Effects of melatonin on orphanin FQ/nociceptin-induced hyperalgesia in mice

Supraspinal melatonin MT2 receptor agonism alleviates pain via a neural circuit that recruits mu opioid receptors

Melatonin, through its G protein-coupled receptor (GPCR) (MTNR1B gene) MT₂ , is implicated in analgesia, but the relationship between MT₂ receptors and the opioid system remains elusive. In a model of rodent neuropathic pain (spared nerve injured [SNI]), the selective melatonin MT₂ agonist UCM924 reversed the allodynia (a pain response to a non-noxious stimulus), and this effect was nullified by the pharmacological blockade or genetic inactivation of the mu opioid receptor (MOR), but not the delta opioid receptor (DOR). Indeed, SNI MOR, but not DOR knockout mice, did not respond to the antiallodynic effects of the UCM924. Similarly, the nonselective opioid antagonist naloxone and the selective MOR antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) blocked the effects of UCM924 in SNI rats, but not the DOR antagonist naltrindole (NTI). Electrophysiological recordings in the rostral-ventromedial medulla (RVM) revealed that the typical reduction of the firing activity of pronociceptive ON-cells, and the enhancement of the firing of the antinociceptive OFF-cells, induced by the microinjection of the MT₂ agonist UCM924 into the ventrolateral periaqueductal gray (vlPAG) were blocked by MOR, but not DOR, antagonism. Immunohistochemistry studies showed that MT₂ receptors are expressed in both excitatory (CaMKIIα⁺ ) and inhibitory (GAD65⁺ ) neuronal cell bodies in the vlPAG (~2.16% total), but not RVM. Only 0.20% of vlPAG neurons coexpressed MOR and MT₂ receptors. Finally, UCM924 treatment induced an increase in the enkephalin precursor gene (PENK) in the PAG of SNI mice. Collectively, the melatonin MT₂ receptor agonism requires MORs to exert its antiallodynic effects, mostly through an interneuronal circuit involving MOR and MT₂ receptors.

The Role of Nutrient Supplementation in the Management of Chronic Pain in Fibromyalgia: A Narrative Review

INTRODUCTION

The multifaceted clinical presentation of fibromyalgia (FM) supports the modern understanding of the disorder as a more global condition than one simply affecting pain sensation. The main pharmacologic therapies used clinically include anti-epileptics and anti-depressants. Conservative treatment options include exercise, myofascial release, psychotherapy, and nutrient supplementation.

METHODS

Narrative review.

RESULTS

Nutrient supplementation is a broadly investigated treatment modality as numerous deficiencies have been linked to FM. Additionally, a proposed link between gut microbiome patterns and chronic pain syndromes has led to studies investigating probiotics as a possible treatment. Despite positive results, much of the current evidence regarding this topic is of poor quality, with variable study designs, limited sample sizes, and lack of control groups.

CONCLUSIONS

The etiology of FM is complex, and has shown to be multi-factorial with genetics and environmental exposures lending influence into its development. Preliminary results are promising, however, much of the existing evidence regarding diet supplementation is of poor quality. Further, more robust studies are needed to fully elucidate the potential of this alternative therapeutic option.

Nociceptive responses in melatonin MT2 receptor knockout mice compared to MT1 and double MT1 /MT2 receptor knockout mice

