Effects of melatonin on neurotransmitter uptake and release by synaptosome-rich homogenates of the rat hypothalamus

Delirium REduction after administration of melatonin in acute ischemic stroke (DREAMS): A propensity score-matched analysis

BACKGROUND AND PURPOSE

Poststroke delirium (PSD) comprises a common and severe complication after stroke. However, treatment options for PSD remain insufficient. We investigated whether prophylactic melatonin supplementation may be associated with reduced risk for PSD.

METHODS

Consecutive patients admitted to the Tübingen University Stroke Unit, Tübingen, Germany, with acute ischemic stroke (AIS), who underwent standard care between August 2017 and December 2017, and patients who additionally received prophylactic melatonin (2 mg per day at night) within 24 h of symptom onset between August 2018 and December 2018 were included. Primary outcomes were (i) PSD prevalence in AIS patients and (ii) PSD risk and PSD-free survival in patients with cerebral infarction who underwent melatonin supplementation compared to propensity score-matched (PSM) controls. Secondary outcomes included time of PSD onset and PSD duration.

RESULTS

Out of 465 (81.2%) patients with cerebral infarction and 108 (18.8%) transient ischemic attack (TIA) patients, 152 (26.5%) developed PSD (median time to onset [IQR]: 16 [8-32] h; duration 24 [8-40] h). Higher age, cerebral infarction rather than TIA, and higher National Institutes of Health Stroke Scale score and aphasia on admission were significant predictors of PSD. After PSM (164 melatonin-treated patients with cerebral infarction versus 164 matched controls), 42 (25.6%) melatonin-treated patients developed PSD versus 60 (36.6%) controls (odds ratio, 0.597; 95% confidence interval, 0.372-0.958; p = 0.032). PSD-free survival differed significantly between groups (p = 0.027), favoring melatonin-treated patients. In patients with PSD, no between-group differences in the time of PSD onset and PSD duration were noted.

CONCLUSIONS

Patients prophylactically treated with melatonin within 24 h of AIS onset had lower risk for PSD than patients undergoing standard care. Prospective randomized trials are warranted to corroborate these findings.

Cellular Mechanisms of Melatonin: Insight from Neurodegenerative Diseases

Neurodegenerative diseases are the second most common cause of death and characterized by progressive impairments in movement or mental functioning in the central or peripheral nervous system. The prevention of neurodegenerative disorders has become an emerging public health challenge for our society. Melatonin, a pineal hormone, has various physiological functions in the brain, including regulating circadian rhythms, clearing free radicals, inhibiting biomolecular oxidation, and suppressing neuroinflammation. Cumulative evidence indicates that melatonin has a wide range of neuroprotective roles by regulating pathophysiological mechanisms and signaling pathways. Moreover, melatonin levels are decreased in patients with neurodegenerative diseases. In this review, we summarize current knowledge on the regulation, molecular mechanisms and biological functions of melatonin in neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, vascular dementia and multiple sclerosis. We also discuss the clinical application of melatonin in neurodegenerative disorders. This information will lead to a better understanding of the regulation of melatonin in the brain and provide therapeutic options for the treatment of various neurodegenerative diseases.

Electrochemical detection of exogenously administered melatonin in the brain

Melatonin (MT) is an important electroactive hormone that regulates different physiological actions in the brain, ranging from circadian clock to neurodegeneration. An impressive number of publications have highlighted the effectiveness of MT treatments in different types of sleep and neurological disorders, including Alzheimer's and Parkinson's disease. The ability to detect MT in different regions of the brain would provide further insights into the physiological roles and therapeutic effects of MT. While multiple electrochemical methods have been used to detect MT in biological samples, monitoring MT in the brain of live animals has not been demonstrated. Here, we optimized a square wave voltammetry (SWV) electroanalytical method to evaluate the MT detection performance at CFEs in vitro and in vivo. SWV was able to sensitively detect the MT oxidation peak at 0.7 V, and discriminate MT from most common interferents in vitro. More importantly, using the optimized SWV, CFEs successfully detected and reliably quantified MT concentrations in the visual cortex of anesthetized mice after intraperitoneal injections of different MT doses, offering stable MT signals for up to 40 minutes. To the best of our knowledge, this is the first electrochemical measurement of exogenously administered MT in vivo. This electrochemical MT sensing technique will provide a powerful tool for further understanding MT's action in the brain.

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

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

Melatonin receptors: molecular pharmacology and signalling in the context of system bias

Melatonin, N-acetyl-5-methoxytryptamine, an evolutionally old molecule, is produced by the pineal gland in vertebrates, and it binds with high affinity to melatonin receptors, which are members of the GPCR family. Among the multiple effects attributed to melatonin, we will focus here on those that are dependent on the activation of the two mammalian MT1 and MT2 melatonin receptors. We briefly summarize the latest developments on synthetic melatonin receptor ligands, including multi-target-directed ligands, and the characterization of signalling-biased ligands. We discuss signalling pathways activated by melatonin receptors that appear to be highly cell- and tissue-dependent, emphasizing the impact of system bias on the functional outcome. Different proteins have been demonstrated to interact with melatonin receptors, and thus, we postulate that part of this system bias has its molecular basis in differences of the expression of receptor-associated proteins including heterodimerization partners. Finally, bias at the level of the receptor, by the expression of genetic receptor variants, will be discussed to show how a modified receptor function can have an effect on the risk for common diseases like type 2 diabetes in humans. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Melatonin ameliorates brain oxidative stress and upregulates senescence marker protein-30 and osteopontin in a rat model of vascular dementia

The aim of this study was to investigate the effect of melatonin on oxidative stress and senescence marker protein-30 (SMP30) as well as osteopontin (OPN) expression in the hippocampus of rats subjected to vascular dementia (VD). A total of 72 male rats were divided into six groups (n = 12 each) as follows: (i) untreated control (CON), (ii) sham-operated group, (iii) sham-operated + melatonin, (iv) rats exposed to VD induced by permanent bilateral occlusion of the common carotid arteries (BCCAO) leading to chronic cerebral hypoperfusion, (v) rats exposed to VD + melatonin, and (vi) rats exposed to VD + donepezil (DON). At the end of experiment, the hippocampal levels of acetylcholine (ACh), norepinephrine (NE), and dopamine (Dop) were measured. Expression of OPN was determined using immunohistochemistry, and SMP30 expression was determined using real-time PCR in the hippocampus. Hippocampal thiobarbituric acid reactive substances (TBARS) and total antioxidant capacity (TAC) were evaluated. The BCCAO group showed significantly decreased TAC (p < 0.05) and significantly increased in TBARS levels compared with the CON group. In addition, BCCAO significantly decreased (p < 0.05) the expression of both OPN and SMP30 and the levels of ACh, NE, and Dop in the hippocampus compared with CON treatment. Treatment with melatonin significantly increased OPN and SMP30 expression and ACh, NE, and Dop levels in the hippocampus with amelioration of the oxidative stress compared with BCCAO rats. Melatonin might produce a neuroprotective effect through its antioxidant action and by increasing the expression of SMP30 and OPN that is not comparable with that of DON.

