Chemoreceptor Afferents Research Articles

Contraction-sensitive skeletal muscle afferents inhibit arterial baroreceptor signalling in the nucleus of the solitary tract: role of intrinsic GABA interneurons

Arterial baroreceptor and skeletal muscle receptor afferents relay sensory information to the nucleus of the solitary tract (NTS) during exercise. Previous studies have suggested that skeletal muscle afferent input inhibits baroreflex function; however, detailed information on the role of muscle afferents and GABAergic mechanisms in the NTS is limited. Furthermore, identification of specific afferent modalities that activate GABAergic neurons in the NTS remains unknown. In the present study, we examined the neuroanatomical and physiological interactions between spinal dorsal horn cells that transmit contraction-sensitive input from skeletal muscle and GABAergic interneurons in the NTS. Biotinylated dextran amine (BDA, 10%, 25–100 nL) microinjection into dorsal horn of the cervical spinal cord was combined with glutamate decarboxylase (GAD) immunohistochemistry to visualize the nature of the relationship of BDA-labeled fibers in the NTS with GAD immunoreactivity (GAD-ir). BDA-labeled axons and terminal processes were localized in the medial, commissural, dorsomedial and dorsolateral subdivisions of the caudal NTS. Moreover, BDA-labeled fibers were observed in close proximity to GAD-ir structures throughout these regions of the NTS. The physiological interaction between skeletal muscle receptor and arterial baroreceptor afferents was investigated using an arterially perfused, decerebrate rat preparation. Activation of skeletal muscle afferents by electrically evoked twitch contraction of the forelimb attenuated baroreflex responsiveness (BR, calculated as the ratio of changes in heart rate to systemic pressure) from −1.5±0.3 bpm·mm Hg −1 to −0.1±0.1 bpm·mm Hg −1 (control versus contraction, P<0.05, n=15). However, forelimb contraction failed to inhibit the reflex bradycardia evoked by activation of peripheral chemoreceptor afferents, indicating a reflex-specific action. Bilateral microinjection of bicuculline methiodide (BIC, 10 μM, 40–60 nL) into the caudal NTS restored baroreflex responsiveness during contraction (−1.6±0.2 versus −0.1±0.1 versus −1.5±0.2 bpm·mmHg −1, control versus contraction versus contraction+BIC P<0.05, n=8). We conclude that activation of ascending spinal neurons from the cervical dorsal horn by contraction-sensitive skeletal muscle afferents selectively inhibits arterial baroreceptor signaling in the NTS via activation of a GABAergic mechanism.

Nasal Oxygen and Muscle Sympathetic Nerve Activity in Heart Failure

To evaluate the effects of mild hyperoxia on sympathetic activity during quiet breathing in patients with chronic heart failure (CHF) and, hence, to investigate whether tonic activation of excitatory chemoreceptor afferents contributes to the elevated sympathetic activity in these patients. Sympathetic activation in patients with CHF may result in part from increased chemoreflex sensitivity. Previous studies using microneurography did not demonstrate deactivation of the chemoreceptors while the patients were breathing 100% O(2). However, 100% O(2) may decrease cardiac output, thereby offsetting the effects on the chemoreflexes. University hospital. Ten patients with moderate-to-severe CHF (mean [+/-SD] age, 53.9 +/- 9.2 years; mean ejection fraction, 21.3 +/- 4.7%) were assigned to breathing 20 min of O(2) as well as room air (3 L/min) applied by nasal prongs. Muscle sympathetic nerve activity (MSNA) was evaluated by microneurography of the peroneal nerve. The application of O(2) resulted in an increase of arterial O(2) saturation but no significant change in MSNA during resting ventilation. Although voluntary apneas were no longer with O(2) (25.3 +/- 5.8 vs 32.6 +/- 8.6 s, respectively; p = 0.014), MSNA during the last 10 s of voluntary apnea was lower while breathing O(2) (63.5 +/- 15.0 vs 59.9 +/- 13.9 bursts per minute, respectively; p = 0.02). The increased MSNA in the patients studied could not be reduced by mild hyperoxia, suggesting that the tonic activation of chemoreflex afferents is unlikely to contribute to the elevated sympathetic activity. That nasal O(2) reduces MSNA during apnea may explain the beneficial effects of nocturnal O(2) therapy in CHF patients with Cheyne-Stokes respiration.

