Binding characteristics and regional distribution of [125I]iodomelatonin binding sites in the brain of the human fetus

Melatonin: an underappreciated player in retinal physiology and pathophysiology

In the vertebrate retina, melatonin is synthesized by the photoreceptors with high levels of melatonin at night and lower levels during the day. Melatonin exerts its influence by interacting with a family of G-protein-coupled receptors that are negatively coupled with adenylyl cyclase. Melatonin receptors belonging to the subtypes MT(1) and MT(2) have been identified in the mammalian retina. MT(1) and MT(2) receptors are found in all layers of the neural retina and in the retinal pigmented epithelium. Melatonin in the eye is believed to be involved in the modulation of many important retinal functions; it can modulate the electroretinogram (ERG), and administration of exogenous melatonin increases light-induced photoreceptor degeneration. Melatonin may also have protective effects on retinal pigment epithelial cells, photoreceptors and ganglion cells. A series of studies have implicated melatonin in the pathogenesis of age-related macular degeneration, and melatonin administration may represent a useful approach to prevent and treat glaucoma. Melatonin is used by millions of people around the world to retard aging, improve sleep performance, mitigate jet lag symptoms, and treat depression. Administration of exogenous melatonin at night may also be beneficial for ocular health, but additional investigation is needed to establish its potential.

Significance and application of melatonin in the regulation of brown adipose tissue metabolism: relation to human obesity

A worldwide increase in the incidence of obesity indicates the unsuccessful battle against this disorder. Obesity and the associated health problems urgently require effective strategies of treatment. The new discovery that a substantial amount of functional brown adipose tissue (BAT) is retained in adult humans provides a potential target for treatment of human obesity. BAT is active metabolically and disposes of extra energy via generation of heat through uncoupling oxidative phosphorylation in mitochondria. The physiology of BAT is readily regulated by melatonin, which not only increases recruitment of brown adipocytes but also elevates their metabolic activity in mammals. It is speculated that the hypertrophic effect and functional activation of BAT induced by melatonin may likely apply to the human. Thus, melatonin, a naturally occurring substance with no reported toxicity, may serve as a novel approach for treatment of obesity. Conversely, because of the availability of artificial light sources, excessive light exposure after darkness onset in modern societies should be considered a potential contributory factor to human obesity as light at night dramatically reduces endogenous melatonin production. In the current article, the potential associations of melatonin, BAT, obesity and the medical implications are discussed.

Prenatal melatonin and its interaction with tachykinins in the hypothalamic-pituitary-gonadal axis

The pineal gland, through its hormone melatonin, influences the function of the hypothalamic-pituitary-gonadal axis. Tachykinins are bioactive peptides whose presence has been demonstrated in the pineal gland, hypothalamus, anterior pituitary gland and the gonads, in addition to other central and peripheral structures. Tachykinins have been demonstrated to influence the function of the hypothalamic-pituitary-gonadal axis, acting as paracrine factors at each of these levels. In the present review, we examine the available evidence supporting a role for melatonin in the regulation of reproductive functions, the possible role of tachykinins in pineal function and the possible interactions between melatonin and tachykinins in the hypothalamic-pituitary-gonadal axis. Evidence is presented showing that melatonin, given to pregnant rats, influences the developmental pattern of tachykinins in the hypothalamus and the anterior pituitary gland of the offspring during postnatal life. In the gonads, the effects of melatonin on the tachykinin developmental pattern were rather modest. In particular, in the present review, we have included a summary of our own work performed in the past few years on the effect of melatonin on tachykinin levels in the hypothalamic-pituitary-gonadal axis.

Neural stem cells express melatonin receptors and neurotrophic factors: colocalization of the MT1 receptor with neuronal and glial markers

Background

In order to optimize the potential benefits of neural stem cell (NSC) transplantation for the treatment of neurodegenerative disorders, it is necessary to understand their biological characteristics. Although neurotrophin transduction strategies are promising, alternative approaches such as the modulation of intrinsic neurotrophin expression by NSCs, could also be beneficial. Therefore, utilizing the C17.2 neural stem cell line, we have examined the expression of selected neurotrophic factors under different in vitro conditions. In view of recent evidence suggesting a role for the pineal hormone melatonin in vertebrate development, it was also of interest to determine whether its G protein-coupled MT₁ and MT₂ receptors are expressed in NSCs.

