Vol. 280, Issue 6, R1704-R1712, June 2001
Sympathetic nerve and cardiovascular responses to chemical
activation of the midbrain defense region
Peter D.
Larsen,
Sheng
Zhong,
Gerard L.
Gebber, and
Susan M.
Barman
Department of Pharmacology and Toxicology, Michigan State
University, East Lansing, Michigan 48824
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ABSTRACT |
The changes in mean arterial
pressure (MAP), renal (RBF) and femoral (FBF) blood flows, and inferior
cardiac (CN) and vertebral nerve (VN) sympathetic nerve discharges
(SND) produced by chemical activation
(D,L-homocysteic acid) of the midbrain
periaqueductal gray (PAG) were compared in baroreceptor-denervated and
-innervated cats anesthetized with urethan. Defenselike cardiovascular
responses in both states were similar in magnitude and consisted of
increased MAP and FBF and decreased RBF; however, the nerve responses
differed. In baroreceptor-denervated cats, PAG activation increased CN
10-Hz activity, decreased VN 10-Hz activity, and lengthened the CN-VN phase angle. In baroreceptor-innervated cats in which the rhythm in SND
was cardiac related, PAG activation increased CN activity, but VN
activity and the CN-VN phase angle were unchanged. These results
demonstrate that chemical activation of PAG neurons induces differential patterns of sympathetic outflow generally consistent with
accompanying defenselike cardiovascular responses. However, the
mechanisms responsible for the changes in 10-Hz and cardiac-related SND
appear to be different.
periaqueductal gray; regional blood flows; sympathetic nerve
discharge
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INTRODUCTION |
TWO STUDIES FROM OUR
LABORATORY (10, 11) described the patterns of
sympathetic outflow evoked by electrical stimulation of the defense
region of the subtentorial midbrain periaqueductal gray (PAG). In
baroreceptor-denervated cats, we observed immediate and sustained
increases in the 10-Hz discharges of the inferior cardiac (CN) and
renal (RN) postganglionic sympathetic nerves, but a decrease in
vertebral sympathetic nerve (VN) 10-Hz activity. The CN, RN, and VN
provide sympathetic outflow to the heart, kidney, and forelimb
vasculature, respectively. The reciprocal changes in VN vs. CN and RN
discharges were accompanied by significant lengthening of the CN-VN
phase angle and shortening of the VN-RN phase angle in the 10-Hz band.
Using the work of Haken (12) and Kelso (16)
as a guide, we proposed that the changes in phase angle reflected
alterations in the coupling of multiple brain stem 10-Hz oscillators,
which, in turn, led to the differential responses of the three nerves
(10, 11). This proposal was supported by the temporal
correlation of the changes in phase angle and nerve activities and the
fact that the magnitude of changes in phase angle was directly related
to the extent to which PAG stimulation reciprocally affected the 10-Hz
discharges of the CN and VN. Moreover, the response pattern to PAG
stimulation was switched to one of increased activity in all three
nerves when the frequency of stimulation was reduced to just below that of the free-running 10-Hz rhythm in sympathetic nerve discharge (SND).
Importantly, phase angles were unchanged when CN, VN, and RN 10-Hz
discharges were uniformly increased. This observation and the results
obtained with partial phase spectral analysis (10)
virtually ruled out the possibility that the phase angle between the
discharges of nerve pairs simply reflected the difference in conduction
times to the nerves from the site of PAG stimulation.
The pattern of changes in CN, RN, and VN discharges was different in
baroreceptor-innervated cats in which the predominant rhythm in SND was
at the frequency of the heart beat (near 3 Hz) rather than near 10 Hz.
