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Logo of jphysiolThe Journal of Physiology SiteMembershipSubmissionJ Physiol
J Physiol. Aug 1, 2003; 550(Pt 3): 973–983.
Published online Jun 6, 2003. doi:  10.1113/jphysiol.2003.042200
PMCID: PMC2343062

Inspiration-Promoting Vagal Reflex in Anaesthetized Rabbits after Rostral Dorsolateral Pons Lesions

Abstract

The centrally generated respiratory rhythm is under strong modulation by peripheral information, such as that from the slowly adapting pulmonary stretch receptors (SA-PSRs) conveyed via the vagus nerve. We have already demonstrated that vagal afferent stimulation at a low frequency (5–40 Hz), or holding the lung volume at the end-expiratory level (no-inflation test) prevents spontaneous termination of the inspiratory (I) phase or initiates I activity in anaesthetized rabbits in which the NMDA receptors (NMDA-Rs) are pharmacologically blocked. Here we show that this I-promoting vagal reflex also becomes manifest in animals where the pontine respiratory groups are ablated. Following lesions of the rostral dorsolateral pons, including the nucleus parabrachialis medialis and Kölliker-Fuse nucleus, with radio-frequency current or local injection of kainic acid, low-frequency stimulation of the vagus nerve and the no-inflation test significantly prolonged the I phase in a manner highly similar to that observed in rabbits with NMDA-R block. Brief stimuli at low frequency during the mid-expiratory (E) phase evoked I discharge with a latency significantly smaller and less variable than that before the lesions. It is concluded that low-frequency input from the SA-PSR suppresses I-to-E phase transition and promotes central I activity when the medullary respiratory network is released from pontine influence, which involves NMDA-R-mediated signalling.

The respiratory rhythm in vertebrates is automatically generated in the central nervous system without afferent information (Bianchi et al. 1995). Various afferent signals arising from peripheral receptors, such as mechano- and chemoreceptors, produce ‘reflexogenic’ respiratory responses through modulation of this automatic medullary rhythm (Feldman, 1986). Of these afferent signals, the mechanosensory information reflecting transpulmonary pressure in the bronchi is most influential in determining the depth and rate of breathing (Clark & von Euler, 1972). As first described by Hering and Breuer, this afferent information arising from bronchial slowly adapting pulmonary stretch receptors (SA-PSRs) induces bimodal responses of the central respiratory network to changes in the lung volume. They are (1) the deflation reflex, an inspiratory (I) promotion with expiratory (E) inhibition at reduced lung volume, and (2) the inflation reflex, an I suppression with expiratory promotion in response to the inflation of the lung (reviewed in Widdicombe, 1961; Paintal, 1973; Comroe, 1974). While the neural mechanism underlying the Hering-Breuer inflation reflex is now well understood owing to the detailed analysis of the neuronal activities at the vagally induced I off-switch (see Denavit-Saubié & Foutz, 1996; Haji et al. 2000), that underlying the Hering-Breuer deflation reflex is still undetermined.

We have demonstrated that maintaining the tidal volume at end-expiratory low levels or stimulating the vagal afferent nerves at a low frequency prevents the central I off-switch and promotes the I on-switch in anaesthetized rabbits (Takano & Kato, 1999). It was concluded that this I-promoting vagal reflex is the neuronal basis for the Hering-Breuer deflation reflex that might be important to assure the I movement when the lung volume is too low (Takano & Kato, 1999). In support of this notion, it has been demonstrated that the Hering-Breuer deflation reflex is operational in human neonates, ensuring the I efforts in response to non-pathological chest compression (Marsh et al. 1994; Hannam et al. 2000)

Interestingly, the I-promoting vagal reflex becomes manifest under pharmacological blockade of N-methyl-D-aspartate (NMDA) receptors (NMDA-Rs), suggesting that this reflex is usually masked by more predominant influences from other respiratory mechanisms involving NMDA-R-mediated synaptic transmission (Takano & Kato, 1999). One of these mechanisms is the pontine pneumotaxic mechanism (Ling et al. 1993). It is well known that pharmacological blockade of NMDA-Rs induces an apneusis in vagotomized animals very similar to that observed following lesion of the rostral dorsolateral pons including the nucleus parabrachialis medialis (NPBM) and the Kölliker-Fuse nucleus (KF) (Foutz et al. 1989), suggesting that the pontine respiratory control involves NMDA-R-mediated signalling. However, NMDA-R-mediated excitatory transmission is operational not only in the pontine pneumotaxic mechanisms but also in the medullary respiratory network (Pierrefiche et al. 1991; Aylwin et al. 1997; Haji et al. 1998). Here one question arises: does the I-promoting reflex we described under NMDA-R blockade result from elimination of pontine influence to the medullary respiratory pattern generator or from additional blockade of NMDA-R-mediated signalling in the medullary respiratory network?

