Phrenic Nerve Discharge Research Articles

Depressor and Tachypneic Responses to Chemical Stimulation of the Ventral Respiratory Group Are Reduced by Ablation of Neurokinin-1 Receptor-Expressing Neurons

Our goal was to investigate whether the neurokinin-1 receptor (NK1R)-expressing cells of the rostral ventrolateral medulla (RVLM) regulate respiration and arterial pressure (AP). We examined the consequences of their ablation on the cardiorespiratory responses [phrenic nerve discharge (PND) and AP] caused by injecting dl-homocysteic acid (DLH) into the ventral respiratory group (VRG). In intact rats, DLH produced tachypnea only when injected into the pre-Bötzinger complex (pre-BötC). Injections into pre-BötC and rostral VRG (rVRG) caused hypotension, whereas injections into the Bötzinger region elevated AP. Selective unilateral ablation of RVLM NK1R-immunoreactive cells (97% loss within the pre-BötC and rVRG without loss of catecholaminergic neurons) was done by injecting saporin (SAP) conjugated with a selective NK1R agonist [Sar9, Met(O2)11]-substance P (SSP). Free SAP produced no lesion. Resting AP was normal in SAP- and SSP-SAP-treated rats, but the PND rate was slightly elevated in SSP-SAP-treated rats. The response of SAP-treated rats to DLH injection into VRG was normal and identical on each side, but tachypnea could not be elicited in the pre-BötC of SSP-SAP-treated rats on the toxin-injected side, and DLH caused a long-lasting apnea on the untreated side. The hypotension produced by DLH injection into pre-BötC and rVRG of SSP-SAP-treated rats was reduced on the lesioned side only. In conclusion, NK1R-expressing cells of the rostral ventrolateral medulla control both respiratory rhythm and blood pressure. However, there is no evidence yet that these two functions are regulated by the same NK1R-expressing neurons.

Integration of cornea and cardiorespiratory afferents in the nucleus of the solitary tract of the rat.

We determined the activity of neurons within the nucleus of the solitary tract (NTS) after stimulation of the cornea and assessed whether this input affected the processing of baroreceptor and peripheral chemoreceptor inputs. In an in situ, unanesthetized decerebrate working heart-brain stem preparation of the rat, noxious mechanical or electrical stimulation was applied to the cornea, and extracellular single unit recordings were made from NTS neurons. Cornea nociceptor stimulation evoked bradycardia and an increase in the cycle length of the phrenic nerve discharge. Of 90 NTS neurons with ongoing activity, corneal stimulation excited 51 and depressed 39. There was a high degree of convergence to these NTS neurons from either baroreceptors or chemoreceptors. The excitatory synaptic response in 12 of 19 baroreceptive and 10 of 15 chemoreceptive neurons was attenuated significantly during concomitant electrical stimulation of the cornea. This inhibition was GABA(A) receptor mediated, being blocked by pressure ejection of bicuculline. Thus the NTS integrates information from corneal receptors, some of which converges onto neurons mediating reflexes from baroreceptors and chemoreceptors to inhibit these inputs.

Trigeminal reflex regulation of the glottis depends on central glycinergic inhibition in the rat.

In an unanesthetized decerebrate in situ arterially perfused brain stem preparation of mature rat, strychnine (0.05-0.2 microM) blockade of glycine receptors caused postinspiratory glottal constriction to occur earlier, shifting from early expiration to inspiration. This resulted in a paradoxical inspiratory-related narrowing of the upper airway. Stimulation of the trigeminal ethmoidal nerve (EN5; 20 Hz, 100 micros, 0.5-2 V) evoked a diving response, which included a reflex apnea, glottal constriction, and bradycardia. After strychnine administration, this pattern was converted to a maintained phrenic nerve discharge and a reduced glottal constriction that was interrupted intermittently by transient abductions. The onset of firing of postinspiratory neurons shifted from early expiration into neural inspiration in the presence of strychnine, but neurons maintained their tonic activation during EN5 stimulation, as observed during control. Inspiratory neurons that were hyperpolarized by EN5 stimulation in control conditions were powerfully excited after loss of glycinergic inhibition. Thus the integrity of glycinergic inhibition within the pontomedullary respiratory network is critical for the coordination of cranial and spinal motor outflows during eupnea but also for protective reflex regulation of the upper airway.

