Effect of ramelteon (TAK-375), a selective MT1/MT2 receptor agonist, on motor performance in mice

Enhanced analgesia: Synergistic effects of melatonin and tramadol on acute thermal nociception in Wistar rats via tail-flick and hot-plate tests

BACKGROUND/AIM

The aim of the study is to evaluate the antinociceptive effects of Tramadol+Melatonin [using Hot-plate (HP) and Tail-flick (TF) tests] at the behavioral level and to investigate its effects on motor coordination (using the Rotarod test) in acute pain.

MATERIALS AND METHODS

Thirty-two male Wistar albino rats, 8 weeks old and weighing 250-300 g, were used. The rats were randomly divided into four groups of eight animals each [Control, Tramadol (20 mg/kg), Melatonin (120 mg/kg), and Tramadol+Melatonin (20 mg/kg+120 mg/kg)]. The rats were trained to walk on the rotarod for 3 days prior to the experiment. Thermal acute nociception was assessed in rats using two TF tests and one HP test. Measurements were recorded before drug administration (baseline) and at 15, 30, 60, 90, and 120 minutes post-administration.

RESULTS

Significantly higher values were obtained in both the HP and TF tests compared to the control, Melatonin, and Tramadol groups at various time points. In the Rotarod test, the baseline values of the Melatonin and Tramadol+Melatonin groups were significantly longer at certain post-administration time points.

CONCLUSION

The findings indicate a potential synergistic interaction between melatonin and tramadol, as evidenced by improved pain sensitivity thresholds. These results underscore the necessity for further clinical investigations to elucidate the therapeutic advantages and underlying mechanisms of this combination. Such research could significantly contribute to the development of more efficacious pain management protocols.

ZYKR1, a novel, potent, and peripherally selective kappa opioid receptor agonist reduces visceral pain and pruritus in animal models

Opioid receptor agonists are effective analgesic agents. Central activation of the mu and/or kappa opioid receptors (KOR) is associated with CNS side effects, which limits their effectiveness. Recent studies indicated that peripherally restricted, selective KOR agonists were potent analgesics and devoid of CNS-related side effects. To confirm this hypothesis, we designed a novel, potent, and peripherally restricted KOR-selective agonist, ZYKR1. The analgesic efficacy, brain penetration and safety of ZYKR1 were assessed in pre-clinical models. ZYKR1 showed KOR agonistic activity in the cAMP assay, with an EC₅₀ of 0.061 nM and more than 10⁵-fold selectivity over the mu and delta opioid receptors (EC₅₀ > 10 μM). ZYKR1 was not found to bind mu, delta opioid, and NOP receptors in radioligand binding assays. ZYKR1 produced concentration-dependent inhibition of electrically evoked contractions in isolated mouse vas deferens with an IC₅₀ of 1.6 nM ZYKR1 showed peripheral restriction and potent analgesic efficacy in various in-vivo animal models (acetic acid induced visceral pain mouse model, ED₅₀: 0.025 mg/kg, IV; ovariohysterectomy induced postoperative pain rat model, ED₅₀: 0.023 mg/kg, IV; and C48/80 induced pruritus mouse model, ED₅₀: 0.063 mg/kg, IV). In addition, ZYKR1 was devoid of motor coordination, physical dependence, dysphoria, and respiratory depression at 30, 400, 10 and 10-fold of efficacy dose, respectively. In conclusion, ZYKR1 has potent antinociceptive action in visceral pain and pruritus with limited CNS side effects in preclinical models owing to its peripheral restriction.

