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2-Adrenergic receptors in NTS facilitate
baroreflex function in adult spontaneously hypertensive rats
Cardiovascular Center and Department of Internal Medicine, University of Iowa and Research Service of the Department of Veterans Affairs Medical Center, Iowa City, Iowa 52242; and Department of Physiological Sciences, University of Florida, Gainesville, Florida 32610
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We
examined the effect of 
2-adrenoreceptor blockade in the
nucleus of the solitary tract (NTS) on baroreflex responses elicited by
electrical stimulation of the left aortic depressor nerve (ADN) in
urethane-anesthetized spontaneously hypertensive rats (SHR, n = 11) and normotensive Wistar-Kyoto rats (WKY,
n = 11). ADN stimulation produced a frequency-dependent
decrease in mean arterial pressure (MAP), renal sympathetic nerve
activity (RSNA), and heart rate (HR). In SHR, unilateral microinjection
of idazoxan into the NTS markedly reduced baroreflex control of MAP,
RSNA, and HR and had a disproportionately greater influence on
baroreflex control of MAP than of RSNA. In WKY, idazoxan
microinjections did not significantly alter baroreflex function
relative to control vehicle injections. These results suggest that
baroreflex regulation of arterial pressure in SHR is highly dependent
on NTS adrenergic mechanisms. The reflex regulation of sympathetic
outflow to the kidney is less influenced by the altered
2-adrenoreceptor mechanisms in SHR.
hypertension; medulla; norepinephrine
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IN HYPERTENSION, the arterial baroreflex is often characterized by a decrease in reflex sensitivity and/or a resetting of reflex function to higher than normal pressures (9, 10, 15, 31, 34). In the spontaneously hypertensive rat (SHR), for example, resting blood pressure may be 40-60 mmHg higher than in normotensive rats, and the sigmoid curve describing baroreflex control of mean arterial pressure (MAP) and sympathetic nerve activity is shifted upward but remains parallel to the normal baroreflex curve (9, 12, 38). In contrast, baroreflex control of heart rate (HR) is markedly attenuated in the SHR (2, 15, 21, 34), despite a resting HR that is comparable to that of its normotensive control, the Wistar-Kyoto rat (WKY). Because baroreflex impairment is present in many forms of hypertension and may contribute to the development and maintenance of the disease (9, 10, 15, 31, 34), an understanding of the neurochemical mechanisms involved is of great importance. Neurochemical derangements in hypertension might alter afferent and efferent signaling or integration of baroreceptor information within the central nervous system.
In the brain, the medial nucleus of the solitary tract (NTS) in the
caudal medulla is the primary central termination site for baroreceptor
afferents (16). The integrity of this region is necessary
for normal baroreflex control of blood pressure (29). In
the SHR and other rat models of hypertension with altered baroreflex function, declines in norepinephrine content and
2-adrenoreceptor density within the NTS have been
documented (27, 33, 36, 37). Because selective blockade of
2-adrenoreceptors in the medial NTS of normotensive
subjects can increase resting blood pressure and attenuate baroreflex
function (19, 30, 35), this decline in
2-adrenoreceptor density in the NTS of hypertensive animals has been hypothesized to contribute to autonomic dysfunction in
these animals. However, the physiological impact of these neurochemical changes on blood pressure regulation in the SHR is poorly understood. To date, only a few studies have directly investigated the influence of
2-adrenoreceptor stimulation in the NTS on blood
pressure regulation in the adult SHR. The results of these studies
demonstrate that 2-adrenoreceptor stimulation in NTS of
both SHR and WKY produces a gradual decline in resting blood pressure
and HR, which peaks 10-20 min after microinjection (6, 20,
36). Although the time course for these cardiovascular changes
appears to be similar between strains, 2-adrenoreceptor
stimulation in NTS of SHR tends to produce an equal or greater decline
in both MAP and HR compared with WKY (6, 20, 36). This
suggests that, although receptor numbers are reduced in the NTS of the
SHR, the sensitivity of 2-adrenoreceptors on neurons
involved in blood pressure regulation may be enhanced. However, changes
in baroreflex function associated with 2-adrenoreceptor
modulation in NTS of SHR remain to be determined. In normotensive rats,
modulation of baroreflex function after 2-adrenoreceptor
manipulation in the NTS has been reported to occur on a much shorter
time scale, on the order of 1-2 min (30). Therefore,
it remains to be identified whether neurons specifically involved in
baroreceptor afferent processing in NTS of the SHR also demonstrate
increased sensitivity to 2-adrenoreceptor modulation.
