Some reflex cardioinhibitory responses in the cat and their modulation by central inspiratory neuronal activity

High-Intensity Inspiratory Protocol Increases Heart Rate Variability in Myocardial Revascularization Patients

Objective:

To evaluate heart rate variability during an inspiratory muscle endurance protocol at three different load levels [30%, 60% and 80% of maximal inspiratory pressure], in patients who had previously undergone coronary artery bypass grafting.

Methods:

Nineteen late postoperative myocardial revascularization patients participating in a cardiovascular rehabilitation program were studied. Maximal inspiratory pressure maneuvers were performed. An inspiratory muscle endurance protocol at 30%, 60% and 80% of maximal inspiratory pressure was applied for four minutes each, in random order. Heart rate and RR intervals were recorded and heart rate variability was analyzed by time (RMSSD-the mean of the standard deviations for all R-R intervals, and RMSM-root-mean square differences of successive R-R intervals) and frequency domains indices (high and low frequency) in normalized units. ANOVA for repeated measurements was used to compare heart rate variability indices and Student t-test was used to compare the maximal inspiratory pressure and maximal expiratory pressure values.

Results:

Heart rate increased during performance of maximal respiratory pressures maneuvers, and the maximal inspiratory pressure and maximal expiratory pressure mean values were significantly lower than predicted values (P<0.05). RMSSD increased significantly at 80% in relation to rest and 30% of maximal inspiratory pressure and RMSM decreased at 30% and 60% of maximal inspiratory pressure in relation to rest (P<0.05). Additionally, there was significant and progressive decrease in low frequency and increase in high frequency at 30%, 60% and 80% of maximal inspiratory pressure in relation to the resting condition.

Conclusion:

These results suggest that respiratory muscle training at high intensities can promote greater parasympathetic activity and it may confer important benefits during a rehabilitation program in post-coronary artery bypass grafting.

Cardiorespiratory coupling in health and disease

Cardiac and respiratory activities are intricately linked both functionally as well as anatomically through highly overlapping brainstem networks controlling these autonomic physiologies that are essential for survival. Cardiorespiratory coupling (CRC) has many potential benefits creating synergies that promote healthy physiology. However, when such coupling deteriorates autonomic dysautonomia may ensue. Unfortunately there is still an incomplete mechanistic understanding of both normal and pathophysiological interactions that respectively give rise to CRC and cardiorespiratory dysautonomia. Moreover, there is also a need for better quantitative methods to assess CRC. This review addresses the current understanding of CRC by discussing: (1) the neurobiological basis of respiratory sinus arrhythmia (RSA); (2) various disease states involving cardiorespiratory dysautonomia; and (3) methodologies measuring heart rate variability and RSA.

Chronic fluoxetine reduces autonomic control of cardiac rhythms in rats with congestive heart failure

Up to 40% of patients with heart failure develop depression, and depression is an independent risk factor for cardiovascular mortality in this patient population. Consequently, increasing numbers of patients with heart failure are treated with antidepressants. Selective serotonin reuptake inhibitors are typically the antidepressant of choice since this drug class has limited cardiovascular toxicity. However, little is known about the effects of selective serotonin reuptake inhibitors on autonomic cardiac regulation in congestive heart failure (CHF). Here, indexes of cardiac autonomic control were evaluated before and during chronic fluoxetine (FLX) treatment (20 mg·kg−1·day−1, 5 wk) in rats that developed CHF after coronary artery ligation. FLX reduced the low-frequency (LF) component of heart rate variability (HRV; P < 0.01) as well as the sympathetic contribution to LF HRV ( P < 0.01) in both CHF and sham-operated rats. Both FLX and CHF reduced high-frequency HRV ( P < 0.01). Spontaneous baroreflex gain was decreased in CHF rats 8 wk after ligation ( P < 0.01). Cross-spectral coherence between the interbeat interval and mean arterial pressure was reduced in the LF domain 3 wk after ligation in CHF rats ( P < 0.01) and was further reduced after chronic FLX treatment ( P < 0.01). Plasma catecholamines and LF blood pressure variability were not affected by FLX. Chronotropic responses to both efferent vagal nerve stimulation and isoproterenol administration were reduced in CHF rats and by FLX ( P < 0.01), whereas inotropic responses to isoproterenol were reduced only in CHF rats ( P < 0.01). These data indicate that chronic FLX reduces the responsiveness to autonomic output controlling cardiac rhythm and may further compromise autonomic regulation of cardiac function in CHF.

Respiratory modulation of premotor cardiac vagal neurons in the brainstem

The respiratory and cardiovascular systems are highly intertwined, both anatomically and physiologically. Respiratory and cardiovascular neurons are often co-localized in the same brainstem regions, and this is particularly evident in the ventral medulla which contains presympathetic neurons in the rostral ventrolateral medulla, premotor parasympathetic cardioinhibitory neurons in the nucleus ambiguus, and the ventral respiratory group, which includes the pre-Botzinger complex. Anatomical studies of respiratory and cardiovascular neurons have demonstrated that many of these neurons have projections and axon collateral processes which extend into their neighboring cardiorespiratory regions providing an anatomical substrate for cardiorespiratory interactions. As other reports in this Special Issue of Respiratory Physiology & Neurobiology focus on interactions between the respiratory network and baroreceptors, neurons in the nucleus tractus solitarius, presympathetic neurons and sympathetic activity, this report will focus on the respiratory modulation of parasympathetic activity and the neurons that generate parasympathetic activity to the heart, cardiac vagal neurons.