Modulation of gasp frequency by activation of pre-Bötzinger complex in vivo.

Under hyperoxic conditions, both chemical stimulation of neurons and focal hypoxia in the pre-Bötzinger complex (pre-BötC) in vivo modify the eupneic pattern of inspiratory motor output by eliciting changes in the patterning and timing of phrenic bursts, which includes both phasic and tonic excitation. The influence of this region on the gasping pattern of phrenic motor output produced during severe brain hypoxia is unknown. We therefore examined the effects of chemical stimulation of neurons (DL-homocysteic acid; DLH; 10 mM; < or =20 nl) and focal hypoxia (sodium cyanide; NaCN; 1 mM; < or =20 nl) in the pre-BötC on hypoxia-induced gasping in chloralose-anesthetized, vagotomized, mechanically ventilated cats. Unilateral microinjection of DLH into the pre-BötC during hypoxia-induced gasping increased phrenic burst frequency by approximately 630% (P < 0.01) over baseline frequency due predominantly to a reduction in T(E) (from 28.9 +/- 6.2 to 5.2 +/- 1.8 s; mean +/- SE; P < 0.01). No significant changes in T(I) or rate of rise between hypoxia-induced gasps and the DLH-induced bursts were observed; the effects on peak amplitude of integrated phrenic nerve discharge were variable. Similar responses were evoked by unilateral microinjection of NaCN into the pre-BötC. These findings demonstrate that both activation of pre-BötC neurons and focal hypoxia in the pre-BötC not only influence the eupneic pattern of phrenic motor output but also modify the expression of hypoxia-induced gasping in vivo. These findings also provide additional support to the concept of intrinsic hypoxic chemosensitivity of the pre-BötC.

Activation of mu-opioid receptors in rat ventrolateral medulla selectively blocks baroreceptor reflexes while activation of delta opioid receptors blocks somato-sympathetic reflexes

The effects of activation of mu and delta-opioid receptors in the rostral ventrolateral medulla (RVLM) on somato-sympathetic, baroreceptor and chemoreceptor reflexes, as well as respiratory rhythmicity in sympathetic nerves, were examined in urethane anaesthetized (1.1–1.2 g/kg) and artificially ventilated Sprague–Dawley rats. Microinjection of the delta-opioid receptor agonist [ D-Pen 2,5]-enkephalin (DPDPE; 8 mM, 50 nl) bilaterally into the RVLM potently inhibited the post-inspiratory-related burst discharges of lumbar sympathetic nerve activity (LSNA) but had only limited effects on splanchnic sympathetic nerve activity (SSNA) and phrenic nerve discharge. Injection of DPDPE into the RVLM strongly attenuated the somato-sympathetic reflex (∼50–80%) evoked in the lumbar sympathetic nerve and splanchnic sympathetic nerve by tibial nerve stimulation but had no effect on baroreceptor reflexes and chemoreceptor reflexes evoked by aortic nerve stimulation and brief hypoxia, respectively. Injection of the mu-opioid receptor agonist, [ D-Ala 2, N-Me-Phe 4,Gly-ol 5]-enkephalin (DAMGO; 4 mM, 50 nl), also elicited a greater inhibition of LSNA than SSNA accompanied by an abolition of phrenic nerve discharge. Injection of DAMGO inhibited the baroreceptor reflex without significant effect on either the somato-sympathetic or the chemoreceptor reflexes. We propose that opioid peptides diminish specific excitatory and inhibitory inputs to the presympathetic neurons in RVLM via distinct presynaptic receptor subclasses.

Bilateral coordination of inspiratory neurones in the rat.