The present study was specifically undertaken to examine the role of
altered 2-adrenoreceptor function in the NTS on
baroreflex regulation in the SHR. On the basis of knowledge that
blockade of 2-adrenoreceptors in the NTS significantly
attenuates baroreflex control in normotensive animals (30)
and evidence suggesting that 2-adrenoreceptors in the
NTS of SHR may have increased sensitivity to neuromodulation, we
hypothesized that blockade of 2-adrenoreceptors in the
NTS would have a greater impact on baroreflex function in SHR than in
WKY. The results of this study confirm our hypothesis and also
demonstrate a differential influence of 2-adrenergic modulation of renal sympathetic nerve activity (RSNA) vs. MAP in the
SHR. The results of this study were presented previously in a
preliminary form (26).
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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All experiments were performed on age-matched (11-13 wk old) adult male SHR and WKY (265-450 g; Taconic Farms) housed in the University of Iowa animal care facility. All experimental procedures were approved by the University Animal Care and Use Committee before use.
General preparation. All animals were anesthetized with an intraperitoneal injection of urethane (1.2-1.4 g/kg) and instrumented with femoral arterial and venous catheters (PE-50 tubing) for the recording of arterial pressure and intravenous administration of supplemental fluids, respectively. Body temperature was monitored with a rectal temperature probe and kept within normal range (37 ± 1°C) with a heating blanket. Supplemental anesthesia was given when necessary as evidenced by marked changes in blood pressure, HR, or respiration during surgery or in response to a pinch of the hind paw. While the animal was in the supine position, a midline incision was made on the ventral surface of the neck, the trachea was exposed, a tracheostomy was performed, and the animal was intubated. The right carotid sinus nerve (CSN) was identified near its insertion into the glossopharyngeal nerve and sectioned. The right and left aortic depressor nerves (ADN) were identified by their insertion into the superior laryngeal nerve near the bifurcation of the common carotid artery and isolated from the vagus nerve. The right ADN was sectioned. The left CSN and ADN were left intact.
The animal was then placed in the prone position in a stereotaxic head holder (Kopf Instruments, Tujunga, CA) with the nose flexed downward 30-40° to increase the exposure of the caudal medulla and minimize movement of the brain stem. The left renal nerve was exposed retroperitoneally and isolated. An intact branch of the nerve was placed on a silver wire bipolar electrode and covered with warmed mineral oil. A lateral incision was made in the neck, and the intact left ADN was placed on a bipolar stimulating electrode and covered with a dental acrylic mixture to prevent drying. Finally, an incision was made over the occipital bone, the muscles were retracted, the atlantooccipital membrane was removed, and the dorsal surface of the medulla was exposed at the level of obex.Hemodynamic and sympathetic recording. The arterial catheter was connected to a pressure transducer (Statham P10 EZ; Gould, Valley View, OH) that was connected a chart recorder (TA20; Gould) and a computer data sampling system [Cambridge Electronics Design (CED), Cambridge, UK]. RSNA was amplified and band-pass filtered (0.3-1.0 kHz) with a Grass P511 amplifier (Grass Instruments, West Warwick, RI) and passed through a nerve traffic analyzer (model 121; World Precision Instruments, Sarasota, FL), which produced a transistor-transistor logic (TTL) pulse for each spike falling within a voltage window set above the background noise level. The TTL pulses were recorded as digital signals by the computer sampling system (CED 1401). Background noise was determined by baroreflex inhibition of sympathetic drive in response to a rapid rise in arterial pressure produced by an intravenous dose of the potent vasoconstrictor phenylephrine (20-30 µg/kg).
