Specific subnuclei of the nucleus tractus solitarius play a role in determining the duration of inspiration in the rat

Passive limb training modulates respiratory rhythmic bursts

Exercise modifies respiratory functions mainly through the afferent feedback provided by exercising limbs and the descending input from suprapontine areas, two contributions that are still underestimated in vitro. To better characterize the role of limb afferents in modulating respiration during physical activity, we designed a novel experimental in vitro platform. The whole central nervous system was isolated from neonatal rodents and kept with hindlimbs attached to an ad-hoc robot (Bipedal Induced Kinetic Exercise, BIKE) driving passive pedaling at calibrated speeds. This setting allowed extracellular recordings of a stable spontaneous respiratory rhythm for more than 4 h, from all cervical ventral roots. BIKE reversibly reduced the duration of single respiratory bursts even at lower pedaling speeds (2 Hz), though only an intense exercise (3.5 Hz) modulated the frequency of breathing. Moreover, brief sessions (5 min) of BIKE at 3.5 Hz augmented the respiratory rate of preparations with slow bursting in control (slower breathers) but did not change the speed of faster breathers. When spontaneous breathing was accelerated by high concentrations of potassium, BIKE reduced bursting frequency. Regardless of the baseline respiratory rhythm, BIKE at 3.5 Hz always decreased duration of single bursts. Surgical ablation of suprapontine structures completely prevented modulation of breathing after intense training. Albeit the variability in baseline breathing rates, intense passive cyclic movement tuned fictive respiration toward a common frequency range and shortened all respiratory events through the involvement of suprapontine areas. These observations contribute to better define how the respiratory system integrates sensory input from moving limbs during development, opening new rehabilitation perspectives.

Effects of neonatal hyperoxia on the critical period of postnatal development of neurochemical expressions in brain stem respiratory-related nuclei in the rat

We have identified a critical period of respiratory development in rats at postnatal days P12‐13, when inhibitory influence dominates and when the response to hypoxia is at its weakest. This critical period has significant implications for Sudden Infant Death Syndrome (SIDS), the cause of which remains elusive. One of the known risk factors for SIDS is prematurity. A common intervention used in premature infants is hyperoxic therapy, which, if prolonged, can alter the ventilatory response to hypoxia and induce sustained inhibition of lung alveolar growth and pulmonary remodeling. The goal of this study was to test our hypothesis that neonatal hyperoxia from postnatal day (P) 0 to P10 in rat pups perturbs the critical period by altering the normal progression of neurochemical development in brain stem respiratory‐related nuclei. An in‐depth, semiquantitative immunohistochemical study was undertaken at P10 (immediately after hyperoxia and before the critical period), P12 (during the critical period), P14 (immediately after the critical period), and P17 (a week after the cessation of hyperoxia). In agreement with our previous findings, levels of cytochrome oxidase, brain‐derived neurotrophic factor (BDNF), TrkB (BDNF receptor), and several serotonergic proteins (5‐HT_1A and _2A receptors, 5‐HT synthesizing enzyme tryptophan hydroxylase [TPH], and serotonin transporter [SERT]) all fell in several brain stem respiratory‐related nuclei during the critical period (P12) in control animals. However, in hyperoxic animals, these neurochemicals exhibited a significant fall at P14 instead. Thus, neonatal hyperoxia delayed but did not eliminate the critical period of postnatal development in multiple brain stem respiratory‐related nuclei, with little effect on the nonrespiratory cuneate nucleus.

GABA-ergic neurotransmission in the nucleus of the solitary tract modulates cough in the cat

GABA, muscimol, and baclofen were microinjected into the rostral (rNTS) and caudal solitary tract nucleus (cNTS) in 24 anesthetized cats. Electromyograms (EMGs) of diaphragm (DIA) and abdominal muscles (ABD), blood pressure and esophageal pressure (EP) were recorded and analysed. Bilateral microinjections of 1 mM GABA (total 66 ± 4 nl), 1 mM baclofen (64 ± 4 nl) and unilateral microinjections of 0.5 mM muscimol (33 ± 1 nl) in the rNTS significantly reduced cough number (CN), amplitudes of ABD EMGs, expiratory EP, and prolonged the duration of the cough inspiratory phase. GABA microinjections decreased the amplitudes of cough-related DIA EMGs and inspiratory EP; muscimol microinjections decreased the cough DIA EMG on the contralateral side. Only microinjections of GABA into the cNTS suppressed CN. In some cases, microinjections prolonged the inspiratory phase, lowered respiratory rate, changed the depth of breathing, and increased blood pressure and heart rate. Our results confirm that GABA-ergic inhibitory mechanisms in the rNTS can regulate coughing in the anesthetized cat.

