Melatonin, selective and non-selective MT1/MT2 receptors agonists: differential effects on the 24-h vigilance states

Melatonin regulates the circadian rhythm to ameliorate postoperative sleep disorder and neurobehavioral abnormalities in aged mice

BACKGROUND

Postoperative sleep disorder (PSD) and delirium, which may be associated with surgery and inhalational anesthetics, induce adverse effects in old adults. Emerging evidence indicates that circadian rhythm contributes to various neuropathological diseases, including Alzheimer's disease. Thus, we analyzed the potential role of circadian rhythm in PSD and delirium-like behavior in aged mice and determined whether exogenous melatonin could facilitate entrainment of the circadian rhythm after laparotomy under sevoflurane anesthesia.

METHODS

We selected old C57BL/6J mice which receiving laparotomy/sevoflurane anesthesia as model animals. We employed buried food, open field, and Y maze test to assess delirium-like behavior, and electroencephalography/electromyography (EEG/EMG) were used to investigate sleep changes. We analyzed the transcription rhythm of clock genes in superchiasmatic nucleus (SCN) to explore the effects of surgery and melatonin pretreatment on the circadian rhythm. Then, we measured melatonin receptor levels in SCN and ERK/CREB pathway-related proteins in hippocampus and prefrontal cortex to assess their role in PSDs and delirium-like behavior.

RESULTS

Laparotomy under sevoflurane anesthesia had a greater influence than sevoflurane alone, leading to sleep disorder, a shift in sleep-wake rhythm, and delirium-like behavior. Bmal1, Clock, and Cry1 mRNA expression showed a peak shift, MT1 melatonin receptor expression level was increased in the SCN, and p-ERK/ERK and p-CREB/CREB were decreased in hippocampus and prefrontal cortex of aged mice 1 day after laparotomy. Melatonin showed significant efficacy in ameliorating PSD and delirium-like behavior and restoring the circadian rhythm, reversing melatonin receptor and ERK/CREB pathway expression abnormalities. In addition, most of the beneficial effect of melatonin was antagonized by luzindole, a melatonin receptor antagonist.

CONCLUSIONS

Melatonin receptors in SCN, circadian rhythm, and ERK/CREB signaling pathway participate in the pathophysiological processes of PSD and delirium-like behavior. Melatonin intervention could be a potential preventative approach for PSD and delirium.

Mechanism of GW117 antidepressant action: melatonin receptor-mediated regulation of sleep rhythm

Alterations in circadian sleep patterns constitute a salient manifestation in major depressive disorder. GW117, an emergent antidepressant, functions as an agonist for melatonin 1 and melatonin 2 (MT1/MT2) receptors, in tandem with antagonism of the serotonin (5-HT) 2C receptor. The present investigation is dedicated to elucidating the role and underlying mechanisms by which GW117 ameliorates circadian sleep disruptions. Utilizing an adapted chronic unpredictable mild stress protocol, we induced a depressive-like phenotype and perturbed circadian rhythms in rodent models. Our methodological approach integrated quantitative polymerase chain reaction (qPCR) in real-time, enzyme-linked immunosorbent assay (ELISA), and immunoblotting techniques to probe alterations in the expression of core circadian genes and homeostatic sleep markers. The impact of GW117 was assessed across various dosages (10, 20, and 40 mg/kg) on these molecular signatures. In a parallel examination, we evaluated the influence of GW117 (administered at 15, 40, and 60 mg/kg) on the sleep patterns of healthy mice. The results showed that GW117 significantly improved sleep-wake circadian rhythms, altered sleep architecture, and shortened sleep latency. Furthermore, GW117 increased the expression of several clock genes in the hypothalamus of chronic unpredictable mild stress model rats and normal mice. It also regulated circadian biomarkers, including melatonin and cortisol. Based on our findings, we propose that the beneficial effects of GW117 on sleep rhythms may be due to the melatonin system-mediated activation of the Wnt/β-catenin signaling pathway.