Baroreflex testing. Baroreflex testing typically took place 3 h after the initial induction of anesthesia. The ADN bipolar stimulating electrodes were connected to a programmable stimulator (Master8; AMPI, Jerusalem, Israel) in series with a stimulus isolator (DS2; Digitimer). Arterial baroreflex responses were evoked by electrical stimulation (8 V, 2-ms pulse duration, 10 s) of the left ADN, which in the rat contains primarily baroreceptor afferents (18). All data were collected during 1-min trials, which included sampling blood pressure and RSNA for 20 s before ADN stimulation, for 10 s during ADN stimulation at a single frequency, and for 25 s after the offset of ADN stimulation. Stimulation frequency (2, 5, 10, or 15 Hz) was randomly varied between trials. Within each trial, only one stimulation frequency was applied. Trials were separated by ~1-min recovery periods.
2-Adrenoreceptor blockade.
After the collection of baseline baroreflex responses, a five-barrel
microinjection pipette was stereotaxically positioned 400 µm rostral
to the calamus scriptorius, 400 µm lateral to midline, and
300-400 µm ventral from the surface of the medulla for
microinjection into the left NTS. The dorsal medulla and area postrema
were visualized with the help of a surgical microscope (Zeiss). Four of
the barrels of the microinjection pipette were attached to a pressure
injection system (BH-2; Medical Systems), and the pipette was secured
to a micromanipulator (Kopf Instruments). Local blockade of
2-adrenoceptors in the NTS was accomplished by pressure
injection of the 2-receptor antagonist idazoxan
(8, 18, 30) from the microinjection pipette. In test
animals, three of the barrels contained idazoxan in concentrations of
10, 50, and 100 mM, diluted in artificial cerebrospinal fluid (aCSF).
The concentrations of idazoxan to be used were chosen on the basis of
previous studies demonstrating that this dose range significantly
alters baroreflex function in rats when unilaterally microinjected into
the NTS (30) and significantly attenuates
cardiovascular responses to central microinjection of
norepinephrine in other regions of the brain (14). In
control animals, three of the barrels were filled with aCSF containing (in mM) 122 NaCl, 3 KCl, 25.7 NaHCO3, and 1 CaCl2, with pH adjusted to 7.4. In approximately one-half
of the animals, the fourth barrel was filled with the excitatory amino
acid DL-homocysteic acid (DLH; 10 mM, diluted in aCSF). A
test injection of DLH (50 nl) was made to confirm the location of the
cardiovascular region of the NTS. The DLH solution was mixed with a
small amount of fluorescent latex microspheres (LumaFluor, Naples, FL)
to facilitate identification of the microinjection sites. In the
remaining animals, the fourth barrel contained 2% pontamine blue for
histological marking of the injection sites. The volume of solution
that was pressure injected into the NTS was determined by monitoring
the movement of the meniscus in a microinjection pipette through a magnifying monocular microscope equipped with a calibrated eyepiece.
Protocol. Baseline measurements of MAP, HR, and RSNA and baroreflex responses to ADN stimulation were determined. Idazoxan (10, 50, or 100 mM in 100 nl) or aCSF (100 nl) was then microinjected into the left NTS ipsilateral to the ADN stimulating electrode. Two minutes later, baroreflex responses were retested. The animal was then allowed to recover for 1 h before repeat baroreflex testing and the next NTS microinjection. Previous investigators demonstrated that baroreflex responses can recover from central idazoxan injections in the NTS within 15 min (30). The order in which the different concentrations of idazoxan were microinjected into the NTS was randomized among animals. Each animal received all three idazoxan concentrations or three separate injections of aCSF during the course of the 3- to 4-h experiment.
At the end of the experiment the animals were euthanized, and the brain was removed and placed in a 10% formalin solution for 72 h. The brains were then frozen to
12 to 14°C, and the caudal brain stem
was sliced into 40-µm sections with a freezing microtome (Carl Zeiss,
Thornwood, NY). The microinjection site was recovered by imaging the
brain slices with a microscope (Zeiss) equipped with epifluorescence to
detect the location of the fluorescent beads. The microscope was
equipped with a sidearm drawing device so that the microinjection sites
within the NTS could be reconstructed on schematic representations of
the NTS.
