Caudal ventrolateral medullary neurons are elements of the network responsible for the 10-Hz rhythm in sympathetic nerve discharge

Abstract

1. This is the first study to show that caudal ventrolateral medullary (CVLM) neurons play an important role in governing the 10-Hz rhythm in sympathetic nerve discharge (SND). Spike-triggered averaging showed that the naturally occurring discharges of 66 of 246 CVLM neurons located 0–2.5 mm rostral to the obex, 4–4.25 mm lateral to the midline, and within 2 mm of the ventral surface were correlated to the 10-Hz rhythm in inferior cardiac SND of 17 urethan-anesthetized cats. 2. Frequency domain analysis was used to characterize further the relationships between SND and the discharges of 45 CVLM neurons with activity correlated to the 10-Hz rhythm in inferior cardiac nerve activity. The autospectra of the discharges of 22 of these neurons contained a sharp peak near 10 Hz (corresponding to the peak in the autospectra of SND), although the mean firing rate of these neurons was only 5.9 +/- 0.5 (SE) spikes/s. The peak coherence value relating the 10-Hz discharges of these CVLM neurons and the inferior cardiac nerve was 0.42 +/- 0.03. The autospectra for the other 23 CVLM neurons did not contain a peak near 10 Hz. Their mean firing rate was 2.3 +/- 0.5 spikes/s, and the peak coherence value relating their discharges to the 10-Hz rhythm in SND was 0.08 +/- 0.01. The coherence value was significantly different than zero in all but three cases. 3. Importantly, spike-triggered averaging and coherence analysis demonstrated that CVLM neurons with activity correlated to the 10-Hz rhythm did not have activity correlated 1:1 to the cardiac-related rhythm in SND of baroreceptor-innervated cats. Also, their discharges were not correlated to the irregular 2- to 6-Hz oscillations in SND of baroreceptor-denervated cats. These data support the hypothesis that different pools of brain stem neurons generate the 10-Hz rhythm and the 2- to 6-Hz oscillations (or cardiac-related rhythm) in SND. 4. Despite the fact that CVLM neurons with activity correlated to the 10-Hz rhythm did not have activity correlated 1:1 to the cardiac-related rhythm in SND, these neurons were influenced by baroreceptor afferent nerve activity. First, their firing rates could be decreased (n = 12) or increased (n = 2) during the pressor response induced by inflating a balloon in the aorta (aortic obstruction). Second, on occasion, the discharges of CVLM neurons and the 10-Hz rhythm in SND were entrained to a harmonic of the heart rate.(ABSTRACT TRUNCATED AT 400 WORDS)