The neuronal determinants of respiratory rhythm

Correlation analysis of respiratory neuron activity in ventrolateral medulla of brainstem-spinal cord preparation isolated from newborn rat

Cross-correlation analysis was used to study functional connections between one inspiratory (I) neuron and another, and between one pre-inspiratory (Pre-I) neuron and another, in 54 brainstem-spinal cord preparations isolated from newborn rats. Pre-I neurons usually fired in the pre- and post-inspiratory phases. Neurons were recorded extracellularly with pairs of microelectrodes placed on the same or opposite sides of the brainstem. Fourteen pairs of Pre-I neurons recorded bilaterally in the rostral ventrolateral medulla (RVL), 14 pairs of ipsilateral Pre-I neurons in the RVL, 14 pairs of bilateral I neurons in the RVL and 12 pairs of ipsilateral I neurons in the ventrolateral medulla were studied. Cross-correlation histograms (CCHs) were computed. Significantly high peak bin counts were detected in 24 of 54 pairs. Peaks on one side of the origin of the CCHs were observed for one pair of ipsilateral Pre-I neurons, four pairs of bilateral I neurons and five pairs of ipsilateral I neurons. These findings suggest mono- or oligo-synaptic excitatory connections between paired neurons or shared inputs. Only one trough suggesting an oligo-synaptic inhibitory connection was evident in a CCH obtained from the pair of bilateral I neurons. This CCH revealed the peak and the trough on opposite sides of the origin, which was consistent with reciprocal excitatory and inhibitory connections between recorded neurons. Peaks on both sides of the origin were observed for three pairs of bilateral I neurons. From auto-correlation analysis and the latencies of these peaks, two of the three CCHs were consistent with reciprocal excitatory connections between recorded neurons, whereas the other CCH suggests shared inputs. Peaks at the origin were observed for two pairs of ipsilateral Pre-I neurons, four pairs of bilateral I neurons and five pairs of ipsilateral I neurons. These results suggest shared inputs. For Pre-I neurons recorded in opposite sides, no significant bin counts were detected. Peaks on one side were detected for three pairs. Present results suggest short-term synchronisation of I neurons, and of Pre-I neurons via excitatory coupling, and the likelihood of comparatively strong interaction between I neurons, which may be important in maintaining the I burst.

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

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

Behavior of VRG neurons during the atonia of REM sleep induced by pontine carbachol in decerebrate cats

The microinjection of carbachol into the pons of acute decerebrate cats elicits a REM sleep-like atonia and a profound suppression of respiratory motoneuronal activity (J. Appl. Physiol., 69 (1990) 2280-2289). To assess whether this suppression is mediated by medullary neurons that provide respiratory drive to motoneurons of the respiratory pump muscles (diaphragm and intercostals), we studied the effect of pontine carbachol on the activity of neurons of the ventral respiratory group (VRG) in decerebrate, vagotomized, paralyzed and artificially ventilated cats. VRG neurons were recorded extracellularly along with the activity of phrenic and intercostal (external and internal) nerves. Both inspiratory (I) and expiratory (E) VRG neurons had incrementing, ramp-like bursts of activity during their firing periods and were not vagal motoneurons. Carbachol produced a depression of the peak firing rate in most (42/57) neurons studied. However, five cells showed no change and ten had an increase in activity in spite of consistent depression at the motoneuronal level. For the total population of cells (34 I and 23 E), the peak firing was reduced to 88.5% +/- 16.3 (S.D.) of control. The simultaneously recorded phrenic activity was reduced to 77.9% +/- 11.5, while inspiratory intercostal activity fell to 63.4% +/- 21.6 and expiratory to 23.2% +/- 21.2 of control. The carbachol-induced changes in peak firing of both I and E cells were quantitatively similar, and positively correlated to changes in peak phrenic activity. Analysis of this correlation suggested that phrenic and intercostal activities will be depressed to some degree by carbachol even when the average VRG cell activity remains unchanged. In addition, our data show that VRG cells may receive a combination of inhibitory and excitatory inputs during the carbachol-induced depression of respiratory motoneurons. Thus, although some disfacilitation from VRG cells may occur, there must be additional inhibitory or disfacilitatory pathways that mediate the decrease in activity of both phrenic and intercostal motoneurons that accompanies the REM sleep-like atonia.

Calbindin-D28K (CaBP28k)-like Immunoreactivity in Ascending Projections

This study concerns the involvement of calbindin-D28K (CaBP28k)-containing neurons in the efferent projections of both the trigeminal nucleus caudalis and the dorsal vagal complex (nucleus of the solitary tract and area postrema) in rats. Recent evidence has shown that these projections are particularly important for the processing of visceroception and/or nociception at central levels. The trigeminal nucleus caudalis has dense projections to both the nucleus of the solitary tract and the parabrachial area; the dorsal vagal complex is intimately connected to the parabrachial area. CaBP28k is a calcium-binding protein the function of which could be a determining factor in controlling the excitability of cells by acting on intrinsic calcium metabolism. CaBP28k content of projections was ascertained using a double labelling approach that combined the retrograde transport of a protein - gold complex to identify projection cells and immunocytochemistry to identify CaBP28k-positive cells. The trigeminal nucleus caudalis is rich in both CaBP28k-immunoreactive cells and cells projecting to the parabrachial area or the nucleus of the solitary tract. Cells containing both the protein and the retrograde tracer, however, were mostly restricted to the superficial layers (laminae I and outer II) and to their rostral extensions, the dorsal paramarginal and paratrigeminal nuclei. These trigeminal subdivisions are targets for nociceptive, visceroceptive and thermal inputs of peripheral origins. The dorsal vagal complex is rich in CaBP28k. Dense populations of immunoreactive cells are observed in the ventrolateral part of the area postrema and all of the three main subdivisions of the nucleus of the solitary tract (rostral gustatory, ventrolateral respiratory and medial cardiovascular subregions). The subnucleus commissuralis, subnucleus centralis and dorsal subnuclei are particularly densely stained. The subnucleus centralis, which is involved in regulating food and water intake, does not project to the parabrachial area. The area postrema, subnucleus commissuralis and dorsal subnuclei, which are implicated in cardiovascular and/or ingestive behaviours, have dense projections to the parabrachial area, many of which contain CaBP28k. The present results demonstrate that CaBP28k-containing cells form a major part of the solitary and trigeminal projection systems, including subregions that are involved in visceroception and/or nociception processing. The location of solitary nucleus projection cells overlaps those of some neuropeptidergic projecting populations, suggesting colocalization. Consequently, certain neuropeptidergic actions may be CaBP28k-dependent.

