| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1 Department of Anesthesiology and 2 Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157
 |
ABSTRACT |
|---|
 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Ischemic stimulation of cardiac
receptors reflexly excites the cardiovascular system. However, the
supraspinal mechanisms involved in this reflex are not well defined.
This study examined the responses of barosensitive neurons in the
rostral ventrolateral medulla (RVLM) to stimulation of cardiac
receptors and the afferent pathways involved in these responses.
Single-unit activity of RVLM neurons was recorded in
-chloralose-anesthetized rats. Cardiac receptors were stimulated by
epicardial application of 10 µg/ml of bradykinin (BK). Barosensitive
neurons were silenced by stimulation of baroreceptors. Application of
BK increased the mean arterial pressure from 65.2 ± 1.9 to
89.3 ± 2.9 mmHg and excited RVLM barosensitive neurons from
6.2 ± 0.7 to 10.7 ± 0.9 impulses/s (P < 0.05, n = 40). BK had no effect on 21 nonbarosensitive
neurons. Blockade of stellate ganglia abolished the response of
barosensitive neurons to BK. Cervical vagotomy significantly increased
the baseline discharges of RVLM barosensitive neurons but had no effect
on their responses to BK. Thus this study indicates that stimulation of
cardiac receptors selectively activates RVLM barosensitive neurons
through sympathetic afferent pathways. This information suggests that
the RVLM barosensitive neurons are likely involved in the sympathetic
control of circulation during myocardial ischemia.
cardiac nociceptors; angina pectoris; afferent neurons; sympathetic nervous system; cardiovascular reflexes; myocardial ischemia
| |
INTRODUCTION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
STIMULATION OF CARDIAC SENSORY NERVE ENDINGS during myocardial ischemia typically elicits chest pain and disturbance of the autonomic nervous system in patients with ischemic heart disease (13, 17, 23, 27, 40). Many endogenous metabolites, including bradykinin (BK), prostaglandins, and lactic acid, are capable of stimulating cardiac afferents and contribute to activation of cardiac receptors during myocardial ischemia (19, 21, 30, 34). The importance of the sympathetic outflow has been well recognized in the cardiovascular reflex responses to stimulation of cardiac afferents (12, 38-39). Although earlier studies suggest that these cardiac sympathetic reflexes are mediated by a localized spinal circuit (12-13, 38), the cardiac-cardiovascular reflexes are less likely confined to the spinal cord because electrical stimulation of the stellate ganglion or nerves containing cardiac afferents also alters the discharge activity of neurons located in the brain stem nuclei, such as rostral ventrolateral medulla (RVLM) (2, 4, 39). The essential role of RVLM neurons in the neural control of the cardiovascular system has been well demonstrated. In this regard, the RVLM vasomotor neurons contribute a major source of tonic excitatory drive to the preganglionic neurons in the spinal cord that control sympathetic tone to the heart and blood vessels (6, 14, 18, 25). These RVLM neurons are tonically active and are powerfully inhibited by activation of baroreceptors (4, 32, 36). The RVLM vasomotor neurons have also been shown to be involved in various sympathetic reflexes and integration of inputs from a variety of visceral, somatic, and supramedullary structures in a manner that closely correlates the effect of these inputs on sympathetic vasomotor outflow (6, 32, 36). Therefore, these observations suggest that the RVLM neurons are a critical component of central pathways integrating cardiovascular responses to stimulation of both primary afferents and higher brain centers (2, 4, 6-7, 32). Several studies (4, 32-33, 36) have examined the role of RVLM vasomotor neurons in mediating cardiovascular responses to electrical stimulation of somatic afferents, pharmacological stimulation of carotid body chemoreceptors, and electrical stimulation of the hypothalamus, but their responses to stimulation of cardiac receptors have not been studied specifically.
Barosensitive neurons in the RVLM project to the hypothalamus as well as to the spinal cord (4, 6, 36). Although the cardiovascular reflexes induced by stimulation of cardiac receptors during myocardial ischemia have been documented in animals and humans (13, 17, 23, 27, 40), the detailed information on the functional role of RVLM neurons in these reflexes is still lacking. For example, the RVLM is known to contain heterogenous groups of neurons (14, 36). It is not known which type of RVLM neuron (e.g., barosensitive and nonbarosensitive neurons) is involved in the cardiovascular responses to activation of cardiac receptors. Furthermore, both cardiac sympathetic and vagal afferents are stimulated during myocardial ischemia (9, 20-21, 34-35). It remains to be determined which afferent pathways mediate the excitatory responses of RVLM neurons to activation of cardiac afferents. Consequently, in the present study, we examined the influence of activation of cardiac receptors on the single-unit activity of RVLM neurons. An initial protocol examined the differential responses of barosensitive and nonbarosensitive RVLM neurons to epicardial application of BK, a substance known to stimulate cardiac afferent nerve endings and elicit cardiovascular reflexes during myocardial ischemia (19, 29, 34). Furthermore, the primary afferent pathways responsible for the excitation of RVLM barosensitive neurons by stimulation of cardiac receptors were determined. This study provides direct evidence that cardiac sympathetic, but not vagal, afferents mediate the response of RVLM barosensitive neurons to activation of cardiac receptors.
