| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1Department of Life Science, National Taiwan Normal University, 3Department of Sports, Health, and Leisure, Chihlee Institute of Technology, Taipei, 4National Center for High-Performance Computing, Hsinchu, Taiwan; and 2Department of Physical Therapy, University of Florida, Gainesville, Florida
Submitted 11 May 2007 ; accepted in final form 26 September 2007
 |
ABSTRACT |
|---|
 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
facial motoneurons; pulmonary stretch receptors
We recently reported that respiratory bursts recorded in UAW motor nerves persist during phrenic apnea evoked by increasing positive end-expired pressure (PEEP) from the control (baseline) value of 3 cmH2O to 9 cmH2O (28). A similar response can also be induced by hypothermia and hypocapnia (43). In the case of PEEP-induced uncoupling, the response is dependent on vagal afferent inputs and reflects activation of UAW Pre-I and expiratory-inspiratory (EI) neurons during the Pre-I period with concurrent inhibition of UAW I neuron discharge and phrenic bursting (28). The overall effect is that Pre-I UAW activity is dissociated (i.e., "uncoupled") from I phrenic bursting. The dependence of uncoupled UAW bursting on Pre-I neuronal discharge suggests that inhibition of Pre-I activity could abolish or attenuate the uncoupling response to 9 cmH2O PEEP.
Pulmonary C fibers (PCFs) are unmyelinated vagal afferent neurons that innervate the lungs and lower airways (37). PCF receptors are activated during pulmonary edema (8, 9) and can be stimulated by external irritants (e.g., smoke and ozone) (29) and internal chemicals (e.g., histamine, prostaglandin, and bradykinin) (22). Pharmacological activation of PCF receptors in laboratory experiments is often accomplished with capsaicin or phenydiguanide (30, 37). Activation of PCF receptors evokes pulmonary chemoreflexes characterized by apnea, hypotension, and bradycardia (6, 7, 37). In addition, we previously reported that activation of PCF receptors with capsaicin inhibits respiratory-related UAW nerve and/or muscle activity as reflected by a decrease in burst amplitude coupled with a simultaneous delay in burst onset (27, 33). Thus capsaicin injection appears to inhibit Pre-I UAW neuronal activity, and this observation led us to hypothesize that stimulation of vagal PCF receptors could attenuate or abolish Pre-I neuron-dependent, uncoupled UAW respiratory bursting during 9 cmH2O PEEP. To test our hypothesis, we recorded respiratory-related bursting in the facial nerve and studied its response to capsaicin during elevated (9 cmH2O) PEEP.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
Twenty-six Male Wistar rats (372 ± 5 g body wt) were studied. Experimental procedures were similar to those described in our previous studies (26–28) and were approved by the Animal Care and Use Committee at National Taiwan Normal University.
Animals were treated with atropine (0.5 mg/kg im) and then anesthetized with urethane (1.2 g/kg ip; Sigma, St. Louis, MO). The rats were considered to be fully anesthetized when the withdrawal reflex was absent during application of pressure to the paws or tail with a hemostat. Anesthesia was supplemented (0.06 g/kg) if any reflex activity was observed. The rats were placed in a supine position, and the trachea was cannulated with polyethylene (PE-240) tubing below the larynx. Femoral arterial and venous catheters (PE-50) were inserted for blood pressure (BP) measurement and drug administration, respectively. The right jugular vein was cannulated (PE-100) for capsaicin injection. Rats were paralyzed with gallamine triethiodide (5 mg/kg iv) and artificially ventilated with 100% O2 (60–70 breaths/min, 10 ml/kg). Rectal temperature was maintained at 37 ± 1°C by a heating lamp and/or blanket. The end-tidal fractional concentration of CO2 was monitored (Electrochemistry CD3A, Ametek, Pittsburgh, PA) and maintained at 5% by regulation of ventilatory stroke volume and frequency. PEEP was applied by placement of the outlet tube of the ventilator under water and maintained at 3 cmH2O during baseline conditions.
