Fluorescence location of RVLM kainate microinjections that alter the control of breathing.

Kainic acid (4.7 mM) applied to the rostral ventrolateral medulla (RVLM) surface decreases phrenic output, CO2 sensitivity, and blood pressure in chloralose-urethan-anesthetized, vagotomized, paralyzed, glomectomized, servoventilated cats. In this study using the same preparation, bilateral 50- to 100-nl kainate injections just below the RVLM surface better localized these responses topographically. The physiological responses to unilateral 10-nl kainate injections were then correlated with anatomic location determined by fluorescent microbeads (0.5 micron diam). Many sites were associated with no effect, a few rostral and caudal sites with increased phrenic activity, and cluster of sites with decreased phrenic activity often to apnea, decreased CO2 sensitivity, and decreased responses to carotid sinus nerve stimulation. Blood pressure was unaffected. These sites, within 400 microns of the surface, were ventral to the facial nucleus, ventrolateral to the nucleus paragigantocellularis lateralis, caudal to the superior olive, and rostral to the retrofacial nucleus. They appeared to be within the recently described retrotrapezoid nucleus, which contains cells with respiratory-related activity and projections to the dorsal and ventral respiratory groups. Cells within this site appear able to provide tonic input to respiration and to affect peripheral and central chemoreception.