Acute Ramelteon Treatment Maintains the Cardiac Rhythms of Rats during Non-REM Sleep

Sleep curtailment negatively affects cardiac activities and thus should be ameliorated by pharmacological methods. One of the therapeutic targets is melatonin receptors, which tune circadian rhythms. Ramelteon, a melatonin MT1/MT2 receptor agonist, has recently been developed to modulate sleep-wake rhythms. To date, the sleep-promoting effect of ramelteon has been widely delineated, but whether ramelteon treatment physiologically influences cardiac function is not well understood. To address this question, we recorded electrocardiograms, electromyograms, and electrocorticograms in the frontal cortex and the olfactory bulb of unrestrained rats treated with either ramelteon or vehicle. We detected vigilance states based on physiological measurements and analyzed cardiac and muscular activities. We found that during non-rapid eye movement (non-REM) sleep, heartrate variability was maintained by ramelteon treatment. Analysis of the electromyograms confirmed that neither microarousal during non-REM sleep nor the occupancy of phasic periods during REM sleep was altered by ramelteon. Our results indicate that ramelteon has a remedial effect on cardiac activity by keeping the heartrate variability and may reduce cardiac dysfunction during sleep.

Melatonin receptor activation provides cerebral protection after traumatic brain injury by mitigating oxidative stress and inflammation via the Nrf2 signaling pathway

Traumatic brain injury (TBI) is a principal cause of death and disability worldwide. Melatonin, a hormone made by the pineal gland, is known to have anti-inflammatory and antioxidant properties. In this study, using a weight-drop model of TBI, we investigated the protective effects of ramelteon, a melatonin MT1/MT2 receptor agonist, and its underlying mechanisms of action. Administration of ramelteon (10 mg/kg) daily at 10:00 a.m. alleviated TBI-induced early brain damage on day 3 and long-term neurobehavioral deficits on day 28 in C57BL/6 mice. Ramelteon also increased the protein levels of interleukin (IL)-10, IL-4, superoxide dismutase (SOD), glutathione, and glutathione peroxidase and reduced the protein levels of IL-1β, tumor necrosis factor, and malondialdehyde in brain tissue and serum on days 1, 3, and 7 post-TBI. Similarly, ramelteon attenuated microglial and astrocyte activation in the perilesional cortex on day 3. Furthermore, ramelteon decreased Keap 1 expression, promoted nuclear factor erythroid 2-related factor 2 (Nrf2) nuclear accumulation, and increased levels of downstream proteins, including SOD-1, heme oxygenase-1, and NQO1 on day 3 post-TBI. However, in Nrf2 knockout mice with TBI, ramelteon did not decrease the lesion volume, neuronal degeneration, or myelin loss on day 3; nor did it mitigate depression-like behavior or most motor behavior deficits on day 28. Thus, timed ramelteon treatment appears to prevent inflammation and oxidative stress via the Nrf2-antioxidant response element pathway and might represent a potential chronotherapeutic strategy for treating TBI.

Administration of Melatonin and Metformin Prevents Deleterious Effects of Circadian Disruption and Obesity in Male Rats

Circadian disruption and obesity synergize to predispose to development of type 2 diabetes mellitus (T2DM), signifying that therapeutic targeting of both circadian and metabolic dysfunctions should be considered as a potential treatment approach. To address this hypothesis, we studied rats concomitantly exposed to circadian disruption and diet-induced obesity (CDO), a rat model recently shown to recapitulate phenotypical aspects of obese T2DM (eg, circadian disruption, obesity, insulin resistance, and islet failure). CDO rats were subsequently treated daily (for 12 wk) by timed oral gavage with vehicle, melatonin (a known chronobiotic), metformin, or combination treatment of both therapeutics. Melatonin treatment alone improved circadian activity rhythms, attenuated induction of β-cell failure, and enhanced glucose tolerance. Metformin alone did not modify circadian activity but enhanced insulin sensitivity and glucose tolerance. Importantly, the combination of melatonin and metformin had synergistic actions to modify progression of metabolic dysfunction in CDO rats through improved adiposity, circadian activity, insulin sensitivity, and islet cell failure. This study suggests that management of both circadian and metabolic dysfunctions should be considered as a potential preventative and therapeutic option for treatment of obesity and T2DM.

