Retrotrapezoid nucleus and parafacial respiratory group

The rat retrotrapezoid nucleus (RTN) contains about 2000 Phox2b-expressing glutamatergic neurons (ccRTN neurons; 800 in mice) with a well-understood developmental lineage. ccRTN neuron development fails in mice carrying a Phox2b mutation commonly present in the congenital central hypoventilation syndrome. In adulthood, ccRTN neurons regulate the breathing rate and intensity, and may regulate active expiration along with other neighboring respiratory neurons. Prenatally, ccRTN neurons form an autonomous oscillator (embryonic parafacial group, e-pF) that activates and possibly paces inspiration. The pacemaker properties of the ccRTN neurons probably vanish after birth to be replaced by synaptic drives. The neonatal parafacial respiratory group (pfRG) may represent a transitional phase during which ccRTN neurons lose their group pacemaker properties. ccRTN neurons are activated by acidification via an intrinsic mechanism or via ATP released by glia. In summary, throughout life, ccRTN neurons seem to be a critical hub for the regulation of CO(2) via breathing.

Central respiratory chemoreception

By definition central respiratory chemoreceptors (CRCs) are cells that are sensitive to changes in brain PCO(2) or pH and contribute to the stimulation of breathing elicited by hypercapnia or metabolic acidosis. CO(2) most likely works by lowering pH. The pertinent proton receptors have not been identified and may be ion channels. CRCs are probably neurons but may also include acid-sensitive glia and vascular cells that communicate with neurons via paracrine mechanisms. Retrotrapezoid nucleus (RTN) neurons are the most completely characterized CRCs. Their high sensitivity to CO(2) in vivo presumably relies on their intrinsic acid sensitivity, excitatory inputs from the carotid bodies and brain regions such as raphe and hypothalamus, and facilitating influences from neighboring astrocytes. RTN neurons are necessary for the respiratory network to respond to CO(2) during the perinatal period and under anesthesia. In conscious adults, RTN neurons contribute to an unknown degree to the pH-dependent regulation of breathing rate, inspiratory, and expiratory activity. The abnormal prenatal development of RTN neurons probably contributes to the congenital central hypoventilation syndrome. Other CRCs presumably exist, but the supportive evidence is less complete. The proposed locations of these CRCs are the medullary raphe, the nucleus tractus solitarius, the ventrolateral medulla, the fastigial nucleus, and the hypothalamus. Several wake-promoting systems (serotonergic and catecholaminergic neurons, orexinergic neurons) are also putative CRCs. Their contribution to central respiratory chemoreception may be behavior dependent or vary according to the state of vigilance.

Avian nucleus retroambigualis: cell types and projections to other respiratory-vocal nuclei in the brain of the zebra finch (Taeniopygia guttata)

In songbirds song production requires the intricate coordination of vocal and respiratory muscles under the executive influence of the telencephalon, as for speech in humans. In songbirds the site of this coordination is suspected to be the nucleus retroambigualis (RAm), because it contains premotor neurons projecting upon both vocal motoneurons and spinal motoneurons innervating expiratory muscles, and because it receives descending inputs from the telencephalic vocal control nucleus robustus archopallialis (RA). Here we used tract-tracing techniques to provide a more comprehensive account of the projections of RAm and to identify the different populations of RAm neurons. We found that RAm comprises diverse projection neuron types, including: 1) bulbospinal neurons that project, primarily contralaterally, upon expiratory motoneurons; 2) a separate group of neurons that project, primarily ipsilaterally, upon vocal motoneurons in the tracheosyringeal part of the hypoglossal nucleus (XIIts); 3) neurons that project throughout the ipsilateral and contralateral RAm; 4) another group that sends reciprocal, ascending projections to all the brainstem sources of afferents to RAm, namely, nucleus parambigualis, the ventrolateral nucleus of the rostral medulla, nucleus infra-olivarus superior, ventrolateral parabrachial nucleus, and dorsomedial nucleus of the intercollicular complex; and 5) a group of relatively large neurons that project their axons into the vagus nerve. Three morphological classes of RAm cells were identified by intracellular labeling, the dendritic arbors of which were confined to RAm, as defined by the terminal field of RA axons. Together the ascending and descending projections of RAm confirm its pivotal role in the mediation of respiratory-vocal control.

Effects of microinjections of glutamate and glutamate receptor antagonists into A5 zone on generation of respiratory rhythm in ponto-bulbospinal preparations from newborn rats in vitro

Activation of glutamate receptors in A5 neurons enhanced their tonic inhibitory influence on the respiratory rhythm generator in isolated newborn rat ponto-bulbospinal preparations. The stimulating effect of glutamate on A5 neurons is determined by its effect mainly on non-NMDA receptors and less so on NMDA receptors.

Transgenic expression of fluorescent proteins in respiratory neurons

We screened transgenic mouse lines with Thy1.2 promoter-induced expression of fluorescent proteins (FPs) for targeting of respiratory neuronal populations in the medulla oblongata. Respiratory neurons were found to be tagged by FPs within the ventral respiratory column (VRC), the pre-Bötzinger complex (preBötC) and the rostral ventral respiratory group (rVRG) interneurons. A subset of neurons in the preBötC, labeled with the enhanced yellow fluorescent protein (EYFP), showed inspiratory activity during whole cell recordings from rhythmic slice preparations. Additionally, a subpopulation of EYFP-labeled preBötC neurons expressed both NK1- and mu-opioid receptors. Furthermore, the spinal trigeminal nucleus, the lateral reticular nucleus (LRT) and the hypoglossal nucleus demonstrated intense EYFP expression whereas other regions of the medulla were devoid of neuronal EYFP labeling (e.g. the nucleus ambiguous). In conclusion, Thy1.2-FP transgenic mice will facilitate the functional analysis of respiratory-related neurons in the medulla and improve the three dimensional analysis of cells contributing to this important neuronal circuit.