Inspiratory activity on the left and right sides must be coordinated to be effective. We used cross-correlation to examine the hypothesis that the coordination of left and right medullary inspiratory neurones is produced by excitation from common sources and by midline-crossing excitatory connections among these neurones. In adult rats, a total of 185 contralateral pairs of inspiratory neurones ( n=370) were recorded extracellularly, and classified, according to their firing pattern, as augmenting ( n=262), constant ( n=82) or decrementing ( n=26). Of the 262 augmenting inspiratory neurones, 98 were classified as phrenic premotor neurones by cross-correlation with phrenic nerve discharge. The 185 cross-correlograms showed little evidence of common activation, or midline-crossing excitatory connections. Of the 45 cross-correlograms for pairs of augmenting neurones, only 4 (approximately equal to 9%) indicated a common activation, and only one a monosynaptic connection. Of the 45 for pairs of augmenting and phrenic premotor neurones, only 9 (20%) showed a common activation, and only 2 a monosynaptic excitatory connection. Of the 19 pairs of phrenic premotor neurones, 5 from the same rat showed high-frequency oscillations, and 1 a monosynaptic excitatory connection. Cross-correlograms for pair combinations of other types of neurones also exhibited few features. We suggest that, in the adult rat, although both common activation and excitatory cross-connections exist as a means for coordinating left and right ventral group inspiratory neurones to the same respiratory rhythm, they are insufficient to account for it.

Influence of levels of carbon dioxide and oxygen upon gasping in perfused rat preparation

In vivo, the augmenting pattern of integrated phrenic nerve discharge of eupnea is altered to the decrementing pattern of gasping in severe hypoxia or ischaemia. Identical alterations in phrenic discharge are found in perfused in situ preparations of the juvenile rat. In this preparation, gasping was produced by equilibration of the perfusate with various levels of carbon dioxide and oxygen. The duration of the phrenic burst, the interval between bursts and the burst amplitude were not significantly different following equilibration with 21–6%O 2 at 5% CO 2 or with 0–9% CO 2 at 6% O 2, with the exception that the burst amplitude was significantly greater in hypercapnic-hypoxia (9% CO 2 at 6% O 2). It is proposed that hypoxia-induced gasping results from the release of an endogenous pacemaker activity of rostral medullary neurons. This release is caused by cellular mechanisms that change the balance between membrane ionic currents. Moreover, these cellular mechanisms may be explicitly induced by alterations in the ionic and metabolic homeostasis.

Evidence for the involvement of purinergic signalling in the control of respiration

The ventrolateral medulla has a critical role in the generation and patterning of respiration via an extensive network of respiratory neurones. We have investigated the effects of activating purinergic P2 receptors within the ventrolateral medulla of the anaesthetised rat on the overall pattern of respiratory activity. In addition, using immunohistochemical techniques, we have identified the subtypes of P2X receptors in the ventrolateral medulla. Unilateral microinjection of ATP into the ventrolateral medulla reduced in a dose-dependent manner, or abolished, resting phrenic nerve discharge recorded as an indication of central inspiratory drive. ATP also elicited increases in blood pressure and variable changes in heart rate. These effects were mimicked by microinjection of the P2X receptor agonist α,β-methylene ATP into the ventrolateral medulla. Whilst microinjection of suramin, a P2 receptor antagonist, had no effect on resting cardiorespiratory variables it blocked the respiratory and cardiovascular effects of ATP microinjected into the ventrolateral medulla. Immunohistochemical staining using IgG antibodies showed that P2X 1, P2X 2, P2X 5 and P2X 6, but not P2X 3, P2X 4 or receptor subunits were localised in the rostral ventrolateral medulla. Our results indicate that several P2X receptor subtypes are localised within areas of the ventrolateral medulla that are important for cardiorespiratory control (including the pre-Bötzinger and Bötzinger complexes), and that activation of these receptors can have profound effects on both the cardiovascular and the respiratory networks. Our pharmacological data suggest that different P2X subunits in this region may co-assemble to form hetero-oligomeric assemblies as well as homomultimers within this region.

Paraventricular nucleus bicuculline alters frequency components of sympathetic nerve discharge bursts.

Autospectral and coherence analyses were used to determine the effect of paraventricular nucleus (PVN) GABA(A) receptor antagonism [microinfusion or microinjections of bicuculline methiodide (BMI) 100 pmoles] on sympathetic nerve discharge (SND) frequency components (bursting pattern and relationships between discharges in regionally selective nerves) in alpha-chloralose-anesthetized rats. SND was recorded from the renal, splenic, and lumbar nerves. The following observations were made. First, PVN BMI microinjections, but not PVN saline or cortical BMI microinjections, transformed the cardiac-related SND bursting pattern in baroreceptor-innervated rats to one characterized by the presence of low-frequency bursts not synchronized to the cardiac cycle or phrenic nerve discharge bursts. Second, SND pattern changes were similar in the renal, splenic, and lumbar nerves, and peak coherence values relating low-frequency bursts in sympathetic nerve pairs (renal-splenic, renal-lumbar, and splenic-lumbar) were significantly increased from preinjection control after PVN BMI microinjection. Third, PVN BMI microinjections significantly increased the coupling between low-frequency SND bursts in baroreceptor-denervated rats. Finally, PVN BMI-induced changes in the SND bursting pattern were not observed after PVN pretreatment with muscimol (GABA agonist, 1 nmole). We conclude that PVN GABA(A) receptor antagonism profoundly alters the frequency components in sympathetic nerves.

