Vol. 282, Issue 3, H964-H972, March 2002
Increased baroreceptor response in mice deficient in
monoamine oxidase A and B
D. P.
Holschneider1,2,5,
O. U.
Scremin5,6,
K. P.
Roos6,7,
D. R.
Chialvo8,
K.
Chen4, and
J. C.
Shih3,4
1 Departments of Psychiatry and the Behavioral Sciences,
2 Neurology, and 3 Cell and Neurobiology, Keck School of
Medicine, and 4 Department of Molecular Pharmacology and
Toxicology, School of Pharmacy, University of Southern California, Los
Angeles 90089; 5 Greater Los Angeles VA Healthcare System, Los
Angeles 90073; 6 Department of Physiology and
7 Cardiovascular Research Laboratory, School of Medicine,
University of California, Los Angeles, California 90024; and
8 Center for Studies in Physics and Biology, Rockefeller
University, New York, New York 10021
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ABSTRACT |
The recent development of mice doubly
deficient for monoamine oxidase A and B (MAO-A/B, respectively) has
raised questions about the impact of these mutations on cardiovascular
function, in so far as these animals demonstrate increased tissue
levels of the vasoactive amines serotonin, norepinephrine, dopamine, and phenylethylamine. We recorded femoral arterial pressures and electrocardiograms in adult MAO-A/B-deficient mice during
halothane-nitrous oxide anesthesia as well as 30 min postoperatively.
During both anesthesia and recovery, systolic, diastolic, and mean
arterial pressures were 10-15 mmHg lower in MAO-A/B-deficient mice
compared with normal controls (P < 0.01). Mutants also
showed a greater baroreceptor-mediated reduction in heart rate in
response to hypertension after intravenous pulses of phenylephrine or
angiotensin II. Tachycardia elicited in response to hypotension after
nitroprusside was greater in mutants than in controls. Heart rate
responsiveness to changes in arterial pressure was abolished after
administration of glycopyrrolate, with no differences in this
phenomenon noted between genotypes. These data suggest that prevention
of hypertension may occur in chronic states of
catecholaminergic/indoleaminergic excess by increased gain of the baroreflex.
arterial baroreceptor reflex; serotonin; norepinephrine; phenylethylamine; dopamine; blood pressure; heart rate; sympathetic
nervous system
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INTRODUCTION |
SEROTONIN (5-HT),
norepinephrine (NE), and dopamine (DA) play a major role in the control
of arterial blood pressure and heart rate (HR). Most studies suggest
that these neurotransmitters exert an acute pressor effect by acting on
both the central and peripheral nervous system. The autonomic effects
of chronic elevations of the catecholamines or indoleamines, however,
remain controversial. On one hand, hypertension and tachycardia are
frequent features of clinical conditions characterized by chronically
elevated noradrenergic and serotonergic states. Such conditions include
patients undergoing long-term treatment with monoamine oxidase (MAO)
inhibitors (23) and patients suffering from
pheochromocytoma (18, 28). On the other hand, a
substantial number of such patients show a relatively normal resting
blood pressure and additionally may suffer from orthostatic hypotension
and wide blood pressure swings.
One mechanism that may help to reconcile such discrepancies is the
occurrence of alterations in the baroreceptor reflex that serve to
control blood pressure through a negative feedback system acting on
cardiac output and peripheral vasomotor tone. Although evidence
suggests that under normal physiological conditions the baroreflex
plays little to no role in the long-term regulation of arterial blood
pressure (25), recent studies suggest that arterial
baroreceptors may be important in the long-term regulation of arterial
pressure in pathological states characterized by a catecholamine excess
(18, 28, 39), sodium overload (32), or
hypertension (30). Desensitization of the adrenergic
system associated with chronic catecholamine excess has been considered to be one of the determinants for the wide blood pressure fluctuations in patients with pheochromocytoma (18). Indeed, in these
patients, a functional impairment of the baroreflex exists, which is
reversible soon after normalization of catecholamine levels with
removal of the tumor (28). Likewise, patients with a NE
transporter deficiency show elevated levels of plasma NE as well as a
decreased baroreceptor sensitivity (39). Improved
understanding of the relationship between noradrenergic function and
baroreceptor control may have implications also for clinical conditions
such as heart failure, in which poor prognosis has been linked to
sympathetic dysregulation and attenuation of cardiac baroreceptor
control (27).
A major catabolic pathway for bioamines is MAO, whose two isoforms
(MAO-A and MAO-B) are distributed in virtually all mammalian cell types
and serve to metabolize catecholamines as well as indoleamines. Our
laboratory has recently identified a natural mutation of MAO-A (35) occurring in mice with targeted deletion of the MAO-B
gene (17). These mice doubly deficient in both MAO
isoforms [MAO-A/B knockout (KO)] demonstrate brain levels of 5-HT,
NE, DA, and phenylethylamine (PEA) that are respectively increased
8.5-, 2.2-, 1.7-, and 15.7-fold above those noted in adult wild-type
(WT) animals, with levels of the 5-HT metabolite 5-hydroxy-indoleacetic
acid essentially undetectable in both the brain and urine. It is likely
that MAO-A/B KO mice, with their elevated bioamine levels, exhibit
altered autonomic control. This could be important in understanding the mechanisms contributing to the regulation of blood pressure in states
characterized by excesses of the catecholaminergic and serotonergic
tone. In this study, we compared autonomic function of MAO-A/B KO mice
to that of their WT counterparts and evaluated the effects of differing
genotypes on the baroreflex during anesthesia as well as during
recovery. The surprising key findings are a decrease in basal blood
pressure and an increase in the baroreceptor gain of MAO-A/B KO mice
compared with their WT counterparts.
