PRE-SYMPATHETIC NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA OF THE RAT - ELECTROPHYSIOLOGY, MORPHOLOGY AND RELATIONSHIP TO ADJACENT NEURONAL GROUPS

The activity of preganglionic sympathetic neurones largely depends on synaptic excitation from antecendent reticulospinal neurones located i n the rostral ventrolateral medulla (RVLM). Our study, conducted in an aesthetized rats, showed that all RVLM pre-sympathetic neurones displa y a substantial synaptic noise and their action potentials are usually preceded by fast EPSPs. No evidence was found for presence of gradual depolarizations (autodepolarizations) between individual spikes. Ther efore our results are consistent with the ''network'' hypothesis for t he generation of sympathetic vasomotor tone. Axons of some pre-sympath etic neurones intracellularly labelled with Neurobiotin or Lucifer Yel low had collaterals arborizing in several medullary regions. Thus thes e neurones have synaptic inputs not only to preganglionic sympathetic neurones, but also to other, yet unidentified cells in the brainstem. Finally, our results show that anatomically adjacent RVLM pre-sympathe tic and Botzinger respiratory neurones from two functionally distinct neuronal subpopulations, and that some pre-sympathetic neurones have a n adrenergic phenotype.