Bulbar Respiratory Neurons Research Articles

Activity of Different Populations of Neurons of the Bulbar Respiratory Center of Rats in the Dynamics of Hypobaric Hypoxia

We examined the involvement of different groups of inspiratory and expiratory neurons, INs and ENs, respectively, of the bulbar respiratory center, RC, in the control of respiration under conditions of hypobaric hypoxia. Such conditions were created in a laboratory altitude chamber and corresponded to a “lift” up to 8000 m altitude. At the beginning of such a “lift,” at a pressure corresponding to an altitude of 4000–5000 m, a decrease in the pO2 in the inspired air to 98–85 mm Hg induced increases in the frequency of spiking of the examined neurons. Within a phase of extreme hypoxia, at an “altitude” of 7500 to 8000 m (pO2= 64–58 mm Hg), we observed considerable suppression of the activity of bulbar respiratory neurons. Impulse activity of INs and ENs within different stages of hypoxia demonstrated considerable specificity; INs were characterized by relatively higher resistivity with respect to the action of this factor. Among subgroups of these units, “early” and “full” INs were found to be more resistive with respect to the oxygen insufficiency. After “descent” of the animals and recovery of the normal atmospheric pressure, characteristics of spiking of most bulbar respiratory neurons gradually returned to the initial values.

Regulatory Influences of Hippocampal Fields CA1 and CA3 on Bulbar Respiratory Neurons in Conditions of Hypoxia

The influences of hippocampal fields CA1 and CA3 on the spike activity of respiratory neurons in the bulbar respiratory center were studied in normal conditions and hypoxia. In normoxia, stimulation of hippocampal fields CA1 and CA3 had mainly inhibitory influences. During oxygen insufficiency, the activity responses of respiratory neurons in the bulbar respiratory center to stimulation of these structures were phasic. At the beginning of the “elevation” to an “altitude” of 4000–5000 m, the small decrease in pO2 in inspired air induced an increase in neuron spike activity. On this background, the inhibitory influence of stimulation of hippocampal fields CA1 and CA3 was more marked than in normoxia. At an “altitude” of 7500–8000 m, with severe oxygen deficit, the inhibitory influences of hippocampal fields CA1 and CA3 were minor on the background of sharp suppression of baseline neuron activity in the respiratory center. Comparative analysis of the activities of inspiratory (IN) and expiratory (EN) neurons at different stages of hypoxia revealed marked differences in their behavior, in that IN had relatively greater resistance to hypoxia than EN. Among subgroups of IN, early and complete IN were more resistant to oxygen deficit. After the “descent” of the animals to normal atmospheric pressure, there was gradual recovery of the initial values of both spontaneous phasic neuron activity and responses to stimuli.

Comparative study of effect of cortical nucleus of amygdala and pyriform cortex on activity of bulbar respiratory neurons in cats

Comparative microelectrophysiological study of character and peculiarities of effects of the cortical nucleus of amygdala and of the periamygdalar area of pyriform cortex on impulse activity was performed on the same single functionally identified respiratory medullar neurons. A high reactivity of bulbar respiratory neurons on stimulation is established in both studied limbic structures. There is established the qualitatively different character of their response reactions at stimulation of the cortical amygdala nucleus and the periamygdalar cortex. The cortical amygdala nucleus has been shown to produce on the activity of medullar respiratory neurons both facilitating and inhibitory action with predominance of the activating one (without topographical orderliness). The effect of periamygdalar cortex at stimulation of various parts was characterized by topographic differentiation. The suppressing reactions of neurons in the majority of cases were recorded at stimulation of the rostral area of periamygdalar cortex, whereas the excitatory reactions--at stimulation of its caudal part. Functional organization of respiratory control of the studied limbic system structures is discussed.

