Central nervous mechanisms in the generation of the pattern of breathing

The role of the B?tzinger complex (B?tC) and the pre-B?tzinger complex (pre-B?tC) in the genesis of the breathing pattern was investigated in anesthetized, vagotomized, paralysed and artificially ventilated rabbits making use of bilateral microinjections of kainic acid (KA) and excitatory amino acid (EAA) receptor antagonists. KA microinjections into either the B?tC or the pre-B?tC transiently eliminated respiratory rhythmicity in the presence of tonic phrenic activity (tonic apnea). Rhythmic activity resumed as low-amplitude, high-frequency irregular oscillations, superimposed on tonic inspiratory activity and displayed a progressive, although incomplete recovery. Microinjections of kynurenic acid (KYN) and D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) into the B?tC caused a pattern of breathing characterized by low-amplitude, high-frequency irregular oscillations and subsequently tonic apnea. Responses to KYN and D-AP5 in the pre-B?tC were similar, although less pronounced than those elicited by these drugs in the B?tC and never characterized by tonic apnea. Microinjections of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) into the B?tC and the pre-B?tC induced much less intense responses mainly consisting of increases in respiratory frequency. The results show that the investigated medullary regions play a prominent role in the genesis of the normal pattern of breathing through the endogenous activation of EAA receptors.     

Focal CO2/H+ alters phrenic motor output response to chemical stimulation of cat pre-Botzinger complex in vivo.

Microinjection of dl-homocysteic acid (DLH), a glutamate analog, into the pre-B?tzinger complex (pre-B?tC) can produce tonic excitation of phrenic nerve discharge. Although this DLH-induced tonic excitation can be modified by systemic hypercapnia, the role of focal increases in pre-B?tC CO(2)/H(+) in this modulation of the DLH-induced response remains to be determined. Therefore, we examined the effects of unilateral microinjection of DLH (10 mM; 10-20 nl) into the pre-B?tC before and during increased focal pre-B?tC CO(2)/H(+) (i.e., focal tissue acidosis) in chloralose-anesthetized, vagotomized, mechanically ventilated cats. Focal tissue acidosis was produced by blockade of carbonic anhydrase with either focal acetazolamide (AZ) or methazolamide (MZ) microinjection. For these experiments, sites were selected in which unilateral microinjection of DLH into the pre-B?tC produced a nonphasic tonic excitation of phrenic nerve discharge (n = 10). Microinjection of 10-20 nl AZ (50 microM) or MZ (50 microM) into these 10 sites in the pre-B?tC increased the amplitude and/or frequency of eupneic phrenic bursts, as previously reported. Subsequent microinjection of DLH produced excitation in which phasic respiratory bursts were superimposed on tonic discharge. These DLH-induced phasic respiratory bursts had an increased frequency compared with the preinjection baseline frequency (P < 0.05). These findings demonstrate that modulation of phrenic motor activity evoked by DLH-induced activation of the pre-B?tC is influenced by focal CO(2)/H(+) chemosensitivity in this region. Furthermore, these findings suggest that focal increases in pre-B?tC CO(2)/H(+) may have contributed to the modulation of the DLH-induced responses previously observed during systemic hypercapnia.     

Chemical activation of pre-Bötzinger complex in vivo reduces respiratory network complexity

In the in vivo anesthetized adult cat model, multiple patterns of inspiratory motor discharge have been recorded in response to chemical stimulation and focal hypoxia of the pre-B?tzinger complex (pre-B?tC), suggesting that this region may participate in the generation of complex respiratory dynamics. The complexity of a signal can be quantified using approximate entropy (ApEn) and multiscale entropy (MSEn) methods, both of which measure the regularity (orderliness) in a time series, with the latter method taking into consideration temporal fluctuations in the underlying dynamics. The current investigation was undertaken to examine the effects of pre-B?tC-induced excitation of phasic phrenic nerve discharge, which is characterized by high-amplitude, rapid-rate-of-rise, short-duration bursts, on the complexity of the central inspiratory neural controller in the vagotomized, chloralose-anesthetized adult cat model. To assess inspiratory neural network complexity, we calculated the ApEn and MSEn of phrenic nerve bursts during eupneic (basal) discharge and during pre-B?tC-induced excitation of phasic inspiratory bursts. Chemical stimulation of the pre-B?tC using DL-homocysteic acid (DLH; 10 mM; 10-20 nl; n=10) significantly reduced the ApEn from 0.982+/-0.066 (mean+/-SE) to 0.664+/-0.067 (P<0.001) followed by recovery ( approximately 1-2 min after DLH) of the ApEn to 1.014+/-0.067; a slightly enhanced magnitude reduction in MSEn was observed. Focal pre-B?tC hypoxia (induced by sodium cyanide; NaCN; 1 mM; 20 nl; n=2) also elicited a reduction in both ApEn and MSEn, similar to those observed for the DLH-induced response. These observations demonstrate that activation of the pre-B?tC reduces inspiratory network complexity, suggesting a role for the pre-B?tC in regulation of complex respiratory dynamics.     