Modulation of reflexly evoked vagal bradycardias by central 5-HT1A receptors in anaesthetized rabbits.

1. The role of central 5-HT(1A) receptors in the control of the bradycardia and changes in central respiratory drive, renal nerve activity and blood pressure evoked by stimulating cardiopulmonary afferents with phenylbiguanide, baroreceptors by electrical stimulation of the aortic nerve and chemoreceptors by injections of sodium cyanide (NaCN) in atenolol-pretreated anaesthetized rabbits were studied. 2. Buspirone (100 micro g kg(-1); i.c.) potentiated the bradycardia (increase in R-R interval) and the changes in blood pressure and renal nerve activity evoked by all three reflexes. These effects could be attenuated by pretreatment with the 5-HT(1A) receptor antagonist WAY-100635 (100 micro g kg(-1)); i.v.), which alone had no effect on these reflex-evoked changes. However, WAY-100635 (100 micro g kg(-1); i.c.) did attenuate these reflex-evoked responses produced by activation of cardiopulmonary and aortic baroreceptors but not that caused by stimulation of chemoreceptors. When given i.v., buspirone was less effective in modulating the responses evoked by these three reflexes. 3. The present data are consistent with the view that central 5-HT(1A) receptors play a role in the reflex activation of cardiac preganglionic vagal motoneurones. However, although antagonists of 5-HT(1A) receptors affected the responses evoked by cardiopulmonary and aortic nerve afferents, they were not effective on chemoreceptor reflex-evoked changes. This suggests that 5-HT(1A) receptors play a different role in chemoreceptor pathways compared to that for the other reflexes. This may relate to the fact that the chemoreceptor afferents travel in the IXth (glossopharyngeal) nerve whilst the other afferents travel in the Xth (vagus) nerve and thus may use different central circuitry and neurotransmitters.

Cellular distribution of oxygen sensor candidates-oxidases, cytochromes, K+-channels--in the carotid body.

The specific tissue of the carotid body is built up of groups of glomus cells, enveloped by glial-type sustentacular cells, and innervated by sensory nerve fibers. These units sense arterial pO(2) and respond to hypoxia by a variety of reactions that include initiation of the arterial chemoreflex, i.e., increasing firing activity in the carotid sinus nerve. Until now, neither the cellular localization of the initial events that lead to stimulation of chemoreceptor afferents nor the molecular mechanism of oxygen sensing in the carotid body have been unequivocally identified. Proposed molecular candidates for the mechanism of oxygen sensing include: 1). components of the mitochondrial respiratory chain, 2). NADPH oxidases generating reactive oxygen species in an oxygen-dependent manner, 3). oxygen-regulated plasmalemmal K(+)-channels, and 4). nonoxidase iron-proteins. Our still limited knowledge on their cellular distribution within the carotid body is reviewed here. It is evident that: 1). the distribution of at least some oxygen sensor candidates is not ubiquitous but cell-type-specific, and 2). each specific parenchymal cell type of the carotid body contains at least one of the proposed oxygen sensor candidates. This applies also for the glial-type sustentacular cells that exhibit immunoreactivity to the two-pore domain K(+)-channel, TASK-1. These observations fit best with the assumption that each cell type within the carotid body is principally responsive to hypoxia. The differential equipping of glomus cells, nerve endings, and sustentacular cells with sensor proteins might serve to determine different thresholds of sensitivity and/or to connect the process of oxygen sensing to different signaling pathways. It also favors the assumption that several mechanisms of oxygen sensing may act simultaneously. The cellular identification of the cell type initiating the chemoreceptor reflex, however, has to await the molecular identification of the particular oxygen sensor molecule that initiates increased carotid sinus nerve activity.