α-Synuclein reduces acetylserotonin O-methyltransferase mediated melatonin biosynthesis by microtubule-associated protein 1 light chain 3 beta-related degradation pathway

Previous studies have demonstrated that α-synuclein (α-SYN) is closely associated with rapid eye movement sleep behavior disorder (RBD) related to several neurodegenerative disorders. However, the exact molecular mechanisms are still rarely investigated. In the present study, we found that in the α-SYNA53T induced RBD-like behavior mouse model, the melatonin level in the plasma and pineal gland were significantly decreased. To elucidate the underlying mechanism of α-SYN-induced melatonin reduction, we investigated the effect of α-SYN in melatonin biosynthesis. Our findings showed that α-SYN reduced the level and activity of melatonin synthesis enzyme acetylserotonin O-methyltransferase (ASMT) in the pineal gland and in the cell cultures. In addition, we found that microtubule-associated protein 1 light chain 3 beta (LC3B) as an important autophagy adapter is involved in the degradation of ASMT. Immunoprecipitation assays revealed that α-SYN increases the binding between LC3B and ASMT, leading to ASMT degradation and a consequent reduction in melatonin biosynthesis. Collectively, our results demonstrate the molecular mechanisms of α-SYN in melatonin biosynthesis, indicating that melatonin is an important molecule involved in the α-SYN-associated RBD-like behaviors, which may provide a potential therapeutic target for RBD of Parkinson's disease.

Melatonin MT1 receptors as a target for the psychopharmacology of bipolar disorder: A translational study

The treatment of bipolar disorder (BD) still remains a challenge. Melatonin (MLT), acting through its two receptors MT1 and MT2, plays a key role in regulating circadian rhythms which are dysfunctional in BD. Using a translational approach, we examined the implication and potential of MT1 receptors in the pathophysiology and psychopharmacology of BD. We employed a murine model of the manic phase of BD (Clock mutant (ClockΔ19) mice) to study the activation of MT1 receptors by UCM871, a selective partial agonist, in behavioral pharmacology tests and in-vivo electrophysiology. We then performed a high-resolution Nuclear Magnetic Resonance study on isolated membranes to characterize the molecular mechanism of interaction of UCM871. Finally, in a cohort of BD patients, we investigated the link between clinical measures of BD and genetic variants located in the MT1 receptor and CLOCK genes. We demonstrated that: 1) UCM871 can revert behavioral and electrophysiological abnormalities of ClockΔ19 mice; 2) UCM871 promotes the activation state of MT1 receptors; 3) there is a significant association between the number of severe manic episodes and MLT levels, depending on the genetic configuration of the MT1 rs2165666 variant. Overall, this work lends support to the potentiality of MT1 receptors as target for the treatment of BD.

Age-Related Effects of Exogenous Melatonin on Anxiety-like Behavior in C57/B6J Mice

The synthesis of melatonin (MLT) physiologically decreases during aging. Treatment with MLT has shown anxiolytic, hypnotic, and analgesic effects, but little is known about possible age-dependent differences in its efficacy. Therefore, we studied the effects of MLT (20 mg/kg, intraperitoneal) on anxiety-like behavior (open field (OFT), elevated plus maze (EPMT), three-chamber sociability, and marble-burying (MBT) tests), and the medial prefrontal cortex (mPFC)-dorsal hippocampus (dHippo) circuit in adolescent (35-40 days old) and adult (three-five months old) C57BL/6 male mice. MLT did not show any effect in adolescents in the OFT and EPMT. In adults, compared to vehicles, it decreased locomotor activity and time spent in the center of the arena in the OFT and time spent in the open arms in the EPMT. In the MBT, no MLT effects were observed in both age groups. In the three-chamber sociability test, MLT decreased sociability and social novelty in adults, while it increased sociability in adolescents. Using local field potential recordings, we found higher mPFC-dHippo synchronization in the delta and low-theta frequency ranges in adults but not in adolescents after MLT treatment. Here, we show age-dependent differences in the effects of MLT in anxiety paradigms and in the modulation of the mPFC-dHippo circuit, indicating that when investigating the pharmacology of the MLT system, age can significantly impact the study outcomes.

Sleep disturbances in autism spectrum disorder: Animal models, neural mechanisms, and therapeutics

Sleep is crucial for brain development. Sleep disturbances are prevalent in children with autism spectrum disorder (ASD). Strikingly, these sleep problems are positively correlated with the severity of ASD core symptoms such as deficits in social skills and stereotypic behavior, indicating that sleep problems and the behavioral characteristics of ASD may be related. In this review, we will discuss sleep disturbances in children with ASD and highlight mouse models to study sleep disturbances and behavioral phenotypes in ASD. In addition, we will review neuromodulators controlling sleep and wakefulness and how these neuromodulatory systems are disrupted in animal models and patients with ASD. Lastly, we will address how the therapeutic interventions for patients with ASD improve various aspects of sleep. Together, gaining mechanistic insights into the neural mechanisms underlying sleep disturbances in children with ASD will help us to develop better therapeutic interventions.