Control of respiratory and hypotensive response during hypoxic chemoreflex by A5 region neurons in rats

The responses of A5 region neurons, the phrenic nerve, and systemic blood pressure to short-term hypoxia were examined in rats under conditions of spontaneous respiration. Tonic and respiration-modulated neurons increasing their discharge activity during hypoxia were identified. This hypoxia-induced response was more pronounced in the neurons with baseline discharge rate of 0.1-4.5 Hz (electrical activity of neurons increased by 4-5 times) compared to neurons with the baseline activity of 5.4-49.6 Hz (discharge rate increased by 1.4-2.0 times). The latency and duration of activation of all types A5 neurons correlated with the parameters of activation of the phrenic nerve. During hypoxia, activation of A5 neurons corresponded to the period of blood pressure drop (one-third of the reaction time), but not to the period of plateau or recovery phase. Low-, middle, and high-frequency A5 neurons participated in the modulation of hypoxia-provoked respiratory and hypotensive responses. Modulation of the respiratory response by A5 neurons was observed during the entire period of phrenic nerve activation, while modulation of the hypotensive response occurred only during blood pressure decrease.

Glutamatergic neurons in the Kölliker-Fuse nucleus project to the rostral ventral respiratory group and phrenic nucleus: a combined retrograde tracing and in situ hybridization study in the rat

Kölliker-Fuse nucleus (KF) neurons are considered to excite motoneurons in the phrenic nucleus (PhN) during inspiration through its projection to the PhN and/or to the rostral ventral respiratory group (rVRG), which in turn projects to the PhN, probably by releasing glutamate from their axon terminals. Using a combined retrograde tracing and in situ hybridization technique, here we demonstrate that most of the KF neurons projecting to the PhN and rVRG contain vesicular glutamate transporter 2 (VGLUT2) mRNA but not glutamic acid decarboxylase 67 (GAD67) mRNA, providing definitive evidence that these neurons are glutamatergic. Together with previous data by Stornetta et al. [Stornetta, R.L., Sevigny, C.P., Guyenet, P.G., 2003b. Inspiratory argumenting bulbospinal neurons express both glutamatergic and enkephalinergic phenotypes. J. Comp. Neurol. 455, 113-124], indicating that PhN-projecting rVRG neurons are VGLUT2 mRNA-positive, the present results suggest that the glutamatergic KF-PhN pathway and/or the glutamatergic KF-rVRG-PhN pathway transmit excitatory outputs of KF neurons to the PhN neurons during inspiration.

Respiratory rhythms generated in the lamprey rhombencephalon

Brainstem networks generating the respiratory rhythm in lampreys are still not fully characterized. In this study, we described the patterns of respiratory activities and we identified the general location of underlying neural networks. In a semi-intact preparation including the brain and gills, rhythmic discharges were recorded bilaterally with surface electrodes placed over the vagal motoneurons. The main respiratory output driving rhythmic gill movements consisted of short bursts (40.9+/-15.6 ms) of discharge occurring at a frequency of 1.0+/-0.3 Hz. This fast pattern was interrupted by long bursts (506.3+/-174.6 ms) recurring with an average period of 37.4+/-24.9 s. After isolating the brainstem by cutting all cranial nerves, the frequency of the short respiratory bursts did not change significantly, but the slow pattern was less frequent. Local injections of a glutamate agonist (AMPA) and antagonists (6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or D,L-amino-5-phosphonopentanoic acid (AP5)) were made over different brainstem regions to influence respiratory output. The results were similar in the semi-intact and isolated-brainstem preparations. Unilateral injection of AP5 or CNQX over a rostral rhombencephalic region, lateral to the rostral pole of the trigeminal motor nucleus, decreased the frequency of the fast respiratory rhythm bilaterally or stopped it altogether. Injection of AMPA at the same site increased the rate of the fast respiratory rhythm and decreased the frequency of the slow pattern. The activity recorded in this area was synchronous with that recorded over the vagal motoneurons. After a complete transverse lesion of the brainstem caudal to the trigeminal motor nucleus, the fast rhythm was confined to the rostral area, while only the slow activity persisted in the vagal motoneurons. Our results support the hypothesis that normal breathing depends on the activity of neurons located in the rostral rhombencephalon in lampreys, whereas the caudal rhombencephalon generates the slow pattern.

Acute role of the brain-derived neurotrophic factor (BDNF) on the respiratory neural network activity in mice in vitro

In humans, several pathologies are associated with disturbances of the respiratory control, some of them including alteration in the brain-derived neurotrophic factor (BDNF) signalling pathway. BDNF has long been known as a neurotrophic factor involved in survival, differentiation and maintenance of neuronal populations in the peripheral and central nervous system. More recently BDNF has also been discovered to be a potent neuromodulator with acute effects on neuronal excitability and synaptic plasticity. Animals deleted for the gene encoding BDNF exhibit respiratory alteration suggesting an important but yet undefined role of the neurotrophin in respiratory rhythmogenesis either by a trophic and/or an acute action. The possibility that BDNF might exert an acute regulatory role on the rhythmic activity of the respiratory generator of the pre-Bötzinger complex has been recently examined in newborn mice in vitro. Results obtained, reviewed in the present paper, will help getting insights in respiratory rhythm regulatory mechanisms that involve BDNF signalling.

Cluster breathing associated with bihemispheric infarction and sparing of the brainstem

OBJECTIVE

To report cluster breathing pattern associated with a nonbrainstem lesion.

DESIGN

Case report.

SETTING

Neurointensive care unit, St Mary's Hospital, Rochester, Minn.

PATIENT

A patient with subarachnoid hemorrhage developed severe, diffuse, distal bilateral middle cerebral artery vasospasm with resultant cortical laminar necrosis and transient cluster breathing. Intervention Magnetic resonance imaging revealed bihemispheric lesions but no brainstem lesion.

CONCLUSION

Cluster breathing may occur with nonbrainstem lesions.

