Characterization of melatonin receptors and signal transduction system in rat arteries forming the circle of Willis

Empowering Melatonin Therapeutics with Drosophila Models

Melatonin functions as a central regulator of cell and organismal function as well as a neurohormone involved in several processes, e.g., the regulation of the circadian rhythm, sleep, aging, oxidative response, and more. As such, it holds immense pharmacological potential. Receptor-mediated melatonin function mainly occurs through MT1 and MT2, conserved amongst mammals. Other melatonin-binding proteins exist. Non-receptor-mediated activities involve regulating the mitochondrial function and antioxidant cascade, which are frequently affected by normal aging as well as disease. Several pathologies display diseased or dysfunctional mitochondria, suggesting melatonin may be used therapeutically. Drosophila models have extensively been employed to study disease pathogenesis and discover new drugs. Here, we review the multiple functions of melatonin through the lens of functional conservation and model organism research to empower potential melatonin therapeutics to treat neurodegenerative and renal diseases.

A Green and Blue Monochromatic Light Combination Therapy Reduces Oxidative Stress and Enhances B-Lymphocyte Proliferation through Promoting Melatonin Secretion

Artificial illumination may interfere with biological rhythms and distort physiological homeostasis in avian. Our previous study demonstrated that 660 nm red light exacerbates oxidative stress, but a combination of green and blue lights (G→B) can improve the antibody titer in chickens compared with single monochromatic light. Melatonin acts as an antioxidant which is a critical signaling to the coordination between external light stimulation and the cellular response from the body. This study further clarifies the potential role of melatonin in monochromatic light combination-induced bursa B-lymphocyte proliferation in chickens. A total of 192 chicks were exposed to a single monochromatic light (red (R), green (G), blue (B), or white (W) lights) or various monochromatic light combinations (B→G, G→B, and R→B) from P0 to P42. We used qRT-PCR, MTT, western blotting, immunohistochemistry, and Elisa to explore the effect of a combination of monochromatic light on bursa B-lymphocytes and its intracellular signal pathways. With consistency in the upregulation in melatonin level of plasma and antioxidant enzyme ability, we observed increases in organ index, follicle area, lymphocyte density, B-lymphocyte proliferation, PCNA-positive cells, and cyclin D1 expression in bursa of the G→B group compared with other light-treated groups. Melatonin bound to Mel1a and Mel1c and upregulated p-AKT, p-PKC, and p-ERK expression, thereby activating PI3K/AKT and PKC/ERK signaling and inducing B-lymphocyte proliferation. Overall, these findings suggested that melatonin modulates a combination of green and blue light-induced B-lymphocyte proliferation in chickens by reducing oxidative stress and activating the Mel1a/PI3K/AKT and Mel1c/PKC/ERK pathways.

Type 2 diabetes mellitus: Role of melatonin and oxidative stress

Type 2 diabetes mellitus caused by transfer of susceptible immortal gene from parent to progeny in individuals prone, and/or in contribution of factors such as obesity and physical inactivity results in chronic extracellular hyperglycemia due to insulin resistance or impaired glucose tolerance. Hyperglycemia leads to increased production of superoxide radical in mitochondrial electron transport chain, consequently, inhibit glyceraldehyde-3-phosphate dehydrogenase activity, increase the flux of substrates that direct the expression of genes responsible for activation of polyol, hexosamine, advanced glycation end products and protein kinase-C pathways enzymes. Simultaneously, these pathways add-up free radicals in the body, hamper cell redox state, alter genes of insulin sensitivity and are responsible for the diabetic complications like retinopathy, atherosclerosis, cardiovascular diseases, nephropathy and neuropathy. Experimental evidence suggests that the indoleamine hormone melatonin is capable of influencing in development of diabetic complications by neutralizing the unnecessary production of ROS, protection of beta cells, as they possess low antioxidant potential and normalize redox state in the cell. However, studies reported the beneficial effects of pharmacological supplementation of melatonin in humans but it has not been extensively studied in a multicountric, multicentric which should include all ethnic population.