Effects of the ornidazole enantiomers on the central nervous system: Involvement of the GABAA receptor

This is the preliminary study of the sedative and muscle relaxation activity of ornidazole enantiomers, which are widely used in the treatment of susceptible protozoal infections and anaerobic bacterial infections. Adverse effects on the central nervous system (CNS) are the main side effects of ornidazole during its clinical application. The aim of this study was to compare the different central inhibitory effects between S-(-) ornidazole and R-(+) ornidazole in mice and clarify the possible mechanisms. In the present study, central effects of ornidazole were evaluated by open-field test and rota-rod test, and such effects were reversed by pre-treatment with flumazenil (i.p., 10 mg/kg) suggesting that ornidazole exhibits such action by interacting with the GABAergic system. Then, the functional difference between S-(-) ornidazole and R-(+) ornidazole was further explored by evaluating the contents of glutamate (Glu) and γ-aminobutyric acid (GABA) in the brain, and Western blot was used to measure glutamic acid decarboxylase (GAD65/67) expression in the mice cerebral cortex. We found that R-(+) ornidazole mediated an increase in GABA level while decreased the level of glutamate through upregulation of GAD65/67 in the cerebral cortex. Taken together, our study suggests that R-(+) ornidazole mediate stronger central inhibitory effects than S-(-) ornidazole through interaction with the GABAergic system.

The circadian control of sleep

The sleep/wake cycle is arguably the most familiar output of the circadian system, however, sleep is a complex biological process that arises from multiple brain regions and neurotransmitters, which is regulated by numerous physiological and environmental factors. These include a circadian drive for wakefulness as well as an increase in the requirement for sleep with prolonged waking (the sleep homeostat). In this chapter, we describe the regulation of sleep, with a particular emphasis on the contribution of the circadian system. Since their identification, the role of clock genes in the regulation of sleep has attracted considerable interest, and here, we provide an overview of the interplay between specific elements of the molecular clock with the sleep regulatory system. Finally, we summarise the role of the light environment, melatonin and social cues in the modulation of sleep, with a focus on the role of melanopsin ganglion cells.

Circadian system functionality, hippocampal oxidative stress, and spatial memory in the APPswe/PS1dE9 transgenic model of Alzheimer disease: effects of melatonin or ramelteon

Alzheimer disease (AD) is a neurodegenerative disorder that primarily causes β-amyloid accumulation in the brain, resulting in cognitive and behavioral deficits. AD patients, however, also suffer from severe circadian rhythm disruptions, and the underlying causes are still not fully known. Patients with AD show reduced systemic melatonin levels. This may contribute to their symptoms, since melatonin is an effective chronobiotic and antioxidant with neuroprotective properties. Here, the authors critically assessed the effects of long-term melatonin treatment on circadian system function, hippocampal oxidative stress, and spatial memory performance in the APPswe/PS1 double transgenic (Tg) mouse model of AD. To test if melatonin MT1/MT2 receptor activation, alone, was involved, the authors chronically treated some mice with the selective MT1/MT2 receptor agonist ramelteon. The results indicate that many of the circadian and behavioral parameters measured, including oxidative stress markers, were not significantly affected in these AD mice. During the day, though, Tg controls (Tg-CON) showed significantly higher mean activity and body temperature (BT) than wild-type (WT) mice. Overall, BT rhythm amplitude was significantly lower in Tg than in WT mice. Although melatonin treatment had no effect, ramelteon significantly reduced the amplitude of the BT rhythm in Tg mice. Towards the end of the experiment, Tg mice treated with ramelteon (Tg-RAM) showed significantly higher circadian rhythm fragmentation than Tg-CON and reduced circadian BT rhythm strength. The free-running period (τ) for the BT and locomotor activity (LA) rhythms of Tg-CON was <24 h. Whereas melatonin maintained τ at 24 h for BT and LA in both genotypes, ramelteon treatment had no effect. In the behavioral tests, the number of approaches and time spent exploring novel objects were significantly higher in Tg-CON than WT controls. Brain tissue analysis revealed significant reduction in hippocampal protein oxidation in Tg-MEL and Tg-RAM compared with Tg-CON animals. These results suggest that not all aspects of the circadian system are affected in the APPswe/PS1 mice. Therefore, care should be taken when extending the results obtained in Tg mice to develop new therapies in humans. This study also revealed the complexity in the therapeutic actions of melatonin and ramelteon in this mouse model of AD.

