Melatonin inhibits cyclic AMP and cyclic GMP accumulation in the rat pituitary

Exploring the potential use of melatonin as a modulator of tramadol-induced rewarding effects in rats

Background

Melatonin is responsible for regulating the sleep-wake cycle and circadian rhythms in mammals. Tramadol, a synthetic opioid analgesic, is used to manage moderate to severe pain but has a high potential for abuse and dependence. Studies have shown that melatonin could be a potential modulator to reduce tramadol addiction.

Methods

Male Wistar rats were used to investigate the effect of melatonin on tramadol-induced place preference. The rats were divided into four groups: control, tramadol, tramadol + melatonin (single dose), and tramadol + melatonin (repeated doses). Tramadol was administered intraperitoneally at 40 mg/kg, while melatonin was administered at 50 mg/kg for both the single dose and repeated-dose groups. The study consisted of two phases: habituation and acquisition.

Results

Tramadol administration produced conditioned place preference (CPP) in rats, indicating rewarding effects. However, melatonin administration blocked tramadol-induced CPP. Surprisingly, repeated doses of melatonin were ineffective and did not reduce the expression of CPP compared to that of the single dose administration.

Conclusion

The study suggests that melatonin may be a potential therapeutic option for treating tramadol addiction. The results indicate that melatonin attenuates the expression of tramadol-induced CPP, supporting its uses as an adjunct therapy for managing tramadol addiction. However, further studies are needed to investigate its effectiveness in humans.

Metabolic signatures in post-myocardial infarction heart failure, including insights into prediction, intervention, and prognosis

Heart failure (HF) is a prevalent long-term complication of myocardial infarction (MI). The incidence of post-MI HF is high, and patients with the condition have a poor prognosis. Accurate identification of individuals at high risk for post-MI HF is crucial for implementation of a protective and ideally personalized strategy to prevent fatal events. Post-MI HF is characterized by adverse cardiac remodeling, which results from metabolic changes in response to long-term ischemia. Moreover, various risk factors, including genetics, diet, and obesity, can influence metabolic pathways in patients. This review focuses on the metabolic signatures of post-MI HF that could serve as non-invasive biomarkers for early identification in high-risk populations. We also explore how metabolism participates in the pathophysiology of post-MI HF. Furthermore, we discuss the potential of metabolites as novel targets for treatment of post-MI HF and as biomarkers for prognostic evaluation. It is expected to provide valuable suggestions for the clinical prevention and treatment of post-MI HF from a metabolic perspective.

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.

The five dimensions of receptor pharmacology exemplified by melatonin receptors: An opinion

Receptology has been complicated with enhancements in our knowledge of G-protein-coupled-receptor (GPCR) biochemistry. This complexity is exemplified by the pharmacology of melatonin receptors. Here, we describe the complexity of GPCR biochemistry in five dimensions: (a) receptor expression, particularly in organs/tissues that are only partially understood; (b) ligands and receptor-associated proteins (interactome); (c) receptor function, which might be more complex than the known G-protein-coupled systems; (d) ligand bias, which favors a particular pathway; and (e) receptor dimerization, which might concern all receptors coexpressed in the same cell. Thus, receptor signaling might be modified or modulated, depending on the nature of the receptor complex. Fundamental studies are needed to clarify these points and find new ways to tackle receptor functionality. This opinion article emphasizes the global questions attached to new descriptions of GPCRs and aims to raise our awareness of the tremendous complexity of modern receptology.

Characterization of the various functional pathways elicited by synthetic agonists or antagonists at the melatonin MT1 and MT2 receptors

Melatonin is a neurohormone that translates the circadian rhythm to the peripheral organs through a series of binding sites identified as G protein-coupled receptors MT1 and MT2. Due to minute amounts of receptor proteins in target organs, the main tool of studies of the melatoninergic system is recombinant expression of the receptors in cellular hosts. Although a number of studies exist on these receptors, studies of several signaling pathways using a large number of melatoninergic compounds are rather limited. We chose to fill this gap to better describe a panel of compounds that have been only partially characterized in terms of functionality. First, we characterized HEK cells expressing MT1 or MT2, and several signaling routes with melatonin itself to validate the approach: GTPγS, cAMP production, internalization, β-arrestin recruitment, and cell morphology changes (CellKey ® ). Second, we chose 21 compounds from our large melatoninergic chemical library and characterized them using this panel of signaling pathways. Notably, antagonists were infrequent, and their functionality depended largely on the pathway studied. This will permit redefining the availability of molecular tools that can be used to better understand the in situ activity and roles of these receptors.

Are aryl hydrocarbon receptor and G-protein-coupled receptor 30 involved in the regulation of seasonal testis activity in photosensitive rodent-the bank vole (Myodes glareolus)?

Within the reproductive system both aryl hydrocarbon receptor (AHR) and G-protein-coupled receptor 30 (GPR30) contribute to estrogen signaling and controlling of reproductive physiology. The specific question is whether and how AHR and GPR30 are involved in regulation of testis function in seasonally breeding rodents. Bank vole testes were obtained from animals reared under 18 hours light:6 hours dark (LD) and 6 hours light:18 hours dark (SD) conditions. Aryl hydrocarbon receptor and GPR30 expression were analyzed by quantitative reverse transcriptase-polymerase chain reaction, Western blot, and immunohistochemistry and/or immunofluorescent staining. In addition, the activity of enzymes involved in the intracellular signal transduction; extracellular signal-regulated kinase (ERK), protein kinase (PKA), matrix metalloproteinase 9 (MMP 9) and the concentrations of cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), and calcium (Ca(2+)) were examined by immunohistochemical, immunoenzymatic, and colorimetric assays, respectively. Aryl hydrocarbon receptor and GPR30 were expressed in testes of actively reproducing voles and regressed ones although their expression at the messenger RNA and AHR also at protein level appeared to be photoperiod-dependent. A specific cellular localization and expression of AHR and GPR30 correlated with the expression of ERK, PKA, and MMP 9. Moreover, we found robust differences in the levels of cAMP, cGMP, and Ca(2+) in testicular homogenates between LD and SD voles. In the testes of LD voles, the levels of second messengers were always higher compared to SD. In vole testis, AHR and GPR30 can induce signaling pathways that involve ERK, PKA, MMP 9 and cAMP, cGMP, Ca(2+). In addition, in AHR, signaling the engagement of both photoperiod and estrogens, whereas in GPR30, signaling only estrogens is reported. It is likely that in vole, because of a differential activity of signaling molecules, signal transduction via AHR rather than through GPR30 plays a role in regulation of seasonal changes of testis physiology.

Signal transduction of receptor-mediated antiproliferative action of melatonin on human prostate epithelial cells involves dual activation of Gα(s) and Gα(q) proteins

Melatonin has been shown to inhibit the proliferation of malignant and transformed human prostate epithelial cells by transcriptional up-regulation of p27(Kip1) expression via MTNR1A receptor-mediated activation of protein kinase A (PKA) and protein kinase C (PKC) in parallel. Given that melatonin MTNR1A receptor is a G protein-coupled receptor, this study was conducted to identify the specific G proteins that mediate the antiproliferative action of melatonin on human prostate epithelial cells. In 22Rv1 and RWPE-1 cells, knockdown of either Gα(s) or Gα(q) , but not Gα(i2) expression by RNA interference, abrogated the effects of melatonin on p27(Kip1) and cell proliferation. Conversely, cellular overexpression of activated mutants of Gα(s) and Gα(q) in 22Rv1 and RWPE-1 cells mimicked the effects of melatonin on prostate epithelial cell antiproliferation by increasing p27(Kip1) expression through downstream activation of PKA and PKC in parallel. Moreover, melatonin or 2-iodomelatonin induced elevation of adenosine-3',5'-cyclic monophosphate (cAMP) in 22Rv1 and RWPE-1 cells. The effects of 2-iodomelatonin on cAMP were blocked by the nonselective MTNR1A/MTNR1B receptor antagonist luzindole but were not affected by the selective MTNR1B receptor antagonist 4-phenyl-2-propionamidotetraline (4-P-PDOT). Furthermore, knockdown of Gα(s) mitigated the stimulatory effects of 2-iodomelatonin on cAMP. Collectively, the data demonstrated, for the first time, functional coupling of MTNR1A receptor to Gα(s) in cancerous or transformed human cells expressing endogenous melatonin receptors. Our results also showed that dual activation of Gα(s) and Gα(q) proteins is involved in the signal transduction of MTNR1A receptor-mediated antiproliferative action of melatonin on human prostate epithelial cells.

