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

Postsynaptic inhibition of hypoglossal motoneurons produces atonia of the genioglossal muscle during rapid eye movement sleep

STUDY OBJECTIVES

Hypoglossal motoneurons were recorded intracellularly to determine whether postsynaptic inhibition or disfacilitation was responsible for atonia of the lingual muscles during rapid eye movement (REM) sleep.

DESIGN

Intracellular records were obtained of the action potentials and subthreshold membrane potential activity of antidromically identified hypoglossal motoneurons in cats during wakefulness, nonrapid eye movement (NREM) sleep, and REM sleep. A cuff electrode was placed around the hypoglossal nerve to antidromically activate hypoglossal motoneurons. The state-dependent changes in membrane potential, spontaneous discharge, postsynaptic potentials, and rheobase of hypoglossal motoneurons were determined.

ANALYSES AND RESULTS

During quiet wakefulness and NREM sleep, hypoglossal motoneurons exhibited spontaneous repetitive discharge. In the transition from NREM sleep to REM sleep, repetitive discharge ceased and the membrane potential began to hyperpolarize; maximal hyperpolarization (10.5 mV) persisted throughout REM sleep. During REM sleep there was a significant increase in rheobase, which was accompanied by barrages of large-amplitude inhibitory postsynaptic potentials (IPSPs), which were reversed following the intracellular injection of chloride ions. The latter result indicates that they were mediated by glycine; IPSPs were not present during wakefulness or NREM sleep.

CONCLUSIONS

We conclude that hypoglossal motoneurons are postsynaptically inhibited during naturally occurring REM sleep; no evidence of disfacilitation was observed. The data also indicate that glycine receptor-mediated postsynaptic inhibition of hypoglossal motoneurons is crucial in promoting atonia of the lingual muscles during REM sleep.

Respiratory rhythm generation in vivo

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

Descending control of the respiratory neuronal network by the midbrain periaqueductal grey in the rat in vivo

Emotional reactions such as vocalization take place during expiration, and thus expression of emotional behaviour requires a switch from inspiration to expiration. I investigated how the midbrain periaqueductal grey (PAG), a known behavioural modulator of breathing, influences the inspiratory-to-expiratory phase transition. Contemporary models propose that late inspiratory (late-I) and post-inspiratory (post-I) neurones found in the medulla, which are active during the inspiratory-to-expiratory phase transition are involved in converting inspiration to expiration. I examined the effect of excitatory amino acid (d,l-homocysteic acid; DLH) stimulation of the PAG on the discharge function of late-I and post-I neurones. The data show a topographical organization of DLH-induced late-I and post-I neuronal modulation within the PAG. Dorsal PAG stimulation induced tachypnoea and caused excitation of both the late-I and post-I neurones. Lateral PAG induced inspiratory prolongation and caused an excitation of late-I neurones but inhibition of post-I neurones. Ventrolateral PAG induced expiratory prolongation and caused a persistent activation of post-I neurones. As well, PAG stimulation modulated both the late-I and post-I cells for least two-three breaths even prior to the change in respiratory motor pattern. This indicates that the PAG influences the late-I and post-I cells independent of pulmonary or other sensory afferent feedback. I conclude that the PAG modulates the activity of the medullary late-I and post-I neurones, and this modulation contributes to the conversion of eupnoea into a behavioural breathing pattern.

Influence of hypercapnic acidosis and hypoxia on abdominal expiratory nerve activity in the rat

We studied the influence of hypercapnic acidosis and hypoxia on the neural drive to abdominal muscles in anesthetized and decerebrate rats; this information is unavailable despite widespread use of the rat as an experimental model in respiratory physiology and neurobiology. To minimize confounding influences from receptors in the lungs and chest wall, the animals were vagotomized, paralyzed and mechanically ventilated, and electrical activity was recorded from abdominal muscle nerves. In anesthetized and decerebrate rats, both stimuli evoked steady, low amplitude expiratory discharge that persisted throughout the expiratory phase (E-all activity), but was inhibited during inspiration. We also observed late expiratory, high-amplitude bursts (E2 activity) superimposed on this steady activity, but only at the highest levels of respiratory drive. Hypoxia enhanced abdominal motor activity transiently, whereas hypercapnic acidosis caused a sustained increase in activity. Thus, both hypercapnic acidosis and hypoxia activate abdominal muscle motoneurons in the absence of phasic afferent inputs.

Pattern-Specific Synaptic Mechanisms in a Multifunctional Network. I. Effects of Alterations in Synapse Strength

Many neuronal networks are multifunctional, producing different patterns of activity in different circumstances, but the mechanisms responsible for this reconfiguration are in many cases unresolved. The mammalian respiratory network is an example of such a system. Normal respiratory activity (eupnea) is periodically interrupted by distinct large-amplitude inspirations known as sighs. Both rhythms originate from a single multifunctional neural network, and both are preserved in the in vitro transverse medullary slice of mice. Here we show that the generation of fictive sighs were more sensitive than eupnea to reductions of excitatory synapse strength caused by either the P/Q-type (α1A-containing) calcium channel antagonist ω-agatoxin TK or the non- N-methyl-d-aspartate (NMDA) glutamate receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX). In contrast, the NMDA receptor antagonist MK-801, while also inhibiting eupnea, increased the occurrence of sighs. This suggests that among the glutamatergic synapses subserving eupneic rhythmogenesis, there is a specific subset—highly sensitive to agatoxin and insensitive to NMDA receptor blockade—that is essential for sighs. Blockade of N-type calcium channels with ω-conotoxin GVIA also had pattern-specific effects: eupneic activity was not affected, but sigh frequency was increased and postsigh apnea decreased. We hypothesize that N-type (α1B) calcium channels selectively coupled to calcium-activated potassium channels contribute to the generation of the postsigh apnea.

Optical imaging of respiratory neuron activity from the dorsal view of the lower brainstem

1. We visualized respiratory-related neuron network activity in the dorsal part of the pons and medulla of an in vitro preparation from newborn rats by optical recordings using a voltage-sensitive dye. We measured optical signals from several seconds before to several seconds after the inspiratory phase using the inspiratory motor nerve discharge as the trigger signal and we averaged the optical signals of 20-50 respiratory cycles to obtain an optical image correlating specifically to inspiratory activity. 2. Four areas that were excited or inhibited corresponding to the respiratory cycles were detected. (i) The most rostral activity was in the rostral and lateral parts of the pons, with activity mainly in the inspiratory phase, corresponding to the pontine-respiratory group. (ii) In the midpontine level, inspiratory activity followed by long-lasting hyperpolarization appeared in the midlateral parts. This part was presumed to reflect activity in the locus coeruleus. The hyperpolarization became almost negligible after treatment with the alpha-adrenergic antagonist, phentolamine. (iii) In the dorsal medulla, the predominantly inspiratory activity was detected at the rostral level of the area postrema. This part was considered to reflect activity mainly of the hypoglossal nucleus. (iv) At a similar level, we also detected weak and disperse inspiratory activity extending more laterally and caudally than that of the hypoglossal nucleus activity. This might reflect activity of the dorsal respiratory group. 3. In conclusion, the present optical recording study revealed that the dorsal part of the lower brainstem in the in vitro preparation is noticeably active as well as the ventral part shown in the previous study. This method is very useful for analysis of pharmacological properties, as well as the spatio-temporal pattern of respiratory-related network activity in the brainstem.

Chemical activation of C1-C2 spinal neurons modulates intercostal and phrenic nerve activity in rats

Chemical activation of upper cervical spinal neurons modulates activity of thoracic respiratory interneurons in rats. The aim of the present study was to examine the effects of chemical activation of C1-C2 spinal neurons on thoracic spinal respiratory motor outflows. Electroneurograms of left phrenic ( n = 23) and intercostal nerves (ICNs, n = 93) between T3 and T8 spinal segments were recorded from 36 decerebrated, vagotomized, paralyzed, and ventilated male rats. To activate upper cervical spinal neurons, glutamate pledgets (1 M, 1 min) were placed on the dorsal surface of the C1-C2 spinal cord. Glutamate on C1-C2 increased ICN tonic activity in 56/59 (95%) ICNs. The average maximal tonic activity of ICN was increased by 174% ( n = 59). After spinal transection at rostral C1, glutamate on C1-C2 still increased ICN tonic activity in 33/35 ICNs. However, the effects of C1-C2 glutamate on ICN phasic activity were highly variable, with observations of augmentation or suppression of both inspiratory and expiratory discharge. C1-C2 glutamate augmented the average amplitude of phrenic burst by 20%, whereas the increases in amplitude of ICN inspiratory activity, when they occurred, averaged 120%. The burst rate of phrenic nerve discharge was decreased from 34.2 ± 1.6 to 26.3 ± 2.0 (mean ± SE) breaths/min during C1-C2 glutamate. These data suggested that upper cervical propriospinal neurons might play a role in descending modulation of thoracic respiratory and nonrespiratory motor activity.

