Discharge patterns of laryngeal motoneurones in the cat: an intracellular study

Intracellular activity of pharyngeal motoneurons during breathing, swallowing, and coughing

We recorded membrane potentialp changes in 45 pharyngeal motoneurons (PMs) including 33 expiratory modulated and 12 nonrespiratory neurons during breathing, swallowing, and coughing in decerebrate paralyzed cats. Four types of membrane potential changes were observed during swallowing: 1) depolarization during swallowing (n = 27), 2) depolarization preceded by a brief (≤ 0.1 s) hyperpolarization (n = 4), 3) longer term (> 0.3 s) hyperpolarization followed by depolarization (n = 11), and 4) hyperpolarization during the latter period of swallowing (n = 3). During coughing, PMs showed two types of membrane potential changes (n = 10). Nine neurons exhibited a ramp-like depolarization during the expiratory phase of coughing with the potential peak at the end of expiratory phase. This depolarization was interrupted by a transient repolarization just before the potential peak. The membrane potential of the remaining neuron abruptly depolarized at the onset of the expiratory phase and then gradually decreased even after the end of the expiratory phase. Single-shock stimulation of the superior laryngeal nerve (SLN) induced inhibitory postsynaptic potentials in 19 of 21 PMs. Two motoneurons exhibited an SLN-induced excitatory postsynaptic potential. The present study revealed that PMs receive the central drive, consisting of a combination of excitation and inhibition, from the pattern generator circuitry of breathing, swallowing, and coughing, which changes the properties of their membrane potential to generate these motor behaviors of the pharynx. Our data will provide the basis of studies of pharyngeal activity and its control from the medullary neuronal circuitry responsible for the upper airway motor activity.NEW & NOTEWORTHY We have provided the first demonstration of the multifunctional activity of the pharyngeal motoneurons at the level of membrane potential during respiration, swallowing, and coughing.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Congenital Bilateral Adductor Vocal Cord Paralysis

Bilateral adductor vocal cord paralysis (BAdP), presenting with features of laryngeal incompetence, is a rare form of congenital bilateral vocal cord paralysis, and only 2 small series of BAdP have previously been published. Three cases are reported here. The BAdP occurred as an isolated abnormality in 1 child, and was associated with a recognizable syndrome (Robinow's syndrome and 22q deletion) in the other 2 children. Gastrostomy tube feeding was required in 2 children, who both remain gastrostomy tube–dependent at 26 months and 10 years 9 months of age. The child with Robinow's syndrome received parenteral nutrition until 2 months, but was then able to feed orally after partial improvement in vocal cord function. The global impairment in vocal cord constrictor function observed in these 3 children is consistent with the site of lesion's being at the level of the laryngeal constrictor motoneurons in BAdP.

Congenital Bilateral Vocal Cord Paralysis and the Role of Glycine

Objectives:

We sought to modify normal laryngeal constrictor (LC) motoneuron activity to induce a pattern of aberrant LC muscle function that may serve as a model of congenital bilateral vocal cord paralysis.

Methods:

Single unit extracellular recordings of functionally identified LC motoneurons were made in anesthetized Sprague-Dawley rats, and the response to both intravenous and iontophoretic application of the glycine antagonist strychnine was studied.

Results:

The postinspiratory firing pattern of LC motoneurons became inspiratory after intravenous injection of strychnine (4 of 5 rats), but no change was recorded in response to strychnine iontophoresis (7 of 8 rats).

Conclusions:

Blockade of glycinergic inhibitory neurotransmission by strychnine, acting above the level of the LC motoneuron, causes LC motoneurons to fire during inspiration rather than after inspiration. This observation suggests that impaired glycine neurotransmission may be an underlying mechanism that explains the clinical manifestations of congenital bilateral vocal cord paralysis.

Electrophysiological properties of laryngeal motoneurones in rats submitted to chronic intermittent hypoxia

KEY POINTS

The respiratory control of the glottis by laryngeal motoneurones is characterized by inspiratory abduction and post-inspiratory adduction causing decreases and increases in upper airway resistance, respectively. Chronic intermittent hypoxia (CIH), an important component of obstructive sleep apnoea, exaggerated glottal abduction (before inspiration), associated with active expiration and decreased glottal adduction during post-inspiration. CIH increased the inspiratory and decreased the post-inspiratory laryngeal motoneurone activities, which is not associated to changes in their intrinsic electrophysiological properties. We conclude that the changes in the respiratory network after CIH seem to be an adaptive process required for an appropriated pulmonary ventilation and control of upper airway resistance under intermittent episodes of hypoxia.

ABSTRACT

To keep an appropriate airflow to and from the lungs under physiological conditions a precise neural co-ordination of the upper airway resistance by laryngeal motoneurones in the nucleus ambiguus is essential. Chronic intermittent hypoxia (CIH), an important component of obstructive sleep apnoea, may alter these fine mechanisms. Here, using nerve and whole cell patch clamp recordings in in situ preparations of rats we investigated the effects of CIH on the respiratory control of the upper airway resistance, on the electrophysiological properties of laryngeal motoneurones in the nucleus ambiguus, and the role of carotid body (CB) afferents to the brainstem on the underlying mechanisms of these effects. CIH rats exhibited longer pre-inspiratory and lower post-inspiratory superior laryngeal nerve activities than control rats. These changes produced exaggerated glottal abduction (before inspiration) and decreased glottal adduction during post-inspiration, indicating a reduction of upper airway resistance during these respiratory phases after CIH. CB denervation abolished these changes produced by CIH. Regarding choline acetyltransferase positive-laryngeal motoneurones, CIH increased the firing frequency of inspiratory and decreased the firing frequency of post-inspiratory laryngeal motoneurones, without changes in their intrinsic electrophysiological properties. These data show that the effects of CIH on the upper airway resistance and laryngeal motoneurones activities are driven by the integrity of CB, which afferents induce changes in the central respiratory generators in the brainstem. These neural changes in the respiratory network seem to be an adaptive process required for an appropriated pulmonary ventilation and control of upper airway resistance under intermittent episodes of hypoxia.