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METHODS |
Animals.
Our laboratory has recently identified a natural mutation of MAO-A
(35) in mice with targeted deletion of the MAO-B gene (17). MAO-A/B-deficient mice were originally identified by
their marked decreased body size and behavioral hyperreactivity on
handling or threat of being handled, a phenotype not seen in MAO-B KO
mice. Breeding of these animals has revealed an X-linked inheritance with affected male offspring demonstrating an absence of both MAO-A as
well as MAO-B enzymatic activity. Absence of MAO-A mRNA and MAO-A
protein in these animals has been confirmed, respectively, by Northern
blot analysis and Western blot analysis, although the defect in MAO-A
at the gene level has yet to be determined (35).
Subjects of this experiment were 4- to 5-mo-old male MAO-A/B-deficient
mice and their WT counterparts (129 Sv strain). Animals were housed
singly in Plexiglas cages with contact bedding and ad libitum food and
water. A 24-h diurnal light cycle was maintained, with lights on from
07:00 to 19:00 hours each day. All procedures performed were reviewed
and approved by the institutional animal care and use committee. At the
end of the experiment, absence of the MAO-B gene was confirmed in
MAO-A/B KO mice by a polymerase chain reaction of DNA prepared from
tails (17) (data not shown) as well as by measurement of
MAO-A enzymatic activity in the liver (MAO-A/B KO: 0.07 ± 0.02 nmol · 20 min
1 · mg
protein1, n = 8; WT: 5.51 ± 0.70 nmol · 20 min1 · mg
protein1, n = 10) (20).
Surgery.
Mice were anesthetized with halothane (2.0% induction, 1.2%
maintenance) in 30% oxygen-70% nitrous oxide. Rectal temperature was
maintained at 36.5°C with a BAT-12 thermocouple thermometer connected
to a TCAT-1A temperature controller (Physitemp; Clifton, NJ), a heating
pad set at 36.5°C, and a source of radiant heat. For recording of the
electrocardiogram, two platinum needle electrodes were implanted in the
subcutaneous tissue overlying the right scapula and the apex of the
heart. The femoral artery and vein were cannulated with polypropylene
and Silastic Fr-1.2 catheters, respectively, through a femoral cut
down, which was closed with a 6-0 silk suture. Arterial blood pressure
was continuously assessed from the arterial catheter, which was
connected to a Statham strain-gauge pressure transducer. Data were
digitized and recorded on HEM (version 3.3, Notocord; Croissy sur
Seine, France), a computer software package for the recording of
cardiovascular parameters, which derived HR on-line from the arterial
pressure pulses.
Assessment of baroreflex.
The first series of experiments (group 1) was undertaken in
MAO-A/B KO mice (n = 3; age, 30.1 ± 0.8 wk; body
weight, 24.1 ± 0.7 g) and WT mice (n = 3;
age, 29.4 ± 0.4 wk; body weight, 31.5 ± 0.5 g).
Baroreceptor function was examined using intravenous bolus injections
of phenylephrine (PE; an 
1-adrenergic agonist) and
sodium nitroprusside (SNP; a nitric oxide donor) to test the responses
of HR to transient hypertension and hypotension, respectively. All
boluses were administered in a volume of 100 µl of 0.9% aqueous saline heparinized at 7 U/ml and injected over 7 s. Final drug concentrations were adjusted to account for the catheter dead volume of
50 µl. The following sequence of drug administrations was delivered,
with 5-min periods between each bolus: saline 
5 µg/kg PE 25 µg/kg PE 70 µg/kg PE saline 5 µg/kg SNP 15 µg/kg
SNP 30 µg/kg SNP saline.
After this initial series of injections, anesthesia was discontinued,
and the animal was allowed to recover breathing only room air. During
this time, animals remained in the supine position and their movements
were limited by soft paper restraints. This state, beginning 30 min
after discontinuation of the anesthesia, is subsequently referred to as
the "recovery" state.
During the recovery state, animals were administered the following
sequence of pharmacological agents, again with 5-min delays between
each bolus: 25 µg/kg PE saline 30 µg/kg SNP saline. Animals were not manipulated in any way and would rest quietly during
the drug administration, which occurred at a distance through the
immobilized catheters. Subsequently, in the same animals, the effects
of cholinergic or 
-adrenergic blockade on the baroreceptor reflex
were examined, respectively, using glycopyrrolate (a muscarinic antagonist without central effects) and propranolol (a nonselective -adrenergic antagonist). These were administered under the following protocol using single doses: 75 µg/kg glycopyrrolate 25 µg/kg PE saline 750 µg/kg propanolol 30 µg/kg SNP saline.
At the end of this last sequence, animals were euthanized by a high level of halothane (5%) until respiration ceased, followed by cardiotomy.
To examine the effects of a pressor agent that, unlike PE, is
independent of MAO catabolism, we repeated the above experiments using
human angiotensin II (ANG II; Sigma). In a separate group of mice, the
baroreceptor reflex was examined using a drug administration sequence
identical to that described above except that ANG II (0.5, 1.0, and 4.0 µg/kg) replaced the low, medium, and high doses of PE. Experiments
(group 2) were again performed in the anesthetized animals
and 30 min after recovery in the MAO-A/B KO mice (n = 5; age, 24.7 ± 1.5 wk; body weight, 25.7 ± 1.0 g) and
their WT counterparts (n = 7; age, 29.3 ± 2.5 wk;
body weight, 30.7 ± 1.3 g).
Data analysis.