Respiratory cycle timing and fast inspiratory discharge rhythms in the adult decerebrate rat

In supracollicular decerebrate paralyzed adult rats, neural respiration was monitored by bilateral phrenic recordings. In the study of respiratory cycle timing, the effects of vagal afferent input (lung inflation) on respiratory phase durations resembled those seen in decerebrate cats. 1) Withholding lung inflation during neural inspiration (I) produced lengthening of I phase duration by 46% (mean, n = 11). 2) Maintaining lung inflation during neural expiration (E) produced lengthening of E phase duration by 112% (mean, n = 4). In the study of fast rhythms in inspiratory discharges, phrenic nerve autospectra and bilateral (left-right) phrenic coherences in 16 rats revealed two types of fast rhythm: 1) high-frequency oscillation (HFO), which had significant coherence peaks (n = 9, range 106-160 Hz, mean 132 Hz); and 2) medium-frequency oscillation (MFO), which had autospectral peaks but no distinct coherence peaks (n = 11, range 46-96 Hz, mean 66 Hz). These rhythms resembled MFOs and HFOs in the decerebrate cat, but the modal frequency range was about twice as large. In addition, these frequency values differed markedly from the 20-40 Hz of the rhythms found in earlier studies in neonatal in vitro preparations; the difference may be due to developmental immaturity.

Activation of N-methyl-d-aspartate Receptors Induces Endogenous Rhythmic Bursting Activities in Nucleus Tractus Solitarii Neurons: An Intracellular Study on Adult Rat Brainstem Slices

A brainstem slice preparation and intracellular recording techniques were used to examine the effects of N-methyl-d-aspartate (NMDA) application on neurons within the swallowing area of the nucleus tractus solitarii (NTS). According to their cellular properties, NTS neurons were classified into type I and type II neurons. The most striking difference was the occurrence of delayed excitation in type I but not in type II neurons, when they were depolarized from membrane potentials more negative than -60 mV. Bath application of NMDA (30 - 60 microM) elicited depolarization and triggered stable repetitive firing in all the NTS neurons but one. During the NMDA-induced depolarization, hyperpolarization below -60 mV elicited, in some type I neurons, a rhythmic bursting pattern. The duration of the bursts (300 - 1000 ms) and their frequency (0.5 - 2 Hz) depended on the membrane potential. With hyperpolarizations below -75 mV, rhythmic bursting was converted into rhythmic single discharges, a pattern elicited directly in the other type I neurons. In all cases, rhythmic patterns were superimposed on cyclic depolarizations of the membrane potential characterized by an initial ramp-shaped phase. In type II neurons, rhythmic bursting discharges, superimposed on rhythmic oscillations of the membrane potential, were also obtained upon hyperpolarization during the NMDA-induced depolarization. In all type I neurons tested, NMDA-induced cyclic ramp-shaped depolarizations continued after addition of tetrodotoxin to the medium. Rhythmic bursting was not elicited by bath application of kainate (10 - 20 microM). Application of d-2-amino-5-phosphonovalerate (50 microM) blocked NMDA-induced depolarizations without modifying those elicited by kainate, which were selectively depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). Moreover, removal of Mg2+ from the medium suppressed NMDA-induced cyclic depolarizations. Results demonstrate that both NMDA and non-NMDA receptors are present in NTS neurons and that selective activation of NMDA receptors induced rhythmic bursting and/or rhythmic single discharges. Rhythmic patterns were not driven by synaptic mechanisms but originated from endogenous properties of NTS neurons activated by NMDA. Thus, NTS neurons can be considered as conditional pacemakers. According to the location of the neurons, the conditional properties shown in these in vitro experiments might be involved in vivo in the generation of rhythmic motor activities set up at the NTS level, such as swallowing.

Noradrenergic receptors and in vitro respiratory rhythm: possible interspecies differences between mouse and rat neonates

Similar in vitro experiments were performed on brainstem-spinal cord preparations from mouse and rat neonates to compare the noradrenergic regulations of the respiratory network. In preparations retaining the pons, rhythmic phrenic bursts occurred in rats but not in mice. Transection of the pons, electrolytic lesions and noradrenaline applications showed that the pontine noradrenergic A5 group inhibited the respiratory rhythm generator in both species but the inhibition was especially potent in mice. After pons elimination, noradrenaline applications to the medulla decreased the respiratory frequency in rats but increased it in mice. Noradrenergic agent applications revealed that the frequency changes implicated medullary alpha 1 and alpha 2 noradrenergic receptors in mice and rats, respectively. Thus, interspecies differences seem to exist in the noradrenergic regulations of the rat and mouse medullary respiratory networks.

Glutamic acid decarboxylase-immunoreactivity of bulbar respiratory neurons identified by intracellular recording and labeling in rats

To distinguish the GABAergic neuron in the ventral respiratory group (VRG) of rats, immunohistochemical staining of glutamic acid decarboxylase (GAD) was performed in neurons that had been individually identified by in vivo intracellular recording and labeling with neurobiotin. A total of five types of respiratory neurons were identified and labeled; augmenting inspiratory (aug-I, n=12), decrementing or early inspiratory (early-I, n=3), inspiration-expiration phase spanning or late inspiratory (late-I, n=3), decrementing expiratory or postinspiratory (PI, n=8), and augmenting or stage 2 expiratory (E2, n=3). In addition, expiration-inspiration phase-spanning or pre-inspiratory neurons (pre-I, n=2) were recorded, but not labeled. The membrane potential trajectory of each neuron type resembled that previously described in cat, suggesting a comparable neuronal organization between the two species. According to the axonal arborization, those labeled neurons were further classified as propriobulbar (6 aug-I, all early-I, all late-I, and 3 PI), bulbospinal (2 aug-I and all E2) and cranial-motor neurons (4 aug-I and 5 PI). GAD-immunoreactivity was consistently detected in the propriobulbar neurons, while it was not seen in cranial-motor and bulbospinal neurons. In addition, GAD-immunoreactive varicosities were found surrounding the somatic and dendritic surface of all labeled neurons. The present results illustrate that the propriobulbar types of early-I, aug-I, late-I and PI neurons are GABAergic inhibitory neurons and virtually all types of respiratory neurons receive GABAergic inputs in the rat's VRG.

Neuroimaging evidence implicating cerebellum in the experience of hypercapnia and hunger for air

Recent neuroimaging and neurological data implicate cerebellum in nonmotor sensory, cognitive, vegetative, and affective functions. The present study assessed cerebellar responses when the urge to breathe is stimulated by inhaled CO(2). Ventilation changes follow arterial blood partial pressure CO(2) changes sensed by the medullary ventral respiratory group (VRG) and hypothalamus, entraining changes in midbrain, pons, thalamus, limbic, paralimbic, and insular regions. Nearly all these areas are known to connect anatomically with the cerebellum. Using positron emission tomography, we measured regional brain blood flow during acute CO(2)-induced breathlessness in humans. Separable physiological and subjective effects (air hunger) were assessed by comparisons with various respiratory control conditions. The conjoint physiological effects of hypercapnia and the consequent air hunger produced strong bilateral, near-midline activations of the cerebellum in anterior quadrangular, central, and lingula lobules, and in many areas of posterior quadrangular, tonsil, biventer, declive, and inferior semilunar lobules. The primal emotion of air hunger, dissociated from hypercapnia, activated midline regions of the central lobule. The distributed activity across the cerebellum is similar to that for thirst, hunger, and their satiation. Four possible interpretations of cerebellar function(s) here are that: it subserves implicit intentions to access air; it provides predictive internal models about the consequences of CO(2) inhalation; it modulates emotional responses; and that while some cerebellar regions monitor sensory acquisition in the VRG (CO(2) concentration), others influence VRG to adjust respiratory rate to optimize partial pressure CO(2), and others still monitor and optimize the acquisition of other sensory data in service of air hunger aroused vigilance.

