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)     

Behavior of upper cervical inspiratory propriospinal neurons during fictive vomiting

1. The role of upper cervical inspiratory (UCI)-modulated neurons in respiratory muscle control during vomiting was examined by recording the impulse activity of these neurons during fictive vomiting in decerebrate, paralyzed cats. Fictive vomiting was identified by a characteristic series of bursts of coactivation of phrenic and abdominal muscle nerves, elicited either by electrical stimulation of supradiaphragmatic vagal nerve afferents or by emetic drugs, which would be expected to produce expulsion of gastric contents in nonparalyzed animals. 2. Data were recorded from 43 propriospinal UCI neurons, located in the C1-C3 spinal segments near the border of the intermediate gray matter and lateral funiculus, which were antidromically activated with floating pin electrodes placed in the ipsilateral lateral funiculus, usually at T1-T3. Some cells (9/21 tested) were also activated from the upper lumbar cord (L1). During respiration, most neurons (n = 40) had an augmenting discharge pattern during inspiration. In addition, more than one-half (55%) fired tonically during the remainder of the respiratory cycle. About 40% of UCI neurons showed variations in their firing pattern during the noninspiratory portion of respiration. These latter two properties of UCI neurons were not observed in dorsal and ventral respiratory group (DRG and VRG-, respectively) bulbospinal inspiratory (I) neurons previously recorded under similar conditions. 3. During fictive vomiting, the firing pattern of most UCI neurons fell into one of three main categories. More than one-half (53%) were active in phase with bursts of phrenic discharge and were thus classified as Active-type cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of dorsal respiratory group neurons studied with cross-correlation in the decerebrate rat

We examined the role of dorsal respiratory group (DRG) inspiratory neurons as transmitters of respiratory drive to phrenic and intercostal motoneurons and as relays of afferent information to ventral respiratory group (VRG) bulbospinal, inspiratory neurons. Attempts to antidromically activate 76 DRG neurons from the spinal cord at the C7 segment resulted in only 4 (5.3%) successes (3 contralateral, 1 ipsilateral). Cross-correlating DRG neuron discharge with that of the ipsilateral (56) and contralateral (20) phrenic nerve detected common activation peaks in 2 and 3 cases respectively, with no evidence for monosynaptic connections. Cross-correlating DRG neuron discharge with that of bulbospinal, inspiratory VRG neurons found some evidence for interaction. Peaks in 7 of 73 (10%) cross-correlation histograms were attributed to a monosynaptic excitation of DRG neurons by VRG neurons, although a common activation cannot be ruled out; troughs, some with an accompanying peak, in 9 (12.3%) histograms were interpreted as a combined excitation of the DRG neuron and delayed inhibition of the VRG neuron. In addition, 2 cross-correlation histograms showed peaks with latencies and half-amplitude widths consistent with a disynaptic excitation of a DRG neuron by a bulbospinal inspiratory VRG neuron. Cross-correlating the discharge of 57 pairs of DRG inspiratory neurons (6 contralateral) detected common activation peaks in 7 (12.3%) cases (none contralateral) and one case interpreted as evidence for a disynaptic excitation. These findings suggest that the role of the DRG inspiratory neurons in rats differs from that in cats, primarily because they do not act to transmit respiratory rhythmic drive directly to phrenic and intercostal motoneurons. The results offer some support for an excitation of DRG neurons by VRG inspiratory neurons, but no support for a role of DRG inspiratory neurons as mediators of afferent information transfer to VRG bulbospinal inspiratory neurons.     

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)     

Activity of dorsal respiratory group inspiratory neurons during laryngeal-induced fictive coughing and swallowing in decerebrate cats

