived mice, the sensitivity to melatonin was significantly reduced. Thus, it appears that melatonin may control the hippocampal glutamergic system in a complex manner, which may be regulated by the circadian rhythm. This may influence memory formation in the hippocampus.

Endogenous kynurenines as targets for drug discovery and development

The kynurenine pathway is the main pathway for tryptophan metabolism. It generates compounds that can modulate activity at glutamate receptors and possibly nicotinic receptors, in addition to some as-yet-unidentified sites. The pathway is in a unique position to regulate other aspects of the metabolism of tryptophan to neuroactive compounds, and also seems to be a key factor in the communication between the nervous and immune systems. It also has potentially important roles in the regulation of cell proliferation and tissue function in the periphery. As a result, the pathway presents a multitude of potential sites for drug discovery in neuroscience, oncology and visceral pathology.

Physiological concentrations of melatonin inhibit the nitridergic pathway in the Syrian hamster retina

In the present work, the effect of melatonin on the hamster retinal nitridergic pathway was examined. When the retinas were incubated in the presence of low concentrations (1 pM-10 nM) of melatonin for 15 min, a significant decrease of nitric oxide synthase (NOS) activity was observed. However, when crude retinal homogenates were preincubated with melatonin for 15 min, no changes in NOS activity were detected, despite the fact that under the same conditions trifluoperazine, a calmodulin inhibitor, significantly decreased enzymatic activity. Kinetic analysis showed that melatonin decreased the V(max) of retinal NOS without changes in the K(m). On the other hand, low concentrations (100 pM) of melatonin significantly reduced retinal L-arginine influx. A decrease in the V(max) of L-arginine uptake was observed in the presence of melatonin, whereas the K(m) remained unchanged. Melatonin significantly inhibited the accumulation of cyclic guanosine monophosphate (cGMP) levels induced by both L-arginine and sodium nitroprusside (SNP). In summary, the present results indicate that melatonin could be a potent inhibitor of the retinal nitridergic pathway.

Melatonin-dopamine interactions: from basic neurochemistry to a clinical setting

To review the interaction between melatonin and the dopaminergic system in the hypothalamus and striatum and its potential clinical use in dopamine-related disorders in the central nervous system. Medline-based search on melatonin-dopamine interactions in mammals. Melatonin. the hormone produced by the pineal gland at night. influences circadian and seasonal rhythms, most notably the sleep-wake cycle and seasonal reproduction. The neurochemical basis of these activities is not understood yet. Inhibition of dopamine release by melatonin has been demonstrated in specific areas of the mammalian central nervous system (hypothalamus, hippocampus, medulla-pons, and retina). Antidopaminergic activities of melatonin have been demonstrated in the striatum. Dopaminergic transmission has a pivotal role in circadian entrainment of the fetus, in coordination of body movement and reproduction. Recent findings indicate that melatonin may modulate dopaminergic pathways involved in movement disorders in humans. In Parkinson patients melatonin may, on the one hand, exacerbate symptoms (because of its putative interference with dopamine release) and, on the other, protect against neurodegeneration (by virtue of its antioxidant properties and its effects on mitochondrial activity). Melatonin appears to be effective in the treatment of tardive dyskinesia. a severe movement disorder associated with long-term blockade of the postsynaptic dopamine D2 receptor by antipsychotic drugs in schizophrenic patients. The interaction of melatonin with the dopaminergic system may play a significant role in the nonphotic and photic entrainment of the biological clock as well as in the fine-tuning of motor coordination in the striatum. These interactions and the antioxidant nature of melatonin may be beneficial in the treatment of dopamine-related disorders.

