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)

Medullary lateral tegmental field mediates the cardiovascular but not respiratory component of the Bezold-Jarisch reflex in the cat

“…This difference can be explained by the fact that the LTF of the cat contains a functionally heterogeneous population of neurons involved in control of SND (2,3,6,13,37). Whereas the CVLM contains sympathoinhibitory neurons (5,19,32,37), the LTF of the cat contains both sympathoinhibitory and sympathoexcitatory neurons (2,3,6,13,37). LTF sympathoexcitatory neurons are inhibited during activation of the baroreceptor and/or BezoldJarisch reflexes (2,6,13,37), and their axons appear to terminate in the RVLM (2).…”

Medullary lateral tegmental field: control of respiratory rate and vagal lung inflation afferent influences on sympathetic nerve discharge

“…The most rostral injection site in the LTF is ϳ2 mm caudal, 1 mm medial, and ϳ1 mm dorsal to the region of the RVLM that contains neurons with activity correlated to SND (8,30). The most caudal injection site in the LTF is 1-1.5 mm medial and ϳ1 mm dorsal to the most rostral portion of the caudal ventrolateral medulla (CVLM) that contains neurons with activity correlated to SND (11).…”

Role of ventrolateral medulla in generating the 10-Hz rhythm in sympathetic nerve discharge

“…Single neurons with naturally occurring discharges correlated to the 10-Hz rhythm in SND have been identified in three lower brain stem regions: caudal and rostral portions of the ventrolateral medulla (VLM) and medullary raphe nuclei (3,4,7,8,15). As demonstrated with antidromic activation, many of these neurons in the rostral VLM and medullary raphe complex send their axons to the intermediolateral nucleus (IML) of the thoracic spinal cord (4).…”

“…Although the 10-Hz rhythm in sympathetic nerve discharge (SND) is most evident after baroreceptor denervation, it can occur in combination with the cardiac-related rhythm in baroreceptor-innervated cats (4,7,14,32,35). The 10-Hz rhythm is functionally relevant because its sudden appearance in SND is accompanied by an abrupt increase in arterial pressure (5).…”

“…Those in the caudal VLM do not have spinal projections (8). Microinjection of the GABA-A receptor agonist, muscimol, into any one of these regions reduces or eliminates the 10-Hz rhythm in SND (7,49). Nonetheless, information on the extent and nature of the interconnections among these cell groups remains precursory, as does the issue of which group(s) comprise the rhythm generator and which comprise the follower circuits.…”