Melatonin: identification of sites of antigonadal action in mouse brain

Brown adipose tissue and novel therapeutic approaches to treat metabolic disorders

In humans, 2 functionally different types of adipose tissue coexist: white adipose tissue (WAT) and brown adipose tissue (BAT). WAT is involved in energy storage, whereas BAT is involved in energy expenditure. Increased amounts of WAT may contribute to the development of metabolic disorders, such as obesity-associated type 2 diabetes mellitus and cardiovascular diseases. In contrast, the thermogenic function of BAT allows high consumption of fatty acids because of the activity of uncoupling protein 1 in the internal mitochondrial membrane. Interestingly, obesity reduction and insulin sensitization have been achieved by BAT activation-regeneration in animal models. This review describes the origin, function, and differentiation mechanisms of BAT to identify new therapeutic strategies for the treatment of metabolic disorders related to obesity. On the basis of the animal studies, novel approaches for BAT regeneration combining stem cells from the adipose tissue with active components, such as melatonin, may have potential for the treatment of metabolic disorders in humans.

Melatonin supplementation decreases prolactin synthesis and release in rat adenohypophysis: correlation with anterior pituitary redox state and circadian clock mechanisms

In the laboratory rat, a number of physiological parameters display seasonal changes even under constant conditions of temperature, lighting, and food availability. Since there is evidence that prolactin (PRL) is, among the endocrine signals, a major mediator of seasonal adaptations, the authors aimed to examine whether melatonin administration in drinking water resembling in length the exposure to a winter photoperiod could affect accordingly the 24-h pattern of PRL synthesis and release and some of their anterior pituitary redox state and circadian clock modulatory mechanisms. Melatonin (3 µg/mL drinking water) or vehicle was given for 1 mo, and rats were euthanized at six time intervals during a 24-h cycle. High concentrations of melatonin (>2000 pg/mL) were detected in melatonin-treated rats from beginning of scotophase (at 21:00 h) to early photophase (at 09:00 h) as compared with a considerably narrower high-melatonin phase observed in controls. By cosinor analysis, melatonin-treated rats had significantly decreased MESOR (24-h time-series average) values of anterior pituitary PRL gene expression and circulating PRL, with acrophases (peak time) located in the middle of the scotophase, as in the control group. Melatonin treatment disrupted the 24-h pattern of anterior pituitary gene expression of nitric oxide synthase (NOS)-1 and -2, heme oxygenase-1 and -2, glutathione peroxidase, glutathione reductase, Cu/Zn- and Mn-superoxide dismutase, and catalase by shifting their acrophases to early/middle scotophase or amplifying the maxima. Only the inhibitory effect of melatonin on pituitary NOS-2 gene expression correlated temporally with inhibition of PRL production. Gene expression of metallothionein-1 and -3 showed maxima at early/middle photophase after melatonin treatment. The 24-h pattern of anterior pituitary lipid peroxidation did not vary after treatment. In vehicle-treated rats, Clock and Bmal1 expression peaked in the anterior pituitary at middle scotophase, whereas that of Per1 and Per2 and of Cry1 and Cry2 peaked at the middle and late photophase, respectively. Treatment with melatonin raised mean expression of anterior pituitary Per2, Cry1, and Cry2. In the case of Per1, decreased MESOR was observed, although the single significant difference found between the experimental groups when analyzed at individual time intervals was increase at early scotophase in the anterior pituitary of melatonin-treated rats. Melatonin significantly phase-delayed expression of Per1, Per2, and Cry1, also phase-delayed the plasma corticosterone circadian rhythm, and increased the amplitude of plasma corticosterone and thyrotropin rhythms. The results indicate that under prolonged duration of a daily melatonin signal, rat anterior pituitary PRL synthesis and release are depressed, together with significant changes in the redox and circadian mechanisms controlling them.

Acamprosate-responsive brain sites for suppression of ethanol intake and preference

Acamprosate suppresses alcohol intake and craving in recovering alcoholics; however, the central sites of its action are unclear. To approach this question, brain regions responsive to acamprosate were mapped using acamprosate microimplants targeted to brain reward and circadian areas implicated in alcohol dependence. mPer2 mutant mice with nonfunctional mPer2, a circadian clock gene that gates endogenous timekeeping, were included, owing to their high levels of ethanol intake and preference. Male wild-type (WT) and mPer2 mutant mice received free-choice (15%) ethanol/water for 3 wk. The ethanol was withdrawn for 3 wk and then reintroduced to facilitate relapse. Four days before ethanol reintroduction, mice received bilateral blank or acamprosate-containing microimplants releasing ∼50 ng/day into reward [ventral tegmental (VTA), peduculopontine tegmentum (PPT), and nucleus accumbens (NA)] and circadian [intergeniculate leaflet (IGL) and suprachiasmatic nucleus (SCN)] areas. The hippocampus was also targeted. Circadian locomotor activity was measured throughout. Ethanol intake and preference were greater in mPer2 mutants than in wild-type (WT) mice (27 g·kg(-1)·day(-1) vs. 13 g·kg(-1)·day(-1) and 70% vs. 50%, respectively; both, P < 0.05). In WTs, acamprosate in all areas, except hippocampus, suppressed ethanol intake and preference (by 40-60%) during ethanol reintroduction. In mPer2 mutants, acamprosate in the VTA, PPT, and SCN suppressed ethanol intake and preference by 20-30%. These data are evidence that acamprosate's suppression of ethanol intake and preference are manifest through actions within major reward and circadian sites.

Endocrine mechanisms of seasonal adaptation in small mammals: from early results to present understanding

Seasonal adaptation is widespread among mammals of temperate and polar latitudes. The changes in physiology, morphology and behaviour are controlled by the photoneuroendocrine system that, as a first step, translates day lengths into a hormonal signal (melatonin). Decoding of the humoral melatonin signal, i.e. responses on the cellular level to slight alterations in signal duration, represents the prerequisite for appropriate timing of winter acclimatization in photoperiodic animals. Corresponding to the diversity of affected traits, several hormone systems are involved in the regulation downstream of the neural integration of photoperiodic time measurement. Results from recent studies provide new insights into seasonal control of reproduction and energy balance. Most intriguingly, the availability of thyroid hormone within hypothalamic key regions, which is a crucial determinant of seasonal transitions, appears to be regulated by hormone secretion from the pars tuberalis of the pituitary gland. This proposed neuroendocrine pathway contradicts the common view of the pituitary as a gland that acts downstream of the hypothalamus. In the present overview of (neuro)endocrine mechanisms underlying seasonal acclimatization, we are focusing on the dwarf hamster Phodopus sungorus (long-day breeder) that is known for large amplitudes in seasonal changes. However, important findings in other mammalian species such as Syrian hamsters and sheep (short-day breeder) are considered as well.

