ADRENERGIC AND NONADRENERGIC SPINAL PROJECTIONS OF A CARDIOVASCULAR-ACTIVE PRESSER AREA OF MEDULLA-OBLONGATA - QUANTITATIVE TOPOGRAPHIC ANALYSIS

A cardiovascular-active presser 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 presser responses larger than 10 mmHg wer e localized to a rostral longitudinal cell column of the nucleus retic ularis rostroventrolateralis (n.RVL) extending 450 mu m posteriorly to the facial nucleus. Spinal projections from the ventrolateral medulla were studied with a dual retrograde transport-immunocytochemical meth od. A striking correspondence was observed between the ventrolateral p resser area (VLPA) of n.RVL and rostrocaudal distribution of a circums cribed population of thoracic reticulospinal neurons containing tyrosi ne 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 im munocytochemically positive for TH; 28% of the reticulospinal projecti on cells were immunonegative. Deposits of L-Glu and dye through the sa me micropipettes verified a consistent correlation of vasopressor site s and the rostral subset of catecholaminergic neurons. Since comparabl e numbers of cell bodies in the VLPA contain TH and PNMT all are presu med to be adrenergic. At levels of n.RVL immediately adjacent to the V LPA commencing at a level 450 mu m caudal to the facial nucleus, sites were unresponsive to Glu-stimulation or vasodepressor. At these level s, only non-adrenergic reticulospinal neurons project to cervical or t horacic spinal segments. We conclude that the VLPA is highly restricte d to a narrow column of n.RVL < 0.5 mm in length and corresponds preci sely with a population of predominantly adrenergic thoracic reticulosp inal neurons that project exclusively to sympathoadrenal preganglionic motoneurons [cf 46]. These findings corroborate the idea that an adre nergic-spinal pathway may play a role in controlling sympathetic outfl ow.