[125I]melatonin binding sites in membrane preparations of quail brain: characteristics and diurnal variations

Neuroendocrine regulation of gonadotropin secretion in seasonally breeding birds

Seasonally breeding birds detect environmental signals, such as light, temperature, food availability, and presence of mates to time reproduction. Hypothalamic neurons integrate external and internal signals, and regulate reproduction by releasing neurohormones to the pituitary gland. The pituitary gland synthesizes and releases gonadotropins which in turn act on the gonads to stimulate gametogenesis and sex steroid secretion. Accordingly, how gonadotropin secretion is controlled by the hypothalamus is key to our understanding of the mechanisms of seasonal reproduction. A hypothalamic neuropeptide, gonadotropin-releasing hormone (GnRH), activates reproduction by stimulating gonadotropin synthesis and release. Another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), inhibits gonadotropin synthesis and release directly by acting on the pituitary gland or indirectly by decreasing the activity of GnRH neurons. Therefore, the next step to understand seasonal reproduction is to investigate how the activities of GnRH and GnIH neurons in the hypothalamus and their receptors in the pituitary gland are regulated by external and internal signals. It is possible that locally-produced triiodothyronine resulting from the action of type 2 iodothyronine deiodinase on thyroxine stimulates the release of gonadotropins, perhaps by action on GnRH neurons. The function of GnRH neurons is also regulated by transcription of the GnRH gene. Melatonin, a nocturnal hormone, stimulates the synthesis and release of GnIH and GnIH may therefore regulate a daily rhythm of gonadotropin secretion. GnIH may also temporally suppress gonadotropin secretion when environmental conditions are unfavorable. Environmental and social milieus fluctuate seasonally in the wild. Accordingly, complex interactions of various neuronal and hormonal systems need to be considered if we are to understand the mechanisms underlying seasonal reproduction.

Binding characteristics and daily rhythms of melatonin receptors are distinct in the retina and the brain areas of the European sea bass retina (Dicentrarchus labrax)

Melatonin is synthesized, with a circadian rhythm, in the pineal organ of vertebrates, high levels being produced during the scotophase and low levels during the photophase. The retina also produces melatonin, although in the case of the European sea bass, its secretion pattern appears to be inverted. In the study described here, radioreceptor assay techniques were used to characterize the melatonin binding sites, their regional distribution and their daily variations. Brain and retina membrane preparations were used in all the binding assays and 2-[125I]iodomelatonin ([125I]Mel) as radioligand at 25 degrees C. The specific binding of [125I]Mel was seen to be saturable, reversible, specific and of high affinity. In all the tissues assayed, the power of the ligands to inhibit [125I]Mel binding decreased in the following order: melatonin>>4-P-PDOT>luzindole> or =N-acetylserotonin, which points to the presence of Mel1-like receptors. The inhibition curves of 4-P-PDOT suggested the presence of two different binding sites in the brain areas, but only one type of site of low affinity in the neural retina. No daily variations in [125I]Mel binding capacity (Bmax) or affinity (Kd) were detected in the brain areas, while a clear rhythm in Kd melatonin receptor affinity and Bmax binding capacity was observed in the retina. Kd and Bmax retinal rhythms were out of phase with the lowest Kd and the highest Bmax occurring at scotophase. This result suggests that retinal melatonin is a paracrine factor able to control receptor desensitization during photophase when ocular melatonin is higher in this species.

Factors regulating the influence of melatonin on hippocampal evoked potentials: comparative studies on different strains of mice