α2-Adrenoreceptor mediated sympathoinhibition of heart rate during acute hypoxia is diminished in conscious prostacyclin synthase deficient mice

Acute hypoxia increases ventilatory drive in conscious animals, resulting in tachycardia. Sustained hypoxia changes the initial chemoreflex ventilatory increase to secondary ventilatory depression, which then evokes a gradual secondary heart rate (HR) reduction. Prostacyclin (PGI(2)) release is known to potentiate alpha(2)-adrenoreceptor (alpha(2)-AR) mediated inhibition of sympathoactivation during ischaemia and hypoxia. We examined whether alpha(2)-AR mediated sympathoinhibition was responsible for limiting hypoxic heart rate increases during initial sympathoactivation, and subsequent secondary HR depression, and if PGI(2) is required for sympathoinhibition of HR. The responses of unrestrained PGI(2) synthase deficient (PGID) and wild type (WT) mice to acute hypoxia (10% O(2) for 30 min) were investigated by simultaneous telemetry, whole body plethysmography and open-flow respirometry. PGID mice exhibited potentiated .V(E) (p < 0.007) after intraperitoneal vehicle injection (n = 8), but not so HR responses compared to WT mice during sustained hypoxia. Idazoxan (alpha(2)-AR antagonist, i.p. bolus 3 mg/kg) pretreatment did not change hypoxic ventilatory response in either group, but significantly elevated hypoxic HR in WT mice only (p < 0.013). Sodium meclofenamate (cyclooxygenase inhibition, i.p. bolus 25 mg/kg) pretreatment eliminated the potentiated .V(E) of PGID and caused significant basal hypotension that led to a transient hypertensive response to hypoxia. From these results, we suggest that alpha(2)-AR activation is required for coupling HR to central inspiratory drive during acute hypoxia, and that PGI(2) is required to enhance the inhibition of sympathoactivation.

Dual effects of neurokinin on calcium channel currents and signal pathways in neonatal rat nucleus tractus solitarius

Neurokinins, such as substance P (SP), modulate the reflex regulation of cardiovascular and respiratory function in the CNS, particularly in the nucleus tractus solitarius (NTS). There is considerable evidence of the action of SP in the NTS, but the precise effects have not yet been determined. Voltage-dependent Ca2+ channels (VDCCs) serve as crucial mediators of membrane excitability and Ca2+ -dependent functions such as neurotransmitter release, enzyme activity and gene expression. The purpose of this study was to investigate the effects of neurokinins on VDCCs currents (ICa) in the NTS using patch-clamp recording methods. In 142 of 282 neurons, an application of [Sar(9), Met(O(2)11]-substance P (SSP, NK(1) receptor agonist) caused facilitation of L-type I(Ba). Intracellular dialysis of the Galpha(q/11)-protein antibody attenuated the SSP-induced facilitation of I(Ba). In addition, phospholipase C (PLC) inhibitor, protein kinase C (PKC) inhibitor and PKC activator attenuated the SSP-induced the facilitation of I(Ba). In contrast, in 115 of 282 neurons, an application of SSP caused inhibition of N- and P/Q-types I(Ba). Intracellular dialysis of the Gbetagamma-protein antibody attenuated the SSP-induced inhibition of I(Ba). These results indicate that NK(1) receptor facilitates L-type VDCCs via Galpha(q/11)-protein involving PKC in NTS. On the other hand, NK(1) receptor inhibits N- and P/Q-types VDCCs via Galpha(q/11)-protein betagamma subunits in NTS.

Prenatal nicotine exposure recruits an excitatory pathway to brainstem parasympathetic cardioinhibitory neurons during hypoxia/hypercapnia in the rat: implications for sudden infant death syndrome

Maternal cigarette smoking and prenatal nicotine exposure increase the risk for sudden infant death syndrome (SIDS) by 2- to 4-fold, yet despite adverse publicity, nearly one of four pregnant women smoke tobacco. Infants who succumb to SIDS typically experience a severe bradycardia that precedes or is accompanied by centrally mediated life-threatening apneas and gasping. Although the causes of the apnea and bradycardia prevalent in SIDS victims are unknown, it has been hypothesized that these fatal events are exaggerated cardiorespiratory responses to hypoxia or hypercapnia. Changes in heart rate are primarily determined by the activity of cardiac vagal neurons (CVNs) in the brainstem. In this study, we tested whether hypoxia/hypercapnia evokes synaptic pathways to CVNs and whether these cardiorespiratory interactions are altered by prenatal exposure to nicotine. Spontaneous rhythmic inspiratory-related activity was recorded from the hypoglossal rootlet of 700- to 800-microm medullary sections. CVNs were identified in this preparation by retrograde fluorescent labeling, and excitatory synaptic inputs to CVNs were isolated and studied using patch-clamp electrophysiologic techniques. Hypoxia/hypercapnia did not elicit an increase in excitatory neurotransmission to CVNs in unexposed animals, but in animals that were exposed to nicotine in the prenatal period, hypoxia/hypercapnia recruited an excitatory neurotransmission to CVNs. This study establishes a likely neurochemical mechanism for the exaggerated decrease in heart rate in response to hypoxia/hypercapnia that occurs in SIDS victims.

