Download Effects of Sodium Pentobarbital on Electrical and Reflex Activation of

Effects of Sodium Pentobarbital on Electrical
and Reflex Activation of the Cardiovascular
System
By Clarence N. Peiss, Ph.D., and John W. Manning, Ph.D.
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• The effects of barbiturates on the cardiovascular system have been reported by many
investigators. These effects include reduction
of vagal inhibition of the heart rate,1-2 direct
myocardial depression,3-4 and depression of
pressor responses to sciatic nerve stimulation."
Bouckaert and Heymansfi reported that the
carotid sinus reflex in dogs is depressed by
barbiturates. Several investigators have concluded that these anesthetic agents have a
significant effect on central neural mediation
of cardiovascular reflexes. Masserman7 showed
that sympathetic responses to faradic stimulation of the hypothalamus in the cat are
markedly decreased by sodium Amytal in
doses of 20 to 50 mg/kg. He also demonstrated a decrease in blood pressure when
small amounts of this drug were injected directly into the hypothalamus. Similar results
were reported by Dikshit,8 who also demonstrated depression of chloroform-induced arrhythmias by barbiturates. Redgate and Gellhorn n reported that injection of sodium thiopental into the posterior hypothalamus of
cats produced a fall in blood pressure and
heart rate, with recovery to control levels
occurring 25 to 30 minutes later. Reduction
of pressor responses and reversal to depressor
responses during electrical stimulation of the
brain stem was shown recently by Gutman
et al.10 Peiss n suggested a hypothalamic
site of action for the depressant effects of
pentobarbital on cardiovascular responses in
the cat. Since it is known that large doses of
From the Departments of Physiology, Stritch School
of Medicine, Loyola University, Chicago, Illinois, and
Emory University, Atlanta, Georgia.
Supported by a grant from the Chicago Heart Association and Grant H-6705 from the National Heart
Institute.
Received for publication August 14, 1963.
228
barbiturates have direct myocardial actions,
much of the previous work is subject to question in relation to specific site(s) of action
of these compounds. Even the microinjection
results are inconclusive due to possible neural
effects of tissue compression, pH, etc. It is
known that pentobarbital in small doses
blocks ascending somatic afferents in the
reticular formation and depresses evoked potentials in the thalamus and hypothalamus.12'13
Bradley14 has shown that the minimum dose
of sodium pentobarbital which causes elevation of the threshold for both behavioral and
EEG arousal is 3.5 mg/kg. It is the purpose of
this paper, therefore, to describe experiments
illustrating differential effects of small doses of
sodium pentobarbital on central nervous structures mediating cardiovascular reflexes in
dogs, under conditions in which peripheral
cardiovascular effector status can be assessed.
Methods
Experiments were carried out on 22 adult
mongrel dogs weighing between 8.1 and 10.5
kg. Dogs were selected for size and head shape
in order to minimize variations in stereotaxic
placement of electrodes. The animals were given
2 mg/kg of phencyclidine hydrochloride (Semylan °) intramuscularly. Alpha-chloralose was
then administered intravenously until the first
sign of pupillary constriction was observed (60
to 80 mg/kg). Bilateral cervical vagotomy was
performed in order to confine short-latency heart
rate changes to those mediated by sympathetic
outflow, and to eliminate compensatory vagal reflex effects on the heart. All animals were ventilated with intermittent positive pressure. In all
experiments sodium pentobarbital was injected
into the femoral vein at a dose of 5 mg/kg. At
the termination of the experiments the brains
were fixed with formalin, sectioned, and stained
for determination of electrode locations.
• Generously supplied by Parke, Davis and Company.
Circulation Research, Volume XIV, March 1964
229
PENTOBARBITAL AND CARDIOVASCULAR RESPONSES
10 cycles/sec, using a Grass S4 stimulator.
STELLATE STIMULATION
The chest was opened in the midsternal line
and electrodes placed around each stellate ganglion at the caudal pole. Stimulation parameters
were: 1.5 to 3.0 v, 5 msec pulse duration, and
SCIATIC NERVE STIMULATION
The sciatic nerve was dissected free for a
length of two inches. It was then sectioned and
POSTERIOR
HYPOTHALAMUS
(AI8.LL2.H9)
26V"2Msec- 100 Cps
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DORSOLATERAL
MEDULLA
(C7, RL3.H7)
3IV-2MMC- 100 Cp*
RIGHT STELLATE
GANGLION
3.0 V - 5 Msec - 8Cpi
FIGURE 1
Effect of sodium pentobarbital on cardiovascular responses to electrical stimulation of the
hypothalamus (top), medulla (center) and right stellate ganglion (bottom) in a vagotomized dog.
