J Physiol Volume 514, Number 3, 821-828, February 1, 1999
The Journal of Physiology (1999), 514.3, pp. 821-828
© Copyright 1999 The Physiological Society
Activity of aortic chemoreceptors in the anaesthetized rat
Sarah Brophy, Tim W. Ford *, Michael Carey ¹ and James F. X. Jones
Department of Human Anatomy and Physiology, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland, * Autonomic Neuroscience Institute, Department of Physiology, Royal Free Hospital School of Medicine, Rowland Hill Street, London NW3 2PF, UK and ¹ Department of Anaesthetics, Coombe Womens Hospital and St James Hospital, Dublin, Ireland
MS 8429 Received 4 July 1998; accepted after revision 6 October 1998.
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ABSTRACT |
- It has been widely accepted that the rat aortic depressor nerve contains only baroreceptors. However, the experiments which have provided these negative data have employed whole aortic nerve recording. In the present study, the technical difficulties associated with recording single fibres in vivo, from the rat aortic nerve (diameter 25-50 µm), have been surmounted using a small tip, glass suction electrode technique.
- Upon switching from normocapnic hyperoxia to hypercapnic hypoxia, irregularly firing units (n = 13) appeared and these were significantly excited by intravenous injections of sodium cyanide (20 µg) but not by rises in arterial blood pressure induced by methoxamine (an
1-adrenoreceptor agonist; 10 µg). Inhalation of 100 % oxygen rapidly and reversibly silenced, or profoundly reduced, ongoing activity.
- Intravenous injection of phenylbiguanide (PBG; a 5-HT3 receptor agonist; 8 µg) strongly stimulated the chemoreceptors and was followed by a period of chemodepression (3-21 s). In contrast none of the single fibre baroreceptors recorded (n = 15) were excited by PBG but all significantly increased their discharge in response to the increases in arterial blood pressure associated with methoxamine and cyanide. Both the excitatory and inhibitory effects of PBG on the chemoreceptor fibres were abolished by ondansetron (a 5-HT3 receptor antagonist: 1 mg kg-1 i.v.; n = 5 animals) whilst the chemoexcitatory action of cyanide was preserved.
- It is concluded that there are chemoreceptor afferents contained in the aortic nerve of the Sprague-Dawley rat. The 5-HT3 receptor appears not to be a pre-requisite for aortic body chemoexcitation.
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INTRODUCTION |
There is an extensive literature supporting the concept that the aortic nerve of the rat contains only baroreceptor fibres. Thus, electrical stimulation of the central cut end of the nerve at different frequencies and intensities produces inhibition of respiration characteristic of baroreceptor stimulation (Sapru et al. 1981; Numao et al. 1985). This is in marked contrast to the effect of electrical stimulation of the aortic nerve in other species (Neil & Redwood, 1949). Whole nerve recording shows only pulse-rhythmic activity and no discharge increase following injection of cyanide, 5-HT, lobeline and doxapram (Sapru & Krieger 1977a,b).
Whilst thoracic chemoreceptor tissue has been described in the cat and dog, it has been reported to be sparse or absent in rat, rabbit and mouse (Easton & Howe, 1983). In agreement with this anatomical work, carotid sinus nerve section abolishes hypoxic hyperventilation in the rat (for extensive review, see Daly, 1997). In the rat there is some recovery of this reflex with time which appears not to involve aortic nerve afferents but is more probably attributable to abdominal chemoreceptors (Martin-Body et al. 1985, 1986). There appears to be general agreement, therefore, that the aortic bodies of the rat are not functional and the aortic nerve is purely barosensory. This has led to the use of the rat aortic nerve as a neuroanatomical and electrophysiological probe of central baroreflex pathways (e.g. Jeske et al. 1993; Lipski et al. 1996). However, aortic body chemoreceptors are potent in their effects on the cardiovascular system and this facet of the aortic body chemoreceptor reflex probably requires further exploration in the rat.
A major impetus for the present work, which re-examines the question of aortic body chemoreceptors in the rat, is the recent anatomical study by Cheng et al. (1997). These authors provide compelling evidence not only for the existence of aortic bodies in the rat, but also for an afferent innervation from the aortic nerve.