Microinjection of kynurenic acid in the rostral nucleus of the tractus solitarius disrupts spatiotemporal aspects of mechanically induced tracheobronchial cough

The importance of neurons in the nucleus of the solitary tract (NTS) in the production of coughing was tested by microinjections of the nonspecific glutamate receptor antagonist kynurenic acid (kyn; 100 mM in artificial cerebrospinal fluid) in 15 adult spontaneously breathing anesthetized cats. Repetitive coughing was elicited by mechanical stimulation of the intrathoracic airway. Electromyograms (EMG) were recorded from inspiratory parasternal and expiratory transversus abdominis (ABD) muscles. Bilateral microinjections of kyn into the NTS rostral to obex [55 ± 4 nl total in 2 locations (n = 6) or 110 ± 4 nl total in 4 locations (n = 5)], primarily the ventrolateral subnucleus, reduced cough number and expiratory cough efforts (amplitudes of ABD EMG and maxima of esophageal pressure) compared with control. These microinjections also markedly prolonged the inspiratory phase, all cough-related EMG activation, and the total cough cycle duration as well as some other cough-related time intervals. In response to microinjections of kyn into the NTS rostral to the obex respiratory rate decreased, and there were increases in the durations of the inspiratory and postinspiratory phases and mean blood pressure. However, bilateral microinjections of kyn into the NTS caudal to obex as well as control vehicle microinjections in the NTS location rostral to obex had no effect on coughing or cardiorespiratory variables. These results are consistent with the existence of a critical component of the cough rhythmogenic circuit located in the rostral ventral and lateral NTS. Neuronal structures of the rostral NTS are significantly involved specifically in the regulation of cough magnitude and phase timing.NEW & NOTEWORTHY The nucleus of the solitary tract contains significant neuronal structures responsible for control of 1) cough excitability, 2) motor drive during cough, 3) cough phase timing, and 4) cough rhythmicity. Significant elimination of neurons in the solitary tract nucleus results in cough apraxia (incomplete and/or disordered cough pattern). The mechanism of the cough impairment is different from that for the concomitant changes in breathing.

Repetitive Diving in Trained Rats Still Increases Fos Production in Brainstem Neurons after Bilateral Sectioning of the Anterior Ethmoidal Nerve

This research was designed to investigate the role of the anterior ethmoidal nerve (AEN) during repetitive trained diving in rats, with specific attention to activation of afferent and efferent brainstem nuclei that are part of this reflexive response. The AEN innervates the nose and nasal passages and is thought to be an important component of the afferent limb of the diving response. Male Sprague-Dawley rats (N = 24) were trained to swim and dive through a 5 m underwater maze. Some rats (N = 12) had bilateral sectioning of the AEN, others a Sham surgery (N = 12). Twelve rats (6 AEN cut and 6 Sham) had 24 post-surgical dive trials over 2 h to activate brainstem neurons to produce Fos, a neuronal activation marker. Remaining rats were non-diving controls. Diving animals had significantly more Fos-positive neurons than non-diving animals in the caudal pressor area, ventral medullary dorsal horn, ventral paratrigeminal nucleus, nucleus tractus solitarius, rostral ventrolateral medulla, Raphe nuclei, A5, Locus Coeruleus, and Kölliker-Fuse area. There were no significant differences in brainstem Fos labeling in rats diving with and without intact AENs. Thus, the AENs are not required for initiation of the diving response. Other nerve(s) that innervate the nose and nasal passages, and/or suprabulbar activation of brainstem neurons, may be responsible for the pattern of neuronal activation observed during repetitive trained diving in rats. These results help define the central neuronal circuitry of the mammalian diving response.