Unusual cytochrome a592 with low PO2 affinity correlates as putative oxygen sensor with rat carotid body chemoreceptor discharge.

Light-absorption spectra and afferent chemoreceptor discharge were simultaneously recorded on superfused rat carotid bodies (CBs) under the influence of cytochrome a3-CuB ligands (O2, CN-, CO) in order to identify the primary mitochondrial cytochrome c oxidase (CCO) oxygen sensor. Spectra could be described on the basis of weighted light-absorption spectra of cytochrome b558 of the NAD(P)H oxidase and mitochondrial cytochromes b and c, CCO, cytochrome a3, and an unusual cytochrome a peaking at 592 nm. Discharge signals were deconvoluted into phasic and tonic activity for comparing different CB responses. The spectral weight of cytochrome a592 decreased significantly starting at high PO2 (100 mm Hg) and low sodium cyanide (CN-, 10 mM) accompanied by increasing phasic peak discharge. Combined CO-hypoxia or CO-CN- application inhibited photolysis of CO-stimulated chemoreceptor discharge, revealing photometrically cytochrome a592 as central in oxygen sensing. Control spectra in tissue from sympathetic and nodose ganglia did not show any cytochrome a592 contribution. According to these results, cytochrome a592 is assumed as a unique component of CB CCO, revealing in contrast to other cytochromes an apparent low PO2 and high CN- affinity, probably due to a shortcut of electron flow within CCO between CuA and cytochrome a3-CuB.

Nociceptive afferents selectively modulate the cardiac component of the peripheral chemoreceptor reflex via actions within the solitary tract nucleus

Our previous findings showed that the nucleus of the solitary tract (NTS) mediated part of the tachycardia evoked during somatic noxious stimulation. Here, we investigated the interaction between somatic nociceptor- and peripheral chemoreceptor-evoked cardiac changes. We sought to determine whether this interaction occurred within the NTS, the primary site of termination of chemoreceptor afferents. In a working heart–brainstem preparation of rat, mechanical noxious activation of a forelimb evoked a tachycardia of 17.5±3 (mean±S.E.M.) b.p.m., whereas sodium cyanide (7–30 μg) stimulation of peripheral chemoreceptors produced a sub-maximal bradycardia of −140±15 b.p.m. During nociceptor stimulation the sodium cyanide-evoked bradycardia was attenuated to −42.6±12 b.p.m. but could be prevented by a multiple bilateral NTS microinjection of bicuculline (i.e. −173±18 b.p.m.). Furthermore, the activity of NTS neurones responding to peripheral chemoreceptor stimulation increased from 2.8±1.3 to 9.4±1.9 Hz during sodium cyanide injection ( n=7; P<0.01). The latter response was attenuated reversibly to 2.9±0.9 Hz during simultaneous stimulation of the brachial nerve. Pressure ejection of bicuculline abolished this inhibitory action of brachial-nerve stimulation on the chemoreceptor-evoked excitatory synaptic response. We conclude that somatic noxious stimulation attenuates the chemoreceptor reflex-evoked bradycardia via a GABA Aergic mechanism in the NTS.

Evidence for alpha-2 adrenoreceptor modulation of arterial chemoreflexes in the caudal solitary nucleus of the rat.

The caudal region of the nucleus of the solitary tract (cNTS) is the primary central termination site for arterial chemoreceptor afferents originating from the carotid body. The purpose of the present study was to investigate the role of endogenous activation of alpha-2 adrenoreceptors in the cNTS on arterial chemoreflex function. Arterial chemoreflex responses to intravenous injections of potassium cyanide (KCN; 75 microg/kg) were recorded before and following blockade of alpha-2 adrenoreceptors in the cNTS of urethane-anesthetized rats. KCN alone elicited a reflex increase in arterial pressure, renal sympathetic nerve activity, and respiration. After bilateral cNTS microinjection of alpha-2 receptor antagonists (2 nmol idazoxan or 0.2 nmol yohimbine), arterial chemoreflex responses were markedly attenuated. Attenuation of chemoreflex function was not accompanied by any significant change in resting blood pressure or respiratory rate. The results suggest that the endogenous activation of alpha-2 adrenoreceptors facilitates central processing of chemoreceptor afferent inputs in the cNTS of the rat.