A number of possible reasons could explain why Sapru & Krieger (1977a) did not record chemoreceptors in the rat aortic nerve. First, the whole nerve recording approach might have missed small, less abundant chemoreceptor fibres. Second, the level of arterial PO2 (Pa,O2) was not stated in the experiments of Sapru & Krieger (1977a) but may have been too high. The response of carotid chemoreceptors to cyanide is dependent upon the prevailing Pa,O2, a phenomenon first described by von Euler et al. (1939), and more intensively investigated by Mulligan & Lahiri (1981). The Pa,O2 dependency of cyanide excitation of the aortic bodies has not been examined previously. The present experiments were designed to perform single fibre recordings in vivo, and to test whether cyanide, injected intravenously during hypoxia, stimulates afferents in the aortic nerve of the rat.
Some of our findings have been reported briefly elsewhere (Brophy et al. 1998).
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METHODS |
Male Sprague-Dawley rats (n = 14) weighing 264-447 g (328 ± 50 g, mean ± S.D.) were anaesthetized with pentobarbitone sodium (Sagatal, 60 mg kg-1 I.P. and 3-6 mg kg-1 h-1 I.V.). Following induction of anaesthesia, supplementary doses were given when necessary as assessed by the flexion withdrawal reflex and blood pressure response to paw pinch.
A rectal thermometer was used to monitor temperature, which was maintained at 37°C by a Harvard homeothermic blanket system. The femoral vein and artery were cannulated for administration of supplementary anaesthetic/drugs and for recording of blood pressure, respectively. Arterial pressure was measured with a Statham P23Db transducer (Statham Ltd, Puerto Rico). A cervical tracheostomy was performed and the trachea cannulated low in the neck. The animals were artificially ventilated with humidified, O2-enriched air at an appropriate respiratory frequency, which was adjusted to maintain arterial blood PCO2 between 35 and 40 mmHg (Harvard rodent ventilator, model 683). A triple lumen cannula was advanced via the right external jugular vein to the superior vena cava. Successive lumina of the tubing were prefilled with phenylbiguanide (PBG, 400 µg ml-1, Aldrich Chemicals), sodium cyanide (CN, 1 mg ml-1, Aldrich Chemicals), and methoxamine hydrochloride (1 mg ml-1, Sigma) each dissolved in phosphate-buffered saline (Sigma; 10 mM phosphate buffer, 2·7 mM KCl, 137 mM NaCl). Hamilton syringes were used to administer 10-20 µl volumes of these solutions. A minimum of 5 min was left between succeeding doses of phenylbiguanide to avoid tachyphylaxis. All blood gas variables were measured from samples obtained from the femoral artery, using a Corning Blood Gas Analyser (model 158). Fractional inspired oxygen (FI,O2) was adjusted by altering the relative flow of nitrogen and oxygen using a set of rotameters. The inspired PO2 was measured using the Corning Blood Gas Analyser to ensure the correct FI,O2 levels were administered. At the end of each experiment, the animal was killed by an overdose of the anaesthetic agent.
Aortic nerve recording
All experiments were carried out on the right aortic nerve (Fig. 1). The superior laryngeal nerve was cut central and peripheral to its junction with the aortic nerve. The vagus and cervical sympathetic nerves were also sectioned and the peripheral ends separated as a bundle from the underlying tissues. The nerve bundle was transposed to a small Perspex recording chamber and sealed in place using dental impression material (President, Coltene/ Whaledent Ltd, UK). The Perspex bath was filled with Krebs solution (mM: 118·2 NaCl, 4·75 KCl, 2·54 CaCl2, 1·19 KH2PO4, 1·18 MgSO4.7H2O, 11·1 glucose, 25 NaHCO3) and was equilibrated with 95 % O2-5 % CO2 at 20°C.
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Figure 1. Schematic drawing of the experimental preparation
At the arrow negative pressure is applied to obtain a high resistance seal between the suction electrode and the nerve fascicle of the right aortic nerve. SLN, superior laryngeal nerve.