| |
METHODS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Surgical Preparation and Procedures
Experiments were performed on male Sprague-Dawley rats (280-350 g). The experimental procedures and protocols were approved by the Institutional Animal Care and Use Committee of the Wake Forest University School of Medicine and adhered to the Guide for the Care and Use of Laboratory Animals (NIH, US Public Health Service). Rats were anesthetized initially with halothane in an induction chamber. The halothane was discontinued after-chloralose (50-60 mg/kg ip) and pentobarbital sodium (20 mg/kg ip) were
administered. Adequate depth of anesthesia was verified by the absence
of responses to noxious pinch of the paw. Supplemental doses of
-chloralose (20-25 mg/kg iv) were administered to maintain an
appropriate level of anesthesia. The trachea was cannulated, and the
rats were ventilated artificially with 100% O2 using a
rodent ventilator (SAR-830/A, IITC/Life Science Instruments, Woodland
Hills, CA). The left carotid artery was cannulated, and the arterial
blood pressure was measured with a pressure transducer (PT300, Grass Instruments, Quincy, MA). The femoral vein was cannulated for intravenous injection of drugs. A midline thoracectomy was performed to
expose the heart. A snare was placed around the descending thoracic
aorta above the diaphragm and was used to elevate blood pressure by
constricting the aorta to stimulate the baroreceptors. For rats
subjected to vagotomy, we isolated the cervical vagal nerve and placed
a suture around it while the rat was under surgical anesthesia before
recording of RVLM neurons. Under direct vision, the cervical vagal
nerve can be easily identified and separated from the aortic nerve
depressor nerve before it enters the nodose ganglia. Vagotomy was
accomplished by lifting the vagal nerve and cutting it with scissors.
The rat was fixed on a stereotaxic frame (David-Kopf Instruments, Tujunga, CA) in a supine position. The medulla was approached from the ventral surface, as described previously (4). A midline incision was made on the ventral surface of the neck. After retraction of the bilateral longus capitis muscles, we carefully cut away the basal portion of the occipital bone from the ventral surface of the rostral medulla. The exposed ventral surface of the medulla was then covered with warm mineral oil. After an appropriate level of anesthesia was ensured, the rat was paralyzed with pancuronium bromide (1 mg/kg iv). The adequacy of the anesthesia level during neuromuscular blockade was judged by the stability of the arterial blood pressure and was tested once paralysis wore off and before the next dose of pancuronium was given. Lidocaine (2%) was infiltrated around the surgical wound to minimize the nociceptive afferent input after surgery. Body temperature was maintained in the range of 37-38°C with a heating lamp. Animals were killed at the end of experiments by an intravenous injection of an overdose of pentobarbital sodium.
Extracellular Single-Unit Recordings
Extracellular single-unit recordings were made with a tungsten microelectrode (FHC, Bowdoinham, ME; or World Precision Instruments, Sarasota, FL) with an impedance of 2-10 M
. Because of the
prolonged experimental protocols, the metal electrode was chosen for
this study due to the stability for its in vivo extracellular
recordings. The electrode was advanced using a hydraulic microdrive
(Stoelting, Wood Dale, IL) into the RVLM at a speed of 1-2 µm/s
until the single-unit activity was recorded. The electrode was inserted into the RVLM (coordinates: 1.9-2.3 mm lateral to the midline, 2.6-3.3 mm caudal to interaural line, and 0.3-0.9 mm from the ventral surface), an area verified in our preliminary experiments that
increased the mean arterial pressure 20-40 mmHg after
microinjection of glutamate (10-20 nl, 50 mM) into this
region. The signals were amplified and filtered with an
alternating current amplifier (DAM 80, World Precision Instruments),
processed through an audio amplifier (AM8, Grass Instruments, Quincy,
MA), and then displayed on a storage oscilloscope (DSO 450, Gould,
Essex, UK). The neurogram and blood pressure were
simultaneously monitored and recorded on a thermal sensitive recorder
(model K2G, Astro-Med, West Warwich, RI). In addition, the activity of
the neurons was fed into a Pentium computer through an
analog-to-digital interface card for subsequent off-line quantitative
analysis. Discharge frequency was quantified by using a data
acquisition and analysis software (Experimental Workbench, DataWave
Technology, Longmont, CO), and a histogram was created using a graphic
software (SigmaPlot, SPSS, Chicago, IL) for the unit discharges of
individual neurons. Accurate counting of the single-unit discharge
frequency was verified for each neuron by comparing the constructed
histogram with the original neurogram.