Data analysis. All data were analyzed off-line with SPIKE2 software (CED). HR was derived from the intervals between peak systolic pressure pulses in the arterial pressure trace. Baseline MAP, RSNA, and HR were calculated from 5-s averages measured immediately preceding each ADN stimulation period or DLH microinjection trials. Peak changes in MAP, RSNA, and HR during baroreflex testing were calculated as the difference between the preceding baseline value and the peak drop in MAP, RSNA, or HR during 10 s of ADN stimulation. The peak drop in RSNA and HR was measured from a 5-s window starting at the onset of stimulation. The peak drop in MAP was measured from a 5-s average taken over the last 5 s of stimulation. Peak changes in MAP, HR, and RSNA after microinjection of DLH were detected within the first 15 s of microinjection; 5-s averages were taken around the nadir of the response. Peak changes in MAP, RSNA, and HR during DLH stimulation were then calculated as the difference between the preceding baseline value (a 5-s average) and the peak drop in MAP, RSNA, and HR after DLH microinjection. In all experiments measurements of RSNA were quantified with the TTL pulses generated from windowed spike activity. For the present set of experiments this method of quantifying RSNA was chosen over other methods of nerve analysis, such as full-wave rectification and integration, because the use of the window discriminator afforded elimination of stimulus artifacts associated with electrical stimulation of the ADN. In addition, previous comparisons in our lab (13) detected no significant difference between methods of quantifying changes in RSNA when comparing data averaged over similar (5-10 s) time intervals.
Significant changes in baroreflex responses after2-blockade in the NTS were compared between strains with
a three-way ANOVA (1st factor was ADN stimulation frequency, 2nd factor
was central microinjection drug, and 3rd factor was strain). When
appropriate, differences either between or within strains were analyzed
next with a two-way ANOVA (1st factor was either strain or
within-strain effect of drug, and 2nd factor was ADN stimulation
frequency). A post hoc Scheffé's F-test was performed
when the ANOVA showed a significant effect. Significant differences in
baseline RSNA, MAP, HR, and weight between strains were determined with
a one-factor ANOVA. Significant differences in baseline RSNA, MAP, or
HR after central microinjections were tested with a one-way ANOVA with repeated measures (comparing 5-s averages before and after NTS microinjection). Significant differences in reflex function over time
were examined with a one-way ANOVA with repeated measures. Differences
between mean values were considered significant when P < 0.05. Statistical comparisons of DLH responses between strains, however, were not made because of the small sample size and relatively large standard deviations. All data are reported as means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The mean age of all rats used in this study was 12 ± 0.2 wk.
Although they were age matched, the mean weight of the SHR
(n = 11) was significantly less than that of the WKY
(n = 11) (298 ± 6 vs. 412 ± 11 g;
P < 0.001). Table 1
shows baseline variables for both rat strains before treatment and at 1 and 2 h into the experimental protocol. Under urethane anesthesia,
resting MAP of the SHR was also significantly different from that of
the WKY. There was, however, no significant difference in resting HR or RSNA.
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Baroreflex control in normotensive vs. hypertensive rats.
Figure 1 shows a comparison of the
averaged baroreflex responses curves from 11 SHR vs. 11 WKY before NTS
microinjection. In both SHR and WKY, electrical stimulation of the left
ADN elicited a frequency-related decrease in both MAP and RSNA.
Increasing ADN stimulation frequencies in WKY also elicited a
frequency-dependent decrease in HR. In contrast, electrical stimulation
of the ADN in SHR produced no significant reflex bradycardia.
Baroreflex control of RSNA and HR in the SHR was significantly
attenuated relative to that in the WKY (P <0.01; see Figs.
3 and 4). Conversely, reflex control of MAP was similar between strains
[i.e., 15-Hz ADN stimulation produced a 37 ± 4 mmHg drop in MAP
in SHR (n = 11) vs. a 40 ± 4 mmHg drop in WKY
(n = 11); P > 0.1].
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Responses to DLH stimulation.