Cardiac vagal tone, exercise performance and the effect of respiratory training

Heart rate variability (HRV) at rest and heart rate recovery after exercise reflect cardiac vagal activity. The aim of this study was to determine whether increasing HRV during involuntary respiratory training induced by rebreathing air using a Hepburn heart and lung exerciser (HHALE) could, like exercise, improve vagal tone. Eighteen subjects (36-88 years) underwent a 6-week control period, then a 6-week training period with the HHALE following which half continued training for 6 weeks and half ceased training. Measurements were made of HRV, work at 60% predicted heart rate max for 15 min, heart rate recovery after exercise, resting blood pressure, heart rate, vital capacity and forced expiratory volume. After the first 6-week HHALE training, there was a significant increase of 13.2 +/- 5.7 nu in the high frequency peak of the power spectrum of HRV at rest, whereas, the low frequency peak decreased. Similarly, exercise performance showed a significant improvement of 0.031 +/- 0.012 J per heartbeat from a pre-training 0.128 +/- 0.022. Also, heart rate recovery after exercise significantly faster (drop in the first 20 s improving by 3.3 +/- 1.5 beats from a pre-training 12.9 +/- 1.6). The subgroup that continued training maintained or slightly improved these values. In those that ceased training the speed of heart rate recovery at the end of the exercise test returned to pre-trained levels, whereas, other responses were either maintained or decreased slightly. We conclude that training with the HHALE can, without additional exercise, increase cardiac vagal tone and exercise performance.

Respiratory sinus arrhythmia in freely moving and anesthetized rats

Heart rate increases during inspiration and slows during postinspiration; this respiratory sinus arrhythmia helps match pulmonary blood flow to lung inflation and maintain an appropriate diffusion gradient of oxygen in the lungs. This cardiorespiratory pattern is found in neonatal and adult humans, baboons, dogs, rabbits, and seals. Respiratory sinus arrhythmia occurs mainly due to inhibition of cardioinhibitory parasympathetic cardiac vagal neurons during inspiration. Surprisingly, however, a recent study in anesthetized rats paradoxically found an enhancement of cardiac vagal activity during inspiration, suggesting that rats have an inverted respiratory sinus arrhythmia (Rentero N, Cividjian A, Trevaks D, Pequignot JM, Quintin L, and McAllen RM. Am J Physiol Regul Integr Comp Physiol 283: R1327-R1334, 2002). To address this controversy, this study examined respiratory sinus arrhythmia in conscious freely moving rats and tested whether the commonly used experimental anesthetics urethane, pentobarbital sodium, or ketamine-xylazine alter respiratory sinus arrhythmia. Heart rate significantly increased 21 beats/min during inspiration in conscious rats, a pattern similar to the respiratory sinus arrhythmia that occurs in other species. However, anesthetics altered normal respiratory sinus arrhythmia. Ketamine-xylazine (87 mg/kg and 13 mg/kg) depressed and pentobarbital sodium (60 mg/kg) abolished normal respiratory sinus arrhythmia. Urethane (1 g/kg) inverted the cardiorespiratory pattern so that heart rate significantly decreased during inspiration. Our study demonstrates that heart rate normally increases during inspiration in conscious, freely moving rats, similar to the respiratory sinus arrhythmia pattern that occurs in other species but that this pattern is disrupted in the presence of general anesthetics, including inversion in the case of urethane. The presence and consequences of anesthetics need to be considered in studying the parasympathetic control of heart rate.

Ionotropic glutamate receptor subunit immunoreactivity of vagal preganglionic neurones projecting to the rat heart

The ionotropic glutamate receptor subunits expressed by vagal preganglionic neurones in the rat medulla oblongata were examined by using fluorescence immunolabelling combined with retrograde neuronal tracing. The general population of these neurones in the medulla was identified by intraperitoneal injections of Fluorogold and also with choline acetyltransferase antibodies. Cardiac projecting neurones were specifically identified by applying the fluorescent tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine (DiI) to the heart or by injecting cholera toxin B-subunit into the pericardium. Both tracers labelled populations of neurones lying in the dorsal vagal nucleus, intermediate reticular formation and nucleus ambiguus, and when both tracers were applied simultaneously, approximately 50% of cells were dual-labelled. Control experiments established that the labelling was specific for neurones projecting to the heart. Most vagal preganglionic neurones, including those projecting to the heart, irrespective of their location in the medulla, had a similar profile of glutamate receptor immunoreactivity. Labelling of somata for the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) subunit GluR1 was weak or absent, while labelling with antibodies directed to GluR2, a common sequence of GluR2 and GluR3, and GluR4 was moderate or intense. All neurones studied appeared to express the N-methyl-D-aspartate (NMDA) receptor subunit NR1, and while antibodies recognising the NR2A and NR2B splice variants gave strong labelling, immunoreactivity with a NR2B specific antibody was weaker. Weak to moderate labelling was seen in some neurones using antibodies to the kainate receptor subunits KA2 and GluR5-7. These results are consistent with neurophysiological data indicating the presence of AMPA, NMDA and kainate responses in cardiac vagal preganglionic neurones, and suggest that these neurones are similar to other vagal parasympathetic preganglionic neurones in expressing mainly AMPA receptor subunits GluR2-4.

Effect of upper airway negative pressure and lung inflation on laryngeal motor unit activity in rabbit

Distortion of the upper airway by negative transmural pressure (UANP) causes reflex vagal bradycardia. This requires activation of cardiac vagal preganglionic neurons, which exhibit postinspiratory (PI) discharge. We hypothesized that UANP would also stimulate cranial respiratory motoneurons with PI activity. We recorded 32 respiratory modulated motor units from the recurrent laryngeal nerve of seven decerebrate paralyzed rabbits and recorded their responses to UANP and to withholding lung inflation using a phrenic-triggered ventilator. The phasic inspiratory (n = 17) and PI (n = 5) neurons detected were stimulated by -10 cmH(2)O UANP and by withdrawal of lung inflation (P < 0.05, Friedman's ANOVA). Expiratory-inspiratory units (n = 10) were tonically active but transiently inhibited in postinspiration; this inhibition was more pronounced and prolonged during UANP stimuli and during no-inflation tests (P < 0.05). We conclude that, in addition to increasing inspiratory activity in the recurrent laryngeal nerve, UANP also stimulates units with PI activity.