Identification of deflation-sensitive inspiratory neurons in the dorsal respiratory group of the rat

It has been well established that inspiratory neurons of the dorsal respiratory group (DRG) are classified into two types based on whether they receive inputs from slowly-adapting pulmonary stretch receptors (SARs) or not. Inspiratory neurons with SAR inputs are called Ibeta and the others are called Ialpha neurons. In this study, we identified a novel group of inspiratory neurons in Nembutal-anesthetized, paralyzed, and artificially-ventilated rats. (1) These DRG inspiratory neurons were activated characteristically by lung deflation. (2) They were orthodromically activated by electrical stimulation of the vagus nerve at a low intensity just above the threshold for afferents from SARs or rapidly-adapting pulmonary stretch receptors (RARs). (3) The orthodromic latencies (ranged from 1.9 to 2.5 ms) indicated that they receive direct inputs from low-threshold vagal afferents. (4) Unlike Ibeta neurons, they hardly responded to lung inflation and never exhibited tonic firing in response to maintained lung inflation. (5) The majority (92%) of them were antidromically activated by electrical stimulation of the cervical spinal cord. These deflation-sensitive inspiratory neurons clearly form a distinct group, and their firing pattern is consistent with the hypothesis that they receive inputs from RAR afferents as well as the central inspiratory drive. The results indicating that DRG inspiratory neurons are classified into at least three groups provide new insights into the organization and role of the DRG.

Respiratory activity in neonatal rats

In neonatal animals in vitro preparations have been employed widely to study the central control of respiration. These preparations have limitations in that reflex afferent inputs and kinesiological studies cannot be performed. Here, we describe an alternative in situ experimental model for studying both peripheral and central control of the respiratory system in neonatal rats. Using technology based on adult mammals, we introduce an intra-arterially perfused working heart-brainstem preparation (WHBP) that permits studies on eupnoeic respiration in neonatal rats from within a few hours of birth. Using this preparation we demonstrate a three-phase respiratory rhythm as revealed by the activity in phrenic and recurrent laryngeal motor nerves, the respiratory modulation of laryngeal resistance and the firing patterns of respiratory neurones recorded from the ventrolateral medulla. We conclude that the neonatal rat WHBP is an in situ preparation because it produces a respiratory rhythm similar to that of adult in vivo mammal preparations but distinct from in vitro preparations.

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

Our previous data obtained in the cat suggest that the neurons of the ventrolateral subnucleus of the tractus solitarius (vlNTS) act as an inspiratory off-switch and terminate the inspiratory phase of the respiratory cycle (Berger et al., Eur. J. Pharmacol. 277 (1995) 195-208; Gillis et al., Neurosci. Abstr. 23 (1997) 725). The purpose of the present study was to determine whether inhibition of the region of the vlNTS of the rat using drugs that hyperpolarize, disfacilitate or block both axonal conduction and action potential generation would alter the inspiratory phase of the respiratory cycle. Experiments were conducted in anesthetized, vagotomized and spontaneously breathing rats while monitoring diaphragmatic electromyogram activity. Vagus nerves were sectioned in order to rule out prolongation of inspiration evoked by microinjection of agents into the vlNTS which block excitatory drive from lung afferent inputs. Bilateral microinjection of the inhibitory amino acid gamma-aminobutyric acid (GABA) 25 nmol/45 nl produced an immediate prolongation of inspiratory duration (484+/-18 to 1291+/-84 ms) and an apneustic pattern of breathing. Other effects observed were a significant shortening of expiratory duration (778+/-36 to 432+/-38 ms), rise in blood pressure (83+/-4 to 108+/-6 mmHg) and a small but significant increase in heart rate (439+/-17 to 452+/-18 beats/min). Bilateral microinjection of the ionotropic glutamate receptor antagonist kynurenic acid (1 nmol) and the Na(+) channel blocker tetrodotoxin (10 pmol) into the region of the vlNTS consistently produced a similar prolongation of inspiratory duration and an apneustic pattern of breathing. These results support the hypothesis that neurons in the region of the vlNTS promote the transition from inspiration to expiration and function as part of the 'Inspiratory Off Switch'.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

The medullary dorsal reticular nucleus facilitates pain behaviour induced by formalin in the rat

The influence of the dorsal reticular nucleus (DRt) on pain behaviour during the formalin test was studied in the rat by lesioning the nucleus through local application of electrical current or quinolinic acid. Animals in which the DRt was lesioned ipsilaterally to the paw injected with formalin spent less time in focused (licking, biting or scratching the injected paw) and total (focused pain behaviour plus protection of the injected paw during movements) pain behaviour, and showed paw-jerks less frequently than non-lesioned animals in both phases 1 and 2 of the test. Animals in which the DRt was lesioned contralaterally to the injected paw presented a decrease in total pain behaviour and number of paw-jerks only during phase 2. The number of superficial (laminae I-II) and deep (laminae III-VI) spinal dorsal horn cells expressing the c-fos proto-oncogene 2 h after subcutaneous injection of formalin was reduced by 34% and 50%, respectively, in animals with an ipsilateral DRt lesion as compared to non-lesioned rats. No differences in c-fos expression were observed after lesioning the DRt contralateral to the formalin injection. The results indicate that the DRt is involved in the facilitation of nociception during the formalin test by enhancing the response capacity of dorsal horn neurons to noxious stimulation. It is suggested that the pronociceptive action of the DRt is mediated by the reciprocal connections it establishes with the spinal dorsal horn.

Connections of the rostral ventral respiratory neuronal cell group: an anterograde and retrograde tracing study in the rat

The connections of the rostral ventral respiratory cell group (VRG) were retrogradely and anterogradely determined after discrete injections of a mixture of the fluorescent tracers Fast Blue (FB) and Fluoro Ruby (FR) into the physiologically identified rostral inspiratory cell group. Retrogradely FB-labeled neurons and/or anterogradely FR-labeled fibers and terminal fields were located bilaterally in a variety of brain areas. Both retrograde and anterograde labelings were mainly found in: 1) the deep cerebellar nuclei; 2) the lateral lemniscus and paralemniscal nuclei, deep gray, and white intermediate layers of the superior colliculus, tegmental (laterodorsal and microcellular) nuclei, and central gray; and 3) the septohypothalamic nucleus, and lateral and posterior hypothalamic areas. The FR-labeled terminal-like elements were found in: 1) Crus 2 of the ansiform lobule, and the simple, 2, and 3 cerebellar lobules; 2) the subcoeruleus, deep mesencephalic, and Edinger-Westphal nuclei; and 3) the premammillary, lateral, and medial mammillary nuclei, retrochiasmatic part of the supraoptic nucleus, and the zona incerta. The FB-labeled neurons were found in: 1) the parapedunculopontine tegmental and cuneiform nuclei, caudal linear nucleus of the raphe, and adjacent area of the cerebral peduncle; 2) the thalamic posterior nuclear group and subparafascicular, parafascicular, and gelatinosus thalamic nuclei; 3) the parastrial amygdaloid and subthalamic nuclei; and 4) the olfactory tubercle, granular, and agranular insular cortex, parietal and lateral orbital cortices. The connections of the rostral VRG with several cerebellar, midbrain, diencephalic, and telencephalic regions could provide an anatomical substrate for a role of these regions in the control of respiratory-related functions.

Brainstem neurons projecting to the rostral ventral respiratory group (VRG) in the medulla oblongata of the rat revealed by co-application of NMDA and biocytin

Groups of neurons in the medulla and pons are essential for the rhythm generation, pattern formation and modulation of respiration. The rostral Ventral Respiratory Group (rVRG) is thought to be a crucial area for rhythm generation. Here we co-applied biocytin and NMDA in the rVRG to label retrogradely brainstem neurons reciprocally connected to a population of inspiratory neurons in the rat rVRG. The procedure excited rVRG neurons in multi-unit recordings and led to a Golgi-like labelling of distant cells presumably excited by efferents from the rVRG. Injection of biocytin without NMDA did not label neurons in distant structures. Several brainstem ipsi- and contralateral structures were found to project to the rVRG, but three major respiratory-related structures, the nucleus of the solitary tract (NTS), the parabrachialis medialis and Kölliker-Fuse nuclei (PB/KF) and the caudal VRG, which are known to project bilaterally to the rVRG, were exclusively labelled ipsilaterally, suggesting an ipsilateral excitation of these structures by the rVRG. The pathways of efferent axons from labelled neurons in the rVRG were traced rostrally towards the pons and caudally to the spinal cord. Terminal axonal arborizations were seen in the same regions where retrogradely filled neurons were found as well as in a few other motor nuclei (the dorsal vagal motor nucleus and XII nucleus). Moreover, in the NTS and the PB/KF, efferent terminal varicosities were seen closely apposed to the soma and proximal dendrites of labelled neurons, suggesting monosynaptic connections between the rVRG and these nuclei.