Segments marked A represent control responses. Those marked B represent responses after
intravenous administration of S mg/kg sodium pentobarbital. VF indicates ventricular force
recorded from Walton strain gauge arch on left ventricle. Solid bars indicate periods of stimulation.
Circulation Research, Volume XIV, March 1964
230
PEISS, MANNING
secured with a ligature to a stand in such a way
that it was free of adjacent muscle. Bipolar platinum electrodes were placed on the proximal end
and the nerve was covered with mineral oil to
prevent drying. Stimulation parameters were:
4 to 8 v, 1 msec pulse duration and 50 cycles/sec.
CAROTID OCCLUSION
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In some experiments the carotid arteries were
exposed with a small midline incision in the
neck, and were occluded with artery clamps. In
other experiments a long incision was made and
the carotid arteries dissected free up to the bifurcation of the internal and external carotids.
Umbilical tapes were placed under each common
carotid artery just below the bifurcation. In these
experiments occlusion was performed with artery
clamps and the sinus walls were stretched by
pulling caudally on the tapes. Occlusions were
maintained for 45 to 60 seconds.
STIMULATION OF MEDULLA AND
HYPOTHALAMUS
After making skin incisions in convenient locations on the skull, the muscles were retracted
from the skull and the periosteum scraped off
with a surgical chisel. Small bur holes were made
with an osteological drill mounted in the stereotaxic carrier. Bipolar concentric electrodes were
positioned under stereotaxic control in the posterior hypothalamus (R16-19, RL or LL1-3 and
H6-10) and in the dorsal reticular formation of
the medulla (C4-7, RL or LL 3 and H5-8). lr '
Stimulation parameters were: 1.3 to 4.2 v, 2
msec pulse duration and 100 cycles/sec.
RECORDINGS
Blood pressure was recorded from the abdominal aorta via a large catheter inserted into
the femoral artery. The catheter was connected
to a Statham P23Db pressure transducer through
a 1" 22-gauge needle for critical damping of
the transducer response. Heart rate was registered
continuously by an integrating cardiotachometer.16 In 10 animals myocardial force was recorded with a Walton strain gauge arch sutured
to the left ventricle. All recordings were made on
a Midwest model 591 oscillograph using galvanometers with a flat frequency response of 180
cycles/sec. The recorded signals were simultaneously monitored on a Rycom 17" oscilloscope. Electrical stimulation parameters were adjusted to produce a moderate but far from
maximal response.
Results
Figure 1 is a composite photograph of original recordings showing the effects of 5 m g A g
sodium pentobarbital on the cardiovascular
responses to electrical stimulation of the hypothalamus, medulla and right stellate ganglion.
The control record (A) shows a typical response to stimulation of the posterior hypothalamus, with striking increases in blood
pressure, heart rate, and myocardial contractility, all of which were markedly reduced when
the identical stimulus was applied five minutes after administration of sodium pentobarbital ( B ) . Both systolic and diastolic pressure responses to hypothalamic stimulation
were reduced after pentobarbital, but the
effect on systolic pressure was greater.
The middle portion of figure 1 shows a
moderate depression of cardiovascular responses to stimulation of the dorsal medullary
reticular formation after administration of
pentobarbital. There was no reduction in the
response of diastolic pressure to medullary
stimulation in this dog. In other experiments
in which lower intensities of stimulation were
used, reversal of pressor to depressor responses
has been noted from both hypothalamus and
medulla. This confirms the recent report of
Gutman et al.10 The bottom segment of
figure 1 indicates that sodium pentobarbital
had no significant effect on cardiovascular
responses to electrical stimulation of the right
stellate ganglion. At this dosage, therefore,
sodium pentobarbital does not appear to have
direct effects on the peripheral sympathetic
system or the myocardium itself. In all of the
above experimental procedures it was possible
to override the depressant effects of the pentobarbital by increasing stimulation intensity.
For this reason it is imperative that stimulation voltages be constantly monitored by
oscilloscope.
Table 1 summarizes the data for stimulation
of these three structures in all animals. At least
two, and usually three, stimulations of each
area were performed before and after administration of tlie pentobarbital. Values in table 1
are the average response of all stimulations in
all animals. The P values in the right hand
column indicate that the effects of sodium
pentobarbital on heart rate, blood pressure
and diastolic pressure were significant for
both medulla and hypothalamus. There was
no significant difference in responses to stelCirculalion Research, Volume XIV, March 1964
231
PENTOBARBITAL AND CARDIOVASCULAR RESPONSES
TABLE 1
Effect of Sodium Pentobarbital (5 mg/kg) on Cardiovascular Responses to Stimulation of
Hypothalamus, Medulla and Stellate Ganglion
Changes in heart rate
After
Before
pentobarbital
pentobarbital
beats/min
beats/min
Procedure
decrease
after
p
Stimulation of
hypothalamus
(12)
Cont.
