Atrial Injections Of Phenylbiguanide Research Articles

Contribution of central μ-receptors to switching pulmonary C-fibers-mediated rapid shallow breathing into an apnea by fentanyl in anesthetized rats

Our previous study has shown that activating peripheral μ-receptors is necessary for switching the bronchopulmonary C-fibers (PCFs)-mediated rapid shallow breathing (RSB) into an apnea by systemic administration of fentanyl. The brainstem nuclei, such as the medial nucleus tractus solitarius (mNTS) and the pre-Botzinger complex (PBC), are required for completing the PCF-mediated respiratory reflexes. Moreover, these areas contain abundant μ-receptors and their activation prolongs expiratory duration (TE). Thus, we asked if central μ-receptors, especially those in the mNTS and PBC, are involved in fully expressing this RSB-apnea switch by fentanyl. In anesthetized rats, the cardiorespiratory responses to right atrial injection of phenylbiguanide (PBG, 3–6μg/kg) were repeated after: (1) fentanyl (iv), a μ-receptor agonist, alone (8μg/kg, iv); (2) fentanyl following microinjection of naloxone methiodide (NXM, an opioid receptor antagonist) into the cisterna magna (10μg/4μl); (3) the bilateral mNTS (10mM, 20nl); or (4) PBC (10mM, 20nl). Our results showed that PBG shortened TE by 37±6% (RSB, from 0.41±0.05 to 0.26±0.03s, P<0.01), but it markedly prolonged TE by 5.8-fold (an apnea, from 0.50±0.04s to 2.9±0.57s, P<0.01) after fentanyl (iv). Pretreatment with NXM injected into the cisterna magna or the PBC, but not the mNTS, prevented the fentanyl-induced switch. This study, along with our previous results mentioned above, suggests that although peripheral μ-receptors are essential for triggering the fentanyl-induced switch, central μ-receptors, especially those in the PBC, are required to fully exhibit such switch.Summary statement: Our results suggest that the activation of central μ-receptors, especially those in the pre-Botzinger complex, is required for switching the pulmonary C-fiber-mediated rapid shallow breathing into an apnea by systemic administration of fentanyl.

Mu‐Opioid Receptors (MORs) in Pre‐Botzinger Complex (PBC) Are Involved in Fentanyl Switching of Pulmonary C‐Fibers (PCFs)‐Mediated Rapid Shallow Breathing (RSB) into An Apnea in Anesthetized Rats

Our previous study showed that fentanyl (iv), a selective MOR agonist, could switch the PCFs‐mediated RSB into an apnea and that peripheral MORs are necessary for triggering this switch. Here we further tested if central MORs, especially those in the medial nucleus tractus solitarius (mNTS) receiving PCFs’ inputs and the PBC responsible for respiratory rhythmogenesis, were also involved in such switching. Right atrial injection of phenylbiguanide (3–5 μg/kg) was performed to stimulate PCFs in anesthetized rats before and after fentanyl (8 μg/kg, iv) without or with prior central administration of naloxone methiodide (M‐NLX, an opioid receptor antagonist). The central administration was achieved by injecting M‐NLX into the cisterna magna (8 μg/4μl), the mNTS or the PBC (10 mM, 15 nl in each side). Our results showed that phenylbiguanide alone shortened expiratory duration (TE) by 32 ± 6 % (RSB), while it markedly prolonged TE by 5.5‐fold (an apnea) after fentanyl. M‐NLX injected into the cisterna magna or the PBC, but not the mNTS, prevented the fentanyl switching with the TE of RSB slightly but significantly prolonged. This study, along with our previous results, suggests that although peripheral MORs are critical to trigger the fentanyl switching of PCF‐mediated RSB into an apnea, central MORs, especially those in the PBC, are necessary to establish this switching (Supported by ALA RG‐191095‐N and HL 107462).