Rhythmic activities in the rat solitary complex in vitro

In adult rat brainstem slices, rhythmic discharge of action potentials occurred spontaneously in 10 out of 197 cells of the solitary complex. In 6 neurones, fast rhythms (2-6 per min) were characterized by volleys of synaptic activity presenting abrupt onset denoting synchronized discharge of presynaptic elements. Synchronizing signals may be generated by cells discharging bursts of high-frequency action potentials and presenting extensive axonal arborization, as observed in one cell. Slower rhythms (0.3-0.8 per min) monitored in three cells did not involve synchronizing processes and could be evoked in non-rhythmic cells by 15-30 min bath application of the cholecystokinin octapeptide (100 nM). These results suggest distinct operating mechanisms of fast and slow rhythms in the solitary complex in vitro.

Evidence for sex steroid receptors in feline brainstem

Specific binding sites for estrogens, androgens, and progestins were found in cytosol from perfused cat brainstem with the use of a steroid binding assay. These sites resemble 'classical' steroid receptors in ligand specificity as well as in sedimentation properties (sedimentation constant 8-10S on sucrose density gradient). The presence of estrogen receptor in brainstem extracts was also confirmed by an enzyme immunoassay employing monoclonal antibodies directed against human estrogen receptor. This finding may be relevant to known influences of sex steroids on respiration.

Synaptic inputs to medullary respiratory neurons from superior laryngeal afferents in the cat

Synaptic inputs from afferents in the superior laryngeal nerve (SLN) to medullary respiratory neurons (n = 154) in the dorsal respiratory group (DRG), ventral respiratory group (VRG) and the region of the Bötzinger complex (BOT) were studied in anesthetized cats. Single pulse stimulation of the SLN-evoked monosynaptic EPSPs in most inspiratory bulbospinal (I-BS) neurons in the DRG, and disynaptic or oligosynaptic chloride-dependent IPSPs in other I-BS neurons in the DRG and VRG. Stimulation of laryngeal afferents also inhibited oligosynaptically expiratory bulbospinal neurons in the VRG, and all types of respiratory neurons recorded in the BOT region. Oligosynaptic potentials (usually EPSPs) were recorded in inspiratory and expiratory laryngeal motoneurons. These results provide evidence of a processing of SLN-evoked synaptic responses by all tested groups of medullary respiratory neurons. The pathways mediating these synaptic responses are discussed.

The bulbar network of respiratory neurons during apneusis induced by a blockade of NMDA receptors

Our aim was to study the mechanisms producing the transition from the inspiratory phase to the expiratory phase of the breathing cycle. For this purpose we observed the changes affecting the discharge patterns and excitabilities of the different types of respiratory neurons within the respiratory network in cat medulla, after inducing an apneustic respiration with the N-methyl-D-aspartate (NMDA) antagonist MK-801 given systemically. Respiratory neurons were recorded extracellularly through the central barrel of multibarrelled electrodes, in the ventral respiratory area of pentobarbital-anesthetized, vagotomized, paralyzed and ventilated cats. Inhibitions exerted on each neuron by the pre-synaptic pools of respiratory neurons were revealed when the neuron was depolarized by an iontophoretic application of the excitatory amino-acid analogue quisqualate. Cycle-triggered time histograms of the spontaneous and quisqualate-increased discharge of respiratory neurons were constructed in eupnea and in apneusis induced with MK-801. During apneustic breathing, the activity of the respiratory neuronal network changed throughout the entire respiratory cycle including the post-inspiratory phase, and the peak discharge rates of all types of respiratory neurons, except the late-expiratory type, decreased. During apneusis, the activity of the post-inspiratory neuronal pool, the post-inspiratory depression of other respiratory neurons, and the phrenic nerve after-discharge were reduced (but not totally suppressed), whereas the discharge of some post-inspiratory neurons shifted into the apneustic plateau. The shortened post-inspiration (stage 1 of expiration) altered the organization of the expiratory phase. Late-expiratory neurons (stage 2 of expiration) discharged earlier in expiration and their discharge rate increased. The inspiratory on-switching was functionally unaffected. Early inspiratory neurons of the decrementing type retained a decrementing pattern followed by a reduced discharge rate in the apneustic plateau, whereas early-inspiratory neurons of the constant type maintained a high discharge rate throughout the apneustic plateau. Inspiratory augmenting neurons, late-inspiratory and "off-switch" neurons also discharged throughout the apneustic plateau. During the apneustic plateau, the level of activity was constant in the phrenic nerve and in inspiratory neurons of the early-constant, augmenting, and late types. However, progressive changes in the activity of other neuronal types demonstrated the evolving state of the respiratory network in the plateau phase. There was a slowed but continued decrease of the activity of early-inspiratory decrementing neurons, accompanied by an increasing activity and/or excitability of "off-switch", post-inspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytochrome oxidase activity in the nucleus of the tractus solitarius of the cat

We studied the cytochrome oxidase (CO) activity in the nucleus of the tractus solitarius (NTS) of normal cats and in animals subjected to unilateral removal of vagal and glossopharyngeal afferents. In normal cats CO activity was higher in the ventrolateral, dorsolateral, interstitial and ventral NTS subnuclei. The dorsal, medial, commissural and gelatinosus subdivisions showed lower levels of CO activity. The peripheral deafferentation up to 47 days did not reduce the CO activity, suggesting an important role for the central inputs in determining the neural activity of the NTS.

Synaptic response of bulbar respiratory neurons to hypercapnic stimulation in peripherally chemodenervated cats

Effects of hypercapnia on the membrane potential and synaptic activity of bulbar respiratory neurons were studied in decerebrate, vagotomized, glomectomized and artificially ventilated cats. Coaxial multibarrelled electrodes were used for intracellular recording and extracellular iontophoresis of drugs. Hypoventilation with oxygen-enriched air (hyperoxic hypercapnia) produced an increase of depolarization together with an increase of spiking during the active phase and an increase of hyperpolarization during the inactive phase of each respiratory cycle in the inspiratory, postinspiratory and expiratory neurons of the ventral respiratory group. Both depolarizing and hyperpolarizing effects were associated with a decrease in input resistance. Intracellular injection of Cl- reversed the polarity of the hyperpolarizing synaptic wave to depolarization during the inactive phase, and hypercapnia increased the depolarization at that phase. Iontophoresis of tetrodotoxin eliminated the CO2-induced changes in membrane potential and input resistance. In 20 out of 58 neurons examined, iontophoretically applied atropine partly or totally suppressed the depolarizing response to hypercapnia. For these neurons, iontophoresed acetylcholine produced a sustained depolarization that was antagonized by atropine, but not by hexamethonium. The present study shows that both depolarizing and hyperpolarizing responses of medullary respiratory neurons to hyperoxic hypercapnia are synaptically mediated. A muscarinic mechanism is involved in part of the respiratory neuronal excitation evoked by hypercapnia.

