Quantitative analysis of bulbospinal projections from the rostral ventrolateral medulla: Contribution of C1‐adrenergic and nonadrenergic neurons

The contribution of C1‐adrenergic and nonadrenergic neurons to the spinal projection from the rostral ventrolateral medulla (RVLM) and their relative innervation density throughout thoracic spinal segments were examined by combining the Fluorogold (FG) retrograde tracing technique with immunofluorescent labeling for the epinephrine‐synthesis enzyme phenylethanolamine N‐methyltransferase (PNMT). The results indicate that the RVLM‐spinal projection is comprised of both PNMT‐positive and PNMT‐negative neurons located in the subretrofacial area of the RVLM, approximately 1 to 1.7 mm rostral to obex. The bulbospinal projection from the RVLM is predominantly ipsilateral, and bulbospinal neurons do not appear to be organized within the RVLM in a manner indicating their segmental termination site. Eighty‐one percent (4%, n = 2) of the PNMT‐positive cells in the ipsilateral subretrofacial RVLM were retrogradely labeled after unilateral FG injections into multiple thoracic levels of the intermediolateral cell column (IML). Following, single level FG injections, the number of retrogradely labeled PNMT‐positive neurons in the subretrofacial RVLM decreased with injections in more caudal thoracic segments, indicating a heavier innervation of the upper thoracic IML by C1 neurons. PNMT‐negative neurons were the main component of the RVLM‐spinal population with 63 · 8% (n = 7) of the non‐PNMT‐containing neurons within the ipsilateral subretrofacial RVLM innervating all thoracic levels of the IML. The results indicate that both C1‐adrenergic and nonadrenergic neurons in the RVLM make a substantial contribution to the innervation of the IML.