Melatonin receptors: current status, facts, and hypotheses

Great progress has been made in the identification of melatonin binding sites, commonly identified as melatonin receptors by many authors, in recent years. The bulk of these studies have investigated the sites using either autoradiographic and biochemical techniques with the majority of the experiments being done on the rat, Djungarian and Syrian hamster, and sheep, although human tissue has also been employed. Many of the studies have identified melatonin binding in the central nervous system with either tritium- or iodine-labelled ligands. The latter ligand seems to provide the most reproducible and consistent data. Of the central neural tissues examined, the suprachiasmatic nuclei are most frequently mentioned as a location for melatonin binding sites although binding seems to be widespread in the brain. The other tissue that has been prominently mentioned as a site for melatonin binding is the pars tuberalis of the anterior pituitary gland. There may be time-dependent variations in melatonin binding densities in both neural and pituitary gland tissue. Very few attempts have been made to identify melatonin binding outside of the central nervous system despite the widespread actions of melatonin. Preliminary experiments have been carried out on the intracellular second messengers which mediate the actions of melatonin.     

Ancestral TSH mechanism signals summer in a photoperiodic mammal

In mammals, day-length-sensitive (photoperiodic) seasonal breeding cycles depend on the pineal hormone melatonin, which modulates secretion of reproductive hormones by the anterior pituitary gland [1]. It is thought that melatonin acts in the hypothalamus to control reproduction through the release of neurosecretory signals into the pituitary portal blood supply, where they act on pituitary endocrine cells [2]. Contrastingly, we show here that during the reproductive response of Soay sheep exposed to summer day lengths, the reverse applies: Melatonin acts directly on anterior-pituitary cells, and these then relay the photoperiodic message back into the hypothalamus to control neuroendocrine output. The switch to long days causes melatonin-responsive cells in the pars tuberalis (PT) of the anterior pituitary to increase production of thyrotrophin (TSH). This acts locally on TSH-receptor-expressing cells in the adjacent mediobasal hypothalamus, leading to increased expression of type II thyroid hormone deiodinase (DIO2). DIO2 initiates the summer response by increasing hypothalamic tri-iodothyronine (T3) levels. These data and recent findings in quail [3] indicate that the TSH-expressing cells of the PT play an ancestral role in seasonal reproductive control in vertebrates. In mammals this provides the missing link between the pineal melatonin signal and thyroid-dependent seasonal biology.     

Pinealectomy increases and exogenous melatonin decreases leptin production in rat anterior pituitary cells: an immunohistochemical study

Melatonin, the main hormone of the pineal gland, informs the body about the environmental light and darkness regimen, which in turn contributes to the photoperiodic adaptation of several physiological functions. Leptin, the hormone secreted mainly by adipocytes and some other tissues including the pituitary, informs the brain about the mass of adipose tissue, which plays an important role in energy homeostasis. Melatonin has been shown to decrease circulating leptin levels. It is currently not known whether melatonin has an effect on leptin synthesis in the pituitary. The aim of this study was to immunohistochemically examine the effects of pinealectomy and administration of melatonin on leptin production in the rat anterior pituitary. The pituitary samples obtained from 18 male Wistar rats including sham-pinealectomized, pinealectomized and melatonin-injected pinealectomized groups were immunohistochemically evaluated. Immunostaining of leptin was moderate (3+) in sham-pinealectomized rats, heavy (5+) in pinealectomized rats and low (1+) in melatonin-treated pinealectomized rats, respectively. The present results indicate that pinealectomy induces leptin secretion in anterior pituitary cells, and this increase of leptin synthesis can be prevented by administration of melatonin. Thus, melatonin seems to have both physiological and pharmacological effects on leptin production in the anterior pituitary of male rats.     

