Anatomical substrates for baroreflex sympathoinhibition in the rat

The fundamental neuronal substrates of the arterial baroreceptor reflex hav e been elucidated by combining anatomical, neurophysiological, and pharmaco logical approaches. A serial pathway between neurons located in the nuclei of the solitary tract (NTS), the caudal ventrolateral medulla (CVL), and th e rostral ventrolateral medulla (RVL) plays a critical role in inhibition o f sympathetic outflow following stimulation of baroreceptor afferents, In t his paper, we summarize our studies using tract-tracing and electron micros copic immunocytochemistry to define the potential functional sites for syna ptic transmission within this circuitry. The results are discussed as they relate to the literature showing: (1) baroreceptor afferents excite second- order neurons in NTS through the release of glutamate; (2) these NTS neuron s in turn send excitatory projections to neurons in the CVL; (3) GABAergic CVL neurons directly inhibit RVL sympathoexcitatory neurons; and (4) activa tion of this NTS-->CVL-->RVL pathway leads to disfacilitation of sympatheti c preganglionic neurons by promoting withdrawal of their tonic excitatory d rive, which largely arises from neurons in the RVL, Baroreceptor control ma y also be regulated over direct reticulospinal pathways exemplified by a ne wly recognized sympathoinhibitory region of the medulla, the gigantocellula r depressor area. This important autonomic reflex may also be influenced by parallel, multiple, and redundant networks. (C) 2000 Elsevier Science Inc.