Control Of Tonic Sympathetic Nerve Activity By The Brainstem In Rats

Control of tonic firing of sympathetic nerves by the rostral ventrolateral medulla (RVLM) and the pontine reticular formation (PRF) was investigated. Electrical activity of renal, splenic, mesenteric and preganglionic greater splanchnic nerves and T{dollar}\sb{lcub}13{rcub}{dollar} white rami was recorded in artificially respired, urethane-anaesthetized rats. Glycine (1.0M;180nl) was microinjected into the RVLM to inhibit neuronal discharge and effects on these nerves were compared. To study descending inhibitory influences on these nerves, effects of RVLM blockade were compared to effects of cervical spinal cord transection. RVLM blockade caused greater decreases in discharge of renal ({dollar}-44\ \pm\ 3\%{dollar}) than splenic ({dollar}-31\ \pm\ 2\%{dollar}) nerves, had no effect on mesenteric nerves and decreased discharge of T{dollar}\sb{lcub}13{rcub}{dollar} white rami ({dollar}-34\ \pm\ 8\%{dollar}) more than that of splanchnic nerves ({dollar}-18\ \pm\ 4\%{dollar}). RVLM blockade and cord transection caused similar decreases in splanchnic ({dollar}-34\ \pm\ 5\%{dollar} and {dollar}-32\ \pm\ 5\%{dollar}) and splenic ({dollar}-45\ \pm 6\%{dollar} and {dollar}-48\ \pm\ 7\%{dollar}) nerve firing. However, RVLM blockade caused greater decreases than cord transection in discharge of renal nerves ({dollar}-53\ \pm\ 5\%{dollar} vs. {dollar}-27\ \pm\ 2\%{dollar}) and T{dollar}\sb{lcub}13{rcub}{dollar} white rami ({dollar}-34\ \pm\ 8\%{dollar} vs. {dollar}+12\ \pm\ 28\%{dollar}). These findings demonstrate that neurons controlling the kidney depend more on excitation from the RVLM than do those controlling the spleen and intestine and that renal neurons may also receive tonic descending inhibition.;Next, glycine (1.0M;65nl) was injected into the PRF to locate regions providing tonic control of arterial pressure, heart rate and activity of renal and splenic nerves in artificially respired, Saffan-anaesthetized rats. PRF blockade caused large, short-lasting decreases in arterial pressure and equal decreases in renal ({dollar}-47\ \pm\ 4\%{dollar}) and splenic ({dollar}-45\ \pm\ 4\%{dollar}) nerve activity. PRF blockade also inhibited ongoing firing of 10 RVLM cardiovascular neurons, so characterized because their discharge was inhibited by baroreceptor activation and was synchronized to the cardiac cycle. These findings indicate that PRF neurons contribute to resting control of vasomotor discharge and provide tonic excitation to RVLM cardiovascular neurons. Finally, the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was iontophoresed into PRF sites in which glycine injections caused cardiovascular responses. PHA-L-labelled axons and terminals were identified primarily within the medullary reticular formation and RVLM. Therefore, PRF neurons could influence tonic sympathetic outflow by synaptic connections in either of these regions. The generation of sympathetic nerve activity probably includes an interaction between the PRF and RVLM