To address this issue, we examined whether surgical and pharmacological ablation of the rostral dorsolateral pons in the absence of the NMDA-R blockade gives rise to the I-promoting vagal reflex similarly to that observed under NMDA-R blockade (Takano & Kato, 1999). The data presented in this study clearly indicate that the attenuation of pontine influence to the medullary respiratory pattern generator causes manifest promotion of I activities and inhibition of E activities in response to low-level PSR inputs.

METHODS

Surgical procedures

All experiments were performed in accordance with the Guiding Principles for the Care and Use of Animals in the Field of Physiological Sciences of the Physiological Society of Japan (1998). Adult Japanese white rabbits of either sex weighing 2.7–3.2 kg were anaesthetized with urethane (0.5 g kg−1, I.P., then 0.5 g kg−1, I.V.). After tracheal cannulation, the animals were immobilized with gallamine triethiodide (6 mg kg−1, I.V.; supplemental dose, 3 mg kg−1 h−1) and artificially ventilated with room air using a constant-volume animal ventilator (Shinano, SN-480-5, Tokyo, Japan). The tidal volume and ventilation rate were manually adjusted so that the end-tidal CO2 concentration monitored with a gas analyser (NEC-San-ei, 1H31, Tokyo, Japan) was between 4.0 % and 4.5 %. The vagus, depressor and sympathetic nerves were severed at the neck for experiments in which the vagus nerve was stimulated. These nerves were kept intact for experiments in which the lung no-inflation test was performed. The femoral vein and artery were cannulated for intravenous injection of drugs and for measurement of arterial blood pressure, respectively. The rectal temperatures were maintained at 37 °C with a thermostatically controlled heating pad (Nihon-Koden, AD-1100, Tokyo, Japan). The blood pressure, heart rate and electroencephalogram (EEG) were recorded simultaneously to monitor the animal's general state, especially the level of anaesthesia. Additional injection of urethane was never required as judged from these parameters in the course of the present experiments, which usually lasted 6–10 h. At the end of experiments, the animals were killed by intravenous injection of an overdose (>100 mg kg−1) of pentobarbitone.

Lesion of the rostral dorsolateral pons

The head of the rabbit was fixed to a stereotaxic apparatus and a part of the cerebellum was removed by suction.

Radio-frequency lesion

The tip of a lesion electrode (0.7 mm diameter, 1.5 mm polar length) with a local temperature sensor was inserted into the region between the medial parabrachial nucleus and KF nucleus (1.0 mm caudal from the inferior colliculus; 3.5 mm lateral from the midline; 2.0 mm deep from the surface of the brain). An indifferent electrode was attached to the dissected head skin. Radio-frequency current (500 kHz, ≈25 mA) was applied with a Lesion Generator (Radionics Inc., RFG-4, MA, USA) with the local temperature at the tip of the lesion electrode between 75 and 85 °C for 60 to 180 s.

Cytotoxic lesion with kainic acid (KA)

A 1 μl solution of KA (1 mg in 1 ml PBS; pH 7.4) was slowly injected into the bilateral rostral dorsolateral pons with a micromanipulator-controlled microsyringe (Kloehen, Whittier, USA). The KA solution contained 1 % eosin Y for the post hoc histological examination of the location and spread of the solution in the brain tissue.

Recording of neural activities, vagus nerve stimulation and lung volume holding

The bilateral phrenic nerves were sectioned in the neck, and the central cut end of a nerve was hooked on a bipolar platinum electrode for recording. The compound phrenic nerve discharge, EEG, arterial blood pressure, concentration of the expired CO2 and normalized pulses of the stimulation were recorded on digital audio tapes (Sony, PC108M, Tokyo, Japan). The Aα and Aβ afferent fibre groups in the vagus nerve arising from the SA-PSR were selectively stimulated (see Methods in Takano & Kato, 1999) with a bipolar platinum electrode (0.5 V; 100 μs; Nihon-Kohden, SEN-7203). The lung volume was held in animals with intact vagi as follows: the ventilation was stopped at the end-expiration level with a three-way cock and a known volume of room air was injected directly into the lung via the tracheal catheter using a 10 ml or 50 ml syringe. Passive ventilatory movements of the chest were recorded with a tension transducer attached to the chest wall of the rabbit, which was held in the supine position.

