Melatonin administration increases the affinity of D2 dopamine receptors in the rat striatum

Assessment of Biochemical and Neuroactivities of Cultural Filtrate from Trichoderma harzianum in Adjusting Electrolytes and Neurotransmitters in Hippocampus of Epileptic Rats

BACKGROUND

Epilepsy is a serious chronic neurological disorder, which is accompanied by recurrent seizures. Repeated seizures cause physical injuries and neuronal dysfunction and may be a risk of cancer and vascular diseases. However, many antiepileptic drugs (AEDs) have side effects of mood alteration or neurocognitive function, a reduction in neuron excitation, and the inhibition of normal activity. Therefore, the present study aimed to evaluate the effect of secondary metabolites of Trichoderma harzianum cultural filtrate (ThCF) when adjusting different electrolytes and neurotransmitters in the hippocampus of epileptic rats.

METHODS

Cytotoxicity of ThCF against LS-174T cancer cells was assessed using a sulforhodamine B (SRB) assay. Quantitative estimation for some neurotransmitters, electrolytes in sera or homogenate of hippocampi tissues, and mRNA gene expression for ion or voltage gates was assessed by quantitative Real-Time PCR.

RESULTS

Treatment with ThCF reduces the proliferative percentage of LS-174T cells in a concentration-dependent manner. ThCF administration improves hyponatremia, hyperkalemia, and hypocalcemia in the sera of the epilepticus model. ThCF rebalances the elevated levels of many neurotransmitters and reduces the release of GABA and acetylcholine-esterase. Also, treatments with ThCF ameliorate the downregulation of mRNA gene expression for some gate receptors in hippocampal homogenate tissues and recorded a highly significant elevation in the expression of SCN1A, CACNA1S, and NMDA.

CONCLUSION

Secondary metabolites of Trichoderma (ThCF) have cytotoxic activity against LS-174T (colorectal cancer cell line) and anxiolytic-like activity through a GABAergic mechanism of action and an increase in GABA as inhibitory amino acid in the selected brain regions and reduced levels of NMDA and DOPA. The present data suggested that ThCF may inhibit intracellular calcium accumulation by triggering the NAADP-mediated Ca2+ signaling pathway. Therefore, the present results suggested further studies on the molecular pathway for each metabolite of ThCF, e.g., 6-pentyl-α-pyrone (6-PP), harzianic acid (HA), and hydrophobin, as an alternative drug to mitigate the side effects of AEDs.

PDK4 Inhibition Ameliorates Melatonin Therapy by Modulating Cerebral Metabolism and Remyelination in an EAE Demyelinating Mouse Model of Multiple Sclerosis

We recently showed that melatonin ameliorates the severity of experimental autoimmune encephalomyelitis (EAE), an animal model of MS. However, efficiency of melatonin therapy was associated with side effects, manifested by slowing down of remyelination, through increasing the inhibitory effects of brain pyruvate dehydrogenase kinase-4 (PDK-4) on pyruvate dehydrogenase complex (PDC), a key enzyme in fatty acid (FA) synthesis during remyelination. In this study, we investigated the metabolic profile of FA synthesis using combination therapy of melatonin and diisopropylamine dichloroacetate (DADA), a PDK4 inhibitor, in EAE mice. Disease progression was monitored by recording the disability scores. Immunological, oligodendrogenesis and metabolic factors were also evaluated. Results showed that combination therapy of melatonin and DADA significantly reduced EAE disability scores, compared to melatonin, whereas DADA alone did not have any effect. In addition, co-therapy inhibited pro-inflammatory while increasing anti-inflammatory cytokines, significantly better than melatonin alone. Moreover, administration of combination drugs recovered the declined expression of oligodendrocytic markers in EAE, more potently than melatonin. Furthermore, co-therapy affected cerebral energy metabolism by significantly reducing lactate levels while increasing N-acetylaspartate (NAA) and 3-hydroxy-3-methyl-glutaryl-coenzyme-A reductase (HMGCR) levels. Finally, while melatonin increased lactate and PDK4 expression levels and greatly reduced PDC activity, co-therapy significantly restored PDC function while reducing the lactate levels. In summary, administration of melatonin with DADA increased the efficiency of melatonin treatment by eliminating the inhibitory effects of PDK4 on PDC’s function, a critical step for proper FA synthesis during remyelination.