Melatonin: a pleiotropic molecule regulating inflammation

Melatonin is a neurohormone produced by the pineal gland that regulates sleep and circadian functions. Melatonin also regulates inflammatory and immune processes acting as both an activator and inhibitor of these responses. Melatonin demonstrates endocrine, but also paracrine and autocrine effects in the leukocyte compartment: on one side, leukocytes respond to melatonin in a circadian fashion; on the other side, leukocytes are able to synthesize melatonin by themselves. With its endocrine and paracrine effects, melatonin differentially modulates pro-inflammatory enzymes, controls production of inflammatory mediators such as cytokines and leukotrienes and regulates the lifespan of leukocytes by interfering with apoptotic processes. Moreover, its potent antioxidant ability allows scavenging of oxidative stress in the inflamed tissues. The interesting timing of pro- and anti-inflammatory effects, such as those affecting lipoxygenase activity, suggests that melatonin might promote early phases of inflammation on one hand and contribute to its attenuation on the other hand, in order to avoid complications of chronic inflammation. This review aims at giving a comprehensive overview of the various inflammatory pathways regulated by this pleiotropic hormone.

Hypothalamic gonadotropin-releasing hormone receptor activation stimulates oxytocin release from the rat hypothalamo-neurohypophysial system while melatonin inhibits this process

The present study was undertaken to investigate the influence of gonadotropin-releasing hormone (GnRH) and its agonist and antagonist on oxytocin (OT) release from the rat hypothalamo-neurohypophysial (H-N) system. An additional aim was to determine whether the possible response of oxytocinergic neurons to these peptides could be modified by melatonin through a cAMP-dependent mechanism. The results show that the highly selective GnRH agonist (i.e., [Des-Gly(10),d-His(Bzl)(6),Pro-NHEt(9)]-LHRH; Histrelin) stimulates the secretion of OT from an isolated rat H-N system. Melatonin significantly inhibited basal and histrelin-induced release of OT in vitro, and displayed no significant influence on OT release in the presence of GnRH or its antagonist. Addition of melatonin to a medium containing forskolin resulted in significant reduction of OT secretion from the H-N system. On the other hand, addition of forskolin to a medium containing both histrelin and melatonin did not further alter the inhibitory influence of melatonin on the histrelin-dependent secretion of OT in vitro. Intracerebroventricular (icv) infusion (experiment in vivo) of a GnRH antagonist resulted in substantial inhibition of OT release, thus revealing the stimulatory action of endogenous GnRH. In melatonin-treated animals, blood plasma OT levels were not changed in comparison to the vehicle. Our present data strongly suggests that activation of the GnRH receptor in the hypothalamus is involved in stimulation of OT secretion from the rat H-N system. It has also been shown, under experimental in vitro conditions, that melatonin fully suppresses the response of oxytocinergic neurons to the GnRH agonist - histrelin. The effect of melatonin on OT release is mediated by the cAMP-dependent mechanism, although other mechanisms of action are also possible.

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.

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.

The Galphai and Galphaq proteins mediate the effects of melatonin on steroid/thyroid hormone receptor transcriptional activity and breast cancer cell proliferation

Melatonin, via its MT1 receptor, but not the MT2 receptor, can modulate the transcriptional activity of various nuclear receptors - estrogen receptor alpha (ERalpha) and retinoic acid receptor alpha (RARalpha), but not ERbeta- in MCF-7, T47D, and ZR-75-1 human breast cancer cell lines. The anti-proliferative and nuclear receptor modulatory actions of melatonin are mediated via the MT1 G protein-coupled receptor expressed in human breast cancer cells. However, the specific G proteins and associated pathways involved in the nuclear receptor transcriptional regulation by melatonin are not yet clear. Upon activation, the MT1 receptor specifically couples to the G(alphai2), G(alphai3), G(alphaq), and G(alphall) proteins, and via activation of G(alphai2) proteins, melatonin suppresses forskolin-induced 3',5'-cyclic adenosine monophosphate production, while melatonin activation of G(alphaq), is able to inhibit phospholipid hydrolysis and ATP's induction of inositol triphosphate production in MCF-7 breast cancer cells. Employing dominant-negative and dominant-positive) forms of these G proteins, we demonstrate that G(alphai2) proteins mediate the suppression of estrogen-induced ERalpha transcriptional activity by melatonin, while the G(q) protein mediates the enhancement of retinoid-induced RARalpha transcriptional activity by melatonin. However, the growth-inhibitory actions of melatonin are mediated via both G(alphai2) and G(alphaq) proteins.

Melatonin receptors, heterodimerization, signal transduction and binding sites: what's new?

Melatonin is a neurohormone that has been claimed to be involved in a wide range of physiological functions. Nevertheless, for most of its effects, the mechanism of action is not really known. In mammals, two melatonin receptors, MT1 and MT2, have been cloned. They belong to the G-protein-coupled receptor (GPCR) superfamily. They share some specific short amino-acid sequences, which suggest that they represent a specific subfamily. Another receptor from the same subfamily, the melatonin-related receptor has been cloned in different species including humans. This orphan receptor also named GPR50 does not bind melatonin and its endogenous ligand is still unknown. Nevertheless, this receptor has been shown to behave as an antagonist of the MT1 receptor, which opens new pharmacological perspectives for GPR50 despite the lack of endogenous or synthetic ligands. Moreover, MT1 and MT2 interact together through the formation of heterodimers at least in cells transfected with the cDNA of these two receptors. Lastly, signalling complexes associated with MT1 and MT2 receptors are starting to be deciphered. A third melatonin-binding site has been purified and characterized as the enzyme quinone reductase 2 (QR2). Inhibition of QR2 by melatonin may explain melatonin's protective effect that has been reported in different animal models and that is generally associated with its well-documented antioxidant properties.

Melatonin potentiates rod signals to ON type bipolar cells in fish retina

Melatonin is involved in regulation of a variety of physiological functions through activation of specific G-protein coupled receptors. However, the neuromodulatory role of melatonin, released from photoreceptors in the retina, is poorly understood. Here we show that melatonin enhances the sensitivity of the rod signal pathway by potentiating signal transfer from rod photoreceptors to ON bipolar cells (Rod-ON-BCs). Whole-cell patch-clamp recordings showed that melatonin induced a sustained inward current from Rod-ON-BCs, through activation of the melatonin MT2 receptor, which was identified as one mediated by a cGMP-dependent cation channel. Consistent with this, melatonin was found, using immunocytochemistry, to increase intracellular cGMP levels, which was identified due to an inhibition of phosphodiesterase. Physiologically, melatonin potentiated responses of Rod-ON-BCs to simulated light flashes (brief puffs of CPPG, an mGluR6 antagonist, in the presence of l-AP4, an mGluR6 agonist), which was mediated by cGMP-dependent kinase, and increased the amplitude of the scotopic electroretinographic b-wave, a reflection of Rod-ON-BC activity. These results suggest that melatonin, being at a higher level at night, may improve the signal/noise ratio for rod signals in the outer retina by enhancing signal transfer from rods to BCs.

Modulation by melatonin of glutamatergic synaptic transmission in the carp retina

Melatonin is involved in a variety of physiological functions through activating specific receptors coupled to GTP-binding protein. Melatonin and its receptors are abundant in the retina. Here we show for the first time that melatonin modulates glutamatergic synaptic transmission from cones to horizontal cells (HCs) in carp retina. Immunocytochemical data revealed the expression of the MT1 receptor on carp HCs. Whole-cell recordings further showed that melatonin of physiological concentrations potentiated glutamate-induced currents from isolated cone-driven HCs (H1 cells) in a dose-dependent manner, by increasing the efficacy and apparent affinity of the glutamate receptor. The effects of melatonin were reversed by luzindole, but not by K 185, indicating the involvement of the MT1 receptor. Like melatonin, methylene blue (MB), a guanylate cyclase inhibitor, also potentiated the glutamate currents, but internal infusion of cGMP suppressed them. The effects of melatonin were not observed in cGMP-filled and MB-incubated HCs. These results suggest that the melatonin effects may be mediated by decreasing the intracellular concentration of cGMP. Consistent with these observations, melatonin depolarized the membrane potential of H1 cells and reduced their light responses, which could also be blocked by luzindole. These effects of melatonin persisted in the presence of the antagonists of receptors for dopamine, GABA and glycine, indicating a direct action of melatonin on H1 cells. Such modulation by melatonin of glutamatergic transmission from cones to HCs is thought to be in part responsible for circadian changes in light responsiveness of cone HCs in teleost retina.

