Brainstem projections to the phrenic nucleus: a HRP study in the cat

The excitation by neurotensin of nucleus tractus solitarius neurons induces apneustic breathing

The possible involvement of neurotensin in the regulation of respiratory drive has been tested on single brainstem respiratory related neurons and on the global respiratory output. The neuropeptide was locally applied either by microiontophoresis or by pressure injection in the dorsal and ventral respiratory areas of the anesthetized bivagotomized cat. Effects of neurotensin applications were studied, on the one hand on the firing discharge of respiratory related neurons and on the other hand on the phrenic nerve activity and on arterial blood pressure. An increase of the firing frequency of respiratory related neurons was induced by neurotensin applied by iontophoresis or by pressure injection (0.005-33.5 fmol/s) on single neurons. In the latter case, neurotensin was active at concentration 10(3) times lower than glutamate. A bilateral apneustic pattern was induced on the phrenic nerve activities by microinjection of neurotensin (0.23-0.54 pmol/s) in one ventrolateral nucleus tractus solitarius without alteration of arterial blood pressure. These results suggest that the release of neurotensin in the nucleus tractus solitarius regulates respiratory rhythmogenesis by increasing the inspiratory duration.

Projections of supraspinal structures to the phrenic motor nucleus in cats studied by a horseradish peroxidase microinjection method

The activity of phrenic motor neurons is influenced by the cardiovascular control system of the supraspinal structures. In order to obtain the basic data for analyzing the anatomical relations between the cardiovascular and the pulmonary control system, supraspinal structures projecting to the phrenic motor nucleus of the cat spinal cord were studied using a horseradish peroxidase method. A double-barrel coaxial electrode was employed. To determine the site of the phrenic motor neurons, the inner barrel electrode, filled with 3 M NaCl solution, was used for recording the activity of these neurons. The outer barrel electrode, filled with a 20% HRP solution, was used for injecting HRP iontophoretically into the phrenic motor nucleus. Out of 13 experiments, 5 showed that the HRP-injection sites were centered in and almost confined to the phrenic motor nucleus. Some 1798 HRP-labeled cells were thus identified in the selected 5 experiments. They were distributed in the medulla oblongata (93.5%), pons (6.0%) and midbrain (0.5%). The majority were concentrated in the nucleus para-ambiguus (48.9%), nucleus tractus solitarii (21.5%) and in or around the nucleus retrofacialis (9.8%). A few labeled cells were scattered throughout the nucleus raphe (1.1%) and the parabrachial and Koelliker-Fuse nuclei (0.3%), suggesting that these nuclei may be, if any, only minor sources of input to the phrenic motor nucleus.

Ultrastructural studies of the nucleus ambiguus in cat and monkey following injection of HRP into the vagus nerve

The fine structure of retrogradely labelled neurons in the nucleus ambiguus (NA) has been examined in cat and monkey (Macaca fascicularis) following injections of horseradish peroxidase (HRP) into the vagus nerve. Many retrogradely labelled neurons were observed in the NA although unlabelled neurons were also present within the boundaries of the NA as identified by the distribution of retrogradely labelled cells. In both species a wide range of sizes of labelled neurons (20-60 microns in long axis) was observed from rostral to caudal levels of the NA. Large labelled neurons were generally oval or spindle-shaped while smaller neurons were oval in cross-section. Unlabelled neurons observed among labelled NA neurons tended to be smaller on average than the labelled neurons and ranged in size from 15 to 30 microns in long axis. The unlabelled neurons typically had nuclei which were more invaginated than those of the labelled neurons. Quantitative analyses of the synaptic organization in the NA revealed high proportions of terminals containing round vesicles and making asymmetrical or symmetrical contact with the somata in both monkey and cat. Terminals containing pleomorphic vesicles and making symmetrical contact with somata were slightly less numerous. Retrogradely labelled neurons exhibited a positive correlation between the size of neuron and density of synapses on the surface. There tended to be a greater synaptic density on monkey NA neurons than on cat NA neurons of comparable size.

