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  • 1
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The nocturnal peak in pineal and serum melatonin content was reduced following administration of the selective alpha-1 adrenoceptor antagonist prazosin in a dose of 0.25 mg/kg. The effect was pronounced one hour post treatment during the late dark phase of the daily photocycle. These data confirm the reported findings that the hamster pineal sensitivity to adrenergic challenge is confined to the second part of the dark phase and indicate that postsynaptically located alpha-1 adrenoceptors are also involved in the physiological control of melatonin synthesis and/or release in that species.
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  • 2
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: It has been difficult, so far, to obtain melatonin analogs possessing high selectivity for the respective melatonin receptors, mt1 and MT2. In the present work, we report the synthesis and pharmacological characterization of a new compound N-{2-[5-(2-hydroxyethoxy)-1H-indol-3-yl)] ethyl} acetamide or 5-hydroxyethoxy-N-acetyltryptamine (5-HEAT). To assess the activity of the compound, the following tests were performed: affinity determination for the high- and low-affinity receptor states (2-[ 〈displayedItem type="mathematics" xml:id="di-fml-1" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:07423098:JPI290406:JPI_290406_m1"/〉I]iodomelatonin binding), potency and intrinsic activity in inducing G protein activation ([〈displayedItem type="mathematics" xml:id="di-fml-2" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:07423098:JPI290406:JPI_290406_m2"/〉 S]GTPγS binding assay). 5-HEAT showed little selectivity for the mt1 receptor, with pKi values of 7.77 for mt1 and 7.12 for the MT2 receptors, respectively. 5-HEAT was able to differentiate between the high- and the low-affinity receptor states in the mt1 but not in the MT2 receptor. 5-HEAT induced a high level of G protein activation when acting through the mt1 receptor, with a relative intrinsic activity of 0.92. On the contrary, it elicited only minimal MT2 receptor-mediated G protein activation, with a relative intrinsic activity of 0.16, and was also able to inhibit the melatonin-induced MT2 receptor-mediated G protein activation, with a pKB value of 7.4. In conclusion, it appears that 5-HEAT possesses very different efficacies at the two melatonin receptors, behaving as a full melatonin receptor agonist at the mt1 and as an antagonist/weak partial agonist at the MT2 receptor. Therefore, it is a promising ligand for use in functional studies aimed at distinguishing between the effects mediated by the different melatonin receptors in the human.
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  • 3
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Male Syrian hamsters that were exposed for 8 weeks to short photoperiod (LD 10:14) or treated with melatonin in the late afternoon under long photoperiod conditions (LD 14:10) had a significantly higher content of androgen receptors in the Lipidex-purified soluble fractions isolated from the Harderian glands as compared to the long photoperiod (LD 14:10) exposed controls. Simultaneous computer-assisted analyses of all series of saturation and competition experiments revealed that the numerical value of the apparent Kd, as determined by using the synthetic androgen R-1881 (methyltrienolone), was not different between the experimental groups, and ranged from 0.050 to 0.067 nM. Of the principal natural androgens, testosterone (T) was most potent in inhibiting methyltrienolone binding to the receptor (Ki values from 0.33 to 0.55 nM), and 5α-dihydrotestosterone (DHT) and α4-androstenedione (AD) were less effective (Ki values between 1 and 1.9 nM). In the hypothalami and pituitaries of the same animals, used in parallel control assays, DHT was twice as potent as T. Short-term castration (24 hr post-orchidectomy) did not result in significant changes in the receptor binding characteristics. Following 8 weeks exposure to a long photoperiod (LD 14:10) the Bmax values demonstrated a four-fold increase in castrated animals (179 fmoles/mg protein vs. 47 fmoles/mg protein) over intact controls. The relative binding affinity of the major androgens under these conditions remained unchanged, with the exception of AD, where a five-fold increase in the numerical Ki values (decrease in the binding affinity) was recorded (Ki = 9.6 nM). Studies on in vitro metabolism of the natural androgens by the cytosol during incubation under conditions identical to those employed in the binding studies revealed that in the normal intact animal AD is metabolized to T in elevated amounts. The ability of the Harderian gland to make this conversion is dramatically diminished following long-term castration. This could explain the apparent “shift” in the high affinity of AD toward a much lower one. Testosterone, however, was not further metabolized. No DHT was formed and small amounts of AD were recovered. Thus, in the androgen receptor system of the hamster Harderian gland testosterone appears to be a strong androgen per se, binding to the receptor with affinity higher than that of DHT. This receptor system is apparently flexible in its response to changes in the photoperiod and to removal of the natural steroids from the endogenous milieu following long-term castration. Both situations are accompanied by a significant increase in receptor numbers, though castration seemingly results in concomitant changes in the androgen metabolism in the gland.