Histological procedures

The brainstem was carefully removed and fixed in a paraformaldehyde solution (4 %) for more than 48 h at room temperature (22–25 °C). Pontomedullary coronal slices (150 μm thick) were made with a vibrating slicer and stained with cresyl violet, dehydrated with ethanol, mounted on glass slides and examined by light microscopy (SZX9, Olympus).

Data analysis

The phrenic nerve discharge was sampled at 1000 Hz after low-cut filtration at 200 or 500 Hz for the measurement of inspiratory time (TI), expiratory time (TE) and total respiratory duration (TTOT) with a CED-1401 (Cambridge Electronic Design Ltd, Cambridge, UK). The TI/TTOT ratio was estimated as previously described (Takano & Kato, 1999). In brief, the sum of the TI values of all I phases appearing within the 15 s sample window was expressed as a percentage of whole window duration. Traces in the figures were made with Igor Pro version 4.07 (WaveMetrics) after acquisition of the nerve discharge at 5000 Hz. For the data after ablation of the rostral dorsolateral pons, a sample window of 30 s instead of 15 s was used because TI became longer than 15 s in many animals. The values are presented as means ± s.d. Differences were compared using one-way ANOVA and the Mann-Whitney U test. Differences with a probability (P) less than 0.05 were considered significant.

RESULTS

Effects of radio-frequency lesions of the rostral dorsolateral pons

The central side of the cut end of a vagus nerve was stimulated at frequencies from 5 to 160 Hz in six vagotomized animals following the removal of the cerebellum. The low-frequency stimulation (5–40 Hz) slightly shortened the respiratory cycle (Fig. 1A, left; Table 1), whereas high-frequency stimulation (80–160 Hz) prolonged the respiratory cycle by increasing the TE (Fig. 1A, left; Table 1). These effects of vagal afferent stimulation were identical to those observed before NMDA-R blockade in rabbits with intact cerebellum in our previous study (Takano & Kato, 1999), suggesting little involvement of the cerebellum in this vagal reflex.

Figure 1
Effect of radio-frequency lesion of the rostral dorsolateral pons on changes in phrenic nerve discharge induced by vagal afferent stimulation
Table 1
Effects of vagal stimulation and radio-frequency lesion of the rostral dorsolateral pons on respiratory phase duration

The rostral dorsolateral pons was then lesioned with local radio-frequency current application. Unilateral lesion resulted in only a slight prolongation of TI but not TE (data not shown). Bilateral lesions of the rostral dorsolateral pons immediately revealed an apneustic breathing pattern with extremely prolonged TI (Fig. 1A, right; Table 1), which was quite similar to the that observed under NMDA-R blockade (e.g. Fig. 1A and Fig. 3A in Takano & Kato, 1999). Unlike the NMDA-R blockade, bilateral lesion of rostral dorsolateral pons also prolonged TE (Fig. 1A, right; Table 1). After the bilateral lesion, low-frequency (5–40 Hz) stimulation of the vagus nerve markedly prolonged the I phase (Fig. 1A, right; Table 1). The I discharge did not cease for more than 30 s when stimulation at 10–40 Hz was continued for more than 30 s (Fig. 1A, right; Table 1). In contrast, a high-frequency stimulation (160 Hz) suppressed the I activity in a manner similar to that before lesions (Fig. 1A). Figure 1B depicts the frequency dependence of the response of the central rhythm to vagal stimulation. TI/TTOT was not significantly affected with low-frequency stimulation (0–40 Hz) before the lesion, whereas it was significantly increased after the lesion of the rostral dorsolateral pons. In contrast, the lesions of the rostral dorsolateral pons did not affect the effect of high-frequency stimulation on TI/TTOT. These effects of radio-frequency pontine lesion are highly similar to the results before dizocilpine administration in our previous study (Fig. 1C and Fig. 3C in Takano & Kato, 1999). Table 1 summarizes the effects of radio-frequency lesion on respiratory cycle duration. These responses to vagal stimulations at various frequencies were stably observed for more than 2 h after lesion of the bilateral rostral dorsolateral pons.

