| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Department of Physiology and Biophysics, State University of New York at Stony Brook, Stony Brook, New York 11794-8661
 |
ABSTRACT |
|---|
 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
We examined the effects of focal tissue acidosis in the pre-Bötzinger complex (pre-BötC; the proposed locus of respiratory rhythm generation) on phrenic nerve discharge in chloralose-anesthetized, vagotomized, paralyzed, mechanically ventilated cats. Focal tissue acidosis was produced by unilateral microinjection of 10-20 nl of the carbonic anhydrase inhibitors acetazolamide (AZ; 50 µM) or methazolamide (MZ; 50 µM). Microinjection of AZ and MZ into 14 sites in the pre-BötC reversibly increased the peak amplitude of integrated phrenic nerve discharge and, in some sites, produced augmented bursts (i.e., eupneic breath ending with a high-amplitude, short-duration burst). Microinjection of AZ and MZ into this region also reversibly increased the frequency of eupneic phrenic bursts in seven sites and produced premature bursts (i.e., doublets) in five sites. Phrenic nerve discharge increased within 5-15 min of microinjection of either agent; however, the time to the peak increase and the time to recovery were less with AZ than with MZ, consistent with the different pharmacological properties of AZ and MZ. In contrast to other CO2/H+ brain stem respiratory chemosensitive sites demonstrated in vivo, which have only shown increases in amplitude of integrated phrenic nerve activity, focal tissue acidosis in the pre-BötC increases frequency of phrenic bursts and produces premature (i.e., doublet) bursts. These data indicate that the pre-BötC has the potential to play a role in the modulation of respiratory rhythm and pattern elicited by increased CO2/H+ and lend additional support to the concept that the proposed locus for respiratory rhythm generation has intrinsic chemosensitivity.
central respiratory chemoreceptors; control of breathing
| |
INTRODUCTION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
MULTIPLE FOCI IN THE BRAIN STEM have been shown to be CO2/H+ chemosensitive and have the potential to play a modulatory role in respiratory control. Work by Nattie and colleagues (3, 7, 8, 24) has established that multiple central chemosensitive regions modulating respiration may be identified in vivo by producing focal tissue acidosis with small-volume microinjections of the carbonic anhydrase inhibitor acetazolamide (AZ). These injections produce a circumscribed region of reduced tissue pH that leads to an increase in phrenic nerve discharge. As systemic PCO2 is maintained constant, the evoked increases in respiratory output can be ascribed to a central chemoreceptor response. The technique has revealed central CO2/H+ chemosensitivity modulating respiration in the rostral, caudal, and intermediate chemosensitive areas of the ventral medullary surface and the nucleus tractus solitarii, locus coeruleus, medullary raphe, and rostral ventral respiratory group (RVRG).
Results obtained from in vitro studies have recently suggested that chemosensitive neurons within the ventrolateral medulla may play a key role in the generation and modulation of respiratory rhythm (13-16, 42). The pre-Bötzinger complex (pre-BötC) located in the rostral ventrolateral medulla (RVLM) has been postulated to be essential for respiratory rhythm generation in neonatal mammals (for a recent review, see Ref. 28) and has been demonstrated to play a role in the generation and modulation of respiratory rhythm in adult mammals (1, 27, 37).
Recent work from our laboratory has also shown that the respiratory rhythm generator located in the pre-BötC is hypoxia chemosensitive (38). Focal hypoxia, produced by microinjection of small volumes of sodium cyanide, in the pre-BötC elicited modulation of phrenic burst pattern, increases in respiratory burst frequency, and tonic inspiratory discharge. Preliminary data involving GABAergic disinhibition suggest that the respiratory rhythm generator located in the pre-BötC can function as an O2 sensor in the brain over the physiological range (when disinhibited), acting as a medullary analog to chemoreceptors of the carotid bodies. Because peripheral chemoreceptors exhibit CO2/H+ chemosensitivity as well as hypoxic chemosensitivity and because other respiratory-related neurons in the brain stem manifest CO2/H+ sensitivity, we examined the effects of focal tissue acidosis in the pre-BötC on phrenic nerve discharge and arterial blood pressure.
| |
METHODS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
General. All experiments were performed under protocols approved by the Institutional Animal Care and Use Committee in compliance with the Animal Welfare Act and in accordance with Public Health Service Policy on Humane Care and Use of Laboratory Animals.
Experiments were conducted in 26 adult cats weighing 3.4-5.4 kg. The cats were anesthetized initially by placing them inside a sealed, plastic induction chamber into which a gaseous mixture of halothane (5%) and O2 was introduced. After the cats were anesthetized, they were removed from the chamber, and anesthesia was maintained by delivering halothane (1.5-4%) and O2 through a face mask placed over the cat's nose and mouth. The right brachial vein was cannulated, and
-chloralose (35 mg/kg iv) was
administered. The gaseous anesthetic was gradually reduced over a
30-min period as the -chloralose took effect. Supplemental
-chloralose (3-5 mg/kg iv) was given as needed. The adequacy of
anesthesia was regularly verified by firmly pinching a toe. If an
increase in blood pressure was evoked or, during the absence of
paralysis (see below), if the cat withdrew its limb, additional
anesthetic was given. Dexamethasone (2 mg iv) was administered to
minimize brain swelling. Both brachial arteries were cannulated. The
left brachial cannula was connected to a Statham transducer (P23XL) for
measurement of arterial blood pressure; the right brachial cannula was
used for sampling arterial blood.
The trachea was cannulated low in the neck, and the lungs were
mechanically ventilated with room air enriched with O2. The cats were then paralyzed with gallamine triethiodide (2-4 mg/kg iv) or vecuronium bromide (0.2-0.4 mg/kg iv), with supplemental doses administered as needed. The chest was opened bilaterally through
the sixth or seventh intercostal spaces, and the expiratory outlet of
the ventilator was placed under 2-3 cmH2O to prevent collapse of the lungs during expiration. End-tidal CO2 was
monitored continuously through a side port in the tracheal cannula and
was maintained constant (±0.2%) between 4.0 and 6.0% by adjusting the tidal volume and frequency of the ventilator. Arterial
PO2, PCO2, and pH were measured at hourly
intervals (Radiometer ABL-500), and, when necessary, blood-gas values
were corrected by either adjusting the ventilator or intravenous
infusion of sodium bicarbonate (8.5%). No microinjections were made
within 30 min of bicarbonate infusion. At the start of the experimental protocol (see below), arterial PO2,
PCO2, and pH averaged 181 ± 5 (SE)
Torr, 39 ± 1 Torr, and 7.34 ± 0.01, respectively. Body temperature
was maintained at 36-38°C with the use of a heating pad and a
heat lamp.