Catecholamine inputs to expiratory laryngeal motoneurons in rats

Many respiration-related interneurons and motoneurons receive a catecholaminergic input, but the extent and distribution of this input to recurrent laryngeal motoneurons that innervate intrinsic muscles of the larynx are not clear. In the present study, we examined the catecholaminergic input to expiratory laryngeal motoneurons in the caudal nucleus ambiguus by combining intracellular labeling of single identified motoneurons, with immunohistochemistry to reveal tyrosine hydroxylase immunoreactive (catecholaminergic) terminal varicosities. Close appositions were found between the two structures, with 18 ± 5 close appositions per motoneuron (n = 7). Close appositions were more frequently observed on distal rather than proximal dendrites. Axosomatic appositions were not seen. In order to determine the source of this input, microinjections of cholera toxin B subunit (1%, 20 nl) were made into the caudal nucleus ambiguus. Retrogradely labeled neurons, located in the ipsilateral nucleus tractus solitarius and the area postrema, were tyrosine hydroxylase-positive. Our results not only demonstrate details of the extent and distribution of potential catecholamine inputs to the expiratory laryngeal motoneuron, but further indicate that the inputs, at least in part, originate from the dorsomedial medulla, providing a potential anatomical basis for previously reported catecholaminergic effects on the laryngeal adductor reflex.

Bidirectional plasticity of pontine pneumotaxic postinspiratory drive: implication for a pontomedullary respiratory central pattern generator

The "pneumotaxic center" in the rostral dorsolateral pons as delineated by Lumsden nine decades ago is known to play an important role in promoting the inspiratory off-switch (IOS) for inspiratory-expiratory phase transition as a fail-safe mechanism for preventing apneusis in the absence of vagal input. Traditionally, the pontine pneumotaxic mechanism has been thought to contribute a tonic descending input that lowers the IOS threshold in medullary respiratory central pattern generator (rCPG) circuits, but otherwise does not constitute part of the rCPG. Recent evidence indicates that descending input from the Kölliker-Fuse nucleus (KFN) within the pneumotaxic center is essential for gating the postinspiratory phase of the three-phase respiratory rhythm to control the IOS in vagotomized animals. A critical question arising is whether such a descending pneumotaxic input from KFN that drives postinspiratory activity is tonic (null hypothesis) or rhythmic with postinspiratory phase modulation (alternative hypothesis). Here, we show that multifarious evidence reported in the literature collectively indicates that the descending pneumotaxic input may exhibit NMDA receptor-dependent short-term plasticity in the form of a biphasic neural differentiator that bidirectionally and phase-selectively modulates postinspiratory phase duration in response to vagal and peripheral chemoreceptor inputs independent of the responses in inspiratory and late-expiratory activities. The phase-selectivity property of the descending pneumotaxic input implicates a population of pontine early-expiratory (postinspiratory/expiratory-decrementing) neurons as the most likely neural correlate of the pneumotaxic mechanism that drives post-I activity, suggesting that the pontine pneumotaxic mechanism may be an integral part of a pontomedullary rCPG that underlies the three-phase respiratory rhythm.

Computational models and emergent properties of respiratory neural networks

Computational models of the neural control system for breathing in mammals provide a theoretical and computational framework bringing together experimental data obtained from different animal preparations under various experimental conditions. Many of these models were developed in parallel and iteratively with experimental studies and provided predictions guiding new experiments. This data-driven modeling approach has advanced our understanding of respiratory network architecture and neural mechanisms underlying generation of the respiratory rhythm and pattern, including their functional reorganization under different physiological conditions. Models reviewed here vary in neurobiological details and computational complexity and span multiple spatiotemporal scales of respiratory control mechanisms. Recent models describe interacting populations of respiratory neurons spatially distributed within the Bötzinger and pre-Bötzinger complexes and rostral ventrolateral medulla that contain core circuits of the respiratory central pattern generator (CPG). Network interactions within these circuits along with intrinsic rhythmogenic properties of neurons form a hierarchy of multiple rhythm generation mechanisms. The functional expression of these mechanisms is controlled by input drives from other brainstem components,including the retrotrapezoid nucleus and pons, which regulate the dynamic behavior of the core circuitry. The emerging view is that the brainstem respiratory network has rhythmogenic capabilities at multiple levels of circuit organization. This allows flexible, state-dependent expression of different neural pattern-generation mechanisms under various physiological conditions,enabling a wide repertoire of respiratory behaviors. Some models consider control of the respiratory CPG by pulmonary feedback and network reconfiguration during defensive behaviors such as cough. Future directions in modeling of the respiratory CPG are considered.