Experimental Protocols
Responses of RVLM neurons to epicardial application of BK. The responses of RVLM barosensitive and nonbarosensitive neurons to epicardial application of BK were studied in 53 rats. The vasomotor neurons were identified if their activity was silenced by an increase in blood pressure (130-200 mmHg) caused by gradual occlusion of the descending thoracic aorta. After a RVLM neuron was identified, a stabilization period of at least 15 min was allowed. BK (10 µg/ml, Sigma Chemical, St. Louis, MO), dissolved in normal saline, was applied to the anterior surface of the heart using a cotton applicator (20, 34). The heart surface was rinsed twice using cotton-tipped applicators soaked with normal saline to remove residual BK. Responses of some RVLM barosensitive neurons to application of BK was repeated three times, each separated by 15-20 min, to allow the discharge activity of the neuron and the blood pressure to return to the control levels to prevent tachyphylaxis (16, 20, 34).
Cardiac afferent pathways mediating response of RVLM barosensitive neurons to BK. The effect of vagotomy on the baseline activity of the RVLM vasomotor neurons and on the responses of these neurons to epicardial application of BK were studied in 14 animals. After a barosensitive neuron was identified, the response of the neuron to epicardial application of 10 µg/ml of BK was examined twice, separated by 15-20 min, to ensure that the neuronal response was reproducible. Subsequently, cervical vagal nerves were cut bilaterally. BK was topically applied to the surface of the heart again 15 min later after vagotomy. Both blood pressure and the discharge activity of the RVLM neurons were recorded simultaneously. In addition, the barosensitivity of the RVLM barosensitive neurons was tested again after vagotomy.
The effect of blockade of stellate ganglia on the response of RVLM barosensitive neurons to application of BK was studied in eight additional rats. After recording a single-unit discharge activity, we identified a barosensitive neuron by 100% inhibition of the spontaneous activity in response to an increase in blood pressure. The response of the barosensitive neurons to epicardial application of 10 µg/ml of BK was tested twice, as described above. Blockade of stellate ganglia was accomplished by local infiltration of lidocaine (2%, 0.3-0.5 ml) into the stellate ganglia bilaterally because the cardiac sympathetic afferents pass through the stellate ganglia and enter the spinal cord through the upper thoracic sympathetic chain (10, 17, 20). Lidocaine was injected into the stellate ganglia under direct vision when the chest was open. Both the pressor response and the response of the RVLM neurons to BK application were examined before and after blockade of stellate ganglia. Also, the barosensitivity of the RVLM vasomotor neurons was tested again after blockade of the stellate ganglia.Histological Localization of Recording Sites
The location of recorded RVLM neurons was marked by passing direct current (50 µA for 30 s) at the site of the electrode tip. Only one RVLM barosensitive neuron was studied in each rat. After the lesion was made, the rat was given a lethal injection of pentobarbital, and the brain stem was quickly removed and fixed in 10% buffered Formalin. The brain stem was cut in 40-µm coronal sections on a freezing microtome, mounted on slides, and then stained with cresyl violet (Sigma Chemical). The lesion sites were identified and plotted on standardized sections from the Paxinos and Watson's Atlas (22). Data were excluded if the recorded neurons were not located in the RVLM area.Data Analysis
Results are presented as means ± SE. The discharge rate of RVLM neurons was averaged during a 2-min control period and 1-min response periods. The RVLM neurons were considered to be responsive if their peak discharge frequency after BK application was increased at least 30% above the baseline. The baseline activity of RVLM neurons was measured before and after vagotomy and blockade of stellate ganglia. Comparisons between control and experimental interventions were made by either Student's paired t-test or repeated measures analysis of variance followed by the Tukey's post hoc test. Differences were considered to be statistically significant when P < 0.05.| |
RESULTS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
A total of 75 spontaneously active neurons were studied.
Four neurons were not located in the RVLM, and the data from these experiments were excluded. Among the neurons recorded in the RVLM, there were 40 barosensitive and 31 nonbarosensitive neurons (including 21 neurons not responsive to an increase in blood pressure and 10 neurons excited by an increase in blood pressure). Figure
1 shows the locations of the recorded
neurons in the RVLM. Figure 2 provides an
example of brain stem slice showing the actual recording and lesion
site in one rat. There was no clear difference in topographical distributions between barosensitive and nonbarosensitive neurons within
the RVLM. The resting mean arterial pressure of the animals studied was
75.2 ± 2.9 mmHg, and the baseline heart rate was 352 ± 11 beats/min. The baseline activity of these 71 spontaneously active
neurons was 6.4 ± 0.5 impulses/s. Although it was not difficult to find RVLM neurons, the success rate for identifying RVLM
barosensitive neurons and completion of prolonged recordings in
anesthetized rats was only ~40% during the course of this study.
