Melatonin activates Nrf2/HO-1 signalling pathway to antagonizes oxidative stress-induced injury via melatonin receptor 1 (MT1) in cryopreserved mice ovarian tissue.

Our previous research has shown that melatonin (MLT) can reduce cryopreserved ovarian damage in mice. Yet, the molecular mechanism of MLT protection is still unclear. Some studies have shown that melatonin receptor 1 (MT1) is very important for animal reproductive system. To evaluate whether MLT exerts its protective effect on cryopreserved mice ovarian tissue via MT1, we added antagonist of MT1/MT2 (Luzindor) or antagonist of MT2 (4P-PDOT) to the freezing solution, followed by cryopreservation and thawing of ovarian tissue. The levels of total superoxide dismutase (T-SOD), catalase (CAT), nitric oxide (NO) and malondialdehyde (MDA) were detected. Besides, by using RT-PCR and Western blotting, the expression of Bcl-2, Bax and Nrf2/HO-1 signalling pathway-related proteins was detected. These findings demonstrated that compared with the melatonin group, the addition of Luzindor increased apoptosis, NO and MDA activities, decreased CAT and T-SOD activities and inhibited Nrf2/HO-1 signalling pathway. In conclusion, melatonin can play a protective role in cryopreserved ovarian tissue of mice through MT1 receptor.

Differential effect of melatonin on ram spermatozoa depending on the allelic variant of the RsaI polymorphism of the MTR1A gene, incubation medium and season.

Context The Rsa I polymorphism of the melatonin receptor MTNR1A gene affects seasonal reproduction in sheep, but its effect on ram spermatozoa and their response to melatonin is unknown. Aims This study aims to evaluate whether Rsa I polymorphism of the MTNR1A gene influences the response of ram spermatozoa to in vitro added melatonin. Methods Spermatozoa from rams carrying different Rsa I allelic variants were incubated with melatonin in a TALP medium or a capacitation-triggering medium during the reproductive and non-reproductive seasons. After incubation, sperm motility, membrane integrity, mitochondria activity, oxidative damage, apoptotic markers and capacitation status were assessed. Key results In the reproductive season, the T/T genotype was related to some adverse effects of melatonin when spermatozoa were incubated in TALP medium, whereas the C/C genotype was linked with adverse effects when the hormone was added in a capacitation-triggering medium. The decapacitating effect of melatonin on spermatozoa was also different depending on genotype. Conclusions The melatonin effect on spermatozoa from rams carrying different Rsa I genotypes differed depending on the season and the medium. Implications The knowledge of the Rsa I allelic variant of the MTNR1A gene of rams could be helpful when carrying out in vitro reproductive techniques in the ovine species.

Genotyping of rams based on melatonin receptor 1A gene polymorphisms: a tool in sire selection?

Context Several polymorphisms in the melatonin receptor 1A gene (MTNR1A ) have been related to reproductive performance in ovine. Aims To investigate the effect of the Rsa I and Mnl I polymorphisms on ram seminal quality. Methods Eighteen Rasa Aragonesa rams were genotyped for the Rsa I (C/C, C/T, T/T) and Mnl I (G/G, G/A, A/A) allelic variants of the MTNR1A gene. Individual ejaculates were analysed once a month throughout the whole year. Sperm motility, morphology, membrane integrity, levels of reactive oxygen species (ROS), phosphatidylserine (PS) inversion, DNA fragmentation and capacitation status were assessed. The effect of the season and polymorphisms on seminal quality was evaluated by mixed ANOVA. Key results Both polymorphisms had an effect on membrane integrity and viable spermatozoa with low levels of ROS and without PS translocation, and Rsa I also on motile and DNA-intact spermatozoa. An interaction between both polymorphisms was found, pointing to a negative effect on seminal quality of carrying the T or A allele in homozygosity. Differences were higher in the reproductive than in the non-reproductive season. Conclusions Mutations substituting C by T and G by A at Rsa I and Mnl I polymorphic sites, respectively, in the MTNR1A gene in rams could decrease the seminal quality. Implications Genotyping of rams based on melatonin receptor 1A could be a powerful tool in sire selection.

Sperm melatonin receptors, seminal plasma melatonin and semen freezability in goats.