Melatonin, a neurohormone that binds to two G protein-coupled receptors MT₁ and MT_2, is involved in pain regulation, but the distinct role of each receptor has yet to be defined. We characterized the nociceptive responses of mice with genetic inactivation of melatonin MT₁ (MT₁ ^-/- ), or MT₂ (MT₂ ^-/- ), or both MT₁ /MT₂ (MT₁ ^-/- /MT₂ ^-/- ) receptors in the hot plate test (HPT), and the formalin test (FT). In HPT and FT, MT₁ ^-/- display no differences compared to their wild-type littermates (CTL), whereas both MT₂ ^-/- and MT₁ ^-/- /MT₂ ^-/- mice showed a reduced thermal sensitivity and a decreased tonic nocifensive behavior during phase 2 of the FT in the light phase. The MT₂ partial agonist UCM924 induced an antinociceptive effect in MT₁ ^-/- but not in MT₂ ^-/- and MT₁ ^-/- /MT₂ ^-/- mice. Also, the competitive opioid antagonist naloxone had no effects in CTL, whereas it induced a decrease of nociceptive thresholds in MT₂ ^-/- mice. Our results show that the genetic inactivation of melatonin MT₂ , but not MT₁ receptors, produces a distinct effect on nociceptive threshold, suggesting that the melatonin MT₂ receptor subtype is selectively involved in the regulation of pain responses.

Melatonin and morphine: potential beneficial effects of co-use

Morphine is a potent analgesic agent used to control acute or chronic pain. Chronic administration of morphine results in analgesic tolerance, hyperalgesia, and other side effects including dependence, addiction, respiratory depression, and constipation, which limit its clinical usage. Therefore, identifying the new analgesics with fewer side effects which could increase the effect of morphine and reduce its side effects is crucial. Melatonin, a multifunctional molecule produced in the body, is known to play an important role in pain regulation. The strong anti-inflammatory effect of melatonin is suggested to be involved in the attenuation of the pain associated with inflammation. Melatonin also increases the anti-nociceptive actions of opioids, such as morphine, and reverses their tolerance through regulating several cellular signaling pathways. In this review, published articles evaluating the effect of the co-consumption of melatonin and morphine in different conditions were investigated. Our results show that melatonin has pain-killing properties when administered alone or in combination with other anti-nociceptive drugs. Melatonin decreases morphine consumption in different pathologies. Furthermore, attenuation of morphine intake can be accompanied by reduction of morphine-associated side-effects, including physical dependence, morphine tolerance, and morphine-related hyperalgesia. Therefore, it is reasonable to believe that the combination of melatonin with morphine could reduce morphine-induced tolerance and hyperalgesia, which may result from anti-inflammatory and antioxidant properties of melatonin. Overall, we underscore that, to further ameliorate patients' life quality and control their pain in various pathological conditions, melatonin deserves to be used with morphine by anesthesiologists in clinical practice.

Efficacy of Melatonin in prevention of radiation-induced oral mucositis: A randomized clinical trial

OBJECTIVES

Evaluating the effectiveness of melatonin in prevention of radiation-induced oral mucositis.

MATERIAL AND METHODS

A randomized controlled clinical study was conducted on forty head and neck cancer (HNC) patients undergoing radiotherapy at the Department of Clinical Oncology, Alexandria University, Egypt. Patients were assigned equally to either control group who received conventional treatment or test group who received 20 mg of melatonin along with the conventional treatment. All patients were clinically evaluated for oral mucositis severity and pain at three and six weeks after the start of radiotherapy. Additionally, the total antioxidant capacity (TAC) in patients' saliva samples was assessed at the start of radiotherapy and six weeks later.

RESULTS

92.5% of all patients have experienced oral mucositis with more severity reported in the control group (30%) compared with the test group (5%). Mean pain scores decreased significantly, in the second assessment, in test group rather than the controls. TAC values showed a significant difference between the test and controls with a significant decrease in TAC in the control group.

CONCLUSION

The administration of melatonin with conventional treatment has reduced severe oral mucositis development. It aided in decreasing pain and hindering the reduction of TAC resulting from radiotherapy among the test group compared with controls.

Current understanding of pineal gland structure and function in headache

PURPOSE

The pineal gland plays an important role in biological rhythms, circadian and circannual variations, which are key aspects in several headache disorders.

OVERVIEW

Melatonin, the main pineal secreting hormone, has been extensively studied in primary and secondary headache disorders. Altered melatonin secretion occurs in many headache syndromes. Experimental data show pineal gland and melatonin both interfere in headache animal models, decreasing trigeminal activation. Melatonin has been shown to regulate CGRP and control its release.

