**5.1 The relationship between melatonin and blood pressure**

It has been reported in previous studies that blood pressure and catecholamine levels are related to circadian rhythm. Blood serum melatonin levels of patients with hypertension were found to be low [46]. In addition, melatonin administration has been found to decrease blood pressure in hypertensive and normotensive patients [47, 48]. Yıldız and Akdemir investigated the endogenous role of melatonin on arterial elasticity and blood pressure for arterial expandability as assessed by aortic pulse wave velocity [49]. Melatonin reduces vascular pressure through its receptor on the arterial wall or by modulation of autonomic activity [50]. It has been reported that the vasodilator effect of melatonin can be achieved by inhibiting the methylation of endothelial nitric oxide synthase [51]. In some studies, melatonin has been reported to cause vasoconstriction by inducing norepinephrine signaling as a result of melatonin binding to MT1 receptors on smooth muscle cells [52].

Doolen et al. demonstrated a vasodilator effect on rat caudal arteries by administering 4-phenyl-2-acetamidotetralin, a selective MT2 agonist [53]. According to our current knowledge, MT1 receptor activation causes vasoconstriction, while MT2 receptor activation causes vasodilation [53]. Differential responses of the vascular bed to melatonin uptake are associated with different distribution of melatonin receptors.
