Diaphragmatic and external intercostal muscle control during vomiting: behavior of inspiratory bulbospinal neurons

1. The role of dorsal and ventral respiratory group (DRG and VRG) bulbospinal inspiratory (I) neurons in the control of diaphragmatic and external intercostal (inspiratory) muscle activity during vomiting was examined by recording from these neurons during fictive vomiting in decerebrate, paralyzed cats. Fictive vomiting was defined by a characteristic series of bursts of coactivation of phrenic and abdominal muscle nerves, elicited either by electrical stimulation of abdominal vagal afferents or by emetic drugs, which would be expected to produce vomiting if the animals were not paralyzed. 2. Data were recorded from 22 DRG and 29 VRG I neurons that were antidromically activated from the fourth cervical spinal segment (C4). Only 10% (5/51) of these neurons started to fire near the beginning of phrenic discharge during fictive vomiting and thus had the appropriate discharge pattern to contribute to the initial activation of the diaphragm and coactive external intercostal muscles during vomiting. The frequency of occurrence of these Active neurons was not significantly different in the DRG (3/22) and VRG (2/29) (chi 2 test). Most remaining neurons were either totally silent (n = 7) or had only sporadic, infrequent firing (n = 16) (Silent neurons, 23/51 = 45%), or else fired near the end of phrenic discharge during fictive vomiting (End neurons, 21/51 = 41%). Two neurons were categorized as having miscellaneous (Misc) behavior. 3. No differences were found among neurons having different response patterns during fictive vomiting in regard to the following: the manner in which fictive vomiting was elicited: cell location: conduction velocity; and neuronal firing onset, rate, and pattern during respiration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Control of abdominal and expiratory intercostal muscle activity during vomiting: role of ventral respiratory group expiratory neurons

The role of ventral respiratory group (VRG) expiratory (E) neurons in the control of abdominal and internal intercostal (expiratory) muscle activity during vomiting was examined in decerebrate cats by recording from these neurons during fictive vomiting in paralyzed animals and comparing abdominal muscle activity during vomiting before and after sectioning the axons of these descending neurons. Fictive vomiting was defined by a series of bursts of coactivation of abdominal and phrenic nerves elicited by either subdiaphragmatic vagus nerve stimulation or emetic drugs. Such coordinated activity would be expected to produce vomiting if the animals were not paralyzed. Data were recorded from 27 VRG E neurons that were antidromically activated from the lower thoracic (T13) or lumbar spinal cord. During fictive vomiting, almost two-thirds of these neurons (17/27) were mainly active in between periods of abdominal and phrenic nerve coactivation, when the internal intercostal motoneurons are known to be active. This group of neurons was termed INT neurons. INT neurons were subdivided according to whether they were active between every burst of phrenic and abdominal nerve coactivation (INTa neurons, n = 10) or only between some bursts (INTb neurons, n = 7). Another one-third of the VRG E neurons had normal or increased levels of activity when the abdominal nerves were active during fictive vomiting (ABD neurons). The one remaining neuron was mainly silent throughout fictive vomiting. ABD neurons were indistinguishable from INT neurons on the basis of their location in the VRG, type of firing pattern (ramp versus step ramp), conduction velocity, or extent of projection in the lumbar cord. However, INTa neurons had a significantly higher discharge rate during respiration than either ABD or INTb neurons. Abdominal muscle EMG and nerve activity were recorded from six unparalyzed cats before and after cutting the axons of VRG E neurons as they cross the midline between C1 and the obex. The lesions abolished or almost eliminated expiratory modulation of abdominal muscle activity. In contrast, the abdominal muscles were always active during vomiting; however, the amplitude of postlesion abdominal activity varied from approximately 70-100% of prelesion values in three cats to 60-70% of normal in a fourth animal to only approximately 20% of prelesion values in two other cats.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pontine respiratory group neuron discharge is altered during fictive cough in the decerebrate cat