Discharge patterns of dorsal and ventral respiratory group neurons during spontaneous augmented breaths observed in pentobarbital anesthetized rats

To clarify the difference between the firing patterns of the dorsal respiratory group (DRG) and ventral respiratory group (VRG) neurons during spontaneous augmented breaths, extracellular single unit recording of 139 respiratory-related neurons (inspiratory: 98/139, expiratory: 41/139) was performed in pentobarbital anesthetized rats. Both the I and E neurons were further classified into six groups: (1) I-augmenting, (2) I-decrementing, (3) I-other cells, (4) E-augmenting, (5) E-decrementing and (6) E-other cells. During the augmented breaths, most inspiratory neurons in the DRG (25/26) and VRG (67/72) show an increase in their discharge frequency irrespective of the cell type, but the discharges after an augmented breath were inhibited. Changes in these inspiratory neurons coincided with those of diaphragm electromyogram activity. With regard to relative changes in the mean firing rate during the inspiratory phase II of augmented breaths, there was a significant difference between the I-augmenting DRG and VRG neurons (353.5 +/- 56.9% vs. 237.5 +/- 17.1%, P < 0.01), but not in the I-decrementing and I-other neurons. On the other hand, the activities of the expiratory DRG (9/9) and VRG (27/32) neurons decreased during the augmented breath. A significant difference in the relative mean firing rate during the expiratory phase of augmented breaths was observed between the E-decrementing DRG and VRG neurons (27.3 +/- 5.2% vs. 58.0 +/- 6.3%, P < 0.05), but not between the E-augmenting and E-other neurons. These results suggested that during spontaneous augmented breaths the firing patterns of the DRG neurons were not qualitatively different from those of the VRG neurons.

Pontomedullary efferent projections of the ventral respiratory neuronal subsets of the rat

The pontomedullary trajectories of projections efferent from the ventral respiratory cell group were anterogradely labelled after discrete injections of Fluoro Ruby into three morphophysiologically identified subdivisions (Bötzinger complex, rostral inspiratory, and caudal expiratory cell groups). The anterogradely labelled varicosities were located in a variety of areas involved in cardiorespiratory function: other subdivisions of the ventral respiratory cell group, the parabrachial (medial, central, and external lateral), Kölliker-Fuse, and lateral paragigantocellular nuclei, A5, and perifacial areas. Although the target areas were similar for the three studied subdivisions, some differences of the location and densities of labelled varicosities were found. Anterogradely labelled fibre bundles were found bilaterally after all of the tracer injections. Three caudally efferent bundles passed through the ventral respiratory cell group, dorsal medullary, and paramedian reticular nuclei. A labelled fibre bundle also took an ascending route through the ventral respiratory cell group: it surrounded the facial nucleus, and then followed two different pathways, one coursing towards forebrain areas and the other to the parabrachial and Kölliker-Fuse complex. Bundles of efferent axons decussated mainly at medullary levels and to a lesser extent in the pons. In the contralateral medulla and pons these labelled fibre bundles followed pathways similar to those observed ipsilaterally. The three ventral respiratory neuronal subsets sent axonal projections through similar tracts, but within them they were topographically organized. The present data are discussed with respect to the circuitry involved in the mechanisms of cardiorespiratory and other visceral functions.

Respiratory neurons mediating the Breuer-Hering reflex prolongation of expiration in rat

Afferent input from pulmonary stretch receptors is important in the control of the timing of inspiratory and expiratory phases of the respiratory cycle. The current study was undertaken to identify neurons within a column of respiratory neurons in the ventrolateral medulla (termed the ventral respiratory group, VRG) that, when activated by lung inflation, produce the Breuer-Hering (BH) reflex in which lung inflation causes inspiratory termination and expiratory prolongation. Intracellular recordings of VRG neurons revealed three groups of inspiratory (I) and two groups of expiratory (E) neurons similar to previous descriptions: I-augmenting (I-Aug), I-decrementing (I-Dec), I-plateau (I-All), E-augmenting (E-Aug), and E-decrementing (E-Dec) neurons. Low-intensity, low-frequency stimulation of a vagus nerve elicited paucisynaptic EPSPs in E-Dec, I-Aug, and I-All neurons that could be divided into two groups on the basis of latency (2.8 +/- 0.1 msec, n = 10; 4.0 +/- 0.1 msec, n = 17). IPSPs were elicited in I-Aug and I-All neurons (4.8 +/- 0.1 msec, n = 12). However, only E-Dec neurons were depolarized when the BH reflex was activated by lung inflation (7.5 cm H2O) or mimicked by vagus nerve stimulation (50 Hz). All other neurons were hyperpolarized and ceased firing during BH reflex-mediated expiratory prolongation. A subset of E-Dec neurons (termed E-Decearly) discharged before inspiratory termination and could contribute to inspiratory termination. The findings are consistent with the hypothesis that a group of E-Dec neurons receives a paucisynaptic (probably disynaptic) input from pulmonary afferents and, in turn, inhibits inspiratory neurons, thereby lengthening expiration.

Central chemoreception in the region of the ventral respiratory group in the rat

We injected acetazolamide (AZ; 5 x 10(-6) M, 1 nl) into the region of the ventral respiratory group (VRG) of anesthetized paralyzed ventilated rats. Control injections (mock cerebrospinal fluid, n = 6, or the inactive AZ analogue 2-acetylamino-1,3, 4-thiadiazole-5-sulfon-t-butylamide, n = 6) did not increase the integrated phrenic neurogram [phrenic nerve amplitude (PNA)]. The AZ injections produced a focal region of tissue acidosis with a radius < 300-400 microns and are used as a probe for sites of central chemosensitivity. Injection location is determined by anatomic analysis. Of 22 VRG injections of AZ, 14 increased the amplitude of the PNA over 15-90 min; 8 had no effect. In 17 cases, we measured medullary tissue pH at the injection center and/or at a distant site and reaffirmed the size of the acidotic region produced by such small AZ injections. Of injections with pH electrodes within 300-400 microns of the injection center, all responders showed an acid pH; three nonresponders showed an acid pH, and one an alkaline pH. In a subgroup of five rats, at VRG sites with known respiratory effects identified by prior glutamate injection (10 nl, 100 mM), all subsequent AZ injections produced a PNA response. Simultaneous measurement of PNA and tissue pH responses at the injection center of eight rats did not show a uniform correlation in time; initially, both changed with a similar time course, but PNA recovered more quickly. We conclude that 1) the region of the VRG contains sites of ventilatory chemoreception, 2) ineffective AZ injections do produce a tissue acidosis but at sites with minimal impact on breathing, and 3) tissue pH does not uniquely represent the chemoreceptor stimulus.

Proprioceptive representation of the levator veli palatini muscle in the solitary nucleus of the rat

The levator veli palatini muscle is innervated by motoneurons of the glossopharyngeal nerve, which are located within the ambiguus nucleus; however, little is known about the afferent fibers of this muscle. A horseradish peroxidase study was conducted in rats following injection into the levator veli palatini muscle branch to reveal the location and the distribution of dendrites of the afferent fibers of the muscle. Terminal labels were densely distributed in the lateral region of the solitary nucleus, which receives afferents of the glossopharyngeal nerve, ipsilateral and contralateral to the injection site. The relationship of the levator veli palatini muscle with respiration was suggested by the localization of labeled terminals at sites where the afferents from the respiratory organs project densely, and by the demonstrated proprioceptive role of the afferent fibers passing through the muscle spindles contained in the levator veli palatini muscle.