Melatonin receptor type 1 signals to extracellular signal-regulated kinase 1 and 2 via Gi and Gs dually coupled pathways in HEK-293 cells

The pineal gland hormone melatonin exerts its regulatory roles in a variety of physiological and pathological responses through two G protein-coupled receptors, melatonin receptor type 1 (MT1) and melatonin receptor type 2 (MT2), which have been recognized as promising targets in the treatment of a number of human diseases and disorders. The MT1 receptor was identified nearly 20 years ago; however, the molecular mechanisms by which MT1-mediated signaling affects physiology remain to be further elucidated. In this study, using HEK293 cells stably expressing the human MT1 receptor, melatonin induced a concentration-dependent activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2). The melatonin-mediated phosphorylation of ERK1/2 at later time points (≥5 min) was strongly suppressed by pretreatment with pertussis toxin, but only a slight, if any, inhibition of ERK1/2 activation at early time points (≤2 min) was detected. Further experiments demonstrated that the Gβγ subunit, phosphoinositide 3-kinase, and calcium-insensitive protein kinase C were involved in the MT1-mediated activation of ERK1/2 at later time points (≥5 min). Moreover, results derived from cAMP assays combined with a MT1 mutant indicated that the human MT1 receptor could also couple to Gs protein, stimulating intracellular cAMP formation, and that the MT1-induced activation of ERK1/2 at early time points (≤2 min) was mediated by the Gs/cAMP/PKA cascade. Our findings may provide new insights into the pharmacological effects and physiological functions modulated by the MT1-mediated activation of ERK1/2.

Uterine infusion of melatonin or melatonin receptor antagonist alters ovine feto-placental hemodynamics during midgestation

Dietary melatonin supplementation from mid- to late gestation increases umbilical artery blood flow and causes disproportionate fetal growth. Melatonin receptors have been described throughout the cardiovascular system; however, there is a paucity of data on the function of placental melatonin receptors. The objectives of the current experiment were to determine fetal descending aorta blood flow, umbilical artery blood flow, and placental and fetal development following a 4-wk uterine infusion of melatonin (MEL), melatonin receptor 1 and 2 antagonist (luzindole; LUZ), or vehicle (CON) from Day 62 to Day 90 of gestation. After 4 wk of infusion, umbilical artery blood flow and umbilical artery blood flow relative to placentome weight were increased (P < 0.05) in MEL- versus CON- and LUZ-infused dams. Fetal descending aorta blood flow was increased (P < 0.05) in MEL- versus CON- and LUZ-infused dams, while fetal descending aorta blood flow relative to fetal weight was increased in MEL- versus CON-infused dams and decreased in LUZ- versus CON-infused dams. Following the 4-wk infusion, we observed an increase in placental efficiency (fetal-placentome weight ratio) in MEL- versus LUZ-infused dams. The increase in umbilical artery blood flow due to chronic uterine melatonin infusion is potentiated by an increased fetal cardiac output through the descending aorta. Moreover, melatonin receptor antagonism decreased fetal descending aorta blood flow relative to fetal weight. Therefore, melatonin receptor activation may partially mediate the observed increase in fetal blood flow following dietary melatonin supplementation.

Melatonin membrane receptors in peripheral tissues: distribution and functions

Many of melatonin's actions are mediated through interaction with the G-protein coupled membrane bound melatonin receptors type 1 and type 2 (MT1 and MT2, respectively) or, indirectly with nuclear orphan receptors from the RORα/RZR family. Melatonin also binds to the quinone reductase II enzyme, previously defined the MT3 receptor. Melatonin receptors are widely distributed in the body; herein we summarize their expression and actions in non-neural tissues. Several controversies still exist regarding, for example, whether melatonin binds the RORα/RZR family. Studies of the peripheral distribution of melatonin receptors are important since they are attractive targets for immunomodulation, regulation of endocrine, reproductive and cardiovascular functions, modulation of skin pigmentation, hair growth, cancerogenesis, and aging. Melatonin receptor agonists and antagonists have an exciting future since they could define multiple mechanisms by which melatonin modulates the complexity of such a wide variety of physiological and pathological processes.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, classification, and pharmacology of G protein-coupled melatonin receptors

The hormone melatonin (5-methoxy-N-acetyltryptamine) is synthesized primarily in the pineal gland and retina, and in several peripheral tissues and organs. In the circulation, the concentration of melatonin follows a circadian rhythm, with high levels at night providing timing cues to target tissues endowed with melatonin receptors. Melatonin receptors receive and translate melatonin's message to influence daily and seasonal rhythms of physiology and behavior. The melatonin message is translated through activation of two G protein-coupled receptors, MT(1) and MT(2), that are potential therapeutic targets in disorders ranging from insomnia and circadian sleep disorders to depression, cardiovascular diseases, and cancer. This review summarizes the steps taken since melatonin's discovery by Aaron Lerner in 1958 to functionally characterize, clone, and localize receptors in mammalian tissues. The pharmacological and molecular properties of the receptors are described as well as current efforts to discover and develop ligands for treatment of a number of illnesses, including sleep disorders, depression, and cancer.

