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-2 adrenoreceptor modulation of
arterial chemoreflexes in the caudal solitary nucleus of the
rat
Department of Physiological Sciences, University of Florida College of Veterinary Medicine, Gainesville, Florida 32610-0144
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The caudal region of
the nucleus of the solitary tract (cNTS) is the primary central
termination site for arterial chemoreceptor afferents originating from
the carotid body. The purpose of the present study was to investigate
the role of endogenous activation of 
-2 adrenoreceptors in the cNTS
on arterial chemoreflex function. Arterial chemoreflex responses to
intravenous injections of potassium cyanide (KCN; 75 µg/kg) were
recorded before and following blockade of -2 adrenoreceptors in the
cNTS of urethane-anesthetized rats. KCN alone elicited a reflex
increase in arterial pressure, renal sympathetic nerve activity, and
respiration. After bilateral cNTS microinjection of -2 receptor
antagonists (2 nmol idazoxan or 0.2 nmol yohimbine), arterial
chemoreflex responses were markedly attenuated. Attenuation of
chemoreflex function was not accompanied by any significant change in
resting blood pressure or respiratory rate. The results suggest that
the endogenous activation of -2 adrenoreceptors facilitates central
processing of chemoreceptor afferent inputs in the cNTS of the rat.
idazoxan; peripheral chemoreflex; arterial baroreflex; respiratory control; potassium cyanide
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE CAUDAL REGIONS of the nucleus of the solitary tract (NTS) have been identified as the primary central termination site of arterial chemoreceptor afferents (4-6, 15, 36). In the rat, electrical or chemical activation of the caudal NTS (cNTS), ~600 µm caudal to the most caudal tip of area postrema, elicits a rapid increase in sympathetic drive, blood pressure, and respiratory rate (25, 36), a response similar to that evoked by direct activation of arterial chemoreceptors (16, 36). Conversely, electrolytic lesions or chemical blockade of the cNTS selectively attenuates chemoreflex responses (27, 36, 38). Although the cNTS and other critical regions of the brain, including the rostral ventrolateral medulla and the ventrolateral pons, have been identified to be essential components of the central arterial chemoreflex pathway (17-19, 29), surprisingly little is known about the neurochemicals important in modulating this central reflex arc.
Present evidence suggests that glutamate may be the primary
neurotransmitter released by chemoreceptor afferents to activate second-order neurons in the NTS (10, 36, 38). There is
also evidence that release of norepinephrine within the cNTS may be important to central processing of arterial chemoreceptor inputs. First, the cNTS is richly innervated by noradrenergic neurons, originating from both within the NTS (A2), the ventrolateral medulla (A1), and the ventrolateral pons (A5) (21, 35). Second,
immunohistochemical studies have shown that brief exposure to hypoxia
in rats activates noradrenergic neurons in the A1 and A5 regions that
may potentially project to the cNTS (5, 14), and, finally,
changes in norepinephrine release and biosynthesis have been documented
in the cNTS following prolonged exposure to hypoxia (28, 31,
32). Therefore, there is good evidence to suggest that
norepinephrine may be an important neuromodulator of chemoreceptor
afferent processing in the cNTS. The functional implication of
modulating norepinephrine levels or -2 receptor function in the cNTS
on arterial chemoreflex function, however, remains to be fully defined.
The present study was undertaken to identify the effects of -2
adrenoreceptor blockade in the cNTS on arterial chemoreflex function.
On the basis of previous findings that norepinephrine is an important
neuromodulator of visceral afferent processing in other regions of the
NTS, including arterial baroreflex function (20, 33), in
the present study it was hypothesized that blockade of the -2
adrenoreceptors in cNTS would significantly attenuate chemoreflex
function. The results of this study confirm this hypothesis. Portions
of this work have been presented in a preliminary format (11).
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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All experiments were performed on adult male Sprague-Dawley rats (320-420 g; obtained from Harlan) housed in the university animal care facility and exposed to a normal 12:12-h light-dark cycle (6 AM to 6 PM and 6 PM to 6 AM). All experimental procedures were preapproved by the University of Florida Institutional Animal Care and Use Committee.