Effects of focal KA injection to the rostral dorsolateral pons

The radio-frequency lesions shown above might have damaged the fibres passing through or near the rostral dorsolateral pons. To clarify whether the observed change caused by lesion of the rostral dorsolateral pons can occur by more selective ablation of the somatodendritic structure of the neurons in the pontine respiratory group, we examined the effect of KA injection. Similarly to the radio-frequency lesions, bilateral injection of KA into the rostral dorsolateral pons revealed an apneustic breathing in 1–5 min (Fig. 2A, right). Two hours after the KA injection, the responses of central respiratory rhythm to vagal stimulation at various frequencies changed markedly (Fig. 2A and B) in a manner highly similar to that observed after radio-frequency lesion (compare filled circles in Fig. 1B with those in Fig. 2B; Table 1), that is, a prolonged I phase during low-frequency stimulation and cessation of I discharge immediately following high-frequency stimulation (Fig. 2A). Again, TI/TTOT during vagal stimulation at lower (< 40 Hz) frequency was significantly increased after KA injection, while that at higher (> 80 Hz) frequency was not significantly affected (Fig. 2B; Table 2). These results indicate that ablation of the rostral dorsolateral pons by both surgical and chemical lesions completely alters the response of the central respiratory rhythm to low-frequency vagal inputs but not high-frequency ones.

Figure 2
Effect of the rostral dorsolateral pons lesion by kainic acid (KA) on changes in phrenic nerve discharge induced by vagal stimulation
Table 2
Effects of vagal stimulation and lesion of the rostral dorsolateral pons by KA injection on respiratory phase duration

Effect of ablation of the rostral dorsolateral pons on PSR-mediated reflex

We have previously demonstrated that the central respiratory responses to vagal afferent stimulation at various frequencies are reproduced by physiological changes in the levels of the PSR excitation elicited by changing the lung volume with the ventilator (Takano & Kato, 1999). In particular, the isobaric holding of lung volume at the end-tidal level (no-inflation test) induced a complete prevention of spontaneous I phase termination, which was highly similar to the effect of low-frequency (10–20 Hz) stimulation of the vagal afferents in rabbits with pharmacological blockade of NMDA-Rs (see Fig. 4 in Takano & Kato, 1999). We therefore examined whether lesion of the rostral dorsolateral pons changes the responses of the central respiratory rhythm to the no-inflation test in rabbits with intact vagi. Figure 3 shows a result of a no-inflation test performed before and after bilateral radio-frequency lesion of the rostral dorsolateral pons. Holding the tidal volume at the end-tidal level (inflation (–) in Fig. 3A and B), which only slightly accelerated the respiratory rhythm before the lesion (Fig. 3A), completely prevented spontaneous termination of the central I discharge after the lesion (Fig. 3B). Inflation of the lung with 15 ml air immediately terminated the I phase and maintained the E phase (open bar in Fig. 3B). After these manoeuvres, we severed the vagus nerves bilaterally in the neck, after which the no-inflation test had no effect on the phrenic discharge (top trace, Fig. 3C1). Five minutes after the vagotomy, vagus nerve stimulation at 20 Hz prevented spontaneous I phase termination in a manner very similar to that observed with the no-inflation test before vagotomy (Fig. 3C, 2a-d) indicating that the no-inflation test produced a response equivalent to the vagal afferent excitation at ≈20 Hz in the rabbits with the rostral dorsolateral pons lesion. These results indicate that, in the animals with an ablated rostral dorsolateral pons, similarly to those with NMDA-R blockade (Takano & Kato, 1999), the I-promoting reflex is elicited by physiological afferent information that the lung volume is almost at the functional residual capacity (FRC) level.