The generation of pharyngeal phase of swallow and its coordination with breathing: interaction between the swallow and respiratory central pattern generators

Swallowing and breathing utilize common muscles and an anatomical passage: the pharynx. The risk of aspiration of ingested material is minimized not only by the laryngeal adduction of the vocal folds and laryngeal elevation but also by the precise coordination of swallows with breathing. Namely, swallows: (1) are preferentially initiated in the postinspiratory/expiratory phase, (2) are accompanied by a brief apnea, and (3) are often followed by an expiration and delay of the next breath. This review summarizes the expiratory evidence on the brainstem regions comprising the central pattern generator (CPG) that produces the pharyngeal stage of swallow, how the motor acts of swallowing and breathing are coordinated, and lastly, brainstem regions where the swallowing and respiratory CPGs may interact in order to ensure "safe" swallows.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Protective action of tetramethylpyrazine on the medulla oblongata in rats with chronic hypoxia

Tetramethylpyrazine (TMP), one of the active ingredients of the Chinese herb Lingusticum Wallichii Frantchat (Chuan Xiong), plays an important role in neuroprotection. However, the protective effect of TMP on the medulla oblongata, the most important region of the brain for cardiovascular and respiratory control, during chronic hypoxia remains unclear. In this study, we examined the neuroprotective effect of TMP on the medulla oblongata after chronic hypoxic injury in rats. Male Sprague-Dawley rats were randomly divided into four groups: control group, TMP group, chronic hypoxia group, and chronic hypoxia+TMP group. Rats were exposed to hypoxia (10% (v/v) O₂) or normoxia for 6 h daily for 14 days. TMP (80 mg/kg) or vehicle (saline) was injected intraperitoneally 30 min before experimentation. Loss of neurons in the pre-Bötzinger complex, the nucleus ambiguus, the nucleus tractus solitarius, the hypoglossal nucleus and the facial nucleus were evaluated by Nissl staining. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were measured, and apoptosis was monitored using the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) method. The level of Bcl-2 mRNA and Bax mRNA was quantitatively measured by RT-PCR analysis. TMP protected Nissl bodies of neurons from injury in all nuclei observed, and reduced the loss of neurons in the nucleus ambiguus, the nucleus tractus solitarius, and the hypoglossal nucleus in rats subjected to chronic hypoxia. TMP upregulated SOD activity and inhibited the increase in MDA content in the medulla oblongata of hypoxic rats. In addition, TMP decreased the rate of apoptosis index (the percentage of apoptotic cells against the total number of cells) in all medullary structures examined, excepting the nucleus ambiguus and inhibited the decrease in Bcl-2 mRNA levels in the medulla oblongata following hypoxia. Our findings indicate that TMP may protect the medullary structures that are involved in cardiovascular and respiratory control from injury induced by chronic hypoxia in rats via its anti-oxidant and anti-apoptotic effects.

Decrease in the expression of N-methyl-D-aspartate receptors in the nucleus tractus solitarii induces antinociception and increases blood pressure

N-methyl-D-aspartate receptors (NMDAR) have a role in cardiovascular control at the nucleus tractus solitarii (NTS), eliciting increases or decreases in blood pressure (BP), depending on the area injected with the agonists. In spite of the association between cardiovascular control and pain modulation, the effects of manipulating NMDAR in pain responses have never been evaluated. In this study, we decreased the expression of NMDAR in the NTS using gene transfer to target receptor subunits and evaluate long-term effects. Seven days after the injection of lentiviral vectors containing the NR1a subunit cDNA of NMDAR, in antisense orientation, into the intermediate NTS of Wistar rats, BP was measured, and the formalin test of nociception was performed. The antisense vector induced a decrease of NR1 expression in the NTS and elicited BP rises and hypoalgesia. Antisense vectors inhibited formalin-evoked c-Fos expression in the spinal cord, indicating decreased nociceptive activity of spinal neurons. Using a time-course approach, we verified that the onset of both the increases in BP and the hypoalgesia was at 4 days after vector injection into the NTS. The injection of NMDA into the NTS reversed the effects of antisense vectors in pain behavioral responses and spinal neuronal activation and decreased BP and heart rate. The present study shows that the NR1 subunit of the NMDAR at the NTS is critical in the regulation of tonic cardiovascular and nociceptive control and shows an involvement of the nucleus in the modulation of sustained pain.