A network of neurons in the rostral dorsal lateral pons and pons/mescencephalic junction constitute the pontine respiratory group (PRG) and is essential for reflex cough. As a next step in understanding the role of the PRG in the expression of the cough reflex, we examined neuron firing rates during fictive cough in cats. Decerebrated, thoracotomized, paralyzed, cycle-triggered ventilated adult cats were used. Extracellular activity of many single neurons and phrenic and lumbar neurograms were monitored during fictive cough produced by mechanical stimulation of the intrathoracic trachea. Neurons were tested during control periods for respiratory modulation of firing rate by cycle-triggered histograms and statistical tests. Most respiratory modulated cells were continuously active with various superimposed respiratory patterns; major categories included inspiratory decrementing (I-Dec), expiratory decrementing (E-Dec) and expiratory augmenting (E-Aug). There were alterations in the discharge patterns of respiratory, as well as, non-respiratory modulated neurons during cough. The results suggest an involvement of the PRG in the configuration of the cough motor pattern.     

Membrane potential changes of phrenic motoneurons during fictive vomiting, coughing, and swallowing in the decerebrate cat.

1. The patterns of membrane potential changes of phrenic motoneurons were compared during fictive vomiting, fictive coughing, and fictive swallowing in decerebrate, paralyzed cats. These fictive behaviors were identified by motor nerve discharge patterns similar to those recorded from the muscles of nonparalyzed animals. Phrenic motoneurons (n = 54) were identified by antidromic activation from the thoracic phrenic nerve. Intracellular recordings were obtained from 27 motoneurons during fictive vomiting, 40 during fictive coughing, and 27 during fictive swallowing. Sixteen motoneurons were recorded during both fictive coughing and fictive swallowing, eight during both fictive coughing and fictive vomiting, and two during both fictive vomiting and fictive swallowing. Seven motoneurons were studied during all three behaviors. 2. Fictive vomiting, typically evoked by electrical stimulation of abdominal vagal afferents, was characterized by a series of bursts of coactivation of phrenic and abdominal motor nerves, culminating in an expulsion phase in which abdominal discharge was prolonged both with respect to phrenic discharge and to abdominal discharge during the preceding retching phase. During fictive vomiting, phrenic motoneurons depolarized abruptly, and the amplitude of depolarization was significantly greater than during control inspirations. They then repolarized slowly throughout the phrenic burst, rapidly repolarizing at the end of each phrenic burst during retching and reaching a level similar to that observed during expiration. During the expulsion phase, the pattern was initially the same. However, after the cessation of phrenic discharge, the membrane potential repolarized slowly until the end of the abdominal burst, exhibiting greater synaptic noise than during expiration. One phrenic motoneuron, presumably innervating the periesophageal region of the diaphragm, received a strong hyperpolarization just before the onset of the emetic episode and fired for shorter periods during fictive vomiting than did other phrenic motoneurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Swallowing-related activities of respiratory and non-respiratory neurons in the nucleus of solitary tract in the rat

Swallowing-related activity was examined in respiratory (   n = 60  ) and non-respiratory (   n = 82  ) neurons that were located in and around the nucleus of the solitary tract (NTS) in decerebrated, neuromuscularly blocked and artificially ventilated rats. Neurons that were orthodromically activated by electrical stimulation of the superior laryngeal nerve (SLN) were identified, and fictive swallowing was evoked by SLN stimulation. The pharyngeal phase of swallowing was monitored by hypoglossal nerve activity. Two types of non-respiratory neurons with swallowing-related bursts were identified: 'early' swallowing neurons (   n = 24  ) fired during periods of hypoglossal bursts, and 'late' swallowing neurons (   n = 8  ) fired after the end of hypoglossal bursts. The remaining non-respiratory neurons were either suppressed (   n = 21  ) or showed no change in activity (   n = 29  ) during swallowing. On the other hand, respiratory neurons with SLN inputs included 56 inspiratory and four expiratory neurons. Inspiratory neurons were classified into two major types: a group of neurons discharged simultaneously with hypoglossal bursts (type 1 neurons,   n = 19  ), while others were silent during bursts but were active during inter-hypoglossal bursts when swallowing was provoked repetitively (type 2 neurons,   n = 34  ). Three of the expiratory neurons fired during hypoglossal bursts. Many of the swallowing-related non-respiratory neurons and the majority of the inspiratory neurons received presumed monosynaptic inputs from the SLN. Details of the distribution and firing patterns of these NTS neurons, which have been revealed for the first time in a fictive swallowing preparation in the rat, suggest their participation in the initiation, pattern formation and mutual inhibition between swallowing and respiration.     

