Expiratory Neurons Research Articles

Presympathetic neuron dysfunction--time to reconsider increased intrinsic activity as the cause of neurogenic hypertension.

Presympathetic neuron dysfunction--time to reconsider increased intrinsic activity as the cause of neurogenic hypertension.

Orexin induces excitation of respiratory neuronal network in isolated brainstem spinal cord of neonatal rat

Endogenous neuropeptides known as orexins (hypocretins) play important roles in the regulation of feeding, drinking, endocrine function, and sleep/wakefulness. Orexin neuron projection sites include the rostral ventrolateral medulla of brainstem, which is related to the control of breathing. Previous studies suggest that orexins modulate the central CO2 ventilatory response during wakefulness in rodent. In the present study, we examined the effects of the orexinergic system on central respiratory control by adding orexin into a superfusion medium in the isolated brainstem–spinal cord of neonatal rat. Exposure to orexin B resulted in dose-dependent increases in C4 burst rate via brainstem, but not spinal cord. These increases in C4 burst rate induced concomitant increases in the depolarizing cycle rate of pre-inspiratory (Pre-I) and inspiratory (Insp) neurons. Tonic discharge was induced on C4 recording, although the rhythmic bursts of Pre-I and Insp neurons were maintained. Expiratory (Exp) neurons were also depolarized on administration of orexin B. Our findings indicate that orexin B activates central respiratory activity, mainly through depolarization and decreases in membrane resistance in Pre-I and Insp neurons, and possibly through early initiation of the expiratory phase induced by depolarization of Exp neurons.

Intrinsic properties of rostral ventrolateral medulla presympathetic and bulbospinal respiratory neurons of juvenile rats are not affected by chronic intermittent hypoxia.

The presympathetic neurons in the rostral ventrolateral medulla (RVLM) are considered to be the source of the sympathetic activity, and there is experimental evidence that these cells present intrinsic autodepolarization. There is also evidence that an important respiratory neuronal population located in the RVLM/Bötzinger complex (BötC) corresponds to augmenting expiratory neurons (aug-E), which send projections to the phrenic nucleus in the spinal cord. However, the pacemaker activity of presympathetic neurons and the intrinsic properties of aug-E neurons had not been evaluated in brainstem slices of juvenile rats (postnatal day35). Chronic intermittent hypoxia (CIH) is a sympathetic-mediated hypertension model, which seems to produce an associated increase in the activity of aug-E neurons. In this study, we evaluated the effects of CIH on the intrinsic properties of RVLM/BötC presympathetic and phrenic nucleus-projecting neurons (aug-E) in brainstem slices of juvenile rats (postnatal day35). We observed that all presympathetic neurons presented spontaneous action potential firing (n=18), which was not abolished by ionotropic receptor antagonism. In addition, exposure to 10days of CIH produced no changes in their intrinsic passive properties, firing pattern or excitability. Most aug-E neurons presented spontaneous firing in control conditions (13 of 15 neurons), and this characteristic was preserved after blocking fast synaptic transmission (12 of 15 neurons), clearly demonstrating their intrinsic pacemaker activity. Chronic intermittent hypoxia also produced no changes in intrinsic passive properties, frequency and pattern of discharge or excitability of the aug-E neurons. The present study shows that: (i)it is possible to record the electrophysiological properties of RVLM/BötC presympathetic and aug-E neurons in brainstem slices from juvenile rats; (ii)these neurons present characteristics of intrinsic pacemakers; and (iii)their intrinsic properties were not altered by chronic intermittent hypoxia.

