Effect of hypercapnic blood injection into the vertebral artery on the phrenic nerve activity in cats

Hypercapnic blood was injected into the vertebral artery in anesthetized and paralyzed cats. The stimulating effect on the phrenic nerve activity was dependent on the injection rate, duration, and PCO2 level of the injected blood. The time delay from the start of injection to the onset of increase in phrenic nerve activity was inversely proportional to both the injection rate and the PCO2 of injected blood.     

Central chemoreceptors and sympathetic vasomotor outflow

The present study explores how elevations in brain   P _CO2  increase the sympathetic nerve discharge (SND). SND, phrenic nerve discharge (PND) and putative sympathoexcitatory vasomotor neurons of the rostral ventrolateral medulla (RVLM) were recorded in anaesthetized sino-aortic denervated and vagotomized rats. Hypercapnia (end-expiratory CO₂ from 5% to 10%) increased SND (97 ± 6%) and the activity of RVLM neurons (67 ± 4%). Injection of kynurenic acid (Kyn, ionotropic glutamate receptor antagonist) into RVLM or the retrotrapezoid nucleus (RTN) eliminated or reduced PND, respectively, but did not change the effect of CO₂ on SND. Bilateral injection of Kyn or muscimol into the rostral ventral respiratory group (rVRG-pre-Bötzinger region, also called CVLM) eliminated PND while increasing the stimulatory effect of CO₂ on SND. Muscimol injection into commissural part of the solitary tract nucleus (commNTS) had no effect on PND or SND activation by CO₂. As expected, injection of Kyn into RVLM or muscimol into commNTS virtually blocked the effect of carotid body stimulation on SND in rats with intact carotid sinus nerves. In conclusion, CO₂ increases SND by activating RVLM sympathoexcitatory neurons. The relevant central chemoreceptors are probably located within or close to RVLM and not in the NTS or in the rVRG-pre-Bötzinger/CVLM region. RVLM sympathoexcitatory neurons may be intrinsically pH-sensitive and/or receive excitatory synaptic inputs from RTN chemoreceptors. Activation of the central respiratory network reduces the overall sympathetic response to CO₂, presumably by activating barosensitive CVLM neurons and inhibiting RTN chemoreceptors.     

Responses of simultaneously recorded respiratory-related medullary neurons to stimulation of multiple sensory modalities.

This study addresses the hypothesis that multiple afferent systems share elements of a distributed brain stem network that modulates the respiratory motor pattern. Data were collected from 18 decerebrate, bilaterally vagotomized, paralyzed, artificially ventilated cats. Up to 28 neurons distributed in the rostral and caudal ventral respiratory group, nucleus tractus solitarius, and raphe obscurus were recorded simultaneously with microelectrode arrays. Phases of the respiratory cycle and inspiratory drive were assessed from integrated efferent phrenic nerve activity. Carotid chemoreceptors were stimulated by injection of CO2-saturated saline solution via the external carotid artery. Baroreceptors were stimulated by increased blood pressure secondary to inflation of an embolectomy catheter in the descending aorta. Cutaneous nociceptors were stimulated by pinching a footpad. Four hundred seventy-four neurons were tested for respiratory modulated firing rates and responses; 403 neurons were tested with stimulation of all 3 modalities. Chemoreceptor stimulation and pinch, perturbations that tend to increase respiratory drive, caused similar responses in 52 neurons; 28 responded oppositely. Chemoreceptor and baroreceptor stimulation resulted in similar primary responses in 45 neurons; 48 responded oppositely. Similar responses to baroreceptor stimulation and pinch were recorded for 38 neurons; opposite effects were measured in 26 neurons. Among simultaneously recorded neurons, distinct combinations of firing rate changes were evoked in response to stimulation of the different modalities. The results show a functional convergence of information from carotid chemoreceptors, baroreceptors, and cutaneous nociceptors on respiratory-modulated neurons distributed in the medulla. The data are consistent with the hypothesis that brain stem neurons have overlapping memberships in multifunctional groups that influence the respiratory motor pattern.     

