Melatonin decreases brain serotonin release, arterial pressure and heart rate in rats

The effects of intravenous administration of melatonin (30-60 mg/kg) or vehicle (10% alcohol) on arterial pressure, heart rate, blood gases or brain serotonin release were assessed in rats under urethane anesthesia. Administration of melatonin, but not the vehicle, produced a dose-related fall in mean arterial pressure, heart rate, or serotonin release in both the corpus striatum and the hypothalamus. Melatonin treatment had an insignificant effect on either P(a)CO₂, P(a)O₂ or pH. In addition, the melatonin-induced depressor responses were abolished by pretreatment with spinal transection, whereas melatonin-induced bradycardia was abolished by pretreatment with bilateral vagotomy. These results suggest that melatonin decreases brain serotonin release and results in sympathetic inhibition or parasympathetic stimulation which leads to hypotension and bradycardia in rats.