CAUDAL VENTROLATERAL MEDULLARY NEURONS ARE ELEMENTS OF THE NETWORK RESPONSIBLE FOR THE 10-HZ RHYTHM IN SYMPATHETIC-NERVE DISCHARGE

1. This is the first study to show that caudal ventrolateral medullary (CVLM) neurons play an important role in governing the 10-Hz rhythm i n sympathetic nerve discharge (SND). Spike-triggered averaging showed that the naturally occurring discharges of 66 of 246 CVLM neurons loca ted 0-2.5 mm rostral to the obex, 4-4.25 mm lateral to the midline, an d within 2 mm of the ventral surface were correlated to the 10-Hz rhyt hm in inferior cardiac SND of 17 urethan-anesthetized cats. 2. Frequen cy domain analysis was used to characterize further the relationships between SND and the discharges of 45 CVLM neurons with activity correl ated to the 10-Hz rhythm in inferior cardiac nerve activity. The autos pectra of the discharges of 22 of these neurons contained a sharp peak near 10 Hz (corresponding to the peak in the autospectra of SND), alt hough 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 th ese 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 w as 0.08 +/- 0.01. The coherence value was significantly different than zero in all but three cases. 3. Importantly, spike-triggered averagin g 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. Al so, their discharges were not correlated to the irregular 2- to 6-Hz o scillations in SND of baroreceptor-denervated cats. These data support the hypothesis that different pools of brain stem neurons generate th e 10-Hz rhythm and the 2- to 6-Hz oscillations( or cardiac-related rhy thm) in SND. 4. Despite the fact that CVLM neurons with activity corre lated to the 10-Hz rhythm did not have activity correlated 1:1 to the cardiac-related rhythm in SND, these neurons were influenced by barore ceptor afferent nerve activity. First, their firing rates could be dec reased (n = 12) or increased (n = 2) during the presser response induc ed by inflating a balloon in the aorta (aortic obstruction). Second, o n occasion, the discharges of CVLM neurons and the 10-Hz rhythm in SND were entrained to a harmonic of the heart rate. 5. The interval betwe en CVLM neuronal activity and the peak of the next 10-Hz slow wave in the spike-triggered average of inferior cardiac SND was 59 +/- 2 ms. T his value was significantly longer than that (41 +/- 3 ms) for 43 raph e neurons with activity correlated to the 10-Hz rhythm in SND. Thus CV LM neurons fired earlier than raphe neurons during the 10-Hz slow wave in SND. 6. Chemical inactivation (muscimol microinjections) of the CV LM reversibly eliminated the 10-Hz rhythm in SND of five baroreceptor- denervated cats. This observation supports the view that CVLM neurons are elements of he central network responsible for the 10-Hz rhythm in SND.