Goat bucks are seasonal breeders that show variation in sperm quality, endogenous melatonin (MLT), and presumably in the expression of MLT receptors on the sperm throughout the year, which may modify sperm freezability. The aim of this study was to determine whether sperm freezability is associated with (i) endogenous melatonin levels in seminal plasma and (ii) the expression of sperm plasma membrane melatonin receptors (MT1, MT2). To evaluate this, spermatozoa from seven Saanen goat bucks were cryopreserved throughout the year in Mexico using a standard freezing protocol. Seminal plasma MLT concentrations were determined by ELISA and the expression and localization of MT1 and MT2 were detected by immunocytochemistry and confirmed by western blotting. The recovery rate of progressive motility after thawing was higher in spring than autumn and winter; in contrast, the F pattern (CTC assay) was higher in winter than in the other seasons. A proportional increase in the AR pattern (CTC assay) was smaller in winter than in the other seasons and the proportion of sperm showing high plasma membrane fluidity was higher in spring than in summer and autumn. The seminal plasma MLT concentrations showed no significant interseasonal differences. The MT1 receptor was immunolocalised at the apical region of the sperm head, while MT2 was mainly localised in the neck. The relative expression of MLT receptors showed significant differences between summer and winter for all bands, except at 75 kDa of MT2. In conclusion, there was an association between the relative expression of MT1 and MT2 receptors throughout the year and sperm freezability in goat bucks in México. Post-thaw sperm quality is enhanced in semen samples collected during breeding season.

Thirty-seven years of MT1 and MT2 melatonin receptor localization in the brain: Past and future challenges.

Identifying the target cells of a hormone is a key step in understanding its function. Once the molecular nature of the receptors for a hormone has been established, researchers can use several techniques to detect these receptors. Here I will review the different tools used over the years to localize melatonin receptors and the problems associated with each of these techniques. The radioligand 2-[125I] iodomelatonin was the first tool to allow localization of melatonin receptors on tissue sections. Once the MT1 and MT2 receptors were cloned, in situ hybridization could be used to detect the messenger RNA for these receptors. The deduced amino acid sequences for MT1 and MT2 receptors allowed the production of peptide immunogens to generate antibodies against the MT1 and MT2 receptors. Finally, transgenic reporters driven by the promoter elements of the MT1 and MT2 genes have been used to map the expression of MT1 and MT2 in the brain and the retina. Several issues have complicated the localization of melatonin receptors and the characterization of melatonin target cells over the last three decades. Melatonin receptors are expressed at low levels, leading to sensitivity issues for their detection. The second problem are specificity issues with antibodies directed against the MT1 and MT2 melatonin receptors. These receptors are G protein-coupled receptors and many antibodies directed against such receptors have been shown to present similar problems concerning their specificity. Despite these specificity problems which start to be seriously addressed by recent studies, antibodies will be important tools in the future to identify and phenotype melatonin target cells. However, we will have to be more stringent than previously when establishing their specificity. The results obtained by these antibodies will have to be confronted and be coherent with results obtained by other techniques.

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.

Melatonin receptor 1A (MTNR1A) gene linkage and association to type 2 diabetes in Italian families.

OBJECTIVE: Melatonin regulates the mammalian circadian rhythm and plays metabolic functions such as glucose homeostasis. Both melatonin receptors (MTNR1A and MTNR1B, encoded by the MTNR1A and MTNR1B genes, respectively) are expressed in pancreatic beta cells and mediate the glucometabolic roles of melatonin as well as insulin secretion. The MTNR1B gene is a well-known genetic risk factor in type 2 diabetes (T2D); however, little is known about the involvement of the MTNR1A gene in here T2D. We aimed to investigate whether MTNR1A is linked to and/or associated with familial T2D. SUBJECTS AND METHODS: We genotyped 14 single nucleotide polymorphisms within the MTNR1A gene in 212 peninsular Italian families with T2D. We performed parametric linkage and linkage disequilibrium analyses to investigate the role of MTNR1A variants in conferring T2D risk. We considered variants statistically significant if conferring linkage or linkage disequilibrium with p < 0.05. RESULTS: We found 3 novel variants (rs62350392, rs2119883, and rs13147179) significantly linked to and/or associated with T2D in multigenerational Italian families. CONCLUSIONS: This is the first study to report MTNR1A as a novel risk gene in T2D. Functional studies are needed to confirm these results.