Melatonin regulates delayed embryonic development in the short-nosed fruit bat, Cynopterus sphinx

The aim of the present study was to evaluate the seasonal variation in serum melatonin levels and their relationship to the changes in the serum progesterone level, ovarian steroidogenesis, and embryonic development during two successive pregnancies of Cynopterus sphinx. Circulating melatonin concentrations showed two peaks; one coincided with the period of low progesterone synthesis and delayed embryonic development, whereas the second peak coincided with regressing corpus luteum. This finding suggests that increased serum melatonin level during November–December may be responsible for delayed embryonic development by suppressing progesterone synthesis. The study showed increased melatonin receptors (MTNR1A and MTNR1B) in the corpus luteum and in the utero–embryonic unit during the period of delayed embryonic development. The in vitro study showed that a high dose of melatonin suppressed progesterone synthesis, whereas a lower dose of melatonin increased progesterone synthesis by the ovary. The effects of melatonin on ovarian steroidogenesis are mediated through changes in the expression of peripheral-type benzodiazepine receptor, P450 side chain cleavage enzyme, and LH receptor proteins. This study further showed a suppressive impact of melatonin on the progesterone receptor (PGR) in the utero–embryonic unit; this effect might contribute to delayed embryonic development in C. sphinx. The results of the present study thus suggest that a high circulating melatonin level has a dual contribution in retarding embryonic development in C. sphinx by impairing progesterone synthesis as well as by inhibiting progesterone action by reducing expression of PGR in the utero–embryonic unit.

Influence of the olfactory bulbs on blood leukocytes and behavioral responses to infection in Siberian hamsters

Surgical removal of the olfactory bulb alters several aspects of immunological activity. This study investigated the role of the olfactory bulbs in the control of behavioral responses to simulated infection, and the environmental modulation of sickness behaviors by changes in day length. Adult male Siberian hamsters (Phodopus sungorus) were subjected to bilateral olfactory bulbectomy (OBx) or a sham surgical procedure, and were then exposed to long(15 h light/day; LD) or short (9 h light/day; SD) photoperiods for 8–12 weeks, after which circulating leukocytes and behavioral responses (anorexia, anhedonia, cachexia) to simulated gram-negative bacterial infections (i.p. lipopolysaccharide [LPS] treatment;0.625 mg/kg) were quantified. OBx treatment altered the effects of photoperiod on immune function in a trait-specific manner. LPS-induced anorexia was exacerbated in SD-OBx hamsters; LPS-induced anhedonia was exacerbated in LD-OBx hamsters; and photoperiodic differences in circulating leukocytes and LPS-induced cachexia were eliminated by OBx. Plasma cortisol concentrations did not differ between LD and SD hamsters, irrespective of olfactory bulb integrity. The data indicate that photoperiod affects immune function via OB-dependent and -independent mechanisms, and that changes in cortisol production are not required for photoperiodic changes in sickness behaviors to manifest.

6-MBOA affects testis size, but not delayed-type hypersensitivity, in white-footed mice (Peromyscus leucopus)

Many rodents use day length to time reproduction to occur when resources are abundant, but some species also use supplementary environmental cues. One supplementary cue is the plant-derived compound, 6-methoxy-2-benzoxazolinone (6-MBOA). Most rodents grow their gonads in response to 6-MBOA in their diets, but it is presently unknown whether they also use 6-MBOA to adjust other aspects of physiology, specifically their immune systems. 6-MBOA is structurally similar to melatonin, and seasonal changes in rodent immune activities are often mediated by melatonin. We therefore predicted that white-footed mice (Peromyscus leucopus), which breed seasonally and are reproductively sensitive to melatonin, would adjust their immune systems when fed 6-MBOA. 6-MBOA treated mice in long day lengths regressed their testes to a greater extent than mice fed a standard diet, or mice kept in short day lengths and fed 6-MBOA or a standard diet. One type of immune activity (delayed-type hypersensitivity) was not affected by 6-MBOA, however, although responses were greater in short versus long day mice. In sum, P. leucopus responded reproductively to 6-MBOA, although differently than other species; immune activity was unaffected. Other aspects of the immune system, especially in herbivorous rodents, may be affected by 6-MBOA and thus warrant further study.

Response to selection for photoperiod responsiveness on the density and location of mature GnRH-releasing neurons

Natural variation in neuroendocrine traits is poorly understood, despite the importance of variation in brain function and evolution. Most rodents in the temperate zones inhibit reproduction and other nonessential functions in short winter photoperiods, but some have little or no reproductive response. We tested whether genetic variability in reproductive seasonality is related to individual differences in the neuronal function of the gonadotropin-releasing hormone network, as assessed by the number and location of mature gonadotropin-releasing hormone-secreting neurons under inhibitory and excitatory photoperiods. The experiments used lines of Peromyscus leucopus previously developed by selection from a wild population. One line contained individuals reproductively inhibited by short photoperiod, and the other line contained individuals nonresponsive to short photoperiod. Expression of mature gonadotropin-releasing hormone (GnRH) immunoreactivity in the brain was detected using SMI-41 antibody in the single-labeled avidin-biotin-peroxidase-complex method. Nonresponsive mice had 50% more immunoreactive GnRH neurons than reproductively inhibited mice in both short- and long-day photoperiods. The greatest differences were in the anterior hypothalamus and preoptic areas. In contrast, we detected no significant within-lines differences in the number or location of immunoreactive GnRH neurons between photoperiod treatments. Our data indicate that high levels of genetic variation in a single wild population for a specific neuronal trait are related to phenotypic variation in a life history trait, i.e., winter reproduction. Variation in GnRH neuronal activity may underlie some of the natural reproductive and life history variation observed in wild populations of P. leucopus. Similar genetic variation in neuronal traits may be present in humans and other species.

Determination of endogenous melatonin in the individual pineal glands of inbred mice using precolumn oxidation reversed-phase micro-high-performance liquid chromatography

The amount of endogenous melatonin in the individual pineal glands of inbred mice has been determined using reversed-phase micro-high-performance liquid chromatography after precolumn oxidation of melatonin to a compound having strong fluorescence. The fluorescent compound was identified as N-[(6-methoxy-4-oxo-1,4-dihydroquinolin-3-yl)methyl]acetamide. The excitation and emission wavelengths of this compound are 245 and 380 nm, respectively, and the fluorescence intensity is 6.8 times greater than that of melatonin. Molar absorptivity and fluorescence quantum yield of this compound are 46,300[L mol(-1)cm(-1)] and 0.31 (245 nm), respectively. The lower quantification limit of melatonin in biological samples using this precolumn oxidation method is 200 amol, and the calibration curve of spiked melatonin is linear from 200 amol to 50 fmol (r>0.999). The sensitivity of the present method is almost 10 times higher than that of the previous method. The values of endogenous melatonin obtained for ICR, C57BL, BALB/c, and AKR mice are 4.7, 6.1, 7.4, and 18.8 fmol/pineal gland, respectively. The amounts of endogenous pineal melatonin of these strains had not been clearly reported due to the poor enzymatic activities for melatonin biosynthesis; this is the first report that clearly demonstrates the existence of endogenous melatonin in these inbred mice.

