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

Brainstem-mediated sniffing and respiratory modulation during odor stimulation

The trigeminal and olfactory systems interact during sensory processing of odor. Here, we investigate odor-evoked modulations of brainstem respiratory networks in a decerebrated perfused brainstem preparation of rat with intact olfactory bulbs. Intranasal application of non-trigeminal odors (rose) did not evoke respiratory modulation in absence of cortico-limbic circuits. Conversely, trigeminal odors such as menthol or lavender evoked robust respiratory modulations via direct activation of preserved brainstem circuits. Trigeminal odors consistently triggered short phrenic nerve bursts (fictive sniff), and the strong trigeminal odor menthol also triggered a slowing of phrenic nerve frequency. Phrenic and vagal nerve recordings reveal that fictive sniffs transiently interrupted odor evoked tonic postinspiratory vagal discharge. This motor pattern is significantly different from normal (eupneic) respiratory activity. In conclusion, we show for the first time the direct involvement of brainstem circuits in primary odor processing to evoke protective sniffs and respiratory modulation in the complete absence of forebrain commands.

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.

Testing the hypothesis of neurodegeneracy in respiratory network function with a priori transected arterially perfused brain stem preparation of rat

Degeneracy of respiratory network function would imply that anatomically discrete aspects of the brain stem are capable of producing respiratory rhythm. To test this theory we a priori transected brain stem preparations before reperfusion and reoxygenation at 4 rostrocaudal levels: 1.5 mm caudal to obex (n = 5), at obex (n = 5), and 1.5 (n = 7) and 3 mm (n = 6) rostral to obex. The respiratory activity of these preparations was assessed via recordings of phrenic and vagal nerves and lumbar spinal expiratory motor output. Preparations with a priori transection at level of the caudal brain stem did not produce stable rhythmic respiratory bursting, even when the arterial chemoreceptors were stimulated with sodium cyanide (NaCN). Reperfusion of brain stems that preserved the pre-Bötzinger complex (pre-BötC) showed spontaneous and sustained rhythmic respiratory bursting at low phrenic nerve activity (PNA) amplitude that occurred simultaneously in all respiratory motor outputs. We refer to this rhythm as the pre-BötC burstlet-type rhythm. Conserving circuitry up to the pontomedullary junction consistently produced robust high-amplitude PNA at lower burst rates, whereas sequential motor patterning across the respiratory motor outputs remained absent. Some of the rostrally transected preparations expressed both burstlet-type and regular PNA amplitude rhythms. Further analysis showed that the burstlet-type rhythm and high-amplitude PNA had 1:2 quantal relation, with burstlets appearing to trigger high-amplitude bursts. We conclude that no degenerate rhythmogenic circuits are located in the caudal medulla oblongata and confirm the pre-BötC as the primary rhythmogenic kernel. The absence of sequential motor patterning in a priori transected preparations suggests that pontine circuits govern respiratory pattern formation.

Defining inhibitory neurone function in respiratory circuits: opportunities with optogenetics?

Pharmacological and mathematical modelling studies support the view that synaptic inhibition in mammalian brainstem respiratory circuits is essential for generating normal and stable breathing movements. GABAergic and glycinergic neurones are known components of these circuits but their precise functional roles have not been established, especially within key microcircuits of the respiratory pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes involved in phasic control of respiratory pump and airway muscles. Here, we review briefly current concepts of relevant complexities of inhibitory synapses and the importance of synaptic inhibition in the operation of these microcircuits. We highlight results and limitations of classical pharmacological studies that have suggested critical functions of synaptic inhibition. We then explore the potential opportunities for optogenetic strategies that represent a promising new approach for interrogating function of inhibitory circuits, including a hypothetical wish list for optogenetic approaches to allow expedient application of this technology. We conclude that recent technical advances in optogenetics should provide a means to understand the role of functionally select and regionally confined subsets of inhibitory neurones in key respiratory circuits such as those in the pre-BötC and BötC.