Respiratory-modulated and phrenic afferent-driven neurons in the cervical spinal cord (C4–C6) of the fluorocarbon-perfused guinea pig

The potential contributions of cervical spinal interneurons to the neural control of respiration have been investigated by extracellularly recording the patterns of activity of neurons in the C4–C6 spinal cord during fictive respiration in the fluorocarbon-perfused, adult guinea pig. Two types of neurons were recorded: respiratory-modulated neurons, whose activity was modulated with respiration, and phrenic-driven neurons, which were excited by electrical stimulation of the phrenic nerve. Respiratory-modulated neurons (n = 20) could be divided into inspiratory, expiratory, and phase-spanning neurons, based on their patterns of activity during fictive respiration. Respiratory-modulated neurons showed varying dependencies on the type of breathing; during spontaneous augmented breaths, one-half exhibited patterns of activity that were significantly different to those seen during normal, fictive respiration, whereas the other half of the respiratory-modulated neurons showed similar patterns of activity during both normal and augmented breaths. Phrenic-driven neurons (n = 22) could be divided into short-latency (7–12 ms), moderate-latency (12–25 ms), and inhibited neurons, but were only occasionally and weakly modulated with respiration. The results suggest that respiratory-modulated C4–C6 spinal neurons may contribute to the neural control of respiration, with different subpopulations specialized for different types of respiratory tasks, and that phrenic-driven neurons may be interposed in sensory or reflex pathways, such as the spinothalamic tract or phrenic-to-phrenic inhibitory reflex.     

Converse motor output of inspiratory bulbospinal premotoneurones during vomiting

Intracellular recordings of the activities of 10 inspiratory bulbospinal neurones of the medulla were performed in decerebrate cats. Fictive vomiting was induced by repetitive stimulation of the supra-diaphragmatic vagus nerves and was defined by series of synchronous large bursts in both the phrenic (inspiratory) and abdominal (expiratory) nerves. During these synchronous bursts the inspiratory bulbospinal neurones of both the dorsal and ventral respiratory groups were strongly hyperpolarized by chloride-dependent inhibitory postsynaptic potentials (IPSPs). We concluded that during vomiting the central pattern generator is inhibited, and that another pattern generator drives the spinal respiratory motoneurones.     

Respiratory interneurons of the lower cervical (C4-C5) cord: membrane potential changes during fictive coughing,vomiting, and swallowing in the decerebrate cat

The possible roles of interneurons in the C4-C5 cervical spinal cord in conveying central drives to phrenic motoneurons during different behaviour patterns were investigated using intracellular recordings in decerebrate, paralysed, artificially ventilated cats. Eleven cells were tentatively classified as respiratory interneurons since they: (i) could not be antidromically activated from the ipsilateral whole intrathoracic phrenic nerve, and (ii) exhibited large membrane potential changes during eupnea (7.3 mV±3.6, range 2–13.5 mV) or non-respiratory behaviour patterns. Six neurons depolarized in phase with phrenic discharge; four others depolarized during the expiratory phase; one neuron exhibited depolarization during the end of both expiration and inspiration. A variety of responses was observed during fictive coughing, vomiting, and swallowing. The results are consistent with C4-C5 expiratory interneurons conveying inhibition to phrenic motoneurons during different behaviour patterns. The responses of inspiratory and multiphasic neurons suggest that the roles of these interneurons are mode complex than simply relaying central excitatory or inhibitory drive to phrenic motoneurons.     