Short‐term sustained hypoxia induces changes in the coupling of sympathetic and respiratory activities in rats

Individuals experiencing sustained hypoxia (SH) exhibit adjustments in the respiratory and autonomic functions by neural mechanisms not yet elucidated. In the present study we evaluated the central mechanisms underpinning the SH-induced changes in the respiratory pattern and their impact on the sympathetic outflow. Using a decerebrated arterially perfused in situ preparation, we verified that juvenile rats exposed to SH (10% O2) for 24h presented an active expiratory pattern, with increased abdominal, hypoglossal and vagal activities during late-expiration (late-E). SH also enhanced the activity of augmenting-expiratory neurones and depressed the activity of post-inspiratory neurones of the Bötzinger complex (BötC) by mechanisms not related to changes in their intrinsic electrophysiological properties. SH rats exhibited high thoracic sympathetic activity and arterial pressure levels associated with an augmented firing frequency of pre-sympathetic neurones of the rostral ventrolateral medulla (RVLM) during the late-E phase. The antagonism of ionotropic glutamatergic receptors in the BötC/RVLM abolished the late-E bursts in expiratory and sympathetic outputs of SH rats, indicating that glutamatergic inputs to the BötC/RVLM are essential for the changes in the expiratory and sympathetic coupling observed in SH rats. We also observed that the usually silent late-E neurones of the retrotrapezoid nucleus/parafacial respiratory group became active in SH rats, suggesting that this neuronal population may provide the excitatory drive essential to the emergence of active expiration and sympathetic overactivity. We conclude that short-term SH induces the activation of medullary expiratory neurones, which affects the pattern of expiratory motor activity and its coupling with sympathetic activity.

Relative contributions of NMDA and AMPA receptor‐mediated excitation to the spontaneous discharge of canine pontine respiratory group neurons (712.5)

Neurons within the parabrachial complex appear to play a key role in controlling breathing frequency since local application of opioid receptor agonists produces bradypnea and apnea. To characterize the excitatory neurotransmission underlying the discharge of these PRG neurons, we antagonized glutamatergic inputs to AMPA and NMDA receptors with microejections of 2,3‐dihydroxy‐6‐nitro‐7‐sulfamoylbenzo‐(f)quinoxaline (NBQX) and 2‐amino‐5‐phosphonovalerate (AP5) in a decerebrate, vagotomized, ventilated and paralyzed canine model. Histograms (CTHs) triggered from the phrenic neurogram were used to quantify the activity of simultaneously recorded PRG neurons. NBQX reduced the discharge frequency (Fn) of all subtypes of PRG neurons by 25‐30% while AP5 decreased Fn by an additional 35‐45%, with the greatest depression occurring in non‐respiratory modulated (NRM) and expiratory (E) neurons after AP5. These data suggest that most (~70%) of the excitation of PRG neurons is mediated via AMPA and NMDA receptors with the latter contributing ~33% more drive than the former.Grant Funding Source: Supported by VA Medical Research Funds I01 BX000721‐01

CrossTalk opposing view: The pre‐Bötzinger complex is not essential for respiratory depression following systemic administration of opioid analgesics

Opioid receptors and enkephalinergic nerve terminals are widely distributed throughout respiratory-related regions of the brainstem and in the phrenic motor nucleus of the spinal cord (Xia & Haddad, 1991; Laferriere et al. 1999; Wang et al. 2002; Haji et al. 2003a; Lonergan et al. 2003a,b; Strornetta et al. 2003). Since opiate drugs given systemically will act on opioid receptors with conjoint selectivity in all respiratory regions, respiratory depression is unlikely to be dependent on actions at a single site. Therapeutic doses of opioids given to most mammalian species depress respiratory rate, minute ventilation, alveolar–arterial gas exchange and respiratory responsiveness to hypoxia and hypercapnia (Jaffe & Martin, 1990). Opioid-mediated depression of respiration is due at least in part to direct effects on the brainstem respiratory network, which includes several sites of action in medullary and pontine regions (reviewed by Pattison, 2008; Lalley, 2008). The degree of opioid-mediated respiratory depression depends on agonist dose, opioid receptor density and the subtypes of opioid receptor in various respiratory regions. Species variability and stage of development are also factors (Santiago & Edelman, 1985). In the paragraphs to follow, we review results of studies that indicate that the pre-Botzinger complex (preBotC) is not essential for respiratory depression by systemically administered opioid analgesics.