Melatonin and mammary pathological growth

In this article we review the state of the art on the role of the pineal gland and melatonin in mammary cancer tumorigenesis in vivo as well as in vitro. The former hypothesis of a possible role of the pineal gland in mammary cancer development was based on the evidence that the pineal, via its main secretory product, melatonin, downregulates some of the pituitary and gonadal hormones which control mammary gland development and are also responsible for the growth of hormone-dependent mammary tumors. Furthermore, melatonin could act directly on tumoral cells, thereby influencing their proliferative rate. Other possible origins of melatonin's antitumoral actions could be found in its antioxidant or immunoenhancing properties. The working hypotheses of most experiments were that the activation of the pineal gland, or the administration of melatonin, should give rise to antitumoral behavior; conversely, suppression of the pineal gland or melatonin deficits should stimulate mammary tumorigenesis. From in vivo studies on animal models of tumorigenesis, the general conclusion is that experimental manipulations activating the pineal gland, or the administration of melatonin, enlarge the latency and reduce the incidence and growth rate of chemically induced mammary tumors, while pinealectomy usually has the opposite effects. The direct actions of melatonin on mammary tumors have been suggested because of its ability to inhibit, at physiological doses (1 nM), the in vitro proliferation and invasiveness of MCF-7 human breast cancer cells. The fact that most studies have been performed on two models, chemically induced mammary adenocarcinoma in rats (in vivo studies) and the cell tumor line MCF-7 (in vitro studies), makes the generalization of the results somewhat difficult. However, the characteristics of these actions, comprising different aspects of tumor biology such as initiation, proliferation, and metastasis, as well as the doses (physiological range) at which the effect is accomplished, give special value to these findings. On the strength of these data, the small number of clinical studies focusing on the possible therapeutic value of melatonin on breast cancer is surprising.

Differences in hypothalamic 2-[125I]iodomelatonin binding in photoresponsive and non-photoresponsive white-footed mice, Peromyscus leucopus

Photoperiod is an environmental cue used by many temperate-zone species to regulate their reproductive timing. Within species, the degree of reproductive photoresponsiveness can vary widely both among and within populations. The neuroendocrine mechanisms causing this individual variation in photoresponsiveness are unknown. Using selected lines from a population of white-footed mice known to vary genetically in reproductive photoresponsiveness, we tested the hypothesis that variation in the number and/or location of melatonin receptors is the basis for individual differences in reproductive photoresponsiveness. The brains and pars tuberalis of the pituitary from sixteen mice, (eight mice from each of two lines selected for two generations to respond strongly or weakly to photoperiod), were processed for autoradiography using the radioligand 2-[125I]-iodomelatonin (IMEL). We found significantly higher specific IMEL binding in the medial preoptic area and the bed nucleus of the stria terminalis of non-responsive mice than responsive mice. There were no differences between groups in specific IMEL binding in the suprachiasmatic and dorsomedial nuclei of the hypothalamus, pars tuberalis, or paraventricular nucleus of the thalamus. These results provide support for the hypothesis that individual variation in photoresponsiveness is due in part to differences in the density or affinity of melatonin receptors.

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.

Estrogenic effects on urinary 6-sulphatoxymelatonin excretion in the female rat

The pattern of melatonin production during the estrous cycle of the rat was measured by monitoring urinary 6-sulphatoxymelatonin (aMT.6S) excretion. Adult rats were maintained under a 14L:10D photoperiod and urine was collected at hourly intervals over a 5-day period using an automated collection system; the concentration of aMT.6S was assayed by RIA and hourly outputs were calculated. Each nightly collection of urine was assigned to an estrous cycle stage as determined by the vaginal smear of the preceding morning. Total aMT.6S excretions (mean +/- SEM) during estrous, metestrous, diestrous, and proestrous stages were 493 +/- 49, 539 +/- 44, 562 +/- 40, and 646 +/- 51 pmol/night, respectively (n = 7). The excretion of aMT.6S was significantly higher on the night of proestrus compared to each of the other stages (P < 0.05). To determine whether estrogen was responsible for the increased aMT.6S excretion during proestrus, rats were studied before and after ovariectomy and following implantation with estradiol implants. Total overnight aMT.6S excretion was reduced by 31% in ovariectomized animals relative to the intact state (P < 0.05) and restored to the intact levels by administration of estradiol (P < 0.05). It was concluded that estradiol can modulate melatonin production in adult rats, and that the changing pattern of aMT.6S excretion throughout the estrous cycle may provide a basis for a functional relationship between pineal activity and reproduction in this species.

Effects of melatonin on estrogen receptor expression in the forebrain of outbred (Lak.LVG) golden hamsters

Studies have shown that the pineal gland via its hormone, melatonin, induces the involution of male and female reproductive systems in seasonally reproducing animals. Melatonin has direct inhibitory effects on both hypothalamic and pituitary functions, which are also exquisitely sensitive to the feedback effects of estradiol. Since melatonin can modulate estrogen receptor (ER) expression in other tissues, immunocytochemical and ribonuclease protection analyses were used to examine the effects of 12 weeks of daily late afternoon injections of melatonin on ER protein and mRNA levels in the hypothalamus of Lak.LVG golden hamsters. Significant decreases in ER-immunoreactivity were noted in the medial preoptic area (MPOA) and bed nucleus of the stria terminalis (BNST) in response to melatonin, while other hypothalamic areas which express ER, e.g. the anterior hypothalamus, showed less dramatic changes. Hypothalamic ER mRNA was decreased in response to melatonin in both intact and ovariectomized animals by 25%. In intact, cycling female hamsters, there was a significant reduction in uterine weight after melatonin treatment. These results suggest that melatonin exerts its anti-reproductive effects in hamsters by modulating ER levels in neurons of the MPOA and BNST, thereby influencing steroid feedback mechanisms.