Membrane potential changes and/or discharges from 36 inspiratory neurons were recorded intracellularly in the dorsal respiratory group (DRG; i.e., the ventrolateral subdivision of the nucleus tractus solitarii) in decerebrate, paralyzed, and ventilated cats. Electrical activities were recorded from both somata (n=10) and axons (n=26). Activities during quiet breathing were compared with those observed during fictive coughing and swallowing evoked by repetitive electrical stimulation of afferent fibers of the superior laryngeal nerve (SLN). These nonrespiratory behaviors were evident in paralyzed animals as characteristic discharge patterns of the phrenic, abdominal, and hypoglossal nerves. Twenty-six neurons exhibiting antidromic action potentials in response to electrical stimuli applied to the cervical (C3–5) spinal cord were classified as inspiratory bulbospinal neurons (IBSNs). These neurons were considered as premotoneurons. The remaining 10 inspiratory neurons (INAA) were not antidromically activated by electrical stimuli applied to either cervical spinal cord or ipsilateral cervical vagus. These neurons are thought to be propriobulbar neurons. We recorded the activity of 31 DRG inspiratory neurons (24 IBSNs and 7 I-NAA) during coughing. All but one (a late-recruited IBSN) discharged a burst of action potentials during the coughing-related phrenic nerve activity. Typically, ramp-like membrane depolarization trajectories and discharge frequencies during coughing were similar to those observed during inspiration. We recorded the activity of 33 DRG inspiratory neurons (23 IBSNs and 10 I-NAA) during swallowing. Most (28/33) neurons were briefly activated, i.e., discharged a burst of action potentials during swallowing, but peak discharge frequency decreased compared with that measured during inspiration. The membrane potentials of nine somata exhibited a brief bell-shaped depolarization during swallowing, the amplitude of which was similar to that observed during inspiration. These results suggest that some inspiratory premotoneurons and propriobulbar neurons of the DRG might be involved in nonrespiratory motor activities, even if clearly antagonistic to breathing (e.g., swallowing). We postulate the existence in the medulla oblongata of adult mammals of neurons exhibiting a functional flexibility.     

Effects of graded focal cold block in the solitary and para-ambigual regions of the medulla in the cat

Unilateral focal cold blocks in the region of the nucleus tractus solitarius and the dorsal respiratory group of neurons, DRG, of anaesthetized cats consistently caused apneustic-type breathing. There was no concomitant change in the initial rate of rise of inspiratory activity. The apneustic prolongation of inspiratory duration, TI, was most pronounced in, but was not confined to, the DRG. The apneustic effects were more marked after vagotomy. In cats with intact vagus nerves being given artificial ventilation, focal cooling at certain sites of the DRG region could produce 'unlocking' of the respiratory rhythm from that of the respiratory pump. At other sites in this region, focal cooling could selectively block the effects of the inspiration-facilitating reflex induced by deflation without blocking the inspiration-inhibiting Hering-Breuer reflex. Unilateral focal cold blocks in the region of the intermediate part of the ventral respiratory group of neurons, VRG, generally caused depression of the rate of rise of inspiratory activity, but almost never apneustic effects. All effects of unilateral focal cooling both in the DRG and VRG were bilaterally symmetrical. No systematic differences between the effects on phrenic and external intercostal inspiratory activity were found in response to focal cooling either of the DRG or VRG suggesting that differential control of phrenic and external intercostal motoneurons is not exerted mainly at the level of these medullary structures. The results suggest that the DRG and VRG areas exert somewhat different effects on the respiratory pattern: DRG appears to be more concerned with integration of vagal and other inputs contributing to the inspiratory off-switch mechanisms which, however, are not confined only to the DRG. The VRG inspiratory mechanisms, on the other hand, appear to be more involved in the gain control of the inspiratory output intensity.     

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.     

Monosynaptic excitation of medullary inspiratory neurons by bulbospinal inspiratory neurons of the ventral respiratory group in the cat

Summary In Nembutal-anesthetized, immobilized, and artificially ventilated cats, we studied the connectivity of medullary collaterals of bulbospinal inspiratory (BS-I) neurons in the ventral respiratory group (VRG). BS-I neurons which projected to the contralateral spinal cord were isolated extracellularly and intracellular recordings were made from the respiratory neurons in the contralateral VRG. The intracellular membrane potentials were averaged using extracellular spikes of the BS-I neurons as triggers (spike-triggered averaging method). Fast-rising and short-lasting depolarizing potentials locked to the triggering spikes were obtained and shown to be unitary EPSPs induced monosynaptically by the medullary collaterals of BS-I neurons. A total of 137 pairs were analyzed and unitary EPSPs were found in 11 pairs. Four BS-I neurons and 7 inspiratory vagal motoneurons received EPSPs from the medullary collaterals of BS-I neurons. These findings suggest that 1) BS-I neurons in the VRG drive medullary motoneurons of accessory respiratory muscles and phrenic or intercostal motoneurons simultaneously, 2) BS-I neurons on both sides synchronize via the excitatory connections, and 3) the augmenting firing pattern of BS-I neurons might partly be produced by this reexcitatory connection within the population of BS-I neurons.     