Looking for inspiration: new perspectives on respiratory rhythm

Recent experiments in vivo and in vitro have advanced our understanding of the sites and mechanisms involved in mammalian respiratory rhythm generation. Here we evaluate and interpret the new evidence for two separate brainstem respiratory oscillators and for the essential role of emergent network properties in rhythm generation. Lesion studies suggest that respiratory cell death might explain morbidity and mortality associated with neurodegenerative disorders and ageing.

Visceral-related area in the rat insular cortex

The insular cortex interacts with the bulbar autonomic nuclei providing autonomic manifestations accompanying several neurological and psychosomatic disorders. The aim of our study was to identify the sites within the insular cortex, which could be responsible for the gastrointestinal, respiratory and cardiovascular responses. The main methods used were microinjections of HRP into several parts of the bulbar dorsal vagal complex and microstimulation of the insular cortex. It has been found that a compact group of neurons located in the middle level of the rat insular cortex projects directly to the specific "gastric" part of the dorsal vagal complex. Retrograde labelled cell bodies were revealed in the V layer of the disgranular and agranular insular cortex. Microstimulation of the sites within the middle level of the rat insular cortex produced gastric motor responses and a decrease in inspiratory airflow without significant alteration in respiratory cycle duration. More caudal microstimulation produced an increase in respiratory airflow and decreased respiratory cycle duration. These responses were usually accompanied by changes in the level of blood pressure. It is concluded that autonomic representation in the rat insular cortex is organised in a viscerotopic manner. The inhibitory respiratory zone overlaps with the gastrointestinal control area in the middle part of the insular cortex. More caudally, the excitatory respiratory zone overlaps with the cardiovascular area. On the basis of these experimental results and the data of others authors the general scheme of autonomic representation in the rat insular cortex is discussed.

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.

Nitric Oxide in the A5 Region Modulates Reaction of the Respiratory Center and Blood Pressure to Hypoxia in Rats

Hypoxia was followed by more pronounced activation of the respiratory center and pronounced hypotensive response after unilateral injection of nitric oxide synthase blocker L-NAME into the A5 region. Microinjection of exogenous nitric oxide donor sodium nitroprusside into the A5 region abolished the effect of L-NAME on hypoxia-induced changes in activity of the respiratory center and blood pressure. Bilateral transection of the vagal and sinocarotid nerves suppressed the response of the respiratory center to hypoxia. However, the hypotensive response to hypoxia in these rats did not differ from that in intact animals. Under conditions of peripheral chemoreceptor deafferentation, the hypotensive response to hypoxia did not differ before and after blockade of nitric oxide synthase in the A5 region. The regulation of respiratory center activity and blood pressure during hypoxia was modulated by A5 neurons with the involvement of nitric oxide.

Neuroanatomical evidence for indirect connections between the medial preoptic nucleus and the song control system: possible neural substrates for sexually motivated song

In European starlings ( Sturnus vulgaris) as in other seasonally breeding songbirds, a major function of song during the breeding season is mate attraction, and song in this context is highly sexually motivated. Song learning, perception, and production are regulated by nuclei of the song control system, but there is no evidence that these nuclei participate in the motivation to sing. Evidence suggests that the medial preoptic nucleus (POM), a diencephalic nucleus outside of the song control system, might regulate the motivation to sing, at least in a sexual context. If the POM is involved in the regulation of sexually motivated song, then this structure must interact with the song control system. To examine possible neuroanatomical connections between the POM and song control nuclei a tract-tracing study was performed in male starlings using the antero- and retrograde tract tracer, biotinylated dextran amine (BDA). No direct connections were identified between the POM and song control nuclei; however, labeled fibers were found to terminate in a region bordering dorsal-medial portions of the robust nucleus of the archistriatum (RA). Additionally, several indirect routes via which the POM might communicate with the song control system were identified. Specifically, POM projected to dorsomedial nucleus intercollicularis (DM), mesencephalic central gray (GCt), area ventralis of Tsai (AVT), and locus ceruleus (LoC), structures projecting directly to nuclei involved in song production (DM --> vocal-patterning and respiratory nuclei; GCt, AVT, LoC --> RA and HVC, and the context in which song is sung (AVT --> area X). These results are consistent with the possibility that the POM regulates sexually motivated song through interactions with brain regions involved in vocal production.

Preliminary study on the cytoarchitecture of the human parabrachial/Kölliker-fuse complex, with reference to sudden infant death syndrome and sudden intrauterine unexplained death

The parabrachial/Kölliker-Fuse complex has been defined, in different animal species, to lie in the dorsolateral part of the pontine tegmentum and to be subdivided into three well-defined regions: the medial parabrachial nucleus, the lateral parabrachial nucleus, and the Kölliker-Fuse nucleus. Experimental studies have shown that the parabrachial/Kölliker-Fuse complex is involved in a variety of functional activities and above all plays an important role in respiratory modulation. In human brainstem, the cytoarchitecture and physiology of this complex have not yet been fully characterized. The aim of the present study was to examine fetal and infant human brainstems in order to define the precise morphology of the three nuclei of the parabrachial/Kölliker-Fuse complex, and to determine whether this nervous center shows morphologic alterations in sudden infant death syndrome (SIDS) and in sudden intrauterine unexplained death (SIUD). In serial sections of 31 brainstems of subjects aged from 32 gestational wk to 10 months of life, we studied, by morphologic and morphometric analyses, the cytoarchitecture and the extension of the three nuclei of the parabrachial/Kölliker-Fuse complex. All the morphometric parameters were very similar in SIUD and SIDS cases to those of the respective control group, as shown by the absence of significant statistical differences between the two fetus and infant groups. We observed that the features of both the lateral and the medial parabrachial nuclei are largely consistent with those reported in experimental studies. In contrast, the Kölliker-Fuse nucleus appears to be more developed in human beings than in other animal species, showing a greater extension and a more complex structure, as well as subdivision into two subnuclei (compactus and dissipatus).