The ovine pars tuberalis does not appear to be targeted by melatonin to modulate luteinizing hormone secretion, but may be important for prolactin release

The pineal hormone, melatonin, transduces photoperiodic information to the neuroendocrine axis of seasonally breeding mammals to regulate reproduction. It is not known where or how melatonin achieves this effect, but the recent identification of the pars tuberalis (PT) as the area with the highest density of melatonin binding sites suggests that this pituitary subdivision may be an important target for the actions of this indoleamine on luteinizing hormone (LH) and prolactin release. The present study was designed to test this hypothesis. Ovariectomized oestradiol-implanted ewes were exposed to inhibitory long days for 85 days and then received melatonin micro-implants (Day 0) in the mediobasal hypothalamus (MBH; n = 7) or PT (Melatonin-PT; n = 5). The effect of these micro-implants was compared to ewes receiving empty micro-implants in the PT (Sham-PT; n = 5). For LH, bi-weekly jugular blood samples were collected and for prolactin, samples were collected every 20 min for 5 h, with the first hour discarded, on Days -4, 26 and 69. Melatonin implanted in the MBH stimulated LH secretion in 3 ewes by Day 46 +/- 0 after implantation, and one ewe by Day 67 after implantation. In contrast, no Melatonin-PT or Sham-PT ewes exhibited an increase in LH secretion by the end of the study (Day 70). A subsequent experiment, in which the Sham-PT ewes were implanted with melatonin both subcutaneously and in the PT showed that the micro-implants did not impair the ability of the ovine reproductive neuroendocrine axis to respond to melatonin.(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.

Differential seasonal regulation of melatonin receptor density in the pars tuberalis and the suprachiasmatic nuclei: a study in the hedgehog (Erinaceus europaeus, L.)

Using quantitative autoradiography, we have studied the seasonal changes of high affinity melatonin receptor density in both the SCN and PT of the hedgehog, a seasonal breeder and an hibernator. Animals in 3 different physiological states were studied: sexually active animals, and sexually inactive animals during the hibernation period, being then either euthermic or hypothermic. In sexually active animals, Bmax were 75.8 +/- 7.1 fmol/mg protein in PT and 9.1 +/- 0.5 fmol/mg protein in SCN; and Kd values were: 94 +/- 22 pM in the PT and 101 +/- 15 pM in the SCN. This specific binding was strongly decreased in the PT of sexually inactive animals. Moreover, this decrease was significantly stronger in hypothermic than in euthermic hedgehogs. Saturation studies and Scatchard analysis revealed that the observed decrease in the PT resulted from change in the Bmax but not in the Kd, Bmax values being respectively 56.4 +/- 5.9 and 29.5 +/- 1.9 fmol/mg protein in euthermic and hypothermic sexually at rest animals. In none of the different physiological states, did the density of melatonin receptors of the SCN show any changes, Bmax values being respectively 9.8 +/- 0.5 and 9.8 +/- 0.4 fmol/mg protein in euthermic and hypothermic sexually at rest animals. This shows for the first time a tissue-specific regulation of melatonin receptor density occurring in the PT but not in the SCN. Furthermore, this decrease of binding in the PT is correlated with both sexual inactivity and hibernation period. This strongly suggests that the mediation of the photoperiodic effect on seasonal functions like seasonal hypothermia and reproduction involves an effect of melatonin on the PT rather than on the SCN.

The timed infusion paradigm for melatonin delivery: what has it taught us about the melatonin signal, its reception, and the photoperiodic control of seasonal responses?

This review summarizes the evidence showing that the duration of the nocturnal secretory profile of pineal melatonin (MEL) is critical for eliciting seasonally appropriate reproductive physiological and behavioral responses in mammals. We review experiments using the timed infusion paradigm (TIP) to deliver MEL either systemically or centrally to pinealectomized hamsters and sheep. In this paradigm, MEL is infused, usually once daily, for a specific number of hours and at a predetermined time of day. This experimental strategy tests most directly those features of the MEL signal that are necessary to trigger photoperiodic responses. The data suggest that the duration of the MEL stimulation is the critical feature of the MEL signal for both inhibitory and stimulatory effects of the hormone on the photoperiodic control of reproductive development in juvenile Siberian hamsters, and for the photoperiodic control of reproductive and metabolic responses in adult Siberian and Syrian hamsters and sheep. The use of the TIP reveals the importance of the frequency of the signal presentation of MEL and suggests the importance of a period of low-to-absent circulating concentrations of the hormone. The TIP also reveals that the characteristics of the MEL signal that regulate male sexual behavior are similar to those that are critical for reproductive and metabolic responses in Syrian hamsters. We summarize the locations of possible functional MEL target sites identified by combining the TIP with traditional brain lesion techniques. Evidence from such studies suggests that the integrity of the suprachiasmatic nucleus (SCN) region in Siberian hamsters and the anterior hypothalamus in Syrian hamsters is necessary for the response to short-day MEL signals. The TIP has been used to deliver MEL to putative target sites for the hormone in the brain of juvenile and adult Siberian hamsters. The results of these preliminary experiments suggest that the regions of specific MEL binding in this species, especially the SCN, are effective sites where MEL may stimulate short-day-type responses. In contrast, results from intracranial application of MEL in sheep suggest the medial basal hypothalamus as a critical site of action. Finally, we also discuss potential applications of the TIP for identification of brain MEL target sites, understanding of other photoperiodic phenomena and responses, and resolution of the cellular/molecular basis underlying the reception and interpretation of MEL signals.(ABSTRACT TRUNCATED AT 400 WORDS)

Central sites mediating reproductive responses to melatonin in juvenile male Siberian hamsters

Juvenile male Siberian hamsters received infusions of varying doses of melatonin (MEL), or saline vehicle, via microdialysis probes implanted in brain regions which have previously been shown to contain MEL receptors. Daily infusions were 10 h in length and occurred during exposure to constant light on days 22-34 of age. All animals were sacrificed on day 35 and paired testis weights recorded prior to preparation of the brain tissue for histological evaluation of the infusion site. Some animals were also blood-sampled prior to sacrifice for determination of circulating levels of prolactin (PRL). Saline infusions did not have a significant effect upon gonadal maturation, regardless of the infusion site, when compared with unoperated control animals reared under similar photoperiod conditions. In contrast, animals which received infusions of 75 pg MEL into the suprachiasmatic nucleus (SCN), paraventricular nucleus of the thalamus, or nucleus reuniens regions, showed a marked inhibition of gonadal growth. Infusions of this dose of MEL into various other neural regions (e.g. lateral hypothalamus, ventromedial nucleus of the hypothalamus, paraventricular nucleus of the hypothalamus) did not result in decreased testis weights at the time of sacrifice. Daily administration of 20 pg MEL inhibited gonadal maturation and resulted in decreased circulating PRL levels only when infused into the SCN region. For animals receiving the 7.5 pg dose, infusions into the midline thalamic nuclei were not successful in inhibiting testis growth, and infusions in the SCN region had only a marginal effect.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of the pineal gland on migraine and cluster headaches and effects of treatment with picoTesla magnetic fields

