Melatonin receptors in the spinal cord.
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The pineal hormone, melatonin, plays an important role in the regulation of diurnal and seasonal rhythms in animals. In addition to the well established actions on the brain, the possibility of a direct melatonin action on the spinal cord has to be considered. In our laboratory, we have obtained data suggesting that melatonin receptors are present in the spinal cords of birds and mammals. Using radioreceptor binding and quantitative autoradiography assays with 2-[125I]iodomelatonin as the specific melatonin agonist, melatonin binding sites have been demonstrated in the rabbit and chicken spinal cords. These sites are saturable, reversible, specific, guanosine nucleotide-sensitive, of picomolar affinity and femtomolar density. The linearity of Scatchard plots of saturation data and the unity of Hill coefficients indicate that a single class of melatonin binding sites is present in the spinal cord membranes studied. The picomolar affinity of these sites is in line with the circulating levels of melatonin in these animals suggesting that these sites are physiologically relevant. Autoradiography studies in the rabbit spinal cord show that melatonin binding sites are localized in the central gray substance (lamina X). In the chicken spinal cord, these binding sites are localized in dorsal gray horns (laminae I-V) and lamina X. As lamina X and laminae I-II have similar functions, melatonin may have comparable roles in the chicken and rabbit spinal cords. Moreover, in the chicken spinal cord, the density of 2-[125I]iodomelatonin binding in the lumbar segment was significantly higher than those of the cervical and thoracic segments. The densities of these binding sites changed with environmental manipulations. When chickens were adapted to a 12L/12D photoperiod and sacrificed at mid-light and mid-dark, there was a significant diurnal variation in the density (maximum number of binding sites; Bmax) of melatonin binding sites in the spinal cord. After constant light treatment or pinealectomy, the Bmax of melatonin receptors in the chicken spinal cord increased significantly in the subjective mid-dark period. Moreover, there was an age-related decrease in the 2-[125I]iodomelatonin binding to the chicken spinal cord. Our results suggest that melatonin receptors in the chicken spinal cord are regulated by environmental lighting and change with development. These receptors may play an important role in the chronobiology of spinal cord function. The biological responses of melatonin on spinal cords have also been demonstrated in vitro. Melatonin decreased the forskolin-stimulated cAMP production in the chicken spinal cord explant. Preincubation with pertussis toxin blocked the melatonin effect. Our results suggest that melatonin receptors in the chicken spinal cord are linked to the adenylate cyclase via a pertussis toxin-sensitive G protein and that melatonin binding sites in spinal cords are melatonin receptors with biological functions. These receptors may be involved in the regulation of spinal cord functions related to sensory transmission, visceral reflexes and autonomic activities.
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