Both cervical vagus nerves were exposed and cut bilaterally. In 12 cats, the carotid sinus nerves were also exposed and cut bilaterally.
The cat's head was then placed in a stereotaxic instrument, and the
dorsal surface of the medulla was exposed by separating the nuchal
musculature along the midline, removing the basioccipital bone, and
opening the atlantooccipital membrane. The dorsal surface of the brain
stem was covered with saline or mineral oil to prevent drying.
The C5 rootlet of one or both phrenic nerves was isolated
in the neck via a lateral approach, cut, and desheathed, and the central end was placed on a bipolar platinum-rod electrode. The nerve
was then covered with a mixture of mineral oil and petroleum jelly.
Phrenic nerve discharge was amplified (×1,000-10,000) and filtered (10-10,000 Hz), and a moving average was obtained by using a third-order Paynter filter with a 100-ms time constant. Both
the raw and averaged nerve outputs were recorded on digital tape (model
4000A, A. R. Vetter) and on a chart recorder throughout the
experimental protocol.
Experimental protocol. We examined the effects of focal tissue acidosis in the pre-BötC on phrenic nerve discharge and arterial blood pressure. Focal tissue acidosis was produced by unilateral microinjection of AZ or methazolamide (MZ), both of which inhibit carbonic anhydrase. Responses to unilateral microinjection of AZ and MZ from a total of 14 sites in the pre-BötC were recorded. Only one pre-BötC site was examined per animal. In six sites, we also recorded responses to unilateral microinjection of vehicle (see below) into the pre-BötC. To control for spread of injectate, we also recorded responses to unilateral microinjection of AZ from an additional 10 sites adjacent (within 300-500 µm) to the pre-BötC, including the RVRG. Only one microinjection of AZ or MZ was made into a single site, and no more than two microinjections were made in a single animal. All sites in the pre-BötC were initially localized by using predetermined stereotaxic coordinates relative to the calamus scriptorius (3.4-4.2 mm rostral), midline (3.8-4.1 mm lateral), and dorsal surface (4.2-4.5 mm ventral). The range in values for the coordinates used to find the pre-BötC reflects the variability in the dimensions of the brain stems in cats of different sizes. Sites were then functionally identified as pre-BötC by using DL-homocysteic acid (DLH), as previously described (37), and were histologically confirmed after completion of the experiments (see Location of injection sites). The functional criteria for identification of the pre-BötC were production of either a series of high-amplitude, short-inspiratory-duration bursts, tonic excitation, or augmented bursts (i.e., eupneic breath ending with a high-amplitude, short-duration burst) of phrenic nerve discharge.
Microinjections into the pre-BötC were made with the use of a triple-barreled glass pipette (12- to 20-µm tip diameter) attached to a pressure injection device (General Valve Picospritzer II). One barrel of the pipette contained 10 mM DLH. The second barrel contained 50 µM AZ or 50 µM MZ. The third barrel contained Fast Green dye (2%), which was used to mark the injection sites (
100 nl). AZ and MZ were
initially dissolved in DMSO (0.6 µl/ml) and brought to their final
concentration by adding distilled water or saline. A vehicle solution
using the same concentration of DMSO was used in six additional
experiments instead of AZ or MZ as a control for nonspecific effects.
DLH and Fast Green dye were dissolved in a saline solution. The pH of
all microinjected chemicals was adjusted to 7.29-7.37.
Microinjection volumes of DLH, AZ, and MZ were 10-20 nl, and
microinjection typically required 1-2 s to complete.
Microinjection of vehicle was 20 nl, and microinjection similarly
required 1-2 s to complete. The volume of each injection was
monitored by observing the displacement of the fluid meniscus by using
a microscope equipped with an eyepiece reticule.
Histology. At the conclusion of the experiment, the cat was killed under deep anesthesia by an injection of saturated KCl solution. The brain stem was removed and placed in 4% Formalin for at least 48 h. The brain stem was then frozen, sectioned coronally (40 µm), mounted on slides, and stained for cell bodies by using 1% Neutral Red dye. With the use of a microprojector, we made drawings of tissue sections containing sites marked with Fast Green dye.
Data analysis. We measured peak amplitude of integrated phrenic nerve activity, frequency of phrenic bursts, arterial blood pressure, and arterial blood gases before, at 5 min after, and every 15 min after microinjection of AZ or MZ into the pre-BötC until phrenic nerve output returned toward control levels. Amplitude of integrated phrenic nerve discharge and frequency of phrenic bursts were determined from the phrenic neurogram off-line. Baseline and response values for these variables were determined by averaging the values obtained from 1-min intervals at each time point. These data are reported as a percent change from preinjection baseline levels of discharge, which were set at 100%. For mean arterial pressure (MAP), baseline and response values were taken as the steady-state values reached at each time point.
All values are reported as means ± SE. Responses before and after stimulation were analyzed by a repeated-measures ANOVA to determine statistical significance, followed by Fisher's least significant difference post hoc test, for which the criterion level was set at P < 0.05.| |
RESULTS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
General effects of microinjection of AZ and MZ into the pre-BötC. Six sites in the pre-BötC received AZ, and eight sites in the pre-BötC received MZ. The effects on phrenic nerve discharge were bilaterally symmetrical (n = 12 animals with bilateral recordings). Unilateral microinjection of AZ and MZ (50 µM, 10-20 nl) into the pre-BötC reversibly increased phrenic nerve discharge in each of the sites examined within 5-15 min of microinjection of either agent. The magnitude of the increase in phrenic nerve discharge in response to AZ and MZ was similar with each agent; however, the overall time course for these responses was different. In addition, similar phrenic neurogram and blood pressure responses were observed in cats with carotid sinus nerves intact (AZ, n = 2; MZ, n = 5) or cut (AZ, n = 4; MZ, n = 3); therefore, these data will not be considered separately. The effects on amplitude of integrated phrenic activity and frequency of phrenic bursts are described below.