Protein interactome mining defines melatonin MT1 receptors as integral component of presynaptic protein complexes of neurons

In mammals, the hormone melatonin is mainly produced by the pineal gland with nocturnal peak levels. Its peripheral and central actions rely either on its intrinsic antioxidant properties or on binding to melatonin MT1 and MT2 receptors, belonging to the G protein-coupled receptor (GPCR) super-family. Melatonin has been reported to be involved in many functions of the central nervous system such as circadian rhythm regulation, neurotransmission, synaptic plasticity, memory, sleep, and also in Alzheimer's disease and depression. However, little is known about the subcellular localization of melatonin receptors and the molecular aspects involved in neuronal functions of melatonin. Identification of protein complexes associated with GPCRs has been shown to be a valid approach to improve our understanding of their function. By combining proteomic and genomic approaches we built an interactome of MT1 and MT2 receptors, which comprises 378 individual proteins. Among the proteins interacting with MT1 , but not with MT2 , we identified several presynaptic proteins, suggesting a potential role of MT1 in neurotransmission. Presynaptic localization of MT1 receptors in the hypothalamus, striatum, and cortex was confirmed by subcellular fractionation experiments and immunofluorescence microscopy. MT1 physically interacts with the voltage-gated calcium channel Cav 2.2 and inhibits Cav 2.2-promoted Ca(2+) entry in an agonist-independent manner. In conclusion, we show that MT1 is part of the presynaptic protein network and negatively regulates Cav 2.2 activity, providing a first hint for potential synaptic functions of MT1.

Electrophysiological characterization of dopamine neuronal activity in the ventral tegmental area across the light-dark cycle

Direct evidence that dopamine (DA) neurotransmission varies during the 24 h of the day is lacking. Here, we have characterized the firing activity of DA neurons located in the ventral tegmental area (VTA) using single-unit extracellular recordings in anesthetized rats kept on a standard light-dark cycle. DA neuronal firing activity was measured under basal conditions and in response to intravenous administration of increasing doses of amphetamine (AMPH: 0.5, 1, 2, 5 mg/kg), apomorphine (APO: 25, 50, 100, 200 µg/kg) and melatonin (MLT: 0.1, 1, 10 mg/kg) at different time intervals of the light-dark cycle. DA firing activity peaked between 07:00 and 11:00 h (3.5 ± 0.3 Hz) and between 19:00 and 23:00 h (4.1 ± 0.7 Hz), with lowest activity occurring between 11:00 and 15:00 h (2.4 ± 0.2 Hz) and between 23:00 and 03:00 h (2.6 ± 0.2 Hz). The highest number of spontaneously active neurons was observed between 03:00 and 06:00 h (2.5 ± 0.3 neurons/track), whereas the lowest was between 19:00 and 23:00 h (1.5 ± 0.2 neurons/track). The inhibitory effect of AMPH on DA firing rate was similar in both phases. The inhibitory effect of low dose of APO (25 μg/kg, dose selective for D2 autoreceptor) was more potent in the dark phase, whereas APO effects at higher doses were similar in both phases. Finally, MLT administration (1 mg/kg) produced a moderate inhibition of DA cell firing in both phases. These experiments demonstrate the existence of an intradiurnal rhythmic pattern of VTA DA neuronal firing activity and a higher pharmacological response of D2 autoreceptors in the dark phase.

Selected biomarkers as predictive tools in testing efficacy of melatonin and coenzyme Q on propionic acid - induced neurotoxicity in rodent model of autism

BACKGROUND

Exposures to environmental toxins are now thought to contribute to the development of autism spectrum disorder. Propionic acid (PA) found as a metabolic product of gut bacteria has been reported to mimic/mediate the neurotoxic effects of autism. Results from animal studies may guide investigations on human populations toward identifying environmental contaminants that produce or drugs that protect from neurotoxicity. Forty-eight young male Western Albino rats were used in the present study. They were grouped into six equal groups 8 rats each. The first group received a neurotoxic dose of buffered PA (250 mg/Kg body weight/day for 3 consecutive days). The second group received only phosphate buffered saline (control group). The third and fourth groups were intoxicated with PA as described above followed by treatment with either coenzyme Q (4.5 mg/kg body weight) or melatonin (10 mg/kg body weight) for one week (therapeutically treated groups). The fifth and sixth groups were administered both compounds for one week prior to PA (protected groups). Heat shock protein70 (Hsp70), Gamma amino-butyric acid (GABA), serotonin, dopamine, oxytocin and interferon γ-inducible protein 16 together with Comet DNA assay were measured in brain tissues of the six studied groups.

RESULTS

The obtained data showed that PA caused multiple signs of brain toxicity revealed in depletion of GABA, serotonin, and dopamine, are which important neurotransmitters that reflect brain function, interferon γ-inducible protein 16 and oxytocin. A high significant increase in tail length, tail DNA% damage and tail moment was reported indicating the genotoxic effect of PA. Administration of melatonin or coenzyme Q showed both protective and therapeutic effects on PA-treated rats demonstrated in a remarkable amelioration of most of the measured parameters.

CONCLUSION

In conclusion, melatonin and coenzyme Q have potential protective and restorative effects against PA-induced brain injury, confirmed by improvement in biochemical markers and DNA double strand breaks.

Melatonin

Melatonin (MEL) is a hormone synthesized and secreted by the pineal gland deep within the brain in response to photoperiodic cues relayed from the retina via an endogenous circadian oscillator within the suprachiasmatic nucleus in the hypothalamus. The circadian rhythm of melatonin production and release, characterized by nocturnal activity and daytime quiescence, is an important temporal signal to the body structures that can read it. Melatonin acts through high-affinity receptors located centrally and in numerous peripheral organs. Different receptor subtypes have been cloned and characterized: MT(1) and MT(2) (transmembrane G-protein-coupled receptors), and MT(3). However, their physiological role remains unelucidated, although livestock management applications already include the control of seasonal breeding and milk production. As for potential therapeutic applications, exogenous melatonin or a melatonin agonist and selective 5-hydroxytrypiamine receptor (5-HT(2c)) antagonist, eg, S 20098, can be used to manipulate circadian processes such as the sleep-vake cycle, which are frequently disrupted in many conditions, most notably seasonal affective disorder.