Effect of pulmonary C-fibre afferent stimulation on cardiac vagal neurones in the nucleus ambiguus in anaesthetized cats

It has been demonstrated previously that the vagal bradycardia evoked by activation of pulmonary C-fibres is not respiratory modulated. Experiments were carried out in alpha-chloralose anaesthetized cats to determine if these cardiac vagal preganglionic neurones (CVPNs) in the nucleus ambiguus (NA), which have respiratory modulated activity, can be activated when pulmonary C-fibre afferents are stimulated by right atrial injections of phenylbiguanide (PBG). Eleven CVPNs with B-fibre axons in the right cardiac vagal branches were identified and found to be localized within or ventrolateral to the nucleus ambiguus. Ionophoretic application of a high current of dl-homocysteic acid (DLH) induced a vagally mediated bradycardia and hypotension in six of eight sites from which CVPNs were recorded. The activity of B-fibre CVPNs, whether spontaneous (n = 4) or induced by ionophoresis of DLH (n = 7) was respiratory modulated, firing perferentially during post-inspiration and stage 2 expiration. This activity also correlated with the rising phase of the arterial blood pressure wave consistent with these CVPNs receiving an arterial baroreceptor input. Right atrial injections of PBG excited nine of eleven CVPNs tested. In eight of these activated neurones the onset latency of the excitation was within the pulmonary circulation time, consistent with being activated only by pulmonary C-fibre afferents. In two neurones the PBG-evoked excitation still occurred when central inspiratory drive was inhibited, as indicated by the disappearance of phrenic nerve activity. In conclusion, B-fibre respiratory modulated CVPNs can be activated following stimulation of pulmonary C-fibre afferents.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Enhanced peripheral chemoreflex function in conscious rabbits with pacing-induced heart failure

The present study aimed to determine whether peripheral and/or central chemoreflex function is altered in chronic heart failure (CHF) and whether altered chemoreflex function contributes to sympathetic activation in CHF. A rabbit model of pacing-induced CHF was employed. The development of CHF (3-4 wk of pacing) was characterized by an enlarged heart, an attenuated contractility, and an elevated central venous pressure. Renal sympathetic nerve activity (RSNA) and minute volume (MV) of ventilation in response to stimulation of peripheral chemoreceptors by isocapnic/hypoxic gases were measured in the conscious state. It was found that the baseline RSNA at normoxia was higher in CHF rabbits than in sham rabbits (35. 00 +/- 4.03 vs. 20.75 +/- 2.87% of maximum, P < 0.05). Moreover, the magnitudes of changes in RSNA and MV in response to stimulation of the peripheral chemoreceptors and the slopes of RSNA-arterial PO2 and MV-arterial PO2 curves were greater in CHF than in sham rabbits. Inhibition of the peripheral chemoreceptors by inhalation of 100% O2 decreased RSNA in CHF but not in sham rabbits. The central chemoreflex function, as evaluated by the responses of RSNA and MV to hyperoxic/hypercapnic gases, was not different between sham and CHF rabbits. These data suggest that an enhancement of the peripheral chemoreflex occurs in the rabbit model of pacing-induced CHF and that the enhanced peripheral chemoreflex function contributes to the sympathetic activation in the CHF state.

Trigeminal and carotid body inputs controlling vascular resistance in muscle during post-contraction hyperaemia in cats

1. In anaesthetized cats, the effects of stimulation of the receptors in the nasal mucosa and carotid body chemoreceptors on vascular resistance in hindlimb skeletal muscle were studied to see whether the responses were the same in active as in resting muscle. The measurements of vascular resistance were taken, first, in resting muscle, and second, in the immediate post-contraction hyperaemic phase that followed a 30 s period of isometric contractions. 2. Stimulation of the receptors in the nasal mucosa caused reflex apnoea and vasoconstriction in muscle. The latter response was attenuated when the test was repeated during post-contraction hyperaemia. 3. Stimulations of the carotid bodies were made during a period of apnoea evoked reflexly by electrical stimulation of both superior laryngeal nerves. This apnoea prevented any effects of changes in respiration on the carotid body reflex vascular responses. Stimulation of the carotid bodies evoked hindlimb muscle vasoconstriction. In the post-contraction hyperaemic period, the response was reduced or abolished. A similar attenuation of the reflex vasoconstrictor responses occurred in decentralized muscles stimulated through their motor roots in the cauda equina. 4. Evidence is presented that the attenuation of the vasoconstrictor responses evoked by the two reflexes is a phenomenon localized to the contracting muscles themselves resulting from an interaction between sympathetic neuronal activity and the local production of metabolites. 5. The results are discussed in relation to the metabolic needs of tissues in relation to asphyxial defence mechanisms such as occur in the diving response.