Connections from upper cervical inspiratory neurons to phrenic and intercostal motoneurons studied with cross-correlation in the decerebrate rat

We examined the synaptic connections from upper cervical inspiratory neurons to phrenic and intercostal motoneurons in decerebrate rats using cross-correlation. Upper cervical inspiratory neurons (n=79) were recorded from the C1 and C2 segments of the spinal cord in 38 vagotomized, paralyzed, ventilated, and decerebrate rats. The neurons were identified by their inspiratory firing pattern and antidromic activation from the ipsilateral spinal cord at C7. Whole-nerve recordings were made using bipolar electrodes from the central cut ends of the C5 phrenic nerve and the external and internal intercostal nerves at various thoracic levels. Cross-correlation histograms were computed between these recordings to detect short time-scale synchronizations indicative of synaptic connections. The 55 cross-correlation histograms computed between the upper cervical inspiratory neurons and the ipsilateral phrenic nerve showed seven (13%) narrow peaks (mean half-amplitude width±SD, 1.09±0.15 ms) at short latencies (mean latency±SD, 1.29±0.26 ms) suggestive of monosynaptic excitation, and four (7%) broader peaks (mean half-amplitude width±SD, 1.50±0.17 ms) at short latencies (mean latency±SD, 1.40±0.24 ms) suggestive of oligosynaptic excitation. Another 14 (25%) cross-correlation histograms displayed a central broad peak (mean half-amplitude width±SD, 1.59±0.23 ms) suggestive of common activation. The eight cross-correlation histograms computed between the upper cervical inspiratory neurons and the contralateral phrenic nerve were featureless. The 77 cross-correlation histograms computed between the upper cervical inspiratory neurons and the internal and external intercostal nerves at various thoracic levels (T2–8) showed no peaks suggestive of synaptic connections. We conclude that some upper cervical inspiratory neurons make monosynaptic and paucisynaptic connections to phrenic motoneurons but not to intercostal motoneurons.     

Characteristic behavior of the respiratory muscles, esophagus, and external anal and urethral sphincters during straining, retching, and vomiting in the decerebrate dog

To ascertain differences in the brainstem pattern generators for straining and retching, discharges of 230 respiratory single motor units from the intercostal nerves and discharges of the nerves to accessory respiratory muscles, the esophagus, and external anal and urethral sphincters were observed during straining, retching, and vomiting (fictive expulsion) in decerebrate and paralyzed dogs. Straining and retching were identified with coactivation of the phrenic nerve and the nerve to the rectus abdominis, which was elicited by distension of the rectum and stomach, respectively. Synchronous discharges with each retch and straining were always exhibited by the phrenic nerve, the nerves innervating the abdominal part of the rectus abdominis and the external anal and urethral sphincters. In contrast, different behaviours were constantly observed in four nerves. 1) The nerves to the serratus dorsalis cranialis was inhibited during straining, but activated synchronously with each retch and vomiting. 2) Vagal pharyngeal branches innervating the upper esophagus and branches of the recurrent nerve innervating the lower part of the cervical esophagus fired synchronously with straining. The esophageal nerves, on the other hand, did not fire with each retch, but did fire strongly between retches. 3) The nerve to the sacrocaudalis dorsalis lateralis discharged concomitantly with straining but did not with retches. Straining and retching behaviors exhibited by the nerves innervating other thoracic accessory inspiratory muscles (the intercartilagineus, rectus thoracis, scalenus) varied from dog to dog. The nerves innervating the thoracic parts of the rectus abdominis and obliquus externus abdominis always discharged synchronously with straining, and discharged with retches in the majority of dogs, but did not in a minority of dogs. The expiratory units from the internal intercostal nerves showed intense discharges synchronously with coactivation in straining (96%), retching (65%), and vomiting (100%). In contrast, none of the inspiratory units from the external intercostal nerves exhibited such intense discharges with straining and retching. These results suggest that the functional difference in straining and retching mainly depends on the differences in the behavior of the serratus dorsalis cranialis and esophagus. The forms of this dependence were discussed.     

Excitation and inhibition of medullary inspiratory neurons by two types of burst inspiratory neurons in the cat

In Nembutal-anesthetized and artificially ventilated cats, we studied the connectivity of burst inspiratory (I) neurons in the B?tzinger complex and the ventral respiratory group (VRG) with spike-triggered averaging methods. Burst I neurons exhibited tonic (I-TON) or decrementing (I-DEC) firing patterns. Spikes of I-TON neurons induced monosynaptic EPSPs in intracellularly recorded I neurons of both the VRG and the dorsal respiratory group (DRG). Spikes of I-DEC neurons induced monosynaptic inhibitory postsynaptic potentials (IPSPs) in both VRG and DRG I neurons.     