Molecular characterization of the long-day response in the Soay sheep, a seasonal mammal

In mammals, seasonal timekeeping depends on the generation of a nocturnal melatonin signal that reflects nightlength/daylength. To understand the mechanisms by which the melatonin signal is decoded, we studied the photoperiodic control of prolactin secretion in Soay sheep, which is mediated via melatonin responsive cells in the pars tuberalis of the pituitary. We demonstrate that the phases of peak expression of the clock genes Cryptochrome1 (Cry1), Period1 (Per1), and RevErbalpha respond acutely to altered melatonin secretion after a switch from short to long days. Cry1 is activated by melatonin onset, forming the dusk component of the molecular decoder, while Per1 expression at dawn reflects the offset of melatonin secretion. The Cry1-Per1 interval immediately adjusts to the melatonin signal on the first long day, and this is followed within 24 hr by an increase in prolactin secretion. The timing of peak RevErbalpha expression also responds to a switch to long days due to altered melatonin secretion but does not immediately reset to an entrained long-day state. These data suggest that effects of melatonin on clock gene expression are pivotal events in the neuroendocrine response and that pars tuberalis cells can act as molecular calendars, carrying a form of "photoperiodic memory."     

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.     

Effects of melatonin on dopamine metabolism in the hypothalamus and the pituitary of the rainbow trout, Oncorhynchus mykiss

The present paper discusses the effect of a single melatonin treatment (0.5 mg/kg, i.p.) on the dopaminergic metabolism in the hypothalamus and pituitary of the rainbow trout. The effects of exogenous melatonin on dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) contents were compared with the variations in the content of these catecholamines associated to the natural increase in the endogenous melatonin from daytime (3 hr before lights off) to nighttime (3 hr after lights off). Animals treated with melatonin showed a rapid (maximal values at 30 min post-injection) and relatively sustained rise in plasma melatonin levels, which reached supraphysiological ranges. The increase in circulating melatonin was accompanied by a reduction in the amount of DOPAC in both the hypothalamus (30, 60, and 120 min after i.p. melatonin) and the pituitary (120 min after i.p. melatonin) as well as in the pituitary DOPAC/DA ratio (60 and 120 min after i.p. melatonin). Similarly, the increase in circulating melatonin levels from the daytime to nighttime was associated with decreases in the contents of DOPAC in both the hypothalamus and pituitary and in the DOPAC/DA ratio in the pituitary. These data suggest that the inhibition of the hypothalamic-pituitary dopaminergic metabolism may be a specific mechanism of melatonin action in the trout brain that might operate following changes in the secretion of the hormone from the pineal gland.     

Mode of action and functional significance of avian gonadotropin-inhibitory hormone (GnIH): a review

Neuropeptide control of gonadotropin secretion at the level of the anterior pituitary gland is primarily through the stimulatory action of the hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH). However, a hypothalamic neuropeptide acting at the level of the pituitary to negatively regulate gonadotropin secretion has, until recently, remained unknown in any vertebrate. In 2000, we discovered a novel hypothalamic neuropeptide inhibiting gonadotropin release at the level of the pituitary in quail and termed it gonadotropin-inhibitory hormone (GnIH). A gonadotropin-inhibitory system is an intriguing concept and provides us with an unprecedented opportunity to study the regulation of avian reproduction from an entirely novel standpoint. To elucidate the mode of action of GnIH, we further identified the receptor for GnIH and characterized its expression and binding activity in quail. The identified GnIH receptor possessed seven transmembrane domains and specifically bound to GnIH in a concentration-dependent manner. The expression of GnIH receptor was found in the pituitary and several brain regions including the hypothalamus. These results suggest that GnIH acts directly on the pituitary via GnIH receptor to inhibit gonadotropin release. GnIH may also act on the hypothalamus to inhibit GnRH release. To understand the functional significance of GnIH in avian reproduction, we also investigated the mechanism that regulates GnIH expression. Interestingly, melatonin induced dose-dependently GnIH expression and melatonin receptor (Mel(1c)) was expressed in GnIH neurons. Thus melatonin appears to act directly on GnIH neurons via its receptor to induce GnIH expression. Based on these studies, GnIH is likely an important neuropeptide for the regulation of avian reproduction.     