Melatonin as a Chronobiotic with Sleep-promoting Properties

The use of exogenous melatonin (exo-MEL) as a sleep-promoting drug has been under extensive debate due to the lack of consistency of its described effects. In this study, we conduct a systematic and comprehensive review of the literature on the chronobiotic, sleep-inducing, and overall sleep-promoting properties of exo-MEL. To this aim, we first describe the possible pharmacological mechanisms involved in the sleep-promoting properties and then report the corresponding effects of exo-MEL administration on clinical outcomes in: a) healthy subjects, b) circadian rhythm sleep disorders, c) primary insomnia. Timing of administration and doses of exo-MEL received particular attention in this work. The exo-MEL pharmacological effects are hereby interpreted in view of changes in the physiological properties and rhythmicity of endogenous melatonin. Finally, we discuss some translational implications for the personalized use of exo-MEL in the clinical practice.

Physiological and pharmacological perspectives of melatonin

The pineal gland is a interface between light-dark cycle and shows neuro-endocrine functions. Melatonin is the primary hormone of pineal gland, secreted at night. The night-time melatonin peak regulates the physiological functions at dark. Melatonin has several unique features as it synchronises internal rhythm with daily and seasonal variations, regulates circadian rhythm and sleep-wake cycle. Physiologically melatonin involves in detoxification of free radicals, immune functions, neuro-protection, oncostatic effects, cardiovascular functions, reproduction, and foetal development. The precise functions of melatonin are exhibited by specific receptors. In relation to pathophysiology, impaired melatonin secretion promotes sleep disorder, cancer progression, type-2 diabetes, and neurodegenerative diseases. Several reports have highlighted the therapeutic benefits of melatonin specially related to cancer protection, sleep disorder, psychiatric disorders, and jet lag problems. This review will touch the most of the area of melatonin-oriented health impacts and its therapeutic aspects.

Night shift hormone: How does melatonin affect depression?

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

Melatonin and aggressive behavior: A systematic review of the literature on preclinical and clinical evidence

The melatonin system and circadian disruption have well-established links with aggressive behaviors; however, the biological underpinnings have not been thoroughly investigated. Here, we aimed at examining the current knowledge regarding the neurobiological and psychopharmacological involvement of the melatonin system in aggressive/violent behaviors. To this end, we performed a systematic review on Embase and Pubmed/MEDLINE of preclinical and clinical evidence linking the melatonin system, melatonin, and melatoninergic drugs with aggressive/violent behaviors. Two blinded raters performed an independent screening of the relevant literature. Overall, this review included 38 papers distributed between clinical and preclinical models. Eleven papers specifically addressed the existing evidence in rodent models, five in fish models, and 21 in humans. The data indicate that depending on the species, model, and timing of administration, melatonin may exert a complex influence on aggressive/violent behaviors. Particularly, the apparent contrasting findings on the link between the melatonin system and aggression/violence (with either increased, no, or decreased effect) shown in preclinical models underscore the need for further research to develop more accurate and fruitful translational models. Likewise, the significant heterogeneity found in the results of clinical studies does not allow yet to draw any firm conclusion on the efficacy of melatonin or melatonergic drugs on aggressive/violent behaviors. However, findings in children and in traits associated with aggressive/violent behavior, including irritability and anger, are emerging and deserve empirical attention given the low toxicity of melatonin and melatonergic drugs.

Hypnotic Effects of Melatonergic Compounds Measured in Mice or Rats

Melatonin is a pleiotropic compound mostly acting through its two G protein-coupled receptors named MT₁ and MT₂ and known for its role in regulating the sleep/wake cycle and circadian rhythms. Although this is common belief, there has been long debate in the scientific community on whether melatonin or melatonergic compounds are endowed with hypnotic effects. One of the reasons for this debate relied on contrasting findings in the literature in part due to non-standardized methods across studies in examining at preclinical level the effect of melatonin or melatonergic compounds on the sleep/wake cycle. Here, we describe a method in mice and rats to evaluate the effect of melatonin or melatonergic compounds on the different stages of the sleep/wake cycle, providing details concerning the administration of the drug and the analytical procedure for scoring the electroencephalogram/electromyogram (EEG/EMG) recording.