General preparation. 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 recording of arterial pressure and administration of intravenous fluids, respectively. While the rat was in the supine position, a midline incision was made on the ventral surface of the neck, a tracheostomy was performed, and the animal was intubated. Two small (0.003-mm diameter) Teflon-coated, stainless steel wires with bared tips were inserted into the right side of the diaphragm through the abdominal musculature for measurement of inspiratory activity. Next, the renal nerve was exposed retroperitoneally, isolated, and covered with warmed mineral oil. The animal was placed in the prone position in a stereotaxic head holder (Kopf, Tugunga, CA), and the dorsal surface of the medulla was exposed by a limited craniotomy and removal of the atlantooccipital membrane.
The arterial catheter was attached to a calibrated pressure transducer (Statham) connected to an amplifier (Stoelting). The analog output from the blood pressure amplifier was connected to a computer data-sampling system [Cambridge Electronics Design (CED), 1401 computer interface]. The intact branch of the renal nerve was placed on a bipolar silver wire electrode attached to a Grass preamplifier probe (H1P1) and signal amplifier (P511). The diaphragm electromyographic (dEMG) wires were connected to a second Grass preamplifier probe and P511 amplifier. Both dEMG and renal sympathetic nerve activity (RSNA) signals were amplified (200,000 to 500,000 times), band-pass filtered (0.3 to 1.0 kHz), rectified, and integrated (50- to 20-ms time constants, respectively; Paynter Filter, BAK Electronics). Rectified and integrated dEMG and RSNA signals were then connected to the CED system. Baseline dEMG and RSNA amplitudes were arbitrarily adjusted to a value of 2.0-2.5 (arbitrary U) at the beginning of the experiment and recorded simultaneously along with arterial pressure. Body temperature was kept within normal range of 38 ± 1°C with a heating blanket and a rectal temperature probe (Harvard Apparatus). Supplemental anesthesia was given when necessary (0.1 g/kg iv), as evidenced by marked changes in blood pressure, heart rate (HR), or respiration during surgery or in response to a pinch of the hindpaw. In some experiments, animals were vagotomized (vagal nerves were isolated and sectioned bilaterally in the cervical region), paralyzed, and ventilated. Before administration of the paralytic (20 mg/kg pancuronium bromide), animals were mechanically ventilated (Harvard Small Animal Ventilator). Mechanical ventilation rate was set between 65 and 75 breaths/min. The paralytic was then administered, and absence of dEMG was used as an indicator of the effectiveness of the paralytic. With the return of dEMG activity, an additional dose of the paralytic was given. In all animals, reflex testing typically took place 2 h following the initial induction of anesthesia.Protocol. In the present set of experiments, arterial chemoreflex responses were elicited by intravenous bolus injections of potassium cyanide (KCN; 75 µg/kg). The intertrial interval for KCN administration was ~10 min. KCN was chosen as a stimulus because it provides a brief potent stimulus for arterial chemoreceptors and elicits reproducible reflex responses when repeated administration occurs at 5- to 10-min intervals (12, 13, 18).
Baroreflex responses were also recorded by monitoring RSNA and HR in response to intravenous bolus injections of phenylephrine (PE; 20-30 µg/kg). The testing of baroreflex responses occurred in separate trials from chemoreflex testing. Intravenous PE injections were administered ~5 min following each KCN injection and 10 min following the preceding PE injection. All data were collected during 90-s trials, during which time blood pressure, dEMG, and RSNA were sampled continuously for 20 s before and 70 s following intravenous drug administration. After the collection of baseline chemoreflex and baroreflex responses (typically, two reflex responses each), a single-barrel microinjection pipette was attached to a pressure injection system (BH-2, Medical Systems) and secured to a micromanipulator (Kopf). Next, the micropipette was positioned 500-600 µm caudal to the most caudal tip of area postrema, ±200 µm lateral from midline, and 200-400 µm ventral to the surface of the brain. All drugs were diluted in artificial cerebrospinal fluid (aCSF) (in mM): 122 NaCl, 3 KCl, 25.7 NaHCO
-2 receptor
antagonists (9), or kynurenic acid (10 mM; Sigma), a
broad-spectrum excitatory amino acid (EAA) receptor antagonist
(38). In separate experiments, aCSF alone was
microinjected into the cNTS as a control for potential volume-related
effects produced by microinjection process alone.