Chapter 3 - Morphological Characterization of Respiratory Neurons in the Pre-Bötzinger Complex

Although the pre-Bötzinger complex (preBötC) was defined as the inspiratory rhythm generator long ago, the functional-anatomical characterization of its neuronal components is still being achieved. Recent advances have identified the expression of molecular markers in the preBötC neurons that, however, are not exclusive to specific respiratory neuron subtypes and have not always been related to specific cell morphologies. Here, we evaluated the morphology and the axonal projections of electrophysiologically defined respiratory neurons in the preBötC using whole-cell recordings and intracellular biocytin labeling. We found that respiratory pacemaker neurons are larger than expiratory neurons and that inspiratory neurons are smaller than pacemaker and expiratory neurons. Other morphological features such as somata shapes or dendritic branching patterns were not found to be significantly different among the preBötC neurons sampled. We also found that both pacemaker and inspiratory nonpacemaker neurons, but not expiratory neurons, show extensive axonal projections to the contralateral preBötC and show signs of electrical coupling. Overall, our data suggest that there are morphological differences between subtypes of preBötC respiratory neurons. It will be important to take such differences in consideration since morphological differences would influence synaptic responses and action potential propagation.

Activity of respiratory neurons in the rostral medulla during vocalization, swallowing, and coughing in guinea pigs

To examine the relationship between the neuronal networks underlying respiration and non-respiratory behaviors such as vocalization and airway defensive reflexes, we compared the activity of respiratory neurons in the ventrolateral medulla during breathing with that during non-respiratory behaviors including vocalization, swallowing, and coughing in guinea pigs. During fictive vocalization the activity of augmenting expiratory neurons ceased, whereas the other types of expiratory neurons did not show a consistent tendency of increasing or decreasing activity. All inspiratory neurons discharged in synchrony with the phrenic nerve activity. Most of the phase-spanning neurons were activated throughout the vocal phase. During fictive swallowing, many expiratory and inspiratory neurons were silent, whereas many phase-spanning neurons were activated. During fictive coughing, many expiratory neurons were activated during the expiratory phase of coughing. Most inspiratory neurons discharged in parallel with the phrenic nerve activity during coughing. Many phase-spanning neurons were activated during the expiratory phase of coughing. These findings indicate that the medullary respiratory neurons help shape respiratory muscle nerve activity not only during breathing but also during these non-respiratory behaviors, and thus suggest that at least some of the respiratory neurons are shared among the neuronal circuits underlying the generation of breathing and non-respiratory behaviors.

Specificity in monosynaptic and disynaptic bulbospinal connections to thoracic motoneurones in the rat

The respiratory activity in the intercostal nerves of the rat is unusual, in that motoneurones of both branches of the intercostal nerves, internal and external, are activated during expiration. Here, the pathways involved in that activation were investigated in anaesthetised and in decerebrate rats by cross-correlation and by intracellular spike-triggered averaging from expiratory bulbospinal neurones (EBSNs), with a view to revealing specific connections that could be used in studies of experimental spinal cord injury. Decerebrate preparations, which showed the strongest expiratory activity, were found to be the most suitable for these measurements. Cross-correlations in these preparations showed monosynaptic connections from 16/19 (84%) of EBSNs, but only to internal intercostal nerve motoneurones (24/37, 65% of EBSN/nerve pairs), whereas disynaptic connections were seen for external intercostal nerve motoneurones (4/19, 21% of EBSNs or 7/25, 28% of EBSN/nerve pairs). There was evidence for additional disynaptic connections to internal intercostal nerve motoneurones. Intracellular spike-triggered averaging revealed excitatory postsynaptic potentials, which confirmed these connections. This is believed to be the first report of single descending fibres that participate in two different pathways to two different groups of motoneurones. It is of interest compared with the cat, where only one group of motoneurones is activated during expiration and only one of the pathways has been detected. The specificity of the connections could be valuable in studies of plasticity in pathological situations, but care will be needed in studying connections in such situations, because their strength was found here to be relatively weak.