An arterially perfused nose-olfactory bulb preparation of the rat

A main feature of the mammalian olfactory bulb network is the presence of various rhythmic activities, in particular, gamma, beta, and theta oscillations, with the latter coupled to the respiratory rhythm. Interactions between those oscillations as well as the spatial distribution of network activation are likely to determine olfactory coding. Here, we describe a novel semi-intact perfused nose-olfactory bulb-brain stem preparation in rats with both a preserved olfactory epithelium and brain stem, which could be particularly suitable for the study of oscillatory activity and spatial odor mapping within the olfactory bulb, in particular, in hitherto inaccessible locations. In the perfused olfactory bulb, we observed robust spontaneous oscillations, mostly in the theta range. Odor application resulted in an increase in oscillatory power in higher frequency ranges, stimulus-locked local field potentials, and excitation or inhibition of individual bulbar neurons, similar to odor responses reported from in vivo recordings. Thus our method constitutes the first viable in situ preparation of a mammalian system that uses airborne odor stimuli and preserves these characteristic features of odor processing. This preparation will allow the use of highly invasive experimental procedures and the application of techniques such as patch-clamp recording, high-resolution imaging, and optogenetics within the entire olfactory bulb.

Respiratory rhythm generation in vivo

The cellular and circuit mechanisms generating the rhythm of breathing in mammals have been under intense investigation for decades. Here, we try to integrate the key discoveries into an updated description of the basic neural processes generating respiratory rhythm under in vivo conditions.

Parasympathetic preganglionic cardiac motoneurons labeled after voluntary diving

A dramatic bradycardia is induced by underwater submersion in vertebrates. The location of parasympathetic preganglionic cardiac motor neurons driving this aspect of the diving response was investigated using cFos immunohistochemistry combined with retrograde transport of cholera toxin subunit B (CTB) to double-label neurons. After pericardial injections of CTB, trained rats voluntarily dove underwater, and their heart rates (HR) dropped immediately to 95 ± 2 bpm, an 80% reduction. After immunohistochemical processing, the vast majority of CTB labeled neurons were located in the reticular formation from the rostral cervical spinal cord to the facial motor nucleus, confirming previous studies. Labeled neurons caudal to the rostral ventrolateral medulla were usually spindle-shaped aligned along an oblique line running from the dorsal vagal nucleus to the ventrolateral reticular formation, while those more rostrally were multipolar with extended dendrites. Nine percent of retrogradely-labeled neurons were positive for both cFos and CTB after diving and 74% of these were found rostral to the obex. CTB also was transported transganglionically in primary afferent fibers, resulting in large granular deposits in dorsolateral, ventrolateral, and commissural subnuclei of the nucleus tractus solitarii (NTS) and finer deposits in lamina I and IV-V of the trigeminocervical complex. The overlap of parasympathetic preganglionic cardiac motor neurons activated by diving with those activated by baro- and chemoreceptors in the rostral ventrolateral medulla is discussed. Thus, the profound bradycardia seen with underwater submersion reinforces the notion that the mammalian diving response is the most powerful autonomic reflex known.

Animal models for investigating the central control of the Mammalian diving response

Pioneering studies by Per Scholander indicated that the diving response consists of reflexly induced apnea, bradycardia and an alteration of blood flow that maintains perfusion of the heart and brain. More recently field physiological studies have shown that many marine animals can adjust cardiorespiratory aspects of their diving response depending upon the behavioral situation. This could suggest that the very labile heart rate during diving is under direct cortical control. However, the final control of autonomic nervous system functioning resides within the brainstem and not the cortex. Many physiologists regard the brain as a "black box" where important neuronal functioning occurs, but the complexity of such functioning leaves systematic investigation a daunting task. As a consequence the central control of the diving response has been under-investigated. Thus, to further advance the field of diving physiology by understanding its central neuronal control, it would be first necessary to understand the reflex circuitry that exists within the brainstem of diving animals. To do this will require an appropriate animal model. In this review, two animals, the muskrat and rat, will be offered as animal models to investigate the central aspects of the diving response. Firstly, although these rodents are not marine animals, natural histories indicate that both animals can and do exploit aquatic environments. Secondly, physiological recordings during natural and simulated diving indicate that both animals possess the same basic physiological responses to underwater submersion that occur in marine animals. Thirdly, the size and ease of housing of both animals makes them attractive laboratory research animals. Finally, the enormous amount of scientific literature regarding rodent brainstem autonomic control mechanisms, and the availability of brain atlases, makes these animals ideal choices to study the central control of the mammalian diving response.

