Dorsal Respiratory Group Neurons Research Articles

Effects of treadmill running exercise on neuronal expression of c-Fos protein in the medulla oblongata after unilateral phrenicotomy in Wistar rats

The purpose of this study was to examine whether treadmill running exercise after phrenicotomy resulted in change in neuronal expression of c-Fos protein in the medulla oblongata in Wistar rats. All rats were subjected to left-sided unilateral phrenicotomy. In test experiments, rats were trained with treadmill running for 4 weeks, while in control experiments rats were not provided with the opportunity to exercise. c-Fos protein, a transient transcription factor, was immunohistochemically stained by the ABC method, and surveyed in the medulla oblongata. Numbers of c-Fos-immunoreactive (c-Fos-ir) neurons were counted microscopically, and their distribution in the medulla oblongata was mapped. In the dorsal respiratory group (DRG), numbers of c-Fos-ir neurons were found to be significantly increased in the test rats compared with control rats, while in the ventral respiratory group (VRG) no significant differences were found in numbers of c-Fos-ir neurons between test and control rats. These findings suggest that DRG neurons may play more important roles than VRG neurons in recovery of respiration in treadmill exercise after unilateral phrenicotomy in Wistar rats.

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.

Discharge patterns of dorsal and ventral respiratory group neurons during spontaneous augmented breaths observed in pentobarbital anesthetized rats

To clarify the difference between the firing patterns of the dorsal respiratory group (DRG) and ventral respiratory group (VRG) neurons during spontaneous augmented breaths, extracellular single unit recording of 139 respiratory-related neurons (inspiratory: 98 139 , expiratory: 41 139 ) was performed in pentobarbital anesthetized rats. Both the I and E neurons were further classified into six groups: (1) I-augmenting, (2) I-decrementing, (3) I-other cells, (4) E-augmenting, (5) E-decrementing and (6) E-other cells. During the augmented breaths, most inspiratory neurons in the DRG ( 25 26 ) and VRG ( 67 72 ) show an increase in their discharge frequency irrespective of the cell type, but the discharges after an augmented breath were inhibited. Changes in these inspiratory neurons coincided with those of diaphragm electromyogram activity. With regard to relative changes in the mean firing rate during the inspiratory phase II of augmented breaths, there was a significant difference between the I-augmenting DRG and VRG neurons (353.5 ± 56.9% vs. 237.5 ± 17.1%, P < 0.01), but not in the I-decrementing and I-other neurons. On the other hand, the activities of the expiratory DRG ( 9 9 ) and VRG ( 27 32 ) neurons decreased during the augmented breath. A significant difference in the relative mean firing rate during the expiratory phase of augmented breaths was observed between the E-decrementing DRG and VRG neurons (27.3 ± 5.2% vs. 58.0 ± 6.3%, P < 0.05), but not between the E-augmenting and E-other neurons. These results suggested that during spontaneous augmented breaths the firing patterns of the DRG neurons were not qualitatively different from those of the VRG neurons.

Electrophysiological study of dorsal respiratory neurons in the medulla oblongata of the rat

There has been controversy whether the dorsal respiratory group (DRG), identified in the cat and several other species as a concentration of mainly inspiratory neurons located in the ventrolateral subnucleus of the solitary tract, also exists in the rat. The aim of this study was to re-examine this question by systematically exploring this region with extracellular microelectrodes, in anesthetized and artificially ventilated rats. One-hundred and forty-two units were recorded which fired in phase with central respiratory cycles (determined by recording from the phrenic nerve) and/or lung inflations. One-hundred and nineteen recordings were thought to be from neuronal cell bodies (confirmed in some cases by excitatory responses to microelectrophoretic administration of dl-homocysteic acid), while the remaining 23 were from lung vagal afferents. Most neurons in the former group (87/119) were inspiratory. Out of 96 neurons tested for spinal projections only 14 (12 inspiratory, 2 expiratory) responded antidromically following stimulation at C 3 segment. These results confirm the existence of the DRG in the rat and demonstrate that neurons located in this region have firing patterns generally similar to those previously described in the cat. The main difference is the relative paucity in the rat of neurons projecting spinally below the C 2 level, which indicates that most DRG neurons in this species do not project directly to phrenic and intercostal motoneurons, but to other, as yet unidentified, neuronal groups within the brainstem or upper cervical segments.