Insights into Potential Targets for Therapeutic Intervention in Epilepsy

Epilepsy is a chronic brain disease that affects approximately 65 million people worldwide. However, despite the continuous development of antiepileptic drugs, over 30% patients with epilepsy progress to drug-resistant epilepsy. For this reason, it is a high priority objective in preclinical research to find novel therapeutic targets and to develop effective drugs that prevent or reverse the molecular mechanisms underlying epilepsy progression. Among these potential therapeutic targets, we highlight currently available information involving signaling pathways (Wnt/β-catenin, Mammalian Target of Rapamycin (mTOR) signaling and zinc signaling), enzymes (carbonic anhydrase), proteins (erythropoietin, copine 6 and complement system), channels (Transient Receptor Potential Vanilloid Type 1 (TRPV1) channel) and receptors (galanin and melatonin receptors). All of them have demonstrated a certain degree of efficacy not only in controlling seizures but also in displaying neuroprotective activity and in modifying the progression of epilepsy. Although some research with these specific targets has been done in relation with epilepsy, they have not been fully explored as potential therapeutic targets that could help address the unsolved issue of drug-resistant epilepsy and develop new antiseizure therapies for the treatment of epilepsy.

Haloperidol-induced hypokinesia in rats is differentially affected by the light/dark phase, age, and melatonin

It has been well established that the striatal dopaminergic system is compromised with aging, namely D2 receptor function. Also well documented is the age related decline of the neurohormone, melatonin, in both humans and nonhuman animals. What has not been well studied is the possible interaction between the D2 receptor system and the age related decline in melatonin with its unmistakable pattern of synthesis and release exclusively during the dark phase. We tested the effect of the D2 antagonist, haloperidol (1.0 mg/kg ip), in adolescent (2 mo old) and adult rats (10 mo old) in the light (ZT3) and dark phases (ZT 15) in rats kept in a 12 L/12D cycle and the effect of exogenous melatonin (15 mg/kg ip/day x 4 days for a total of 60 mg/kg) on D2 antagonism. Using the bar test, measuring the extrapyramidal side-effect of hypokinesia, we report haloperidol to work differentially depending on both age and phase. Adult rats experienced the effect of the D2 antagonist in both the light and dark phases, while younger rats did not show hypokinetic affects in the dark. By manipulated lighting, we were able to restore the effect of haloperidol in younger rats in the dark phase. We also found ameliorating effects of melatonin lessening time on the bar after treatment with haloperidol, however, this effect was only found in older rats. These data demonstrate the importance of the light/dark cycle and age in the susceptibility of extrapyramidal effects with use of drugs that target D2 receptor function.

Melatonin Therapy Modulates Cerebral Metabolism and Enhances Remyelination by Increasing PDK4 in a Mouse Model of Multiple Sclerosis

