Drug Insight: the use of melatonergic agonists for the treatment of insomnia-focus on ramelteon

Significance of Melatonin in the Regulation of Circadian Rhythms and Disease Management

Melatonin, the 'hormone of darkness' is a neuronal hormone secreted by the pineal gland and other extra pineal sites. Responsible for the circadian rhythm and seasonal behaviour of vertebrates and mammals, melatonin is responsible for regulating various physiological conditions and the maintenance of sleep, body weight and the neuronal activities of the ocular sites. With its unique amphiphilic structure, melatonin can cross the cellular barriers and elucidate its activities in the subcellular components, including mitochondria. Melatonin is a potential scavenger of oxygen and nitrogen-reactive species and can directly obliterate the ROS and RNS by a receptor-independent mechanism. It can also regulate the pro- and anti-inflammatory cytokines in various pathological conditions and exhibit therapeutic activities against neurodegenerative, psychiatric disorders and cancer. Melatonin is also found to show its effects on major organs, particularly the brain, liver and heart, and also imparts a role in the modulation of the immune system. Thus, melatonin is a multifaceted candidate with immense therapeutic potential and is still considered an effective supplement on various therapies. This is primarily due to rectification of aberrant circadian rhythm by improvement of sleep quality associated with risk development of neurodegenerative, cognitive, cardiovascular and other metabolic disorders, thereby enhancing the quality of life.

Melatonin Treatment in Kidney Diseases

Melatonin is a neurohormone that is mainly secreted by the pineal gland. It coordinates the work of the superior biological clock and consequently affects many processes in the human body. Disorders of the waking and sleeping period result in nervous system imbalance and generate metabolic and endocrine derangements. The purpose of this review is to provide information regarding the potential benefits of melatonin use, particularly in kidney diseases. The impact on the cardiovascular system, diabetes, and homeostasis causes melatonin to be indirectly connected to kidney function and quality of life in people with chronic kidney disease. Moreover, there are numerous reports showing that melatonin plays a role as an antioxidant, free radical scavenger, and cytoprotective agent. This means that the supplementation of melatonin can be helpful in almost every type of kidney injury because inflammation, apoptosis, and oxidative stress occur, regardless of the mechanism. The administration of melatonin has a renoprotective effect and inhibits the progression of complications connected to renal failure. It is very important that exogenous melatonin supplementation is well tolerated and that the number of side effects caused by this type of treatment is low.

Redox Biology of Melatonin: Discriminating Between Circadian and Noncircadian Functions

Melatonin has not only to be seen as a regulator of circadian clocks. In addition to its chronobiotic functions, it displays other actions, especially in cell protection. This includes antioxidant, anti-inflammatory, and mitochondria-protecting effects. Although protection is also modulated by the circadian system, the respective actions of melatonin can be distinguished and differ with regard to dose requirements in therapeutic settings. It is the aim of this article to outline these differences in terms of function, signaling, and dosage. Focus has been placed on both the nexus and the dissecting properties between circadian and noncircadian mechanisms. This has to consider details beyond the classic view of melatonin's role, such as widespread synthesis in extrapineal tissues, formation in mitochondria, effects on the mitochondrial permeability transition pore, and secondary signaling, for example, via upregulation of sirtuins and by regulating noncoding RNAs, especially microRNAs. The relevance of these findings, the differences and connections between circadian and noncircadian functions of melatonin shed light on the regulation of inflammation, including macrophage/microglia polarization, damage-associated molecular patterns, avoidance of cytokine storms, and mitochondrial functions, with numerous consequences to antioxidative protection, that is, aspects of high actuality with regard to deadly viral and bacterial diseases. Antioxid. Redox Signal. 37, 704-725.

Divergent Importance of Chronobiological Considerations in High- and Low-dose Melatonin Therapies

Melatonin has been used preclinically and clinically for different purposes. Some applications are related to readjustment of circadian oscillators, others use doses that exceed the saturation of melatonin receptors MT1 and MT2 and are unsuitable for chronobiological purposes. Conditions are outlined for appropriately applying melatonin as a chronobiotic or for protective actions at elevated levels. Circadian readjustments require doses in the lower mg range, according to receptor affinities. However, this needs consideration of the phase response curve, which contains a silent zone, a delay part, a transition point and an advance part. Notably, the dim light melatonin onset (DLMO) is found in the silent zone. In this specific phase, melatonin can induce sleep onset, but does not shift the circadian master clock. Although sleep onset is also under circadian control, sleep and circadian susceptibility are dissociated at this point. Other limits of soporific effects concern dose, duration of action and poor individual responses. The use of high melatonin doses, up to several hundred mg, for purposes of antioxidative and anti-inflammatory protection, especially in sepsis and viral diseases, have to be seen in the context of melatonin's tissue levels, its formation in mitochondria, and detoxification of free radicals.

Melatonergic agonist regulates circadian clock genes and peripheral inflammatory and neuroplasticity markers in patients with depression and anxiety

OBJECTIVE

Circadian dysfunction is a core manifestation and a risk factor for psychiatric disorders. Ramelteon (RMT), a melatonin receptor agonist, has been shown to induce sleep phase shifts and has been used to normalize sleep onset time. RMT has been used in sleep disorders, depression and anxiety. In this study, we aimed to investigate the effects of RMT in regulating gene expression profiles of the circadian clock and peripheral markers of inflammation and neuroplasticity.

METHODS

Sixteen patients with a diagnosis of primary insomnia comorbid with depression and anxiety and ten healthy controls were recruited in an 8-week open-label trial. The patients with primary insomnia received RMT 8 mg/day. The morning expression profiles of 15 core clock genes from peripheral blood mononuclear cells (PBMCs), urine and plasma levels of melatonin and its metabolite levels, and plasma inflammatory markers and neurotrophin levels were evaluated at baseline, 4th and 8th week of RMT treatment.

RESULTS

RMT treatment was associated with significant clinical improvement in depression scores at 8th week (Hamilton depression rating scale scores (Mean ± SEM) from 21.5 ± 2.44 to 14.31 ± 2.25, p ≤ 0.05). The overall poor sleep quality (Pittsburgh sleep quality index) of the patient group significantly improved (p ≤ 0.05) following RMT treatment. The mRNA level analysis showed a significant association between RMT treatment and alterations of the nine core circadian genes (CLOCK, PER1, PER2, CRY1, CRY2, NR1D1, NR1D2, DEC1 and TIMELESS) in the patient group when compared with the control group (p ≤ 0.05). Compared with the controls, the patient group had a decrease in neurotrophins (brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor and beta-nerve growth factor; p ≤ 0.05) but an increase in pro-inflammatory cytokine levels (interleukin-6, interleukin-1b, tumour necrosis factor-alpha and interferon gamma; p ≤ 0.05); RMT treatment normalized the levels of neurotrophins and cytokine levels.

