The respiratory role of the ventral surface of the medulla studied in the anaesthetized rat

The caudal solitary complex is a site of central CO(2) chemoreception and integration of multiple systems that regulate expired CO(2)

The solitary complex is comprised of the nucleus tractus solitarius (NTS, sensory) and dorsal motor nucleus of the vagus (DMV, motor), which functions as an integrative center for neural control of multiple systems including the respiratory, cardiovascular and gastroesophageal systems. The caudal NTS-DMV is one of the several sites of central CO(2) chemoreception in the brain stem. CO(2) chemosensitive neurons are fully responsive to CO(2) at birth and their responsiveness seems to depend on pH-sensitive K(+) channels. In addition, chemosensitive neurons are highly sensitive to conditions such as hypoxia (e.g., neural plasticity) and hyperoxia (e.g., stimulation), suggesting they employ redox and nitrosative signaling mechanisms. Here we review the cellular and systems physiological evidence supporting our hypothesis that the caudal NTS-DMV is a site for integration of respiratory, cardiovascular and gastroesophageal systems that work together to eliminate CO(2) during acute and chronic respiratory acidosis to restore pH homeostasis.

Hyperventilation evoked by activation of the vicinity of the caudal inferior olivary nucleus depends on the fastigial nucleus in anesthetized rats

Several studies have demonstrated that cerebellar deep nuclei, particularly the rostral fastigial nucleus (FNr), are involved in respiratory modulation. These nuclei receive inputs from the contralateral caudal inferior olivary nuclei of the medulla. The objectives of this study were to determine whether electrical and chemical activation of the vicinity of the caudal inferior olivary nuclei (vIOc) affected respiration and, if true, whether the FNr was involved in the vIOc stimulation-evoked ventilatory responses. Experiments were conducted in 30 anesthetized and spontaneously breathing rats. Our results showed that 1) electrical (25 or 100 μA at 10 or 20 Hz for 10 s) and chemical (1 or 100 mM, 25–50 nl N-methyl-d-aspartate) stimulation of the vIOc augmented ventilation predominantly via increasing tidal volume; 2) the responses to the electrical stimulation were almost eliminated by lesion of the contralateral FNr via microinjection of ibotenic acid; and 3) the respiratory responses to electrical stimulation in the vicinity of the rostral IO were 65–70% smaller compared with that evoked by vIOc stimulation. These findings strongly suggest that vIOc neurons play a significant role in modulation of respiratory activity, largely depending on their projections to the FNr.

Electrical stimulation of the rabbit pulmonary artery increases respiratory output

This study was conducted to test the hypothesis that the pulmonary artery is involved in neural respiratory control and to identify the involved topographical region, if any. Six adult rabbits were anesthetized, artificially ventilated, and the chest was opened. The outer surface of the extra-pulmonary portion of the pulmonary artery was electrically stimulated by monitoring phrenic nerve activity. Phrenic nerve activity increased in three of the six rabbits when the proximal dorsal surface of the pulmonary trunk was stimulated. This positive response was abolished after bilateral vagotomy. In histological examinations we found densely grouped cells, i.e. pulmonary glomic tissue, with a fine nerve bundle in the tissue adjacent to the dorsal surface of the pulmonary trunk where electrical stimulation elicited respiratory augmentation. We suggest that there is a neural substrate which is involved in respiratory control inside the wall of or in the region adjacent to the proximal dorsal surface of the pulmonary trunk. Further studies to anatomically identify the neural substrate and clarify its physiological role in respiratory control are necessary.

Do circadian rhythms in respiratory control contribute to sleep-related breathing disorders?

Sleep-related respiratory dysfunction compromises the health and quality of life of millions of people worldwide, underscoring the need for a full understanding of the mechanisms by which the respiratory control system is altered at night. This paper suggests the hypothesis that the circadian timing system may play a role in the pathogenesis of some types of sleep-related breathing disorders. Recent studies have provided evidence that the circadian timing system has an influence on respiration and respiratory control, even in the absence of sleep. These new data are reviewed and potential mechanisms underlying the circadian modulation of breathing are outlined, identifying important gaps in our knowledge. It is proposed that circadian rhythms in respiratory control may increase the propensity for nocturnal respiratory instability and recurrent apnea. Importantly, circadian and sleep mechanisms appear to have additive effects on breathing, suggesting that the circadian timing system can potentially amplify or suppress sleep-related breathing abnormalities, depending upon the characteristics of the circadian output and the time of day at which sleep occurs.

