Frequency responses to hypoxia and hypercapnia in carotid body-denervated conscious rats

Time-dependent changes in cardiorespiratory functions of anesthetized rats exposed to sustained hypoxia

Although cardiovascular responses may be altered by respiratory changes under prolonged hypoxia, the relationship between respiratory and cardiovascular changes remains unknown. The aim of the present study is to clarify cardiorespiratory changes in anesthetized rats during and after hypoxic conditions using simultaneous recordings of cardiorespiratory variables with 20-sec recording intervals. After air breathing for 20 min (pre-exposure period), rats were subjected to 10% O₂ for 2 h (hypoxic exposure period) and then air for 30 min (recovery period). Minute ventilation (V_E), respiratory frequency, tidal volume, arterial blood pressure (BP), and heart rate (HR) were continuously monitored during the experimental period. Just after hypoxic exposure, V_E, BP, and HR exhibited an overshoot, undershoot, and overshoot followed by a decrease, respectively. During the remaining hypoxic exposure period, continuous high V_E and low BP were observed, whereas HR re-increased. In the recovery period, V_E, BP, and HR showed an undershoot, increase, and decrease followed by an increase, respectively. These results suggest that the continuation of enhanced V_E and re-increased HR, probably, due to carotid body excitation and accompanying sympathetic activation, during the late period of hypoxic exposure are protective responses to avoid worsening hypoxemia and further circulatory insufficiencies under sustained hypoxia.

Effect of pyridostigmine on in vivo and in vitro respiratory muscle of mdx mice

The current work was conducted to verify the contribution of neuromuscular transmission defects at the neuromuscular junction to Duchenne Muscular Dystrophy disease progression and respiratory dysfunction. We tested pyridostigmine and pyridostigmine encapsulated in liposomes (liposomal PYR), an acetylcholinesterase inhibitor to improve muscular contraction on respiratory muscle function in mdx mice at different ages. We evaluated in vivo with the whole-body plethysmography, the ventilatory response to hypercapnia, and measured in vitro diaphragm strength in each group. Compared to C57BL10 mice, only 17 and 22 month-old mdx presented blunted ventilatory response, under normocapnia and hypercapnia. Free pyridostigmine (1mg/kg) was toxic to mdx mice, unlike liposomal PYR, which did not show any side effect, confirming that the encapsulation in liposomes is effective in reducing the toxic effects of this drug. Treatment with liposomal PYR, either acute or chronic, did not show any beneficial effect on respiratory function of this DMD experimental model. The encapsulation in liposomes is effective to abolish toxic effects of drugs.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Is plasticity within the retrotrapezoid nucleus responsible for the recovery of the PCO2 set-point after carotid body denervation in rats?

KEY POINTS

Arterial PCO2 is kept constant via breathing adjustments elicited, at least partly, by central chemoreceptors (CCRs) and the carotid bodies (CBs). The CBs may be active in a normal oxygen environment because their removal reduces breathing. Thereafter, breathing slowly returns to normal. In the present study, we investigated whether an increase in the activity of CCRs accounts for this return. One week after CB excision, the hypoxic ventilatory reflex was greatly reduced as expected, whereas ventilation and blood gases at rest under normoxia were normal. Optogenetic inhibition of Phox2b-expressing neurons including the retrotrapezoid nucleus, a cluster of CCRs, reduced breathing proportionally to arterial pH. The hypopnoea was greater after CB excision but only in a normal or hypoxic environment. The difference could be simply explained by the loss of fast feedback from the CBs. We conclude that, in rats, CB denervation may not produce CCR plasticity. We also question whether the transient hypoventilation elicited by CB denervation means that these afferents are active under normoxia.