Association of rotating night shift work, CLOCK, MTNR1A, MTNR1B genes polymorphisms and their interactions with type 2 diabetes among steelworkers: a case-control study.

BACKGROUND: The purpose of this study is to investigate the association of rotating night shift work, CLOCK, MTNR1A, MTNR1B genes polymorphisms and their interactions with type 2 diabetes among steelworkers. METHODS: A case-control study was conducted in the Tangsteel company in Tangshan, China. The sample sizes of the case group and control group were 251 and 451, respectively. The logistic regression, log-linear model and generalized multifactor dimensionality (GMDR) method were used to investigate the interaction between circadian clock gene, melatonin receptor genes and rotating night shift work on type 2 diabetes among steelworkers. Relative excess risk due to interaction (RERI) and attributable proportions (AP) were used to evaluate additive interactions. RESULTS: Rotating night shift work, current shift status, duration of night shifts, and average frequency of night shifts were associated with an increased risk of type 2 diabetes after adjustment for confounders. Rs1387153 variants in MTNR1B was found to be associated with an increased risk of type 2 diabetes, which was not found between MTNR1A gene rs2119882 locus, CLOCK gene rs1801260 locus and the risk of type 2 diabetes. The association between rotating night shift work and risk of type 2 diabetes appeared to be modified by MTNR1B gene rs1387153 locus (RERI = 0.98, (95% CI, 0.40-1.55); AP = 0.60, (95% CI, 0.07-1.12)). The interaction between MTNR1A gene rs2119882 locus and CLOCK gene rs1801260 locus was associated with the risk of type 2 diabetes (RERI = 1.07, (95% CI, 0.23-1.91); AP = 0.77, (95% CI, 0.36-1.17)). The complex interaction of the MTNR1A-MTNR1B-CLOCK-rotating night shift work model based on the GMDR methods may increase the risk of type 2 diabetes (P = 0.011). CONCLUSIONS: Rotating night shift work and rs1387153 variants in MTNR1B were associated with an increased risk of type 2 diabetes among steelworkers. The complex interaction of MTNR1A-MTNR1B-CLOCK-rotating night shift work may increase the risk of type 2 diabetes.

The Uterine Melatonergic Systems of AANAT and Melatonin Membrane Receptor 2 (MT2) Are Essential for Endometrial Receptivity and Early Implantation in Mice.

In the current study, using Aanat and Mt2 KO mice, we observed that the preservation of the melatonergic system is essential for successful early pregnancy in mice. We identified that aralkylamine N-acetyltransferase (AANAT), melatonin receptor 1A (MT1), and melatonin receptor 1B (MT2) were all expressed in the uterus. Due to the relatively weak expression of MT1 compared to AANAT and MT2, this study focused on AANAT and MT2. Aanat and Mt2 KO significantly reduced the early implantation sites and the abnormal morphology of the endometrium of the uterus. Mechanistical analysis indicated that the melatonergic system is the key player in the induction of the normal nidatory estrogen (E2) response for endometrial receptivity and functions by activating the STAT signaling pathway. Its deficiency impaired the interactions between the endometrium, the placenta, and the embryo. The reduction in melatonin production caused by Aanat KO and the impairment of signal transduction caused by Mt2 KO reduced the uterine MMP-2 and MMP-9 activity, resulting in a hyperproliferative endometrial epithelium. In addition, melatonergic system deficiency also increased the local immunoinflammatory reaction with elevated local proinflammatory cytokines leading to early abortion in the Mt2 KO mice compared to the WT mice. We believe that the novel data obtained from the mice might apply to other animals including humans. Further investigation into the interaction between the melatonergic system and reproductive effects in different species would be worthwhile.

MTNR1A and MTNR1B Gene Variants of the Melatonin Receptor and Arterial Stiffness in Persons without Arterial Hypertension.

A comparative analysis of vascular stiffness indices and the results of blood test was carried out in 85 healthy donors aged 19-64 years, carriers of polymorphic variants of type 1 and type 2 melatonin receptor genes. The associations of polymorphic markers of type 1 MTNR1A (rs34532313) and type 2 MTNR1B (rs10830963) melatonin receptor genes with parameters of vascular stiffness and blood parameters in healthy patients were studied. Genotyping was performed using allele-specific PCR. In all patients, 24-h BP monitoring with assessment of arterial stiffness was performed. Allele C homozygotes of MTNR1A differed significantly from carriers of the major T allele by elevated triglyceride, LDL, and fibrinogen levels. The major allele C of the rs10830963 polymorphic variant of the MTNR1B gene is associated with elevated LDL and triglycerides, as well as with individual differences in the elastic properties of the vascular wall in the examined subjects.

