Differing activities of medullary respiratory neurons in eupnea and gasping

Our purpose was to compare further eupneic ventilatory activity with that of gasping. Decerebrate, paralyzed, and ventilated cats were used; the vagi were sectioned within the thorax caudal to the laryngeal branches. Activities of the phrenic nerve and medullary respiratory neurons were recorded. Antidromic invasion was used to define bulbospinal, laryngeal, or not antidromically activated units. The ventilatory pattern was reversibly altered to gasping by exposure to 1% carbon monoxide in air. In eupnea, activities of inspiratory neurons commenced at various times during inspiration, and for most the discharge frequency gradually increased. In gasping, the peak discharge frequency of inspiratory neurons was unaltered. However, all commenced activities at the start of the phrenic burst and reached peak discharge almost immediately. The discharge frequencies of all groups of expiratory neurons fell in gasping, with many neurons ceasing activity entirely. These data are consistent with the hypothesis that brain stem mechanisms controlling eupnea and gasping differ fundamentally.     

Changes in antidromic latencies of medullary respiratory neurons in hypercapnia and hypoxia

We evaluated mechanisms underlying changes in discharge frequencies of medullary respiratory neurons. This evaluation was made by determining variations in antidromic latencies; these variations reflect changes in membrane potentials. In decerebrate, vagotomized, paralyzed, and ventilated cats, activities of the phrenic nerve and single respiratory neurons were monitored in hyperoxic normocapnia, hyperoxic hypercapnia, and/or normocapnic hypoxia. Axonal projections were defined as bulbospinal or laryngeal by antidromic activation. At normocapnic hyperoxia, antidromic latencies fell to minima during periods of spontaneous neuronal activity, with maxima occurring between neuronal bursts. In hypercapnia or hypoxia, these minima were not altered, whereas maximum latencies typically rose for neurons whose discharge frequencies increased. However, the increased frequencies most strongly correlated with increases in the difference between maximum and minimum latencies. No such correlation was evident for neurons whose discharge frequencies declined. We conclude that the overall change of membrane potential primarily defines neuronal discharge frequencies. Changes in membrane potentials induced by peripheral and central chemoreceptor afferents and by direct actions of hypercapnia and hypoxia are discussed.     

Activity of medullary respiratory neurons during ventilator-induced apnea in sleep and wakefulness

Mechanical ventilation of cats in sleep andwakefulness causes apnea, often within two to three cycles of theventilator. We recorded 137 medullary respiratory neurons in four adultcats during eupnea and during apnea caused by mechanical ventilation. We hypothesized that the residual activity of respiratory neurons during apnea might reveal its cause(s). The results showed that residual activity depended on 1) theamount of nonrespiratory inputs to the cell (cells with morenonrespiratory inputs had greater amounts of residual activity);2) the cell type (expiratory cellshad more residual activity than inspiratory cells); and 3) the state of consciousness (moreresidual activity in wakefulness and rapid-eye-movement sleep than innon-rapid-eye-movement sleep). None of the cells showed an activationduring ventilation that could explain the apnea. Residual activity ofapproximately one-half of the cells was modulated in phase with theventilator. The strength of this modulation was quantified by using aneffect-size statistic and was found to be weak. The patterns ofmodulation did not support the idea that mechanoreceptors excite somerespiratory cells that, in turn, inhibit others. Indeed, most cells,inspiratory and expiratory, discharged during the deflation-inflationtransition of ventilation. Residual activity failed to reveal the causeof apnea but showed that during apnea respiratory neurons act as ifthey were disinhibited and disfacilitated.

     

Localization of the medullary respiratory neurons in rats by microelectrode recording.

Neuronal activity has been recorded extracellularly from the medulla of anesthetized rats. Units whose discharge frequency varied in phase with respiratory airflow were located bilaterally between 1.5 and 2 mm lateral to midline, extending from 1 mm caudal to 1.5 mm rostral to the obex, in the ventral two-thirds of the medulla. Expiratory units predominated and were intermingled with inspiratory units. Ten different patterns of discharge were distinguished, varying from a short burst at the beginning of expiration to a resting discharge which increased in frequency during either inspiration or expiration. Evidence was also obtained that fiber tracts from other areas of the brain cross midline just caudally to the obex and pass to the respiratory centers on which they apparently exert and excitatory action.     