CONCLUSION

RMT treatment is able to restore phase-shifted melatonin markers, normalized the altered expression of the circadian genes, the levels of inflammatory cytokines and neurotrophins in patients with insomnia comorbid anxiety and depression.

Effects of hypnotics on prefrontal cortex activity during a verbal fluency task in healthy male subjects: A near-infrared spectroscopy study

OBJECTIVE

To assess the effects of hypnotics on prefrontal cortex activity in healthy subjects using near-infrared spectroscopy (NIRS) in a double-blind, placebo-controlled crossover trial.

METHODS

Eighteen healthy males received acute doses of ramelteon (8 mg), triazolam (0.125 mg), or placebo in a predetermined randomization schedule, with a washout period of more than 1 week. All subjects performed a verbal fluency task during NIRS assessments at baseline and at 1 and 4 hr post-dose. The number of words correctly generated during the task (behavioral performance) and scores on the Stanford Sleepiness Scale (SSS) were also recorded at each test time.

RESULTS

Compared with the placebo, triazolam (0.125 mg) significantly decreased oxyhemoglobin (oxy-Hb) concentration change in NIRS during the posttask period and significantly increased behavioral performance, whereas triazolam (0.125 mg) and ramelteon (8 mg) significantly increased SSS scores.

CONCLUSIONS

The differential effects of two types of hypnotics on oxy-Hb change measured by NIRS were observed in acute dosing, suggesting that when assessing brain activity of patients with psychiatric disorders, researchers should consider how certain types of hypnotics can influence brain function. This would also provide useful information to clinicians when prescribing hypnotics suitable for their patients' conditions.

Epigenetic regulation of melatonin receptors in neuropsychiatric disorders

Melatonin, the primary indoleamine hormone of the mammalian pineal gland, is known to have a plethora of neuroregulatory, neuroprotective and other properties. Melatonergic signalling is mediated by its two GPCRs, MT₁ and MT₂ , which are widely expressed in the mammalian CNS. Melatonin levels and receptor expression often show a decrease during normal ageing, and this reduction may be accelerated in some disease states. Depleted melatonergic signalling has been associated with neuropsychiatric dysfunction and impairments in cognition, memory, neurogenesis and neurorestorative processes. The anticonvulsant and mood stabilizer, valproic acid (VPA), up-regulates melatonin MT₁ and/or MT₂ receptor expression in cultured cells and in the rat brain. VPA is known to affect gene expression through several mechanisms, including the modulation of intracellular kinase pathways and transcription factors, as well as the inhibition of histone deacetylase (HDAC) activity. Interestingly, other HDAC inhibitors, such as trichostatin A, which are structurally distinct from VPA, can also up-regulate melatonin receptor expression, unlike a VPA analogue, valpromide, which lacks HDAC inhibitory activity. Moreover, VPA increases histone H3 acetylation along the length of the MT₁ gene promoter in rat C6 cells. These findings indicate that an epigenetic mechanism, linked to histone hyperacetylation/chromatin remodelling and associated changes in gene transcription, is involved in the up-regulation of melatonin receptors by VPA. Epigenetic induction of MT₁ and/or MT₂ receptor expression, in areas where these receptors are lost because of ageing, injury or disease, may be a promising therapeutic avenue for the management of CNS dysfunction and other disorders. 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.

Dietary Sources and Bioactivities of Melatonin

Insomnia is a serious worldwide health threat, affecting nearly one third of the general population. Melatonin has been reported to improve sleep efficiency and it was found that eating melatonin-rich foods could assist sleep. During the last decades, melatonin has been widely identified and qualified in various foods from fungi to animals and plants. Eggs and fish are higher melatonin-containing food groups in animal foods, whereas in plant foods, nuts are with the highest content of melatonin. Some kinds of mushrooms, cereals and germinated legumes or seeds are also good dietary sources of melatonin. It has been proved that the melatonin concentration in human serum could significantly increase after the consumption of melatonin containing food. Furthermore, studies show that melatonin exhibits many bioactivities, such as antioxidant activity, anti-inflammatory characteristics, boosting immunity, anticancer activity, cardiovascular protection, anti-diabetic, anti-obese, neuroprotective and anti-aging activity. This review summaries the dietary sources and bioactivities of melatonin, with special attention paid to the mechanisms of action.

Zolpidem's use for insomnia

Zolpidem is a short-acting non-benzodiazepine hypnotic drug that belongs to the imidazopyridine class. In addition to immediate-release (IR) and extended-release (ER) formulations, the new delivery forms including two sublingual tablets [standard dose (SD) and low dose (LD)], and an oral spray form have been recently developed which bypass the gastrointestinal tract. So far, Zolpidem has been studied in several clinical populations: cases poor sleepers, transient insomnia, elderly and non-elderly patients with chronic primary insomnia, and in comorbid insomnia. Peak plasma concentration (Tmax) of zolpidem-IR occurs in 45 to 60min, with the terminal elimination half-life (t½) equating to 2.4h. The extended-release formulation results in a higher concentration over a period of more than 6h. Peak plasma concentration is somewhat shorter for the sublingual forms and the oral spray, while their t½ is comparable to that of zolpidem-IR. Zolpidem-IR reduces sleep latency (SL) at recommended doses of 5mg and 10mg in elderly and non-elderly patients, respectively. Zolpidem-ER at doses of 6.25mg and 12.5mg, improves sleep maintenance in elderly and non-elderly patients, respectively, 4h after its administration. Sublingual zolpidem-LD (5mg) and zolpidem oral spray are indicated for middle-of-the-night (MOTN) wakefulness and difficulty returning to sleep, while sublingual zolpidem-SD (10mg) is marketed for difficulty falling asleep. With their array of therapeutic uses and their popularity among physicians and patients; this review describes the clinical pharmacology, indications and uses, identifying withdrawal symptoms, abuse and dependence potentials, and adverse drug reactions are discussed.