DISCUSSION

Melatonin has been used successfully as a treatment for migraine, cluster headaches and other headaches. There is a rationale for including the pineal gland as a relevant brain structure in the mechanisms of headache pathophysiology, and melatonin as a treatment option in primary headache.

Melatonin and non-small cell lung cancer: new insights into signaling pathways

Non-small-cell lung cancer (NSCLC) is a type of malignancy with progressive metastasis having poor prognosis and lowered survival resulting from late diagnosis. The therapeutic approaches for the treatment of this incurable cancer are chemo- and radiotherapy. Since current treatments are insufficient and because of drug-induced undesirable side effects and toxicities, alternate treatments are necessary and critical. The role of melatonin, produced in and released from the pineal gland, has been documented as a potential therapy for NSCLC. Melatonin prevents tumor metastasis via inducing apoptosis processes and restraining the autonomous cell proliferation. Moreover, melatonin inhibits the progression of tumors due to its oncostatic, pro-oxidant and anti-inflammatory effects. As a result, the combined treatment with melatonin and chemotherapy may have a synergistic effect, as with some other tumors, leading to a prolonged survival and improved quality of life in patients with NSCLC. This review summarizes the available data, based on the molecular mechanisms and related signaling pathways, to show how melatonin and its supplementation function in NSCLC.

Dose-Dependent Effect of Hyperbaric Oxygen Treatment on Burn-Induced Neuropathic Pain in Rats

Hyperbaric oxygen treatment (HBOT) has been used to reduce neuropathic pain. Melatonin and opioid receptors are involved in neuropathic pain, but it is not known if HBOT works through these pathways to achieve its antinociceptive effect. We divided anesthetized rats into two treatment and three sham groups. The two treatment groups received third-degree burns on their right hind paws, one treated in a hyperbaric chamber for a week and the other for two weeks. We evaluated the mechanical paw-withdrawal threshold (MWT) and expression of melatonin receptor 1 (MT1), melatonin receptor 2 (MT2), μ (MOR) and κ (KOR) opioid receptor, brain-derived neurotrophic factor (BDNF), Substance P, and calcitonin gene-related peptide (CGRP) in cuneate nucleus, dorsal horn, and hind paw skin by immunohistochemical, immunofluorescence assays and real-time quantitative polymerase chain reaction (RT-PCR). The group receiving one-week HBOT had increased expressions of MT1, MT2, MOR and KOR and decreased expressions of BDNF, Substance P, and CGRP. Their mechanically measured pain levels returned to normal within a week and lasted three weeks. This anti-allodynia effect lasted twice as long in those treated for two weeks. Our findings suggest that increasing the duration of HBOT can reduce burn-induced mechanical allodynia for an extended period of time in rats. The upregulation of melatonin and opioid receptors observed after one week of HBOT suggests they may be partly involved in attenuation of the mechanical allodynia. Downregulation of BDNF, substance P and CGRP may have also contributed to the overall beneficial effect of HBOT.

Melatonin Suppresses Neuropathic Pain via MT2-Dependent and -Independent Pathways in Dorsal Root Ganglia Neurons of Mice

