The role of descending monoaminergic systems in central control of blood pressure.

The descending projections to the intermediolateral cell column arise from several sources. The serotonergic inputs come from the raphe pallidus, raphe obscurus, raphe magnus nuclei, and ventral medulla. The A5 cell group provides a norepinephrine input to the intermediolateral cell column and the A1 cell group appears to be the main source of the epinephrine input. The functional role of monoamines on sympathetic preganglionic neurons is unclear. Iontophoresis studies indicate that serotonin excites and norepinephrine inhibits sympathetic preganglionic neurons. Pharmacological studies indicate that an alpha-2 receptor is located on or near these neurons. In contrast, electrical stimulation of the monoamine cell bodies produces opposite results. Stimulation of the raphe nuclei causes an inhibition of the sympathetic outflow whereas stimulation of some of the noradrenergic cell groups causes pressor responses. The role of monoamines in various cardiovascular reflexes and in experimental hypertension is discussed.