Modulation of respiratory responses to chemoreflex activation by L-glutamate and ATP in the rostral ventrolateral medulla of awake rats

Presympathetic neurons in the different anteroposterior aspects of rostral ventrolateral medulla (RVLM) are colocalized with expiratory [B?tzinger complex (B?tC)] and inspiratory [pre-B?tzinger complex (pre-B?tC)] neurons of ventral respiratory column (VRC), suggesting that this region integrates the cardiovascular and respiratory chemoreflex responses. In the present study, we evaluated in different anteroposterior aspects of RVLM of awake rats the role of ionotropic glutamate and purinergic receptors on cardiorespiratory responses to chemoreflex activation. The bilateral ionotropic glutamate receptors antagonism with kynurenic acid (KYN) (8 nmol/50 nl) in the rostral aspect of RVLM (RVLM/B?tC) enhanced the tachypneic (120 ± 9 vs. 180 ± 9 cpm; P < 0.01) and attenuated the pressor response (55 ± 2 vs. 15 ± 1 mmHg; P < 0.001) to chemoreflex activation (n = 7). On the other hand, bilateral microinjection of KYN into the caudal aspect of RVLM (RVLM/pre-B?tC) caused a respiratory arrest in four awake rats used in the present study. Bilateral P2X receptors antagonism with PPADS (0.25 nmol/50 nl) in the RVLM/B?tC reduced chemoreflex tachypneic response (127 ± 6 vs. 70 ± 5 cpm; P < 0.001; n = 6), but did not change the chemoreflex pressor response. In addition, PPADS into the RVLM/B?tC attenuated the enhancement of the tachypneic response to chemoreflex activation elicited by previous microinjections of KYN into the same subregion (188 ± 2 vs. 157 ± 3 cpm; P < 0.05; n = 5). Our findings indicate that: 1) L-glutamate, but not ATP, in the RVLM/B?tC is required for pressor response to peripheral chemoreflex and 2) both transmitters in the RVLM/B?tC are required for the processing of the ventilatory response to peripheral chemoreflex activation in awake rats.     

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

We examined the effects of focal tissue acidosis in the pre-B?tzinger complex (pre-B?tC; 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-B?tC 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-B?tC increases frequency of phrenic bursts and produces premature (i.e., doublet) bursts. These data indicate that the pre-B?tC 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.     

Ionotropic excitatory amino acid receptors in pre-Botzinger complex play a modulatory role in hypoxia-induced gasping in vivo.

Activation of ionotropic excitatory amino acid (EAA) receptors in pre-B?tzinger complex (pre-B?tC) not only influences the eupneic pattern of phrenic motor output but also modifies hypoxia-induced gasping in vivo by increasing gasp frequency. Although ionotropic EAA receptor activation in this region appears to be required for the generation of eupneic breathing, it remains to be determined whether similar activation is necessary for the production and/or expression of hypoxia-induced gasping. Therefore, we examined the effects of severe brain hypoxia before and after blockade of ionotropic EAA receptors in the pre-B?tC in eight chloralose-anesthetized, deafferented, mechanically ventilated cats. In each experiment, before blockade of ionotropic EAA receptors in the pre-B?tC, severe brain hypoxia (6% O2 in a balance of N2 for 3-6 min) produced gasping. Although bilateral microinjection of the broad-spectrum ionotropic EAA receptor antagonist kynurenic acid (20-100 mM; 40 nl) into the pre-B?tC eliminated basal phrenic nerve discharge, severe brain hypoxia still produced gasping. Under these conditions, however, the onset latency to gasping was increased (P < 0.05), the number of gasps was reduced for the same duration of hypoxic gas exposure (P < 0.05), the duration of gasps was prolonged (P < 0.05), and the duration between gasps was increased (P < 0.05). These findings demonstrate that hypoxia-induced gasping in vivo does not require activation of ionotropic EAA receptors in the pre-B?tC, but ionotropic EAA receptor activation in this region may modify the expression of the hypoxia-induced response. The present findings also provide additional support for the pre-B?tC as the primary locus of respiratory rhythm generation.     