Parkinson's disease as a neuroendocrine disorder of circadian function: dopamine-melatonin imbalance and the visual system in the genesis and progression of the degenerative process

For more than 50 years, Parkinson's disease (PD) has been conceptualized as a product of nigro-striatal dopamine (NSD) system degeneration. In spite of a growing body of evidence depicting the mammalian brain as an interrelated complexity of circuitous systems, dopamine (DA) deficiency of the NSD is still regarded as the main problem, with DA replacement being the purpose of therapeutic intervention. For at least 191 years circadian involvement in various aspects of PD, including depression and insomnia, has been recognized as an integral part of the symptom matrix of PD and yet attempts to elucidate the involvement of this system is uncharted territory. The present review attempts a major reorganization of mammalian brain into a coordinated complex involving the NSD and the retinal hypothalamic tract (RHT) as the primary systems involved in the retino-diencephalic/mesencephalic-pineal (RDMP) axis. Secondary systems including the lateral hypothalamus (LH), the area postraema (AP) and the subthalamic nucleus (STN) also form an integral part of this system as they have been shown to be either intimately related to the primary systems of the RDMP axis or have been shown to be significantly involved in the expression and treatment of PD. A large volume of evidence suggests that the RDMP axis is activated during the course of PD and during therapeutic intervention. Four types of neurotoxicity associated with melatonin are identified and the susceptibility of various parts of the RDMP axis to undergo neuropathological change, the tendency for melatonin to induce PD-like behavioural toxicity, and the relationship of this to PD symptomotology are described. This includes adverse effects of melatonin on motor function, hypotension, the adjuvant use of benzodiazepines, depression, insomnia, body weight regulation and various biochemical effects of melatonin administration: all problems currently facing the proposal to introduce melatonin as an adjuvant. It is suggested further that traditional DA replacement may well work by exerting its effect upon the circadian system, rather than simply replacing deficient DA. Activation of the circadian function by antagonizing melatonin with bright light not only has therapeutic value in treating the primary symptoms of PD but it shares a common mechanism with L-dopa in reducing the occurrence of seborrheic dermatitis. Concepts at the centre of understanding pineal function in PD, including pineal calcification, melatonin deficiency, symptomatic versus protective features of melatonin and antioxidative effects, are explained in a counterintuitive context. Intriguing propositions including the role of the retina in the aetiology of PD and that the nigra functions as a retina in this disorder are presented with the intention to provide a new understanding of the underlying compromised function in PD and to provide new treatment strategies. For the first time, abundant evidence is presented describing PD as an endocrine disorder of melatonin hyperplasia. The role of circadian interventive therapies and internal desynchrony in the aetiology and progression of PD provides a new direction for understanding the underlying physiology of a disease which is currently in a state of impasse and provides new hope for those who suffer from its debilitating effects.

The therapeutic effects of dopamine replacement therapy and its psychiatric side effects are mediated by pineal function

There are reports that melatonin secretion from the pineal gland gradually diminishes with advancing age. It has been suggested that various forms of neuropsychiatric disease, in particular, Parkinson's disease (PD), is consequentially related to this decrease by virtue of increased oxidative stress which enhances the process of dopamine (DA) degeneration. There is, however, considerable disagreement on this theme as very little is generally known about the role of the pineal gland in the aetiology and treatment of PD. To assess the role of the pineal gland in PD and in dopamine replacement therapy (DART), the effect of three anti-Parkinsonian drugs on motor and psychiatric function was assessed in normal, pinealectomized (PX) and DA deficient, PX rats. In the first study, rats underwent PX or sham operation and were then injected (IP) with Amantadine (30 or 50 mg/kg), Bromocriptine (5 or 10 mg/kg) or L-Dopa (30 or 60 mg/kg plus 50 mg/kg of R-044602) 3-8 weeks after surgery. Open field performance and motor reflex tests were assessed during the light and dark phases of the L/D cycle. In a second study, clinically effective doses of Bromocriptine (10 mg/kg) and L-Dopa (30 and 100 mg/kg with 50 mg/kg R-044602) were injected into depleted, PX or sham operated rats. In study I, sham operated and PX rats responded differently to Bromocriptine and L-Dopa, while Amantadine did not differentially effect motor performance in the two groups. In study II, 6-OHDA induced degeneration of the nigro-striatal system abolished the effects of Bromocriptine and dramatically altered the effects of L-Dopa seen in study I, in sham operated versus PX rats. DART significantly altered emotionality, as measured by escape attempts, agitation and rage in sham operated animals, compared to PX rats. DA deficiency abolished the tendency to escape in all groups except those treated with 100mg/kg of L-Dopa. Conversely, agitation and rage scores were greater after 100 mg/kg of L-Dopa, in rats with intact pineal function, than in PX rats. These results provide compelling evidence that altered pineal function plays a major role in the aetiology of PD, the therapeutic effect of anti-Parkinsonian drugs and in the psychiatric side effects of DART.

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

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

Melatonin inhibition of nicotine-stimulated dopamine release in PC12 cells

Melatonin, a pineal hormone, modifies numerous physiologic processes including circadian rhythms and sleep. In specific tissues, melatonin appears to have an inverse relationship with dopamine. To examine this relationship, a pheochromocytoma cell line (PC12) was used to determine the extent of melatonin's ability to inhibit nicotine-stimulated dopamine release. Multiple experiments were conducted that examined: (1). the dose response of acute melatonin (5 min); (2). the effects of chronic melatonin (16 h pre-exposure); (3). the effects of prior nicotine or melatonin exposure (5 min) on melatonin's ability to alter dopamine release from a second 5-min nicotine exposure; and (4). the role of melatonin receptors (by pertussis toxin inhibition) on nicotine-stimulated dopamine release. In the dose response studies, melatonin inhibited nicotine-stimulated dopamine release with an ED50 of 8.6 microM. Chronic exposure to melatonin had no effect on melatonin's acute inhibition of nicotine-stimulated dopamine release. Prior nicotine or melatonin exposure had little effect on subsequent melatonin or nicotine exposure, except that the cells exposed to nicotine were not responsive to a second exposure to nicotine. Blockade of melatonin receptor function by pre-exposure to pertussis toxin (16 h) did not prevent melatonin's inhibition of nicotine-stimulated dopamine release. However, the toxin-treated cells were less inhibited by melatonin when compared to control cells suggesting a partial role for melatonin receptors. These results indicate that melatonin can acutely inhibit nicotine-stimulated dopamine release in PC12 cells. This model system allows detailed examination of melatonin's cellular actions as well as supporting a role for melatonin on neuronal dopamine release.

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

Serotonin (5-HT) participates as a neurotransmitter in the control of the circadian sleep/wake rhythm, feeding and sexual behaviours, and emotional and affective states. The present study investigated whether melatonin affects the circadian rhythm of 5-HT neurotransmission in the hippocampus, a major target for serotoninergic antidepressants. The present results show a daytime dependency of [3H]5-HT uptake insensitive to melatonin, with a peak from 14.00 h to 22.00 h and a trough from 02.00 h to 06.00 h. They also indicate that melatonin reduced the spontaneous efflux of [3H]5-HT as well as KCl-evoked release of [3H]5-HT during the dark phase, while it increased the evoked release during the light phase. Both effects were concentration-dependent; the facilitatory effect was maximum at high nanomolar concentrations of melatonin, whereas the inhibition preferentially occurred at low concentrations. Finally, nifedipine, an effective antagonist of L-type voltage-sensitive calcium channels, prevented the effects of melatonin on KCl-evoked [3H]5-HT release during the light but not the dark phase. Together, these data suggest the involvement of two distinct mechanisms by which melatonin might regulate both spontaneous efflux and evoked release of 5-HT in the hippocampus.