Convergence of central respiratory and locomotor rhythms onto single neurons of the lateral reticular nucleus

We have analyzed the behavior of neurons of the lateral reticular nucleus (LRN) during fictive respiration and locomotion and found that some LRN neurons have both central respiratory and locomotor rhythms. Experiments were conducted on decrebrate, decerebellate, immobilized, and artificially ventilated cats, with the spinal cord transected at the lower thoracic cord. Fictive respiration and fictive forelimb locomotion were ascertained by monitoring activities from the phrenic nerve and forelimb extensor and flexor nerves, respectively. Fictive locomotion was evoked by electrical stimulation of the mesencephalic locomotor region (MRL) or sometimes occurred spontaneously. During fictive locomotion many LRN neurons fired in certain phases of the locomotion cycle; i.e., with respect to the nerve discharge of the ipsilateral forelimb they fired in either the extensor, flexor, extensor-flexor, or flexor-extensor phase. Firing of some LRN neurons was modulated synchronously with central respiratory rhythm. Neurons with inspiratory activity and those with expiratory activity were both found. More than half of these respiration-related LRN neurons had locomotor rhythm as well. The majority of the three types of LRN neurons, i.e., neurons with only locomotor rhythm, those with only respiratory rhythm, and those with both respiratory and locomotor rhythm, were antidromically activated by electrical stimulation of the ipsilateral inferior cerebellar peduncle. Electrical stimulation of the upper cervical cord showed that these LRN neurons, not only locomotion-related but also respiration-related neurons, received short latency inputs from the spinal cord. The LRN neurons studied were distributed widely in the LRN, relatively densely in the caudal two-thirds of the nucleus. No particular differences were detected between the three types of LRN neurons with respect to their location in the nucleus. These results indicate that the information about central respiratory and locomotor rhythms that is necessary for cerebellar control of the coordination between respiration and locomotion converges, at least partly, at the level of the LRN.     

Activity of Brainstem Respiratory Neurones just before the Expiration-Inspiration Transition in the Rat

Inspiratory activity of the hypoglossal nerve (XIIn) often precedes that of the phrenic nerve (PHRn). By manipulating artificial respiration, this preceding activity (pre-I XIIn activity) can be lengthened or isolated prematurely (decoupled XIIn activity) without developing into overt PHRn-associated inspiratory bursts. We hypothesized that these pre-I and decoupled XIIn activities, collectively termed 'XIIn-w/o-PHRn activity', reflect certain internal states of the respiratory centre at the period just prior to the transition from the expiratory phase to the inspiratory phase. In decerebrate, neuromuscularly blocked and artificially ventilated rats, the firing properties of medullary respiratory neurones were examined during the period of the XIIn-w/o-PHRn activity. The majority of the inspiratory neurones examined could be classified into two types: one was active (XIIn-type) and the other was inactive (PHRn-type) during the XIIn-w/o-PHRn period. On the other hand, augmenting expiratory (E-AUG) neurones of the Bötzinger complex (BOT) and the caudal ventral respiratory group (VRG) fired intensively during this period. Their firing stopped at the onset of the overt inspiratory bursts in the XIIn and PHRn, suggesting that BOT E-AUG neurones inhibit PHRn-type, but not XIIn-type, inspiratory neurones. We hypothesize that XIIn-type inspiratory activity facilitates the phase change from expiration to inspiration, through activation of certain inspiratory neurones that inhibit the firing of BOT E-AUG neurones and generation of the overt inspiratory bursts in XIIn-type and PHRn-type inspiratory neurones.     

Non-respiratory neurons in the B?tzinger complex exhibiting appropriate firing patterns to generate the emetic act in dogs.