Melatonin (Mel) and its receptors (MT1 and MT2) have a well-documented efficacy in treating different pain conditions. However, the anti-nociceptive effects of Mel and Mel receptors in neuropathic pain (NP) are poorly understood. To elucidate this process, pain behaviors were measured in a dorsal root ganglia (DRG)-friendly sciatic nerve cuffing model. We detected up-regulation of MT2 expression in the DRGs of cuff-implanted mice and its activation by the agonist 8-M-PDOT (8MP). Also, Mel attenuated the mechanical and thermal allodynia induced by cuff implantation. Immunohistochemical analysis demonstrated the expression of MT2 in the DRG neurons, while MT1 was expressed in the satellite cells. In cultured primary neurons, microarray analysis and gene knockdown experiments demonstrated that MT2 activation by 8MP or Mel suppressed calcium signaling pathways via MAPK1, which were blocked by RAR-related orphan receptor alpha (RORα) activation with a high dose of Mel. Furthermore, expression of nitric oxide synthase 1 (NOS1) was down-regulated upon Mel treatment regardless of MT2 or RORα. Application of Mel or 8MP in cuff-implanted models inhibited the activation of peptidergic neurons and neuro-inflammation in the DRGs by down-regulating c-fos, calcitonin gene-related peptide [CGRP], and tumor necrosis factor-1α [TNF-1α] and interleukin-1β [IL-1β]. Addition of the MT2 antagonist luzindole blocked the effects of 8MP but not those of Mel. In conclusion, only MT2 was expressed in the DRG neurons and up-regulated upon cuff implantation. The analgesic effects of Mel in cuff-implanted mice were closely associated with both MT2-dependent (MAPK-calcium channels) and MT2-independent (NOS1) pathways in the DRG.

Effect of melatonin injection into the periaqueductal gray on antinociception and tonic immobility in male rats

Melatonin (MLT) is a neurohormone with significant involvement in several biological functions, of which antinociception and tonic immobility (TI) may be the key neurobehavioral components to survive in adverse conditions such as a predator attack. TI-induced antinociception can be elicited, facilitated, or increased through opioid and γ-aminobutyric acid (GABA) among other chemical mediators at several levels of the central nervous system, mainly in the periaqueductal gray (PAG). The aim of this study was to assess the effect of the microinjection of MLT into the main PAG regions that are related to different integrated defensive responses, namely dorsal (D) and ventrolateral (VL), on both antinociception through the tail-flick (TF) test and TI duration as single behavioral response and on combined behavioral responses (TF/TI). We found that the microinjection of MLT into the main PAG areas produced antinociception but did not affect the TI duration. The microinjection of MLT into the D-PAG decreased TF latency during TI in the combined trial (TF/TI), which implies that TI-induced antinociception was blocked. The microinjection of MLT into the VL-PAG maintained the antinociceptive capability of the TI without addition or increase in the antinociceptive effects, implying a permissive effect by MLT on the TI-induced antinociception. MLT administration into the D-PAG decreased the TI duration on the TF/TI, whereas MLT administration into the VL-PAG had the opposite effect of significantly increasing TI duration with the TF/TI trial.

Melatonin 4 mg as prophylactic therapy for primary headaches: a pilot study

There is growing evidence that headaches are connected to melatonin secretion. Our aim was to assess the potential effectiveness of melatonin for primary headache prevention. Forty-nine patients (37 with migraine and 12 with chronic tension-type headache, TTH) were prescribed oral melatonin, 4 mg, 30 minutes before bedtime for six months. Forty-one (83.6%) of the 49 patients completed the study, while eight dropped out for personal reasons. A statistically significant reduction in headache frequency was found between baseline and final follow-up after six months of treatment (p=0.033 for TTH patients and p<0.001 for migraineurs). The Headache Impact Test score was significantly reduced in both groups of headache patients (p=0.002 and p<0.001, respectively). At baseline, melatonin levels, measured both during a headache attack and a pain-free period, did not differ between patients with TTH and migraineurs (p=0.539 and p=0.693, respectively), and no statistically significant differences in Hamilton Depression Rating Scale scores were found between the two groups. This pilot study shows promising results, in terms of headache frequency reduction and daily quality of life improvement, in both groups.

