Melatonin modulates [3h]serotonin release in the rat hippocampus: effects of circadian rhythm

Effect of high-endurance exercise intervention on sleep-dependent procedural memory consolidation in individuals with schizophrenia: a randomized controlled trial

BACKGROUND

Little is known about the effects of physical exercise on sleep-dependent consolidation of procedural memory in individuals with schizophrenia. We conducted a randomized controlled trial (RCT) to assess the effectiveness of physical exercise in improving this cognitive function in schizophrenia.

METHODS

A three-arm parallel open-labeled RCT took place in a university hospital. Participants were randomized and allocated into either the high-intensity-interval-training group (HIIT), aerobic-endurance exercise group (AE), or psychoeducation group for 12 weeks, with three sessions per week. Seventy-nine individuals with schizophrenia spectrum disorder were contacted and screened for their eligibility. A total of 51 were successfully recruited in the study. The primary outcome was sleep-dependent procedural memory consolidation performance as measured by the finger-tapping motor sequence task (MST). Assessments were conducted during baseline and follow-up on week 12.

RESULTS

The MST performance scored significantly higher in the HIIT (n = 17) compared to the psychoeducation group (n = 18) after the week 12 intervention (p < 0.001). The performance differences between the AE (n = 16) and the psychoeducation (p = 0.057), and between the AE and the HIIT (p = 0.999) were not significant. Yet, both HIIT (p < 0.0001) and AE (p < 0.05) showed significant within-group post-intervention improvement.

CONCLUSIONS

Our results show that HIIT and AE were effective at reverting the defective sleep-dependent procedural memory consolidation in individuals with schizophrenia. Moreover, HIIT had a more distinctive effect compared to the control group. These findings suggest that HIIT may be a more effective treatment to improve sleep-dependent memory functions in individuals with schizophrenia than AE alone.

Night shift hormone: How does melatonin affect depression?

Melatonin is the main hormone secreted by the pineal gland that modulates the circadian rhythm and mood. Previous studies have shown the therapeutic effects of melatonin, or its important analogue, agomelatine, on depression. In this review study, we aimed to discuss the potential mechanisms of melatonin involved in the treatment of depression. It was noted that disrupted circadian rhythm can lead to depressive state, and melatonin via regulating circadian rhythm shows a therapeutic effect. It was also noted that melatonin induces antidepressant effects via promoting antioxidant system and neurogenesis, and suppressing oxidative stress, neuroinflammation, and apoptosis. The interaction effect between melatonin or agomelatine and serotonergic signaling has a significant effect on depression. It was noted that the psychotropic effects of agomelatine are induced by the synergistic interaction between melatonin and 5-HT_2C receptors. Agomelatine also interacts with glutamatergic signaling in brain regions involved in regulating mood and circadian rhythm. Interestingly, it was concluded that melatonin exerts both pro- and anti-inflammatory effects, depending on the grade of inflammation. It was suggested that synergistic interaction between melatonin and 5-HT_2C receptors may be able to induce therapeutic effects on other psychiatric disorders. Furthermore, dualistic role of melatonin in regulating inflammation is an important point that can be examined at different levels of inflammation in animal models of depression.

Chronic Piromelatine Treatment Alleviates Anxiety, Depressive Responses and Abnormal Hypothalamic-Pituitary-Adrenal Axis Activity in Prenatally Stressed Male and Female Rats

The prenatal stress (PNS) model in rodents can induce different abnormal responses that replicate the pathophysiology of depression. We applied this model to evaluate the efficacy of piromelatine (Pir), a novel melatonin analog developed for the treatment of insomnia, in male and female offspring. Adult PNS rats from both sexes showed comparable disturbance associated with high levels of anxiety and depressive responses. Both males and females with PNS demonstrated impaired feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis compared to the intact offspring and increased glucocorticoid receptors in the hippocampus. However, opposite to female offspring, the male PNS rats showed an increased expression of mineralocorticoid receptors in the hippocampus. Piromelatine (20 mg/kg, i.p., for 21 days injected from postnatal day 60) attenuated the high anxiety level tested in the open field, elevated plus-maze and light-dark test, and depressive-like behavior in the sucrose preference and the forced swimming tests in a sex-specific manner. The drug reversed to control level stress-induced increase of plasma corticosterone 120 min later in both sexes. Piromelatine also corrected to control level the PNS-induced alterations of corticosteroid receptors only in male offspring. Our findings suggest that the piromelatine treatment exerts beneficial effects on impaired behavioral responses and dysregulated HPA axis in both sexes, while it corrects the PNS-induced changes in the hippocampal corticosteroid receptors only in male offspring.

Activation of the glucocorticoid receptor rapidly triggers calcium-dependent serotonin release in vitro

Aims

Glucocorticoids rapidly provoke serotonin (5‐HT) release in vivo. We aimed to investigate molecular mechanisms of glucocorticoid receptor (GR)‐triggered 5‐HT release.