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Sharp dissecting forceps were used to separate the aortic nerve from the vagus and cervical sympathetic nerves. Collagenase (1 mg ml-1; Sigma; C-7657; 1280-1330 units of collagenase activity mg-1) was dissolved in Krebs solution and placed in the bath for 5 min. This was sufficient time to loosen the epineurium and permit forceps stripping of the residual connective tissue. Suction electrodes were made from borosilicate glass (World Precision Instruments) pulled to a tip with a vertical two-stage puller (Narishige). They were broken back and fire-polished in a microforge (Intracel) to an internal tip diameter of 10 µm and backfilled with the collagenase-Krebs solution. The increase in signal/noise ratio which occurred for 1-2 min after sealing onto the nerve or fascicles of the nerve strongly suggested that a thin epineurium or perineurium still persisted after mechanical and chemical 'stripping' of the whole nerve. There is a potential danger in the use of collagenase if the correct quantity or activity is not administered or if the enzyme is used at 37°C. The axonal activity patterns at 20°C reported in this paper appear consistent with healthy nerve fibres and this activity could be recorded for many hours. Of course, the glomus tissue lies undisturbed within the chest, in the dark and at 37°C.
Aortic nerve activity was amplified and filtered (0·5-5 kHz; Neurolog, NL104, NL125) and displayed on two computers using 1401 interfaces and CHART or SPIKE2 software (Cambridge Electronic Design (CED), Cambridge, UK). On-line analysis of spike height and shape was made using spike template-matching software (SPIKE2). Only discriminated units with a distinctive shape and height were included for analysis. Sampling rate was 15-25 kHz and the width of data capture around each spike was between 2·0 and 10 ms.
Protocol
Animals were rendered slightly hypoxic and hypercapnic during the recording procedure because only baroreceptors could be detected during hyperoxia. In order to identify chemoreceptors the units were tested with 20 µl PBG (8 µg), 20 µl CN (20 µg) and 10 µl methoxamine (10 µg), and the drugs were injected quickly (within 100 ms). Baroreceptors were identified by their pulse-rhythmic activity and obvious barosensitivity to rises or falls in arterial blood pressure induced by the chemical agents. Chemoreceptors displayed a characteristic irregular basal discharge unrelated to the pressure pulse and were identified by their chemosensitivity to cyanide and PBG, but their activity showed no correlation to blood pressure changes induced by cyanide, PBG or methoxamine. In some animals confirmatory tests such as ventilator disconnection or inhalation of 100 % O2 were carried out. PO2 response curves were constructed by plotting the mean discharge (averaging over 10 s) during ventilation with an FI,O2 of 0·2, 0·15 and 0·1. Arterial blood gases were analysed to determine the Pa,O2 which these three different inspired oxygen concentrations produced in each animal.
Following characterization of chemoreceptor fibres, a 1 mg kg-1 dose of ondansetron hydrochloride dihydrate (Glaxo) was administered via the femoral vein. The PBG and CN tests were then repeated 5 min after administration of ondansetron.
Histological techniques
At the end of the nerve recording experiments the aortic bodies were visualized by application of 1 % Neutral Red (BDH) to the distal aortic nerve, and then the area was excised and examined under a light microscope.
In addition, three rats were perfused with paraformaldehyde (4 %), picric acid (0·2 %) and glutaraldehye (0·1 %) in 0·1 M phosphate buffer. Both the right and left aortic nerves were removed and examined as whole mounts using a fluorescence microscope (New Vanox, AHBT, Olympus Optical Co. Ltd, London, UK).
Analysis
The baseline and peak discharges attained during the tests were compared. Repeated measures ANOVA (with the Tukey-Kramer post hoc test), and Student's paired t tests were used as appropriate. To ascertain whether the distributions of spike intervals for the chemoreceptor fibres were similar to a random series two criteria were tested (Biscoe & Taylor 1963). The first was that the mean/standard deviation ratio (coefficient of variation) of the intervals should be close to one. The second criterion was that the intervals should be distributed according to an exponential law. A single decay exponential curve was fitted to the interval time histogram and the goodness of fit was analysed with a 
2 (chi-squared) test.
For all tests statistical significance was taken as P < 0·05. Results are given as means ± S.E.M. unless otherwise stated.