Factors regulating the influence of melatonin on the hippocampal glutamergic system in mouse hippocampal slices were evaluated. The sensitivity of hippocampal pyramidal neurons to melatonin (Sigma) was highest at 2 h following slice preparation and then declined with time. This pattern of sensitivity to melatonin correlated well with a reduced binding of melatonin to its receptors. The slices obtained from older animals remained sensitive to melatonin through the entire incubation period. Most of the experiments evaluating the influence of melatonin on hippocampal evoked potentials were performed within 2 h following slice preparation. The effect of melatonin was biphasic: an initial depression of the potential was followed by a recovery/amplification phase. The recovery phase was not a result of melatonin decomposition. The effect of melatonin was similar in three different strains of mice tested: CD-1, C57J/B6, and Swiss Webster. While the melatonin from another vendor (Regis) gave similar results, it was effective at much lower concentrations. In slices obtained from CD-1 light-deprived mice, the sensitivity to melatonin was significantly reduced. Thus, it appears that melatonin may control the hippocampal glutamergic system in a complex manner, which may be regulated by the circadian rhythm. This may influence memory formation in the hippocampus.

Sexual differences and effect of photoperiod on melatonin receptor in avian brain

Several data suggest that melatonin may influence avian reproduction by acting at the level of the hypothalamic-hypophisial-gonadal axis, and/or on neural circuits controlling reproductive behaviours. The action of melatonin is exerted through specific receptors whose distribution and pharmacological properties have been extensively investigated. This review will focus on the distribution, sexual dimorphism, and dependence upon the photoperiod of melatonin binding sites in avian species with a special emphasis on Japanese quail. Melatonin receptors are widely distributed in avian brain. They are mostly present in the visual pathways of all the investigated species and in the song controlling nuclei of oscine birds. Sexual dimorphism of melatonin binding sites (higher density in males than in females) was detected in some telencephalic nuclei of songbirds, in the visual pathways, and in the preoptic area of quail. The last region plays a key role in the activation of male quail copulatory behaviour and it hosts a large population of gonadotropin-releasing hormone-containing neurons. Sexual dimorphism of melatonin-binding sites in the above-mentioned regions suggests a differential role for this hormone in the modulation of visual perception, gonadotropin production, and seasonally activated behaviours in male and female quail. Further studies are necessary to understand interrelationships among photic cues, gonadal steroids, density, and sexually dimorphic distribution of melatonin receptors.

Regulation of the expression of serotonin N-acetyltransferase gene in Japanese quail (Coturnix japonica): I. Rhythmic pattern and effect of light

Serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AANAT); EC2.3.1.87] is the rate-limiting enzyme in melatonin synthesis, and its activity exhibits a diurnal rhythm similar to that of the melatonin content in the pineal gland and retina of Japanese quail. Studies were conducted to characterize the Japanese quail AANAT cDNA, and to evaluate the expression of AANAT mRNA in the pineal gland, the retina, and other peripheral tissues. The nucleic acid sequence of a 400 bp cDNA clone obtained by RT-PCR manifested 78 and 95% homology compared to the rat and chicken AANAT cDNA, respectively, while the deduced amino acid sequence homology was 82 and 99%, respectively. AANAT mRNA content in a single pineal gland or an aliquot of eye lysate was measured by a micro-lysate protection assay. The expression of AANAT mRNA in the pineal gland and the retina exhibited circadian rhythm with peak levels at night. AANAT mRNA was also detected in the testis, but did not display a rhythmic change over a 24 hr period. AANAT mRNA was not detected in other tissues studied. Darkness during the day did not increase the pineal AANAT mRNA levels. However, unexpected light-exposure for 2 hr just after lights-off blocked the increase in AANAT mRNA, and at midnight remarkably decreased AANAT mRNA by 50%.

Diurnal variation in the effect of melatonin on neurohypophysial hormone release from the rat hypothalamus

Secretion of neurohypophysial hormones can show a diurnal variation. This has been investigated further in rats maintained on 14 h light:10 h dark using a previously validated in vitro technique employing hypothalami obtained at three different times, 2-3 h after lights on (group A), 13-14 h after lights on (group B), and at 4-5 h after lights off (group C). Hormone release under basal conditions and following stimulation with 40 mM KCl was monitored with or without added melatonin in the concentration range 4.3-43 nM. Basal release of hormone was not influenced by the time of day when the animals were taken, although stimulated release was elevated at midnight. In groups A and B both doses of melatonin significantly reduced basal and stimulated release of vasopressin and basal release of oxytocin (p < 0.01), although no effect was seen in group C animals. Inhibition of stimulated oxytocin release was only produced in group B. These findings suggest that the inhibitory effect of melatonin depends on the time of day and are consistent with the suggestion that melatonin secretion during the dark period may acutely downregulate binding sites in the brain.