Methods

Employing 1C11 cells to model 5‐HT neurotransmission, immunofluorescence and Pearson's Correlation Coefficient were used to analyze colocalization of GR, 5‐HT, vesicle membrane protein synaptotagmin 1 and vesicle dye FM4‐64FX. FFN511 and FM4‐64FX dyes as well as calcium imaging were used to visualize vesicular 5‐HT release upon application of GR agonist dexamethasone, GR antagonist mifepristone and voltage‐gated calcium channel (VGCC) inhibitors.

Results

GR, 5‐HT, synaptotagmin 1 and FM4‐64FX showed overlapping staining patterns, with Pearson's Correlation Coefficient indicating colocalization. Similarly to potassium chloride, dexamethasone caused a release of FFN511 and uptake of FM4‐64FX, indicating vesicular 5‐HT release. Mifepristone, calcium depletion and inhibition of L‐type VGCC significantly diminished dexamethasone‐induced vesicular 5‐HT release.

Conclusions

In close proximity to 5‐HT releasing sites, activated GR rapidly triggers L‐type VGCC‐dependent vesicular 5‐HT release. These findings provide a better understanding of the interrelationship between glucocorticoids and 5‐HT release.

Sleep well. Untangling the role of melatonin MT1 and MT2 receptors in sleep

The pharmacological potential of targeting selectively melatonin MT1 or MT2 receptors has not yet been exploited in medicine. Research using selective MT1/MT2 receptor ligands and MT1/MT2 receptor knockout mice has indicated that the activation of MT2 receptors selectively increases non-rapid eye movement (NREM) sleep whereas MT1 receptors seem mostly implicated in the regulation of REM sleep. Moreover, MT1 knockout mice show an increase in NREM sleep, while MT2 knockout a decrease, suggesting an opposite role of these two receptors. A recent paper in mice by Sharma et al (J Pineal Res, 2018, e12498) found that MT1 but not MT2 receptors are expressed on orexin neurons in the perifornical lateral hypothalamus (PFH). Moreover, after injecting melatonin or luzindole into the mouse PFH, the authors suggest that melatonin promotes NREM sleep because activates PFH MT1 receptors, which in turn inhibit orexin neurons that are important in promoting arousal and maintaining wakefulness. In this commentary, we have critically commented on some of these findings on the bases of previous literature. In addition, we highlighted the fact that no conclusions could be drawn on the melatonin receptor subtype mediating the effects of melatonin on sleep because the authors used the non-selective MT1/MT2 receptors antagonist luzindole. More solid research should further characterize the pharmacological function of these two melatonin receptors in sleep.

Differential Function of Melatonin MT1 and MT2 Receptors in REM and NREM Sleep

The pathophysiological function of the G-protein coupled melatonin MT1 and MT2 receptors has not yet been well-clarified. Recent advancements using selective MT1/ MT2 receptor ligands and MT1/MT2 receptor knockout mice have suggested that the activation of the MT1 receptors are mainly implicated in the regulation of rapid eye movement (REM) sleep, whereas the MT2 receptors selectively increase non-REM (NREM) sleep. Studies in mutant mice show that MT1 knockout mice have an increase in NREM sleep and a decrease in REM sleep, while MT2 knockout mice a decrease in NREM sleep. The localization of MT1 receptors is also distinct from MT2 receptors; for example, MT2 receptors are located in the reticular thalamus (NREM area), while the MT1 receptors in the Locus Coeruleus and lateral hypothalamus (REM areas). Altogether, these findings suggest that these two receptors not only have a very specialized function in sleep, but that they may also modulate opposing effects. These data also suggest that mixed MT1-MT2 receptors ligands are not clinically recommended given their opposite roles in physiological functions, confirmed by the modest effects of melatonin or MT1/MT2 non-selective agonists when used in both preclinical and clinical studies as hypnotic drugs. In sum, MT1 and MT2 receptors have specific roles in the modulation of sleep, and consequently, selective ligands with agonist, antagonist, or partial agonist properties could have therapeutic potential for sleep; while the MT2 agonists or partial agonists might be indicated for NREM-related sleep and/or anxiety disorders, the MT1 agonists or partial agonists might be so for REM-related sleep disorders. Furthermore, MT1 but not MT2 receptors seem involved in the regulation of the circadian rhythm. Future research will help further develop MT1 and/or MT2 receptors as targets for neuropsychopharmacology drug development.

Repetitive Neonatal Erythropoietin and Melatonin Combinatorial Treatment Provides Sustained Repair of Functional Deficits in a Rat Model of Cerebral Palsy