Effects on amplitude of integrated phrenic activity.
Unilateral microinjection of AZ and MZ (50 µM, 10-20 nl) into
the pre-BötC reversibly increased peak amplitude of phrenic nerve
discharge in each of the sites examined. Figure
1 shows an example depicting these
amplitude effects. In this example, unilateral microinjection of AZ
(Fig. 1, A and C) and MZ (Fig. 1, B and
D) increased peak amplitude of integrated phrenic nerve discharge with no change in the frequency of phrenic bursts. This increase in amplitude had an onset latency of ~15 min for AZ and 5 min for MZ. Both AZ and MZ elicited similar peak (maximal) increases in
amplitude; however, this peak (maximal) response followed slightly different time courses (75 min for AZ; 105 min for MZ). In addition, the duration of this increase in amplitude was less for AZ (135 min)
than for MZ (180 min).
|
|
|
|
Effects on frequency of phrenic bursts. Unilateral microinjection of AZ and MZ (50 µM, 10-20 nl) into the pre-BötC reversibly increased the frequency of eupneic phrenic bursts in 7 of the 14 sites examined. In these seven sites, microinjection of AZ and MZ also produced an increase in amplitude of integrated phrenic nerve discharge (as described above). In five of these sites, microinjection of AZ and MZ also evoked augmented bursts. Figure 4 shows an example depicting these frequency effects. In this example, unilateral microinjection of AZ (Fig. 4, A and C) and MZ (Fig. 4, B and D) increased both amplitude of integrated phrenic nerve discharge and frequency of phrenic bursts. The increases in amplitude and frequency followed a similar time course and were similar in magnitude. Both agents elicited an increase in amplitude and frequency of ~10% within 5 min from the beginning of microinjection, and the peak (maximal) responses occurred at 60 and 75 min for AZ and MZ, respectively. Although the onset latency and time to peak for the increases in amplitude and frequency were similar, the duration of these increases was less for AZ (105 min) than for MZ (165 min).
Unilateral microinjection of AZ and MZ elicited an increase in the frequency of eupneic phrenic bursts in 50% of the sites examined; in the remaining sites, no change in the frequency of eupneic bursts was evoked by microinjection of AZ or MZ. Overall for the group as a whole (including both frequency responders and nonresponders; n = 14), however, there was an increase in the frequency of eupneic bursts, which averaged 26.0 ± 11.2% (P < 0.05). Averaged data for the frequency of eupneic phrenic bursts for preinjection baseline, onset, peak response, and recovery evoked by unilateral microinjection of AZ and MZ in those sites that showed a frequency response (n = 7) are shown in Fig. 5. Because there were no differences in the magnitude of the increases in frequency in response to AZ (range = 27-105% increase above preinjection baseline; n = 3) and MZ (range = 25-123% increase above preinjection baseline; n = 4), these data have been combined. In these sites, unilateral microinjection of AZ and MZ increased the frequency of phrenic bursts by 55.6 ± 15.7% (P < 0.01) above preinjection baseline.
|
|
Effects on MAP. Unilateral microinjection of AZ and MZ (50 µM, 10-20 nl) into the pre-BötC reversibly increased arterial blood pressure in 6 of the 14 sites examined; no change in MAP was seen in the remaining 8 sites. There were no differences in the magnitude of the increases in MAP produced by microinjection of AZ (n = 3) and MZ (n = 3) into these sites; therefore, these data have been combined. In these sites, unilateral microinjection of AZ and MZ reversibly increased MAP from 134 ± 11 (baseline) to 152 ± 12 (peak response) to 136 ± 11 mmHg (recovery) (P < 0.01). The time course for the onset and peak blood pressure and phrenic neurogram responses was similar; however, MAP usually recovered 15-30 min before the recovery of phrenic nerve discharge.
Control injections. Unilateral microinjections of vehicle (DMSO control, 20 nl) were made into six sites in the pre-BötC to control for nonspecific effects. Vehicle microinjection was ineffective in producing an increase in peak amplitude of integrated phrenic nerve discharge or frequency of phrenic bursts in five of these sites. In the remaining site, there was a small increase in phrenic neurogram amplitude (<10%); however, arterial PCO2 also increased in this animal.
We also examined the effects of unilateral microinjection of AZ into 10 sites adjacent (within 300-500 µm) to the pre-BötC including the RVRG as a control for the spread of injectate. In these experiments, a site in the pre-BötC was functionally identified by using DLH, and then the pipette was moved dorsal, medial, or caudal in increments of 100-500 µm. Subsequent microinjection of DLH into sites 300-500 µm dorsal or medial was ineffective in altering phrenic nerve discharge. For RVRG sites, subsequent microinjection of DLH produced a small, transient increase in the amplitude of integrated phrenic nerve discharge. In each of the sites examined in the RVRG (n = 5), unilateral microinjection of AZ (50 µM, 20 nl) reversibly increased peak amplitude of integrated phrenic nerve discharge with no change in the frequency of phrenic bursts. In addition, unilateral microinjection of AZ into these sites was ineffective in producing either augmented bursts or premature (doublet) bursts in the phrenic neurogram. Figure 7 shows the averaged data for preinjection baseline, onset, peak response, and recovery evoked by unilateral microinjection of AZ into the RVRG. Unilateral microinjection of AZ into the RVRG increased the amplitude of integrated phrenic nerve discharge by 38.1 ± 9.2% (P < 0.05) above preinjection baseline. The onset latency for this increase in amplitude averaged ~15 min. The peak increase in amplitude typically occurred between 30 and 60 min (mean ± SE = 41.3 ± 7.2 min), after which the amplitude began to return toward baseline values. Recovery was usually noted within 75 min (mean ± SE = 71.3 ± 9.4 min). In the remaining five sites adjacent to the pre-BötC, unilateral microinjection of AZ was ineffective in producing an increase in peak amplitude of integrated phrenic nerve discharge or frequency of phrenic bursts.
|
Location of injection sites.
The distribution of sites in which AZ and MZ were microinjected into
the pre-BötC is shown in Fig.