Pain control by melatonin: Physiological and pharmacological effects

Pain and anxiety are the most common neurological responses to many harmful or noxious stimuli and their management clinically is often challenging. Many of the frequently used morphine-based drugs, non-steroid anti-inflammatory drugs and acetaminophen, while efficient for treating pain, lead to patients suffering from several unwanted side effects. Melatonin, produced from the pineal body is a hormone of darkness, is involved in the control of circadian rhythms, and exerts a number of pharmacological effects. Melatonin mediates its actions through MT1/MT2 melatonin receptors on the cell membrane and also through RZR/ROR nuclear orphan receptors. Chronic pain syndromes are often associated with the desynchronization of circadian and biological rhythms, which also cause disturbances in the sleep-wake cycle. Melatonin-mediated analgesic effects seem to involve β-endorphins, GABA receptor, opioid receptors and the nitric oxide-arginine pathway. The effectiveness of melatonin as an analgesic and anxiolytic agent has been demonstrated in various animal models of pain and this led to the use of melatonin clinically in different pathological conditions and also in patients undergoing surgery. Melatonin was found to be effective in many of these cases as an anxiolytic and analgesic agent, indicating its clinical application.

Functional plasticity of the N/OFQ-NOP receptor system determines analgesic properties of NOP receptor agonists

Despite high sequence similarity between NOP (nociceptin/orphanin FQ opioid peptide) and opioid receptors, marked differences in endogenous ligand selectivity, signal transduction, phosphorylation, desensitization, internalization and trafficking have been identified; underscoring the evolutionary difference between NOP and opioid receptors. Activation of NOP receptors affects nociceptive transmission in a site-specific manner, with antinociceptive effects prevailing after peripheral and spinal activation, and pronociceptive effects after supraspinal activation in rodents. The net effect of systemically administered NOP receptor agonists on nociception is proposed to depend on the relative contribution of peripheral, spinal and supraspinal activation, and this may depend on experimental conditions. Functional expression and regulation of NOP receptors at peripheral and central sites of the nociceptive pathway exhibits a high degree of plasticity under conditions of neuropathic and inflammatory pain. In rodents, systemically administered NOP receptor agonists exerted antihypersensitive effects in models of neuropathic and inflammatory pain. However, they were largely ineffective in acute pain while concomitantly evoking severe motor side effects. In contrast, systemic administration of NOP receptor agonists to non-human primates (NHPs) exerted potent and efficacious antinociception in the absence of motor and sedative side effects. The reason for this species difference with respect to antinociceptive efficacy and tolerability is not clear. Moreover, co-activation of NOP and μ-opioid peptide (MOP) receptors synergistically produced antinociception in NHPs. Hence, both selective NOP receptor as well as NOP/MOP receptor agonists may hold potential for clinical use as analgesics effective in conditions of acute and chronic pain.

Short- but not long-term melatonin administration reduces central levels of brain-derived neurotrophic factor in rats with inflammatory pain

OBJECTIVE

To evaluate the effect of short- and long-term administration of melatonin on central brain-derived neurotrophic factor (BDNF) levels in rats with acute and chronic inflammatory pain.

METHODS

The animals were allocated to one of two experiments: experiment 1 or experiment 2. In experiment 1, all animals were injected with complete Freund's adjuvant (CFA) to induce inflammation and were randomly allocated to receiving melatonin (60 mg/kg) or vehicle. Injections were administered 1 h after CFA and once daily for 2 more days (for a total of 3 days of melatonin administration). In experiment 2, fifteen days after CFA injection, the animals were treated with melatonin (50 mg/kg) or vehicle for 8 days. The animals were killed by decapitation 24 h after the last melatonin or vehicle administration, and an ELISA assay was performed to detect BDNF expression in the spinal cord, brainstem, and prefrontal cortex of the rats in both groups. Data were analyzed using Student's t test and the results are expressed as means ± SEM.

RESULTS

In the first experiment, the BDNF levels of the melatonin group were reduced in the prefrontal cortex (Student's t test, p = 0.01) and increased in the spinal cord (Student's t test, p = 0.04). In experiment 2, BDNF levels were similar in both groups for all structures (Student's t test, p > 0.00 for all). A two-way ANOVA reveled a significant effect of structures (p = 0.0001) but not of treatment (p > 0.05). The prefrontal cortex presented higher BDNF levels than other structures (ANOVA/Student-Newman-Keuls test, p = 0.0001). Considering the relationship between BDNF levels in all three structures, we found an effect of central nervous system structures (p = 0.01) and an interaction between treatment and structures (p = 0.04).

