Negative chronotropism of the heart is inhibited with lesions of the caudal medulla in the rat

Neurons in the ventrolateral medulla are essential for cardiorespiratory re gulation. It has been suggested that neurons in the caudal ventrolateral me dulla are responsible for the negative chronotropic effect of the heart, at least in carnivores, because injection of glutamate into this area decreas es heart rate significantly. In the present study, we monitored heart rate both before and after injections of the excitotoxin ibotenic acid into the most caudal part of the ventrolateral medulla in rats. We found that restin g heart rate increased significantly by more than 53% (P<0.0001) after the ibotenic acid injections. This result suggests that neurons located in the caudal ventrolateral medulla are responsible for the negative chronotropic effect of the heart in the rat, especially its most caudal part. (C) 2001 P ublished by Elsevier Science B.V.