The cardiovascular response to neuronal excitation with the excitatory
amino acid agonist DLH was measured in approximately one-half of the
animals. The other animals received no central injection before the
onset of the experiment. In both SHR (n = 6) and WKY
(n = 6), local microinjection of 50 nl of the
excitatory amino acid antagonist DLH (0.5 nmol) into the NTS produced a
small depressor response (MAP 25 ± 7 vs. 14 ± 4 mmHg,
RSNA 27 ± 9 vs. 28 ± 9 spikes/s, SHR vs. WKY) and
bradycardia (HR 27 ± 9 vs. 32 ± 11 beats/min, SHR vs.
WKY). Baroreflex responses were retested after DLH microinjections.
Baroreflex responses recorded before and after DLH
microinjection were not significantly different in either strain
(P > 0.85).
Effects of idazoxan in NTS on baroreflex function.
Figure 2 illustrates the effect of
unilateral microinjection of the highest dose of idazoxan (10 nmol)
into the NTS on baroreflex control in an SHR and a WKY. In this
example, baroreflex control of MAP in the SHR was blocked after
idazoxan microinjection (see Fig. 2A, left vs.
right). Baroreflex control of RSNA, however, was not
markedly altered, and the HR response at baseline was too small to
observe an effect. In contrast, this dose of idazoxan had a pronounced
effect only on baroreflex control of HR in the WKY, with minimal
effects on the baroreflex-mediated change in MAP and RSNA (see Fig. 2B,
left vs. right). Microinjection of 1, 5, or 10 nmol idazoxan and of aCSF into the left NTS had no effect
(P > 0.3) on baseline MAP, RSNA, or HR in the SHR
(n = 11) or the WKY (n = 11) relative
to preinjection values.
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2-adrenoreceptor blockade in the SHR, microinjection of
idazoxan (at 1, 5, or 10 nmol) into the NTS of the WKY had no
significant effect (P > 0.1) on baroreflex control of
MAP, RSNA, or HR beyond the effects of the microinjection alone. Group
data from the WKY that received 10 nmol of idazoxan are shown in Fig.
4. Although microinjection of 10 nmol of
idazoxan unilaterally into the NTS appeared to attenuate baroreflex
control of HR relative to preinjection controls (n = 11), a comparable effect was observed after microinjection of aCSF
alone. A significant decrease in baroreflex control of HR was observed
across all three groups of aCSF microinjections (n = 5/group; P < 0.05) relative to preinjection controls
(n = 11).
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Recovery of baroreflex function after repeated microinjection in
NTS.
Approximately 1 h after each microinjection trial of either
idazoxan or aCSF in the NTS, the recovery of baroreflex function was
tested. Within strains, baroreflex responses measured 1 h after
central microinjection of either idazoxan or aCSF were not significantly different from control responses (P > 0.2); thus the data were combined. Figure
5 illustrates the effect of time and
repeated microinjection on baseline baroreflex control. In both SHR
(n = 11) and WKY (n = 11) there was a
small decline in baseline MAP and HR over time (see Table 1), but these
changes were not significant. Associated with the small shift in
baseline MAP there was a downward shift of the MAP baroreflex response curves in both SHR and WKY. In WKY there was also a significant downward shift of the HR baroreflex curve. The downward shift of the
MAP and HR baroreflex curves over time, however, was not accompanied by
any significant change in baroreflex sensitivity, such that the
absolute drop in MAP or HR evoked by ADN stimulation remained constant
over time in both strains of rats (P > 0.1; data not
shown).
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Histological verification of microinjection sites.
