n-pentanoyl-2-benzyltryptamine and 2-iodomelatonin

The effects of melatonin on reproductive function were examined using hamster spermatozoa. When 1 pM to 1 microM melatonin was added to the mTALP medium, hyperactivation was significantly enhanced. Antagonists and agonists of the melatonin receptor (i.e., MT1 and MT2) were added to the medium. Luzindole, an MT1 and MT2 competitive antagonist, significantly inhibited melatonin-induced hyperactivation, whereas the MT2-specific antagonists, 4-phenyl-2-propionamidotetralin and N-pentanoyl-2-benzyltryptamine, had no effect. Moreover, hyperactivation was significantly enhanced when non-specific agonists, such as 6-chloromelatonin and 2-iodomelatonin, were added to the medium. 8-Methoxy-2-propionamidotetralin, which is a strong MT2 agonist and a weak MT1 agonist, significantly increased hyperactivation, although the effect was weak. Therefore, it is likely that melatonin enhances sperm hyperactivation via the MT1 receptor.

Topics: Animals; Calcium; Cells, Cultured; Cricetinae; Culture Media; Dose-Response Relationship, Drug; Male; Melatonin; Mesocricetus; Receptor, Melatonin, MT1; Receptor, Melatonin, MT2; Serum Albumin; Sperm Motility; Spermatozoa; Stimulation, Chemical; Tetrahydronaphthalenes; Time; Tryptamines

Melatonin attenuates carotid chemoreceptor response to hypercapnic acidosis and may contribute to the effect of circadian rhythms on the chemoreflex. The purpose of this study was to test the hypothesis that melatonin modulates rat carotid chemoreceptor response to hypoxia. To examine the effect of melatonin on the hypoxic response of the chemosensitive cells, cytosolic calcium ([Ca2+]i) was measured by spectrofluorometry in fura-2-loaded type-I (glomus) cells dissociated from rat carotid bodies. Melatonin (0.01-10 nm) did not change the resting Ca2+]i level of the glomus cells but it concentration-dependently increased peak Ca2+]i response to cyanide or deoxygenated buffer. An agonist of melatonin receptors, iodomelatonin also enhanced the Ca2+]i response to hypoxia. The melatonin-induced enhancement of the Ca2+]i response was abolished by pretreatment with nonselective mt1/MT2 antagonist, luzindole, and by MT2 antagonists, 4-phenyl-2-propionamidotetraline or DH97. These findings suggest that melatonin receptors in the glomus cells mediate the effect of melatonin on the chemoreceptor response to hypoxia. In addition, melatonin increased the carotid afferent response to hypoxia in unitary activities recorded from the sinus nerve in isolated carotid bodies superfused with bicarbonate-buffer saline. Furthermore, plethysmographic measurement of ventilatory activities in unanesthetized rats revealed that melatonin (1 mg/kg, i.p.) increased the ventilatory response to hypoxia. Hence, the circadian rhythm of melatonin in arterial blood can modulate the carotid chemoreceptor response to hypoxia. This modulation may be a physiological mechanism involved in the day-light differences in ventilatory activities.

Topics: Animals; Calcium; Carotid Body; Chemoreceptor Cells; Circadian Rhythm; Hypoxia; In Vitro Techniques; Melatonin; Rats; Rats, Sprague-Dawley; Receptors, Melatonin; Respiration; Tetrahydronaphthalenes; Tryptamines

Respiratory activity is under circadian modulation and the physiological mechanisms may involve the pineal secretory product, melatonin, and the carotid chemoreceptor. We hypothesized that melatonin modulates the carotid chemoreceptor response to hypercapnic acidosis. To determine whether the effect of melatonin on the chemoreceptor response to hypercapnic acidosis is mediated by melatonin receptors in the chemosensitive cells, cytosolic calcium ([Ca2+]i) was measured by spectrofluorometry in fura-2-loaded glomus cells dissociated from rat carotid bodies. Melatonin (0.01-10 nm) per se did not change the [Ca2+]i levels of the glomus cells but it concentration-dependently attenuated the peak [Ca2+]i response to hypercapnic acidosis in the glomus cells. In addition, the [Ca2+]i response was attenuated by 2-iodomelatonin, an agonist of melatonin receptors. The melatonin-induced attenuation of the [Ca2+]i response to hypercapnic acidosis was abolished by pretreatment with an non-selective mt1/MT2 antagonist, luzindole, and by MT2 antagonists, 4-phenyl-2-propionamidotetraline or DH97. In situ hybridization study with antisense mt1 and MT2 receptor mRNA oligonucleotide probes showed an expression of mt1 and MT2 receptors in the rat carotid body. Also, melatonin attenuated the carotid afferent response to hypercapnic acidosis in single- or pauci-fibers recorded from the sinus nerve in isolated carotid bodies superfused with bicarbonate-buffer saline. Results suggest that an activation of the melatonin receptors expressed in the glomus cells of the rat carotid body reduces the chemoreceptor response to hypercapnic acidosis. This modulation may play a physiological role in the influence of the circadian rhythms on the chemoreflex.

Topics: Acidosis, Respiratory; Animals; Calcium; Carotid Body; Chemoreceptor Cells; Electrophysiology; Fura-2; Hypercapnia; Melatonin; Rats; Rats, Sprague-Dawley; Receptor, Melatonin, MT1; Receptor, Melatonin, MT2; Receptors, Melatonin; Tetrahydronaphthalenes; Tryptamines

The neurohormone melatonin is the central switch of the circadian rhythm and presumably exerts its activities through a series of receptors among which MT1 and MT2 have been widely studied. The third binding site of melatonin, MT3, has been recently characterized as a melatonin-sensitive form of the quinone reductase 2 (QR2, EC 1.6.99.2). In the present work, we showed that the binding of melatonin at MT3/QR2 was better described with 2-[125I]-iodomethoxy-carbonylamino-N-acetyltryptamine (2-[125I]-I-MCA-NAT) and, most importantly, that it was measurable at 20 degrees while it has been initially described and thoroughly studied using 2-[125I]-iodomelatonin at 4 degrees. Under these novel conditions, binding to MT3 could be traced without cross-reactivity with MT1 and MT2 receptors and, moreover, under conditions similar to those used to measure MT3/QR2 catalytic activity. The pharmacology established here on hamster kidney samples using the reference compounds remained essentially as already described using other experimental conditions. A new series of compounds with nanomolar affinity for the MT3 binding site and a high MT3 selectivity versus MT1 and MT2 is reported. In addition, we further document the MT3/QR2 binding site by demonstrating that it was widely distributed among mammals, although inter-species and inter-tissues differences exist. The present report details new experimental conditions for the pharmacological study of melatonin-sensitive QR2 isoforms, and suggests that, in addition to an already demonstrated inter-species difference, inter-tissues differences in QR2 sensitivity to melatonin may exist in primates and, therefore, represent an original and interesting route of investigation on the effect of melatonin on MT3/QR2.

Topics: Animals; Binding, Competitive; Cells, Cultured; Cricetinae; Humans; Iodine Radioisotopes; Kinetics; Melatonin; Metallothionein 3; Mice; Rabbits; Radioligand Assay; Rats; Receptors, Cell Surface; Receptors, Cytoplasmic and Nuclear; Receptors, Melatonin; Species Specificity; Temperature; Tissue Distribution; Tryptamines