CONCLUSION

The high spinal cord BDNF levels and the low prefrontal cortical BDNF levels observed in rats with acute CFA-induced inflammation following short-term melatonin administration may be related to the pain-modulating and neuroprotective effects of this protein. Long-term melatonin administration did not alter BDNF levels in chronic inflammation.

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.

Melatonin treatment entrains the rest-activity circadian rhythm in rats with chronic inflammation

We assessed the therapeutic effect of exogenous melatonin (MEL), dexamethasone (DEXA), and a combination of both on nociceptive response induced by chronic inflammation and on the rest-activity circadian rhythm in rats. A total of 64 animals were randomly divided into eight groups of eight rats each: one control group and seven groups with complete Freund's adjuvant-inflamed animals (CFA; injection into the footpad). One of the CFA-inflamed groups did not receive any treatment; the other six were treated with melatonin (MEL), dexamethasone (DEXA), melatonin plus dexamethasone (MELDEXA), and their respective vehicles. Fifteen days after CFA injection, animals were treated with intraperitoneal injection of MEL (50 mg/kg) or its vehicle (8% ethanol in saline), DEXA (0.25 mg/kg) or its vehicle (saline), and MEL plus DEXA or their vehicles, for 8 days. The von Frey test was performed 24 h after the last administration of each treatment regimen. Hind paw thickness was measured using a pachymeter during the treatment days. The degree of swelling and histological findings were analyzed. All treated groups significantly reduced the severity of inflammation when compared with their vehicles (repeated-measures analysis of variance [ANOVA], p < 0.05 for all analyses). Inflamed animals treated with dexamethasone alone or associated with melatonin showed marked inhibition of histological findings. On the other hand, the group treated with melatonin remained with moderate inflammation. The CFA group showed a decrease in the mean rest-activity circadian rhythm, determined by the number of touch-detections per hour during water intake in comparison with the control group; only the group treated with melatonin showed a synchronized rest-activity rhythm. At the end of treatment, a significant increase was observed in hind paw withdrawal threshold on the von Frey test in the treated groups (one-way ANOVA, p < 0.05 for all). Our findings showed that melatonin (50 mg/kg) has strong chronobiotic and antinociceptive effects, but only mild anti-inflammatory effects. This evidence supports the hypothesis that melatonin can induce phase advance and circadian rhythm synchronization in rats with chronic inflammation.

Quantitation of melatonin and n-acetylserotonin in human plasma by nanoflow LC-MS/MS and electrospray LC-MS/MS

Melatonin (MEL) and its chemical precursor N-acetylserotonin (NAS) are believed to be potential biomarkers for sleep-related disorders. Measurement of these compounds, however, has proven to be difficult due to their low circulating levels, especially that of NAS. Few methods offer the sensitivity, specificity and dynamic range needed to monitor MEL and its precursors and metabolites in small blood samples, such as those obtained from pediatric patients. In support of our ongoing study to determine the safety, tolerability and PK dosing strategies for MEL in treating insomnia in children with autism spectrum disorder, two highly sensitive LC-MS/MS assays were developed for the quantitation of MEL and precursor NAS at pg/mL levels in small volumes of human plasma. A validated electrospray ionization (ESI) method was used to quantitate high levels of MEL in PK studies, and a validated nanospray (nESI) method was developed for quantitation of MEL and NAS at endogenous levels. In both assays, plasma samples were processed by centrifugal membrane dialysis after addition of stable isotopic internal standards, and the components were separated by either conventional LC using a Waters SymmetryShield RP18 column (2.1 × 100  mm, 3.5 µm) or on a polyimide-coated, fused-silica capillary self-packed with 17 cm AquaC18 (3 µm, 125 Å). Quantitation was done using the SRM transitions m/z 233 → 174 and m/z 219 → 160 for MEL and NAS, respectively. The analytical response ratio versus concentration curves were linear for MEL (nanoflow LC: 11.7-1165  pg/mL, LC: 1165-116,500  pg/mL) and for NAS (nanoflow LC: 11.0-1095  pg/mL).