Synthesis of a novel series of tricyclic indan derivatives as melatonin receptor agonists

To develop a new therapeutic agent for sleep disorders, we synthesized a novel series of tricyclic indan derivatives and evaluated them for their binding affinity to melatonin receptors. In our previous paper, we proposed a conformation of the methoxy group favorable for the binding of the MT(1) receptor. To fix the methoxy group in an active conformation, we decided to synthesize conformationally restricted tricyclic indan analogues with the oxygen atom in the 6-position incorporated into a furan, 1,3-dioxane, oxazole, pyran, morpholine, or 1,4-dioxane ring system. Among these compounds, indeno[5,4-b]furan analogues were found to be the most potent and selective MT(1) receptor ligands and to have superior metabolic stability. The optimization of substituents led to (S)-(-)-22b, which showed very strong affinity for human MT(1) (K(i) = 0.014 nM), but no significant affinity for hamster MT(3)() (K(i) = 2600 nM) or other neurotransmitter receptors. The pharmacological effects of (S)-(-)-22b were studied in experimental animals, and it was found that a dose of 0.1 mg/kg, po promoted a sleep in freely moving cats, as demonstrated by a decrease in wakefulness and increases in slow wave sleep and rapid eye movement sleep, which lasted for 6 h after administration. Melatonin (1 mg/kg, po) also had a sleep-promoting effect, though it lasted only 2 h. A new chiral method for the synthesis of (S)-(-)-22b starting from 60, which was prepared from 59 employing asymmetric hydrogenation with the (S)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl-Ru complex, was developed. (S)-(-)-22b (TAK-375) is currently under clinical trial for the treatment of insomnia and circadian rhythm disorders.

2[125I]Iodomelatonin binding and interaction with beta-adrenergic signaling in chick heart/coronary artery physiology

2[125I]Iodomelatonin ([125I]Mel) binding sites were characterized on membrane preparations of young chick hearts. [125I]Mel binding was rapid, saturable, stable, reversible, specific and of picomolar affinity and femtomolar density. Guanosine 5'-O-(3-thiotriphosphate) significantly lowered the binding affinity by one- to twofold, supporting G-protein linkage of melatonin receptors. Binding was detected as early as embryonic day-9 (E9), and increased steadily peaking at E13 before it slowly declined to about 15% of the peak level a week posthatch. Specific [125I]Mel binding was significantly increased by in ovo administration of inotropic agents dopamine and isoproterenol. Melatonin or 2-iodo-N-butanoyl-tryptamine inhibited isoproterenol-stimulated cAMP accumulation in primary heart cell cultures and the effect was attenuated after pretreatment with pertussis toxin (PTX). Localization of melatonin receptors using autoradiography showed intense labeling in the coronary arteries in all age groups whereas those in the myoblasts decreased as the heart matured. While the myoblasts and undifferentiated developing coronary arteries expressed melatonin MT1 receptor subtype in E11 hearts as detected by immunostaining with anti-MT1 receptor serum, immunoreactivities were observed mostly on the endothelium/subendothelium and smooth muscle cells of the well developed coronary vessels in posthatch hearts. Collectively, our data suggest the presence of PTX-sensitive, G protein-coupled melatonin receptors, whose expression is up-regulated by dopamine and isoproterenol, in the chick heart. Activation of these receptors, which include MT1 subtype, may modulate beta-adrenergic receptor-mediated cAMP signaling in the control of chick heart and coronary artery physiology.

Dual coupling of MT(1) and MT(2) melatonin receptors to cyclic AMP and phosphoinositide signal transduction cascades and their regulation following melatonin exposure

In this investigation, we wanted to determine whether MT(1) or MT(2) melatonin receptors are capable of coupling to the phosphoinositide (PI) signal transduction cascade. In addition, we wanted to assess the effects of chronic melatonin exposure on MT(1) and MT(2) melatonin receptor-mediated stimulation of PI hydrolysis. We also assessed the effects of chronic melatonin exposure on other parameters of the MT(2) melatonin receptor function including total specific 2-[125I]-iodomelatonin binding, the affinity of melatonin for the receptor, and melatonin (1nM)-mediated inhibition of cyclic 3',5'-adenosine monophosphate (cAMP) accumulation. Investigation of the PI signal transduction cascade activated by either the MT(1) or MT(2) melatonin receptor expressed in Chinese hamster ovary (CHO) cells showed that melatonin (1pM to 1mM) was able to stimulate the formation of PIs to approximately 40-60% over basal [EC(50): MT(1)=29nM (2-300nM) and MT(2)=1.1nM (0.32-3.5nM), N=5]. This response was mediated via receptors based upon the findings that melatonin did not stimulate the formation of PIs in CHO cells devoid of receptor and that antagonism of MT(2) melatonin receptors by 4P-PDOT (AH 024; 4-phenyl-2-propionamidotetralin) attenuated melatonin-mediated stimulation of PI hydrolysis in CHO cells expressing the MT(2) melatonin receptor. The consequence of chronic melatonin exposure on MT(1) and MT(2) receptor function was also examined. Pretreatment of either MT(1)- or MT(2)-CHO cells with melatonin (1 microM for 5hr) resulted in: (a) a complete loss of melatonin-mediated stimulation of PI hydrolysis, and (b) an attenuation of melatonin (1nM)-mediated inhibition of forskolin-induced cAMP accumulation by approximately 20-40%. The desensitization of the PI hydrolysis signal transduction cascades coupled to either MT(1) or MT(2) melatonin receptors following chronic melatonin exposure was not due to depleted phospholipid pools, to elevated basal levels, or to decreases in receptor affinity and density. This dual coupling of melatonin receptors to different signal transduction cascades may contribute to the diversity of melatonin receptor function in vivo.

mt(1) Receptor-mediated antiproliferative effects of melatonin on the rat uterine antimesometrial stromal cells

It has been shown that melatonin regulates uterine function. Our previous studies have demonstrated the presence of melatonin receptors in the rat uterine endometrium, indicating that melatonin may act directly on the uterus. In the present study, the histological localization of the rat uterine melatonin binding was revealed by autoradiography and the molecular subtyping was studied by in situ hybridization in the stromal cells. The signal transduction process and effects of melatonin on stromal cell proliferation was also investigated. Our autoradiograms showed that 2[(125)I]iodomelatonin binding sites were localized in the antimesometrial endometrial stroma. In situ hybridization with specific mt(1) receptor cDNA probe in the primary culture of antimesometrial stromal cells demonstrated the expression of mt(1) receptor mRNAs. Melatonin dose-dependently inhibited forskolin-stimulated cAMP accumulation, which was reversed by pertussis toxin. This indicates that the rat uterine melatonin receptors are negatively coupled to adenylate cyclase via pertussis toxin sensitive G(i) protein. Melatonin also inhibited the incorporation of [(3)H]thymidine in the rat uterine antimesometrial stromal cells, showing that melatonin has an anti-proliferative effect on the uterus. Our results suggest that melatonin may act directly on the mt(1) melatonin receptors in the rat uterine antimesometrial stromal cells to inhibit their proliferation. Its action may be mediated through a pertussis toxin-sensitive adenylate cyclase coupled G(i)-protein.