Altered responses of MeCP2-deficient mouse brain stem to severe hypoxia

Rett syndrome (RTT) patients suffer from respiratory arrhythmias with frequent apneas causing intermittent hypoxia. In a RTT mouse model (methyl-CpG-binding protein 2-deficient mice; Mecp2−/ y) we recently discovered an enhanced hippocampal susceptibility to hypoxia and hypoxia-induced spreading depression (HSD). In the present study we investigated whether this also applies to infant Mecp2−/ y brain stem, which could become life-threatening due to failure of cardiorespiratory control. HSD most reliably occurred in the nucleus of the solitary tract (NTS) and the spinal trigeminal nucleus (Sp5). HSD susceptibility of the Mecp2−/ y NTS and Sp5 was increased on 8 mM K+-mediated conditioning. 5-HT1A receptor stimulation with 8-hydroxy-2-(di-propylamino)tetralin (8-OH-DPAT) postponed HSD by up to 40%, mediating genotype-independent protection. The deleterious impact of HSD on in vitro respiration became obvious in rhythmically active slices, where HSD propagation into the pre-Bötzinger complex (pre-BötC) immediately arrested the respiratory rhythm. Compared with wild-type, the Mecp2−/ y pre-BötC was invaded less frequently by HSD, but if so, HSD occurred earlier. On reoxygenation, in vitro rhythms reappeared with increased frequency, which was less pronounced in Mecp2−/ y slices. 8-OH-DPAT increased respiratory frequency but failed to postpone HSD in the pre-BötC. Repetitive hypoxia facilitated posthypoxic recovery only if HSD occurred. In 57% of Mecp2−/ y slices, however, HSD spared the pre-BötC. Although this occasionally promoted residual hypoxic respiratory activity (“gasping”), it also prolonged the posthypoxic recovery, and thus the absence of central inspiratory drive, which in vivo would lengthen respiratory arrest. In view of the breathing disorders in RTTs, the increased hypoxia susceptibility of MeCP2-deficient brain stem potentially contributes to life-threatening disturbances of cardiorespiratory control.

Nitric oxide synthase expression in the medullary respiratory related nuclei and its involvement in CO-mediated central respiratory effects in neonatal rats

The present study was conducted in order to observe the potential participation of the nitric oxide synthase-NO pathway in CO-mediated regulation of respiration of neonatal rats. An immunofluorescent histochemical technique was used to examine the existence of the neuronal nitric oxide synthase, a key enzyme of synthesizing NO, in medullary respiratory nuclei. The rhythmic respiratory-like discharges of hypoglossal rootlets of medullary slices were recorded to test the role of the nitric oxide synthase in CO-mediated respiratory effects. We observed neuronal nitric oxide synthase expressed in the medullary respiratory nuclei in conjunction with CO lengthened expiratory duration, decreased respiratory frequency, and increased inspiratory amplitude. These CO-mediated respiratory effects could be partially eliminated by prior treatment of the slices with Nω-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase. The results suggest that nitric oxide synthase-NO pathway might be involved in the CO-mediated central regulation of respiration at the level of medulla oblongata in neonatal rats.

Glutamatergic Antagonism in the NTS Decreases Post-Inspiratory Drive and Changes Phrenic and Sympathetic Coupling During Chemoreflex Activation