Nerve Recording
The phrenic nerve was exposed at C4–C5 with a ventral approach. The facial nerve innervates the muscles of facial expression, including the alae nasi muscles (41), and its peripheral end was isolated near the maxilla. After an incision was made between the nostril and the angle of the jaw, the facial nerve was observed in apposition to the dorsal labial vein, cut distally, and desheathed as previously described (20). The compound action potentials (i.e., neurograms) in the facial and phrenic nerves were recorded using bipolar electrodes, amplified (AC preamplifier P511, Grass Instruments, Quincy, MA), filtered (0.3–3 kHz), and integrated (time constant = 50 ms). Facial nerve filaments were dissected using no. 5 forceps under a surgical microscope (20, 28). Electrical activity in facial nerve filaments was recorded using single or bipolar electrodes and amplified by an AC preamplifier (model P511, Grass Instruments). Action potentials were confirmed to represent the bursting of a single facial motoneuron by superimposition of the traces on an oscilloscope (model 5111A, Tektronix) using the external trigger mode. All neural signals were recorded and digitized with a PowerLab system (ADInstrument) and stored in the hard drive for subsequent offline analysis.
Experimental Protocols
The protocols used to activate PCF receptors and induce uncoupled activity were adapted from our previously published studies (26–28, 33). Two separate protocols were performed.
Protocol 1. Phrenic and facial nerve responses to capsaicin injection were examined at two levels of PEEP (n = 12 rats). Baseline was established at 3 cmH2O PEEP, and two doses of capsaicin (0.25 and 0.625 µg/kg) were randomly injected into the right atrium via a microsyringe (model 710RN, Hamilton, Reno, NV) connected to the right jugular vein catheter. To avoid tachyphylaxis, a 20-min period was allowed between successive injections. These capsaicin doses were lower than those used in our previous studies (26, 27), because a capsaicin dose that would inhibit Pre-I facial nerve activity (FNA) with minimal impact on I FNA was needed. In our previous work, high-dose capsaicin resulted in complete inhibition of UAW (hypoglossal) Pre-I and I bursting and also initiated tonic bursting (26). If a similar response occurred in the facial nerve, we would have been unable to distinguish whether inhibition of I activity reflected the influence of capsaicin injection or PEEP application.
In the second part of protocol 1, uncoupled facial activity from phrenic nerve bursting was induced by increasing PEEP from 3 to 9 cmH2O for 30 s. We previously demonstrated that Pre-I UAW activity was uncoupled from phrenic bursting by 9 cmH2O PEEP but was totally inhibited by 15 cmH2O PEEP (28). PEEP was therefore set at 9 cmH2O to produce uncoupled bursting, and we examined whether this uncoupled bursting could be abolished or attenuated by capsaicin-induced activation of PCF receptors. After confirmation of uncoupled bursting, phrenic nerve and facial nerve bursting was allowed to return to baseline conditions (3 cmH2O PEEP), and then a capsaicin trial with 9 cmH2O PEEP was performed. All rats received a capsaicin dose of 0.25 µg/kg and, in some cases, an additional dose of 0.625 µg/kg. Capsaicin was delivered within 5 s after the onset of 9 cmH2O PEEP (i.e., immediately after PEEP application). The percentage of uncoupled bursting (see below) was determined only from the data obtained after low-dose capsaicin injection.
Experiments were conducted in additional rats (n = 3) to examine the importance of PCF receptor activation relative to bronchial C-fiber and/or extravagal C-fiber activation to the facial nerve capsaicin response. Thus the low and high doses of capsaicin were injected into the left ventricle (instead of the right ventricle in the studies described above). This was accomplished by placing the tip of the catheter in the left ventricle via the carotid artery and repeating the protocol described above.
Protocol 2. The response of single facial motoneurons to capsaicin was examined at 3 and 9 cmH2O PEEP (n = 14). The protocol for capsaicin injection and PEEP manipulation for these single-fiber experiments was identical to that described above. In these experiments, we specifically examined the behavior of Pre-I and EI facial motoneurons, because the discharge of these cell populations can be uncoupled from phrenic bursting during PEEP manipulations (28).
Preparation of Chemicals
Stock solution of capsaicin (Tocris, Bristol, UK) was prepared as follows: 5 mg of capsaicin were dissolved in 1 ml of 95% ethanol, Tween 80 (1 ml) was added, and the solution was diluted with saline (pH 7.4) to achieve a total volume of 10 ml (500 µg/ml). This stock solution was prepared once per month and frozen. During the experiment, the stock solution was thawed and diluted with saline to 0.25 or 0.625 µg/kg according to each animal's body weight (26). The vehicle solution consisted of 1 ml of 95% ethanol, 1 ml of Tween 80, and 8 ml of saline.