Melatonin inhibits estrogen receptor transactivation and cAMP levels in breast cancer cells

We have previously demonstrated that the pineal hormone, melatonin, can inhibit the growth of estrogen receptor-alpha (ERalpha)-positive breast cancer cells and suppress ERalpha gene transcription. To investigate the relationship between the estrogen response pathway and melatonin's growth inhibition, ERalpha-positive MCF-7 human breast cancer cells were transiently transfected with an estrogen response element (ERE) luciferase reporter construct and then treated with melatonin (10(-9)-10(-6) M) for 30 min followed by 10(-9) M 17-beta-estradiol (E2) or treated with each compound alone. Melatonin pre-treatment significantly reduced E2-induced ERalpha transactivation and ERalpha-ERE binding activity. We also conducted experiments to determine if melatonin modulates cAMP levels in MCF-7 cells. Melatonin inhibited the forskolin-induced and E2-induced elevation of cAMP levels by 57 and 45%, respectively. These data indicate that melatonin can act as a biological modifier to affect ERalpha transcriptional activity by regulating signal transduction pathways which impinge on the ERalpha and by altering E2-mediated ERalpha transactivation and ERalpha DNA binding activity.

Dual effect of melatonin on gonadotropin-releasing-hormone-induced Ca(2+) signaling in neonatal rat gonadotropes

In neonatal rat gonadotropes, melatonin inhibits gonadotropin-releasing-hormone (GnRH)-stimulated increase in intracellular Ca(2+) concentration ([Ca(2+)](i)); in cells transfected with the Mel1a melatonin receptor, however, melatonin has been shown to potentiate agonist-stimulated [Ca(2+)](i) increase. To elucidate this discrepancy, we investigated the effects of melatonin in neonatal gonadotropes over a wide range of melatonin concentrations. Nystatin perforated patch recording of Ca(2+)-dependent potassium currents was used to monitor GnRH-induced [Ca(2+)](i) changes. In 32% of cells, increasing melatonin concentrations in the range of 1 pM to 100 nM prolonged the latency of, and inhibited GnRH (10 nM)-stimulated [Ca(2+)](i) increases in a concentration-dependent manner. In the remaining 68% of cells, the Ca(2+) increase elicited by exposure to 10 nM GnRH was also inhibited by picomolar concentrations of melatonin, but at nanomolar concentrations the inhibitory effect disappeared and melatonin was only able to prolong the latency of the response. This dual effect of melatonin however was not observed in cells stimulated with lower (2 nM) GnRH concentrations; in that case, melatonin was inhibitory at all concentrations tested with an IC(50) of about 30 pM. In contrast, application of nanomolar concentrations of melatonin resulted in potentiation of the GnRH-induced Ca(2+) increase in a small population of gonadotropes which did not respond by inhibition or prolonged latency. These results indicate that in neonatal gonadotropes, melatonin has both inhibitory and potentiating effects on GnRH-stimulated [Ca(2+)](i) increases. Ranges of concentrations needed to produce either effect suggest that two distinct G proteins may be involved, as already observed in transfected cells.

The influence of 1.2 microT, 60 Hz magnetic fields on melatonin- and tamoxifen-induced inhibition of MCF-7 cell growth

We independently examined the findings of Harland and Liburdy, who reported that 1.2 microT(rms), 60 Hz magnetic fields could significantly reduce the inhibitory action of physiological levels of melatonin (10(-9) M) and of pharmacological levels of tamoxifen (10(-7) M) on the growth of MCF-7 human breast cancer cells in vitro. We used two testing protocols. In the melatonin study, the cell numbers per dish on day 7 of treatment were determined using a hemocytometer assay. In the tamoxifen study we used an expanded protocol, employing an alternative cell counting assay to characterize the cell numbers per dish on days 4, 5, 6, and 7. In both the melatonin and tamoxifen studies, cells were plated on 35 mm dishes and placed in each of two exposure chambers inside 5% CO(2) incubators. One exposure chamber was energized to produce 1.2 microT(rms), 60 Hz magnetic fields and the other chamber was not energized. Treatment was continuous until assays were performed. Cells were harvested at selected times, and enumerated without knowledge of treatment. In the melatonin study, the experiment was repeated three times, whereas in the tamoxifen study, each experiment was repeated nine times. In the melatonin study, cell numbers per dish were significantly reduced (by 16.7%) in the melatonin treated cultures after 7 days of incubation compared to control cultures, whereas in the presence of 1.2 microT(rms), 60 Hz magnetic fields, the melatonin treated cultures had the same cell populations as the control cultures. In the tamoxifen study, tamoxifen reduced the cell growth by 18.6 and 25% on days 6 and 7, respectively, in the chamber not energized, while in 1.2 microT(rms), 60 Hz fields, tamoxifen reduced the cell growth only by 8.7 and 13.1%, respectively. These results are consistent with those reported by Harland and Liburdy. A critical element of this successful replication effort was the constructive communication established and maintained with the original investigators. Bioelectromagnetics 22:122-128, 2001. Published 2001 Wiley-Liss, Inc.

Melatonin potentiates contractile responses to serotonin in isolated porcine coronary arteries

The present study was designed to determine the effects of melatonin on coronary vasomotor tone. Porcine coronary arteries were suspended in organ chambers for isometric tension recording. Melatonin (10(-10)-10(-5) M) itself caused neither contraction nor relaxation of the tissues. Serotonin (10(-9)-10(-5) M) caused concentration-dependent contractions of coronary arteries, and in the presence of melatonin (10(-7) M) the maximal response to serotonin was increased in rings with but not without endothelium. In contrast, melatonin had no effect on contractions produced by the thromboxane A(2) analog U-46619 (10(-10)-10(-7) M). The melatonin-receptor antagonist S-20928 (10(-6) M) abolished the potentiating effect of melatonin on serotonin-induced contractions in endothelium-intact coronary arteries, as did treatment with 1H-[1, 2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10(-5) M), methylene blue (10(-5) M), or N(G)-nitro-L-arginine (3 x 10(-5) M). In tissues contracted with U-46619, serotonin caused endothelium-dependent relaxations that were inhibited by melatonin (10(-7) M). Melatonin also inhibited coronary artery relaxation induced by sodium nitroprusside (10(-9)-10(-5) M) but not by isoproterenol (10(-9)-10(-5) M). These results support the hypothesis that melatonin, by inhibiting the action of nitric oxide on coronary vascular smooth muscle, selectively potentiates the vasoconstrictor response to serotonin in coronary arteries with endothelium.

Melatonin receptors in the pituitary of a teleost fish: mRNA expression, 2-[(125)I]iodomelatonin binding and cyclic AMP response

Melatonin has for a long time been involved in the photoperiodic control of fish physiology (growth, reproduction) and behavior (locomotor activity); but its mechanisms of action are not understood. We show here that 2-[(125)I]iodomelatonin binds specifically to membrane preparations from Pike (Esox lucius, L.) pituitaries (K(D): 556 pM; B(max): 2.8 fmol/mg proteins). Radioautography indicated that the binding was restricted to a part of the pituitary only. Using polymerase chain reaction from pike genomic DNA, we subcloned two partial cDNAs encoding the P1.4 (Mel(1a)) and the P2.6 (Mel(1b)) melatonin receptor subtypes. The two corresponding transcripts were expressed in the pituitary as revealed by RT-PCR assay and Southern blot hybridization. In culture, melatonin inhibited in a time- and dose-dependent manner cyclic AMP levels in pituitaries cultured in the presence of forskolin, an adenylyl cyclase activator. This is the first evidence for the expression of melatonin receptors and binding sites, and for the modulation of a second messenger by melatonin in the pituitary of a nonmammalian species. We suggest that in fish, the melatonin-mediated photoperiodic control of neuroendocrine functions involves, at least, a direct effect on the pituitary.

