SYMPATHOINHIBITORY RESPONSES TO 2-METHYLSEROTONIN DURING CHANGES IN SODIUM-INTAKE

The vagal-mediated reflex responses elicited by the selective serotonin type 3 receptor agonist 2-methylserotonin were examined by administration (6.25, 12.5, 25, and 50 mug/kg i.v.) of 2-methylserotonin to sinoaortic-denervated rats with either intact or sectioned vagi. To study the influence of dietary sodium intake on 2-methylserotonin-induced vagal reflex responses, we performed experiments in rats fed either a high or low sodium diet. Left ventricular end-diastolic pressure was significantly higher in animals on high than low salt diet. However, mean arterial pressure and heart rate were similar in high and low salt groups. In rats with intact vagi, 2-methylserotonin produced a dose-dependent increase in afferent vagal nerve activity and a dose-dependent decrease in efferent renal sympathetic nerve activity, mean arterial pressure, and heart rate. The sympathoinhibitory responses of decreased efferent renal sympathetic nerve activity, mean arterial pressure, and heart rate were abolished by vagotomy and were not affected by changes in dietary sodium intake. We conclude that the sympathoinhibitory effect of 2-methylserotonin is due to stimulation of vagal afferents with inhibitory action on peripheral sympathetic nerve activity and that the sympathoinhibitory responses are unaffected by changes in dietary sodium intake.