Opiate slowing of feline respiratory rhythm and effects on putative medullary phase-regulating neurons

Opiates have effects on respiratory neurons that depress tidal volume and air exchange, reduce chest wall compliance, and slow rhythm. The most dose-sensitive opioid effect is slowing of the respiratory rhythm through mechanisms that have not been thoroughly investigated. An in vivo dose-response analysis was performed on medullary respiratory neurons of adult cats to investigate two untested hypotheses related to mechanisms of opioid-mediated rhythm slowing: 1) Opiates suppress intrinsic conductances that limit discharge duration in medullary inspiratory and expiratory neurons, and 2) opiates delay the onset and lengthen the duration of discharges postsynaptically in phase-regulating postinspiratory and late-inspiratory neurons. In anesthetized and unanesthetized decerebrate cats, a threshold dose (3 microg/kg) of the mu-opioid receptor agonist fentanyl slowed respiratory rhythm by prolonging discharges of inspiratory and expiratory bulbospinal neurons. Additional doses (2-4 microg/kg) of fentanyl also lengthened the interburst silent periods in each type of neuron and delayed the rate of membrane depolarization to firing threshold without altering synaptic drive potential amplitude, input resistance, peak action potential frequency, action potential shape, or afterhyperpolarization. Fentanyl also prolonged discharges of postinspiratory and late-inspiratory neurons in doses that slowed the rhythm of inspiratory and expiratory neurons without altering peak membrane depolarization and hyperpolarization, input resistance, or action potential properties. The temporal changes evoked in the tested neurons can explain the slowing of network respiratory rhythm, but the lack of significant, direct opioid-mediated membrane effects suggests that actions emanating from other types of upstream bulbar respiratory neurons account for rhythm slowing.     

Pacemaker properties of respiratory neurons of the ventrolateral medullary regions in early postnatal rats

Spike activity of respiratory neurons of the ventrolateral medullary regions was studied under conditions of blocking of synaptic transmission. The experiments were carried out on superfusedin situ semi-isolated medullo-spinal preparations (SIMSP) of newborn (1st day of life) and 4- to 5-day-old rats. Part of the pre-inspiratory and (to a somewhat lesser extent) expiratory neurons of newborn rats appeared most resistive to superfusion of preparations with a low-Ca²⁺ (0.2 mM) and Mg²⁺-rich (5.0 mM) solution. Spike activity in some neurons of these groups was preserved up to 40 and 25 min, respectively, after mass inspiratory discharges in then. phrenicus had disappeared. Similar neurons in 4- to 5-day-old SIMSP were less resistive. Inspiratory neurons in animals of both age groups demonstrated no pacemaker properties. Coagulation of the regions where pre-inspiratory neurons are localized (the retrofacial zone) did not evoke irreversible blockade of respiratory rhythm in all SIMSP of 4- to-5-day-old rats and in most SIMSP of newborn animals. At the same time, coagulation of the zone where inspiratory neurons are concentrated (the pre-Bötzinger complex) resulted in the blockade of respiratory rhythm in all SIMSP, with no exceptions.Neirofiziologiya/Neurophysiology, Vol. 28, No. 6, pp. 273–284, November–December, 1996.     

Two types of P neurons in the medullary dorsal respiratory cellular group in cats

Electrophysiological properties of P neurons localized in the medullary dorsal respiratory cellular group and of vagal afferent fibers innervating these neurons were studied in acute experiments on nembutal-anesthetized cats with preserved spontaneous respiration. P neurons were shown to form a non-homogeneous cellular population. They generated phasic discharges during the whole inspiration period, but differed in their responses to lung inflation. These findings allowed us to classify P neurons as slowly adapting and rapidly adapting units, probably activated by slowly and rapidly adapting pulmonary receptors, respectively. Sensitivity of the slowly adapting P neurons to activation by the corresponding receptors and the mechanisms underlying the participation of the two types of P neurons in the reflex feedback between the respiratory center and lungs are discussed.Neirofiziologiya/Neurophysiology, Vol. 26, No. 3, pp. 211–217, May–June, 1994.     