Ventilatory response to asphyxia in conscious rats: effect of ambient and body temperatures

In many mammals the ventilatory response to hypoxia depends on ambient temperature (Ta), largely because of the hypometabolic effects of hypoxia below thermoneutrality. We questioned whether the ventilatory response to asphyxia also depends upon Ta, and the role played by metabolism and body temperature (Tb). Oxygen consumption (VO2) and pulmonary ventilation (VE) were measured in conscious rats at Ta = 27 degrees C (warm) and 11 degrees C (cold), breathing air or two levels of asphyxic gases, moderate (10% O2-4% CO2), or severe (10% O2-8% CO2), for approximately 30 min each. In the cold, the pattern of the VE response to moderate asphyxia was qualitatively similar to that seen in hypoxia alone, i.e the attained VE/VO2 was similar in warm and cold conditions, with, in the latter, a major drop in VO2 and little or no hyperpnea. During severe asphyxia, however, the VE/VO2 attained in the cold was less than in the warm, and it was accompanied by a large drop in Tb (approximately 6 degrees C). Blood gases confirmed the lower asphyxic hyperventilation in the cold. By maintaining Tb at 38 degrees C with an implanted abdominal heat exchanger, the VE/VO2 levels attained during asphyxia were the same between cold and warm conditions. We conclude that (a) the VE response to asphyxia is Ta-dependent, largely because of the hypometabolic effect of the hypoxic component in the cold, (b) during moderate asphyxia the hypercapnic component is qualitatively unimportant, and (c) with severe asphyxia the hypercapnia becomes an important contributor to the Ta-sensitivity by aggravating the decrease in Tb in the cold and lowering VE sensitivity.

Retrotrapezoid nucleus lesions decrease phrenic activity and CO2 sensitivity in rats

In chloralose-urethane anesthetized, paralyzed, and ventilated rats, we measured the effects of unilateral lesions in the region ventral and ventromedial to the facial nucleus, the retrotrapezoid nucleus (RTN), on eucapnic phrenic activity and the response to increased end-tidal CO2. Chemical (kainic acid injections; 4.7 mM; 10-100 nl) and electrolytic (5-15 mA; 5-15 sec) lesions, anatomically demonstrated to be in the RTN, resulted in a progressive decrease in the amplitude of the integrated phrenic nerve activity from baseline levels of 49-59% of maximum to values of 21-32% of maximum over 30 to 120 min. There were no consistent effects on frequency or on blood pressure. The initial slope of the response to hypercapnia was decreased by 86-92%. Bilateral carotid body ablation did not alter the general pattern of the responses. As in the cat, unilateral RTN lesions decrease baseline phrenic amplitude and virtually abolish the response to hypercapnia. We hypothesize that the RTN region provides; (1) a source of tonic activity which maintains eucapnic ventilatory output, and (2) allows expression of the response to hypercapnia.

Respiratory suppression by focal cooling of ventral medullary surface in anesthetized rats; functional and neuroanatomical correlate

Bilateral cooling of the parapyramidal region in the rostral ventral medullary surface (VMS) elicited a reduction in respiratory frequency and phrenic inspiratory activity in halothane anesthetized rats. A distinct cluster of neurons (nucleus parapyramidalis superficialis) was found in a superficial layer (10-15 microns from the surface) just beneath the area where cooling produced suppression of respiration. The rat VMS layer contains neural substrates which regulate the respiratory rhythm generation and inspiratory neural output.

Endothelin-sensitive areas in the ventral surface of the rat medulla

In urethane-anesthetized rats, subregions of the ventral surface of the medulla (VSM) in which endothelin (ET) caused cardiorespiratory effects were mapped by topically applying 1 pmol of ET-1. Two distinct subregions, termed the rostral and caudal ET-sensitive areas, were identified. The rostral area was also sensitive to L-glutamate and glycine. It extended between the caudal end of the trapezoid body and the rootlet of the XIIth nerve partly overlying the pyramidal tract. In this position ET-1 caused the type I response consisting of an initial increase (excitatory component) in arterial pressure (AP), renal sympathetic nerve activity (RSNA), heart rate (HR), phrenic nerve activity (PNA) and the number of bursts of PNA (burst rate) followed by a sustained decrease (inhibitory component) in them. The caudal ET-sensitive area was located near the rootlet of the XIIth nerve. In this position ET-1 caused the type II response consisting of a decrease in PNA and an increase in burst rate. Part of this area responded to nicotine but not to glutamate or glycine. ET-3 (10 pmol) applied to the two ET-sensitive areas produced responses similar to those elicited by ET-1. The dose-response relationship was investigated by delivering ETs to the rostral area. The excitatory component of most of the variables was elicited at a dose of 1 fmol of ET-1 or 1 pmol of ET-3, whereas the inhibitory component was produced at 10 fmol of ET-1 or 10 pmol of ET-3. These results suggest that subregions of the rat's VSM may participate in the central cardiorespiratory control by ET.