Differing effects of the cannabinoid agonist, CP 55,940, in an alcohol or Tween 80 solvent, on prepulse inhibition of the acoustic startle reflex in the rat

It has been suggested that cannabinoid agonists increase dopamine (DA) transmission in the mesolimbic dopamine system. However, evidence for such an effect is inconsistent. Prepulse inhibition (PPI) of the acoustic startle reflex is a behavioural paradigm that is modulated by an increase of mesolimbic dopamine. This study sought to ascertain whether or not a cannabinoid agonist, CP 55,940, mimicked the effects of amphetamine (a drug which increases dopamine release) on PPI. The first experiment measured the PPI of 16 male Wistar rats injected (i.p.) with different doses of CP 55,940 in a Latin-square design. A second experiment replicated the effects of the first experiment in a between-subjects design, and also examined the effects of using a 5% alcohol solution as a solvent for cannabinoid agonists, in comparison to the more inert detergent, Tween 80. In both experiments, CP 55,940 in Tween 80 significantly reduced basal activity, increased startle onset latencies and increased PPI, effects opposite to those of amphetamine. These results suggest that the net behavioural effects of cannabinoids are opposite to those of amphetamine. In addition, it was found that 1 ml/kg of a 5% alcohol solution has significant behavioural effects on its own, and reverses the effects of CP 55,940 on PPI.

Melatonin induces tyrosine hydroxylase mRNA expression in the ventral mesencephalon but not in the hypothalamus

We have evaluated the effect of chronic administration of melatonin in terms of mRNA expression for tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, and in the terms of dopamine (DA) transporter (DAT) by means of in situ hybridization. Experimental rats received daily late afternoon injections of 1.5 mg/kg melatonin for 30 days and analysis were performed in the ventral mesencephalon including the substantia nigra (SN) and ventral tegmental area (VTA), and hypothalamus. In the ventral mesencephalon, melatonin treatment significantly induced TH mRNA levels in individual dopaminergic neurons in SN and VTA. In contrast, DAT mRNA levels remained at control levels. Striatal synaptosomal DA uptake was not modified by melatonin treatment as compared with controls. Analysis of glutamic acid decarboxylase (GAD) mRNA in SN, the biosynthetic enzyme for GABAergic neurons, revealed no effect of melatonin treatment on mRNA levels for this marker. In the hypothalamus, we performed mRNA quantitation for TH in arcuate nucleus (Arc) and supraoptic nucleus (SO). Melatonin treatment failed to alter mRNA levels in either area. We detected weak but significant mRNA levels for DAT in Arc, SO, zona incerta (ZI) and periventricular hypothalamic nucleus (Pe). However, because of the low levels of mRNA in hypothalamic areas we were unable to perform a reliable measurement of DAT mRNA levels in response to melatonin treatment. We conclude that melatonin administration, that combines antioxidant capacity and a tissue-specific TH inducing effect, may be useful as a pharmacological agent to protect dopaminergic neurons from degeneration.

Entrainment of rat circadian rhythms by melatonin does not depend on the serotonergic afferents to the suprachiasmatic nuclei

Daily administration of melatonin (MEL) can entrain rat circadian rhythms free-running in constant darkness. The high MEL doses needed to obtain entrainment suggest the implication of other neural mechanisms than simply an effect on the hormone's specific receptors detected in the SCN. Administration of serotonin receptor agonists can phase-shift the rodent circadian clock, and MEL is known to modulate release and reuptake of serotonin in nerve endings. This raises the question of a critical involvement of 5-HT-fibres in the entraining properties of MEL. The aim of the present study was to test this hypothesis. Bilateral neurotoxic (5,7-dihydroxytryptamine) lesions of the serotonergic fibres in the SCN were performed in animals kept in LD 12:12. Following the post-operative period, the animals were transferred to constant darkness to free-run. MEL was then administered by a 1 h daily infusion. Both well lesioned and intact animals entrained to MEL. No differences were observed between lesioned and control animals on parameters such as the phase-angles between MEL onset and activity onset, and core body temperature acrophase, respectively. Entrainment of rat circadian rhythms to exogenous MEL is thus not directly dependent on the 5-HT fibres in the SCN.

Organization of rat circadian rhythms during daily infusion of melatonin or S20098, a melatonin agonist

Daily administration of melatonin or S20098, a melatonin agonist, is known to entrain the free-running circadian rhythms of rats. The effects of the duration of administration on entrainment were studied. The animals demonstrated free-running circadian rhythms (running-wheel activity, body temperature, general activity) in constant darkness. Daily infusions of melatonin or S20098 for 1, 8, or 16 h entrained the circadian rhythms to 24 h. Two daily infusions of 1 h (separated by 8 h) entrained the activity peak within the shorter time interval. The entraining properties of melatonin and S20098 were similar and were affected neither by pinealectomy nor by infusion of 1- or 8-h duration. However, with 16-h infusion, less than half of the animals became entrained. Once entrained, the phase angle between the onset of infusion and the rhythms (onset of activity or acrophase of body temperature) increased with the duration of infusion. Before entrainment, the free-running period increased with the duration of infusion, an effect that was not predictable from the phase response curve.

I. Role of the pineal gland in multiple sclerosis: a hypothesis

Despite intensive research over the past several decades, the etiology and pathogenesis of multiple sclerosis (MS) remain elusive. The last 20 years have seen only meager advances in the treatment of the disease in part because too much attention has been devoted to the process of demyelination and its relationship to the neurologic symptoms and recovery of the disease. A host of biological phenomena associated with the disease involving interactions among genetic, environmental, immunologic, and hormonal factors, cannot be explained on the basis of demyelination and, therefore, require refocusing attention on alternative explanations, one of which implicates the pineal gland as the pivotal mover of the disease. This review summarizes the evidence linking dysfunction of the pineal gland with the epidemiology, pathogenesis, clinical manifestations, and course of the disease. The pineal hypothesis of MS also provided the impetus for the development of a novel and highly effective therapeutic modality, one that involves the transcranial application of AC pulsed electromagnetic fields in the picotesla flux density.

Melatonin effects on serotonin synthesis and metabolism in the striatum, nucleus accumbens, and dorsal and median raphe nuclei of rats

This work examined the influence of the pineal gland and its hormone melatonin on the metabolism of serotonin (5-HT) in discrete areas of the forebrain, such as the striatum and the nucleus accumbens, and the midbrain raphe. The content of 5-HT and its major oxidative metabolite, the 5-hydroxyindoleacetic acid (5-HIAA), as well as the in-vivo tryptophan hydroxylation rate were examined after long-term pinealectomy (one month) and daily melatonin treatment (500 micrograms/kg; twice daily for ten days) in pinealectomized rats. Pinealectomy did not alter 5-HT content in any of these brain areas, but it significantly increased the content of 5-HIAA in striatum and the 5-HIAA/5-HT ratio in nucleus accumbens. The normal values of these parameters were recuperated after administration of exogenous melatonin, but it also increased the rate of tryptophan hydroxylation in both areas. In addition, melatonin treatment decreased the levels of 5-HIAA in dorsal raphe nucleus. These data suggest that the pineal gland, through the secretion of melatonin, modulates the local metabolism of 5-HT in forebrain areas by acting on the oxidative deamination. Moreover, melatonin injected in pinealectomized rats derives in a more extended effect than pinealectomy and induces a stimulation of 5-HT synthesis in the striatum, probably due to a pharmacological effect. These results point to the striatum as a target area for the interaction between pineal melatonin and the serotonergic function, and suggest a differential effect of the melatonin injected on areas containing serotonergic terminals and cell bodies, which may relevant for the mode of action of melatonin and its behavioral effects.