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RESULTS |
Morphology and blood supply of aortic bodies
Aortic bodies were seen along the course of both the right and left aortic nerves in all animals examined (n = 8). Small vascular glomera were evident at the bifurcation of the brachiocephalic artery and the venous drainage was seen to enter the superior vena cava but the arterial supply was too small to be distinguished. The formaldehyde-induced fluorescence technique showed several aggregations of small, intensely fluorescent cells (diameter < 10 µm) along the course of both aortic nerves (Fig. 2).
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Figure 2. Formaldehyde-induced fluorescence of the right aortic nerve
The arrow is pointing to one small intensely fluorescent (SIF) cell. Note the blood vessel seen entering the caudal pole of the glomus. Scale bar: 50 µm.
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Aortic nerve recording
After switching to room air and reducing the frequency of ventilation, the initial control values (means ± S.D.) for cardiovascular and arterial blood gas variables were: heart rate, 373 ± 58 beats min-1; mean arterial blood pressure, 100 ± 18 mmHg; haematocrit, 42 ± 4 %; pH, 7·4 ± 0·04; PCO2, 47 ± 6 mmHg; PO2, 73 ± 12 mmHg.
Chemoreceptor fibres
Thirteen single chemoreceptor afferent fibres were recorded from nine animals. The basal discharge of eight of these fibres was analysed to see if the discharge was a random (Poissonian) process. The average coefficient of variation was 0·977 (range 0·78-1·129). All these fibres could be fitted with a single decay exponential curve and in 7/8 fibres there was no statistical difference between the observed and predicted histograms. The chemoreceptors significantly increased their discharge (from 2 ± 0·6 to 19 ± 4 Hz) with a latency of 3·7 ± 0·5 s following the injection of 20 µg of sodium cyanide into the superior vena cava (Fig. 3). The mean blood pressure rose in some animals and fell in others but on average the pressure fell slightly from 86 ± 4 to 82 ± 9 mmHg. Injection of 10 µg of methoxamine raised pressure significantly from 82 ± 4 to 135 ± 6 mmHg but had a small and insignificant effect on the firing of the chemoreceptors (firing increased from 1·6 ± 0·3 Hz to 3·1 ± 0·8 Hz). PBG (8 µg) produced a prompt (latency 2·1 ± 0·2 s) increase in the chemoreceptor discharge (from 1·8 ± 0·4 Hz to 21 ± 3 Hz), comparable to that achieved with cyanide. The latency to excitation following PBG injection was significantly shorter than that following cyanide (P = 0·002) (Fig. 5). The burst response to PBG lasted 2·2 ± 0·7 s (mean ± S.D.) and this was followed by a period of chemodepression lasting 3-21 s. PBG caused mean blood pressure to fall from 93 ± 5 mmHg to 60 ± 4 mmHg. Inhalation of 100 % oxygen was tested in six animals and this rapidly and reversibly silenced, or profoundly reduced, the ongoing activity of all chemoreceptor fibres tested. PO2-response curves were constructed for the aortic chemoreceptor fibres in three animals (Fig. 6). The effect of 100 % oxygen on cyanide-evoked excitation was tested in three animals and abolished the response of all fibres (n = 3) tested. The cyanide excitation recovered in all cases when the animals were returned to their original hypoxic condition (Fig. 7).
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Figure 3. Single chemoreceptor fibre of the aortic nerve
Integrated activity of a single chemoreceptor afferent fibre following injection (10-20 µl) of phenylbiguanide (PBG, a 5-HT3 receptor agonist), cyanide and methoxamine into the superior vena cava. (Blood gas variables: pH 7·43; Pa,O2 64 mmHg; Pa,CO2 51 mmHg; haematocrit 40 %.)
