Autonomic Neuroscience: Basic and Clinical 142 (2008) 32–39
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Autonomic Neuroscience: Basic and Clinical
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a u t n e u
Dominant role of aortic baroreceptors in the cardiac baroreflex of the rat in situ
Anthony E. Pickering a,b,⁎, Annabel E. Simms a, Julian F.R. Paton a
a
b
Department of Physiology & Pharmacology, Bristol Heart Institute, School of Medical Sciences, University Walk, University of Bristol, Bristol, BS8 1TD, UK
Department of Anaesthesia, Bristol Royal Infirmary, Bristol, BS2 8HW, UK
A R T I C L E
I N F O
Article history:
Received 29 January 2008
Received in revised form 25 March 2008
Accepted 26 March 2008
Keywords:
Arterial baroreflex
Carotid sinus
Aortic arch
Parasympathetic
Sympathetic
A B S T R A C T
The arterial baroreceptors detect changes in blood pressure and form the afferent limb of the baroreflex
which acts to buffer changes in pressure through reciprocal regulation of the sympathetic and
parasympathetic outflow. We have previously shown that the sympathetic and parasympathetic limbs of
the baroreflex operate over different pressure ranges and hypothesised that these differences in regulation of
heart rate and sympathetic activity could originate from the baroafferents. We tested this hypothesis using
sequential baroafferent denervations in the decerebrate, arterially perfused rat preparation. We found that
baroreflex control of heart rate is critically dependent upon the aortic arch afferents with relatively little
contribution from the carotid sinuses. Indeed the baroreflex bradycardia was attenuated by 85% (n = 7) when
only one aortic depressor nerve was cut indicating a strongly synergistic interaction between aortic
baroafferents. By contrast baroreflex sympathoinhibition was dependent on inputs from all four sites, and the
stimulation of any single site could elicit robust sympathoinhibition. These findings were independent of the
sequence of baroafferent nerve resection (n = 15). Perfusion of the isolated carotid sinus (n = 5) showed that it
was possible to elicit baroreflex sympathoinhibition (and changes in vascular resistance) without any
significant change in heart rate despite the use of strong stimuli (N 100 mmHg) or repeated pulsatile stimuli.
These results indicate fundamental differences in the responses elicited by stimulation of the afferents from
the carotid and aortic barosensor sites and suggest that their actions within the nucleus of the solitary tract
are functionally specified (sympathetic versus parasympathetic).
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The arterial baroreceptors, located in the aortic arch and the
carotid sinus, detect changes in blood pressure and form the afferent
limb of the baroreflex, which is a key buffer of acute changes in blood
pressure (Sagawa, 1983). This buffering is predominantly mediated by
a reciprocal modulation of the sympathetic and parasympathetic
nervous systems. We have recently shown, in the rat, that these limbs
of the baroreflex operate with different pressure–function curves such
that the sympathetic limb operates over a lower range of pressures
(Simms et al., 2007). This has led us to propose that there is a hierarchy
of recruitment of the baroreflex limbs. The site of origin, within the
baroreflex arc, of these differences in the operating pressure ranges of
the autonomic effectors is unknown. Differences in neural excitability
could exist at the level of either the respective sympathetic and
parasympathetic motoneurones, within the brainstem interneurones
or indeed it could originate within the baroafferents. With this last
possibility in mind, it is interesting that the aortic baroafferents in
dogs have been reported to have a higher pressure operating range
than the carotids for the regulation of hindlimb vascular resistance
and that the threshold for activation of heart rate responses appeared
higher than that for the vasomotor effects (Donald and Edis, 1971,
reviewed by Sagawa, 1983).
To address the hypothesis that the aortic and carotid baroafferents
may have different functional roles in the control of heart rate and
sympathetic activity, we have performed sequential acute baroafferent denervations in the decerebrate, arterially perfused rat (Pickering
and Paton, 2006). We show that the aortic arch baroreceptors provide
the dominant input to the cardiac (parasympathetic) limb of the
baroreflex whereas the sympathetic limb of the baroreflex is activated
by inputs from all four barosensors. This observation is supported by
our ability to obtain a baroreflex sympathoinhibition without
bradycardia by direct stimulation of an isolated carotid sinus. Some
of these data have been communicated previously in abstract form
(Pickering et al., 2004).