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

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

Segmental, coronal and subcellular distribution of 2-[125I]iodomelatonin binding sites in the chicken spinal cord

Radioreceptor and autoradiography studies using chicken spinal cords demonstrated that the binding density of 2-[125I]iodomelatonin ([125I]MEL) was significantly higher in the lumbar segment and the specific binding of [125I]MEL was localized in the gray matter. Subcellularly, different densities of binding sites were localized in the following order: nuclear > microsomal > mitochondrial > cytosolic. Localization of [125I]MEL binding sites in the dorsal gray matter of the chicken spinal cord suggests that melatonin plays a role in regulating the spinal cord functions which may associate with the modulation of temperature and pain transmission and/or visceral and autonomic functions.

Localization and characterization of [125I]iodomelatonin binding sites in duck gonads

The characterization and localization of [125I]iodomelatonin binding sites in the gonads advances the understanding of possible regulatory sites of melatonin action. With the availability of [125I]iodomelatonin as a biologically active radioligand, our study utilized a combined approach of autoradiography for anatomical resolution together with an established radioligand binding assay to assess mid-light [125I]iodomelatonin binding in the testes and ovaries of ducks. In the autoradiography study, specific [125I]iodomelatonin binding was shown to be homogeneous throughout the testes, while in the ovaries, specific [125I]iodomelatonin binding appeared to be concentrated around the follicle. Radioligand binding assay results indicated a single class of binding sites with the maximum number of [125I]iodomelatonin binding sites measured at 1.91 +/- 0.70 fmol/mg protein in testicular membrane and 4.54 +/- 0.64 fmol/mg protein in ovarian membrane. [125I]Iodomelatonin binding affinity, characterized by equilibrium dissociation constants of 29 +/- 6 pmol/L in testicular membrane and 53 +/- 9 pmol/L in ovarian membrane, was in accordance with circulating melatonin levels, suggesting an appropriate concentration for eliciting a physiological response. [125I]Iodomelatonin binding in duck gonads satisfied all the criteria for a binding site, being rapid, stable, saturable, reversible, specific, and of high affinity, and may indicate a direct pineal-gonadal connection.

Characterization of high-affinity melatonin binding sites in purified cell nuclei of rat liver

High-affinity 2-125I-iodomelatonin binding sites in homogenates of purified cell nuclei from rat liver were localized and characterized using biochemical binding techniques. Binding at these sites was found to be rapid, reversible, saturable, and to demonstrate pharmacological selectivity. At 0 degrees C, binding reached equilibrium in about 10 min. Scatchard analysis of the data at equilibrium revealed a single class of binding sites with a dissociation constant of KD = 190 +/- 47 pM, Bmax = 9.8 +/- 0.6 fmol/mg protein, and a Hill coefficient of nH = 1.02 +/- 0.034. Kinetic analysis of the association and dissociation curves indicated a kinetic KD = 148 +/- 41 pM, which is in good agreement with the value obtained at equilibrium. The specific binding of 2-125I-iodomelatonin (45 pM) (0.51 +/- 0.04 fmol/mg protein) was significantly improved (0.79 +/- 0.04 fmol/mg protein) when the homogenates of purified liver cell nuclei were preincubated with DNase (2 micrograms/ml at 37 degrees C for 20 min) before being used in binding experiments. After the addition of either proteinase K or trichloroacetic acid to DNase-treated purified cell nuclear homogenates, the specific binding disappeared. This suggests that the specific binding of 2-125I-iodomelatonin in liver cell nuclei is associated with nuclear protein. Competition experiments show that N-acetyl-serotonin (Ki = 81.3 nM) was more potent than 5-hydroxytryptamine (Ki > 1 microM) and 5-methoxytryptamine (Ki >> 10 microM) in inhibiting 2-125I-iodomelatonin binding (Ki melatonin = 146 pM). These data indicate that specific 2-125I-iodomelatonin binding sites exist in the cell nuclei of rat liver, and that they may comprise a locus for the intracellular action of melatonin. The correlation between the KD and Bmax values with melatonin concentrations in nuclei suggest that these binding sites may be a physiological melatonin receptor, which could explain the described genomic effects of the pineal hormone.