Importance of neurokinin-1 receptors in the nucleus tractus solitarii of mice for the integration of cardiac vagal inputs

Unmyelinated vagal afferents from the heart terminate within the nucleus tractus solitarii (NTS) located in the dorsomedial medulla. The neurotransmitter and postsynaptic receptors mediating information from cardiac vagal receptors to the NTS are unknown. This study determined the effects of neurokinin-1 (NK1) receptor blockade on: (i) the reflex response evoked following aortic root injection of either veratridine (1-3 microg/kg) or bradykinin (80-300 ng/kg) to stimulate cardiac receptors in in vivo anaesthetized mice; and (ii) the evoked synaptic response of cardioreceptive NTS neurons following both intraleft-ventricular injection of veratridine or bradykinin, and electrical stimulation of the ipsilateral vagus nerve in an arterially perfused working heart-brainstem preparation of mouse. Administration of CP-99,994 (0.75-1.5 mg/kg i.v.), a specific NK1 antagonist, attenuated significantly the evoked reflex bradycardia and depressor response following cardiac receptor (n = 6), but not pulmonary chemoreflex stimulation in vivo. From extracellular recordings of cardioreceptive NTS neurons, CP-99,994 reduced reversibly the total number of evoked spikes, peak firing frequency and response duration evoked by intraventricular injections of veratridine (n = 5) or bradykinin (n = 5). The number of evoked action potentials following electrical stimulation of the vagus nerve was also reduced. In five whole cell recordings of NTS neurons, both the evoked depolarization following cardiac receptor stimulation, and the peak amplitude and duration of vagus nerve-evoked EPSPs were reduced by CP-99,994; synaptic inputs from both peripheral chemoreceptors or pulmonary C-fibres were unaffected. These data support a selective involvement of NK1 receptors in the transmission of cardiac vagal afferent inputs to NTS neurons integrating cardiorespiratory information.

Pattern of cardiorespiratory afferent convergence to solitary tract neurons driven by pulmonary vagal C-fiber stimulation in the mouse

The central integration of signals from pulmonary vagal C-fibers (or type-J receptors) with those arising from cardiac, peripheral chemoreceptor, and baroreceptor afferents to neurons within the nucleus of the solitary tract (NTS) was studied in an arterially perfused working heart-brain stem preparation of adult mouse. Pulmonary vagal C-fibers were excited by right atrial injection of phenylbiguanide (PBG) while cardiac receptors were stimulated by left ventricular injection of veratridine (1-3 micrograms/kg) or mechanically by distension of the left ventricle (20-50 microl perfusate) using an indwelling cannula. Carotid body chemoreceptors were activated by aortic injection of Na cyanide, whereas baroreceptors were stimulated by increasing arterial perfusion pressure. Stimulation of pulmonary C-fibers and cardiac, chemo-, and baroreceptors all produced a reflex bradycardia (23-133 bpm). Central respiratory activity, as recorded from the phrenic nerve, was depressed by stimulating pulmonary C-fibers and cardiac and baroreceptors but enhanced in amplitude and frequency during chemoreceptor stimulation. Twenty-seven NTS neurons were excited and three were inhibited after pulmonary C-fiber stimulation displaying decrementing discharges with a peak firing frequency of up to 42 Hz (15 +/- 2.2 Hz, mean +/- SE) that lasted for 8.8 +/- 0.9 s. These responses occurred <1 s from the end of the PBG injection that was within the pulmonary circulation time. None of these cells responded to increases in right atrial pressure. All cells excited by PBG were also driven synaptically after electrical stimulation of the ipsilateral cervical vagus nerve at a latency of 32.9 +/- 3.2 ms (range 20-62 ms). None of these neurons had ongoing activity related to central respiratory activity. Convergence from cardiorespiratory afferents to 21 neurons driven by pulmonary C-fibers was tested. Twenty-five percent of cells were selectively excited by chemical stimulation of cardiac receptors alone, 19% were driven by peripheral chemoreceptors, and 38% responded to both cardiac and chemoreceptor activation. In contrast, only 13% of the cells activated by PBG injection responded to stimulation of baroreceptors and only 6% to cardiac mechanoreceptor stimulation. None of these neurons were activated by increasing right atrial pressure. The data indicate a high proportion of afferent convergence from pulmonary C-fibers, cardiac receptors, and peripheral chemoreceptors in the NTS. However, these neurons appear not to integrate inputs from cardiovascular mechanoreceptors. The significance of the data is discussed in relation to pathological disease states such as pulmonary congestion and cardiac failure.

Convergence properties of solitary tract neurons responsive to cardiac receptor stimulation in the anesthetized cat

The convergence pattern of cardiac receptors, pulmonary C-fibers, carotid chemoreceptor, and baroreceptor afferents onto neurons within the nucleus of the solitary tract (NTS) was studied in the anesthetized (pentobarbitone sodium, 40 mg/kg,) paralyzed and artificially ventilated cat. Extra- and intracellular recordings were made from NTS neurons while stimulating both cardiac receptors by aortic root injections of veratridine (1-3 micrograms/kg) and pulmonary C-fibers by a right atrial injection of phenylbiguanide (10-20 micrograms/kg). The ipsilateral carotid body was stimulated by using arterial injection of CO2-saturated bicarbonate solution, whereas inflation of the ipsilateral carotid sinus was used to activate baroreceptors. The ipsilateral cardiac vagal branch, cervical vagus, and carotid sinus nerves were stimulated electrically (1 Hz, 0.2-1 ms, 1-35 V). In 78 NTS neurons recorded either extracellularly (n = 47) or intracellularly (n = 31), electrical stimulation of the cardiac branch of the vagus nerve evoked synaptic potentials (spikes and/or excitatory postsynaptic potentials) with an onset latency between 4 and 220 ms. Some neurons displayed both short and long latency inputs(15.5 +/- 1.8 and 160.0 +/- 8.5 ms; n = 14). Of these 78 neurons, 24 responded to veratridine stimulation of cardiac receptors (i.e., cardioreceptive neurons) by exhibiting an augmenting-decrementing discharge of 37 +/- 4 s in duration with a peak frequency of 30 +/- 5 Hz. Convergence from other cardiorespiratory receptors was noted involving either carotid chemoreceptors (n = 7) or pulmonary C-fibers (n = 4) or from both carotid chemoreceptors and pulmonary C-fibers (n = 6). In contrast, only one cardioreceptive NTS neuron was activated by distension of the carotid sinus. Recording sites recovered were confined to the medial NTS at the level of the area postrema and extended caudally into the commissural subnucleus. Our results indicate a convergence of carotid chemoreceptor and pulmonary C-fiber afferent inputs to cardioreceptive NTS neurons. With the paucity of baroreceptor inputs to these neurons it is suggested that sensory integration within the NTS may reflect regulatory versus defensive or protective reflex control.