2. Methods
⁎ Corresponding author. Department of Physiology & Pharmacology, Bristol Heart
Institute, School of Medical Sciences, University Walk, University of Bristol, Bristol, BS8
1TD, UK. Tel.: +44 117 3312311; fax: +44 117 331 2288.
E-mail address: [email protected] (A.E. Pickering).
1566-0702/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.autneu.2008.03.009
All procedures conformed to the UK Animals (Scientific Procedures)
Act 1986 and were approved by the University of Bristol ethical review
committee. Experiments employed the decerebrate arterially perfused
rat (DAPR, Pickering and Paton, 2006). In brief, male Wistar (n = 20) rats
A.E. Pickering et al. / Autonomic Neuroscience: Basic and Clinical 142 (2008) 32–39
(60–100 g, post-natal age 28–35 days) were deeply anaesthetised with
halothane, until loss of withdrawal to paw pinch. The stomach,
intestines and spleen were ligated and removed via a midline
laparotomy. After sternotomy the ribcage was retracted to allow access
to the mediastinum. The animal was cooled by immersion in
33
carbogenated Ringer's at 10 °C and decerebrated pre-collicularly.
Anaesthesia was discontinued after decerebration as the animal was
insentient. The animal was positioned supine in the recording chamber
and a double lumen perfusion cannula was introduced into the
ascending aorta, via an incision in the left ventricle, for anterograde
Fig. 1. Sequential denervation of aortic arch followed by carotid sinuses. A. Control baroreflex response to increased perfusion pressure with baroafferents intact showing bradycardia and
pronounced sympathoinhibition. B. Same preparation after section of both aortic depressor nerves showing that the baroreflex bradycardia is completely abolished while the
sympathoinhibition is attenuated but is still clearly present. C. Pooled data showing the effect of sequential loss of baroafferents (n = 7). The cardiac gain is significantly attenuated following
the loss of just one aortic depressor nerve and after the loss of both ADN the baroreflex has no effect on heart rate. By contrast the sympathetic gain is only attenuated significantly after the
loss of both aortic and one carotid sinus baroafferent input. Bars show mean ± SEM, significance assessed by one-way repeated measures ANOVA (⁎ — P b 0.01, ⁎⁎ — P b 0.001). D. By using a
ramp change in perfusate flow to generate a biphasic pressure challenge it was possible to strongly activate the baroreflex producing a large bradycardia and sympathoinhibition. After the
loss of one aortic depressor nerve, an equivalent ramp produced relatively little change in heart rate but a similar degree of sympathoinhibition.
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A.E. Pickering et al. / Autonomic Neuroscience: Basic and Clinical 142 (2008) 32–39
perfusion using a peristaltic roller pump (Watson Marlow 505D) with a
carbogenated Ringer's solution at 32 °C (for constituents, see below).
The second lumen of the cannula was used to monitor aortic perfusion
pressure. The pump head speed was controlled using custom written
scripts (Spike2 driving micro1401, CED) allowing the generation of
flexible flow change protocols to alter perfusion pressure.
Phrenic nerve activity (along with ECG) was recorded via a suction
electrode (filtered 80 Hz–3 kHz). The baseline perfusate flow was
adjusted until the respiratory motor pattern consisted of an
augmenting burst discharge indicating eupnoea (Paton, 1996).
Vasopressin (200–400 pM) was added to the perfusate to increase
vascular resistance and hence baseline perfusion pressure (Pickering
and Paton, 2006). Instantaneous heart rate (HR) was derived by
triggering from the R wave of the ECG with a window discriminator.
2.1. Cardiorespiratory afferent stimulation
The baroreflex was activated by perfusion pressure challenges
(generated by altering the perfusate flow). Most studies used flow
steps to increase perfusion pressure (by ~30 mmHg) into the linear range
of the baroreflex function curve (Paton and Kasparov, 1999; Pickering et
al., 2003). In some recordings a flow ramp was also employed (typically
flow was dropped to 0.1× basal flow and then increased to 3× basal flow
over 15–60 s) to change flow linearly and thus produce biphasic
perfusion pressure challenges (Pickering and Paton, 2006).