Sleep, Melatonin, and the Menopausal Transition: What Are the Links?

The pineal hormone Melatonin plays an important role in the regulation of the circadian sleep/wake cycle, mood, and perhaps immune functions, carcinogensis and reproduction. The human circadian rhythm of melatonin release from the pineal gland is tightly synchronized with the habitual hours of sleep. Peri- and postmenopausal women often complain of difficulties initiating and/or maintaining sleep, with frequent nocturnal and early morning awakenings. In this review we discuss the pathophysiology of melatonin function as it relates to sleep disorders in menopausal women, highlighting the potential use of exogenous melatonin during the menopausal transition and beyond.

A Review of Melatonin, Its Receptors and Drugs

After a Turkish scientist took Nobel Prize due to his contributions to understand clock genes, melatonin, closely related to these genes, may begin to shine. Melatonin, a hormone secreted from the pineal gland at night, plays roles in regulating sleep-wake cycle, pubertal development and seasonal adaptation. Melatonin has antinociceptive, antidepressant, anxiolytic, antineophobic, locomotor activity-regulating, neuroprotective, anti-inflammatory, pain-modulating, blood pressure-reducing, retinal, vascular, anti-tumor and antioxidant effects. It is related with memory, ovarian physiology, and osteoblast differentiation. Pathologies associated with an increase or decrease in melatonin levels are summarized in the review. Melatonin affects by four mechanisms: 1) Binding to melatonin receptors in plasma membrane, 2) Binding to intracellular proteins such as calmoduline, 3) Binding to Orphan nuclear receptors, and 4) Antioxidant effect. Receptors associated with melatonin are as follows: 1) Melatonin receptor type 1a: MT1 (on cell membrane), 2) Melatonin receptor type 1b: MT2 (on cell membrane), 3) Melatonin receptor type 1c (found in fish, amphibians and birds), 4) Quinone reductase 2 enzyme (MT3 receptor, a detoxification enzyme), 5) RZR/RORα: Retinoid-related Orphan nuclear hormone receptor (with this receptor, melatonin binds to the transcription factors in nucleus), and 6) GPR50: X-linked Melatonin-related Orphan receptor (it is effective in binding of melatonin to MT1). Melatonin agonists such as ramelteon, agomelatine, circadin, TIK-301 and tasimelteon are introduced and side effects will be discussed. In conclusion, melatonin and related drugs is a new and promising era for medicine. Melatonin receptors and melatonin drugs will take attention with greater interest day by day in the future.

The effects of acute treatment with ramelteon, triazolam, and placebo on driving performance, cognitive function, and equilibrium function in healthy volunteers

RATIONALE

Hypnotics are widely used to treat insomnia but adverse effects of different hypnotics, especially benzodiazepine receptor agonists, are getting more attention lately. The effects of novel hypnotics have not been fully examined.

OBJECTIVE

This study aims to assess the effects of two hypnotics, ramelteon and triazolam, on driving performance, cognitive function, and equilibrium function.

METHODS

In this double-blinded, three-way crossover trial, 17 healthy males received acute doses of 8 mg ramelteon, 0.125 mg triazolam, and placebo. The subjects were administered three driving tasks-road-tracking, car-following, and harsh-braking-using a driving simulator and three cognitive tasks-Continuous Performance Test, N-back Test, and Trail-Making Test-at baseline and at 1 and 4 h post-dosing. The Stanford Sleepiness Scale scores and computerized posturography were also assessed.

RESULTS

In the driving simulations, ramelteon and triazolam increased the number of subjects who slid off the road. Triazolam increased the standard deviation of lateral position compared to ramelteon and placebo at 1 h post-dosing. Ramelteon and triazolam significantly increased the time to complete of Trail-Making Test part A and the environmental area in posturography compared to placebo at 1 and 4 h post-dosing. Ramelteon and triazolam significantly increased subjective sleepiness compared to placebo at 1 h post-dosing.

CONCLUSIONS

Ramelteon may affect road-tracking performance, visual attention and/or psychomotor speed measured by Trail-Making Test part A, and body balance in acute dosing. Lower dose of triazolam also impaired performance worse than ramelteon. Physicians should consider risks and benefits when prescribing both drugs, especially in the initial period of administration.

Melatonergic drugs in development

Melatonin (N-acetyl-5-methoxytryptamine) is widely known as "the darkness hormone". It is a major chronobiological regulator involved in circadian phasing and sleep-wake cycle in humans. Numerous other functions, including cyto/neuroprotection, immune modulation, and energy metabolism have been ascribed to melatonin. A variety of studies have revealed a role for melatonin and its receptors in different pathophysiological conditions. However, the suitability of melatonin as a drug is limited because of its short half-life, poor oral bioavailability, and ubiquitous action. Due to the therapeutic potential of melatonin in a wide variety of clinical conditions, the development of new agents able to interact selectively with melatonin receptors has become an area of great interest during the last decade. Therefore, the field of melatonergic receptor agonists comprises a great number of structurally different chemical entities, which range from indolic to nonindolic compounds. Melatonergic agonists are suitable for sleep disturbances, neuropsychiatric disorders related to circadian dysphasing, and metabolic diseases associated with insulin resistance. The results of preclinical studies on animal models show that melatonin receptor agonists can be considered promising agents for the treatment of central nervous system-related pathologies. An overview of recent advances in the field of investigational melatonergic drugs will be presented in this review.

Antinociceptive effects of novel melatonin receptor agonists in mouse models of abdominal pain

AIM: To characterize the antinociceptive action of the novel melatonin receptor (MT) agonists, Neu-P11 and Neu-P12 in animal models of visceral pain.

METHODS: Visceral pain was induced by intracolonic (ic) application of mustard oil or capsaicin solution or by intraperitoneal (ip) administration of acetic acid. Neu-P11, Neu-P12, or melatonin were given ip or orally and their effects on pain-induced behavioral responses were evaluated. To identify the receptors involved, the non-selective MT1/MT2 receptor antagonist luzindole, the MT2 receptor antagonist 4-P-PDOT, or the μ-opioid receptor antagonist naloxone were injected ip or intracerebroventricularly (icv) prior to the induction of pain.