Modulatory Effect of Fentanyl on Bronchopulmonary C‐fiber (PCF)‐mediated Ventilatory (VE) Responses Is Age‐dependent in Anesthetized Rats

In adult rats, opioids administered into the pulmonary circulation produces an apnea and given systemically turns the phenylbiguanide (PBG)‐evoked rapid shallow breaths (RSBs) to an apnea by stimulating and facilitating PCFs, respectively. Fentanyl, an opioid μ‐receptor agonist, clinically has a lesser inhibitory effect on ventilation in children (&lt; 3 years old) than in adults (Barker, Pediatric Anesthesia 2007). Thus, we tested if the modulatory effect of opioids on the PCF‐mediated VE responses in rats were age‐dependent. VE responses to a right atrial bolus injection of PBG (before and after systemic administration of fentanyl) or fentanyl were recorded in anesthetized rat pups (3 week‐old) and adults. We found that: 1) PBG (5 μg/kg) evoked 5–10 RSBs in adults, but caused no change or 2–3 rapid breaths in pups; 2) systemic fentanyl (6 μg/kg, iv) reduced VE more greatly in adults (60%) than pups (27%, P &lt; 0.01) and switched the PBG‐induced RSB to an apnea in adults, but failed to alter the PBG effects on VE in pups; 3) right atrial injection of fentanyl (2 μg/kg) alone produced an apnea in adults but not in pups; and 4) the VE responses to right atrial injection of PBG or fentanyl were eliminated by bilateral vagotomy. Our data suggest that the PCF‐mediated RSBs and inspiratory inhibition in response to PBG and fentanyl are age‐dependent in rats, indicating a plasticity of PCFs as development. (ALA RT‐83131‐N).

Cough reflex responses during pulmonary C-fibre receptor activation in anesthetized rabbits

We investigated the changes induced by pulmonary C-fibre receptor activation in the cough reflex evoked by mechanical stimulation of the tracheobronchial tree in pentobarbitone anesthetized, spontaneously breathing rabbits. Phrenic nerve and abdominal muscle activities were monitored along with tracheal and arterial blood pressures. The activation of pulmonary C-fibre receptors by means of right atrial injection of phenylbiguanide (PBG) caused the pulmonary chemoreflex characterized by tachypnea, bradycardia and hypotension. During the pulmonary chemoreflex, the time components (total cycle duration, inspiratory and expiratory times) of the cough motor pattern significantly decreased, whereas no consistent changes in peak phrenic and abdominal activity, peak tracheal pressure and number of coughs evoked by each stimulation trial were observed. At variance with previous findings in cats and dogs, the results show that tracheobronchial cough is not significantly reduced in the rabbit during PBG-induced chemoreflex. This study is the first to provide evidence supporting the hypothesis that the time components of the cough motor pattern are, to some extent, dependent upon the timing characteristics of the ongoing respiratory activity and suggests a novel mechanism leading to cough depression.

Cardiac vagal preganglionic neurones in the intermediate zone of the brainstem in anaesthetized cats

Cardiac vagal preganglionic neurones (CVPNs) show respiratory modulation in the nucleus ambiguus but not in the dorsal vagal nucleus. Both types of neurones can be activated by pulmonary C fibre afferents. Another brainstem area that has been identified as containing CVPNs is the intermediate zone between the dorsal vagal nucleus and nucleus ambiguus. Experiments were carried out in alpha-chloralose-anaesthetized cats to determine the physiological properties of these CVPNs and their responses to pulmonary C fibre afferent activation. Seven CVPN axons in the right cardiac vagal branches were identified and found to be localized in the intermediate zone with a conduction velocity of between 1.2 and 1.6 m s(-1), in the C fibre range. These seven CVPNs [either showing spontaneous activity (n = 1) or having activity induced by dl-homocysteic acid applied ionophoretically (n = 3)] were neither respiratory modulated nor did they receive a baroreceptor input, thus being similar to those found in the dorsal vagal nucleus. Right atrial injections of phenylbiguanide excited all four CVPNs tested. In conclusion, CVPNs located in the intermediate zone have similar properties to those in the dorsal vagal nucleus but not the nucleus ambiguus.