This work was performed to prove the hypothesis that the pattern generator for the emetic act exists in the B?tzinger complex (BOT) and is driven by vagal afferents via the subpostrema portion of the nucleus of the solitary tract (mNST). Non-respiratory neurons (78) intermingling with BOT respiratory neurons in decerebrate dogs responded to pulse train stimulation of vagal afferents with a mean latency of 387 ms. During retching induced by vagal stimulation, one-half of the non-respiratory neurons exhibited high frequency burst firings synchronous with each retch (SH-firing, SH-neurons) and one-third of these neurons showed similar firings synchronous with the periods between retches (BH-firing, BH-neurons). Two-thirds of the SH-neurons and one-half of the BH-neurons fired with gradually augmenting frequencies (augmenting firing) during the period prior to retching, which may correspond to the period of prodromal signs of vomiting. Three SH-neurons were observed at fictive expulsion: all 3 exhibited burst firings concomitant with expulsion. During cooling block of transmission in the mNST, stimulation of the vagus nerve ipsilateral to the cooling failed to induce not only retching but also augmenting firing and SH-firing in all 11 BOT SH-neurons observed. In contrast, contralateral vagal stimulation induced retching and neuronal firings which had been observed before the cooling. These results support the hypothesis mentioned above. Respiratory firings were changed during retching in all BOT respiratory neurons observed. Respiratory firings were depressed during retching in the majority (15/25) of inspiratory (I) neurons and in a few expiratory (E) neurons (6/45). SH-firing was exhibited by 3 I- and 13 E-neurons. A few (2) I- and half (23) E-neurons showed BH-firing. These results indicate that all BOT respiratory neurons participate in central patterning of the emetic act.     

Blockade of brain stem gap junctions increases phrenic burst frequency and reduces phrenic burst synchronization in adult rat

Recent investigations have examined the influence of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in vitro using transverse medullary slice and en bloc brain stem-spinal cord preparations obtained from neonatal (1-5 days postnatal) mice. Gap junction proteins, however, have been identified in both neurons and glia in brain stem regions implicated in respiratory control in both neonatal and adult rodents. Here, we used an in vitro arterially perfused rat preparation to examine the role of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents. We recorded rhythmic inspiratory motor activity from one or both phrenic nerves before and during pharmacological blockade (i.e., uncoupling) of brain stem gap junctions using carbenoxolone (100 microM), 18alpha-glycyrrhetinic acid (25-100 microM), 18beta-glycyrrhetinic acid (25-100 microM), octanol (200-300 microM), or heptanol (200 microM). During perfusion with a gap junction uncoupling agent, we observed an increase in the frequency of phrenic bursts (~95% above baseline frequency; P < 0.001) and a decrease in peak amplitude of integrated phrenic nerve discharge (P < 0.001). The increase in frequency of phrenic bursts resulted from a decrease in both T(I) (P < 0.01) and T(E) (P < 0.01). In addition, the pattern of phrenic nerve discharge shifted from an augmenting discharge pattern to a "bell-shaped" or square-wave discharge pattern in most experiments. Spectral analyses using a fast Fourier transform (FFT) algorithm revealed a reduction in the peak power of both the 40- to 50-Hz peak (corresponding to the MFO) and 90- to 110-Hz peak (corresponding to the HFO) although spurious higher frequency activity (> or =130 Hz) was observed, suggesting an overall loss or reduction in inspiratory-phase synchronization. Although additional experiments are required to identify the specific brain stem regions and cell types (i.e., neurons, glia) mediating the observed modulations in phrenic motor output, these findings suggest that gap junction communication modulates generation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents in vitro.     