To verify that the cardiac vagal efferent limb was still intact after each
nerve section (glossopharyngeal or superior laryngeal nerve) we
activated the oculo-cardiac reflex by applying traction to an extra-ocular
muscle using a glass probe. This manoeuvre reliably evoked a bradycardia
that was dependent on vagal activity. Carotid sinus nerve (CSN) function
was verified by activating the peripheral chemoreflex with NaCN (0.01%
solution; 100 μl intra-aortic bolus) to produce a bradycardia (Paton and
Kasparov, 1999). Such activation of the peripheral chemoreceptor reflex
produced an increase in central respiratory drive accompanied by
bradycardia and an increase in sympathetic nerve activity.
laterally. It was looped with fine thread and cut just proximal to its
entry site into the skull to section carotid sinus afferents.
2.4. Carotid sinus perfusion
The right carotid artery was mobilised, tied proximally and a double
lumen perfusion cannula inserted until its tip was just below the
carotid sinus. The external carotid artery was tied off and perfusate
flow initiated to maintain oxygenation (2–3 ml/min). Care was taken to
avoid damage to the nerves lying close to the carotid in the neck
including vagus, ADN, SLN and the cervical sympathetic trunk. Carotid
sinus pressure was monitored through the second lumen of the
cannula. Pressure ramps were generated by injecting boluses of
oxygenated perfusate (0.25–2 ml, 32 °C) through the perfusion line.
CSN function was tested by injecting boluses of NaCN into the sinus to
stimulate the ipsilateral carotid body and activate the peripheral
chemoreflex (0.03%, 50 μl).
2.5. Data analysis
For the transient pressor challenges the cardiac baroreflex gain was
calculated from the ratio of Δheart rate/Δperfusion pressure (bpm/
mmHg). Because the SNA showed respiratory modulation, the pressor
challenges were applied during the same phase of the respiratory
cycle (end inspiration). The sympathetic baroreflex gain was calculated by ratioing the change in ∫SNA during the perfusion pressure
ramp against the average of two equivalent control periods of ∫SNA
taken from the corresponding phase of preceding respiratory cycles
(expressed as %sympathoinhibition/mmHg, see Pickering et al., 2003).
Significance of data was assessed using one-way repeated
measures ANOVA with post hoc Fisher least significant difference
test (Prism4, Graphpad, San Diego, CA, USA). All values quoted are the
mean ± SEM and differences were considered significant at the 95%
confidence limit.
2.6. Drugs and solutions
2.2. Nerve recordings
Sympathetic nerve recordings were made using bipolar suction
electrodes from the superior mesenteric nerve. The sympathetic nerve
activity exhibited marked respiratory modulation and was profoundly
attenuated by an increase in perfusion pressure (activating arterial
baroreceptors). Nerve recordings were AC amplified (custom built
amplifier), filtered (100 Hz–2 kHz), rectified and integrated (time
constant of 200 ms).
2.3. Baroafferent nerve sections
With the animal supine, both carotid arteries were exposed from
the sternum to above the bifurcation by resection of the overlying
strap muscles. The aortic depressor nerve (ADN) was identified as a
fine, myelinated nerve running parallel and medial to the vagus low in
the neck that ascends to join the superior laryngeal nerve (SLN) close
to the nodose ganglion. The ADN was looped with fine thread and cut
before it joined the SLN and to ensure a complete section the SLN was
traced to its union with the vagus and cut at this point.
The glossopharyngeal nerve was identified, running between the
internal and external carotid arteries, and traced rostrally and
The composition of the modified Ringer's solution was (mM): NaCl
(125); NaHCO3 (24); KCl (5); CaCl2 (2 · 5); MgSO4 (1 · 25); KH2PO4
(1 · 25); dextrose (10); pH 7·35–7.4 after carbogenation. The perfusion
solution also contained Ficoll 70 (1 · 25%) as an oncotic agent, and
heparin (1 IU/ml). All chemicals were from Sigma (UK).
3. Results
In all preparations (n = 15), prior to baroafferent section, application of systemic perfusion pressure ramps evoked a bradycardia
(cardiac gain = −2.3 ± 0.3 bpm/mmHg, n = 15) and sympathoinhibition
(gain = − 2.1 ± 0.2%/mmHg). Similarly, all intact preparations upon
activation of the peripheral chemoreflex showed tachypnoea, bradycardia and sympathoexcitation and activation of the oculo-cardiac
reflex produced bradypnoea, bradycardia and sympathoexcitation.