Ventral respiratory column tonic expiratory neurons: Nodes for convergent afferent modulation of inspiratory drive

The medullary ventral respiratory column (VRC) has an essential role in generating the motor pattern for breathing and inspiratory (I) drive. Contemporary models include a rostral‐to‐caudal excitatory I neuron chain extending to premotor populations that also acts upon inhibitory I neurons with recurrent and feed‐forward connections upon other VRC excitatory and inhibitory I neurons and expiratory (E) neurons. Prior work suggests (i) baroreceptor stimulation inhibits inspiratory drive in part via evoked tonic E (t‐E) neuron activity and (ii) central chemoreceptor stimulation enhances I drive via disinhibition through reduced t‐E neuron activity. A recent computational neuromechanical model (O'Connor et al. 2012) predicted that inhibition of t‐E neurons amplifies I drive during coughing ‐ a result supported by in vivo data (Segers et al. 2012). We now report recording sites of pericolumnar t‐E neurons (n=140) and show that selective stimulation of carotid chemoreceptors via close injection of CO2‐saturated saline reduces the I‐phase activity of t‐E cells coincident with increased I drive in vagotomized decerebrate cats. Spike train cross correlation and gravity clustering methods revealed short‐time scale impulse synchrony in 24 t‐E neuron pairs. We conclude that multiple afferent systems tune inspiratory drive via actions of coordinated groups of VRC t‐E neurons. Support: NIH grant NS19814.

Persistent activity and interactions in the brainstem respiratory network during hyperventilatory apnea

Brainstem respiratory neuron interactions during hyperventilatory apnea (HA) and the dissolution and reemergence of the motor pattern for breathing remain poorly understood. We used multi‐array recording, cross‐correlation, and gravity clustering methods to evaluate spike train data from the ventral respiratory column (VRC), pontine respiratory group, and medullary raphé nuclei of decerebrate vagotomized neuromuscularly‐blocked cats. Phrenic activity, blood pressure, end‐tidal CO2, and arterial blood gas measures of O2, CO2 and pH were monitored. During HA: (i) tonic activities persisted in subsets of VRC inspiratory (I) and expiratory (E) neurons; (ii) short‐time scale correlations indicated interactions among neurons that expressed E and non‐respiratory modulated discharge patterns during control conditions, including neurons with functional inhibitory actions on I neurons; (iii) selective stimulation of carotid chemoreceptors by close injection of CO2‐saturated saline transiently evoked rhythmic respiratory network activity and suppressed tonic E neuron activity; (iv) respiratory and slower (0.04 – 0.1 Hz) network oscillations were disrupted. We conclude that VRC tonic E neurons contribute to I drive inhibition during HA (cf. Sears et al. 1982) and document the loss of multiple rhythms during HA. Support: NIH grant NS19814

Connections between expiratory bulbospinal neurons and expiratory motoneurons in thoracic and upper lumbar segments of the spinal cord

Cross-correlation of neural discharges was used to investigate the connections between expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross-correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve branches innervating internal intercostal muscle and external abdominal oblique muscle and a more distal branch of the internal intercostal nerve; and at L1, a nerve branch innervating internal abdominal oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 nerve innervating external abdominal oblique, but a paucity of presumed α-motoneuron discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the thoracic and abdominal musculature.

Dopamine receptor 1 modulates the discharge activities of inspiratory and biphasic expiratory neurons via cAMP-dependent pathways.

Dopamine receptor 1 (D(1)R) plays an essential role in regulating respiratory activity in mammals, however, little is known about how this receptor acts to modulate the basic respiratory rhythmogenesis. Here, by simultaneously recording the discharge activities of biphasic expiratory (biphasic E) neurons/inspiratory (I) neurons and the XII nerve rootlets from brainstem slices, we found that the application of D(1)R agonist cis-(±)-1-(aminomethyl)-3,4-dihydro-3-phenyl-1H-2-benzopyran-5,6-diolhydrochloride (A68930, 5 μM), or forskolin, an intracellular cAMP-increasing agent, substantially decreased respiratory cycle and expiratory time of both types of neurons, and elevated the integral amplitude and frequency of XII nerve rootlets discharge. These changes were reversed by subsequent application of their antagonists SCH-23390 and Rp-Adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt hydrate (Rp-cAMPS), respectively. Importantly, after pretreatment with Rp-cAMPS, the effects of A68930 in both types of neurons were blocked, suggestive of a cAMP-dependent action of A68930. Thus, the current study indicates that D(1)R may modulate basic breathing rhythmogenesis via cAMP-dependent mechanisms.