Under this condition, CN and RN discharges were dramatically increased
during the first 20 s of PAG stimulation, but VN activity was
either unchanged or modestly increased. Later, as blood pressure rose,
VN activity usually was reduced, whereas the increases in CN and RN
discharges were somewhat blunted. At no time during PAG stimulation was
there a change in the CN-VN or VN-RN phase angles at the cardiac
frequency. It was proposed that the late decrease in VN cardiac-related
activity reflected increased baroreceptor nerve discharge attendant to
the rise in blood pressure (11). Thus we attributed the
differential changes in the 10-Hz vs. cardiac-related discharges of the
CN, RN, and VN to different central mechanisms.
The experiments with electrical stimulation of the PAG left at least
two important questions unanswered. First, did the sympathetic nerve
responses involve the activation of neuronal cell bodies in the PAG or
fibers of passage? Second, were the differential sympathetic nerve
responses accompanied by regional changes in blood flows characteristic
of the defense reaction? These include decreased renal blood flow and
increased skeletal muscle blood flow (1, 5, 13, 20). In
the current study, we addressed these two issues by recording renal and
femoral blood flows together with CN and VN discharges during chemical
activation of the defense region of the PAG with the excitatory amino
acid D,L-homocysteic acid (DLH).
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METHODS |
Experimental subjects and general procedures.
The protocols used on 10 cats of either sex (2.75-4.25 kg) were
approved by the All-University Committee on Animal Use and Care of
Michigan State University. The cats were initially anesthetized with
2.5-3.5% isoflurane in oxygen. Urethan (1.2-1.8 g/kg iv) was
administered via a catheter inserted into the right femoral vein, and
isoflurane inhalation was terminated. This dose range of urethan has
been reported (8) to maintain a surgical level of
anesthesia for a period (8-10 h) that exceeded the duration of our
experiments. Noxious stimuli (pinch, cauterization of muscle, surgery)
failed to desynchronize the electroencephalogram (EEG) at any time
during the experiment. The EEG was recorded with an electrode placed on
the skull overlying the frontal-parietal cortex (3). Blood
pressure was measured from a catheter inserted into the right femoral
artery. The cats were paralyzed (gallamine triethiodide, 4 mg/kg iv,
initial dose) and artificially ventilated with room air; a bilateral
pneumothoracotomy was performed. End-tidal CO2 was
maintained near 4.0% by adjusting the parameters of ventilation. Rectal temperature was maintained near 38°C with a heat lamp.
Baroreceptor denervation was performed in six of the cats by bilateral
section of the carotid sinus, aortic depressor, and vagus nerves
(3). Sympathetic nerve recordings were begun before baroreceptor denervation in two of these cats. In these preparations, section of the baroreceptor and vagus nerves eliminated the
cardiac-related rhythm in SND and the inhibition of SND produced by
raising blood pressure with a bolus intravenous injection of
norepinephrine bitartrate (2 µg/kg). Intravenous norepinephrine also
failed to inhibit SND in the four cats in which baroreceptor
denervation was performed before sympathetic nerve recordings were
begun. SND contained a variable mixture of the 10-Hz rhythm and
irregular low-frequency (
6 Hz) oscillations after baroreceptor denervation.
Blood flow was measured ultrasonically with a Transonic T206 small
animal flowmeter and perivascular 2S flow probes (Transonic Systems).
The blood flow probes were placed around the left renal artery and the
left femoral artery. Zero flow was determined by occlusion of the
arteries just distal to the probes. The left hindlimb was skinned, and
the paw was tied so that femoral blood flow was largely directed to
skeletal muscle. As previously described (3), bipolar
platinum electrodes were used to record monophasically from the central
ends of the cut postganglionic sympathetic CN and VN near their exits
from the left stellate ganglion. The CN and VN provide sympathetic
outflow to the heart and forelimb, respectively. Nerve recordings were
made with the band-pass filter set at 1-1,000 Hz (Grass
Instruments 7P3 preamplifier), so that envelopes of multiunit spikes
appeared as slow waves (3, 6). Data were stored on
magnetic tape.