Oxidation of melatonin and tryptophan by an HRP cycle involving compound III

We recently described that horseradish peroxidase (HRP) and myeloperoxidase (MPO) catalyze the oxidation of melatonin, forming the respective indole ring-opening product N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) (Biochem. Biophys. Res. Commun. 279, 657-662, 2001). Although the classic peroxidatic enzyme cycle is expected to participate in the oxidation of melatonin, the requirement of a low HRP:H(2)O(2) ratio suggested that other enzyme paths might also be operative. Here we followed the formation of AFMK under two experimental conditions: predominance of HRP compounds I and II or presence of compound III. Although the consumption of substrate is comparable under both conditions, AFMK is formed in significant amounts only when compound III predominates during the reaction. Using tryptophan as substrate, N- formyl-kynurenine is formed in the presence of compound III. Both, melatonin and tryptophan efficiently prevents the formation of p-670, the inactive form of HRP. Since superoxide dismutase (SOD) inhibits the production of AFMK, we proposed that compound III acts as a source of O(-*)(2) or participates directly in the reaction, as in the case of enzyme indoleamine 2,3-dioxygenase.

Calcium-dependent effects of melatonin inhibition of glutamatergic response in rat striatum

The effects of melatonin, amlodipine, diltiazem (L-type Ca2+ channel blockers) and omega-conotoxin (N-type Ca2+ channel blocker) on the glutamate-dependent excitatory response of striatal neurones to sensory-motor cortex stimulation was studied in a total of 111 neurones. Iontophoresis of melatonin produced a significant attenuation of the excitatory response in 85.2% of the neurones with a latency period of 2 min. Iontophoresis of either L- or N-type Ca2+ channel blocker also produced a significant attenuation of the excitatory response in more than 50% of the recorded neurones without significant latency. The simultaneous iontophoresis of melatonin + amlodipine or melatonin + diltiazem did not increase the attenuation produced by melatonin alone. However, the attenuation of the excitatory response was significantly higher after ejecting melatonin + omega-conotoxin than after ejecting melatonin alone. The melatonin-Ca2+ relationship was further supported by iontophoresis of the Ca2+ ionophore A-23187, which suppressed the inhibitory effect of either melatonin or Ca2+ antagonists. In addition, in synaptosomes prepared from rat striatum, melatonin produced a decrease in the Ca2+ influx measured by Fura-2AM fluorescence. Binding experiments with [3H]MK-801 in membrane preparations from rat striatum showed that melatonin did not compete with the MK-801 binding sites themselves although, in the presence of Mg2+, melatonin increased the affinity of MK-801. The results suggest that decreased Ca2+ influx is involved in the inhibitory effects of melatonin on the glutamatergic activity of rat striatum.

Weak vasoconstrictor activity of melatonin in human umbilical artery: relation to nitric oxide-scavenging action

We evaluated the nitric oxide (NO)-scavenging property of melatonin, demonstrated in a recent in vitro study, on vascular reactivity in the human umbilical artery. Helical sections of human umbilical artery were prepared following elective Cesarean deliveries near term. Changes in maximal tension induced by prostaglandin F(2 alpha)(5 x 10(-5) M) were measured in artery sections with an intact endothelium. Melatonin at concentrations higher than 10(-6) M increased prostaglandin F(2 alpha)-induced vascular tension. The vasospastic effect of melatonin was much less than that of L-N(G)-monomethylarginine (L-NMA, 2 x 10(-4) M), an inhibitor of NO synthesis (2.8+/-1.4%, 9.1+/-1.7%, 16.5+/-2.5%, and 29.6+/-5.9% of the L-NMA effect at melatonin concentrations of 10(-8), 10(-7), 10(-6), and 10(-5) M, respectively). Removal of the endothelium significantly reduced the vasoconstrictive effect of melatonin. Treatment with L-NMA (2 x 10(-4) M) prior to addition of prostaglandin F(2 alpha) also significantly reduced the vasoconstrictive effect of melatonin (10(-5) M). Treatments with melatonin (10(-5) M) did not affect calcium ionophore A 23187-induced relaxation or 5-hydroxytryptamine-induced constriction. The findings indicate that melatonin may potentiate vascular tension in human umbilical artery by scavenging endogenous endothelial NO, but not by inhibiting NO synthesis. However, the NO-scavenging vasoconstrictive effect of melatonin may be negligible at physiologic concentrations and very weak at pharmacologic concentrations below 10(-7) M.