Results

RT-PCR analysis revealed robust expression of glial cell-line derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in undifferentiated cells maintained for two days in culture. After one week, differentiating cells continued to exhibit high expression of BDNF and NGF, but GDNF expression was lower or absent, depending on the culture conditions utilized. Melatonin MT₁ receptor mRNA was detected in NSCs maintained for two days in culture, but the MT₂ receptor was not seen. An immature MT₁ receptor of about 30 kDa was detected by western blotting in NSCs cultured for two days, whereas a mature receptor of about 40 – 45 kDa was present in cells maintained for longer periods. Immunocytochemical studies demonstrated that the MT₁ receptor is expressed in both neural (β-tubulin III positive) and glial (GFAP positive) progenitor cells. An examination of the effects of melatonin on neurotrophin expression revealed that low physiological concentrations of this hormone caused a significant induction of GDNF mRNA expression in NSCs following treatment for 24 hours.

Conclusions

The phenotypic characteristics of C17.2 cells suggest that they are a heterogeneous population of NSCs including both neural and glial progenitors, as observed under the cell culture conditions used in this study. These NSCs have an intrinsic ability to express neurotrophic factors, with an apparent suppression of GDNF expression after several days in culture. The detection of melatonin receptors in neural stem/progenitor cells suggests involvement of this pleiotropic hormone in mammalian neurodevelopment. Moreover, the ability of melatonin to induce GDNF expression in C17.2 cells supports a functional role for the MT₁ receptor expressed in these NSCs. In view of the potency of GDNF in promoting the survival of dopaminergic neurons, these novel findings have implications for the utilization of melatonin in neuroprotective strategies, especially in Parkinson's disease.

Melatonin receptors in human fetal brain: 2-[(125)I]iodomelatonin binding and MT1 gene expression

The purpose of this study was to identify sites of action of melatonin in the human fetal brain by in vitro autoradiography and in situ hybridization. Specific, guanosine triphosphate (GTP) sensitive, binding of 2-[(125)I]iodomelatonin was localized to the leptomeninges, cerebellum, thalamus, hypothalamus, and brainstem. In the hypothalalmus, specific binding was present in the suprachiasmatic nuclei (SCN) as well as the arcuate, ventromedial and mammillary nuclei. In the brainstem specific binding was present in the cranial nerve nuclei including the oculomotor nuclei, the trochlear nuclei, the motor and sensory trigeminal nuclei, the facial nuclei, and the cochlear nuclei. The localization of MT1 receptor subtype gene expression as determined by in situ hybridization matched the localization of 2-[(125)I]iodomelatonin binding. No MT2 receptor subtype gene expression was detected using this technique. Thus, melatonin may act on the human fetus via the MT1 receptor subtype at a number of discrete brain sites. A major site of action of melatonin in both fetal and adult mammals is the pars tuberalis of the pituitary gland. However, no 2-[(125)I]iodomelatonin binding or melatonin receptor gene expression was detected in the pituitary gland in the present study, indicating that the pituitary, particularly the pars tuberalis, is not a site of action of melatonin in the human fetus.

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 on chloride secretion by human colonic T84 cells