Metabolic disturbances have been implicated in demyelinating diseases including multiple sclerosis (MS). Melatonin, a naturally occurring hormone, has emerged as a potent neuroprotective candidate to reduce myelin loss and improve MS outcomes. In this study, we evaluated the effect of melatonin, at both physiological and pharmacological doses, on oligodendrocytes metabolism in an experimental autoimmune encephalomyelitis (EAE) mouse model of MS. Results showed that melatonin decreased neurological disability scores and enhanced remyelination, significantly increasing myelin protein levels including MBP, MOG, and MOBP. In addition, melatonin attenuated inflammation by reducing pro-inflammatory cytokines (IL-1β and TNF-α) and increasing anti-inflammatory cytokines (IL-4 and IL-10). Moreover, melatonin significantly increased brain concentrations of lactate, N-acetylaspartate (NAA), and 3-hydroxy-3-methylglutaryl-coenzyme-A reductase (HMGCR). Pyruvate dehydrogenase kinase-4 (PDK-4) mRNA and protein expression levels were also increased in melatonin-treated, compared to untreated EAE mice. However, melatonin significantly inhibited active and total pyruvate dehydrogenase complex (PDC), an enzyme under the control of PDK4. In summary, although PDC activity was reduced by melatonin, it caused a reduction in inflammatory mediators while stimulating oligodendrogenesis, suggesting that oligodendrocytes are forced to use an alternative pathway to synthesize fatty acids for remyelination. We propose that combining melatonin and PDK inhibitors may provide greater benefits for MS patients than the use of melatonin therapy alone.

Altered melatonin MT1 receptor expression in the ventral midbrain following 6-hydroxydopamine lesions in the rat medial forebrain bundle

The indoleamine hormone melatonin protects dopamine neurons in the rat nigrostriatal pathway following 6-hydroxydopamine lesioning, and an increase in striatal melatonin levels has been detected in this model of Parkinson's disease. Melatonin induces the expression of tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, in the ventral midbrain, where G protein-coupled melatonin receptors are present. Based on the interaction between the melatonergic and dopaminergic systems, we hypothesized that 6-hydroxydopamine-induced degeneration of dopamine neurons would affect the expression of melatonin receptors in the nigrostriatal pathway. Following unilateral injection of 6-hydroxydopamine into the rat striatum or medial forebrain bundle, there was a significant increase in apomorphine-induced contralateral rotations in lesioned animals as compared to sham controls. A loss of tyrosine hydroxylase immunoreactivity and/or immunofluorescence in the striatum and substantia nigra was seen in animals lesioned in either the striatum or medial forebrain bundle, indicating degeneration of dopamine neurons. There were no significant differences in melatonin MT₁ receptor protein expression in the striatum or substantia nigra, between intrastriatally lesioned animals and sham controls. In contrast, lesions in the medial forebrain bundle caused a significant increase in MT₁ receptor mRNA expression (p<0.03) on the lesioned side of the ventral midbrain, as compared with the contralateral side. Given the presence of MT₁ receptors on neurons in the ventral midbrain, these results suggest that a compensatory increase in MT₁ transcription occurs to maintain expression of this receptor and neuroprotective melatonergic signaling in the injured brain.

Common variants in DRD2 are associated with sleep duration: the CARe consortium

Sleep duration is implicated in the etiologies of chronic diseases and premature mortality. However, the genetic basis for sleep duration is poorly defined. We sought to identify novel genetic components influencing sleep duration in a multi-ethnic sample. Meta-analyses were conducted of genetic associations with self-reported, habitual sleep duration from seven Candidate Gene Association Resource (CARe) cohorts of over 25 000 individuals of African, Asian, European and Hispanic American ancestry. All individuals were genotyped for ∼50 000 SNPs from 2000 candidate heart, lung, blood and sleep genes. African-Americans had additional genome-wide genotypes. Four cohorts provided replication. A SNP (rs17601612) in the dopamine D2 receptor gene (DRD2) was significantly associated with sleep duration (P = 9.8 × 10(-7)). Conditional analysis identified a second DRD2 signal with opposite effects on sleep duration. In exploratory analysis, suggestive association was observed for rs17601612 with polysomnographically determined sleep latency (P = 0.002). The lead DRD2 signal was recently identified in a schizophrenia GWAS, and a genetic risk score of 11 additional schizophrenia GWAS loci genotyped on the IBC array was also associated with longer sleep duration (P = 0.03). These findings support a role for DRD2 in influencing sleep duration. Our work motivates future pharmocogenetics research on alerting agents such as caffeine and modafinil that interact with the dopaminergic pathway and further investigation of genetic overlap between sleep and neuro-psychiatric traits.