Analgesic effects of melatonin: a review of current evidence from experimental and clinical studies

Melatonin is an endogenous indoleamine, produced mainly by the pineal gland. Melatonin has been proven to have chronobiotic, antioxidant, antihypertensive, anxiolytic and sedative properties. There are also experimental and clinical data supporting an analgesic role of melatonin. In experimental studies, melatonin shows potent analgesic effects in a dose-dependent manner. In clinical studies, melatonin has been shown to have analgesic benefits in patients with chronic pain (fibromyalgia, irritable bowel syndrome, migraine). The physiologic mechanism underlying the analgesic actions of melatonin has not been clarified. The effects may be linked to G(i) -coupled melatonin receptors, to G(i) -coupled opioid μ-receptors or GABA-B receptors with unknown downstream changes with a consequential reduction in anxiety and pain. Also, the repeated administration of melatonin improves sleep and thereby may reduce anxiety, which leads to lower levels of pain. In this paper, we review the current evidence regarding the analgesic properties of melatonin in animals and humans with chronic pain.

Potential use of melatonergic drugs in analgesia: mechanisms of action

Melatonin is a remarkable molecule with diverse physiological functions. Some of its effects are mediated by receptors while other, like cytoprotection, seem to depend on direct and indirect scavenging of free radicals not involving receptors. Among melatonin's many effects, its antinociceptive actions have attracted attention. When given orally, intraperitoneally, locally, intrathecally or through intracerebroventricular routes, melatonin exerts antinociceptive and antiallodynic actions in a variety of animal models. These effects have been demonstrated in animal models of acute pain like the tail-flick test, formalin test or endotoxin-induced hyperalgesia as well as in models of neuropathic pain like nerve ligation. Glutamate, gamma-aminobutyric acid, and particularly, opioid neurotransmission have been demonstrated to be involved in melatonin's analgesia. Results using melatonin receptor antagonists support the participation of melatonin receptors in melatonin's analgesia. However, discrepancies between the affinity of the receptors and the very high doses of melatonin needed to cause effects in vivo raise doubts about the uniqueness of that physiopathological interpretation. Indeed, melatonin could play a role in pain through several alternative mechanisms including free radicals scavenging or nitric oxide synthase inhibition. The use of melatonin analogs like the MT(1)/MT(2) agonist ramelteon, which lacks free radical scavenging activity, could be useful to unravel the mechanism of action of melatonin in analgesia. Melatonin has a promising role as an analgesic drug that could be used for alleviating pain associated with cancer, headache or surgical procedures.

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.

Possible participation of the nitric oxide-cyclic GMP-protein kinase G-K+ channels pathway in the peripheral antinociception of melatonin

The possible participation of the nitric oxide (NO)-cyclic GMP-protein kinase G (PKG)-K(+) channel pathway on melatonin-induced local antinociception was assessed during the second phase of the formalin test. The local peripheral ipsilateral, but not contralateral, administration of melatonin (150-600 microg/paw) produced a dose-related antinociception during both phases of the formalin test in rats. Moreover, local pretreatment with N(G)-L-nitro-arginine methyl ester (L-NAME, NO synthesis inhibitor, 10-100 microg/paw), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor, 5-50 microg/paw), (9S, 10R, 12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo [1,2,3-fg:3',2',1'-kl]pyrrolo [3,4-i][1,6] benzodiazocine-10-carboxylic acid methyl ester (KT-5823, specific PKG inhibitor, 50-500 ng/paw), glibenclamide (ATP-sensitive K(+) channel blocker, 5-50 microg/paw), apamin (small-conductance Ca(2+)-activated K(+) channel blocker, 0.1-1 microg/paw) or charybdotoxin (large- and intermediate-conductance Ca(2+)-activated K(+) channel blocker, 0.03-0.3 microg/paw), but not N(G)-D-nitro-arginine methyl ester (D-NAME, inactive isomer of L-NAME, 100 microg/paw) or vehicle, significantly prevented melatonin (300 microg/paw)-induced antinociception. Data suggest that melatonin-induced local peripheral antinociception during the second phase of the test could be due to activation of the NO-cyclic GMP-PKG-ATP-sensitive and Ca(2+)-activated K(+) channels pathway.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Melatonin reduces formalin-induced nociception and tactile allodynia in diabetic rats