Evidence of a role for melatonin in fetal sheep physiology: direct actions of melatonin on fetal cerebral artery, brown adipose tissue and adrenal gland

Although the fetal pineal gland does not secrete melatonin, the fetus is exposed to melatonin of maternal origin. In the non-human primate fetus, melatonin acts as a trophic hormone for the adrenal gland, stimulating growth while restraining cortisol production. This latter physiological activity led us to hypothesize that melatonin may influence some fetal functions critical for neonatal adaptation to extrauterine life. To test this hypothesis we explored (i) the presence of G-protein-coupled melatonin binding sites and (ii) the direct modulatory effects of melatonin on noradrenaline (norepinephrine)-induced middle cerebral artery (MCA) contraction, brown adipose tissue (BAT) lypolysis and ACTH-induced adrenal cortisol production in fetal sheep. We found that melatonin directly inhibits the response to noradrenaline in the MCA and BAT, and also inhibits the response to ACTH in the adrenal gland. Melatonin inhibition was reversed by the melatonin antagonist luzindole only in the fetal adrenal. MCA, BAT and adrenal tissue displayed specific high-affinity melatonin binding sites coupled to G-protein (K(d) values: MCA 64 +/- 1 pm, BAT 98.44 +/- 2.12 pm and adrenal 4.123 +/- 3.22 pm). Melatonin binding was displaced by luzindole only in the adrenal gland, supporting the idea that action in the MCA and BAT is mediated by different melatonin receptors. These direct inhibitory responses to melatonin support a role for melatonin in fetal physiology, which we propose prevents major contraction of cerebral vessels, restrains cortisol release and restricts BAT lypolysis during fetal life.

Melatonin inhibits nitric oxide production by microvascular endothelial cells in vivo and in vitro

BACKGROUND AND PURPOSE

We have previously shown that melatonin inhibits bradykinin-induced NO production by endothelial cells in vitro. The purpose of this investigation was to extend this observation to an in vivo condition and to explore the mechanism of action of melatonin.

EXPERIMENTAL APPROACH

RT-PCR assays were performed with rat cultured endothelial cells. The putative effect of melatonin upon arteriolar tone was investigated by intravital microscopy while NO production by endothelial cells in vitro was assayed by fluorimetry, and intracellular Ca(2+) measurements were assayed by confocal microscopy.

KEY RESULTS

No expression of the mRNA for the melatonin synthesizing enzymes, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase, or for the melatonin MT(2) receptor was detected in microvascular endothelial cells. Melatonin fully inhibited L-NAME-sensitive bradykinin-induced vasodilation and also inhibited NO production induced by histamine, carbachol and 2-methylthio ATP, but did not inhibit NO production induced by ATP or alpha, beta-methylene ATP. None of its inhibitory effects was prevented by the melatonin receptor antagonist, luzindole. In nominally Ca(2+)-free solution, melatonin reduced intracellular Ca(2+) mobilization induced by bradykinin (40%) and 2-methylthio ATP (62%) but not Ca(2+) mobilization induced by ATP.

CONCLUSIONS AND IMPLICATIONS

We have confirmed that melatonin inhibited NO production both in vivo and in vitro. In addition, the melatonin effect was selective for some G protein-coupled receptors and most probably reflects an inhibition of Ca(2+) mobilization from intracellular stores.

Seasonal changes in adiposity: the roles of the photoperiod, melatonin and other hormones, and sympathetic nervous system

