Ventrolateral medulla and sympathetic chemoreflex in the rat.

Splanchnic sympathetic nerve discharge (SND), phrenic nerve activity (PND) and putative sympathetic premotor neurons of the rostral ventrolateral medulla (RVL) were recorded in urethane-anesthetized vagotomized rats without aortic baroreceptor afferents. Carotid chemoreceptor stimulation with brief N2 inhalation increased SND by 101 +/- 7%, raised mean arterial pressure (MAP) and increased the discharge rate of RVL premotor neurons by 46 +/- 12% (N = 32). During chemoreceptor activation. SND and most RVL neurons displayed pronounced central respiratory rhythmicity with maximal firing probability immediately after cessation of the PND (postinspiratory phase) and lowest probability during PND (inspiratory phase). Bilateral microinjection of the breed spectrum glutamate receptor antagonist kynurenic acid (Kyn, 5 nmol in 100 nl) into RVL blocked the sympathetic chemoreflex but left the sympathetic baroreflex intact. In contrast, bilateral microinjection of the same dose of Kyn into the caudal ventrolateral medulla (at obex level CVL) blocked the baroreflex but left the sympathetic chemoreflex intact. Bilateral microinjection of the GABAA agonist muscimol (87.5 pmol in 50 nl) into CVL produced effects identical to those of Kyn. These results confirm that the caudal ventrolateral medulla contains an essential relay of the sympathetic baroreflex and demonstrate that the same area plays no role in the sympathetic chemoreflex. The data suggests that these two reflexes could have a largely independent course through the medulla oblongata and that integration between the baroreceptor and chemoreceptor information used for sympathetic vasomotor control may occur as late as the premotor neuronal stage in RVL.