Pre-Bötzinger complex functions as a central hypoxia chemosensor for respiration in vivo

Recently, we identified a region located in the pre-B?tzinger complex (pre-B?tC; the proposed locus of respiratory rhythm generation) in which activation of ionotropic excitatory amino acid receptors using DL-homocysteic acid (DLH) elicits a variety of excitatory responses in the phrenic neurogram, ranging from tonic firing to a rapid series of high-amplitude, rapid rate of rise, short-duration inspiratory bursts that are indistinguishable from gasps produced by severe systemic hypoxia. Therefore we hypothesized that this unique region is chemosensitive to hypoxia. To test this hypothesis, we examined the response to unilateral microinjection of sodium cyanide (NaCN) into the pre-B?tC in chloralose- or chloralose/urethan-anesthetized vagotomized, paralyzed, mechanically ventilated cats. In all experiments, sites in the pre-B?tC were functionally identified using DLH (10 mM, 21 nl) as we have previously described. All sites were histologically confirmed to be in the pre-B?tC after completion of the experiment. Unilateral microinjection of NaCN (1 mM, 21 nl) into the pre-B?tC produced excitation of phrenic nerve discharge in 49 of the 81 sites examined. This augmentation of inspiratory output exhibited one of the following changes in cycle timing and/or pattern: 1) a series of high-amplitude, short-duration bursts in the phrenic neurogram (a discharge similar to a gasp), 2) a tonic excitation of phrenic neurogram output, 3) augmented bursts in the phrenic neurogram (i.e., eupneic breath ending with a gasplike burst), or 4) an increase in frequency of phrenic bursts accompanied by small increases or decreases in the amplitude of integrated phrenic nerve discharge. Our findings identify a locus in the brain stem in which focal hypoxia augments respiratory output. We propose that the respiratory rhythm generator in the pre-B?tC has intrinsic hypoxic chemosensitivity that may play a role in hypoxia-induced gasping.     

Dopamine1 receptor agonists reverse opioid respiratory network depression, increase CO2 reactivity

In adult pentobarbital-anesthetized and unanesthetized decerebrate cats, the D(1)R agonists (6-chloro-APB, SKF-38393, dihydrexidine) given intravenously restored phrenic nerve and vagus nerve respiratory discharges and firing of bulbar post-inspiratory neurons after the discharges were abolished by the micro-opioid receptor agonist fentanyl given intravenously. Reversal of opioid-mediated discharge depression was prevented by the D(1)R antagonist SCH23390. Iontophoresis of the micro-opioid receptor agonist DAMGO depressed firing of medullary bulbospinal inspiratory neurons. Co-iontophoresis of SKF-38393 did not restore firing and had no effect on bulbospinal inspiratory neuron discharges when applied alone. The D(1)R agonists given intravenously prolonged and intensified phrenic nerve and bulbospinal inspiratory neuron discharges. They also increased reactivity to CO(2) by lowering the phrenic nerve apnea threshold and shifting the phrenic nerve-CO(2) response curve to lower et(CO(2)) levels. Intravenous fentanyl on the other hand decreased CO(2) reactivity by shifting the phrenic nerve apnea threshold and the response curve to higher et(CO(2)) levels. Fentanyl effects on reactivity were partially reversed by D(1)R agonists.     

Foreword

High frequency spinal cord stimulation (HF-SCS) is a method of inspiratory muscle activation resulting in phrenic motoneuron activation via stimulation of spinal cord pathways. The specific pathways mediating this response, however, are unknown. The aim of this study was to assess the potential role of upper cervical (C1–C4) pre-phrenic interneurons (UCI) and localize the pathways in the thoracic spinal cord mediating activation of phrenic motoneurons during HF-SCS. In 7 anesthetized, spinalized (C1 level) dogs, HF-SCS was applied at the T2 level. Diaphragm EMG, inspired volume and airway pressure generation were monitored before and following sequential spinal cord sections at the C4 and C8 levels. Section at the C4 level and dorsal columns at C8 resulted in no significant changes. However, lateral funiculi section (C8 level) resulted in significant reductions in each parameter. We conclude that during upper thoracic HF-SCS, the phrenic motoneuron pools are activated via spinal pathways located in the lateral funiculus but UCI are not involved.     

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.     