Influence of respiratory network drive on phrenic motor output evoked by activation of cat pre-Botzinger complex

We have previously demonstrated that microinjection of dl-homocysteic acid (DLH), a glutamate analog, into the pre-B?tzinger complex (pre-B?tC) can produce either phasic or tonic excitation of phrenic nerve discharge during hyperoxic normocapnia. Breathing, however, is influenced by input from both central and peripheral chemoreceptor activation. This influence of increased respiratory network drive on pre-B?tC-induced modulation of phrenic motor output is unclear. Therefore, these experiments were designed to examine the effects of chemical stimulation of neurons (DLH; 10 mM; 10-20 nl) in the pre-B?tC during hyperoxic modulation of CO2 (i.e., hypercapnia and hypocapnia) and during normocapnic hypoxia in chloralose-anesthetized, vagotomized, mechanically ventilated cats. For these experiments, sites were selected in which unilateral microinjection of DLH into the pre-B?tC during baseline conditions of hyperoxic normocapnia [arterial PCO2 (PaCO2) = 37-43 mmHg; n = 22] produced a tonic (nonphasic) excitation of phrenic nerve discharge. During hypercapnia (PaCO2 = 59.7 +/- 2.8 mmHg; n = 17), similar microinjection produced excitation in which phasic respiratory bursts were superimposed on varying levels of tonic discharge. These DLH-induced phasic respiratory bursts had an increased frequency compared with the preinjection baseline frequency (P < 0.01). In contrast, during hypocapnia (PaCO2 = 29.4 +/- 1.5 mmHg; n = 11), microinjection of DLH produced nonphasic tonic excitation of phrenic nerve discharge that was less robust than the initial (normocapnic) response (i.e., decreased amplitude). During normocapnic hypoxia (PaCO2 = 38.5 +/- 3.7; arterial Po2 = 38.4 +/- 4.4; n = 8) microinjection of DLH produced phrenic excitation similar to that seen during hypercapnia (i.e., increased frequency of phasic respiratory bursts superimposed on tonic discharge). These findings demonstrate that phrenic motor activity evoked by chemical stimulation of the pre-B?tC is influenced by and integrates with modulation of respiratory network drive mediated by input from central and peripheral chemoreceptors.     

Influences of superior laryngeal afferent stimulation on expiratory activity in cats

The effects of superior laryngeal nerve (SLN) stimulation on the activity of the expiratory muscles and medullary expiration-related (ER) neurons were investigated in 24 pentobarbital-anesthetized cats. In some experiments the animals were also paralyzed and artificially ventilated. Sustained tetanic stimulation of SLN consistently caused an apneic response associated with the appearance of tonic CO2-dependent activity in the expiratory muscles and in ER neurons located in the caudal ventral respiratory group (VRG) and the B?tzinger complex. Single shocks or brief tetani at the same stimulation intensities failed to evoke excitatory responses in the expiratory muscles and in the vast majority of ER neurons tested. At higher stimulation strengths, single shocks or short tetani elicited excitatory responses in the expiratory muscles (20- to 35-ms latency) and in the majority of ER neurons of the caudal VRG (7.5- to 15.5-ms latency). These responses were obtained only during the expiratory phase and proved to be CO2 independent. On the contrary, only inhibitory responses were evoked in the activity of B?tzinger complex neurons. The observed tonic expiratory activity most likely represents a disinhibition phenomenon due to the suppression of inspiratory activity; activation of expiratory muscles at higher stimulation intensities appears to be a polysynaptic reflex mediated by ER neurons of the caudal VRG but not by B?tzinger complex neurons.     