All solutions were microinjected bilaterally in volumes of 45-65
nl/side. The volume microinjected was determined by carefully monitoring the movement of the meniscus in the micropipette with a
monocular microscope equipped with a calibrated eyepiece (Titan Tools).
Thirty seconds following completion of the central injections, the
micropipette was retracted from the brain. Five minutes following completion of bilateral microinjection, arterial chemoreflex and baroreflex responses were retested. Reflex responses were then retested
90 min following the central microinjection.
At the end of the experiment, the animal was euthanized, and the brain
was removed and placed in a 4% paraformaldehyde solution for
24-72 h. The brains were then frozen to 
14°C, and the caudal brain stem was sliced into 40-µm transverse sections with a cryostat (Zeiss, HM101), mounted on slides, and sealed with a coverslip (Antifade, Molecular Probes). The microinjection site was then recovered by imaging the brain slices with a microscope equipped with
epifluorescence (Zeiss) and a computer-enhanced imaging system (Axiovision).
Data analysis. All data were analyzed offline using SPIKE2 software (CED). Peak changes in mean arterial pressure (MAP) and RSNA during chemoreceptor or baroreceptor stimulation were calculated from the difference between the baseline (a 5-s average measured 10 s before each bolus injection) and the peak deviation from baseline during chemoreceptor or baroreceptor stimulation (a 3-s average). Background noise levels in the RSNA signal were quantified (during baroreflex-evoked inhibition of sympathetic drive elicited by the PE bolus injections) and then subtracted from the total rectified and integrated RSNA signal. Peak changes in RSNA during KCN or PE administration were represented as a percentage change from the immediately preceding baseline. To help maintain consistent measurement time windows between trials, changes in HR were always calculated from within same time window used for calculation of the peak MAP response. HR was derived from the average interval between peak systolic pressure pulses in the arterial pressure trace.
Respiratory parameters, including dEMG burst duration, interburst interval, burst amplitude, and respiratory frequency, were calculated from individual bursts and then averaged. Baseline respiratory activity was averaged over the same 5-s period as MAP, RSNA, and HR (i.e., 10 s before intravenous bolus injections). Peak changes in respiratory parameters were calculated by subtracting the 5-s baseline average from the average peak respiratory reflex response averaged from within one of two consecutive 3-s time windows that began at the onset of a change in respiratory pattern (typically identified by an increase in dEMG burst amplitude; see RESULTS). For baroreflex responses, the peak respiratory response was measured during the same time window used to calculate peak changes in MAP and HR. Significant changes in chemoreflex or baroreflex responses following central microinjection of idazoxan, yohimbine, kynurenic acid, or aCSF were analyzed using paired Student's t-test with Bonferroni corrections for multiple comparisons. Changes were considered significant when P < 0.05. All data are means ± SE.| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The mean weight of the adult male rats used in this study was 348 ± 5 g (n = 31). In the spontaneously breathing rats, the average resting MAP, HR, and respiratory rates were 100 ± 2 mmHg, 372 ± 7 beats/min, and 106 ± 3 breaths/min (n = 26), respectively.
Figure 1A shows the typical
response to intravenous injection of KCN (75 µg/kg) observed in a
spontaneously breathing animal. The onset of the arterial chemoreflex
response was characterized by an increase in the burst amplitude of
both RSNA and dEMG and typically began 3 s following the bolus
injection of KCN. The initial increase in dEMG amplitude was identified
as the first phase of a two-phase respiratory response to KCN injection
and was characterized by a peak increase in dEMG amplitude during the
first 3 s of onset of the response. This first phase was labeled peak-1 (Pk-1; see Fig. 1 for an example of the Pk-1 time window). The
average increase in dEMG burst amplitude during this time was 100 ± 13% (n = 26) above the preceding baseline and was
coupled with small initial increase in respiratory rate (Pk-1 mean
increase in rate was 29 ± 4 breaths/min; n = 26).
But the greatest change in respiratory rate typically occurred during
the next 3-s time window following KCN injection. This second phase of
the response to KCN was labeled peak-2 (Pk-2; see Fig. 1 for an example
of the Pk-2 time window), and the average increase in respiratory rate
during Pk-2 was 47 ± 4 breaths/min (n = 26). Peak
changes in RSNA occurred during Pk-1 (98 ± 16% above baseline;
n = 26), and peak changes in MAP (18 ± 2 mmHg;
n = 26) followed the RSNA response. KCN-evoked changes
in HR were averaged during the peak change in MAP and were minimal
(5 ± 1 beats/min; n = 26). MAP, HR, and RSNA
returned to baseline within 30 s following KCN injections. Respiration rate returned to baseline levels within 60 s following the initial onset of the reflex response.