|
|
Responses of RVLM Barosensitive and Nonbarosensitive Neurons to Epicardial BK
Figure 3 shows representative responses of a barosensitive neuron to brief occlusion of aorta and epicardial application of 10 µg/ml of BK. The discharge activity of 40 RVLM vasomotor neurons was silenced by elevation of the mean arterial blood pressure at a level of 142.3 ± 8.2 mmHg. Topical application of vehicle (saline) on the surface of the heart had no effect on the discharge activity of RVLM barosensitive neurons. Among 40 RVLM barosensitive neurons, 34 (85%) were activated by BK application (Figs. 3 and 4). The discharge activity of two neurons (5%) was inhibited after BK application, and the remaining four cells (10%) showed no response to epicardially applied BK. The discharge activity of those neurons activated by BK significantly increased from 6.2 ± 0.7 to 10.7 ± 0.9 impulses/s (P < 0.05) after an onset latency of 7.5 ± 0.4 s. Topical application of BK (10 µg/ml) also significantly increased the mean arterial blood pressure from 65.2 ± 1.9 to 89.3 ± 2.9 mmHg (P < 0.05) after a latency of 16 ± 5 s. The repeatability of the responses of RVLM neurons to epicardial application of BK was determined in 18 of 34 vasomotor neurons. Responses of these 18 RVLM neurons to repeat applications of BK, separated by 15-20 min, were highly reproducible (Figs. 3 and 4).
|
|
For those RVLM neurons that were either not sensitive to an increase in
blood pressure (n = 21) or were excited by an increase in blood pressure (n = 10), epicardial application of
10 µg/ml of BK failed to alter their baseline discharge activity
(Fig. 5). The baseline discharge
activity of these neurons (9.6 ± 1.5 impulses/s) was higher
than that of barosensitive neurons (6.9 ± 1.1 impulses/s,
P < 0.05). These barosensitive neurons did not have a
tonic discharge or a respiration-related rhythm.
|
Effect of Vagotomy on Response of RVLM Barosensitive Neurons to Epicardial BK
Figure 6 is a representative tracing showing the effect of bilateral vagotomy on the baseline activity and the response of a RVLM barosensitive neuron to epicardial application of 10 µg/ml of BK. Before vagotomy, topical application of bradykinin (10 µg/ml) significantly increased the discharge rate of 14 RVLM neurons (Figs. 6 and 7). The mean arterial blood pressure increased from 66.5 ± 2.5 to 89.6 ± 3.8 mmHg (P < 0.05) after application of BK after an onset latency of 17 ± 5 s. Bilateral vagotomy significantly increased the baseline discharge activity of 14 RVLM barosensitive neurons (P < 0.05) but did not significantly alter the responses of these neurons to BK application (Figs. 6 and 7). The increase in discharge rate of these neurons induced by epicardial application of BK was not significantly altered by vagotomy (3.8 ± 1.4 impulses/s before vagotomy and 3.2 ± 1.1 impulses/s after vagotomy, P > 0.05). The baseline blood pressure and the pressor response to BK (from 67.5 ± 1.3 to 88.7 ± 3.5 mmHg, n = 14) after vagotomy were not significantly changed compared with those seen before vagotomy. In addition, the RVLM barosensitive neurons were silenced by elevation of blood pressure at a level of 136.5 ± 6.9 mmHg (n = 14) before vagotomy. After vagotomy, the blood pressure level required to silence the discharge activity of these neurons was increased to 162.5 ± 9.8 mmHg, which was significantly different from that observed before vagotomy.
|
|
For those RVLM barosensitive neurons showing either an inhibitory response (n = 2) or lack of response (n = 4) to the epicardial application of BK described above, vagotomy neither altered their baseline discharges nor the patterns of their original responses to BK.
Effect of Blockade of Stellate Ganglia on Response of RVLM Barosensitive Neurons to BK
Figure 8 is an original tracing showing the effect of blockade of stellate ganglia on the response of a RVLM barosensitive neuron to epicardial application of BK. Bilateral infiltration of lidocaine into the stellate ganglia did not alter the baseline discharge activity of the RVLM barosensitive neurons. However, blockade of the stellate ganglia abolished the response of RVLM vasomotor neurons (Figs. 8 and 9) and the pressor response (62.5 ± 2.3 to 63.4 ± 1.5 mmHg, n = 8, P > 0.05) to epicardial application of BK. Figure 9 summarizes the responses of eight RVLM barosensitive neurons to epicardial application of BK before and after blockade of stellate ganglia. The blood pressure level required to silence the discharges of these RVLM barosensitive neurons before and after blockade of stellate ganglia was 142.5 ± 4.3 and 146.5 ± 5.7 mmHg (n = 8, P > 0.05), respectively.
|
|
| |
DISCUSSION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Two important observations were made from the present study
about the potential role of RVLM neurons in cardiovascular reflex responses to stimulation of cardiac receptors. We observed that epicardial application of BK excited a majority of barosensitive, but
not nonbarosensitive, neurons located in the RVLM in
-chloralose-anesthetized rats. Furthermore, our study demonstrated
directly that cardiac sympathetic afferents are essential for the
excitatory responses of RVLM barosensitive neurons to activation of
cardiac receptors. In this regard, bilateral vagotomy failed to
attenuate the responses of RVLM barosensitive neurons to epicardial
application of BK, whereas blockade of the sympathetic afferent
pathways by local infiltration of lidocaine into the stellate ganglia
completely eliminated the pressor response and the response of RVLM
vasomotor neurons to epicardial application of BK. Therefore, this
study demonstrates, for the first time, that stimulation of cardiac receptors selectively excites RVLM barosensitive neurons through sympathetic afferent pathways.