Changes in serotonin level and turnover in discrete hypothalamic nuclei after pinealectomy and melatonin administration to rats

The influence of the pineal gland on the hypothalamic serotonergic function was examined by studying the effects of long-term pinealectomy (1 month) and melatonin replacement (500 micrograms/kg; 10 days) on serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content as well as on the in vivo 5-HT synthesis rate in discrete hypothalamic nuclei. Pinealectomy was followed by a significant decrease of 5-HT content in the anterior hypothalamic nuclei (AHN) and the ventromedial hypothalamic nuclei (VMHN), and also in 5-HIAA content in lateral (LPON) and medial preoptic nuclei (MPON). The 5-HT synthesis rate, estimated from the accumulation of 5-hydroxytryptophan after blockade of the 1-amino acid decarboxylase activity, were also decreased in the AHN and the paraventricular hypothalamic nuclei (PVHN) of pinealectomized rats. In contrast, an enhanced 5-HT synthesis rate and basal 5-HIAA content were found in the suprachiasmatic nuclei (SCN) after pinealectomy. Daily treatment with melatonin for 10 days reversed most of the effects induced by pinealectomy. Thus, melatonin increased the levels of 5-HT in the AHN and VMHN, and slightly increased the 5-HIAA content in preoptic nuclei. In addition, melatonin increased the 5-HT synthesis rate in the AHN and VMHN, but also in the MPON, VMHN and dorsomedial hypothalamic nuclei (DMHN) where pinealectomy had no effect. By contrast, melatonin treatment did not affect SCN 5-HT synthesis rate, although it decreased 5-HIAA levels. The results demonstrate that melatonin is able to stimulate 5-HT metabolism in most of the hypothalamic areas, but inhibits SCN 5-HT function. Some of the effects of melatonin seems to be exerted by modulating the synthesis of the amine, although melatonin likely also interacts with other regulatory processes of 5-HT function (i.e. release/uptake). The well defined presence of melatonin receptors in the rat SCN, and its absence in other hypothalamic structures, suggest that this may be the mechanism mediating the differential response to endogenous melatonin. Moreover, the larger effect of exogenous melatonin in relation to pinealectomy suggests the presence of melatonin unespecific effects possibly owing to supraphysiological doses. The present findings may be relevant for the mode of action of melatonin and its implication in several endocrine and behavioral functions mediated by serotonergic neurons.

Weak electromagnetic fields increase the amplitude of the pattern reversal VEP response in patients with multiple sclerosis

Visual evoked potential (VEP) studies are widely used for the diagnosis of multiple sclerosis (MS) and are also useful in monitoring the effects of various therapeutic modalities in the disease. Brief, extracerebral applications of picotesla (pT) range flux intensity electromagnetic fields (EMFs) of low frequency have been shown efficacious in the treatment of motor and cognitive symptoms in MS implying that this treatment modality improves action potential transmission in demyelinating pathways. This report documents three MS patients with a remitting-progressive course in whom two successive brief extracerebral applications of pT range EMFs caused an immediate increase (and normalization) of the amplitudes of the visual evoked response in the eye previously affected by optic neuritis. However, the pretreatment prolonged latencies of the evoked responses remained essentially unchanged after the administration of EMFs. Since the latency of the VEP reflects the degree of conduction velocity and the amplitude the degree of conduction block in demyelinating optic pathways, the report demonstrates that extracerebral applications of these EMFs may rapidly reverse conduction block in demyelinating fibers. Reversal of the conduction block, which is though to be related to changes in axonal Na+ and K+ channels and synaptic neurotransmitter release, accounts for the immediate improvement of vision and other neurological deficits observed in MS patients following exposure to these EMFs.

The pineal gland, cataplexy, and multiple sclerosis

Since the discovery of melatonin as the principal hormone of the pineal gland in 1963, scientists have come to recognize that melatonin is a "master hormone" involved in the control of circadian rhythms and other biological functions. Although little is known about the influence of the pineal gland on motor control, important clues may be obtained by considering the pattern of melatonin secretion during the sleep cycles and particularly during rapid eye movement (REM) sleep when melatonin plasma levels are at their lowest. Since REM sleep is characterized by the occurrence of profound atonia which results in an almost complete paralysis of striated muscles, it is suggested that there might be a causal relationship between inhibition of melatonin secretion during REM sleep and the development of REM sleep atonia. This relationship is supported by the findings that melatonin regulates the activity of brainstem serotonin (5-HT) neurons which characteristically cease to fire during REM sleep and which faciliate the development of REM sleep atonia. Moreover, as the muscular atonia of REM sleep is physiologically and pharmacologically indistinguishable from cataplexy, it is possible that the pineal gland also influences to the development of cataplexy. Cataplexy is an ancillary symptom of narcolepsy and also occurs in multiple sclerosis (MS). In fact, it is believed that several of the neurological symptoms experienced by patients with MS such as weakness in the legs, feeling of collapsing knees, paroxysmal sudden falling, weakness in the neck, extreme fatigue, intermittent paresthesias, slurring of speech and intermittent blurring of vision, which often are exacerbated by stress and other emotional influences, may reflect the manifestations of cataplexy. Thus, several of the clinical features of MS may reflect a dissociated state of wakefulness and sleep and may improve by the administration of anticataplectic drugs.

Evidence for a regulatory role of melatonin on serotonin release and uptake in the pineal gland

Melatonin has been proposed to exert some regulatory actions within the pineal gland itself. The present study examined the effect of melatonin on the release of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) from rat pineal glands by using an in vitro perifusion system. Melatonin induced a concentration-dependent stimulatory effect on 5-HT secretion from 10(-6) M to 10(-3) M. Maximal effects were obtained with melatonin 10(-3) M and concentrations lower than 10(-6) M were without effect. The secretion of 5-HIAA was inhibited by melatonin 10(-3) and 10(-4) M, but it was increased when pineals were incubated with 10(-5) and 10(-6) M of melatonin. The indoleamine secretion was also studied on peripherally denervated rat pineal glands. Basal output of 5-HT from these glands was increased when compared with those from control rats. In contrast, the secretion of 5-HIAA was strongly reduced after removal of the sympathetic input to the pineal gland. Melatonin 10(-3) M failed to stimulate 5-HT release from denervated pineal glands, although it inhibited 5-HIAA secretion. In contrast, melatonin 10(-5) M enhanced 5-HT release without altering 5-HIAA output. Fluoxetine, a 5-HT uptake inhibitor, produced similar effects than mM concentrations of melatonin on the indoleamine secretion from control pineal glands, but it had no effect on glands taken from peripherally denervated rats. These data suggest that mM concentrations of the pineal hormone are able to stimulate 5-HT release from the pinealocyte, while mM concentrations of melatonin increase extracellular 5-HT by inhibiting its reuptake in the adrenergic nerve endings. These findings are discussed in relation to the possible role of melatonin regulating the intra- and extracellular availability of 5-HT in the pineal gland and its significance as an autocrine factor.