Figure 6 shows the recovered
microinjection sites for both idazoxan and aCSF in SHR and WKY. In both
SHR and WKY, the injection sites were primarily located in the NTS at
the level of area postrema.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In the present study we investigated for the first time the direct
effects of 2-adrenoreceptor blockade in the NTS on
baroreflex function in SHR. Baroreflex responses were evoked by
electrical stimulation of the left ADN. Before experimental
intervention, we observed a significantly higher resting blood pressure
and an upward resetting of baroreflex function in SHR relative to age-matched WKY. As previously described by others, the baroreflex control of MAP in SHR paralleled that in WKY, with similar absolute decline in MAP during ADN stimulation (9, 12), but
baroreflex control of RSNA and HR was attenuated (4, 9, 12, 15, 24, 34). These alterations in baroreflex function in the SHR have been associated with a reduction in both catecholamine content in
the NTS and 2-adrenoreceptor numbers within the NTS
(27, 36, 37). Moreover, stimulation of
2-adrenoreceptors in normotensive rats lowers resting
pressure and augments arterial baroreflex function (3, 7, 19,
20) and blockade of these receptors significantly attenuates
baroreflex control of MAP, HR, and RSNA and can raise resting pressures
(19, 30, 35). These observations suggest that
altered 2-adrenoreceptor function within the
NTS may play a role in baroreflex dysfunction in the SHR. In our study, immediately after 2-adrenoreceptor blockade in the NTS,
baroreflex control of MAP was markedly attenuated in SHR but was
unchanged in WKY. These findings are in agreement with the results of
other studies demonstrating that modulation of
2-adrenoreceptor function in the NTS produces greater
changes in cardiovascular regulation in SHR compared with WKY (6,
20). Furthermore, because 2-adrenoreceptor numbers may be reduced in the NTS of SHR (27, 33, 36, 37), our findings suggest that 2-adrenoreceptor sensitivity
is enhanced in the SHR.
To our knowledge, only one other study has investigated the influence
of 2-adrenoreceptor modulation on baroreflex function in
SHR vs. WKY. In that study, Hayashi and colleagues (12)
used electrical stimulation of the left ADN to elicit baroreflex
responses, similar to the experimental protocol we used. They
demonstrated that systemic administration of the
2-adrenoreceptor agonist clonidine (5 µg/kg)
significantly augmented baroreflex control of MAP and splanchnic
sympathetic nerve activity in SHR but had no effect on baroreflex
control in WKY. In addition, the effects of clonidine administration in
SHR were shown to eliminate the differences in baroreflex function
between the two strains. Although systemic administration of drugs can
target multiple regions of the brain, the findings of Hayashi and
colleagues suggest that a central change in
2-adrenoreceptor stimulation contributes in part to
shifts in baroreflex control of MAP and sympathetic nerve activity in
the SHR. The results of our study corroborate these results,
demonstrating that modulation of central -adrenergic receptors has
an impact on baroreflex control of MAP, RSNA, and HR in SHR but not in
WKY. The results of our study extend the results of Hayashi and
colleagues and raise the possibility that the central change in
2-adrenoreceptor function, which strongly contributes to
baroreflex dysfunction in the SHR, may be localized to the NTS.
Moreover, as mentioned above, if 2-adrenoreceptor sensitivity is indeed enhanced in the NTS of the SHR, our findings, in
combination with those of Hayashi et al., suggest that this change in
receptor sensitivity is not sufficient to overcome the reduction in
local norepinephrine levels reported by others (27, 33, 36,
37). As a result, baroreflex function is normally blunted and
shifted to higher pressures in the SHR. Furthermore, NTS neurons
involved in baroreflex control of MAP, and possibly HR, in the SHR
appear to demonstrate a greater dependence on existing 2-adrenoreceptor stimulation than do neurons involved in
reflex control of RSNA.
The observation that higher doses of idazoxan in the NTS had a
disproportionate effect on baroreflex control of MAP vs. RSNA in the
SHR is novel. This finding suggests that differential baroreflex modulation of vascular beds by 2-adrenoreceptors can
occur early in the central baroreflex arc of the SHR. The possibility
that 2-adrenoreceptors in the SHR would have a
predominant influence on sympathetic outflow to specific vascular beds
has been suggested before. For example, Kapusta and colleagues
(17) demonstrated that intracerebroventricular
administration of the 2-adrenoreceptor agonist guanabenz
resulted in a selective modulation of mesenteric vascular responses to
air jet stress in the SHR without altering stress-induced changes in
renal vascular resistance. Furthermore, in the study of Hayashi and
colleagues (12) mentioned above, intravenous clonidine
augmented MAP responses to ADN stimulation in the SHR by only
5-10%, whereas splanchnic nerve responses increased by
10-30%. These findings are in agreement with our results,
suggesting that the central blockade of
2-adrenoreceptors in the SHR may have differential
effects on sympathetic drive to specific vascular beds. Unfortunately,
in both our study and the study by Hayashi and colleagues, the effects
of higher doses of 2-adrenoreceptor agonists or
antagonists, sufficient to modulate baroreflex function in WKY, were
not tested. Thus it remains to be determined whether this
characteristic of differential control is linked to hypertension, strain differences, or simply a characteristic of
2-adrenoreceptor modulation of baroreflex function in
the rat NTS.