Activation of Melatonin Receptors Reduces Relapse-Like Alcohol Consumption

Melatonin is an endogenous synchronizer of biological rhythms and a modulator of physiological functions and behaviors of all mammals. Reduced levels of melatonin and a delay of its nocturnal peak concentration have been found in alcohol-dependent patients and rats. Here we investigated whether the melatonergic system is a novel target to treat alcohol addiction. Male Wistar rats were subjected to long-term voluntary alcohol consumption with repeated abstinence phases. Circadian drinking rhythmicity and patterns were registered with high temporal resolution by a drinkometer system and analyzed by Fourier analysis. We examined potential antirelapse effect of the novel antidepressant drug agomelatine. Given that agomelatine is a potent MT1 and MT2 receptor agonist and a 5-HT2C antagonist we also tested the effects of melatonin itself and the 5-HT2C antagonist SB242084. All drugs reduced relapse-like drinking. Agomelatine and melatonin administered at the end of the light phase led to very similar changes on all measures of the post-abstinence drinking behavior, suggesting that effects of agomelatine on relapse-like behavior are mostly driven by its melatonergic activity. Both drugs caused a clear phase advance in the diurnal drinking pattern when compared with the control vehicle-treated group and a reduced frequency of approaches to alcohol bottles. Melatonin given at the onset of the light phase had no effect on the circadian phase and very small effects on alcohol consumption. We conclude that targeting the melatonergic system in alcohol-dependent individuals can induce a circadian phase advance, which may restore normal sleep architecture and reduce relapse behavior.

The melatonergic system in mood and anxiety disorders and the role of agomelatine: implications for clinical practice

Melatonin exerts its actions through membrane MT1/MT2 melatonin receptors, which belong to the super family of G-protein-coupled receptors consisting of the typical seven transmembrane domains. MT1 and MT2 receptors are expressed in various tissues of the body either as single ones or together. A growing literature suggests that the melatonergic system may be involved in the pathophysiology of mood and anxiety disorders. In fact, some core symptoms of depression show disturbance of the circadian rhythm in their clinical expression, such as diurnal mood and other symptomatic variation, or are closely linked to circadian system functioning, such as sleep-wake cycle alterations. In addition, alterations have been described in the circadian rhythms of several biological markers in depressed patients. Therefore, there is interest in developing antidepressants that have a chronobiotic effect (i.e., treatment of circadian rhythm disorders). As melatonin produces chronobiotic effects, efforts have been aimed at developing agomelatine, an antidepressant with melatonin agonist activity. The present paper reviews the role of the melatonergic system in the pathophysiology of mood and anxiety disorders and the clinical characteristics of agomelatine. Implications of agomelatine in "real world" clinical practice will be also discussed.

Sleep and circadian rhythm dysregulation in schizophrenia

Sleep-onset and maintenance insomnia is a common symptom in schizophrenic patients regardless of either their medication status (drug-naive or previously treated) or the phase of the clinical course (acute or chronic). Regarding sleep architecture, the majority of studies indicate that non-rapid eye movement (NREM), N3 sleep and REM sleep onset latency are reduced in schizophrenia, whereas REM sleep duration tends to remain unchanged. Many of these sleep disturbances in schizophrenia appear to be caused by abnormalities of the circadian system as indicated by misalignments of the endogenous circadian cycle and the sleep-wake cycle. Circadian disruption, sleep onset insomnia and difficulties in maintaining sleep in schizophrenic patients could be partly related to a presumed hyperactivity of the dopaminergic system and dysfunction of the GABAergic system, both associated with core features of schizophrenia and with signaling in sleep and wake promoting brain regions. Since multiple neurotransmitter systems within the CNS can be implicated in sleep disturbances in schizophrenia, the characterization of the neurotransmitter systems involved remains a challenging dilemma.