The role of central 5‐HT3 receptors in vagal reflex inputs to neurones in the nucleus tractus solitarius of anaesthetized rats

Brainstem 5-hydroxytryptamine (5-HT, serotonin)-containing neurones modulate cardiovascular reflex responses but the differing roles of the many 5-HT receptors have not been thoroughly investigated. The present experiments on anaesthetized rats investigated the role of 5-HT3 receptors in modulating vagal afferent evoked activity of nucleus tractus solitarius (NTS) neurones. Recordings were made from 301 NTS neurones receiving an input at long (> 20 ms) minimum onset latency from stimulation of the vagus nerve. These included 140 neurones excited by activating non-myelinated cardiopulmonary afferents by right atrial injection of phenylbiguanide (PBG). Ionophoretic application of PBG, a highly selective 5-HT3 receptor agonist, significantly increased activity (from 2.4 +/- 0.4 to 5.5 +/- 0.8 spikes s(-1)) in 96 of 106 neurones tested and in all 17 neurones tested the increase in activity (3.4 +/- 1.1 to 7.0 +/- 1.9 spikes s(-1)) was significantly attenuated (3.0 +/- 0.9 to 3.8 +/- 1.1 spikes s(-1)) by the selective 5-HT3 receptor antagonist granisetron. Ionophoretic application of PBG potentiated responses to vagus nerve and cardiopulmonary afferent stimulation, and granisetron significantly attenuated this cardiopulmonary input (20.2 +/- 5.7 to 10.6 +/- 4.1 spikes burst(-1)) in 9 of 10 neurones tested. Ionophoretic application of AMPA and NMDA also excited NTS neurones and these excitations could be selectively antagonized by the non-NMDA and NMDA receptor antagonists DNQX and AP-5, respectively. At these selective currents, DNQX and AP-5 also attenuated PBG- and cardiopulmonary input-evoked increases in NTS activity. These data are consistent with the hypothesis that vagal inputs, including non-myelinated cardiopulmonary inputs to the NTS, utilize a 5-HT-containing pathway which activates 5-HT3 receptors. This excitatory response to 5-HT3 receptor activation may be partly a direct postsynaptic action but part may also be due to facilitation of the release of glutamate which in turn acts on either non-NMDA or NMDA receptors to evoke excitation.

Alveolar hypercapnia augments pulmonary C-fiber responses to chemical stimulants: role of hydrogen ion.

To determine whether the excitabilities of pulmonary C fibers to chemical and mechanical stimuli are altered by CO(2)-induced acidosis, single-unit pulmonary C-fiber activity was recorded in anesthetized, open-chest rats. Transient alveolar hypercapnia (HPC) was induced by administering CO(2)-enriched gas mixture (15% CO(2), balance air) via the respirator inlet for 30 s, which rapidly lowered the arterial blood pH from a baseline of 7.40 +/- 0.01 to 7.17 +/- 0.02. Alveolar HPC markedly increased the responses of these C-fiber afferents to several chemical stimulants. For example, the C-fiber response to right atrial injection of the same dose of capsaicin (0.25-1.0 microg/kg) was significantly increased from 3.07 +/- 0.70 impulses/s at control to 8.48 +/- 1.52 impulses/s during HPC (n = 27; P < 0.05), and this enhanced response returned to control within approximately 10 min after termination of HPC. Similarly, alveolar HPC also induced significant increases in the C-fiber responses to right atrial injections of phenylbiguanide (4-8 microg/kg) and adenosine (0.2 mg/kg). In contrast, HPC did not change the response of pulmonary C fibers to lung inflation. Furthermore, the peak response of these C fibers to capsaicin during HPC was greatly attenuated when the HPC-induced acidosis was buffered by infusion of bicarbonate (1.36-1.82 mmol. kg(-1). min(-1) for 35 s). In conclusion, alveolar HPC augments the responses of these afferents to various chemical stimulants, and this potentiating effect of CO(2) is mediated through the action of hydrogen ions on the C-fiber sensory terminals.

