Ascending collaterals of medullary barosensitive neurons and C1 cells in rats.

Triple-label experiments were conducted in rats to determine whether reticulospinal C1 adrenergic neurons of rostral ventrolateral medulla (RVLM) also innervate dorsolateral pons (LPBN/KF), mesencephalic central gray area (CG), and hypothalamus (LH). Ten percent of RVLM neurons that project to LPBN/KF were immunoreactive for tyrosine hydroxylase (TH), and 50% of those projecting to either pariaqueductal gray (PAG) or LH were TH immunoreactive. A large proportion of RVLM TH-immunoreactive neurons that were retrogradely labeled from either LPBN/KF or PAG (65.5 and 28.5%, respectively) were retrogradely labeled from the spinal cord as well. A smaller fraction of RVLM TH-immunoreactive neurons that were labeled from LH were also labeled from the spinal cord (13.1%). The vast majority of the cohaterized RVLM neurons were TH immunoreactive (LPBN/KF 50%; PAG 82%; LH 84%), and the remaining ones may be nonadrenergic neurons. Antidromic (AD) mapping techniques were also used to determine if RVLM reticulospinal barosensitive neurons (putative vasomotor sympathoexcitatory cells, total sample of 106) send axonal collaterals to the supramedullary areas investigated anatomically. Twenty percent of the cells sampled (n = 21) were also AD activated from 1 of 20 supramedullary sites. The combined approaches support the following conclusions: 1) RVLM barosensitive neurons include phenotypically adrenergic cells, and 2) RVLM barosensitive cells do not represent an output pathway solely dedicated to providing an excitatory drive to sympathetic preganglionic neurons.