Pineal melatonin modulates the mammalian immune system. In vivo studies showed that melatonin enhanced the natural and acquired immunity while in vitro studies demonstrated its inhibitory influence. The mechanism of melatonin action on the immune system remains unknown. Actions through lymphokines or opioid release or via other endocrine changes have been proposed. In this paper, a direct action of melatonin on the lymphoid tissue is hypothesized. 2-[¹²⁵I]Iodomelatonin binding sites have been identified in the membrane homogenates of thymus, bursa of Fabricius and spleens of a number of birds and mammals. The bindings were stable, saturable, reversible, specific and of high affinity. The Bmax ranged from 0.6 to 3.9 fmol/mg protein. The Kd was in the physiological range of circulating melatonin levels, about 30–70 pmol/l. The binding sites in the primary lymphoid organs demonstrated diurnal variation in density, with higher levels found at the middle of the light period. However, those in the spleen did not vary with the time of the day. An age-dependent decrease in the density was also found in the chicken bursa of Fabricius. In addition, when the noctural melatonin secretion was suppressed by constant light exposure, the density of the binding sites increased in the guinea pig spleen. Immunosuppression with Cortisol injection in young ducks decreased the density of the melatonin binding sites in the thymus. The regulation of the binding characteristics by physiological variation in melatonin levels and/or immunological status of the animals provide evidence that these 2-[¹²⁵I]iodomelatonin binding sites in the lymphoid tissues may be physiologically significant and represent true melatonin receptors. The melatonin receptors in the lymphoid organs may be coupled to a G protein as Guanosine 5''-O-(3-thiotri-phosphate inhibited 2-[¹²⁵I]iodomelatonin binding in the spleen by increasing the Kd and decreasing the Bmax.

No abstract

The pineal neurohormone melatonin modulates a variety of physiological processes through different receptors. It has recently been reported that the cloned melatonin receptors (MT1, MT2 and Mel1c) exhibit differential abilities to stimulate phospholipase C (PLC) via G 16 . Here we examined the molecular basis of such differences in melatonin receptor signaling. Coexpression of MT1 or MT2 with the a subunit of G 16 (Ga 16 ) allowed COS-7 cells to accumulate inositol phosphates in response to 2-iodomelatonin. In contrast, Mel1c did not activate Ga 16 even though its expression was demonstrated by radioligand binding and agonist-induced inhibition of adenylyl cyclase. As Mel1c possesses an exceptionally large C-terminal tail, we further asked if this structural feature prevented productive coupling to Ga 16 . Eleven chimeric melatonin or mutant receptors were constructed by swapping all or part of the C-terminal tail between MT1, MT2 and Mel1c. The primary roles of melatonin, a hormone synthesized and secreted by the pineal gland, are to regulate behavioral and neuroendocrine processes, as well as circadian rhythms in mammals. High affinity melatonin receptors cloned from various organisms have been found to be homologs of one of three receptor subtypes, MT1, MT2 and Mel1c, which make up the G protein-coupled melatonin receptor family (Shiu et al. 1997). The mammalian MT1 and MT2 receptors are 60% identical to each other and they share 60% sequence homology with Xenopus Mel1c (Reppert et al. , 1995a. The receptor subtypes also have distinct expression patterns. Mammalian MT1 is expressed in the hypophyseal pars tuberalis and hypothalamic suprachiasmatic nuclei, regions of the brain that have been implicated as being important for reproduction and circadian rhythms, respectively . The mammalian MT2, on the other hand, is found in the retina (Reppert et al. 1995a). Furthermore, the three receptor subtypes may not necessarily occur in the same organisms. Initial studies indicate that only the MT1

Melatonin, the pineal neurohormone, is an evolutionarily conserved photoperiodic signaling molecule with diverse functions that include the entrainment of human circadian rhythms. Although evidence supporting a direct inhibitory action of melatonin on human cancer cell proliferation exists in the literature, the molecular and cellular signaling mechanisms involved are largely undefined. In our study, significant inhibition of human choriocarcinoma JAr cell proliferation at physiological and pharmacological concentrations of melatonin was observed. 2-Iodomelatonin, a high affinity melatonin receptor agonist, was more potent than melatonin in inhibiting JAr cell proliferation. In addition, the presence of putative melatonin receptors in choriocarcinoma was suggested by the demonstration of specific 2-[125I]iodomelatonin binding to the tumor. Interestingly, the selective MT2 melatonin receptor ligand, 4-phenyl-2-propionamidotetraline (4-P-PDOT), was found to exert not only concentration-dependent anti-proliferative actions on JAr cells, but also additive effects with melatonin in inhibiting JAr cell proliferation. Furthermore, MT2 melatonin receptor gene expression by JAr cells was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization (ISH). Taken together, our data suggest that the reported anti-proliferative action of melatonin on human choriocarcinoma JAr cells may be mediated, in part, by MT2 melatonin receptor. Moreover, analysis of melatonin effect on cell cycle kinetics indicated that G1/S transition delay may underlie the observed inhibition of choriocarcinoma cell proliferation by melatonin.