Role of the Rostroventrolateral Medulla in the Generation of Rhythmicities of the Sympathetic Activity

Abstract

In anaesthetized, paralyzed, bilaterally vagotomized, and artificially ventilated rats, the ventral medullary surface was exposed. Spontaneous sympathetic activity (SA) of the preganglionic cervical -and postganglionic 1unbar nerves was recorded and integrated. For the evaluation of respiratory-related synchronization in the time domain, phrenic nerve activity and SA were integrated and averaged. In the frequency domain, the analysis of the periodicities was determined by Fast Fourier Transform (FFT). Ipsi-, contra- or bilateral microinjections of calcium channel blockers, CoCl₂ or MgCl₂ into the rostroventrolateral medulla (RVLM) abolished respiratory frequency and cardiac frequency related peaks in the power density spectrum (PDS), and significantly reduced the power of other periodicities. After bilateral microinjections of Ca-channel blockers into the RVLM, the magnitude of PDS peaks was reduced to those which remained in spinal animals. A similar effect was produced by kynurenate. An increase in blood pressure and an augmented desynchronized sympathetic discharge accompanied this central effect produced by Ca-channel blockers. SA desynchronization was reversible, lasted for 15-25 minutes, and could be reproduced by repetitive microinjections. It is concluded that in the time domain a restricted neuronal pool within the rostral para-ambigual area of the RVLM determines the respiratory-related rhythmical SA pattern occurring in each respiratory cycle. In the frequency domain, the RVLM neurons are critical for supraspinal rhythmogenesis of SA.