Modulation of respiratory reflexes by an excitatory amino acid mechanism in the ventrolateral medulla

Results from several studies suggest that the ventrolateral medulla (VLM) is involved in modulating the respiratory response to central and/or peripheral chemoreceptor stimulation. Furthermore, the excitatory amino acid (EAA) glutamate has been shown to have marked effects on respiration when administered to VLM sites. The purpose of this study was to determine if an excitatory amino acid mechanism in the VLM modulates the respiratory responses to hypoxia or hypercapnia in anesthetized rats. Exposure to hypoxic or hypercapnic gas under control conditions elicited increases in respiratory activity (diaphragmatic EMG activity and breathing frequency). Bilateral injection of kynurenic acid (KYN), an EAA antagonist, into rostral VLM sites evoked significant increases in breathing frequency; injections more caudal in the VLM typically caused apnea. Significantly larger increases in respiratory output were elicited by both hypoxia and hypercapnia after rostral VLM microinjections of KYN. The accentuated responses returned to control levels after a recovery of approximately 100 min. Microinjection of xanthurenic acid (XAN), an inactive analog of kynurenic acid, into the VLM prior to KYN had only slight effects on resting respiratory activity and no effects on the responses to hypoxia or hypercapnia. These results suggest two separate VLM sites which modulate respiration by EAA mechanisms. A more rostral site tonically inhibits respiratory activity and the respiratory responses to chemoreceptor stimulation and more caudal VLM sites may be required for the maintenance of respiratory activity.

Kainic acid on the rat ventral medullary surface depresses hypoxic and hypercapnic ventilatory responses

Kainic acid, topically applied to the ventral surface of the medulla immediately caudal to the trapezoid body in the urethane/chloralose anaesthetised rat, led to a depression of ventilation and a sustained rise in blood pressure; ventilatory responses to hypercapnia (10% carbon dioxide) and hypoxia (11% oxygen) were slightly depressed. Widespread application of kainic acid to an area at and slightly rostral to the rootlets of the hypoglossal nerve produced a stimulation of ventilation and an unsustained rise in blood pressure. Apnea ensued 12-28 min after application. Ventilatory responses to hypercapnia and hypoxia were markedly attenuated; more discrete bilateral application revealed two regions, one immediately rostral and lateral to the hypoglossal rootlets and the other over the point of exit of the hypoglossal nerve rootlets, which specifically contributed to the diminution of the chemosensory responses. These results raise questions about the medullary circuitry which mediates the chemoreflex regulation of breathing.

Depolarization and stimulation of neurons in nucleus tractus solitarii by carbon dioxide does not require chemical synaptic input

The effects of elevated CO2 (i.e. hypercapnia) on neurons in the nucleus tractus solitarii were studied using extracellular (n = 82) and intracellular (n = 33) recording techniques in transverse brain slices prepared from rat. Synaptic connections from putative chemosensitive neurons in the ventrolateral medulla were removed by bisecting each transverse slice and discarding the ventral half. In addition, the response to hypercapnia in 20 neurons was studied during high magnesium-low calcium synaptic blockade. Sixty-five per cent of the neurons (n = 75) tested were either insensitive or inhibited by hypercapnia. However, 35% (n = 40) were depolarized and/or increased their firing rate during hypercapnia. Nine out of 10 CO2-excited neurons retained their chemosensitivity to CO2 in the presence of high magnesium-low calcium synaptic blockade medium. Our findings demonstrate that many neurons in the nucleus tractus solitarii were depolarized and/or increased their firing rate during hypercapnia. These neurons were not driven synaptically by putative chemosensitive neurons of the ventrolateral medulla since this region was removed from the slice. Furthermore, because chemosensitivity persisted in most neurons tested during synaptic blockade, we conclude that some neurons in the nucleus tractus solitarii are inherently CO2-chemosensitive. Although the function of dorsal medullary chemosensitive neurons cannot be determined in vitro, their location and their inherent chemosensitivity suggest a role in cardiorespiratory central chemoreception.