It appears advantageous for many non-human animals to store energy body fat extensively and efficiently because their food supply is more labile and less abundant than in their human counterparts. The level of adiposity in many of these species often shows predictable increases and decreases with changes in the season. These cyclic changes in seasonal adiposity in some species are triggered by changes in the photoperiod that are faithfully transduced into a biochemical signal through the nightly secretion of melatonin (MEL) via the pineal gland. Here, we focus primarily on the findings from the most commonly studied species showing seasonal changes in adiposity-Siberian and Syrian hamsters. The data to date are not compelling for a direct effect of MEL on white adipose tissue (WAT) and brown adipose tissue (BAT) despite some recent data to the contrary. Thus far, none of the possible hormonal intermediaries for the effects of MEL on seasonal adiposity appear likely as a mechanism by which MEL affects the photoperiodic control of body fat levels indirectly. We also provide evidence pointing toward the sympathetic nervous system as a likely mediator of the effects of MEL on short day-induced body fat decreases in Siberian hamsters through increases in sympathetic drive on WAT and BAT. We speculate that decreases in the SNS drive to these tissues may underlie the photoperiod-induced seasonal increases in body fat of species such as Syrian hamsters. Clearly, we need to deepen our understanding of seasonal adiposity, although, to our knowledge, this is the only form of environmentally induced changes in body fat where the key elements of its external trigger have been identified and can be traced to and through their transduction into a physiological stimulus that ultimately affects identified responses of white adipocyte physiology and cellularity. Finally, the comparative physiological approach to the study of seasonal adiposity seems likely to continue to yield significant insights into the mechanisms underlying this phenomenon and for understanding obesity and its reversal in general.

Receptor-mediated modulation of avian caecal muscle contraction by melatonin: role of tyrosine protein kinase

Abstract: Melatonin receptors in the quail caecum were studied by 2[125I]iodomelatonin binding assay and the involvement of tyrosine protein kinase in the melatonin-induced contraction was explored. The binding of 2[125I]iodomelatonin in the quail caecum membrane preparations was saturable, reversible and of high affinity with an equilibrium dissociation constant (Kd) of 24.6 +/- 1.1 pm (n = 7) and a maximum number of binding sites (Bmax) of 1.95 +/- 0.09 fmol (mg/protein) (n = 7). The relative order of potency of indoles in competing for 2[125I]iodomelatonin binding was: 2-iodomelatonin > melatonin > 2-phenylmelatonin > 6-chloromelatonin > 6-hydroxymelatonin > N-acetylserotonin, indicating that ML(1) receptors are involved. The binding was inhibited by Mel1b melatonin receptor antagonists, luzindole and 4-phenyl-2-propionamidotetralin (4-P-PDOT) as well as by non-hydrolyzable analogs of GTP like GTPgammaS and Gpp(NH)p but not by adenosine nucleotides. The latter suggests that the action of melatonin on the caecum is G-protein linked. Cumulative addition of melatonin (1-300 nM) potentiated both the amplitude and frequency of spontaneous contractions in the quail caecum. The potentiation of rhythmic contractions was blocked by both luzindole and 4-P-PDOT. Antagonists of tyrosine kinase, genistein(2 microM) and erbstatin(4 microM) suppressed the modulation of spontaneous contractions by melatonin, but not inhibitors of protein kinase C (PKC) or protein kinase A (PKA). Melatonin-induced increment in spontaneous contraction was blocked by nifedipine (0.4 nM). Thus, we suggest that melatonin potentiates spontaneous contraction in the quail caecum via interacting with G-protein-coupled Mel(1b) receptor which may activate L-type Ca2+ channels by mobilizing tyrosine kinases.

24h variation in the expression of the mt1 melatonin receptor subtype in coronary arteries derived from patients with coronary heart disease

Previous studies presented evidence for impaired nocturnal secretion and synthesis of melatonin in patients with coronary heart disease (CHD). This study aimed to investigate whether the melatonin receptor subtype mt1 is differentially expressed in coronary arteries derived from patients with CHD (n = 9) compared to patients with dilative cardiomyopathy (CMP; n = 10) who served as controls. Expression of the mt1 receptor was studied in sections of isolated coronary arteries by a reverse transcriptase-polymerase chain reaction (RT-PCR) and a Western immunoblot technique. In addition, the data from the Western blotting of 15 patients were interpolated against the exact time of aortic clamp to study the 24h expression of the mt1 receptor. The analyses of the results from both methods indicated the presence of the mt1 receptor in all of the individuals. No statistically significant difference was observed in the receptor expression between patients with CHD and those with CMP (in arbitrary units: 3.39 +/- 3.08 versus 3.91 +/- 2.78). Expression of the melatonin receptor in the coronary arteries of the whole patient group presented a 24h variation, with the lowest values detectable after 02:00 up to the late morning hours and a progressive increase beginning after 13:00 until 00:00 (mesor = 3.66, amplitude = 3.23, acrophase = 20.45, P = .0003). When studying the 24h variation in patients with CHD and CMP separately, a nearly similar circadian course was observed. In conclusion, we demonstrated for the first time a 24h variation of a melatonin receptor subtype in human vessels. Furthermore, in relation to our results, we suggest that the expression of the mt1 melatonin receptor in the coronary arteries is probably not impaired in patients with CHD.