Basal levels of HR, mean arterial pressure (MAP), systolic pressure
(SBP), and diastolic pressure (DBP) were examined 1) during anesthesia before intravenous drug administration and 2) 30 min after discontinuation of the anesthesia (before acute drug
intervention). Differences between MAO-A/B KO (n = 8)
and WT mice (n = 10) were examined using
t-tests (two-tailed, P < 0.05).
To assess the baroreceptor gain, we plotted the peak HR as a function
of the peak SBP before and after each drug administration. Analysis
included the saline flushes that followed drug delivery and cleared the
catheter of any residual drug. A regression of peak HR on peak SBP was
calculated using the least-squares method. Because baroreceptor gain is
reflected in the relative decrease/increase in HR in response to either
hypertension/hypotension, our analysis for group 1 for the
observed changes in HR versus SBP pooled the responses to PE and SNP of
all animals under study. Likewise, in group 2, changes in HR
versus SBP after ANG II were pooled with those after SNP. Analysis was
performed separately for each genotype and separately for the
anesthetized or recovery state. Significance between the regression
lines was assessed by calculating the ratio of the regression sum of
squares over groups by the residual sum of squares within groups
(7). This approach, although powerful, does not provide
information of whether the difference resides in the slope, the
intercept, or both. A second approach was used to obtain this
information. Regressions of peak HR on peak SBP were obtained as
described above for every animal. These individual values of slope and
intercept were analyzed by ANOVA with the factors genotype (KO or WT),
drugs (PE or ANG II), and anesthesia (anesthetized or recovering).
The effects of cholinergic blockade on attenuating the baroreceptor
response was examined by statistical comparison of the regression of
peak HR on peak SBP during the response to PE or ANG II before and
after administration of glycopyrrolate. Similarly, the effects of
-adrenergic blockade on attenuating the baroreceptor response were
examined by statistical comparison of the regression of peak HR on peak
SBP during the response to SNP before and after administration of
propanolol. Measurements of the time delay between the occurrence of
the peak of the SBP and the trough of the HR after administration of PE
(25 µg/kg) or ANG II (1.0 µg/kg ) were determined for each animal
during recovery using the HEM software. The time delay between the SBP
trough and the HR peak after administration of SNP (30 µg/kg) was
similarly determined, with comparison between the genotypes made using
t-tests (2-tailed, P < 0.05).
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RESULTS |
Basal state.
We examined the "basal" autonomic state of the mice 1)
during anesthesia before intravenous drug administration and
2) 30 min after discontinuation of the anesthesia. During
both conditions, MAO-A/B KO compared with WT mice showed significantly
lower MAP, SBP, and DBP, with differences ranging from 10 to 15 mmHg
for each parameter (Table 1). HR were
lower in MAO-A/B KO mice compared with WT mice both during the recovery
state as well as during anesthesia, although differences were not
statistically significant.
MAP, SBP, and DBP increased 14-17 mmHg during recovery compared
with the baseline 30 min earlier during anesthesia, with no significant
difference in this effect between genotypes (Table 1). HR did not
differ between anesthesia and the recovery state for both genotypes.
Baroreflex response to PE and SNP.
Responses of blood pressure to drug injections were well developed and
were followed after a short delay by changes in HR (Fig.
1). In the recovery state, ANOVA of
regression demonstrated a statistically significant
dependence of HR on blood pressure for MAO-A/B KO and WT mice
(Table 2 and Fig.
2A). For the statistical analysis in which data from all observations were pooled, slopes were
more negative for MAO-A/B KO mice than WT mice, with significant differences between genotypes (Table 2). Results in anesthetized animals also showed a statistically significant dependence of HR on
blood pressure for MAO-A/B KO and WT mice (Table 2). Comparison across
genotypes demonstrated a significant difference, with a more negative
slope in MAO-A/B KO mice than in WT mice.
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Fig. 1.
Baroreceptor response in a wild-type (WT) mouse. Depicted are the
systolic, mean, and diastolic arterial blood pressure (BP;
top) as well as heart rate [HR; calculated from the
electrocardiogram (ECG; bottom)] versus time. Transient
hyper- and hypotension were induced with phenylephrine (25 µg/kg) and
nitroprusside (30 µg/kg), respectively.
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Fig. 2.
Baroreceptor reflex 30 min after discontinuation of
anesthesia. Depicted are the changes in HR [in beats/min (bpm)] and
systolic BP (SBP; in mmHg) for WT mice (n = 3) and
monoamine oxidase A and B (MAO-A/B) knockout (KO) mice
(n = 3) in response to intravenous administration of
phenylephrine or nitroprusside (A) as well as WT mice
(n = 7) and MAO-A/B KO mice (n = 5) in
response to intravenous administration of angiotensin II or
nitroprusside (B). Baseline values (means ± SD) before
pharmacological challenge are represented for WT mice
( ) and MAO-A/B KO mice ( ).
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When regression slopes and intercepts were calculated separately for
every animal, the factors genotype (P < 0.001) and
anesthesia (P < 0.00001) were found significant, but
not the factor drugs (P = 0.9) in the case of slopes.
In the case of intercepts, only the factor anesthesia was significant
(P < 0.00001). When the same analysis was conducted
separately for the anesthesia condition, it was found that the average
of the slopes of MAO-A/B KO mice was significantly different from WT
mice in the recovery condition (MAO-A/B KO: 
4.14 ± 0.16; WT:
3.275899 ± 0.14, P = 0.003). The same analysis
performed in animals under anesthesia indicated no significance between
the genotypes (MAO-A/B KO: 1.93 ± 0.18; WT: 1.48 ± 0.17). These results confirmed the conclusions of the initial
statistical analysis in which data from all observations were pooled
and indicated, in addition, that those differences resided (for the
animals recovering from anesthesia) in the slopes only.