Botzinger complex region role in phrenic-to-phrenic inhibitory reflex of cat

Neuronal recordings, microstimulation, and electrolytic and chemical lesions were used to examine the involvement of the B?tzinger Complex (B?tC) in the bilateral phrenic-to-phrenic inhibitory reflex. Experiments were conducted in decerebrate cats that were paralyzed, ventilated, thoracotomized, and vagotomized. Microelectrode recordings within the B?tC region revealed that some neurons were activated by phrenic nerve stimulation (15 of 69 expiratory units, 9 of 67 inspiratory units, and 19 nonrespiratory-modulated units) at average latencies similar to the onset latency of the phrenic-to-phrenic inhibition. In addition, microstimulation within the B?tC caused a short latency transient inhibition of phrenic motor activity. In 17 cats phrenic neurogram responses to threshold and supramaximal (15 mA) stimulation of phrenic nerve afferents were recorded before and after electrolytic B?tC lesions. In 15 animals the inhibitory reflex was attenuated by bilateral lesions. Because lesion of either B?tC neurons or axons of passage could account for this attenuation, in eight experiments the phrenic-to-phrenic inhibitory responses were recorded before and after bilateral injections of 5 microM kainic acid (30-150 nl) into the B?tC. After chemical lesions, the inhibitory response to phrenic nerve stimulation remained; however, neuronal activity typical of the B?tC could not be located. These results suggest that axons important in producing the phrenic-to-phrenic reflex pass through the region of the B?tC, but that B?tC neurons themselves are not necessary for this reflex.     

Large lesions in the pre-B?tzinger complex area eliminate eupneic respiratory rhythm in awake goats.

In awake goats, 29% bilateral destruction of neurokinin-1 receptor-expressing neurons in the pre-B?tzinger complex (pre-B?tzC) area with saporin conjugated to substance P results in transient disruptions of the normal pattern of eupneic respiratory muscle activation (Wenninger JM, Pan LG, Klum L, Leekley T, Bastastic J, Hodges MR, Feroah T, Davis S, and Forster HV. J Appl Physiol 97: 1620-1628, 2004). Therefore, the purpose of these studies was to determine whether large or total lesioning in the pre-B?tzC area of goats would eliminate phasic diaphragm activity and the eupneic breathing pattern. In awake goats that already had 29% bilateral destruction of neurokinin-1 receptor-expressing neurons in the pre-B?tzC area, bilateral ibotenic acid (10 microl, 50 mM) injection into the pre-B?tzC area resulted in a tachypneic hyperpnea that reached a maximum (132 +/- 10.1 breaths/min) approximately 30-90 min after bilateral injection. Thereafter, breathing frequency declined, central apneas resulted in arterial hypoxemia (arterial Po2 approximately 40 Torr) and hypercapnia (arterial Pco2 approximately 60 Torr), and, 11 +/- 3 min after the peak tachypnea, respiratory failure was followed by cardiac arrest in three airway-intact goats. However, after the peak tachypnea in four tracheostomized goats, mechanical ventilation was initiated to maintain arterial blood gases at control levels, during which there was no phasic diaphragm or abdominal muscle activity. When briefly removed from the ventilator (approximately 90 s), these goats became hypoxemic and hypercapnic. During this time, minimal, passive inspiratory flow resulted from phasic abdominal muscle activity. We estimate that 70% of the neurons within the pre-B?tzC area were lesioned in these goats. We conclude that, in the awake state, the pre-B?tzC is critical for generating a diaphragm, eupneic respiratory rhythm, and that, in the absence of the pre-B?tzC, spontaneous breathing reflects the activity of an expiratory rhythm generator.     