Cerebral palsy (CP) is the leading cause of motor impairment for children worldwide and results from perinatal brain injury (PBI). To test novel therapeutics to mitigate deficits from PBI, we developed a rat model of extreme preterm birth (<28 weeks of gestation) that mimics dual intrauterine injury from placental underperfusion and chorioamnionitis. We hypothesized that a sustained postnatal treatment regimen that combines the endogenous neuroreparative agents erythropoietin (EPO) and melatonin (MLT) would mitigate molecular, sensorimotor, and cognitive abnormalities in adults rats following prenatal injury. On embryonic day 18 (E18), a laparotomy was performed in pregnant Sprague–Dawley rats. Uterine artery occlusion was performed for 60 min to induce placental insufficiency via transient systemic hypoxia-ischemia, followed by intra-amniotic injections of lipopolysaccharide, and laparotomy closure. On postnatal day 1 (P1), approximately equivalent to 30 weeks of gestation, injured rats were randomized to an extended EPO + MLT treatment regimen, or vehicle (sterile saline) from P1 to P10. Behavioral assays were performed along an extended developmental time course (n = 6–29). Open field testing shows injured rats exhibit hypermobility and disinhibition and that combined neonatal EPO + MLT treatment repairs disinhibition in injured rats, while EPO alone does not. Furthermore, EPO + MLT normalizes hindlimb deficits, including reduced paw area and paw pressure at peak stance, and elevated percent shared stance after prenatal injury. Injured rats had fewer social interactions than shams, and EPO + MLT normalized social drive. Touchscreen operant chamber testing of visual discrimination and reversal shows that EPO + MLT at least partially normalizes theses complex cognitive tasks. Together, these data indicate EPO + MLT can potentially repair multiple sensorimotor, cognitive, and behavioral realms following PBI, using highly translatable and sophisticated developmental testing platforms.

An earlier time of scan is associated with greater threat-related amygdala reactivity

Time-dependent variability in mood and anxiety suggest that related neural phenotypes, such as threat-related amygdala reactivity, may also follow a diurnal pattern. Here, using data from 1,043 young adult volunteers, we found that threat-related amygdala reactivity was negatively coupled with time of day, an effect which was stronger in the left hemisphere (β = -0.1083, p-fdr = 0.0012). This effect was moderated by subjective sleep quality (β = -0.0715, p-fdr = 0.0387); participants who reported average and poor sleep quality had relatively increased left amygdala reactivity in the morning. Bootstrapped simulations suggest that similar cross-sectional samples with at least 300 participants would be able to detect associations between amygdala reactivity and time of scan. In control analyses, we found no associations between time and V1 activation. Our results provide initial evidence that threat-related amygdala reactivity may vary diurnally, and that this effect is potentiated among individuals with average to low sleep quality. More broadly, our results suggest that considering time of scan in study design or modeling time of scan in analyses, as well as collecting additional measures of circadian variation, may be useful for understanding threat-related neural phenotypes and their associations with behavior, such as fear conditioning, mood and anxiety symptoms, and related phenotypes.

The antidepressant effect of melatonin and fluoxetine in diabetic rats is associated with a reduction of the oxidative stress in the prefrontal and hippocampal cortices

In the past few years possible mechanisms that link diabetes and depression have been found. One of these mechanisms is the increase in lipid peroxidation and decrease in antioxidant activity in the hippocampal and prefrontal cortices, which are brain areas involved in mood. The goal of the present study was to evaluate the effect of an antidepressant and of an antioxidant on behavior and oxidative activity in brains of diabetic rats. Rats rendered diabetic after a treatment with streptozotocin (STZ) (60mg/kg) were treated with fluoxetine (15mg/kg), melatonin (10mg/kg), or vehicle for 4 weeks. All animals were tested for signs of depression and anxiety using the elevated plus maze (EPM), open field test (OFT) and the forced swim test (FST). Four groups were compared: (1) normoglycemic, (2) hyperglycemic vehicle treated, and hyperglycemic (3) fluoxetine or (4) melatonin treated rats. On the last day of the study, blood samples were obtained to determine the levels of hemoglobin A1c (HbA1c). Also, brain samples were collected to measure the oxidative stress in the hippocampal and prefrontal cortices using the thiobarbituric acid reactive substances (TBARS) assay. The activity of the antioxidant enzymes catalase (CAT), glutathione peroxidase (GPx), and glutathione S-transferase (GST) were also measured on the brain samples. The results show that both fluoxetine and melatonin decrease the signs of depression and anxiety in all tests. Concomitantly, the levels of HbA1c were reduced in drug treated rats, and to a greater degree in the fluoxetine group. In the cerebral cortex of diabetic rats, TBARS was increased, while the activity of CAT, GPx and GST were decreased. Fluoxetine and melatonin treatments decreased TBARS in both cortices. In the prefrontal cortex, fluoxetine and melatonin restored the activity of CAT, while only melatonin improved the activity of GPx and GST. In the hippocampus, the activity of GPx alone was restored by melatonin, while fluoxetine had no effect. These results suggest that antidepressants and antioxidants can counter the mood and oxidative disorders associated with diabetes. While these effects could result from a decreased production of reactive oxygen species (ROS) remains to be established.

Piromelatine exerts antinociceptive effect via melatonin, opioid, and 5HT1A receptors and hypnotic effect via melatonin receptors in a mouse model of neuropathic pain

RATIONALE

An effective and safe treatment of insomnia in patients with neuropathic pain remains an unmet need. Melatonin and its analogs have been shown to have both analgesic and hypnotic effects; however, capacity of them on sleep disturbance with neuropathic pain as well as the precise mechanism is unclear.

OBJECTIVE

The present study evaluated effects of piromelatine, a novel melatonin receptor agonist, on sleep disturbance in a neuropathic pain-like condition as well as the underlying mechanisms.