Medullary visceral reflex circuits: local afferents to nucleus tractus solitarii synthesize catecholamines and project to thoracic spinal cord

Visceral feedback circuits in lower brainstem were elucidated with retrograde tracers by mapping neurons that issue local projections to the general visceral afferent division of the nucleus tractus solitarii (NTS) and dorsomotor vagal nucleus (DMX) in adult male rats. In study 1, spinal and intramedullary afferents to the visceral-sensorimotor complex (NTS-X) were traced to contiguous populations of cell bodies arranged in cylindrical segmental organization. NTS-X afferents derive from curvilinear arrays of neurons that parallel the efferent radiations of the solitariotegmental tract. Newly discovered afferents arise from circumscribed cell groups in the dorsal reticular formation and periventricular zone. Another source was traced to a paraambigual cell column in the apex of the rostral ventrolateral reticular nucleus (n.RVL). In study 2, catecholaminergic afferents were initially defined with combined retrograde transport-immunocytochemical methods. Deposits of retrograde tracers into NTS-X transported to neurons containing tyrosine hydroxylase (TH) in the A1, C1, and C3 areas or phenylethanolamine N-methyltransferase (PNMT) in the C1 area of the n.RVL and C3 area. In study 3, it was revealed that NTS-X afferents arise, in part, as collaterals of thoracic reticulospinal neurons. Deposits of the retrograde fluorescent tracer Fluorogold into the upper thoracic cord and rhodamine-labeled microbeads into NTS-X transported to the same neurons within a subambigual locus in n.RVL and parts of nucleus raphe magnus. In study 4, dual retrograde tracer-immunocytochemical analysis demonstrated that catecholamines are synthesized by a subset of neurons in the n.RVL that issue collaterals to the NTS-X and thoracic cord. Double retrogradely labeled TH- or PNMT-immunoreactive cell bodies were restricted to the C1 area within a 450-microns column bordered rostrally by the facial nucleus and ventrally by the medullary subpial surface. We conclude that visceral reflex arcs are reciprocally organized. Targets of NTS projection are also sources of local NTS-X afferent innervation. Catecholaminergic and other local afferents from reticular formation, periventricular, and spinal gray may, via collaterals, simultaneously modulate visceral reflex excitability at the level of NTS and the outflow of autonomic and respiratory motoneurons.

Brain stem projections by axonal collaterals to the rostral and caudal ventral respiratory group in the rat

The location of neurons projecting by axonal collaterals to the rostral and caudal ventral respiratory group (VRG) regions was determined after discrete injections of Fast blue and Diamidino yellow into the physiologically identified rostral inspiratory VRG and the caudal expiratory VRG areas, respectively. In contrast with single fluorochrome labeled neurons found throughout the rostro-caudal extent of the medulla and pons (in a variety of areas known to have cardiorespiratory function), double-labeled neurons were located in discrete ponto-medullary regions. The largest number of the double-labeled neurons was counted within the peripheral facial area, lateral paragigantocellular nucleus, and the VRG region, ipsi- and contralaterally to the injected side. Only a few double-labeled neurons were found within the ventrolateral and intermediate subnuclei of the solitary tract, medial parabrachial, and Kölliker-Fuse nuclei. The possible physiological implications of this neuronal network have also been emphasized.

Respiratory-related activity of the pharyngeal nerves in the rat

In anesthetized (n = 26) or decerebrate (n = 15) rats, vagotomized, paralyzed and artificially ventilated, we recorded efferent respiratory-related discharges of nerves supplying the pharyngeal muscles, i.e. the glossopharyngeal nerve (IX; n = 30) and the pharyngeal ramus of the vagus nerve (PH.X; n = 33). In both types of preparation, all IX nerves fired during inspiration (I); most PH.X fired either during I (n = 11), expiration (E, n = 12), or both phases (n = 4); some of them were continuously active, without any respiratory modulation (n = 6). We also examined the timing of inspiratory pharyngeal bursts in relation to the phrenic (inspiratory) bursts. We found that the burst onset occurred significantly earlier in pharyngeal nerves than in phrenic ones (pharyngeal-to-phrenic delay). In anesthetized animals, this pharyngeal-to-phrenic delay was long enough to reveal that inspiratory activity appeared first on IX, then on PH.X, and then on phrenic nerves. Since hypoxia did not significantly alter the pharyngeal-to-phrenic delay, we conclude that a reliable organization of the inspiratory drive on pharyngeal and diaphragmatic muscles should exist, in the rat.

A midline area in the nucleus commissuralis of NTS mediates the phrenic nerve responses to carotid chemoreceptor stimulation

The carotid body chemoreceptor afferents have been reported to project to a discrete area located in the nucleus commissuralis of nucleus tractus solitarius [A. Vardhan et al., Am. J. Physiol., 264 (1993) R41-R50]. The afore-mentioned study was done in spontaneously breathing rats and the afferents and efferents located in the chest wall and the respiratory tract of these animals were intact. In order to exclude the role, if any, of these afferents and efferents, in the present experiments respiratory changes were monitored by recording the phrenic nerve activity instead of tracheal airflow. Experiments were carried out in pentobarbital-anesthetized, bilaterally vagotomized, paralyzed and artificially ventilated rats with a pneumothorax. The carotid body chemoreceptors were stimulated with tracheal administration of nitrogen for 7-10 s. The chemoreceptor stimulation induced an increase in the frequency and amplitude of phrenic nerve bursts. A decrease in the duration of inspiratory (T1), expiratory (TE) and total cycles (TTOT) was observed in the phrenic nerve activity. Inhibition of neuronal cell bodies by microinjections of muscimol (140 pmol/20 nl) into a discrete area in the commissural subnucleus of the nucleus tractus solitarius (coordinates in mm: 0.3 rostral to 0.5 caudal, 0 to 0.5 lateral and 0.3 to 0.5 deep with respect to the calamus scriptorius), attenuated the phrenic nerve responses to the carotid body stimulation. On the other hand, control injections of saline (0.9%) into this site did not alter the phrenic nerve response to the carotid body stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Brainstem network controlling descending drive to phrenic motoneurons in rat

Contraction of the diaphragm is controlled by phrenic motoneurons that receive input from sources that are not fully established. Bulbospinal (second-order) neurons projecting to phrenic motoneurons and propriobulbar (third-order) neurons projecting to these bulbspinal neurons were investigated in rat by transsynaptic transport of the neuroinvasive pseudorabies virus. Bulbospinal neurons were located predominantly in the medullary lateral tegmental field in two functionally described regions, the ventral respiratory group and Bötzinger complex. An intervening region, the pre-Bötzinger complex, contained essentially no phrenic premotoneurons. Bulbospinal neurons were also located in ventral, interstitial, and ventrolateral subnuclei of the solitary tract, and gigantocellular, Kölliker-Fuse, parabrachial, and medullary raphe nuclei. A monosynaptic pathway to phrenic motoneurons from the nucleus of the solitary tract was confirmed; monosynaptic pathways from upper cervical spinal cord, spinal trigeminal nucleus, medical and lateral vestibular nuclei, and medial pontine tegmentum were not verified. Locations of third-order neurons were consistent with described projections to the ventral respiratory group, from contralateral ventral respiratory group, Bötzinger complex, A5 noradrenergic cell group, and the following nuclei; solitary, raphe, Kölliker-Fuse, parabrachial, retrotrapezoid, and paragigantocellular. Novel findings included a projection from locus coeruleus to respiratory premotoneurons and the lack of previously described pathways from area postrema and spinal trigeminal nucleus. These second- and third-order neurons from the output network for diphragm motor control which includes numerous behaviors (e.g., respiration, phonation, defecation). Of the premotoneurons, the rostral ventral respiratory group is the primary population controlling phrenic motoneurons.