Role de la glande pineale dans la regulation de l’axe hypothalamo-hypophyso-testiculaire

The role of the pineal gland and of its main hormone, melatonin, has not yet been clearly defined. The best known activity of the gland is antigonadal, at least in the experimental animals. In fact, the administration of melatonin or the modification of the light/darkness ratio, that is considered the most important regulatory mechanism of the melatonin-synthetic activity, leads to a gonadal regression by which all the morphological and hormonal parameters are modified. Such a mechanism is not operating in humans. However, data exist indicating a role for the pineal in this species. Our experimental data and those of the litterature indicate that the action of melatonin seems to be principally at the hypothalamic level, even though a direct action at the pituitary and gonadal levels cannot be excluded     

Effects of diazepam and its metabolites on nocturnal melatonin secretion in the rat pineal and Harderian glands. A comparative in vivo and in vitro study

We investigated the effects of diazepam (DZP) and its three metabolites: nordiazepam (NZP), oxazepam (OZP), and temazepam (TZP) on pineal gland nocturnal melatonin secretion. We looked at the effects of benzodiazepines on pineal gland melatonin secretion both in vitro (using organ perifusion) and in vivo in male Wistar rats sacrificed in the middle of the dark phase. We also examined the effects of these benzodiazepines on in vivo melatonin secretion in the Harderian glands. Neither DZP (10^-5-10^-6 M) nor its metabolites (10^-4-10^-5 M) affected melatonin secretion by perifused rat pineal glands in vitro. In contrast, a 10^-4 M suprapharmacological concentration of DZP increased melatonin secretion of perifused pineal glands by 70%. In vivo, a single acute subcutaneous administration of DZP (3 mg/kg body weight) significantly affected pineal melatonin synthesis and plasma melatonin levels, while administration of the metabolites under the same conditions did not. DZP reduced pineal melatonin content (-40%), N-acetyltransferase activity (-70%), and plasma melatonin levels (-40%), but had no affects on pineal hydroxyindole-O-methyltransferase activity. Neither DZP nor its metabolites affected Harderian gland melatonin content. Our results indicate that the in vivo inhibitory effect of DZP on melatonin synthesis is not due to the metabolism of DZP. The results also show that the control of melatonin production in the Harderian glands differs from that observed in the pineal gland.     

Morphofunctional and molecular bases of pineal gland aging

The review analyzed morphology, molecular and functional aspects of pineal gland aging and methods of it correction. The pineal gland is central organ, which regulates activity of neuroimmunoendocrine, antioxidant and other organisms systems. Functional activity of pineal gland is discreased at aging, which is the reason of melatonin level changing. The molecular and morphology research demonstrated, that pineal gland hadn't strongly pronounced atrophy at aging. Long-term experience showed, that peptides extract of pineal gland epithalamin and synthetic tetrapeptide on it base epithalon restored melatonin secretion in pineal gland and had strong regulatory activity at neuroimmunoendocrine and antioxidant organism systems.     

Importance of photoperiodic signal quality to entrainment of the circannual reproductive rhythm of the ewe

An endogenous circannual rhythm drives the seasonal reproductive cycle of a broad spectrum of species. This rhythm is synchronized to the seasons (i.e., entrained) by photoperiod, which acts by regulating the circadian pattern of melatonin secretion from the pineal gland. Prior work has revealed that melatonin patterns secreted in spring/summer entrain the circannual rhythm of reproductive neuroendocrine activity in sheep, whereas secretions in winter do not. The goal of this study was to determine if inability of the winter-melatonin pattern to entrain the rhythm is due to the specific melatonin pattern secreted in winter or to the stage of the circannual rhythm at that time of year. Either a summer- or a winter-melatonin pattern was infused for 70 days into pinealectomized ewes, centered around the summer solstice, when an effective stimulus readily entrains the rhythm. The ewes were ovariectomized and treated with constant-release estradiol implants, and circannual cycles of reproductive neuroendocrine activity were monitored by serum LH concentrations. Only the summer-melatonin pattern entrained the circannual reproductive rhythm. The inability of the winter pattern to do so indicates that the mere presence of a circadian melatonin pattern, in itself, is insufficient for entrainment. Rather, the characteristics of the melatonin pattern, in particular a pattern that mimics the photoperiodic signals of summer, determines entrainment of the circannual rhythm of reproductive neuroendocrine activity in the ewe.     