Expression of the MT1 melatonin receptor subtype in human coronary arteries

Previous experimental data suggest a possible influence of melatonin on the circulatory system of animals after binding to G-protein coupled melatonin receptors. The present study sought to investigate whether the melatonin receptor, mt1, is expressed in human coronary arteries derived from healthy heart donors (n = 8). Expression of the mt1-receptor was studied in sections of isolated coronary arteries by a reverse transcriptase-polymerase chain reaction (RT-PCR) and Western immunoblot technique. The analyses of the results from both methods indicated the presence of the mt1-receptor in all of the subjects. Referring to these data we assume that melatonin regulates physiological processes in human coronary arteries after receptor binding.

Melatonin potentiates NE-induced vasoconstriction without augmenting cytosolic calcium concentration

Because little is known of the intracellular mechanisms involved in the vasoconstrictor effect of melatonin (Mel), we examined the in vitro effects of Mel by using perfused cylindrical segments of the rat tail artery loaded with the intracellular Ca(2+) concentration ([Ca(2+)](i))-sensitive fluorescent dye, fura 2. Mel (10(-14) to 10(-4) M) had no effect on baseline perfusion pressure or [Ca(2+)](i) but increased, at submicromolar concentrations, the vasoconstrictor effect of norepinephrine (NE) (P = 0.0029). Mel did not modify NE-induced [Ca(2+)](i) mobilization, and thus the [Ca(2+)](i) sensitivity of NE-induced contraction increased in the presence of Mel. Mel consistently increased KCl-induced vasoconstriction and [Ca(2+)](i) sensitivity of contraction, but differences were not statistically significant. In conclusion, Mel increases the [Ca(2+)](i) sensitivity of vasoconstriction evoked by NE suggesting that Mel may amplify endogenous vasoconstrictor responses to sympathetic outflow.

Cardiovascular effects of melatonin in hypertensive patients well controlled by nifedipine: a 24-hour study

AIMS

As melatonin has been found to play a role in the mechanisms of cardiovascular regulation, we designed the present study to evaluate whether the evening ingestion of the pineal hormone might interfere with the antihypertensive therapy in hypertensive patients well-controlled by nifedipine monotherapy.

METHODS

Forty-seven mild to moderate essential hypertensive outpatients taking nifedipine GITS 30 or 60 mg monotherapy at 08.30 h for at least 3 months, were given placebo or melatonin 5 mg at 22.30 h for 4 weeks according to a double-blind cross-over study. At the end of each treatment period patients underwent a 24 h noninvasive ambulatory blood pressure monitoring (ABPM) during usual working days; sleeping period was scheduled to last from 23.00 to 07.00 h.

RESULTS

The evening administration of melatonin induced an increase of blood pressure and heart rate throughout the 24 h period (DeltaSBP = + 6.5 mmHg, P < 0.001; DeltaDBP = + 4.9 mmHg, P < 0.01; DeltaHR = + 3.9 beats min-1, P < 0.01). The DBP as well as the HR increase were particularly evident during the morning and the afternoon hours.

CONCLUSIONS

We hypothesize that competition between melatonin and nifedipine, is able to impair the antihypertensive efficacy of the calcium channel blocker. This suggests caution in uncontrolled use of melatonin in hypertensive patients. As the pineal hormone might interfere with calcium channel blocker therapy, it cannot be considered simply a dietary supplement.

More than a marker: interaction between the circadian regulation of temperature and sleep, age-related changes, and treatment possibilities