The purpose of this study was to assess the antinociceptive and antiallodynic effect of melatonin as well as its possible mechanism of action in diabetic rats. Streptozotocin (50 mg/kg) injection caused hyperglycemia within 1 week. Formalin-evoked flinching was increased in diabetic rats as compared to non-diabetic rats. Oral administration of melatonin (10-300 mg/kg) dose-dependently reduced flinching behavior in diabetic rats. In addition, K-185 (a melatonin MT(2) receptor antagonist, 0.2-2 mg/kg, s.c.) completely blocked the melatonin-induced antinociception in diabetic rats, whereas that naltrexone (a non-selective opioid receptor antagonist, 1 mg/kg, s.c.) and naltrindole (a selective delta opioid receptor antagonist, 0.5 mg/kg, s.c.), but not 5'-guanidinonaltrindole (a selective kappa opioid receptor antagonist, 1 mg/kg, s.c.), partially reduced the antinociceptive effect of melatonin. Given alone K-185, naltrexone, naltrindole or 5'-guanidinonaltrindole did not modify formalin-induced nociception in diabetic rats. Four to 8 weeks after diabetes induction, tactile allodynia was observed in the streptozotocin-injected rats. On this condition, oral administration of melatonin (75-300 mg/kg) dose-dependently reduced tactile allodynia in diabetic rats. Both antinociceptive and antiallodynic effects were not related to motor changes as melatonin did not modify number of falls in the rotarod test. Results indicate that melatonin is able to reduce formalin-induced nociception and tactile allodynia in streptozotocin-injected rats. In addition, data suggest that melatonin MT(2) and delta opioid receptors may play an important role in these effects.

Oral and spinal melatonin reduces tactile allodynia in rats via activation of MT2 and opioid receptors

The antiallodynic effect of melatonin after intrathecal (it) and oral administration as well as the possible participation of MT(2) and opioid receptors in melatonin-induced antiallodynia in neuropathic rats were assessed. Ligation of the L5/L6 spinal nerves produced a clear-cut tactile allodynia in the rats. Intrathecal (3-100 microg) and oral (37.5-300 mg/kg) administration of melatonin decreased tactile allodynia induced by spinal nerve ligation. Intrathecal administration of the preferential MT(2) receptor antagonist luzindole (1-100 microg), but not vehicle, significantly diminished in a dose-dependent manner the antiallodynic effect induced by melatonin (100 microg, it). Oral (0.01-1mg/kg) or intrathecal (0.1-10 microg) administration of the highly selective MT(2) receptor antagonist 4P-PDOT diminished the antiallodynic activity induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin, respectively. Subcutaneous (1mg/kg) or intrathecal (0.5-50 microg) treatment with naltrexone, but not vehicle, significantly diminished the antiallodynic effect induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin. Oral melatonin (150 mg/kg)-induced antiallodynia was partially reduced by the spinal administration of 4P-PDOT (10 microg). Moreover, the spinal effect of melatonin (100 microg) was significantly reduced by the combination 4P-PDOT (0.1 microg)-naltrexone (0.5 microg). At the greatest tested doses, the antagonist drugs did not modify tactile allodynia in neuropathic rats. Melatonin (100 microg or 300 mg/kg) did not affect motor co-ordination in the rotarod test. Results indicate that melatonin reduces tactile allodynia in neuropathic rats after intrathecal and oral administration. Moreover, data suggest the participation of spinal MT(2) and opioid receptors in the melatonin-induced antiallodynic effect in this model.