Data Analysis
Data were retrieved from the hard disk and analyzed as previously described (27, 28). I duration (TI) was defined as the period of the phrenic burst, and expiratory (E) duration (TE) was computed as the interval between two successive phrenic bursts. I activity of the phrenic and facial nerves was quantified as the peak integrated height during TI. Pre-I facial nerve activity was assessed by determining the peak integrated height of the facial neurogram during TE. The onset difference of the facial nerve burst relative to the phrenic burst was computed by subtracting the total duration of the phrenic burst (i.e., TI) from the total duration of the facial burst.
Ten neural respiratory cycles were averaged during the baseline (3 cmH2O PEEP) condition and served as the control value subsequent to capsaicin injections and/or increases in PEEP. The higher dose of capsaicin generally induced an apnea, which was defined as the prolonged TE after capsaicin treatment. After phrenic bursting resumed, the first 15 neural respiratory cycles, along with the response 1, 3, and 5 min after apnea, were analyzed. Facial nerve or neuron activity was considered to be uncoupled from phrenic nerve bursting if rhythmic discharge persisted during periods of phrenic apnea (i.e., prolonged TE) associated with PEEP. Respiratory discharges during the 30-s PEEP exposure were grouped in bins associated with every five ventilatory cycles (as assessed by tracheal pressure fluctuations). The grouped data were then averaged for each condition (i.e., with and without capsaicin). The relative degree of "uncoupled facial activity" was assessed during each group of five tracheal pressure fluctuations. As described in our recent report (see Fig. 1 in Ref. 28), the relative percentage of uncoupled bursting was calculated by dividing the number of facial nerve bursts during periods of phrenic apnea by the total number of facial nerve bursts during PEEP (9 cmH2O) application.
|
Facial motoneurons were classified as EI, Pre-I, or I according to their discharge pattern during the respiratory cycle (28). The interspike interval was used to compute motoneuron discharge rate (Hz) to construct a discharge histogram (see Figs. 5F and 7F). The mean discharge rate was expressed as burst number divided by TI or TE. The duration of spike activity (i.e., time between the first and the last spike during a respiratory cycle) and the time difference between the onset of facial motoneuron bursting and the phrenic inspiratory burst were also calculated. These variables were calculated over 10 respiratory cycles before capsaicin (i.e., control) and after bursting resumed following capsaicin treatment.
|
|
|
|
|
|
|
RESULTS |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
Intrajugular delivery of capsaicin to the right ventricle of a low (0.25 µg/kg) or high (0.625 µg/kg) capsaicin dose evoked phrenic responses similar to those described in our previous reports (Fig. 1 ) (27, 33). The high, but not low, dose evoked apnea (mean duration = 1.74 ± 0.22 s). When bursting resumed, TI was reduced (P < 0.05 vs. control value of 0.32 ± 0.01 s) and TE was prolonged from 0.56 ± 0.02 s (control) to 0.66 ± 0.09 s during the first breath following apnea (P < 0.05). After the low capsaicin dose, phrenic nerve burst amplitude (PNA) remained below control values for the first three neural breaths (P < 0.05; Figs. 1A and 2A). After the higher dose of capsaicin, PNA was reduced from neural breath 1 (75.2 ± 3.5%) through 15 (91.5 ± 2.1%, P < 0.05 vs. control; Fig. 2A).
Capsaicin also reduced FNA during Pre-I and I (Figs. 1 and 2). The low dose reduced Pre-I FNA to 37 ± 18% of control at neural breath 1 (P < 0.05; Fig. 2C), and FNA gradually returned to control values (Fig. 2C). Pre-I FNA was virtually abolished with high-dose capsaicin but returned to control values by 5 min after injection (Figs. 1B, 1Ec, and 2C). Low-dose capsaicin induced an immediate decrease in inspiratory FNA (breath 1 = 78 ± 9% of control, P < 0.05; Fig. 2B). The higher dose caused a larger reduction in I FNA (breath 1 = 52 ± 7% of control, P < 0.05; Fig. 2B), and the attenuation in I FNA was more persistent than that observed with low-dose capsaicin (P < 0.05; Fig. 2B). Figure 3 presents the average of Pre-I and I FNA recorded over the initial 10 breaths after apnea. This analysis indicates that Pre-I facial discharge is decreased to a greater extent by capsaicin (P < 0.01 vs. I; Fig. 3).