Beta-asarone inhibits synaptic inputs to airway preganglionic parasympathetic motoneurons

Therapeutic application of Asarum, a herbal medicine that has been used for centuries, reportedly causes acute respiratory disturbance. The responsible constituents, the sites of action, and the mechanisms involved in this side effect are unclear. We investigated the effects of β-asarone, a volatile constituent of Asarum, on neurotransmission in the medullary respiratory neuronal network using extracellular recording of the rhythmic hypoglossal activity and voltage clamp recordings of the postsynaptic activity of the airway preganglionic parasympathetic motoneurons (APPMs) in vitro. β-Asarone caused progressive decreases in the duration and area of the hypoglossal bursts in a concentration-dependent manner. The frequency and amplitude of the bursts were initially unaltered or temporarily increased, but were then inhibited progressively after prolonged exposure. As with the inhibition of the hypoglossal bursts, the tonic and the phasic excitatory and inhibitory postsynaptic currents in the APPMs were attenuated. These data suggest that the Asarum-caused acute respiratory disturbance involves β-asarone-induced inhibition of neurotransmission in the medullary respiratory neuronal network.

Multifunctional laryngeal premotor neurons: their activities during breathing, coughing, sneezing, and swallowing

To examine whether motor commands of two or more distinct laryngeal motor patterns converge onto a common premotor network, we conducted dual recordings from the laryngeal adductor motoneuron and its premotor neuron within the brainstem respiratory circuitry during fictive breathing, coughing, sneezing, and swallowing in decerebrate paralyzed cats. Expiratory neurons with an augmenting firing pattern (EAUG), whose action potentials evoked monosynaptic IPSPs in the adductor motoneurons, sharply fired during the expulsive phases of fictive coughing and sneezing, during which the adductor motoneurons transiently repolarized. In contrast, these premotor neurons were silent during the swallow-related hyperpolarization in adductor motoneurons. These results show that one class of medullary respiratory neuron, EAUG, is multifunctional and shared among the central pattern generators (CPGs) for breathing, coughing, and sneezing. In addition, although the CPGs underlying these three behaviors and the swallowing CPG do overlap, EAUG neurons are not part of the swallowing CPG and, in contrast to the other three behaviors, are not a source of inhibitory input to adductor motoneurons during swallowing.

The airway-related parasympathetic motoneurones in the ventrolateral medulla of newborn rats were dissociated anatomically and in functional control

The respiratory-related synaptic control of the airway-related preganglionic parasympathetic motoneurones (APPMs) has not been investigated, and whether differently targeted APPMs receive differential respiratory-related synaptic modulation is unknown. In this study, putative APPMs in the ventrolateral medulla of newborn rats were retrogradely traced with fluorescent tracer and were examined using the patch-clamp method in brainstem slices with respiratory rhythm. The results indicate that tracer application directly to the recurrent laryngeal nerve only labelled the putative APPMs within the compact portion of nucleus ambiguus (cNA), while tracer injection into the trachea wall labelled the putative APPMs both in cNA and in the area ventral/ventrolateral to cNA (vNA). The putative APPMs within cNA received mainly inhibitory inputs, which in some (9 of 20) neurones showed an inspiratory-related attenuation and in others (7 of 20) showed an inspiratory-related augmentation. At least some putative APPMs within cNA, of which the inhibitory synaptic inputs showed inspiratory-related changes, might be related to the control of laryngeal muscles. The putative APPMs in vNA receive both excitatory and inhibitory inputs, and central inspiratory activity excited some (11 of 19) neurones via augmentation of their excitatory inputs and inhibited others (8 of 19) via augmentation of their inhibitory inputs. At least some putative APPMs in vNA might be trachea-related motoneurones. These results provide evidence that APPMs controlling different segments of the airway might be dissociated in the ventrolateral medulla both anatomically and in functional control.

Synaptic origin of the respiratory-modulated activity of laryngeal motoneurons

To determine the synaptic source of the respiratory-related activity of laryngeal motoneurons, spike-triggered averaging of the membrane potentials of laryngeal motoneurons was conducted using spikes of respiratory neurons located between the Bötzinger complex and the rostral ventral respiratory group as triggers in decerebrate, paralyzed cats. We identified one excitatory and two inhibitory sources for inspiratory laryngeal motoneurons, and two inhibitory sources for expiratory laryngeal motoneurons. In inspiratory laryngeal motoneurons, monosynaptic excitatory postsynaptic potentials were evoked by spikes of inspiratory neurons with augmenting firing patterns, and monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked by spikes of expiratory neurons with decrementing firing patterns and by spikes of inspiratory neurons with decrementing firing patterns. In expiratory laryngeal motoneurons, monosynaptic IPSPs were evoked by spikes of inspiratory neurons with decrementing firing patterns and by spikes of expiratory neurons with augmenting firing patterns. We conclude that various synaptic inputs from respiratory neurons contribute to shaping the respiratory-related trajectory of membrane potential of laryngeal motoneurons.

Coordination of cranial motoneurons during mastication

Mastication is the first stage of digestion and involves several motor processes such as food intake, intra-oral food transport, bolus formation and chewing in its broad sense. These complicated motor functions can be accomplished by the well-coordinated activities in various cranial motoneurons innervating the jaw, hyoid, tongue and facial muscles. The brainstem masticatory central pattern generator (CPG) plays a crucial role in generating basic activity patterns of these cranial motoneuron groups. However, descending inputs from higher brain (e.g., cerebral cortex) and mastication-generated peripheral sensory inputs also play important roles in modulating the activity pattern of each motoneuron so that the final motor outputs fit the environmental demand. In this review, we focus on the coordination of the trigeminal, facial and hypoglossal motoneurons during mastication. We first summarize findings showing the activity patterns of muscles innervated by these motoneurons during natural mastication, and then discuss the possible neural mechanisms underlying their coordinated activities during mastication.