Mechanisms of melatonin action in the pituitary and SCN

We have compared melatonin effects in two different cell types in order to determine general intracellular mechanisms of its action. In neonatal rat pituitary, melatonin acts via the specific membrane receptors to inhibit GnRH-induced LH release. The melatonin effect disappears in adulthood due to the disappearance of the receptors. The mechanism of the melatonin action involves inhibition of the GnRH induced increase of intracellular calcium ([Ca2+])i. Our observations indicate that melatonin has dual inhibitory effect on GnRH-induced [Ca2+]i: it inhibits mobilisation of Ca2+ from endoplasmic reticulum as well as Ca2+ influx through voltage sensitive channels. Besides, melatonin also decreases basal and GnRH- or forskolin-induced increase of cAMP concentration in the pituitary. Although cAMP is not of primary importance for regulation of LH release, the cAMP decrease may participate in the mechanism of inhibitory melatonin action on LH release. Rat suprachiasmatic nuclei (SCN) have a high density of the melatonin receptors throughout the postnatal life. Cultures of dispersed SCN cells show circadian rhythm of vasopressin (AVP) release, with several fold increase in the middle of the day and decrease during night. Melatonin inhibits the spontaneous AVP release. Melatonin also inhibits the AVP release induced by vasoactive intestinal peptide (VIP). Intracellular mechanisms of the melatonin effect may involve cAMP, because melatonin inhibits the VIP-induced increase of cAMP and increase of cAMP formation by forskolin stimulates AVP release from the cultures. On the other hand, involvement of intracellular calcium in the regulation of AVP release may not be excluded. VIP induces [Ca2+]i increase in 14% of the SCN cells and AVP release is stimulated by Ca2+ ionophore ionomycin. Our observations indicate that some of the mechanisms of melatonin action are similar in the pituitary and SCN.

Melatonin inhibits pituitary adenylyl cyclase-activating polypeptide-induced increase of cyclic AMP accumulation and [Ca2+]i in cultured cells of neonatal rat pituitary

The effects of melatonin on pituitary adenylyl cyclase-activating polypeptide-induced increase of cyclic AMP and [Ca2+]i were studied in neonatal rat pituitary cells. The polypeptide increased cyclic AMP accumulation. In the presence of melatonin the increase of cyclic AMP was inhibited in a dose-dependent manner, the maximal inhibition was achieved with 1-10 nM melatonin. Pituitary adenylyl cyclase-activating polypeptide also increased [Ca2+]i in 30% of the pituitary cells and melatonin inhibited the effect. Most of the cells sensitive to adenylyl cyclase-activating polypeptide (77%) were also sensitive to GnRH, suggesting they are gonadotrophs. The remaining cells were not identified. The polypeptide-induced [Ca2+]i increase was inhibited in Ca2+-free medium in 2/3 of the cells indicating that Ca2+ influx was involved. To examine causal relationship between cyclic AMP and [Ca2+]i increase, we have studied the effect of adenylyl cyclase activation by forskolin on intracellular Ca2+ concentration. Forskolin had similar effects as adenylyl cyclase-activating polypeptide: it increased [Ca2+]i in the pituitary cells and the increase was dependent on presence of Ca2+ in the medium. Melatonin inhibited the forskolin induced [Ca2+]i increase. Our observations indicate that increase of cyclic AMP stimulates Ca2+ influx in the pituitary cells of neonatal rat and that this mechanism is involved in [Ca2+]i increase induced by the pituitary adenylyl cyclase-activating polypeptide. Because melatonin inhibits increase of cyclic AMP induced by pituitary adenylyl cyclase-activating polypeptide or forskolin, the inhibitory effect of melatonin on Ca2+-influx may be mediated by the decrease of cyclic AMP concentration. This mechanism of melatonin action has not been described previously. Because melatonin inhibits the polypeptide- or forskolin-induced [Ca2+]i also in the cells not sensitive to GnRH, melatonin receptors seem to be present on both gonadotrophs and non-gonadotrophic pituitary cells.

Differential signaling of human Mel1a and Mel1b melatonin receptors through the cyclic guanosine 3'-5'-monophosphate pathway

Cyclic guanosine 3'-5'-monophosphate (cGMP) has recently been shown to constitute a second messenger for Xenopus laevis melatonin Mel1c receptors. To verify whether cGMP levels are also modulated by mammalian melatonin receptors, we cloned the genes encoding the human Mel1a and Mel1b receptor subtypes and expressed them in human embryonic kidney cells. Pharmacological profiles and inhibition of forskolin-stimulated adenosine 3'-5'-cyclic monophosphate levels by melatonin confirmed functional expression of high-affinity melatonin receptors. Mel1b receptor-transfected cells modulated cGMP levels in a dose-dependent manner via the soluble guanylyl cyclase pathway. In contrast, Mel1a receptors had no effect on cGMP levels. These results demonstrate that mammalian melatonin receptors modulate cGMP levels and reveal for the first time differences in signaling between melatonin receptor subtypes, which may explain the necessity to express different receptor subtypes.

Melatonin receptors in PC3 human prostate tumor cells

Melatonin, secreted nocturnally by the pineal gland, can bind to human benign prostate epithelial cells and attenuate their growth and viability. In the present study, melatonin binding and responses were explored in the human steroid-independent PC3 prostatic tumor cells. PC3 cells bound 125I-melatonin with low affinity (Kd ca. 0.9 nM) at high as well as low cell density. Melatonin enhanced cGMP and 3H-thymidine incorporation at low, but attenuated them at high cell density. In addition, melatonin inhibited cAMP at low, but augmented it at high cell density. These effects were associated with an increase in cell count at low- but not high-density cultures. Pertussis toxin treatment suppressed 125I-melatonin binding and ablated all the effects of melatonin on 3H-thymidine incorporation, cAMP, and cGMP at both cell densities. Cholera toxin treatment failed to block the effects of melatonin on 3H-thymidine incorporation, but prevented the modulation by melatonin of cAMP at low and cGMP at high cell density. The cGMP analog 8-Br-cGMP, inhibited melatonin's effects on 3H-thymidine incorporation at both cell densities. H89, a protein kinase A inhibitor, prevented melatonin's effects on 3H-thymidine incorporation at low but not high cell density. These results provide the first demonstration of direct interaction of melatonin with hormone-insensitive prostate tumor cells. The melatonin receptors in the PC3 cells are coupled to pertussis toxin-sensitive G proteins to induce cell density-dependent changes in cGMP, cAMP, and cell growth.

Melatonin inhibits in vitro serotonergic phase shifts of the suprachiasmatic circadian clock

The suprachiasmatic (SCN) circadian pacemaker generates 24 h rhythms of spontaneous neuronal activity when isolated in an acute brain slice preparation. The isolated pacemaker also retains its capacity to be reset, or phase-shifted by exogenous stimuli. For example, serotonin (5-HT) agonists advance the SCN pacemaker when applied during mid subjective day, while neuropeptide Y (NPY) agonists and melatonin advance the pacemaker when applied during late subjective day. Previous work has demonstrated interactions between NPY and 5-HT agonists, such that NPY can block 5-HTergic phase advances, while 5-HT agonists do not prevent NPY-induced advances. Due to a number of similarities in the actions of melatonin and NPY in the SCN, it seemed possible that melatonin and 5-HT might interact in the SCN as well. Therefore, in this study potential interactions between melatonin and 5-HT agonists were explored. Melatonin inhibited phase advances by the 5-HT agonist, (+)DPAT, and this inhibition was decreased by co-application of tetrodotoxin. Conversely, melatonin was unable to block phase advances by the cyclic AMP analog, 8BA-cAMP. Finally, neither 5-HT agonists nor 8BA-AMP were able to block melatonin-induced phase advances. These results demonstrate a clear interaction between melatonin and 5-HT in the SCN, and suggest that melatonin and NPY may play similar roles with respect to modulating the phase of the SCN circadian pacemaker in rats.

Cell and molecular biology of the pars tuberalis of the pituitary

The pars tuberalis of the adenohypophysis is mainly composed of a special type of endocrine cells, pars tuberalis-specific cells, lining the primary capillary plexus of the hypophysial portal system. Dense expression of melatonin receptors and marked changes in morphological appearance, production pattern, and secretory activity during annual cycle show that these cells are highly sensitive to changes in photoperiod. This leads to the hypothesis that the pars tuberalis is involved in the transmission of photoperiodic stimuli to endocrine targets. Several investigations support the theory that pars tuberalis-specific cells are multipotential cells exerting a modulatory influence on the secretory activity of the pars distalis. Specifically, there is accumulating evidence that seasonal modulation of prolactin secretion, independent of hypothalamic input, is due to melatonin-regulated activity of pars tuberalis-specific cells. The exact nature of secretory products and their effects within neuroendocrine regulation, however, remain rather enigmatic. Accordingly, molecular mechanisms regulating gene expression under the influence of photoperiod, respectively, circulating melatonin levels are still incomplete. Recent cloning of melatonin receptor genes and new data on intracellular signal transduction will probably lead to new insights on melatonin action and pars tuberalis-specific cell physiology.