Repetitive paired stimulation of nasotrigeminal and peripheral chemoreceptor afferents cause progressive potentiation of the diving bradycardia

Hallmarks of the mammalian diving response are protective apnea and bradycardia. These cardiorespiratory adaptations can be mimicked by stimulation of the trigeminal ethmoidal nerve (EN5) and reflect oxygen-conserving mechanisms during breath-hold dives. Increasing drive from peripheral chemoreceptors during sustained dives was reported to enhance the diving bradycardia. The underlying neuronal mechanisms, however, are unknown. In the present study, expression and plasticity of EN5-bradycardias after paired stimulation of the EN5 and peripheral chemoreceptors was investigated in the in situ working heart-brain stem preparation. Paired stimulations enhanced significantly the bradycardic responses compared with EN5-evoked bradycardia using submaximal stimulation intensity. Alternating stimulations of the EN5 followed by paired stimulation of the EN5 and chemoreceptors (10 trials, 3-min interval) caused a progressive and significant potentiation of EN5-evoked diving bradycardia. In contrast, bradycardias during paired stimulation remained unchanged during repetitive stimulation. The progressive potentiation of EN5-bradycardias was significantly enhanced after microinjection of the 5-HT(3) receptor agonist (CPBG hydrochloride) into the nucleus tractus solitarii (NTS), while the 5-HT(3) receptor antagonist (zacopride hydrochloride) attenuated the progressive potentiation. These results suggest an integrative function of the NTS for the multimodal mediation of the diving response. The potentiation or training of a submaximal diving bradycardia requires peripheral chemoreceptor drive and involves neurotransmission via 5-HT(3) receptor within the NTS.

Laryngeal response to nasal ventilation in nonsedated newborn lambs

Although endoscopic studies in adult humans have suggested that laryngeal closure can limit alveolar ventilation during nasal intermittent positive pressure ventilation (nIPPV), there are no available data regarding glottal muscle activity during nIPPV. In addition, laryngeal behavior during nIPPV has not been investigated in neonates. The aim of the present study was to assess laryngeal muscle response to nIPPV in nonsedated newborn lambs. Nine newborn lambs were instrumented for recording states of alertness, electrical activity [electromyograph (EMG)] of glottal constrictor (thyroarytenoid, TA) and dilator (cricothyroid, CT) muscles, EMG of the diaphragm (Dia), and mask and tracheal pressures. nIPPV in pressure support (PS) and volume control (VC) modes was delivered to the lambs via a nasal mask. Results show that increasing nIPPV during wakefulness and quiet sleep led to a progressive disappearance of Dia and CT EMG and to the appearance and subsequent increase in TA EMG during inspiration, together with an increase in trans-upper airway pressure (TUAP). On rare occasions, transmission of nIPPV through the glottis was prevented by complete, active glottal closure, a phenomenon more frequent during active sleep epochs, when irregular bursts of TA EMG were observed. In conclusion, results of the present study suggest that active glottal closure develops with nIPPV in nonsedated lambs, especially in the VC mode. Our observations further suggest that such closure can limit lung ventilation when raising nIPPV in neonates.

Laryngeal function and nasal ventilatory support in the neonatal period

Nasal application of positive airway pressure, either intermittently or continuously, is increasingly used in the neonatal period. An important difference however when using a nasal interface as opposed to an endotracheal tube for ventilatory support is the interposition of the larynx. Recent animal studies from our laboratory showed that nasal ventilatory support in the neonatal period can significantly impact laryngeal function. This includes active laryngeal closure against intermittent positive pressure ventilation, which can limit lung ventilation, and inhibition of non-nutritive swallowing, which may delay swallowing maturation. Those novel findings are highly relevant to neonatal respiratory care. Additional studies are underway to uncover both the mechanisms involved and consequences on lung ventilation and swallowing function.