Autophagy in neurons: a review

Macroautophagy is a process of regulated turnover of cellular constituents that occurs during development and under conditions of stress such as starvation. Defects in autophagy have serious consequences, as they have been linked to neurodegenerative disease, cancer, and cardiomyopathy. This process, which exists in all eukaryotic cells, is tightly controlled, but in extreme cases results in the death of the cell. While major insights into the molecular and biochemical pathways involved have come from genetic studies in yeast, little is known about autophagic pathways in mammalian cells, particularly in neurons. Recently, research in neuronal culture models has begun to identify some characteristics of neuronal macroautophagy. The results suggest that macroautophagy in neurons may provide a neuroprotective mechanism. Here, we review the defining characteristics of autophagy with special attention to its role in neurodegenerative disorders, and recent efforts to delineate the pathway of autophagic protein degradation in neurons.     

Parameters of discharges of respiratory neurons of the ventrolateral medullary regions in early postnatal rats

In experiments on superfusedin situ semi-isolated medullo-spinal preparations (SIMSP) of newborn (1st day of life) and 4- to 5-day-old rats, we studied the parameters of extracellularly recorded spike activity of respiratory neurons of the ventrolateral medullary regions (VLMR). In SIMSP of 4- to 5-day-old rats, the frequency of discharges of pre-inspiratory, inspiratory, and expiratory neurons is shown to be significantly higher, while the dispersion of its values is considerably lower, as compared with the corresponding values for newborn animals. In the majority of pre-inspiratory and inspiratory neurons of SIMSP of newborn rats, irregular low-frequency discharges are usually generated within the interinspiration phase. The relative intensity of suppression of discharges of pre-inspiratory and expiratory neurons within an inspiration phase is much lower in SIMSP of newborn rats, as compared with that in 4- to 5-day-old preparations. The activity of most pre-inspiratory neurons manifests a trend toward transformation from a two-phase pattern in newborn rats (with two frequency peaks, pre- and post-inspiratory) to a monophasic pattern (with one pre-inspiratory frequency peak) typical of 4- to 5-day-old animals. The effects of electrical stimulation of the site of localization of pre-inspiratory neurons showed that in SIMSP of both age groups of rats an inspiratory response could be evoked in then. phrenicus only in the case when stimulation was applied within the second half of an interinspiratory phase. Therefore, it can be supposed that the respiratory network in newborn animals is to a considerable extent immature in the morphofunctional aspect. It seems probable that in early postnatal rats pre-inspiratory neurons are involved in the medullary mechanisms foron-off switching of the inspiratory and expiratory phases.Neirofiziologiya/Neurophysiology, Vol. 28, No. 4/5, pp. 207–217, July–October, 1996.     

The histological localization of DNA in rabbit medullary neurons

The nuclei of unfixed isolated rabbit neurons cleared on incubation with DNAse (10 mg/ml), but not RNAse (10 mg/ml). The nuclei stained for DNA with eight chromosomal or nuclear stains more intensely than the cytoplasm, and less intensely after treatment with DNAse (10 mg/ml). On the other hand, when the whole tissue was embedded and sectioned, DNA did not appear to be stained in the nucleus; the nucleolus and the cytoplasm were more heavily stained than the nucleoplasm. Possible explanations for this apparent anomaly are considered. It was concluded that DNA diffused out of the nucleus during embedding and sectioning, and that the colouration of the nucleolus and cytoplasm with the eight staining systems used was due to other nucleotides present.     