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To identify whether the cardiorespiratory responses described above
were indeed associated with KCN stimulation of arterial chemoreceptors,
in three animals reflex responses to KCN were recorded before and 10 min following bilateral sectioning of the carotid sinus nerves, which
contain the primary source of peripheral chemoreceptor afferents in the
rat (16). As illustrated in Fig. 2, carotid sinus denervation eliminated
both cardiovascular and respiratory responses to intravenous KCN. Acute
denervation also resulted in a small decrease in resting MAP (82 ± 5 vs. 73 ± 11 mmHg, pre- vs. postdenervation, respectively;
n = 3), HR (359 ± 9 vs. 339 ± 17 beats/min), and respiratory rate (101 ± 8 vs. 88 ± 4 breaths/min).
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Effects of idazoxan microinjection into the cNTS on arterial
chemoreflex and baroreflex responses.
To identify the potential role of endogenous catecholamines in the
central processing of arterial chemoreceptor input, the -2 receptor
antagonist idazoxan (9, 33) was microinjected bilaterally
into the cNTS of a group of spontaneously breathing animals
(n = 10). As illustrated in Fig. 1B,
arterial chemoreflex responses were markedly attenuated following
microinjection of a total of 2 nmol of idazoxan into the cNTS. Figures
3 and 4
illustrate the average change in chemoreflex responses recorded before,
5 min, and 90 min following idazoxan microinjection into the cNTS. Peak
reflex-evoked changes in both MAP and RSNA were significantly attenuated following idazoxan microinjection (Fig. 3) as were the peak
respiratory responses (Pk-1 dEMG burst amplitude and Pk-2 respiratory
rate; see Fig. 4). There was also a trend for the increase in
respiratory rate measured during Pk-1 to be blunted, but the change was
not significant (Table 1).
Reflex-evoked changes in HR were minimal before and were not markedly
altered following idazoxan microinjection into the cNTS.
Idazoxan-induced attenuation of chemoreflex responses was not coupled
with any significant change in resting MAP, RSNA, HR, or respiratory
rate (Table 2).
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-2 Receptor blockade in the nucleus gracilus produced no noticeable
change in chemoreflex responses. For example, the peak KCN-evoked
increase in MAP was 15 ± 1 mmHg before microinjection vs. 14 ± 1 mmHg after idazoxan microinjection in the nucleus gracilus. Reflex
responses from these two animals were not included in the analysis of
those animals described above in which idazoxan microinjections were
localized to the cNTS.
Effects of yohimbine in the cNTS on chemoreflex responses in
ventilated, vagotomized rats.
To identify whether changes in respiratory pattern associated with
-2 receptor blockade in the cNTS contributed in any way to the
simultaneous attenuation of the cardiovascular response, chemoreflex
responses were recorded in a second group of vagotomized, artificially
ventilated, and paralyzed animals (n = 5). In these animals, chemoreflex responses were recorded before and after bilateral
microinjection of a different -2 receptor antagonist, yohimbine
(0.2-nmol total dose), into the cNTS. Similar to the effects of
idazoxan, yohimbine microinjected into the cNTS significantly attenuated KCN-evoked increases in MAP and RSNA (Fig.
6A). HR responses were minimal
before and remained unchanged following cNTS yohimbine microinjection.
All reflex responses returned to control levels 90 min following
central yohimbine microinjection (Fig. 6, B and
C).
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Effects of kynurenic acid in the cNTS.