A vasodepressor effect of pentobarbitone sodium

1. In anaesthetized cats under artificial ventilation, a few milligrams of pentobarbitone sodium injected into the cerebral ventricles produced a pronounced fall in arterial blood pressure, which was central in origin and resulted from inhibition of vasomotor tone.2. Pentobarbitone sodium was more effective in lowering blood pressure when injected into the cerebral ventricles than when injected into the cisterna magna, yet the pentobarbitone sodium did not act on structures in the ventricular wall, but acted on structures reached from the subarachnoid space.3. To produce its vasodepressor effect, the pentobarbitone sodium had to pass through the foramina of Luschka into the subarachmoid space beneath the medulla oblongata and to penetrate its ventral surface in a region caudal to the trapezoid bodies and lateral to the pyramids. This was the outcome of experiments in which the pentobarbitone sodium was injected into or perfused through the cerebral ventricles with or without an outflow cannula inserted into the aqueduct or into the fourth ventricle, and of experiments in which pentobarbitone sodium solutions were applied by means of Perspex rings to this region of the exposed ventral surface of the medulla. Whereas the application of pentobarbitone sodium to this region on one side had a weak vasodepressor effect only, its application on both sides produced a pronounced fall in arterial blood pressure.4. The region where pentobarbitone acted on topical application covers the region where nerve cells are found in the marginal glia immediately under the pia mater. The possibility is discussed that these cells are the morphological substrate on which the pentobarbitone acts, that arterial blood pressure is maintained by their activity which is suppressed by the pentobarbitone sodium.     

The role of the solitary and paramedian reticular nuclei in mediating cardiovascular reflex responses from carotid baro- and chemoreceptors

1. With dye-filled micro-electrodes single neurones in the medulla of anaesthetized paralysed cats were identified which: (a) fired rhythmically in synchrony with or were modulated by the cardiac cycle, and which ceased firing with occlusion of the ipsilateral common carotid artery (carotid sinus baroreceptor neurones); (b) were excited by stimulation of carotid body chemoreceptors by close intra-arterial injection of lobeline into the thyroid artery (carotid body chemoreceptor neurones).2. Twelve carotid baroreceptor neurones were identified, in thirty-three cats, nine of which were localized in the intermediate area of the nucleus of the solitary tract (NTS) within 1 mm ahead of or behind the obex; three units were located either in the parahypoglossal area or the dorsal portion of the paramedian reticular nucleus (PRN).3. Of the twenty-one carotid chemoreceptor neurones which were identified, thirteen were localized in the NTS, three in the parahypoglossal area and four in the dorsal PRN.4. Bilateral lesions of the paramedian reticular area of medulla destroying the PRN, abolished or reversed the depressor response to electrical stimulation of myelinated fibres of the carotid sinus nerve (CSN), attenuated the depressor response to carotid sinus stretch and augmented the pressor response to chemoreceptor stimulation by lobeline. Such lesions did not significantly alter the reflex heart rate responses.5. Small lesions of the NTS within an area 1 mm rostral to the obex abolished all reflex blood pressure and heart rate responses to electrical stimulation of the CSN or natural stimulation of carotid baro- or chemoreceptors.6. Baroreceptors and chemoreceptors of the CSN project both to the intermediate zone of the NTS and to more medial areas of the medulla, particularly the dorsal PRN and parahypoglossal area.7. The PRN serves to mediate the reflex depressor, but not cardio-vagal, response from myelinated baroreceptors and buffers the pressor responses from chemoreceptors; it may serve as an important area integrating cardiovascular activity descending from forebrain, brain stem and cerebellum with baroreceptor reflexes.8. Cardiovascular reflex responses arising from non-myelinated baroreceptors and all chemoreceptors are mediated by neurones in the intermediate area of the NTS.     

