Bulbospinal serotonergic pathways in the control of blood pressure.

The anatomy, physiology, and pharmacology of bulbospinal 5-HT neurons innervating spinal sympathetic cell groups are reviewed. On the basis of the location of 5-HT cell bodies, size of axons, type of synapse, and diversity of chemical coding, it is argued that the group of bulbospinal neurons is anatomically heterogenous. The evidence for both sympathoexcitatory and sympathoinhibitory actions of these neurons is discussed, and, although unequivocal evidence is still lacking, the idea of a functional heterogeneity is attractive. This is supported by observations that 5-HT also has both sympathoexcitatory and sympathoinhibitory actions in the spinal cord. These actions are mediated via two receptors, 5-HT1-like and 5-HT2, respectively. The question of whether or not these receptors are tonically activated is discussed, and it is shown that most evidence indicates they are not.