48 W
gland. The anode was clipped to the muscle near the to RVLM stimulation and their differential sensitivity to
adrenal nerve. the baroreceptor reflex. In the upper trace of Fig. 1A, the
To activate the baroreceptor reflex, the left aortic peristimulus time interval histogram for an EPI ADR SPN
depressor nerve ADN was identified in the neck and the responding to paired-pulse stimulation of the RVLM shows
central cut end was placed on a pair of platinum hook the characteristic early inhibition of spontaneous discharge
electrodes spaced 2 mm apart. Electrode stimuli were triad during the first 50–80 ms following RVLM stimulation,
square-wave pulses 7 ms interpulse interval, 10 mA, 1 ms, followed by a prominent excitation at a longer latency
0.25 Hz. Natural stimulation of arterial baroreceptors mean response latency, 150 ms. In contrast, the RVLM
consisted of the rise in arterial pressure following an stimulus-evoked responses of NE ADR SPNs Fig. 1B,
intravenous bolus of phenylephrine 50–100 mg ml in a upper trace consisted of an early, marked increase in
volume of 0.1 ml of saline over 1–2 s. discharge probability mean response latency, 25 ms,
which was often followed by a period of reduced firing lasting 100–200 ms. The average mean response latency
2.5. Data analysis for the 23 EPI ADR SPNs in this study was 14164.3 ms
range, 87 to 172 ms, which was significantly P,0.001 The action potentials of adrenal SPNs and the arterial
longer than that of the 25 NE ADR SPNs average, 3061.9 blood pressure were digitized at 22 kHz and recorded on
ms; range, 17–53 ms. VCR tape, along with trigger pulses coincident with
As illustrated in the lower traces of Fig. 1, the sponta- stimulus delivery. Computer-aided data analysis consisted
neous discharge probability of EPI ADR SPNs usually of peristimulus and perisystolic time interval histograms of
exhibited no modulation over the time course of the the discharges of adrenal SPNs. Results were expressed as
cardiac cycle, indicating little or no influence of the means6S.E.M. Significant differences between population
baroreceptor reflex on the neurons that determine the means were assessed with Student’s t-test P,0.05.
spontaneous activity of EPI ADR SPNs. In contrast, the perisystolic time interval histogram of the spontaneous
activity of an NE ADR SPN Fig. 1B, lower trace illustrates a pronounced cardiac cycle-related modulation
3. Results
of the unit’s discharge probability. Nineteen of the 20 EPI ADR SPNs recorded under conditions in which the mean
In 19 rats, 48 neurons in the IML region of thoracic AP was greater than 80 mmHg i.e., above the expected
spinal segments, T –T , were antidromically activated
6 8
threshold pressure for activation of baroreceptors had no with electrical stimuli applied to the left adrenal nerve. The
discernible modulation of their spontaneous discharge by responses of individual adrenal SPNs consisted of single
the baroreceptor reflex. In contrast, 14 of the18 NE ADR action potentials with constant onset latencies that collided
SPNs recorded at AP greater than 80 mmHg exhibited a with spontaneous action potentials. The evoked responses
discharge pattern with a marked reduction in discharge in adrenal SPNs followed paired adrenal nerve stimuli
probability during a consistent portion of the cardiac cycle. separated by 4 ms or less. The mean onset latency of the
Similarly, 13 of 15 NE ADR SPNs that were tested adrenal SPN responses to adrenal nerve stimulation was
showed a complete inhibition of their spontaneous dis- 3461.6 ms range, 13 to 59 ms, corresponding to an
charge during the rise in AP caused by intravenous approximate conduction velocity of 1.2 m s average
injection 10 mg of phenylephrine data not shown. Only estimated distance from the adrenal nerve stimulation site
four of 14 tested EPI ADR SPNs, by contrast, were to the spinal recording site, 33 mm. At mean arterial
completely inhibited during similarly evoked pressor re- pressures between 70 and 135 mmHg mean, 10062.8
sponses. Electrical stimulation of the baroreceptor afferents mmHg, the mean spontaneous discharge rates of adrenal
in the ipsilateral aortic depressor nerve ADN with a burst SPNs ranged from 0.9 to 10.8 spikes s mean, 5.960.5
of three pulses produced a transient inhibition in 19 out of spikes s.