Melatonin inhibits the increase of cyclic AMP in rat suprachiasmatic neurons induced by vasoactive intestinal peptide

The effects of melatonin on basal and vasoactive intestinal peptide (VIP)-induced cAMP concentration was studied in dispersed cells of the rat suprachiasmatic nuclei (SCN). Our data indicate, that VIP induces a rapid increase of cAMP concentration in the cells followed by a slow and prolonged increase in the medium. The VIP-induced increase was dose-dependent in the range of 1-100 nM. Melatonin had no effect on basal cAMP but inhibited the cAMP increase induced by VIP in a dose-dependent manner (EC50 = 0.21 nM). Our observations indicate that melatonin acts through the inhibition of cAMP in the SCN cells similar as shown in other tissues.

Cellular mechanisms of melatonin action

The pineal hormone melatonin is involved in photic regulations of various kinds, including adaptation to light intensity, daily changes of light and darkness, and seasonal changes of photoperiod lengths. The melatonin effects are mediated by the specific high-affinity receptors localized on plasma membrane and coupled to GTP-binding protein. Two different G proteins coupled to the melatonin receptors have been described, one sensitive to pertussis toxin and the other sensitive to cholera toxin. On the basis of the molecular structure, three subtypes of the melatonin receptors have been described: Mel1A, Mel1B, and Mel1C. The first two subtypes are found in mammals and may be distinguished pharmacologically using selective antagonists. Melatonin receptor regulates several second messengers: cAMP, cGMP, diacylglycerol, inositol trisphosphate, arachidonic acid, and intracellular Ca2+ concentration ([Ca2+]i). In many cases, its effect is inhibitory and requires previous activation of the cell by a stimulatory agent. Melatonin inhibits cAMP accumulation in most of the cells examined, but the indole effects on other messengers have been often observed only in one type of the cells or tissue, until now. Melatonin also regulates the transcription factors, namely, phosphorylation of cAMP-responsive element binding protein and expression of c-Fos. Molecular mechanisms of the melatonin effects are not clear but may involve at least two parallel transduction pathways, one inhibiting adenylyl cyclase and the other regulating phospholipide metabolism and [Ca2+]i.

Melatonin receptors in benign prostate epithelial cells: evidence for the involvement of cholera and pertussis toxins-sensitive G proteins in their signal transduction pathways

BACKGROUND

Melatonin, the hormone secreted nocturnally by the pineal gland, binds to epithelial cells from the human benign prostate, and can reduce their growth and viability. The possible involvement of GTP binding proteins cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in melatonin responses in these cells were investigated.

METHODS

The effects of melatonin on cAMP and cGMP were assessed in prostate cells untreated or pretreated with pertussis toxin (PTX) or cholera toxin (CTX).

RESULTS

Melatonin augmented cAMP but reduced cGMP in the epithelial cells (maximal responses at 10 nM). The increase in cAMP was attenuated by PTX, but not by CTX, whereas the decrease in cGMP was attenuated by CTX, but not by PTX. CTX, but not PTX, abolished the melatonin-mediated suppression of 3H-thymidine incorporation. In addition, melatonin facilitated the CTX- and PTX-mediated ADP ribosylation of 44- and 41-kilodalton proteins, respectively. The cGMP analogue 8-bromo-cGMP, negated the melatonin-mediated decrease in 3H-thymidine incorporation, whereas H89, a protein kinase A inhibitor, did not inhibit melatonin's effect.

CONCLUSIONS

Melatonin receptors in the human benign prostate epithelial cells enhance cAMP and inhibit cGMP through PTX- and CTX-sensitive G proteins, respectively. The decrease in DNA synthesis may be secondary to the melatonin-mediated decrease in cGMP.

Studies of melatonin effects on epithelia using the human embryonic kidney-293 (HEK-293) cell line

The expression of melatonin receptors (MR) of the Mel1a subtype in basolateral membrane of guinea pig kidney proximal tubule suggests that melatonin plays a role in regulating epithelial functions. To investigate the cellular basis of melatonin action on epithelia, we sought to establish an appropriate in vitro culture model. Epithelial cell lines originating from kidneys of dog (MDCK), pig (LLC-PK1), opossum (OK), and human embryo (HEK-293) were each tested for the presence of MR using 2-[125I]iodomelatonin (125I-MEL) as a radioligand. The HEK-293 cell line exhibited the highest specific 125I-MEL binding. By intermediate filament characterization, the HEK-293 cells were determined to be of epithelial origin. Binding of 125I-MEL in HEK-293 cells demonstrated saturability, reversibility, and high specificity with an equilibrium dissociation constant (Kd) value of 23.8 +/- 0.5 pM and a maximum number of binding sites (Bmax) value of 1.17 +/- 0.11 fmol/mg protein (n = 5), which are comparable with the reported Kd and Bmax values in human kidney cortex. Coincubation with GTPgammaS (10 microM) and pertussis toxin (100 ng/ml) provoked a marked decrease in binding affinity (Kd was increased by a factor of 1.5-2.0), with no significant difference in Bmax. Melatonin (1 microM) decreased the forskolin (10 microM) stimulated cAMP level by 50%. HEK-293 cells do not express dopamine D1A receptor. Following transient transfection of HEK-293 cells with human dopamine D1A receptor (hD1A-R), exposure of the cells to dopamine stimulated an increase in the level of cAMP. Similarly, transient transfection of HEK-293 cells with rat glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and PTH type 1 receptors, each resulted in an hormone inducible increase in cAMP levels. Surprisingly, only the stimulatory effect of dopamine could be inhibited by exposure to melatonin. The inhibitory effect of melatonin on dopamine D1-induced increase in cAMP was completely inhibited by pertussis toxin (100 ng/ml, 18 h). Immunoblot and immunocytochemical studies were carried out using two polyclonal antibodies raised against the extra and cytoplasmic domains of Mel1a receptor. Immunoblot studies using antibody against the cytoplasmic domain of Mel1a receptor confirmed the presence of a peptide blockable 37 kDa band in HEK-293 cells. Indirect immunofluorescent studies with both antibodies revealed staining predominantly at the cell surface, but staining with the antibody directed against the cytoplasmic domain required prior cell permeabilization. By RT-PCR, HEK-293 cells express both Mel1a and Mel1b messenger RNAs, but the messenger RNA level for Mel1b is several orders of magnitude lower than for Mel1a. We conclude that HEK-293 cells express MR predominantly of the Mel1a subtype. Our evidence suggests that one of the ways that melatonin exerts its biological function is through modulation of cellular dopaminergic responses.

Novel isoforms of Mel1c melatonin receptors modulating intracellular cyclic guanosine 3',5'-monophosphate levels

Two cDNAs encoding novel isoforms of Xenopus laevis melatonin receptors were cloned using PCR primers specific for the X. laevis-melanophore Mel1c melatonin receptor described in a recent publication. The novel isoforms were highly homologous to the described frog Mel1c cDNA, although the C-terminal tail of both was shorter by 65 amino acid residues. Nucleotide sequences of these novel isoforms, called Mel1c(alpha) and Mel1c(beta), differed from each other by only 35 nucleotides and six amino acid residues. Studies on several animals of various Xenopus species indicate that Mel1c(alpha) and Mel1c(beta) receptors may correspond to allelic variants of the same locus. Studies on cells transfected with both receptor cDNAs showed the expression of high-affinity 2-[125I]iodomelatonin binding sites. Agonist stimulation of Mel1c(alpha) receptor was associated with the inhibition of cAMP accumulation stimulated by forskolin (IC50 approximately 10(-10) M) in HeLa, Ltk-, and human embryonic kidney 293 (HEK 293) cells. Mel1c(beta) receptor modulated cAMP in HeLa and HEK 293 cells but not in Ltk- cells. Both receptors inhibited, in a dose-dependent manner, cGMP accumulation in all three cell lines incubated with a phosphodiesterase inhibitor. This effect was localized upstream of soluble guanylyl cyclase and was blocked by pertussis toxin treatment. However, IC50 values (approximately 10(-10) M for Mel1c(beta) and 10(-9) to 10(-7) M for Mel1c(alpha)) and maximal inhibition levels showed that Mel1c(alpha) receptors are much less efficiently coupled to the cGMP pathway. Coupling differences may be explained by the fact that five of the six amino acid substitutions between Mel1c(alpha) and Mel1c(beta) receptors are located within cytoplasmic regions potentially involved in signal transduction. The existence of coupling differences is in agreement with the observation that expression of both receptors is evolutionally conserved in native tissue. In conclusion, two novel, potentially allelic, isoforms of Xenopus Mel1c melatonin receptors display identical ligand-binding characteristics, but different potencies in modulating cAMP and cGMP levels through G(i)/G(o)-dependent pathways. Furthermore, to our knowledge, this study provides the first data on the modulation of intracellular cGMP levels by cloned melatonin receptors.