Serotonin inputs to laryngeal constrictor motoneurons in the rat

OBJECTIVES/HYPOTHESIS

The objective was to demonstrate close appositions between serotonin-immunoreactive boutons and laryngeal constrictor (LCon) motoneurons in Sprague-Dawley rats.

STUDY DESIGN

Animal experimental.

METHODS

LCon motoneurons were identified functionally by their antidromic responses to stimulation of the recurrent laryngeal nerve and postinspiratory modulation and were filled by intracellular injection of biotin amide (n = 6). The medulla was sectioned and, using immunohistochemical analysis, examined by light microscopy.

RESULTS

Serotonin appositions were found on all 6 LCon motoneurons, with an average number of 17 +/- 6 close appositions per neuron.

CONCLUSION

In comparison with the authors' previous study of inspiratory laryngeal motoneurons, the number of serotonin close appositions with LCon motoneurons was similar to that found with posterior cricoarytenoid motoneurons, but significantly less than that found with cricothyroid motoneurons. This finding may represent a basis for differences in tonic activity of laryngeal muscles observed in relation to the sleep-wake cycle.

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 Bötzinger and pre-Bötzinger 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.

Membrane potential changes in vocal cord tensor motoneurons during breathing, vocalization, coughing and swallowing in decerebrate cats

We studied the patterns of membrane potential changes in vocal cord tensor motoneurons, i.e. cricothyroid muscle motoneurons (CTMs), during fictive breathing, vocalization, coughing, and swallowing in decerebrate paralyzed cats to determine the nature of central drives to CTMs during these behaviors. CTMs were identified by antidromic activation from the superior laryngeal nerve. During breathing, CTMs always depolarized during the inspiratory phase, and sometimes depolarized during the expiratory phase as well. During vocalization, CTMs strongly depolarized. During coughing, CTMs exhibited depolarizations during both inspiratory and expiratory phases, but it was interrupted by a transient repolarization between the last part of the inspiratory phase and the first part of the abdominal burst during which chloride-dependent inhibitory postsynaptic potentials were revealed. During swallowing, most CTMs hyperpolarized, and this hyperpolarization was sometimes followed by a weak depolarization. We conclude that the main role of the cricothyroid muscle is vocalization but the functional roles in coughing and swallowing are minor, and that the CTM activity during resting breathing and vocalization are primarily controlled by excitatory inputs, while during coughing and swallowing, inhibitory inputs play roles in shaping membrane potential trajectories.

Inspiratory Activation of the Vocal Cord Adductor, Part I: Human Study in Patients With Restricted Abduction of the Vocal Cords

OBJECTIVES/HYPOTHESIS

In patients with restricted abduction of the vocal cords, it has generally been accepted that glottis narrowing with laryngeal stridor during inspiration is attributed to static and passive obstruction of the glottis. However, active glottis narrowing can also be contributory. We tested the hypothesis that the vocal cord adductor is activated during inspiration in patients with restricted abduction of the vocal cords.

STUDY DESIGN

Electromyographic evaluation of vocal cord adductor activity in patients with restricted abduction of the vocal cords.

METHODS

Five patients with restricted abduction of the vocal cords who had stridor with mild to severe dyspnea during wakefulness were anesthetized with propofol. We recorded the adductor muscle electromyogram during breathing through a laryngeal mask airway while observing the vocal cord movement endoscopically. In three patients who had undergone tracheostomy, we also recorded adductor firing patterns not only while closing but also while opening the tracheostoma.

RESULTS

The adductor was activated during inspiration, and the glottis was narrowed in accordance with inspiratory stridor. This adductor inspiratory activity was abolished by opening the tracheostoma in the tracheostomized patients.

CONCLUSION

Not only static or passive glottis narrowing but also active narrowing may contribute to inspiratory flow limitation in patients with restricted abduction of the vocal cords. This active glottis narrowing is probably induced by an airway reflex.

Central control of upper airway resistance regulating respiratory airflow in mammals

This article reviews recent studies on the importance of glycine receptors for both the spontaneous and the reflex respiratory modulation of the laryngeal abductors and adductors. Our findings show that strychnine blockade of glycine receptors within the brainstem changes the eupneic three-phase respiratory pattern into two phases. This has major implications for glottal control: (i) the inspiratory glottic abduction and early expiratory adduction were both compromised--a finding mimicked by 5% hypoxia; (ii) closure of the glottis during defensive upper airway reflexes became intermittent and the reflex apnoea reversed to sustained inspiratory discharge. Based on these data, we predict that periods of prolonged hypoxia, such as those that occur during sleep apnoeas, will constrain inspiratory glottic abduction thereby impeding inhalation.