Effect of chronic L-dopa or melatonin treatments after dopamine deafferentation in rats: dyskinesia, motor performance, and cytological analysis

The present study examines the ability of melatonin to protect striatal dopaminergic loss induced by 6-OHDA in a rat model of Parkinson's disease, comparing the results with L-DOPA-treated rats. The drugs were administered orally daily for a month, their therapeutic or dyskinetic effects were assessed by means of abnormal involuntary movements (AIMs) and stepping ability. At the cellular level, the response was evaluated using tyrosine hydroxylase immunoreactivity and striatal ultrastructural changes to compare between L-DOPA-induced AIMs and Melatonin-treated rats. Our findings demonstrated that chronic oral administration of Melatonin improved the alterations caused by the neurotoxin 6-OHDA. Melatonin-treated animals perform better in the motor tasks and had no dyskinetic alterations compared to L-DOPA-treated group. At the cellular level, we found that Melatonin-treated rats showed more TH-positive neurons and their striatal ultrastructure was well preserved. Thus, Melatonin is a useful treatment to delay the cellular and behavioral alterations observed in Parkinson's disease.

Analysis of genetic variations in the human melatonin receptor (MTNR1A, MTNR1B) genes and antipsychotics-induced tardive dyskinesia in schizophrenia

OBJECTIVES

Antipsychotics-induced tardive dyskinesia (TD) has been suggested to be related to altered dopaminergic neurotransmission in the striatum. Melatonin has a modulating effect on dopaminergic neurotransmission in the brain; therefore, the hypothesis of an association between the melatonin receptor genes (MTNR1A, MTNR1B) and antipsychotics-induced TD was examined in this study.

METHODS

Schizophrenic inpatients receiving long-term antipsychotic treatment were assessed using the Abnormal Involuntary Movement Scale, and only patients who were either free of any abnormal involuntary movement (non-TD group) or who demonstrated persistent TD (TD group) were enrolled. Genotyping of six tagging single nucleus polymorphisms (SNPs) in the melatonin receptor genes (MRNR1A, MTNR1B) was then performed for each subject.

RESULTS

Four hundred and eighteen inpatients (TD=256, non-TD=162) fitted the study criteria and underwent TD assessment and genotyping. Individual haplotype analysis showed that the haplotype ATG was significantly associated with non-TD (permutation P=0.037), and the association was also found to be significant by global haplotype analyses (permutation P=0.045).

CONCLUSIONS

Our results indicated a significant association between the haplotype ATG in the MTNR1A gene and non-TD. Further replication in other countries or other populations is indicated.

Mechanisms involved in the antinociception caused by melatonin in mice

The present study assesses the antinociceptive effect of melatonin in chemical behavioral models of nociception and investigates some of the mechanisms underlying this effect. Melatonin administered by intraperitoneal (i.p., 10-100 mg/kg), intracerebroventricular (i.c.v., 250-500 pmol/site) and intraplantar (i.pl., 30-100 ng/i.pl.) routes, reduced in a dose-dependent manner the nociception caused by i.pl. injection of glutamate (10 micromol/paw), with mean ID50 values of 32.6 mg/kg, 200 pmol/site and 59 ng/i.pl., respectively. Furthermore, melatonin in the dose range of 10-100 mg/kg, i.p., reduced the neurogenic pain caused by i.pl. injection of capsaicin (5.2 nmol/paw) with inhibition of 48 +/- 4%. The antinociceptive effect of melatonin (100 mg/kg, i.p.) on glutamate-induced nociception was completely prevented by the pretreatment of animals with naloxone (a nonselective opioid receptor antagonist, 1 mg/kg, i.p.), ketanserin (a preferential 5-HT2A receptor antagonist, 1 mg/kg, i.p.), sulpiride (a D2 receptor antagonist, 50 mg/kg, i.p.), L-arginine (a precursor of nitric oxide, 600 mg/kg, i.p.), yohimbine (an alpha2-adrenoceptor antagonist, 0.15 mg/kg, i.p.) and luzindole (a preferential MT2 receptor antagonist, 10 mg/kg, i.p.), but was not affected by the pretreatment with D-arginine (an inactive isomer of L-arginine, 600 mg/kg, i.p.), prazosin (an alpha1-adrenoceptor antagonist, 0.15 mg/kg, i.p.) or after bilateral adrenalectomy. Collectively, present results suggest that melatonin produces peripheral and central antinociception when assessed on capsaicin- or glutamate-induced pain in mice through mechanisms that are likely mediated by interaction with plasma membrane-bound melatonin receptors and modulated by opioid, serotonergic (5-HT2A receptors), dopaminergic (D2-receptors), adrenergic (alpha2-adrenoceptors) systems as well as the L-arginine-nitric oxide pathway.