Stim.
Incr.
161
208
47
155
173
18
62
<
Stimulation of
medulla
(10)
Cont.
Stim.
Incr.
157
197
40
155
182
27
32
<.O2
Stimulation of
stellate
(10)
Cont.
Stim.
Incr.
156
197
41
150
190
40
2
m
>.»
Stimulation of
hypothalamus
(12)
Cont.
Stim.
Incr.
39
66
27
35
46
11
59
<„,
Stimulation of
medulla
(10)
Cont.
Stim.
Incr.
37
55
18
34
46
12
33
<.O2
Stimulation of
stellate
(10)
Cont.
Stim.
Incr.
35
56
21
32
52
20
5
>.,
110
160
50
Stimulation of
medulla
(10)
Cont.
Stim.
Incr.
108
150
42
Stimulation of
stellate
(10)
Cont.
Stim.
Incr.
104
119
15
r-^
Cont.
Stim.
Incr.
H-1
Stimulation of
hypothalamus
(12)
o
o
Changes in diastolic pressure
mmHg
mmHg
to to
to to
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Changes in pulse pressure
mmHg
mmHg
56
<»,
99
131
32
24
<.M
101
116
15
0
Figures in parentheses indicate number of animals in each group.
Cont. = control; Stim. = during stimulation; Incr. = increment.
late stimulation after the pentobarbital. Pentobarbital depression of hypothalamic responses
in the three cardiovascular parameters shown
in table 1 was significantly greater than depression of medullary responses (P = 0.001).
Although responses to stimulation of the
stellate ganglia remain undiminished after administration of 5 mgAg of sodium pentobarbital, a depressant effect occurred when doses
of 10 mg/kg were used in a series of six
animals. It has also been noticed that smaller
Circnlalinn Reiearch, Volume XIV, March 1964
doses will cause depression of responses to
stellate stimulation if administered to an open
chest animal after several hours of experimentation.
The data on pulse pressure changes were
qualitatively confirmed by simultaneous recording with the Walton strain gauge arch.
For numerous reasons, however, it does not
appear to be valid to attempt qiiantitation of
the latter measurement.
Figure 2 illustrates the results of reflex
PEISS, MANNING
232
BILATERAL CAROTID
OCCLUSION
B
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LEFT SCIATIC
NERVE
4 V - I Msec- 5OCps
FIGURE 2
Effect of sodium pentobarbital on reflex activation of the cardiovascular system by carotid,
artery occlusion and sciatic nerve stimulation in a vagotomized dog. Segments marked A represent control responses. Those marked B represent responses after intravenous administration
of 5 mg/kg sodium pentobarbital. Solid bars indicate durations of carotid occlusion and sciatic
nerve stimulation.
activation of cardiovascular responses by stimulation of the sciatic nerve and bilateral carotid artery occlusion before (A) and after (B)
intravenous administration of 5 mg/kg sodium
pentobarbital. In the top segments of this figure the solid line represents the total time
during which both carotid arteries were occluded. At the arrow S, with occlusion maintained, the carotid sinuses were stretched by
pulling up on the tapes which had been
placed around them. The stretch was maintained until the arrow R, at which time it was
released. A marked depression of the responses in heart rate, pulse pressure, and
diastolic pressure occurred after pentobarbital.
It is difficult to determine, however, whether
the inhibitory response to stretching of the
sinus walls was affected by the drug. Other
experiments have shown that there is not a
marked effect of this drug on sympathetic in-
hibitory responses elicited by stimulation in
the anterior hypothalamus and preoptic region.
The lower segments of figure 2 show the
results of stimulation of the proximal sciatic
nerve. A small pressor and augmentor reponse was elicited, with a negligible response
in heart rate. After pentobarbital most of the
pressor and augmentor response was eliminated and a decrease in heart rate was observed. Since the animal was bilaterally vagotomized, this decrease in heart rate must
presumably have been mediated by central
inhibition of sympathetic outflow to the heart.
The decreased heart rate resulting from sciatic
nerve stimulation after pentobarbital was not
abolished by atropine.
In another series of control studies it was
shown that additional doses of Sernylan (1
mgAg) or chloralose (30 mgAg) did not
Circulation Research, Volume XIV, March 1964
233
PENTOBARBITAL AND CARDIOVASCULAR RESPONSES
cause depression of these reflexly induced
cardiovascular responses.