Effects of basolateral amygdalar nuclei on neuronal activity in the medullary respiratory center under hypoxia conditions

The effect of stimulation of the basolateral nuclei of the amygdala (ABL) on the impulse activity of respiratory neurons (RNs) of the rat medulla and the respiratory function was studied in the norm and under conditions of oxygen deficiency. Electrical stimulation of the ABL under conditions of normal atmospheric pressure exerted ambivalent effects on bulbar RNs; both activation and inhibition of these neurons were observed, but inhibitory effects noticeably prevailed. Electrical stimulation of the ABL within an initial phase of hypobaric hypoxia corresponding to ascent to a 4,000 to 5,000 m altitude exerted mostly inhibitory effects on the RN activity (similarly to what was observed under normoxia conditions). Stimulation of these nuclei within a phase of intensive hypoxia (7,500 to 8,000 m) evoked no typical responses of such neurons against the background of hypoxic suppression of their activities.

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.

Responses of respiratory neurons in the medulla oblongata to stimulation of the septal nuclei during hypoxia.

Dynamic studies were performed in conditions of hypoxia on the effects of stimulation of the ventral, lateral, and medial nuclei of the septum on the spike activity of bulbar respiratory neurons and respiration. The various phases of hypoxia provided a model experiment over which the overall effects of the septal neurons were summed. Electrical stimulation of these septal nuclei in conditions of normal atmospheric pressure had both facilitatory and inhibitory effects on the spike activity of respiratory neurons in the respiratory center of the medulla oblongata, inhibitory effects being predominant. The ventral nucleus had the most effective inhibitory effect on the activity of respiratory neurons. Electrical stimulation of the septal nuclei in the initial phase (4000-5000 m) of hypoxia, on the background of activation, had a predominantly inhibitory influence on the activity of respiratory neurons. During the phase of severe hypoxia (7500-8000 m), on the background of marked hypoxic suppression of respiratory neuron activity, stimulation of the septal nuclei produced no characteristic changes in the activity of these neurons.

Distribution of μ receptors in the ventral respiratory group neurons; immunohistochemical and pharmacological studies in decerebrate cats

Immunoreactivity for μ receptors was investigated in 21 bulbar respiratory neurons, individually identified by intracellular recording and labeling with neurobiotin. In 14 of these neurons, effects of iontophoresed morphine were examined. Morphine hyperpolarized the membrane and decreased spike discharges in 4/6 augmenting inspiratory (aug-I), 4/5 postinspiratory (post-I) and 3/3 augmenting expiratory (aug-E) neurons. It had no effect on two aug-I and one post-I neurons. Strong immunoreactivity for μ receptor was detected in the soma and dendrites of 5/8 aug-I, 5/7 post-I and 6/6 aug-E neurons. In the remaining three aug-I and two post-I neurons that included cells unresponsive to morphine, weak immunoreactivity was detected only in the dendrites. These results demonstrated wide, but uneven, distribution of μ receptors in bulbar respiratory neurons and suggest their contribution to respiratory depression by opioids.

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.

Effects of a κ-Receptor Agonist U-50488 on Bulbar Respiratory Neurons and Its Antagonistic Action Against the μ Receptor-Induced Respiratory Depression in Decerebrate Cats

The function of κ receptor-mechanisms in bulbar respiratory network was investigated in decerebrate cats. Intravenous injection of U-50488 (0.3 -3.0 mg /kg) dose-dependently decreased the phrenic nerve discharge and shortened inspiration and expiration. U-50488 caused hyperpolarization, and decreased input resistance and the action potential discharge in respiratory neurons. The effects of U-50488 were antagonized by nor-binaltorphimine. DAMGO (0.3 mg/kg,i.v.) decreased the phrenic discharge and prolonged inspiration and expiration. U-50488 partially reversed the respiratory depression induced by DAMGO. These results suggest that the activation of κ receptors by itself depresses the central respiratory activity, while it opposes the μ receptor-mediated respiratory depression.

Effect of penicillin on impulse activity of medullar neurons.

Acute experiments on narcotized rats showed that intracerebral injection of penicillin increased the mean discharge rate in neurons of the bulbar respiratory center and neurons with rhythmic activity in the gigantocellular nucleus of the medulla oblongata. It also increased the number of neurons with high-frequency bursting activity. In most inspiratory neurons penicillin prolonged the discharges and increased the number of spikes in bursts. The population of inspiratory neurons became hyperactive and suppressed activity of expiratory neurons.