3.1. Sequential denervation of aortic arch followed by carotid sinus
Sequential section of the ADN had a profound effect on the cardiac
baroreflex with the loss of a single ADN causing an 85% reduction in
the gain (− 2.6 ± 0.5 versus − 0.4 ±0.1 bpm/mmHg, n = 7, P b 0.001,
Fig. 2. Sequential denervation of carotid sinuses followed by aortic arch. A. Control baroreflex response to increased perfusion pressure with baroafferents intact showing bradycardia
and pronounced sympathoinhibition. B. Same preparation after section of both glossopharyngeal nerves (denervating the carotid sinus) showing that the baroreflex bradycardia and
sympathoinhibition are relatively unaffected. C. Pooled data showing the effect of sequential loss of baroafferents (n = 5 DAPR). Both the cardiac and the sympathetic baroreflex gains
are not significantly altered by carotid sinus denervation. However, the cardiac gain was significantly attenuated by 86% following the loss of the first aortic depressor nerve. By contrast
although there was a trend for the degree of sympathoinhibition to be attenuated with each successive denervation the sympathetic gain was only significantly attenuated after
denervation of all the baroafferent inputs. Bars show mean ± SEM, significance assessed by one-way repeated measures ANOVA (⁎⁎ — P b 0.001). D. Before denervation, activation of the
peripheral chemoreflex (NaCN, 0.01%, 0.05 ml) produces tachypnoea, sympathoactivation and bradycardia. Sequential glossopharyngeal nerve section to denervate the carotid sinus
attenuates and then completely abolishes all three responses to peripheral chemoreflex activation indicating successful denervation (Data from the same preparation as A and B).
A.E. Pickering et al. / Autonomic Neuroscience: Basic and Clinical 142 (2008) 32–39
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A.E. Pickering et al. / Autonomic Neuroscience: Basic and Clinical 142 (2008) 32–39
Fig. 3. Pressure pulses applied to a single carotid sinus evoke sympathoinhibition without bradycardia. The carotid sinus was cannulated with a double lumen cannula and pressure
pulses generated by injecting oxygenated aCSF (0.25–1 ml). A. Series of pressure pulses (bars) of increasing amplitude generating graded baroreflex sympathoinhibitions and
consequent falls in systemic perfusion pressure reflecting a reduction in vascular resistance secondary to the loss of sympathetic output. However, even the largest amplitude pulse
(N 120 mmHg) failed to produce a significant bradycardia. B. In a different preparation the application of a series of pressure pulses (asterisked dotted lines mark peaks) evoked graded
sympathoinhibition accompanied by a pronounced summating fall in perfusion pressure. However this repeated pulsatile stimulus was also insufficient to evoke a baroreflex
bradycardia.
ANOVA, Fig. 1) and it was completely abolished after loss of both ADN.
In all preparations both chemoreflex and oculo-cardiac reflex
activation produced a bradycardia after section of both ADN. By
contrast the sympathetic baroreflex gain was unaffected by the loss of
both ADN (−1.9 ± 0.3 versus −1.7 ±0.3%Si/mmHg, n = 7) and was only
significantly attenuated when just a single carotid sinus was left intact
(−0.9 ± 0.1%Si/mmHg, P b 0.01, ANOVA, Fig. 1).
3.2. Sequential denervation of carotid sinuses followed by aortic arch
Reversing the sequence of denervations showed that the carotid
sinuses appear to have relatively little influence on the cardiac
baroreflex with no reduction in the amplitude of the bradycardia
being seen after both carotid sinuses were denervated (−2.4 ± 0.5
versus −2.6 ±0.8 bpm/mmHg, Fig. 2). However, the loss of the first ADN
produced an 86% reduction in the cardiac baroreflex gain (−0.3 ± 0.1,
n = 5, P b 0.001, ANOVA). The sympathetic baroreflex showed a gradual
reduction in gain with each loss of baroafferent that became
significant when all four afferents were denervated. The sequential
loss of the glossopharyngeal nerves first attenuated and then
completely obtunded the response to activation of the peripheral
chemoreflex (Fig. 2D).