ABSTRACT

Carotid body denervation (CBD) causes hypoventilation and increases the arterial PCO2 set-point; these effects eventually subside. The hypoventilation is attributed to reduced CB afferent activity and the PCO2 set-point recovery to CNS plasticity. In the present study, we investigated whether the retrotrapezoid nucleus (RTN), a group of non-catecholaminergic Phox2b-expressing central respiratory chemoreceptors (CCRs), is the site of such plasticity. We evaluated the contribution of the RTN to breathing frequency (FR ), tidal volume (VT ) and minute volume (VE ) by inhibiting this nucleus optogenetically for 10 s (archaerhodopsinT3.0) in unanaesthetized rats breathing various levels of O2 and/or CO2 . The measurements were made in seven rats before and 6-7 days after CBD and were repeated in seven sham-operated rats. Seven days post-CBD, blood gases and ventilation in 21% O2 were normal, whereas the hypoxic ventilatory reflex was still depressed (95.3%) and hypoxia no longer evoked sighs. Sham surgery had no effect. In normoxia or hypoxia, RTN inhibition produced a more sustained hypopnoea post-CBD than before; in hyperoxia, the responses were identical. Post-CBD, RTN inhibition reduced FR and VE in proportion to arterial pH or PCO2 (ΔVE : 3.3 ± 1.5% resting VE /0.01 pHa). In these rats, 20.7 ± 8.9% of RTN neurons expressed archaerhodopsinT3.0. Hypercapnia (3-6% FiCO2 ) increased FR and VT in CBD rats (n = 4). In conclusion, RTN regulates FR and VE in a pH-dependent manner after CBD, consistent with its postulated CCR function. RTN inhibition produces a more sustained hypopnoea after CBD than before, although this change may simply result from the loss of the fast feedback action of the CBs.

Ventilatory chemosensory drive is blunted in the mdx mouse model of Duchenne Muscular Dystrophy (DMD)

Duchenne Muscular Dystrophy (DMD) is caused by mutations in the DMD gene resulting in an absence of dystrophin in neurons and muscle. Respiratory failure is the most common cause of mortality and previous studies have largely concentrated on diaphragmatic muscle necrosis and respiratory failure component. Here, we investigated the integrity of respiratory control mechanisms in the mdx mouse model of DMD. Whole body plethysmograph in parallel with phrenic nerve activity recordings revealed a lower respiratory rate and minute ventilation during normoxia and a blunting of the hypoxic ventilatory reflex in response to mild levels of hypoxia together with a poor performance on a hypoxic stress test in mdx mice. Arterial blood gas analysis revealed low PaO2 and pH and high PaCO2 in mdx mice. To investigate chemosensory respiratory drive, we analyzed the carotid body by molecular and functional means. Dystrophin mRNA and protein was expressed in normal mice carotid bodies however, they are absent in mdx mice. Functional analysis revealed abnormalities in Dejours test and the early component of the hypercapnic ventilatory reflex in mdx mice. Together, these results demonstrate a malfunction in the peripheral chemosensory drive that would be predicted to contribute to the respiratory failure in mdx mice. These data suggest that investigating and monitoring peripheral chemosensory drive function may be useful for improving the management of DMD patients with respiratory failure.

Hypoxic ventilatory response in Tac1-/- neonatal mice following exposure to opioids

Morphine is the dominating analgetic drug used in neonates, but opioid-induced respiratory depression limits its therapeutic use. In this study, we examined acute morphine effects on respiration during intermittent hypoxia in newborn Tac1 gene knockout mice (Tac1-/-) lacking substance P and neurokinin A. In vivo, plethysmography revealed a blunted hypoxic ventilatory response (HVR) in Tac1-/- mice. Morphine (10 mg/kg) depressed the HVR in wild-type animals through an effect on respiratory frequency, whereas it increased tidal volumes in Tac1-/- during hypoxia, resulting in increased minute ventilation. Apneas were reduced during the first hypoxic episode in both morphine-exposed groups, but were restored subsequently in Tac1-/- mice. Morphine did not affect ventilation or apnea prevalence during baseline conditions. In vitro, morphine (50 nM) had no impact on anoxic response of brain stem preparations of either strain. In contrast, it suppressed the inspiratory rhythm during normoxia and potentiated development of posthypoxic neuronal arrest, especially in Tac1-/-. Thus this phenotype has a higher sensitivity to the depressive effects of morphine on inspiratory rhythm generation, but morphine does not modify the reactivity to oxygen deprivation. In conclusion, although Tac1-/- mice are similar to wild-type animals during normoxia, they differed by displaying a reversed pattern with an improved HVR during intermittent hypoxia both in vivo and in vitro. These data suggest that opioids and the substance P-ergic system interact in the HVR, and that reducing the activity in the tachykinin system may alter the respiratory effects of opioid treatment in newborns.

