Effects And Mechanism Of Melatonin On Bp Of Stress-induced Hypertensive Rats

Melatonin, an important hormone secreted by the pineal gland exerts widely biological function such as sleep promotion, biological rhythm regulation,immune reinforcement, exterminating free radical,delaying aging and anti-stress. Forty years before melatonin has been found to regulate the blood pressure in mammals. Several laboratories have reported that surgical removal of the pineal gland elevated the blood pressure and even induced hypertension in rats. Another laboratory reported that melatonin had antihypertensive action in the spontaneously hypertensive rat. The hypotensive effect of melatonin also has been observed in human beings. Melatonin can reduce the blood pressure not only in healthy men, but also in patients with essential hypertension. Recently it was shown that microinjection of melatonin into the anterior hypothalamic area (AHA) produced a dose-related fall in MAP of stress-induced hypertension rats.Therefore,melatonin maybe act as a central depressor factor to regulate the cardiovascular activity. Nowadays,rapid development,quick living tempo and acute competition cause that the intensity and quantity of stress on one's mind and spirit are obviously much more that before.We considered that the stimulation of stress was the important reason to induce hypertension,and recent years the prevalence rate of hypertension appears obviously tendency. There are some reports related to the anti-stress effect of melatonin by enhancing the central inhibitory effects of GABA, but rarely do papers describe whether melatonin inhibit the forming of the stress-induced hypertension. In the brain of mammals there is a high level of GABA which is the major inhibitory neurotransmitter in the central nervous system (CNS). It has been shown that GABA can exert modulatory effects on cardiovascular activities. In addition, GABA/BZD is one of anti-stress system of brain and activated GABA receptor can inhibit or revert physiological and behavior changes caused by stress. Recently study shows that GABA can inhibit angiotensin I-converting enzyme (ACE) which is the synthetic rate enzyme of angiotensin II. The level of angiotensin II in stress-induced hypertensive rats is significantly elevated after stress and intracerebroventricular injection of angiotensin II can elevate the blood pressure of rats. These reports support that angiotensin II acts as a hypertensive factor during the stress and GABA can inhibit the synthesis of angiotensin II. Several laboratories reported that there was a binding site of melatonin on the GABAA receptor. Therefore, melatonin may act as an endogenous hypotensive factor and exert its anti-stress effect by the GABAA receptor. Electrical field stimulation(EFS) of pineal slices released angiotensin II and reproduced many of the actions of exogenous angiotensin II on serotonin and melatonin biosynthesis and release.That means melatonin can not only inhibit the action of angiotensin II but also reduce angiotensin II biosynthesis by the GABAA receptor enhancing the inhibitive function of GABA.