Convergence properties of solitary tract neurones driven synaptically by cardiac vagal afferents in the mouse

1. Cardiac vagal receptors are chemically and/or mechanically sensitive but it is unknown if this information is preserved centrally within the nucleus of the solitary tract (NTS). The present study had two aims: first, to investigate qualitatively whether both mechanically and chemically sensitive cardiac vagal encoding were preserved within the NTS, and second, to determine the patterns of convergence from other cardiorespiratory afferents to NTS neurones receiving cardiac vagal inputs. 2. The extracellular activity of single NTS neurones was investigated during stimulation of both chemically and mechanically sensitive cardiac vagal receptors in a working heart-brainstem preparation of mouse. Chemically sensitive cardiac receptors were stimulated using intra-left ventricular injections of either veratridine (1-3 microg kg-1), bradykinin (0.25-1 microg) or prostaglandin E2 (100-200 ng), whereas the left ventricle was distended to activate cardiac mechanoreceptors. 3. Forty-three NTS neurones were activated both synaptically by electrical stimulation of the ipsilateral vagus nerve (latency, 35 +/- 3 ms), and by intra-left ventricular injection of veratridine and also, in some cases, by bradykinin and/or PGE2. These NTS neurones were delineated into two populations based on their response to left ventricular distension and convergence properties. Left ventricular distension-insensitive neurones (n = 30) were excited by stimulation of carotid body chemoreceptors (81 %) but not arterial baroreceptors (3 %; i.e. n = 1 neurone), whereas distension-sensitive cells (n = 13) were activated mainly by baroreceptors (86 %) rather than peripheral chemoreceptors (14 %; i.e. n = 1 neurone). 4. The data reveal two distinct populations of NTS neurones receiving cardiac vagal inputs: (a) cells responsive to veratridine stimulation only, and (b) neurones activated by both veratridine and mechanical stimuli. The specific convergence pattern of baroreceptors and chemoreceptors to these cardioreceptive NTS neurones is discussed in relation to a common afferent modality integration within the NTS.

Activity of C fibre cardiac vagal efferents in anaesthetized cats and rats

1. Bradycardia can be evoked by stimulation of both myelinated and non-myelinated vagal efferent fibres. The on-going activity and synaptic inputs to cardiac vagal preganglionic neurones with myelinated axons have been studied in detail, but there is little information regarding cardiac preganglionic neurones with non-myelinated axons. In the present study, the on-going discharge and afferent inputs to cardiac vagal efferents with non-myelinated axons were studied in anaesthetized rats and cats. 2. Extracellular recordings were made from vagal preganglionic neurones in the dorsal vagal motor nucleus (DVMN) of anaesthetized rats following electrical stimulation of the cervical vagus nerve. Based on calculated axonal conduction velocities, fifty-six neurones had non-myelinated axons. Sixteen of these C fibre neurones had on-going activity but this showed no relationship to central respiratory drive, lung inflation or the cardiac cycle. Activity of twenty-one of these fifty-six neurones was increased at short latency following right atrial injections of phenylbiguanide (PBG). 3. Eight presumed cardiac vagal preganglionic neurones were also recorded in the DVMN. Five of these were activated by PBG administration, and the one neurone with resting activity showed no indication of respiratory-related activity. 4. Finally, twenty-one single C fibres were recorded in peripheral branches of the cardiac vagus. They had a low rate of on-going activity, and in twelve fibres this on-going discharge was analysed in detail. On-going activity in seven of these fibres showed no relationship to central respiratory drive, lung inflation or the cardiac cycle, whereas the other five had prominent relationships with both central respiratory drive and lung inflation, and two of these also showed a relationship to heart period. PBG administration evoked an increased activity in fourteen of the fifteen fibres tested, and the latencies of nine of these responses (1.3 +/- 0.5 s) were within the pulmonary circulation time. 5. In anaesthetized cats, extracellular recordings were made in the DVMN from thirty-three cardiac vagal preganglionic neurones with C fibre axons. The on-going activity of these neurones, when present, never exhibited an on-going- or reflexly induced respiratory rhythm, nor any indication of an input from the arterial baroreceptors. Right atrial injection of PBG evoked a short-latency response in four of the five spontaneously active neurones but was without effect on the eight neurones tested which did not exhibit on-going activity. 6. In conclusion, the majority of cardiac vagal preganglionic neurones located in the DVMN have C fibre axons, show no obvious input from central or peripheral respiratory- or cardiac-related inputs, but are activated by stimulation of pulmonary C fibre afferent fibres.

Cardiorespiratory reflexes in mice

The various transgenic strains of mice make this species an attractive experimental model. We compared qualitatively some cardiorespiratory reflexes in two different preparations of mouse: in vivo urethane anaesthetised and a working heart-brainstem preparation (WHBP). Cardiorespiratory reflexes were evoked by stimulating baroreceptors, pulmonary vagal C fibres and cardiac receptors in both preparations, while peripheral chemoreceptors were also stimulated in the WHBP. In anaesthetised mice, activation of baroreceptors, pulmonary C fibres and cardiac receptors evoked an atropine-sensitive bradycardia (range: 21-414 bts/min) and depressed ventilation. A reflex fall in arterial pressure was also observed during pulmonary C fibre and cardiac receptor stimulation. Similar reflex bradycardia (range 81-164 bts/min) and respiratory responses were observed in the WHBP following stimulation of baro-, pulmonary C fibre and cardiac receptors. Additionally, sodium cyanide stimulation of peripheral chemoreceptors in the WHBP produced an atropine-sensitive bradycardia and increased respiratory frequency and amplitude. Thus, the cardiorespiratory reflex responses elicited in the mouse are similar to those reported in other species. It is concluded that the qualitatively similar reflex performances between the in vivo anaesthetised mouse and the WHBP make the latter an adequate model for studying central mechanisms controlling the cardiorespiratory system.