Analysis of excitatory and inhibitory interactions at high temporal resolution in core circuits of the respiratory CPG

The pre-Botzinger complex (pre-BotC) is the essential core component of the brainstem respiratory central pattern generator (rCPG) in mammals. Phasic excitatory and inhibitory synaptic inputs during the respiratory cycle are thought to dynamically shape membrane potential trajectories and spiking patterns of pre-BotC neurons. These synaptic inputs reflect the functionally interacting neuron populations involved in rhythm and inspiratory-expiratory pattern generation. The dynamic patterns of these synaptic inputs have not been systematically studied and characterized experimentally. Accurate temporal reconstruction of the synaptic input patterns is important for testing current models of the rCPG that incorporate specific excitatory and inhibitory circuit mechanisms for rhythm and pattern generation. A recent computational model of the rCPG generating a normal “three-phase” respiratory pattern [1] specified patterns of synaptic excitation and inhibition in the preBotC during the respiratory cycle required to produce the behaviorally distinct phases of inspiration (I) and two phases of expiration (E1, or post-inspiration, and E2). This model has not been rigorously tested by experimental data on dynamic changes in phasic postsynaptic excitatory and inhibitory conductances in different populations of pre-BotC inspiratory and expiratory neurons. We developed analytical techniques that allow quantitative reconstruction of the patterns of synaptic conductances at high temporal resolution from intracellular recordings of membrane potential trajectories. Our approach extends previously proposed methods for extracting dynamic patterns of synaptic input conductances [2,3] and provides nearly continuous readouts of inhibitory and excitatory synaptic conductances in electrophysiologically different cell types throughout the respiratory cycle. The membrane potential trajectories of pre-BotC respiratory neurons were obtained by sharp microelectrode current-clamp recording within in situ perfused brainstem-spinal cord preparations of mature rats, which generate a three-phase respiratory pattern and provide mechanically stable conditions for intracellular recordings in functionally intact brainstem circuits. We distinguished different types of inspiratory and expiratory pre-BotC neurons and analyzed the dynamical changes of excitatory (Ge )a nd inhibitory (Gi) conductances. Inspiratory neurons exhibited strong excitatory inputs (large dynamic increases in Ge) during their spiking phase followed by a wave(s) of inhibitory inputs during the expiratory period with large increases in Gi and temporal patterns consistent with two populations of inhibitory neurons providing inputs that coordinate inspiratory to expiratory and expiratory to inspiratory phase transitions. Expiratory neurons exhibited strong inhibition during the inspiratory phase, consistent with the rhythmic alternation of inspiration and expiration, and changes in Ge indicating reciprocal interactions between two major populations of inhibitory neurons coordinating the generation of two (E1 and E2) phases of expiration. The reconstructed patterns of Ge and Gi are generally consistent with previously proposed circuit architecture [1] and also suggest its extension. The techniques that we have developed for high temporal resolution of postsynaptic conductance changes are likely to have broad application for analysis of synaptic interactions in circuit components whenever intracellular recordings of membrane potentials can be

Regulatory Influences of Hippocampal Fields CA1 and CA3 on Bulbar Respiratory Neurons in Conditions of Hypoxia