Glycinergic inhibition is essential for co-ordinating cranial and spinal respiratory motor outputs in the neonatal rat

Eupnoeic breathing in mammals is dependent on the co-ordinated activity of cranial and spinal motor outputs to both ventilate the lungs and adjust respiratory airflow, which they do by regulating upper-airway resistance. We investigated the role of central glycinergic inhibition in the co-ordination of cranial and spinal respiratory motor outflows. We developed an arterially perfused neonatal rat preparation (postnatal age 0-4 days) to assess the effects of blocking glycine receptors with systemically administered strychnine (0.5-1 microM). We recorded respiratory neurones located within the ventrolateral medulla, inspiratory phrenic nerve activity (PNA) and recurrent laryngeal nerve activity (RLNA), as well as dynamic changes in laryngeal resistance. Central recordings of postinspiratory neurones revealed an earlier onset in firing relative to the onset of inspiratory PNA after exposure to strychnine (260 +/- 38.9 vs. 129 +/- 26.8 ms). After glycine receptor blockade, postinspiratory neurones discharged during the inspiratory phase. Strychnine also evoked a decrease in PNA frequency (from 38.6 +/- 4.7 to 30.7 +/- 2.8 bursts min(-1)), but amplitude was unaffected. In control conditions, RLNA comprised inspiratory and postinspiratory discharges; the amplitude of the latter exceeded that of the former. However, after administration of strychnine, the amplitude of inspiratory-related discharge increased (+65.2 +/- 15.2 %) and exceeded postinspiratory activity. Functionally this change in RLNA caused a paradoxical, inspiratory-related glottal constriction during PNA. We conclude that during the first days of life in the rat, glycine receptors are essential for the formation of the eupnoeic-like breathing pattern as defined by the co-ordinated activity of cranial and spinal motor inspiratory and postinspiratory activities.

Inhibitory synaptic mechanisms regulating upper airway patency

The breathing cycle of vertebrates comprises three phases (inspiration, postinspiration and expiration) that are apparent in the activities generated in the ponto-medullary respiratory network. A large body of evidence now indicates that in adult mammals generation of this three-phase pattern is based on reciprocal synaptic inhibition between distinct subsets of respiratory neurones. This review summarises our recent experiments focused on the role of glycinergic inhibition in respiratory pattern formation: e.g. in co-ordinating the activity of spinal and cranial motor outputs that drive the ventilatory pump (thoracic and abdominal muscles) and adjust airflow by regulating laryngeal resistance (laryngeal abductors and adductors). We used arterially perfused in situ preparations of neonatal and mature rat and show that specific blockade of glycine receptors within the ponto-medullary network caused a severe disruption of the co-ordination of spinal and cranial motor outputs: postinspiratory neurones lose their characteristic inspiratory inhibition revealing excitatory synaptic drive coincident with inspiratory phrenic nerve activity. The resulting simultaneous discharge of inspiratory and postinspiratory neurones caused co-activation of both glottal abductors and adductors during neural inspiration. The latter resulted in a paradoxical inspiratory adduction of the vocal fold and severe disruption of the eupneic breathing pattern. The effect of blocking glycine receptors was the same in both mature and newborn rats suggesting that glycinergic inhibition is essential for co-ordinating cranial and spinal motor outputs from birth.

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.

Medullary respiratory neurones and control of laryngeal motoneurones during fictive eupnoea and cough in the cat

1. This study addressed the hypothesis that ventrolateral medullary respiratory neurones participate in the control of laryngeal motoneurones during both eupnoea and coughing. 2. Data were obtained from 28 mid-collicular decerebrated, artificially ventilated cats. Cough-like motor patterns (fictive cough) in phrenic, lumbar and recurrent laryngeal nerves were elicited by mechanical stimulation of the intrathoracic trachea. Microelectrode arrays were used to monitor simultaneously several neurones in the ventral respiratory group, including the Bötzinger and pre-Bötzinger complexes. Spike trains were evaluated for responses during fictive cough and evidence of functional connectivity with spike-triggered averages of efferent recurrent laryngeal nerve activity. 3. Primary features were observed in averages triggered by 94 of 332 (28 %) neurones. An offset biphasic wave with a positive time lag was present in the unrectified average for 10 inspiratory and 13 expiratory neurones. These trigger neurones were respectively identified as inspiratory laryngeal motoneurones with augmenting, decrementing, plateau and "other" discharge patterns, and expiratory laryngeal motoneurones with decrementing firing patterns. 4. Rectified averages triggered by inspiratory neurones included 37 offset peaks, 11 central peaks and one offset trough. Averages triggered by expiratory neurones had 12 offset peaks, six central peaks and four offset troughs. Relationships inferred from these features included premotor actions of inspiratory neurones with augmenting, decrementing, plateau and "other" patterns on inspiratory laryngeal motoneurones, and premotor actions of decrementing and "other" expiratory neurones on expiratory laryngeal motoneurones. Corresponding changes in neuronal firing patterns during fictive cough supported these inferences. 5. The data confirm and extend previous results on the control of laryngeal motoneurones during eupnoea and support the hypothesis that the same premotor neurones help to shape motoneurone firing patterns during both eupnoea and coughing.

Respiratory activity in glossopharyngeal, vagus and accessory nerves and pharyngeal constrictors in newborn rat in vitro