Rhythmic bursting of pre- and post-inspiratory neurones during central apnoea in mature mice

1. Stimulation of pulmonary vagal C fibres (PCFs) inhibits inspiration but the response pattern of respiratory rhythm-generating neurones is unknown. This study provides the first description of the effects of PCF stimulation on six different types of respiratory neurones located in the ventrolateral medulla of the mature mouse. 2. Studies were performed in both urethane-anaesthetized (1.5 g kg-1 I.P.) mature mice and in an arterially perfused working heart-brainstem preparation (WHBP). In both preparations the respiratory motor pattern of phrenic and recurrent laryngeal nerves were comparable. Stimulation of PCFs, using phenylbiguanide (2-5 micrograms) injected into the right atrium, evoked a similar respiratory and cardiac response pattern in both anaesthetized and perfused mice, which included: (i) a significant prolongation of the inter-inspiratory interval; (ii) an increase in the duration and amplitude of post-inspiratory (PI) activity; and (iii) an atropine-sensitive bradycardia (50-260 beats min-1). 3. In the WHBP, PCF stimulation evoked a depolarization (11 +/- 1 mV) and high frequency tonic discharge (up to 64 Hz) in ten out of twenty-one PI neurones. During the PCF-induced prolongation of PI activity all other PI neurones (n = 11), as well as pre-inspiratory neurones (PreI; n = 11), displayed oscillations in membrane potential and/or rhythmic bursting at a similar frequency of 0.7-1.0 Hz. Other respiratory neurones recorded, including stage II expiratory neurones (n = 7), early- (n = 6), ramp- (n = 16) and late-inspiratory neurones (n = 4), ceased firing rhythmically during PCF stimulation. 4. The firing behaviour of PI and PreI neurones was assessed after switching to a low Ca2+ (0.2 mM)-high Mg2+ (5.25 mM) perfusate to block synaptic transmission in the WHBP. In the absence of synaptic transmission, PreI neurones (n = 7/8) continued to discharge rhythmically, whereas all other respiratory cell types (including PI neurones, n = 5) fired tonically. 5. In conclusion, stimulation of PCFs elicits a reflex-evoked prolongation of the PI phase of the respiratory cycle and excitation of PI neurones including rhythmic discharging. It is suggested that this rhythmic bursting depends on inhibitory connections from PreI neurones. The functional significance of these central 'apnoeic rhythms' are discussed.

Ventral medullary neuronal responses to peripheral chemoreceptor stimulation

Recent findings suggest that carotid chemoreceptor input into the ventral medullary surface intermediate area during hypoxia is inhibitory (Gozal et al., (1994) Neurosci. Lett. 178, 73-76. However, systemic hypoxia is a complex stimulus, and effects of carotid chemoreceptor stimulation per se on intermediate ventral medullary surface neuronal activity are difficult to isolate. Therefore, we studied neural activation of the intermediate ventral medullary surface during peripheral chemoreceptor stimulation by intravenous sodium cyanide using optical procedures in seven pentobarbital-anesthetized cats. Control recordings were also acquired in the suprasylvian cortex of three cats. Images of reflected 660 nm light were collected at l/s with a charge-coupled device camera, triggered by the cardiac R wave, after 0.0, 0.5, 2, 5, 10, 20 and 40 micrograms/kg i.v. sodium cyanide administration before and following carotid sinus denervation. Sodium cyanide doses > 5 micrograms/kg significantly increased ventilation, an effect which was eliminated following carotid sinus denervation. A pronounced, dose-dependent activity decrease within the intermediate ventral medullary surface occurred within seconds of sodium cyanide administration, with subsequent return to baseline. Carotid sinus denervation eliminated rapid-onset neural responses to all sodium cyanide doses. However, at the 40 micrograms/kg dose, a smaller, slower onset (25 s), activity decrease occurred both pre- and postdenervation. In the neocortex, the sodium cyanide-induced fast responses were absent. Intravenous cyanide, acting via a carotid sinus nerve pathway, results in a dose-dependent decrease in neural activity within the intermediate ventral medullary surface of cats. High-dose sodium cyanide also appears to decrease intermediate ventral medullary surface neural activity directly.