Similar to previous reports (26, 27, 33), capsaicin-induced activation of PCF receptors produced a reflex inhibition reflected as a decrease in FNA with a concomitant delay of burst onset. Specifically, during baseline recordings, the respiratory facial nerve burst commenced 
200 ms before the phrenic burst (Fig. 1, A and B), and the low dose of capsaicin caused a delay in facial nerve burst onset, such that it occurred closer to the phrenic burst (P < 0.05; Fig. 2D). This relative delay in burst onset was significantly greater after high-dose capsaicin (P < 0.05 vs. low dose; Fig. 2D). Indeed, the onset of FNA occurred after the phrenic burst onset following the high dose of capsaicin. Vehicle injections did not alter PNA or FNA (data not shown).
Capsaicin injection into the left ventricle (rather than the right ventricle in the studies described above) had no discernable effect on Pre-I and I facial nerve bursting (Fig. 1, F and G). However, injection of capsaicin into the left ventricle caused tonic FNA, as indicated by the upward displacement of the facial neurogram (Fig. 1G). The phrenic burst showed a mild decrease following the high left ventricular dose of capsaicin. These results were consistent across the three animals studied.
Capsaicin Responses During 9 cmH2O PEEP
The influence of PEEP and capsaicin on phrenic and facial nerve output is summarized in Table 1. Increasing PEEP to 9 cmH2O reduced TI and PNA and increased TE (Fig. 1, C and Ed). The reductions in TI and PNA were maintained throughout PEEP application (P < 0.01; Table 1). However, after the initial increase, TE was progressively reduced during 9 cm H2O PEEP and was not statistically different from control over the last 10 breaths (P > 0.05; Table 1). Capsaicin did not alter the PEEP-induced attenuation in TI. However, capsaicin injection during PEEP prolonged the elongation of TE and induced a greater reduction in PNA (both P < 0.05 vs. PEEP alone; Table 1).
|
20% during neural breaths 6–20 following PEEP application (Fig. 4A). Capsaicin inhibited Pre-I FNA (Fig. 1, A, Ea, and Eb) and, therefore, significantly attenuated uncoupled facial bursting during 9 cmH2O PEEP (Fig. 4A). Specifically, PEEP-induced uncoupled bursting was only observed during the initial five ventilation cycles (indicated by tracheal pressure fluctuations) after capsaicin injection. Data were grouped in bins associated with five ventilation cycles, because the phrenic nerve was transiently silent during uncoupled bursting but discharged in phase with the ventilator during coupled bursting (Fig. 1Ed).
The PEEP-induced change in facial nerve burst onset was also significantly attenuated by capsaicin (Fig. 4B). Finally, capsaicin blunted the PEEP-induced increases in Pre-I and I FNA (P < 0.05; Table 1). The responses to PEEP were totally abolished after bilateral vagotomy (data not shown).
Behavior of Individual Facial Motoneurons During Capsaicin and PEEP
Extracellular recordings were made from EI (n = 8), Pre-I (n = 3), and I (n = 3) facial motoneurons. All motoneurons were examined during capsaicin-induced PCF receptor activation. However, the influence of PEEP application on the response to capsaicin was examined only in EI and Pre-I neurons, because previous work indicates that I neurons contributed minimally, if at all, to uncoupled UAW bursting during PEEP (28).
EI facial motoneurons. During baseline conditions (3 cmH2O PEEP), facial motoneurons classified as EI began to burst during late E and continued to discharge until the end of I (Fig. 5). EI motoneuron discharge rate (Hz) during TI and TE was reduced in response to low and high doses of capsaicin (Fig. 6A). Capsaicin also decreased the overall EI motoneuron discharge duration (Fig. 6, Ba and Bb). Relative to control (i.e., pre-capsaicin) burst rates, inhibition of EI motoneuron discharge rate and duration was stronger during TE than during TI (P < 0.01; Fig. 6, Ac and Bc). During control conditions, EI motoneuron bursting began 191.0 ± 30.6 ms before phrenic burst onset. The low dose of capsaicin shifted EI neuron onset, such that it occurred 63.3 ± 32.7 ms before the phrenic burst. In contrast, the higher dose of capsaicin resulted in EI neuron onset 22.4 ± 15.6 s after the onset of the corresponding phrenic burst (P < 0.01). Thus EI motoneuron discharge changed to an I pattern after capsaicin treatment (i.e., a loss of Pre-I activity in EI motoneurons; Fig. 5Eb).