1. Previously, in a brainstem-spinal cord-rib preparation from neonatal rats we demonstrated that a decrement in extracellular pH (from about 7.4 to 7.1) caused expiratory activity in an internal intercostal muscle (IIM) during the first half of the expiratory phase (Ea). As the initial step in finding nerves or muscles firing during the second half of the expiratory phase (Eb), the patterns of activity in the glossopharyngeal, vagus and accessory nerves were examined in the present study. 2. Since the emerging motor rootlets of these three nerves (> 20; collected into about 10 bundles before the jugular foramen) are distributed in a continuous fashion from rostral to caudal levels of the brainstem, visual identification was impossible. Therefore, antidromic compound action potentials evoked by stimulation of the glossopharyngeal nerve (IX), the pharyngeal branch of the vagus nerve (PhX), the superior laryngeal nerve (SLN), the cervical vagus nerve (CX) and the accessory nerve (XI) were recorded from the peripheral stumps of the various rootlets. Nerve rootlets could be categorised into rostral, intermediate and caudal groups (rostIX-XI, intIX-XI, caudIX-XI). The rostIX-XI rootlets showed their largest potential on IX stimulation, while the intIX-XI and caudIX-XI rootlets showed their largest potentials on CX stimulation. The intIX-XI rootlets showed larger potentials on PhX and SLN stimulation than the caudIX-XI rootlets. 3. Activity was recorded simultaneously from the central stumps of the rootlets in the above three groups. Most rootlets showed inspiratory bursts. Under low pH conditions, all representatives of group rostIX-XI, most of intIX-XI and about half of caudIX-XI showed additional bursts during the Ea phase. Groups intIX-XI and caudIX-XI but not rostIX-XI also showed discrete bursts during the Eb phase in some preparations. In general, expiratory activity was prominent in intIX-XI. The spinal branch of XI showed no consistent respiratory activity. 4. Since the intIX-XI rootlets showed Eb bursts and large antidromic potentials on stimulation of PhX and SLN (which innervate the inferior pharyngeal constrictor muscle (IPC)), electromyograms were recorded from the rostral and caudal parts of IPC (rIPC and cIPC). Under low pH conditions, cIPC showed bursts during the Ea and Eb phases, while rIPC showed bursts predominantly during the Eb phase. 5. These results indicate that recording from rIPC would be a useful way of examining the neuronal mechanisms responsible for Eb phase activity.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

Intracellular recording from posterior cricoarytenoid motoneurons in the rat

The ability to maintain coordinated vocal cord abduction and upper airway patency is dependent on the integrity of the posterior cricoarytenoid (PCA) motoneurons and their multiple neural connections. Study of the PCA motoneurons represents the initial step in understanding the complex mechanisms responsible for coordinated vocal cord abduction and may provide an insight into the possible pathological processes underlying the various clinical presentations of vocal cord dysfunction. Intracellular recordings were made from 11 PCA motoneurons in Sprague-Dawley rats, which all showed an inspiratory augmenting discharge pattern that is also characteristic of phrenic nerve activity. The resting membrane potential was -56+/-11 mV. Two PCA motoneurons were injected with Neurobiotin to demonstrate neuronal morphology, which was found to be similar to that obtained by retrograde labeling with cholera toxin B subunit. The technique described for intracellular recording of PCA motoneurons should allow more detailed morphological, electrophysiological, and immunohistochemical information to be obtained, to thereby identify some of the factors responsible for maintaining normal function of the PCA muscle.

Identification of posterior cricoarytenoid motoneurons in the rat

The posterior cricoarytenoid (PCA) muscle is the sole abductor of the larynx and is controlled by motoneurons located in the nucleus ambiguus. These motoneurons receive inputs from a variety of interneurons, including those that impart respiratory modulation, and are responsible for the phasic inspiratory activity of the PCA muscle. Identification of PCA motoneurons is therefore an essential initial step in understanding the mechanisms responsible for coordinated vocal cord abduction. We identified PCA motoneurons in the rat model by retrograde labeling, and following antidromic activation. A total of 194 neurons were identified by retrograde labeling with cholera toxin B subunit (CTB). Labeling was exclusively ipsilateral where the contralateral vagus and superior laryngeal nerves had been divided. The neurons were multipolar, with dimensions of 33.2 +/- 6.4 microm (mean +/- standard deviation) in length and 22.4 +/- 3.4 microm in width. The neurons were located within a range of 0.6 to 2.4 mm caudal to the caudal pole of the facial nerve, 1.2 to 1.7 mm lateral to the midline, and 1.5 to 2.3 mm deep to the dorsal surface of the medulla. The PCA motoneurons were antidromically activated by focal stimulation of the PCA muscle. The extracellular field was recorded in 5 rats, and the PCA motoneurons were found within a range of 0.8 to 1.7 mm caudal to the caudal pole of the facial nerve, 1.5 to 2.0 mm lateral to the midline, and 1.9 to 2.4 mm deep to the dorsal surface of the medulla. The mean conduction velocity ranged from 37.0 +/- 5.8 to 68.6 +/- 5.0 m/s. An extracellular antidromic field potential, which corresponds to the distribution of the PCA motoneuron pool demonstrated by retrograde labeling with CTB, can be reliably obtained in a rat model following focal PCA muscle stimulation.

Multifunctional laryngeal motoneurons: an intracellular study in the cat

We studied the patterns of membrane potential changes in laryngeal motoneurons (LMs) during vocalization, coughing, swallowing, sneezing, and the aspiration reflex in decerebrate paralyzed cats. LMs, identified by antidromic activation from the recurrent laryngeal nerve, were expiratory (ELMs) or inspiratory (ILMs) cells that depolarized during their respective phases in eupnea. During vocalization, most ELMs depolarized and most ILMs hyperpolarized. Some ILMs depolarized slightly during vocalization. During coughing, ELMs depolarized abruptly at the transition from the inspiratory to the expiratory phase. In one-third of ELMs, this depolarization persisted throughout the abdominal burst. In the remainder ("type A"), it was interrupted by a transient repolarization. ILMs exhibited a membrane potential trajectory opposite to that of type A ELMs during coughing. During swallowing, the membrane potential of ELMs decreased transiently at the onset of the hypoglossal burst and then depolarized strongly during the burst. ILMs hyperpolarized sharply at the onset of the burst and depolarized as hypoglossal activity ceased. During sneezing, ELMs and ILMs exhibited membrane potential changes similar to those of type A ELMs and ILMs during coughing. During the aspiration reflex, ELMs and ILMs exhibited bell-shaped hyperpolarization and depolarization trajectories, respectively. We conclude that central drives to LMs, consisting of complex combinations of excitation and inhibition, vary during vocalization and upper airway defensive reflexes. This study provides data for analysis of the neuronal networks that produce these various behaviors and analysis of network reorganization caused by changes in dynamic connections between the respiratory and nonrespiratory neuronal networks.