Effect of blocking medial area of nucleus retrofacialis on respiratory rhythm

Experiments were performed on anaesthetized, vagotomized rabbits. Respiratory movement and phrenic rhythmical discharge were reversibly abolished by the symmetrical injection of 1% procaine into the medial area of the nucleus retrofacialis (mNRF). Blocking other areas of the medulla had no obvious effect on respiratory rhythm, with the exception of the rostral portion of the ventral respiratory group (VRG), which overlaps with the mNRF. When the mNRF was blocked, most inspiratory and expiratory neurons recorded in the VRG and DRG (dorsal respiratory group) gradually started to fire continuously, and no longer exhibited respiratory rhythm. A minority of respiratory neurons was inactivated during apnea. Stimulation of the caudal portion of the DRG and VRG evoked only a short cluster of phrenic discharges instead of rhythmical firing, indicating that the respiratory neurons situated in these areas cannot generate rhythmic activity by themselves. This suggests that the mNRF plays an important role in the genesis and maintenance of basic respiratory rhythm.

GABAA receptor mediated fast synaptic inhibition in the rabbit brain-stem respiratory system

The involvement of GABA mediated neurotransmission in the central control of respiration was investigated by administration of the specific GABAA receptor agonist muscimol and the specific GABAA receptor antagonist biculline into the fourth cerebral ventricle of the rabbit. Cycle-triggered averaging of the phrenic nerve activity (PNA) was used to quantify drug-induced changes of the central respiratory pattern. Muscimol reduced the peak amplitude of PNA and increased the duration of the respiratory phases. High amounts of muscimol led to a long-lasting but reversible central apnea. Bicuculline very effectively blocked the effects of externally applied muscimol. Blockade of intrinsically active GABAergic neurotransmission by bicuculline resulted in a multitude of effects. Peak amplitude of PNA increased whereas the duration of both inspiration and expiration decreased. In this respect, effects of bicuculline and muscimol were complementary. Bicuculline reduced the slope of the inspiratory ramp, increased postinspiratory activity and induced an augmenting type of discharge activity in the last part of expiration resulting in a smooth transition between expiration and inspiration. In some cases the respiratory modulation was completely lost and PNA became perfectly tonic. This 'apneustic' type of respiratory pattern could be transformed into rhythmic breathing by increasing the respiratory drive. We conclude that neurotransmission via GABAA receptors is important for the maintenance of respiratory rhythm as well as the generation of normal respiratory pattern.

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

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

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

We analyzed the membrane potential of 161 respiratory neurons in the medulla of decerebrate rats which were paralyzed and ventilated. Three types of inspiratory (I) neurons were observed: those displaying progressive depolarization in inspiration (augmenting I neurons), those which gradually repolarized after maximal depolarization at the onset of inspiration (decrementing I neurons) and those exhibiting a plateau or bell-shaped membrane potential trajectory throughout inspiration (I-all neurons). Three types of expiratory (E) neurons were also encountered: those in which the membrane potential progressively depolarized (augmenting E neurons), those in which the membrane potential repolarized during the interval between phrenic bursts (decrementing E or post-I neurons) and those exhibiting a plateau or bell-shaped membrane potential trajectory throughout expiration (E-all neurons). Axonal projections of these medullary neurons were identified in the cranial nerves (n = 34), or in the spinal cord (n = 19) as revealed by antidromic stimulation and/or by reconstruction following horseradish peroxidase (HRP) labeling. The other 108 neurons were not antidromically activated (NAA) by the stimulations tested, or had their axons terminating inside the medulla as revealed by HRP labeling. All these respiratory neurons, except for 3 which were hypoglossal motoneurons, had their somata within the ventrolateral medulla, in the region of the nucleus ambiguus, homologous to the ventral respiratory group (VRG) of the cat. No dorsal respiratory group (DRG) was detected within the medulla of the rats. Due to this absence of a DRG, it is concluded that the neural organization of respiratory centers is quite different in cats and rats.

The adenosine analog, 5'-N-ethylcarboxamidoadenosine, exerts mixed agonist action on cardiorespiratory parameters in the intact but not decerebrate rat following microinjections into the nucleus tractus solitarius

A limited occipital craniotomy was conducted on intact and decerebrate urethane-anesthetized, spontaneously breathing rats to expose the caudal medulla in the region of the obex. Microinjections of 5'-N-ethylcarboxamidoadenosine (NECA), a metabolically stable adenosine analog which exhibits mixed agonist properties for adenosine receptor subtypes, were made into the medial region of the caudal nucleus tractus solitarius (NTS) at the level of the caudal tip of the area postrema, an area of the NTS in which there is known to be a functional co-existence of cardiovascular and respiratory-related neuronal elements. Cardiorespiratory responses were subsequently recorded for a 30-min test period. In the intact rat, microinjections of NECA produced significant dose-related reductions in respiratory rate which were accompanied by dose-dependent increases in tidal volume and these pronounced effects on respiration persisted throughout the test period. On the other hand, microinjections of NECA into this region of the NTS of the intact rat elicited complex, bi-directional cardiovascular responses, producing hypotension (at lower doses) and pressor responses (at higher doses) in addition to bradycardia (at lower doses). In an effort to examine the functional interactions between the NTS and forebrain structures involved in cardiorespiratory control, microinjections of NECA in the identical dose range were made into the same NTS sites of a separate group of urethane-anesthetized, spontaneously breathing rats in which reciprocal connections between forebrain areas and the brainstem had been disrupted by acute supracollicular decerebration. A simulating electrode, placed in the paraventricular nucleus of the hypothalamus (PVH), was used to confirm complete transection during the experiment and to ascertain the integrity of reciprocal connections between the brainstem and rostral brain regions involved in cardiorespiratory control. Although decerebration at the supracollicular level negligibly affected basal cardiorespiratory parameters, microinjections of NECA into the NTS revealed dramatic differences in the cardiovascular response patterns between intact and decerebrate rats. Whereas cardiovascular responses elicited by microinjections into the NTS were significantly affected by supracollicular decerebration, respiratory responses were highly similar for both intact and decerebrate animals. Indeed, repeated measures MANOVA indicated that there were no significant differences in the time-related or dose-related responses in the depression of respiration between decerebrate and intact rats following NECA microinjections.(ABSTRACT TRUNCATED AT 400 WORDS)

Expiration-related neurons in the caudal ventral respiratory group of the cat: influences of the activation of Bötzinger complex neurons

The functional role of Bötzinger complex (Böt. c.) projections to the expiration-related (ER) area of the caudal ventral respiratory group (cVRG) was investigated in anesthetized, vagotomized, paralyzed and artificially ventilated cats. ER neurons in both the ipsi- and the contralateral cVRG displayed excitatory responses to Böt. c. electrical microstimulation. They were also activated by microinjections of D,L-homocysteic acid into the Böt. c. region. We propose that at least part of the Böt. c. projections to the cVRG have an excitatory function.