A functional study of uncrossed and crossed pulmonary afferent fibres in the cervical vagus nerves of the cat

The functional distribution of uncrossed and crossed pulmonary afferent fibres in the cervical vagus nerves has been studied in the anaesthetized cat using acute and chronic unilateral pneumonectomized preparations. The heart and lungs were sympathectomized routinely. The vagal afferent pathways of three pulmonary reflexes were investigated: the Hering–Breuer respiratory reflex, the lung inflation cardio-accelerator reflex, and the pulmonary chemoreflex. Inflation of the remaining lung caused temporary inhibition of inspiration. It also resulted in acceleration of the heart, but only when the background cardiac vagal tone was augmented. These respiratory and cardiac responses were abolished in most animals by ipsilateral cervical vagotomy; however, in some, a small response persisted and this was abolished by contralateral vagotomy. Stimulation of pulmonary C-fibre endings with right atrial injections of phenylbiguanide caused a reduction in respiration, bradycardia and systemic hypotension, responses which occurred with a latency of 2.9±0.15 s. They were mostly abolished by ipsilateral cervical vagotomy, but reduced responses persisted in a few animals. The residual responses were abolished by contralateral cervical vagotomy and by selective denervation of the lung. These results indicate that most afferent fibres subserving the three pulmonary reflexes studied run in the ipsilateral cervical vagus, representing the uncrossed pathway. Some afferent fibres, however, cross to the contralateral cervical vagus. Degenerative changes in cells of the contralateral nodose ganglion in chronic unilateral pneumonectomized animals support these findings.

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.

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.

Comparison of cardiorespiratory reflexes in NK 1 receptor knockout, heterozygous and wild-type mice in vivo

Neurokinin-1 receptors (NK 1) are present within the nucleus of the solitary tract, a nucleus which plays a vital role in cardiovascular and respiratory homeostasis. We compared the efficacy of the baroreceptor and pulmonary chemoreflexes between NK 1 knockout, heterozygous and wild-type urethane-anaesthetised mice. The magnitude of the baroreceptor reflex mediated bradycardia, induced by a phenylephrine induced pressor response, was significantly greater in NK 1 knockout mice ( P<0.001) compared to heterozygous and wild-type animals. In comparison, administration of an NK 1 antagonist, CP-99,994 (1.5 mg/kg i.v.) to wild-type animals, had no significant effect on baroreceptor reflex performance. In contrast to the baroreceptor reflex, there were no significant differences in the magnitude of the reflex evoked falls in heart rate, arterial pressure, or respiratory depression between the three groups of mice when the pulmonary chemoreflex was evoked with right atrial injections of phenylbiguanide. It is concluded that the baroreceptor reflex pathway over-compensates for the lack of NK 1 receptors in knockout mice. Plausible mechanisms accounting for the enhanced baroreceptor reflex responsiveness in NK 1 knockout animals are discussed.

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.

K+ channel blockade in the NTS alters efficacy of two cardiorespiratory reflexes in vivo.

We investigated the role of potassium conductances in the nucleus of the solitary tract (NTS) in determining the efficacy of the baroreceptor and cardiopulmonary reflexes in anesthetized rats. The baroreceptor reflex was elicited with an intravenous injection of phenylephrine to evoke a reflex bradycardia, and the cardiopulmonary reflex was evoked with a right atrial injection of phenylbiguanide. Microinjection of two Ca-dependent potassium channel antagonists (apamin and charybdotoxin) into the NTS potentiated the baroreceptor reflex bradycardia. This may reflect the increased neuronal excitability observed previously in vitro with these blockers. In contrast, the Ca-dependent potassium channel antagonists attenuated the cardiopulmonary reflex, whereas voltage-dependent potassium channel antagonists (4-aminopyridine and dendrotoxin) attenuated both the baro- and cardiopulmonary reflexes when microinjected into the NTS. The possibility that the reflex attenuation observed indicates a predominant distribution of certain potassium channels on gamma-aminobutyric acid interneurons is discussed.