Spinal connections of ventral-group bulbospinal inspiratory neurons studied with cross-correlation in the decerebrate rat

We examined the synaptic connections from ventral-group bulbospinal inspiratory neurons to upper cervical inspiratory neurons and phrenic and intercostal motoneurons in decerebrate rats using cross-correlation. Inspiratory neurons were recorded in the medulla (n=28) at the level of the obex and from the upper-cervical segments (C1 and C2) of the spinal cord (n=29) in 18 vagotomized, paralyzed, ventilated, and decerebrated rats. The neurons were identified by their inspiratory firing pattern and antidromic activation from the spinal cord at C7. Whole-nerve recordings were made using bipolar electrodes from the central cut ends of the C5 phrenic nerve and the external and internal intercostal nerves at various thoracic levels. Cross-correlation histograms were computed between these recordings to detect short time scale synchronizations indicative of synaptic connections. Cross-correlation histograms (n=20), computed between the activities of ventral-group bulbospinal inspiratory neurons and the phrenic nerve, all showed peaks (mean half-amplitude width±SD, 1.1±0.3 ms) at short latencies (mean latency±SD, 2.0±0.6 ms) suggestive of monosynaptic excitation. Cross-correlation histograms (n=33), computed between the activities of ventral-group bulbospinal inspiratory neurons and upper-cervical inspiratory neurons, displayed four (12%) peaks (mean halfamplitude width±SD, 0.9±0.1 ms) at short latencies(mean latency±SD, 1.8±0.6 ms) suggestive of monosynaptic excitation, and six (18%) peaks (mean half-amplitude width±SD, 1.4±0.4 ms) at latencies near zero suggestive of excitation fro m a common source. Cross-correlation histograms (n=34), computed between the activities of ventral-group bulbospinal inspiratory neurons and the internal and external intercostal nerves at various thoracic levels (T2-8), showed six (18%) peaks (mean half-amplitude width±SD, 2.5±0.5 ms) at short latency (mean latency±SD, 4.5±1.1 ms) suggestive of oligosynaptic connections. Cross-correlation histograms (n=42) computed between activities of intercostal nerves at various levels of the thoracic spinal cord showed central peaks suggestive of excitation from a common source. Although the size of the peaks decreased with segmental separation, the displacement of the peaks from time zero did not increase with segmental separation (mean displacement±SD, 0.6±0.6 ms) as would be expected if the common excitation resulted from a descending monosynaptic excitation by a source such as the ventral-groupbulbospinal inspiratory neurons. We conclude that all ventral-group bulbospinal inspiratory neurons make monosynaptic connections to phrenic motoneurons, a few make monosynaptic connections to upper-cervical inspiratory neurons, but connections to intercostal motoneurons are made via interneurons.     

Bulbospinal augmenting inspiratory neurons may participate in contractions of the diaphragm during vomiting in decerebrate dogs

Contradictory results have been reported regarding the participation of bulbospinal inspiratory (BSI) neurons in vomiting contractions of the diaphragm in dogs and cats maintained at low and high end-tidal CO₂ levels, respectively. To resolve this discrepancy, effects of CO₂ levels on vomiting activities were observed in 159 BSI-neurons of paralyzed decerebrate dogs. About half of the BSI-neurons exhibited a vigorous burst with each fictive retch regardless of the CO₂ level and 28 of 51 BSI-neurons produced a burst during the early phase of expulsion. These BSI-neurons were not concentrated in any particular area. While these results do not resolve the above contradiction, they do suggest that some BSI-neurons participate in the vomiting contraction of the diaphragm.     

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.     

Discharge patterns of hypoglossal motoneurons during fictive breathing, coughing, and swallowing

We performed a series of experiments to study the intracellular activity of 58 hypoglossal motoneurons (HMs) in decerebrate, paralyzed, and ventilated cats. Changes in membrane potentials (MP) and discharge activities were evaluated during fictive breathing (FB), swallowing (FS), and coughing (FC). FS and FC were elicited by electrical stimulation of the superior laryngeal nerves. FB, FS, and FC all exhibited characteristic discharge patterns of the phrenic, abdominal, pharyngeal branch of the vagus, and hypoglossal nerves. Thirty-nine HMs displayed respiratory modulation, and 19 were nonrespiratory modulated. Nine HMs did not exhibit MP changes during FB, FS, and FC. During FS, 49 HMs exhibited MP changes consisting of depolarization, hyperpolarization or hyperpolarization-depolarization. HMs involved in FS were either respiratory modulated (n = 38) or not (n = 11). Only 20 HMs displayed MP changes and/or discharge activity during FC. All but two HMs fired during the expiratory phase of FC or at the end of this reflex. All HMs involved in FC (n = 20) were also modulated during both FB and FS. Our results suggest that the XII nucleus is functionally divided into common and distinct subsets of HMs based on their spontaneous activities and responses observed during FS and FC. The changes in MP and discharge frequencies observed during the three behaviors also suggest that HMs are driven by specific premotor neurons during FS, whereas a common premotor pathway is involved during FB and FC.     