The distribution of monosynaptic connexions from inspiratory bulbospinal neurones to inspiratory motoneurones in the cat

Monosynaptic connexions were sought between bulbospinal inspiratory neurones and inspiratory motoneurones of the C5 phrenic nerve or the intercostal nerves of up to six segments in the anaesthetized cat; they were identified by the presence of narrow peaks in cross-correlation histograms constructed from the spontaneous discharges of these neurones. About half of the bulbospinal neurones tested with the phrenic gave a monosynaptic connexion, as did just under half of those tested with the intercostal nerves. Some neurones gave connexions to both. For pairs of bulbospinal neurones and intercostal nerves, connexions were seen in 50 out of 295 instances (17%). No evidence was found of systematic patterns of connexions to different segments. Some neurones gave connexions to adjacent segments, others up to five segments apart. Bulbospinal neurones were not equipotent at giving connexions to the intercostal motoneurones. If a neurone gave a connexion to one segment, it was more likely to give connexion to another segment than chance would predict. The neurones with strong connexions were concentrated rostral to obex in the ventral respiratory group; neurones in both the caudal part of the ventral group and in the dorsal group gave weaker connexions. Regional variation within the medulla in the proportions of neurones giving connexions to the phrenic motoneurones was not seen, in particular neurones of the dorsal group (eight out of sixteen) and the ventral group (five out of eleven) were equipotent. When the strength of the connexions was assessed by k, the ratio of the peak count in the cross-correlation histogram to the base-line count, the mean strength was similar for the peaks seen for the phrenic (k = 1.113) and the intercostal (k = 1.114) motoneurones. However, because, on a segment-by-segment basis, fewer connexions were seen for the intercostals, the over-all mean value of k (absence of a narrow peak taken as k = 1.0) was stronger for the phrenic (k = 1.055) than for the intercostals (k = 1.019). For the phrenic, the apparent strength of connexion (assessed by k) was considerably stronger for the motoneurones giving large spikes than for those giving small ones. The same result was probably also true for the intercostals. With several assumptions, the total depolarization during inspiration derived monosynaptically in phrenic or intercostal motoneurones was calculated and found to be low, about 1 mV for the intercostals and 3-4 mV for the phrenic.(ABSTRACT TRUNCATED AT 400 WORDS)

Cells of origin of corticospinal projections to phrenic and thoracic respiratory motoneurones in the cat as shown by retrograde transport of HRP

A combined electrophysiological and histological approach was employed to identify neurones within the motor cortex which project to the vicinity of spinal respiratory motoneurones, and which may be involved in the alteration of the pattern of breathing under certain conditions. Recording of respiratory phased activity from phrenic, or from thoracic motoneurones within either the upper (T3-4) or lower (T8-9) segments, was followed by the iontophoretic injection of HRP at these recording sites. After injections within the cervical or thoracic ventral horn, 219 cells were retrogradely labelled in 14 experiments. The majority of these cells (88%) were labelled contralateral to the injection site. Following the injection of HRP into the phrenic nucleus, labelling was observed at two major sites within the anterior sigmoid gyrus (ASG), one along the anterolateral edge of the cruciate sulcus, and the other along the ventrolateral border of the ASG. In contrast, cells labelled after injections into the thoracic ventral grey matter were located more medially within the ASG and the posterior sigmoid gyrus (PSG). The populations of cells labelled following phrenic and thoracic injections overlapped, primarily at the lateral edge of the cruciate sulcus. The somas of labelled cells were pyramidal, round or oval. The mean diameters of cortical cells labelled after injections into the lower or upper thoracic segments were 30.5 +/- 6.2 and 31.5 +/- 5.6 respectively, which were not significantly different in size. However, they were significantly larger than the mean diameter of the cells labelled from injections into the phrenic nucleus (22.7 +/- 4.2 micron).(ABSTRACT TRUNCATED AT 250 WORDS)

The identification of brainstem neurones projecting to thoracic respiratory motoneurones in the cat as demonstrated by retrograde transport of HRP

Brainstem neurones which project to the immediate vicinity of the spinal motoneurones which supply the intercostal and abdominal respiratory muscles were identified by means of the retrograde transport of horseradish peroxidase (HRP). A combined electrophysiological and histological technique was used in which recording of phasic inspiratory or expiratory motoneurone activity within upper (T3-T4) or lower (T8-T9) thoracic segments was followed by the ion-tophoretic injection of HRP at these recording sites. HRP labelled cells were concentrated in those brainstem regions known to contain phasic respiratory neurones, namely the ventrolateral nucleus of the solitary tract (vl-NTS) or dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine (PB) nuclei. In 18 cats, 248 cells were labelled in these three respiratory regions of the brainstem while 668 were much more diffusely distributed in other regions of the medulla and pons. The ipsilateral and contralateral contributions within the respiratory regions were respectively; 23%:77% (DRG), 33%:67% (VRG), 95%:5% (PB). These results are considered in the general context of previous electrophysiological and histological findings, but also with particular reference to a related study of the projections from brainstem neurones to the phrenic nucleus [32].