Ventrolateral medullary respiratory network and a model of cough motor pattern generation

The primaryhypothesis of this study was that the cough motor pattern is produced,at least in part, by the medullary respiratory neuronal network inresponse to inputs from "cough" and pulmonary stretch receptorrelay neurons in the nucleus tractus solitarii. Computer simulations ofa distributed network model with proposed connections from the nucleustractus solitarii to ventrolateral medullary respiratory neuronsproduced coughlike inspiratory and expiratory motor patterns. Predictedresponses of various "types" of neurons (I-DRIVER, I-AUG, I-DEC,E-AUG, and E-DEC) derived from the simulations were tested in vivo.Parallel and sequential responses of functionally characterizedrespiratory-modulated neurons were monitored during fictive cough indecerebrate, paralyzed, ventilated cats. Coughlike patterns in phrenicand lumbar nerves were elicited by mechanical stimulation of theintrathoracic trachea. Altered discharge patterns were measured in mosttypes of respiratory neurons during fictive cough. The resultssupported many of the specific predictions of our cough generationmodel and suggested several revisions. The two main conclusions were asfollows: 1) TheBötzinger/rostral ventral respiratory group neurons implicated inthe generation of the eupneic pattern of breathing also participate inthe configuration of the cough motor pattern.2) This altered activity ofBötzinger/rostral ventral respiratory group neurons istransmitted to phrenic, intercostal, and abdominal motoneurons via thesame bulbospinal neurons that provide descending drive during eupnea.

     

Endogenous 5-HT2B receptor activation regulates neonatal respiratory activity in vitro

An implication of 5-HT(2B) receptors in central nervous system has not yet been clearly elucidated. We studied the role of different 5-HT(2) receptor subtypes in the medullary breathing center, the pre-B?tzinger complex, and on hypoglossal motoneurons in rhythmically active transversal slice preparations of neonatal rats and mice. Local microinjection of 5-HT(2) receptor agonists revealed tonic excitation of hypoglossal motoneurons. Excitatory effects of the 5-HT(2B) receptor agonist BW723C86 could be blocked by bath application of LY272015, a highly selective 5-HT(2B) receptor antagonist. Excitatory effects of the 5-HT(2A/B/C) receptor agonist alpha-methyl 5-HT could be blocked by the preferential 5-HT(2A) receptor antagonist ketanserin. Therefore, 5-HT-induced excitation of hypoglossal motoneurons is mediated by convergent activation of 5-HT(2A) and 5-HT(2B) receptors. Local microinjection of BW723C86 in the pre-B?tzinger complex increased respiratory frequency. Bath application of LY272015 blocked respiratory activity, whereas ketanserin had no effect. Therefore, endogenous 5-HT appears to support tonic action on respiratory rhythm generation via 5-HT(2B) receptors. In preparations of 5-HT(2B) receptor-deficient mice, respiratory activity appeared unaltered. Whereas BW723C86 and LY272015 had no effects, bath application of ketanserin disturbed and blocked rhythmic activity. This demonstrates a stimulatory role of endogenous 5-HT(2B) receptor activation at the pre-B?tzinger complex and hypoglossal motoneurons that can be taken up by 5-HT(2A) receptors in the absence of 5-HT(2B) receptors. The presence of functional 5-HT(2B) receptors in the neonatal medullary breathing center indicates a potential convergent regulatory role of 5-HT(2B) and -(2A) receptors on the central respiratory network.     

Parvalbumin in respiratory neurons of the ventrolateral medulla of the adult rat

A column of parvalbumin immunoreactive neurons is closely associated with the location of respiratory neurons in the ventrolateral medulla of the rat. The majority (66%) of bulbospinal neurons in the medullary ventral respiratory column (VRC) that were retrogradely labeled by tracer injections in the phrenic nucleus were also positive for parvalbumin. In contrast, only 18.8% of VRC neurons retrogradely labeled after a tracer injection in the VRC, also expressed parvalbumin. The average cross-sectional area of VRC neurons retrogradely labeled after VRC injections was 193.8 μm² ± 6.6 SE. These were significantly smaller than VRC parvalbumin neurons (271.9 μm² ± 12.3 SE). Parvalbumin neurons were found in the Bötzinger Complex, the rostral ventral respiratory group (VRG), and the caudal VRG, areas which all contribute to the bulbospinal projection. In contrast, parvalbumin neurons were sparse or absent in the preBötzinger Complex and in the vicinity of the retrotrapezoid nucleus, areas that have few bulbospinal projections. Parvalbumin was rarely colocalized within Neurokinin-1 receptor positive (NK1R) VRC neurons, which are found in the preBötzinger complex and in the anteroventral part of the rostral VRG. Parvalbumin neurons in the Bötzinger Complex and rostral VRG help define the rostrocaudal extent of these regions. The absence of parvalbumin neurons from the intervening preBötzinger complex also helps establish the boundaries of this region. Regional boundaries described in this manner are in good agreement with earlier physiological and anatomical studies. Taken together, the distributions of parvalbumin, NK1R and bulbospinal neurons suggest that the rostral VRG may be subdivided into distinct, anterodorsal, anteroventral, and posterior subdivisions.     