RESULTS: Orally and ip administered melatonin, Neu-P11, and Neu-P12 reduced pain responses in a dose-dependent manner. Neu-P12 was more effective and displayed longer duration of action compared to melatonin. The antinociceptive effects of Neu-P11 or Neu-P12 were antagonized by ip or icv. administered naloxone. Intracerebroventricularly, but not ip administration of luzindole or 4-P-PDOT blocked the antinociceptive actions of Neu-P11 or Neu-P12.

CONCLUSION: Neu-P12 produced the most potent and long-lasting antinociceptive effect. Further development of Neu-P12 for future treatment of abdominal pain seems promising.

Characterization and engineering of an o-xylene dioxygenase for biocatalytic applications

Depending on the size and position of the substituent groups on the aromatic ring, the o-xylene dioxygenase from Rhodococcus sp. strain DK17 possesses the unique ability to perform distinct regioselective hydroxylations via differential positioning of substrates within the active site. The substrate-binding pocket of the DK17 o-xylene dioxygenase is large enough to accommodate bicyclics and can be divided into three regions (distal, central, and proximal), and hydrophobic interactions in the distal position are important for substrate binding. Current molecular and functional knowledge contribute insights into how to engineer this enzyme to create tailor-made properties for chemoenzymatic syntheses.

Sleep medicine pharmacotherapeutics overview: today, tomorrow, and the future (Part 1: insomnia and circadian rhythm disorders)

Over the past 10 years, significant strides have been made in the understanding, development, and availability of sleep disorder therapeutics. In this review series, we discuss the current evidence surrounding the mechanisms of actions, indications, efficacy, and adverse side effects associated with the available armamentarium of sleep over-the-counter and pharmacotherapeutics. This article is the first of a two-part series that covers the therapeutics for insomnia and circadian rhythm disorders.

Chronobiology of Melatonin beyond the Feedback to the Suprachiasmatic Nucleus-Consequences to Melatonin Dysfunction

The mammalian circadian system is composed of numerous oscillators, which gradually differ with regard to their dependence on the pacemaker, the suprachiasmatic nucleus (SCN). Actions of melatonin on extra-SCN oscillators represent an emerging field. Melatonin receptors are widely expressed in numerous peripheral and central nervous tissues. Therefore, the circadian rhythm of circulating, pineal-derived melatonin can have profound consequences for the temporal organization of almost all organs, without necessarily involving the melatonin feedback to the suprachiasmatic nucleus. Experiments with melatonin-deficient mouse strains, pinealectomized animals and melatonin receptor knockouts, as well as phase-shifting experiments with explants, reveal a chronobiological role of melatonin in various tissues. In addition to directly steering melatonin-regulated gene expression, the pineal hormone is required for the rhythmic expression of circadian oscillator genes in peripheral organs and to enhance the coupling of parallel oscillators within the same tissue. It exerts additional effects by modulating the secretion of other hormones. The importance of melatonin for numerous organs is underlined by the association of various diseases with gene polymorphisms concerning melatonin receptors and the melatonin biosynthetic pathway. The possibilities and limits of melatonergic treatment are discussed with regard to reductions of melatonin during aging and in various diseases.

Cardiovascular effects of melatonin receptor agonists

INTRODUCTION

Melatonin synchronizes circadian rhythms with light/dark period and it was demonstrated to correct chronodisruption. Several melatonin receptor agonists with improved pharmacokinetics or increased receptor affinity are being developed, three of them are already in clinical use. However, the actions of melatonin extend beyond chronobiology to cardiovascular and metabolic systems as well. Given the high prevalence of cardiovascular disease and their common occurrence with chronodisruption, it is of utmost importance to classify the cardiometabolic effects of the newly approved and putative melatoninergic drugs.

AREAS COVERED

In the present review, the available (although very sparse) data on such effects, in particular by the approved (circadin, ramelteon, agomelatine) or clinically advanced (tasimelteon, piromelatine = Neu-P11, TIK-301) compounds are summarized. The authors have searched for an association with blood pressure, vascular reactivity, ischemia, myocardial and vascular remodeling and metabolic syndrome.

EXPERT OPINION

The data suggest that cardiovascular effects of melatonin are at least partly mediated via MT(1)/MT(2) receptors and associated with its chronobiotic action. Therefore, despite the sparse direct evidence, it is believed that these effects will be shared by melatonin analogs as well. With the expected approval of novel melatoninergic compounds, it is suggested that the investigation of their cardiovascular effects should no longer be neglected.

Neurobiology, pathophysiology, and treatment of melatonin deficiency and dysfunction

Melatonin is a highly pleiotropic signaling molecule, which is released as a hormone of the pineal gland predominantly during night. Melatonin secretion decreases during aging. Reduced melatonin levels are also observed in various diseases, such as types of dementia, some mood disorders, severe pain, cancer, and diabetes type 2. Melatonin dysfunction is frequently related to deviations in amplitudes, phasing, and coupling of circadian rhythms. Gene polymorphisms of melatonin receptors and circadian oscillator proteins bear risks for several of the diseases mentioned. A common symptom of insufficient melatonin signaling is sleep disturbances. It is necessary to distinguish between symptoms that are curable by short melatonergic actions and others that require extended actions during night. Melatonin immediate release is already effective, at moderate doses, for reducing difficulties of falling asleep or improving symptoms associated with poorly coupled circadian rhythms, including seasonal affective and bipolar disorders. For purposes of a replacement therapy based on longer-lasting melatonergic actions, melatonin prolonged release and synthetic agonists have been developed. Therapies with melatonin or synthetic melatonergic drugs have to consider that these agents do not only act on the SCN, but also on numerous organs and cells in which melatonin receptors are also expressed.

Melatonin and its analogs in insomnia and depression

Benzodiazepine sedative-hypnotic drugs are widely used for the treatment of insomnia. Nevertheless, their adverse effects, such as next-day hangover, dependence and impairment of memory, make them unsuitable for long-term treatment. Melatonin has been used for improving sleep in patients with insomnia mainly because it does not cause hangover or show any addictive potential. However, there is a lack of consistency on its therapeutic value (partly because of its short half-life and the small quantities of melatonin employed). Thus, attention has been focused either on the development of more potent melatonin analogs with prolonged effects or on the design of slow release melatonin preparations. The MT(1) and MT(2) melatonergic receptor ramelteon was effective in increasing total sleep time and sleep efficiency, as well as in reducing sleep latency, in insomnia patients. The melatonergic antidepressant agomelatine, displaying potent MT(1) and MT(2) melatonergic agonism and relatively weak serotonin 5HT(2C) receptor antagonism, was found effective in the treatment of depressed patients. However, long-term safety studies are lacking for both melatonin agonists, particularly considering the pharmacological activity of their metabolites. In view of the higher binding affinities, longest half-life and relative higher potencies of the different melatonin agonists, studies using 2 or 3mg/day of melatonin are probably unsuitable to give appropriate comparison of the effects of the natural compound. Hence, clinical trials employing melatonin doses in the range of 50-100mg/day are warranted before the relative merits of the melatonin analogs versus melatonin can be settled.