Increasing PEEP to 9 cmH2O prolonged TE in phrenic recordings (see above) and also transformed EI motoneuron bursting to a tonic pattern (Fig. 5, C, Ec, and Ed). The tonic discharge of EI neurons appeared to be moderated by lung inflation (Fig. 5, Ec, 5Ed, and F). Specifically, EI neurons discharged tonically, with a higher frequency during periods of low than high tracheal pressure (Fig. 5, Ec, Ed, and F). In contrast, when capsaicin (0.625 µg/kg) was given in concert with PEEP, EI neurons did not burst during phrenic apnea (Fig. 5D). After phrenic recovery, EI motoneurons discharged only during the phrenic burst (i.e., an I pattern; Fig. 5Ee). Thus, during increased PEEP, EI neurons were initially completely inhibited by capsaicin but gradually resumed bursting with an altered (I) pattern.
Pre-I facial motoneurons. Firing of Pre-I facial motoneurons began during late E, but bursting ceased at the onset of I (Fig. 7 Ea). The discharge rate of Pre-I motoneurons was reduced by the low dose of capsaicin (Fig. 7, Ab and Eb), and these cells were totally inhibited by the high dose of capsaicin (Fig. 7B). Elevation of PEEP to 9 cmH2O increased Pre-I motoneuron discharge rate (Fig. 7, C and Ec). Figure 7F depicts the discharge rate (Hz) of Pre-I neurons relative to lung inflations (as indicated by low and high tracheal pressure, respectively). Data are presented during control (baseline) bursting, uncoupled bursting (indicated by 1–3 in Fig. 7C), and after resumption of coupled bursting. It is evident from this analysis that 9 cmH2O PEEP transformed the Pre-I neuron bursting to a primary tonic discharge pattern. This excitatory effect was reversed by a subsequent intrajugular injection of high-dose capsaicin (Fig. 7, D and Ee). Thus, immediately after capsaicin, Pre-I neurons did not discharge during increased PEEP. In Pre-I neurons, after a period of quiescence, bursting began during elevations in tracheal pressure associated with lung inflation (Fig. 7Ee), and then a predominantly tonic discharge pattern was resumed (Fig. 7D). Similar results were observed in each of the three Pre-I motoneurons examined.
I facial motoneurons. I motoneurons discharged only during TI, and capsaicin resulted in a decrease in the discharge rate and duration (Table 2). In addition, onset of I motoneuron discharge was delayed in response to capsaicin injection and shifted from early I toward the middle of TI.
|
Both doses of capsaicin significantly decreased mean arterial pressure (BP) and heart rate (P < 0.01; Table 3). Elevation of PEEP to 9 cmH2O did not change heart rate but reduced BP, (P < 0.01; Table 3). During application of 9 cmH2O PEEP, capsaicin further reduced BP and heart rate (P < 0.01; Table 3).
|
|
DISCUSSION |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
Critique of Methods
A detailed critique of our experimental methods related to PEEP and capsaicin is provided in two recent publications (27, 28). Prior work indicates that capsaicin injection activates PCF receptors (5, 6, 31) to induce a pulmonary chemoreflex producing apnea, hypotension, and bradycardia (6, 25, 26, 30, 33). The present data are consistent with these earlier reports. PCF receptors have been reported to be activated by high (30 cmH2O) PEEP (16) and, thus, were probably not excited by the 9 cmH2O PEEP stimulus used in the present study.
The ability of PEEP to induce uncoupled UAW vs. phrenic bursting appears to be dependent on vagal afferent input associated with mechanical ventilation (28). Accordingly, the "uncoupling response" may reflect Hering-Breuer reflex inhibition of phrenic I discharge coupled with simultaneous PEEP-induced excitation of Pre-I UAW motor output (28). However, the mechanism of the PEEP-induced increase in Pre-I discharge is not clear.
Capsaicin Injection Inhibits Facial Motor Output
Prior work has described how facial motor output is modulated by chemoreceptor and/or pulmonary stretch receptor activation (19, 20, 44, 48). Here we extend this work by describing the impact of PCF receptor activation on facial motor output throughout the respiratory cycle. The capsaicin-evoked reduction in FNA (Figs. 1 and 2) is consistent with the response of the hypoglossal, recurrent laryngeal (RLN), and superior laryngeal nerves to capsaicin (26, 27, 33). Activation of PCF receptors with capsaicin also delayed the onset of facial nerve bursting, as has been reported for other UAW motor nerves (26, 27, 33). Accordingly, the reduction in burst amplitude and delay in burst onset are consistent responses across different UAW motor pools in response to the activation of PCF receptors with capsaicin.