Melatonin recovers sleep phase delayed by MK-801 through the melatonin MT2 receptor- Ca2+ -CaMKII-CREB pathway in the ventrolateral preoptic nucleus

Melatonin (MLT) is widely used to treat sleep disorders although the underlying mechanism is still elusive. In mice, using wheel-running detection, we found that exogenous MLT could completely recover the period length prolonged by N-methyl-D-aspartate receptor (NMDAR) impairment due to the injection of the NMDAR antagonist MK-801, a preclinical model of psychosis. The analysis of the possible underlying mechanisms indicated that MLT could regulate the homeostatic state in the ventrolateral preoptic nucleus (VLPO) instead of the circadian process in the suprachiasmatic nucleus (SCN). In addition, our data showed that MK-801 decreased Ca²⁺ -related CaMKII expression and CREB phosphorylation levels in the VLPO, and MLT could rescue these intracellular impairments but not NMDAR expression levels. Accordingly, Gcamp6 AAV virus was injected in-vivo to further monitor intracellular Ca²⁺ levels in the VLPO, and MLT demonstrated a unique ability to increase Ca²⁺ fluorescence compared with MK-801-injected mice. Additionally, using the selective melatonin MT₂ receptor antagonist 4-phenyl-2-propionamidotetralin (4P-PDOT), we discovered that the pharmacological effects of MLT upon NMDAR impairments were mediated by melatonin MT₂ receptors. Using electroencephalography/electromyography (EEG/EMG) recordings, we observed that the latency to the first nonrapid eye movement (NREM) sleep episode was delayed by MK-801, and MLT was able to recover this delay. In conclusion, exogenous MLT by acting upon melatonin MT₂ receptors rescues sleep phase delayed by NMDAR impairment via increasing intracellular Ca²⁺ signaling in the VLPO, suggesting a regulatory role of the neurohormone on the homeostatic system.

Dysfunction of serotonergic activity and emotional responses across the light-dark cycle in mice lacking melatonin MT2 receptors

Melatonin (MLT) levels fluctuate according to the external light/dark cycle in both diurnal and nocturnal mammals. We previously demonstrated that melatonin MT₂ receptor knockout (MT₂ ^-/- ) mice show a decreased nonrapid eye movement sleep over 24 hours and increased wakefulness during the inactive (light) phase. Here, we investigated the role of MT₂ receptors in physiological light/dark cycle fluctuations in the activity of dorsal raphe nucleus (DRN) serotonin (5-HT) neurons and anxiety- and depression-like behavior. We found that the 5-HT burst-firing activity was tonically reduced across the whole 24 hours in MT₂ ^-/- mice compared with MT₂ ^+/+ mice.  Importantly, the physiological changes in the spontaneous firing activity of DRN 5-HT neurons during the light/dark cycle were nullified in MT₂ ^-/- mice, with a higher DRN 5-HT neural firing activity during the light phase in MT₂ ^-/- than in MT₂ ^+/+  mice. The role of MT₂ receptors over DRN 5-HT neurons was confirmed by acute pharmacological studies in which the selective MT₂ receptors agonist UCM1014 dose dependently inhibited DRN 5-HT activity, mostly during the dark phase. Compared with MT₂ ^+/+ , MT₂ ^-/- mice displayed an anxiety-like phenotype in the novelty-suppressed feeding and in the light/dark box tests; while anxiety levels in the light/dark box test were lower during the dark than during the light phase in MT₂ ^+/+ mice, the opposite was seen in MT₂ ^-/- mice. No differences between MT₂ ^+/+ and MT₂ ^-/- mice were observed for depression-like behavior in the forced swim and in the sucrose preference tests. These results suggest that MT₂ receptor genetic inactivation impacts 5-HT neurotransmission and interferes with anxiety levels by perturbing the physiologic light/dark pattern.

Agomelatine treatment corrects impaired sleep-wake cycle and sleep architecture and increases MT1 receptor as well as BDNF expression in the hippocampus during the subjective light phase of rats exposed to chronic constant light

RATIONALE

Exposure to chronic constant light (CCL) has a detrimental impact on circadian rhythms of motor activity and sleep/wake cycles. Agomelatine is an atypical antidepressant showing a chronotropic activity.