Photostimulation of retrotrapezoid nucleus phox2b-expressing neurons in vivo produces long-lasting activation of breathing in rats

The retrotrapezoid "nucleus" (RTN), located in the rostral ventrolateral medullary reticular formation, contains a bilateral cluster of approximately 1000 glutamatergic noncatecholaminergic Phox2b-expressing propriobulbar neurons that are activated by CO(2) in vivo and by acidification in vitro. These cells are thought to function as central respiratory chemoreceptors, but this theory still lacks a crucial piece of evidence, namely that stimulating these particular neurons selectively in vivo increases breathing. The present study performed in anesthetized rats seeks to test whether this expectation is correct. We injected into the left RTN a lentivirus that expresses the light-activated cationic channel ChR2 (channelrhodopsin-2) (H134R mutation; fused to the fluorescent protein mCherry) under the control of the Phox2-responsive promoter PRSx8. Transgene expression was restricted to 423 +/- 38 Phox2b-expressing neurons per rat consisting of noncatecholaminergic and C1 adrenergic neurons (3:2 ratio). Photostimulation delivered to the RTN region in vivo via a fiberoptic activated the CO(2)-sensitive neurons vigorously, produced a long-lasting (t(1/2) = 11 s) increase in phrenic nerve activity, and caused a small and short-lasting cardiovascular stimulation. Selective lesions of the C1 cells eliminated the cardiovascular response but left the respiratory stimulation intact. In rats with C1 cell lesions, the mCherry-labeled axon terminals originating from the transfected noncatecholaminergic neurons were present exclusively in the lower brainstem regions that contain the respiratory pattern generator. These results provide strong evidence that the Phox2b-expressing noncatecholaminergic neurons of the RTN region function as central respiratory chemoreceptors.

Lateral parabrachial nucleus mediates shortening of expiration and increase of inspiratory drive during hypercapnia

We have previously shown that unilateral or bilateral lesions of the lateral parabrachial nucleus (LPBN) in anesthetized, vagotomized rats markedly and selectively attenuate the shortening of expiratory duration (T(E)) during hypoxia without appreciably affecting all other hypoxic response components. Here, we report that unilateral LPBN lesion by kainic acid in the same group of animals not only abolished normal T(E)-shortening during central chemoreceptors activation by hyperoxic hypercapnia, but led to paradoxical T(E)-prolongation and corresponding decrease of respiratory frequency. Furthermore, LPBN lesion significantly attenuated the increase in phrenic activity during hyperoxic hypercapnia, without appreciably affecting the corresponding shortening of inspiratory duration (T(I)). These findings provide the first evidence indicating that central chemoafferent inputs are organized in parallel and segregated pathways that separately modulate inspiratory drive, T(I), and T(E) in conjunction with similar parallel and segregated central processing of peripheral chemoafferent inputs reported previously [Young, D.L., Eldridge, F.L., Poon, C.S., 2003. Integration-differentiation and gating of carotid afferent traffic that shapes the respiratory pattern. J. Appl. Physiol. 94, 1213-1229].

Influence of hypercapnic acidosis and hypoxia on abdominal expiratory nerve activity in the rat

We studied the influence of hypercapnic acidosis and hypoxia on the neural drive to abdominal muscles in anesthetized and decerebrate rats; this information is unavailable despite widespread use of the rat as an experimental model in respiratory physiology and neurobiology. To minimize confounding influences from receptors in the lungs and chest wall, the animals were vagotomized, paralyzed and mechanically ventilated, and electrical activity was recorded from abdominal muscle nerves. In anesthetized and decerebrate rats, both stimuli evoked steady, low amplitude expiratory discharge that persisted throughout the expiratory phase (E-all activity), but was inhibited during inspiration. We also observed late expiratory, high-amplitude bursts (E2 activity) superimposed on this steady activity, but only at the highest levels of respiratory drive. Hypoxia enhanced abdominal motor activity transiently, whereas hypercapnic acidosis caused a sustained increase in activity. Thus, both hypercapnic acidosis and hypoxia activate abdominal muscle motoneurons in the absence of phasic afferent inputs.