[125I]iodomelatonin binding sites in the chicken spinal cord: binding characteristics and diurnal variation

[125I]Iodomelatonin ([125I]MEL) binding in chicken spinal cords was characterized. The specific binding of [125I]MEL to the membrane preparation of spinal cord was rapid, stable, saturable, reversible, specific and of high affinity. Scatchard analysis of the specific binding data indicated an equilibrium dissociation constant (Kd) of 30.7 +/- 3.43 pM and a maximum number of binding sites (Bmax) of 1.69 +/- 0.14 fmol/mg protein in spinal cords collected at mid-light. The diurnal variation study showed that the Bmax was 32% higher (P < 0.05) at mid-light compared to mid-dark. The pharmacological characteristics demonstrated that [125I]MEL binding sites was highly specific for melatonin. Our results suggest that melatonin may exert a direct action on the spinal cord.

Putative melatonin receptors in the male guinea pig kidney

The direct action of pineal melatonin on the renal system is supported by our demonstration of 2-[125I]iodomelatonin binding sites in the male guinea pig kidney. Scatchard analyses and Hill coefficients revealed a single type of binding site with an equilibrium dissociation constant (Kd) of 22.3 +/- 1.6 pmol/l and a maximum binding density (Bmax) of 0.99 +/- 0.03 fmol/mg protein (n = 7) at mid-light. There was no significant difference in the Kd and Bmax values between kidney tissues collected at the middle of light and dark periods. The pharmacological profile of these 2-[125I]iodomelatonin binding sites indicated high specificity for melatonin, 2-iodomelatonin and 6-chloromelatonin while kinetic studies generated a Kd value of 28.4 +/- 7.3 pmol/l (n = 5) which was comparable to that determined from Scatchard transformations. Our results suggest that these binding sites are stable, reversible, saturable, specific, and of high affinity. Regional distribution study showed that specific binding of 2-[125I]iodomelatonin was 8-fold higher in the cortical region than that in the medullary region. Studies of subcellular distribution showed that 59.3% of binding sites were localized in crude nuclear fractions followed by crude mitochondrial fractions (22.3%) and crude microsomal fractions (18.3%) with no detectable binding in cytosolic fractions. Our present findings suggest the presence of putative melatonin receptors in the guinea pig kidney, which support the hypotheses of melatonin-regulated renin secretion together with renal excretory functions via melatonin receptors.

Effect of pinealectomy and the light/dark cycle on 2-[125I]iodomelatonin binding in the chick optic tectum

1. These studies investigated the regulation of melatonin receptor sites within the chick brain by the light/dark cycle and by endogenous melatonin using quantitative autoradiography. Saturation analyses performed in coronal brain sections of the optic tectum revealed a single class of high-affinity binding sites. 2. For diurnal rhythm studies, chicks were maintained on a 14:10 L:D cycle (light on 0400) and sacrificed at 4-h intervals. No significant variation was observed in either the affinity (KD = 67.8-76.4 pM) or the density (BMAX = 88.9-118.9 fmol/mg protein) of 2-[125I]iodomelatonin binding sites as a function of time of day. 3. In pinealectomized or age-matched sham-operated chicks, no changes were found in either the affinity (KD = 66.7-89.1 pM) or the density (BMAX = 112.8-180.4 fmol/mg protein) of 2-[125I]iodomelatonin binding sites at either 1, 7, or 14 days following surgery. These data suggest that endogenous pineal melatonin may not play a role in the regulation of melatonin receptor site affinity or density in the chick optic tectum.