Ventrolateral medullary respiratory network participation in the expiration reflex in the cat.

The expiration reflex is a distinct airway defensive response characterized by a brief, intense expiratory effort and coordinated adduction and abduction of the laryngeal folds. This study addressed the hypothesis that the ventrolateral medullary respiratory network participates in the reflex. Extracellular neuron activity was recorded with microelectrode arrays in decerebrated, neuromuscular-blocked, ventilated cats. In 32 recordings (17 cats), 232 neurons were monitored in the rostral (including B?tzinger and pre-B?tzinger complexes) and caudal ventral respiratory group. Neurons were classified by firing pattern, evaluated for spinal projections, functional associations with recurrent laryngeal and lumbar nerves, and firing rate changes during brief, large increases in lumbar motor nerve discharge (fictive expiration reflex, FER) elicited during mechanical stimulation of the vocal folds. Two hundred eight neurons were respiratory modulated, and 24 were nonrespiratory; 104 of the respiratory and 6 of the nonrespiratory-modulated neurons had altered peak firing rates during the FER. Increased firing rates of bulbospinal neurons and expiratory laryngeal premotor and motoneurons during the expiratory burst of FER were accompanied by changes in the firing patterns of putative propriobulbar neurons proposed to participate in the eupneic respiratory network. The results support the hypothesis that elements of the rostral and caudal ventral respiratory groups participate in generating and shaping the motor output of the FER. A model is proposed for the participation of the respiratory network in the expiration reflex.     

Anatomical and functional development of the pre-B?tzinger complex in prenatal rodents.

Developmental anomalies of central respiratory neural control contribute to newborn mortality and morbidity. Elucidation of the cellular, molecular, trophic, and genetic mechanisms involved in the formation and function of respiratory nuclei during prenatal development will provide a foundation for understanding pathologies. The pre-B?tzinger Complex (pre-B?tC) is a specific group of neurons located in the ventrolateral medulla that is critical for respiratory rhythmogenesis. Thus it has become a major focus of research. Here, we provide an overview of current knowledge regarding the anatomical and functional emergence of the rodent pre-B?tC during the prenatal period.     

Recovery of breathing pattern after 15 min of cerebral ischemia in rabbits

The study was undertaken to ascertain the neural control of breathing and vagal reflexes during and after cerebral ischemia. The experiments were performed on anesthetized, paralyzed, and artificially ventilated rabbits. Cerebral ischemia was induced by reversible intrathoracic occlusion of the brachiocephalic trunk and the left subclavian and both internal thoracic arteries for 15 min. The effect of cerebral ischemia on breathing pattern was assessed by monitoring the integrated activities of phrenic and recurrent laryngeal nerves. Ischemia produced enhancement of breathing followed by apnea and gasping. During enhanced breathing as well as during gasping, the inspiratory-inhibiting effect of lung inflation (Breuer-Hering reflex) was abolished. When brain circulation was restored, respiratory activity started with gasps, which later were intermingled with eupneic type of inspirations. During the onset of a eupneic breath, lung inflation produced inspiratory facilitation but never an inhibition. However, after 30 min of recovery from cerebral ischemia, the Breuer-Hering reflex was restored. Results show that precise analysis of vagal reflexes and respiratory pattern during ischemia and resuscitation may be used as an indicator of resumption of autonomic activity in the brain stem.     