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  • 4
    ISSN: 1520-4804
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
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  • 5
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The gonads of male hamsters exposed to short photoperiod (LD, 10: 14) or treated with melatonin in the late afternoon under long photoperiod (LD, 14: 10) had undergone complete regression by the end of the treatments (8 weeks). Animals treated for the same period of time with prazosin (a putative melatonin analogue) under the same conditions failed to show a difference in their gonadal status as compared to the long photoperiod controls. Prazosin was unable to prevent melatonin-induced gonadal atrophy when injected either in the morning or 30 min before melatonin. Moreover, prazosin was without any effect on (and unable to prevent the melatonin-stimulated) progesterone production by rat adrenals under in vitro conditions. These data demonstrate that prazosin, which reportedly inhibits 2-[1251]iodomelatonin binding in the hamster brain in an affinity-related manner, does not possess properties of a biological melatonin analogue under conditions of two different model systems in two species.
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  • 6
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Reverse transcription-polymerase chain reaction (RT-PCR) analysis performed on total RNA from different tissues of Xenopus laevis showed that the melatonin receptor gene cloned from dermal melanophores is expressed in the whole brain, skin, and retina, and that apart from the ovary, there is no expression in tissues having origin outside the central nervous system. Comparative studies using in vitro autoradiography and in situ hybridization demonstrated that the melatonin receptor is expressed with discrete allocation in Xenopus brain. Though the distribution pattern of the specific messenger RNA conforms well with that of the corresponding receptor protein, it is not always coincident.
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  • 7
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: With some exceptions, in most of the mammals the pituitary pars tuber-alis and the hypothalamic suprachiasmatic nuclei are reportedly the main targets for the pineal hormone melatonin. However, it is not known if the conspicuous diversity in the distribution pattern of melatonin binding sites in these areas depicts differences in reproductive behavior observed in the seasonally breeding species in the temperate zones. We explored the distribution and the characteristics of melatonin binding sites in the hypothalamus and pituitary of three species (bovine, horse, and donkey) different in terms of seasonal reproductive competence. The topographical localization, investigated by in vitro autoradiography, revealed 2-[125I]iodomelatonin binding sites only in the pituitary gland in all three species, primarily in the pars tuberalis (PT), but also in the pars distalis (PD) and pars intermedia (PI). Kinetic, inhibition, and saturation studies, performed by means of in vitro binding, revealed presence of a single class high affinity binding sites. The Kd values, melatonin, and 2-iodomelatonin Kj values were in the low picomo-lar range. Coincubation with GTP7S inhibited 2-[125 I]iodomelatonin binding, demonstrating that these putative receptors are linked to a G protein in their signal-transduction pathway. The hypothalamus was devoid of specific binding. In conclusion, the results suggest that in these species, the hypophysis may be a principal target for the melatonin action on the reproductive system.
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  • 8
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract:  The aim of the present study was to identify the distribution of the second melatonin receptor (MT2) in the human hippocampus of elderly controls and Alzheimer's disease (AD) patients. This is the first report of immunohistochemical MT2 localization in the human hippocampus both in control and AD cases. The specificity of the MT2 antibody was ascertained by fluorescence microscopy using the anti-MT2 antibody in HEK 293 cells expressing recombinant MT2, in immunoblot experiments on membranes from MT2 expressing cells, and, finally, by immunoprecipitation experiments of the native MT2. MT2 immunoreactivity was studied in the hippocampus of 16 elderly control and 16 AD cases. In controls, MT2 was localized in pyramidal neurons of the hippocampal subfields CA1-4 and in some granular neurons of the stratum granulosum. The overall intensity of the MT2 staining was distinctly decreased in AD cases. The results indicate that MT2 may be involved in mediating the effects of melatonin in the human hippocampus, and this mechanism may be heavily impaired in AD.
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  • 9
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Thirteen patients with tumors in the pineal region were submitted to pre- and post-operative blood sampling (08:00, 14:00, 20:00, and 02:00 hr) for three or four consecutive days. A single cerebrospinal fluid (CSF) sample was collected at surgery, and melatonin levels determined. In all patients, serum and CSF beta subunit of human chorionic gonadotrophin (βHCG), carcino embryonic antigen (CEA), and alpha-fetoprotein (AFP) levels were measured. Histology revealed four pineocytomas, one pineoblastoma, four germinomas, one immature teratoma, one pilocytic astrocytoma, one lymphoma, and one meningioma. Serum and CSF levels of serological biomarkers were normal, except for one of the germinoma cases. In most patients, alteration either in the circadian rhythm or in the melatonin concentration was observed before surgery. In benign neoplasms the circadian rhythm was conserved. In pineoblastoma, lymphoma, and three out of four germinomas, melatonin concentrations were undetectable. In one case of germinoma, melatonin levels were high, with the circadian rhythm being abolished. According to conventional histology, all germinomas were similar. Therefore, in a rare case of pineal germinoma with high melatonin levels, the tissue was subjected to an in depth investigation (immunohistochemical and ultrastructural) in order to determine the pathology and the possible differences from the other typical germinomas. Results were compared to those provided from other pineal neoplasms. Electron microscopy examination detected the presence of clusters of intermediate filaments and numerous electrondense granules only in the case of a germinoma producing melatonin.
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