Capsaicin-induced activation of pulmonary vagal C fibers produces reflex laryngeal closure in the rat

Our recent studies show that intravenous administration of capsaicin induces enhancement of the intralaryngeal thyroarytenoid (TA) branch but a reduction of the intralaryngeal abducent branch, suggesting that the glottis is likely closed by capsaicin. The aim of the present study was to examine whether the glottis is adducted by intravenous administration of capsaicin. Electromyographic (EMG) activity of the TA muscle, subglottal pressure (SGP), and glottal behavior were evaluated before and after intravenous administration of capsaicin in male Wistar rats that were anesthetized and tracheostomized. Catheters were placed in the femoral artery and vein, as well as in the right jugular vein. Low and high doses of capsaicin (0.625 and 1.25 microg/kg) produced apnea and increases in the amplitude of the TA EMG. This enhancement of the TA EMG was observed during apnea as well as during recovery from apnea. Moreover, the onset of the TA EMG was advanced such that it commenced earlier during inspiration. Concomitantly, the SGP substantially increased. Increases in both the TA EMG and SGP were abolished after bilateral sectioning of the recurrent laryngeal nerve. In some animals, movement of the vocal folds was recorded by taking a motion picture with a digital camera under a surgical microscope. With intravenous administration of capsaicin, a tight glottal closure, decreases in blood pressure, and bradycardia were observed. These results strongly suggest that glottal closure is reflexively induced by intravenous administration of capsaicin and that closure of the glottis is beneficial for the defense of the airway and lungs when an animal is exposed to environmental irritants.

The anterior ethmoidal nerve is necessary for the initiation of the nasopharyngeal response in the rat

Stimulation of the nasal passages with ammonia vapors can initiate a nasopharyngeal response that resembles the diving response. This response consists of a sympathetically mediated increase in peripheral vascular resistance, parasympathetically mediated bradycardia and an apnea. The current study investigated the role of the anterior ethmoidal nerve (AEN) in the nasopharyngeal response in the rat, as it is thought that the AEN provides the main sensory innervation of the nasal passages. When both AENs were intact, nasal stimulation caused significant bradycardia, hypertension, and apnea and produced Fos label ventrally within the ipsilateral medullary dorsal horn (MDH) and paratrigeminal nucleus just caudal to the obex. This labeling presumably represents activation of second-order trigeminal neurons. When only one AEN was intact, the nasopharyngeal response was slightly attenuated, and a similar pattern of Fos labeling was only seen in the trigeminal nucleus ipsilateral to the intact AEN. The trigeminal labeling contralateral to the intact AEN was significantly reduced. When both AENs were cut, the nasopharyngeal response to nasal stimulation consisted of only a slight apnea and an increase in arterial pressure; the resultant Fos labeling within the trigeminal nucleus was significantly reduced. Cutting both AENs but not stimulating the nasal passages also produced some Fos labeling within the trigeminal nucleus. These findings suggest that a single AEN can provide sufficient afferent input to initiate the cardiorespiratory changes consistent with the nasopharyngeal response. We conclude that the AEN provides a unique afferent contribution that is capable of producing the diving response.

Capsaicin administration inhibits the abducent branch but excites the thyroarytenoid branch of the recurrent laryngeal nerves in the rat