Effects of morphine on membrane potentials of bulbar respiratory neurons in cats

Effects of morphine on membrane potentials of bulbar respiratory neurons in cats

GAD-immunoreactivity of bulbar respiratory neurons identified by intracellular recording and labeling in anesthetized rats

GAD-immunoreactivity of bulbar respiratory neurons identified by intracellular recording and labeling in anesthetized rats

Synaptic interactions between respiratory neurons during inspiratory on-switching evoked by vagal stimulation in decerebrate cats.

To elucidate neuronal mechanisms underlying phase-switching from expiration to inspiration, or inspiratory on-switching (IonS), postsynaptic potentials (PSPs) of bulbar respiratory neurons together with phrenic nerve discharges were recorded during IonS evoked by vagal stimulation in decerebrate and vagotomized cats. A single shock stimulation of the vagus nerve applied at late-expiration developed an inspiratory discharge in the phrenic neurogram after a latency of 79+/-11 ms (n = 11). Preceding this evoked inspiratory discharge, a triphasic response was induced, consisting of an early silence (phase 1 silence), a transient burst discharge (phase 2 discharge) and a late pause (phase 3 pause). During phase 1 silence, IPSPs occurred in augmenting inspiratory (aug-I) and expiratory (E2) neurons, and EPSPs in postinspiratory (PI) neurons. During phase 2 discharge, EPSPs arose in aug-I neurons and IPSPs in PI and E2 neurons. These initial biphasic PSPs were comparable with those during inspiratory off-switching evoked by the same stimulation applied at late-inspiration. In both on- and off-switching, phase-transition in respiratory neuronal activities started to arise concomitantly with the phrenic phase 3 pause. These results suggest that vagal inputs initially produce a non-specific, biphasic response in bulbar respiratory neurons, which consecutively activates a more specific process connected to IonS.

Behaviors of bulbar respiratory interneurons during fictive swallowing and vomiting

Behaviors of the same individual medullary respiratory interneurons were examined during both swallowing and vomiting. In 8 decerebrated and paralyzed cats, 18 neurons having either augmenting expiratory (E-AUG), decrementing expiratory (E-DEC), decrementing inspiratory (I-DEC), or constant inspiratory (I-CON) firing patterns were recorded near the most rostral part of the nucleus ambiguus. All neurons exhibited elementary reflexes to single-shock stimulation of the superior laryngeal nerve. During fictive swallowing elicited by superior laryngeal nerve stimulation, all neurons were basically inactive. During fictive vomiting induced by vagal stimulation and/or emetic drugs, all E-AUG and E-DEC neurons tested either were silent or fired weakly between successive retches, whereas I-DEC and I-CON neurons tested exhibited burst activity during the retching and early-expulsion phases. These results indicate that these bulbar respiratory neurons, which may be involved in respiratory rhythmogenesis, are multifunctional neurons that could also be involved in vomiting but not likely in swallowing. (Otolaryngol Head Neck Surg 1999;120:412-8.)

Reaction of bulbar cardiovascular and respiratory neurons to intra-atrial laser irradiation

Unilateral changes in activity of afferent and intercalary cardiovascular neurons were observed during intra-atrial laser irradiation. Most respiratory neurons shortened the burst duration and the number of spikes in it. These changes in neuronal activity seem to play an important role in the antiarrhythmic effect of laser irradiation, preventing discoordination of activity of the bulbar cardiovascular center.

Characterization of respiratory-related neurons in the isolated brainstem of the frog

Using intra- and extracellular recording techniques we examined the spontaneous discharge and membrane properties of respiratory-related neurons in isolated brainstem preparations of the frogs Rana catesbeiana and Rana pipiens that display spontaneous respiratory related activity in vitro. We observed neurons that depolarize during the fictive lung ventilation cycle as well as neurons that depolarize during the non-lung ventilation phase. Respiratory-related neurons demonstrated significant decreases in membrane input resistance during the fictive lung ventilation cycle but showed no evidence of voltage-dependent membrane conductances activated near resting membrane potential. Furthermore, respiratory neurons showed little spike frequency adaptation, their oscillatory activity was not dissociated from the global respiratory motor output following imposed changes in membrane potential, and spontaneous fluctuations in membrane potential were not observed following reversible interruption of respiratory burst activity by application of solutions low in calcium and high in magnesium. Taken together these results suggest that bulbar respiratory neurons in the isolated frog brainstem sampled in our study do not display endogenous bursting characteristics. Rather, they are strongly influenced by synaptic input.