METHODS

A mouse model of chronic neuropathic pain induced by partial sciatic nerve ligation (PSL) was employed. The antinociceptive and hypnotic effects of piromelatine were evaluated by measurement of thermal hyperalgesia, mechanical allodynia, and electroencephalogram (EEG) recordings in PSL mice. Pharmacological approaches were used to clarify the mechanisms of action of piromelatine.

RESULTS

PSL significantly lowered thermal and mechanical latencies and decreased non-rapid eye movement (NREM) sleep, and PSL mice exhibited sleep fragmentation. Treatment with 25, 50, or 100 mg/kg of piromelatine significantly prolonged thermal and mechanical latencies and increased NREM sleep. Moreover, the antinociceptive effect of piromelatine was prevented by melatonin antagonist luzindole, opioid receptor antagonist naloxone, or 5HT1A receptor antagonist WAY-100635. The hypnotic effect of piromelatine was blocked by luzindole but neither by naloxone nor WAY-100635.

CONCLUSIONS

These data indicate that piromelatine is an effective treatment for both neuropathic pain and sleep disturbance in PSL mice. The antinociceptive effect of piromelatine is likely mediated by melatonin, opioid, and 5HT1A receptors; however, the hypnotic effect of piromelatine appears to be mediated by melatonin receptors.

Therapeutic effects of melatonin receptor agonists on sleep and comorbid disorders

Several melatonin receptors agonists (ramelteon, prolonged-release melatonin, agomelatine and tasimelteon) have recently become available for the treatment of insomnia, depression and circadian rhythms sleep-wake disorders. The efficacy and safety profiles of these compounds in the treatment of the indicated disorders are reviewed. Accumulating evidence indicates that sleep-wake disorders and co-existing medical conditions are mutually exacerbating. This understanding has now been incorporated into the new Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5). Therefore, when evaluating the risk/benefit ratio of sleep drugs, it is pertinent to also evaluate their effects on wake and comorbid condition. Beneficial effects of melatonin receptor agonists on comorbid neurological, psychiatric, cardiovascular and metabolic symptomatology beyond sleep regulation are also described. The review underlines the beneficial value of enhancing physiological sleep in comorbid conditions.

Treatment with melatonin after status epilepticus attenuates seizure activity and neuronal damage but does not prevent the disturbance in diurnal rhythms and behavioral alterations in spontaneously hypertensive rats in kainate model of temporal lobe epilepsy

Melatonin is involved in the control of circadian and seasonal rhythmicity, possesses potent antioxidant activity, and exerts a neuroprotective and anticonvulsant effect. Spontaneously hypertensive rats (SHRs) are widely accepted as an experimental model of essential hypertension with hyperactivity, deficient sustained attention, and alterations in circadian autonomic profiles. The purpose of the present study was to determine whether melatonin treatment during epileptogenesis can prevent the deleterious consequences of status epilepticus (SE) in SHRs in the kainate (KA) model of temporal lobe of epilepsy (TLE). Spontaneous recurrent seizures (SRSs) were EEG- and video-recorded during and after the treatment protocol. Melatonin (10mg/kg diluted in drinking water, 8weeks) increased the seizure-latent period, decreased the frequency of SRSs, and attenuated the circadian rhythm of seizure activity in SHRs. However, melatonin was unable to affect the disturbed diurnal rhythms and behavioral changes associated with epilepsy, including the decreased anxiety level, depression, and impaired spatial memory. Melatonin reduced neuronal damage specifically in the CA1 area of the hippocampus and piriform cortex and decreased hippocampal serotonin (5-HT) levels both in control and epileptic SHRs. Although long-term melatonin treatment after SE shows a potential to attenuate seizure activity and neuronal loss, it is unable to restore epilepsy-associated behavioral abnormalities in SHRs.

Melatonin protects against diazinon-induced neurobehavioral changes in rats

Diazinon is an organophosphorous pesticide with a prominent toxicity on many body organs. Multiple mechanisms contribute to diazinon-induced deleterious effects. Inhibition of acetyl-cholinesterase, cholinergic hyperstimulation, and formation of reactive oxygen species may play a role. On the other hand, melatonin is a pineal hormone with a well-known potent antioxidant activity and a remarkable modulatory effect on many behavioral processes. The present study revealed that oral diazinon administration (25 mg/kg) increased anxiety behavior in rats subjected to elevated plus maze and open-field tests possibly via the induction of changes in brain monoamines levels (dopamine, norepinephrine, and serotonin). Additionally, brain lipid peroxides measured as malondialdehyde (MDA) and tumor necrosis factor alpha (TNF-α) levels were elevated, while the activity of brain glutathione peroxidase enzyme was reduced by diazinon. Co-administration of oral melatonin (10 mg/kg) significantly attenuated the anxiogenic activity of diazinon, rebalanced brain monoamines levels, decreased brain MDA and TNF-α levels, and increased the activity of brain glutathione peroxidase enzyme.