Location of neurones with cardiovascular and respiratory function, at the ventral surface of the cat's medulla

A study has been made of the ventral surface of the medulla, to identify neurones with cardiovascular and respiratory functions. Experiments were performed on chloralose-anaesthetized, artificially ventilated cats. Ventral medullary neurones were stimulated by microinjections of excitant amino acid (which selectively activates cell bodies), and responses measured in blood pressure, heart rate, renal sympathetic and phrenic nerve activity. A small region of ventral medulla was found, corresponding to the "glycine-sensitive area", from which large increases in blood pressure and renal nerve activity were evoked by amino acid injections. More caudally, another cell group was localized lateral to the hypoglossal nerve roots, and these neurones depressed blood pressure and renal nerve activity. Two distinct regions were found to increase phrenic nerve activity: rostral to the pressor neurones, encroaching on the trapezoid body (roughly corresponding to area "M"), and a caudal group, close to the depressor neurones (i.e. lateral to the hypoglossal roots). No respiratory response could be evoked from medial to the hypoglossal roots (area "L") and stimulation of neurones in area "S" generally depressed phrenic activity. Neurones with cardiovascular and respiratory actions could be distinguished anatomically. Their locations have been mapped and compared with previous studies.     

Transient and steady state responses of pulmonary ventilation to the medullary extracellular pH after approximately rectangular changes in alveolarP_{CO_2 }

The extracellular pH (pHe) either on the ventral surface of the medulla oblongata or the parietal cortex, the tidal volume, the expiratory PCO2 and the arterial blood pressure were continuously recorded in anaesthetized or unanaesthetized decerebrate cats. The concentration of the inspired CO2 was manipulated in order to obtain a nearly rectangular increase in the end-tidal PCO2. The responses of VT.f, VE and pH to such a change in PCO2 were observed. The observations from such a preparation were: 1. pHe responded with a delay of 5-7 s to a rectangular variation in end-tidal PCO2. 2. The time constant of the change in the medullary extracellular pH was in the range of 50 s and a similar value was found for VT and VE. 3. The response of VT or VE to a change in the medullary pHe was approximately linear in anaesthetized as well as in unanaesthetized decerebrate cats. The slope of the respiratory response of VT to pHe in decerebrate cats was about 3 times greater than that in anaesthetized cats. There were only slight differences in the relation of VT to pHe between the transient and steady state responses. This means that the "upstroke" of the on-transient of VT was approximately the same as the 'downstroke' of the off-transient. On the other hand, a slight delay was observed when VE was plotted against pHe. Pronounced delay occurred when respiratory frequency was plotted against pHe for on- and off-CO2 transients. 4. A marked hyteresis was observed when VT or VE was plotted against the cortical pHe for on- and off-CO2 inhalation. 5. Such a precise time correlation of the medullary surface pH and VT changes could only be possible if the pH on the ventral medullary surface is representative for the pH at the sensor.     

猫延髓吻侧腹外侧区与孤束核的纤维联系—WGA—HRP法研究

将WGA-HRP经背路或腹路注入成年家猫一侧延髓吻侧腹外区(RVL),在双侧孤束核(NTS)观察到逆行标记的细胞.标记细胞主要位于NTS尾侧份的内侧亚核、外侧亚核和腹外侧亚核。并见RVL注射区有顺行标记纤维投射至双侧NTS及对侧RVL。标记纤维在NTS吻侧半最为明显。     

An integrated model of the human ventilatory control system: the response to hypercapnia.