CO2 decreases membrane conductance and depolarizes neurons in the nucleus tractus solitarii

To identify central sites of potential CO2/H+-chemoreceptive neurons, and the mechanism responsible for neuronal chemosensitivity, intracellular recordings were made in rat tissue slices in two cardiopulmonary-related regions (i.e., nucleus tractus solitarii, NTS; nucleus ambiguus, AMBc) during exposure to high CO2. When the NTS was explored slices were bisected and the ventral half discarded. Utilizing such "dorsal" medullary slices removed any impinging synaptic input from putative chemoreceptors in the ventrolateral medulla. In the NTS, CO2-induced changes in firing rate were associated with membrane depolarizations ranging from 2-25 mV (n = 15). In some cases increased e.p.s.p. activity was observed during CO2 exposure. The CO2-induced depolarization occurred concomitantly with an increased input resistance ranging from 19-23 M omega (n = 5). The lower membrane conductance during hypercapnia suggests that CO2-induced depolarization is due to a decreased outward potassium conductance. Unlike neurons in the NTS, AMBc neurons were not spontaneously active and were rarely depolarized by hypercapnia. Eleven of 12 cells tested were either hyperpolarized by or insensitive to CO2. Only 1 neuron in the AMBc was depolarized and it also showed an increased input resistance during CO2 exposure. Our findings suggest that CO2/H+-related stimuli decrease potassium conductance which depolarizes the cell and increases firing rate. Although our in vitro studies cannot guarantee the specific function of these cells, we believe they may be involved with brain pH homeostasis and cardiopulmonary regulation.

Respiratory and vasomotor responses to focal cooling of the ventral medullary surface (VMS) of the rat

Experiments were performed on anesthetized, paralyzed and artificially ventilated rats after denervation of the vagus and carotid sinus nerves. The electrical activity of the phrenic and cervical sympathetic nerves (CS) along with the arterial blood pressure (BP) were monitored. Graded unilateral cooling of the ventral lateral surface (VMS) from 37 degrees C to 10 degrees C between 6th and 12th nerve rootlets did not affect the phrenic activity. Whereas, a significant depression or apnea was seen with cooling of an area between 1st cervical and 12th nerve rootlets. Bilateral cooling also produced similar respiratory responses. Respiratory depression could also be obtained during higher respiratory drive (7% CO2 in O2). On the other hand, a significant fall in BP and reduction in CS activity were observed with unilateral cooling in any of these VMS areas. However, the magnitude of BP decrease was less with 7% CO2 in O2 compared to 100% O2 breathing.

Modulation of the centrally-evoked visceral alerting/defence response by changes in CSF pH at the ventral surface of the medulla oblongata and by systemic hypercapnia

In the present study on nine cats, repeated tests were made of the effects of superfusion of the ventral surface of the medulla oblongata with acid or alkaline CSF. Only two animals showed slight hyperventilation, tachycardia, mesenteric vasoconstriction and variable changes in hindlimb vascular conductance when the ventral surface was superfused with acid CSF; alkaline CSF produced opposite effects. These changes are qualitatively similar to, but much smaller than, published results which support the idea that the central chemoreceptor areas for CO2 are near the surface of the ventral medulla. But, in accord with those who have disputed this idea, the remaining 7 animals showed no response to superfusion with acid or alkaline CSF. Yet, all 9 animals showed marked hyperventilation in response to inhalation of 5% or 8% CO2. These findings accord with the view that chemosensitive structures on the ventral medulla represent part, but not all of the central chemosensitive mechanism for CO2. Inhalation of CO2 also induced bradycardia, mesenteric vasodilatation and either vasodilatation or vasoconstriction in hindlimb, attributable to a predominance of the direct myocardial depressant and local vasodilator effects of CO2, over the increase in sympathetic activity produced by central hypercapnia. But, despite the different effects of acid CSF and inhaled CO2 on baselines, they produced comparable effects on the visceral altering/defence response evoked by electrical stimulation in the ventral amygdalo-hypothalamic pathway viz, the magnitude of the characteristic hindlimb dilatation was reduced while that of the mesenteric constriction was increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Central chemosensitivity to H+ and CO2 in the rat respiratory center in vitro

The brainstem, cervical cord and attached phrenic nerve were excised from newborn rats and superfused in vitro. Respiratory output was measured by integration of phrenic nerve discharges. Respiratory output was enhanced by an increase in pCO2 at constant pH as well as by decreased pH with constant pCO2. It is concluded that the adequate stimulus to central chemoreceptors is not restricted only to H+.

