BAROSENSITIVE NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA OF THE RATIN-VIVO - MORPHOLOGICAL PROPERTIES AND RELATIONSHIP TO C1 ADRENERGIC-NEURONS

The aim of this study, conducted in anaesthetized rats, was to examine the morphology of barosensitive neurons in the rostral ventrolateral medulla and their immunoreactivity for a catecholamine synthesizing en zyme, tyrosine hydroxylase. Thirty neurons displaying inhibitory posts ynyptic potentials following stimulation of the aortic depressor nerve were intracellularly labelled with Lucifer Yellow or Neurobiotin. Som e of these neurons could be excited antidromically from the second tho racic segment of the spinal cord, with conduction velocities of spinal axons ranging from 1.9 to 7.2 m/s. The filled somas were found immedi ately caudal to the facial nucleus and ventral or ventromedial to comp act formation of the nucleus ambiguus. Some dendrites reached the vent ral medullary surface. Axons usually projected dorsomedially and then made a sharp rostral and/or caudal turn. The caudally projecting axon could, in some cases, be followed to the first cervical segment of the spinal cord. Seven cells issued fine axon collaterals on the ipsilate ral side. These were identified mainly in two areas: in the rostral ve ntrolateral medulla (or immediately dorsomedial to that region), and w ithin the dorsal vagal complex. Seven of 27 examined cells (26%) were tyrosine hydroxylase-immunoreactive and were classified as C1 adrenerg ic neurons. No clear relationship was found between the presence or ab sence of adrenergic phenotype and the morphology of filled cells. Howe ver, the amplitude of aortic nerve-evoked inhibitory postsynaptic pote ntials was significantly larger in tyrosine hydroxylase-positive neuro ns. Possible reasons for the low percentage of barosensitive cells wit h tyrosine hydroxylase immunoreactivity found in this study, in compar ison with previously published estimates, are discussed. This is the f irst study describing the morphology of neurons in this part of the me dulla identified as barosensitive in vivo, and directly demonstrating adrenergic phenotype in a subset of these neurons.