Pharmacological properties of peripherally induced postsynaptic potentials in bulbar respiratory neurons of decerebrate cats

Intracellular recordings of bulbar inspiratory and post-inspiratory neurons, combined with extracellular iontophoresis of antagonists of putative neurotransmitters, were performed in decerebrate cats. Inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of the superior laryngeal nerve or vagus nerve were depressed by bicuculline in all 22 neurons tested, but not modified by strychnine. The non- N-methyl- d-aspartate (NMDA) glutamate antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the neurally evoked excitatory postsynaptic potentials (EPSPs) in 23 out of 26 neurons tested, while the NMDA antagonist dizocilpine had no notable effect. The present results suggest that the peripherally induced IPSPs are mediated through γ-aminobutyric acid (GABA) A receptors and the EPSPs through non-NMDA glutamate receptors in bulbar respiratory neurons.

Effects of iontophoretically applied acetylcholine on membrane potential and synaptic activity of bulbar respiratory neurones in decerebrate cats

Intracellular recordings were made from bulbar respiratory neurones of decerebrate cats, together with iontophoretic application of acetylcholine and its specific antagonists to assess cholinergic mechanisms involved in the central respiratory network in vivo. Of 126 respiratory-related neurones impaled in the ventral respiratory group, iontophoresis of acetylcholine produced depolarization in 67 cells (53%), hyperpolarization in 36 cells (29%), and no effect in the remaining 23 cells (18%). Depolarization occurred predominantly in laryngeal motoneurones ( 31 40 ) and bulbo-spinal neurones ( 4 5 ), while a comparable number of non-antidromically-activated respiratory neurones displayed either depolarization ( 33 81 ) or hyperpolarization ( 31 81 ). Acetylcholine had no significant effect on excitatory and inhibitory postsynaptic potentials in all types of neurones tested. Both depolarizing and hyperpolarizing effects of acetylcholine were antagonized by coiontophoresis of atropine, but not by hexamethonium. Input resistance was increased ( 7 9 ) or unchanged ( 2 9 ) in depolarized cells, whereas it was unaltered in all hyperpolarized cells tested ( n = 6). The present results suggest that the distribution and functions of cholinergic muscarinic receptors are different for the laryngeal and bulbo-spinal types of respiratory neurones and the non-antidromically-activated respiratory neurones in the cat.

Inhibitions mediated by glycine and GABA A receptors shape the discharge pattern of bulbar respiratory neurons

Experiments were performed to identify the glycinergic or GABAergic nature, and the timing of discharge, of the neurons which produce chloride-dependent inhibitions on other bulbar respiratory neurons (RNs) during their silent and active phases. RNs recorded extracellularly in pentobarbital-anesthetized or decerebrate cats, were subjected to iontophoretic applications of glutamate, of the glycine antagonist strychnine, and of the GABA A receptor antagonist bicuculline. Both antagonists induced discharge or increased discharge frequency in restricted parts of the respiratory cycle without affecting the discharge frequency in other parts of the cycle. Strychnine most often elicited activity in late-inspiration and early-expiration, but also in early inspiration and in late expiration. Bicuculline was most often effective throughout the entire discharge period of each neuron with no effect during the silent period, although it also acted selectively during late-inspiration in inspiratory neurons, an effect attributed to GABA A receptor blockade. The convergence of glycinergic afferent inputs during late inspiration and early expiration suggests that glycinergic neurons may play an important role in the inspiratory to expiratory phase transition.

P-461 - Identification of calcium currents in bulbar respiratory neurones of anaethetized cats.

P-461 - Identification of calcium currents in bulbar respiratory neurones of anaethetized cats.