Neurochemical and neuropharmacological aspects of circadian disruptions: an introduction to asynchronization

Circadian disruptions are common in modern society, and there is an urgent need for effective treatment strategies. According to standard diagnostic criteria, most adolescents showing both insomnia and daytime sleepiness are diagnosed as having behavioral-induced sleep efficiency syndrome resulting from insomnia due to inadequate sleep hygiene. However, a simple intervention of adequate sleep hygiene often fails to treat them. As a solution to this clinical problem, the present review first overviews the basic neurochemical and neuropharmachological aspects of sleep and circadian rhythm regulation, then explains several circadian disruptions from similar viewpoints, and finally introduces the clinical notion of asynchronization. Asynchronization is designated to explain the pathophysiology/pathogenesis of exhibition of both insomnia and hypersomnia in adolescents, which comprises disturbances in various aspects of biological rhythms. The major triggers for asynchronization are considered to be a combination of light exposure during the night, which disturbs the biological clock and decreases melatonin secretion, as well as a lack of light exposure in the morning, which prohibits normal synchronization of the biological clock to the 24-hour cycle of the earth and decreases the activity of serotonin. In the chronic phase of asynchronization, involvement of both wake- and sleep-promoting systems is suggested. Both conventional and alternative therapeutic approaches for potential treatment of asynchronization are suggested.

Melatonin agonists in primary insomnia and depression-associated insomnia: are they superior to sedative-hypnotics?

Current pharmacological treatment of insomnia involves the use of sedative-hypnotic benzodiazepine and non-benzodiazepine drugs. Although benzodiazepines improve sleep, their multiple adverse effects hamper their application. Adverse effects include impairment of memory and cognitive functions, next-day hangover and dependence. Non-benzodiazepines are effective for initiating sleep but are not as effective as benzodiazepines for improving sleep quality or efficiency. Furthermore, their prolonged use produces adverse effects similar to those observed with benzodiazepines. Inasmuch as insomnia may be associated with decreased nocturnal melatonin, administration of melatonin is a strategy that has been increasingly used for treating insomnia. Melatonin can be effective for improving sleep quality without the adverse effects associated with hypnotic-sedatives. Ramelteon, a synthetic analog of melatonin which has a longer half life and a stronger affinity for MT1 and MT2 melatonergic receptors, has been reportedly effective for initiating and improving sleep in both adult and elderly insomniacs without showing hangover, dependence, or cognitive impairment. Insomnia is also a major complaint among patients suffering from depressive disorders and is often aggravated by conventional antidepressants especially the specific serotonin reuptake inhibitors. The novel antidepressant agomelatine, a dual action agent with affinity for melatonin MT1 and MT2 receptors and 5-HT2c antagonistic properties, constitutes a new approach to the treatment of major depressive disorders. Agomelatine ameliorates the symptoms of depression and improves the quality and efficiency of sleep. Taken together, the evidence indicates that MT1/MT2 receptor agonists like ramelteon or agomelatine may be valuable pharmacological tools for insomnia and for depression-associated insomnia.

Pharmacotherapy of insomnia with ramelteon: safety, efficacy and clinical applications

Ramelteon is a tricyclic synthetic analog of melatonin that acts specifically on MT₁ and MT₂ melatonin receptors. Ramelteon is the first melatonin receptor agonist approved by the Food and Drug Administration (FDA) for the treatment of insomnia characterized by sleep onset difficulties. Ramelteon is both a chronobiotic and a hypnotic that has been shown to promote sleep initiation and maintenance in various preclinical and in clinical trials. The efficacy and safety of ramelteon in patients with chronic insomnia was initially confirmed in short-term placebo-controlled trials. These showed little evidence of next-day residual effects, withdrawal symptoms or rebound insomnia. Other studies indicated that ramelteon lacked abuse potential and had a minimal risk of producing dependence or adverse effects on cognitive or psychomotor performance. A 6-month placebo-controlled international study and a 1-year open-label study in the USA demonstrated that ramelteon was effective and well tolerated. Other potential off-label uses of ramelteon include circadian rhythm sleep disorders such as shift-work and jet lag. At the present time the drug should be cautiously prescribed for short-term treatment only.

Investigational melatonin receptor agonists

IMPORTANCE OF THE FIELD

Melatonin is a major chronobiological regulator involved in circadian phasing, sleep, and numerous other functions including cyto-/neuroprotection, immune modulation, and energy metabolism. The suitability of melatonin as a drug is limited because of its short half-life. Therefore, various indolic and non-indolic melatonergic agonists have been developed. Frequent health problems such as sleep disturbances, neuropsychiatric disorders related to circadian dysphasing, and metabolic diseases associated with insulin resistance are targeted by melatonergic agonists.

AREAS COVERED IN THIS REVIEW

Various synthetic melatonergic drugs are compared with regard to receptor affinities, selectivity, effects on sleep, endogenous melatonin, circadian phase and insulin-related metabolism.

WHAT THE READER WILL GAIN

The chemical design of melatonin receptor agonists is discussed in relation to consequences for receptor affinity, selectivity, metabolism, and spectrum of effects.

TAKE HOME MESSAGE

Melatonergic agonists are suitable for phase-shifting circadian rhythms, and may be used for treating disorders related to circadian dysfunction including sleep difficulties. Facilitation of sleep onset is a general property, whereas promotion of sleep maintenance is demonstrable but not always fully sufficient. Details are especially available for tasimelteon. Support of insulin sensitivity may become a new area of application for compounds such as NEU-P11. Some drugs acting additionally as serotonergic antagonists display antidepressant properties.