For a better understanding of the processing at the nucleus tractus solitarius (NTS) level of the autonomic and respiratory responses to peripheral chemoreceptor activation, herein we evaluated the role of glutamatergic neurotransmission in the intermediate (iNTS) and caudal NTS (cNTS) on baseline respiratory parameters and on chemoreflex-evoked responses using the in situ working heart-brain stem preparation (WHBP). The activities of phrenic (PND), cervical vagus (cVNA), and thoracic sympathetic (tSNA) nerves were recorded before and after bilateral microinjections of kynurenic acid (Kyn, 5 nmol/20 nl) into iNTS, cNTS, or both simultaneously. In WHBP, baseline sympathetic discharge markedly correlated with phrenic bursts (inspiration). However, most of sympathoexcitation elicited by chemoreflex activation occurred during expiration. Kyn microinjected into iNTS or into cNTS decreased the postinspiratory component of cVNA and increased the duration and frequency of PND. Kyn into iNTS produced no changes in sympathoexcitatory and tachypneic responses to peripheral chemoreflex activation, whereas into cNTS, a reduction of the sympathoexcitation, but not of the tachypnea, was observed. The pattern of phrenic and sympathetic coupling during the chemoreflex activation was an inspiratory-related rather than an expiratory-related sympathoexcitation. Kyn simultaneously into iNTS and cNTS produced a greater decrease in postinspiratory component of cVNA and increase in frequency and duration of PND and abolished the respiratory and autonomic responses to chemoreflex activation. The data show that glutamatergic neurotransmission in the iNTS and cNTS plays a tonic role on the baseline respiratory rhythm, contributes to the postinspiratory activity, and is essential to expiratory-related sympathoexcitation observed during chemoreflex activation.

Learning to breathe: control of the inspiratory-expiratory phase transition shifts from sensory- to central-dominated during postnatal development in rats

The hallmark of the dynamic regulation of the transitions between inspiration and expiration is the timing of the inspiratory off-switch (IOS) mechanisms. IOS is mediated by pulmonary vagal afferent feedback (Breuer-Hering reflex) and by central interactions involving the Kölliker-Fuse nuclei (KFn). We hypothesized that the balance between these two mechanisms controlling IOS may change during postnatal development. We tested this hypothesis by comparing neural responses to repetitive rhythmic vagal stimulation, at a stimulation frequency that paces baseline breathing, using in situ perfused brainstem preparations of rats at different postnatal ages. At ages < P15 (P, postnatal days), phrenic nerve activity (PNA) was immediately paced and entrained to the afferent input and this pattern remained unchanged by repetitive stimulations, indicating that vagal input stereotypically dominated the control of IOS. In contrast, PNA entrainment at > P15 was initially insignificant, but increased after repetitive vagal stimulation or lung inflation. This progressive adaption of PNA to the pattern of the sensory input was accompanied by the emergence of anticipatory centrally mediated IOS preceding the stimulus trains. The anticipatory IOS was blocked by bilateral microinjections of NMDA receptor antagonists into the KFn and PNA was immediately paced and entrained, as it was seen at ages < P15. We conclude that as postnatal maturation advances, synaptic mechanisms involving NMDA receptors in the KFn can override the vagally evoked IOS after 'training' using repetitive stimulation trials. The anticipatory IOS may imply a hitherto undescribed form of pattern learning and recall in convergent sensory and central synaptic pathways that mediate IOS.

Release of ATP and glutamate in the nucleus tractus solitarii mediate pulmonary stretch receptor (Breuer-Hering) reflex pathway

The Breuer-Hering inflation reflex is initiated by activation of the slowly adapting pulmonary stretch receptor afferents (SARs), which monosynaptically activate second-order relay neurones in the dorsal medullary nucleus of the solitary tract (NTS). Here we demonstrate that during lung inflation SARs release both ATP and glutamate from their central terminals to activate these NTS neurones. In anaesthetized and artificially ventilated rats, ATP- and glutamate-selective microelectrode biosensors placed in the NTS detected rhythmic release of both transmitters phase-locked to lung inflation. This release of ATP and glutamate was independent of the centrally generated respiratory rhythm and could be reversibly abolished during the blockade of the afferent transmission in the vagus nerve by topical application of local anaesthetic. Microionophoretic application of ATP increased the activity of all tested NTS second-order relay neurones which receive monosynaptic inputs from the SARs. Unilateral microinjection of ATP into the NTS site where pulmonary stretch receptor afferents terminate produced central apnoea, mimicking the effect of lung inflation. Application of P2 and glutamate receptor antagonists (pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid, suramin and kynurenic acid) significantly decreased baseline lung inflation-induced firing of the second-order relay neurones. These data demonstrate that ATP and glutamate are released in the NTS from the central terminals of the lung stretch receptor afferents, activate the second-order relay neurones and hence mediate the key respiratory reflex - the Breuer-Hering inflation reflex.