8A. As landmarks for identifying the rostrocaudal level of the pre-BötC, we identified the caudal pole of the retrofacial nucleus, nucleus ambiguus, the rostral pole of
the lateral reticular nucleus, and the rostral pole of the hypoglossal
nucleus. All sites in the pre-BötC were identified with reference
to the caudal pole of the retrofacial nucleus, not the obex.
|
| |
DISCUSSION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
We have demonstrated that blockade of carbonic anhydrase in the pre-BötC elicits a long-lasting but reversible increase in phrenic nerve discharge. As do others, we attribute the AZ- and MZ-induced responses to focal tissue acidosis. The increase in phrenic nerve discharge consisted of an increase in peak amplitude of integrated activity, production of augmented bursts (i.e., eupneic breath ending with a high-amplitude, short-duration burst), and an increase in the frequency of bursts (including premature bursts). These responses did not require input from the peripheral chemoreceptors, as similar results were obtained from animals with the carotid sinus intact or cut. In addition, it is unlikely that these responses resulted from nonspecific effects of the injectate, because they were not produced by microinjection of vehicle. We, therefore, suggest that the pre-BötC has intrinsic chemosensitivity mediated by focal changes in CO2 and/or H+ in this region.
Mechanisms of central chemoreception. The exact chemical stimulus by which central chemoreceptors are activated to drive ventilation is not completely resolved, although molecular CO2, extracellular pH, and intracellular pH have all been implicated (for reviews see Refs. 19 and 23). Both in vivo and in vitro studies have proposed that CO2 and H+ exert independent and differential effects on central respiratory chemoreceptors (5, 11, 13, 17, 20, 32, 33, 42). For example, Borison et al. (5) reported that, in the decerebrate, peripherally denervated cat, the respiratory response to an increase in arterial H+ was greater with respiratory-induced acidosis than with metabolically induced acidosis. Similarly, in the anesthetized, peripherally denervated cat, Eldridge et al. (11) found that raising PCO2 increased phrenic nerve activity to a greater extent than did infusion of HCl for the same change in medullary extracellular pH.
Data obtained from in vitro studies further suggest that CO2 and H+ exert differential effects with respect to frequency and amplitude of inspiratory motor output (13, 39, 42). In the isolated neonatal rodent brain stem-spinal cord preparation, for example, alteration of superfusate pH at constant PCO2 elicits changes in both respiratory burst frequency and amplitude of phrenic nerve bursts, whereas alteration of superfusate PCO2 at constant pH produces a transient change in amplitude but not frequency of phrenic bursts (13). It is not clear whether these reflect differences in the effect of CO2/H+ at one or more receptors or differential access of CO2 and H+ to receptors in the brain stem.AZ- and MZ-induced tissue acidosis. The exact mechanism by which microinjection of AZ and MZ produces focal tissue acidosis is not completely understood. It has been demonstrated, however, that inhibition of carbonic anhydrase by intravenous administration of AZ in both rabbits (4) and cats (41) produces an increase in brain PCO2 and medullary extracellular fluid H+ concentration. This increase in brain PCO2 has been suggested to result from interference with the hydration of CO2 and impairment of transport in brain capillaries and red blood cells (4, 31, 41), whereas the increase in extracellular fluid H+ concentration has been suggested to result from an accumulation of metabolically produced H+ (4, 31). Thus the extracellular fluid H+ concentration and brain PCO2 can increase independently in response to inhibition of brain carbonic anhydrase (4). Because both of these mechanisms contribute to the production of focal tissue acidosis, and PCO2 and H+ may independently stimulate central chemoreceptors (5, 11, 13, 20, 32, 33, 40), it is unclear whether our responses were evoked by focal increases in PCO2 or changes in H+ concentration.
In our experiments, focal tissue acidosis was produced pharmacologically by unilateral microinjection of AZ and MZ. Although both AZ and MZ are cell-permeable carbonic anhydrase inhibitors, MZ differs from AZ because of its slightly higher solubility, lower plasma protein binding, and longer activity (21, 22). In our experiments, we found that both agents were effective in eliciting a similar increase in peak amplitude of integrated phrenic nerve activity and frequency of phrenic bursts; however, the time course for these responses was dependent on the agent microinjected. In general, the onset latency was slightly shorter for MZ than for AZ, although this difference was not statistically significant. Furthermore, the time to the peak increase and the time to recovery were generally shorter with AZ than with MZ. These findings appear to be consistent with the pharmacological properties of AZ vs. MZ discussed above.Widespread sites of central CO2/H+ chemosensitivity using AZ. Previous studies have used injection of AZ to demonstrate central CO2/H+ chemosensitivity for respiratory control in widespread medullary sites (3, 7, 8, 24). In these studies, AZ injection produced a region of sustained focal tissue acidosis that was sufficient to induce an increase in phrenic nerve discharge. The acidosis produced at the center of the injection site, as measured by pH electrodes, was reported to be similar to that evoked by increasing end-tidal PCO2 by ~40 Torr (range = 36-47 Torr), with no detectable pH change noted at distances 300-400 µm from the injection center (8, 24). In our experiments, we did not measure the medullary tissue pH; however, we believe that the changes in phrenic nerve activity obtained in our experiments are likely to be specific to focal tissue acidosis within the pre-BötC and not simply spread to other chemosensitive sites, although some effects from spread cannot be excluded. We do note that the spread of injectate, and presumably tissue acidosis, in our experiments most likely exceeded that reported in some other studies (3, 8, 24), as our injection volumes were larger. However, microinjection of AZ into sites located 300-500 µm dorsal or medial to the pre-BötC was ineffective in increasing phrenic nerve activity, and microinjection of AZ into sites in the RVRG (within 500 µm of the pre-BötC) produced only small increases in the amplitude of integrated phrenic nerve discharge, consistent with the increases in magnitude previously reported in other studies (24). Furthermore, examination of the data in the studies by Coates et al. (8) and Nattie and Li (24), in which focal tissue acidosis was produced in the rostral and caudal chemosensitive areas, the intermediate area of the ventral medullary surface, and the RVRG, (i.e., regions that are adjacent to the pre-BötC), indicates that there was an increase in the amplitude of integrated phrenic nerve activity, with little or no modulation of respiratory rhythm. In our experiments, microinjection of AZ and MZ into the pre-BötC dramatically increased the frequency of eupneic phrenic bursts in one-half of the sites examined and produced premature bursts (i.e., phase resetting of the respiratory rhythm), consistent with stimulation of the respiratory rhythm generator located in the pre-BötC.