There is suggestive evidence that the altered responsiveness to
2-adrenoreceptor blockade that we observed in SHR
compared with WKY may indeed be related, in part, to strain
differences. For example, the same concentration of idazoxan (5 nmol)
we used in our study has been shown to be sufficient to significantly attenuate baroreflex control of MAP in normotensive Sprague-Dawley rats
(30). The fact that this same dose had no effect on
baroreflex control in the WKY raises the possibility that WKY, rather
than SHR, may have an altered response to
2-adrenoreceptor modulation. If WKY
2-adrenoreceptor responsiveness is truly reduced, this would suggest that the results of the present study may primarily be
due to strain differences and not linked to hypertension.
Unfortunately, at the present time this issue cannot be addressed
further without performing additional tests comparing WKY and
Sprague-Dawley rats under the same conditions, because the previous
study in Sprague-Dawley rats was performed in chloralose-anesthetized,
artificially ventilated rats (30). Both anesthesia and
ventilation have been shown to influence baroreflex responsiveness
(24, 28), and either of these methodological differences
could have accounted for the differences observed in the present study
for WKY and those reported for Sprague-Dawley rats. Still, in support
of our findings suggesting a relationship between hypertension and
central changes in 2-adrenoreceptor modulation of the
baroreflex, there has been one report that norepinephrine microinjected
into the NTS of the SHR produces a more pronounced bradycardia compared
with both WKY and Sprague-Dawley rats tested under the same
experimental conditions (20). Nevertheless, further studies are needed to sort out these differences.
The potential importance of methodological differences in identifying
the effects of 2-adrenoreceptor modulation on central baroreflex function is further highlighted by two studies performed in
anesthetized, ventilated rabbits. One study reported no effect of
2-adrenoreceptor blockade in the NTS on baroreflex
control of RSNA (11), whereas the second study observed a
significant attenuation of reflex control of RSNA (35).
Differences in anesthesia, the dose of 2-adrenoreceptor
antagonist used, the type of receptor antagonist chosen, or differences
in the timing of reflex measurement after receptor blockade may all
have contributed to the lack of effect of
2-adrenoreceptor blockade on baroreflex function
observed in one study. There is also evidence from the brain slice
preparation that the effects of 2-adrenoreceptor
modulation may be best observed when submaximal stimuli are used
(1, 5), suggesting that the method used to elicit
baroreflex responses or NTS neuronal activation may also play an
important role in detecting central changes in reflex function.
Overall, current evidence strongly supports the hypothesis that
2-adrenoreceptor function within the NTS is important to reflex regulation of blood pressure. However, the mechanisms through which these receptors modulate baroreceptor afferent processing within
the NTS remain to be clearly defined. Single-unit recording studies
have demonstrated that local application of
2-adrenoreceptor agonists in the NTS typically inhibits
the activity of single NTS neurons (23). Accordingly, it
might be expected that application of an
2-adrenoreceptor antagonist would facilitate afferent
processing. Yet the exact opposite effect has been observed at the
level of the whole reflex, both in our study and in others (19,
30, 35). One possible explanation for this phenomenon comes from pharmacological studies using tissue slice preparations from both the
spinal cord (22) and the NTS (5). These
studies suggest that neurons receiving catecholaminergic inputs may
contain both 1- and 2-adrenergic
receptors. These receptors may be differentially expressed on the cell
soma vs. dendritic processes, and selective activation of the two
receptor subtypes may have opposing effects on membrane excitation.
Moreover, activation of either receptor type can increase or decrease
the excitability of NTS neurons (5). Therefore, the
discrepancy between the effects of catecholaminergic receptor
activation on sensory processing in the NTS at the level of a single
neuron vs. a network of neurons may be a function of the combined and
as yet poorly understood influences of 1- and
2-adrenoreceptor interactions in this region of the brain.