Portions of the occipital and parietal bones and medial cerebellum were
removed. The cerebral aqueduct and inferior colliculus were used as
landmarks for placement of microinjection needles or stimulating
electrodes. The brain stem was removed and fixed in 10% buffered
formalin. Paraffin-embedded frontal sections of 30-µm thickness were
cut and stained with cresyl violet, and sites of microinjection and
electrical stimulation were identified with reference to electrode
tracts. Relative to the stereotaxic coordinates of Snider and Niemer
(21), placements in the PAG were beneath the bony
tentorium at posterior (P) 0 to P1, lateral (L) 0.5 to L2.0, and height
(H) +3 to H+0. This region of the PAG is referred to as the
subtentorial PAG from which cardiovascular and behavioral components of
the flight component of the defense response are elicited (1, 2,
4, 5, 20).
Chemical activation was performed by injecting 200 nl of a 500-mM
solution of DLH over 2-5 s through the needle (outside diameter of
0.5 mm) of a 1-µl Hamilton syringe mounted on a David Kopf Instruments (DKI) stereotaxic instrument. We made one or two injections per track, with two to four tracks per animal. At least 20 min elapsed
between injections that were made on either the left or right side of
the PAG. Electrical stimulation was performed by using a Grass
Instruments S8800 quartz-timed digital stimulator and PSIU6
constant-current unit to deliver 1-ms square-wave pulses of variable
intensity at 25 Hz through concentric bipolar stainless steel
electrodes (Rhodes model SNE-100, with 0.25-mm tip exposures separated
by 0.75 mm) mounted on a DKI stereotaxic instrument.
Data analysis.
We measured mean arterial pressure (MAP, mmHg) and mean blood flows
(ml/min) through the femoral and renal arteries. Changes in mean blood
flow are expressed as a percentage of control. Mean vascular
conductance was calculated as mean blood flow divided by MAP. We
measured the time from drug injection to the onset and peak of the rise
in MAP.
Time series analysis was performed by using software written in our
laboratory by Lewis (see Refs. 10 and 18). We extracted 10-Hz and cardiac-related activity in SND from the original recordings using a digital band-pass filter (symmetric, nonrecursive type with a
Lanczos smoothing function; RC Electronics, Santa Barbara, CA) with a
width of 4 Hz centered at the frequency of the sharp peak in the
autospectrum of SND. This filtering caused minimal phase distortion of
the signal (10). The roll-off slope of the filter was
39%/Hz outside of the band pass. The slow waves in SND extracted by
digital filtering are smoother and more sinusoidal-like than the
originals, thus aiding in the accurate detection of peaks and troughs.
We made cycle-by-cycle measurements of the peak-to-trough amplitudes of
the filtered 10-Hz or cardiac-related slow waves (normalized on a scale
of 0 to 1.0). The phase lag (in degrees) of VN activity relative to CN
activity was calculated as a three-point (cardiac-related SND) or
ten-point (10-Hz SND) moving average. Cycle-by-cycle values of the
CN-VN phase angle were derived from intervals between the peaks of
corresponding slow waves in the two nerves (10).
The method of autospectral analysis used in this study has been
described in detail (3, 10). Briefly, autospectral
analysis was performed by using fast Fourier transform (FFT) after SND had been low-pass filtered at 100 Hz (original recordings). The sampling rate of 200 Hz gave a resolution of 0.2 Hz/bin. The power spectra (autospectra) derived from 40-s data segments were averages of
29 5-s data windows with 75% overlap. The autospectrum of a signal
shows how much power is present at each frequency. Although FFT was
performed over a bandwidth of 0-100 Hz, the spectra are displayed
on a scale of 0-15 Hz, which contained >90% of the power in SND
(3). Total power in SND was defined as the sum of the absolute values in the bins between 0 and 15 Hz. A macro written in
Microsoft Excel 7.0 was used to measure the power above background activity in the 10-Hz and cardiac-related bands of SND. A line was
fitted to connect the left and right limits of the sharp peak near 10 Hz or at the cardiac frequency in the autospectrum of SND, and power in
these bands was calculated as the area above this line. In
baroreceptor-denervated cats, low-frequency power was calculated as the
sum of the values in the bins from 0 to 6 Hz.
