Caudal midline medulla mediates behaviourally-coupled but not baroreceptor-mediated vasodepression.

Within the caudal medulla there are two regions whose activation leads to vasodepression and bradycardia, the caudal ventrolateral medulla and a discrete region of the caudal midline medulla. This study investigated, in the halothane anaesthetized rat, the contribution of these two vasodepressor regions to "homeostatic" and "behaviourally-coupled" cardiovascular regulation. In an initial set of experiments the contribution of each of these two regions to the hypotension and bradycardia evoked by acute hypovolaemia (15% haemorrhage) was investigated. It was found that inactivation of the caudal midline medulla significantly attenuated (cobalt chloride) or completely blunted (lignocaine) the hypotension and bradycardia evoked by acute hypovolaemia. In contrast, inactivation of the caudal ventrolateral medulla using cobalt chloride, although attenuating the magnitude of the hypotension and completely blocking the bradycardia, did not delay the onset of the hypotension evoked by acute hypovolaemia. The caudal ventrolateral medulla is known to be critical in homeostatic cardiovascular control through the expression of the "baroreceptor reflex" and the hypotension and bradycardia evoked by activation of cardiopulmonary afferents. In a second series of experiments we found inactivation of the caudal midline medulla played no role in baroreflex-evoked bradycardia (i.v. phenylephrine) or the hypotension and bradycardia evoked by cardiopulmonary afferent activation (i.v. 5-hydroxytryptamine). These data suggest that the caudal midline medulla and caudal ventrolateral medulla play different roles in cardiovascular control. The caudal ventrolateral medulla is involved in mediating cardiovascular changes associated with a variety of stimuli including "homeostatic" and "behaviourally-coupled" cardiovascular changes, whereas the caudal midline medulla is critical for mediating "behaviourally-coupled" changes in arterial pressure and heart rate.