For over half a century the generally accepted views on the pathogenesis of migraine were based on the theories of Harold Wolff implicating changes in cerebral vascular tone in the development of migraine. Recent studies, which are based on Leao's concept of spreading depression, favor primary neuronal injury with secondary involvement of the cerebral circulation. In contrast to migraine, the pathogenesis of cluster headache (CH) remains entirely elusive. Both migraine and CH are cyclical disorders which are characterised by spontaneous exacerbations and remissions, seasonal variability of symptoms, and a relationship to a variety of environmental trigger factors. CH in particular has a strong circadian and seasonal regularity. It is now well established that the pineal gland is an adaptive organ which maintains and regulates cerebral homeostasis by "fine tuning" biological rhythms through the mediation of melatonin. Since migraine and CH reflect abnormal adaptive responses to environmental influences resulting in heightened neurovascular reactivity, I propose that the pineal gland is a critical mediator in their pathogenesis. This novel hypothesis provides a framework for future research and development of new therapeutic modalities for these chronic headache syndromes. The successful treatment of a patient with an acute migraine attack with external magnetic fields, which acutely inhibit melatonin secretion in animals and humans, attests to the importance of the pineal gland in the pathogenesis of migraine headache.

Melatonin binding sites in the ovine brain and pituitary: characterization during the oestrous cycle

The distribution of putative melatonin receptors in the sheep has been investigated using in vitro autoradiography and the high affinity, high specific activity ligand 2-[(125) l]iodomelatonin. A wide distribution of specific labelling was found in both the ovine brain and pituitary gland as previously reported. Several novel areas of binding were also identified in the present study, including a fine layer of labelling at the medial edge of the diagonal band of Broca, the trigeminal nucleus, laminae II and III of the substantia gelatinosa, the molecular layer of the cerebellum as well as a scattered labelling in the pars distalis of the pituitary. There was no evidence of specific labelling in any of the peripheral tissues examined. Characterization studies performed on both neuronal and pituitary melatonin binding sites revealed that binding was time- and temperature-dependent and reversible on addition of 1 μM melatonin. The binding of 2-[(125) l]iodomelatonin was also competitively inhibited by increasing concentrations of 2-iodomelatonin and melatonin. The inhibition constants (K(i) ) estimated for each of these substances were similar for both neuronal and pituitary sites. Saturation studies also revealed similarities between neuronal and pituitary tissues with 2-[(125) l]iodomelatonin binding specifically to a single class of high affinity binding sites. Values for equilibrium constants (K(d) ) were within a range of 28 to 48 pM, and values were found to be not significantly different amongst the four regions of the brain investigated and the pars tuberalis of the pituitary. In contrast, the concentration of 2-[(125) l]iodomelatonin binding sites (B(max) ) ranged from 3 to 218fmol/mg protein and were maximal for the pars tuberalis. Saturation studies on brain and pituitary tissues taken from ewes killed either on the day of oestrus or during the luteal phase of the oestrous cycle, indicated that no differences exist in the affinity or concentration of 2-[(125) l]iodomelatonin binding in any region between the two times of the cycle investigated.

The pineal gland and the menstrual cycle

The menstrual cycle reflects the expression of a cyclical process involving the interaction between the hypothalamic-pituitary axis and the ovaries. This complex process requires an integrated neural and humoral control mechanism. It is now well established that a hypothalamic "transducer" located in the medial basal hypothalamus integrates neural and humoral information and translates it into an oscillatory signal which eventually results in the release of the gonadotropin releasing hormone (GnRH), triggering the secretion of gonadotropins from the pituitary gland. Recent animal studies indicate that melatonin influences the functions of the hypothalamic-pituitary-gonadal axis by modifying the firing frequency of the hypothalamic GnRH pulse generator. Consequently, the pineal gland, through the action of melatonin, may exert an important modulatory effect on the mechanisms controlling menstrual cyclicity. Furthermore, abnormal melatonin functions may be involved in the pathogenesis of several disorders of the menstrual cycle including some forms of hypothalamic amenorrhea such as exercise and malnutrition-induced amenorrhea. Consideration of pineal melatonin functions provides a new dimension into the understanding of the neuroendocrine mechanisms governing the cyclical phenomena of the female reproductive system.

Inhibition of sexual maturation in male rats by melatonin: evidence linking the mechanism of action to changes in the regulation of hypothalamic neuropeptide y

Activation of gonadotrophin-releasing hormone (GnRHJ pathways is a pivotal event in the process of sexual maturation, however the regulatory influences that precipitate this change and lead to the onset of puberty remain poorly understood. Recent studies indicate that neuropeptide Y (NPY) may participate in the regulation of luteinizing hormone secretion by modulating the pattern of GnRH secretion and by directly altering the pituitary responsiveness to GnRH stimulation. To determine whether NPY plays a role in puberty-associated changes in hypothalamic function, levels of NPY-like immunoreactivity (NPY-IR) were measured in a fragment of the hypothalamus encompassing the median eminence and medial portion of the arcuate nucleus (ME-AN), and also in the remainder of the hypothalamus from male rats of different ages. To identify changes in hypothaiamic NPY linked to the process of sexual development, the effect of delaying sexual maturation by daily afternoon administration of 100 μg melatonin (MT) from 20 to 40 days was investigated. In the hypothalamus and ME-AN, total NPY content increased progressively with age. Expressed as a concentration (fmol/μg extracted protein), peak values for the ME-AN (55.4 ± 7.0) were observed at 30 days of age followed by a decline to lower levels (30.2 ± 1.9) at 40 days. Daily afternoon administration of MT from 20 days of age resulted in significant increases (P<0.01) in the levels of NPY-IR in the ME-AN compared to control values at 30 and 40 days of age. MT was without effect on NPY-IR levels in the remainder of the hypothalamus. When MT was administered in the early morning, a procedure which does not delay sexual maturation, NPY-IR values for the ME-AN region were not different from control rats indicating that the MT-induced changes in NPY were related to the effects on sexual maturation. Using pituitary luteinizing hormone content and seminal vesicle weight as indices of sexual development, significant inverse correlation coefficients (P<0.001) between these parameters and the NPY concentration in the ME-AN were observed (r =-0.79 and -0.70, respectively). From published data it is not possible to conclude whether the main effects of NPY are exerted at the hypothalamic or pituitary level. However, the changes in the NPY content of the ME-AN observed during the onset of puberty, and the influence of MT on these changes, support assertions that NPY is involved in the regulation of sexual maturation.