Extensive monosynaptic inhibition of ventral respiratory group neurons by augmenting neurons in the Bötzinger complex in the cat

Axonal projections and synaptic connectivity of expiratory Bötzinger neurons with an augmenting firing pattern (Bot-Aug neurons) to neurons in the ipsilateral ventral respiratory group (VRG) were studied in anaesthetized cats. Antidromic mapping revealed extensive axonal arborizations of Bot-Aug neurons (24 of 45) to the rostral or caudal VRG, with some having arbors in both regions. Of 234 pairs of neurons studied with intracellular recording and spike-triggered averaging, monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked in 49/221 VRG neurons by 38/98 Bot-Aug neurons. The highest incidence of monosynaptic inhibition was found in inspiratory bulbospinal neurons (10 of 23 tested). Evidence was also found for monosynaptic inhibition, by a separate group of Bot-Aug neurons, of expiratory bulbospinal neurons (12/58), while excitatory postsynaptic potentials (EPSPs) were identified in another two of these neurons. In addition, monosynaptic IPSPs were recorded from 13 of 53 identified laryngeal motoneurons, and from 14 of 100 respiratory propriobulbar neurons. Presumptive disynaptic IPSPs were recorded from 11 of the 221 VRG neurons. We conclude that Bot-Aug neurons exert widespread inhibition on all major neuron categories in the ipsilateral VRG, and should be regarded as an important element in shaping the spatiotemporal output pattern of both respiratory motoneurons and premotor neurons.

Precursor of respiratory pattern in the early gestation mammalian fetus

Recordings of respiratory muscle activity in fetal lambs from early in gestation provide insight into the organization of the central pattern generator for respiration in mammals. Evidence presented here is consistent with the recent hypothesis that production of the respiratory pattern involves two separate neural modules: one, the 'rhythm' module, which specifies the respiratory cycle and another, the 'form' module, which creates the characteristic shape of each burst of activity within this cycle. The rhythm module is already functional when gestation is 35% complete while the form module appears to be constructed gradually over the second half of gestation.

Serotonin immunoreactive boutons form close appositions with respiratory neurons of the dorsal respiratory group in the cat

The aim of this study was to examine the location of serotonin immunoreactive boutons on both the soma and dendrites of neurons in the dorsal respiratory group by using a combination of intracellular recording and labelling and immunohistochemistry. Inspiratory neurons in the ventrolateral nucleus of the solitary tract (vl-NTS) were intracellularly labelled with horseradish peroxidase (HRP) in anaesthetised adult cats. The morphology of 23 neurons, all antidromically activated from the contralateral C3 spinal segment, was examined. Six neurons displayed pronounced dendritic arborizations outside the vl-NTS, with prominent dorsal and/or medial projections. The dendrites of the remaining neurons were almost entirely confined to the vl-NTS. Intramedullary axon collaterals were detected in four of nineteen examined axons. Serotoninergic fibres were immunohistochemically demonstrated in the NTS, and the apposition of immunoreactive boutons to the HRP-filled neurons examined at the light microscopic level. Boutons were identified in close apposition with the somata, proximal and distal dendrites of these neurons. However, the density of contacts was found to be substantially less than in a previous study of phrenic motoneurons (Lipski et al: Soc. Neurosci. Abst. 14:379, '88; Pilowsky et al: J. Neurosci. in press, '90). The relative paucity of contacts of serotonin immunoreactive boutons with premotor inspiratory neurons of the dorsal respiratory group indicates that the serotoninergic system affects respiratory pathways mainly at the level of respiratory motoneurons or at brainstem sites outside the vl-NTS.

Candidate cell populations for respiratory chemosensitive fields in the human infant medulla

The histology and location of human respiratory chemosensitive fields are not known. In contrast, the physiology of respiratory chemosensory areas in the ventral medulla of cats has been studied extensively, and their anatomy has been partially described. Using basic principles of comparative cytoarchitecture and computer-aided reconstructions of serial-sectioned medullae, we describe the histology and three-dimensional distribution of putative respiratory chemosensors in the feline and human infant medulla. We found that ventrolateral neurons of the human nucleus conterminalis are homologous to neurons identified in the feline L chemosensitive field by Trouth and others, and that ventrolaterally situated neurons in the human arcuate nucleus correspond to neurons predominating in the feline S and M fields. In addition, there are foci of thickened marginal glia along the feline ventral medullary surface that colocalize with chemosensitive fields identified by physiologic studies reported by others; we identify similar foci in the infant medulla. Thickened marginal glia are intermixed with neuronal fibers, often adjacent to neurons of the feline chemosensitive fields and their human counterparts, suggesting that they constitute a chemosensory neuropil at the medullary surface. Computer-aided reconstructions provide insight into the three-dimensional topography of putative respiratory chemosensors and their relationships to other brainstem structures in ways not obvious in single or even multiple sections. This delineation of candidate human respiratory chemosensors is a first step toward their postmortem analysis in patients with central ventilatory control disorders where finding histological abnormalities in these sites would support their role in human ventilation.