The neurobiological mechanisms of both sleep and circadian regulation have been unraveled partly in the last decades. A network of brain structures, rather than a single locus, is involved in arousal state regulation, whereas the suprachiasmatic nucleus (SCN) has been recognized as a key structure for the regulation of circadian rhythms. Although most models of sleep regulation include a circadian component, the actual mechanism by which the circadian timing system promotes--in addition to homeostatic pressure--transitions between sleep and wakefulness remains to be elucidated. Little more can be stated presently than a probable involvement of neuronal projections and neurohumoral factors originating in the SCN. This paper reviews the relation among body temperature, arousal state, and the circadian timing system and proposes that the circadian temperature rhythm provides an additional signaling pathway for the circadian modulation of sleep and wakefulness. A review of the literature shows that increased brain temperature is associated with a type of neuronal activation typical of sleep in some structures (hypothalamus, basal forebrain), but typical of wakefulness in others (midbrain reticular formation, thalamus). Not only local temperature, but also skin temperature are related to the activation type in these structures. Warming of the skin is associated with an activation type typical of sleep in the midbrain reticular formation, hypothalamus, and cerebral cortex (CC). The decreasing part of the circadian rhythm in core temperature is mainly determined by heat loss from the skin of the extremities, which is associated with strongly increased skin temperature. As such, alterations in core and skin temperature over the day could modulate the neuronal activation state or "preparedness for sleep" in arousal-related brain structures. Body temperature may thus provide a third signaling pathway, in addition to synaptic and neurohumoral pathways, for the circadian modulation of sleep. A proposed model for the effects of body temperature on sleep appears to fit the available data better than previous hypotheses on the relation between temperature and sleep. Moreover, when the effects of age-related thermoregulatory alterations are introduced into the model, it provides an adequate description of age-related changes in sleep, including shallow sleep and awakening closer to the nocturnal core temperature minimum. Finally, the model indicates that appropriately timed direct (passive heating) or indirect (bright light, melatonin, physical activity) manipulation of the nocturnal profile of skin and core temperature may be beneficial to disturbed sleep in the elderly. Although such procedures could be viewed by researchers as merely masking a marker for the endogenous rhythm, they may in fact be crucial for sleep improvement in elderly subjects.

Effects of melatonin on vascular reactivity, catecholamine levels, and blood pressure in healthy men

In this study it was investigated whether the oral administration of melatonin (1 mg) in comparison to placebo was able to reduce blood pressure, vascular reactivity, and catecholamines in men, as previously reported in young women. The administration of melatonin significantly reduced blood pressure, the pulsatility index in the internal carotid artery, and catecholamines levels within 90 minutes. The effect of melatonin on the artery pulsatility index was related to baseline values, being greater in men with higher baseline values. The present data indicate that melatonin may blunt the activity of the cardiovascular system and may have both physiopathologic and clinical implications.

Effects of melatonin on ionic currents in cultured ocular tissues

The effects of melatonin on ionic conductances in a cultured mouse lens epithelial cell line (alpha-TN4) and in cultured human trabecular meshwork (HTM) cells were measured using the amphotericin perforated patch whole cell voltage-clamp technique. Melatonin stimulated a voltage-dependent Na+-selective current in lens epithelial cells and trabecular meshwork cells. The effects of melatonin were observed at 50 pM and were maximal at 100 microM. Melatonin enhanced activation and inactivation kinetics, but no change was observed in the voltage dependence of activation. The results are consistent with an increase in the total number of ion channels available for activation by membrane depolarization. Melatonin was also found to stimulate a K+-selective current at high doses (1 mM). Melatonin did not affect the inwardly rectifying K+ current or the delayed rectifier type K+ current that has been described in cultured mouse lens epithelial cells. The results show that melatonin specifically stimulated the TTX-insensitive voltage-dependent Na+ current by an apparently novel mechanism.

Mechanisms of melatonin-induced vasoconstriction in the rat tail artery: a paradigm of weak vasoconstriction

1. Vasoconstrictor effects of melatonin were examined in isolated rat tail arteries mounted either in an isometric myograph or as cannulated pressurized segments. Melatonin failed by itself to mediate observable responses but preactivation of the arteries with vasopressin (AVP) reliably uncovered vasoconstriction responses to melatonin with maxima about 50% of maximum contraction. Further experiments were conducted with AVP preactivation to 5-10% of the maximum contraction. 2. Responses to melatonin consisted of steady contractions with superimposed oscillations which were large and irregular in isometric but small in isobaric preparations. Nifedipine (0.3 microM) reduced the responses and abolished the oscillations. Charybdotoxin (30 nM) increased the magnitude of the oscillations with no change in the maximum response. 3. Forskolin (0.6 microM) pretreatment increased the responses to melatonin compared to control and sodium nitroprusside (1 microM) treated tissues. The AVP concentration required for preactivation was 10 fold higher than control in both the forskolin and nitroprusside treated groups. 4. In isometrically-mounted arteries treated with nifedipine, melatonin receptor agonists had the potency order 2-iodomelatonin > melatonin > S20098 > GR196429, and the MT2-selective antagonist luzindole antagonized the effects of melatonin with a low pK(B) of 6.1+/-0.1. 5. It is concluded that melatonin elicits contraction of the rat tail artery via an mt1 or mt1-like receptor that couples via inhibition of adenylate cyclase and opening of L-type calcium channels. Calcium channels and charybdotoxin-sensitive K channels may be recruited into the responses via myogenic activation rather than being coupled directly to the melatonin receptors. 6. It is proposed that the requirement of preactivation for overt vasoconstrictor responses to melatonin results from the low effector reserve of the melatonin receptors together with the tail artery having threshold inertia. Potentiative interactions between melatonin and other vasoconstrictor stimuli probably also result from the threshold inertia. A simple model is presented and a general framework for consideration of interactions between weak vasoconstrictor agonists and other vasoconstrictor stimuli is discussed.