Prophylactic treatment with melatonin after status epilepticus: effects on epileptogenesis, neuronal damage, and behavioral changes in a kainate model of temporal lobe epilepsy

Melatonin is a potent antioxidant which showed anticonvulsant activities both in experimental and clinical studies. In the present study, we examined the effect of melatonin treatment (10mg/kg/day, diluted in drinking water, 8 weeks) during epileptogenesis on the consequences of a kainate (KA)-induced status epilepticus (SE) in rats. Melatonin increased the latency in the appearance of spontaneous recurrent seizures (SRSs) and decreased their frequency only during the treatment period. The behavioral alterations associated with hyperactivity, depression-like behavior during the light phase, and deficits in hippocampus-dependent working memory were positively affected by melatonin treatment in rats with epilepsy. Melatonin reduced the neuronal damage in the CA1 area of the hippocampus and piriform cortex and recovered the decrease of hippocampal serotonin (5-HT) level in rats with epilepsy. Taken together, long-term melatonin treatment after SE was unable to suppress the development of epileptogenesis. However, it showed a potential in reducing some of the deleterious alterations that develop during the chronic epileptic state in a diurnal phase-dependent mode.

Hippocampal body changes in pure partial onset sleep and pure partial onset waking epileptic patients

The aim of the current study was to evaluate for the first time the hippocampal changes in patients with pure sleep and pure waking epilepsy. A total of 35 patients with pure partial onset sleep epilepsy and 35 patients with pure partial onset waking epilepsy matched for age and sex ratio were enrolled. MR images were analyzed to determine hippocampal body changes. Rounding ratio of hippocampal body was defined as short axis divided by long axis and hippocampal bodies with ratios ≥ 0.70 were considered rounded. Hippocampal sclerosis and atrophy were found in nine (25.7 %) and seven (20.0 %) patients with pure sleep epilepsy, and in 12 (34.3 %) and 11 (31.4 %) patients with pure waking epilepsy, respectively (P > 0.05 for the comparison between sleep and waking epilepsy). However, proportion of subjects with rounded hippocampal bodies (15, 42.9 % vs. 3, 8.6 % for patients with sleep and waking epilepsy, respectively) and rounding ratios of both left and right hippocampal bodies (0.66 ± 0.13 and 0.61 ± 0.12, respectively for left and right hippocampal bodies in sleep epileptic patients vs. 0.57 ± 0.11 and 0.55 ± 0.11, respectively for left and right hippocampal bodies in waking epileptic patients) were increased in patients with sleep epilepsy (P < 0.05). Further, in sleep epileptic patients with left sided hippocampal body rounding, epileptiform discharges were more readily lateralized to the left temporal lobe (P < 0.05). In conclusion, hippocampal sclerosis and atrophy are not different between pure partial onset sleep and waking epileptic patients. However, rounding ratio and frequency of hippocampal body rounding are increased in sleep epileptic patients.

Molecular mechanism of circadian rhythmicity of seizures in temporal lobe epilepsy

The circadian pattern of seizures in people with epilepsy (PWE) was first described two millennia ago. However, these phenomena have not received enough scientific attention, possibly due to the lack of promising hypotheses to address the interaction between seizure generation and a physiological clock. To propose testable hypotheses at the molecular level, interactions between circadian rhythm, especially transcription factors governing clock genes expression, and the mTOR (mammalian target of rapamycin) signaling pathway, the major signaling pathway in epilepsy, will be reviewed. Then, two closely related hypotheses will be proposed: (1) Rhythmic activity of hyperactivated mTOR signaling molecules results in rhythmic increases in neuronal excitability. These rhythmic increases in excitability periodically exceed the seizure threshold, displaying the behavioral seizures. (2) Oscillation of neuronal excitability in SCN modulates the rhythmic excitability in the hippocampus through subiculum via long-range projections. Findings from published results, their implications, and proposals for new experiments will be discussed. These attempts may ignite further discussion on what we still need to learn about the rhythmicity of spontaneous seizures.

Short-term effects of melatonin and pinealectomy on serotonergic neuronal activity across the light-dark cycle

Melatonin (MLT) and serotonin (5-HT) are two biosynthetically related compounds implicated in several common physiological functions and the etiology of mood disorders. How they interact, though, is not yet fully understood. In this study, single-unit extracellular recordings were used to monitor dorsal raphe nucleus (DR) 5-HT neuronal activity in anesthetized rats, under basal conditions (CTRL), in response to MLT administration, and after pinealectomy (PX) across the light-dark cycle. Under basal conditions, the number of spontaneously active 5-HT neurons and their firing rate were both significantly lower in the dark phase. In the light phase, administration of MLT at low doses (0.5-1 mg/kg, i.v.) decreased 5-HT firing activity. This inhibitory effect of MLT was completely blocked by the MT₁/MT₂ receptor antagonist luzindole, but not by the selective MT(2) receptor antagonist 4P-PDOT, the selective 5-HT(1A) receptor antagonist WAY100635, or by the α₂ adrenoceptor antagonist idazoxan. In the opposite experiment, PX increased 5-HT firing activity in the dark phase, and this was reversed by MLT administration (1 mg/kg, i.v.). Finally, in a forced swim test, MLT (1 mg/kg, i.p.) increased immobility time and decreased swimming behavior. Together, these results suggest that nocturnal MLT secretion imposes tonic inhibitory control over a sub-population of DR 5-HT neurons. This MLT-induced decrease in 5-HT neurotransmission may represent a biological mechanism underlying mood disorders characterized by increased MLT secretion, such as seasonal affective disorder.