Cardiorespiratory response patterns elicited by microinjections of neuropeptide Y in the nucleus tractus solitarius

A limited occipital craniotomy was conducted on anesthetized, spontaneously breathing rats to expose the caudal medulla in the region of the obex. Microinjections of neuropeptide Y (NPY), a putative neuromodulator associated with catecholaminergic (CA) synapses, were made into the medial region of the caudal nucleus tractus solitarius (NTS) at the level of the posterior portion of the area postrema, an area of the NTS in which there is known to be a functional coexistence of cardiovascular and respiratory-related neuronal elements. This region of the caudal NTS in the rat is not only the principal site of termination of baro- and chemoreceptor afferents, but it also has profuse reciprocal connections with NPY-containing cardiorespiratory control regions in the hypothalamus and with other brainstem regulatory nuclei. Moreover, this same region of the rat NTS also shows very high densities of NPY binding sites. Cardiorespiratory responses were subsequently recorded for a 60-min test period following NPY administration. Microinjections of NPY, in the dose range of 10-100 pmol/rat, into the caudal NTS of intact rats produced significant dose-related reductions in mean arterial blood pressure, pulse pressure and minute volume. To a lesser extent, NPY microinjections also produced significant reductions in heart rate, respiratory rate and tidal volume. In a series of separate experiments, in an effort to ascertain the modulatory influences of rostral brain regions on these NPY-evoked, NTS-mediated cardiorespiratory response patterns, microinjections of NPY were made under identical anesthetic and experimental conditions in a group of rats wherein reciprocal connections between the NTS and rostral brain regions had been disrupted via supracollicular decerebration. In addition, since NPY microinjections were made into specific loci wherein afferent inputs from cardiopulmonary receptors are known to converge in the rat NTS, the effects of bilateral vagotomy on NPY-evoked, NTS-mediated cardiorespiratory response patterns were also examined in otherwise intact rats and under the same experimental conditions. The effects of NPY microinjections at the same dosage on NTS-mediated cardiorespiratory response patterns were subsequently compared among the intact, decerebrate and vagotomized rats. The results showed that whereas the hypotensive actions of NPY were not affected by decerebration, vagotomy significantly increased the magnitude of the hypotension elicited by NPY microinjections in comparison to the intact and decerebrate groups of rats. On the other hand, vagotomy abolished the NPY-evoked bradycardia which had a similar magnitude in both intact and decerebrate rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that neurons of the central amygdaloid nucleus directly project to the site concerned with circulatory and respiratory regulation in the ventrolateral nucleus of the cat: a WGA-HRP study

Projections from the central amygdaloid nucleus (ACE) to the ventrolateral nucleus (VLN) of the medulla oblongata were studied using the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) method. WGA-HRP was injected into the rostral VLN, where a pressor response associated with bradycardiac and apneustic responses was elicited by microinjection of glutamate, and into the caudal VLN, where a depressor response associated with bradycardiac and apneustic responses was elicited. In both experiments, HRP-labeled cells were found in the ACE. These results suggest that the ACE may send direct projections to the VLN and may play a role in cardiovascular and/or respiratory control mechanisms.

Cardiorespiratory function is altered by picomole injections of 5'-N-ethylcarboxamidoadenosine into the nucleus tractus solitarius of rats

A limited occipital craniotomy was conducted on urethane-anesthetized, spontaneously breathing rats to expose the caudal medulla in the region of the obex. Microinjections of 5'-N-ethylcarboxamidoadenosine (NECA), an adenosine analog, were made into the medial region of the caudal nucleus tractus solitarius (NTS) at the level of the caudal tip of the area postrema, an area of the NTS in which there is known to be a functional co-existence of cardiovascular and respiratory-related neuronal elements. Cardiorespiratory responses were subsequently recorded for a 60 min test period. Microinjections of NECA, in the dose range of 0.35-350 pmol per rat, produced significant dose-related reductions in respiratory rate which were accompanied by dose-dependent increases in tidal volume and these pronounced effects on respiration persisted throughout the test period. In contrast, the effects of NECA microinjections on cardiovascular parameters in this region of the NTS were bidirectional and elicited considerably more complex responses during the test period. During the initial period (2-5 min) following injection, NECA elicited significant hypotension (at lower doses) and pressor responses (at higher doses) in addition to significant bradycardia (at lower doses) whereas by the end of the 60 min test period, almost all doses of NECA had resulted in hypertension and tachycardia. Multivariate analysis of variance (MANOVA) and correlation statistics indicated that the effects of NECA on blood pressure during the initial 2-5 min were dose-dependent and unlikely related to depression of respiratory frequency. A further examination of the data by MANOVA indicated that the pharmacological effects of NECA during the 60 min test period exhibited a highly significant and specific dose-dependent and time-related response pattern for the respiratory, but not the cardiovascular, parameters. Taken together, these manifold response patterns suggest that the respiratory effects of NECA may be mediated by different intrinsic mechanisms in the NTS than are the cardiovascular effects of NECA. At the end of the 60 min test period following the administration of NECA, the respiratory rate remained profoundly depressed. In view of previous studies showing that microinjections of cyclic AMP analogs, forskolin, isoproterenol and adenosine into the same NTS sites elicit a similar depression of respiration, the results with NECA in the present study further support the notion that cyclic AMP may serve as a second messenger in NTS respiratory control regions and these respiratory depressant effects may be mediated by a single adenosine receptor subtype.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiorespiratory effects of inositol hexakisphosphate following microinjections into the nucleus tractus solitarii

Microinjections of inositol hexakisphosphate (IP6), a metabolite of inositol recently found to occur in high concentrations in the brainstem, were made into the caudal portion of the nucleus tractus solitarii (NTS) of spontaneously breathing rats and cardiorespiratory parameters recorded for a 30 min test period. Microinjections of IP6, in the dose range of 100-500 pmol/rat, produced significant dose-related reductions in mean arterial blood pressure and respiratory rate. The onset for hypotensive action and respiratory depression following microinjections of IP6 was very rapid and a transient apnea could be elicited at the higher doses. Moreover, the sodium and calcium salts of IP6 were relatively equipotent in depressing cardiorespiratory parameters, with the exception of heart rate wherein the sodium salt elicited a much more pronounced bradycardia. These results confirm and extend the findings of a previous study suggesting that IP6 and closely related metabolites may act on extracellular receptors. Taken together, these data provide further support to the notion that inositol lipid signalling pathways may generate both intracellular and extracellular signals in the brain.