PCF receptors may sense internal stimuli such as pulmonary congestion or edema (8, 9, 37). However, the UAW motor response to these "natural stimuli" is unclear. In this regard, we recently observed that activation of PCF receptors with anandamide, a lipid metabolite synthesized in the central nervous system and the lungs (2, 10) and also a capsaicin receptor agonist (39), initiates a response similar to the response to capsaicin (e.g., reduced amplitude and delayed burst onset) in the abducent branch of the RLN. However, anandamide also induced a concomitant increase in burst amplitude with earlier onset in the adducent branch of the RLN. This coordinated response of the RLN branches resulted in a short period of glottal closure (47), similar to that reported in response to capsaicin (34). Moreover, a similar response occurs during pulmonary edema in lambs (8, 9).
Consistent with previous studies, we observed Pre-I, EI, and I motoneurons in the facial nerve (20). Indeed, Pre-I facial motor activity has been noted in rats, mice, dogs, and cats (19–21, 27, 36, 44). The onset of facial and other UAW motoneuron bursting before the phrenic burst may be beneficial for UAW patency. For example, Pre-I discharge may dilate and/or stiffen the airways and, thus, facilitate UAW airflow during I (15, 41, 45). Accordingly, inhibition of Pre-I UAW discharge may put the UAW at risk for narrowing and/or collapse.
Capsaicin-Induced Inhibition of Pre-I Facial Activity
A detailed analysis of facial neurogram bursting patterns suggested that the decrease in facial activity after capsaicin injection was greater during the Pre-I than during the I period (Fig. 3). Consistent with this observation, the capsaicin-induced inhibition of EI motoneuron discharge rate and duration was greater during the Pre-I than during the I period (Fig. 6, Ac and Bc), and Pre-I motoneurons were totally inhibited by capsaicin injection (Fig. 7). We previously hypothesized that UAW motoneurons receive two distinct respiratory-related modulatory inputs (28). On the basis of this hypothesis, the greater inhibitory effects of capsaicin during the Pre-I (than during the I) phase might reflect the presence of different premotor systems driving the Pre-I and I FNA. Activation of PCF receptors may contribute to UAW narrowing and/or collapse during I (27). Consistent with this suggestion, capsaicin exacerbates sleep-disordered breathing in rats (3). Moreover, smokers have been reported to suffer from sleep apnea syndrome (24), which could potentially reflect smoke-induced activation of PCF receptors (29, 30).
Because of the inhibition of Pre-I activity, facial nerve bursting recorded during PEEP-induced phrenic apnea (i.e., uncoupled bursting) was substantially attenuated by capsaicin injection (Fig. 4; see below). Thus activation of PCF receptors with the low dose of capsaicin appears to "override" PEEP-induced excitation of Pre-I neurons, and the impact of vagal afferents on the overall activity of Pre-I facial motor output may represent balance among excitatory and inhibitory influences. Consistent with this suggestion, E abdominal activity can be inhibited by PCF receptor activation but excited by SARs (17). This result raises the intriguing possibility that pharmacologically antagonizing PCF receptor activation, or activating SARs, may benefit patients with sleep-disordered breathing, such as sleep apnea syndrome.
Although the detailed anatomic substrate underlying the PEEP and/or capsaicin effects reported here remains to be determined, insights can be derived from previous work. Vagal afferent inputs activated by capsaicin and PEEP project to the nucleus of the tractus solitarius (NTS) in the medulla (1). Second-order neurons have wide-ranging projections to "respiratory-related regions of the medulla, pons, and spinal cord" (25). Neurons sensitive to PCF activation have been described in the medullary ventral respiratory group (50). Interestingly, activation of PCF receptors excites decrementing E neurons in the ventral respiratory group but inhibits augmenting E and I VRG neurons (50). Vagally mediated apnea caused by intravenous serotonin was potentiated by microinjection of the glutamate receptor antagonist kynurenic acid into the pontine intertrigeminal region (38). Since serotonin does not cross the blood-brain barrier and may activate PCF receptors (5), the results of Radulovacki et al. (38) suggest that signals from PCF receptors terminate first in the NTS and then project to the intertrigeminal region to modulate PCF-induced apnea. Moreover, signals from SAR activation also travel to the NTS and, thereafter, to the pontine region (13).