Multimodal medullary neurons and correlational linkages of the respiratory network.

This study addresses the hypothesis that multiple sensory systems, each capable of reflexly altering breathing, jointly influence neurons of the brain stem respiratory network. Carotid chemoreceptors, baroreceptors, and foot pad nociceptors were stimulated sequentially in 33 Dial-urethan-anesthetized or decerebrate vagotomized adult cats. Neuronal impulses were monitored with microelectrode arrays in the rostral and caudal ventral respiratory group (VRG), nucleus tractus solitarius (NTS), and n. raphe obscurus. Efferent phrenic nerve activity was recorded. Spike trains of 889 neurons were analyzed with cycle-triggered histograms and tested for respiratory-modulated firing rates. Responses to stimulus protocols were assessed with peristimulus time and cumulative sum histograms. Cross-correlation analysis was used to test for nonrandom temporal relationships between spike trains. Spike-triggered averages of efferent phrenic activity and antidromic stimulation methods provided evidence for functional associations of bulbar neurons with phrenic motoneurons. Spike train cross-correlograms were calculated for 6,471 pairs of neurons. Significant correlogram features were detected for 425 pairs, including 189 primary central peaks or troughs, 156 offset peaks or troughs, and 80 pairs with multiple peaks and troughs. The results provide evidence that correlational medullary assemblies include neurons with overlapping memberships in groups responsive to different sets of sensory modalities. The data suggest and support several hypotheses concerning cooperative relationships that modulate the respiratory motor pattern. 1) Neurons responsive to a single tested modality promote or limit changes in firing rate of multimodal target neurons. 2) Multimodal neurons contribute to changes in firing rate of neurons responsive to a single tested modality. 3) Multimodal neurons may promote responses during stimulation of one modality and "limit" changes in firing rates during stimulation of another sensory modality. 4) Caudal VRG inspiratory neurons have inhibitory connections that provide negative feedback regulation of inspiratory drive and phase duration.     

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)     

Behavior of hypoglossal inspiratory premotor neurons during the carbachol-induced, REM sleep-like suppression of upper airway motoneurons

In individuals with compromised upper airway anatomy, genioglossus (GG), the main protruder muscle of the tongue, is an important upper airway dilator which helps prevent upper airway obstructions. During rapid eye movement (REM) sleep, both the tonic and inspiratory-modulated components of GG activity are suppressed in parallel with the characteristic postural atonia. We tested whether the REM sleep-related reduction in the respiratory activity of GG may, in part, result from a reduced inspiratory drive relayed to hypoglossal (XII) motoneurons from their premotor medullary inspiratory neurons. In 15 urethane-anesthetized, paralyzed, vagotomized and artificially ventilated rats, we recorded XII nerve activity and the extracellular activity of medullary inspiratory-modulated neurons antidromically activated with latencies of 0.8 ms +/- 0.3(SD) from within (n = 19) or adjacent to (n = 11) the XII nucleus. Carbachol (10-20 nl, 10 mM), a cholinergic agonist, was microinjected into the dorsomedial pons. Such injections trigger a REM sleep-like state in chronically instrumented, intact animals and, in anesthetized rats, produce respiratory and electrocortical changes similar to those of REM sleep. Following the injections, the respiratory component of XII nerve activity was depressed by 51 +/- 22%, while the mean inspiratory firing rate of the neurons decreased by only 7.4 +/- 13.8% (from 69 +/- 34 Hz to 65 +/- 37 Hz; P < 0.02; n = 30). The activity of ventral respiratory group (VRG) and reticular formation inspiratory neurons with axons within the XII nucleus was reduced by 10 +/- 14% (P < 0.005; n = 19), whereas the activity of neurons located near the VRG that had axons passing below the XII nucleus did not change (n = 5). Thus, the depressant effect of carbachol on medullary inspiratory neurons was slightly more pronounced in reticular formation and VRG cells premotor to XII motoneurons than in other medullary inspiratory cells. For all cells, the magnitude of the decrease of cell activity was not related to the magnitude of depression of XII nerve activity, the simultaneously occurring decrease in respiratory rate or the cell's control firing rate. Since the magnitude of this depressant effect on all cell types was disproportionately small when compared with the depression of XII nerve activity, the REM sleep-like decrease in GG activity must be mainly mediated by non-respiratory premotor pathways.     