Antidepressant- and anxiolytic effects of the novel melatonin agonist Neu-P11 in rodent models

AIM

To investigate the potential antidepressant and anxiolytic effects of Neu-P11, a novel melatonin agonist, in two models of depression in rats and a model of anxiety in mice.

METHODS

In the learned helplessness test (LH), Neu-P11 or melatonin (25-100 mg/kg, ip) was administered to rats 2 h before the beginning of the dark phase once a day for 5 days and the number of escape failures and intertrial crossings during the test phase were recorded. In the forced swimming test (FST), rats received a single or repeated administration of Neu-P11 (25-100 mg/kg, ip). The total period of immobility during the test phase was assessed. In the elevated plus-maze test (EPM), mice were treated with Neu-P11 (25-100 mg/kg, ip) or melatonin in the morning or in the evening and tested 2 h later. The percentage of time spent in the open arms and the open arms entries were assessed.

RESULTS

In the LH test, Neu-P11 but not melatonin significantly decreased the escape deficit and had no effect on the intertrial crossings. In the FST, a single or repeated administration of Neu-P11, either in the morning or in the evening, significantly decreased the duration of immobility. In the EPM test, Neu-P11 significantly increased the percentage of time spent in the open arms and the open arms entries irrespective to the time of administration. Melatonin was effective only when administered in the afternoon.

CONCLUSION

The results demonstrate that Neu-P11 exerts antidepressant and anxiolytic activities in rodent models.

Chronic melatonin treatment and its precursor L-tryptophan improve the monoaminergic neurotransmission and related behavior in the aged rat brain

Melatonin has an important role in the aging process as a potential drug to relieve oxidative damage, a likely cause of age-associated brain dysfunction. As age advances, the nocturnal production of melatonin decreases potentially causing physiological alterations. The present experiments were performed to study in vivo the effects of exogenously administered melatonin chronically on monoaminergic central neurotransmitters serotonin (5-HT), dopamine (DA) and norepinephrine (NE) and behavioral tests in old rats. The accumulation of 5-hydroxy-tryptophan (5-HTP) and L-3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in rat brain. Also neurotransmitters 5-HT, DA and NE and some metabolites were quantified by HPLC. In control rats, an age-related decline was observed in neurochemical parameters. However, chronic administration of melatonin (1 mg/kg/day, diluted in drinking water, 4 wk) significantly reversed the age-induced deficits in all the monoaminergic neurotransmitters studied. Also, neurochemical parameters were analyzed after administration of melatonin biosynthesis precursor L-tryptophan (240 mg/kg/day, i.p., at night for 4 wk) revealing similar improvement effects to those induced by melatonin. Behavioral data corresponded well with the neurochemical findings since spatial memory test in radial-maze and motor coordination in rota-rod were significantly improved after chronic melatonin treatment. In conclusion, these in vivo findings suggest that melatonin and L-tryptophan treatments exert a long-term effect on the 5-HT, DA and NE neurotransmission by enhancing monoamine synthesis in aged rats, which might improve the age-dependent deficits in cognition and motor coordination.

Headache endocrinological aspects

In this chapter we review the current understanding of how hormones, neurohormones, and neurotransmitters participate in the pain modulation of primary headaches. Stressful conditions and hormones intimately implicated in headache neurobiology are also discussed. With the recent progress in neuroimaging techniques and the development of animal models to study headache mechanisms, the physiopathology of several of the primary headaches is starting to be better understood. Various clinical characteristics of the primary headaches, such as pain, autonomic disturbances, and behavioral changes, are linked to hypothalamic brainstem activation and hormonal influence. Headache is greatly influenced by the circadian circle. Over the millennia the nervous system has evolved to meet changing environmental conditions, including the light-dark cycle, in order to ensure survival and reproduction. The main elements for synchronization between internal biological events and the external environment are the pineal gland and its main secretory product, melatonin. Melatonin is believed to be a significant element in migraine and in other headache disorders, which has implications for treatment. A potential therapeutic use of melatonin has been considered in several headache syndromes. In short, primary headaches are strongly influenced by physiological hormonal fluctuations, when nociceptive and non-nociceptive pathways are differentially activated to modulate the perception of pain.