The effect of melatonin on human colonic T84 cells was studied using the short-circuit current (I(SC)) technique. Basolateral, as well as apical, addition of melatonin stimulated I(SC) in a concentration-dependent manner (EC50 at about 100 microM). The I(SC) response to melatonin was nearly abolished when external Cl- was removed. The increase in I(SC) was also blocked by apical addition of two Cl- channel blockers, 4,4'-diisothiocyanatostibene-2,2'-disulfonic acid (DIDS) and diphenylamine-2-carboxylic acid (DPC), indicating that melatonin stimulated Cl- secretion. The effect of different melatonin analogs was compared and the order of potency appeared to be 2-iodomelatonin > melatonin > 6-hydroxymelatonin, indicating the involvement of melatonin receptors. However, inhibitors for Gi-protein, adenylate cyclase or phospholipase C were found to be ineffective in inhibiting the melatonin-induced I(SC). Pretreatment of the cells with melatonin was also found to exert little effect on subsequent forskolin- or VIP-induced I(SC), further excluding its interaction with adenylate cyclase. Our data suggest that melatonin may play a role in regulating colonic Cl- secretion via melatonin receptors; however, the signal transduction pathway(s) involved remains to be elucidated.

Identification and characterisation of 2-[125I]iodomelatonin binding and Mel1a melatonin receptor expression in the human fetal leptomeninges

Melatonin binding sites were identified over the leptomeninges surrounding the human fetal brain using quantitative in vitro autoradiography and the melatonin agonist, 2-[125I]iodomelatonin. Binding was found to be saturable and of high affinity (dissociation constant (Kd) = 54 pM and maximal theoretical binding (Bmax) = 13 fmol/mg protein), and inhibited by guanosine-5'-o-(3-thiotriphosphate) (GTPgammaS) suggesting that these binding sites represent G protein-coupled melatonin receptors. RT-PCR performed on mRNA isolated from the human fetal leptomeninges detected expression of the G protein-coupled melatonin receptor Mel1a, but not Mel1b. In situ hybridisation confirmed the localisation of Mel1a mRNA transcripts over the leptomeninges of the fetal brain. The identification of 2-[125I]iodomelatonin and Mel1a melatonin receptor expression in the fetal leptomeninges implies that melatonin may play a role in the early growth and development of the human brain.

Melatonin and 2[125I]iodomelatonin binding sites in the human colon

2[125I]Iodomelatonin binding sites were identified in the mucosa of the human colon from Chinese patients with carcinoma of the rectum or colon using biochemical receptor assay and autoradiography. Melatonin in the colonic mucosa/submucosa and muscle layers were quantitated by radioimmunoassay. The binding of 2[125I]iodomelatonin to the membrane preparations of the human colonic mucosa/submucosa was stable, saturable, reversible and of high affinity. Rosenthal analysis from saturation studies performed at 21 degrees C yielded an equilibrium dissociation constant (Kd) of 61.7 +/- 4.48 pmol/L (n = 3) and maximum number of binding sites (B(max)) of 1.65 +/- 0.51 fmol/mg protein (n = 3). The linearity of the Rosenthal plots and unity of the Hill coefficient suggested that 2[125I]iodomelatonin was bound to a single class of binding sites. The radioligand binding was displaced by 2-iodomelatonin (Ki = 0.02 nmol/L), melatonin (0.65 nmol/L), 6-chloromelatonin (Ki = 5.33 nmol/L), 6-hydroxymelatonin (Ki = 33.8 nmol/L) and N-acetylserotonin (Ki = 122 nmol/L). The characteristic of the binding sites were similar to those reported in the jejunum of duck, chicken, and human but of higher affinity than those in the mouse colon. Autoradiography localizes the binding to the mucosa of the human colon. Radioimmunoassay revealed a melatonin concentration of 467 +/- 99 pg/g wet tissue of human colon (n = 6). Our findings suggest that melatonin may influence the human colonic functions through interaction with its receptors in the mucosa.

Changes in circadian rhythms and sleep quality with aging: mechanisms and interventions

Literature is reviewed indicating that aging is characterized by changes in circadian rhythms and sleep quality. The most marked change is an attenuation of amplitude. An advance of phase, a shortening of period, and a desynchronization of rhythms are also evident. The mechanisms underlying these changes are unknown. However, age-related changes in the retina, suprachiasmatic nucleus, and pineal gland seem relevant along with behavioral changes such as a reduction in physical activity and exposure to photic stimulation. Changes in circadian rhythms are frequently associated with a reduction in nighttime sleep quality, a decrease in daytime alertness, and an attenuation in cognitive performance; reversing such changes could enhance the quality of life for a large and rapidly increasing percentage of the population. Reversal appears possible by increasing melatonin levels with either appropriately timed exposure to photic stimulation and/or appropriately timed administration of exogenous melatonin. These interventions may increase aspects of genetic expression that have changed with aging. A hypothesis concerning the potential benefits of enhanced circadian amplitude is also offered.