Sympathetic and vagal nerves innervating the heart contain not only
autonomic efferent axons but also afferent fibers that transmit signals
generated by cardiac sensory receptors (1, 10,
19-21). Cardiac primary afferents running in the
sympathetic nerves, especially finely myelinated A
-fiber afferents
and unmyelinated C fiber afferents, are generally considered to be the
essential pathways for transmission of cardiac nociception to the
central nervous system during myocardial ischemia (1, 5, 19,
21). Axonal tracing studies have shown that cardiac sympathetic
afferents project to the dorsal horn of the upper thoracic spinal cord
through the stellate ganglia and the sympathetic chain (8,
10). Electrophysiological studies also have shown that
stimulation of cardiac afferents excites neurons in the spinal
ascending pathways, such as those located in the spinothalamic tract
(3). The RVLM is a critical site for generating
sympathetic output (6, 15, 18, 25, 28). The neurons in the
RVLM, particularly the vasomotor (or presympathetic) neurons receiving
a powerful baroreflex inhibitory input, are believed to play an
important role in reflex control of circulation (6, 15,
33). The origins of afferent inputs to RVLM neurons have been
extensively studied using electrophysiological and neuroanatomical
techniques (2, 6, 26, 31). An important feature of RVLM
neurons is the degree of convergence of inputs onto individual RVLM
neurons (2, 6-7). RVLM neurons integrate inputs from
many peripheral (somatic, visceral, and other sensory afferents) and
central sources (2, 6, 31). We observed that noxious pinch
of the handpaw also increased the discharge activity of some RVLM
barosensitive neurons (data not presented). The RVLM vasomotor neurons
are inhibited by baroreceptor inputs and project to the spinal cord,
indicating that they provide tonic sympathoexcitatory outputs to spinal
preganglionic neurons in the intermediolateral cell column and the
central autonomic area of the spinal cord (6, 18, 24). The
differential responses of barosensitive and nonbarosensitive neurons to
stimulation of cardiac receptors observed in this study indicate that
these two classes of RVLM neurons are functionally distinct in
cardiac-cardiovascular reflexes. However, it should be acknowledged
that we only studied those spontaneously active neurons in the RVLM.
Thus the functional significance of silent RVLM neurons in
cardiac-cardiovascular reflexes remains to be established in the future
studies. In this study, we were unable to antidromically stimulate the
RVLM neurons to verify their spinal projection because the rats were
placed in a supine position to expose the heart. We suspect that many recorded RVLM barosensitive neurons in our study likely function as
vasomotor neurons. In this regard, a recent study has revealed that
there are six types of neurons in the RVLM with different phenotypes,
projection sites, and response patterns to stimulation of baroreceptors
(36). However, it is important to note that RVLM neurons
silenced by the baroreceptor activation project to both the
hypothalamus and the spinal cord (36).
Few studies are available about the functional roles of RVLM neurons in cardiovascular reflexes originating from the heart. Many previous studies (2, 4, 25, 32-33, 36) have used either electrical stimulation of stellate ganglia and vagal nerves or intravenous injection of phenyl biguanide to stimulate cardiopulmonary afferent nerves. Thus the physiological relevance of these reflexes is difficult to interpret because these maneuvers nonspecifically activate the afferent fibers in the nerve. Furthermore, these nonnatural stimuli are not restricted to the afferent nerves of a particular organ, and the types of nerve fibers stimulated and sensory modalities cannot be distinguished with these maneuvers. For these reasons, either excitatory or inhibitory responses of RVLM neurons to these stimuli have been reported (2, 32, 36). Additionally, the functional properties of RVLM neurons often are not examined in previous studies (7, 39). In the present study, the RVLM neurons were first identified as barosensitive neurons by verifying their barosensitivity. Epicardial application of BK was employed in the present study to selectively stimulate cardiac receptors because endogenous BK is known to stimulate ischemically sensitive cardiac afferents through kinin B2 receptors and may play an important role in ischemia-induced chest pain and cardiovascular reflexes (19-20, 30). The fact that most of the barosensitive neurons in the RVLM were excited by epicardial application of BK indicates that these barosensitive neurons are involved in the excitatory cardiovascular reflex responses to stimulation of cardiac receptors. In contrast, we found none of the RVLM nonbarosensitive neurons responded to epicardial application of BK. This observation strongly suggests that nonbarosensitive neurons in the RVLM do not participate directly in the cardiovascular response to activation of cardiac receptors. Therefore, our study provides additional important evidence for the role of RVLM barosensitive neurons in cardiovascular reflexes originating from the heart.
We observed that epicardial application of BK induced the pressor
response in -chloralose-anesthetized rats, which is consistent with
the results of a previous study (16). The pressor response also closely correlated with the responses of RVLM vasomotor neurons to
BK. The increased discharge activity of RVLM neurons induced by
epicardial application of BK is unlikely due to the hemodynamic alterations induced by BK because an increase in blood pressure would
be expected to exert an inhibitory action on the discharge activity of
RVLM barosensitive neurons. Furthermore, the pressor response was
clearly preceded by the response of RVLM barosensitive neurons to BK.