Activity of respiratory laryngeal motoneurons during fictive coughing and swallowing

Membrane potential changes and discharges from 28 laryngeal motoneurons were recorded intracellularly in the caudal nucleus ambiguus of decerebrate, paralyzed and ventilated cats. Electrical activities were recorded from 17 expiratory laryngeal motoneurons (ELMs) with maximal depolarizing membrane potential in early expiration, and from 11 inspiratory laryngeal motoneurons (ILMs) with maximal depolarizing membrane potential in inspiration. Activities during breathing were compared with those observed during fictive coughing and swallowing evoked by electrical stimulation of the superior laryngeal nerves. These non-respiratory behaviors were evidenced in paralyzed animals by characteristic discharge patterns of the phrenic, abdominal nerves and pharyngeal branch of the vagus nerve. We recorded the activity of 11 ELMs and 5 ILMs during coughing in which ELMs, but not ILMs, exhibited increased membrane depolarization and discharge frequencies. Membrane depolarization and discharge frequencies of all ELMs were also significantly increased during swallowing. In addition, membrane depolarization of most ELMs (15/17) was preceded by a short-lasting hyperpolarization due to chloride-dependent inhibitory mechanisms occurring at the onset of swallowing. Out of 10 ILMs tested during swallowing, 7 exhibited membrane depolarization, preceded in 5 cases by a short-lasting hyperpolarization. Discharge frequencies of ILMs were significantly reduced during swallowing. The same pattern of phasic activities of ILMs and ELMs was observed during coughing and breathing, suggesting the involvement of similar excitatory pathways in both behaviors. These results imply that the duration of activation and the discharge frequency of neurons of the central generator for breathing that drive laryngeal motoneurons are enhanced during coughing. During swallowing, in addition to central excitatory mechanisms, laryngeal motoneurons are subjected to an initial inhibition of unknown origin. This inhibition probably contributes to the temporal organization of the swallowing motor sequence. Received: 3 December 1998 / Accepted: 26 June 1999     

Presynaptic delta opioid receptors differentially modulate rhythm and pattern generation in the ventral respiratory group of the rat

The specific role of the Delta opioid receptor (DOR), in opioid-induced respiratory depression in the ventral respiratory group (VRG) is largely unknown. Here, we sought to determine (1) the relationship between DOR-immunoreactive (ir) boutons, bulbospinal and functionally identified respiratory neurons in the VRG and (2) the effects of microinjection of the selective DOR agonist, D-Pen 2,5-enkephalin (DPDPE), into different subdivisions of the VRG, on phrenic nerve discharge and mean arterial pressure. Following injections of retrograde tracer into the spinal cord or intracellular labelling of respiratory neurons, in Sprague-Dawley rats, brainstem sections were processed for retrograde or intracellular labelling and DOR-ir. Bulbospinal neurons were apposed by DOR-ir boutons regardless of whether they projected to single (cervical or thoracic ventral horn) or multiple (cervical and thoracic ventral horn) targets in the spinal cord. In the VRG, a total of 24 +/- 5% (67 +/- 13/223 +/- 49) of neurons projecting to the cervical ventral horn, and 37 +/- 3% (96 +/- 22/255 +/- 37) of neurons projecting to the thoracic ventral horn, received close appositions from DOR-ir boutons. Furthermore, DOR-ir boutons closely apposed six of seven intracellularly labelled neurons, whilst the remaining neuron itself possessed boutons that were DOR-ir. DPDPE was microinjected (10 mM, 60 nl, unilateral) into regions of respiratory field activity in the VRG of anaesthetised, vagotomised rats, and the effects on phrenic nerve discharge and mean arterial pressure were recorded. DPDPE depressed phrenic nerve amplitude, with little effect on phrenic nerve frequency in the B?tzinger complex, pre-B?tzinger complex and rVRG, the greatest effects occurring in the B?tzinger complex. The results indicate that the DOR is located on afferent inputs to respiratory neurons in the VRG. Activation of the DOR in the VRG is likely to inhibit the release of neurotransmitters from afferent inputs that modulate the pattern of activity of VRG neurons.