Firing patterns of pre-Bötzinger and Bötzinger neurons during hypocapnia in the adult rat

Controversy exists about how a coordinated respiratory rhythm is generated in the brainstem. Some authors suggest that neurons in the pre-Bötzinger complex are key to initiation of all types of breathing. While, on the other hand, it has been reported that some pre-Bötzinger neurons fail to maintain a rhythmic discharge in phase with phrenic nerve discharge during mechanical hyperventilation. Extracellular recordings were made from respiratory units in the pre-Bötzinger and Bötzinger complexes of 13 anaesthetised, paralysed and vagotomised rats. Central respiratory activity was monitored from the C5 phrenic nerve. During mechanical hyperventilation, several changes were observed in the phrenic neurogram. Firstly, the frequency and amplitude of integrated phrenic nerve discharge were reduced and reversibly stopped. Secondly, the patterned discharges changed from an augmenting to a variety of non-augmenting patterns in 53 of 60 cases. In some cases ( n=9) we observed that the pattern appeared to have two components, an early short duration discharge followed by a longer duration discharge. Respiratory units also started to show different firing patterns during mechanical hyperventilation. In general, they were divided into those units that fired tonically ( n=28) and units that became silent ( n=32), before phrenic nerve discharge ceased coincidently with complete apnoea. Of particular interest were those expiratory–inspiratory units in the pre-Bötzinger complex ( n=8) that narrowed their firing period towards late expiration and early inspiration during mechanical hyperventilation. Given their firing features, it is possible that these expiratory–inspiratory units may participate in generation of the early inspiratory component of phrenic nerve discharge.

Respiratory response to baroreceptor stimulation and spontaneous contractions of the urinary bladder

Spontaneous contractions of the urinary bladder (SBCs) and experimental elevations of carotid sinus pressure (CSP) have been shown to result in respiratory inhibition with preferential reduction in hypoglossal (HG) nerve activity as compared with that of phrenic nerve discharge. We assessed the interaction between these respiratory inhibitory stimuli in decerebrate, vagotomized, paralyzed and artificially ventilated cats. We denervated the right carotid sinus and pressurized the isolated left carotid sinus region within the linear range of the baroreflex, while maintaining systemic arterial pressure at ∼100 mmHg. We monitored the HG and phrenic nerve responses to SBCs, to elevations in CSP between SBCs, and to elevations in CSP during SBCs. Our results show that superimposing these stimuli results in respiratory inhibition, especially of HG activity, that exceeds that resultant from either stimulus alone. We speculate that the combined presence of SBCs and episodic hypertension may contribute to the development of periodic breathing or obstructive apnea, particularly during sleep.

Effects of a κ-Receptor Agonist U-50488 on Bulbar Respiratory Neurons and Its Antagonistic Action Against the μ Receptor-Induced Respiratory Depression in Decerebrate Cats

The function of κ receptor-mechanisms in bulbar respiratory network was investigated in decerebrate cats. Intravenous injection of U-50488 (0.3 -3.0 mg /kg) dose-dependently decreased the phrenic nerve discharge and shortened inspiration and expiration. U-50488 caused hyperpolarization, and decreased input resistance and the action potential discharge in respiratory neurons. The effects of U-50488 were antagonized by nor-binaltorphimine. DAMGO (0.3 mg/kg,i.v.) decreased the phrenic discharge and prolonged inspiration and expiration. U-50488 partially reversed the respiratory depression induced by DAMGO. These results suggest that the activation of κ receptors by itself depresses the central respiratory activity, while it opposes the μ receptor-mediated respiratory depression.