Statistical analysis.
Comparisons before and after microinjection of DLH into the PAG were
made by using the Wilcoxon-signed rank test; comparisons between
baroreceptor-innervated and -denervated conditions were made by using
the Mann-Whitney U test. Statistical tests were performed by
using Statmost 3.2 (DataMost).
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RESULTS |
We made a total of 45 microinjections of DLH into the PAG; 22 of
the injections were in baroreceptor-denervated cats and 23 in
baroreceptor-innervated cats. The responses to 18 of the injections were classified as defenselike on the basis of an increase in MAP, an
increase in femoral blood flow (FBF), and a decrease in renal blood
flow (RBF). Eight defenselike responses were noted in five
baroreceptor-denervated cats, and ten defenselike responses were
observed in six baroreceptor-innervated cats. No more than two
defenselike responses were seen in a given experiment. There was only
one cat in which a defenselike response was observed both before and
after baroreceptor denervation. The sites in the PAG from which the
defenselike responses were chemically induced are illustrated in Fig.
1. These sites were in the same vicinity as those (not shown) from which decreases in both FBF and RBF (n = 6) or no cardiovascular responses
(n = 21) were observed on microinjection of DLH.
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Fig. 1.
Sites from which microinjection of
D,L-homocysteic acid (DLH) into the midbrain
periaqueductal gray (PAG) elicited a defenselike cardiovascular
response as defined by increases in mean arterial pressure (MAP) and
femoral blood flow (FBF) and a decrease in renal blood flow (RBF). IC,
inferior colliculus;  , microinjection in
baroreceptor-denervated cats;  , microinjection in
baroreceptor-innervated cats; scale bar is 1 mm. P0.2 and P0.9
correspond to stereotaxic planes of Snider and Niemer
(21).
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Responses to chemical activation of the PAG in
baroreceptor-denervated cats.
A defenselike response elicited by microinjection of DLH into the PAG
of a baroreceptor-denervated cat is shown in Fig.
2. Within 10 s of completing the
microinjection of DLH, MAP began to rise from 114 mmHg, reaching a peak
of 125 mmHg ~25 s later. Subsequently, MAP slowly decreased toward
the control level. RBF decreased to a minimum of 66% of control, and
FBF increased to a maximum of 328% of control. As reported by others
(1, 5), the maximal changes in RBF and FBF occurred after
MAP reached its peak. Renal arterial conductance was reduced to a
minimum of 60% of control, and femoral arterial conductance was
increased to a maximum of 298% of control. DLH microinjection also
increased the amplitude of CN 10-Hz slow waves and decreased the
amplitude of VN 10-Hz slow waves. The reciprocal changes in nerve
activity were accompanied by an increase in the phase lag of VN 10-Hz
activity relative to CN 10-Hz activity from near 50 to near 80°.
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Fig. 2.
Defenselike response elicited by microinjection of DLH
into the PAG of a baroreceptor-denervated cat. Time series
(top to bottom) show MAP (mmHg), mean RBF (% of
control), mean FBF (% of control), cycle-by-cycle measurements of
10-Hz slow-wave amplitude (normalized, scale of 0-1.0) in the
inferior cardiac (CN) and vertebral (VN) sympathetic nerves, and CN-VN
phase angle (10-point moving average in degrees). The arrow indicates
the time at which the microinjection of DLH was completed. The bars
labeled 1 and 2 indicate the 40-s periods used
for autospectral analysis of CN and VN discharges in Fig. 3.