To confirm that our microinjection sites were in the same region of the
NTS previously identified to be critical for chemoreceptor afferent
processing (36, 39), we tested the effect of bilateral EAA
receptor blockade (1 nmol kynurenic acid, total dose) in the cNTS on
arterial chemoreflex function in a third group of spontaneously breathing animals (n = 8). Similar to those findings
reported by other investigators, 5 min following blockade of EAA
receptors in the cNTS, arterial chemoreflex responses were markedly
attenuated (Fig. 7), including a
significant decrease in reflex-evoked changes in MAP and RSNA (Fig. 3)
and a significant reduction in peak dEMG burst amplitude (Pk-1) and
respiratory rate (Pk-2; Fig. 4). The initial increase in respiratory
rate (Pk-1) was also attenuated, but the change was not significant
(Table 1). Reflex-associated changes in HR were minimal before and were
not altered following kynurenic acid microinjection. All components of
the cardiorespiratory response to KCN (Table 1 and Figs. 3 and 4)
returned to control levels when remeasured 90 min following kynurenic
microinjection of the cNTS.
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-2 adrenoreceptor blockade in the cNTS, local
blockade of EAA receptors also resulted in a significant reduction in
both the resting respiratory rate and HR, yet there was no significant
change in resting MAP (Table 2). By 90 min postkynurenic
microinjection, resting HR and respiratory rate had returned to
preinjection levels (Table 2).
Effects of aCSF in the cNTS on chemoreflex responses. To control for the effects of microinjection alone, in a fourth group of animals chemoreflex responses were tested before and following bilateral microinjections into the cNTS of aCSF (90- to 120-nl total volume). aCSF alone had no effect on resting cardiorespiratory parameters (Table 2), and neither arterial chemoreflex responses (Figs. 3 and 4) nor baroreflex responses (Fig. 5) were significantly altered by aCSF microinjection. Chemoreflex responses retested at 90 min were also unchanged (Table 1 and Figs. 3 and 4).
Central microinjection sites.
Figure 8 shows the reconstructed
microinjection sites from all microinjection studies. Histological
analysis of the brain stem verified the localization of all
microinjection sites within the boundaries of the cNTS, except for the
two idazoxan microinjection sites placed in the overlying nucleus
gracilus to test the specificity of the microinjection location of
reflex function (Fig. 8).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The major finding of this study was that blockade of
-2 adrenoreceptors in the region of the cNTS selectively attenuates arterial chemoreflex responses. The effects of -2 adrenoreceptor blockade on the cardiovascular component of the reflex remained in the
ventilated animals, suggesting that reflex attenuation by -2
adrenoreceptor antagonists was primarily related to altered chemoreceptor afferent processing in the cNTS and was not dependent on
changes in the respiratory component of the reflex. Consequently, the
results of the present study provide the first piece of evidence to
suggest that tonic activation of -2 adrenoreceptors in the cNTS may
facilitate the central processing of chemoreceptor afferent inputs.
In the present study, KCN was used to briefly stimulate arterial chemoreceptors. Intravenous administration of KCN elicited a rapid increase in respiration rate, dEMG amplitude, RSNA, and MAP, with little change in HR. This response was similar to that reported by other investigators using KCN to elicit chemoreflex responses in urethane-anesthetized rats (8, 30, 36, 38). Unlike previous studies, however, we characterized the respiratory response to KCN administration as having two components. To our knowledge, this type of analysis has not been previously used to describe the respiratory response to KCN in rats, although a similar biphasic response has previously been noted in conscious animals, but not analyzed as such (8). Yet, in conscious animals, the phase of increased dEMG frequency was reported to precede the increase in burst amplitude (8), suggesting that the temporal effects of KCN on respiration that we reported may reflect the combined effects of brief stimulation of the carotid chemoreceptors and the urethane anesthesia. Moreover, the specificity of the cardiovascular and respiratory components of the response to KCN was confirmed by complete elimination of the responses following bilateral denervation of the carotid sinus nerves. This finding is in agreement with the findings of other studies (8, 12, 30) and supports recent data, which suggest that the chemoreflex response in the rat is primarily mediated through carotid chemoreceptor afferents (16).
A putative role for norepinephrine in the central processing of
arterial chemoreceptor afferent input in the cNTS has been suggested
for many years by studies demonstrating a link between long-term
exposure to hypoxia and altered norepinephrine levels in the cNTS in
rats (28, 31, 32). Furthermore, single-unit recordings
from the NTS have identified groups of chemoreceptor afferent-sensitive
neurons that are sensitive to local application of catecholamine
receptor agonists and antagonists (24), suggesting chemoreflex function may be altered by -2 adrenoreceptor modulation of chemoreceptor afferent processing within the cNTS. Although previous
studies have suggested a role for norepinephrine in the cNTS in the
modulation of chemoreflex function, to our knowledge the specific
action of norepinephrine or -2 adrenergic receptor modulation within
the cNTS on chemoreflex function has not been previously described.