Effects on iontophoretically applied acetylcholine on membrane potential and synaptic activity of bulbar respiratory neurones in decerebrate cats

Intracellular recordings were made from bulbar respiratory neurones of decerebrate cats, together with iontophoretic application of acetylcholine and its specific antagonists to assess cholinergic mechanisms involved in the central respiratory network in vivo. Of 126 respiratory-related neurones impaled in the ventral respiratory group, iontophoresis of acetylcholine produced depolarization in 67 cells (53%), hyperpolarization in 36 cells (29%), and no effect in the remaining 23 cells (18%). Depolarization occurred predominantly in laryngeal motoneurones (31/40) and bulbo-spinal neurones (4/5), while a comparable number of non-antidromically-activated respiratory neurones displayed either depolarization (33/81) or hyperpolarization (31/81). Acetylcholine had no significant effect on excitatory and inhibitory postsynaptic potentials in all types of neurones tested. Both depolarizing effects of acetylcholine were antagonized by co-iontophoresis of atropine, but not by hexamethonium. Input resistance was increased (7/9) or unchanged (2/9) in depolarized cells, whereas it was unaltered in all hyperpolarized cells tested (n = 6). The present results suggest that the distribution and functions of cholinergic muscarinic receptors are different for the laryngeal and bulbo-spinal types of respiratory neurones and the non-antidromically-activated respiratory neurones in the cat.

Thyrotropin-releasing hormone inputs are preferentially directed towards respiratory motoneurons in rat nucleus ambiguus

In the present study, we assessed the extent of the thyrotropin-releasing hormone (TRH) input to motoneurons in the ambigual, facial, and hypoglossal nuclei of the rat using a combination of intracellular recording, dye filling, and immunohistochemistry. Twelve motoneurons in the rostral nucleus ambiguus were labelled by intracellular injection in vivo of Neurobiotin (Vector). Seven out of 12 ambigual motoneurons displayed rhythmic fluctuations of their membrane potential in phase with phrenic nerve discharge, whereas the other five had no modulations of any kind. Seven facial motoneurons and seven hypoglossal motoneurons were also filled with Neurobiotin. All three motor nuclei contained TRH-immunoreactive varicosities, with the largest numbers found in the nucleus ambiguus. Close appositions were seen between TRH-immunoreactive boutons and every labelled motoneuron. Respiratory-related motoneurons in the nucleus ambiguus received the largest number of TRH appositions with 74 +/- 38 appositions/neuron (mean +/- S.D.; n = 7). In contrast, nonrespiratory ambigual motoneurons received significantly fewer TRH appositions (11 +/- 5; n = 5; P < 0.05; Mann-Whitney U test). Facial motoneurons received about the same number of TRH appositions as nonrespiratory ambigual motoneurons, with 13 +/- 4 (n = 7). Hypoglossal motoneurons received the fewest appositions from TRH-containing boutons, with 8 +/- 2 (n = 7). There were no differences in the TRH inputs to respiratory and nonrespiratory motoneurons in the facial and hypoglossal nuclei. These results demonstrate that, among motoneurons in the medulla, respiratory motoneurons in the rostral nucleus ambiguus are preferentially innervated by the TRH-immunoreactive boutons.

Variations in membrane potential trajectory of post-inspiratory neurons in the ventrolateral medulla of the cat

In decerebrate cats, intracellular recordings were made in 124 expiratory neurons displaying either a plateau-phase of depolarization during post-inspiration or a steadily decrementing depolarization throughout expiration. Both groups consisted of vagal motoneurons and non-antidromically activated neurons. Five neurons were antidromically activated by both vagal and spinal cord stimuli. The pattern in membrane potential was changed from one type to another either spontaneously or experimentally. The present results suggest that the variable appearance of the membrane potential trajectory does not represent the different functional category of bulbar post-inspiratory neurons.

The bulbar network of respiratory neurons during apneusis induced by a blockade of NMDA receptors