Nap and melatonin-induced changes in hippocampal activation and their role in verbal memory consolidation

Overnight sleep contributes to memory consolidation; even a short nap improves memory performance. Such improvement has been linked to hippocampal activity during sleep. Melatonin has been shown to affect the human hippocampus and to induce 'sleep like' changes in brain activation. We therefore conducted and compared two functional magnetic resonance imaging studies: the first study assessed the effect of a 2-hr mid-day nap versus an equal amount of wakefulness on a verbal memory task (unrelated word pair association); the second assessed the effect of melatonin versus placebo (both conditions without nap) on a similar task. We report that following a nap relative to wakefulness, successful retrieval-related activation in the parahippocampus is decreased. A smaller decrease is seen in wakefulness with melatonin but not placebo. In parallel, an improvement in verbal memory recall was found after a nap compared with wakefulness but not with melatonin during wakefulness compared with placebo. Our findings demonstrate effects of melatonin that resemble those of sleep on verbal memory processing in the hippocampus thus suggesting that melatonin, like sleep, can initiate offline plastic changes underlying memory consolidation; they also suggest that concomitant rest without interferences is necessary for enhanced performance.

The influence of daylight exposure on platelet 5-HT levels in patients with major depression and schizophrenia

Platelet serotonin (5-HT) can be used as a limited, peripheral model for the central 5-HT synaptosomes. Altered platelet 5-HT concentrations have been associated with psychiatric disorders like depression and schizophrenia. The aim of the present study was to compare platelet 5-HT concentrations during long, medium and short period of natural daylight exposure in a large number of medication-free male and female schizophrenic and depressed patients and sex-matched healthy controls. Platelet 5-HT concentration was determined spectrofluorimetrically in 240 (97 female, 143 male) schizophrenic and 258 (153 female, 105 male) nonpsychotic, nonsuicidal depressed medication-free patients and 328 (149 women, 179 men) healthy subjects during periods with short (<12), long (>12) and medium (average 12) hours of the natural daylight. Platelet 5-HT concentration was significantly lower in women compared to men in all groups. Healthy male subjects had significantly higher (p=0.011) platelet 5-HT concentrations during long compared to medium period. There were no significant differences (p>0.05) in platelet 5-HT concentration between different periods in healthy women. The significant increase in platelet 5-HT values were found in female (p=0.01) and male (p=0.029) depressed patients during long compared to short period. There were no significant associations between platelet 5-HT concentrations and different periods in both male and female schizophrenic patients. The results indicate the sex-related differences in the serotonergic system. The alterations of platelet 5-HT concentrations, observed across period with different durations of daylight exposure, point to a direct or indirect effect of light on peripheral 5-HT system that could be related to different sensitivity of the pineal gland to light and/or melatonin influence on 5-HT metabolism.

Melatonin and the human hippocampus, a time dependent interplay

Melatonin serves as a signal of darkness and participates in sleep/wake regulation. Animal studies demonstrated effects of melatonin in the hippocampus, particularly suggesting involvement in synaptic plasticity. We used functional magnetic resonance imaging to identify and investigate effects of melatonin in the human hippocampus. Activity in the hippocampal complex during a memory task was examined at 22:00 hr (when endogenous melatonin levels are normally increasing) and compared with 16:00 hr (when endogenous melatonin levels are minimal). The relationship between observed activation patterns and endogenous melatonin was assessed. Finally, the effects of exogenous melatonin administered at 22:00 hr were studied in a double-blind, placebo-controlled crossover manner. Our findings indicate that activation in the left hippocampus at 22:00 hr is significantly reduced compared with afternoon hours compatible with diurnal variation in hippocampal activity. Exogenous melatonin further reduced activation in this region, only in subjects who already crossed the melatonin onset phase at this hour and in correlation with endogenous melatonin levels. As such an effect was not demonstrated with afternoon administration of melatonin, a time depended effect is suggested. Contrary, activation in the left para-hippocampus at 22:00 hr was higher in subjects that crossed the melatonin onset phase. Parahippocampal activation correlated with individual endogenous melatonin levels and was not further affected by exogenous melatonin. These results demonstrate that memory related activation in the hippocampus and para-hippocampus are affected by time of day and melatonin in a differential manner and may implicate the circadian clock and melatonin in human memory processing during the night.

Orally Administered Melatonin Improves Nocturnal Rest in Young and Old Ringdoves (Streptopelia risoria)

Melatonin possesses chronobiotic properties, which affects sleep/wake rhythms. We investigated a 7-day administration of melatonin (0.25, 2.5 and 5 mg/kg body weight) on the activity/rest rhythms of a diurnal animal (the ringdove, Streptopelia risoria), aged 2-3 (young) and 10-12 (old) years, and its possible relationship with the serum levels of melatonin and serotonin. Total nocturnal and diurnal activity pulses were logged at basal, during, and up to 7 days after the treatments. The animals received 0.1 ml of melatonin orally 1 hr before lights off. The results showed that the administration of whichever melatonin dose decreased both diurnal and nocturnal old ringdove activity, the reduction being larger at night. The young animals also reduced their nocturnal activity with all three melatonin concentrations, whereas their diurnal activity only decreased with the 2.5 and 5 mg/kg body weight treatments. We chose those treatments that gave the best results in terms of nocturnal rest and the least affected diurnal activity (0.25 mg/kg body weight and 2.5 mg/kg in the young and old animals, respectively). Serum melatonin was measured by radioimmunoassay and serotonin by ELISA. In both age groups, the treatment increased both nocturnal and diurnal melatonin levels, with the effect continuing until 1 day after the last dose. Serum serotonin levels were unaffected by the treatments in either age group. The treatment restored the amplitude of the serum melatonin rhythm in the old animals to that of the young group. In summary, treatment with melatonin may be appropriate to improve nocturnal rest, and beneficial as a therapy for sleep disorders.

