Inhibition of baroreflex vagal bradycardia by activation of the rostral ventrolateral medulla in rats

In stressful conditions, baroreflex vagal bradycardia (BVB) is often suppre ssed while blood pressure is increased. To address the role of the rostral ventrolateral medulla (RVL), a principal source of sympathetic tone, in inh ibition of BVB, we microinjected DL-homocysteic acid (DLH, 6 nmol) into the RVL of chloralose-urethan-anesthetized, sinoaortic-denervated rats to exam ine the effect on BVB. The BVB was provoked by electrical stimulation of th e aortic depressor nerve ipsilateral to the injection sites. DLH microinjec tion was found to suppress BVB while increasing blood pressure. The inhibit ion of BVB was observed even during the early phase in which DLH transientl y suppressed central inspiratory activity. The inhibition was not affected either by upper spinal cord transection or suprapontine decerebration. Simi lar results were obtained by microinjection of bicuculline methiodide (160 pmol), a GABA antagonist, into the RVL of carotid sinus nerve-preserved rat s due to withdrawal of a tonic GABA-mediated, inhibitory influence includin g the input from arterial baroreceptors. In conclusion, activation of the R VL inhibits BVB at brain stem level independently of central inspiratory dr ive.