It should be noted that, in our experiments, microinjection of AZ and MZ into the pre-BötC increased the amplitude of integrated phrenic nerve discharge in all of the sites examined. In contrast, previous studies have found that ~40% of the AZ microinjections are ineffective in eliciting an increase in phrenic nerve discharge (3, 8, 24). One possible explanation for our findings is related to the concentration and volume of AZ and MZ used in our experiments. Because our concentration of AZ was higher and our volume of AZ and MZ was larger, it is possible that microinjection of AZ and MZ produced a greater intensity and spread of tissue acidosis in our experiments. If so, then our microinjections had the potential to produce a more intense stimulation of CO2/H+ chemosensitive neurons in the region under investigation as well as to affect a greater number of neurons involved in respiratory control. It should be noted, however, when Coates et al. (7) microinjected just beneath the medullary surface even higher concentrations and larger volumes than those used in our study, only 79% of the sites examined showed an increase in phrenic nerve activity. Another possibility for the difference between these previous studies and our present findings results from the methods employed to select injection sites. In the studies by Bernard et al. (3) and Coates et al. (7), no attempt was made to functionally identify respiratory regions. In contrast, all sites used in our study were selected on the basis that microinjection of DLH modified respiratory rhythm and pattern. Thus sites in which AZ injections are ineffective may reflect sites within the medulla that do not contain sufficient numbers of neurons involved in respiratory control. In support of this idea, Nattie and Li (24) found in a recent study that, when sites in the RVRG were functionally identified by using glutamate, microinjection of AZ increased phrenic nerve activity in all sites examined, whereas, in functionally unidentified sites in the RVRG, microinjection of AZ elicited an increase in phrenic nerve activity in ~64% of the sites studied. These studies raise the idea that CO2/H+ chemosensitivity may be a widely distributed property of premotor respiratory neurons.CO2/H+ chemosensitivity and respiratory rhythm generation. It is well accepted that central chemosensitivity provides a major source of tonic input to the neurons that make up the respiratory rhythm generator, thus mediating basal respiration as well as changes in breathing in response to changes in CO2 and H+. Recent in vitro studies, moreover, suggest that chemosensitive neurons within the ventrolateral medulla play a key role in the generation and modulation of respiratory rhythm (13-16, 39, 42). For example, perfusing the isolated neonatal rat brain stem-spinal cord with a Krebs solution of high pH (addition of NaOH) markedly depresses the rate of respiratory rhythmic oscillations recorded from the C4 ventral root, whereas perfusion with low pH (addition of HCl) markedly enhances activity (39). Furthermore, CO2-induced reductions in pH of the superfusate bathing the isolated neonatal rat brain stem-spinal cord elicits a substantial increase in the frequency of respiration-related spinal (C2) or phrenic (C4 or C5) nerve bursts (16, 42). In addition, elevating the H+ in the superfusate bathing the neonatal rat medullary slice containing the pre-BötC increases hypoglossal burst frequency (14).
In our experiments, we found that focal tissue acidosis in the pre-BötC, the primary locus of respiratory rhythm generation (34), modifies both respiratory rhythm and pattern in addition to increasing the amplitude of eupneic bursts. We interpret the phase resetting of respiratory rhythm in our study, demonstrated by increases in the frequency of phrenic bursts and the production of premature bursts, as an indication of intrinsic chemosensitivity of the rhythm generating "kernel." This interpretation is supported by recent work from Johnson et al. (15) that demonstrated an increase in burst frequency of synaptically isolated pre-BötC pacemaker neurons in response to increasing H+ (at a constant CO2) of the superfusate bathing the neonatal rat medullary slice. These pacemaker neurons in the pre-BötC exhibit conditional bursting properties and are thought to provide rhythmic drive to the rest of the respiratory network during the inspiratory phase of network activity (6, 12, 18, 28, 34, 35). In the hybrid pacemaker-network model for respiratory rhythm generation, synaptic interactions synchronize and modify the basic rhythm generator by modulating the intrinsic membrane conductances of the conditional bursting (i.e., voltage-dependent) pacemaker neurons. Although Johnson et al. (15) have demonstrated that pre-BötC pacemaker neurons are intrinsically chemosensitive, it is unclear from their study or our present findings whether other populations of neurons in this region are also CO2/H+ chemosensitive. For example, in the adult cat, the pre-BötC has been shown to contain a high concentration of preinspiratory (pre-I) (9, 29) or inspiratory-driver (I-driver) (30) neurons, which are proposed to be key elements in respiratory phase transition (35, 36). These pre-I or I-driver neurons are also known to provide excitatory drive to inspiratory neurons that exhibit augmenting and decrementing patterns of discharge in the respiratory network (30). It is possible that stimulation of these neurons elicited the modulations in respiratory rhythm reported in our study. In our experiments, we found that, in addition to the modulation of respiratory rhythm, there was also an increase in the amplitude of integrated phrenic nerve discharge and the production of augmented bursts (i.e., eupneic breath ending with a high-amplitude, short-duration burst). As the conditional bursting pacemaker neurons located in the pre-BötC are known to provide excitatory drive to other inspiratory neurons in the respiratory network, the patterning changes evoked by focal tissue acidosis in the pre-BötC could have resulted from CO2/H+-mediated excitation of these chemosensitive neurons. However, as noted above, chemosensitivity of other than pacemaker neurons (i.e., pre-I or I-driver neurons) can also explain the patterning (i.e., amplitude and augmented bursts) changes observed.CO2/H+ chemosensitivity and arterial blood pressure. In addition to the increase in phrenic nerve discharge, in six sites we also evoked a reversible increase in MAP. Although stimulation of central chemoreceptors by increasing systemic CO2 is known to increase sympathetic nerve activity and arterial blood pressure (26), the increase in blood pressure evoked in our study most likely resulted from respiratory modulation of sympathetic activity (2, 10) mediated by the effects of focal tissue acidosis of the respiratory rhythm generator located in the pre-BötC. Previous neuroanatomic studies with the use of intracellular biocytin injections have demonstrated that inspiratory neurons originating in the RVRG (some in the vicinity of the pre-BötC) project directly onto tyrosine hydroxylase immunoreactive neurons located in the RVLM (25), suggesting that these respiratory neurons may provide direct synaptic input to RVLM sympathoexcitatory neurons.