Melatonin agonists induce phosphoinositide hydrolysis in Xenopus laevis melanophores

Melatonin, the principal hormone of the vertebrate pineal gland, has been implicated in a variety of neurobiological processes such as circadian rhythmicity and reproductive function. One of the earliest described actions of melatonin was its ability to cause pigment translocation in the dermal melanophores of amphibians. Melatonin binding sites have been identified in the brain of many species and in pigmented tumour cell lines; however, the dermal melanophores of the frog Xenopus Laevis possess the highest known density of melatonin binding sites. These cells are the source from which a melatonin receptor has been cloned and provide an excellent model to study melatonin-mediated signal transduction in an isolated cell system. In Xenopus melanophores, melatonin induces a rapid perinuclear aggregation of intracellular pigment which is associated with a pertussis toxin-sensitive inhibition of cAMP. We have previously demonstrated that a subtype of melatonin binding sites found in selected regions of the pigeon brain and in Syrian Hamster RPMI 1846 melatonin cells are functionally coupled to phosphoinositide hydrolysis as a second messenger. Here we now present evidence to suggest that Xenopus Laevis melanophores also possess melatonin binding sites which are functionally linked to phosphoinositide hydrolysis. Melatonin agonists induced phosphoinositide hydrolysis in melanophores in a concentration-dependent manner with a rank order of potency of 2-iodomelatonin > 6-chloromelatonin > N-acetylserotonin > melatonin. Stimulatory response of 2-iodomelatonin was blocked by the melatonin antagonist N-acetyltryptamine and the alpha-adrenergic antagonist prazosin, which has been shown to have high affinity for melatonin binding sites. Phosphoinositide hydrolysis induced by melatonin agonists was not blocked by the serotonin antagonist ketanserin or by phentolamine, an alpha-adrenergic antagonist, indicating that the response observed was not due to stimulation of 5-HT2a/2c receptors or alpha-adrenergic receptors. Furthermore, incubation of melanophores with the non-hydrolyzable G-protein source GTP-gamma-S attenuated the phosphoinositide dose response induced by 2-iodomelatonin, and pre-incubation of the cells with pertussis toxin had no effect on 2-iodomelatonin-induced phosphoinositide hydrolysis. The present data suggest that Xenopus Laevis Melanophores possess G-protein linked pertussis toxin-insensitive melatonin binding sites which are functionally coupled to phosphoinositide hydrolysis as a signal transduction mechanism.

The high affinity melatonin binding site probed with conformationally restricted ligands--II. Homology modeling of the receptor

We present the first 3-D model of the melatonin receptor based on the recently published amino acid sequence of the cloned melatonin receptor. The seven trans membrane helices were positioned using the helices found in the structure of Bacterio Rhodopsine. From the results of an indirect modeling study with six melatonergic agents, an alignment of these compounds was found directing towards common interaction points. These points are suggested to be the two serines in helix three and the histidine in helix five, forming hydrogen bonds with the amide function and the methoxy-oxygen in melatonin, respectively. The ligands were docked into these binding sites and the receptor-ligand complexes were energy minimized. Considering the position of the active and inactive ligands in the receptor and their respective occupied volumes, the structure-activity relationships are rationalized by the suggested model. This model can be of use as a pharmacological test model in molecular biological studies and as a basis to develop compounds being active as synchronizing circadian agents.

Comparative aspects of the pineal/melatonin system of poikilothermic vertebrates

The pineal gland of poikilothermic vertebrates originates as an evagination from the diencephalic roof between the habenular and the posterior commissures, and associates with a parapineal organ to form the so-called pineal complex. The pinealocytes may be photosensitive, secretory or intermediate cells between both. Melatonin, the indoleamine secreted by the pineal, exhibits a circadian secretory rhythm that conveys environmental information to the organism. The peak melatonin secretion occurs during the night, although there are a few examples of an increase in indoleamine secretion during the day. Melatonin is also synthesized in other sites such as the retina, and it has been found in many invertebrates and unicellular organisms. The rhythmic secretory pattern of melatonin is responsible for many biological rhythms exhibited by lower vertebrates. These rhythms are abolished by pinealectomy in some species, but not in others, suggesting the existence of an extra-pineal pacemaker. The photoperiod and the temperature (especially in reptiles) are the main environmental factors affecting the secretory rhythm of melatonin. Poikilothermic vertebrates exhibit a circadian rhythmic color change, with nocturnal blanching, usually related to melatonin secretion. In amphibians, melatonin exhibits a potent skin lightening activity. However, in fishes and reptiles the melatonin effects vary with the species, the developmental stage, and the pigment cell location. Melatonin also exerts inhibitory or excitatory activity on the amphibian reproductive system, regulation of circadian locomotory activity in reptiles, and modulation of the amphibian metamorphosis. Melatonin has also a modulatory effect on the response of target cells to different hormones and high concentrations or prolonged exposure to the indoleamine may cause autodesensitization in various tissues. Binding sites of melatonin have been detected in the central nervous system and peripheral tissues of various vertebrates. The relative potencies of melatonin analogues demonstrated two subtypes of melatonin receptors (ML-1 and ML-2). A transmembrane melatonin receptor has been cloned from Xenopus laevis melanophores; it belongs to the family of the G protein-coupled receptors and exhibits 85% homology with the mammalian nervous system receptor. Melatonin binding sites in the nucleus of many cell types and its potent intracellular anti-oxidant action suggest mechanisms of action other than through the G-protein coupled receptor.

Melatonin receptors couple through a cholera toxin-sensitive mechanism to inhibit cyclic AMP in the ovine pituitary

The nature of melatonin receptor-G-protein coupling in ovine pars tuberalis (PT) cells of the pituitary was addressed using cholera (CTX) and pertussis (PTX) toxins. ADP-ribosylation of ovine PT membrane proteins using 32P-NAD in the presence of CTX radiolabelled several substrates including 44, 51, and 60 kD proteins. Each were clearly distinct from the 40 kD substrate radiolabelled in the presence of PTX. Acute incubation of PT membranes with either toxin reduced the number of high affinity binding sites for 125I-MEL, although the magnitude of the inhibition was much greater for CTX (56%) than for PTX (20%). A CTX-sensitive component also mediates the inhibition of forskolin-stimulated cyclic AMP accumulation as pre-treatment of PT cells with CTX (5 micrograms/ml) for 16 h blocked this response. Gs alpha is a major substrate for ADP-ribosylation by CTX, and 16 h pre-treatment of PT cells with CTX (5 micrograms/ml) caused a down-regulation of Gs alpha. Northern analysis showed only one major transcript of Gs alpha of about 2 kb, which would encompass all of the known splice variants of the Gs gene. Screening of a cDNA library from ovine PT for Gs-related genes and sequencing of clones, combined with RT-PCR of PT mRNA, revealed no novel products. On this basis it is concluded that the CTX substrate is unlikely to be a novel splice variant or related gene product of the Gs class of G-protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Melatonin inhibits increase of intracellular calcium and cyclic AMP in neonatal rat pituitary via independent pathways

In neonatal rat pituitary, melatonin inhibits GnRH-induced increase of cAMP and [Ca2+]i. Both effects are transduced by specific high-affinity melatonin receptors coupled with pertussis toxin-sensitive G-protein. We have attempted to determine whether melatonin acts via independent pathways on both messengers or whether the indole directly inhibits only one of the messengers and the second is decreased as a secondary consequence. Melatonin inhibition of cAMP accumulation was not prevented by agents known to block melatonin effect on [Ca2+]i such as Na(+)- or Ca2(+)-free medium, Bay K, nifedipine, KCl or gramicidin. Melatonin effect on [Ca2+]i was not prevented by forskolin or 8-bromo-cAMP. We therefore conclude that melatonin inhibits cAMP accumulation and [Ca2+]i increase independently by separate pathways.