We found that four barosensitive neurons exhibited an inhibitory
response to epicardial application of BK. Although it has been reported
that some RVLM vasomotor neurons are inhibited by stimulation of
cardiopulmonary afferents (2), the functional roles played
by these neurons remain uncertain. Our study indicates that the
inhibitory response of these four RVLM neurons to BK is not mediated by
vagal afferent nerves because vagotomy had no effect on their baseline
activity, and their inhibitory response to epicardial application of BK
was not affected by vagotomy.
Stimulation of cardiac vagal afferents has been found to induce inhibitory cardiovascular reflexes (11, 37). On the other hand, cardiac afferent fibers that travel in sympathetic nerves constitute the major pathway for cardiac nociception and excitatory cardiovascular reflexes (8, 10, 17). Because epicardial application of BK or myocardial ischemia stimulates both cardiac sympathetic and vagal afferents (9, 13, 20-21, 34-35), the afferent pathways mediating the response of RVLM barosensitive neurons to activation of cardiac receptors were further determined in this study. We demonstrated that blockade of the stellate ganglia eliminated their response to BK, and bilateral cervical vagotomy had no effect on the response of RVLM barosensitive neurons to epicardial application of BK. Therefore, the present study provides substantial evidence that cardiac sympathetic afferents mediate the excitatory response of the RVLM barosensitive neurons to stimulation of cardiac receptors. We found that vagotomy increased the baseline activity of barosensitive, but not of nonbarosensitive, neurons in the RVLM. Because the level of blood pressure increase required to silence the RVLM neurons significantly increased after vagotomy, our data suggest that some tonic inhibitory baroreceptor inputs are likely relayed to RVLM vasomotor neurons through the vagal nerves. The fact that the discharge activity of RVLM neurons was still silenced by an increase in blood pressure suggests that the aortic depressor nerve was intact after vagotomy. Thus increased baseline activity of RVLM vasomotor neurons after vagotomy is likely due to the removal of the inhibitory inputs from the cardiopulmonary afferents traveling in the vagal nerve.
In summary, we found that only barosensitive neurons in the RVLM are excited by stimulation of cardiac receptors with epicardial application of BK. The cardiac sympathetic, but not vagal, afferent pathways mediate the response of RVLM neurons to activation of cardiac receptors. These findings are important for our understanding of the mechanisms by which cardiac pain may enhance sympathetic tone to the heart and aggravate coronary events during myocardial ischemia.
| |
ACKNOWLEDGEMENTS |
|---|
This study was supported by grants from the American Heart Association, Mid-Atlantic Affiliate (GS-30), and by National Heart, Lung, and Blood Institute Grants HL-60026 and HL-04419. H.-L. Pan is a recipient of an Independent Scientist Career Award from the National Heart, Lung, and Blood Institute.
| |
FOOTNOTES |
|---|
Address for reprint requests and present address of H.-L. Pan: Dept. of Anesthesiology (H187), Penn State Univ. College of Medicine, 500 Univ. Dr., Hershey, PA 17033 (E-mail: hpan{at}psu.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 8 February 2000; accepted in final form 20 June 2000.
| |
REFERENCES |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
1.
Barber, MJ,
Mueller TM,
Davies BG,
and
Zipes DP.
Phenol topically applied to canine left ventricular epicardium interrupts sympathetic but not vagal afferents.
Circ Res
55:
532-544,
1984
2.
Blair, RW.
Convergence of sympathetic, vagal, and other sensory inputs onto neurons in feline ventrolateral medulla.
Am J Physiol Heart Circ Physiol
260:
H1918-H1928,
1991
3.
Blair, RW,
Weber RN,
and
Foreman RD.
Responses of thoracic spinothalamic neurons to intracardiac injection of bradykinin in the monkey.
Circ Res
51:
83-94,
1982
4.
Brown, DL,
and
Guyenet PG.
Electrophysiological study of cardiovascular neurons in the rostral ventrolateral medulla in rats.
Circ Res
56:
359-369,
1985
5.
Cervero, F.
Sensory innervation of the viscera: peripheral basis of visceral pain.
Physiol Rev
74:
95-138,
1994
6.
Dampney, RA.
Functional organization of central pathways regulating the cardiovascular system.
Physiol Rev
74:
323-364,
1994
7.
Ermirio, R,
Ruggeri P,
Molinari C,
and
Weaver LC.
Somatic and visceral inputs to neurons of the rostral ventrolateral medulla.
Am J Physiol Regulatory Integrative Comp Physiol
265:
R35-R40,
1993
8.
Hopkins, DA,
and
Armour JA.
Ganglionic distribution of afferent neurons innervating the canine heart and cardiopulmonary nerves.
J Auton Nerv Syst
26:
213-222,
1989[Web of Science][Medline].
9.