Brainstem and spinal projections of augmenting expiratory neurons in the rat

There are two types of expiratory neurons with augmenting firing patterns (E-AUG neurons), those in the Bötzinger complex (BOT) and those in the caudal ventral respiratory group (cVRG). We studied their axonal projections morphologically using intracellular labeling of single E-AUG neurons with Neurobiotin, in anesthetized, paralyzed and artificially-ventilated rats. BOT E-AUG neurons (n = 11) had extensive axonal projections to the brainstem, but E-AUG neurons (n = 5) of the cVRG sent axons that descended the contralateral spinal cord without medullary collaterals. In addition to these somewhat expected characteristics, the present study revealed a number of new projection patterns of the BOT E-AUG neurons. First, as compared with the dense projections to the ipsilateral brainstem, those to the contralateral side were sparse. Second, several BOT E-AUG neurons sent long ascending collaterals to the pons, which included an axon that reached the ipsilateral parabrachial and Kölliker-Fuse nuclei and distributed boutons. Third, conspicuous projections from branches of these ascending collaterals to the area dorsolateral to the facial nucleus were found. Thus, the present study has shown an anatomical substrate for the extensive inhibitory projections of single BOT E-AUG neurons to the areas spanning the bilateral medulla and the pons.

Microinjections of glycine into the pre-Bötzinger complex inhibit phrenic nerve activity in the rat

Microinjections of L-glutamate were used to identify the pre-Bötzinger complex in urethane-anesthetized, immobilized, bilaterally vagotomized, artificially ventilated, adult male Wistar rats. Unilateral microinjections (20-30 nl) of L-glutamate into the pre-Bötzinger complex on either side elicited a bilateral continuous phrenic nerve discharge superimposed on which was an increase in burst-frequency. Neurokinin-1 receptor immunoreactivity in the semi-compact region of the nucleus ambiguus and the area immediately ventral to it indicated that the site of microinjections was in the general region of pre-Bötzinger complex. Unilateral microinjections of glycine into the pre-Bötzinger complex caused an inhibition of phrenic nerve activity bilaterally in a concentration-dependent manner. At lower concentrations (1 and 3 mM) phrenic nerve burst-frequency as well as burst-amplitude were decreased. At higher concentrations (6 mM), complete bilateral cessation of phrenic nerve activity was observed. The effects of glycine were prevented by a prior microinjection of strychnine (0.5 mM) into the pre-Bötzinger complex. The specificity of strychnine as an antagonist for glycine receptors was established by its lack effect on GABA(A) receptors; muscimol was used as a GABA(A) receptor agonist. Unilateral microinjections of muscimol (0.01 and 0.1 mM) into previously identified pre-Bötzinger complex also caused a bilateral decrease in phrenic nerve burst-frequency and burst-amplitude. At higher concentrations (0.3 and 1 mM) muscimol microinjections into the pre-Bötzinger elicited a complete bilateral cessation of phrenic nerve activity. The effects of muscimol were not altered by prior microinjections of strychnine (0.5 mM) at the same site. These results demonstrate pharmacologically the presence of glycine receptors in the pre-Bötzinger complex. The role of these receptors in the regulation of respiration remains to be elucidated.

Projections between the medial region of the nucleus retrofacialis and other respiratory regions in rats brainstem

OBJECTIVE

To study the projections between the medial region of the nucleus retrofacialis (mNRF) and other respiratory regions in rat brainstem.

METHODS

Twenty-five adult SD rats were used in this study, which received anesthesia with sodium pentobarbitone. Retrograde/anterograde tracing methods were employed by unilateral microinjection into mNRF with 30% horseradish peroxidase (HRP, 0.5-1.0 microliter1) in 13 rats. Another 5 rats with injections of HRP and 2 with WGA-HRP into the regions 2 mm off mNRF served as control groups. and with 5% wheat germ agglutinin-conjugated HRP (WGA-HRP, 20-60 nl) in 5 rats (with another 2 receiving the injections as control).

RESULTS

It was found that HRP-labeled neuronal somas and WGA-HRP labeled terminal fibers were found in many respiration-related regions in rat brainstem such as the nucleus tractus solitarius, nucleus ambiguous, gigantocellular reticular nucleus and so forth.

CONCLUSIONS

There are comprehensive projections between mNRF and other respiratory regions in rat brainstem, and these respiratory regions may be involved in basic respiratory rhythm regulation via these connections with mNRF.

[Nerve tissue morphological study of tutin microinjection into pontine NPBM in two hours]

OBJECTIVE

To observe whether Tutin microinjection into the pontine NPBM respiratory area of rabbit will cause morphological damage to that area two hours later.

METHODS

At two hours after the microinjection of Tutin into NPBM, the experimental effects on respiration came to be remarkable and the physiological condition was well, the rabbit was subjected to morphological sampling then. The sample was cut into slices for LM (Nissl dyeing) and transmission EM observation and photography.

RESULTS

Under the LM and EM examination, no remarkable morphological damage done by Tutin microinjection into the pontine NPBM was observed. By comparing the Tutin-injected side with the other side of NPBM where equal normal saline microinjection was given, no apparent morphological difference could be found.

CONCLUSION

In our experiment condition, there was no morphological damage caused by Tutin microinjection into pontine NPBM respiratory area of the rabbit.