Patterns of phrenic motor output evoked by chemical stimulation of neurons located in the pre-Bötzinger complex in vivo

The pre-B?tzinger complex (pre-B?tC) has been proposed to be essential for respiratory rhythm generation from work in vitro. Much less, however, is known about its role in the generation and modulation of respiratory rhythm in vivo. Therefore we examined whether chemical stimulation of the in vivo pre-B?tC manifests respiratory modulation consistent with a respiratory rhythm generator. In chloralose- or chloralose/urethan-anesthetized, vagotomized cats, we recorded phrenic nerve discharge and arterial blood pressure in response to chemical stimulation of neurons located in the pre-B?tC with DL-homocysteic acid (DLH; 10 mM; 21 nl). In 115 of the 122 sites examined in the pre-B?tC, unilateral microinjection of DLH produced an increase in phrenic nerve discharge that was characterized by one of the following changes in cycle timing and pattern: 1) a rapid series of high-amplitude, rapid rate of rise, short-duration bursts, 2) tonic excitation (with or without respiratory oscillations), 3) an integration of the first two types of responses (i.e., tonic excitation with high-amplitude, short-duration bursts superimposed), or 4) augmented bursts in the phrenic neurogram (i.e., eupneic breath ending with a high-amplitude, short-duration burst). In 107 of these sites, the phrenic neurogram response was accompanied by an increase or decrease (>/=10 mmHg) in arterial blood pressure. Thus increases in respiratory burst frequency and production of tonic discharge of inspiratory output, both of which have been seen in vitro, as well as modulation of burst pattern can be produced by local perturbations of excitatory amino acid neurotransmission in the pre-B?tC in vivo. These findings are consistent with the proposed role of this region as the locus for respiratory rhythm generation.     

Opposing effects on the phrenic motor pathway attributed to dopamine-D1 and -D3/D2 receptor activation

Previous in vivo studies revealed that dopamine-D1-agonists elevate excitability of ventral respiratory column (VRC) neurons and increase discharge activity in the phrenic motor output through actions in the brainstem. In this in vivo study performed on pentobarbital-anesthetized cats, we show that D1-agonists (SKF-38393, dihydrexidine) given intravenously enhanced discharge activity in VRC inspiratory neurons and the phrenic nerve in two stages; discharge intensity first increased to a peak and then discharge duration increased. Cross-correlation analysis of VRC inspiratory neuron and phrenic nerve discharges showed that both stages increased strength of coupling between medullary inspiratory neurons and the phrenic motoneuron output. Intracellular recording and microiontophoresis experiments indicated that D1-agonists produced their stimulatory effects indirectly through actions on synaptic inputs to VRC inspiratory neurons. Because other laboratories have provided evidence that dopamine acting on other types of receptors depresses respiratory neuron excitability we tested the effects of piribedil, an agonist that activates receptors of the generally depressant D3/D2-dopamine receptor family, on phrenic nerve activity. Piribedil depressed phrenic nerve inspiratory discharge intensity, prolonged discharge duration, slowed burst frequency and slowed rate of action potential augmentation. The effects of piribedil were partially counteracted by intravenous injection of dihydrexidine. We propose that under normal, steady state conditions, D1-receptor-mediated excitatory modulation of phrenic motor output overrides D3/D2-receptor mediated inhibition.     

Nimodipine increases excitability of rabbit CA1 pyramidal neurons in an age- and concentration-dependent manner.