Finally, we also reported a small attenuation of baroreflex control of HR in WKY after microinjection of the vehicle into the NTS. Although we do not have a good explanation for this observation, it should be noted that other investigators have also reported a greater influence of volume on modulation of HR compared with MAP in both normotensive and hypertensive rats (6, 32, 36). We did not observe a similar effect on baroreflex control of HR in SHR after vehicle injections, but reflex control of HR was so markedly attenuated in the SHR that a change would be difficult to detect. Conversely, we did observe a progressive increase in baseline RSNA in SHR over time, which was not observed in WKY. Again, we do not have a specific explanation for these results, but important to this study is the fact that baroreflex control of RSNA did not change over time. Thus there was no apparent change in central processing of afferent inputs associated with the time intervals required for repeated microinjections in the NTS of either SHR or WKY.
In summary, the results of the present study describe for the first
time the effects of 2-adrenoreceptor modulation in the medial NTS on baroreflex function in the SHR. First, we observed a
greater change in baroreflex regulation of MAP and RSNA after 2-adrenoreceptor blockade in the NTS of SHR vs. WKY.
This observation is in agreement with one previous study suggesting
that central baroreflex processing in the SHR may have an increased
sensitivity to modulation of 2-adrenoreceptors
(12) and supports our original hypothesis that
2-adrenoreceptor blockade would have a greater impact on
baroreflex function in SHR than in WKY. This finding also suggests that
neurons involved in baroreflex control of MAP in the NTS of the SHR are
extremely dependent on endogenous catecholaminergic stimulation.
Second, we demonstrated that baroreflex regulation of RSNA in SHR was
less sensitive to 2-adrenergic modulation than
baroreflex control of MAP. This finding is consistent with one previous
report, suggesting that modulation of central
2-adrenoreceptors in the SHR may have a greater
influence over sympathetic drive to the mesenteric vascular bed
(17). It also suggests that the baroreflex control of RSNA
in the SHR is preserved despite a reduction in
2-adrenoreceptor function, raising the possibility that
alternate neurotransmitters may be more important to the central
modulation of baroreflex control of RSNA. Still, the link between our
findings and hypertension per se remains to be established because the responses of other normotensive rat strains to comparable doses of
2-adrenoreceptor agonists or antagonists in NTS have not
yet been tested. Finally, our data suggest that differential modulation of vascular resistance by 2-adrenoreceptors can occur
relatively early on in the central processing of baroreceptor inputs in
the SHR. These results provide new insights into the potential
mechanisms by which antihypertensive agents that rely on central
adrenoreceptor modulation may selectively influence different vascular beds.
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ACKNOWLEDGEMENTS |
|---|
This work was supported by American Heart Association Grant-in-Aid Award 95014480 to L. F. Hayward and National Heart, Lung, and Blood Institute Grant HL-29302 to R. B. Felder.
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FOOTNOTES |
|---|
Address for reprint requests and other correspondence: L. F. Hayward, Dept. of Physiological Sciences, College of Veterinary Medicine, Univ. of Florida, PO Box 100144, Gainesville, FL 32610-0144 (E-mail: lindah{at}ufl.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.
First published January 31, 2002;10.1152/ajpheart.00167.2001
Received 31 December 2001; accepted in final form 24 January 2002.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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) and 11 WKY
(
). Resting values are represented by 0-Hz stimulation.
Baroreflex control of MAP, RSNA, and HR was significantly different in
SHR compared with WKY (*P < 0.01, 2-way ANOVA) across
all ADN stimulation frequencies tested. Comparison of effects at
individual ADN frequencies was not made. bpm, Beats per minute.
MAP; top), RSNA (
,
solid lines), immediately after artificial cerebrospinal fluid
microinjection (aCSF, n = 5;
, dotted
lines), and after 10 nmol of idazoxan (n = 6;
, dashed lines). B: reflex response evoked
before NTS microinjections (n = 11; 
, Averaged baroreflex
responses elicited 1 h after the 1st microinjection trial.