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Spectral analysis of CN and VN discharges before and after injection of
DLH into the PAG is shown in Fig. 3. In
this experiment, the frequency of the free-running rhythm in SND was
9.4 Hz. Chemical activation of the PAG produced an increase in CN 10-Hz
power (248% of control) and a decrease in VN 10-Hz power (72% of
control). Power is the square of voltage (10), thus the
changes in 10-Hz power are larger than the changes in slow-wave
amplitudes in the corresponding time series (Fig. 2). CN total power
increased to 210% of control, and VN total power decreased to 75% of
control. There was no change in low-frequency power for either the CN
or VN.
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Fig. 3.
Autospectra of CN (A) and VN (B)
discharges before (1, thin line) and during (2,
thick line) the defenselike response elicited by microinjection of DLH
into the PAG of a baroreceptor-denervated cat (same as Fig. 2).
Relative power is plotted against frequency. The autospectra are
averages of 29 5-s data windows with 75% overlap, and frequency
resolution is 0.2 Hz/bin.
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Summary data for the eight defenselike responses elicited by DLH
microinjection in the PAG of five baroreceptor-denervated cats are
presented in Tables 1 and
2. The responses elicited by
microinjections into the left and right sides of the PAG were indistinguishable; thus the data were pooled. Table 1 shows that the
increase in MAP, decrease in RBF and increase in FBF, and corresponding
changes in conductances were statistically significant. Table 2 shows
that DLH microinjection into the PAG produced a statistically
significant increase in CN 10-Hz power and CN total power, but VN 10-Hz
and VN total power were decreased significantly. There was no change in
low-frequency power for either nerve. The reciprocal changes in 10-Hz
power for the two nerves were accompanied by a statistically
significant increase in the CN-VN phase angle in this band.
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Table 1.
Defenselike cardiovascular responses to microinjection of DLH into the
midbrain PAG of baroreceptor-denervated and
baroreceptor-innervated cats
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Table 2.
Sympathetic nerve responses to microinjection of DLH into
the midbrain PAG of baroreceptor-denervated and baroreceptor-innervated
cats
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We electrically activated the PAG in three baroreceptor-denervated
cats. Figure 4 shows the defenselike
response to electrical stimulation of the PAG for the same
baroreceptor-denervated cat as is shown in Figs. 2 and 3. Electrical
stimuli (60 µA, 25 Hz) were applied for 40 s (between the 2 arrows). There was an increase in MAP of 15 mmHg that was not
maintained during the period of stimulation. Peak MAP was reached
20 s after PAG stimulation was begun. The maximal decrease in RBF
(71% of control) and peak increase in FBF (270% of control) were
reached later than the maximal increase in MAP. Electrical stimulation
of the PAG produced a sustained increase in CN 10-Hz slow-wave
amplitude and a sustained decrease in VN 10-Hz slow-wave amplitude
whose onsets preceded the increase in MAP. These changes were
associated with an increase in the phase lag of VN 10-Hz activity
relative to CN 10-Hz activity from near 90 to near 160°.
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Fig. 4.
Defenselike response elicited by electrical stimulation
(60 µA, 25 Hz) of PAG in a baroreceptor-denervated cat. Sequence of
traces and scales are as in Fig. 2. CN-VN phase angle is a 10-point
moving average. Arrows denote onset (left) and end
(right) of stimulation.
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Responses to chemical activation of the PAG in
baroreceptor-innervated cats.
A defenselike response produced by microinjection of DLH into the PAG
of a baroreceptor-innervated cat is shown in Fig.
5. After completion of the microinjection
of DLH (indicated by the arrow), there was an increase in MAP and FBF
and a decrease in RBF. Peak MAP was not reached until 55 s after
the microinjection of DLH and represented a rise of 20 mmHg. RBF
decreased maximally to 84% of control (a decrease in conductance to
76% of control), and FBF increased to 239% of control (an increase in
conductance to 215% of control). Whereas CN cardiac-related slow-wave
amplitude was increased after microinjection of DLH, the amplitude of
VN cardiac-related slow waves and the CN-VN phase angle were
essentially unchanged.