Effects of pinealectomy and melatonin treatments on serotonin uptake and release from synaptosomes of rat hypothalamic regions

This study examined the effects induced by long-term pinealectomy, daily melatonin treatment to pinealectomized and intact rats, and a single melatonin injection on [14C]-serotonin (5-HT) uptake and release from synaptosomes obtained of hypothalamic regions. Pinealectomy inhibited the accumulation of labeled 5-HT by synaptosomes of the preoptic area-anterior hypothalamus (POA-AH), but it failed to alter the [K+]-evoked 5-HT release. Melatonin treatment for 10 consecutive days to pinealectomized rats restored 5-HT uptake in POA-AH, and also increased 5-HT release in medial and posterior hypothalamus. These results suggest that pineal melatonin plays a stimulatory role on the serotoninergic terminals of the hypothalamus. Moreover, when daily melatonin treatment was administered to intact rats a significant increase in 5-HT uptake activity by synaptosomes of all the hypothalamic regions was observed, but 5-HT release was unaffected. In contrast, a single melatonin injection induced a significant decrease in 5-HT release from synaptosomes of the POA-AH was observed. The results suggest the existence of a differential sensitivity in the mechanisms mediating melatonin actions on 5-HT uptake/release, which depends on the presence of the pineal gland in the animals and on the frequency of the treatments with the pineal hormone.

Effects of single doses and daily melatonin treatments on serotonin metabolism in rat brain regions

The acute effects of two doses (0.5 and 1 mg/kg) of melatonin on the levels of tryptophan, serotonin, and 5-hydroxyindoleacetic acid in several rat brain regions were studied. Tryptophan content in the brain regions was unchanged by the treatments. Melatonin at a dosage of 0.5 mg/kg increased medial hypothalamic serotonin levels at 60 and 90 min after the injection. However, the dose of 1 mg/kg increased the levels of this amine or its metabolite in the preoptic area-anterior hypothalamus, medial and posterior hypothalamus, amygdala, and midbrain. These results suggest a specific regional sensitivity to melatonin as well as a dose-dependent response. The stimulatory melatonin effect on the serotoninergic system was also observed after a daily treatment with this hormone (0.5 mg/kg, twice daily during 10 days) in both intact or pinealectomized rats. In intact rats, melatonin treatment increased the levels of 5-hydroxyindole-3-acetic acid in the preoptic area-anterior hypothalamus and medial hypothalamus, while in pinealectomized rats melatonin increased the serotonin content in the medial hypothalamic region. The data support the idea that melatonin has a selective action on serotonin metabolism in regions that contain serotoninergic terminals, especially at medial hypothalamic level.

Melatonin effect on serotonin uptake and release in rat platelets: diurnal variation in responsiveness

The present study was conducted to examine whether melatonin impairs serotonin (5HT) release and uptake in rat platelets. Exposure of platelet-rich plasma samples (PRP) to melatonin induced a concentration-dependent inhibition of 5HT uptake and the value of IC50 was 1.3 x 10(-3) M. We have also investigated the melatonin effect on the kinetic parameters of platelet 5HT uptake. Transport capacity was inhibited (Vmax;

CONTROL

2.28 +/- 0.52, Melatonin: 0.74 +/- 0.13 pmol/10(7) platelet.min; p < 0.05) while the affinity of 5HT for its uptake carriers remained unaltered, thus indicating a non-competitive effect. Studies carried out to determine the existence of a differential morning (8:00h)-evening (21:00h) melatonin effect showed a higher platelet uptake sensitivity at 8:00h (two-way ANOVA, p < 0.001). Spontaneous 5HT release was not impared by the hormone and no daily variation in sensitivity was detected. The possible mechanism of action of melatonin on platelet transport is discussed, and the results support the suitability of the platelet model for studying sensitivity changes in target cells to the hormone.

Modification by oxazepam of the diurnal variations in brain 125I-melatonin binding sites in sham-operated and pinealectomized rats

Sham-operated and pinealectomized male rats were maintained at 14 h light:10 h dark cycles (lights-on 5.00 h) and injected daily, for 14 days, with oxazepam or vehicle. 125I-melatonin binding was recorded in synaptosomes prepared at 10.00, 18.00, and 24.00 h from the hypothalamus, hippocampus and medulla-pons of the rats. In the sham-operated, vehicle treated rats, specific 125I-melatonin binding in all brain areas studied was higher at 18.00 h, whereas in the oxazepam-treated animals, binding was higher at 24.00 h than at the other times tested. In the pinealectomized, vehicle-treated rats, the binding recorded at 18.00 h in all three brain areas, was lower than at the other times of day tested. Oxazepam treatment decreased 125I-melatonin binding at 24.00 h in the hippocampus and medulla-pons of the pinealectomized rats and did not significantly affect the binding in the hypothalamus. These results indicate the ability of oxazepam, pinealectomy and their combination, to manipulate the diurnal variations in 125I-melatonin binding sites in the rat brain.

Melatonin inhibits ACh release from rabbit retina

Previous studies have suggested that melatonin, released from photoreceptors, may modulate retinal dark-adaptive responses by inhibition of dopamine release from retinal interneurons. We have broadened these studies to examine the effect of melatonin on release of another retinal neurotransmitter, acetylcholine (ACh). The ACh system in rabbit retina has been localized to starburst amacrine cells, which release ACh in response to a variety of experimental stimuli, including direct potassium depolarization, flashing light, and glutamatergic as well as GABAergic inputs. The effect of melatonin on release of endogenously synthesized [3H]-ACh was measured in perfusates from retinas or retinal synaptosomes preloaded with [3H]-choline chloride. Melatonin significantly inhibited ACh release stimulated by potassium in intact retina but not in synaptosomes. Stimulation of intact retina by flashing light or by the glutamate receptor agonist, kainic acid, was also inhibited by melatonin. In contrast, there was no significant effect of melatonin on picrotoxin-induced release. These findings suggest that melatonin does have an inhibitory effect on ACh release, either by direct interaction with the cholinergic amacrine cell, or indirectly via GABAergic but not glutamatergic neurons.

The therapeutic potential for tryptophan and melatonin: possible roles in depression, sleep, Alzheimer's disease and abnormal aging

Evidence suggests that stress and/or a dietary lack of tryptophan may make deficiencies of serotonin and melatonin common. In addition, older animals and human beings have a reduced ability to synthesize melatonin. Disorders of melatonin levels and rhythms are suggested to be a cause of affective disease, abnormal sleep, Alzheimer's disease, and some age related disorders. If these ideas prove to be true, then preventive measures are possible.