Kaufman, MP,
Baker DG,
Coleridge HM,
and
Coleridge JC.
Stimulation by bradykinin of afferent vagal C-fibers with chemosensitive endings in the heart and aorta of the dog.
Circ Res
46:
476-484,
1980
10.
Kuo, DC,
Oravitz JJ,
and
DeGroat WC.
Tracing of afferent and efferent pathways in the left inferior cardiac nerve of the cat using retrograde and transganglionic transport of horseradish peroxidase.
Brain Res
321:
111-118,
1984[Web of Science][Medline].
11.
Lombardi, F,
Casalone C,
Della Bella P,
Malfatto G,
Pagani M,
and
Malliani A.
Global versus regional myocardial ischaemia: differences in cardiovascular and sympathetic responses in cats.
Cardiovasc Res
18:
14-23,
1984[Web of Science][Medline].
12.
Malliani, A,
Peterson DF,
Bishop VS,
and
Brown AM.
Spinal sympathetic cardiocardiac reflexes.
Circ Res
30:
158-166,
1972
13.
Malliani, A,
Schwartz PJ,
and
Zanchetti A.
A sympathetic reflex elicited by experimental coronary occlusion.
Am J Physiol
217:
703-709,
1969.
14.
McAllen, RM,
and
Dampney RA.
Vasomotor neurons in the rostral ventrolateral medulla are organized topographically with respect to type of vascular bed but not body region.
Neurosci Lett
110:
91-96,
1990[Web of Science][Medline].
15.
McAllen, RM,
and
May CN.
Differential drives from rostral ventrolateral medullary neurons to three identified sympathetic outflows.
Am J Physiol Regulatory Integrative Comp Physiol
267:
R935-R944,
1994
16.
McDermott, DA,
Meller ST,
Gebhart GF,
and
Gutterman DD.
Use of an indwelling catheter for examining cardiovascular responses to pericardial administration of bradykinin in rat.
Cardiovasc Res
30:
39-46,
1995[Web of Science][Medline].
17.
Meller, ST,
and
Gebhart GF.
A critical review of the afferent pathways and the potential chemical mediators involved in cardiac pain.
Neuroscience
48:
501-524,
1992[Web of Science][Medline].
18.
Morrison, SF,
Callaway J,
Milner TA,
and
Reis DJ.
Rostral ventrolateral medulla: a source of the glutamatergic innervation of the sympathetic intermediolateral nucleus.
Brain Res
562:
126-135,
1991[Web of Science][Medline].
19.
Nerdrum, T,
Baker DG,
Coleridge HM,
and
Coleridge JC.
Interaction of bradykinin and prostaglandin E1 on cardiac pressor reflex and sympathetic afferents.
Am J Physiol Regulatory Integrative Comp Physiol
250:
R815-R822,
1986
20.
Pan, HL,
and
Longhurst JC.
Lack of a role of adenosine in activation of ischemically sensitive cardiac sympathetic afferents.
Am J Physiol Heart Circ Physiol
269:
H106-H113,
1995
21.
Pan, HL,
Longhurst JC,
Eisenach JC,
and
Chen SR.
Role of protons in activation of cardiac sympathetic C-fibre afferents during ischaemia in cats.
J Physiol (Lond)
518:
857-866,
1999
22.
Paxinos, G,
and
Watson C.
The Rat Brain in Stereotaxic Coordinates (4th ed.). San Diego, CA: Academic, 1999.
23.
Perez-Gomez, F,
Martin de Dios R,
Rey J,
and
Garcia Aguado A.
Prinzmetal's angina: reflex cardiovascular response during episode of pain.
Br Heart J
42:
81-87,
1979
24.
Ross, CA,
Ruggiero DA,
Joh TH,
Park DH,
and
Reis DJ.
Rostral ventrolateral medulla: selective projections to the thoracic autonomic cell column from the region containing C1 adrenaline neurons.
J Comp Neurol
228:
168-185,
1984[Web of Science][Medline].
25.
Ross, CA,
Ruggiero DA,
Park DH,
Joh TH,
Sved AF,
Fernandez-Pardal J,
Saavedra JM,
and
Reis DJ.
Tonic vasomotor control by the rostral ventrolateral medulla: effect of electrical or chemical stimulation of the area containing C1 adrenaline neurons on arterial pressure, heart rate, and plasma catecholamines and vasopressin.
J Neurosci
4:
474-494,
1984[Abstract].
26.
Ross, CA,
Ruggiero DA,
and
Reis DJ.
Projections from the nucleus tractus solitarii to the rostral ventrolateral medulla.
J Comp Neurol
242:
511-534,
1985[Web of Science][Medline].
27.
Roughgarden, JW,
and
Newman EV.
Circulatory changes during the pain of angina pectoris. 1772-1965-a critical review.
Am J Med
41:
935-946,
1966[Web of Science][Medline].
28.
Sato, A,
and
Schmidt RF.
Spinal and supraspinal components of the reflex discharges into lumbar and thoracic white rami.
J Physiol (Lond)
212:
839-850,
1971
29.