Effects of the melatonin MT-1/MT-2 agonist ramelteon on daytime body temperature and sleep

STUDY OBJECTIVES

A reduction in core temperature and an increase in the distal-proximal skin gradient (DPG) are reported to be associated with shorter sleep onset latencies (SOL) and better sleep quality. Ramelteon is a melatonin MT-1/MT-2 agonist approved for the treatment of insomnia. At night, ramelteon has been reported to shorten SOL. In the present study we tested the hypothesis that ramelteon would reduce core temperature, increase the DPG, as well as shorten SOL, reduce wakefulness after sleep onset (WASO), and increase total sleep time (TST) during a daytime sleep opportunity.

DESIGN

Randomized, double-blind, placebo-controlled, cross-over design. Eight mg ramelteon or placebo was administered 2 h prior to a 4-h daytime sleep opportunity.

SETTING

Sleep and chronobiology laboratory.

PARTICIPANTS

Fourteen healthy adults (5 females), aged (23.2 +/- 4.2 y).

MEASUREMENTS AND RESULTS

Primary outcome measures included core body temperature, the DPG and sleep physiology (minutes of total sleep time [TST], wake after sleep onset [WASO], and SOL). We also assessed as secondary outcomes, proximal and distal skin temperatures, sleep staging and subjective TST. Repeated measures ANOVA revealed ramelteon significantly reduced core temperature and increased the DPG (both P < 0.05). Furthermore, ramelteon reduced WASO and increased TST, and stages 1 and 2 sleep (all P < 0.05). The change in the DPG was negatively correlated with SOL in the ramelteon condition.

CONCLUSIONS

Ramelteon improved daytime sleep, perhaps mechanistically in part by reducing core temperature and modulating skin temperature. These findings suggest that ramelteon may have promise for the treatment of insomnia associated with circadian misalignment due to circadian sleep disorders.

The effect of melatonergic and non-melatonergic antidepressants on sleep: weighing the alternatives

In DSM-IV the occurrence of disturbed sleep is one of the principal diagnostic criteria for major depressive disorder (MDD). Further, there is evidence of reciprocity between the two conditions such that, even in the absence of current depressive symptoms, disturbed sleep often predicts their development. The present review discusses the effects of antidepressants on sleep and evaluates the use of the recently developed melatonin agonist-selective serotonin antagonists on sleep and depression. Although many antidepressants such as the tricyclics, monoamine oxidase inhibitors, serotonin-norepinephrine reuptake inhibitors, several serotonin receptor antagonists and selective serotonin reuptake inhibitors (SSRIs) have all been found successful in treating depression, their use is often associated with a disruptive effect on sleep. SSRIs, currently the most widely prescribed of the antidepressants, are well known for their instigation or exacerbation of insomnia. The recently introduced novel melatonin agonist and selective serotonin antagonist antidepressant, agomelatine, which has melatonin MT(1) and MT(2) receptor agonist and 5-HT(2c) antagonist properties, has been useful in treating patients with MDD. Its rapid onset of action and effectiveness in improving the mood of depressed patients has been attributed to its ability to improve sleep quality. These properties underline the use of melatonin analogues as a promising alternative for the treatment of depression.

MT1 melatonin receptors mediate somatic, behavioral, and reproductive neuroendocrine responses to photoperiod and melatonin in Siberian hamsters (Phodopus sungorus)

Environmental day length drives nocturnal pineal melatonin secretion, which in turn generates or entrains seasonal cycles of physiology, reproduction, and behavior. In mammals, melatonin (MEL) binds to a number of receptor subtypes including high-affinity (MT1 and MT2) and low-affinity (MT3, nuclear orphan receptors) binding sites, which are distributed throughout the central nervous system and periphery. The MEL receptors that mediate photoperiodic reproductive and behavioral responses to MEL have not been identified in a reproductively photoperiodic species. Here I tested the hypothesis that MT1 receptors are necessary and sufficient to engage photoperiodic responses by challenging male Siberian hamsters (Phodopus sungorus), a species that does not express functional MT2 receptors, with ramelteon (RAM), a specific MT1/MT2 receptor agonist. In hamsters housed in a long-day photoperiod, late-afternoon RAM treatment inhibited gonadotropin secretion, induced gonadal regression, and suppressed food intake and body mass, mimicking effects of MEL. In addition, chronic (24 h/d) RAM infusions were sufficient to obscure endogenous MEL signaling, and these treatments attenuated gonadal regression in short days. Together, the outcomes indicate that signaling at the MT1 receptor is sufficient and necessary to mediate the effects of photoperiod-driven changes in MEL on behavior and reproductive function in a reproductively photoperiodic mammal.

Influence of the novel antidepressant and melatonin agonist/serotonin2C receptor antagonist, agomelatine, on the rat sleep-wake cycle architecture

RATIONALE

The novel antidepressant, agomelatine, behaves as an agonist at melatonin MT(1) and MT(2) receptors and as an antagonist at serotonin (5-HT)(2C) receptors. In animal models and clinical trials, agomelatine displays antidepressant properties and re-synchronizes disrupted circadian rhythms.

OBJECTIVES

The objectives of this study were to compare the influence of agomelatine upon sleep-wake states to the selective melatonin agonists, melatonin and ramelteon, and to the selective 5-HT(2C) receptor antagonist, S32006.

METHODS

Rats were administered with vehicle, agomelatine, ramelteon, melatonin, or S32006, at the onset of either dark or light periods. Polygraphic recordings were performed and changes determined over 24 h, i.e., number and duration of sleep-wake episodes, latencies to rapid eye movement (REM) and slow-wave (SWS) sleep, power band spectra of the electroencephalogram (EEG), and circadian changes.

RESULTS

Administered at light phase onset, no changes were induced by agomelatine. In contrast, administered shortly before dark phase, agomelatine (10 and 40 mg/kg, per os) enhanced duration of REM and SWS sleep and decreased wake state for 3 h. Melatonin (10 mg/kg, per os) induced a transient enhancement in REM sleep followed by a reduction in REM and SWS sleep and an increase in waking. Ramelteon (10 mg/kg, per os) provoked a transient increase in REM sleep. Finally, S32006 (10 mg/kg, intraperitoneally), administered at dark phase onset, mimicked the increased SWS provoked by agomelatine, yet diminished REM sleep.

CONCLUSIONS

Agomelatine possesses a distinctive EEG profile compared with melatonin, ramelteon, and S32006, possibly reflecting co-joint agonist and antagonist properties at MT(1)/MT(2) and 5-HT(2C) receptors, respectively.

