IDENTIFICATION OF C1 PRESYMPATHETIC NEURONS IN RAT ROSTRAL VENTROLATERAL MEDULLA BY JUXTACELLULAR LABELING IN-VIVO

The rostral ventrolateral medulla (RVLM) contains barosensitive, bulbo spinal neurons that provide the main supraspinal excitatory input to s ympathetic vasomotor preganglionic neurons. However, the phenotype of the critical RVLM cells has not been conclusively determined. The goal of the current study was to identify the proportion of electrophysiol ogically defined, putative, presympathetic RVLM neurons that are C1 ce lls. We used a juxtacellular labeling technique to individually fill s pontaneously active, barosensitive, bulbospinal RVLM neurons with biot inamide following electrophysiological characterization in chloralose- anesthetized rats. To determine whether these neurons could be classif ied as C1 cells, the biotinamide-labeled cells were processed for dete ction of tyrosine hydroxylase. The majority of barosensitive bulbospin al RVLM neurons were tyrosine hydroxylase immunoreactive (TH-ir: 28 of 39). All of the barosensitive bulbospinal RVLM neurons with axonal co nduction velocities in the C fiber range (<1 m/second) were TH-ir (n = 16), whereas faster conducting cells (1 to 7 m/second) were either li ghtly TH-ir (n = 12) or not detectably TH-ir (n = 11). Adjacent respir atory-related RVLM units labeled with biotinamide were not detectably TH-ir (n = 10). To verify that TH-ir cells were indeed adrenergic, a s ubset of barosensitive bulbospinal cells labeled with biotinamide were examined for phenylethanolamine N-methyltransferase immunoreactivity (PNMT-ir). Three slowly conducting cells had detectable PNMT-ir, and t wo fast-conducting cells had no detectable PNMT-ir. These results indi cate that the majority of bulbospinal RVLM neurons with putative sympa thoexcitatory function are C1 cells. J. Comp. Neurol. 387:524-536, 199 7. (C) 1997 Wiley-Liss, Inc.