Effects induced by Apis mellifera venom and its components in experimental models of nociceptive and inflammatory pain

The effects induced by Apis mellifera venom (AMV), melittin-free AMV, fraction with molecular mass < 10 kDa (F<₁₀) or melittin in nociceptive and inflammatory pain models in mice were investigated. Subcutaneous administration of AMV (2, 4 or 6 mg/kg) or melittin-free AMV (1, 2 or 4 mg/kg) into the dorsum of mice inhibited both phases of formaldehyde-induced nociception. However, F<₁₀ (2, 4 or 6 mg/kg) or melittin (2 or 3 mg/kg) inhibited only the second phase. AMV (4 or 6 mg/kg), but not F<₁₀, melittin-free AMV or melittin, induced antinociception in the hot-plate model. Paw injection of AMV (0.05 or 0.10 mg), F<₁₀ (0.05 or 0.1 mg) or melittin (0.025 or 0.050 mg) induced a nociceptive response. In spite of inducing nociception after paw injection, scorpion (Tityus serrulatus) or snake (Bothrops jararaca) venom injected into the dorsum of mice did not inhibit formaldehyde-induced nociception. In addition, AMV (6 mg/kg), but not F<₁₀ (6 mg/kg) or melittin (3 mg/kg), inhibited formaldehyde paw oedema. Concluding, AMV, F<₁₀ and melittin induce two contrasting effects: nociception and antinociception. AMV antinociception involves the action of different components and does not result from non-specific activation of endogenous antinociceptive mechanisms activated by exposure to noxious stimuli.

Critical appraisal of ramelteon in the treatment of insomnia

Ramelteon is the first member of a novel class of hypnotics and acts as a selective melatonin receptor agonist. In 2005, ramelteon was approved by the US Food and Drug Administration for the treatment of insomnia characterized by sleep onset problems. Its unique mechanism of action made it a promising candidate compared with the widely used hypnotics that act on the benzodiazepine receptor complex. Several studies have examined its efficacy and safety as a hypnotic agent. The primary efficacy of ramelteon was found to lie in a decrease in latency to persistent sleep, as measured by polysomnographic tests. Other sleep-related measures, such as total sleep time and number of nightly awakenings, show less pronounced improvement when treated with ramelteon. In addition, no rebound insomnia or abuse potential was observed in clinical studies. Although additional studies are necessary, current data on the acute and next-morning effects of ramelteon did not indicate cognitive or psychomotor impairment. Overall, ramelteon is safe and well tolerated, although some questions remain regarding its long-term efficacy and safety. These issues and possibilities for use in other patient groups should be addressed in future research.

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.

Efficacy and clinical safety of ramelteon: an evidence-based review

Ramelteon is a novel hypnotic compound that is FDA-approved for the treatment of sleep-onset difficulty. It is a melatonin 1/2 receptor agonist with rapid absorption, extensive first-pass metabolism and an elimination half-life of just over 1h. Clinical efficacy data indicate moderate efficacy in reduction of sleep latency in adults of all ages with chronic insomnia, with estimated effect sizes roughly comparable to other standard hypnotic agents. Objective studies show minimal increases in total sleep time and no significant impact on wake after sleep-onset or sleep-stage distribution. Subjective reports demonstrate comparable, if slightly smaller, improvements. The recommended dosage is 8mg but studies suggest a flat response across dosage ranges from 4 to 32mg. Safety data indicate no evidence of clinically significant next-day performance effects and a reasonably low side effect profile. Animal studies, and a single human study suggest low abuse potential. Single-blind run-out from clinical trials have demonstrated no evidence of rebound insomnia.