Is ventilatory acclimatization to hypoxia a phenomenon that arises through mechanisms that have an intrinsic role in the regulation of ventilation at sea level?

The purpose of this article is to set out the hypothesis that arterial PO2 may play a significant role in the regulation of breathing at sea level. The following points are made: 1) Although CO2 is clearly the dominant feedback signal in the acute setting, there is evidence, particularly clinical observation, that the ventilatory response to CO2 may adapt. 2) Although the ventilatory response to an acute variation in alveolar PO2 around sea-level values is feeble, studies at altitude have shown that over longer-time periods alveolar PO2 is a more powerful regulator of ventilation. 3) Recent evidence suggests that mechanisms associated with ventilatory acclimatization to hypoxia are active at sea-level values for PO2, and indeed affect the acute ventilatory response to hypoxia. 4) While most evidence suggests that the peripheral and central chemoreflexes are independent and additive in their contributions to ventilation, experiments over longer durations suggest that peripheral chemoreceptor afferents may play an important role in regulating central chemoreflex sensitivity to CO2. This is potentially an important mechanism by which oxygen can alter the acute chemoreflex responses to CO2. In conclusion, the mechanisms underlying ventilatory acclimatization to hypoxia may have an important role in regulating the respiratory system at sea level.

Reflex patterns in preganglionic sympathetic neurons projecting to the superior cervical ganglion in the rat

Reflex patterns in preganglionic neurons projecting in the cervical sympathetic trunk (CST) were analyzed in response to stimulation of various afferent systems. We focused on the question whether these preganglionic neurons can be classified into functionally distinct subpopulations. Reflex responses were elicited by stimulation of trigeminal and spinal nociceptive, thermoreceptive as well as baroreceptor and chemoreceptor afferents. Multi- and single fiber preparations were studied in baroreceptor intact and sino-aortically denervated animals. Spontaneous activity of 36 preganglionic single neurons ranged from 0.2 to 3.5 imp/s (median=1.11 imp/s). The degree of cardiac rhythmicity (CR) in the activity of sympathetic neurons was 69.5±13% (mean±S.D.; N=52; range=39–95%). Noxious stimulation of acral skin activated the majority (67%) of sympathetic preparations by 37±25% ( N=35) above pre-stimulus activity; 15% were inhibited. In these neurons the response to noxious stimulation of acral skin was significantly correlated with the degree of CR ( P<0.001, N=52) in that neurons showing the strongest excitation to noxious stimulation displayed the strongest CR. Noxious mechanical stimulation of body trunk skin ( N=60) inhibited the majority (80%) of fiber preparations tested (by 34±18% of pre-stimulus activity, N=48); an activation was not observed. Cold stimulation of acral ( N=9) and body trunk skin ( N=42) activated most fiber preparations. Trigeminal stimulation evoked a uniform reflex activation of preganglionic neurons (+79±73% of pre-stimulus activity, N=32). Chemoreceptor stimulation by systemic hypercapnia elicited inhibitory (−31±19%, N=8) as well as excitatory (+59±5%, N=4) responses. These results show that preganglionic sympathetic neurons projecting to target organs in the head exhibit distinct reflex patterns to stimulation of various afferent systems; however, a clear classification into different functional subgroups did not emerge. Furthermore, reflex patterns showed a segmental organization to noxious cutaneous stimulation of acral parts and body trunk reflecting a differential central integration of spinal afferent input. Compared with the cat the reflex organization of sympathetic neurons projecting to the head seems to be less differentiated in the anesthetized rat.