Distribution and characterization of melatonin receptors in the brain of the Japanese quail, Coturnix japonica

2-[125I]iodomelatonin was used to study the distribution and properties of the melatonin receptor in the Japanese quail brain. High receptor density was detected in the major targets of direct retinal input (optic tectum, nucleus of the optic basal rout, ventrolateral geniculate nucleus), as well as areas representing terminals in the visual pathways (nucleus rotundus, ectostriatum, thalamo-hyperstriatal pathway). Binding was also found in the piriform cortex, the hypophyseal pars tuberalis, the oculomotorius nucleus and the associated Edinger-Westphal nucleus, and in the nuclei of the third, fourth and sixth cranial nerves. A comparison of the receptor pharmacological profile to that of the mammalian brain demonstrated pharmacological identity of the two binding sites. In the saturation experiments, GPT gamma S decreased the binding affinity, numerical Kd values increasing from approximately 35 pM to approximately 150 pM.

Melatonin receptor density is regulated in rat pars tuberalis and suprachiasmatic nuclei by melatonin itself

The pineal hormone melatonin interacts with the neuroendocrine system via high affinity binding sites. The density of these receptors is increased after 72 h constant light or pinealectomy in both rat pars tuberalis and suprachiasmatic nuclei. The present results show that, in rat PT and SCN, this increase can be inversed by a single melatonin injection (50 micrograms s.c. in 0.1 ml NaCl, 4 h before sacrifice). At the time of sacrifice, plasma melatonin concentrations of injected animals were no longer significantly different when compared to control values. Thus, this observed effect is not a result of a receptor occupation, but demonstrates a direct regulation of melatonin receptors by melatonin itself.

Characterization of 2-[125I]iodomelatonin-binding sites in quail testes at mid-light and mid-dark

The binding and pharmacological characteristics of 2-[125I]iodomelatonin binding sites in testis membrane preparations of quails were examined. Scatchard analyses yielded an equilibrium dissociation constant (Kd) of 46.6 +/- 8.6 pmol/l and maximum binding capacity (Bmax) of 2.77 +/- 0.20 fmol/mg protein for the gonadal 2-[125I]iodomelatonin binding sites. Except for melatonin, 6-chloromelatonin, 2-iodomelatonin and N-acetylserotonin, all compounds tested elicited no significant inhibition of radioligand binding. Significant diurnal variations were detected in serum melatonin levels of 24-week-old quails while no diurnal difference was detected in the affinities or densities of the gonadal 2-[125I]iodomelatonin binding sites in quails. Results of the present study suggest possible direct gonadal action by pineal melatonin in birds.

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

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

2[125I]iodomelatonin binding sites in spleens of guinea pigs

2-[125I]Iodomelatonin was found to bind specifically to the membrane preparations of the spleens of guinea pigs with high affinity. The binding was rapid, stable, saturable and reversible. Scatchard analysis of the binding assays revealed an equilibrium dissociation constant (Kd) of 49.8 +/- 4.12 pmol/l and binding site density (Bmax) of 0.69 +/- 0.082 fmol/mg protein at mid-light (n = 10). There was no significant change in the Kd (41.8 +/- 3.16 pmol/l) or the Bmax (0.58 +/- 0.070 fmol/mg protein) at mid-dark (n = 10). Kinetic analysis showed a Kd of 23.13 +/- 4.81 pmol/l (mean +/- SE, n = 4), in agreement to that derived from the saturation studies. The 2-[125I]iodomelatonin binding sites have the following order of potency: 2-iodomelatonin greater than melatonin greater than 6-chloromelatonin much greater than N-acetylserotonin, 6-hydroxymelatonin greater than 5-methoxytryptamine, 5 methoxytryptophol greater than serotonin, 5-methoxyindole-3-acetic acid greater than 5-hydroxytryptophol, 3-acetylindole, 1-acetylindole-3-carboxyaldehyde, L-tryptophan greater than tryptamine, 5-hydroxyindole-3-acetic acid. Differential centrifugation studies showed that the binding sites are localized mainly in the nuclear fraction (65.5%), the rest are distributed in the microsomal fraction (17.4%), mitochondrial fraction (14.7%) and cytosolic fraction (0.3%). The demonstration of 2-[125I]iodomelatonin binding sites in the spleen suggests the presence of melatonin receptors and a direct mechanism of action of melatonin on the immune system.