Ventilatory acclimatization to chronic hypoxia: relationship to noradrenaline metabolism in the rat solitary complex

1. The relationship between ventilatory acclimatization to chronic hypoxia (10% O2-90% N2) and noradrenaline metabolism was examined in two regions located immediately caudal and rostral to the obex within the rat solitary complex. 2. Three experimental protocols were established. In protocol 1, the percentage changes in respiratory tidal volume, frequency and minute ventilation elicited by 4, 7, 10 and 14 days of hypoxia were assessed by flow plethysmography in awake rats, and then the content of tyrosine hydroxylase was measured in the solitary complex. In protocol 2, the time course response of tyrosine hydroxylase protein level was determined after 3, 7, 14 and 22 days of hypoxia by using a quantitative immunoblotting method for the protein assay. In protocol 3, the turnover of noradrenaline was estimated in the solitary complex after 14 days of hypoxia. 3. A progressive increase in ventilation was observed to reach a maximum (+105 +/- 15%, mean +/- S.E.M.) above normoxic control after 10 days of hypoxia, at which time it stabilized. Furthermore, tyrosine hydroxylase protein increased progressively and reached a maximal level at 14 days of hypoxia (+36 +/- 4%, mean +/- S.E.M.). Return to the basal level of tyrosine hydroxylase was observed after 22 days of hypoxia. 4. Tyrosine hydroxylase content (+36 +/- 4%) and noradrenaline turnover (+394 +/- 3%) increased exclusively in the caudal part of the solitary complex. 5. The ventilatory acclimatization to chronic hypoxia preceded the increase in tyrosine hydroxylase and these two parameters were significantly correlated. 6. These data suggest that ventilatory acclimatization to chronic hypoxia is associated with topical modifications of the brainstem catecholamine metabolism.

Morphological study of long axonal projections of ventral medullary inspiratory neurons in the rat

The aim of this study was to examine medullary and spinal axonal projections of inspiratory bulbospinal neurons of the rostral ventral respiratory group (VRG) in the rat. A direct visualization of long (9.8-33 mm) axonal branches, including those projecting to the contralateral side of the medulla oblongata and the spinal cord, was possible due to intracellular labeling with neurobiotin and long survival times (up to 22 h) after injections. Seven of the nine labeled neurons had bilateral descending axons, which were located in discrete regions of the spinal white matter; ipsilateral axons in the lateral and dorsolateral funiculus, contralateral in the ventral and ventromedial funiculus. The collaterals issued by these axons at the mid-cervical level formed close appositions with dendrites of phrenic motoneurons, which had also been labeled with neurobiotin. None of these collaterals crossed the midline. The significance of this finding is discussed in relation to the crossed-phrenic phenomenon. Additional spinal collaterals were identified in the C1 and T1 segments. Within the medulla, collaterals with multiple varicosities were identified in the lateral tegmental field and in the dorsomedial medulla (in the hypoglossal nucleus and in the nucleus of the solitary tract). These results demonstrate that inspiratory VRG neurons in the rat have some features which have not been previously described in the cat, including frequent bilateral spinal projection and projection to the nucleus of the solitary tract. In addition, this study shows that intracellular labeling with neurobiotin offers an effective way of tracing long axonal projections, supplementing results previously obtainable only with antidromic mapping, and providing morphological details which could not be observed in previous studies using labeling with horseradish peroxidase.

The medullary rostral ventrolateral pressor region: an electrophysiological study in decerebrate rat

Decerebrate rat was used to study the discharge pattern of barosensitive neurons. This preparation avoids general anesthesia and suppresses painful sensations of the animal. Twenty-eight spontaneously active units were recorded in the rostral ventrolateral region of the medulla (RVLM). Six units had projections to the spinal cord (bulbospinal) and 22 were not antidromically activated by spinal cord stimulation (NAA). Transient hypertension induced by intravenous injection of noradrenaline depressed the activity of 21 units, and increased activity of 7, regardless of axonal destination. Unlike anesthetized rat, the decerebrate rat possesses spontaneously active neurons with excitatory response to hypertension.

Electrophysiological study of dorsal respiratory neurons in the medulla oblongata of the rat

There has been controversy whether the dorsal respiratory group (DRG), identified in the cat and several other species as a concentration of mainly inspiratory neurons located in the ventrolateral subnucleus of the solitary tract, also exists in the rat. The aim of this study was to re-examine this question by systematically exploring this region with extracellular microelectrodes, in anesthetized and artificially ventilated rats. One-hundred and forty-two units were recorded which fired in phase with central respiratory cycles (determined by recording from the phrenic nerve) and/or lung inflations. One-hundred and nineteen recordings were thought to be from neuronal cell bodies (confirmed in some cases by excitatory responses to microelectrophoretic administration of DL-homocysteic acid), while the remaining 23 were from lung vagal afferents. Most neurons in the former group (87/119) were inspiratory. Out of 96 neurons tested for spinal projections only 14 (12 inspiratory, 2 expiratory) responded antidromically following stimulation at C3 segment. These results confirm the existence of the DRG in the rat and demonstrate that neurons located in this region have firing patterns generally similar to those previously described in the cat. The main difference is the relative paucity in the rat of neurons projecting spinally below the C2 level, which indicates that most DRG neurons in this species do not project directly to phrenic and intercostal motoneurons, but to other, as yet unidentified, neuronal groups within the brainstem or upper cervical segments.

Close appositions between tyrosine hydroxylase immunoreactive boutons and respiratory neurons in the rat ventrolateral medulla

The extent of the adrenergic input to respiratory neurons in the ventrolateral medulla oblongata of rats was assessed by using a combination of intracellular recording, dye filling, and immunohistochemistry. Twenty-two neurons that displayed a pronounced respiration-related modulation of their membrane potential, and could not be antidromically activated by electrical stimulation of the superior laryngeal, vagus, or facial nerves, were labelled by intracellular injection of biocytin. Three types of respiration-related neurons were labelled: small neurons located in the Bötzinger complex between 0.5 and 1.0 mm caudal to the facial nucleus; medium-sized neurons located in the ventral respiratory group 1.0 to 2.0 mm caudal to the facial nucleus; and large motoneurons located within the nucleus ambiguus 0.5 to 2.0 mm caudal to the facial nucleus. Small Bötzinger neurons [length = 22 +/- 5 microns, width = 13 +/- 3 microns, area = 222 +/- 79 microns2; (mean +/- SD, n = 5)] had membrane potentials of -15 to -27 mV during the recording period. Four of five of these cells had profuse axonal terminations between 50 microns caudal and 450 microns rostral to their somata, suggesting that they may form part of local networks responsible for generating respiratory activity. Medium-sized ventral respiratory group neurons (length = 26 +/- 5 microns, width = 18 +/- 4 microns, area = 377 +/- 141 microns2; n = 5) were found in the vicinity of the nucleus ambiguus dorsal to the lateral reticular nucleus. Three of five of these neurons had an axon that crossed the midline and travelled caudally. One axon had a collateral with varicosities close to its soma. The somata of motoneurons (length = 29 +/- 6 microns, width = 21 +/- 4 microns, area = 485 +/- 142 microns2; n = 12) were located within the nucleus ambiguus, and had axons that could be traced to exist points from the medulla. Tyrosine hydroxylase immunoreactive cells and their terminal fibres within the medulla were localised by immunocytochemistry. Small Bötzinger neurons received the largest number of close appositions from tyrosine hydroxylase immunoreactive boutons (13 +/- 2 appositions/neuron; n = 5). Medium-sized ventral respiratory group neurons received fewer appositions (8 +/- 4 appositions/neuron; n = 5). Most motoneurons (n = 10) received few appositions from tyrosine hydroxylase immunoreactive boutons, while two received none. The average number was 3 +/- 3 appositions/neuron (n = 12).(ABSTRACT TRUNCATED AT 400 WORDS)

Projections from the nucleus tractus solitarii to the spinal cord

Projections from the nucleus tractus solitarii (NTS) to the spinal cord were demonstrated in the male Sprague-Dawley rat. In retrograde transport studies, a horseradish peroxidase conjugate or a fluorescent dye, FluoroGold, were injected into midcervical or upper thoracic spinal segments. Most solitariospinal neurons were multipolar or bipolar and located between the obex and spinomedullary junction. Solitariospinal neurons were concentrated in proximity to the ventral border of the solitary tract and extended dorsally into the intermediate division and ventrolaterally into the intermediate reticular zone (IRt) of the lateral tegmental field. This subgroup predominantly projects to midcervical spinal segments. A subset of small neurons was retrogradely labeled from cervical or thoracic spinal segments in the medial commissural nucleus and contiguous with a periventricular group surrounding the central canal. In anterograde transport studies, iontophoretic deposits of Phaseolus vulgaris leucoagglutinin were centered stereotaxically on sites in NTS identified by retrograde transport data. The lectin was incorporated by neurons of the solitary complex and transported bilaterally by axons that emerged from the nucleus and entered the reticular formation. The solitario-reticular (transtegmental) pathway irradiated diagonally across the IRt and extended caudally into the cervical lateral funiculus and spinal gray. A small periventricular-spinal pathway also descended longitudinally to the neuraxis. Solitariospinal neurons project to superficial lamina of the dorsal horn, laminae VII and X and ventral horn. The projections are predominantly contralateral to phrenic and intercostal motor nuclei and ipsilateral to the intermediolateral cell column. The solitariospinal projection represents the shortest route in the central nervous system, other than the local intraspinal reflex, through which first order visceral afferents signal cardiorespiratory and alimentary motor nuclei.