Melatonin: perspectives in the life sciences

The pineal gland hormone melatonin is now considered an important neuroendocrine component of animal physiology. Although the functional status of melatonin has been well described for subhuman species, there is a paucity of data concerning the physiological role of this hormone in man. This paucity of data has much to do with the limitations of experimental design imposed by the practical and ethical difficulties associated with the study of a nocturnally secreted hormone. The recent advent of salivary melatonin assay has provided a very practical means of monitoring melatonin secretion in long-term longitudinal type community based studies of pineal gland function in human health and disease. The efforts to describe key chronobiological changes in melatonin secretion of possible functional significance have been accompanied by a seemingly less enthusiastic search to describe the nature of the melatonin receptor, another highly important component of the 'melatonin message'. The functional relevance of specific chronobiological changes in melatonin secretion cannot be completely understood without an increased knowledge of melatonin action at the receptor level. The present work describes the recent methodological advance in the investigation of human pineal gland physiology represented by salivary melatonin assay, and discusses the present status of our knowledge of the melatonin receptor.     

Melatoninergic receptor agonists and antagonists: therapeutic perspectives

The chronobiotic neurohormone melatonin, synthetized in the pineal gland during darkness periods governs the circadian and seasonal biological rhythms. Physiologically, melatonin regulates the sleep/activity alternance, together with the circadian cycle of body temperature and cortisol secretion, and influences various immune, endocrine and metabolic functions. Dysfunction of the endogenous melatonin secretion is associated with mood and behavioral disorders including body weight. Patients with severe depression exhibit desynchronized and reduced melatonin secretion, in parallel with marked sleep disturbances whereas exogenous melatonin administration and antidepressive drugs restore melatonin secretion. A dysregulated melatonin secretion is also observed in obese subjects. Implication of melatonin in these disorders stimulated the search for melatonin analogues with enhanced antidepressive and body weight control effects. The melatoninergic agonist S 20098, or agomelatin, disclosed a potent antidepressive and anxiolytic activity in preclinical studies, which was confirmed in clinical trials in patients with major depression. The antagonist S 20928 was shown to limit seasonal weight gain in an hibernating rodent model. Thus, development of melatoninergic agonists and antagonists appear as an innovative approach in the treatment of depression and obesity, two major public health problems.     

Effects of inhibition of thyroid function and of cold on melatonin synthesis and porphyrin content in the Harderian glands of male Syrian hamsters, Mesocricetus auratus

1. Indole metabolism and porphyrin content of the Harderian glands of the male Syrian hamster were measured as functions of drug-induced hypothyroidism and exposure to cold conditions. 2. Harderian gland N-acetyltransferase (NAT) activity was reduced from control levels by hypothyroidism induced by methimazole; exposure to cold had no effect on NAT activity. 3. Immunoreactive melatonin in the Harderian glands was unaffected by the state of thyroid secretion. However, immunoreactive melatonin content declined after 180 and 270 min, at 4 degrees C, suggesting that Harderian gland melatonin may be involved in thermoregulation. 4. Porphyrin content of the Harderian glands was not affected by either thyroid secretion or cold.     

Peripheral and CNS Effects of the Pineal Gland: Target Enzymes Common to Tissues and Species

The pineal gland is an important transducer mediating environmentalinfluences on endocrine organs. It has direct effects on thehypothalamus and pancreas and indirect effects on the pituitary,adrenal, thyroid, and testes. Thus, specific endocrine rhythmsare affected by alterations in trophic hormone secretion bythe hypothalamus-pituitary complex, direct effects on biochemicaltransformations within target organs, or by alterations in metabolismand excretion of hormones by the liver. Target enzymes commonto the above organs for melatonin and arginine vasotocin (AVT)are 5-reductase, monoamine oxidase (MAO), and smooth muscleenzymes. Melatonin selectively inhibited 17rß ol-dehydrogenaseactivity and 5-reductase activity while 17rß ol-dehydrogenasewas stimulated by serotonin (5-HT). Other steroid biotransformationswere inhibited by both 5-HT and melatonin. Evidence from thepancreas and insulin secretion, liver and glucuronosyl transferaseactivity, and hypothalamic and pituitary studies indicate thatmelatonin mediated some of its effects on these organs throughMAO activity and 5-HT levels. There were some species and tissuedifferences with respect to the effects of melatonin and AVTon MAO activity and steroid biotransformations. Melatonin stimulatedsteroid biotransformations in the duck, while MAO activity and5-reductase activity in the hamster responded differently tomelatonin than did similar preparations from the rat.     