3.3. Carotid sinus perfusion
The right carotid sinus was cannulated and separately perfused
(n = 5). Pressure pulses evoked graded sympathoinhibitions in all
preparations, and this sympathoinhibition provoked corresponding
falls in the systemic perfusion pressure (Fig. 3). However, despite the
application of large carotid pressure pulses (N100 mm Hg) no
significant change in heart rate was seen (n = 5/5). We further
investigated using brief trains of pressure pulses to mimic the effect
of a pulsatile circulation. This produced graded sympathoinhibition
and a fall in systemic vascular resistance, but failed to produce
significant bradycardia (n = 3, Fig. 3). Injection of NaCN (0.03%, 0.05 ml,
Supplementary Fig. 1) to the carotid sinus produced strong peripheral
chemoreflex responses in all preparations indicating that the carotid
sinus nerve was intact. Furthermore, application of systemic pressure
pulses was always able to evoke both baroreflex sympathoinhibition
and bradycardia in all preparations (n = 5/5, Supplementary Fig. 1).
A.E. Pickering et al. / Autonomic Neuroscience: Basic and Clinical 142 (2008) 32–39
4. Discussion
In this study using sequential acute denervations of baroreceptors,
we have shown that the sensors in the aortic arch play a dominant role in
the cardiac baroreflex. By contrast, both the carotid and aortic
baroreceptors contribute towards the generation of baroreflex sympathoinhibition. In support of these observations, robust pressure
stimuli applied to an isolated carotid sinus produced sympathoinhibition
and decreased systemic vascular resistance yet were unable to produce
an accompanying bradycardia. Thus, in our in situ rat preparation, there
is a marked difference in the characteristics of the baroreflex responses
evoked from the cardiac and aortic baroreceptor sites.
In support of this functional difference between the baroafferents, it
has been reported that stimulation of the carotid sinus in the conscious
rat is around 10 fold less effective at producing a bradycardia than
stimulation of the ADN (Dworkin et al., 2000). Although their study
employed different stimulus techniques (balloon for carotid and
electrical for ADN), the differences between the baroafferent sites
were apparent when using stimulus intensities that produced equivalent depressor effects on systolic blood pressure. A similar relative lack of
effect of carotid sinus baroafferents on heart rate has been clearly
demonstrated in conscious unrestrained normotensive rats (McKeown
and Shoukas,1998). Using a bilaterally isolated carotid sinus preparation
in animals with intact vagi these authors showed that increasing carotid
baroafferent loading from 50–200 mmHg produced a small, 10 bpm fall
in heart rate (4%, not statistically significant) compared to a 30% change
in systemic arterial pressure (comparable to previous isolated carotid
sinus studies in anaesthetised, vagotomised rat preparations (Nosaka
and Wang,1972; Shoukas et al.,1991)). They concluded that the majority
of the carotid sinus baroafferent influence on arterial pressure is
mediated through a change in peripheral vascular resistance. Our study
has extended these findings by applying the same stimulus modality
(increase in perfusion pressure) to both the carotid and aortic
barosensors, as they were serially, acutely disconnected from the
system, which has highlighted the functional differences in their
response properties. When taken together these studies indicate that
the role of the carotid and aortic arch baroreceptors in the rat are
fundamentally different with respect to the control of heart rate.
We have previously reported that the operating pressure range of
the sympathetic and parasympathetic limbs of the baroreflex are quite
distinct in the rat with the sympathetic arm being active over a lower
range of pressures (Simms et al., 2007). This was observed for pressure
stimuli applied to all four barosensor regions and was a consequence
of differences in the regulation of the sympathetic and parasympathetic outflows. It is apparent from our denervation study that the
generation of bradycardia requires a synergistic interaction between
the aortic arch afferents whereas sympathoinhibition can be produced
from any of the baroafferent sites in isolation, as has been previously
reported for other species (reviewed in Sagawa, 1983). Indeed, robust
sympathoinhibition can be obtained from any one remaining
barosensor or from stimuli applied to a single isolated carotid sinus,
emphasising that the intensity of stimulus required to generate
sympathoinhibition appears smaller than that required to produce
heart rate changes. It is worth noting that we still observed a degree of
sympathoinhibition in response to pressure challenges after sectioning both ADN and glossopharyngeal nerves, however as the vagi were
still intact this residual inhibition is likely to originate from
cardiopulmonary receptors.