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.

Effect of melatonin in the rat tail artery: role of K+ channels and endothelial factors

1. The role of endothelial factors and potassium channels in the action of the pineal hormone melatonin to potentiate vasoconstrictor responses was investigated in the isolated perfused tail artery of the rat. 2. Melatonin (100 nM) potentiated contractile responses to both adrenergic nerve stimulation and alpha1-adrenoceptor stimulation by phenylephrine. After removal of the endothelium, melatonin no longer caused potentiation. 3. The potentiating effect of melatonin was also lost when nitric oxide synthase was inhibited with L-NAME (10 nM). Thus potentiating effects depend on the presence of nitric oxide released by the endothelium. However, melatonin did not affect relaxation responses to acetylcholine in endothelium-intact arteries, nor did melatonin modulate relaxing responses to sodium nitroprusside in endothelium-denuded arteries. While melatonin does not appear to modulate agonist-induced release of nitric oxide nor its effect, melatonin may modulate nitric oxide production induced by flow and shear stress. 4. When the Ca2+-activated K+ channel opener, NS 1619 (10 microM), was present, potentiating effects of melatonin were restored in endothelium-denuded vessels. However, addition of the opener of ATP-sensitive K+ channels, cromakalim (3 microM), did not have the same restorative effect. Furthermore, addition of a blocker of Ca2+-activated K+ channels, tetraethylammonium (1 mM), significantly attenuated potentiating effects of melatonin. These findings support the hypothesis that melatonin inhibits the activity of large conductance Ca2+-activated K+ channels to produce its potentiating effects. 5. Thus in the rat perfused tail artery, potentiation of constriction by melatonin depends on the activity of both endothelial factors and Ca2+-activated K+ channels. Our findings suggest that melatonin inhibits endothelial K+ channels to decrease flow-induced release of nitric oxide as well as block smooth muscle K+ channels to enhance vascular tone.

Melatonin receptor genes

Melatonin is produced rhythmically by the pineal gland and the retina with increased synthesis during darkness. Pineal melatonin serves as the 'chemical expression of darkness' conveying information on the ambient light-dark cycle into rhythmic bodily functions. On-going debate on modes and sites of action ranges from views of melatonin affecting each and every cell ('cure-all') to those of melatonin having restricted actions through specific high-affinity receptors. The present review deals with the latter view. The use of 2-[125I]-iodomelatonin has allowed the exact localization and characterization of high-affinity melatonin receptors that signal through the G(i/o) class of G proteins. Molecular cloning of melatonin receptor genes has confirmed that most, if not all, high-affinity melatonin-binding sites represent the G-protein-coupled melatonin receptors. Based on sequence dissimilarities, melatonin receptors are classified into three subtypes, Mel1a, Mel1b and Mel1c. A distribution wider than originally thought of melatonin receptors in the human brain and peripheral sites has brought these receptors into focus of several drug companies, promising exciting times for research on melatonin and new therapeutic possibilities.

Influences of melatonin administration on the circulation of women

The cardiovascular effects induced by the daytime administration of melatonin (1 mg) were compared with those of placebo in 17 young, healthy, early follicular-phase women. Compared with placebo, the administration of melatonin modified, within 90 min, the pulsatility index (PI), evaluated by color Doppler ultrasound, of the internal carotid artery, abdominal aorta, and axillary artery. The effect was linearly related to baseline PI, higher baseline PI being associated with greater PI declines. Melatonin administration significantly decreased mean PI of internal carotid artery (P < 0.02), systolic and diastolic blood pressure (P < 0.01), and norepinephrine levels evaluated after 5 min of standing position (P < 0.02). Heart rate and supine catecholamine levels were not modified. These data indicate that in young, healthy women the administration of 1 mg of melatonin greatly influences artery blood flow, decreases blood pressure, and blunts noradrenergic activation. Clinical implications of present data are worthy to be fully explored.