OBJECTIVES

In this study, we explored the role of melatonin (MT) receptors and brain-derived neurotrophic factor (BDNF) in the brain in the mechanism underlying the effects of agomelatine on diurnal variations of motor activity, sleep/wake cycle, and sleep architecture in a rat model of CCL.

METHODS

In Experiment #1, home cage activity was monitored automatically with cameras for a period of 24 h. The diurnal rhythm of MT_1, MT₂ receptors, and BDNF expression in the hippocampus and frontal cortex (FC), was tested using the ELISA test. In Experiment #2, rats were equipped with electroencephalographic (EEG) and electromyographic (EMG) electrodes and recordings were made under basal conditions (12:12 LD cycle + vehicle), LL + vehicle and LL + agomelatine (40 mg/kg/day for 21 days).

RESULTS

The rats exposed to CCL showed an impaired diurnal rhythm of motor activity and sleep/wake cycle with reduced NREM sleep and delta power and increased REM sleep and theta power. The duration and number of episodes of the wake were diminished during the subjective dark phase in this group. The circadian rhythm of MT₁ and MT₂ receptors and their expression did not change in the hippocampus and FC under CCL exposure, while the BDNF levels in the hippocampus decreased during the subjective light phase. Agomelatine restored the diurnal rhythm of motor activity, disturbed sleep/wake cycle, and sleep architecture, which effect was accompanied by an increase in MT₁ receptor and BDNF expression in the hippocampus at 10:00 in CCL rats.

CONCLUSIONS

These findings support the value of agomelatine as an antidepressant that can adjust circadian homeostasis of motor activity and sleep/wake cycle in a CCL model.

Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle

Melatonin (MLT) is a neurohormone that regulates many physiological functions including sleep, pain, thermoregulation, and circadian rhythms. MLT acts mainly through two G-protein-coupled receptors named MT₁ and MT₂, but also through an MLT type-3 receptor (MT₃). However, the role of MLT receptor subtypes in thermoregulation is still unknown. We have thus investigated the effects of selective and non-selective MLT receptor agonists/antagonists on body temperature (T_b) in rats across the 12/12-h light-dark cycle. Rectal temperature was measured every 15 min from 4:00 a.m. to 9:30 a.m. and from 4:00 p.m. to 9:30 p.m., following subcutaneous injection of each compound at either 5:00 a.m. or 5:00 p.m. MLT (40 mg/kg) had no effect when injected at 5 a.m., whereas it decreased T_b during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT₂ receptor antagonist 4P-PDOT and the non-selective MT₁/MT₂ receptor antagonist, luzindole, but not by the α₁/MT₃ receptors antagonist prazosin. However, unlike MLT, neither the selective MT₁ receptor partial agonist UCM871 (14 mg/kg) nor the selective MT₂ partial agonist UCM924 (40 mg/kg) altered T_b during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased T_b at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased T_b at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT₃ receptor agonist GR135531 (10 mg/kg) did not affect T_b. These data suggest that the simultaneous activation of both MT₁ and MT₂ receptors is necessary to regulate T_b during the light phase, whereas in a complex but yet unknown manner, they regulate T_b differently during the dark phase. Overall, MT₁ and MT₂ receptors display complementary but also distinct roles in modulating circadian fluctuations of T_b.

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

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

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

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

Drugs for Insomnia beyond Benzodiazepines: Pharmacology, Clinical Applications, and Discovery