Indeed, prior physiological evidence supporting an important role for
catecholamines in the central modulation of the arterial chemoreflex
has been relatively sparse. In the early 1980s, pharmacological depletion of both brain and peripheral catecholamines was reported to
have little effect on the resting respiratory rate or ventilatory responses to hypoxia in conscious rats (22). Furthermore,
no profound effect on the respiratory component of the arterial
chemoreflex has been observed following either intracerebroventricular
or intravenous administration of -2 receptor antagonists in rats (1, 3). Alternatively, intravertebral administration of the adrenoreceptor agonist clonidine has been reported to facilitate chemoreflex-induced vasoconstriction of the hindlimb in anesthetized dogs (34). Thus it appears that the method of drug
delivery and the brain regions targeted by the drugs may make a
difference in identifying the role of these receptors in chemoreflex function.
In our study, we used approximately one-half of the dose of idazoxan
reported previously to modulate baroreflex function (33) when microinjected into the more rostral region of the NTS.
Approximately 5 min following central -2 adrenergic receptor
blockade, we observed a marked attenuation of all components of the
arterial chemoreflex. Reflex responses returned within 90 min,
suggesting that tonic activation of -2 adrenoreceptors in the cNTS
is important for the central processing of arterial chemoreceptor
afferent inputs. The similarities between our findings and the effects
observed by others on baroreflex function in the intermediate regions
of the NTS (20, 33) strongly suggest that tonic activation
of -2 adrenoreceptors throughout the NTS may be extremely important to the initial processing of visceral afferent inputs.
We also confirmed that use of a broad-spectrum EAA receptor antagonist
in the same region markedly reduced arterial chemoreflex responses.
This finding is in agreement with previous reports investigating the
role of EAA receptors in the NTS on chemoreflex function (36,
38). Although reflex function was markedly attenuated following
blockade of EAA receptors in the cNTS, we did not observe any
significant shift in baseline pressure. Similarly, blockade of -2
adrenoreceptors in the cNTS did not significantly alter baseline blood
pressure. These findings suggest that tonic input from the arterial
chemoreceptors does not significantly contribute to the mechanisms
controlling baseline pressure in the anesthetized normotensive rat.
This is in marked contrast to the 20- to 50-mmHg increase in resting
pressure reported to follow -2 adrenoreceptor blockade in the
intermediate NTS and blockade of baroreflex function (33).
Although blockade of EAA receptors and -2 adrenergic receptors in
the cNTS produced similar effects on chemoreflex function and baseline
blood pressure, the two different receptor antagonists had markedly
different effects on resting respiratory rate. On the one hand, EAA
receptor blockade in the region of the cNTS produced a significant
decrease in respiratory rate and prolongation of dEMG burst duration.
In contrast, no change in resting respiratory rate was observed
following -2 receptor blockade. Because inspiratory burst duration
is heavily influenced by input from pulmonary stretch receptors and
these receptors terminate in cNTS, it is possible that the change in
respiratory rate observed following EAA receptor blockade was due to a
change in the central processing of these sensory afferents and a
subsequent change in inputs to the respiratory central pattern
generator. Indeed, it has been shown that the termination of
inspiration mediated by these receptors depends on activation of
glutamate receptors in the cNTS (2). Interestingly, the
lack of influence of -2 receptor blockade on baseline respiratory rate suggests the possibility that catecholamines may not be involved in the central processing pulmonary afferents. This, however, remains
to be tested.
Finally, the results of the present study and other studies (20,
33) suggest that -2 adrenoreceptors play a
significant role in modulating sensory processing within the NTS.
Despite this, the mechanisms through which -2 adrenoreceptors
modulate afferent processing in the NTS remain to be identified.