Dorsal motor nucleus of the vagus: a site for evoking simultaneous changes in crural diaphragm activity, lower esophageal sphincter pressure, and fundus tone

The sphincter mechanism at the esophagogastric junction includes smooth muscle of the lower esophagus and skeletal muscle of the crural diaphragm (CD). Smooth muscle is known to be under the control of the dorsal motor nucleus of the vagus (DMV), while central nervous system (CNS) control of the CD is unknown. The main purposes of our study were to determine the CNS site that controls the CD and whether simultaneous changes in lower esophageal sphincter (LES) pressure and CD activity occur when this site is activated. Experiments were performed on anesthetized male ferrets whose LES pressure, CD activity, and fundus tone were monitored. To activate DMV neurons, L-glutamate was microinjected unilaterally into the DMV at three areas: intermediate, rostral, and caudal. Stimulation of the intermediate DMV decreased CD activity (-4.8 +/- 0.1 bursts/min and -0.3 +/- 0.01 mV) and LES pressure (-13.2 +/- 2.0 mmHg; n = 9). Stimulation of this brain site also produced an increase in fundus tone. Stimulation of the rostral DMV elicited increases in the activity of all three target organs (n = 5). Stimulation of the caudal DMV had no effect on the CD but did decrease both LES pressure and fundus tone (n = 5). All changes in LES pressure, fundus tone, and some DMV-induced changes in CD activity (i.e., bursts/min) were prevented by ipsilateral vagotomy. Our data indicate that simultaneous changes in activity of esophagogastric sphincters and fundus tone occur from rostral and intermediate areas of the DMV and that these changes are largely mediated by efferent vagus nerves.

Differential expression of vesicular glutamate transporters by vagal afferent terminals in rat nucleus of the solitary tract: projections from the heart preferentially express vesicular glutamate transporter 1

The central projections and neurochemistry of vagal afferent neurones supplying the heart in the rat were investigated by injecting cholera toxin B-subunit into the pericardium. Transganglionically transported cholera toxin B-subunit was visualized in the medulla oblongata in axons and varicosities that were predominantly aggregated in the dorsomedial, dorsolateral, ventrolateral and commissural subnuclei of the caudal nucleus of the solitary tract. Unilateral vagal section in control rats prevented cholera toxin B-subunit labeling on the ipsilateral side of the nucleus of the solitary tract. Fluorescent and electron microscopic dual labeling showed colocalization of immunoreactivity for vesicular glutamate transporter 1, but only rarely vesicular glutamate transporters 2 or 3 with cholera toxin B-subunit in terminals in nucleus of the solitary tract, suggesting that cardiac vagal axons release glutamate as a neurotransmitter. In contrast, populations of vagal afferent fibers labeled by injection of cholera toxin B-subunit, tetra-methylrhodamine dextran or biotin dextran amine into the aortic nerve, stomach or nodose ganglion colocalized vesicular glutamate transporter 2 more frequently than vesicular glutamate transporter 1. The presence of other neurochemical markers of primary afferent neurones was examined in nucleus of the solitary tract axons and nodose ganglion cells labeled by pericardial cholera toxin B-subunit injections. Immunoreactivity for a 200-kDa neurofilament protein in many large, cholera toxin B-subunit-labeled nodose ganglion cells indicated that the cardiac afferent fibers labeled are mostly myelinated, whereas binding of Griffonia simplicifolia isolectin B4 to fewer small cholera toxin B-subunit-labeled ganglion cells suggested that tracer was also taken up by some non-myelinated axons. A few labeled nucleus of the solitary tract axons and ganglion cells were positive for substance P and calcitonin gene-related peptide, which are considered as peptide markers of nociceptive afferent neurones. These data suggest that the population of cardiac vagal afferents labeled by pericardial cholera toxin B-subunit injection is neurochemically varied, which may be related to a functional heterogeneity of baroreceptive, chemoreceptive and nociceptive afferent fibers. A high proportion of cardiac neurones appear to be glutamatergic, but differ from other vagal afferents in expressing vesicular glutamate transporter 1.