1. Cellular properties were studied before and after bath application of the dihydropyridine L-type calcium channel antagonist nimodipine in aging and young rabbit hippocampal CA1 pyramidal cells in vitro. Various concentrations of nimodipine, ranging from 10 nM to 10 microM, were tested to investigate age- and concentration-dependent effects on cellular excitability. Drug studies were performed on a population of neurons at similar holding potentials to equate voltage-dependent effects. The properties studied under current-clamp conditions included steady-state current-voltage relations (I-V), the amplitude and integrated area of the postburst afterhyperpolarization (AHP), accommodation to a prolonged depolarizing current pulse (spike frequency adaptation), and single action-potential waveform characteristics following synaptic activation. 2. Numerous aging-related differences in cellular properties were noted. Aging hippocampal CA1 neurons exhibited significantly larger postburst AHPs (both the amplitude and the integrated area were enhanced). Aging CA1 neurons also exhibited more hyperpolarized resting membrane potentials with a concomitant decrease in input resistance. When cells were grouped to equate resting potentials, no differences in input resistance were noted, but the AHPs were still significantly larger in aging neurons. Aging CA1 neurons also fired fewer action potentials during a prolonged depolarizing current injection than young CA1 neurons. 3. Nimodipine decreased both the peak amplitude and the integrated area of the AHP in an age- and concentration-dependent manner. At concentrations as low as 100 nM, nimodipine significantly reduced the AHP in aging CA1 neurons. In young CA1 neurons, nimodipine decreased the AHP only at 10 microM. No effects on input resistance or action-potential characteristics were seen. 4. Nimodipine increased excitability in an age- and concentration-dependent manner by decreasing spike frequency accommodation (increasing the number of action potentials during prolonged depolarizing current injection). In aging CA1 neurons, this effect was significant at concentrations as low as 10 nM. In young CA1 neurons, nimodipine decreased accommodation only at higher concentrations (> or = 1.0 microM). 5. We conclude that aging CA1 neurons were less excitable than young neurons. In aging hippocampus, nimodipine restores excitability, as measured by size of the AHP and degree of accommodation, to levels closely resembling those of young adult CA1 neurons. These actions of nimodipine on aging CA1 hippocampal neurons may partly underlie the drug's notable ability to improve associative learning in aging rabbits and other mammals. Reversal of inhibitory postsynaptic potentials (IPSPs) by chloride ion and/or current injections into six motoneurons revealed the presence of inhibition during the period between phrenic bursts during fictive vomiting and also during the final phase of expulsion when phrenic discharge ceased by abdominal discharge continued. 3. Fictive coughing, evoked by repetitive electrical stimulation of superior laryngeal nerve afferents, was characterized by a large phrenic discharge followed immediately by a large abdominal nerve discharge. During fictive coughing, phrenic motoneurons retained their ramplike depolarizations throughout phrenic discharge; however, the amplitude of depolarization was greater than during inspiration. During the subsequent abdominal nerve discharge, the phrenic membrane potential usually underwent an initial rapid, transient hyperpolarization followed by a gradual repolarization associated with increased synaptic noise.(ABSTRACT TRUNCATED AT 400 WORDS)     

Patterns of spontaneous and reflexly-induced activity in phrenic and intercostal motoneurons

Summary The discharge frequencies of 35 single phrenic and 13 inspiratory intercostal motoneurons were recorded in anaesthetised paralysed cats. Chemical stimulation by asphyxia or hypercapnia increased the discharge frequency and number of motoneurons active within each inspiratory discharge without altering the general pattern of respiratory activity, but mechanical stimulation of the epipharynx and electrical stimulation of the pharyngeal branch of the glossopharyngeal nerve caused repetitive bursts of very high frequency (up to 400 impulses/ sec) in inspiratory motoneurons, with disruption of their normal phasic activity. The latency of the motoneuron response to electrical stimulation of the glossopharyngeal nerve ranged from 15–30 msec and varied with respiratory phase, being shorter during spontaneous inspiratory activity.Phrenic motoneurons were divided according to their order of recruitment during inspiratory activity, and the later recruited (high-threshold) units had significantly larger spike amplitudes than motoneurons which discharged throughout inspiration. High-threshold motoneurons also achieved higher maximum discharge frequencies in response to electrical stimulation of the glossopharyngeal nerve, and it is suggested that these properties are important in increasing the tension developed by respiratory muscles near the end of inspiration when there is greater elastic resistance to lung inflation.