Our aim was to study the mechanisms producing the transition from the inspiratory phase to the expiratory phase of the breathing cycle. For this purpose we observed the changes affecting the discharge patterns and excitabilities of the different types of respiratory neurons within the respiratory network in cat medulla, after inducing an apneustic respiration with the N-methyl-D-aspartate (NMDA) antagonist MK-801 given systemically. Respiratory neurons were recorded extracellularly through the central barrel of multibarrelled electrodes, in the ventral respiratory area of pentobarbital-anesthetized, vagotomized, paralyzed and ventilated cats. Inhibitions exerted on each neuron by the pre-synaptic pools of respiratory neurons were revealed when the neuron was depolarized by an iontophoretic application of the excitatory amino-acid analogue quisqualate. Cycle-triggered time histograms of the spontaneous and quisqualate-increased discharge of respiratory neurons were constructed in eupnea and in apneusis induced with MK-801. During apneustic breathing, the activity of the respiratory neuronal network changed throughout the entire respiratory cycle including the post-inspiratory phase, and the peak discharge rates of all types of respiratory neurons, except the late-expiratory type, decreased. During apneusis, the activity of the post-inspiratory neuronal pool, the post-inspiratory depression of other respiratory neurons, and the phrenic nerve after-discharge were reduced (but not totally suppressed), whereas the discharge of some post-inspiratory neurons shifted into the apneustic plateau. The shortened post-inspiration (stage 1 of expiration) altered the organization of the expiratory phase. Late-expiratory neurons (stage 2 of expiration) discharged earlier in expiration and their discharge rate increased. The inspiratory on-switching was functionally unaffected. Early inspiratory neurons of the decrementing type retained a decrementing pattern followed by a reduced discharge rate in the apneustic plateau, whereas early-inspiratory neurons of the constant type maintained a high discharge rate throughout the apneustic plateau. Inspiratory augmenting neurons, late-inspiratory and "off-switch" neurons also discharged throughout the apneustic plateau. During the apneustic plateau, the level of activity was constant in the phrenic nerve and in inspiratory neurons of the early-constant, augmenting, and late types. However, progressive changes in the activity of other neuronal types demonstrated the evolving state of the respiratory network in the plateau phase. There was a slowed but continued decrease of the activity of early-inspiratory decrementing neurons, accompanied by an increasing activity and/or excitability of "off-switch", post-inspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration.(ABSTRACT TRUNCATED AT 400 WORDS)

Are the post-inspiratory neurons in the decerebrate rat cranial motoneurons or interneurons?

We examined the membrane potentials of 63 respiratory neurons in the ventrolateral medulla of decerebrate rats, whose trajectories had the characteristics of the post-inspiratory neurons, i.e. exhibiting hyperpolarization during inspiration, rapid depolarization at end-inspiration and progressive repolarization with a decrementing pattern during the intervals between phrenic bursts. Synaptic responses of 6 post-inspiratory neurons which were tested by stimulation of cervical vagus or superior laryngeal nerves were excitatory. Eleven of these 63 post-inspiratory neurons were labeled by intracellular injection of horseradish peroxidase (HRP). Ten of these 11 labeled neurons were motoneurons since their axons exited the medulla after joining the roots of cranial nerves. However, only one of these motoneurons was antidromically activated by stimulation of the ipsilateral cervical vagus nerve. We assumed that most of the post-inspiratory medullary neurons of the present study were motoneurons, but not interneurons, although antidromic invasion was not possible after stimulation of the cervical vagus and superior laryngeal nerves. Two post-inspiratory neurons of this sample had bulbospinal axons, which were revealed by antidromical activation of spinal cord and HRP labeling, respectively. The axon of the labeled bulbospinal neuron had axonal collaterals which were distributed within the region of the nucleus ambiguous of the ipsilateral medulla. The functional significance of this type of post-inspiratory neuron is discussed.

Patterns of membrane potentials and distributions of the medullary respiratory neurons in the decerebrate rat

We analyzed the membrane potential of 161 respiratory neurons in the medulla of decerebrate rats which were paralyzed and ventilated. Three types of inspiratory (I) neurons were observed: those displaying progressive depolarization in inspiration (augmenting I neurons), those which gradually repolarized after maximal depolarization at the onset of inspiration (decrementing I neurons) and those exhibiting a plateau or bell-shaped membrane potential trajectory throughout inspiration (I-all neurons). Three types of expiratory (E) neurons were also encountered: those in which the membrane potential progressively depolarized (augmenting E neurons), those in which the membrane potential repolarized during the interval between phrenic bursts (decrementing E or post-I neurons) and those exhibiting a plateau or bell-shaped membrane potential trajectory throughout expiration (E-all neurons). Axonal projections of these medullary neurons were identified in the cranial nerves (n = 34), or in the spinal cord (n = 19) as revealed by antidromic stimulation and/or by reconstruction following horseradish peroxidase (HRP) labeling. The other 108 neurons were not antidromically activated (NAA) by the stimulations tested, or had their axons terminating inside the medulla as revealed by HRP labeling. All these respiratory neurons, except for 3 which were hypoglossal motoneurons, had their somata within the ventrolateral medulla, in the region of the nucleus ambiguus, homologous to the ventral respiratory group (VRG) of the cat. No dorsal respiratory group (DRG) was detected within the medulla of the rats. Due to this absence of a DRG, it is concluded that the neural organization of respiratory centers is quite different in cats and rats.

Coexistence of glutamate and choline acetyltransferase in a major subpopulation of laryngeal motoneurons of the rat

Glutamate immunoreactivity was found in 88% of the laryngeal motoneurons which were located in the ambiguus nucleus and the retrofacial nucleus. The glutamate-containing laryngeal motoneurons were identified by double labeling using the retrograde tracing of True blue from the cut end of the recurrent laryngeal nerve and immunocytochemistry of glutamate. On the other hand, choline acetyltransferase (ChAT) immunoreactivity was found in all the laryngeal motoneurons which were similarly identified as the origin of the recurrent laryngeal nerve using retrograde tracing of True blue. Since glutamate coexists richly with ChAT in a major subpopulation of laryngeal motoneurons, it is probable that glutamate may play a role as a co-transmitter in the cholinergic motoneurons.