There was no significant genotypic difference in the delay between the
peak SBP and the occurrence of the lowest HR value after administration
of PE (MAO-A/B KO: 2.44 ± 1.77 s; WT: 2.62 ± 1.21 s, P > 0.05). Similarly, there was no significant
genotypic difference in the delay between the lowest SBP and the
occurrence of the highest HR value after administration of SNP (MAO-A/B
KO: 4.16 ± 1.28 s; WT: 2.41 ± 0.78 s,
P > 0.05).
Baroreflex response to ANG II and SNP.
The results obtained with ANG II-SNP were similar to those obtained
using PE-SNP. In the recovery state, ANOVA of regression in the pooled
data analysis demonstrated a statistically significant dependence of HR
on blood pressure for MAO-A/B KO and WT mice (Table 2 and Fig.
2B). Slopes were more negative for MAO-A/B KO mice than for
WT mice, with significant differences between genotypes. Results in
anesthetized animals also showed a statistically significant dependence
of HR on blood pressure for MAO-A/B KO and WT mice (Table 2).
Comparison across genotypes demonstrated a significant difference, with
a more negative slope in MAO-A/B KO mice than in WT mice.
MAO-A/B KO mice showed a significantly shortened delay between the peak
SBP and the occurrence of the lowest HR value after administration of
ANG II (MAO-A/B KO: 7.58 ± 2.42 s; WT: 23.13 ± 3.81 s, P < 0.007). There was no significant
genotypic difference in the delay between the lowest SBP and the
occurrence of the highest HR value after administration of SNP (MAO-A/B
KO: 4.16 ± 1.28 s; WT: 2.41 ± 0.78 s,
P > 0.05).
Effect of peripheral cholinergic blockade on the baroreflex
response.
Muscarinic blockade with glycopyrrolate effectively blocked the cardiac
effects of the baroreflex response to PE in both strains. Comparison of
the regression slopes of pre- versus postglycopyrrolate treatment
showed significant changes for MAO-A/B KO mice (F = 88.09, P < 0.00005) and WT mice (F = 102.16, P < 0.00005) (data not shown). Muscarinic
blockade with glycopyrrolate also effectively blocked the occurrence of
the baroreflex response to ANG II in MAO-A/B KO mice (F = 96.61, P < 0.00005) and WT mice (F = 136.64, P < 0.00005) (Fig.
3). The blockade increased the slope of
the graph of HR on SBP from negative to near zero (Table
3). There was no significant genotypic
difference in the effect of glycopyrrolate on the baroreflex to PE or
ANG II.
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Fig. 3.
Effect of glycopyrrolate on blocking the reflex
bradycardia resulting after angiotensin II-induced BP increases.
Depicted are the data for the response to angiotensin II of WT mice
(A; n = 7) and MAO-A/B KO mice
(B; n = 5) before (gray squares) and after
(open circles) glycopyrrolate. Baseline starting values (means ± SD) before angiotensin II challenge are represented for
preglycopyrrolate (solid squares) and postglycopyrrolate (solid
circles) treatment states.
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Effect of -adrenergic blockade on the baroreflex response.
-Adrenergic blockade with propranolol did not significantly alter
the baroreflex response to SNP in either MAO-A/B KO mice (F = 1.38, P > 0.26) or WT mice
(F = 0.02, P > 0.98), with minimal changes noted in the slope of the graph of HR on SBP (Fig.
4 and Table 3).
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Fig. 4.
Effect of propranolol on attenuating the reflex
tachycardia resulting after nitroprusside-induced hypotension. Depicted
are the data for the response to nitroprusside of WT mice
(A; n = 7) and MAO-A/B KO mice
(B; n = 5) before (gray squares) and after
(open circles) propranolol. Baseline starting values (means ± SD)
before nitroprusside challenge are represented for the prepropanolol
(solid squares) and postpropranolol (solid circles) treatment states.
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DISCUSSION |
The following findings were revealed by our study: First, the
basal autonomic state of MAO-A/B KO mice was characterized by significantly lower blood pressures (MAP, SBP, and DBP) than those observed in WT mice; these findings were seen during anesthesia and to
a greater extent after the 30-min postsurgical recovery period. Second,
MAO-A/B KO mice compared with WT mice demonstrated a significantly
increased gain of the baroreflex, with exaggerated responses to both
PE-SNP as well as ANG II-SNP noted during the recovery state as well as
during anesthesia. Third, the response after administration of ANG II
between occurrence of the SBP peak and subsequent trough of the HR was
significantly more rapid in the MAO-A/B KO mice than in the WT mice.
Finally, cholinergic blockade with glycopyrrolate effectively blocked
the cardiac effects of the baroreflex activation in both MAO-A/B KO and
WT mice, with no significant difference between genotypes. The
baroreflex recovered partially, minutes after glycopyrrolate treatment,
but was not affected after this by propranolol treatment in both strains.
A trend toward lower blood pressure in MAO-deficient mice has been
previously reported by our group in mice singly deficient for either
the MAO-A or MAO-B gene (21, 37). In addition, our
findings are consistent with anecdotal reports of altered peripheral
autonomic function and resting hypotension in patients with Norrie
disease, an X-linked recessive disorder encompassing deletions in the
genes for MAO-A and/or MAO-B (29, 41). The decreased blood
pressure and trend toward bradycardia in the present MAO-A/B KO mice
appears counterintuitive based on the known sympathetic effects of NE,
5-HT, and DA. One possible explanation for the lower basal blood
pressure of MAO-A/B KO mice examined in the present study is the
occurrence of an increased gain of the baroreflex. Baroreceptor input,
even under normotensive conditions, tonically inhibits sympathetic
effects on blood vessels and the heart. An increased gain of this
reflex could serve to lower blood pressure in response to the excessive
basal levels of pressor amines. Results of the present study are
consistent with this claim, although we cannot rule out the possibility
that lower basal blood pressure may be determined by an altered set
point, acting possibly through serotonergic mechanisms felt to be
independent of the baroreceptor reflex (31, 44).