Comparison of [125I]iodomelatonin binding sites in infant cerebellum of sudden infant death syndrome and non-sudden infant death syndrome

[125I]Iodomelatonin bindings sites in human infant cerebellum were studied by radioligand receptor binding assay and in vitro quantitative autoradiography. The binding sites characterized in membrane preparations revealed saturable, reversible and highly specific binding sites with a Kd value of 21.2 +/- 8.33 pM and a Bmax value of 2.02 +/- 0.52 fmol/mg protein. 10 microM of GTP gamma S significantly reduced the binding capacity, suggesting the possible G-protein coupling of the binding sites. Autoradiographic study showed that the labelling was mainly located in the cerebellar cortex. On comparison of the binding parameters from cerebellum of sudden infant death syndrome (SIDS) and non-SIDS infants, no significant change in binding capacity and binding affinity was detected. These findings suggest that the binding sites in cerebellum may not be related to the etiology of SIDS.

Identification and characterization of 2[125I]-iodomelatonin binding sites in the rat epididymis

Putative melatonin receptors in different parts of the male reproductive system of rats (Sprague-Dawley), mice (ICR), hamsters (Syrian) and guinea pigs (Dunkin-Hartley), rat epididymal sperm, and boar and human semen were studied by a radioreceptor assay using 2[125I]iodomelatonin as the radioligand. There was limited or no detectable binding of 2[125I]iodomelatonin to membrane preparations of rat testis, seminal vesicles, prostate, or sperm from rat, human, and boar. However, significant bindings of 2[125I]iodomelatonin to the epididymides of rat, mouse, hamster, and guinea-pig were demonstrated. The relative binding capacities of 2[125I]iodomelatonin to the distal epididymal segment in different rodent species was of the order rat mouse hamster guinea pig. The relative number of binding sites was much lower in the proximal segment than in the distal segment of epididymis. 2[125I]iodomelatonin binding to the distal segment of rat epididymis was studied in detail. The binding sites fulfilled all criteria for a receptor site; being stable, saturable, reversible, and of high affinity. The binding had an equilibrium dissociation constant (Kd) of 62.6 +/- 7.79 pmol/l (n = 7) and a density (Bmax) of 1.55 +/- 0.16 fmol/mg protein (n = 7). The Hill coefficient approached 1.0, suggesting a single class of 2[125I]iodomelatonin binding sites. Pharmacological studies revealed that these 2[125I]iodomelatonin binding sites were specific for melatonin receptors. In addition, there was an age-related change in the 2[125I]iodomelatonin binding sites in the rat distal epididymal segment. The binding increased from a lower value in 1-month-old rats to a higher adult value in the 1 1/2- to 24-month-old animals.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

G-protein linked melatonin binding sites in the chicken lung

[125I]Iodomelatonin binding was first demonstrated in the chicken lung membrane preparations. The binding was saturable, reversible, rapid, and of high affinity. The sites of binding distributed widely in different subcellular fractions except the cytosol fraction. Scatchard plots are linear indicating a dissociation constant (Kd) of 8.11 +/- 0.73 pmol/l and a maximum number of binding sites of 1.29 +/- 0.16 fmol/mg protein (n = 9). The Kd estimated from the kinetic study was 11.2 pmol/l. Guanosine 5'-O-(3-thiotriphosphate) significantly increased the Kd values while the density of binding sites was not affected indicating that the binding sites may be linked to a guanine nucleotide binding protein.