Melatonin modifies the spontaneous multiunit activity recorded in several brain nuclei of freely behaving rats

Melatonin, a pineal hormone, released photoperiodically, was administered systemically in rats, previously implanted with semimicroelectrodes into six different brain structures. The multiunit electrical activity of these structures was recorded for 10 min before and 60 min after melatonin administration in unanesthetized, freely moving rats. Different melatonin doses (100, 200, 500, and 1000 micrograms/kg) produced changes in the electrical activity of all tested structures. However, amygdala, rostral hypothalamus and mesencephalic reticular formation showed the most important changes. The main effect induced by melatonin was a dose-related decrease of the spontaneous electrical activity. The significance of these effects is discussed within the context of the behavioral and endocrinological effects of melatonin.

Effect of testosterone replacement on the alteration of steroid metabolism in the hypothalamic-preoptic area of male hamsters treated with melatonin

Adult male hamsters were maintained under 14 hours of light per day and randomly assigned to groups that received daily afternoon melatonin (25 micrograms) or vehicle injections. Animals from both groups were killed following 4, 8, and 12 weeks of treatment. By 12 weeks, the melatonin-treated hamsters had significant reductions in the weights of the testes and seminal vesicles, serum testosterone levels, and activities did not differ between groups. In a second experiment, hamsters were hypothalamic-preoptic area (HPOA) aromatase activities. Hypothalamic-preoptic area 5 alpha-reductase activities did not differ between groups. In a second experiment, hamsters were again treated with melatonin or vehicle for 12 weeks prior to being killed. After 10 weeks of treatment, groups of melatonin-treated animals received subcutaneous silastic capsules (5, 10, or 20 mm) filled with testosterone. Animals in two other groups were given blank implants or no implants at all. Two weeks later, at autopsy, reproductive organ weights, serum testosterone levels, and HPOA aromatase activities were significantly suppressed by melatonin administration. 5 alpha-Reductase activity in the HPOA was not affected. Hamsters that had been given the 10- and 20-mm testosterone implants exhibited normal seminal vesicle weights and HPOA aromatase activities. These results suggest that melatonin-induced reduction of HPOA aromatase activity is mediated by decreased circulating levels of testosterone.

Melatonin inhibitory effect on luteinizing hormone release is potentiated after long pretreatment with the indole

The effect of melatonin on luteinizing-hormone releasing-hormone stimulation of cyclic AMP accumulation and luteinizing hormone (LH) release from neonatal rat hemipituitaries was studied in vitro. Melatonin inhibited LH-release from pituitaries of animals kept previously on long but not on short photoperiods; cyclic AMP accumulation was, however, inhibited on both photoperiods. There were no daily changes in the melatonin effect. Inhibition of LH-release was strongly potentiated after 6 h preincubation with melatonin as compared with 20 min preincubation; cyclic AMP was not significantly affected by the length of preincubation.

Neuroendocrine rhythms

Hormones are secreted with circhoral, circadian and seasonal periodicities. Circhoral pulsatility is a temporal code, many chronic and acute changes in neuroendocrine status being mediated by changes in the frequency of circhoral release. The identity of the neuronal circuits controlling circhoral release is not known. Circadian release of hormones occurs with a precise temporal order entrained to the light-dark cycle, synchronized to the activity/rest rhythm and generated by circadian oscillators, of which the suprachiasmatic nuclei are the most important. Seasonal rhythms are driven either by an endogenous circannual clock mechanism or by a process of photoperiodic time measurement which is dependent upon the duration of the nocturnal peak of the pineal hormone melatonin.

Neurophysiological responses to melatonin in the SCN of short-day sensitive and refractory hamsters

The pineal hormone melatonin plays a central role in the regulation of seasonal reproductive cycles in mammals and several studies have implicated the suprachiasmatic nucleus (SCN) as a target on which melatonin acts. The Syrian hamster is a long-day breeder which exhibits gonadal regression when housed in short (less than 12.5 h) daily photoperiods or injected daily with melatonin in long photoperiods. In the present paper we address the question whether melatonin affects firing rates of SCN neurones and whether these effects change as the animals become refractory to short photoperiods. In long-day (LD14:10) hamsters SCN neurones were suppressed (31%), activated (15%) or unaffected (54%) by melatonin. In contrast, there was an increased proportion of melatonin insensitive cells (88%) in short-day (refractory) hamsters. Melatonin-responsive cells were found primarily during the late projected day and early projected night in both long-day and short-day animals. This reduced responsiveness of SCN neurones to melatonin in hamsters refractory to short-day exposure may represent part of the mechanism underlying the development of gonadal refractoriness and the onset of gonadal growth in anticipation of long spring photoperiods.

Melatonin receptors and signal transduction in melatonin-sensitive and melatonin-insensitive populations of white-footed mice (Peromyscus leucopus)

The pineal hormone melatonin times seasonal alterations in reproductive function in photoperiodic mammals. In white-footed mice, there is variation in responsiveness to the reproductive effects of melatonin between populations originating in different locations; mice from Connecticut (CT) respond normally to melatonin, while mice from Georgia (GA) appear insensitive to melatonin. In the present paper, we compare melatonin receptor distribution and a second messenger response to melatonin in white-footed mice from CT and GA. Specific binding of 125I-labeled melatonin (I-MEL) was observed in a variety of brain regions in each population, but there were no consistent differences in the distribution or intensity of I-MEL binding between the populations. Furthermore, melatonin inhibited forskolin-stimulated cAMP accumulation in median eminence/pars tuberalis explants from both populations. These results suggest that insensitivity to melatonin in GA mice is not due to a gross defect in melatonin receptors or receptor-effector coupling.

Pharmacological characterization of melatonin binding sites in Syrian hamster hypothalamus

The radioligand [125I]iodomelatonin was used to study melatonin binding sites in Syrian hamster hypothalamus and hippocampus. Scatchard analysis revealed a single binding site with nanomolar affinity in hypothalamus (Kd = 1.8 +/- 0.3 nM, Bmax = 75 +/- 7 fmol/mg protein; n = 4) and hippocampus (Kd = 2.2 +/- 0.2 nM, Bmax = 49 +/- 5 fmol/mg protein; n = 4). The Kd value calculated from the association and dissociation rate constants in hypothalamus was (k-1/k1) = 2.4 nM. Regional studies revealed that the highest binding of [125I]iodomelatonin occurs in the hypothalamus. Only indoles structurally related to melatonin exhibited significant affinity at this site. Prazosin was found to be a potent inhibitor of [125I]iodomelatonin binding in all brain regions studied. The pharmacological profile of this binding site indicated it to be unique, since serotonergic, dopaminergic and adrenergic drugs (other than prazosin) did not have appreciable affinity for it. Although saturation studies revealed only one binding site, the low Hill coefficients obtained for several inhibitors suggest that [125I]iodomelatonin labels multiple sites (or affinity states) in the hypothalamus.