Ventilatory roll off during sustained hypercapnia is gender specific in pekin ducks

The objective of the present study was to examine the relative roles of peripheral versus central mechanisms in producing ventilatory adjustments in pekin ducks during prolonged (5 h) hypercapnia (5% inspired CO2), and to determine whether these adjustments differed between male and female ducks. After 20 min of CO2 exposure, intact ducks increased total ventilation (VE) 2.5-3-fold above control values, due to large increases (approximately 200%) in tidal volume (VT) and slightly smaller increases (approximately 140%) in breathing frequency (fR). This response was accompanied by respiratory acidosis (pHa fell from approximately 7.46 to approximately 7.41) and hypercapnia (PaCO2 increased from approximately 35 to approximately 40 Torr). In males, VE fell progressively thereafter due exclusively to a fall in fR, in parallel with a rapid partial recovery of pH (to 7.44) while PaCO2 continued to climb (to approximately 42 Torr). In females, VE remained elevated during hypercapnia, and no pH recovery occurred. This suggests that a respiratory decline resulting from acid-base compensation (probably due to HCO3- mobilization) occurred in males but not in females. Bicarbonate mobilization, and thus pH compensation, may have been reduced in females due to the CaCO3 requirements of eggshell formation. In males, the acute ventilatory response was reduced slightly by denervation of the carotid bodies or intrapulmonary chemoreceptors, but there was no effect of denervation of either receptor group on the responses to prolonged CO2. We conclude that pH compensation triggered by constant or increasing PaCO2, acting at central chemoreceptors, likely mediates the respiratory adjustments seen in male pekin ducks during hypercapnia. Furthermore, we suggest that this ventilatory response be considered a gender-specific hypercapnic ventilatory roll off, in the context of the various time domains of the hypercapnic ventilatory response.

Lamotrigine and phenytoin, but not amiodarone, impair peripheral chemoreceptor responses to hypoxia

Amiodarone, lamotrigine, and phenytoin, common antiarrhythmic and antiepileptic drugs, inhibit a persistent sodium current in neurons (I(NaP)). Previous results from our laboratory suggested that I(NaP) is critical for functionality of peripheral chemoreceptors. In this study, we determined the effects of therapeutic levels of amiodarone, lamotrigine, and phenytoin on peripheral chemoreceptor and ventilatory responses to hypoxia. Action potentials (APs) of single chemoreceptor afferents were recorded using suction electrodes advanced into the petrosal ganglion of an in vitro rat peripheral chemoreceptor complex. AP frequency (at Po(2) approximately 150 Torr and Po(2) approximately 90 Torr), conduction time, duration, and amplitude were measured before and during perfusion with therapeutic dosages of the drug or vehicle. Hypoxia-induced catecholamine secretion within the carotid body was measured using amperometry. With the use of whole body plethysmography, respiration was measured in unanesthesized rats while breathing room air, 12% O(2), and 5% CO(2), before and after intraperitoneal administration of amiodarone, lamotrigine, phenytoin, or vehicle. Lamotrigine (10 microM) and phenytoin (5 microM), but not amiodarone (5 microM), decreased chemoreceptor AP frequency without affecting other AP parameters or magnitude of catecholamine secretion. Similarly, lamotrigine (5 mg/kg) and phenytoin (10 mg/kg) blunted the hypoxic but not the hypercapnic ventilatory response. In contrast, amiodarone (2.5 mg/kg) did not alter the ventilatory response to hypoxia or hypercapnia. We conclude that lamotrigine and phenytoin at therapeutic levels impair peripheral chemoreceptor function and ventilatory response to acute hypoxia. These are consistent with I(NaP) serving an important function in AP generation and may be clinically important in the care of patients using these drugs.