Our recent study showed that both inspiratory and expiratory activities of the recurrent laryngeal nerve (RLN) were enhanced by capsaicin administration in the rat (Lu IJ, Ku LC, Lin JT, Lee KZ, and Hwang JC. Chin J Physiol 45: 143–154, 2002). There are two intralaryngeal branches of the RLN: one innervates the thyroarytenoid (TA) muscle and the other innervates the abductor (Abd) muscles. To examine whether these two intralaryngeal branches respond similarly to capsaicin administration, their discharges as well as activities of the phrenic nerve (PNA) and the superior laryngeal nerve (SLNA) were monitored in anesthetized and ventilated rats at normocapnia in hyperoxia. The low dose of capsaicin (0.625 μg/kg) produced a cardiopulmonary chemoreflex, showing apnea, a decrease in PNA, hypotension, and bradycardia, and significant decreases in SLNA and the activity of the Abd branch. Concurrently, there was an increase in the intralaryngeal TA activity during both apnea and the recovery from apnea. The high dose of capsaicin (1.25 μg/kg) evoked larger chemoreflexive responses and laryngeal nerve activities. In addition, both doses of capsaicin initiated a similar delay in the onset of Abd activity and SLNA but an earlier onset for the TA branch to commence during inspiration. A bilateral vagotomy abolished the laryngeal responses to capsaicin administration. However, PNA and blood pressure were enhanced with capsaicin administration after the vagotomy. These results suggest that laryngeal adduction in response to capsaicin administration is vagal afferent dependent and that it may also represent reflexive protection for the airway and lungs.

Distribution of glycine transporter 2 mRNA-containing neurons in relation to glutamic acid decarboxylase mRNA-containing neurons in rat medulla

We studied the distribution of medullary glycinergic neurons in relation to GABAergic neurons, by using in situ hybridization method for mRNA encoding either glycine transporter 2 (GLYT2) or glutamic acid decarboxylase isoform 67 (GAD67). GLYT2 mRNA-positive (GLYT2+) neurons were distributed widely and clustered in (1). the respiration-related area of the ventrolateral medulla called the Bötzinger complex, (2). the nucleus retroambiguus caudal to the obex or the caudal ventral respiratory group, (3). the spinal trigeminal nucleus, (4). a small area immediately dorsal to the inferior olivary nucleus, and (5). the border zone between the hypoglossal nucleus and the surrounding reticular formation. It was characteristic that in the dorsomedial medulla, GLYT2+ neurons were distributed only sparsely in contrast to dense GAD67+ neurons. Only few GLYT2+ neurons were distributed in the medial and interstitial subnuclei of the nucleus tractus solitarii. In particular virtually no GLYT2+ neurons were found in the area postrema. Furthermore, in the reticular formation and the spinal trigeminal nucleus, GAG67+ neurons tended to be distributed in the area where GLYT2+ neurons were sparse, and vice versa. These results provide useful information for the effort of determining neurotransmitters involved in the medullary neurons.

Glycine is used as a transmitter by decrementing expiratory neurons of the ventrolateral medulla in the rat

The medullary respiratory network involves various types of respiratory neurons. The present study focused on possible inhibitory neurons called decrementing expiratory (E-DEC) neurons and aimed to determine whether their transmitter is glycine or GABA. In Nembutal-anesthetized, neuromuscularly blocked, and artificially ventilated rats we labeled E-DEC neurons with Neurobiotin and processed the tissues for detection of mRNA encoding either glycine transporter 2 (GLYT2) as a marker for glycinergic neurons or glutamic acid decarboxylase isoform 67 (GAD67) as a marker for GABAergic neurons, using in situ hybridization. Of 38 E-DEC neurons that were labeled, cranial motoneurons (n = 14), which were labeled as control, were negative for either GLYT2 mRNA (n = 10) or GAD67 mRNA (n = 4). The other E-DEC neurons (n = 24) were non-motoneurons. Sixteen of them were examined for GLYT2 mRNA, and the majority (11 of 16) was GLYT2 mRNA-positive. The remaining E-DEC neurons (n = 8) were examined for GAD67 mRNA, and all of them were GAD67 mRNA-negative. The GLYT2 mRNA-positive E-DEC neurons were located in the ventrolateral medulla spanning the Bötzinger complex (BOT), the rostral ventral respiratory group (VRG), and the caudal VRG. We conclude that not only E-DEC neurons of the BOT but also many E-DEC neurons of the VRG are inhibitory and use glycine as a transmitter. Although the present negative data cannot rule out completely the release of GABA or co-release of glycine and GABA from E-DEC neurons, several lines of evidence suggest that the glycinergic process is primarily responsible for the phasic inhibition of the respiratory network during the expiratory phase.

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.