The roles of medullary extracellular and cerebrospinal fluid pH in control of respiration

To determine the effective stimulus to the central chemoreceptors, we measured CSF and medullary extracellular fluid (ECF) pH and phrenic activity in 11 anesthetized, paralyzed, vagotomized and glomectomized cats. Flat-tipped pH electrodes (2 mm diam.) were used to measure ECF pH on the ventral surface of the medulla and CSF pH 2 mm above the surface. Changes in alveolar/arterial PCO2 were produced by airway occlusions of 10-20 sec durations. Changes in CSF PCO2 and pH were made by infusing 100% CO2 or an acid buffer into the CSF. Airway occlusion caused an increase of alveolar/arterial PCO2. ECF pH began to fall 6-10 sec later, with a maximum decrease of 0.032 pH unit at 21.9 sec. Phrenic activity increased as ECF pH decreased, the greatest activity occurring when ECF pH was most acid. CSF pH decreased after a longer delay. Its maximum decrease at 54.1 sec was smaller (0.026 pH unit) than ECF pH and did not correlate with the increase of phrenic activity. Addition of 100% CO2 or an acid buffer into the CSF produced an acid shift in the CSF pH but no change in ECF pH or phrenic activity. Prolonged (greater than 30 min) increase of acidity of CSF did not alter phrenic activity until ECF pH developed a delayed acid shift. Even then, the change of ECF pH was much smaller than that of CSF. We conclude that medullary chemoreceptors do not respond to changes of CSF pH or PCO2 and that change of pH of CSF minimally affects ECF pH. On the other hand, respiratory responses are closely linked to changes in ECF pH.

Projections from C2 and C3 nerves supplying muscles and skin of the cat neck: a study using transganglionic transport of horseradish peroxidase

Transganglionic transport of HRP has been used to trace the pathways and termination sites of cutaneous and muscle afferent axons entering from the C2 and C3 dorsal rami. The muscle afferent projection in the spinal cord is restricted and (apart from the ventral horn) largely confined to the intermediate gray matter. There is a muscle afferent projection to the ventrolateral main cuneate nucleus and a complex pattern of projection through the extent of the external cuneate nucleus. In contrast, the cutaneous spinal projection is abundant with extensive filling of axons in the tract of Lissauer and many termination sites in the lateral substantia gelatinosa. Axons enter the lateral gray matter of the cervical spinal cord from the dorsal columns and the dorsolateral funiculus and terminate in the lateral one-third of the dorsal horn as far rostral as the spinomedullary junction. Axons of the tract of Lissauer form a complex web around the dorsal horn and many penetrate rostrally to the region of the spinomedullary junction, where they terminate among clusters of interstitial cells on and close to the dorsal medullary surface. Cutaneous afferent axons from the dorsal columns turn into the main cuneate nucleus and enter a dense mass of HRP-reaction product which occupies the most ventrolateral part of the nucleus for its entire length.

Effects of medullary area I(s) cooling on respiratory response to chemoreceptor inputs

The effect on respiration, as measured by phrenic nerve activity, of bilateral graded cooling of the intermediate, or I(s), areas of the ventral medulla was determined in anesthetized, vagotomized, glomectomized and paralyzed cats. In addition the effect of cooling the I(s) areas on the responses to central and peripheral chemoreceptor afferent test stimuli were studied. When end-tidal PCO2 was kept constant, graded cooling of the I(s) areas led to graded reductions of phrenic activity and arterial pressure. Furthermore, the respiratory response to test stimuli (carotid sinus nerve or CO2) was decreased progressively during graded cooling of the I(s) areas from 40 degrees C to 20 degrees C. We conclude that area I(s) is part of a common pathway for afferent input from both the central and peripheral chemoreceptors and that it is involved in the initial integration of both inputs.

Effects of urethane-chloralose anaesthesia on respiration in the rat

1. Respiratory effects were measured in rats during six hours' anaesthesia with urethane and chloralose. 2. One-hundred min from urethane administration, minute ventilation (VE) was minimal, arterial PO2 was low, arterial PCO2 was high; tidal volume (VT) and respiratory frequency (f) were relatively constant; hypercarbic and hypoxic responses were substantial. 3. Between 100 and 400 min from urethane administration, minute ventilation and frequency increased and became more variable, tidal volume remained relatively constant, arterial PO2 rose to 100 mmHg, PCO2 fell to 37 mmHg; hypercarbic sensitivity increased and hypoxic sensitivity decreased. 4. We conclude that the anaesthetic regime produced initial depression of respiration relative to metabolism but without great loss of respiratory chemosensitivity. The respiratory depression was prolonged by increased dosage with urethane and chloralose. 5. The variations between hypercarbic and hypoxic responses confirm that they operate through separate mechanisms.