Peripheral chemoreceptor activation enhances 5-hydroxytryptamine release in the locus coeruleus of conscious rats

Intravenous bolus injection of KCN (40 μg) elicited brief but pronounced tachypnea, bradycardia and pressor response, and led to a 37% increase in 5-hydroxytryptamine (serotonin) (5-HT) release in the locus coeruleus (LC) of freely moving rats. Slow infusion of KCN (15 μg/min) for 10 min induced only a slight pressor response, but increased the respiration rate (+39 breaths/min), as well as 5-HT release in the LC (+60%) throughout the infusion. In rats with transected chemoreceptor afferents, neither injection or infusion of KCN changed 5-HT release, suggesting that in intact animals, the effect on extracellular 5-HT was due to activation of peripheral chemoreceptors. In summary, we report that peripheral chemoreceptor activation enhances 5-HT release in the LC, indicating that 5-HT might be involved in the modulation of LC activity by ascending chemosensory information.

Carotid and cardiopulmonary chemoreceptor activity increases hippocampal theta rhythm in conscious rabbits.

We have examined whether activation of carotid artery chemoreceptors causes alerting in conscious rabbits. Injection of phenylbiguanide (a 5-hydroxytryptamine(3)-receptor agonist) into the common carotid artery of conscious rabbits increased the proportion of theta rhythm in the hippocampal EEG, commencing in the first 5-s epoch after the injection. Intravenous injection of phenylbiguanide also increased the proportion of theta rhythm in the hippocampal electroencephalogram (EEG), but the onset of the change was not until the second 5-s epoch following injection. Injection of Ringer solution, either into the common carotid artery or into the marginal ear vein, did not affect the hippocampal EEG. Results suggest that phenylbiguanide-mediated activation of carotid and cardiopulmonary chemoreceptor afferents alerts the animal, as assessed by induction of theta rhythm in the hippocampal EEG. This alerting response presumably reflects the action of neural inputs that enter the brain with the carotid sinus nerve at the level of the medulla oblongata.

Connections between retrotrapezoid nucleus and nucleus tractus solitarii in cat

The retrotrapezoid nucleus (RTN), a part of the rostral ventrolateral medulla, is involved in the control of breathing. The mechanisms by which the RTN modulate the activity of respiratory neurons during chemoreceptor stimulation are not fully understood. This electrophysiological study performed in the cat demonstrates that 18 out of 22 RTN neurons receive inputs from the commissural subnucleus of the solitary tract (cNTS), the peripheral chemoreceptor afferents projection site. Moreover, six RTN neurons are found to present interconnection between RTN and the ventrolateral subnucleus of the solitary tract, an area containing mainly bulbo-spinal respiratory neurons. The present data suggest that RTN play a key role by concomitantly integrating chemosensitive informations relayed by the cNTS and providing an influence on the respiratory network.

Important role of carotid chemoreceptor afferents in control of breathing of adult and neonatal mammals

This review provides a summary and prospective on the importance of carotid/peripheral chemoreceptors to the control of breathing during physiologic conditions. For several days after carotid body denervation (CBD), adult mammals hypoventilate (+10 mmHg increase in Pa CO 2 ) at rest and during exercise and CO 2 sensitivity is attenuated by about 60%. In addition, if the rostral ventrolateral medulla is cooled during NREM sleep after CBD, a sustained apnea is observed. Eventually, days or weeks after CBD, a peripheral ventilatory chemoreflex redevelops and there is a normalization of breathing (rest and exercise) and CO 2 sensitivity. The site (s) of the regained chemosensitivity has not been established. This plasticity/redundancy after CBD appears greater in neonates than in adult mammals. These data suggest the carotid and other peripheral chemoreceptors provide an important excitatory input to medullary respiratory neurons that is essential for breathing when wakeful stimuli and central chemoreceptors are absent.

Is the aortic depressor nerve involved in arterial chemoreflexes in rats?