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Baroreceptor fibres
Fifteen single baroreceptor afferent fibres were recorded from 10 animals. None of the baroreceptors tested was excited by PBG and their discharge fell from 7·3 ± 1·5 Hz to 0·3 ± 0·2 Hz following the decline in mean pressure from 103 ± 4 to 80 ± 5 mmHg. Methoxamine significantly raised mean pressure from 79 ± 3 to 141 ± 4 mmHg and significantly raised the baroreceptor discharge from 2·6 ± 0·7 to 25 ± 3·4 Hz (Fig. 4). Cyanide injection raised pressure from 91 ± 3·4 to 99·5 ± 4·2 mmHg and significantly increased the baroreceptor discharge from 4·4 ± 0·9 to 14·3 ± 3·1 Hz.
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Figure 4. Single baroreceptor fibre of the aortic nerve
Integrated activity of a single baroreceptor afferent fibre following injection (10-20 µl) of phenylbiguanide (PBG, a 5-HT3 receptor agonist), cyanide and methoxamine into the superior vena cava.
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Figure 5. Latency to excitation of a chemoreceptor fibre
Raw and integrated activity of a single chemoreceptor afferent fibre following injection (20 µl) of phenylbiguanide (PBG, a 5-HT3 receptor agonist) and cyanide into the superior vena cava. Note the difference in latency to the excitation of the fibre following injection of the two different chemicals. (Blood gas variables: pH 7·46; Pa,O2 94 mmHg; Pa,CO2 42 mmHg; haematocrit 46·5 %.)
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Figure 6. PO2-response curves of single ( ) and few fibre ( ) aortic chemoreceptors
The FI,O2 was set to 0·2, 0·15 and 0·1 and the Pa,CO2 was 41 ± 2·3 mmHg (mean ± S.D.) for all tests; n = 3 animals.
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Figure 7. Cyanide excitation of aortic chemoreceptor is PO2 sensitive
Left panel shows four superimposed sweeps of a single aortic chemoreceptor afferent recorded with a suction electrode. Right panel shows integrated activity of a single chemoreceptor (illustrated in the left panel) during hypercapnic hypoxia (pH 7·37, Pa,O2 55 mmHg; Pa,CO2 56 mmHg; haematocrit 34 %). The figure is a complete data file with two episodes where the data was paused. The spaces between the panels are temporally accurate with regard to the time bar. At the arrows CN indicates the injection times of 20 µg cyanide into the superior vena cava. The horizontal bar indicates the time of application of 100 % oxygen to the ventilator inlet. Note the subsequent silencing of the chemoreceptor and rise in blood pressure. During the pause in the recording the discharge was tracked on the oscilloscope and audio-amplifier, the fibre was silenced for almost two minutes before the second injection of CN. The response to cyanide is almost abolished but recovers after the animal is returned to original hypoxic conditions (third panel).
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5-HT3 receptor role in chemotransduction
In five animals in which single chemoreceptors were discriminated (1 fibre per animal) PBG and cyanide tests were performed before and 5 min after application of ondansetron (1 mg kg-1 I.V.). PBG increased firing from 1·4 ± 0·8 to 22 ± 7 Hz but, after ondansetron, no longer significantly affected the discharge (before PBG, 0·6 ± 0·3; after PBG, 2·2 ± 1·6 Hz). The response to cyanide was, however, preserved: before ondansetron, cyanide injection increased firing from 1·3 ± 0·7 to 20·8 ± 7 Hz; after ondansetron, the cyanide-induced increase was from 0·5 ± 0·3 to 14 ± 3·7 Hz. The changes in the baseline discharges after ondansetron were also not significant.
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DISCUSSION |
The results of these experiments demonstrate the existence of chemoreceptors in the aortic nerve of the Sprague-Dawley rat. These fibres exhibit irregular and random discharge during hypercapnic hypoxia and are rapidly and reversibly silenced, or profoundly inhibited, by hyperoxia. The fibres are intensely excited by intravenous injection of cyanide and not significantly affected by pressor doses of methoxamine. The cyanide excitation of the fibres exhibits a Pa,O2 dependency. These phenomena help distinguish these fibres from sympathetic efferents or irregularly firing baroreceptors.