Effect of cardiopulmonary C fibre activation on the firing activity of ventral respiratory group neurones in the rat.

1. Cardiopulmonary C fibre receptor stimulation elicits apnoea and rapid shallow breathing, but the effects on the firing activity of central respiratory neurones are not well understood. This study examined the responses of ventral respiratory group neurones: decrementing expiratory (Edec), augmenting expiratory (Eaug), and inspiratory (I) neurones during cardiopulmonary C fibre receptor-evoked apnoea and rapid shallow breathing. 2. Extracellular neuronal activity, phrenic nerve activity and arterial pressure were recorded in urethane-anaesthetized rats. Cardiopulmonary C fibre receptors were stimulated by right atrial injections of phenylbiguanide. Neurones were tested for antidromic activation from the contra- and ipsilateral ventral respiratory group (VRG), spinal cord and cervical vagus nerve. 3. Edec neurones discharged tonically during cardiopulmonary C fibre-evoked apnoea and rapid shallow breathing, displaying increased burst durations, number of impulses per burst, and mean impulse frequencies. Edec neurones recovered either with the phrenic nerve activity (25 s) or much later (3 min). 4. By contrast, the firing activity of Eaug and most I neurones was decreased, featuring decreased burst durations and number of impulses per burst and increased interburst intervals. Eaug activity recovered in approximately 3 min and inspiratory activity in approximately 1 min. 5. The results indicate that cardiopulmonary C fibre receptor stimulation causes tonic firing of Edec neurones and decreases in Eaug and I neuronal activity coincident with apnoea or rapid shallow breathing.

Excitatory amino acid receptors in the nucleus tractus solitarius mediate the responses to the stimulation of cardio-pulmonary vagal afferent C fiber endings

Stimulation of cardio-pulmonary vagal afferent C fiber endings by right atrial injections of phenylbiguanide (PBG, 40 μg/kg) elicited apnea, bradycardia and hypotension. These responses were abolished by bilateral vagotomy low in the neck. Stimulation of neurons in a specific nucleus tractus solitarius site (0.5 mm rostral, 0.5 mm lateral and 0.5 mm deep with reference to the calamus scriptorius) by injections of L-glutamate produced responses similar to those following the stimulation of cardio-pulmonary vagal afferent C fiber endings by PBG. Inhibition of neurons in this NTS site by injections of muscimol abolished the responses to PBG. Specific blockade of NMDA receptors by microinjections of AP-7 (100 pmol) or non-NMDA receptors (KA and AMPA) by injections of DNQX (10–25 pmol) into this NTS site did not block the responses to PBG. Microinjections of kynurenic acid (1–4 nmol) into this NTS site blocked the responses to right atrial injections of PBG. These results indicate that: (1) blockade of either NMDA receptors or non-NMDA receptors (KA and AMPA) alone in the above-mentioned NTS site does not abolish the responses to the stimulation of cardio-pulmonary vagal afferent C fiber endings; (2) it is necessary to block NMDA as well as non-NMDA receptors in this NTS site for abolishing the responses to the stimulation of these nerve endings.

Pulmonary C-fibers elicit both apneusis and tachypnea in the rabbit

The effects of phenylbiguanide (PBG) on phrenic nerve and pulmonary C-fibers were studied in anesthetized spontaneously breathing rabbits with unilateral vagotomy. Right atrial injections of PBG at low (10 μg/kg) and high (100 μg/kg) dose resulted in a shallow tachypnea and apneusis followed by tachypnea, respectively, and these effects were blocked by procaine treatment of the vagus nerve. Also, the injection of PBG (100 μg/kg) still evoked the rapid shallow breathing preceded by apneusis in carotid chemoreceptor-denervated animals. Vigorous stimulation of pulmonary C-fibers by PBG (100 μg/kg) coincided with apneusis and the response was followed by a more modest increase in activity associated with tachypnea. Administration of PBG (10 μg/kg) into the right atrium caused an increase in pulmonary C-fiber activity associated with tachypnea. However, a small dose of PBG injected into the aortic circulation had no effect on the C-fiber activity but did inhibit respiration. These results suggest that the stimulation of pulmonary C-fibers via PBG injection can produce both inspiratory apnea and tachypnea.