Structural requirements at the melatonin receptor

1. High affinity, specific binding sites for the pineal hormone, melatonin (5-methoxy N-acetyltryptamine) can be detected in chick brain membranes by use of the radiolabelled agonist, 2-[125I]-iodomelatonin (2-[125I]-aMT). 2. The affinity of a number of analogues of melatonin at the 2-[125I]-aMT binding site was determined and compared with an analysis of their electronic structure and significant quantitative relationships obtained. 3. The best correlations indicated that binding affinity was correlated with delta E, the difference between the frontier orbital energies, and QNH, the electron density in the highest occupied molecular orbital of the side-chain nitrogen atom. 4. These findings suggest that ligand binding may involve hydrogen bonding between the 5-methoxy and amide moieties of melatonin and complementary amino acid residues, and charge transfer interactions between the indole ring of melatonin and an aromatic amino acid in the receptor binding site. 5. A molecular model of a putative binding site is proposed based on the predicted amino acid sequence of the cloned Xenopus laevis melanophore melatonin receptor and the quantitative structure-affinity relationships observed in the present study.

Melatonin inhibits luteinizing hormone releasing hormone (LHRH) induction of LH release from fetal rat pituitary cells

The in vitro effect of melatonin on the release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from fetal rat pituitary cells was investigated. A significant inhibition of LH release induced by 10(-9) M luteinizing hormone releasing hormone (LHRH) was seen when cells were incubated with 10(-9) M melatonin. FSH release was unaffected by either LHRH alone or LHRH in combination with melatonin. In addition, the significant inhibitory effect of melatonin was reduced by pretreatment of the pituitary cells with 10(-10) M melatonin. These findings indicate that melatonin can act directly on the fetal pituitary gland to suppress LHRH-induced release of LH perhaps by a mechanism which eventually involves down-regulation of the melatonin receptors.

Melatonin regulates the phosphorylation of CREB in ovine pars tuberalis

This study investigated whether melatonin could modulate the phosphorylation of the calcium/cyclic AMP response-element binding-protein (CREB) within primary cell cultures of ovine pars tuberalis (oPT) and pars distalis (oPD). Gel shift assays confirmed the presence of nuclear factors able to alter the electrophoretic mobility of a 32P-labelled CRE oligonucleotide. Two shifted bands were observed probably due to monomer and dimer binding to the CRE. Each band was supershifted by antisera directed against both CREB and the phosphorylated form of CREB (P-CREB), consistent with a specific role of CREB proteins in transcriptional regulation. To study the physiological role of CREB, the nuclear immunoreactivity for P-CREB was followed in primary cultures of oPT given different pharmacological treatments. Cells stimulated with forskolin responded with a robust time- and dose-dependent increase in nuclear phospho-CREB immunoreactivity (P-CREB-ir), confirming that activation of this transcription factor occurred through the cyclic AMP-PKA pathway. Maximal stimulation was achieved within 15 min and persisted for up to 1 h. Treatment with melatonin alone did not alter basal P-CREB-ir levels, yet melatonin inhibited the forskolin-induced increase in P-CREB-ir in a dose-dependent manner (IC50 of between 10(-10) M and 10(-8) M melatonin when tested against 1 microM forskolin). In contrast, in primary cultures of oPD, melatonin failed to block forskolin-stimulated increases in either the content of cyclic AMP or the intensity of nuclear P-CREB-ir, confirming that the action of melatonin upon P-CREB-ir is tissue specific. These results demonstrate that, consistent with its inhibitory effect on the activation of PKA within oPT, melatonin prevents or reverses the phosphorylation of CREB induced by activation of the cyclic AMP signal transduction pathway. Therefore melatonin has the potential to regulate gene expression in the oPT by acting upon the CREB transcription factor. However, this paper also shows that 12-O-tetradecanoylphorbol-13-acetate (TPA) which activates PKC also leads to the phosphorylation of CREB in oPT cells, suggesting the potential involvement of other signal transduction pathways in the transcriptional regulation of these cells.

Pharmacologic characterization of melatonin-mediated phosphoinositide hydrolysis in pigeon brain

High densities of [125I]-iodomelatonin binding sites have been demonstrated in pigeon brain. Melatonin binding sites have been shown to be linked to signal transduction mechanisms in other species. The present study investigated the melatonin-mediated second messenger response of phosphoinositide hydrolysis in slices of telencephalon, optic tectum, cerebellum, hypothalamus, and pons medulla of pigeon brain. The highest rates of melatonin-mediated phosphoinositide hydrolysis were observed in telencephalon and pons/medulla. Relative potencies of melatonin agonists to induce phosphoinositide hydrolysis were as follows: 2-iodomelatonin > 6-chloromelatonin > N-acetylserotonin > melatonin > > serotonin (5-HT). Agonist-induced phosphoinositide hydrolysis was blocked by N-acetyltryptamine (NAT), a melatonin antagonist, but not by ketanserin, a 5HT2A/2C receptor antagonist, demonstrating that phosphoinositide hydrolysis did not result from 5HT2A or 5HT2C receptor stimulation. In addition, the effects of melatonin agonists were sensitive to prazosin, an alpha-adrenergic antagonist reported to exhibit nanomolar affinity for melatonin binding sites in hamster brain, but not to phentolamine, an alpha-adrenergic antagonist that shows no affinity for melatonin binding sites. These data provide evidence that signal transduction associated with melatonin in pigeon brain involves the induction of phosphoinositide hydrolysis as a second messenger.

Differential effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) on the 2-[125I]iodomelatonin binding sites in the chicken bursa of Fabricius and spleen

Effects of 10 and 50 mumol/l guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) on 2-[125I]iodomelatonin binding in the chicken bursa of Fabricius and spleen were tested. In the chicken bursa of Fabricius, GTP gamma S increased the Kd of 2-[125I]iodomelatonin binding sites without affecting the Bmax value. In contrast, GTP gamma S caused both an increase in Kd and a reduction in Bmax of 2-[125I]iodomelatonin binding sites in the chicken spleen. Our results suggest the existence of subtypes of melatonin receptors with different receptor-G protein-effector complexes in the avian primary and secondary lymphoid systems.

Melatonin receptors: localization, molecular pharmacology and physiological significance

A pre-requisite to understanding the physiological mechanisms of action of melatonin is the identification of the target sites where the hormone acts. The radioligand 2-[125I]iodo-melatonin has been used extensively to localize binding sites in both the brain and peripheral tissues. In general these binding sites have been found to be high affinity, with Kd in the low picomolar range, and selective for structural analogues of melatonin. Also the affinity of these sites can generally be modulated by guanine nucleotides, consistent with the notion that they are putative G-protein coupled receptors. However, only a few studies have demonstrated that these putative receptors mediate biochemical and cellular responses. In the pars tuberalis (PT) and pars distalis (PD) of the pituitary, the amphibian melanophore and vertebrate retina, evidence indicates that melatonin acts to inhibit intracellular cyclic AMP through a G-protein coupled mechanism, demonstrating that this is a common signal transduction pathway for many melatonin receptors. However in the pars distalis the inhibition of calcium influx and membrane potential are also important mediators of melatonin effects. How many different forms or states of the melatonin receptor exist is unknown, but clearly the identification of the structure of the melatonin receptor(s) and its ability to interact with different G-proteins and signal transduction pathways are quintessential to our understanding of the physiological mechanisms of action of melatonin. In parallel the recent development of new melatonin analogues will greatly aid our understanding of the pharmacology of the melatonin receptor both in terms of the development of potent melatonin receptor antagonists and for the definition of receptor sub-types. The wide species and phylogenic diversity of melatonin binding sites in the brain has probably generated more questions than answers. Nevertheless the localization of melatonin receptors to the suprachiasmatic nucleus of the hypothalamus is at least consistent with circadian effects within the foetus and the adult. In contrast the PT of the pituitary presents an enigma in relation to the seasonal effects of melatonin. A model of how melatonin might mediate the timing of the circannual events through the PT is proposed.