Cardiorespiratory responses following electrical stimulation of caudal sites in the rat medulla

A limited occipital craniotomy was conducted on urethane-anesthetized rats to expose the caudal medulla in the region of the obex. Discrete bipolar electrical stimulation was administered at sites in the dorsal medulla of spontaneously breathing rats in the vicinity of the caudal nucleus tractus solitarius (NTS) and adjacent reticular formation. Cardiorespiratory responses were recorded during microstimulation at three separate stimulating frequencies to examine the functional interaction of cardiovascular and respiratory-related neuronal elements in the NTS. Microstimulation was conducted at sites in the dorsal and medial regions of the NTS beginning at the level of the area postrema and extending posteriorly through the rostrocaudal course of the NTS; microstimulation was also conducted at midline sites in the commissural region of the NTS and the ventral and ventrolateral regions of the caudal NTS. Microstimulation of loci in the reticular formation adjacent to these NTS sites did not elicit any cardiorespiratory responses whereas stimulation of individual NTS regions elicited specific patterns of cardiorespiratory responses. Specifically, microstimulation of the dorsal and medial NTS at the level of the area postrema elicited pressor responses associated with apneic/hypopneic responses whereas stimulation of midline sites in the commissural region, dorsomedial sites caudal to the area postrema and the ventral and ventrolateral areas of the caudal regions of the NTS elicited depressor responses associated with bradycardic and apneic/hypopneic responses. The most profound respiratory effects (i.e., apnea) and heart rate responses (i.e., bradycardia) were seen following stimulation of the ventral and ventrolateral regions of the caudal NTS. These findings support the notion that the caudal NTS is a major site for coordinating cardiorespiratory afferent information in the rat and it is also apparent from this study that specific regions of the caudal NTS demonstrate a functional coexistence of cardiovascular and respiratory-related neurons. Finally, the results from this study showing the regional specificity and frequency-dependent characteristics of cardiorespiratory response patterns elicited by microstimulation suggest that the local microcircuitry and intrinsic neuronal networks in the more caudal regions of the rat NTS are more complex and heterogeneous than hitherto revealed.

Inhibition of inspiratory neurons of the nucleus retroambigualis by expiratory neurons of the Botzinger complex in the cat

The connection between expiratory neurons of the Botzinger Complex and contralateral inspiratory neurons of the nucleus retroambigualis was examined using the technique of spike-triggered averaging of intracellular potentials. Out of a total of 34 expiratory neurons found in the Botzinger Complex, 25 (73%) could be antidromically activated from the inspiratory region of the contralateral nucleus retroambigualis. The spike activities of 15 of these antidromically activated expiratory neurons were used as triggers for the averaging of the intracellular potentials recorded from 39 inspiratory neurons in the region of the contralateral nucleus retroambigualis. Unitary, inhibitory, postsynaptic potentials were observed in 11 of the 39 (28%) averages, from 6 of the 15 (40%) trigger neurons. It was concluded that these experiments demonstrate a monosynaptic inhibitory connection from expiratory neurons in the Botzinger Complex to inspiratory neurons in the contralateral nucleus retroambigualis.

Peripheral chemoreceptor inputs to medullary inspiratory and postinspiratory neurons of cats

The effect of peripheral chemoreceptor activation on inspiratory and postinspiratory medullary neurons was investigated using intracellular recording techniques. Peripheral chemoreceptors were activated by injecting CO2 saturated 1 N bicarbonate solution into the lingual artery or by electrically stimulating the carotid sinus nerve. Injections of 20-300 microliters bicarbonate solution evoked changes in respiratory frequency and in peak phrenic nerve discharge. The membrane potential of inspiratory alpha neurons, whether bulbospinal or not and independent of their anatomic location, was decreased during inspiration. A sequence of compound excitatory and inhibitory effects were observed when the stimulus was given during the postinspiratory and expiratory phases of the respiratory cycle. Inspiratory beta- and late-inspiratory neurons, however, were inhibited by peripheral chemoreceptor activation. Postinspiratory neurons were strongly activated during postinspiration. Neither class of respiratory neurons were shown to receive direct synaptic inputs from the peripheral chemoreceptors as tested by electrical stimulation of the carotid sinus nerve and signal averaging of the respiratory neuron membrane potential. The experiments revealed differential influences of afferent chemoreceptor activity on various components of the respiratory network. We conclude that chemoreceptor afferents activate non-respiratory modulated medullary neurons which, in turn, activate or inhibit various neurons of the medullary respiratory control network. The responses of each type of respiratory neuron to chemoreceptors afferents may then be considered in the context of this direct interaction as well as the network interactions of the various cells.

Axonal trajectory and terminal distribution of inspiratory neurons of the dorsal respiratory group in the cat's medulla

In Nembutal-anesthetized and artificially ventilated cats, we studied the morphological properties of the inspiratory neurons of the dorsal respiratory group (DRG) with HRP intracellular staining. A total of 37 neurons were stained and their axonal trajectories and terminal distribution in the medulla were analyzed. The somata were located predominantly in the ventrolateral region of the nucleus of the solitary tract and were distributed between 2,300 mum rostral and 700 mum caudal to the obex. Most (26/33) of the neurons tested were antidromically activated by the stimulation of the contralateral (n = 24) or ipsilateral (n = 2) cervical cord. To examine the existence of collateral branches in the brainstem, we traced axonal trajectories in 28 neurons. In most cases, the stem axons issuing from the cells of origin coursed ventrally and then turned medially to cross the midline without giving off any axon collaterals. However, six neurons had axonal collaterals in the brain stem ipsilateral to the somata. At least four types of collateralization were observed. The stem axon of the first type bifurcated at the area ipsilateral and ventral to the cell body. One branch crossed the midline to project to the spinal cord, and the other, thinner branch descended caudally in the ipsilateral medullary reticular formation without distributing any terminals. The axon of the second type projected to the contralateral spinal cord and distributed collateral branches with terminal boutons in the ipsilateral ventral respiratory group (VRG). The third type projected to the contralateral spinal cord and distributed terminal boutons in the medial part of the nucleus of the solitary tract (NTS) and its vicinity. The fourth type distributed numerous branches with terminal boutons in and around the ventral part of the NTS and the VRG area. This study indicates that some inspiratory neurons of the DRG influence not only spinal respiratory neurons but also medullary respiratory neurons in the vicinity of the DRG and the VRG.

Immunocytochemical localization of GABA in neurons projecting to the ventrolateral nucleus of the solitary tract

To determine the origin of gamma-aminobutyric acidergic (GABAergic) input to the ventrolateral solitary tract nucleus (vlnTS), we used a double-labeling procedure for retrogradely transported horseradish peroxidase (HRP) and the immunocytochemical localization of GABA. Following HRP injections into the vlnTS, double-labeled neurons were found within the Bötzinger Complex. We conclude that these double-labeled cells are the inhibitory Bötzinger neurons and that GABA is a likely transmitter in this respiratory nucleus.