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Fig. 5.
Defenselike response elicited by microinjection of DLH
into the PAG of a baroreceptor-innervated cat. Sequence of traces and
scales are as in Fig. 2. CN-VN phase angle is a 3-point moving average.
The arrow indicates the time at which the microinjection of DLH was
completed. The bars labeled 1 and 2 indicate the
40-s periods used for autospectral analysis of CN and VN discharges in
Fig. 6.
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Spectral analysis of CN and VN discharges before and after
microinjection of DLH is shown in Fig. 6
(same experiment as Fig. 5). The autospectra of CN and VN discharges
show a peak at the frequency of the heart beat (3.6 Hz), indicating
that the predominant rhythm in SND was cardiac related. There was a
significant increase in CN cardiac-related power (173% of control) and
total power (156% of control) after microinjection of DLH. VN
cardiac-related power (93% of control) and total power (98% of
control) were little affected.
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Fig. 6.
Autospectra of CN (A) and VN (B)
discharges before (1, thin line) and during (2,
thick line) the defenselike response elicited by microinjection of DLH
into the PAG of a baroreceptor-innervated cat (same as Fig. 5). The
autospectra are averages of 29 5-s data windows with 75% overlap, and
frequency resolution is 0.2 Hz/bin.
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Summary data for the 10 defenselike responses elicited by DLH
microinjection in the PAG of six baroreceptor-innervated cats are
presented in Tables 1 and 2. There was a significant rise in MAP,
decrease in RBF and renal conductance, and increase in FBF and femoral
conductance (Table 1). CN cardiac-related power and total power were
significantly increased, but VN cardiac-related and total power were
not significantly affected (Table 2). There was no significant change
in CN-VN phase angle within the cardiac-related band of activity in
response to microinjection of DLH in the caudal PAG.
Comparison of the responses elicited by DLH microinjection in
baroreceptor-innervated and -denervated cats shows that the increase in
MAP was significantly greater in baroreceptor-innervated preparations
(Table 1). On the average, the onset and peak of the increase in MAP
occurred later in baroreceptor-innervated cats. There were no
significant differences in the maximal changes in regional blood flow
or conductance for either the renal or femoral arteries. The increase
in total power for CN was not different in the two states, but VN total
power was significantly decreased only in the baroreceptor-denervated
state (Table 2).
We electrically activated the PAG in three baroreceptor-innervated
cats. One of these cases is shown in Fig.
7. Stimuli (100 µA, 25 Hz) were applied
for 40 s (between the 2 arrows). MAP in this experiment showed
large fluctuations on the time scale of respiratory activity. Soon
after the onset of stimulation, MAP increased, reaching a peak of 120 mmHg 20 s after stimulation began. RBF decreased to a minimum of
85% of control (71% of control conductance), and FBF increased to a
maximum of 298% of control (250% of control conductance). Maximal
blood flow changes lagged that of MAP. CN cardiac-related slow-wave
amplitude barely increased in this case, whereas VN slow-wave amplitude
clearly decreased. CN-VN phase angle was unchanged. Note that the
decrease in VN cardiac-related slow-wave amplitude did not occur until
MAP began to rise.
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Fig. 7.
Defenselike response elicited by electrical stimulation
(100 µA, 25 Hz) of PAG in a baroreceptor-innervated cat. Sequence of
traces and scales are as in Fig. 2. CN-VN phase angle is a 3-point
moving average. Arrows denote onset (left) and end
(right) of stimulation.