Role of the pre-Bötzinger complex in respiratory rhythm generation in vivo: influence of respiratory network drive

We have previously demonstrated that microinjection of DL-homocysteic acid (DLH), a glutamate analog, into the pre-Botzinger complex (pre-BotC) can produce either phasic or tonic excitation of phrenic nerve discharge. Our initial findings were obtained during hyperoxic normocapnia; thus, the influence of respiratory network drive, including intrinsic pre-BotC neuronal excitability, on the DLH-induced modulation of phrenic motor output requires clarification. We, therefore, examined the effects of chemical stimulation of the pre-BotC during increased respiratory network drive elicited by hypercapnia, hypoxia (ie, peripheral chemoreflex), or focal pre-BotC tissue acidosis in chloralose-anesthetized, vagotomized, mechanically ventilated cats. For these experiments, we selected sites in which unilateral microinjection of DLH into the pre-BotC during hyperoxic normocapnia (PaCO2 = 37–43 mmHg) produced a non-rhythmic tonic excitation of phrenic nerve discharge. During hypercapnia (PaCO2 = 59.7 ± 2.8 mmHg; n = 17), similar microinjection produced excitation in which respiratory rhythmic oscillations were superimposed on varying levels of tonic discharge. A similar pattern of modulation was observed in response to microinjection of DLH into the pre-BotC during normocapnic hypoxia (PaCO2 = 38.5 ± 3.7; PaO2 = 38.4 ± 4.4; n = 8) and during focal pre-BotC tissue acidosis (n = 7). Further, during increased respiratory network drive, these DLH-induced respiratory rhythmic oscillations had an increased frequency compared to the preinjection baseline frequency of phrenic bursts (P < 0.05). These findings demonstrate that tonic inspiratory motor activity evoked by chemical stimulation of the pre-BotC is influenced by and integrates with modulation of respiratory network drive mediated by input from both central and peripheral chemoreceptors, as well as focal pre-BotC CO2/H+ chemosensitivity.

Effects of phrenicotomy and exercise on hypoxia-induced changes in phrenic motor output.

To investigate models of plasticity in respiratory motor output, we determined the effects of chronic unilateral phrenicotomy and/or exercise on time-dependent responses to episodic hypoxia in the contralateral phrenic nerve. Anesthetized (urethane), ventilated, and vagotomized rats were presented with three, 5-min episodes of isocapnic hypoxia (11% O(2)), separated by 5 min of hyperoxia (50% O(2)). Integrated phrenic (and hypoglossal) nerve discharge were recorded before and during each hypoxic episode, for the first 5 min after the first hypoxic episode, and at 30 and 60 min after the final episode. Of 36 rats, one-half were sedentary while the other one-half had free access to a running wheel; each of these groups was split into three subgroups: 1) unoperated, 2) chronic left phrenicotomy (27-37 days), and 3) sham operated. Neither unilateral phrenicotomy nor running wheel activity influenced the short-term hypoxic phrenic response (during hypoxia) or long-term facilitation (posthypoxia). Posthypoxia frequency decline was exaggerated in phrenicotomized-sedentary rats relative to unoperated-sedentary rats (change in burst frequency = -23+/-4 vs. -11 +/-5 bursts/min, respectively; 5 min posthypoxia; P<0.05), an effect that was eliminated by spontaneous exercise. The results indicate that neither voluntary running nor unilateral phrenicotomy has major effects on time-dependent hypoxic phrenic responses, with the exception of an unexpected effect of phrenicotomy on posthypoxia frequency decline in sedentary rats.

Substance P in the nucleus of the solitary tract augments bronchopulmonary C fiber reflex output.

Bronchopulmonary C fibers defend the lungs against injury from inhaled agents by a central nervous system reflex consisting of apnea, cough, bronchoconstriction, hypotension, and bradycardia. Glutamate is the putative neurotransmitter at the first central synapses in the nucleus of the solitary tract (NTS), but substance P, also released in the NTS, may modulate the transmission. To test the hypothesis that substance P in the NTS augments bronchopulmonary C fiber input and hence reflex output, we stimulated the C fibers with left atrial capsaicin (LA CAP) injections and compared the changes in phrenic nerve discharge, tracheal pressure (TP), arterial blood pressure (ABP), and heart rate (HR) in guinea pigs before and after substance P injections (200 microM, 25 nl) in the NTS. Substance P significantly augmented LA CAP-evoked increases in expiratory time by 10-fold and increases in TP and decreases in ABP and HR by threefold, effects prevented by neurokinin-1 (NK1) receptor antagonism. Thus substance P acting at NTS NK1 receptors can exaggerate bronchopulmonary C fiber reflex output. Because substance P synthesis in vagal airway C fibers may be enhanced in pathological conditions such as allergic asthma, the findings may help explain some of the associated respiratory symptoms including cough and bronchoconstriction.