An important functional role of persistent Na+ current in carotid body hypoxia transduction

Systemic hypoxia in mammals is sensed and transduced by the carotid body into increased action potential (AP) frequency on the sinus nerve, resulting in increased ventilation. The mechanism of hypoxia transduction is not resolved, but previous work suggested that fast Na(+) channels play an important role in determining the rate and timing of APs (Donnelly, DF, Panisello JM, and Boggs D. J Physiol. 511: 301-311, 1998). We speculated that Na(+) channel activity between APs, termed persistent Na(+) current (I(NaP)), is responsible for AP generation that and riluzole and phenytoin, which inhibit this current, would impair organ function. Using whole cell patch clamp recording of intact petrosal neurons with projections to the carotid body, we demonstrated that I(NaP) is present in chemoreceptor afferent neurons and is inhibited by riluzole. Furthermore, discharge frequencies of single-unit, chemoreceptor activity, in vitro, during normoxia (Po(2) 150 Torr) and during acute hypoxia (Po(2) 90 Torr) were significantly reduced by riluzole concentrations at or above 5 microM, and by phenytoin at 100 microM, without significant affect on nerve conduction time, AP magnitude (inferred from extracellular field), and AP duration. The effect of both drugs appeared solely postsynaptic because hypoxia-induced catecholamine release in the carotid body was not altered by either drug. The respiratory response of unanesthetized, unrestrained 2-wk-old rats to acute hypoxia (12% inspired O(2) fraction), which was measured with whole body plethysmography, was significantly reduced after treatment with riluzole (2 mg/kg ip) and phenytoin (20 mg/kg ip). We conclude that I(NaP) is present in chemoreceptor afferent neurons and serves an important role in peripheral chemoreceptor function and, hence, in the ventilatory response to hypoxia.

Effects of carotid body excision on recovery of respiratory function in C2 hemisected adult rats

In a previous study, we described the spontaneous recovery of respiratory motor function in adult rats subjected to a left C2 hemisection 6-16 weeks post-injury without any therapeutic intervention. We extend the previous findings by demonstrating in the present study that rats subjected to a left C2 hemisection with bilateral carotid body excision will also recover respiratory-related activity in the paralyzed ipsilateral hemidiaphragm. However, in this instance, recovery is significantly accelerated; i.e., it is evident as early as 2 weeks after spinal cord injury. Two experimental groups (and noninjured and sham-operated controls) of rats were employed in the study. H-CBE animals were subjected to a left C2 hemisection plus bilateral carotid body excision while H-CBI animals were subjected to a left C2 hemisection only. Carotid body excision was confirmed by the sodium cyanide test. The animals were allowed to survive for 2 weeks after hemisection. Thereafter, electrophysiologic assessment of respiratory activity was conducted in all animals. Spontaneous recovery of respiratory-related activity in the paralyzed hemidiaphragm (indicated by left phrenic nerve activity) was detected in all H-CBE animals while H-CBI animals did not express spontaneous recovery of diaphragmatic activity. The magnitude of recovered activity when expressed as a function of contralateral phrenic nerve activity was 48.8 +/- 3.8%. When expressed as a function of the homolateral phrenic nerve in noninjured animals, the magnitude amounted to 25.6 +/- 2.8%. Although the mechanisms responsible for the apparent early onset of spontaneous recovery are unknown, it is likely that a reorganization of the respiratory circuitry in the CNS may be involved. The significance of the findings is that it may be feasible to modulate the onset of functional recovery following cervical spinal cord injury by specifically targeting peripheral chemoreceptors.

Chronic hypoxia abolishes expiratory prolongation following carotid sinus nerve stimulation in the anesthetized rat

In anesthetized rats, increases in phrenic nerve (PN) amplitude and frequency during brief periods of hypoxia or electrical stimulation of the carotid sinus nerve (CSN) are followed by an increase in expiratory duration. We investigated the effects of chronic exposure to hypoxia on PN responses to CSN stimulation. In Inactin anesthetized (100 mg/kg) Sprague-Dawley rats PN discharge and arterial pressure responses to 10-120 s of CSN stimulation (20 Hz, 0.2 ms duration pulses) were recorded after 7-10 days exposure to hypoxia (10 +/- .5% O2). In normoxic rats, the degree of CSN-evoked expiratory prolongation was dependent upon the duration of CSN stimulation. CSN-evoked increases in PN burst amplitude were not different comparing chronic hypoxic rats to rats maintained at normoxia while CSN-evoked increases in PN burst frequency were greater in chronic hypoxic rats (p<.05). CSN-evoked expiratory prolongation was abolished in chronic hypoxic rats. Following chronic hypoxia, changes occur within the central processing of arterial chemoreceptor inputs so that CSN stimulation evokes an enhanced PN frequency response and no expiratory prolongation.