Lesions in suprachiasmatic nuclei simulate effects of pinealectomy on prolactin release in ovariectomized and sulpiride-treated female rats

Previous studies indicate that the pineal gland alters prolactin secretion, and it was suggested that at least part of the effect of the pineal hormone melatonin on prolactin release may be mediated by the hypothalamic structures. In this study, pinealectomy and lesions of the suprachiasmatic nuclei were found to alter serum levels of prolactin in the same direction, an effect that was counteracted by daily afternoon melatonin administration. Melatonin, but not other pineal indoles, also prevented sulpiride-induced prolactin secretion in pinealectomized or suprachiasmatic nuclei-lesioned and ovariectomized rats, which suggested that the pineal gland can modulate prolactin secretion by acting through a dopamine mechanism independent of hypothalamic suprachiasmatic structures.

The effects of melatonin and pinealectomy upon local cerebral glucose utilization in awake unrestrained rats are restricted to a few specific regions

The effects of the infusion of melatonin (MT) and/or of pinealectomy upon glucose utilization in anatomically discrete regions of the brain of freely moving rats have been studied by the quantitative autoradiographic 2-deoxy-D-[1-14C]glucose technique. The experiments were made from 14.00 to 16.00 h, when MT is normally not secreted by the pineal gland, in order to compare the local cerebral glucose utilization (LCGU) response to MT from animals with long-term (pinealectomized) or short-term (pineal intact) absence of MT secretion. The majority of the 98 brain areas examined showed no change in LCGU after MT administration and/or pinealectomy. Pinealectomy increased the LCGU in the median mammillary nucleus only, whereas following MT administration, an increase in LCGU was observed in 3 cerebral regions of intact rats (paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, choroid plexus of the third ventricle) and in 5 cerebral regions of pinealectomized rats (paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, choroid plexuses of the lateral ventricles, third and fourth ventricles). Except for the choroid plexuses of the fourth ventricle, there was no difference in LCGU response to MT between pinealectomized and intact animals. This does not favor the hypothesis of receptor supersensitivity under the conditions of this experiment. Our results suggest that the hypothalamus, nucleus of the solitary tract and choroid plexuses represent a neural substrate through which MT could influence the activity of the central nervous system when administered at a low dose under near-physiological conditions.

The electrophysiological effects of melatonin and a putative melatonin antagonist (N-acetyltryptamine) on rat suprachiasmatic neurones in vitro

Several studies have implicated the suprachiasmatic nuclei (SCN) as a target for the action of melatonin in its regulation of seasonal and circadian behaviour. Single-unit activity from the SCN and adjacent paraventricular area and anterior hypothalamus was recorded using the in vitro rat hypothalamic slice preparation. Neurones were tested for responses to iontophoresed or pressure ejected melatonin (n = 62) and serotonin (5-HT, n = 80). The majority (n = 24-26) of melatonin-sensitive SCN neurones were inhibited by melatonin in a dose-dependent manner during the latter portion of the projected light phase of the circadian light-dark cycle. A putative melatonin antagonist, N-acetyltryptamine, exhibited concentration dependent mixed agonist-antagonist effects on melatonin-evoked responses.

Radiosynthesis of NCA [carbonyl-11C]6-fluoromelatonin

A method is described for the preparation of [carbonyl-11C]6-fluoromelatonin for intravenous injection and potential study of the melatonin system in vivo by positron emission tomography. The preparation is based on the acetylation of 6-fluoro-5-methoxytryptamine with NCA [1-11C]acetyl chloride (itself prepared from cyclotron-produced [11C]carbon dioxide) and purification by HPLC. It gives chemically and radiochemically pure [carbonyl-11C]6-fluoromelatonin in 35% radiochemical yield (from [11C]CO2, decay-corrected) within 35 min from the end of radionuclide production and with high specific activity e.g. 1.6 GBq/mumol (43 mCi/mumol) at the end of synthesis from 1.1 GBq (30 mCi) of [11C]carbon dioxide.

Subneuronal fate of intracerebroventricular injected 3H-melatonin

The fate of 3H-melatonin after its intracerebroventricular administration was studied both in different brain regions and in subcellular fractions. The rate of disappearance of 3H-melatonin from the brain was found to be multiphasic. Forty-eight h after a 3H-melatonin injection, radioactivity was still present in the brain. Nonlinear regression analysis of the data confirmed a very rapid half-life component and (t1/2 = 3.04 min) a slower one (t1/2 = 36 min). We also found a much slower component (t1/2 = 24 h), however. Considerable metabolism of melatonin was detected since only 36.5% of administered radioactivity remained as melatonin at 45 min. The subcellular distribution of the radioactivity present in the brain at all times studied showed that a major proportion of the radioactivity remained in the cytosol and respectively decreasing proportions in the 900g pellet, mitochondrial pellet, and the microsomes. The radioactivity remaining in the cytosol at 45 min was found to coelute with a macromolecule that was resolved by gel filtration and could be displaced by previous melatonin administration. Purified nuclei retained 0.71% of the radioactivity at 45 min; of this total, 73% was KCl extractable. Our data suggest the presence of a binding site in the cytosol and in the nucleus. The presence of 3H-melatonin up to 48 h after its administration may account for melatonin's long-term effects on brain function.

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

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

The role of melatonin and serotonin in aging

The hypothesis presented in this paper defines aging as a pathological process originating in the pineal gland. This results in a diminished output of melatonin, along with a diminished melatonin to serotonin ratio, leading to a decline in adaptive processes and a predictable syndrome manifested by the "diseases of the aged" (DOA) and subsequent death of the organism. That is, aging is a syndrome of relative melatonin deficiency resulting from the gradual failure of the pineal gland.

Melatonin: parallels in pineal gland and retina

It is apparent that several relationships exist between the pineal gland and retina. The similarities in development and morphology have been obvious for many years. A recent resurgence of interest in this field has led to a further understanding of many functional similarities between these two organs. A notable feature of the pineal gland and retina is their common ability to synthesize the indolamine hormone, melatonin. Many investigators suspect that the cyclic rhythm of retinal melatonin synthesis may be related to other cyclic events which normally occur in the retina.

The influence of melatonin on the content of vasopressin and oxytocin in the hypothalamus and neurohypophysis in euhydrated and dehydrated male rats

Melatonin injected in a single intraperitoneal dose of 100 micrograms/100 g b.w. to euhydrated rats resulted in a decrease of neurohypophysial oxytocin content but the hypothalamic oxytocin storage as well as the hypothalamo-neurohypophysial storage of vasopressin were not changed. Following 8 d of once-daily melatonin treatment the hypothalamic and neurohypophysial oxytocin and vasopressin content was decreased. It might be therefore suggested that melatonin increases the release of neurohypophysial hormones and/or decreases their synthesis. Melatonin did not significantly modify the neurohypophysial vasopressin depletion rate in animals deprived of water up to 8 days. No consistent effects of melatonin on the decrease of hypothalamo-neurohypophysial content of oxytocin were noted under conditions of dehydration and simultaneous administration of melatonin up to 8 d.