Somatosympathetic reflex in a working heart-brainstem preparation of the rat

The purpose of the present study was to examine the cardiorespiratory responses (CR) evoked by a somatosympathetic reflex (SSR) in the working heart-brainstem preparation (WHBP). Sprague–Dawley rats (75–100 g) were anesthetized with halothane, bisected sub-diaphramatically and decerebrated pre-collicularly ( n = 15). The preparation was transferred to a recording chamber and perfused via the thoracic aorta with Ringer’s solution containing an oncotic agent (Ficoll, 1.25%). SSR was activated by electrical stimulation (5 s) of the brachial nerve (0.5–40 Hz, 1–20 V, 0.1 ms) or the forelimb (0.5–40 Hz, 5–60 V, 2 ms). Stimulation at 40 Hz significantly increased heart rate (HR, 366 ± 10 to 374 ± 9 beats/min), systemic perfusion pressure (PP, 83 ± 5 to 89 ± 6 mmHg) and phrenic nerve discharge (PND, 0.4 ± 0.1 to 1.4 ± 0.3 Hz). Ganglionic blockade with hexamethonium (300 μM) eliminated the tachycardia and pressor response but did not alter the tachypnea to forelimb stimulation ( n = 3). Transection of the brachial nerve plexus abolished the increase in PP and PND ( n = 4). This indicates that a neural reflex mediated these responses. Spinal transection (C1–C2) completely abolished all responses indicating that they were mediated via a supraspinal pathway ( n = 2). Based upon these findings, we conclude that activation of somatosensory afferent fibers in the WHBP evokes a programmed pattern of autonomic responses altering the activity-state of both the cardiovascular and respiratory systems. The WHBP provides a unique opportunity to investigate the medullary circuits and neuronal mechanisms that may be involved in coupling cardiorespiratory and somatomotor activity during locomotion/exercise.

Spinal GABA(A) receptors do not mediate the sympathetic baroreceptor reflex in the rat.

Activation of baroreceptors causes efferent sympathetic nerve activity (SNA) to fall. Two mechanisms could account for this sympathoinhibition: disfacilitation of sympathetic preganglionic neurons (SPN) and/or direct inhibition of SPN. The roles that spinal GABA and glycine receptors play in the baroreceptor reflex were examined in anesthetized, paralyzed, and artificially ventilated rats. Spinal GABA(A) receptors were blocked by an intrathecal injection of bicuculline methiodide, whereas glycine receptors were blocked with strychnine. Baroreceptors were activated by stimulation of the aortic depressor nerve (ADN), and a somatosympathetic reflex was used as control. After an intrathecal injection of vehicle, there was no effect on any measured variable or evoked reflex. In contrast, bicuculline caused a dose-dependent increase in arterial pressure, SNA, phrenic nerve discharge, and it significantly facilitated the somatosympathetic reflex. However, bicuculline did not attenuate either the depressor response or sympathoinhibition evoked after ADN stimulation. Similarly, strychnine did not affect the baroreceptor-induced depressor response. Thus GABA(A) and glycine receptors in the spinal cord have no significant role in baroreceptor-mediated sympathoinhibition.

CO(2)/H(+) chemoreception in the cat pre-Bötzinger complex in vivo.