The ventral medullary respiratory network of the mature mouse studied in a working heart-brainstem preparation

1. This report provides the first description of respiratory network activity within the ventrolateral medulla of the mature mouse obtained from a unique working heart-brainstem preparation (WHBP). 2. In the WHBP three distinct respiratory phases were evident in recordings of both phrenic and vagal efferent nerves. These included a ramp inspiratory (I) discharge, post-inspiratory (PI) activity and a silent or expiratory interval (E2). 3. Extracellular recordings were made from different types of respiratory neurones located within, or in close proximity to, the nucleus ambiguus. Based on firing patterns and phase relative to phrenic nerve discharge, respiratory neurone types, including pre-inspiratory (PreI), early-inspiratory, throughout inspiratory (I), late-inspiratory, post-inspiratory (PI) and stage II expiratory or E2 neurones were characterized. 4. Intracellular recordings were made from four types of respiratory neurones (PreI, I, PI and E2 neurones). PreI neurones were depolarized maximally during the E2-inspiratory transition. I neurones exhibited a ramp depolarization which started either before or at the onset of phrenic discharge. Based on the kinetics of the inspiratory-related hyperpolarizations and duration of discharge, two types of PI neurones were found (rapidly adapting and slowly adapting). E2 neurones were hyperpolarized during both the inspiratory and post-inspiratory phases. 5. Phase-dependent chloride-mediated inhibition was studied in PreI, PI and E2 neurones and included: late inspiratory inhibition of PreI neurones; inspiratory-related inhibition of PI and E2 neurones; and post-inspiratory inhibition of PreI and E2 neurones. In addition, pre-inspiratory inhibition of PI neurones was also demonstrated. 6. The WHBP appears to be viable for analysing reflex, synaptic and cellular mechanisms regulating respiratory activity in an in vitro milieu. The synaptic organization of the respiratory network of the mouse appears comparable to that of the rat and cat. The possibility of a mutual inhibitory interaction between PreI and PI neurones is discussed in terms of the functional organization of the respiratory network in the mouse.

Heart rate responses to selective stimulation of cardiac vagal C fibres in anaesthetized cats, rats and rabbits

1. The contribution of cardiac vagal C fibres to vagal chronotropic control in anaesthetized cats, rats and rabbits was analysed using electrical stimulation of the vagus nerve with a selective anodal block technique. 2. After bilateral vagotomy and pretreatment with atenolol, 10 Hz continuous selective stimulation of unmyelinated fibres in the cut peripheral end of the cervical vagus evoked a bradycardia in anaesthetized rats, cats and rabbits. With this stimulation protocol the three species exhibited a similar lengthening of the heart period (R-R interval) when expressed as a percentage of their basal cardiac interval. 3. The mechanism of action of the selective blocking technique was analysed by recording eighty-nine single A- (n = 12), B- (n = 22) and C-fibre (n = 55) vagal-projecting neurones in the medulla of the rat. This demonstrated that the technique can selectively block conduction in myelinated fibres and that 'break excitation' is seen mainly in unmyelinated fibres. Although thirty C fibres showed break excitation sixteen did not and this difference could not be correlated with their axonal conduction velocity, chronaxie or initial segment frequency following. 4. Using the anodal block technique the vagal effects on heart rate were reanalysed in the cat by incorporating a collision technique. B fibres were activated orthodromically to evoke cardioinhibition and simultaneously antidromically to collide with errant B-fibre spikes activated at the electrode producing anodal block. With this protocol it was noted that the B- and C-fibre bradycardias were not additive. Using a double anodal block and collision technique, it was demonstrated that this phenomenon was likely to be due to occlusion of the effects of B and C fibres. 5. In conclusion, in addition to the well-defined effects of vagal B fibres on heart rate, selective stimulation of vagal C fibres also had a cardioinhibitory effect in all three species studied. However, since the effects of cardiac C fibres on heart rate was small, these neurones alone cannot account for the cardioinhibition of the pulmonary chemoreflex. It is likely that activation of both B- and C-fibre cardiac vagal preganglionic neurones accounts for this reflex cardioinhibition.

Absence of respiration modulation of carotid sinus nerve inputs to nucleus tractus solitarius neurons receiving arterial chemoreceptor inputs

The reflex responses to activation of the arterial chemoreceptors are dependent upon when in the respiratory cycle the chemoreceptor stimulus is given. To determine if the respiratory modulation of the chemoreflex occurs within the nucleus tractus solitarius (NTS), intracellular recordings were obtained in pentobarbital-anesthetized, paralyzed and mechanically ventilated cats, from 22 non-respiratory NTS cells which were depolarized following activation of the ipsilateral carotid body chemoreceptors (by close arterial injection of < 100 microliters CO2 saturated bicarbonate). Activation of the ipsilateral carotid body chemoreceptors evoked depolarizations with amplitudes of 2.9-4.6 mV and durations of 2.1-5.9 s. Three of these cells also received a convergent excitatory input from the carotid sinus baroreceptors. Carotid sinus nerve (CSN) stimulation evoked either an excitatory post-synaptic potential (EPSPs) (n = 14, 8 monosynaptic) or an excitatory/inhibitory sequence (EPSP/IPSPs) (n = 8, 1 monosynaptic). CSN evoked PSPs were separately averaged (25-50 sweeps) during periods of phrenic nerve activity and phrenic nerve silence and during periods when the lungs were inflated and when the lungs were deflated. No parameter of the CSN evoked PSPs (latency, peak amplitude, duration) was altered during periods of phrenic nerve activity or lung inflation (all P values > 0.12, Wilcoxon signed-rank test). The results suggest that there is no respiratory modulation of arterial chemoreceptor inputs by either central respiratory drive or lung stretch receptor afferent inputs at this early stage of the reflex arc.

Carotid chemoreceptor reflex cardioinhibitory responses: comparison of their modulation by central inspiratory neuronal activity and activity of pulmonary stretch afferents

The differential nature of the modulation of excitatory inputs from various cardiovascular receptor groups by pulmonary stretch afferents, driven by lung inflation, and the central inspiratory neuronal activity, has been demonstrated. At levels of activity of pulmonary stretch afferents and of central inspiratory drive that almost completely suppress the bradycardia of stimulation of the carotid bodies, the response to stimulation of the arterial baroreceptors and cardiac C fibre endings was reduced by 30-58%. In contrast, the bradycardia evoked by pulmonary C fibre stimulation was not significantly affected by either respiratory mechanism.