Figure 3
Effect of no-inflation test and low-frequency vagal stimulation on the phrenic nerve discharge

I on-switch by low-frequency vagal stimulation during the E phase

In our previous article we demonstrated that a very short train of vagal stimulation at a low frequency delivered during the E phase induces immediate transition from the E to the I phase in animals with blocked NMDA-Rs. We examined whether the same stimulation can induce E-to-I phase transition after pontine respiratory group lesions. Figure 4 depicts the effects of two-pulse train (20 Hz) stimulation of the vagus nerve before (left) and after (right) bilateral radio-frequency lesion of the rostral dorsolateral pons. As seen clearly in Fig. 4A, the latency from stimulation to the I onset was large and variable before the rostral dorsolateral pons lesion (left), whereas it became shorter and more stable after the lesion of the rostral dorsolateral pons (right). Figure 4B summarizes the effects of the rostral dorsolateral pons lesion on the latency from stimulation to the I onset. Despite large increase in the TI and TE after rostral dorsolateral lesion (see Table 1), the stimulation-to-I onset latency became significantly shorter and more stable after lesions (P = 0.021; Mann-Whitney U test; n = 4), indicating that a short-train low-frequency stimulation of the vagus nerve became more effective in triggering the I activity in the animals with an ablated rostral dorsolateral pons. Similar rapid and stable initiation of I discharge following brief low-frequency stimulation was also observed in the animals in which KA was injected into the rostral dorsolateral pons (Fig. 5). After experiments, the location of the radio-frequency lesion was confirmed to be limited to the dorsolateral structure including the NPBM and KF in all seven animals in which the lesion clearly revealed vagally induced I-promoting responses to low-frequency vagal stimulation and/or to the no-inflation test (Fig. 6).

Figure 4
Effect of low-frequency vagal stimulation delivered at the mid-E phase
Figure 5
Effect of low-frequency vagal stimulation delivered at mid-E phase after rostral dorsolateral pons lesion by kainic acid
Figure 6
Histological verification of the loci lesioned by radio-frequency current

DISCUSSION

Ablation of the pontine pneumotaxic centre reveals vagally induced I promotion

In the preceding paper (Takano & Kato, 1999), we demonstrated a novel I-promoting reflex in response to low-frequency vagal afferent stimulation and small-volume lung holding under pharmacological blockade of NMDA-Rs in anaesthetized rabbits. Here we demonstrate that this vagal I-promoting reflex is also similarly reproduced in rabbits where the rostral dorsolateral pons is ablated bilaterally. This is the first study showing strong suppression of the spontaneous I termination and effective E-to-I phase switching by a low-frequency PSR input in animals with a pontine pneumotaxic centre lesion. This did not depend on the method of ablation of the rostral dorsolateral pons; KA injection into the NPBM and the KF, and radio-frequency lesion of these structures were equally effective in facilitating these I-promoting reflexes.

It has been demonstrated that both the systemic blockade of NMDA-Rs and the ablation of the rostral dorsolateral pons result in a prolonged I phase (apneustic breathing) in vagotomized animals (Feldman & Gautier, 1976; Bianchi et al. 1995; Denavit-Saubié & Foutz, 1996; Haji et al. 2000). The present results clearly indicate that not only the apneustic breathing but also the vagally induced I-promoting reflex is revealed when the medullary respiratory rhythm generator is released from the influence of the pontine pneumotaxic systems. In other words, responses to low-level vagal afferents are under strong control of NMDA-R-dependent pontine respiratory mechanisms.

By contrast, the I-terminating effect of the high-frequency vagal stimulation or lung inflation was not affected either by pharmacological (Takano & Kato, 1999) or surgico-chemical (this study) deprivation of the pontine influence. These results are in accordance with previous reports indicating that the vagal I phase termination or shortening is not under control of the pontine descending pathway (Karius et al. 1991b; Bianchi et al. 1995; Haji et al. 2000) and is independent of NMDA-R-mediated signalling (Feldman et al. 1992). These reports used high-frequency vagal stimulation (e.g. 100 Hz) or strong lung inflation immediately reaching the end-I level in order to elicit the I off-switch. This might be the reason why the I-promoting response to the low-intensity vagal afferent input, as found in our studies, could not be detected in these studies. However, Feldman & Gautier (1976) suggested that the pneumotaxic centre is crucial for the pulmonary afferent influence to the respiratory rhythm, based on their observation that the lung volume threshold for I termination was increased to about 200 % after an electrical lesion of the pneumotaxic centre in anaesthetized cats. As their study used a constant flow cycle-triggered ventilator with a lung volume ‘ramp’, in the light of the present study this result might be attributed partly to the enhanced I-promoting response after the pontine lesion during the early phase of the ramp at which the smaller lung volume facilitated I activity (bottom trace in Fig. 1 in Feldman & Gautier, 1976).