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

Subnanomolar concentrations of melatonin inhibit cyclic AMP and cyclic GMP accumulation in neonatal rat anterior pituitary stimulated in vitro with luteinizing-hormone releasing-hormone. Melatonin also inhibited forskolin-stimulated cyclic AMP accumulation in pars tuberalis. Inhibition of cyclic AMP accumulation is specific for melatonin, since its analogs N-acetylserotonin and 5-methoxytryptamine are 1000 times less potent. Cyclic nucleotides may thus serve as second messengers transducing the effect of melatonin on cellular level.

Mammalian pineal melatonin: a clock for all seasons

The central role of the pineal gland and its hormone melatonin (MEL) in mammalian photoperiodic responses is discussed in terms of: 1) evidence for the involvement of MEL in photoperiodism, 2) which feature of the MEL secretion profile might be most important for regulating photoperiodic responses, 3) evidence for the modulation of responses to changes in daylength based on previous photoperiod exposure (i.e., photoperiodic history) and 4) how the MEL signal might be processed at its target sites to elicit physiological responses.

Neural systems underlying photoperiodic time measurement: a blueprint

This paper briefly reviews the formal properties of the photoperiodic time measurement apparatus of mammals and presents a hypothetical model for the operation of the neural systems responsible for reading and responding to the nocturnal pineal melatonin signal. The primary melatonin readout mechanism is held to be common to all species responsive to melatonin. It seems likely that this mechanism responds to relative changes in the duration and amplitude of the melatonin signal, rather than the absolute levels of melatonin encountered. A series of neural systems which exploit the calendar information provided by the primary readout is envisaged to vary between and within species, depending upon the neuroendocrine response under consideration. Of particular importance is a mechanism for comparing the relative duration of successive melatonin signals. These more complex elements are responsible for phenomena such as the effects of photoperiodic history and photorefractoriness. The brain may be able to encode an accumulated memory of melatonin signals and thereby define longer term intervals within the annual cycle. A series of response elements within the hypothalamus are engaged by the appropriately processed photoperiodic stimuli. For all elements of this model, their anatomical representations are poorly understood or, in certain cases, completely unknown.

Central melatonin receptors: implications for a mode of action

The influence of melatonin on circadian and photoperiodic functions in numerous species is well documented. It is known that the effect of melatonin on circadian rhythmicity is mediated via the suprachiasmatic nucleus (SCN), the biological clock of the brain. It is not known however where the photoperiodic effects of melatonin are mediated. Evidence from brain lesioning and melatonin implant studies point to a site in or near the medial hypothalamus. In contrast to these studies, melatonin receptors have been reported in widespread areas of the brain, the pituitary and in peripheral tissues. The characteristics of the reported melatonin receptors vary widely between studies and consequently no definitive description of a physiologically relevant melatonin receptor has received universal recognition. This review marshals recent evidence for the localization and characterization of the melatonin receptor and discusses these findings in the context of the known effects of the hormone in different species.

Melatonin Receptor Sites in the Syrian Hamster Brain and Pituitary. Localization and Characterization Using [|]lodomelatonin*

Abstract A high-affinity, discretely localized melatonin receptor has been characterized and mapped within the brain and pituitary of the Syrian hamster using the high specific activity ligand [(125)|]iodomelatonin and a combination of in vitro autoradiography and membrane homogenate receptor assays. Specific binding of radioligand was found in regions of the epithalamus and hypothalamus in the brain and the pars tuberalis of the pituitary. Excitatory amino-acid lesions destroyed [(125)|]iodomelatonin binding within the brain, demonstrating that binding sites are located on neurons. Analysis of [(125)|]iodomelatonin binding to membrane homogenates of the pars tuberalis revealed a linear relationship between specific ligand binding and the amount of tissue. The time-course of specific binding at 37 degrees C reached equilibrium after 30 min and remained stable thereafter. The addition of increasing concentrations of [(125)|]iodomelatonin alone and in the presence of 1 muM melatonin showed that specific binding reached equilibrium at 80 to 100 pM. Analysis of the saturation isotherm using a one-site binding model was consistent with a single receptor site with a K(d) of 29.3 (+/-5.9 SEM) pM and B(max) of 2.54 (+/-0.19 SEM) fmol/mg protein.

Evidence that short photoperiod-induced gonadal regression in the Mongolian gerbil is mediated by the action of melatonin in the medial hypothalamus

The present study was undertaken to examine the effects of exposure to short photoperiod (SD) and treatment with subcutaneous (s.c.) or intrahypothalamic melatonin-containing beeswax implants on reproduction in the Mongolian gerbil Meriones unguiculatus. Exposure of adult female gerbils to SD (8 h light: 16 h dark) caused a significant decrease in weight of the reproductive tract (ovaries, oviducts, uterus and vagina; RTW) compared to animals maintained under stimulatory photoperiod (12 h light: 12 h dark; 81.4 +/- 8.0 mg vs 151.8 +/- 16.0 mg, respectively (P less than 0.01]. Treatment with two large s.c. implants, each containing 3 mg melatonin, mimicked the antigonadal effect of SD (68.7 +/- 3.9 mg vs 118.1 +/- 19.5 mg for the blank (melatonin-free) implant controls; P less than 0.05). Gerbils with a small melatonin-beeswax pellet (containing 0.2 mg melatonin) in the anterior hypothalamus (AH) underwent a significant reduction in RTW compared to gerbils with a blank intrahypothalamic implant (73.0 +/- 5.7 mg vs 134.6 +/- 14.5 mg, respectively; P less than 0.01). Melatonin pellets in the mediobasal hypothalamus (MBH) also induced gonadal regression, but to a lesser degree (88.7 +/- 12.6 mg; P less than 0.05 vs blank controls). Small melatonin pellets placed elsewhere in the hypothalamus or s.c. had little effect on RTW (143.4 +/- 20.9 mg and 129.2 +/- 19.7 mg, respectively). Only 25% of the gerbils with melatonin pellet in the AH or MBH had a corpus luteum compared to 73% of the blank controls (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Short days induce changes in specific melatonin binding in hamster median eminence and anterior pituitary

Autoradiography of 125I-melatonin binding to hamster brain sections revealed a competitive 125I-melatonin binding in median eminence only. Saturation studies on crude membrane fraction revealed high-affinity melatonin binding sites in median eminence (Kd = 59 pM) and anterior pituitary (Kd = 97 pM). In the hamsters maintained on LD 14:10, the concentration of the binding sites was 10.4 and 6.0 fmol/mg protein in median eminence and anterior pituitary respectively; long-term exposure to LD 8:16 decreased the concentration to less than a half.