Recent anatomical and physiological studies showed that chemoreceptor afferent fibers are present in the rat aortic depressor nerve (ADN), which has been considered to contain exclusively baroreceptor afferent fibers. However, it remains to be proven whether the chemoreceptor afferents of the ADN are practically involved in chemoreflexes. The present study was performed in chloralose/urethane-anesthetized rats of either Sprague–Dawley (SD) or Wistar strain to examine whether the ADN carries sufficient information regarding arterial hypoxia and hypercapnia, and whether the ADN indeed participates in chemoreflexes, the circulatory and respiratory components. It was found in either strain that afferent discharges of the ADN were not affected at all by hypoxia or hypercapnia, whereas those of the carotid sinus nerve (CSN) markedly increased due to these stimuli. Hypoxia produced hypertension, transient bradycardia followed by tachycardia, and respiratory facilitation, which characterize the chemoreflexes. Any of these responses was not affected at all by the ADN section, but all were abolished by the CSN section. Intraaortic injection of cyanide also induced transient bradycardia and respiratory facilitation, but any of them was not affected by the ADN section while all were abolished by the CSN section. Furthermore, electrical stimulation of the ADN produced solely baroreflex responses, i.e. hypotension and respiratory suppression, whereas that of the CSN provoked chemoreflex responses, i.e. early, transient hypertension and respiratory facilitation. In conclusion, the rat ADN does not contain a functionally significant number of chemoreceptor afferent fibers, if at all, and does not appreciably contribute to generation of chemoreflexes.

Activity of aortic chemoreceptors in the anaesthetized rat.

1. It has been widely accepted that the rat aortic depressor nerve contains only baroreceptors. However, the experiments which have provided these negative data have employed whole aortic nerve recording. In the present study, the technical difficulties associated with recording single fibres in vivo, from the rat aortic nerve (diameter 25-50 microm), have been surmounted using a small tip, glass suction electrode technique. 2. Upon switching from normocapnic hyperoxia to hypercapnic hypoxia, irregularly firing units (n = 13) appeared and these were significantly excited by intravenous injections of sodium cyanide (20 microg) but not by rises in arterial blood pressure induced by methoxamine (an alpha1-adrenoreceptor agonist; 10 microg). Inhalation of 100 % oxygen rapidly and reversibly silenced, or profoundly reduced, ongoing activity. 3. Intravenous injection of phenylbiguanide (PBG; a 5-HT3 receptor agonist; 8 microg) strongly stimulated the chemoreceptors and was followed by a period of chemodepression (3-21 s). In contrast none of the single fibre baroreceptors recorded (n = 15) were excited by PBG but all significantly increased their discharge in response to the increases in arterial blood pressure associated with methoxamine and cyanide. Both the excitatory and inhibitory effects of PBG on the chemoreceptor fibres were abolished by ondansetron (a 5-HT3 receptor antagonist: 1 mg kg-1 i.v.; n = 5 animals) whilst the chemoexcitatory action of cyanide was preserved. 4. It is concluded that there are chemoreceptor afferents contained in the aortic nerve of the Sprague-Dawley rat. The 5-HT3 receptor appears not to be a pre-requisite for aortic body chemoexcitation.

Respiratory sensations during heavy exercise in subjects without respiratory chemosensitivity

Breathlessness arises from increased medullary respiratory center activity projecting to the forebrain (respiratory corollary discharge hypothesis). Subjects with congenital central hypoventilation syndrome (CCHS) lack the normal hyperpnea and breathlessness during hypercapnia. The corollary discharge hypothesis predicts that if CCHS subjects have normal hyperpnea during exercise, they will experience normal breathlessness during exercise. To test this, we studied four CCHS subjects and six matched controls during an exhausting constant-load cycling test requiring substantial anaerobiosis. CCHS subjects rated significantly less breathlessness at the end of the test than controls, but ventilation (index of respiratory corollary discharge) was also somewhat lower in CCHS (not significant). In both groups, breathlessness increased disproportionately more than ventilation towards the end of exercise. These data failed to disprove the corollary discharge hypothesis of breathlessness, but do suggest that the relationship between ventilation and breathlessness is non-linear and/or that projections of chemoreceptor afferents to the forebrain (presumed lacking in CCHS) is one source of breathlessness in normals.