Viscerotopic representation of the upper alimentary tract in the rat: sensory ganglia and nuclei of the solitary and spinal trigeminal tracts

The aim of this study was to map the viscerotopic representation of the upper alimentary tract in the sensory ganglia of the IXth and Xth cranial nerves and in the subnuclei of the solitary and spinal trigeminal tracts. Therefore, in 172 rats 0.5-65 microliters of horseradish peroxidase (HRP), wheat germ agglutinin-HRP, or cholera toxin-HRP were injected into the trunks and major branches of the IXth and Xth cranial nerves as well as into the musculature and mucosa of different levels of the upper alimentary and respiratory tracts. The results demonstrate that the sensory ganglia of the IXth and Xth nerves form a fused ganglionic mass with continuous bridges of cells connecting the proximal and distal portions of the ganglionic complex. Ganglionic perikarya were labeled in crude, overlapping topographical patterns after injections of tracers into nerves and different parts of the upper alimentary tract. After injections into the soft palate, pharynx, esophagus, and stomach, anterograde labeling was differentially distributed in distinct subnuclei in the nucleus of the tractus solitarius (NTS). Palatal and pharyngeal injections resulted primarily in labeling of the interstitial and intermediate subnuclei of the NTS and in the paratrigeminal islands (PTI) and spinal trigeminal complex. Esophageal and stomach wall injections resulted in labeling primarily of the subnucleus centralis and subnucleus gelatinosus, respectively. The distribution of upper alimentary tract vagal-glossopharyngeal afferents in the medulla oblongata has two primary groups of components, i.e., a viscerotopic distribution in the NTS involved in ingestive and respiratory reflexes and a distribution coextensive with fluoride-resistant acid-phosphatase-positive regions of the PTI and spinal trigeminal nucleus presumably involved in visceral reflexes mediated by nociceptive or chemosensitive C fibers.

Respiratory effects of 5'-ethylcarboxamidoadenosine, an analog of adenosine, following microinjections into the nucleus tractus solitarius of rats

Microinjections of 5'-ethylcarboxamidoadenosine (NECA), an adenosine analog, were made into respiratory-related regions of the nucleus tractus solitarius (NTS) of spontaneously breathing rats and cardiorespiratory parameters were recorded during a 60-min test period. Microinjections of NECA, in the dose range of 0.35-350 picomol per rat, produced significant dose-related reductions in respiratory rate which were accompanied by correlative increases in tidal volume. At the end of the 60-min test period following the administration of NECA, the respiratory rate remained profoundly depressed, whereas blood pressure and heart rate were not significantly affected compared to preinjection control values. The data from this study suggest that adenosine may exert modulatory influences in brainstem respiratory control regions.

Electrophysiological properties of rostral medullary respiratory neurones in the cat: an intracellular study

1. We recorded the membrane potentials of sixty-three respiratory neurones in the rostral, ventral medulla of decerebrate vagotomized cats. Stable recordings were obtained in thirty-eight expiratory and twenty-five inspiratory neurones. Axonal projections were identified by antidromic invasion after electrical stimulation of the region of the dorsal respiratory group (DRG), spinal cord, and the cervical vagus, superior laryngeal and pharyngeal nerves. 2. Two types of expiratory neurones were encountered: those in which the membrane potential progressively depolarized (augmenting neurons, n = 22) and those in which the membrane potential repolarized (decrementing or post-inspiratory neurones, n = 16) during the interval between phrenic bursts. Both types were hyperpolarized during inspiration by chloride-dependent, inhibitory postsynaptic potentials (IPSPs) which decreased membrane resistance. In augmenting neurones two waves of IPSPs appeared, one early and one late in inspiration. 3. Five out of seventeen augmenting expiratory neurones tested were antidromically activated by contralateral stimulation of the spinal cord (n = 3) or the DRG (n = 2). Spinal axons were not detected in any of the sixteen decrementing expiratory neurones tested. Of thirteen expiratory neurones tested with pharyngeal nerve stimulation, one (an augmenting type) was antidromically activated. Superior laryngeal or vagal axons could not be demonstrated for any expiratory neurones. 4. Two types of inspiratory neurones were also encountered: those displaying progressive depolarization throughout inspiration (n = 5) and those which gradually repolarized after maximal depolarization at the onset of inspiration (n = 10). None of the former had identifiable spinal or medullary axons, but superior laryngeal axons were demonstrated in three and pharyngeal axons were found in three. None of the latter was antidromically activated from any of the sites stimulated. 5. Stimulation of the superior laryngeal or pharyngeal nerves evoked excitatory postsynaptic potentials (EPSPs) in all neurones except in post-inspiratory neurones. In these, stimulation of the superior laryngeal or pharyngeal nerves evoked IPSPs in five of twelve neurones tested. 6. We conclude that a spectrum of respiratory neurones lie within or ventral to the retrofacial nucleus. These neurones may control upper-airway muscles or may play a role in chemoreception.

Respiratory interneurones in the thoracic spinal cord of the cat

1. The discharges of spontaneously firing neurones, whose activity was modulated in phase with the central respiratory cycle, were recorded in the thoracic ventral horn (T3-T9) of anaesthetized, paralysed cats. 2. Out of 310 neurones, forty-six were positively identified as motoneurones by antidromic activation or spike-triggered averaging, fifty-four were positively identified as interneurones by antidromic activation from other spinal cord segments and ninety were indirectly identified as interneurones by virtue of their positions or firing rates as compared to the motoneurones. 3. Units were classified as inspiratory (64%), expiratory (25%) or post-inspiratory (7%) according to the times of their maximum firing rates. The remaining 4% consisted of units whose discharges were either strongly locked to the respiratory pump cycle or did not fit into the other categories. All but one of the motoneurones were classified as inspiratory or expiratory. 4. Inspiratory and expiratory units were further classified as early, late or tonic according to the starting times of their discharges in the respiratory cycle. The interneurones (both positively and indirectly identified) included more of the early and tonic categories and more fast-firing units than did the motoneurones in both the inspiratory and expiratory groups. 5. The locations of the motoneurones corresponded to the known positions of the intercostal and interchondral motor nuclei, including clear segregation of inspiratory and expiratory populations. The locations of neither the interneurones nor the unidentified units were segregated according to their firing patterns. These neurones were concentrated in the medial half of the ventral horn and were found generally more dorsally than the positions of the motoneurones, though their positions overlapped considerably with this group. 6. The axons of the positively identified interneurones were identified from one to five segments caudally and mostly contralaterally, but were not traced to their terminations. Some axons were located by microstimulation and found to run in the ventral or ventromedial white matter. Conduction velocities covered a wide range, 8 to around 100 m/s, mean 53 m/s. 7. Preliminary calculations indicate that there may be almost 10 times more respiratory thoracic interneurones as respiratory bulbospinal neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitation of upper cervical inspiratory neurons by inspiratory neurons of the nucleus retroambigualis in the cat