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

1. Cats were anaesthetized with a mixture of chloralose and urethane, and were artifically ventilated. 2. An open pneumothorax was provided by two large-bore tubes which were sealed in the sixth intercostal space on each side. They were connected to a Fleisch pneumotachograph. Phasic changes in central inspiratory neuronal activity were measured quantitatively as changes in the volume of the pneumothorax during temporary interruption of artificial respiration, the volume of the lungs being held constant at their end-expiratory level. In this way the activity of slowly adapting pulmonary stretch receptors was maintained constant. 3. Reflex cardioinhibitory responses were elicited by stimulation of (a) the carotid body chemoreceptors by intracarotid injections of cyanide; (b) the arterial baroreflex by controlled elevations of the blood pressure; (c) cardiac receptors by left atrial injections of veratridine; and (d) pulmonary C fibres (including J receptors) by right atrial injections of phenylbiguanide. 4. The effects of central inspiratory neuronal activity on pulse interval were assessed by comparing the values observed during the inspiratory and expiratory phases of the respiratory cycle in the control state and during stimulation of each cardiovascular receptor group. 5. The carotid chemoreceptor-induced bradycardia measured during the expiratory phase of respiration was reduced during inspiration to a value of about 15% of control. The central inspiratory drive was less effective in altering the reflex responses from the arterial baroreceptors and cardiac receptors, the corresponding values being 42 and 51% respectively. 6. In contrast, the bradycardia evoked by pulmonary C fibre stimulation was not significantly affected by the central inspiratory drive. 7. The differential nature of the modulation by the central inspiratory drive occurred independently of the integrity of the sympathetic nerve supply to the heart indicating that the cardiac efferents involved were largely fibres in the vagus nerves. 8. The possible explanation of these results in terms of central mechanisms is discussed.

Differential modulation by pulmonary stretch afferents of some reflex cardioinhibitory responses in the cat.

1. Cats were anaesthetized with a mixture of chloralose and urethane, and were artificially ventilated, the thorax being opened via a medial sternotomy. 2. Various cardiovascular receptors were stimulated during reflex inhibition of the central inspiratory neurones produced by electrical stimulation of both superior laryngeal nerves simultaneously, while the afferent input from the lungs was held constant by temporarily interrupting artificial respiration, the lungs being held static in their expiratory position. 3. Reflex cardioinhibitory responses were elicited by stimulation of (a) the carotid body chemoreceptors by intracarotid injections of cyanide; (b) the arterial baroreflex by controlled elevations of the blood pressure; (c) the carotid sinus baroreceptors by raising the pressure in isolated perfused carotid sinus preparations; (d) cardiac receptors by left atrial injections of veratridine, and (e) pulmonary C fibres (including J receptors) by right atrial injections of phenyl biguanide. 4. The effects of single inflations of the lungs on these reflex cardioinhibitory responses were studied and compared with the effects of the various inputs alone. 5. Stepwise increases in lung volume, while having no consistent effect on arterial blood pressure, progressively diminished the arterial chemoreceptor-induced bradycardia to a value of about 7% of the control response without lung inflation. The pulmonary afferents were less effective on reflex responses from other inputs, the corresponding values being: arterial baroreflex, 66%; carotid baroreceptors, 66%, and cardiac receptors, 70%. These effects of lung inflation were abolished by selective denervation of the lungs. 6. In contrast, the size of the bradycardia evoked by pulmonary C fibre stimulation was, on average, unaffected by inflation of the lungs. In some tests the response was actually increased. 7. The differential modulation by lung inflation of these reflex cardioinhibitory responses were the same after upper thoracic sympathectomy indicating that pulmonary afferents and cardiac efferents involved fibres in the vagus nerves. 8. The possible central mechanisms responsible for the differential modulation by lung inflation of cardioinhibitory reflexes are discussed.