Expiratory neurons of the Bötzinger Complex in the rat: a morphological study following intracellular labeling with biocytin

The term "Bötzinger Complex" (BOT) refers to a distinct group of neurons, located near the rostral portion of the nucleus ambiguus, which are known to play an important role in the control of respiratory movements. Previous studies conducted in cats have demonstrated that most of these neurons are active during expiration, exerting a monosynaptic inhibitory action on several subpopulations of inspiratory neurons in the medulla and spinal cord. The aim of this study was to examine morphological properties and possible synaptic targets of BOT neurons in the rat. Forty-one expiratory neurons were labeled intracellularly with biocytin; 12 were interneurons (BOT neurons) and 29 were motoneurons. The latter could not be antidromically activated following stimulation of the superior laryngeal or vagal nerves. BOT neurons showed extensive axonal arborisations in the ipsilateral medulla, with some projections to the contralateral side. Bouton-like axon varicosities mainly clustered in two areas: near the parent cell bodies, and in the area corresponding to the rostral part of the ventral respiratory group (VRG). In five pairs of labeled neurons, each consisting of one BOT neuron and one inspiratory neuron in the rostral VRG, no appositions were identified at the light microscopic level between axons of BOT neurons and dendrites or cell bodies of inspiratory neurons. These results demonstrate that some features of BOT expiratory neurons in the rat are similar to those previously described in cats. The differences include their more ventral location in relation to the compact formation of nucleus ambiguus (retrofacial nucleus), and the relative paucity in the rat of neurons displaying an augmenting pattern of activity and of neurons with spinally projecting axons. In addition, we were unable to find morphological evidence for contacts between labeled BOT neurons and ipsilateral inspiratory neurons near the obex level, a finding not consistent with previous electrophysiological studies in the cat in which such synaptic connections have been identified.

Pulmonary stretch receptor afferents activate excitatory amino acid receptors in the nucleus tractus solitarii in rats

1. The goal of the present study was to identify potential neurotransmitter candidates in the Breuer-Hering (BH) reflex pathway, specifically at synapses between the primary afferents and probable second-order neurones (pump cells) within the nucleus tractus solitarii (NTS). We hypothesized that if activation of specific receptors in the NTS is required for production of the BH reflex, then (1) injection of the receptor agonist(s) would mimic the reflex response (apnoea), (2) injection of appropriate antagonists would impair the apnoea produced by either lung inflation or agonist injection, and (3) second-order neurones in the pathway would be excited by either lung inflation or agonists while antagonists would prevent the response to either. 2. Studies were carried out either in spontaneously breathing or in paralysed, thoracotomized and ventilated rats in which either diaphragm EMG or phrenic nerve activity, expired CO2 concentration and arterial pressure were continuously monitored. The BH reflex was physiologically activated by inflating the lungs. 3. Pressure injections (0.03-15 pmol) of selective excitatory amino acid (EAA) receptor agonists, quisqualic acid (Quis) and N-methyl-D-aspartic acid (NMDA) into an area of the NTS shown previously to contain neurones required for production of the BH reflex produced dose-dependent apnoeas that mimicked the response to lung inflation. Injection of substance P (0.03-4 pmol) did not alter baseline respiratory pattern. 4. Injections of the EAA antagonists, kynurenic acid (Kyn; 0.6-240 pmol), 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) or 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the BH region of the NTS reversibly impaired the apnoea produced by lung inflation. All three antagonists reduced or abolished the apnoeas resulting from injection of Quis or NMDA, and slowed baseline respiratory frequency. In contrast, injections of the highly selective NMDA receptor antagonist, D-2-amino-5-phosphonovaleric acids (AP5), in doses sufficient to block the apnoeic response to NMDA, neither altered the reflex apnoea evoked by lung inflation nor the baseline respiratory pattern. 5. Pump cells located within the BH region were excited by pressure injections of the broad spectrum EAA agonist, DL-homocysteic acid (DLH). Kyn reversibly blocked the excitation of pump cells in response to either lung inflation or DLH injection. 6. These findings suggest that EAAs mediate primary afferent excitation of second-order neurones in the Breuer-Hering reflex pathway, primarily through the activation of non-NMDA EAA receptor subtypes.

Central respiratory modulation of facial motoneurons in rats

Facial motoneurons (FMN) were recorded intracellularly in Sprague-Dawley rats anesthetized with halothane. The animals were vagotomized, paralyzed, and artificially ventilated. The average membrane potential of the cells was 62.6 +/- 1.9 mV and their input impedance ranged from 5 to 30 M omega (9.8 +/- 1.1 M omega, n = 38). The membrane potential of most FMNs varied throughout the central respiratory cycle and four distinct patterns were detected. Type I (post-inspiratory) cells (21/44) showed a two-phase Cl(-)-mediated hyperpolarization during the respiratory cycle, one during central inspiration and the second during late expiration. Type II cells (early inspiratory, n = 10) showed early inspiratory depolarization. Type III (n = 2, stage-2 expiratory) cells displayed late expiratory depolarization and one cell (type IV or throughout inspiratory) exhibited expiratory Cl(-)-mediated hyperpolarization. The remaining 10 cells showed no detectable respiratory modulation. The results reflect the heterogeneity of the central respiratory modulation of FMNs and suggest that these cells receive both excitatory and inhibitory inputs from elements of the central respiratory pattern generating network.

Microenvironment of respiratory neurons in the in vitro brainstem-spinal cord of neonatal rats

1. O2-, K(+)- and pH-sensitive microelectrodes were used to measure extracellular oxygen pressure (PO2), K+ activity (aKo) and pH (pHo) in ventral regions of the medulla oblongata containing respiratory neurons in the in vitro brainstem-spinal cord preparation from 0 to 4-day-old rats. 2. The location of respiratory neurons was mapped by extracellular recordings with conventional microelectrodes, or with the reference barrel of ion-sensitive microelectrodes. The major populations of respiratory neurons were distributed in the ventrolateral reticular formation near the nucleus ambiguus at depths of 300-600 microns. In this area, aKo baseline increased from 3.2 to 3.8 mM whereas steady-state values of PO2 and pHo fell from 120 to 7 mmHg and from 6.9 to 6.7, respectively. 3. During rhythmic inspiratory discharges recorded with suction electrodes from ventral roots of spinal (C3-C5) and cranial (IX, X, XII) nerves, aKo transiently increased by up to 100 microM, and PO2 fell maximally by 0.4 mmHg. During episodes of non-rhythmic neuronal discharge, aKo increased by as much as 0.4 mM and PO2 decreased by about 10 mmHg. In contrast, no variations in pHo could be detected during such activities. 4. Activation of medullary neurons by tetanic electrical stimulation of axonal tracts in the ventrolateral column of the spinal cord at the level of the phrenic motoneuron pool produced aKo elevations of up to 5 mM, decreases of PO2 by up to 50 mmHg, and pHo increases by a maximum of 0.07 pH units. These aKo and PO2 transients were reduced by more than 80% during blockade of synaptic transmission with 5 mM manganese (Mn2+) and completely blocked by 1 microM tetrodotoxin (TTX). 5. The tissue PO2 gradient as well as activity-related decreases of PO2 were completely abolished after block of oxidative cellular metabolism by addition of 2-10 mM cyanide (CN-) to the bathing solution. 6. Inhibition of the Na(+)-K+ pump by addition of 3-50 microM ouabain (3-10 min) caused a reversible increase of aKo by 0.8-3 mM, a delayed recovery of stimulus-induced aKo elevations, and produced a disturbance of the respiratory rhythm. 7. The sensitivity of the respiratory network to oxygen depletion was tested by superfusing the neuraxis with hypoxic solutions gassed with N2 instead of O2 (5-20 min).(ABSTRACT TRUNCATED AT 400 WORDS)

Possible mutual excitatory couplings between inspiratory neurons in caudal ventrolateral medulla of brainstem-spinal cord preparation isolated from newborn rat

In in vitro brainstem-spinal cord preparations, projection of inspiratory neurons in the caudal ventrolateral medulla (CVL) was examined electrophysiologically, and connectivity between bilateral inspiratory neurons in the CVL was analyzed by pulse-cross correlation (PCC) analysis. CVL inspiratory neurons were found to project to the contralateral CVL and/or mainly ipsilateral spinal cord. PCC analysis revealed significant peaks with different latency on both sides of time zero in 3 of 8 pairs. Results were consistent with mono- or oligo-synaptic excitatory connections between bilateral inspiratory neurons in the CVL.