Conclusion. We have demonstrated that focal tissue acidosis in the pre-BötC increases peak amplitude of integrated phrenic nerve activity, produces augmented bursts, and, in some sites, increases frequency of phrenic bursts (including production of premature bursts). The increase in frequency is in contrast to other CO2/H+ brain stem chemosensitive sites demonstrated in vivo, which have only showed increases in amplitude, but is consistent with the in vitro response to focal tissue H+ in the pre-BötC. Our findings suggest that the pre-BötC has the potential to play a role in the modulation of respiratory rhythm and pattern elicited by increased CO2/H+ and lends additional support to the concept that the locus for respiratory rhythm generation has intrinsic chemosensitivity.
| |
ACKNOWLEDGEMENTS |
|---|
We thank Dr. Eugene Nattie for suggestions and thoughtful review of the manuscript and Tami Halat for excellent technical assistance.
| |
FOOTNOTES |
|---|
This work was supported by National Heart, Lung, and Blood Institute Grant HL-16022 and American Lung Association (ALA) Grant RG-008-N. I. C. Solomon is an Edward Livingston Trudeau Scholar from the ALA.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: I. C. Solomon, Dept. of Physiology and Biophysics, Health Sciences Center, Basic Science Tower, Level 6, Rm. 140, State Univ. of New York at Stony Brook, Stony Brook, NY 11794-8661 (E-mail: ICSolomon{at}physiology.pnb.sunysb.edu).
Received 27 December 1999; accepted in final form 31 January 2000.
| |
REFERENCES |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
1.
Abrahams, TP,
Hornby PJ,
Dalton DP,
DaSilva AMT,
and
Gillis RA.
An excitatory amino acid(s) in the ventrolateral medulla is (are) required for breathing to occur in the anesthetized cat.
J Pharmacol Exp Ther
259:
1388-1395,
1991
2.
Bachoo, M,
and
Polosa C.
Properties of the inspiration-related activity of sympathetic preganglionic neurons of the cervical trunk in the cat.
J Physiol (Lond)
385:
545-564,
1987
3.
Bernard, DG,
Li A,
and
Nattie EE.
Evidence for central chemoreception in the midline raphé.
J Appl Physiol
80:
108-115,
1996
4.
Bickler, PE,
Litt L,
Banville DL,
and
Severinghaus JW.
Effects of acetazolamide on cerebral acid-base balance.
J Appl Physiol
65:
422-427,
1988
5.
Borison, HL,
Hurst JH,
McCarthy LE,
and
Rosenstein R.
Arterial hydrogen ion vs. CO2 on depth and rate of breathing in decerebrate cats.
Respir Physiol
30:
311-325,
1977[Web of Science][Medline].
6.
Butera, RJ, Jr,
Rinzel J,
and
Smith JC.
Models of respiratory rhythm generation in the pre-Bötzinger complex. II. Populations of coupled pacemaker neurons.
J Neurophysiol
81:
398-415,
1999.
7.
Coates, EL,
Li A,
and
Nattie EE.
Acetazolamide on the ventral medulla of the cat increases phrenic output and delays the ventilatory response to CO2.
J Physiol (Lond)
441:
433-451,
1991
8.
Coates, EL,
Li A,
and
Nattie EE.
Widespread sites of brain stem ventilatory chemoreceptors.
J Appl Physiol
75:
5-14,
1993
9.
Connelly, CA,
Dobbins EG,
and
Feldman JL.
Pre-Bötzinger complex in cats: respiratory neuronal discharge patterns.
Brain Res
590:
337-340,
1992[Web of Science][Medline].
10.
Darnall, RA,
and
Guyenet P.
Respiratory modulation of pre- and postganglionic lumbar vasomotor sympathetic neurons in the rat.
Neurosci Lett
119:
148-152,
1990[Web of Science][Medline].
11.
Eldridge, FL,
Kiley JP,
and
Millhorn DE.
Respiratory responses to medullary hydrogen ion changes in cats: different effects of respiratory and metabolic acidosis.
J Physiol (Lond)
358:
285-297,
1985
12.
Feldman, JL,
and
Smith JC.
Neural control of respiratory pattern in mammals: an overview.
In: Regulation of Breathing (2nd ed.), edited by Dempsey JA,
and Pack AI.. New York: Dekker, 1995, p. 39-69.
13.
Harada, Y,
Kuno M,
and
Wang YZ.
Differential effects of carbon dioxide and pH on central chemoreceptors in the rat in vitro.
J Physiol (Lond)
368:
679-693,
1985
14.
Johnson, SM,
Koizumi H,
Trouth CO,
and
Smith JC.
Chemosensory responses of the isolated respiratory network in medullary slices in vitro (Abstract).
Neuroscience
23:
1252,
1997.
15.
Johnson, SM,
Trouth CO,
and
Smith JC.
Chemosensitivity of respiratory pacemaker neurons in the pre-Bötzinger complex in vitro (Abstract).
Neuroscience
24:
875,
1998.
16.
Kawai, A,
Ballantyne D,
Mückenhoff K,
and
Scheid P.
Chemosensitive medullary neurones in the brainstem-spinal cord preparation of the neonatal rat.
J Physiol (Lond)
492:
277-292,
1996
17.
Kogo, N,
and
Arita H.
In vivo study on medullary H+-sensitive neurons.
J Appl Physiol
69:
1408-1412,
1990
18.
Koshiya, N,
and
Smith JC.
Neuronal pacemaker for breathing visualized in vitro.
Nature
400:
360-363,
1999[Medline].
19.
Lassen, NA.
Is central chemoreceptor sensitive to intracellular rather than extracellular pH?
Clin Physiol
10:
311-319,
1990[Web of Science][Medline].
20.
Loeschcke, HH.
Central chemosensitivity and the reaction theory.
J Physiol (Lond)
332:
1-24,
1982
21.
Maren, TH.