Thus, it would appear that some of the functional differences in the
patterns of activation of the baroreflex sympathetic and parasympathetic limbs that we have reported (Simms et al., 2007) originate in the
properties of the baroafferents. It has previously been reported that
aortic baroafferents in dogs have higher pressure operating range than
the carotids for the regulation of hindlimb vascular resistance and that
the threshold for activation of heart rate responses appeared higher
than that for the vasomotor effects (Donald and Edis, 1971). The
37
carotid baroafferents in the dog have been split into type I and type II
groups (largely corresponding to A- and C-fibre types) with different
pressure encoding properties (Seagard et al., 1990) and different
functional roles in baroreflex regulation of blood pressure (Seagard et
al., 1993). Functional differences have also been reported in rats where
electrical stimulation of the ADN has indicated that A-fibre baroafferents have different frequency–response characteristics and less
influence on heart rate control than C-fibre baroafferents (Fan et al.,
1999). These studies indicate that there may be differences in the roles
of particular baroafferents in the regulation of the cardiovascular
system. There is little data comparing the pressure encoding properties of baroafferent fibres originating in the aortic and carotid sites in
the rat, however a previous study in the cat showed little difference in
their properties (Samodelov et al., 1979). Intriguingly, a comparison of
the effects of electrical stimulation of the ADN and CSN in
anaesthetised rats (Fan et al., 1996) has shown clear differences,
with CSN stimulation producing much smaller effects on heart rate
than comparable ADN stimulation. This is a rather surprising finding,
and interpretation is complicated by the fact that the CSN contains
both baroafferent fibres from the sinus and peripheral chemoreflex
fibres from the carotid bodies, but is consistent with our proposed
difference in baroafferent function.
Leaving aside the pressure encoding properties of the baroafferents it is entirely plausible to envisage functional differences in their
influence on the cardiovascular system that would be a consequence
of their pattern of central connectivity within the circuits of the
nucleus of the solitary tract (NTS), the first central site for integration
of the baroreflex. It has been shown that the distribution of the
baroafferent terminals from CSN and ADN in the rat NTS are partly
overlapping but have distinct regional differences (Chan et al., 2000)
consistent with the electrophysiological observation that not all
barosensitive NTS neurones receive convergent inputs from both the
aortic and carotid afferents (in rat (Nosaka et al., 1995), rabbit (Paton et
al., 1990) and cat (Donoghue et al., 1985)). Chan et al. (2000) also
showed that selective denervation of the aortic arch or carotid sinus
produced different changes in the pattern of c-fos expression within
the NTS evoked in response to phenylephrine infusion, implying that
these baroafferents target different subgroups of NTS neurones.
Furthermore, the expression of NADPH-diaphorase by a population
of barosensitive NTS neurones was dependent on inputs from the
aortic arch (and not the carotid sinus). Given that this NADPHdiaphorase activity is associated with the ability of neurones to
synthesise NO and there is evidence to support a role of NO in the
baroreflex regulation of heart rate (Paton et al., 2001a, 2006; Pontieri
et al., 1998; Waki et al., 2003, 2006) it is possible to speculate that
these specific neurones targeted by the ADN form a discrete part of the
baroreflex circuit controlling heart rate. This argues that within the
NTS there are neurones that are functionally specified for a particular
component of the baroreflex i.e. parasympathetic versus sympathetic.
This concept that the NTS barosensitive neurones are functionally
specified for particular baroreflex outputs is supported by the
observation that barosensitive neurones possess different single cell
response properties (Paton et al., 2001b; Zhang and Mifflin, 2000) and
the demonstration that the limbs of the baroreflex have differential
pharmacological sensitivity within the NTS (Pickering et al., 2003;
Simms et al., 2006). In addition, some recent in vitro data (Bailey et al.,
2006) shows differences in the responses to afferent stimulation
between NTS neurones projecting to different targets (paraventricular
nucleus and caudal ventrolateral medulla) and thus presumably
subserving different functional roles.
Our findings in the decerebrate, arterially perfused rat preparation
are comparable to those reported from the conscious rat (Dworkin et
al., 2000; McKeown and Shoukas, 1998) thus we can conclude that
these differences in the responses evoked from the baroafferents are
mediated through hindbrain autonomic centres and do not require
higher centres for their expression. Our in situ preparation offers some
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A.E. Pickering et al. / Autonomic Neuroscience: Basic and Clinical 142 (2008) 32–39
advantages for this functional study such as straightforward surgical
access to nerves and arteries, ability to control perfusion pressure
independent of cardiac output, the presence of basal autonomic tone
which is coupled to autonomic effectors (heart – respiratory sinus
arrhythmia and vessels – Traube-Hering waves, (see Fig. 3, Pickering
and Paton, 2006)) and the lack of anaesthesia. In particular for this
study we have been able to acutely denervate the baroafferents
sequentially, in an unanaesthetised preparation. This acute approach
reduces the confounding issues of central reorganisation and reinnervation that can be found after chronic baroreflex deafferentation
(Chan et al., 2000; O'Leary and Scher, 1988). However, there are also
some aspects of our experimental methodology that merit closer
consideration as they are different from some previous approaches
employed to study the baroreflex:
Using acute sequential baroafferent denervation in the rat we have
identified distinct differences in the contribution of the receptor sites
to the baroreflex control of heart rate and the sympathetic outflow.