Effects of haloperidol and melatonin on the in situ activity of nigrostriatal tyrosine hydroxylase in male Syrian hamsters

Haloperidol, an antipsychotic drug, was tested for its effects on the in situ activity of nigrostriatal and hypothalamic tyrosine hydroxylase, in control male Syrian hamsters and in those receiving a high daily dose of melatonin. After receiving daily ip injections (1.25 mg/kg ip) of haloperidol for 21 days, the animals were sacrificed and brain tissue collected for analysis of dopamine and metabolites by HPLC with electrochemical detection. In situ activity of tyrosine hydroyxlase (TH) activity was determined by measuring the accumulation of L-Dopa after administration of the L amino acid decarboxylase inhibitor, mhydroxybenzylhydrazine. Tissue content of dopamine and its metabolites, DOPAC and HVA, was depressed in striatum of animals receiving haloperidol, and tyrosine hydroxylase (TH) activity was significantly decreased 20-24 h after the last injection (from 1823 +/- 63 to 1139 +/- 85 pg l-dopa/mg tissue). The decrease in TH activity in striatum was significantly inhibited by daily injections of a high dose of melatonin (2.5 mg/kg ip) (from 1139 +/- 85 to 1560 +/- 116 pg L-dopa/mg tissue). In the substantia nigra and in the hypothalamus, on the other hand, haloperidol significantly increased the activity of tyrosine hydroxylase. Melatonin administration did not significantly influence TH activity in the substantia nigra, but inhibited TH activity in the hypothalamus and in the pontine brainstem. One explanation for these data is that chronic haloperidol administration in Syrian hamsters increases TH activity in hypothalamus and substantia nigra, but decreases TH activity in striatum by a mechanism involving D2 presynaptic receptors and a melatonin sensitive kinase which regulates TH phosphorylation.

The regional and cellular expression profile of the melatonin receptor MT1 in the central dopaminergic system

The physiological effects of pineal melatonin are primarily mediated by melatonin receptors located in the brain and periphery. Even though there are a number of studies demonstrating the regulatory role of melatonin in the development of dopaminergic behaviors, such as psychostimulant-induced diurnal locomotor sensitization or drug seeking, little is known about the contribution of melatonin receptors (i.e., MT1) to this role. Therefore, as a first step in understanding the functional role of melatonin receptors in dopaminergic behaviors, we focused on determining the expression pattern of MT1 receptors in the dopaminergic system of the human and rodent brain. Regional (e.g., nucleus accumbens shell) and cellular (e.g., tyrosine hydroxylase immunopositive cells) expression of MT1 mRNA was characterized by applying the immuno-laser capture microdissection (immuno-LCM) technique coupled with nested RT-PCR. Moreover, employing quantitative Western immunoblotting and RT-PCR, we found that the mouse MT1 receptor expression presents diurnal variations (i.e., low mRNA and high protein levels at night, ZT21). The dopaminergic system-based presence of MT1 receptor proteins was not limited to rodents; we found these receptors in postmortem human brain as well. Further research is needed to understand the regional/cellular functional role of melatonin receptors in the regulation of dopaminergic behaviors, using models such as melatonin receptor knockout mice.