Projections of the dorsomedial nucleus of the intercollicular complex (DM) in relation to respiratory-vocal nuclei in the brainstem of pigeon (Columba livia) and zebra finch (Taeniopygia guttata)

Injections of neuronal tracers were made into the dorsomedial nucleus of the intercollicular complex (DM) of pigeons and zebra finches in order to investigate the projections of this nucleus which has long been implicated in respiratory-vocal control. Despite the fact that pigeons are nonsongbirds and zebra finches are songbirds, the projections were very similar in both species. Most descended throughout the brainstem, taking ventral and dorsal trajectories, which merged in the medulla. Those descending ventrally terminated upon the ventrolateral parabrachial nucleus (PBvl), the nucleus infraolivaris superior, a nucleus of the rostral ventrolateral medulla (RVL), and the nucleus retroambigualis (RAm). Those taking a dorsal trajectory via the occipitomesencephalic tract terminated in the tracheosyringeal part of the hypoglossal nucleus (XIIts), the suprahypoglossal region, and nucleus retroambigualis. There were also substantial projections throughout an arc extending between XIIts and RVL rostrally, and XIIts and RAm caudally. Neurons throughout this arc, which include inspiratory premotor neurons at levels straddling the obex and expiratory premotor neurons more caudally (in RAm), were retrogradely labeled from spinal injections. The DM projections were predominantly ipsilateral, but there were distinct contralateral projections to all the homologous nuclei in both species. All but the projections to PBvl and XIIts were reciprocal. In summary, the projections of DM suggest that it is able to influence all the key motor and premotor nuclei involved in patterned respiratory-vocal activity.

Brain stem chemosensitivity: its implication in central respiratory regulation

Central brain stem chemosensitivity plays an essential role in acid-base regulation. The site of the chemosensitive neuronal elements has not yet been clearly established. To address this question, we used an in vitro adult guinea pig isolated brain stem preparation, maintained in survival conditions by intrabasilar and bath perfusions of Krebs solution saturated with 95% O2 and 5% CO2. Hypercapnic stimulation, produced by modifying the CO2 concentration of the medium perfusing brain stem structures via the basilar perfusion system, increased the respiratory burst frequency recorded from the hypoglossal nerve. Hypercapnia evoked by ventral surface superfusion increased the respiratory burst amplitude. These data suggest that chemosensitive neuronal elements could be located at the ventral surface as well as in the deeper brainstem structures. However, their characteristics may be different.

Maturation of brain stem neurons involved in respiratory rhythmogenesis: biochemical, bioelectrical and morphological properties

Neonatal and adult respiratory-related functions of brain stem were compared using in vivo or in vitro approaches. The control of inspiratory off-switch by glutamate-like neurotransmitters was found active at birth. However, neurons from the nucleus tractus solitarius (NTS) are immature at birth because they present growth cones and the transient potassium current appears progressively during the first week of life in association with modification of the dendritic tree. These data support the hypothesis that the mechanisms of respiratory rhythmogenesis are different at birth and in the adult.

Pontile regulation of ventilatory activity in the adult rat

Our purpose was to characterize the pontile components of the brain stem ventilatory control system in rats. This study was precipitated by reports that this pontile component might differ fundamentally from that of other species. Efferent activity of the phrenic nerve was recorded in anesthetized, vagotomized, paralyzed, and ventilated adult rats. As in other species, electrical stimulations of the rostral pons caused premature terminations and/or onsets of phrenic activity in eupnea. Electrolytic lesions of rostrolateral pons resulted in apneusis, characterized by significant prolongations of the phrenic burst. Some effective lesions were in the region of the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus, the site of the pneumotaxic center. Other lesions resulting in apneusis were ventral to the pneumotaxic center. As in cats, lesions in the caudal pontile reticular formation caused the duration of the apneustic neural inspiration to return toward that of eupnea. Again, as in other species, gradual alterations from eupnea to gasping in the rat were recorded during hypoxia, which was induced by ventilation with carbon monoxide. We conclude that the brain stem respiratory control system is similarly organized in rats and other mammalian species. These results have implications for contemporary hypotheses concerning the neurogenesis of ventilatory activity.

Medullary respiratory neurons in the guinea pig: localization and firing patterns

The location and firing patterns of medullary respiratory neurons have been described in a small number of species. The cat has been the most widely studied species, but some potentially important differences have recently been noted in others. A more complete survey of species is required to determine the significance of these differences. We describe the location and firing patterns of respiratory neurons in the medulla of anesthetized, paralyzed and mechanically ventilated adult guinea pigs. Extracellular single-unit recordings were made from the medulla, their phase relationship with phrenic nerve activity used to define them as respiratory and their location marked with fast green. Respiratory units were concentrated ventrolateral to the nucleus tractus solitarius (NTS) and within and surrounding the nucleus ambiguus (NA), corresponding to the dorsal respiratory group (DRG) and ventral respiratory group (VRG) of the cat, respectively. Most DRG respiratory units were inspiratory, while the VRG contained equal numbers of inspiratory and expiratory units. The DRG and VRG both contained early, late and constant-frequency inspiratory and expiratory units. In general, these findings are similar to those in other mammalian species examined, consistent with these basic aspects of the respiratory network being highly conserved.

Monosynaptic transmission of respiratory drive to phrenic motoneurons from brainstem bulbospinal neurons in rats

The termination patterns in the rat phrenic nucleus of neurons within two respiratory cell groups of the ventrolateral medulla (Bötzinger Complex and the rostral ventral respiratory group) were determined. The plant lectin, Phaseolus vulgaris leuco-agglutinin, was used as an anterograde tracer to label presynaptic processes of bulbospinal neurons, and horseradish peroxidase was used simultaneously to label phrenic motoneurons. Labeled bulbospinal axons ended with dense terminal arborizations within the phrenic cell column and on radial phrenic motoneuron dendrite bundles, which represented the exclusive site of termination of Bötzinger Complex and rostral ventral respiratory group neurons in the lower cervical spinal cord. Terminals of these descending axons formed presumptive synaptic contacts within longitudinal and radial dendrite bundles, and on the cell somata of phrenic motoneurons.

Localization of respiratory rhythm-generating neurons in the medulla of brainstem-spinal cord preparations from newborn rats

We describe the location of Pre-I neurons, which are important to respiratory rhythm generation, in the rostral medulla of brainstem-spinal cord preparations isolated from newborn rats. This neuronal group was delimited in the reticular formation slightly medial to the caudal area of the facial nucleus and near the ventral surface. The effects of electrical stimulation and lesions in that region were also examined with respect to respiratory rhythm generation. Single shock stimulation induced Pre-I neuron firing and reset the phase of the respiratory rhythm. Electrolytic lesions in the Pre-I neuron region reduced the respiratory rate.