We examined the effects of focal tissue acidosis in the pre-Bötzinger complex (pre-BötC; the proposed locus of respiratory rhythm generation) on phrenic nerve discharge in chloralose-anesthetized, vagotomized, paralyzed, mechanically ventilated cats. Focal tissue acidosis was produced by unilateral microinjection of 10-20 nl of the carbonic anhydrase inhibitors acetazolamide (AZ; 50 microM) or methazolamide (MZ; 50 microM). Microinjection of AZ and MZ into 14 sites in the pre-BötC reversibly increased the peak amplitude of integrated phrenic nerve discharge and, in some sites, produced augmented bursts (i.e., eupneic breath ending with a high-amplitude, short-duration burst). Microinjection of AZ and MZ into this region also reversibly increased the frequency of eupneic phrenic bursts in seven sites and produced premature bursts (i.e., doublets) in five sites. Phrenic nerve discharge increased within 5-15 min of microinjection of either agent; however, the time to the peak increase and the time to recovery were less with AZ than with MZ, consistent with the different pharmacological properties of AZ and MZ. In contrast to other CO(2)/H(+) brain stem respiratory chemosensitive sites demonstrated in vivo, which have only shown increases in amplitude of integrated phrenic nerve activity, focal tissue acidosis in the pre-BötC increases frequency of phrenic bursts and produces premature (i.e., doublet) bursts. These data indicate that the pre-BötC has the potential to play a role in the modulation of respiratory rhythm and pattern elicited by increased CO(2)/H(+) and lend additional support to the concept that the proposed locus for respiratory rhythm generation has intrinsic chemosensitivity.

Pre-Bötzinger complex functions as a central hypoxia chemosensor for respiration in vivo.

Recently, we identified a region located in the pre-Bötzinger complex (pre-BötC; the proposed locus of respiratory rhythm generation) in which activation of ionotropic excitatory amino acid receptors using DL-homocysteic acid (DLH) elicits a variety of excitatory responses in the phrenic neurogram, ranging from tonic firing to a rapid series of high-amplitude, rapid rate of rise, short-duration inspiratory bursts that are indistinguishable from gasps produced by severe systemic hypoxia. Therefore we hypothesized that this unique region is chemosensitive to hypoxia. To test this hypothesis, we examined the response to unilateral microinjection of sodium cyanide (NaCN) into the pre-BötC in chloralose- or chloralose/urethan-anesthetized vagotomized, paralyzed, mechanically ventilated cats. In all experiments, sites in the pre-BötC were functionally identified using DLH (10 mM, 21 nl) as we have previously described. All sites were histologically confirmed to be in the pre-BötC after completion of the experiment. Unilateral microinjection of NaCN (1 mM, 21 nl) into the pre-BötC produced excitation of phrenic nerve discharge in 49 of the 81 sites examined. This augmentation of inspiratory output exhibited one of the following changes in cycle timing and/or pattern: 1) a series of high-amplitude, short-duration bursts in the phrenic neurogram (a discharge similar to a gasp), 2) a tonic excitation of phrenic neurogram output, 3) augmented bursts in the phrenic neurogram (i.e., eupneic breath ending with a gasplike burst), or 4) an increase in frequency of phrenic bursts accompanied by small increases or decreases in the amplitude of integrated phrenic nerve discharge. Our findings identify a locus in the brain stem in which focal hypoxia augments respiratory output. We propose that the respiratory rhythm generator in the pre-BötC has intrinsic hypoxic chemosensitivity that may play a role in hypoxia-induced gasping.

Effects of prestimulus respiratory levels on inhibitory respiratory response by nociceptive muscular afferents.

We have previously shown that the inhibitory respiratory response, which we call post-stimulus suppression, is induced by nociceptive muscular afferents. This phenomenon is thought to be caused by a negative feedback induced by excessive afferent inputs. In the present study, we investigated whether augmented levels of prestimulus respiration would influence the magnitude of poststimulus suppression by recording the phrenic nerve discharges in chloralose-urethane anesthetized, vagotomized, paralyzed and artificially ventilated cats. The respiratory level was augmented by means of either hypercapnia, hypoxia or naloxone administration, all of which markedly facilitated the peak amplitude (PK) of integrated phrenic discharges, neural tidal volume. When the electrical stimulation of thin-fiber muscular afferents was performed at these augmented PK levels, the magnitude of poststimulus suppression in the PK was markedly attenuated without consistently altering the facilitatory response during the stimulation period. It seems that the facilitatory component of the augmented level of resting respiration may reduce the inhibitory component of poststimulus suppression. The results indicate that prestimulus respiratory activity is an important factor in determining the magnitude of poststimulus suppression.