JOURNALOF NEUROPHYSIOUDGY Vol. 68, No. 5, November 1992. Printed

in U.S.A.

Rostral Ventrolateral Medullary and Caudal Medullary Raphe Neurons With Activity Correlated to the lo-Hz Rhythm in Sympathetic Nerve Discharge SUSAN M. BARMAN AND GERARD L. GEBBER Departments of Pharmacology and Toxicology, and Physiology, Michigan State University, East Lansing, Michigan 48824 SUMMARY

AND

CONCLUSIONS

I. The current study is the first to identify medullary neurons whose naturally occurring discharges were correlated to the 1O-Hz rhythm in sympathetic nerve discharge (SND). Spike-triggered averaging showed that 44 of 164 rostra1 ventrolateral medullary (RVLM) and 44 of 174 caudal medullary raphe neurons had activity correlated to the lo-Hz rhythm in inferior cardiac postganglionic SND of 23 baroreceptor-denervated, decerebrate cats. 2. When the frequency of the rhythm in SND was decreased by lowering body temperature, the discharges of the 10 neurons tested (6 RVLM and 4 raphe) remained locked to the peak of the next 1O-Hz sympathetic nerve slow wave rather than to the peak of the preceding slow wave. This observation supports the contention that the lo-Hz rhythm in basal SND was generated in the brain stem rather than in the spinal cord. 3. Frequency-domain analysis was used to characterize further the relationship between the lo-Hz rhythm in SND and the discharges of 30 RVLM and 24 raphe neurons. The autospectra of the discharges of eight RVLM and four raphe neurons contained a sharp peak near 10 Hz, although the mean firing rates of these neurons were lower than the frequency of the rhythm in SND. Coherence values as high as 0.76 characterized the relationship between the discharges of these “rhythmically firing neurons” and the lO=Hz rhythm in SND. A coherence value of 1.O indicates a perfect correlation. The autospectra of the discharges of the 22 RVLM and 20 raphe neurons did not contain a peak near 10 Hz. The mean firing rates and coherence values relating the discharges of these “nonrhythmically firing neurons” and the lo-Hz rhythm in SND were significantly lower than those for the rhythmically firing neurons. Because the frequency of the population rhythm recorded from the inferior cardiac nerve was higher than the firing rates of individual medullary neurons, the 1O-Hz rhythm in SND appears to be an emergent property of a network of neurons whose discharges are probabilistically related to the population rhythm. 4. In addition to the peak near lo-Hz, the autospectrum of SND often contained considerable power at frequencies 64 windowsare averaged(Benignus 1970). OTHER METHODS OF ANALYSIS. Interspike-interval histograms wereconstructedto characterizefurther the dischargesof medullary neurons.In baroreceptor-innervatedcats arterial pulse-triggeredaveragesof SND and histogramsof neuronal activity were alsoconstructed. Data are expressedas means* SE. Student’st testsfor paired and unpaireddata wereused.P 5 0.05 indicated statisticalsignifiCaIlCe. RESULTS

Properties of RVLM and raphe neurons with activity correlated to the IO-Hz rhythm in SND in baroreceptor-denervated cats We recorded from 164 neurons in a region of the RVLM from 4 to 6 mm rostral to the obex and -3.5 mm to the left

FIG. 1. Distribution of rostral ventrolateral medullary and raphe neurons with activity correlated to the lO-Hz rhythm in sympathetic nerve discharge in baroreceptordenervated and -innervated cats. IO, inferior olive; LR, lateral reticular nucleus; Py, pyramid; R.gc., nucleus reticularis gigantocellularis; Rpc., nucleus reticularis parvocellularis; R.v., nucleus reticularis ventralis; r.m., nucleus raphe magnus; r.o., nucleus raphe ob scurus; r.p., nucleus raphe pallidus; 5Sp, spinal nucleus of trigeminal nerve. Sections correspond to the posterior (P) stereotaxic planes of Berman ( 1968). Calibration is 1 mm.

