Effect of 5-HT depletion on cardiovascular vagal reflex sensitivity in awake and anesthetized rats

Antagonism of central 5-HT1A and 5-HT7 receptors inhibits reflex-evoked vagal bradycardias indicating that 5-HT is released during these reflexes. The present experiments examined the effect of 5-HT depletion with para-chlorophenylalanine (p-CPA) on the cardiac vagal baroreflex and cardiopulmonary reflex in awake and anesthetized rats. Immunocytochemistry and neurochemical detection showed that p-CPA depleted the brainstem of 5-HT, but not of norepinephrine or dopamine. Depletion of 5-HT was associated with an increase in mean arterial pressure (MAP) in awake rats. This difference was abolished by anesthesia, which reduced MAP in both groups of animals. The baroreflex gain, whether calculated from the rise in pressure induced by phenylephrine or the fall in pressure evoked by sodium nitroprusside, was significantly attenuated in depleted rats compared to controls. This attenuation of the baroreflex gain was unaffected by subsequent anesthesia. 5-HT depletion also attenuated the cardiopulmonary reflex vagal bradycardias but this only reached statistical significance when the rats were anesthetized. The data support the view that 5-HT is released in the reflex activation of the cardiac vagal pathway.