Circadian-dependent effect of melatonin on dopaminergic D2 antagonist-induced hypokinesia and agonist-induced stereotypies in rats

Although a melatonin/dopamine relationship has been well established in nonmotor systems wherein dopamine and melatonin share an antagonist relationship, less clear is the role melatonin may play in extrapyramidal dopaminergic function. Therefore, the purpose of the present experiments was to examine the relationship between melatonin and the dopaminergic D2 receptor system and behavior. Hypokinesia was induced in male Sprague-Dawley rats with fluphenazine (D2 antagonist, 0.4 mg/kg ip) and stereotypies with apomorphine (D2 agonist, 0.6 mg/kg sc) during the light (1200 h) and dark (2200 h) phases. As expected, fluphenazine induced severe hypokinesia during the light phase (482 +/- 176 s); however, unexpectedly, fluphenazine-induced hypokinesia during the dark was almost nonexistent (25 +/- 6 s). Furthermore, melatonin treatment (30 mg/kg ip) produced a strong interaction with fluphenazine in that it reduced fluphenazine-induced hypokinesia by nearly 80% in the light (112 +/- 45 s) but paradoxically increased the minimal fluphenazine-induced hypokinesia in the dark by more than 60% (70 +/- 17 s). Melatonin also reduced apomorphine-induced stereotypies by nearly 40% in the light but had no effect in the dark. Taken together, these data show (1) a strong and unexpected nocturnal effect of fluphenazine on hypokinesia and (2) provide support for an antagonistic melatonin/dopaminergic interaction in the context of motor behavior and D2 receptor function which appears to be critically dependent on the light/dark status of the dopaminergic system.

Study of circadian melatonin secretion pattern at different stages of Parkinson's disease

To explore changes in melatonin secretion patterns and biologic rhythms in Parkinson's disease patients with or without levodopa-related motor complications (LDRMCs), the authors investigated, in an observational study, circadian rhythms of central temperature, motor activity, plasma cortisol, and melatonin in three groups: de novo untreated patients (group I), patients treated with levodopa + dopamine agonist and without LDRMCs (group II), and patients treated with levodopa + dopamine agonist and with LDRMCs (group III). There were no differences among the three groups for the rhythm of temperature, motor activity, or plasma cortisol. There was a significant (p < 0.05) phase advance in plasma melatonin secretion in patients receiving a dopaminergic treatment compared with untreated patients. The daytime area under the curve (AUC) was increased significantly in group III, and the nighttime AUC-to-daytime AUC ratio of melatonin secretion decreased significantly in group III, suggesting that the nychthemeral pattern of melatonin secretion was changed in patients with LDRMCs. Comparison of the three groups suggests a slight but insignificant phase advance and amplitude decrease of circadian melatonin secretion related to both evolution and treatment of Parkinson's disease. Despite the lack of a global desynchronization in other circadian biologic rhythms, the circadian secretion pattern of melatonin is modified in patients with LDRMCs.

Melatonin alters behavior and cAMP levels in nucleus accumbens induced by cocaine treatment

This study describes the effects of melatonin on cocaine-induced anxiety-like behavior and nucleus accumbens (NAc) cAMP levels in rats. Animals drinking a solution of melatonin (200 ng/ml) at night, either during repeated cocaine administration (15 mg/kg i.p., twice a day for 9 days) or during its withdrawal, showed less anxiety-like behavior in a defensive withdrawal paradigm 48 h after the last injection of cocaine. Melatonin did not alter behavior in control rats treated with saline. Animals exposed for 1 week to unrestricted free-choice oral melatonin self-administration (200 ng/ml) did not show preference for the drinking solution containing melatonin. Pretreatment with melatonin (200 ng/kg i.p. or 200 ng/ml orally) significantly attenuated the augmentation of cAMP levels in NAc following acute cocaine administration (15 mg/kg i.p.). Taken together, these results suggest that a low-dose night-time melatonin treatment results in anxiolytic-like effects in rats withdrawn from repeated cocaine administration, can antagonize cocaine-induced activation of NAc cAMP levels and has low dependence liability.