延髓嘴端包氏复合体与中枢呼吸节律形成

包氏复合体是一群位于延髓嘴端面神经后核腹内侧的呼气神经元。该群神经元为抑制性,轴突广泛投射到脑干呼吸相关结构及膈神经神经元群。本研究室动用微电刺激、逆行兴奋、ＷＧＡ－ＨＲＰ标记、微量注射等技术研究了该结构在呼吸时相转换及呼吸节律形成中的作用。结果显示：包氏复合体怀脑干呼吸神经元群之间存在双向纤维联系,该结构参与吸气向呼气以及呼气向吸气的时相转换,且在呼气相形成中起重要作用。     

NADPH-Diaphorase-containing neurons in the medullary structures involved in generation of the respiratory activity in neonatal rats

Using a histochemical technique, we examined distribution of the neurons containing a marker of nitric oxide synthase (NOS), NADPH-diaphorase (NADPH-d), on frontal slices of the medulla and upper cervical spinal segments of 4-day-old rats. It was demonstrated that NADPH-d-positive cells are present within the dorsal and ventral medullary respiratory groups. The highest density of the labeled middle-size multipolar neurons (27.9±2.6 cells per 0.1 mm² of the slice) was observed in the rostral part of the ventral respiratory group, within the reticular lateral paragigantocellular nucleus. Similar NADPH-d-positive neurons were also observed in other reticular formation structures: rostroventrolateral reticular, gigantocellular, and ventral medullary nuclei, and in the ventral part of the paramedial nucleus. There were no labeled neurons in the lateral reticular nucleus. Single small and medium-size labeled neurons were found at all rostro-caudal levels of thenucl. ambiguous (nuclei retrofacialis, ambiguous, andretroam-biguous). Groups of NADPH-d-positive neurons were also revealed within the dorsal respiratory group, along the whole length of thenucl. tractus solitarii (mostly in its ventrolateral parts). Single labeled neurons were also observed in thenucl. n. hypoglossi, and their groups were observed in the dorsal motor part of thenucl. n. vagus. Involvement of the structures containing NADPH-d-positive neurons in the processes related to generation of the respiratory activity is discussed. Our neuroanatomical experiments prove that in early postnatal mammals NO is actively involved in generation and regulation of the medullary respiratory rhythm. Neirofiziologiya/Neurophysiology, Vol. 32, No. 2, pp. 128–136, March–April, 2000.     

Mu-opioid receptor agonist effects on medullary respiratory neurons in the cat: evidence for involvement in certain types of ventilatory disturbances

Mu-opioid receptor agonists depress tidal volume, decrease chest wall compliance, and increase upper airway resistance. In this study, potential neuronal sites and mechanisms responsible for the disturbances were investigated, dose-response relationships were established, and it was determined whether general anesthesia plays a role. Effects of micro-opioid agonists on membrane properties and discharges of respiratory bulbospinal, vagal, and propriobulbar neurons and phrenic nerve activity were measured in pentobarbital-anesthetized and unanesthetized decerebrate cats. In all types of respiratory neurons tested, threshold intravenous doses of the micro-opioid agonist fentanyl slowed discharge frequency and prolonged duration without altering peak discharge intensity. Larger doses postsynaptically depressed discharges of inspiratory bulbospinal and inspiratory propriobulbar neurons that might account for depression of tidal volume. Iontophoresis of the micro-opioid agonist DAMGO also depressed the intensity of inspiratory bulbospinal neuron discharges. Fentanyl given intravenously prolonged discharges leading to tonic firing of bulbospinal expiratory neurons in association with reduced hyperpolarizing synaptic drive potentials, perhaps explaining decreased inspiratory phase chest wall compliance. Lowest effective doses of fentanyl had similar effects on vagal postinspiratory (laryngeal adductor) motoneurons, whereas in vagal laryngeal abductor and pharyngeal constrictor motoneurons, depression of depolarizing synaptic drive potentials led to sparse, very-low-frequency discharges. Such effects on three types of vagal motoneurons might explain tonic vocal fold closure and pharyngeal obstruction of airflow. Measurements of membrane potential and input resistance suggest the effects on bulbospinal Aug-E neurons and vagal motoneurons are mediated presynaptically. Opioid effects on the respiratory neurons were similar in anesthetized and decerebrate preparations.     