Hypoxic activation of arterial chemoreceptors inhibits sympathetic outflow to brown adipose tissue in rats

In urethane-chloralose anaesthetized, neuromuscularly blocked, artificially ventilated rats, we demonstrated that activation of carotid chemoreceptors inhibits the elevated levels of brown adipose tissue (BAT) sympathetic nerve activity (SNA) evoked by hypothermia, by microinjection of prostaglandin E2 into the medial preoptic area or by disinhibition of neurones in the raphe pallidus area (RPa). Peripheral chemoreceptor stimulation with systemic administration of NaCN (50 microg in 0.1 ml) or with hypoxic ventilation (8% O2-92% N2, 30 s) completely inhibited BAT SNA. Arterial chemoreceptor-evoked inhibition of BAT SNA was eliminated by prior bilateral transections of the carotid sinus nerves or by prior inhibition of neurones within the commissural nucleus tractus solitarii (commNTS) with glycine (40 nmol/80 nl) or with the GABAA receptor agonist muscimol (160 pmol/80 nl; 77 +/- 10% attenuation), or by prior blockade of ionotropic excitatory amino acid receptors in the commNTS with kynurenate (8 nmol/80 nl; 82 +/- 10% attenuation). Furthermore, activation of commNTS neurones following local microinjection of bicuculline (30 pmol/60 nl) completely inhibited the elevated level of BAT SNA resulting from disinhibition of neurones in the RPa. These results demonstrate that hypoxic stimulation of arterial chemoreceptor afferents leads to an inhibition of BAT SNA and BAT thermogenesis through an EAA-mediated activation of second-order, arterial chemoreceptor neurones in the commNTS. Peripheral chemoreceptor-evoked inhibition of BAT SNA could directly contribute to (or be permissive for) the hypoxia-evoked reductions in body temperature and oxygen consumption that serve as an adaptive response to decreased oxygen availability.

Specific carotid body chemostimulation is sufficient to elicit phrenic poststimulus frequency decline in a novel in situ dual-perfused rat preparation

Time-dependent ventilatory responses to hypoxic and hypercapnic challenges, such as posthypoxic frequency decline (PHxFD) and posthypercapnic frequency decline (PHcFD), could profoundly affect breathing stability. However, little is known about the mechanisms that mediate these phenomena. To determine the contribution of specific carotid body chemostimuli to PHxFD and PHcFD, we developed a novel in situ arterially perfused, vagotomized, decerebrate rat preparation in which central and peripheral chemoreceptors are perfused separately (i.e., a nonanesthetized in situ dual perfused preparation). We confirmed that 1) the perfusion of central and peripheral chemoreceptor compartments was independent by applying specific carotid body hypoxia and hypercapnia before and after carotid sinus nerve transection, 2) the PCO(2) chemoresponse of the dual perfused preparation was similar to other decerebrate preparations, and 3) the phrenic output was stable enough to allow investigation of time-dependent phenomena. We then applied four 5-min bouts (separated by 5 min) of specific carotid body hypoxia (40 Torr PO(2) and 40 Torr PCO(2)) or hypercapnia (100 Torr PO(2) and 60 Torr PCO(2)) while holding the brain stem PO(2) and PCO(2) constant. We report the novel finding that specific carotid body chemostimuli were sufficient to elicit several phrenic time-dependent phenomena in the rat. Hypoxic challenges elicited PHxFD that increased with bout, leading to progressive augmentation of the phrenic response. Conversely, hypercapnia elicited short-term depression and PHcFD, neither of which was bout dependent. These results, placed in the context of previous findings, suggest multiple physiological mechanisms are responsible for PHxFD and PHcFD, a redundancy that may illustrate that these phenomena have significant adaptive advantages.