The influences of hippocampal fields CA1 and CA3 on the spike activity of respiratory neurons in the bulbar respiratory center were studied in normal conditions and hypoxia. In normoxia, stimulation of hippocampal fields CA1 and CA3 had mainly inhibitory influences. During oxygen insufficiency, the activity responses of respiratory neurons in the bulbar respiratory center to stimulation of these structures were phasic. At the beginning of the “elevation” to an “altitude” of 4000–5000 m, the small decrease in pO2 in inspired air induced an increase in neuron spike activity. On this background, the inhibitory influence of stimulation of hippocampal fields CA1 and CA3 was more marked than in normoxia. At an “altitude” of 7500–8000 m, with severe oxygen deficit, the inhibitory influences of hippocampal fields CA1 and CA3 were minor on the background of sharp suppression of baseline neuron activity in the respiratory center. Comparative analysis of the activities of inspiratory (IN) and expiratory (EN) neurons at different stages of hypoxia revealed marked differences in their behavior, in that IN had relatively greater resistance to hypoxia than EN. Among subgroups of IN, early and complete IN were more resistant to oxygen deficit. After the “descent” of the animals to normal atmospheric pressure, there was gradual recovery of the initial values of both spontaneous phasic neuron activity and responses to stimuli.

Contribution of the retrotrapezoid nucleus/parafacial respiratory region to the expiratory-sympathetic coupling in response to peripheral chemoreflex in rats

Central mechanisms of coupling between respiratory and sympathetic systems are essential for the entrainment between the enhanced respiratory drive and sympathoexcitation in response to hypoxia. However, the brainstem nuclei and neuronal network involved in these respiratory-sympathetic interactions remain unclear. Here, we evaluated whether the increase in expiratory activity and expiratory-modulated sympathoexcitation produced by the peripheral chemoreflex activation involves the retrotrapezoid nucleus/parafacial respiratory region (RTN/pFRG). Using decerebrated arterially perfused in situ rat preparations (60-80 g), we recorded the activities of thoracic sympathetic (tSN), phrenic (PN), and abdominal nerves (AbN) as well as the extracellular activity of RTN/pFRG expiratory neurons, and reflex responses to chemoreflex activation were evaluated before and after inactivation of the RTN/pFRG region with muscimol (1 mM). In the RTN/pFRG, we identified late-expiratory (late-E) neurons (n = 5) that were silent at resting but fired coincidently with the emergence of late-E bursts in AbN after peripheral chemoreceptor activation. Bilateral muscimol microinjections into the RTN/pFRG region (n = 6) significantly reduced basal PN frequency, mean AbN activity, and the amplitude of respiratory modulation of tSN (P < 0.05). With respect to peripheral chemoreflex responses, muscimol microinjections in the RTN/pFRG enhanced the PN inspiratory response, abolished the evoked late-E activity of AbN, but did not alter either the magnitude or pattern of the tSN reflex response. These findings indicate that the RTN/pFRG region is critically involved in the processing of the active expiratory response but not of the expiratory-modulated sympathetic response to peripheral chemoreflex activation of rat in situ preparations.

Tonic and phasic drive to medullary respiratory neurons during periodic breathing

It is unknown how central neural activity produces the repetitive termination and restart of periodic breathing (PB). We hypothesized that inspiratory and expiratory neural activities would be greatest during the waxing phase and least during the waning phase. We analyzed diaphragmatic and medullary respiratory neural activities during PB in intact unanesthetized adult cats. Diaphragmatic activity was increased and phasic during the waxing phase and was decreased and tonic during the waning phase. Activity of expiratory (n=21) and inspiratory (n=40) neurons was generally increased and phasic during the waxing phase and was decreased and more tonic during the waning phase. During apneas associated with PB, diaphragmatic activity was silent and most, but not all, inspiratory cells were inactive whereas most expiratory cells decreased activity but remained tonically active. We suggest that reduced strength of reciprocal inhibition, secondary to reduced respiratory drive, allows for simultaneous tonic activity of inspiratory and expiratory neurons of the central pattern generator, ultimately resulting in central apnea.

A joint computational respiratory neural network-biomechanical model for breathing and airway defensive behaviors.

