Vasomotor control by subretrofacial neurones in the rostral ventrolateral medulla

In this paper we review our recent work in the rabbit and cat on the role of the rostral ventrolateral medulla in cardiovascular regulation. Microinjection of neuroexcitatory amino acids into a highly circumscribed region, located just ventral to the retrofacial nucleus at the level of the rostral part of the inferior olive, leads to an increase in blood pressure, owing to sympathetic vasoconstriction. Bilateral destruction of this region, which we have termed the subretrofacial nucleus, leads to a profound fall in blood pressure. Anatomical studies show that the subretrofacial nucleus contains a compact group of bulbospinal neurones that project to sympathetic preganglionic nuclei in the thoracolumbar spinal cord. Single-unit recording studies have shown that these bulbospinal neurons are spontaneously active and are powerfully inhibited by baroreceptor inputs. These observations indicate that the subretrofacial bulbospinal cells are sympathoexcitatory and play a major role in the tonic and phasic control of the cardiovascular system. Some important unresolved questions regarding the subretrofacial neurones will be discussed. (i) Are they functionally homogenous, or are they viscerotopically organized with respect to particular end organs? (ii) What are their afferent inputs? (iii) What are their histochemical properties? Specifically, are they part of the group of adrenaline-synthesizing cells, or alternatively, substance P cells?