The baroreflex regulation of heart rate appears to be dominantly
under the control of the aortic arch baroreceptors through a strongly
synergistic interaction. By contrast, all four baroafferent sites
contribute to the regulation of sympathetic nerve activity. These
functional differences in the response properties of the peripheral
baroafferents imply that there is likely to be specialisation in the
baroreflex circuitry within the NTS with segregation and targeting of
baroafferent information to appropriate circuits regulating the
sympathetic and parasympathetic outflows.
– The lack of a pulse waveform in the perfusion pressure is clearly
different from the in vivo situation, this is relevant because pulsatile
arterial inputs to the baroafferents have been shown to produce an
increased sympathetic baroreflex gain in the dog and cat (Chapleau et
al., 1989; Ead et al., 1952; James and Daly, 1970; Schmidt et al., 1971).
The situation in the conscious rat may be different as the replacement
of pulsatile carotid pressure with static pressure has little effect on
resting arterial pressure or heart rate (McKeown and Shoukas, 1998).
Nonetheless, our findings regarding the different roles of the
baroafferents were similar whether the activating pressure stimulus
was brief (1–2 s) or prolonged (15–60 s) suggesting that the rate of
change of pressure was not crucial to our findings. Additionally the
application of repeated pulsatile pressure waves to the isolated carotid
sinus was still unable to evoke a bradycardia.
– Our dynamic pressure stimuli used to activate the baroreflex
evoked robust bradycardia and sympathoinhibition. These stimuli
increased the perfusion pressure from a basal level of 50–70 mmHg to
80–100 mmHg, which covers the operating range of both the cardiac
and non-cardiac sympathetic baroreflex, in these perfused preparations (Simms et al., 2007).
– We routinely included vasopressin in the perfusate to increase
vascular resistance and maintain perfusion pressure in the whole
preparation which enhances viability (Pickering and Paton, 2006). In
addition to this effect on vascular tone vasopressin has been reported
to increase baroreflex sensitivity (Abboud et al., 1990). However, this
recording condition was identical in all experiments, and as our data
are comparing the relative contribution of different baroafferent sites
and are in agreement with previous in vivo studies (Dworkin et al.,
2000; McKeown and Shoukas, 1998) we do not envisage that the
addition of vasopressin accounts for our findings.
This study was supported by the British Heart Foundation and by
the British Journal of Anaesthesia/Royal College of Anaesthetists. AES
was in receipt of the British Heart Foundation Dr W E Parkes PhD
Studentship. AEP is a Wellcome Trust Advanced Fellow and JFRP holds
a Royal Society Wolfson Research Merit Award.
One of the key roles of the carotid barosensors is to maintain
adequate blood pressure to perfuse the brain. It is therefore
interesting that modelling studies of the rat have suggested that
baroreflex alterations in heart rate have relatively little impact on
blood pressure because of opposing changes in ventricular filling
(Rose and Schwaber, 1996). In the rat it appears that the carotid
baroafferents selectively targeting the sympathetic vasomotor outflow to efficiently maintain blood pressure and brain perfusion
(McKeown and Shoukas, 1998). However, our findings in rat are
different from those observed in man where activation of the carotid
baroafferents (e.g. using neck pressure and suction) has a profound
influence on heart rate (Fadel et al., 2003) and similarly in the dog
(Kollai and Koizumi, 1989). This may be a species difference and could
be related to either the lower resting level of heart rate, the higher
level of basal tone in the cardiac vagal outflow or, in man in particular,
this may be a specific adaptation to the challenge of maintaining
adequate brain perfusion in the face of the challenge imposed by an
upright posture (although the role of carotid baroreflex induced heart
rate changes in compensation for such postural challenges has been
questioned (Ogoh et al., 2006)).
Acknowledgements
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.autneu.2008.03.009.
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