Table 2 is a summary of data for carotid
occlusion and sciatic nerve stimulation experiments. Changes in response to sciatic
nerve stimulation after pentobarbital were significantly different from control responses. P
values for carotid occlusion experiments, however, were between 0.05 and 0.02. This is due
to a wide variance in response from animal
to animal, which is discussed below.
the central nervous system involved in cardiovascular regulation. These experiments do
not assess possible actions of sodium pentobarbital on spinal internuncial or preganglionic cells. It is conceivable that depression of
responses to stimulation of the posterior hypothalamus and dorsal medullary reticular formation may be due to drug effects at these
sites. However, under these conditions one
would not expect the marked difference in
pentobarbital depression between these two
areas in the brain. Together with other evidence in the literature, the data support the
conclusion that, at small dosages, sodium
pentobarbital has significantly more depressant effect on the hypothalamus than on the
medulla. It is of interest to note that the
sensitivity of these two areas to cZ-tubocurarine
is just the reverse.17
Further study is necessary to determine
Discussion
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The use of stimulation of the stellate ganglia in these experiments provides a measurement of the effects of sodium pentobarbital on
all structures from the preganglionic fiber out
to and including the cardiac effectors. The
data support the conclusion that small doses
of this drug have effects in several parts of
TABLE 2
Effect of Sodium Pentobarbital (5 mg/kg) on Cardiovascular Responses to Bilateral Carotid
Occlusion and to Sciatic Nerve Stimulation
Changes in heart rate
After
Before
pentobarbital
pentobarbital
beats/min
beats/min
Procedure
decrease
after
p
Carotid
occlusion
(10)
Cont.
Stim.
Incr.
154
182
28
157
173
16
43
<.O5
Stimulation of
sciatic
(8)
Cont.
Stim.
Incr.
153
174
21
151
161
10
52
<.O2
56
<.O5
54
<.O2
<.O2
Changes in pulse pressure
mmHg
mmHg
Carotid
occlusion
(10)
Cont.
Stim.
Incr.
38
54
16
35
42
7
Stimulation of
sciatic
Cont.
Stim.
Incr.
37
48
11
34
39
(8)
5
Changes in diastolic pressure
mmHg
mmHg
Carotid
occlusion
(10)
Cont.
Stim.
Incr.
104
146
42
94
122
28
33
Stimulation of
sciatic
Cont.
Stim.
Incr.
109
138
29
101
115
14
52
(8)
Figures in parentheses indicate number of animals in each group.
Circulation Research, Volume XIV, March 1964
234
PEISS, MANNING
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whether this depressant action of sodium
pentobarbital on central structures mediating
cardiovascular reflexes is comparable or perhaps even more pronounced at higher levels of
the central nervous system. Comparative
studies of this type, dealing with the limbic
system and cerebral cortex, have not been
performed. It is known, however, that dogs
under pentobarbital anesthesia yield many
more depressor responses to electrical stimulation of the sigmoid gyri than dogs under
ether.18
Sodium pentobarbital also depresses reflex
activation of the cardiovascular system as
shown in the carotid occlusion and sciatic
nerve stimulation experiments. Responses to
carotid occlusion in the dog, both before and
after pentobarbital, are very variable. In some
animals no change in heart rate occurred. In
others there was a tremendous cardiac activation involving both acceleration and augmentation. It seems reasonable that the level of
sympathetic tone varies widely in these animals, and that therefore the effective stimulus
delivered when the carotid arteries are occluded is not constant from animal to animal
or even in the same animal from time to time.
For these reasons it is felt that the carotid
occlusion reflex is not a reproducible test
measure unless resort is made to controlled
perfusion of the isolated carotid sinuses. The
magnitude of the reflex responses in those
cardiovascular parameters which were measured, and their per cent depression by pentobarbital, suggest that mediation of these reflex responses in the intact animal is not
confined to the medullary vasomotor area.
This would support the conclusions of Manning,10 who demonstrated that massive lesions
in the dorsal pontomedullary reticular formation produced only insignificant changes in the
responses of this preparation to hypothalamic
stimulation and to bilateral carotid occlusion.
thalamus, stellate ganglia and sciatic nerve,
and to bilateral occlusion of the carotid arteries were studied. It was concluded that: 1)
this drug depressed cardiovascular responses
to all of these procedures except stimulation
of the stellate ganglia; 2) the effects of the
anesthetic in this dose are referable to some
part(s) of the central nervous system; 3)
cardiovascular responses to hypothalamic
stimulation were depressed more than those
elicited by stimulation of the dorsal medullary
reticular formation.
References
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Summary
The effects of small doses of sodium pentobarbital (5 mg/kg) on the cardiovascular
responses to stimulation of the medulla, hypo-
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PENTOBARBITAL AND CARDIOVASCULAR RESPONSES
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Effects of Sodium Pentobarbital on Electrical and Reflex Activation of the Cardiovascular
System
CLARENCE N. PEISS and JOHN W. MANNING
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Circ Res. 1964;14:228-235
doi: 10.1161/01.RES.14.3.228
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