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DISCUSSION |
To our knowledge, this is the first study in which regional blood
flows were recorded together with the discharges of sympathetic nerves
with different targets during chemical activation of the defense region
of the midbrain PAG. In both baroreceptor-denervated and -innervated
cats, microinjection of DLH into the PAG produced a defenselike
cardiovascular response, as defined by increases in MAP and FBF and a
decrease in RBF. The changes in regional blood flows were attributable
to alterations in vascular resistance as reflected by a significant
increase in femoral arterial conductance and a decrease in renal
arterial conductance. Although CN activity was increased in both
baroreceptor-denervated and -innervated cats, there were notable
differences in the responses of the VN and changes in CN-VN phase
angle. VN 10-Hz activity was decreased by chemical activation of the
PAG in baroreceptor-denervated cats, whereas VN cardiac-related
activity was unchanged in baroreceptor-innervated cats. CN-VN phase
angle in the 10-Hz band was lengthened, whereas the phase angle in the
cardiac-related band was unaffected. The sympathetic nerve responses in
baroreceptor-denervated and -innervated cats were similar to those
observed during electrical stimulation of the PAG (10,
11). Thus the changes in SND elicited by electrical stimulation
likely reflected the excitation of neuronal cell bodies in the PAG
rather than fibers of passage.
It is reasonable to propose that the changes in SND in response to
chemical activation of the subtentorial PAG contributed to the
defenselike cardiovascular pattern. The increase in CN 10-Hz and
cardiac-related discharges is consistent with tachycardia occurring
during the defense reaction (1, 4, 13). We did not
quantify the changes in heart rate in our preparations because monophasic recording of CN activity required us to partially denervate the heart. The decrease in VN 10-Hz activity in baroreceptor-denervated cats is consistent with the view (7, 13) that a reduction in vasoconstrictor outflow to skeletal muscle contributed to the increase in FBF. The VN supplies sympathetic outflow to the forelimb rather than hindlimb, but increased skeletal muscle vascular
conductance occurs in both of these body parts during the defense
reaction (2, 4, 13, 17). Although we did not record RN
discharges in the current study, it is likely that an increase in the
10-Hz and cardiac-related discharges of this nerve contributed to the decrease in RBF produced by chemical activation of the PAG. Regarding this point, our laboratory (11) has reported parallel
increases in CN and RN discharges during electrical stimulation of the
PAG defense region in both baroreceptor-denervated and -innervated cats.
Activation of the sympathetic cholinergic vasodilator system and
circulating epinephrine released from the adrenal gland are also
involved in increasing skeletal muscle blood flow during the defense
reaction in the cat (13, 22). These factors may have been
primarily responsible for the increase in FBF produced in
baroreceptor-innervated cats, because neither total VN power nor VN
cardiac-related power was changed by DLH microinjection into the PAG.
Thus reduced vasoconstrictor outflow to skeletal muscle may not have
contributed to the increase in FBF in baroreceptor-innervated cats.
This interpretation is subject to the provisos that VN activity is
generally representative of sympathetic outflow to all skeletal muscle
and increases in vasoconstrictor outflow to forelimb skin, and
sympathetic cholinergic vasodilator outflow to forelimb skeletal muscle
did not obscure a decrease in vasoconstrictor outflow to skeletal
muscle. The latter possibility seems remote, because the
cardiac-related rhythm is only weakly apparent in skin vasoconstrictor discharge (15). Moreover, skeletal muscle sympathetic
vasodilator fibers neither have cardiac-related activity
(14) nor respond to baroreceptor reflex activation
(19, 22). Regarding this issue, almost all of the power in
VN activity was cardiac related in baroreceptor-innervated cats.
The fact that VN cardiac-related activity was not reduced by DLH
microinjection in the PAG does not eliminate reduced vasoconstrictor outflow as a potential mechanism of skeletal muscle vasodilation in the
baroreceptor-innervated cat. We have reported a delayed reduction in VN
cardiac-related activity occurring in parallel with an increase in MAP
during high-frequency electrical stimulation of the PAG defense region
(11). The delayed decrease in VN cardiac-related activity
was attributed to increased baroreceptor nerve discharge accompanying
the pressor response. The increase in MAP (19 mmHg) produced by
chemical activation of the PAG in the current study was only about
one-third of the magnitude reported during electrical stimulation
(11). Thus the enhancement of baroreceptor nerve discharge
in response to chemical activation of the PAG may have been
insufficient to reduce VN cardiac-related activity under the conditions
of our experiments.
