AREAS OF ROSTRAL MEDULLA PROVIDING TONIC CONTROL OF RENAL AND SPLENIC NERVES

The objective of this study was to determine whether nonuniform control of tonic discharge of renal and splenic nerves by the rostral ventrolateral medulla (RVLM) is dependent on viscerotopic representation of the kidney and the spleen in this region. Small (15 nl, 13 pmol) injections of the potent tau-aminobutyric acid agonist muscimol were used to map the RVLM of anesthetized rats, searching for areas that selectively provided tonic excitation of renal or splenic nerves. Most of the muscimol microinjections produced depressor responses and inhibition of sympathetic nerve activity. Activity of renal nerves was inhibited significantly more than that of splenic nerves, but we found no evidence for topographical organization of RVLM influences on tonic activity of these two nerves. However, this investigation did yield an important new observation, since approximately 10% of the injections caused pressor responses and increases in sympathetic discharge. The unexpected increases must have been caused by blockade of small groups of tonically active sympathoinhibitory neurons located throughout the RVLM. This disinhibition was not evoked from any specific region of the RVLM. In conclusion, the RVLM neurons and putative interneurons that tonically control the discharge of postganglionic renal and splenic neurons appear to be distributed homogeneously throughout this area. Neither excitatory nor inhibitory elements were organized topographically to specifically influence the renal or the splenic nerve. The neural organization responsible for the preferential effect on renal nerves may exist in the local microcircuitry within the RVLM and will be revealed only by an even more discrete analysis of these circuits.