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.

Melatonin affects the immobility time of rats in the forced swim test: the role of serotonin neurotransmission

The efficacy of melatonin or its derivatives in depressive patients has been recently considered for clinical application. However, the evidence for its effect on experimental models of depression is not consolidated. Here, the effects of melatonin on the model of forced swim test (FST) paradigm were studied in male rats of the Wistar strain after acute intraperitoneal (i.p.) administration of 0.1, 0.5 or 1 mg/kg of the hormone. Melatonin at doses of 0.5 and 1 mg/kg, but not of 0.1 mg/kg, decreased the immobility of rats in the FST paradigm suggesting a possible antidepressant-like activity. The dose of 0.5 mg/kg appeared to be as potent as clomipramine 50 mg/kg in reducing the immobility time of rats in the FST paradigm. The effect of melatonin on immobility time of rats in the FST paradigm was abolished by the simultaneous injection of the non-selective melatonin antagonist, luzindole (0.25 mg/kg, subcutaneously). Similarly, administration of small quantities of serotonin (5-HT, 5 ng/1 microl) or of the 5-HT(2A)/5-HT(2C) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (2 ng/1 microl) injected into the amygdale totally suppressed the reduction of immobility time in the FST paradigm induced by melatonin 0.5 mg/kg. These results may suggest that effects of melatonin on the behavioral reaction of rats in the FST paradigm are due to an interaction of the hormone with central 5-HT neurotransmission.

Potential therapeutic use of melatonin in migraine and other headache disorders

There is increasing evidence that headache disorders are connected with melatonin secretion and pineal function. Some headaches have a clearcut seasonal and circadian pattern, such as cluster and hypnic headaches. Melatonin levels have been found to be decreased in both migraine and cluster headaches. Melatonin mechanisms are related to headache pathophysiology in many ways, including its anti-inflammatory effect, toxic free radical scavenging, reduction of pro-inflammatory cytokine upregulation, nitric oxide synthase activity and dopamine release inhibition, membrane stabilisation, GABA and opioid analgesia potentitation, glutamate neurotoxicity protection, neurovascular regulation, 5-HT modulation and the similarity in chemical structure to indometacin. The treatment of headache disorders with melatonin and other chronobiotic agents, such as melatonin agonists (ramelteon and agomelatin), is promising and there is a great potential for their use in headache treatment.

Melatonin, the pineal gland and their implications for headache disorders

There is now evidence that melatonin may have a role in the biological regulation of circadian rhythms, sleep, mood, and ageing. Altered melatonin levels in cluster headache and migraine have been documented. Melatonin mechanisms are related to headache pathophysiology in many ways, including its anti-inflammatory effect, toxic free radical scavenging, reduction of proinflammatory cytokine up-regulation, nitric oxide synthase activity and dopamine release inhibition, membrane stabilization, GABA and opioid analgesia potentiation, glutamate neurotoxicity protection, neurovascular regulation, serotonin modulation, and the similarity of chemical structure to that of indomethacin. Treatment of headache disorders with melatonin and other chronobiotic agents is promising. A double-blind, placebo-controlled trial shows melatonin is effective in cluster headache prevention, other studies also show benefit in other disorders. Melatonin plays an important role in headache disorders, offering new avenues for studying their pathophysiology and treatment.

Effect of melatonin on total food intake and macronutrient choice in rats

Melatonin, a hormone secreted in a rhythmic manner over 24 h mainly by the pineal gland, is used to alleviate the symptoms of jetlag and treat sleeping problems. The objective of the present study was to examine the effects of a 7-h phase-shift from the natural peak of melatonin secretion on total food intake and macronutrient selection. Forty-eight adult Wistar rats of both sexes were divided in three dietary groups, each group offered a simultaneous and different choice of a carbohydrate- and a protein-rich diet. Macronutrient intakes following intraperitoneal administration of four doses of melatonin (3000, 6000, 10000 and 15000 pg/ml blood) at dark onset were examined. Melatonin increased short- (4 h postinjection) and long-term (12 h postinjection) nocturnal total food intake in both male and female rats, mainly with the two highest doses. This effect of melatonin was mainly due to a short-term increase of intake across all carbohydrate-rich diet preparations (dextrin/cornstarch, cornstarch, and sucrose/cornstarch) and across genders. This consistent effect of melatonin on the intake of carbohydrate-rich diets with contrasting sensory attributes rules out the possibility that melatonin acts on sensorymotor pathways, thus suggesting that melatonin's effect on food intake is controlled by the carbohydrate content of the diet. In contrast, melatonin could be affecting some sensory or motor processes peculiar to the ingestion of protein since it increased protein-rich diet intake inconsistently across the various preparations (casein, soy isolate, and egg protein) as well as genders. This evidence supports the view that melatonin acts as a time indicator, reinforcing the animals with a "night cue", and favors predominant carbohydrate intake normally occurring at the beginning of the activity period.