Altogether, this examplifies that (1) peripheral information arising from a single group of receptor cells can produce diametrically opposite responses to the central pattern generator, simply depending on the discharge frequency, and (2) these opposite reflexogenic responses are separately regulated by other modulatory networks (in this case, the pontine respiratory group). It has been long conceived that the pontine pneumotaxic centre does not affect the SA-PSR-mediated vagal reflex (Karius et al. 1991b; Bianchi et al. 1995). However, the results of our studies (Takano & Kato, 1999, and this study) indicate that this notion should be reconsidered because the vagal reflex in response to the SA-PSR afferents discharging at low frequency, such as those activated by the ‘no-inflation’ test, is strongly affected by the presence of the pontine pneumotaxic centre. We propose that documented effects of the ‘no-inflation’ test in the animals with pneumotaxic centre lesion should be re-evaluated in the light of the present findings. The ‘no-inflation’ is not merely an absence of afferent input (Feldman et al. 1992), but rather an active signalling that ‘the lung volume is near FRC’, to which the medullary respiratory network responds in distinct manners, depending on the presence of the pontine influence (e.g. compare Fig. 3A and B).

Unlike the effect of NMDA-R blockade, which increases TI but does not affect TE (e.g. Table 1 in Takano & Kato, 1999), the surgical and chemical lesion of the pneumotaxic centre prolonged not only TI but also TE (Fig. 1 and Fig. 2 and Tables 1 and and2),2), suggesting that the neural mechanism underlying spontaneous E-to-I switching is also disturbed by rostral dorsolateral pons ablation. However, in the animals with blocked NMDA-Rs (Takano & Kato, 1999) and those with ablated pontine mechanisms (Fig. 4 and Fig. 5), a brief low-frequency vagal stimulation resulted in immediate and stable transition from the E to I phase. Despite the large difference in the spontaneous E-to-I transition between the NMDA-R blockade and the pontine lesion this may indicate, vagally induced E-to-I transition is mostly free of the pontine influence that does not involve a NMDA-R-mediated process (Fig. 4 and Fig. 5). Altogether, it is can be concluded that the low-frequency vagus input shifts the balance between the central I and E phases toward the direction where the I phase can be more easily attained and sustained, and this biased influence is greater when the pontine pneumotaxic influence is absent or weak. In contrast, the high-frequency vagus input shifts the balance so that the I phase terminates or does not occur, regardless of the pontine influence.

Hypothetical mechanisms underlying frequency-dependent influence of the vagal afferent to the respiratory rhythm

How could this ‘switching’ between the promotion and suppression of the I activity occur simply depending on the afferent frequency? It is unlikely that electrical stimulations at different frequencies recruited de novo distinct sets of nerve fibres due to temporal summation, because a simple graded change in the lung volume within eupnoeic range could also induce diametrically opposite responses of the central respiratory activity (e.g. Fig. 5 in Takano & Kato, 1999, Fig. 3B in this paper). It is not likely that the I-promoting response to the low-frequency vagal stimulation involves activation of the lung irritant receptors (Widdicombe, 1961), as the higher-threshold C fibres should be less activated at lower frequencies where temporal summation is less likely. In addition, the I-promoting response could be elicited not only by a vagal stimulation of minimal intensity near the threshold but also by a lowering of the lung volume to end-expiratory level (Takano & Kato, 1999, and this study). The precise neuronal mechanism underlying this frequency-dependent switching cannot be deduced at this stage. One of the possibilities is that the effect of the summation of repeated postsynaptic potentials (PSPs) in the medullary respiratory neurons may vary greatly, depending on the stimulation frequency. For example, it is thought that the phasic discharge of the post-I neurons plays a key role in the irreversible I-to-E switching (Bianchi et al. 1995; Denavit-Saubié & Foutz, 1996; Haji et al. 2000). A single shock stimulation of the vagal afferent in decerebrate cats produces a biphasic postsynaptic response composed of an early EPSP with a latency of ≈6 ms followed by a late IPSP with a latency of ≈20 ms in the post-I neurons (Haji et al. 1996a, 1998). It is therefore imaginable that summation of these EPSPs and IPSPs, together with the membrane shunting due to GABAA receptor opening during the IPSP, may result in frequency-dependent changes in the neuronal excitability of the post-I neurons. In addition, it has also been shown that dizocilpine hyperpolarizes post-I neurons by ≈5 mV and prominently reduces the excitatory synaptic inputs during post-I phase, while the vagally evoked EPSP and IPSP remain unaffected (Haji et al. 1996a,b, 2001). Altogether, in the absence of descending excitatory inputs from the pneumotaxic centre to the medullary I off-switching neurons, due to pontine lesion or NMDA-R blockade (Karius et al. 1991b; Ling et al. 1994; Haji et al. 1998, 2001; Takano & Kato, 1999, and this study), low-frequency SA-PSR inputs may preferentially inhibit the post-I neurons, resulting in prevention of the spontaneous I termination. In contrast, high-frequency inputs sufficiently depolarize these neurons allowing them to cause I-to-E switching, regardless of the presence of pontine influence (Karius et al. 1991a,b; Takano & Kato, 1999). Direct membrane potential recording from post-I neurons during low-frequency stimulation or a no-inflation test is required to demonstrate whether this hypothetical explanation is adequate.