Studies on the vasoconstrictor action of melatonin and putative melatonin receptor ligands in the tail artery of juvenile Wistar rats

1. In this study we compared the vasoconstrictor activity of melatonin in rat isolated tail artery using two different recording systems, the Halpern pressure myograph and the Halpern-Mulvany wire myograph, with the view to determining a reliable method for obtaining pharmacological data on vascular melatonin receptors. In addition, we characterized the melatonin receptor in this preparation, using analogues of melatonin, and examined the activity of various naphthalenic derivatives with biological activity in non-vascular models of melatonin receptors. 2. Using the Halpern pressure myograph, cumulative addition of melatonin (0.1 nM to 1 microM) produced direct vasoconstriction (19.3+/-6.4% reduction in lumen diameter, n=5) in five of 11 pressurized segments, with pEC50 of 9.14+/-0.17. Similarly, non-cumulative application of melatonin caused vasoconstriction (19.7+/-4.6% reduction in lumen diameter, n=7) in seven of 20 preparations examined with pEC50 of 8.74+/-0.26. The selective alpha2-adrenoceptor agonist, UK-14304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline bitartrate), produced vasoconstriction in all 'melatonin-insensitive' preparations. 3. Melatonin (0.1 nM to 1 microM) failed to elicit isometric contractions of tail artery segments in the Halpern wire myograph, but produced concentration-dependent potentiation of electrically-evoked, isometric contractions (maximum effect of 150-200% enhancement) when applied either noncumulatively (seven of seven preparations) or cumulatively (four of seven preparations). The pEC50 value of melatonin (non-cumulative) was 8.50+/-0.10 (n=7) which was not different from that obtained in the pressure myograph. All further experiments were conducted using a non-cumulative protocol against electrically-evoked, isometric contractions. 4. Based on the pEC50 values for the melatonin analogues examined, the pharmacological profile for the enhancement of electrically-evoked contractions was 2-iodomelatonin > 6-chloromelatonin > or = (-)-AMMTC > or = S21634 > or = melatonin > or = S20098 > S20242 > or = S20304 > 6-hydroxymelatonin > S20932 > (+)-AMMTC > N-acetyl-5-HT. Our data suggests the vascular receptor belongs to the MEL1-like subtype. All the indole-based analogues of melatonin, 2-iodomelatonin, (-)-AMMTC, (+)-AMMTC, S20932, 6-chloromelatonin, 6-hydroxymelatonin and N-acetyl-5-HT, behaved as full agonists. All the naphthalenic derivatives examined, S21634, S20098, S20242 and S20304 behaved as partial agonists relative to melatonin. 5. The naphthalenic-based antagonists, S20928 and S20929, did not modify electrically-evoked, isometric contractions of the tail artery, but produced a parallel, rightward displacement of the melatonin concentration-response curve. Based upon the effect of 1 microM S20928 and S20929, the estimated pK(B) values for these antagonists were 7.18+/-0.25 (n=4) and 7.17+/-0.25 (n=5), respectively. 6. We demonstrated that enhancement of electrically-evoked, isometric contractions of the rat isolated tail artery (using the Halpern-Mulvany wire myograph) is a simple and reproducible model for assessing the activity of putative agonists, partial agonists and antagonists at vascular melatonin receptors. Pharmacological characterization of the receptor suggests the presence of a MEL1-like subtype.

Melatonin modulates vascular smooth muscle tone

The molecular cloning of a family of melatonin receptors has created a renewed interest in the diverse actions of the hormone melatonin. The radioligand 2-[125I]iodomelatonin has identified potential sites of action for melatonin throughout the central nervous system and periphery of numerous species. Interestingly, in addition to the suprachiasmatic nucleus (the "biological clock"), 2-[125I]iodomelatonin binding sites have been localized to the rat caudal and cerebral arteries. Furthermore, in vitro, melatonin has been shown to induce a concentration-dependent vasoconstriction of rat caudal and cerebral arteries, and pig and human coronary arteries. The lack of melatonin receptor subtype-selective agonists and antagonists prevents the full pharmacological characterization of these responses. The physiological significance of the in vitro vasoconstrictive properties is far from clear, however; in rats, melatonin has been shown to reduce cerebral blood flow. The widespread use of melatonin warrants appropriately designed studies to probe the role of melatonin and its receptors in the modulation of in vitro vascular tone.