Pre-I Facial Motor Output During PEEP
The present data confirm reports that Pre-I activity in UAW motor nerves and/or motoneurons can be uncoupled from the phrenic burst with 9 cmH2O PEEP (12, 28, 40). As reported previously, the uncoupled bursting reflected activation of Pre-I and EI facial motoneurons (28). For example, Pre-I facial neurons showed an increase in discharge rate and onset during 9 cmH2O PEEP (Fig. 7), such that Pre-I neurons fired tonically (Fig. 7, C, Ec, and Ed). A similar response was seen in facial EI motoneurons during the Pre-I period (Fig. 5), but, in contrast, I facial motoneurons decrease discharge rate during 9 cmH2O PEEP (28). Thus PEEP induces a differential effect on motoneurons bursting during the E period (i.e., Pre-I and EI neurons) compared with those showing exclusively I discharge (i.e., I neurons). Moreover, the tonic response of Pre-I and EI facial neurons to 9 cmH2O PEEP suggests that these neurons might receive an inhibitory input during the I and the postinspiratory phase that was probably released by 9 cmH2O PEEP.
Several recent studies have provided evidence that the mammalian central nervous system contains separate rhythm generators that are responsible for I and E, respectively (14, 22). The I rhythm generator is postulated to be in the pre-Bötzinger complex (42), whereas the E rhythm generator is presumed to be located at the retrotrapezoid nucleus/parafacial respiratory group (22, 23). Janczewski and Feldman (22) recently observed that application of a selective µ-opiate agonist (fentanyl) or manipulations of continuous positive airway pressure can reduce I (genioglossus muscle), but not E (abdominal muscle), activity. Thus they were able to separately modulate I and E activities in juvenile rats (22). Onimaru et al. (36) suggested that Pre-I facial nerve activity provides an indicator of Pre-I neuron parafacial respiratory group discharge (i.e., from the putative E rhythm generator). The Pre-I neurons described by Onimaru et al. require an intact pons, and, similarly, Jacquin et al. (21) indicated that the rostral pons plays an important role in Pre-I UAW motor discharge.
These concepts of separate rhythm generators (see above) are compatible with the notion that peak discharge frequency is different between Pre-I and I hypoglossal neurons (32). Moreover, Pre-I hypoglossal activity can be separately modulated by activation of orexin B receptors (11) and clonidine (4). Collectively, these data document that Pre-I UAW motor activity can be modulated by central and peripheral inputs. Our present data describing the differential regulation of facial motor output during the I vs. the E phase are consistent with these data and support the hypothesis that UAW motoneurons receive separate synaptic inputs during these phases (28).
Finally, 9 cmH2O PEEP might have produced reflex excitation of I FNA (Table 1). In contrast, 9 cmH2O PEEP inhibited I facial motoneuron activity in our previous study (28). The PEEP-induced increase in I FNA reported here may reflect tonic discharge of Pre-I and EI facial motoneurons (Figs. 5F and 7F). Thus the overall increase in I FNA during PEEP may reflect the combined activities of residual I neurons with tonically discharging Pre-I and EI facial neurons.
|
GRANTS |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
|
FOOTNOTES |
|---|
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. Section 1734 solely to indicate this fact.
|
REFERENCES |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
in C)]. Capsaicin injection after PEEP application attenuated the relative degree of uncoupled facial bursting [i.e., lower uncoupled number and lower uncoupled amplitude (
in D)]. E: cycle-triggered histograms illustrating relationship of FNA and PNA to tracheal pressure (TP) and, therefore, ventilator rate. Overlay plots were derived from data in A–D (a–d). Preinspiratory (Pre-I) FNA precedes phrenic burst during control conditions (downward arrow in Ea) but was abolished by low dose of capsaicin (Eb). Increasing PEEP to 9 cmH2O elevated Pre-I activity in the facial nerve and induced uncoupled activity corresponding to Pre-I FNA during phrenic apnea [Ed (left); downward arrow represents onset of Pre-I and 
) FNA. F and G: capsaicin injection into the left heart has a minimal effect on Pre-I and I FNA, and high dose of capsaicin induces tonic facial bursting. BP, blood pressure. Upward arrows in A, B, D, F, and G indicate capsaicin injection. Horizontal dotted line in C indicates higher FNA during elevated PEEP than during control PEEP.