Resetting of the baroreflex is well known in animal and human subjects
with chronic hypertension. Typically, in these subjects, a
desensitization of the reflex has been reported, with differences more
apparent in younger than in older animals (30). Evidence suggests that changes in sensitivity may be a result rather than a
cause of the increased pressures, in so far as baroreflex function can
be restored by antihypertensive medication (22). However, in any feedback loop in which blood pressure and baroreflex sensitivity modulate each other, identifying causation becomes difficult. A similar
challenge is presented in interpreting the findings of the current study.
Genotypic differences in the baroreceptor response became increasingly
apparent during postsurgical recovery from anesthesia, consistent with
the known attenuation of the baroreflex by halothane (5).
Although the possibility of lingering effects of halothane on the
baroreflex 30 min after its discontinuation cannot be fully ruled out,
at least in human subjects, a rapid return of the baroreflex sensitivity to normal within 5 min has been reported (9).
The genotypic differences in the baroreflex are unlikely to be
explained simply by a differential effect of postoperative stress in
the KO and WT mice. We believe this to be the case, because the
increase in the baroresponse of KO compared with WT mice was observed
not only after discontinuation of the halothane-nitrous oxide but also
during anesthesia, when effects of behavioral stress would presumably
be absent. Furthermore, there was no significant genotypic difference
in HR during anesthesia or recovery.
The time to reach a minimum HR in response to PE was nearly ninefold
more rapid than that after administration of ANG II. Given the near
instantaneous response (2-4 s of lag) to PE, no genotypic
differences were detected in the delay between the peak SBP and HR
trough. In the slower response to ANG II, however, it was apparent that
MAO-A/B KO mice had not only a greater gain in the baroreflex but also
a significantly more rapid response. The reason for the exaggerated
delay in the baroreflex response using ANG II compared with PE remains
a matter for speculation. PE effects immediate vasoconstriction at the
vascular level through direct adrenergic agonism at
1-receptor sites, whereas ANG II, likely acting through
AT1 receptor sites, secondarily results in vasoconstriction
through an indirect adrenergic mechanism. Furthermore, ANG II may in
fact increase HR through a -adrenergic receptor mechanism. Such
increases do not appear to play an important role in mice in mediating
pressor responses to ANG II, because these are largely buffered by a
concomitant decrease in HR mediated by the baroreflex (6).
However, they might counteract, hence delay, the time to reach minimum
HR after the initial pressor effect of ANG II.
Changes in the baroreflex gain might occur at several levels: the
central nervous system, the peripheral nervous system, or else the
associated vasculature, the sinoatrial node, and the heart. It is
possible that MAO-A/B KO mice, due to their chronically elevated levels
of vasoactive amines, compensate by altering the sensitivity of
receptors and signal transduction pathways important in maintaining
vascular tone. Although such mechanisms were not directly measured in
the current study, our group has previously reported presence of a
downregulation in these animals of postsynaptic 5-HT1a,
5-HT2a, and 5-HT2c receptors in the central
nervous system (8, 40), which, as described below, may
play a role in regulation of the baroreflex.
Numerous studies have shown that several neuroactive substances, in
particular 5-HT, NE, and DA, are implicated in the reflex regulation of
arterial blood pressure at the level of the nucleus tractus solitarius
(NTS). Experiments using direct injection of agonists and antagonists
into the NTS have suggested dose-dependent effects on blood pressure
mediated via 2-adrenergic receptors (42,
49) and serotonergic 5-HT1a, 5-HT2, and
5-HT3 receptors (2, 4, 12, 45) and possibly
dopaminergic D2 receptors (26). Of relevance
to the issue of changes in baroreflex gain is the observation that when
NE is increased acutely (15) or chronically (14,
48) after peripheral administration, or if NE is directly
administered into the NTS (11), the result is an increased
gain of the baroreceptor reflex. Clonidine and guanfacine (both central
2-adrenergic agonists) increase the sensitivity of the
baroreflex (16), whereas prazosin (an -adrenergic
blocker) diminishes it (36). The dopamine
D2/D3 agonist quinpirole after intravenous
administration also results in profound increases in the baroreflex
gain (47). That 5-HT does not directly affect baroreflex
gain is suggested by reports that neither acute nor chronic
administration of fluoxetine (a selective 5-HT reuptake inhibitor) nor
administration of ketanserin (a 5-HT2 receptor antagonist)
nor intra-NTS microinjection of 5-HT change the sensitivity of the
reflex arc (1, 13, 31, 33, 43). The mechanism of action of
PEA in effecting autonomic changes remains unknown, although an
inhibition of the reuptake, as well as effects on direct release of DA,
NE, and possibly 5-HT, has been suggested (3, 24, 34).