In vitro characterization of neurons in the ventral part of the nucleus tractus solitarius. I. Identification of neuronal types and repetitive firing properties

1. An in vitro brain stem slice preparation from adult guinea pigs was used to determine the properties of neurons located in the ventral part of the nucleus tractus solitarius (NTS), an area associated with the dorsal respiratory group. Based upon their morphology and their repetitive firing properties, three classes of ventral NTS neurons, termed types I, II, and III, were observed. 2. Type I neurons were multipolar with pyramidal-shaped cell bodies. These neurons responded to prolonged depolarizations from a resting level of -50 mV with a discrete, high-frequency burst of spikes, which rapidly adapted to a low steady-state level. When depolarized from levels more negative than -65 mV, the initial burst was diminished. 3. Type II neurons were multipolar with fusiform-shaped cell bodies. Type II neurons responded to depolarizations from -50 mV with an initial high spike frequency, which gradually adapted to a steady-state level. When depolarized from levels more negative than -60 mV, these neurons displayed a delay between the onset of the stimulus and the first spike. This delay has been termed "delayed excitation." The expression of delayed excitation was modulated by both the size and duration of hyperpolarizing prepulses that preceded depolarization. 4. Type III neurons were multipolar with spherical shaped-cell bodies. In response to depolarizations from -50 mV, these neurons displayed high-frequency firing with little adaptation. The repetitive firing properties of type III neurons were not modulated by hyperpolarization. 5. Bulbospinal neurons in the ventral NTS were identified using retrograde transport of rhodamine-labeled latex beads injected into the region of the phrenic motor nucleus at spinal cord levels C4 through C6. Only type I and type II neurons were labeled in the ventral NTS (0.2-1.0 mm rostral to the obex). Both contralateral and ipsilateral projections were observed. Contralaterally, type I and II neurons were evenly distributed. Ipsilaterally, however, type II neurons accounted for two-thirds of the labeled neurons. 6. Type I and II neurons had similar input resistances and time constants: 97.0 +/- 17.6 M omega and 14.4 +/- 2.2 ms (n = 5) for type I and 107.0 +/- 11.2 M omega and 13.7 +/- 1.6 ms for type II (n = 5).(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology of expiratory neurons of the Bötzinger complex: an HRP study in the cat

In anesthetized and artificially ventilated cats, the physiological and morphological properties of expiratory neurons or their axons of the Bötzinger complex (BOT) were studied using intracellular recording and intracellular HRP labeling techniques. Thirteen expiratory neurons (nine cell somata and four axons) were successfully stained. Four of them were motoneurons, having relatively large cell somata in the retrofacial nucleus (RFN) and axons without any collaterals inside the brainstem. All the motoneurons showed a plateau shape of depolarization potentials during the expiratory phase. Any of the other nine expiratory neurons exhibited augmenting type firing or membrane potential changes during the expiratory phase. In five out of nine augmenting neurons, cell somata were stained and located ventral to the RFN. In four, only axons were stained. The majority of the augmenting neurons had two major axonal branches: one traveling toward the contralateral side and the other descending ipsilaterally in the brainstem. The most striking feature of the axonal trajectory was that all of the stained augmenting expiratory neurons, including the axons, had collateral branches with synaptic boutons in the BOT area, thus indicating that BOT expiratory neurons interact with some respiratory neurons in the BOT area and its vicinity.

Spinal projections of brainstem respiratory related neurons in the cat as revealed by retrograde fluorescent markers

By means of retrograde axonal transport of fluorescent tracers, connections between brainstem respiratory related regions and the spinal cord has been studied in the cat. Neurons at the pneumotaxic center project bilaterally (90% ipsi-, 10% contra-) to cervical and lumbar spinal cord and ipsilaterally to thoracic levels. The ventrolateral nucleus of the tractus solitarius project mainly contralaterally (85%) to cervical levels and only contralaterally to thoracic levels; no efferent projections were found to lumbar levels. The ventral respiratory group showed a great number of neurons projecting to the spinal cord especially from the nucleus retroambiguus. Both nuclei, ambiguus and retroambiguus, project mainly contralaterally (70%) to the spinal cord. The Bötzinger complex showed rather scarce bilateral projections to cervical and only ipsilateral projections to lower cervical, thoracic and lumber levels.

Responses of bulbospinal and laryngeal respiratory neurons to hypercapnia and hypoxia

The purpose was to evaluate activities of medullary respiratory neurons during equivalent changes in phrenic discharge resulting from hypercapnia and hypoxia. Decerebrate, cerebellectomized, paralyzed, and ventilated cats were used. Vagi were sectioned at left midcervical and right intrathoracic levels caudal to the origin of right recurrent laryngeal nerve. Activities of phrenic nerve and single respiratory neurons were monitored. Neurons exhibiting antidromic action potentials following stimulations of the spinal cord and recurrent laryngeal nerve were designated, respectively, bulbospinal or laryngeal. The remaining neurons were not antidromically activated. Hypercapnia caused significant augmentations of discharge frequencies for all neuronal groups. Many of these neurons had no change or declines of activity in hypoxia. We conclude that central chemoreceptor afferent influences are ubiquitous, but excitatory influences from carotid chemoreceptors are more limited in distribution among medullary respiratory neurons. Hypoxia will increase activities of neurons that receive sufficient excitatory peripheral chemoreceptor afferents to overcome direct depression by brain stem hypoxia. The possibility that responses of respiratory muscles to hypoxia are programmed within the medulla is discussed.