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FIG. 2. Relationship between the discharges of a rhythmically firing rostra1 ventrolateral medullary (RVLM) neuron and sympathetic nerve discharge (SND) in a baroreceptordenervated cat. A : original records of inferior cardiac SND (top) and standardized pulses representing the action potentials of the neuron (bottom). Time marker is 100 ms/division; calibration is 54 pV. B: spike-triggered (top) and “dummy” (bottom) averages of SND, each based on 785 triggers. Binwidth is 5 ms; calibration is 30 pV. C: traces are topto bottom: autospectra (AS) of SND, AS of the discharges of the neuron, and the neuron-to-nerve coherence function. Spectra are based on 65 5-s windows, and frequency resolution is 0.2 Hz/bin in this and subsequent figures.

of the midline in 13 baroreceptordenervated cats in which inferior cardiac SND contained a IO-Hz rhythm. Spiketriggered averaging showed that the naturally occurring discharges of 44 of these neurons were correlated to the lo-Hz rhythm in SND. The raphe was searched from 2 to 6 mm rostra1 to the obex in 12 baroreceptordenervated cats in which SND contained a IO-Hz rhythm. Of the 174 neurons located in the medullary raphe complex from 2 to 4.5 mm above the obex, 44 neurons had activity correlated to the lo-Hz rhythm in SND. None of the 6 1 neurons located more rostrally in the medullary raphe complex (4.7-6 mm above the obex) had activity correlated to the lo-Hz rhythm in SND. Figure 1 shows the locations of RVLM and raphe neurons whose discharges were correlated to the loHz rhythm in SND. These distributions are similar to those of neurons with activity correlated to the 2- to ~-HZ rhythm in anesthetized cats (Gebber and Barman 1985 ). Figure 2 shows an example of the relationship between the activity of an RVLM neuron and inferior cardiac SND in a baroreceptor-denervated cat. The spike-triggered average (Fig. 2 B, top) shows SND for 500 ms before and after unit spike occurrence at time 0. The naturally occurring discharges of this neuron were correlated to the lo-Hz rhythm in SND because the deflections in the spike-triggered average were of much greater amplitude than those in the corresponding dummy average of SND (Fig. 2B,

bottom).

Frequency-domain analysis was used to characterize further the relationships between the lo-Hz rhythm in inferior cardiac SND and the discharges of 30 RVLM and 24 raphe neurons in 18 baroreceptor-denervated cats. The autosnec-

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1538

S. M. BARMAN

AND G. L. GEBBER

1. Propertiesof R VLA4 and raphe neuronswith activity correlatedto the IO-Hz rhythm in inferior cardiac sympatheticnerve dischargein baroreceptor-denervated cats

TABLE

Neurons

n

Peak Coherence*

Firing Rate, spikes/s

STA Lag, mst

Rhythmicallyfiring neurons RVLM Raphe

8 4

0.32 -t 0.08 (0.19-0.76) 0.46 k 0.06 (0.28-0.56)

5.50 k 0.66 (2.4-8.6) 3.65 + 0.7 1 (1.8-5.2)

54 +6(25-84) 34 Ik 1 (30-35)

2.82 + 0.42s

61 + 5(15-95) 43 t 3(20-85)

Nonrhythmicallyjringneurons RVLM Raphe

22 20

+ 0.0 l$ (0.00-0.2 1) 0.10 + 0.02$ (0.0 l-0.26)

0.07

(0.8-6.7)

1.5 1 * 0.17$§ (0.4-2.7)

Values are means + SE; n is number of neurons; ranges are in parentheses. RVLM, rostra1 ventrolateral medullary; STA, spike-triggered average. *Peak coherence value near 10 Hz in neuron-to-nerve coherence function. tInterva1 between unit spike occurrence and peak inferior cardiac sympathetic nerve discharge in the spike-triggered average. *Value is significantly different than corresponding value for rhythmically firing neurons. §Value for nonrhythmically firing raphe neurons is significantly different than value for nonrhythmically firing RVLM neurons.