This work presents a mathematical model of the human respiratory control system, based on physiological knowledge. It includes three compartments for gas storage and exchange (lungs, brain tissue and other body tissues), and various kinds of feedback mechanisms. These comprehend peripheral chemoreceptors in the carotid body, central chemoreceptors in the medulla and a central ventilatory depression. The latter acts by reducing the response of the central neural system to the afferent peripheral chemoreceptor activity during prolonged hypoxia of the brain tissue. Furthermore, the model considers local blood flow adjustments in response to O2 and CO2 arterial pressure changes. In this study, the model has been validated by simulating the response to square changes in alveolar PCO2, performed at different constant levels of alveolar PO2. A good agreement with data reported in the literature has been checked. Subsequently, a sensitivity analysis on the role of the main feedback mechanisms on ventilation response to CO2 has been performed. The results suggest that the ventilatory response to CO2 challenges during hyperoxia can be almost completely ascribed to the central chemoreflex, while, during normoxia, the peripheral chemoreceptors provide a modest contribution too. By contrast, the response to hypercapnic stimuli during hypoxia involves a complex superimposition among different factors with disparate dynamics. Hence, results suggest that the ventilatory response to hypercapnia during hypoxia is more complex than that provided by simple empirical models, and that discrimination between the central and peripheral components based on time constants may be misleading.     

Effect of bilateral destruction of the subretrofacial area on central inspiratory activity of the respiratory center and on the respiratory response to hypercapnia

Local bilateral destruction with 1 M glutamic acid of neuronal structures located between ventral surface of the medulla oblongata and the retrofacial nucleus (subretrofacial area) in rats increases the amplitude of pulses from the phrenic nerve and lowers respiratory rate. Hypercapnia does not affect the amplitude of phrenic nerve pulses but increases respiratory rate. It is suggested that the amplitude of central respiratory activity is regulated by the central chemoreceptors with participation of neuronal structures located in the subretrofacial area. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 123, No. 5, pp. 491–493, May, 1997     

Comparison of the hyperglycaemic effect of adrenaline and morphine introduced into the liquor space.

1. In unanaesthetized cats a comparison is made of the hyperglycaemic effects of adrenaline and morphine, when injected or infused through chronically implanted cannulae, into different regions of the cerebral ventricles or of the subarachnoid space, in order to determine their sites of action. 2. On injection into the cerebral ventricles both adrenaline and morphine have to reach the subarachnoid space beneath the ventral surface of the brain stem before they can exert their hyperglycaemic effect. The adrenaline has to reach the region rostral to the pons, i.e. the fossa interpeduncularis, and the morphine the region caudal to the trapezoid bodies. These conclusions are based on the following findings. 3. When adrenaline (55 mug) and morphine (0-75mg) were infused into one or other of these two regions, adrenaline produced strong hyperglycaemia on infusion into the fossa interpeduncularis, but had scarcely any hyperglycaemic effect on infusion into the region caudal to the trapezoid bodies. The reverse result was obtained with morphine. 4. It is concluded that the adrenaline hyperglycaemia is mainly a peripheral effect. It occurs after the adrenaline has been absorbed into the blood stream from the fossa interpeduncularis but an additional central component, an action on brain stem structures reached from the fossa interpeduncularis, cannot be excluded. The morphine hyperglycaemia is a central effect due to an action on superficial structures of the ventral surface of the medulla oblongata, caudal to the trapezoid bodies.     

Cardiovascular effects elicited from the ventral surface of medulla oblongata in the cat

Cardiovascular adjustments induced by topical application of drugs on a restricted area on the ventral surface of the medulla oblongata, corresponding to the caudal part of the rostral chemoreceptor area and the intermediate area, have been studied in chloralose-anesthetized cats. Topical application of GABA or glycine on these structures resulted in blood pressure fall, bradycardia, vasodilatation in the kidney and the skeletal muscles and also depression of respiration. Similar responses except for a slight tachycardia occurred with application of physostigmine. Application of GABA resulted in a marked attenuation of the reflex vasoconstrictor responses to removal of arterial baroreceptor restraint (carotid occlusion), particularly in the kidney, and to disappearance of the reflex renal vasodilatation to baroreceptor stimulation. The findings suggest that GABA application leads to a general diminution of the tonic vasomotor neuron activity, and with regard to renal vasomotor neurons a virtual cessation. Atropine methylnitrate application induced blood pressure rise, increased peripheral resistance in both skeletal muscle and kidney and a strongly potentiated renal vasoconstrictor response to carotid occlusion. The results indicate that the studied superficial medullary structures play an important role for the maintenance of tonic vasomotor neuron activity, especially renal.Deceased January 1980     