Schulz, R,
Post H,
Vahlhaus C,
and
Heusch G.
Ischemic preconditioning in pigs: a graded phenomenon: its relation to adenosine and bradykinin.
Circulation
98:
1022-1029,
1998
30.
Staszewska-Woolley, J,
Woolley G,
and
Regoli D.
Specific receptors for bradykinin-induced cardiac sympathetic chemoreflex in the dog.
Eur J Pharmacol
156:
309-314,
1988[Web of Science][Medline].
31.
Strack, AM,
Sawyer WB,
Hughes JH,
Platt KB,
and
Loewy AD.
A general pattern of CNS innervation of the sympathetic outflow demonstrated by transneuronal pseudorabies viral infections.
Brain Res
491:
156-162,
1989[Web of Science][Medline].
32.
Terui, N,
Saeki Y,
and
Kumada M.
Barosensory neurons in the ventrolateral medulla in rabbits and their responses to various afferent inputs from peripheral and central sources.
Jpn J Physiol
36:
1141-1164,
1986[Web of Science][Medline].
33.
Terui, N,
Saeki Y,
and
Kumada M.
Confluence of barosensory and nonbarosensory inputs at neurons in the ventrolateral medulla in rabbits.
Can J Physiol Pharmacol
65:
1584-1590,
1987[Web of Science][Medline].
34.
Tjen, ALSC,
Pan HL,
and
Longhurst JC.
Endogenous bradykinin activates ischaemically sensitive cardiac visceral afferents through kinin B2 receptors in cats.
J Physiol (Lond)
510:
633-641,
1998
35.
Ustinova, EE,
and
Schultz HD.
Activation of cardiac vagal afferents by oxygen-derived free radicals in rats.
Circ Res
74:
895-903,
1994
36.
Verberne, AJ,
Stornetta RL,
and
Guyenet PG.
Properties of C1 and other ventrolateral medullary neurones with hypothalamic projections in the rat.
J Physiol (Lond)
517:
477-494,
1999
37.
Weaver, LC,
Danos LM,
Oehl RS,
and
Meckler RL.
Contrasting reflex influences of cardiac afferent nerves during coronary occlusion.
Am J Physiol Heart Circ Physiol
240:
H620-H629,
1981.
38.
Weaver, LC,
Fry HK,
Meckler RL,
and
Oehl RS.
Multisegmental spinal sympathetic reflexes originating from the heart.
Am J Physiol Regulatory Integrative Comp Physiol
245:
R345-R352,
1983.
39.
Weaver, LC,
Meckler RL,
Fry HK,
and
Donoghue S.
Widespread neural excitation initiated from cardiac spinal afferent nerves.
Am J Physiol Regulatory Integrative Comp Physiol
245:
R241-R250,
1983.
40.
Webb, SW,
Adgey AA,
and
Pantridge JF.
Autonomic disturbance at onset of acute myocardial infarction.
Br Med J
3:
89-92,
1972.
This article has been cited by other articles:
|
|
 |
|
  M.-K. Zhong, Y.-C. Duan, A.-D. Chen, B. Xu, X.-Y. Gao, W. De, and G.-Q. Zhu Paraventricular nucleus is involved in the central pathway of cardiac sympathetic afferent reflex in rats Exp Physiol, June 1, 2008; 93(6): 746 - 753. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 W.-Z. Wang, L. Gao, Y.-X. Pan, I. H. Zucker, and W. Wang AT1 receptors in the nucleus tractus solitarii mediate the interaction between the baroreflex and the cardiac sympathetic afferent reflex in anesthetized rats Am J Physiol Regulatory Integrative Comp Physiol, March 1, 2007; 292(3): R1137 - R1145. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. R. Zahner and H.-L. Pan Role of paraventricular nucleus in the cardiogenic sympathetic reflex in rats Am J Physiol Regulatory Integrative Comp Physiol, February 1, 2005; 288(2): R420 - R426. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. R Zahner, D.-P. Li, S.-R. Chen, and H.-L. Pan Cardiac vanilloid receptor 1-expressing afferent nerves and their role in the cardiogenic sympathetic reflex in rats J. Physiol., September 1, 2003; 551(2): 515 - 523. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S.-R. Chen and H.-L. Pan Hypersensitivity of Spinothalamic Tract Neurons Associated With Diabetic Neuropathic Pain in Rats J Neurophysiol, June 1, 2002; 87(6): 2726 - 2733. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. C. Tjen-A-Looi, N. T. Phan, and J. C. Longhurst Nitric oxide modulates sympathoexcitatory cardiac-cardiovascular reflexes elicited by bradykinin Am J Physiol Heart Circ Physiol, November 1, 2001; 281(5): H2010 - H2017. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D.-P. Li, D. B. Averill, and H.-L. Pan Differential roles for glutamate receptor subtypes within commissural NTS in cardiac-sympathetic reflex Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2001; 281(3): R935 - R943. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
, Barosensitive neurons;
, nonbarosensitive neurons; 
, units not
located within the RVLM.