The reciprocal changes in CN and VN 10-Hz discharges observed in
response to both chemical or electrical stimulation of the defense
region of the PAG in baroreceptor-denervated cats were accompanied by
significant lengthening of the CN-VN phase angle. The increase in the
phase lag of VN 10-Hz activity relative to CN 10-Hz activity produced
by microinjection of DLH into the PAG was smaller than that observed
during electrical stimulation (10, 11). However, our
laboratory previously demonstrated that the magnitude of the change in
CN-VN phase angle is directly proportional to the extent to which CN
and VN 10-Hz discharges are differentially affected by PAG stimulation
(10). The increase in CN and decrease in VN 10-Hz
discharges produced by chemical activation of the PAG were not as large
as those seen during electrical stimulation (10), and
therefore the smaller changes in CN-VN phase angle were expected.
In baroreceptor-innervated cats, the increase in MAP produced by
chemical activation of the PAG was significantly greater than that in
baroreceptor-denervated cats, and peak pressure was reached later (see
Table 1). These findings are counterintuitive given that the buffering
provided by the baroreceptor reflex would be expected to decrease the
magnitude of the change in MAP and possibly shorten its time course.
The basis for the differences in the magnitude and duration of the
blood pressure responses in baroreceptor-innervated and -denervated
cats remains to be determined.
Hilton (13) proposed that the sympathetic component of the
baroreceptor reflex is suppressed during the defense response. If so,
cardiac-related power in SND should have decreased during activation of
the PAG defense region, because the cardiac-related rhythm is the
consequence of entrainment of centrally generated oscillations by
pulse-synchronous baroreceptor afferent nerve activity (9,
18). This was not found to be the case for either the CN or VN.
Thus Hilton's contention is not supported by the results of our experiments.
Perspectives
Our laboratory has offered a new model to explain the reciprocal
changes in CN and VN 10-Hz discharges induced by electrical stimulation
of the PAG defense region (10, 11). We have proposed that
this pattern arises from reorganization of the coupling of multiple
brain stem oscillators with different targets, as reflected by the
changes in the phase angles between the 10-Hz discharges of sympathetic
nerve pairs. The results of the current study point to the
physiological relevance of this model in explaining the defense
reaction in baroreceptor-denervated cats. First, because reciprocal
changes in CN and VN 10-Hz discharges were induced by chemical
activation of the PAG, this pattern can be attributed to excitation of
neurons in this region rather than fibers of passage. Second,
defenselike changes in regional blood flows accompanied the reciprocal
changes in CN and VN 10-Hz discharges. Third, the reciprocal changes in
the 10-Hz discharges of the two nerves induced by chemical activation
were accompanied by an increase in the CN-VN phase angle. On the basis
of arguments presented in our previous studies (10, 11),
we interpret the lengthening of phase angle as indicating that the
state of coupling of 10-Hz oscillators with different targets is
changed during the defense reaction in baroreceptor-denervated cats. In
contrast, the phase angle between the cardiac-related discharges of the
CN and VN was unchanged by PAG activation in baroreceptor-innervated
cats. This supports the contention that different mechanisms account for the defenselike changes in 10-Hz and cardiac-related SND.
| |
ACKNOWLEDGEMENTS |
This study was supported by National Heart, Lung, and Blood
Institute Grants HL-13187 and HL-33266.
| |
FOOTNOTES |
Address for reprint requests and other correspondence: G. L. Gebber, Dept. of Pharmacology and Toxicology, Michigan State Univ.,
East Lansing, MI 48824-1317 (E-mail: gebber{at}msu.edu).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 20 November 2000; accepted in final form 24 January 2001.
| |
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