Presence of a distinct 24-hour melatonin rhythm in the ventricular cerebrospinal fluid of the goat

Melatonin profiles in the cerebrospinal fluid (CSF) of conscious goats were examined under long-day (16L:8D) and short-day (8L:16D) environments. CSF melatonin, collected from the lateral ventricle, showed distinct 24 h rhythms with high concentrations being restricted to the dark phase, which averaged 1,320.6 pg/ml under 16L:8D and 660.6 pg/ml under 8L:16D. On the contrary, the nocturnal rise in CSF melatonin was totally absent in the pineal sympathetically denervated animals. Night interruption by 1 h lighting (about 400 lux at the height of goat's head) resulted in an abrupt decline of CSF melatonin to the basal level within 30 min and immediate recovery to the previous high level after reestablishment of the dark phase. The CSF/plasma ratio was 10.8-18.4 during the dark phase and 1.7-1.8 during the light phase. CSF and plasma melatonin levels were also examined after exogenous melatonin given either peripherally or intraventricularly. Continuous subcutaneous infusion of melatonin (5 micrograms/h) maintained melatonin levels in the plasma about 3 times higher than that in the CSF during its application. On the other hand, an intraventricular injection of 2 micrograms melatonin elevated plasma melatonin by 100 pg/ml within 1 min. These results indicate that turnover of CSF melatonin is fairly rapid and favor a hypothesis for direct access of pineal melatonin to the brain ventricular system in the goat.

Antigonadal activity of the melatonin analogs 2-iodomelatonin and 2-chloromelatonin in the juvenile Djungarian hamster, Phodopus sungorus campbelli

The antigonadal effects of daily (20 micrograms, s.c.) injection of melatonin and two analogs, 2-iodomelatonin and 2-chloromelatonin, were compared in juvenile Djungarian hamsters housed under long photoperiod (L:D 16:8). Melatonin, 2-iodomelatonin, and 2-chloromelatonin injected 3 h before lights off for 16 days (17-34 days of age) significantly inhibited testis growth compared to vehicle-injected hamsters. In addition, melatonin and both analogs significantly reduced body weight gain. These 2-substituted analogs appear to be melatonin agonists with a potency in vivo similar to the parent compound, melatonin.

Melatonin and pituitary-gonadal function in disorders of eating behavior

In order to study the possible relationships between melatonin secretion and pituitary-gonadal function, the circadian rhythm of plasma melatonin, the basal levels of estradiol-17beta and testosterone and the luteinizing hormone (LH) and follicle-stimulating hormone (FSH) response to luteinizing hormone-releasing hormone (LH-RH) stimulation were evaluated in normally cycling healthy women and in two groups of women with menstrual dysfunctions related to eating disorders (19 patients with anorexia nervosa and 16 with primary obesity). The circadian rhythm of plasma melatonin reached statistical significance in anorectic patients but not in obese patients. The mean 24 h melatonin level was significantly higher in anorectic than in obese patients and in control subjects. However, both groups of patients shared some abnormalities of melatonin circadian pattern, such as increased ratio between day and night melatonin levels, abnormal secretory peaks during the light hours and great interindividual variability for timing, amplitude, and duration of melatonin nocturnal peak. A selective impairment of LH secretion was observed in both anorectic and obese patients. By considering together the two groups of patients and controls, a linear inverse correlation between the circadian mesor of plasma melatonin and the basal and LH-RH stimulated LH levels was found. The persistence of a certain melatonin secretion during the light hours in both anorectic and obese patients could play an inhibitory role on the pituitary gonadal function in these subjects.

Effects of melatonin and thyroxine treatment on reproductive organs and thyroid hormone levels in male hamsters

The effects of exogenous thyroid hormone administration on melatonin-induced gonadal atrophy were studied in young adult male hamsters. Animals were given daily afternoon injections of 25 micrograms melatonin and/or thrice-weekly injections of 7 micrograms thyroxine. A control group received injections of saline vehicle only. No significant effect of either treatment was seen on body weight. Melatonin-treated animals had significantly reduced testicular and seminal vesicle weights, compared with vehicle-treated control animals: serum T4 levels and the FT4I were reduced by melatonin treatment, but serum T3 and the FT3I were increased. No effect of melatonin was seen on in vitro T3 uptake. Thyroxine treatment alone, while leading to elevated serum T4 levels and FT4I, had no effect on T3, the FT3I, or T3 uptake. Thyroxine treatment normalized circulating T4 levels and the FT4I in melatonin-treated animals, without preventing the melatonin-induced testicular regression. The results are taken as evidence that the inhibitory effects of melatonin on gonadal and thyroid function are independently mediated.

The electrophysiological effects of melatonin and a putative melatonin antagonist (N-acetyltryptamine) on rat suprachiasmatic neurones in vitro

Several studies have implicated the suprachiasmatic nuclei (SCN) as a target for the action of melatonin in its regulation of seasonal and circadian behaviour. Single-unit activity from the SCN and adjacent paraventricular area and anterior hypothalamus was recorded using the in vitro rat hypothalamic slice preparation. Neurones were tested for responses to iontophoresed or pressure ejected melatonin (n = 62) and serotonin (5-HT, n = 80). The majority (n = 24-26) of melatonin-sensitive SCN neurones were inhibited by melatonin in a dose-dependent manner during the latter portion of the projected light phase of the circadian light-dark cycle. A putative melatonin antagonist, N-acetyltryptamine, exhibited concentration dependent mixed agonist-antagonist effects on melatonin-evoked responses.

Impact of circulating testosterone on iodomelatonin binding sites in the male rat brain

The effects of castration and subsequent testosterone and estradiol treatment and of a single injection of ethylene-1,2-dimethanesulphonate (EDS) on the distribution of [2-125I]iodomelatonin ([ 125I]melatonin) binding sites in the male rat brain were investigated. Castration produced a marked testosterone-reversible decrease in [125I]melatonin binding in the male rat brain, particularly in the hypothalamus and hippocampus. In contrast, [125I]melatonin binding in the parietal cortex, medulla-pons and cerebellum was generally unaffected by castration. Estradiol did not reverse the effect of castration on [125I]melatonin binding. A single injection of EDS which causes the destruction of Leydig cells led to a marked decrease in [125I]melatonin binding in the brain of the rats between 3 and 7 days after treatment. This decrease correlated with the decline in serum concentrations of testosterone. Specific [125I]melatonin binding and serum concentrations of testosterone subsequently increased to control levels within 37 days after treatment in accord with the repopulation of Leydig cells. The results clearly show that testosterone regulates the density of melatonin receptors in the hypothalamus and hippocampus of the male rat.