The effect of melatonin on sheep endometrial epithelial cell apoptosis through the receptor and non-receptor pathways.

Melatonin potentially regulates the female animal reproductive function, but its regulatory mechanism in the apoptosis of sheep endometrial epithelial cells (SEECs) remains to be elucidated. In the present study, immunofluorescence staining, western blotting, and quantitative real-time polymerase chain reaction were performed to detect the distribution of melatonin receptors (MT1 and MT2) in the uterus of sheep and the effect of melatonin via the receptor and non-receptor pathways on the apoptosis of SEECs in vitro. The results showed that melatonin inhibits the apoptosis of SEECs to varying degrees to regulate the expression of estrogen receptors (ERs) and progesterone receptors (PGR) via its interaction with MT1 and MT2. In addition, the ER antagonist partially relieved the inhibitory effect of melatonin on the apoptosis of SEECs, while the PGR antagonist did not. Thus, melatonin mediates endometrial epithelial apoptosis through the MT receptors and also by regulating estrogen function. This study provides evidence of the regulatory mechanism of melatonin on the physiological function of the sheep uterus.

Association of melatonin receptor 1 A with litter size in sheep: A review.

Sheep are a valuable livestock species worldwide, providing meat, milk, and various dairy products. This article aims to review the latest literature on the melatonin receptor 1A (MTNR1A) gene as a potential candidate gene associated with reproductive traits, particularly the litter size trait in sheep, by searching various databases for available literature. Studies have shown that different parts of the MTNR1A gene play various roles in sheep. By identifying marker genes associated with reproductive traits in MTNR1A polymorphisms linked to the litter size trait, breeders can achieve a faster selection response in sheep breeding by recognizing the genomic region where these genes are located and understanding their physiological functions. Therefore, highlighting the literature on these functions and their association with reproductive traits may contribute to improving the genetic makeup during sheep breeding.

The ubiquitin-specific protease 8 antagonizes melatonin-induced endocytic degradation of MT1 receptor to promote lung adenocarcinoma growth.

INTRODUCTION: The human genome encodes two melatonin receptors (MT1 and MT2) that relay melatonin signals to cellular interior. Accumulating evidence has linked melatonin to multiple health benefits, among which its anticancer effects have become well-established. However, the implications of its receptors in lung adenocarcinoma have so far remained incompletely understood. OBJECTIVES: This study aims to investigate the response of the MT1 receptor to melatonin treatment and its dynamic regulation by ubiquitin-specific protease 8 (USP8) in lung adenocarcinoma. METHODS: The mRNA levels of MT1 and MT2 receptors were analyzed with sequencing data. The expression and localization of the MT1 receptor with melatonin treatment were investigated by immunoblotting, immunofluorescence and confocal microscopy assays. Endocytic deubiquitylases were screened to identify MT1 association. The effects of USP8 were assessed with shRNA-mediated knockdown and small molecule inhibitor. The combined efficacy of melatonin and USP8 suppression was also evaluated using xenograft animal models. RESULTS: Bioinformatic analysis revealed increased expression of the MT1 receptor in lung adenocarcinoma tissues. Melatonin treatment leads to the downregulation of the MT1 receptor in lung adenocarcinoma cells, which is attributed to receptor endocytosis and lysosomal degradation via the canonical endo-lysosomal route. USP8 negatively regulates the endocytic degradation of the MT1 receptor incurred by melatonin exposure and thus protects lung adenocarcinoma cell growth. USP8 suppression by knockdown or pharmacological inhibition effectively deters cancer cell proliferation and sensitizes lung adenocarcinoma cells to melatonin in vitro. Furthermore, USP8 silencing significantly potentiates the anticancer effects of melatonin in xenograft tumor models. CONCLUSION: The MT1 receptor responds to melatonin treatment and is endocytosed for lysosomal degradation that is counteracted by USP8. The inhibition of USP8 demonstrates tumor-suppressive effects and thus can be exploited as potential therapeutic strategy either as monotherapy or combined therapy with melatonin.