Could supplementary dietary tryptophan and taurine prevent epileptic seizures?

Roles for melatonin, taurine, and the pineal gland in epilepsy are examined. Cerebrospinal fluid melatonin and taurine may be natural anticonvulsants. The flow of cerebrospinal fluid may bathe the medial and lateral geniculate ganglia and the superior and inferior colliculli with these anticonvulsant substances. Supplemental dietary taurine and tryptophan could be of value in the treatment and prevention of seizures.

Enhancement of melatonin's antigonadal action by daily injections of the serotonin uptake inhibitor fluoxetine in male hamsters

The antigonadal effects of daily melatonin injections in golden hamsters are known to be dependent on the time of day when injections are given. As the serotonergic system of the brain has been shown to undergo daily variations, and to influence gonadotrophic activity, it was decided to investigate the relationship between melatonin effects and brain serotonergic function by employing the selective inhibitor of serotonin uptake fluoxetine (Lilly 110140). Daily injections of fluoxetine (5 mg/kg) were administered at two different times of day (late scotophase or late photophase) in combination with or without melatonin (25 micrograms) at either early or late photophase. Following nine weeks on a long photoperiod (14L/10D) testicular widths and weights in all groups receiving late-photophase melatonin were significantly diminished. Average testicular widths and weights in groups receiving fluoxetine plus late-photophase melatonin were smaller than those groups receiving late-photophase melatonin alone. Furthermore, animals receiving fluoxetine plus early-photophase melatonin also showed signs of gonadal regression. These results support the hypothesis that altered serotonergic function is a component of melatonin-induced gonadal inhibition in the male hamster.

Effects of P-chlorophenylalanine on pineal and endocrine function in the rat

This study attempted to determine whether brain serotonin (5-HT), which is altered by melatonin administration, is involved in mediating the effects of melatonin on basal endocrine function. Pineal melatonin levels, serum N-acetylserotonin (NAS) levels, adrenocortical activity, and other endocrine parameters were measured following 5-HT depletion by p-chlorophenylalanine (p-CPA) together with either pineal stimulation by blinding or blinding plus pinealectomy. Blinding increased pineal melatonin levels in both saline and p-CPA treated animals. P-CPA treatment increased adrenal weights and morning plasma corticosterone levels in both blinded and blinded-pinealectomized animals. Conversely, p-CPA depressed pineal melatonin levels and serum NAS but elevated morning plasma corticosterone levels in sighted controls. P-CPA also decreased plasma prolactin and growth hormone levels in intact animals. These findings suggest that 5-HT inhibits morning corticosterone secretion and stimulates prolactin and growth hormone release. In addition, melatonin and serotonin may function independently in regulating adrenocortical function, while melatonin's effect is superceded by that of serotonin.

Daily rhythms of serotonin metabolism in the medial hypothalamus of the chicken: effects of pinealectomy and exogenous melatonin

Indoleamine levels in punches of the medial hypothalamus containing the suprachiasmatic nuclei (SCN) of 4-week-old cockerels were determined by HPLC-EC. Melatonin levels in punches were determined by radioimmunoassay (RIA). Daily rhythms of serotonin (5-HT) and of its metabolite 5-hydroxy-3-indoleacetic acid (5-HIAA) were observed; levels were higher at midnight than at midday. A daily rhythm with the same phase in punch melatonin content was also observed. Pinealectomy at 1 week after hatching abolished the 5-HIAA and melatonin rhythm in 4-week-old birds but did not abolish the 5-HT rhythm. Injections of melatonin (0.5 mg/kg) increased 5-HT, 5-HIAA and melatonin levels in the hypothalamic punches. These results indicate that circulating melatonin of pineal origin may act to increase 5-HT turnover and/or release in the SCN. They suggest a link between the circadian secretion of pineal melatonin and the regulation of 5-HT projections to the hypothalamus from the raphe nuclei in the brainstem of the chicken. We have previously shown that the rhythmic secretion of melatonin by the pineal is influenced by oscillators in the brain via the superior cervical ganglia. The results reported here indicate that melatonin in turn may regulate brain oscillators, suggesting a neuroendocrine loop within the avian circadian system.

Regulation of indoleamine N-acetyltransferase activity in the retina: effects of light and dark, protein synthesis inhibitors and cyclic nucleotide analogs

The regulation of indoleamine N-acetyltransferase (NAT) in the posterior eye was investigated in vivo, and in vitro in cultured eye cups. Surgical separation of neural retina from the retinal pigment epithelium-choroid complex indicated that NAT was localized to neural retina. The activity of retinal NAT fluctuated in vivo in a rhythmic fashion, with peak activity in the dark phase of the light-dark cycle. The rhythm of NAT activity persisted for up to 3 days in constant darkness, with a rhythmic period of approximately 25 h. The rhythm was suppressed by constant light, and could be phase-shifted by exposure to a new light-dark cycle. These observations indicate that retinal NAT activity occurs as a circadian rhythm that is entrained by light and dark. Retinas also responded to light and dark in vitro with changes of NAT activity. A significant increase in retinal NAT activity occurred in eye cups cultured in darkness during the dark phase of the light-dark cycle. This increase was completely suppressed in eye cups cultured at the same time of day in light. The dark-induced increase in NAT was completely blocked by protein synthesis inhibitors, and mimicked in light by cyclic AMP analogs. The similarity of the regulation of NAT activity in retina to that in pineal, and the possible relationship of the retinal NAT rhythm to cyclic metabolism in photoreceptors are discussed.

Melatonin increases cGMP and decreases cAMP levels in rat medial basal hypothalamus in vitro

Measurement of cyclic nucleotide accumulation in rat medial basal hypothalamus (MBH) incubated in vitro with methoxyindoles indicated that melatonin (10(-8) M or greater) increased significantly cGMP and depressed cAMP levels. Only the effect on MBH cAMP was shared by 5-methoxytryptophol which exhibited a greater activity than melatonin. 6-Fluoromelatonin (10(-7) M) increased cGMP, whereas the effect of 6-hydroxymelatonin was not significant. Both 6-fluoro- and 6-hydroxymelatonin (10(-5) M) depressed MBH cAMP accumulation while only 6-fluoromelatonin affected it at 10(-7) M concentrations. These data suggest that melatonin and 5-methoxytryptophol affect differently cyclic nucleotide content of MBH at physiological concentrations.

Na+ independent binding of [3H] muscimol to a membrane fraction of the brains of normal and dwarf mice

Although their body weights were decreased by about 77% and their brain weights by about 30%, high-affinity [3H] muscimol binding to a cerebral membrane fraction was not altered in hereditary pituitary dwarf mice. Marked changes in the level of pituitary growth-associated hormones do not appear to be associated with a change in cerebral GABA-receptors.