Carbonic anhydrase inhibition. V. N5-substituted 2-acetylamino-1,3,4-thiadiazole-5-sulfonamides: metabolic conversion and use as control substances.
J Pharmacol Exp Ther
117:
385-401,
1956
22.
Maren, TH.
The general physiology of reactions catalyzed by carbonic anhydrase and their inhibition by sulfonamides.
Ann NY Acad Sci
429:
568-579,
1984[Web of Science][Medline].
23.
Nattie, EE.
Central respiratory chemoreceptors. Cellular mechanisms and developmental aspects.
In: Developmental Neurobiology of Breathing, edited by Haddad GG,
and Farber JP.. New York: Dekker, 1990, p. 341-371.
24.
Nattie, EE,
and
Li A.
Central chemoreception in the region of the ventral respiratory group in the rat.
J Appl Physiol
81:
1987-1995,
1996
25.
Pilowsky, P,
Llewellyn-Smith IJ,
Lipski J,
Minson J,
Arnolda L,
and
Chalmers J.
Projections from inspiratory neurons of the ventral respiratory group to the subretrofacial nucleus of the cat.
Brain Res
633:
63-71,
1994[Web of Science][Medline].
26.
Preiss, G,
and
Polosa C.
The relation between end-tidal CO2 and discharge patterns of sympathetic preganglionic neurons.
Brain Res
122:
255-267,
1977[Web of Science][Medline].
27.
Ramirez, JM,
Schwarzacher SW,
Pierrefiche O,
and
Olivera BM.
Selective lesion of the cat pre-Bötzinger complex in vivo eliminates breathing but not gasping.
J Physiol (Lond)
507:
895-907,
1998
28.
Rekling, JC,
and
Feldman JL.
PreBötzinger complex and pacemaker neurons: hypothesized site and kernel for respiratory rhythm generation.
Annu Rev Physiol
60:
385-405,
1998[Web of Science][Medline].
29.
Schwarzacher, SW,
Smith JC,
and
Richter DW.
Pre-Bötzinger complex in the cat.
J Neurophysiol
73:
1452-1461,
1995
30.
Segers, LS,
Shannon R,
Saporta S,
and
Lindsey BG.
Functional associations among simultaneously monitored lateral medullary respiratory neurons in the cat. I. Evidence for excitatory and inhibitory actions of inspiratory neurons.
J Neurophysiol
57:
1078-1100,
1987
31.
Severinghaus, JW,
Hamilton FN,
and
Cotev S.
Carbonic acid production and the role of carbonic acid anhydrase in decarboxylation in the brain.
Biochem J
114:
703-705,
1969[Web of Science][Medline].
32.
Shams, H.
Differential effects of CO2 and H+ as central stimuli of respiration in the cat.
J Appl Physiol
58:
357-364,
1985
33.
Shams, H,
Ahmad HR,
and
Loeschcke HH.
The dependence of ventilation on H+ activity in the extracellular fluid of the brain in respiratory or metabolic acidosis (Abstract).
Pflügers Arch
389:
R53,
1981.
34.
Smith, JC,
Ellenberger HH,
Ballanyi K,
Richter DW,
and
Feldman JL.
Pre-Bötzinger complex: a brainstem region that may generate respiratory rhythm in mammals.
Science
254:
726-729,
1991
35.
Smith, JC,
Funk GD,
Johnson SM,
and
Feldman JL.
Cellular mechanisms generating respiratory rhythm: insights from in vitro and computational studies.
In: Ventral Brainstem Mechanisms and Control of Respiration and Blood Pressure, edited by Trouth CO,
Millis R,
Kiwull-Schone H,
and Schlaefke M.. New York: Dekker, 1995, p. 463-496.
36.
Smith, JC,
Greer JJ,
Guosong L,
and
Feldman JL.
Neural mechanisms generating respiratory pattern in mammalian brain stem-spinal cord in vitro. I. Spatiotemporal patterns of motor and medullary neuron activity.
J Neurophysiol
64:
1149-1169,
1990
37.
Solomon, IC,
Edelman NH,
and
Neubauer JA.
Patterns of phrenic motor output evoked by chemical stimulation of neurons located in the pre-Bötzinger complex in vivo.
J Neurophysiol
81:
1150-1161,
1999
38.
Solomon IC, Edelman NH, and Neubauer JA. Pre-Bötzinger
functions as a central hypoxia chemosensor for respiration in vivo.
J Neurophysiol In press.
39.
Suzue, T.
Respiratory rhythm generation in the in vitro brain stem-spinal cord preparation of the neonatal rat.
J Physiol (Lond)
354:
173-183,
1984
40.
Tepemma, LJ,
Barts PWJA,
Folgering HT,
and
Evers JAM
Effects of respiratory and (isocapnic) metabolic arterial acid-base disturbances on medullary extracellular fluid pH and ventilation in cats.
Respir Physiol
53:
379-395,
1983[Web of Science][Medline].
41.
Tepemma, LJ,
Rochette F,
and
Demedts M.
Effects of acetazolamide on medullary extracellular pH and PCO2 and on ventilation in peripherally chemodenervated cats.
Pflügers Arch
415:
519-525,
1990[Web of Science][Medline].
42.
Voipio, J,
and
Ballanyi K.
Interstitial PCO2 and pH, and their role as chemostimulants in the isolated respiratory network of neonatal rats.