Functional and structural models of pontine modulation of mechanoreceptor and chemoreceptor reflexes

The dorsolateral and ventrolateral pons (dl-pons, vl-pons) are critical brainstem structures mediating the plasticity of the Hering-Breuer mechanoreflex (HBR) and carotid chemoreflex (CCR). Review of anatomical evidence indicates that dl-pons and vl-pons are connected reciprocally with one another and with medullary nucleus tractus solitarius (NTS) and ventral respiratory group (VRG). With this structural map, functional models of HBR and CCR are proposed in which the respiratory rhythm is modulated by short-term depression (STD) or potentiation (STP) of corresponding primary NTS-VRG and auxiliary pons-VRG excitatory or inhibitory pathways. Behaviorally, STD and STP of respiratory reflexes are akin to non-associative learning such as habituation, sensitization or desensitization to afferent inputs. Computationally, the STD and STP effects amount to signal differentiation and integration in the time domain, or high-pass and low-pass filtering in the frequency domain, respectively. These functional and structural models of pontomedullary signal processing provide a novel conceptual framework that unifies a wealth of experimental observations regarding mechanoreceptor and chemoreceptor reflex control of breathing.

Anatomic relationships of the human nucleus of the solitary tract in the medulla oblongata: a DiI labeling study

The nucleus of the solitary tract (nTS) is a major site of brainstem control of vital functions (e.g., cardiovascular reflexes and respiration). We examined anatomic relationships of the human nucleus of the solitary tract, using a bidirectional lipophilic fluorescent tracer 1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in 10 postmortem human fetal midgestational medullae oblongatae. Labeling by diffusion of DiI from the nucleus of the solitary tract included: (1) neuropil of all future subdivisions of the nucleus of the solitary tract ipsilateral to the DiI crystal; (2) stellate cells in the caudal raphe at the junction of the nucleus raphe pallidus and the arcuate nucleus at the ventral medullary surface, as well as single fibers along the caudal raphe and the arcuate nucleus; (3) cells and fibers in other medullary areas related to autonomic and respiratory control, including the dorsal motor nucleus of the vagus, nucleus ambiguus complex/ventral respiratory group, rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM), and medullary reticular formation. The pattern of connections of the nucleus of the solitary tract already established by midgestation in the human fetus is consistent with the pattern previously demonstrated in adult experimental animals. A major finding of the study is that of the stellate cells at the junction of nucleus raphe pallidus and the arcuate nucleus at the ventral medullary surface, which project to the nucleus of the solitary tract, and could be homologous to chemosensitive serotonergic neurons at the midline ventral medullary surface of experimental animals. This connection between the ventral caudal raphe and the nucleus of the solitary tract may participate in chemoreception and central regulation of cardiorespiratory reflexes during human perinatal development; it is, therefore, relevant to the study of sudden infant death syndrome (SIDS).

Apneusis follows disruption of NMDA-type glutamate receptors in vagotomized ground squirrels

The influences of N-methyl-D-aspartate (NMDA) type glutamate receptor antagonism, by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate (MK-801), on breathing pattern and ventilatory chemoresponses, were assessed in anaesthetized vagotomized spontaneously breathing golden-mantled ground squirrels, Spermophilus lateralis. MK-801 was administered by either bilateral pressure micro-injection into a region of the rostral dorsolateral pons, containing the medial and lateral Parabrachial and Kölliker-Fuse nuclei (the Parabrachial complex, PbC), or by systemic injection. Both treatments induced apneusis. These data indicate that functional NMDA receptor-mediated processes located within the PbC terminate inspiration and actively prevent apneusis in vagotomized ground squirrels. Although both hypercapnia and hypoxia stimulated breathing during the apneusis, the responses were generally slight. The breathing frequency component of the hypercapnic ventilatory response was completely eliminated supporting the hypothesis that the PbC is an integral component of the control network for CO(2) chemoreflex responses. Differences in the results of systemic versus PbC MK-801 illustrate that NMDA receptor-mediated processes outside the PbC do influence ventilation. Our data also show that such processes outside the PbC lengthen both inspiration and expiration in this species, slowing ventilation, and are necessary for the expression of the hypoxic ventilatory response.