Circadian rhythms of dopamine and dihydroxyphenyl acetic acid in the mouse striatum: effects of pinealectomy and of melatonin treatment

The existence of dopamine (DA)-melatonin (aMT) relationships is well documented in several brain areas of the mammalian central nervous system such as the retina and hypothalamus or the nigrostriatal system. For instance, aMT tempers 1 methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal damage in C57BL/6 mice. In this mouse strain however, rhythmic production of aMT and its possible interaction with striatal DA is still unclear. In the present work we investigated circadian variations in pineal production of aMT and striatal DA levels in C57BL/6 mice. Effects of pinealectomy and aMT administration were also assessed. Intact, pinealectomized and pinealectomized + aMT-treated mice and their respective control groups were sacrificed at six different times during the 24-hour period. In control animals, aMT displayed a circadian rhythm with a narrow peak at midnight. The peak of aMT coincided with the nadir of the DA rhythm present in the striatum. Shortly after the decrease of DA levels, an increase in 3,4-dihydroxyphenylacetic acid (DOPAC), the main DA metabolite, was observed. The rhythmic changes of DA and DOPAC levels in the striatum were blunted by pinealectomy, whereas administration of aMT (0.1-10 mg/kg) during 6 days to pinealectomized mice restored the rhythms in a dose-dependent manner. Striatal levels of 3-methoxytyramine and homovanillic acid did not change during the 24-hour cycle. The serotonergic system, assessed by the determination of 5-hydroxytryptamine and 5-hydroxyindole-3-acetic acid concentration in striatum, did not show significant time-dependent changes in control animals and was not affected by pinealectomy or aMT treatment. These data substantiate the existence of a link between pineal function, melatonin secretion and DA circadian rhythm in the mouse striatum.

Effects of maternal-melatonin treatment on open-field behaviors and hypertensive phenotype in spontaneously hypertensive rats' (SHR) offspring

Effects of maternal-melatonin treatment in spontaneously hypertensive rats (SHR) were investigated in their offspring. Pregnant SHR were given drinking water with/without melatonin (20 micrograms melatonin/ml tap water) during pregnancy and the lactation period. Maternal-melatonin treatment did not cause changes in body weights during 7 to 27 weeks. Melatonin administration up to weaning period via mother caused a decrease in systolic blood pressure (BP) during 11 to 27 weeks in their offspring compared with those of control group. Open-field behaviors in the offspring were observed at 24 weeks age. Both the control and treatment groups had ratios of central and peripheral locomotion of 30% and 70%, respectively. The treatment group exhibited less total locomotor activity and rearing than the control group did, whereas more latency was exhibited in the treatment group compared with that of the control group. These findings suggest that maternal-melatonin administration may modify open-field behaviors as well as the hypertensive phenotype in their progeny.

Endogenous melatonin and epileptogenesis: facts and hypothesis

In human epilepsy, diurnal variation in seizure phenomena suggests the involvement of a time-dependent biological signal. Clinical evidence indicates that in some cases, temporal clustering of epileptic seizures is in phase with the nocturnal rise in circulating melatonin. Although this hormone has been reported to stabilize the brain against seizure-producing stimuli, these pharmacological doses are not representative of physiological conditions but would nonetheless facilitate widespread inhibitory neurotransmission characteristic of traditional anticonvulsants. Instead, it is proposed that endogenous melatonin contributes to epileptiform activity through inhibitory actions on dopaminergic activity. Dopamine is considered a natural downregulator of seizure activity in a number of species, including humans, and numerous lines of evidence suggest that melatonin is capable of stimulating a decrease in dopamine output within areas of the brain thought to participate in the control of epileptic seizures. Pharmacological manipulation of the endogenous melatonin rhythm may provide a useful therapeutic strategy against the occurrence of seizures during increased hormone production.