Another means of altering baroreceptor sensitivity is through changes
in vagal tone (10). It has been suggested that activation of presynaptic 5-HT receptors may increase neurotransmitter release from vagal afferent neurons and thus may modulate the cardiac reflex
responses through mechanisms linked to increases in vagal tone
(19, 38). Within the NTS, there have been reported
D2 receptors both pre- and postsynaptic to vagal terminals
(26), and DA has also been shown to facilitate
baroreceptor discharges of the carotid sinus in vitro
(50). In our study, muscarinic peripheral cholinergic
blockade with glycopyrrolate effectively blocked the occurrence of the
baroreflex of the MAO-A/B KO mice, suggesting an intact vagal component
to the reflex response at the level of the heart. However, because of
the possibility that the dose (75 µg/kg) chosen in our study may have
allowed for the occurrence of a maximal response ("ceiling
effect"), absence of a genotypic difference in response to
glycopyrrolate does not rule out alterations within the cholinergic
system of MAO-A/B KO mice.
-Adrenergic blockade with propranolol demonstrated little effect on
the baroreflex of both MAO-A/B KO and WT mice. Our experiment was
designed to examine the effect of -adrenergic blockade after cholinergic blockade. Because of the robust response to glycopyrrolate that was equivalent in both genotypes, the subsequent baseline HR was
high, making it likely that presence of a ceiling effect may have
masked the attenuation in the HR response expected to occur after
administration of propranolol. The doses employed were equivalent to
those used by other investigators (6, 46), although the
possibility must be considered that higher doses in our mouse strain
(129 Sv) might have elicited a response.
MAO-A/B KO mice have been shown to demonstrate elevated brain levels of
catecholamines, 5-HT and PEA, and presumably similar findings are to be
found in blood. The increase in these animals of the baroreceptor
response should be contrasted with the decreased baroreceptor response
noted in patients with pheochromocytoma. In these patients,
desensitization of the adrenergic system associated with chronic
catecholamine excess has been considered to be one of the determinants
for the decreases in baroreceptor gain. In addition, patients with a NE
transporter deficiency show elevated levels of plasma NE as well as a
decreased baroreceptor sensitivity (39). These findings
suggest that increases in the baroreflex noted in our MAO-A/B KO mice
may not relate directly to increases noted in these animals in levels
of NE. Alternatively, decreases in baroreceptor gain noted in the
above-mentioned clinical disorders may relate to factors outside of the
noradrenergic system. Future studies will be needed to evaluate which
of the neurotransmitters that show elevated tissue concentrations in
MAO-A/B KO mice is responsible for the observed changes in HR and blood
pressure regulation and whether such effects take place within the
central nervous system or in the periphery.
In conclusion, our results demonstrate a decreased basal blood pressure
and an increased baroreceptor gain in MAO-A/B KO mice compared with
their WT counterparts. Such a physiological overcompensation that
resets the blood pressure of MAO-A/B KO mice to a lower basal level may
function to attenuate the physiological consequences of acute,
exaggerated metabolic shifts in the concentration of pressor amines.
| |
ACKNOWLEDGEMENTS |
This study was supported by National Institute of Mental Health
Grants RO1 MH-NS-62148 (to D. P. Holschneider), RO1 MH-37020, R37
MH-39085 (Merit Award), and KO5 MH-00796 (Research Scientist Award), by
Whitaker Foundation Research Grant RG-99-0331 (to D. P. Holschneider), by the Elsie Welin Professorship (to J. C. Shih), by the Veterans Administration (to O. U. Scremin), and by the Laubisch Endowment (to K. P. Roos and O. U. Scremin).
| |
FOOTNOTES |
Address for reprint requests and other correspondence: D. P. Holschneider, Univ. of Southern California, Keck School of Medicine, LAC-USC Hosp., Dept. of Psychiatry, 1200 N. State St., Rm. 10-621, Los
Angeles, CA 90033 (E-mail: holschne{at}hsc.usc.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.
10.1152/ajpheart.00309.2001
Received 17 April 2001; accepted in final form 7 November 2001.
| |
REFERENCES |
1.
Alper, RH.
Effects of the selective serotonin reuptake inhibitor fluoxetine on baroreceptor reflex sensitivity and body weight in young and old rats.
J Gerontol B Psychol Sci Soc Sci
47:
B130-B136,
1992.
2.
Bagdy, G,
Szemeredi K,
Kanyicska B,
and
Murphy DL.
Different serotonin receptors mediate blood pressure, heart rate, plasma catecholamine and prolactin responses to m-chlorophenylpiperazine in conscious rats.
J Pharmacol Exp Ther
250:
72-78,
1989.
3.
Bailey, BA,
Philips SR,
and
Boulton AA.
In vivo release of endogenous dopamine, 5-hydroxytryptamine and some of their metabolites from rat caudate nucleus by phenylethylamine.
Neurochem Res
12:
173-178,
1987.
4.
Balasubramaniam, G,
Lee HS,
and
Mah SC.
Characterization of the biphasic blood pressure response to -methyl-5-hydroxytryptamine in anaesthetized rats.
Arch Int Pharmacodyn Ther
329:
360-378,
1995.
5.
Behnia, R,
and
Koushanpour E.
Local versus central effect of halothane on carotid sinus baroreceptor function.
Anesthesiology
61:
161-168,
1984.
6.
Bivalacqua, TJ,
Dalal A,
Champion HC,
and
Kadowitz PJ.
Role of AT1 receptors and autonomic nervous system in mediating acute pressor responses to ANG II in anesthetized mice.
Am J Physiol Endocrinol Metab
277:
E838-E847,
1999.
7.
BMDP.
Statistical Software Manual. Los Angeles, CA: University of California Press, 1992.
8.
Bou-Flores, C,
Lajard AM,
Monteau R,
De Maeyer E,
Seif I,
Lanoir J,
and
Hilaire G.
Abnormal phrenic motoneuron activity and morphology in neonatal monoamine oxidase A-deficient transgenic mice: possible role of a serotonin excess.
