Melatonin and N-acetylserotonin inhibit leukocyte rolling and adhesion to rat microcirculation

The hormone melatonin produced by the pineal gland during the daily dark ph ase regulates a variety of biological processes in mammals. The aim of this study was to determine the effect of melatonin and its precursor N-acetyls erotonin on the microcirculation during acute inflammation, Arteriolar diam eter, blood flow rate, leukocyte rolling and adhesion were measured in the rat microcirculation in situ by intravital microscopy. Melatonin alone or t ogether with noradrenaline did not affect the arteriolar diameter or blood flow rate. Melatonin inhibited both leukocyte rolling and leukotriene B-4 i nduced adhesion while its precursor N-acetylserotonin inhibits only leukocy te adhesion. The rank order of potency of agonists and antagonist receptor selective ligands suggested that the activation of MT2 and MT3 melatonin bi nding sites receptors modulate leukocyte rolling and adhesion, respectively . The effect of melatonin and N-acetylserotonin herein described were obser ved with concentrations in the range of the nocturnal surge, providing the first evidence for a possible physiological role of these hormones in acute inflammation. (C) 2001 Elsevier Science B.V. All rights reserved.