Respiratory Response to Microinjections of GABA and Penicillin into Various Parts of the Ventral Respiratory Group

Experiments on rats showed that local injection of GABA (10^–4 M) into the rostral and caudal compartments of the ventral respiratory groups decreased the respiratory rhythm, but increased lung ventilation (especially injection into the rostral part). Penicillin (10^–7 M) injected into the rostral division increased the tidal volume and practically did not change the respiratory rate, but its injection into the caudal part reduced the tidal volume and increased respiratory rate. These results indicate that GABAergic mechanisms including GABAА sites play an ambiguous role in the regulation of respiration at the level of the rostral and caudal parts of the ventral respiratory group.     

Excitatory and depressant respiratory responses to chemical stimulation of the rostral ventrolateral medulla in the cat

The rostral ventrolateral medulla (rVLM) is known to play an important role in cardiorespiratory control. In the rVLM an ‘apnoea region’, in which unilateral focal blocks induce strong depressant effects on inspiratory activity up to complete apnoea, has been described. This study was designed to systematically investigate the effects provoked by unilateral micro-injections (10–30 nl) of d,l -homocysteic acid 160 mm into this region on respiratory activity and arterial blood pressure in pentobarbitone anaesthetized, vagotomized, paralyzed and artificially ventilated cats. Micro-injections into the rostral portion of this area caused depressant respiratory responses up to complete apnoea, while micro-injections into more caudally located sites induced excitatory respiratory responses. Similar effects were observed in the activity of phrenic nerves and inspiration-related medullary neurons of both the dorsal and ventral respiratory group. The respiratory responses could be accompanied by marked increases in blood pressure (≥30 mmHg), especially at locations ventral to the retrofacial and facial nucleus; however, they could also occur in the absence of appreciable changes or even in association with slight decreases in blood pressure. Similar respiratory and pressor effects were observed in carotid sinus denervated cats. The results indicate that two distinct rVLM neuronal populations, one located more rostrally and the other more caudally, may have an important role in the genesis and/or maintenance of respiratory rhythm by exerting respectively inhibitory and excitatory influences on inspiratory activity. Furthermore, they support the hypothesis that different neural substrates of the rVLM are involved in the regulation of respiratory and cardiovascular functions.     

Chemoreceptor and pulmonary stretch receptor interactions within amphibian respiratory control systems

The hypercapnic drive to breathe in amphibians is generally greater than hypoxic ventilatory drive and a variety of interdependent control systems function to regulate both the hypoxic and hypercapnic ventilatory responses. During exposure to hypercapnic conditions, breathing increases in response to input from central chemoreceptors (sensitive to CSF pH/CO(2) levels) and peripheral chemoreceptors (sensitive to arterial blood O(2) and CO(2)). On the other hand, olfactory CO(2) receptors in the nasal epithelium inhibit breathing during exposure to acute hypercapnia. Further complexity arises from the CO(2)-sensitive nature of the pulmonary stretch receptors (PSR) which provide both tonic (stimulates lung inflation at low lung volumes; deflation at higher volumes) and phasic (generally excitatory) feedback. This review focuses on interactions between the various populations of chemoreceptors and interactions between chemoreceptors and PSR. Differences between various levels of experimental reduction (i.e., in vitro; in situ; in vivo) are highlighted as are the effects of chronic respiratory challenges on acute hypoxic and hypercapnic chemoreflexes.     

CO2, brainstem chemoreceptors and breathing.

The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.     