Differences in time-dependent hypoxic phrenic responses among inbred rat strains

Hypoxic ventilatory responses differ between rodent strains, suggesting a genetic contribution to interindividual variability. However, hypoxic ventilatory responses consist of multiple time-dependent mechanisms that can be observed in different respiratory motor outputs. We hypothesized that strain differences would exist in discrete time-dependent mechanisms of the hypoxic response and, furthermore, that there may be differences between hypoglossal and phrenic nerve responses to hypoxia. Hypoglossal and phrenic nerve responses were assessed during and after a 5-min hypoxic episode in anesthetized, vagotomized, and ventilated rats from four inbred strains: Brown Norway (BN), Fischer 344 (FS), Lewis (LW), and Piebald-viral-Glaxo (PVG). During baseline, burst frequency was higher in PVG than LW rats ( P < 0.05), phrenic burst amplitude was higher in PVG vs. other strains ( P < 0.05), and hypoglossal burst amplitude was higher in PVG and BN vs. FS and LW ( P < 0.05). During hypoxia, burst frequency did not change in BN or LW rats, but it increased in PVG and FS rats. The phrenic amplitude response was smallest in PVG vs. other strains ( P < 0.05), and the hypoglossal response was similar among strains. Short-term potentiation posthypoxia was slowest in FS and fastest in LW rats ( P < 0.05). Posthypoxia frequency decline was absent in PVG, but it was observed in all other strains. Augmented breaths were observed during hypoxia in FS rats only. Thus genetic differences exist in the time domains of the hypoxic response, and these are differentially expressed in hypoglossal and phrenic nerves. Furthermore, genetic diversity observed in hypoxic ventilatory responses in unanesthetized rats may arise from multiple neural mechanisms.

Recovery of respiratory function following C2 hemi and carotid body denervation in adult rats: influence of peripheral adenosine receptors

The efficacy of the methylxanthine, theophylline, as a respiratory stimulant has been demonstrated previously in an animal model of spinal cord injury. In this model, an upper cervical (C2) spinal cord hemi paralyzes the ipsilateral hemidiaphragm. Theophylline restores respiratory-related activity in the paralyzed hemidiaphragm via activation of a latent respiratory motor pathway. Antagonism of central adenosine A1 receptors mediates this action. Theophylline also enhances respiratory frequency, f, defined as breaths per minute. Thus, long-term use may result in respiratory muscle or motoneuron fatigue particularly after spinal cord injury. We assessed the effects of an adenosine A1 receptor agonist, N6-p-sulfophenyladenosine (p-SPA) on theophylline's action in our model under standardized recording conditions. Four groups of rats, classified as hemisected/nonhemisected with the carotid bodies denervated (H-CBD or NH-CBD), and hemisected/nonhemisected with the carotid bodies intact (H-CBI or NH-CBI ) were used in the study. Eight days after recovery from carotid denervation, a left C2 hemi was performed in H-CBD rats. C2 hemi was also performed in H-CBI animals, and 24 h later, electrophysiologic experiments on respiratory activity were conducted in both groups of animals. Two groups using nonhemisected controls were also employed as described above. In H-CBD rats, theophylline significantly (P < 0.05) enhanced f and induced respiratory-related activity in the previously quiescent left phrenic nerve. In NH-CBD rats, theophylline significantly enhanced f. In both H-CBD and NH-CBD rats, p-SPA (0.25 mg/kg) did not significantly change theophylline-induced effects. In H-CBI rats, theophylline significantly (P < 0.05) enhanced f and induced activity in the previously quiescent left phrenic nerve. In H-CBI rats, p-SPA reduced the values to pre-theophylline discharge levels. Recovered activity was not obliterated with the agonist. In NH-CBI rats, p-SPA reduced theophylline-induced effects to pre-drug discharge levels. Adenosine A1 and A2A receptor immunoreactivity was detected in the carotid bodies. The significance of our findings is that theophylline-induced effects can be normalized to pre-drug levels by the selective activation of peripheral adenosine A1 receptors. The therapeutic benefits of theophylline, i.e., recovered respiratory function after paralysis, however, persists. The potential therapeutic impact is that respiratory muscle fatigue associated with long-term theophylline use may be minimized by a novel therapeutic approach.