The experiments reported here examined inputs from inspiratory neurons of the nucleus retroambigualis to upper cervical inspiratory neurons. Antidromic mapping in contralateral C1 demonstrated the existence of axon collaterals for 9 of 19 nucleus retroambigualis axons tested (47%). Forty nucleus retroambigualis neurons were tested with antidromic mapping for a projection to the ipsilateral C1 segment; 9/40 (22%) had an ipsilateral axon (8 of these also projected contralaterally), and 2/40 (5%) had an axon collateral in ipsilateral C1. Cross-correlation histograms suggested monosynaptic excitation of an upper cervical inspiratory neuron by a contralateral nucleus retroambigualis neuron in 4/69 cases (6%) and common input to the pair in 2/69 cases (3%). Six of the 69 cross-correlograms were computed during antidromic activation of the nucleus retroambigualis neuron, and one of these six demonstrated direct excitation of the upper cervical inspiratory neuron by the retroambigualis neuron. We concluded that at least some upper cervical inspiratory neurons receive monosynaptic excitation from the contralateral, and possibly the ipsilateral, nucleus retroambigualis inspiratory neurons. These results, together with those published elsewhere for inputs from inspiratory neurons in the contralateral ventrolateral nucleus tractus solitarius, suggest that the rhythmic, inspiratory firing pattern of the upper cervical inspiratory neurons is due to excitatory inputs from these two inspiratory bulbospinal neuron populations.

High frequency ventilation and afferent vagal activity

Recordings were made from pulmonary afferent fibres in the vagus nerves of anaesthetised dogs during conventional and high frequency ventilation. In single and multiple fibre preparations, the mean spike counts per minute at 14 breaths/minute were in the ranges 36.9-155.3 and 755-1921, respectively. These counts decreased by up to 61 and 44% respectively at 100 breaths/minute, and by 27-89% and 22-51% at 200 breaths/minute. At this frequency there was a further decrease of between 19 and 65% when the positive end expiratory pressure was removed. The findings of this study are intuitively acceptable, since pulmonary stretch receptor activity is proportional to tidal volume, and are in keeping with the clinical impression that high frequency ventilation per se does not eliminate respiratory drive.

Central respiratory depression induced by acetylcholinesterase inhibition: involvement of anaesthesia

We have studied the effects of anaesthesia on the changes in central respiratory activity following the inhibition of acetylcholinesterase in chronically implanted cats. The organophosphate paraoxon was administered to the brainstem respiratory centres by intracerebroventricular (i.c.v.) injection (3 mg) into the IVth ventricle, thus avoiding peripheral effects such as paralysis of respiratory muscles. Paraoxon had opposite effects on respiratory activity depending on whether the cats were anaesthetized or not: it induced respiratory depression and sometimes respiratory arrest during pentobarbital (30 mg/kg i.v.) or halothane anaesthesia, but in the same animals in the waking state, the same dose of paraoxon always stimulated respiration. These results show a strong interaction between anaesthetics and the effects of acetylcholine (ACh) accumulation on central respiratory activity. This study extends previous results showing an interaction between ACh and pentobarbital on single respiratory neurons and stresses the importance of a 'wakefulness stimulus' for sustaining respiratory activity after organophosphate poisoning.

Role of medullary inspiratory neurones in the control of the diaphragm during oesophageal stimulation in cats

1. The effect of oesophageal distension and swallowing on the activity of medullary respiratory neurones was recorded in decerebrate, spontaneously breathing cats. The distension, produced by inflating a balloon in the thoracic portion of the oesophagus, was of sufficient magnitude to induce inhibition of the peri-oesophageal part of the crural diaphragm, with little effect on the respiratory function of the diaphragm as measured by the activity in the C5 branch of the phrenic nerve. 2. 424 neurones were tested. They were located bilaterally, in the region of the nucleus tractus solitarius (dorsal respiratory group) or the ambiguous complex (ventral respiratory group). No cell exhibited a change in activity during periods of strong inhibition of crural electrical activity induced by distension or swallowing. The activity of all cells paralleled that of the C5 phrenic neurogram, which was unaffected by the tests. 3. We conclude that the reflex inhibition of the crural diaphragm during oesophageal distension does not result from an inhibition of medullary premotor inspiratory neurones of the dorsal and ventral groups. Additional central pathways must exist that inhibit motoneurones to the crural diaphragm during gastrointestinal reflexes.

Monosynaptic excitation of thoracic motoneurones by inspiratory neurones of the nucleus tractus solitarius in the cat

1. The connection between inspiratory neurones in the ventrolateral nucleus tractus solitarius (n.t.s.) and intercostal motoneurones was examined. 2. Descending axonal projections to contralateral T3-T5 spinal segments were found for 110 of 142 (77%) ventrolateral n.t.s. neurones examined. 3. Antidromic mapping was used to locate the axons of thirty-nine ventrolateral n.t.s. neurones in T4, and evidence for axon collaterals was found for thirty-two of forty-seven (68%) neurones examined. Axon collaterals were found in both T3 and T4 for four of nine neurones examined and in T3, T4 and T5 for two of three neurones examined. 4. Cross-correlation histograms were calculated for sixty-five ventrolateral n.t.s. neurones with the contralateral intercostal nerves. Peaks in the cross-correlograms were assessed for significance by calculating k, the ratio of the peak bin count to the mean bin count. Significant peaks (k ratios 1.07-1.24, mean 1.15) were found for twenty-eight (39%) cross-correlograms. Twelve of thirty-three (36%) were for the whole external intercostal nerve, ten of twenty-seven (37%) were for the whole internal intercostal nerve and six of eleven (54%) were for external intercostal nerve filaments. 5. Six of the cross-correlogram peaks were less than or equal to 1.2 ms in width at a level half-way between the peak and the mean bin count. The rest ranged from 2.0 to 4.6 ms (mean 3.0 ms). 6. Intracellular recordings from either internal or external intercostal motoneurones were made and averages of the intracellular potentials were computed using ventrolateral n.t.s. neurone spikes as triggers. 7. Thirty-two spike-triggered averages were computed for pairings between nineteen ventrolateral n.t.s. neurones and thirty-two intercostal motoneurones (twenty-five internal, seven external). Fast-rising, short-lasting depolarizations indicative of a monosynaptic e.p.s.p. were found for five ventrolateral n.t.s. neurones. 8. The characteristics of the cross-correlogram peaks were considered with respect to the e.p.s.p. shapes and it was concluded that the intercostal motoneurones receive a significant monosynaptic excitation from ventrolateral n.t.s. neurones.