Identification and characterization of melatonin binding sites in the gastrointestinal tract of ducks

Utilizing 2-[125I]iodomelatonin as the radioligand, melatonin binding sites were identified and characterized in the jejunum of ducks. These sites were found to be reversible, saturable, specific and exhibited high affinity for melatonin. Scatchard analyses have established the equilibrium dissociation constant (Kd) for tissues collected during mid-photophase to be 40.9 +/- 7 pM and the maximum quantity of binding sites (Bmax) to be 2.0 +/- 0.4 fmol/mg protein while Kd of samples collected during mid-scotophase was found to be 54.1 +/- 10 pM with a corresponding Bmax of 1.5 +/- 0.3 fmol/mg protein. These Kd values are in good proximity to the kinetically derived equilibrium dissociation constant of 47.3 +/- 20 pM. No significant difference in Kd or Bmax was detected between the mid-light and mid-dark samples. Pharmacological profile of these binding sites, developed by their interactions with other indoles and compounds, indicated that these binding sites are highly specific for melatonin. Subcellularly, different densities of binding sites were localized to various fractions in the following order: nuclear greater than microsomal greater than mitochondrial greater than cytosolic. These binding sites in the jejunum might be the receptors accountable for promoting paracrine activities for the locally synthesized gastrointestinal melatonin and/or responsible for eliciting hormonal actions via interactions with melatonin of pineal origin.

Retinal [125I]iodomelatonin binding sites in the tree shrew (Tupaiidae)

The characteristics of melatonin-binding sites labelled by [125I]iodomelatonin in membrane preparations from the tree shrew retina were determined. Specific binding of [125I]iodomelatonin to the membrane preparations of tree shrew retina was rapid, stable, saturable, and reversible. Among the indoles tested only 6-chloromelatonin, melatonin and N-acetylserotonin had significant affinities to the [125I]iodomelatonin binding site. Scatchard analysis of the membrane preparations revealed a dissociation constant (Kd) of 51.0 +/- 16 pM and a total number of binding sites (Bmax) of 1.97 +/- 0.6 fmol/mg protein.

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

The binding and pharmacological characteristics of [125I]iodomelatonin binding sites in human fetal brain membrane preparations were determined. Membrane preparations of the whole brain revealed a equilibrium binding constant (Kd) of 17.5 pmol/l with a total number of binding sites (Bmax) at 0.8 fmol/mg protein. Among the various brain regions studied, [125I]iodomelatonin binding was highest in the hypothalamus, and lowest in the mid-brain, pons-medulla, and cerebral cortex. The Kd of the hypothalamus was calculated to be 26.1 pmol/l and the Bmax 5.4 fmol/mg protein. Only 6-chloromelatonin, melatonin and N-acetylserotonin had significant inhibition in the binding. Our results suggest that melatonin receptors are present in the human brain and that melatonin may exert a central action.

[125I]iodomelatonin binding sites in spleens of birds and mammals

The specific binding of [125I]iodomelatonin to duck spleen membrane preparations was studied in detail. These binding sites were stable, saturable, reversible and of high affinity. Scatchard analysis of the binding revealed a equilibrium binding constant (Kd) of 73.1 +/- 5.4 pM and a total number of binding sites (Bmax) of 3.64 +/- 1.38 fmol/mg protein. Studies on the relative binding capacities of [125I]iodomelatonin to the spleen in different species showed the following order: duck greater than chicken greater than guinea pig greater than pigeon greater than mouse. No binding site was detected in the rat spleen. The presence of [125I]iodomelatonin binding sites in the spleen of birds and mammals suggested a direct action of pineal melatonin on the immune system.