家兔面神经后核内侧区在呼吸节律起源中的作用

从腹侧面暴露家兔延髓,脑内微量注射1%普鲁卡因阻滞面神经后核内侧区(mNRF),全部动物(n=20)一次注射(0.3—1.0μl)后即能可逆地消除呼吸节律。区域对照显示此区非常局限,范围约1.0×1.0×1.0mm。组织学检查表明为面神经后核内侧区。本文分析了 mNRF的呼吸相关神经元(RRNs)的放电形式。在 mNRF 有较多的呼气(E)神经元和呼气-吸气跨时相(E-IPS)神经元。在阻滞 mNRF 引起呼吸停止期间,观察到低位延髓背侧呼吸群(DRG)和腹侧呼吸群(VRG)尾端区 RRNs 放电的节律性消失,表现连续放电或停止放电。电刺激DRG,VRG 尾端区,只能诱发短串的膈神经放电,而不能产生节律性发放。说明这些区域的RRNs 无自动节律性活动的能力。结果表明,面神经后核内侧区与呼吸节律发生有关,它可能是呼吸节律发生器的一个重要的所在部位。     

Respiratory rhythm generation in rats: The importance of inhibition

Stimulation-related modifications of activity in the phrenic nerve and external and internal intercostal nerves were studied on urethane-anesthetized rats; the inspiratory medullary structures were stimulated. The activity was recorded either following microinjections of gamma-aminobutyric acid (GABA) or its derivatives into the medial parabrachial nuclei and rostral part of the ventral respiratory group of medullary neurons, or without such microinjections. Gradual dependence of activity in these nerves on the phase of the respiratory cycle was established. It was shown that the higher was the integrated inspiratory activity, the lower became the relative gain in phrenic nerve activity caused by standard stimulation. When stimulation was applied at the postinspiratory phase, the threshold stimulus intensity showed an S-like rise with an increase in integrated inspiratory activity. Microinjections of GABA or its cyclic derivatives into the parabrachial nuclear structures decreased the inhibitory effects of the latter. During the postinspiratory phase, the effect was opposite: an increase in the relative gain of inspiratory activity and drop in the threshold. The resulting data suggest that there is a two-level organization of the respiratory regulatory inhibition and that the whole respiratory neuronal network has a compartmental structure.Neirofiziologiya/Neurophysiology, Vol. 25, No. 6, pp. 420–426, November–December, 1993.     

Small reduction of neurokinin-1 receptor-expressing neurons in the pre-B?tzinger complex area induces abnormal breathing periods in awake goats.

In awake rats, >80% bilateral reduction of neurokinin-1 receptor (NK1R)-expressing neurons in the pre-B?tzinger complex (pre-B?tzC) resulted in hypoventilation and an "ataxic" breathing pattern (Gray PA, Rekling JC, Bocchiaro CM, Feldman JL, Science 286: 1566-1568, 1999). Accordingly, the present study was designed to gain further insight into the role of the pre-B?tzC area NK1R-expressing neurons in the control of breathing during physiological conditions. Microtubules were chronically implanted bilaterally into the medulla of adult goats. After recovery from surgery, the neurotoxin saporin conjugated to substance P, specific for NK1R-expressing neurons, was bilaterally injected (50 pM in 10 microl) into the pre-B?tzC area during the awake state (n = 8). In unoperated goats, 34 +/- 0.01% of the pre-B?tzC area neurons are immunoreactive for the NK1R, but, in goats after bilateral injection of SP-SAP into the pre-B?tzC area, NK1R immunoreactivity was reduced to 22.5 +/- 2.5% (29% decrease, P < 0.01). Ten to fourteen days after the injection, the frequency of abnormal breathing periods was sixfold greater than before injection (107.8 +/- 21.8/h, P < 0.001). Fifty-six percent of these periods were breaths of varying duration and volume with an altered respiratory muscle activation pattern, whereas the remaining were rapid, complete breaths with coordinated inspiratory-expiratory cycles. The rate of occurrence and characteristics of abnormal breathing periods were not altered during a CO2 inhalation-induced hyperpnea. Pathological breathing patterns were eliminated during non-rapid eye movement sleep in seven of eight goats, but they frequently occurred on arousal from non-rapid eye movement sleep. We conclude that a moderate reduction in pre-B?tzC NK1R-expressing neurons results in state-dependent transient changes in respiratory rhythm and/or eupneic respiratory muscle activation patterns.