Effect of brefeldin A on melatonin secretion of chick pineal cells

Melatonin is secreted from the pineal gland in a circadian manner. It is well established that the synthesis of melatonin shows a diurnal rhythm reflecting a daily change in serotonin N-acetyltransferase (NAT) activity, and the overall secretion of melatonin requires a cellular release process, which is poorly understood. To investigate the possible involvement of Golgi-derived vesicles in the release, we examined the effect of brefeldin A (BFA), a reversible inhibitor of Golgi-mediated secretion, on melatonin secretion of cultured chick pineal cells. We show here that treatment with BFA completely disassembles the Golgi apparatus and reduces melatonin secretion. In more detailed time course experiments, however, the inhibition of melatonin secretion is only observed after the removal of BFA in parallel with the reassembly of the Golgi apparatus. This inhibition of melatonin secretion is not accompanied by accumulation of melatonin in the cells. These observations indicate that chick pineal melatonin is released independently of the Golgi-derived vesicles, and suggest inhibition of melatonin synthesis after the removal of BFA. By measuring the activities and mRNA levels of melatonin-synthesizing enzymes, we found that the removal of BFA specifically inhibits NAT activity at the protein level. On the other hand, BFA causes no detectable phase-shift of the chick pineal oscillator regulating the circadian rhythm of melatonin secretion. The results presented here suggest that the Golgi-mediated vesicular transport is involved in neither the melatonin release nor the time-keeping mechanism of the circadian oscillator, but rather contributes to the regulation of NAT activity.     

The pars tuberalis as a target of the central clock

The pars tuberalis (PT) of the pituitary has emerged from being a gland of obscure and unknown function to a tissue of central importance to our understanding of how photoperiod regulates seasonal responses. The discovery of melatonin receptors on this gland first pointed to its involvement in seasonal physiology. However, the more recent demonstration of the expression of clock genes in the PT, such as Per1, has heightened interest in the gland. Recent work shows how photoperiod, through the hormone melatonin, affects the timing and amplitude of expression of the Per1 gene, as well as other genes such as Icer. The effect of photoperiod and melatonin on the expression of Per1 in the PT is distinct to its effects on the SCN, and this probably reflects distinct functions of the clock genes in the two tissues - acting as part of the biological clock in the SCN, but as an interval timing system within the PT. The changes in amplitude of Per1 gene expression in response to altered length of photoperiod have provided the first clues as to how the durational melatonin signal is decoded within the neuroendocrine system.     

Melatonin and the seasonal control of reproduction.

Many mammalian species from temperate latitudes exhibit seasonal variations in breeding activity which are controlled by the annual photoperiodic cycle. Photoperiodic information is conveyed through several neural relays from the retina to the pineal gland where the light signal is translated into a daily cycle of melatonin secretion: high at night, low in the day. The length of the nocturnal secretion of melatonin reflects the duration of the night and it regulates the pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Changes in GnRH release induce corresponding changes in luteinising hormone secretion which are responsible for the alternating presence or absence of ovulation in the female, and varying sperm production in the male. It is not yet known where and how this pineal indoleamine acts to exert this effect. Although melatonin binding sites are preferentially localised in the pars tuberalis (PT) of the adenohypophysis, the hypothalamus contains the physiological target sites of melatonin for its action on reproduction. Melatonin does not seem to act directly on GnRH neurons; rather it appears to involve a complex neural circuit of interneurons that includes at least dopaminergic, serotoninergic and excitatory aminoacidergic neurons.     

Retention of radioactive substances in the hypothalamus, anterior pituitary, and reproductive organs of male rats after 3H-melatonin administration

Radioactive concentrations were determined in serum, lung, hypothalamus, anterior pituitary, testis, and accessory sex organs of adult male rats at 2, 20, 30 and 60 min after intravenous 3H-melatonin administrations. The retention patterns of 3H-melatonin and other radioactive substance (3H-non-melatonin) in these tissues were compared. The anterior pituitary demonstrated best tissue retention with highest concentration of 3H-melatonin. The testis and prostate gland accumulated 3H-non-melatonin in an increasing manner from 2 to 60 min. The radioactive substances were also preferentially and progressively located in the nuclear fraction of the anterior pituitary, hypothalamus, testis, and prostate gland. This study leads to the following suggestions: the anterior pituitary is another target organ of melatonin; melatonin is converted into other active material which exerts its action in the testis and prostate gland; melatonin and its active derivative exert their action through the nuclei of their respective cells.