Adrenergic and non-adrenergic spinal projections of a cardiovascular-active pressor area of medulla oblongata: quantitative topographic analysis

A cardiovascular-active pressor area of medullary reticular formation was defined by mapping changes in arterial blood pressure produced by microinjections of the neuroexcitatory amino acid, l-Glutamate (l-Glu). Sites where l-Glu provoked pressor responses larger than 10 mmHg were localized to a rostral longitudinal cell column of the nucleus reticularis rostroventrolateralis (n.RVL) extending 450 μm posteriorly to the facial nucleus. Spinal projections from the ventrolateral medulla were studied with a dual retrograde transport-immunocytochemical method. A striking correspondence was observed between the ventrolateral pressor area (VLPA) of n.RVL and rostrocaudal distribution of a circumscribed population of thoracic reticulospinal neurons containing tyrosine hydroxylase (TH)- or phenylethanolamine N-methyltransferase (PNMT)-immunoreactivity. Quantitative analysis revealed that 72% of the total number of retrogradely labeled neurons within the active area were immunocytochemically positive for TH; 28% of the reticulospinal projection cells were immunonegative. Deposits of l-Glu and dye through the same micropipettes verified a consistent correlation of vasopressor sites and the rostral subset of catecholaminergic neurons. Since comparable numbers of cell bodies in the VLPA contain TH and PNMT all are presumed to be adrenergic. At levels of n.RVL immediately adjacent to the VLPA commencing at a level 450 μm caudal to the facial nucleus, sites were unresponsive to Glu-stimulation or vasodepressor. At these levels, only non-adrenergic reticulospinal neurons project to cervical or thoracic spinal segments. We conclude that the VLPA is highly restricted to a narrow column of n.RVL < 0.5 mm in length and corresponds precisely with a population of predominantly adrenergic thoracic reticulospinal neurons that project exclusively to sympathoadrenal preganglionic motoneurons [cf 46]. These findings corroborate the idea that an adrenergic-spinal pathway may play a role in controlling sympathetic outflow.