Dopamine Depresses Minute Ventilation in Patients With Heart Failure

Low-dose dopamine is frequently used in patients in the intensive care setting. Dopamine may inhibit chemoreceptor afferents and hence decrease chemoreflex sensitivity to hypoxia. In a double-blind, randomized, crossover study, we determined the effects of dopamine (5 microg x kg(-1) x min(-1)) and placebo infusion on oxygen saturation, minute ventilation, and sympathetic nerve activity during normoxia and 5 minutes of hypoxia in 10 normal young subjects. We further investigated the effects of dopamine and placebo on minute ventilation during normoxic breathing in 8 patients with severe heart failure and in 8 age-matched control subjects. Dopamine did not decrease minute ventilation during normoxia in normal subjects. During hypoxia, minute ventilation was 12.9+/-1.3 L/min on dopamine and 15.8+/-1.5 L/min on placebo (P<0.0001). Oxygen saturation during hypoxia was lower with dopamine (78+/-3%) than placebo (84+/-2%; P<0.0001). Sympathetic nerve activity during hypoxia was not enhanced with dopamine despite the lower O2 saturation. Subjects were able to maintain a voluntary apnea to a lower oxygen saturation on dopamine than on placebo (P<0.05). In heart failure patients breathing room air, but not in age-matched control subjects, dopamine decreased minute ventilation despite decreased oxygen saturation and increased PETCO2 during dopamine (all P< or =0.02). Dopamine inhibits chemoreflex responses during hypoxic breathing in normal humans, preferentially affecting the ventilatory response more than the sympathetic response. Dopamine also depresses ventilation in normoxic heart failure patients breathing room air. Ventilatory inhibition by low-dose dopamine may adversely influence outcome in hypoxic patients, especially in patients with heart failure.

Activation of P2X receptors for adenosine triphosphate evokes cardiorespiratory reflexes in anaesthetized rats.

1. We tested the hypothesis that activation of P2X receptors associated with vagal afferent nerves can evoke a Bezold-Jarisch (B-J) depressor reflex in anaesthetized rats. 2. Injection of alphabeta-methylene ATP (alphabeta-MeATP; 0.6-600 nmol i.v.) evoked a dose-dependent B-J reflex comprising bradycardia, hypotension and apnoea in rats anaesthetized with pentobarbitone. Apnoea was commonly preceded by hyperventilation. Bilateral vagotomy significantly reduced the bradycardia and most of the apnoeic response without affecting hyperventilation, and unmasked a vasopressor response. Hypotension and apnoea were subject to desensitization, and ATP was about 100 times less potent than alphabeta-MeATP in evoking the B-J reflex. 3. ED50 values for responses to alphabeta-MeATP were: bradycardia 14.6 +/- 3.8 nmol; apnoea 47.1 +/- 8.5 nmol; hyperventilation 23.3 +/- 6.0 nmol, n = 14. The ED50 for apnoea was significantly greater than that for bradycardia or hyperventilation (P < 0.05). Atropine (2.8 micromol (kg body wt)-1 i.v.) antagonized the reflex bradycardia and hypotension. 4. The P2 antagonists suramin (14 micromol (kg body wt)-1 i.v.) and PPADS (17 micromol (kg body wt)-1 i.v.) antagonized the bradycardic and apnoeic components of the reflex response to alphabeta-MeATP, without reducing the vasopressor or hyperventilatory responses to the agonist. 5. Recordings from vagal afferents showed that pulmonary inflation receptors were activated by alphabeta-MeATP in 62 % of units recorded (ED50 22 +/- 5 nmol) and this was blocked by PPADS (17 micromol (kg body wt)-1 i.v.); unidentified vagal afferents were also activated. 6. alphabeta-MeATP activated carotid chemoreceptor afferents (ED50 23 +/- 9 nmol), an action that was unaffected by PPADS or suramin. 7. The results support the hypothesis that P2X receptor subtypes for ATP are associated with specific sensory nerves that form part of the homeostatic mechanism for cardiovascular and respiratory regulation and these receptors therefore have physiological, pathological and therapeutic significance.