GABA-mediated neurotransmission in the ventrolateral NTS plays a role in respiratory regulation in the rat

Our purpose was to determine whether endogenously released GABA in the ventrolateral nucleus of the solitary tract (vlNTS) of the rat influences respiration. Experiments were carried out in anesthetized, vagotomized and spontaneously breathing rats, and diaphragm electromyogram activity was measured while drugs affecting GABAergic neurotransmission were microinjected into the vlNTS and medial NTS (mNTS). Bilateral microinjection of nipecotic acid, 5 or 25 nmol, into the vlNTS (but not in the mNTS) produced dose-dependent increases in inspiratory duration (Ti) frequently culminating in apneustic breathing. Neither unilateral microinjection of bicuculline nor CGP-35348 (GABA(B) receptor antagonist) reversed this response; however, a combination of both GABA receptor antagonists effectively reversed apneustic breathing. Bilateral microinjection of either muscimol or baclofen into the vlNTS mimicked the effect of nipecotic acid. Microinjection of the bicuculline produced apnea, whereas microinjection of CGP-35348 produced a decrease in Ti and an increase in expiratory duration. Immunohistochemical analysis of the vlNTS region revealed GABA(A) receptors densely localized to processes, whereas GABA(B) immunoreactivity was localized to cell bodies. Our data indicate that GABA activity in the vlNTS is important for respiratory function.

Postnatal changes in the respiratory response of the conscious rat to serotonin 2A/2C receptor activation are reflected in the developmental pattern of fos expression in the brainstem

The influence on the breathing pattern of the activation of serotonin receptors belonging to the subtypes 2(A) and 2(C) (5-HT(2A/2C)) has been assessed in newborn and adult conscious rats. Rats were given an acute intraperitoneal dose of the agonist DOI (1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane; 5 mg/kg). In newborns, DOI elicited a long-lasting respiratory depression by decreasing both tidal volume and respiratory frequency. In adults, DOI retained a depressant influence, although attenuated, on tidal volume. In contrast, it elicited an increase in respiratory frequency. In separate subsets of newborn and adult rats, immunohistochemistry has been used to monitor c-fos expression induced by DOI in the medullary and pontine regions involved in respiratory control. Counts of immunoreactive neurons indicated a marked increase in the neuronal populations activated in the adult compared to the newborn rat. The response to both experimental factors (newborn vs. adult controls) and drug (injected vs. control age-matched rats) were more pronounced in mature animals. Among developmental changes in the pattern of labeling, DOI elicited Fos expression in the adult but not in the neonate in the ventrolateral subnucleus of the nucleus of the solitary tract, the parabrachial area and the Kölliker-Fuse nucleus. This finding suggested that changes in the respiratory response to DOI might at least partly depend on maturational events within networks involved in the modulation of respiratory timing.

CO2 dialysis in nucleus tractus solitarius region of rat increases ventilation in sleep and wakefulness

To evaluate the function of widely distributed central chemoreceptors during sleep and wakefulness in the rat, we focally stimulate single chemoreceptor sites during naturally occurring sleep-wake cycles by microdialysis of artificial cerebrospinal fluid equilibrated with 25% CO2. In retrotrapezoid nucleus, this increased ventilation (tidal volume) by 24% only in wakefulness (Li A, Randall M, and Nattie E. J Appl Physiol 87: 910-919, 1999). In caudal medullary raphé, it increased ventilation (frequency) by 15-20% only in sleep (Nattie EE and Li A. J Appl Physiol 90: 1247-1257, 2001). Here, in nucleus tractus solitarius (NTS), focal acidification significantly increased ventilation by 11% in sleep and 7% in wakefulness rostrally (n = 5) and by 16% in sleep and 28% in wakefulness caudally (n = 5). The sleep-wake cycle was unaltered. Dialysis with 5% CO2 had no effect. Dialysis with 50% CO2 caudally did not further stimulate ventilation but did disrupt sleep. Central chemoreceptors in the NTS affect breathing in both sleep and wakefulness. The threshold for arousal in caudal NTS is greater than that for the stimulation of breathing.