Relationship of rostral ventrolateral medullary neurons and angiotensin in the central control of blood pressure

The rostral ventrolateral medulla (RVL) in the medulla oblongata contains a group of vasomotor neurons that regulate the level of arterial blood pressure. That these spinal-projecting neurons were spontaneously active, cardiac-locked and baro-sensitive further underlies their important role in generating a tonic sympathoexcitatory outflow. In pentobarbital anesthetized spontaneously hypertensive rats (SHR), the electrophysiological behaviours of these sympathoexcitatory cardiovascular neurons were significantly different from those of normotensive Wistar Kyoto rats (WKY). In SHR, there was an equal occurrence of slow-conducting single discharge units and fast-conducting double discharge units while the vast majority of RVL neurons was single discharge units in WKY. Further, the single discharge units of SHR had a higher firing rate and a more regular discharge pattern than those in WKY. The activity of a subpopulation of these RVL cardiovascular neurons was excited by iontophoretic application of angiotensin II, with the magnitude and duration of the excitation greater in SHR than in WKY. Besides, microinjection of [Sar(1), Ile(8)]-angiotensin II, an angiotensin II antagonist, to RVL caused a greater decrease in the firing rate of a subpopulation of RVL cardiovascular neurons in SHR than in WKY. In both types of rats, the neuronal responses were significantly greater in double discharge units than in single discharge units. This was followed by a subsequent depressor response which was significantly greater in SHR than in WKY. These observations demonstrate that in SHR the altered spontaneous electrophysiological property of sympathoexcitatory neurons in the RVL and their augmented sensitivity to the tonic influence of brain angiotensin may contribute to the manifestation of hypertension in this strain of rats.