tra of SND contained a sharp peak at 10.1 t 0.2 (SE) Hz (range, 7.5-12.7 Hz) in these experiments. Medullary neurons with activity correlated to the lo-Hz rhythm in SND were classified as either “rhythmically firing” or “nonrhythi mically firing” on the basis of the character of the autospectra of their discharges. RHYTHMICALLY FIRING NEURONS. The autospectra of the discharges of eight RVLM and four raphe neurons contained a sharp peak at the same frequency as that of the peak in the autospectra of SND (Figs. 2C and 3C). The term “sharp peak” means that the power at this frequency exceeded background power by at least a factor of two. These neurons are referred to as rhythmically firing neurons. In Fig. 2C the autospectra of SND (top) and RVLM neuronal activity (middle) contained a sharp peak at 10.2 Hz. The peak at the same frequency in the coherence function (coherence value of 0.57) demonstrated that the lo-Hz discharges of these two signals were significantly correlated (Fig. 2C, bottom). This RVLM neuron missed firing during most of the lo-Hz slow waves in SND (see Fig. 2A). Its mean firing rate was only 2.4 spikes/s. Table 1 shows that the peak coherence values near 10 Hz in the single neuron-to-nerve coherence functions and the mean firing rates were similar for rhythmically firing neurons in the RVLM and raphe. Despite the appearance of a sharp peak near 10 Hz in the autospectra of the discharges of rhythmically firing RVLM and raphe neurons, their mean firing rates were lower than the frequency of the rhythm in SND. The data in Fig. 3 are from the only neuron identified whose firing rate (8.6 spikes/s) was within 2 Hz of the frequency of the rhythm in SND ( 10.2 Hz). As shown by the l-s record of spontaneous activity in Fig. 3A, this RVLM neuron discharged once during almost every lo-Hz slow wave in SND. The value of 0.76 in the RVLM neuron-to-nerve coherence function (Fig. 3C, bottom) was the highest noted in this study. Also the ratio of peak-tobackground power in the autospectrum of the discharges of this neuron (Fig. 3C, middle) was the largest of any neuron studied with activity correlated to the lo-Hz rhythm in SND. Interspike-interval histograms were constructed for 11 rhythmically firing neurons (7 RVLM and 4 raphe). The

histograms for four RVLM and three raphe neurons were multimodal or bimodal with regularly spaced peaks. Figure 4, A and B, shows examples of these types of interspike-interval histograms for the RVLM neurons whose relationships to SND were depicted in Figs. 2 and 3, respectively. The histogram in Fig. 4A contains a peak at 100 ms that corresponded to the period of the lo-Hz rhythm in SND. There were also peaks separated by this period, indicating that the neuron often fired in every second, third, etc. cycle of SND (also see Fig. 2A ). Because the number of counts in the second and third peaks was greater than that in the first peak, this neuron was more likely to discharge in every second or third lo-Hz sympathetic nerve slow wave than in consecutive cycles of SND. The largest peak in the interspike-interval histograms for two raphe neurons was also at an interval indicative of a high probability of firing in every second or third cycle of SND. The largest peak in the multimodal or bimodal histograms for three RVLM neurons and

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FIG. 3. Relationship between the discharges of a rhythmically firing rostra1 ventrolateral medullary neuron and sympathetic nerve discharge in a baroreceptor-denervated cat. A-C: same as Fig. 2. Calibrations are 49 pV in A and 28 PV in 8. Averages in Bare based on 2,8 12 triggers; binwidth is 5 ms. See Fig. 2 for abbreviations.

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MEDULLARY

11

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FIG. 4. Interspike-interval histograms for medullary neurons with activity correlated to the lo-Hz rhythm in sympathetic nerve discharge. A-C: histograms for 3 rhythmically firing rostra1 ventrolateral medullary neurons. D: histogram for a nonrhythmically firing raphe neuron. Binwidth is 10 ms. Histograms in A, B, and D are from the same neurons referred to in F&. 2, 3, and 5, respectively.

one raphe neuron was near 100 ms, indicative of a high incidence of firing in consecutive 1O-Hz slow waves in SND (see Fig. 4B). The interspike-interval histograms for the other rhythmically firing neurons (3 RVLM and 1 raphe) were unimodal with a shoulder that replaced the second peak in the bimodal histograms. Figure 4C shows an example for an RVLM neuron whose mean firing rate was 5.9 spikes/s. The modal interspike interval for the three RVLM neurons was similar to the period of the rhythm in SND ( 107 t 18 vs. 108 t 10 ms). The modal interval for the raphe neuron was twice the period of the rhythm in SND. NONRHYTHMICALLYFIRINGNEURONS. Therewasnosharp peak near 10 Hz in the autospectra of the discharges of 22 RVLM neurons and 20 raphe neurons whose activity was correlated to the lo-Hz rhythm in SND (as demonstrated with spike-triggered averaging). They are referred to as nonrhythmically firing neurons, even though the autospectra for some of these neurons contained a peak at a frequency