おう吐における吸気筋群活動の神経支配

Vomiting is a nonrespiratory behavior during which the major respiratory muscles contract in a characteristic pattern which generates changes in the intrathoracic and intra abdominal pressures that lead to expulsion of gastric content. During vomiting, the major inspiratory muscles (the diaphragm and the external intercostal) co-contract with the expiratory abdominal muscles in a series of activity bursts.The question arises whether brain stem respiratory neurons are involved in the control of these muscles during vomiting. In the present study, we undertook to determine whether bulbospinal inspiratory neurons in the dorsal respiratory group (DRG) and in the para-ambiguus region of the ventral respiratory group (VRG), together with the upper cervical inspiratory propriospinal neurons, are important for activation of the diaphragm and external intercostal muscles during vomiting. To analyze the behavior of these neurons, we elicited “fictive vomiting” using either emetic agents or electrical stimulation of the supradiaphragmatic vagal nerve afferents in decerebrate paralyzed cats. This fictive vomiting was identified by a characteristic series of bursts from coactivation of the phrenic nerve and some other nerves innervating abdominal muscles, which would be expected to produce an expulsion of gastric contents in nonparalyzed animals.Only a few (<10%) bulbospinal DRG and VRG inspiratory neurons were found to have a response capable of initiating a discharge of the phrenic and external intercostal motoneurons during fictive vomiting. In contrast to the behavior of the bulbospinal inspiratory neurons, more than one-half of the upper cervical inspiratory propriospinal neurons were active in phase with the phrenic discharge during fictive vomiting. These results indicate that the DRG and VRG bulbospinal inspiratory neurons do not make a major contribution to the activation of the major inspiratory muscles, and that the upper cervical inspiratory neurons may be one candidate for the activation of these inspiratory muscles during fictive vomiting.

Effects of contralateral superior laryngeal nerve stimulation on dorsal medullary inspiratory neurons

The effects of superior laryngeal nerve (SLN) stimulation on phrenic (PHR) nerve activity and on activity of dorsal respiratory group (DRG) inspiratory (I) neurons contralateral to the stimulus were examined in decerebrate, paralyzed cats. Stimulation caused bilateral PHR suppression followed by recovery at ca. 30 ms. Most DRG neurons (70%) contralateral to the stimulus were inhibited, but average onset of inhibition lagged that of PHR suppression. This contrasts sharply with the observation in an earlier study that inhibition of ipsilateral I neurons on the average preceded PHR suppression. The remaining neurons (30%) were not inhibited. Only 22% of contralateral neurons were excited by SLN stimulation, in contrast to 52% of ipsilateral neurons. Thus, contralateral DRG I neurons do not mediate the onset of bilateral PHR suppression by SLN stimulation and the probably inhibited through a longer pathway than that for the ipsilateral unit responses.

Connectivity of slowly adapting pulmonary stretch receptors with dorsal medullary respiratory neurons.