J Physiol (Lond)
499:
527-542,
1997
This article has been cited by other articles:
|
|
 |
|
  N. L. Nichols, D. K. Mulkey, K. A. Wilkinson, F. L. Powell, J. B. Dean, and R. W. Putnam Characterization of the chemosensitive response of individual solitary complex neurons from adult rats Am J Physiol Regulatory Integrative Comp Physiol, March 1, 2009; 296(3): R763 - R773. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. L. Krause, H. V. Forster, T. Kiner, S. E. Davis, J. M. Bonis, B. Qian, and L. G. Pan Normal breathing pattern and arterial blood gases in awake and sleeping goats after near total destruction of the presumed pre-Botzinger complex and the surrounding region J Appl Physiol, February 1, 2009; 106(2): 605 - 619. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. L. Krause, H. V. Forster, S. E. Davis, T. Kiner, J. M. Bonis, L. G. Pan, and B. Qian Focal acidosis in the pre-Botzinger complex area of awake goats induces a mild tachypnea J Appl Physiol, January 1, 2009; 106(1): 241 - 250. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. M. Johnson, M. A. Haxhiu, and G. B. Richerson GFP-expressing locus ceruleus neurons from Prp57 transgenic mice exhibit CO2/H+ responses in primary cell culture J Appl Physiol, October 1, 2008; 105(4): 1301 - 1311. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. G. Guyenet The 2008 Carl Ludwig Lecture: retrotrapezoid nucleus, CO2 homeostasis, and breathing automaticity J Appl Physiol, August 1, 2008; 105(2): 404 - 416. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. B. Dias, A. Li, and E. Nattie Focal CO2 dialysis in raphe obscurus does not stimulate ventilation but enhances the response to focal CO2 dialysis in the retrotrapezoid nucleus J Appl Physiol, July 1, 2008; 105(1): 83 - 90. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. G. Guyenet, R. L. Stornetta, and D. A. Bayliss Retrotrapezoid nucleus and central chemoreception J. Physiol., April 15, 2008; 586(8): 2043 - 2048. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Taccola, L. Secchia, and K. Ballanyi Anoxic persistence of lumbar respiratory bursts and block of lumbar locomotion in newborn rat brainstem spinal cords J. Physiol., December 1, 2007; 585(2): 507 - 524. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. B. Dias, T. B. Nucci, L. O. Margatho, J. Antunes-Rodrigues, L. H. Gargaglioni, and L. G. S. Branco Raphe magnus nucleus is involved in ventilatory but not hypothermic response to CO2 J Appl Physiol, November 1, 2007; 103(5): 1780 - 1788. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. S. Moreira, A. C. Takakura, E. Colombari, and P. G. Guyenet Central chemoreceptors and sympathetic vasomotor outflow J. Physiol., November 15, 2006; 577(1): 369 - 386. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. L. Stornetta, T. S. Moreira, A. C. Takakura, B. J. Kang, D. A. Chang, G. H. West, J. F. Brunet, D. K. Mulkey, D. A. Bayliss, and P. G. Guyenet Expression of Phox2b by Brainstem Neurons Involved in Chemosensory Integration in the Adult Rat J. Neurosci., October 4, 2006; 26(40): 10305 - 10314. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. G. Guyenet, R. L. Stornetta, D. A. Bayliss, and D. K. Mulkey Re: Homing in on the specific phenotype(s) of central respiratory chemoreceptors Exp Physiol, May 1, 2005; 90(3): 266 - 268. [Full Text] [PDF]
|
|
|
|
|
|
P. G. Guyenet, R. L. Stornetta, D. A. Bayliss, and D. K. Mulkey Retrotrapezoid nucleus: a litmus test for the identification of central chemoreceptors Exp Physiol, May 1, 2005; 90(3): 247 - 253. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. W. Putnam, J. A. Filosa, and N. A. Ritucci Cellular mechanisms involved in CO2 and acid signaling in chemosensitive neurons Am J Physiol Cell Physiol, December 1, 2004; 287(6): C1493 - C1526. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. E. Taylor, M. B. Harris, J. C. Leiter, and M. J. Gdovin Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs Am J Physiol Regulatory Integrative Comp Physiol, December 1, 2003; 285(6): R1461 - R1472. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
I. C. Solomon Focal CO2/H+ alters phrenic motor output response to chemical stimulation of cat pre-Botzinger complex in vivo J Appl Physiol, June 1, 2003; 94(6): 2151 - 2157. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
I. C. Solomon Influence of respiratory network drive on phrenic motor output evoked by activation of cat pre-Botzinger complex Am J Physiol Regulatory Integrative Comp Physiol, February 1, 2003; 284(2): R455 - R466. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. E Nattie and A. Li Substance P-saporin lesion of neurons with NK1 receptors in one chemoreceptor site in rats decreases ventilation and chemosensitivity J. Physiol., October 15, 2002; 544(2): 603 - 616. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Mutolo, F. Bongianni, M. Carfi, and T. Pantaleo Respiratory changes induced by kainic acid lesions in rostral ventral respiratory group of rabbits Am J Physiol Regulatory Integrative Comp Physiol, July 1, 2002; 283(1): R227 - R242. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. G. Guyenet, C. P. Sevigny, M. C. Weston, and R. L. Stornetta Neurokinin-1 Receptor-Expressing Cells of the Ventral Respiratory Group Are Functionally Heterogeneous and Predominantly Glutamatergic J. Neurosci., May 1, 2002; 22(9): 3806 - 3816. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Q. Liu and M. T. T. Wong-Riley Postnatal expression of neurotransmitters, receptors, and cytochrome oxidase in the rat pre-Botzinger complex J Appl Physiol, March 1, 2002; 92(3): 923 - 934. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. Xu, Z. Zhang, and D. T. Frazier Microinjection of acetazolamide into the fastigial nucleus augments respiratory output in the rat J Appl Physiol, November 1, 2001; 91(5): 2342 - 2350. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
) and frequency of phrenic bursts (
) evoked by a
single microinjection of AZ (C) and a single microinjection of
MZ (D), respectively. In these sites, unilateral microinjection
of AZ and MZ into pre-BötC reversibly increased amplitude of
integrated phrenic nerve discharge with little or no change in
frequency of phrenic bursts. Note that magnitude of increase in
amplitude was similar for both agents microinjected. Phrenic nerve
discharge (C and D) represents average values obtained
from multiple respiratory cycles over a 1-min interval at each time
point and is expressed as a percentage of preinjection baseline, with
baseline levels of discharge set at 100%.
, RVRG) in which
microinjections of AZ were made. Also shown in B are
corresponding pre-BötC sites (
). Lines in B
connect pre-BötC site to corresponding adjacent site. All sites
in pre-BötC are identified with reference to caudal pole of
retrofacial nucleus (0 mm). Each section is meant to encompass level
indicated ± 0.2 mm (rostrally and caudally). RFN, retrofacial
nucleus; NA, nucleus ambiguus; 5SP, spinal nucleus of the trigeminal
nerve; 5ST, spinal tract of the trigeminal nerve; LRN, lateral
reticular nucleus; ION, inferior olivary nucleus; and P, pyramidal
tract.