Melatonin and melatonergic drugs on sleep: possible mechanisms of action

Pineal melatonin is synthesized and secreted in close association with the light/dark cycle. The temporal relationship between the nocturnal rise in melatonin secretion and the "opening of the sleep gate" (i.e., the increase in sleep propensity at the beginning of the night), coupled with the sleep-promoting effects of exogenous melatonin, suggest that melatonin is involved in the regulation of sleep. The sleep-promoting and sleep/wake rhythm regulating effects of melatonin are attributed to its action on MT(1) and MT(2) melatonin receptors present in the suprachiasmatic nucleus (SCN) of the hypothalamus. Animal experiments carried out in rats, cats, and monkeys have revealed that melatonin has the ability to reduce sleep onset time and increase sleep duration. However, clinical studies reveal inconsistent findings, with some of them reporting beneficial effects of melatonin on sleep, whereas in others only marginal effects are documented. Recently a prolonged-release 2-mg melatonin preparation (Circadin(TM)) was approved by the European Medicines Agency as a monotherapy for the short-term treatment of primary insomnia in patients who are aged 55 or above. Several melatonin derivatives have been shown to increase nonrapid eye movement (NREM) in rats and are of potential pharmacological importance. So far only one of these melatonin derivatives, ramelteon, has received approval from the U.S. Food and Drug Administration to be used as a sleep promoter. Ramelteon is a novel MT(1) and MT(2) melatonergic agonist that has specific effects on melatonin receptors in the SCN and is effective in promoting sleep in experimental animals such as cats and monkeys. In clinical trials, ramelteon reduced sleep onset latency and promoted sleep in patients with chronic insomnia, including an older adult population. Both melatonin and ramelteon promote sleep by regulating the sleep/wake rhythm through their actions on melatonin receptors in the SCN, a unique mechanism of action not shared by any other hypnotics. Moreover, unlike benzodiazepines, ramelteon causes neither withdrawal effects nor dependence. Agomelatine, another novel melatonergic antidepressant in its final phase of approval for clinical use, has been shown to improve sleep in depressed patients and to have an antidepressant efficacy that is partially attributed to its effects on sleep-regulating mechanisms.

Melatonin: signaling mechanisms of a pleiotropic agent

Melatonin acts both as a hormone of the pineal gland and as a local regulator molecule in various tissues. Quantities of total tissue melatonin exceed those released from the pineal. With regard to this dual role, to the orchestrating, systemic action on various target tissues, melatonin is highly pleiotropic. Numerous secondary effects result from the control of the circadian pacemaker and, in seasonal breeders, of the hypothalamic/pituitary hormonal axes. In mammals, various binding sites for melatonin have been identified, the membrane receptors MT(1) and MT(2), which are of utmost chronobiological importance, ROR and RZR isoforms as nuclear receptors from the retinoic acid receptor superfamily, quinone reductase 2, calmodulin, calreticulin, and mitochondrial binding sites. The G protein-coupled receptors (GPCRs) MT(1) and MT(2) are capable of parallel or alternate signaling via different Galpha subforms, in particular, Galpha(i) (2/) (3) and Galpha(q), and via Gbetagamma, as well. Multiple signaling can lead to the activation of different cascades and/or ion channels. Melatonin frequently decreases cAMP, but also activates phospholipase C and protein kinase C, acts via the MAP kinase and PI3 kinase/Akt pathways, modulates large conductance Ca(2+)-activated K(+) and voltage-gated Ca(2+) channels. MT(1) and MT(2) can form homo and heterodimers, and MT(1) interacts with other proteins in the plasma membrane, such as an orphan GPCR, GPR50, and the PDZ domain scaffolding protein MUPP1, effects which negatively or positively influence signaling capacity. Cross-talks between different signaling pathways, including influences of the membrane receptors on nuclear binding sites, are discussed. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Pathophysiology of depression: role of sleep and the melatonergic system

Profound disturbances in sleep architecture occur in major depressive disorders (MDD) and in bipolar affective disorders. Reduction in slow wave sleep, decreased latency of rapid eye movement (REM) sleep and abnormalities in the timing of REM/non-REM sleep cycles have all been documented in patients with MDD. It is thus evident that an understanding of the basic mechanisms of sleep regulation is essential for an analysis of the pathophysiology of depressive disorders. The suprachiasmatic nucleus (SCN), which functions as the body's master circadian clock, plays a major role in the regulation of the sleep/wakefulness rhythm and interacts actively with the homeostatic processes that regulate sleep. The control of melatonin secretion by the SCN, the occurrence of high concentrations of melatonin receptors in the SCN, and the suppression of electrical activity in the SCN by melatonin all underscore the major influence which this neurohormone has in regulating the sleep/wake cycle. The transition from wakefulness to high sleep propensity is associated with the nocturnal rise of endogenous melatonin secretion. Various lines of evidence show that depressed patients exhibit disturbances in both the amplitude and shape of the melatonin secretion rhythm and that melatonin can improve the quality of sleep in these patients. The choice of a suitable antidepressant that improves sleep quality is thus important while treating a depressive disorder. The novel antidepressant agomelatine, which combines the properties of a 5-HT(2C) antagonist and a melatonergic MT(1)/MT(2) receptor agonist, has been found very effective for resetting the disturbed sleep/wake cycle and in improving the clinical status of MDD. Agomelatine has also been found useful in treating sleep problems and improving the clinical status of patients suffering from seasonal affective disorder.

Melatonin: a hormone that modulates pain

AIMS

Melatonin is a hormone synthesized principally in the pineal gland that has been classically associated with endocrine actions. However, several lines of evidence suggest that melatonin plays a role in pain modulation. This paper reviews the available evidence on melatonin's analgesic effects in animals and human beings.

MAIN METHODS

A medline search was performed using the terms "melatonin", "inflammatory pain", "neuropathic pain", "functional pain", "rats", "mice", "human", "receptors", "opioid" and "free radicals" in combinations.

KEY FINDINGS

The antinociceptive effect of melatonin has been evaluated in diverse pain models, and several findings show that melatonin receptors modulate pain mechanisms as activation induces an antinociceptive effect at spinal and supraspinal levels under conditions of acute and inflammatory pain. More recently, melatonin induced-antinociception has been extended to neuropathic pain states. This effect agrees with the localization of melatonin receptors in thalamus, hypothalamus, dorsal horn of the spinal cord, spinal trigeminal tract, and trigeminal nucleus. The effects of melatonin result from activation of MT(1) and MT(2) melatonin receptors, which leads to reduced cyclic AMP formation and reduced nociception. In addition, melatonin is able to activate opioid receptors indirectly, to open several K(+) channels and to inhibit expression of 5-lipoxygenase and cyclooxygenase 2. This hormone also inhibits the production of pro-inflammatory cytokines, modulates GABA(A) receptor function and acts as a free radical scavenger.