Another possible mechanism that might function in tandem with the mechanism above is ‘frequency-dependent filtering’ at the synapses in the nucleus of the solitary tract in which the PSR afferents terminate (Miles, 1986; Chen et al. 1999; Takano & Kato, 1999; Liu et al. 2000; Kato & Shigetomi, 2001). The balance between the early EPSPs and late IPSPs of the post-I neurons following a vagus input (Haji et al. 1996a, 1998) may change also due to this frequency-dependent filtering. For example, the frequency-dependent attenuation of PSPs is more prominent in higher-order neurons than in lower-order neurons (Chen et al. 1999). As the late IPSP is likely to involve more synapses, including an inhibitory one, than the early EPSP, the late IPSP amplitude might be more reduced than the early EPSP when the afferent frequency is high. This may explain the shift from inhibition to excitation of the vagal influence on the I off-switch mechanism as the afferent frequency increases. The involvement of this mechanism in the frequency-dependent switching also deserves to be analysed by direct recording of the membrane potential trajectory of post-I neurons during vagal stimulation at various frequencies.

Functional implication of the vagally induced I promotion and the pontine influence

This study demonstrates that the response of the central respiratory rhythm to the low-level vagal afferent depends largely on the pontine influence. This new finding suggests a novel important function of the pontine respiratory network: regulation of the response intensity of the medullary respiratory network to the low lung volume information. In other words, a low-level vagal afferent becomes a strong activator of the central I activity especially when the pontine influence is weakened. It is known that the influence of the pontine pneumotaxic mechanism is age and state dependent. For example, it has been suggested that the influence of the pontine pneumotaxic mechanism is weaker in neonates (Morin-Surun et al. 1995) or becomes absent during sleep in non-anaesthetized animals (Lydic & Orem, 1979; Sieck & Harper, 1980). This means that a vagal I-promoting reflex might be more facilitated in the neonates and during sleep. Indeed, it has been reported that chest compression induces I effort in the human neonate (Marsh et al. 1994; Hannam et al. 2000), suggesting that the Hering-Breuer deflation reflex has a protective role to avoid excessive lung compression in young infants when the lung volume is at FRC. In anaesthetized cats, about 44 % of the SA-PSR fibres continue to discharge at 10–40 Hz during the E phase (Sant'ambrogio, 1982), which is the most adequate range to provoke the I-promoting vagal reflex, suggesting that this vagal I-promoting reflex could be operational not only during the excessive lung compression but also during eupnoeic breathing. It is therefore possible that, as is the case in animals with a lesioned pneumotaxic centre or NMDA-R blockade, the I-promoting vagal reflex becomes manifest and helps to avoid apnoea by promoting the central I activity during sleep, when the general skeletal muscle tone is reduced. It will be important to examine whether the I-promoting vagal reflex is functional in human eupnoea during sleep, especially in infants with sudden death syndrome risks, because it is suggested that vagal dysfunction might be one of the aetiologies of this syndrome (Shojaei-Brosseau et al. 2003).

Acknowledgments

We thank Dr M. P. Morin-Surun for her critical reading of the manuscript. The technical assistance from T. Matsuo was invaluable. We thank Dr H. Ohashi for his contribution during the experiments. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 13680902) and Grants for the Research on Health Sciences focusing on Drug Innovation from The Japan Health Sciences Foundation (KH21014) to F.K.

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