J Neurosci
20:
4646-4656,
2000.
9.
Carter, JA,
Clarke TN,
Prys-Roberts C,
and
Spelina KR.
Restoration of baroreflex control of heart rate during recovery from anaesthesia.
Br J Anaesth
58:
415-421,
1986.
10.
Cerati, D,
and
Schwartz PJ.
Single cardiac vagal fiber activity, acute myocardial ischemia, and risk for sudden death.
Circ Res
69:
1389-1401,
1991.
11.
Chan, JY,
Tsou MY,
Len WB,
Lee TY,
and
Chan SH.
Participation of noradrenergic neurotransmission in the enhancement of baroreceptor reflex response by substance P at the nucleus tractus solitarii of the rat: a reverse microdialysis study.
J Neurochem
64:
2644-2652,
1995.
12.
Dabire, H.
Central 5-hydroxytryptamine (5-HT) receptors in blood pressure regulation.
Therapie
46:
421-429,
1991.
13.
Davy, M,
Midol-Monnet M,
Heimburger M,
Beslot F,
and
Cohen Y.
Peripheral and central cardiovascular effects of ketanserin in conscious normotensive rats.
Arch Int Pharmacodyn Ther
290:
193-206,
1987.
14.
Dorward, PK,
Saigusa T,
and
Eisenhofer G.
Differential effects of central and peripheral desipramine on sympathoadrenal function and heart rate in conscious rabbits.
J Cardiovasc Pharmacol
17:
519-531,
1991.
15.
Eremeev, VS,
Smirnov VA,
Khrustaleva RS,
Tsyrlin VA,
and
Shcherbin I.
The possible mechanism of the noradrenaline enhancement of the arterial baroreceptor reflex.
Fiziol Zh Im I M Sechenova
82:
48-56,
1996.
16.
Fischetti, F,
Fabris B,
Calci M,
Candido R,
Armini L,
Bardelli M,
Cominotto F,
Biagi A,
and
Carretta R.
Effects of central alpha 2-adrenergic agonists on systemic haemodynamics and on baroreceptor reflex sensitivity in spontaneously hypertensive rats.
Boll Soc Ital Biol Sper
70:
279-286,
1994.
17.
Grimsby, J,
Toth M,
Chen K,
Kumazawa T,
Klaidman L,
Adams JD,
Karoum F,
Gal J,
and
Shih JC.
Increased stress response and beta-phenylethylamine in MAOB-deficient mice.
Nat Genet
17:
206-210,
1997.
18.
Hamada, M,
Shigematsu Y,
Mukai M,
Kazatani Y,
Kokubu T,
and
Hiwada K.
Blood pressure response to the Valsalva maneuver in pheochromocytoma and pseudopheochromocytoma.
Hypertension
25:
266-271,
1995.
19.
Head, GA,
and
Korner PI.
Cardiovascular functions of brain serotonergic neurons in the rabbit as analysed from the acute and chronic effects of 5,6-dihydroxytryptamine.
J Cardiovasc Pharmacol
4:
398-408,
1982.
20.
Holschneider, DP,
Kumazawa T,
Chen K,
and
Shih JC.
Tissue-specific effects of estrogen on monoamine oxidase A and B in the rat.
Life Sci
63:
155-160,
1998.
21.
Holschneider, DP,
Scremin OU,
Huynh L,
Chen K,
Seif I,
and
Shih JC.
Regional cerebral cortical activation in monoamine oxidase A-deficient mice: differential effects of chronic versus acute elevations in serotonin and norepinephrine.
Neuroscience
101:
869-877,
2000.
22.
Ichikawa, M,
Suzuki H,
Kumagai K,
Ryuzaki M,
Kumagai H,
Nishizawa M,
and
Saruta T.
Baroreceptor function is restored by antihypertensive therapy through lowering of blood pressure in adult SHR.
Clin Exp Pharmacol Physiol
22, Suppl1:
S67-S69,
1995.
23.
Keck, PE, Jr,
Carter WP,
Nierenberg AA,
Cooper TB,
Potter WZ,
and
Rothschild AJ.
Acute cardiovascular effects of tranylcypromine: correlation with plasma drug, metabolite, norepinephrine, and MHPG levels.
J Clin Psychiatry
52:
250-254,
1991.
24.
Knoll, J,
Miklya I,
Knoll B,
Marko R,
and
Racz D.
Phenylethylamine and tyramine are mixed-acting sympathomimetic amines in the brain.
Life Sci
58:
2101-2114,
1996.
25.
Kumada, M,
Terui N,
and
Kuwaki T.
Arterial baroreceptor reflex: its central and peripheral neural mechanisms.
Prog Neurobiol
35:
331-361,
1990.
26.
Lawrence, AJ,
and
Jarrott B.
Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius.
Prog Neurobiol
48:
21-53,
1996.
27.
Middlekauff, HR.
Mechanisms and implications of autonomic nervous system dysfunction in heart failure.
Curr Opin Cardiol
12:
265-275,
1997.
28.
Muneta, S,
Kawada H,
Iwata T,
Murakami E,
and
Hiwada K.
Impairment of baroreceptor reflex in patients with phaeochromocytoma.
J Hum Hypertens
6:
77-78,
1992.
29.
Murphy, DL,
Sims KB,
Karoum F,
de la Chapelle A,
Norio R,
Sankila EM,
and
Breakefield XO.
Marked amine and amine metabolite changes in Norrie disease patients with an X-chromosomal deletion affecting monoamine oxidase.
J Neurochem
54:
242-247,
1990.
30.
Nedvidek, J,
and
Zicha J.
Baroreflex control of heart rate in young and adult salt hypertensive inbred Dahl rats.
