Expression of melatonin receptors in arteries involved in thermoregulation.

Abstract

Melatonin binding sites were localized and characterized in the vasculature of the rat by using the melatonin analogue 2-['25I]iodomelatonin (1251-melatonin) and quantitative in vitro autoradiography. The expression of these sites was restricted to the caudal artery and to the arteries that form the circle of Willis at the base of the brain. The arterial 125I-melatonin binding was stable, saturable, and reversible.Saturation studies revealed that the binding represented a single class of high-affinity binding sites with a dissociation constant (Kd) of 3.4 X 10-11 M in the anterior cerebral artery and 1.05 x 10-10 M in the caudal artery. The binding capacities (Bm,) in these arteries were 19 and 15 fmol/mg of protein, respectively. The relative order of potency of indoles for inhibition of 'II-melatonin binding at these sites was typical of a melatonin receptor: 2-iodomelatonin > melatonin > Nacetylserotonin >>> 5-hydroxytryptamine. Norepinephrineinduced contraction of the caudal artery in vitro was significantiy prolonged and potentiated by melatonin in a concentration-dependent manner, suggesting that these arterial binding sites are functional melatonin receptors. Neither primary steps in smooth muscle contraction (inositol phospholipid hydrolysis) nor relaxation (adenylate cyclase activation) were affected by melatonin. Melatonin, through its action on the tone of these arteries, may cause circulatory adjustments in these arteries, which are believed to be involved in thermoregulation.Melatonin, a hormone produced by the pineal gland of vertebrates, is thought to serve as a neuroendocrine transducer of photoperiodic information (1,2). Through use of the radioiodinated melatonin analogue 2-[125lJiodomelatonin (1251-melatonin) putative melatonin receptors have been described in specific areas of the brain (3-12) and retina (13) Horizontal or coronal sections of the brain and cross sections of the caudal artery (16 ,um) were cut in a cryostat at -17TC, thaw-mounted onto gelatin-coated slides, and dried in a desiccator at 40C overnight.Binding Assays. The binding conditions used have been described (14). Tissue sections were incubated for 60 min in 50 mM Tris HCl buffer (pH 7.4) containing 4 mM CaCl2 and 1251I-melatonin (specific activity, 1800-2100 Ci/mmol; Amersham; 1 Ci = 37 GBq). Nonspecific binding was determined in the presence of 1 uM melatonin. After incubation, the sections were washed twice for 5 min in 50 mM Tris-HCl buffer followed by 30 sec in distilled water at 0C. There was no loss of high-affinity binding when tissue sections were washed in Tris HCI for up to 30 min at 00C.In the anterior cerebral artery, 125I-melatonin binding reached equilibrium within 60 min at 220C and was stable for at least 120 min. Excess of melatonin (1 ,uM), added at equilibrium, was able to compete with 1251-melatonin for binding to the arterial sites, demonstrating reversibility ofthe binding. The apparent dissociation constant (Kd) was 1.27 x 10-11 M, calculated as k2/kl (k1 = association rate constant = 7.5...

Melatonin inhibits nitric oxide production by microvascular endothelial cells in vivo and in vitro

“…Putative melatonin receptors in arteries were first identified in rat brain and tail by autoradiography using 2-[ 125 I]-iodomelatonin (Viswanathan et al, 1990;Seltzer et al, 1992;Capsoni et al, 1994). More recently, functional assays in vitro suggested that melatonin is involved in the control of vasomotor tone and acts via receptors located in the myocytes (Masana et al, 2002) and/or in the endothelium (Geary et al, 1998;Yang et al, 2001).…”

Effect of L-NAME-induced hypertension on melatonin receptors and melatonin levels in the pineal gland and the peripheral organs of rats

“…14 In the cardiovascular system, MT 1 and MT 2 receptor proteins were found in the heart 11 and coronary artery. 15 Activation of the MT 1 receptor causes vasoconstriction in the rat caudal artery, which is believed to be involved in the thermoregulation, 16 and activation of MT 2 results in vasodilatation. 17 Currently, the exact mechanism determining the activation of melatonin receptor subtypes by melatonin is obscure.…”

Circadian Clues to Sleep Onset Mechanisms

“…In contrast, all species, independent of temporal niche, sleep during the circadian trough of the core body temperature (CBT) rhythm. In rats, melatonin administration enhances norepinephrine-induced vasoconstriction of the tail (Viswanathan et al 1990); in humans, melatonin induces vasodilation in fingers and toes (Kräuchi et al 1998), suggesting that transduction of the nocturnal melatonin signal is linked to opposite physiological sequelae appropriate to behavioral niche. To further characterize the functional relationships preceding sleep onset, we have analyzed data from a series of experiments designed to phase-shift the circadian system with different putative zeitgebers, using identical methodology of a constant routine (CR) protocol to permit pooling of data.…”