SIGNIFICANCE

Melatonin receptors constitute attractive targets for developing analgesic drugs, and their activation may prove to be a useful strategy to generate analgesics with a novel mechanism of action.

Dosing time-dependent actions of psychostimulants

The concept of the dosing time-dependent (DTD) actions of drugs has been used to describe the effects of diurnal rhythms on pharmacological responsiveness. Notwithstanding the importance of diurnal variability in drug pharmacokinetics and bioavailability, it appears that in the central nervous system (CNS), the DTD actions of psychotropic drugs involve diurnal changes in the CNS-specific expression of genes encoding for psychotropic drug targets and transcription factors known as clock genes. In this review, we focused our discussion on the DTD effects of the psychostimulants cocaine and amphetamines. Both cocaine and amphetamines produce differential lasting behavioral alterations, that is, locomotor sensitization, depending on the time of the day they are administered. This exemplifies a DTD action of these drugs. The DTD effects of these psychostimulants correlate with diurnal changes in the system of transcription factors termed clock genes, for example, Period 1, and with changes in the availability of certain subtypes of dopamine receptors, for example, D2 and D3. Diurnal synthesis and release of the pineal hormone melatonin influence the DTD behavioral actions of cocaine and amphetamines. The molecular mechanism of melatonin's effects on the responsiveness of CNS to psychostimulants appears to involve melatonin receptors and clock genes. It is proposed that the DTD characteristics of psychostimulant action and the contributions of the melatonergic system may have clinical implications that include treatments for the attention deficit hyperactivity disorder and possibly neurotoxicity/neuroprotection.

New approaches in the management of insomnia: weighing the advantages of prolonged-release melatonin and synthetic melatoninergic agonists

Hypnotic effects of melatonin and melatoninergic drugs are mediated via MT(1) and MT(2) receptors, especially those in the circadian pacemaker, the suprachiasmatic nucleus, which acts on the hypothalamic sleep switch. Therefore, they differ fundamentally from GABAergic hypnotics. Melatoninergic agonists primarily favor sleep initiation and reset the circadian clock to phases allowing persistent sleep, as required in circadian rhythm sleep disorders. A major obstacle for the use of melatonin to support sleep maintenance in primary insomnia results from its short half-life in the circulation. Solutions to this problem have been sought by developing prolonged-release formulations of the natural hormone, or melatoninergic drugs of longer half-life, such as ramelteon, tasimelteon and agomelatine. With all these drugs, improvements of sleep are statistically demonstrable, but remain limited, especially in primary chronic insomnia, so that GABAergic drugs may be indicated. Melatoninergic agonists do not cause next-day hangover and withdrawal effects, or dependence. They do not induce behavioral changes, as sometimes observed with z-drugs. Despite otherwise good tolerability, the use of melatoninergic drugs in children, adolescents, and during pregnancy has been a matter of concern, and should be avoided in autoimmune diseases and Parkinsonism. Problems and limits of melatoninergic hypnotics are compared.

Pharmacology of ramelteon, a selective MT1/MT2 receptor agonist: a novel therapeutic drug for sleep disorders

An estimated one-third of the general population is affected by insomnia, and this number is increasing due to more stressful working conditions and the progressive aging of society. However, current treatment of insomnia with hypnotics, gamma-aminobutyric acid A (GABA(A)) receptor modulators, induces various side effects, including cognitive impairment, motor disturbance, dependence, tolerance, hangover, and rebound insomnia. Ramelteon (Rozerem; Takeda Pharmaceutical Company Limited, Osaka, Japan) is an orally active, highly selective melatonin MT(1)/MT(2) receptor agonist. Unlike the sedative hypnotics that target GABA(A) receptor complexes, ramelteon is a chronohypnotic that acts on the melatonin MT(1) and MT(2) receptors, which are primarily located in the suprachiasmatic nucleus, the body's "master clock." As such, ramelteon possesses the first new therapeutic mechanism of action for a prescription insomnia medication in over three decades. Ramelteon has demonstrated sleep-promoting effects in clinical trials, and coupled with its favorable safety profile and lack of abuse potential or dependence, this chronohypnotic provides an important treatment option for insomnia.

Therapeutic potential of melatonin agonists

Melatonin, a hormone secreted by the pineal gland, has been successfully employed to improve sleep in both normal patients and insomniacs, and for the treatment of circadian rhythm sleep disorders. Melatonergic MT₁ and MT₂ receptors exist in high concentrations in the suprachiasmatic nucleus of the hypothalamus and have been shown to be instrumental for the sleep-promoting and circadian rhythm-regulating effects of melatonin. A lack of consistency among reports on the therapeutic efficacy of melatonin has been attributed to differences in melatonin's bioavailability and the short half-life of the hormone. In view of the need for longer acting melatonergic agonists that improve sleep efficiency without causing drug abuse or dependency, ramelteon (Rozerem™, Takeda) was developed. Ramelteon, which acts via MT₁/MT₂ melatonergic agonism, has been found clinically effective for improving total sleep time and sleep efficiency in insomniacs. Agomelatine (Valdoxan™, Servier) is another MT₁/MT₂ melatonergic agonist that also displays antagonist activity at 5-HT_2C serotonin receptors. Agomelatine has been found effective in treating depression and sleep disorders in patients with major depressive disorder. A slow-release preparation of melatonin (Circadin™, Neurim) has been shown to be effective in treating sleep disorders in the elderly population.

Ramelteon: a new approach to managing insomnia

Ramelteon is a novel melatonergic receptor agonist that represents a new and effective approach to sleep promotion in insomnia. By acting specifically on the melatonin MT1 and MT2 receptors, ramelteon decreases latency to persistent sleep and increases total sleep time without causing sedation. Ramelteon has a good profile, with no next-day residual effects, no rebound insomnia or withdrawal, minimal clinically meaningful disruption of sleep architecture, no memory impairment, no evidence of abuse potential and no acute effects on psychomotor function.