20 tested NE ADR SPNs, but a baroreceptor afferent stimulus-evoked inhibitory response was detected in only
3.1. Identification of two distinct groups of adrenal two of the 20 EPI ADR SPNs that were tested.
sympathetic preganglionic neurons The mean antidromic onset latencies of the EPI ADR
SPNs 3161.1 ms; n523; range, 16 to 52 ms were not Based on the criteria described above and detailed in
different P.0. from those of NE ADR SPNs 3762.6 [17], the 48 spontaneously active adrenal SPNs were
ms; n525; range, 13 to 59 ms. The mean spontaneous divided into two groups: 23 48 EPI ADR SPNs and 25
discharge frequency of EPI ADR SPNs 6.260.5 Hz; 52 NE ADR SPNs.
range, 3.1 to 10.8 Hz; mean AP, 10264.0 mmHg was Fig. 1 illustrates two of the criteria allowing classifica-
significantly greater P,0.01 than that of NE ADR SPNs tion of adrenal SPNs as either EPI ADR SPNs or NE ADR
4.360.5 Hz; range, 0.9 to 9.9 Hz; mean AP, 9765.6 SPNs: differences in the characteristics of their responses
mmHg.
W .-H. Cao, S.F. Morrison Brain Research 887 2000 46 –52
49
Fig. 1. Responses to stimulation of the rostral ventrolateral medulla RVLM and cardiac-related activity patterns of an EPI ADR SPN and an NE ADR SPN. A Upper trace: peri-stimulus time histogram of the discharge of an EPI ADR SPN during paired-pulse stimulation of the RVLM 200 mA, 0.25 Hz,
30 stimuli. Bin width is 5 ms. Note the early inhibition lasting about 100 ms and the late excitation of this EPI ADR SPN. Lower trace: post-systolic average of arterial pressure AP and time interval histogram of the spontaneous discharge of an EPI ADR SPN. Note the absence of baroreceptor
modulation of EPI ADR SPN discharge. Histogram based on 100 sweeps and bin width is 10 ms. B Similar traces as in A for an NE ADR SPN. Note the short-latency 25 ms excitation of the NE ADR SPN and the strong cardiac-related modulation of its discharge lower histogram based on 300
sweeps.
3.2. Effects of activation of Bezold-Jarisch reflex on spontaneous discharge rates were reduced from 6.260.5
adrenal SPNs Hz to a minimum of 2.560.4 Hz Fig. 4, accompanied by
a depressor response from a mean AP of 10364.0 to Activation of the Bezold-Jarisch reflex by administration
7064.5 mmHg Fig. 4. Atrial injection of PBG decreased of PBG 100 mg kg into the right atrium produced a
the mean discharge frequency of 19 76 of 25 NE ADR consistent and characteristic cardiopulmonary chemoreflex,
SPNs Figs. 2, 3 from 4.360.5 to 1.260.4 Hz Fig. 4. In consisting of the prompt bradycardia
Dheart rate5 these experiments, the mean AP decreased from 9964.2 to
2272638 bpm and hypotensive response Dmean AP5
7264.9 mmHg Fig. 4. Neither the Bezold-Jarisch reflex- 22963.2 mmHg from a baseline of 10062.8 mmHg;
evoked decreases in firing rate nor the mean depressor n548. This hypotensive response started 2–5 s after the
responses evoked by the PBG injections were significantly injection of PBG, reached peak effects 5 s later, and
different between EPI ADR SPNs and NE ADR SPNs. The completely returned to the control level 10–20 s after the
other six NE ADR SPNs increased their discharge rate by peak Fig. 2. Control injections 50 ml of saline into the
2.760.5 Hz, during a fall in AP of 22369.1 mmHg. right atrium did not elicit any changes in heart rate or AP.
Bilateral cervical vagotomy abolished the response to the right atrial injection of PBG.
4. Discussion