Although the GABAergic benzodiazepines (BZDs) and Z-drugs (zolpidem, zopiclone, and zaleplon) are FDA-approved for insomnia disorders with a strong evidence base, they have many side effects, including cognitive impairment, tolerance, rebound insomnia upon discontinuation, car accidents/falls, abuse, and dependence liability. Consequently, the clinical use of off-label drugs and novel drugs that do not target the GABAergic system is increasing. The purpose of this review is to analyze the neurobiological and clinical evidence of pharmacological treatments of insomnia, excluding the BZDs and Z-drugs. We analyzed the melatonergic agonist drugs, agomelatine, prolonged-release melatonin, ramelteon, and tasimelteon; the dual orexin receptor antagonist suvorexant; the modulators of the α₂δ subunit of voltage-sensitive calcium channels, gabapentin and pregabalin; the H₁ antagonist, low-dose doxepin; and the histamine and serotonin receptor antagonists, amitriptyline, mirtazapine, trazodone, olanzapine, and quetiapine. The pharmacology and mechanism of action of these treatments and the evidence-base for the use of these drugs in clinical practice is outlined along with novel pipelines. There is evidence to recommend suvorexant and low-dose doxepin for sleep maintenance insomnia; there is also sufficient evidence to recommend ramelteon for sleep onset insomnia. Although there is limited evidence for the use of the quetiapine, trazodone, mirtazapine, amitriptyline, pregabalin, gabapentin, agomelatine, and olanzapine as treatments for insomnia disorder, these drugs may improve sleep while successfully treating comorbid disorders, with a different side effect profile than the BZDs and Z-drugs. The unique mechanism of action of each drug allows for a more personalized and targeted medical management of insomnia.

Selective melatonin MT2 receptor ligands relieve neuropathic pain through modulation of brainstem descending antinociceptive pathways

Neuropathic pain is an important public health problem for which only a few treatments are available. Preclinical studies show that melatonin (MLT), a neurohormone acting on MT1 and MT2 receptors, has analgesic properties, likely through MT2 receptors. Here, we determined the effects of the novel selective MLT MT2 receptor partial agonist N-{2-([3-bromophenyl]-4-fluorophenylamino)ethyl}acetamide (UCM924) in 2 neuropathic pain models in rats and examined its supraspinal mechanism of action. In rat L5-L6 spinal nerve ligation and spared nerve injury models, UCM924 (20-40 mg/kg, subcutaneously) produced a prolonged antinociceptive effect that is : (1) dose-dependent and blocked by the selective MT2 receptor antagonist 4-phenyl-2-propionamidotetralin, (2) superior to a high dose of MLT (150 mg/kg) and comparable with gabapentin (100 mg/kg), but (3) without noticeable motor coordination impairments in the rotarod test. Using double staining immunohistochemistry, we found that MT2 receptors are expressed by glutamatergic neurons in the rostral ventrolateral periaqueductal gray. Using in vivo electrophysiology combined with tail flick, we observed that microinjection of UCM924 into the ventrolateral periaqueductal gray decreased tail flick responses, depressed the firing activity of ON cells, and activated the firing of OFF cells; all effects were MT2 receptor-dependent. Altogether, these data demonstrate that selective MT2 receptor partial agonists have analgesic properties through modulation of brainstem descending antinociceptive pathways, and MT2 receptors may represent a novel target in the treatment of neuropathic pain.

Antinociceptive properties of selective MT(2) melatonin receptor partial agonists

Melatonin is a neurohormone involved in the regulation of both acute and chronic pain whose mechanism is still not completely understood. We have recently demonstrated that selective MT2 melatonin receptor partial agonists have antiallodynic properties in animal models of chronic neuropathic pain by modulating ON/OFF cells of the descending antinociceptive system. Here, we examined the antinociceptive properties of the selective MT2 melatonin receptor partial agonists N-{2-[(3-methoxyphenyl)phenylamino]ethyl}acetamide (UCM765) and N-{2-[(3-bromophenyl)-(4-fluorophenyl)amino]ethyl}acetamide (UCM924) in two animal models of acute and inflammatory pain: the hot-plate and formalin tests. UCM765 and UCM924 (5-40 mg/kg, s.c.) dose-dependently increased the temperature of the first hind paw lick in the hot-plate test, and decreased the total time spent licking the injected hind paw in the formalin test. Antinociceptive effects of UCM765 and UCM924 were maximal at the dose of 20mg/kg. At this dose, the effects of UCM765 and UCM924 were similar to those produced by 200 mg/kg acetaminophen in the hot-plate test, and by 3 mg/kg ketorolac or 150 mg/kg MLT in the formalin test. Notably, antinociceptive effects of the two MT2 partial agonists were blocked by the pre-treatment with the MT2 antagonist 4-phenyl-2-propionamidotetralin (4P-PDOT, 10 mg/kg) in both paradigms. These results demonstrate the antinociceptive properties of UCM765 and UCM924 in acute and inflammatory pain models and corroborate the concept that MT2 melatonin receptor may be a novel target for analgesic drug development.

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.