The effect of PBG on the rat aortic chemoreceptors was similar to that described for the cat carotid body chemoreceptors, namely transient chemoexcitation followed by chemodepression (Kirby & McQueen, 1984). PBG failed to excite any of the baroreceptor fibres, in accord with the findings of Goldman & Saum (1984). Since PBG was such a potent activator of the chemoreceptor afferents, it was hypothesized that 5-HT might be a significant neurotransmitter in the aortic body of the rat. Animals were treated with ondansetron to determine the effect of 5-HT3 receptor blockade on the basal firing and cyanide-evoked discharge of the aortic chemoreceptors. At a dose which abolished the effects of the 5-HT3 agonist, neither the baseline level of firing, nor the response to cyanide was significantly reduced. Therefore, it is concluded that the 5-HT3 receptor is not critical for chemoexcitation in the aortic body. This is in agreement with the report by Kirby & McQueen (1984) concerning the carotid body of the cat.
Aortic bodies have been described in Sprague-Dawley, Long-Evans and Wistar rats, using light, fluorescence and electron microscopy (Hansen 1981; McDonald & Blewett, 1981; Habeck & Przybylski 1989; Habeck et al. 1991; Cheng et al. 1997). The aortic paraganglia demonstrated in this study are similar to the aortic bodies described in other strains of rat and to those seen in other species (Hansen & Yates, 1975). Fluorescence microscopy demonstrated periadventitial, small (< 10 µm in diameter) intensely fluorescent cells, possibly indicating the presence of biogenic amines. The aggregations were variable in size and number, although under the operating microscope a principle vascular glomus was sometimes visible and the bright red venous drainage entered the superior vena cava as it does in other species. The latencies of the responses to PBG and cyanide suggest that the afferent vessels to the glomera are derived from the systemic and not the pulmonary circulation as is the case for other adult mammals (Coleridge et al. 1970). The significant difference in the latency of the fibres to PBG and cyanide may be of interest. The sluggish response of aortic chemoreceptors to cyanide in comparison to carotid chemoreceptors has been attributed in the past to differences in blood flow (Lahiri et al. 1980). What is difficult to explain is the very rapid response to PBG. At least for this chemical, there is no vascular or diffusion limitation in the subclavian body, if that is where the 5-HT3 receptors are located.
Many reflex studies highlight the role of the carotid bodies in the hyperventilation associated with hypoxic hypoxaemia and cyanide injection. In human and rat, carotid glomectomy or sinus nerve section abolishes hypoxic hyperventilation (for review, see Daly, 1997). In the rat there is some recovery with time and this does not involve aortic nerve afferents but probably does involve abdominal chemoreceptors (Martin-Body et al. 1985, 1986). Because of these and similar experiments, rat aortic bodies have been dismissed as being either scarce or non-functioning from a whole systems point of view. It must be stressed, however, that aortic bodies are primarily involved in effecting reflex control of the circulation not the respiratory system. Many cats, for instance, show no respiratory response when the aortic bodies are selectively stimulated, although brisk vasoconstrictor responses are readily demonstrated (Daly & Jones, 1998). In addition aortic bodies have a weak effect on heart rate but a powerful effect on atrio-ventricular conduction (Jones & Daly, 1997), and this phenomenon can be easily missed unless pursued by the experimenter. Whether or not the rat possesses active aortic chemoreceptor reflexes is still uncertain because the appropriate cardiovascular experiments have not been performed. If the aortic bodies elicit reflex neuroendocrine changes it is unclear what the appropriate experiments should be.
In conclusion, the current set of experiments confirm the presence of chemoreceptor tissue in the thoracic region of the Sprague-Dawley rat and demonstrate that the aortic nerve contains excitable chemoreceptor afferents. Although PBG is a potent stimulant of the chemoreceptors, the 5-HT3 receptor is not necessary for chemotransduction. The results of this study alongside those of Cheng et al. (1997) demand a reappraisal of experiments that have used the aortic nerve as a pure baroreceptor probe for central baroreflex pathways.
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Acknowledgements
S. Brophy held a science degree Scholarship and a Summer Studentship from the Health Research Board, Ireland. T. W. Ford is supported by the British Heart Foundation. J. F. X. Jones is supported by The Wellcome Trust.
Corresponding author
J. F. X. Jones: Department of Human Anatomy and Physiology, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland.
Email: JFXJones{at}iveagh.ucd.ie
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Introduction