Physiol Res
49:
323-330,
2000.
31.
Nosjean, A,
Franc B,
and
Laguzzi R.
Increased sympathetic nerve discharge without alteration in the sympathetic baroreflex response by serotonin3 receptor stimulation in the nucleus tractus solitarius of the rat.
Neurosci Lett
186:
41-44,
1995.
32.
Osborn, JW,
and
Hornfeldt BJ.
Arterial baroreceptor denervation impairs long-term regulation of arterial pressure during dietary salt loading.
Am J Physiol Heart Circ Physiol
275:
H1558-H1566,
1998.
33.
Pettersson, A,
Gradin K,
Hedner T,
Henning M,
and
Persson B.
Chronic serotonergic blockade with ketanserin in the spontaneously hypertensive rat.
J Cardiovasc Pharmacol
7, Suppl7:
S112-S113,
1985.
34.
Raiteri, M,
Del Carmine R,
Bertollini A,
and
Levi G.
Effect of sympathomimetic amines on the synaptosomal transport of noradrenaline, dopamine and 5-hydroxytryptamine.
Eur J Pharmacol
41:
133-143,
1977.
35.
Rebrin, I,
Chen K,
Geha RM,
and
Shih JC.
Characterization of naturally occuring mutation in the mouse MAO-A gene (Abstract).
Soc Neurosci Abstr
26:
82,
2000.
36.
Sasso, EH,
and
O'Connor DT.
Prazosin depression of baroreflex function in hypertensive man.
Eur J Clin Pharmacol
22:
7-14,
1982.
37.
Scremin, OU,
Holschneider DP,
Chen K,
Li MG,
and
Shih JC.
Cerebral cortical blood flow maps are reorganized in MAOB-deficient mice.
Brain Res
824:
36-44,
1999.
38.
Sevoz, C,
Nosjean A,
Callera JC,
Machado B,
Hamon M,
and
Laguzzi R.
Stimulation of 5-HT3 receptors in the NTS inhibits the cardiac Bezold-Jarisch reflex response.
Am J Physiol Heart Circ Physiol
271:
H80-H87,
1996.
39.
Shannon, JR,
Flattem NL,
Jordan J,
Jacob G,
Black BK,
Biaggioni I,
Blakely RD,
and
Robertson D.
Orthostatic intolerance and tachycardia associated with norepinephrine-transporter deficiency.
N Engl J Med
342:
541-549,
2000.
40.
Shih, JC,
Ridd MJ,
Chen K,
Meehan WP,
Kung MP,
Seif I,
and
De Maeyer E.
Ketanserin and tetrabenazine abolish aggression in mice lacking monoamine oxidase A.
Brain Res
835:
104-112,
1999.
41.
Sims, KB,
de la Chapelle A,
Norio R,
Sankila EM,
Hsu YP,
Rinehart WB,
Corey TJ,
Ozelius L,
Powell JF,
and
Bruns G.
Monoamine oxidase deficiency in males with an X chromosome deletion.
Neuron
2:
1069-1076,
1989.
42.
Sved, AF,
Tsukamoto K,
and
Schreihofer AM.
Stimulation of alpha 2-adrenergic receptors in nucleus tractus solitarius is required for the baroreceptor reflex.
Brain Res
576:
297-303,
1992.
43.
Takata, M,
Zhang X,
Sugimoto T,
Ikeda K,
Tomoda F,
Mikawa M,
Iida H,
and
Mizumura Y.
Cardiovascular effects of ketanserin in conscious rabbits.
Jpn Heart J
27:
95-106,
1986.
44.
Tsukamoto, K,
Kurihara T,
Nakayama N,
Isogai O,
Ito S,
Komatsu K,
and
Kanmatsuse K.
Pressor responses to serotonin injected into the nucleus tractus solitarius of Sprague-Dawley rats and spontaneously hypertensive rats.
Clin Exp Hypertens
22:
63-73,
2000.
45.
Tsukamoto, K,
Sved AF,
Ito S,
Komatsu K,
and
Kanmatsuse K.
Enhanced serotonin-mediated responses in the nucleus tractus solitarius of spontaneously hypertensive rats.
Brain Res
863:
1-8,
2000.
46.
Uechi, M,
Asai K,
Osaka M,
Smith A,
Sato N,
Wagner TE,
Ishikawa Y,
Hayakawa H,
Vatner DE,
Shannon RP,
Homcy CJ,
and
Vatner SF.
Depressed heart rate variability and arterial baroreflex in conscious transgenic mice with overexpression of cardiac Gs.
Circ Res
82:
416-423,
1998.
47.
Van den Buuse, M,
and
Head GA.
Quinpirole treatment increases renal sympathetic nerve activity and baroreflex gain in conscious rabbits: a spectral study.
Eur J Pharmacol
388:
85-88,
2000.
48.
Wang, J,
Ochoa M,
Patel MB,
Zucker IH,
Loud AV,
Zeballos GA,
and
Hintze TH.
Carotid baroreceptor function in dogs with chronic norepinephrine infusion.
Hypertension
17:
745-754,
1991.
49.
Yamazaki, T,
and
Ninomiya I.
Noradrenaline contributes to modulation of the carotid sinus baroreflex in the nucleus tractus solitarii area in the rabbit.
Acta Physiol Scand
149:
1-6,
1993.
50.
Zapata, P.
Effects of dopamine on carotid chemo- and baroreceptors in vitro.
J Physiol (Lond)
244:
235-251,
1975.
Am J Physiol Heart Circ Physiol 282(3):H964-H972
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ABSTRACT
INTRODUCTION