Fos expression in the rat parabrachial and Kölliker-Fuse nuclei after electrical stimulation of the trigeminal ethmoidal nerve and water stimulation of the nasal mucosa.

The present study examined the location of neurons in the lateral parabrachial nucleus (PBL), Kölliker-Fuse nucleus (KF), nucleus of the solitary tract (NTS), spinal trigeminal nucleus (Sp5C) and upper cervical cord possibly involved in the transmission of autonomic responses (apnea, bradycardia and rise in arterial blood pressure) elicited by nasotrigeminal stimulation in the rat. To identify these neurons we employed immunocytochemical detection of the transcription factor Fos. To induce the expression of Fos protein, two kinds of stimuli and experimental controls were performed in chloralose/urethane-anesthetized animals: (i) electrical stimulation of the trigeminal ethmoidal nerve (EN5) and, as sham controls, dissection of the EN5 without electrical stimulation, (ii) stimulation of the nasal mucosa with water and, as control experiments, no stimulation. Both forms of stimulation lead to a consistent pattern of Fos-positive neurons in the PBL and KF. Differences could be observed rostrally in the PBL and KF, where significantly higher numbers of Fos-positive neurons were present after EN5 versus water stimulation. The EN5-stimulated group had a significantly higher number of Fos-immunoreactive neurons in the KF than the sham controls, especially in the midlevel region. In the PBL significant differences in the numbers of activated cells could be observed between EN5-stimulated versus sham controls. In the water-stimulated rats compared with the anesthesia controls, a significantly higher number of Fos-immunoreactive neurons was always observed at all rostrocaudal levels in the KF and in the midlevel PBL. Electrical EN5 stimulation induced Fos expression in the Sp5C at the level of the area postrema and caudally in the upper cervical cord. In contrast, after water stimulation Fos-positive neurons were exclusively found in the Sp5C. In addition, all forms of stimuli and controls induced strong expression of Fos in the medial and commissural NTS. Linear correlations were found between the numbers of Fos-immunoreactive neurons in the Sp5C versus the KF and the NTS versus the PBL. The activated neurons may belong to two functionally discrete pathways: the nasotrigeminal reflex circuit, which is activated by nasal sensory afferents running through the EN5 via the Sp5C to the KF, and a pathway activated most likely by baro- and chemoreceptor afferents running through the NTS to the PBL. Our results indicate that the PB/KF plays a pivotal role in the mediation and maintenance of the autonomic responses induced by the nasotrigeminal reflex.

Effects of vagal and carotid chemoreceptor afferents on the frequency and pattern of spontaneous augmented breaths in rabbits.

We investigated the effects of vagal afferents and carotid chemoreceptors on the frequency and pattern of spontaneous augmented breaths in anesthetized rabbits by analyzing the changes in the phrenic nerve discharge. Blockade of vagal C-fiber conduction by capsaicin treatment greatly reduced the frequency of spontaneous augmented breaths but had no significant effect on the pattern of the augmented breaths. Inhalation of sulfur dioxide, which abolished the Hering-Breuer inflation reflex, suppressed the frequency of spontaneous augmented breaths but did not significantly alter the pattern of the augmented breaths. Carotid sinus denervation diminished both the frequency and amplitude of spontaneous augmented breaths. The occurrence of spontaneous augmented breaths was not observed in rabbits after bilateral vagotomy. In other experiments we examined the relationship between rapidly adapting pulmonary stretch receptors (RARs) and dynamic lung compliance during the spontaneous augmented breath. The stimulation of RARs occurred at the first phase of spontaneous augmented breaths, and augmentation of transpulmonary pressure seen at the second phase coincided with vigorous stimulation of the receptor activity. These results suggest that spontaneous augmented breaths may be mediated by the summation of several facilitatory inputs from vagal and carotid chemoreceptor afferents.