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Am J Physiol Heart Circ Physiol 282: H964-H972, 2002. First published November 1, 2001; doi:10.1152/ajpheart.00309.2001
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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


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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 protein-1, n = 8; WT: 5.51 ± 0.70 nmol · 20 min-1 · mg protein-1, 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 alpha 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 right-arrow 5 µg/kg PEright-arrow 25 µg/kg PE right-arrow 70 µg/kg PEright-arrow saline right-arrow 5 µg/kg SNPright-arrow 15 µg/kg SNP right-arrow 30 µg/kg SNPright-arrow 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 PEright-arrow saline right-arrow 30 µg/kg SNP right-arrow 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 beta -adrenergic blockade on the baroreceptor reflex were examined, respectively, using glycopyrrolate (a muscarinic antagonist without central effects) and propranolol (a nonselective beta -adrenergic antagonist). These were administered under the following protocol using single doses: 75 µg/kg glycopyrrolate right-arrow 25 µg/kg PEright-arrow saline right-arrow 750 µg/kg propanolol right-arrow 30 µg/kg SNP right-arrow 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 beta -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).


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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.

                              
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  		Table 1.   HR and blood pressure in MAO-A/B KO and WT mice

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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  		Table 2.   Baroreflex response to PE-SNP or ANG II-SNP



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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 ().

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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  		Table 3.   Baroreflex response after cholinergic or beta -adrenergic blockade

Effect of beta -adrenergic blockade on the baroreflex response. beta -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.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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 alpha 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 beta -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 alpha 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 alpha 2-adrenergic agonists) increase the sensitivity of the baroreflex (16), whereas prazosin (an alpha -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.

beta -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 beta -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
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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 alpha -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 Gsalpha . 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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