The genomic response of human granulosa cells (KGN) to melatonin and specific agonists/antagonists to the melatonin receptors.

Melatonin is a known modulator of follicle development; it acts through several molecular cascades via binding to its two specific receptors MT1 and MT2. Even though it is believed that melatonin can modulate granulosa cell (GC) functions, there is still limited knowledge of how it can act in human GC through MT1 and MT2 and which one is more implicated in the effects of melatonin on the metabolic processes in the dominant follicle. To better characterize the roles of these receptors on the effects of melatonin on follicular development, human granulosa-like tumor cells (KGN) were treated with specific melatonin receptor agonists and antagonists, and gene expression was analyzed with RNA-seq technology. Following appropriate normalization and the application of a fold change cut-off of 1.5 (FC 1.5, p ≤ 0.05) for each treatment, lists of the principal differentially expressed genes (DEGs) are generated. Analysis of major upstream regulators suggested that the MT1 receptor may be involved in the melatonin antiproliferative effect by reprogramming the metabolism of human GC by activating the PKB signaling pathway. Our data suggest that melatonin may act complementary through both MT1 and MT2 receptors to modulate human GC steroidogenesis, proliferation, and differentiation. However, MT2 receptors may be the ones implicated in transducing the effects of melatonin on the prevention of GC luteinization and follicle atresia at the antral follicular stage through stimulating the PKA pathway.

MT1 Melatonin Receptor Reconstitution in Nanodiscs.

A way to study G protein-coupled receptors in a minimal system is to reconstruct artificial membrane mimics, made of detergent and/or of lipids in which the purified receptor is maintained. In particular, it is now possible to generate lipid nanoparticles, such as nanodiscs, in which a single receptor molecule is included. Such objects offer the invaluable potential of studying an isolated receptor stabilized in a finely controlled membrane-like environment to evaluate its pharmacology, its function, and its structure at the molecular level. In this chapter, we detail the different steps from the extraction and isolation of a recombinant MT1 melatonin receptor in detergent, down to its reconstitution into nanodiscs. A G protein activation test is further described in order to exemplify how the functionality of such particles may be investigated.

Nonradioactive In Situ Hybridization of MT1 Melatonin Receptor for the Identification of Melatonin Target Cells.

Identifying and phenotyping the target cells of a neuroendocrine messenger is one of the key steps to understand neuroendocrine networks and the physiological action of such messengers. In the absence of reliable antibodies directed against the receptor of a neuroendocrine messenger, detecting the expression of the messenger RNA of this receptor is an important tool to identify the target cells of a neuroendocrine messenger such as melatonin. While radioactive in situ hybridization has a higher sensitivity, nonradioactive in situ hybridization has a much better cellular resolution than radioactive in situ hybridization and is therefore better suited for phenotyping the target cells of melatonin. Here we describe a nonradioactive in situ hybridization protocol with its adaptations to various types of histological preparations. This protocol allowed the phenotyping of melatonin target cells in the pars tuberalis of the adenohypophysis, leading to the discovery of photoperiodic melatonin signaling from the pars tuberalis to the hypothalamus.

Beta-Galactosidase as a Transgenic Reporter for the Mapping and Phenotyping of MT1 and MT2 Melatonin Receptor-Expressing Cells.

Genetic technology allows inserting transgenic reporters such as beta-galactosidase (LacZ) into the loci of the Mtnr1a (MT1) and Mtnr1b (MT2) receptor genes to track MT1 and MT2 melatonin receptor expression. Given the limited sensitivity of nonradioactive in situ hybridization and the problematic specificity of existing melatonin receptor antibodies for immunohistochemistry, this new technology is a key tool to study the localization and the phenotypes of cells expressing melatonin receptors. Here we describe two protocols to detect transgenic LacZ expression driven by the MT1 or MT2 promoters either by the enzymatic activity of the transgenic LacZ enzyme or by using specific antibodies against LacZ with immunohistochemistry. This approach has already yielded a detailed mapping of both MT1 and MT2 expression in the mouse brain and retina. Furthermore, we also phenotyped some of the most important types of cells expressing these two melatonin receptors.

Light-Induced Functional Pinealectomy: Expression of MT2 Receptors in Liver Cells of C57BL/6 Mice after Melatonin Treatment.