The identification of brainstem neurones projecting to thoracic respiratory motoneurones in the cat as demonstrated by retrograde transport of HRP

Brainstem neurones which project to the immediate vicinity of the spinal motoneurones which supply the intercostal and abdominal respiratory muscles were identified by means of the retrograde transport of horseradish peroxidase (HRP). A combined electrophysiological and histological technique was used in which recording of phasic inspiratory or expiratory motoneurone activity within upper (T3-T4) or lower (T8-T9) thoracic segments was followed by the ion-tophoretic injection of HRP at these recording sites. HRP labelled cells were concentrated in those brainstem regions known to contain phasic respiratory neurones, namely the ventrolateral nucleus of the solitary tract (vl-NTS) or dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine (PB) nuclei. In 18 cats, 248 cells were labelled in these three respiratory regions of the brainstem while 668 were much more diffusely distributed in other regions of the medulla and pons. The ipsilateral and contralateral contributions within the respiratory regions were respectively; 23%:77% (DRG), 33%:67% (VRG), 95%:5% (PB). These results are considered in the general context of previous electrophysiological and histological findings, but also with particular reference to a related study of the projections from brainstem neurones to the phrenic nucleus [32].

Differentiation of two respiratory areas in the cat medulla using kainic acid

Kainic acid (KA) was used to destroy neuronal perikarya in different areas of the brainstem. Single KA microinjections were performed in 30 anaesthetized, vagotomized, artificially ventilated cats. Consequences were studied on the phrenic nerve activity (PNA) and blood pressure. We observed changes of the PNA unrelated to blood pressure alteration. Destruction of the dorsal respiratory area (DRA) including the nucleus tractus solitarius at the obex level produced a 40% decrease of the PNA frequency. Destruction restricted to the lateral part of the ventral respiratory area (VRA1) including the ambiguus nucleus induced a 60% decrease of the integrated PNA amplitude followed by a 40% increase of PNA frequency. These latter effects were also observed after destruction inside the infra solitary reticular formation (ISRF). No effect was observed after destruction in other brain structures. We concluded that ISRF and VRA1 form a single ventral bulbar respiratory area. This area controls respiration in a way different from that of the dorsal respiratory area (DRA).

Brainstem projections to the phrenic nucleus: a HRP study in the cat

Brainstem neurones which project to the phrenic nucleus were identified using retrogradely transported horseradish peroxidase (HRP) as a marker. Following iontophoretic injection of HRP into the phrenic nucleus, labelled cells were encountered throughout large areas of the medulla and pons, but occurred with characteristic high densities in those regions known to contain phasic respiratory neurones: namely, the ventrolateral solitary tract nucleus (vl-NTS), known as the dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine nuclei (BCM-KF). In 12 cats a total of 1540 cells was identified within these regions, the relative contralateral and ipsilateral contributions were respectively 72%:28% (vl-NTS), 65%:35% for the ambiguus complex, and 5%:95% (BCM-KF). In addition, labelled cells, predominantly ipsilateral, were observed in the pontine and medullary reticular formation and the vestibular nuclei. The labelled cells of the DRG had round, oval or triangular perikarya. Their mean soma diameter was 18.3 micrometers. The HRP-positive cells of the VRG had slightly larger somas (mean 21.2 micrometers) and they were fusiform and triangular. The neurones labelled in the BCM-KF nuclei were more heterogeneous with a mean soma size of 14.9 micrometers. The bilateral projections to the phrenic nucleus from the DRG and the VRG, and the predominantly ipsilateral projection from the BCM-KF are discussed in relation to current electrophysiological and autoradiographic findings.

Morphology of inspiratory neurons located in the ventrolateral nucleus of the tractus solitarius of the cat

The morphology of 11 dorsal respiratory group (DRG) inspiratory neurons located in the ventrolateral nucleus of the solitary tract (vl-NTS) was studied using the technique of intracellular labeling with the enzyme horseradish peroxidase (HRP). Six of these cells were cut in the transverse plane and had a mean somal diameter of 30.4 micron, while five others sectioned in the horizontal plane had a mean of 38.2 micron. These neurons produced an average of 6.2 primary dendrites (range: 4-10), many of which projected rostrally or caudally up to 1.0 mm from the cell bodies. These dendrites were oriented along the longitudinal axis; they ran parallel and ventral to the tractus solitarius. In general, all dendrites possessed numerous spines and appendages. Many axons could be traced for considerable distances within the medulla (in one instance, up to 8 mm). These axons were last discerned in the contralateral ventral medulla rostral to the level of their cell bodies. The axons of three neurons bifurcated in the ipsilateral medulla; one branch remained ipsilateral and projected caudally, while the other crossed the midline. A small number of counterstained cells of size similar to or larger than the HRP-stained neurons formed a column that constituted the vl-NTS. Based upon our observations of stained and counterstained cells, we conclude that the inspiratory neurons of the vl-NTS are few in number and represent a morphologically homogeneous population. The primary orientation of the dendritic arbors of vl-NTS inspiratory neurons appears to optimize the surface area available to receive synaptic contacts from sensory afferents emerging from the tractus solitarius.

[Systems principle for separating out neuron groups as relatively independent working units]

The respiratory center has been studied as an example of the neural center organization. This organization is presented by a number of cellular populations, each of them consisting of several neuronal groups (components of the populations) of various types. These groups are considered as relatively autonomic sets of various neuronal categories, where 1-5 large efferent (phase) neurons are present as a central link. Analyzing the spatial arrangement and functional interrelations of the neurons in the group, it is possible to conclude that the groups revealed (respirons) are functional units of neuronal activity. Applying the theory of functional system (P. K. Anokhin) for analyzing connections between the neurons in the group and the afferent impulsation that gets into action sphere of the group, it is possible to formulate certain criteria on integrity and a relative functional independence of the neuronal groups as working units of neuronal activity, in which the reticular component of the groups as widely represented in all parts of the CNS, a suggestion is made that the respirons are the natural invariant of the structure when the cerebral function is reorganized.