Two different types of apnea induced by focal cold block of ventral medulla in rabbits

The respiratory effects of unilateral focal cold block of ventral medulla were examined in rabbits. Focal cooling of the paraolivar region in the caudal area of ventral medulla induced prolongation of the expiratory time, with little change in the tidal inspiratory activity amplitude, leading to 'temporal apnea'. Whereas focal cooling of the region ventral to the retrofacial nucleus in the rostral part of the ventral medulla induced depression of tidal phrenic nerve activity amplitude, leading to 'suppression apnea'. Thus, two different types and regions of apnea were demonstrated in rabbits.     

Effects of lesions in the parabrachial nucleus on the mechanisms for central and reflex termination of inspiration in the cat.

The effects of PCO2 and body temperature on the time course and peak amplitude of the central inspiratory activity (CIA) and the inspiratory "off-switch" threshold was studied in apneustic and non-apneustic cats. Apneusis resulted from lesions of the inspiratory inhibiting structures of the medial parabrachial nucleus (NPBM) and by interrupting vagal volume feedback. The cats were paralyzed and ventilated either proportionally to their phrenic output or at predetermined rate and volume. The dependence of the rate of rise and maximal amplitude of phrenic activity on PCO2 and body temperature were comparable in apneustic and non-lesioned animals. The Hering-Breuer volume threshold for inspiratory termination was increased following the rostral pontine lesions. Both hyperthermia and hypercapnia caused augmentation of the absolute rate of rise of inspiratory activity but hypercapnia, in contrast to hyperthermia, caused virtually no change in the fractional increment per unit time. With hypercapnia the inspiratory "off-switch" threshold was raised in the apneustic animals in intact ones, whereas hyperthermia did not seem to influence this threshold. In apneustic conditions expiratory duration remained constant, independent of the large variations in the inspiratory durations. Our results suggest that the NPBM merely provides an excitatory, threshold-lowering input to the inspiratory "off-switch" mechanism.     

The effect on respiration of abrupt changes in carotid artery pH and PCO2 in the cat

1. An in vivo pH monitoring technique was used to assess changes in pH, and by inference changes in P(CO2), in the carotid artery of anaesthetized cats. The changes in carotid artery pH and respiration following abrupt injections of various acids into the carotid artery or aorta were investigated.2. Injections of saline equilibrated with 100% CO(2), timed to produce changes at the carotid body chemoreceptors during early inspiration caused an increase in the tidal volume of that breath. The amplitude and rate of change of the pH changes so produced were comparable with those of the oscillations in pH produced by respiration itself.3. The respiratory responses to injection of saline equilibrated with 100% CO(2) occurred whether the animal was breathing air or 100% O(2).4. Injections of lactic or hydrochloric acid were without an effect on respiration, except when pH changes larger than 0.1 pH unit were produced. A NaHCO(3) solution equilibrated with 30% CO(2) stimulated respiration, even though the solution was alkaline to the cat's arterial blood and induced an alkaline change in arterial pH.5. Infiltration of the carotid sinus nerve area with procaine temporarily abolished the respiratory response to injections of saline equilibrated with 100% CO(2).     

Prenatal development of respiratory chemoreceptors in endothermic vertebrates

Respiratory chemoreceptors are neurons that detect PCO(2), PO(2), and/or pH in body fluids and provide sensory feedback for the control of breathing. They play a critical role in coupling pulmonary ventilation to metabolic demand in endothermic vertebrates. During birth in mammals and hatching in birds, the state change from placental or chorioallantoic gas exchange to pulmonary respiration makes acute demands on the neonatal lungs and ventilatory control system, including the respiratory chemoreceptors. Here we review the literature on prenatal development of carotid body chemoreceptors, central chemoreceptors, and airway chemoreceptors, with emphasis on the histology, histochemistry, and neurophysiology of chemosensory cells or their afferents, and their physiological genomics if known. In general, respiratory chemoreceptors develop prenatally and are functional but immature at birth or hatching. Each type of respiratory chemoreceptor has a unique prenatal developmental time course, and all studied to date require a period of postnatal maturation to express the full adult response.