Single-unit recording studies have demonstrated that local application
of -2 adrenoreceptor agonists in the cNTS typically inhibits cNTS neuronal activity (24). Accordingly, it might be expected
that application of the receptor antagonist, such as the ones used in
the present study, would facilitate afferent processing. Yet, the exact
opposite effect was observed on the level of the whole reflex. One
possible explanation is that -2 receptors are primarily located on
and/or primarily modulate the function of inhibitory interneurons
within the NTS. Removal of tonic suppression of these inhibitory
interneurons in the presence of idazoxan would then lead to an increase
in the local release of GABA, an inhibitory neurotransmitter. The
resulting effect would be a marked attenuation of afferent processing
within the NTS and blunting of reflex function. Although this
hypothesis remains to be tested, a similar modulatory effect on
GABAergic interneurons in the cNTS has been proposed to mediate the
effects of serotonin, another central neuromodulator of chemoreflex
function (30). Alternatively, there is some evidence from
pharmacological studies of preganglionic neurons in the spinal cord,
which suggests 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 either receptor subtype may have opposing
effects on membrane excitation (23). Thus 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 simply reflect the combined influence of -1 and -2
receptors in this region of the brain. The role of -1 receptor
modulation in the cNTS on chemoreflex function remains to be tested.
In conclusion, the present study has identified for the first time the
physiological effect of blockade of -2 adrenoreceptors in the cNTS
on arterial chemoreflex function in anesthetized rats. Pharmacological
blockade of -2 receptors with either the selective blocker idazoxan
or yohimbine was shown to markedly attenuate the arterial chemoreflex
response to KCN. The profound influence of -2 adrenoreceptor
blockade on chemoreceptor afferent processing at the level of the cNTS
mimics the effect previously reported at the level of the intermediate
NTS for baroreflex processing. These data support the hypothesis that
endogenous catecholamines are integrally involved in visceral afferent
processing throughout the NTS. The specific mechanism by which -2
adrenoreceptors modulate chemoreceptor afferent processing in the NTS,
however, remains to be addressed. Additional studies are needed to
determine the influence of this neuromodulator on specific types of
chemoreceptor afferent-sensitive cNTS neurons (GABAergic or glutamatergic).
Perspectives
Previous studies investigating a putative role for catecholamines in the central arterial chemoreflex pathway have identified that local levels of norepinephrine within the NTS increase threefold following prolonged exposure to hypoxia (28). In addition, a portion of the respiratory response to long-term exposure to hypoxia has been shown to depend on the integrity central catecholaminergic inputs (1) and local destruction of neurons within the ventrolateral pons, which contains a cluster of noradrenergic neurons and markedly attenuates the sympathetic component of the arterial chemoreflex (18, 29). Thus there is indirect evidence to support a role for central catecholamines in altered cardiorespiratory function following chronic exposure to hypoxia and possibly in the hypoxia-induced development of hypertension. The results of the present study provide a new perspective on the potential role of cNTS neurons in this-2 adrenergic modulation of arterial chemoreflex function. The origin of this potentially important noradrenergic input to the
cNTS, however, remains to be identified as does the possibility that
other adrenoreceptor subtypes may also contribute to chemoreflex modulation in the cNTS. Finally, although we did not observe any significant changes in baseline blood pressure following acute blockade
of -2 adrenoreceptors in the cNTS, it remains to be determined
whether chronic changes in the adrenoreceptor function in the cNTS or
other regions of the brain (37) might significantly contribute to increased baseline sympathetic drive and the development and maintenance of hypertension associated with chronic exposure to
episodic hypoxia.
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ACKNOWLEDGEMENTS |
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The author gratefully acknowledges the technical assistance of M. Castellanos and J. Piascik and dedicates this paper in memory of Dr. Margaret Sullivan for scientific inspiration over the last 10 years.
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FOOTNOTES |
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This work was supported by the National Heart, Lung, and Blood Institute Grant HL-52607.
Address for reprint requests and other correspondence: L. F. Hayward, Univ. of Florida, Dept. of Physiological Sciences, College of Veterinary Medicine, 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.
Received 29 September 2000; accepted in final form 15 June 2001.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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1.
Bach, KB,
Kinkead R,
and
Mitchell GS.
Post-hypoxia frequency decline in rats: sensitivity to repeated hypoxia and -2 adrenoreceptor antagonism.
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Time during which the peak AP response to KCN was
measured. The preceding rise in AP was associated with the 0.2 ml
saline flush used to administer the KCN intravenously.
bilateral idazoxan microinjection sites made in
the cNTS (n = 10).