We performed an immunohistochemical study of MT2 melatonin receptor expression in the liver of C57BL/6 mice with modeled light-induced functional pinealectomy and after melatonin administration by the indirect avidin-biotin peroxidase ABC method. The animals were kept for 14 days under constant lighting. Intragastric administration of melatonin in physiological doses (1 mg/kg body weight for 14 days) to mice with light-induced functional pinealectomy resulted in a 2-fold increase in the relative expression area of MT2 receptors in liver cells in comparison with that in animals kept under standard lighting conditions, 24-h lighting for 14 days, or 24-h lighting receiving placebo (intragastric administration of 200 ml distilled water). Melatonin treatment had practically no effect on MT2 staining intensity. Our results attest to the important role of MT2 receptors in melatonin synthesis disorders and can serve as the basis for the development of therapeutic strategies aimed at melatonin receptors.

Melatonin Reverses the Warburg-Type Metabolism and Reduces Mitochondrial Membrane Potential of Ovarian Cancer Cells Independent of MT1 Receptor Activation.

Ovarian cancer (OC) is the most lethal gynecologic malignancy, and melatonin has shown various antitumor properties. Herein, we investigated the influence of melatonin therapy on energy metabolism and mitochondrial integrity in SKOV-3 cells and tested whether its effects depended on MT1 receptor activation. SKOV-3 cells were exposed to different melatonin concentrations, and experimental groups were divided as to the presence of MT1 receptors (melatonin groups) or receptor absence by RNAi silencing (siRNA MT1+melatonin). Intracellular melatonin levels increased after treatment with melatonin independent of the MT1. The mitochondrial membrane potential of SKOV-3 cells decreased in the group treated with the highest melatonin concentration. Melatonin reduced cellular glucose consumption, while MT1 knockdown increased its consumption. Interconversion of lactate to pyruvate increased after treatment with melatonin and was remarkable in siRNA MT1 groups. Moreover, lactate dehydrogenase activity decreased with melatonin and increased after MT1 silencing at all concentrations. The UCSC XenaBrowser tool showed a positive correlation between the human ASMTL gene and the ATP synthase genes, succinate dehydrogenase gene (SDHD), and pyruvate dehydrogenase genes (PDHA and PDHB). We conclude that melatonin changes the glycolytic phenotype and mitochondrial integrity of SKOV-3 cells independent of the MT1 receptor, thus decreasing the survival advantage of OC cells.

Melatonin-mediated attenuation of fluphenazine-induced hypokinesia in C57BL/6 mice is dependent on the light/dark phase.

Our aims were to assess the effect of melatonin on fluphenazine-induced hypokinesia during the light (ZT 9.5-10.5) and dark (ZT 17.5-18.5) phases in mice lacking endogenous pineal melatonin (C57BL/6 mouse), and to investigate the effects of the manipulation of environmental lighting in mice with a targeted deletion of the MT1 melatonin receptor. In both knockout (C57KO MT1) and wild type (C57WT) mice, fluphenazine (1 mg/kg) induced hypokinesia during the light phase (C57WT: M=105, SEM=31.2 s, n = 31; C57 MT1KO:M=118, SEM = 32.6 s, n = 29). During the light phase melatonin (10 mg/kg, sc) significantly reduced hypokinesia in both genotypes (C57WT: M=33.1, SEM=8.4 s; C57 MT1KO: M=33.3, SEM=13.0 s). In the dark, fluphenazine did not induce a substantial hypokinesia in either C57WT or C57 MT1KO mice. Manipulating the lightning environment during testing, experiments conducted during the light phase in a dark environment served to abolish the hypokinetic effect of fluphenazine in all groups regardless of melatonin treatment. Conversely, experiments conducted during the dark phase in a light environment showed mice to have hypokinetic effects by fluphenazine treatment in both C57WT (M=98.4, SEM=20.2 s) and C57 MT1KO (M=40.4 SEM=9.5 s) groups. These data suggest that fluphenazine-induced hypokinesia is more pronounced under light than dark conditions, and that melatonin is only able to counteract hypokinesia during the light phase. Importantly, our data suggest that the effect of melatonin on hypokinesia was not solely mediated by the MT1 melatonin receptor in the C57BL/6 mouse, leaving the possible activation of MT2 receptor as the mechanism of action which is regulated by the light/dark environment.