Data-driven computational neural network models have been used to study mechanisms for generating the motor patterns for breathing and breathing related behaviors such as coughing. These models have commonly been evaluated in open loop conditions or with feedback of lung volume simply represented as a filtered version of phrenic motor output. Limitations of these approaches preclude assessment of the influence of mechanical properties of the musculoskeletal system and motivated development of a biomechanical model of the respiratory muscles, airway, and lungs using published measures from human subjects. Here we describe the model and some aspects of its behavior when linked to a computational brainstem respiratory network model for breathing and airway defensive behavior composed of discrete “integrate and fire” populations. The network incorporated multiple circuit paths and operations for tuning inspiratory drive suggested by prior work. Results from neuromechanical system simulations included generation of a eupneic-like breathing pattern and the observation that increased respiratory drive and operating volume result in higher peak flow rates during cough, even when the expiratory drive is unchanged, or when the expiratory abdominal pressure is unchanged. Sequential elimination of the model’s sources of inspiratory drive during cough also suggested a role for disinhibitory regulation via tonic expiratory neurons, a result that was subsequently supported by an analysis of in vivo data. Comparisons with antecedent models, discrepancies with experimental results, and some model limitations are noted.

Modulation of the cough reflex by GABAA receptors in the caudal ventral respiratory group of the rabbit

We have previously shown that the caudal ventral respiratory group (cVRG) is a possible site of action of some antitussive drugs and plays a crucial role in determining both the expiratory and inspiratory components of the cough motor pattern. In addition, it has been reported that medullary expiratory neurons of the cVRG are subject to potent GABAergic gain modulation. This study was devoted to investigate the role of cVRG GABAA receptors in the control of baseline respiratory activity and cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. To this purpose, bilateral microinjections (30–50 nl) of bicuculline or muscimol were performed into the cVRG of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bicuculline (1 mM) increased peak abdominal activity and respiratory frequency due to decreases in TE. Cough responses were potentiated mainly owing to increases in the cough number. The recovery was observed within ~2 h. On the contrary, muscimol (0.3 mM) abolished abdominal activity and decreased respiratory frequency due to increases in TE. In addition, cough responses were progressively reduced and completely suppressed within ~20 min. Partial recovery of cough responses was achieved after ~3 h or within ~5 min following bicuculline microinjections at the same locations. The sneeze reflex induced by mechanical stimulation of the nasal mucosa persisted following bicuculline and muscimol microinjections. However, the number and intensity of expiratory thrusts were enhanced by bicuculline and suppressed by muscimol. The results provide evidence that a potent GABAA-mediated inhibitory modulation is exerted at the level of the cVRG not only on respiratory activity, but also on cough and sneeze reflex responses.

Discharge Identity of Medullary Inspiratory Neurons is Altered during Repetitive Fictive Cough

This study investigated the stability of the discharge identity of inspiratory decrementing (I-Dec) and augmenting (I-Aug) neurons in the caudal (cVRC) and rostral (rVRC) ventral respiratory column during repetitive fictive cough in the cat. Inspiratory neurons in the cVRC (n = 23) and rVRC (n = 17) were recorded with microelectrodes. Fictive cough was elicited by mechanical stimulation of the intrathoracic trachea. Approximately 43% (10 of 23) of I-Dec neurons shifted to an augmenting discharge pattern during the first cough cycle (C1). By the second cough cycle (C2), half of these returned to a decrementing pattern. Approximately 94% (16 of 17) of I-Aug neurons retained an augmenting pattern during C1 of a multi-cough response episode. Phrenic burst amplitude and inspiratory duration increased during C1, but decreased with each subsequent cough in a series of repetitive coughs. As a step in evaluating the model-driven hypothesis that VRC I-Dec neurons contribute to the augmentation of inspiratory drive during cough via inhibition of VRC tonic expiratory neurons that inhibit premotor inspiratory neurons, cross-correlation analysis was used to assess relationships of tonic expiratory cells with simultaneously recorded inspiratory neurons. Our results suggest that reconfiguration of inspiratory-related sub-networks of the respiratory pattern generator occurs on a cycle-by-cycle basis during repetitive coughing.