1. Intracellular recordings were made from 50 dorsal respiratory group (DRG) neurons in the region of the ventrolateral nucleus of the solitary tract in anesthetized, paralyzed cats ventilated with a cycle-triggered pump whose inflation stroke was triggered by the onset of phrenic nerve inspiratory (I) discharge. Activity was recorded simultaneously in the ipsilateral nodose ganglion from sensory cell bodies of slowly adapting pulmonary stretch receptors (PSRs). 2. Respiratory cycle-related membrane potential changes of DRG neurons were recorded. Twenty-six neurons that did not exhibit spikes were classified as I alpha, I beta or pump (P)-cells by comparing their membrane potential trajectories during I in the presence of lung inflation with that observed during I, but with lung inflation withheld. The remaining 24 neurons were classified similarly, but the classification was based upon a comparison of their I-phase spike activity responses with and without lung inflation. I phase-related histograms of either membrane potential or spike activity were constructed to facilitate DRG neuronal classification. Additionally, steady lung inflation of varying magnitudes was applied during the expiratory phase. This prolonged expiration and produced different responses in the neurons. Generally, I beta and P-cells were depolarized, whereas I alpha cells were hyperpolarized. 3. Low-intensity electrical stimulation of the ipsilateral vagus nerve evoked excitatory postsynaptic potentials (EPSPs) in all three DRG neuronal types. P-cells and I beta cells exhibited EPSPs in response to the lowest intensity; generally this intensity was below threshold for the simultaneously recorded PSR. Overall, EPSPs in I alpha cells had the highest thresholds, but some EPSPs could be evoked at thresholds similar to those of the I beta cells. The distributions of the average onset latency of the evoked EPSP overlapped considerably. Thus vagal electrical stimulation cannot be used for unequivocal classification of DRG neurons into I alpha, I beta, and P-cell subpopulations. 4. Using intracellular spike-triggered averaging, single PSRs were shown to generate monosynaptic EPSPs in I beta neurons and P-cells but not I alpha cells. Divergence of single PSR afferents also was observed. Relationships between EPSP shape factors, amplitudes, and PSR afferent conduction velocity are similar to those previously observed for monosynaptic EPSPs in hindlimb motoneurons generated by spinal afferents.

Responses of early and late onset phrenic motoneurons to lung inflation

In anesthetized or decerebrate cats that were paralyzed and ventilated with a cycle-triggered pump, we produced changes in activity of the whole phrenic nerve and of individual phrenic motoneurons (fibers or cell in the spinal cord) by withholding lung inflation during the inspiratory (I) phase. The neurons were classified into early- and late-onset types (discharge onset less or greater than 80 msec, respectively, after whole phrenic onset). Both unit and whole phrenic activity exhibited a variety of responses to inflation (excitation, depression, or no effect); but there were no consistent differences between responses of early- and late-onset neurons. The distribution of responses was quite different from that of dorsal respiratory group (DRG) I neurons (Cohen and Feldman, 1984); in particular there was no group of phrenic neurons corresponding to the late-onset I-beta neurons (I neurons excited by inflation). We conclude that the inputs to late-onset phrenic neurons are not predominantly or exclusively from late-onset DRG neurons.

Monosynaptic excitation of dorsal medullary respiratory neurons by slowly adapting pulmonary stretch receptors.

The discharges of individual slowly adapting pulmonary stretch receptors (PSRs) and single respiratory neurons of the dorsal respiratory group (DRG) within the ventrolateral subnucleus of the solitary tract were recorded simultaneously in anesthetized, paralyzed, artificially ventilated cats. DRG neurons were classified as P-cell, I-alpha, or I-beta, based on the alteration in activity during respiratory cycles when lung inflation was withheld compared to activity when the lungs were inflated in phase with central inspiratory drive. In some cases, vagal stimulation was also used to classify respiratory neurons. Synaptic input of PSRs onto DRG neurons was examined by the construction of cross-correlograms for the simultaneously recorded discharge of individual PSRs (reference events) and individual DRG neurons (correlated events). Monosynaptic excitatory connections were demonstrated by peaks in 26% of the cross-correlograms of PSR and P-cell pairs and 20% of the cross-correlograms of PSR and I-beta neuron pairs. The ratio of the peak number of occurrences to the background number of occurrences (k value) was comparable for P-cells and I-beta neurons: 2.96 +/- 2.09 and 1.62 +/- 0.36 (mean +/- SD), respectively. P-cells and I-beta neurons also had similar short latencies for the peak of increased probability of discharge: 2.74 +/- 0.53 and 2.57 +/- 0.63 ms, respectively. No evidence was obtained demonstrating synaptic connectivity between PSRs and I-alpha neurons. Interpretations of this negative result are discussed.