Melatonin potentiates contractile responses to serotonin in isolated porcine coronary arteries

The present study was designed to determine the effects of melatonin on coronary vasomotor tone. Porcine coronary arteries were suspended in organ chambers for isometric tension recording. Melatonin (10(-10)-10(-5) M) itself caused neither contraction nor relaxation of the tissues. Serotonin (10(-9)-10(-5) M) caused concentration-dependent contractions of coronary arteries, and in the presence of melatonin (10(-7) M) the maximal response to serotonin was increased in rings with but not without endothelium. In contrast, melatonin had no effect on contractions produced by the thromboxane A(2) analog U-46619 (10(-10)-10(-7) M). The melatonin-receptor antagonist S-20928 (10(-6) M) abolished the potentiating effect of melatonin on serotonin-induced contractions in endothelium-intact coronary arteries, as did treatment with 1H-[1, 2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10(-5) M), methylene blue (10(-5) M), or N(G)-nitro-L-arginine (3 x 10(-5) M). In tissues contracted with U-46619, serotonin caused endothelium-dependent relaxations that were inhibited by melatonin (10(-7) M). Melatonin also inhibited coronary artery relaxation induced by sodium nitroprusside (10(-9)-10(-5) M) but not by isoproterenol (10(-9)-10(-5) M). These results support the hypothesis that melatonin, by inhibiting the action of nitric oxide on coronary vascular smooth muscle, selectively potentiates the vasoconstrictor response to serotonin in coronary arteries with endothelium.

Structure-related inhibition of calmodulin-dependent neuronal nitric-oxide synthase activity by melatonin and synthetic kynurenines

We recently described that melatonin and some kynurenines modulate the N-methyl-D-aspartate-dependent excitatory response in rat striatal neurons, an effect that could be related to their inhibition of nNOS. In this report, we studied the effect of melatonin and these kynurenines on nNOS activity in both rat striatal homogenate and purified rat brain nNOS. In homogenates of rat striatum, melatonin inhibits nNOS activity, whereas synthetic kynurenines act in a structure-related manner. Kynurenines carrying an NH(2) group in their benzenic ring (NH(2)-kynurenines) inhibit nNOS activity more strongly than melatonin itself. However, kynurenines lacking the NH(2) group or with this group blocked do not affect enzyme activity. Kinetic analysis shows that melatonin and NH(2)-kynurenines behave as noncompetitive inhibitors of nNOS. Using purified rat brain nNOS, we show that the inhibitory effect of melatonin and NH(2)-kynurenines on the enzyme activity diminishes with increasing amounts of calmodulin in the incubation medium. However, changes in other nNOS cofactors such as FAD or H(4)-biopterin, do not modify the drugs' response. These data suggest that calmodulin may be involved in the nNOS inhibition by these compounds. Studies with urea-polyacrylamide gel electrophoresis further support an interaction between melatonin and NH(2)-kynurenines, but not kynurenines lacking the NH(2) group, with Ca(2+)-calmodulin yielding Ca(2+)-calmodulin-drug complexes that prevent nNOS activation. The results show that calmodulin is a target involved in the intracellular effects of melatonin and some melatonin-related kynurenines that may account, at least in part, for the neuroprotective properties of these compounds.

Melatonin and its precursors scavenge nitric oxide

Nitric oxide (NO) scavenging activity of melatonin, N-acetyl-5-hydroxytryptamine, serotonin, 5-hydroxytryptophan and L-tryptophan was examined by the Griess reaction using flow injection analysis. 1-Hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC-7) was used as NO generator. The Griess reagent stoichiometrically reacts with NO2-, which was converted by a cadmium-copper reduction column from the stable end products of NO oxidation. Except for tryptophan, all the compounds examined scavenged NO in a dose-dependent manner. Melatonin, which has a methoxy group in the 5-position and an acetyl side chain, exhibited the most potent scavenging activity among the compounds tested. Serotonin, N-acetyl-5-hydroxytryptamine, and 5-hydroxytryptophan, respectively, showed moderate scavenging activity compared to melatonin. Tryptophan, which has neither a methoxy nor a hydroxyl group in the 5-position, exhibited the least NO scavenging activity.