Mechanisms of respiratory rhythm generation

In mammals, a three-phasic respiratory rhythm is generated by a network of various types of neurons in the lower brainstem. The cellular mechanisms of rhythmogenesis involve cooperative interactions between synaptic processes and specific membrane properties. The network seems to be driven by extrinsic sources in mature animals, whereas in the immature network pacemaker neurons might be involved.

Brainstem connections of the rat ventral respiratory subgroups: afferent projections

Propriobulbar neurons having axonal projections to the Ventral Respiratory Group (VRG) were retrogradely labeled after discrete injections of Fast blue into one of the three physiologically identified subdivisions (Bötzinger Complex, rostral inspiratory and caudal expiratory regions). Neurons that project to these regions were found throughout the rostrocaudal extent of the medulla and the pons in a variety of areas known to have cardio-respiratory function. Labeled somata were located within the nuclei of the solitary tract (commissural, intermediate and ventrolateral), other subdivisions of VRG, parabrachial nuclei (medial, dorsolateral and central lateral), Kölliker-Fuse nucleus, retrotrapezoid nucleus, lateral paragigantocellular nucleus and lateral tegmental field of the pons. Within the nuclei of the solitary tract and the Kölliker-Fuse nucleus, there was a topographical organization with respect to the three subdivisions of the VRG.

Distribution of bulbospinal neurons supplying bilateral innervation to the phrenic nucleus in the rat

The location of bulbospinal neurons with axon collaterals in both phrenic nuclei were determined by injecting two different fluorescent tracers into the right and left C4 cervical spinal cord. In contrast to single-labeled neurons that were found throughout the rostrocaudal extent of the medulla, the majority of double-labeled neurons were located in the rostral ventral respiratory group. Only a few double-labeled neurons were found in the ventrolateral nucleus of the solitary tract. The role of this bilateral pathway in synchronizing the activity of the phrenic nucleus is discussed.

Medullary expiratory neurons in the decerebrate rat: an intracellular study

Intracellular recordings and labelings with horseradish peroxidase (HRP) of expiratory (E) neurons were performed in decerebrate, paralyzed, and ventilated rats. A total of 37 neurons were recorded, from which 4 cells and 1 axon were labeled. They were located in two regions of the ventrolateral medulla. One was in the rostral portion of the nucleus ambiguus just caudal to the facial nucleus, and the other in the nucleus retroambiguus at the level of the caudal medulla. These expiratory neurons had rhythmical changes in membrane potential similar to those reported in cat, i.e., a depolarization in the intervals between phrenic bursts which evolved in an augmenting (E-aug, n = 15), or bell-shaped or 'plateau' (E-all, n = 22) pattern until a rapid hyperpolarization at the start of inspiration. Both types were hyperpolarized during inspiration by chloride-dependent, inhibitory postsynaptic potentials (IPSPs) which were demonstrated in 17 neurons (10 E-aug and 7 E-all) from which reversal was obtained. Such IPSPs also existed during post-inspiration (stage I of expiration) in 4 of the 10 augmenting E neurons. They were identified by antidromic stimulation or HRP labeling, or both, as bulbospinal neurons (n = 2), cranial motoneurons (n = 4), or not antidromically activated (NAA) neurons (n = 31). All the identified bulbospinal neurons and the motoneurons exhibited an E-all pattern. The expiratory neurons of the caudal medulla had various projections as demonstrated with HRP labeling: one bulbospinal neuron with ipsilateral axon giving off intramedullary collaterals, and NAA neurons with rostral medullary projections or with axons crossing the midline.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular electrophysiological and morphological study of the medullary inspiratory neurons of the decerebrate rat

Intracellular recordings and labelings with horseradish peroxidase (HRP) of inspiratory neurons were performed in decerebrate, paralyzed and ventilated rats. A total of 58 neurons were located within the ventrolateral medulla. They were identified as bulbospinal neurons (n = 15), cranial motoneurons (n = 9) and not antidromically activated (NAA) neurons (n = 34) by antidromic stimulation or HRP labeling, or both. These inspiratory neurons had rhythmical changes in membrane potentials similar to those reported in cats, i.e. an abrupt depolarization at the onset of phrenic discharge followed by trajectories of depolarization which evolved into augmenting I, bell-shaped I or decrementing I patterns until a rapid repolarization at the start of expiration. All types were hyperpolarized during expiration by chloride-dependent inhibitory postsynaptic potentials (IPSPs) which were demonstrated in 13 neurons from which the reversal was obtained. Such IPSPs were apparent in two waves throughout expiration, an early one in post-inspiration (stage I of expiration) and a late one in late expiration (stage II of expiration). These properties are also similar to those of feline inspiratory medullary neurons. Four labeled bulbospinal neurons had axonal collaterals which were ipsi- and contralateral to the site of their somata. Two of 6 labeled NAA neurons exhibited profuse axonal arborizations within various medullary nuclei. No medullary axonal collateral was seen from 6 labeled motoneurons. These results indicate that even though in the rat a single concentration of inspiratory neurons within the ventrolateral medulla has been demonstrated, there is no fundamental difference in the organization of the inspiratory neuronal network compared to that of the cat.

Whole cell recordings from respiratory neurons in the medulla of brainstem-spinal cord preparations isolated from newborn rats

In brainstem-spinal cord preparations isolated from newborn rats, a whole cell recording technique was applied to record membrane potentials of inspiratory (Insp) and pre-inspiratory (Pre-I) neurons in the ventrolateral medulla. Labelling of these respiratory neurons with Lucifer Yellow allowed analysis of their locations and morphology. Intracellular membrane potentials from 25 Insp neurons were recorded. Average resting membrane potential was -49 mV (n = 25) and input resistance was 306 M omega. Insp neurons were classified into three types from the patterns of synaptic potentials. Type I neurons (n = 11) had a high probability of excitatory postsynaptic potentials (EPSPs) in the pre- and post-inspiratory phases. Type II neurons (n = 7) showed abrupt transition to the burst phase from the resting potential level without increased EPSPs in the preinspiratory phase. Type III neurons (n = 7) were hyperpolarized by inhibitory postsynaptic potentials (IPSPs) in the pre- and post-inspiratory phases. These Insp neurons, located in the ventrolateral medulla 80-490 microns from the ventral surface, were 10-30 microns in diameter, and had various soma shapes (pyramidal, spherical or fusiform). Intracellular membrane potentials from 24 Pre-I neurons were recorded. The average resting membrane potential was -45 mV (n = 24), and the input resistance was 320 M omega. Typical Pre-I neurons showed fairly great depolarization accompanied by action potentials during their burst phase and repolarization during the inspiratory phase. Most Pre-I neurons appeared to have a high level of synaptic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of respiratory bulbospinal and propriobulbar neurons in the region of the nucleus ambiguus of the rat

The location of neurons within the ventral respiratory group (VRG) of rat was mapped following injections of 3 different fluorochrome tracers into different sites known to receive projections from VRG neurons. Injection sites included muscles innervated by the vagus (X) and glossopharyngeal (IX) nerves, and the sites of expiratory activity in the caudal medulla and of inspiratory activity in the spinal cord at the C4 level. Labeling of vagal motoneurons resulting from fluorochrome injections into muscles innervated by X and IX nerves was always ipsilateral to the site of injection. Both propriobulbar and bulbospinal neurons had primarily ipsilateral projections. No double-labeled cell bodies were observed. The cell bodies of the 3 types of neurons, propriobulbar, bulbospinal and vagal/glossopharyngeal, were unevenly distributed along the rostrocaudal axis of the VRG, suggesting a complex mosaic of neurons which regulate respiratory-related functions such as swallowing and vocalization.

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

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