Intercostal and abdominal muscle afferent influence on medullary dorsal respiratory group neurons

Studies were performed on anesthetized, paralyzed, artificially ventilated cats. Phrenic (C 5) efferent activity, dorsal respiratory group (DRG) neuron activity in the vicinity of the medullary solitary tract complex, and thoracic dorsal root compound action potentials were recorded during electrical stimulation of intercostal and lumbar nerves. DRG neurons were identified by their firing pattern and response to lung inflation. Phrenic activity (PA) was inhibited by stimulating external intercostal nerves T 3−T 10 internal intercostal nerves T 3−T 12, lateral branch of the main intercostal nerves T 6−T 12, or lumbar nerves 1–2. Stimulation of lower (T 9−T 11) intercostal or lumbar nerves produced a short duration (10–20 msec) facilitation of PA prior to the inhibition. Facilitation and inhibition of PA were correlated with recruitment of afferent fibers from muscle proprioceptors. Inspiratory neurons (Iα and Iβ) in the DRG were inhibited simultaneously with PA regardless of the nerves stimulated. DRG neurons which fired in phase with lung inflation (P cells) were unaffected by nerve stimulation evenn though PA was inhibited. Lower intercostal nerve (T 9−T 11) stimulation produced a brief facilitation of medullary neuron activity simultaneous with facilitation of PA. It is concluded that intercostal and abdominal muscle proprioreceptor afferents, and perhaps cutaneous afferents, reflexly alter the activity of DRG inspiratory neurons (Iα, Iβ) which drive the phrenic motoneurons. The inhibitory effect is not via P cells but may be via other interneurons in close proximity to the I cells.

Dorsal respiratory group neurons in the medulla of cat: Spinal projections, responses to lung inflation and superior laryngeal nerve stimulation

Extracellular activity of 200 dorsal respiratory group (DRG) neurons of the ventrolateral nucleus of the tractus solitarius was recorded in anesthetized, paralyzed, artificially ventilated cats. The effects on neural activity of electrically stimulating the contralateral cervical spinal cord, the superior laryngeal nerves (SLN) bilaterally, and inflating the lungs to various levels during the expiratory pause were tested. 1. (1) Of the 200 cells, 157 were inspiratory cells; 32 cells (termed P cells), fired synchronously with lung inflation but did not have inspiratory rhythm of central origin; and 11 were non-inspiratory respiratory cells. Eighty-eight of the inspiratory cells which were inhibited with inhibition of phrenic inspiratory discharge following lung inflation were classified as α-type. However, most also showed short duration activation (gasp reflex) at the onset of lung hyperinflation. Thirty-five inspiratory cells which were excited by lung inflation in addition to showing the gasp reflex were classified as β-type neurons. The discharge of these β-type cells adapted during lung inflation; their excitation threshold was higher and sensitivity lower than that of the Hering-Breuer expiratory facilitatory reflex. I conclude that they are not the interneurons responsible for that reflex, but could possibly receive excitatory inputs from pulmonary irritant receptors. 2. (2) Approximately 80% of both α- and β-type neurons had axonal projections to the cord. A spinal cord collision test error and relative error were calculated for 93 projecting DRG neurons. Only two neurons had a collison error of zero, the rest had positive values. I conclude that these results imply only spinal axonal projections. 3. (3) Fifty-seven (58%) of the inspiratory cells tested were evoked from the ipsilateral SLN, with a mean latency of 3.9 msec ± 0.3 S.E.. All cells that were evoked with short latency followed stimuli delivered at 100 Hz. No cell was evoked from the contralateral SLN. I suggest that some DRG neurons are part of a disynaptic neural circuit involving SLN afferents, ipsilateral DRG neurons, and contralateral phrenic motoneurons.