Role of the rostral ventrolateral medulla in the generation of synchronized sympathetic rhythmicities in the rat.

In artificially ventilated, paralyzed rats anesthetized with Nembutal or urethane, power density spectral analysis (PDS), using direct FFT algorithm, was used to quantify rhythmicities in the sympathetic cervical and lumbar nerves after bilateral microinjections into rostral ventrolateral medulla (RVLM) of CoCl2 and MgCl2--unspecific synaptic transmission blockers. Later overall sympathetic activity, phrenic nerve discharge, heart rate and arterial blood pressure were recorded. Block of synaptic transmission in RVLM was tested by elimination of sympathoinhibitory or sympathoexcitatory reflex responses to aortic nerve and vagal afferents stimulation respectively. In animals vagotomized with bilateral section of carotid sinus nerve the power in all frequency bands was very significantly reduced to a level not different from that which remained after spinal cord transsection. If carotid baroreceptors were intact, a small peak corresponding to cardiac frequency band persisted. Overall, non-synchronized sympathetic activity and arterial blood pressure increased. All effects were transient, lasted up to 15 min, and could be reproduced several times in one experiment. Respiratory rhythmic activity was unchanged yet respiratory-sympathetic synchronization was abolished. It is concluded that RVLM reticulospinal sympathoexcitatory neurons are responsible for non-synchronized tonic sympathetic activity but are not able to generate synchronized sympathetic rhythms. Synaptic input, presumably inhibitory, either from local neuronal circuits within ventral medulla and/or from other brain stem neuronal circuitries is needed to shape out the flexible pattern of sympathetic oscillations.