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

Melatonin, a chronobiotic that participates in the control of the circadian system, is known for its sleep-promoting effects, which include shortening of sleep latency and lengthening of sleep duration. As a result of its short half-life, melatonin does not exhibit undesirable side effects, and its broad applicability for a variety of sleep problems has been the focus of numerous scientific studies. Melatonin has not, however, received regulatory approval from the US FDA as a drug, because it can be sold freely as a food supplement. Consequently, there has been an active search for patentable melatonin receptor ligands in recent years. Ramelteon, an agonist that acts solely on melatonin MT(1) and MT(2) receptors, is of particular interest, and preliminary research indicates that it holds considerable promise for clinical applications. Ramelteon has been shown to induce sleep initiation and maintenance in various animal models and in clinical trials. In chronic insomnia, ramelteon decreases sleep latency and increases total sleep time and sleep efficiency, without causing hangover, addiction or withdrawal effects. Ramelteon is thought to promote sleep by influencing homeostatic sleep signaling mediated by the suprachiasmatic nucleus. Although ramelteon's metabolism and pharmacokinetics differ from those of melatonin, its safety seems to be sufficient for short-term application. Its long-term effects remain to be determined.

Promoting adjustment of the sleep–wake cycle by chronobiotics

Chronobiotics are substances that adjust the timing of internal biological rhythms. Many classes of drugs have been claimed to possess such properties and arouse growing interest as the circumstances for their use in sleep disturbances caused by circadian rhythms alterations (delayed or advanced sleep-phase syndromes, non-24-h sleep-wake disorders, jet lag, shift work sleep disorders and so on) have become progressively more frequent. Amongst the substances potentially presenting chronobiotic properties, a consensus seems to be reached on the possible use of melatonin or its agonists to shift the phase of the human circadian clock, but optimizing the dose, formulation and especially the time of administration require further studies.

Melatonin receptors in the eye: location, second messengers and role in ocular physiology

The pineal hormone melatonin, an important regulator of circadian and seasonal rhythms, has a role in ocular pathophysiology. In addition to the pineal gland, melatonin synthesis is carried out in several ocular structures. Moreover, specific melatonin receptors have been located in the retina, cornea, ciliary body, lens, choroid and sclera, which suggests that cells in these tissues may be targets for melatonin action. This review summarizes the current knowledge about melatonin receptor subtypes with the emphasis on those melatonin receptors, which have been identified in ocular tissues and their possible roles in biochemical and physiological processes in the eye.

Characterization of the antinociceptive and anti-inflammatory activities of riboflavin in different experimental models

Riboflavin, similar to other vitamins of the B complex, presents anti-inflammatory activity but its full characterization has not yet been carried out. Therefore, we aimed to investigate the effect of this vitamin in different models of nociception, edema, fever and formation of fibrovascular tissue. Riboflavin (25, 50 or 100 mg/kg, i.p.) did not alter the motor activity of mice in the rota-rod or the open field models. The second phase of the nociceptive response induced by formalin in mice was inhibited by riboflavin (50 or 100 mg/kg). The first phase of this response and the nociceptive behavior in the hot-plate model were inhibited only by the highest dose of this vitamin. Riboflavin (25, 50 or 100 mg/kg, i.p.), administered immediately and 2 h after the injection of carrageenan, induced antiedema and antinociceptive effects. The antinociceptive effect was not inhibited by the pretreatment with cadmium sulfate (1 mg/kg), an inhibitor of flavokinase. Riboflavin (50 or 100 mg/kg, i.p., 0 and 2 h) also inhibited the fever induced by lipopolysaccharide (LPS) in rats. Moreover, the formation of fibrovascular tissue induced by s.c. implant of a cotton pellet was inhibited by riboflavin (50 or 100 mg/kg, i.p., twice a day for one week). Riboflavin (10 or 25 mg/kg, i.p.) also exacerbated the effect of morphine (2, 4 or 8 mg/kg, i.p.) in the mouse formalin test. In conclusion, the study demonstrates the antinociceptive and anti-inflammatory activities of riboflavin in different experimental models. These results, associated with the fact that riboflavin is a safe drug, is approved for clinical use and exacerbates the antinociceptive effect of morphine, may warrant clinical trials to assess its potential in the treatment of different painful or inflammatory conditions.