Characterization of melatonin binding sites in chicken and human intestines

The radioligand 2-[125I]iodomelatonin was used to study melatonin binding sites in chicken and human intestines. In the chicken duodenum, 2-[125I]iodomelatonin binding sites were enriched in the musculosa layer (Bmax approximately 1 fmol/mg protein) as compared to the mucosa/submucosa layer (Bmax approximately 0.2 fmol/mg protein). 2-[125I]iodomelatonin bound with a Kd of 68 +/- 18 pM (mean +/- S.E.M., n = 13) and was displaced by melatonin with a Ki of 0.3 nM. The Kd value for 2-[125I]iodomelatonin was increased 2- to 4-fold by a GTP analog, suggesting that the binding sites might be coupled to a G-protein. The affinity order of nine melatonin analogs at the enteric binding sites was in agreement with the pharmacological profile of melatonin receptors described in other tissues. In the human jejunum, 2-[125I]iodomelatonin binding could be observed in the mucosa/submucosa layer (Kd = 150-200 pM, Bmax = 0.7 fmol/mg protein). The radioligand was efficiently displaced by melatonin (Ki = 0.6 nM) but only marginally by N-acetyltryptamine (Ki = 22 microM) and serotonin (Ki = 14 microM).

High affinity specific binding of 2-[125I]iodomelatonin by spleen membrane preparations of chicken

The binding sites for 2-[125I]iodomelatonin in chicken spleens were characterized. The binding was rapid, stable, saturable, reversible, and of high affinity. Both melatonin and 6-chloromelatonin strongly inhibited the binding. The dissociation constant (Kd) obtained from the Scatchard analysis was 31.4 +/- 5.19 pmol/l (3-weeks old, n = 4), which was in good agreement with the Kd (50.6 pmol/l) calculated from the kinetic study. The maximum number of binding sites (Bmax) was 1.09 +/- 0.11 fmol/mg protein (3-weeks old, n = 4). Twelve 11-week-old chicks were killed in two groups at mid-light or mid-dark. Saturation studies indicated no significant difference (P greater than 0.05) in the Kd between mid-light (42.1 +/- 3.9 pmol/l) and mid-dark (31.6 +/- 4.9 pmol/l). The maximum number of binding sites (Bmax) at mid-light and mid-dark were 1.52 +/- 0.16 and 1.35 +/- 0.08 fmol/mg protein, respectively, with no significant variation (P greater than 0.05) recorded. However, when the whole spleen was taken into consideration, the Bmax per spleen protein of the mid-light samples (253 +/- 36 fmol/spleen protein) was significantly greater than that (129 +/- 16 fmol/spleen protein) of the mid-dark samples (P less than 0.05). This indicated that in our study a diurnal rhythm of the total number of 2-[125I]iodomelatonin binding sites might exist in the chicken spleen.

Circadian rhythms and the suprachiasmatic nucleus in perinatal development, aging and Alzheimer's disease

Circadian rhythms are already present in the fetus. At a certain stage of pre-natal hypothalamic development (around 30 weeks of gestation) the fetus becomes responsive to maternal circadian signals. Moreover, recent studies showed that the fetal biological clock is able to generate circadian rhythms, as exemplified by the rhythms of body temperature and heart rate of pre-term babies in the absence of maternal or environmental entrainment factors. Pre-term babies that are deprived of maternal entrainment and kept under constant environmental conditions (e.g., continuous light) in the neonatal intensive care unit run the risk of developing a biological clock dysfunctioning. However, the fact should be acknowledged that at least in mice the development of the circadian pacemaker (i.e., SCN) does not depend on environmental influences (Davis and Menaker, 1981), although other data suggest that severe disruption of the maternal circadian rhythm indeed abolishes the circadian rhythm of the fetal SCN (Shibata and Moore, 1988). During aging and in particular in AD circadian rhythms are disturbed. These disturbances include phase advance and reduced period and amplitude, as well as an increased intradaily variability and a decreased interdaily stability of the rhythm. Among the factors underlying these changes the loss of SCN neurons seems to play a central role. Other contributory factors may be reduced amount of light, degenerative changes in the visual system and the level of activity and decreased melatonin.