Anatomy of respiratory rhythmic systems in brain stem and cerebellum of the carp

The afferent and efferent connections of two respiratory rhythmic loci in the dorsal mesencephalic tegmentum were studied by retrograde and anterograde transport of horseradish peroxidase. The injection areas were determined with extracellular activity recording using HRP filled glass micropipettes, and then followed by electrical stimulation and subsequent iontophoretic HRP delivery. One area in the nucleus of the posterior commissure was found to be optic related respiratory in nature and possessed afferents from optic tectum, pretectal nuclei, preoptic nucleus and the bulbar reticular formation. An extensive set of efferents is present to the torus longitudinalis, nucleus rotundus, corpus cerebelli and the various levels of the reticular formation. The second respiratory rhythmic area was localized in the vicinity of the oculomotor nuclei. This area receives afferent information from corpus cerebelli, vestibular nucleus and reticular formation, and has efferent connections to corpus cerebelli, preoptic nucleus and a major projection to the various parts of the reticular formation. Stimulation of both areas resulted in respiratory movements of the lower jaw and the opercula. Several injections in the corpus cerebelli resulted in retrograde labeling in the nucleus of the posterior commissure, which suggests the involvement of cerebellar circuits in optic related respiratory reflexes.

Where are the real respiratory neurons?

Transection experiments establish that the mechanisms responsible for the generation of the basic breathing pattern are located in the medulla. Several populations of neurons with activity patterns related to this motor pattern are readily recorded in the medulla, and much information has been obtained in the past 10 years about the physiology of these medullary respiratory neurons and their possible interconnections, inputs, and interactions. This evidence does not support the hypothesis that the basic alternations between expiration and inspiration is the result of a stable oscillatory network containing only the presently known medullary respiratory neurons. It is proposed that conventional extracellular recording methods have missed important parts of the medullary respiratory mechanism.

Neurohistochemical methods in tracing central respiratory mechanisms

Neurohistochemical methods using the retrograde and transganglionic transport of horseradish peroxidase (HRP) have been used to examine differences in the topographic representation in the brainstem of airway stretch receptors in the extrathoracic trachea and intrathoracic trachea of the cat. HRP neurohistochemistry has also been used to trace connections between brainstem respiratory nuclei, e.g., the inspiratory region of the nucleus of the tractus solitarius (nTS). Microiontophoretic deposits of HRP in functionally homogeneous neuronal populations of the medulla, the inspiratory neuronal group of the ventrolateral nTS, permit the examination of specific anatomical projections; distinct differences between the subnuclei of the nTS receiving projections from the extrathoracic and intrathoracic trachea could be identified. The afferents from the extrathoracic trachea (trachealis muscle stretch receptors) terminate in the main inspiratory subnucleus of the nTS, the ventrolateral nTS, whereas an identical region of the intrathoracic trachea sends its afferents to the dorsolateral nTS. The possible functional effects of such topographic differences are discussed. The inspiratory neuronal population in the ventrolateral nTS receives afferent projections from the contralateral rostral ventrolateral medulla. These afferent projections originate in a recently identified location in the rostral end of the nucleus ambiguous lying ventral to the retrofacial nucleus. This region has been identified as a site for respiratory related activity, which is expiratory in nature and anatomically distinct from the nucleus ambiguus and the retrofacial nucleus. This region has been identified as the Bötzinger complex, which corresponds to a collection of expiratory neurons in the rostral medulla.

Topology of ascending brainstem projections to nucleus parabrachialis in the cat

The afferent projections to nucleus parabrachialis (NPB) and nearby pontine areas from the lower brainstem were studied in cats using retrograde horseradish peoxidase (HRP) and anterograde autoradiographic tracing techniques. Two groups of medullary neurons send major projections to NPB and the Kölliker-Fuse nucleus (KF): 1) the solitary complex, especially the medial nucleus of the solitary tract (SM), nearby smaller cells of the dorsal motor nucleus of the vagus (DMV) and the commissural nucleus; and 2) the lateral tegmental field (FTL), or parvocellular reticular formation. Autoradiographic tracing from these areas demonstrated terminal fields in NPB/KF and emphasized a ventrolateral route to NPB from both sources, with axons ascending between the facial nerve and superior olive and passing rostral to the trigeminal nuclei. Minor projections to NBPB/KF originate in the ventrolateral nucleus of the solitary tract, area subpostrema, the alaminar spinal trigeminal nucleus, the gigantocellular and magnocellular tegmental fields, and an area dorsal to the ipsilateral inferior olive. Topographical features of the major projections were studied by correlating the locus of overlap of injection sites with the locations of HRP-positive cells. Medial areas of SM/DMV project mostly to medial parts of NPB, while lateral areas near the solitary tract project to lateral parts of NPB and KF. Crossing projections from SM/DMV favor dorsolateral NPB and KF. FTL neurons in dorsomedial areas project more to medial NPB, and ventrolateral areas project to lateral NPB/KF. Using a new coordinate system to locate and normalize the positions of FTL neurons, data from many brains were collated. FTL cells projecting to NPB/KF were found to be on discrete longitudinal sheets, running radially with respect to the fourth ventricle. This substructure and related evidence suggest a preferred pattern for neuroanatomical connections and information processing in the lateral reticular areas of the brainstem, and help in understanding the topography of the projections to NPB/KF.

Neural regulation of respiration

We would suggest that during the evolution of the mammalian respiratory neural networks the primitive centers in the cervical cord as well as the ventral respiratory group which evolved in fish have been preserved and are capable of functioning in the absence of the dorsal respiratory group generator which evolved with air breathing. We believe that these pattern generators are separate from the identified respiratory units that have so far been studied and that the apparent reciprocal inhibition observed in the identified cells results from synchronized excitatory and inhibitory inputs arising from the pattern generator itself. We believe that the model of such a system shown in Figure 3 is consistent with the observations cited in the previous section and inconsistent with models involving a single site for a pattern generator or interaction between various populations of known or identified respiratory units.