Function of different medullary neurons in respiration]

1. We have studied the activity of 162 medullary respiratory neurones in the "encephale isole bas" cat. These neurones were classed into three groups : bulbospinal inspiratory (NBSI : 39) or expiratory (NBSE : 15) neurones whose axons enter the spinal cord ; inspiratory or expiratory laryngeal motoneurones (MLI : 17; MLE : 10) antidromically activated by vagus nerve stimulation ; propriobulbar inspiratory (NPBI : 59) or expiratory (NPBE : 22) neurones whose axons lie perhaps entirely within the medulla. 2. Correlation coefficients between number of spikes delivered in each burst and the duration of the corresponding respiratory phase (inspiration for NBSI, MLI, NPBI ; expiration for NBSE, MLE, NPBE) have been calculated for each neurone. 3. The activity of most of the NBSI and MLI is significantly correlated with the duration of the inspiration. These two groups of neurones are probably homogenous. 4. On the basis of this correlation test, NPBI do not constitute an homogeneous population ; 50% of NPBI are not significantly correlated. The same results are obtained if correlations are calculated between the number of spikes delivered and the amplitude of integrated phrenic nerve acitivty. According to the discharge pattern and correlation test, we can consider three groups of NPBI : early recruited neurones with decreasing frequency and non significantly correlated activity (23,7%); early and late neurones with increasing frequency and significantly correlated activity (32,2%); early and late neurones with increasing frequency and non significantly correlated acitivty (44,1%). 5. The activity of most of the NBSE and NPBE with increasing frequency is significantly correlated with the duration of the expiration. Among the MLE and NPBE with a decreasing frequency, a great number of neurones are not significantly correlated. 6. The functional significantion of the different neuronal types is discussed from these correlation tests and from the pattern of activity and axonal pathways.     

Burden of allergic respiratory disease: a systematic review

This meta-analysis compared the health-related quality of life (HRQL) of patients with allergic rhinitis (AR) and/or allergic asthma (AA) caused by perennial house dust mite (HDM) versus AR and/or AA caused by seasonal pollen allergy. Following a systematic search, the identified studies used the disease-specific rhinitis quality of life questionnaire or generic instruments (SF-36 and SF-12). Summary estimates obtained by meta-analysis showed that HRQL in patients with perennial HDM allergy was significantly worse than that of patients with seasonal pollen allergy, when measured by both disease-specific and generic HRQL instruments, and was reflected by an impact on both physical and mental health. A systematic review of cost data on AR and AA in selected European countries demonstrated that the majority of the economic burden was indirectly caused by high levels of absenteeism and presenteeism; there was little or no evidence of increasing or decreasing cost trends. Increased awareness of the detrimental effects of AR and/or AA on patients’ HRQL and its considerable cost burden might encourage early diagnosis and treatment, in order to minimize the disease burden and ensure beneficial and cost-effective outcomes.     

Ventrolateral medullary respiratory network and a model of cough motor pattern generation

The primaryhypothesis of this study was that the cough motor pattern is produced,at least in part, by the medullary respiratory neuronal network inresponse to inputs from "cough" and pulmonary stretch receptorrelay neurons in the nucleus tractus solitarii. Computer simulations ofa distributed network model with proposed connections from the nucleustractus solitarii to ventrolateral medullary respiratory neuronsproduced coughlike inspiratory and expiratory motor patterns. Predictedresponses of various "types" of neurons (I-DRIVER, I-AUG, I-DEC,E-AUG, and E-DEC) derived from the simulations were tested in vivo.Parallel and sequential responses of functionally characterizedrespiratory-modulated neurons were monitored during fictive cough indecerebrate, paralyzed, ventilated cats. Coughlike patterns in phrenicand lumbar nerves were elicited by mechanical stimulation of theintrathoracic trachea. Altered discharge patterns were measured in mosttypes of respiratory neurons during fictive cough. The resultssupported many of the specific predictions of our cough generationmodel and suggested several revisions. The two main conclusions were asfollows: 1) TheBötzinger/rostral ventral respiratory group neurons implicated inthe generation of the eupneic pattern of breathing also participate inthe configuration of the cough motor pattern.2) This altered activity ofBötzinger/rostral ventral respiratory group neurons istransmitted to phrenic, intercostal, and abdominal motoneurons via thesame bulbospinal neurons that provide descending drive during eupnea.