Time-dependent modulation of carotid body afferent activity during and after intermittent hypoxia

The ventilatory response to several minutes of hypoxia consists of various time-dependent phenomena, some of which occur during hypoxia (e.g., short-term depression), whereas others appear on return to normoxia (e.g., posthypoxic frequency decline). Additional phenomena can be elicited by acute, intermittent hypoxia (e.g., progressive augmentation, long-term facilitation). Current data suggest that these phenomena originate centrally. We tested the hypothesis that carotid body afferent activity undergoes time-dependent modulation, consistent with a direct role in these ventilatory phenomena. Using an in vitro rat carotid body preparation, we found that 1) afferent activity declined during the first 5 min of severe (40 Torr Po(2)), moderate (60 Torr Po(2)), or mild (80 Torr Po(2)) hypoxia; 2) after return to normoxia (100 Torr Po(2)) and after several minutes of moderate or severe hypoxia, afferent activity was transiently reduced compared with prehypoxic levels; and 3) with successive 5-min bouts of mild, moderate, or severe hypoxia, afferent activity during bouts increased progressively. We call these phenomena sensory hypoxic decline, sensory posthypoxic decline, and sensory progressive augmentation, respectively. These phenomena were stimulus specific: similar phenomena were not seen with 5-min bouts of normoxic hypercapnia (100 Torr Po(2) and 50-60 Torr Pco(2)) or hypoxic hypocapnia (60 Torr Po(2) and 30 Torr Pco(2)). However, bouts of either normoxic hypercapnia or hypocapnic hypoxia resulted in sensory long-term facilitation. We suggest time-dependent carotid body activity acts in parallel with central mechanisms to shape the dynamics of ventilatory responses to respiratory chemostimuli.

Role of the carotid bodies in chemosensory ventilatory responses in the anesthetized mouse

We examined the effects of carotid body denervation on ventilatory responses to normoxia (21% O2 in N2 for 240 s), hypoxic hypoxia (10 and 15% O2 in N2 for 90 and 120 s, respectively), and hyperoxic hypercapnia (5% CO2 in O2 for 240 s) in the spontaneously breathing urethane-anesthetized mouse. Respiratory measurements were made with a whole body, single-chamber plethysmograph before and after cutting both carotid sinus nerves. Baseline measurements in air showed that carotid body denervation was accompanied by lower minute ventilation with a reduction in respiratory frequency. On the basis of measurements with an open-circuit system, no significant differences in O2 consumption or CO2 production before and after chemodenervation were found. During both levels of hypoxia, animals with intact sinus nerves had increased respiratory frequency, tidal volume, and minute ventilation; however, after chemodenervation, animals experienced a drop in respiratory frequency and ventilatory depression. Tidal volume responses during 15% hypoxia were similar before and after carotid body denervation; during 10% hypoxia in chemodenervated animals, there was a sudden increase in tidal volume with an increase in the rate of inspiration, suggesting that gasping occurred. During hyperoxic hypercapnia, ventilatory responses were lower with a smaller tidal volume after chemodenervation than before. We conclude that the carotid bodies are essential for maintaining ventilation during eupnea, hypoxia, and hypercapnia in the anesthetized mouse.

Responses to GABA(A) receptor activation are altered in NTS neurons isolated from chronic hypoxic rats

The inhibitory amino acid GABA is released within the nucleus of the solitary tract (NTS) during hypoxia and modulates the respiratory response to hypoxia. To determine if responses of NTS neurons to activation of GABA(A) receptors are altered following exposure to chronic hypoxia, GABA(A) receptor-evoked whole cell currents were measured in enzymatically dispersed NTS neurons from normoxic and chronic hypoxic rats. Chronic hypoxic rats were exposed to 10% O(2) for 9-12 days. Membrane capacitance was the same in neurons from normoxic (6.9+/-0.5 pF, n=16) and hypoxic (6.3+/-0.5 pF, n=15) rats. The EC(50) for peak GABA-evoked current density was significantly greater in neurons from hypoxic (21.7+/-2.2 microM) compared to normoxic rats (12.2+/-0.9 microM) (p<0.001). Peak and 5-s adapted GABA currents evoked by 1, 3 and 10 microM were greater in neurons from normoxic compared to hypoxic rats (p<0.05) whereas peak and 5-s adapted responses to 30 and 100 microM GABA were not different comparing normoxic to hypoxic rats. Desensitization of GABA(A)-evoked currents was observed at concentrations greater than 3 microM and, measured as the ratio of the current 5 s after the onset of 100 microM GABA application to the peak GABA current, was the same in neurons from normoxic (0.37+/-0.03) and hypoxic rats (0.33+/-0.04). Reduced sensitivity to GABA(A) receptor-evoked inhibition in chronic hypoxia could influence chemoreceptor afferent integration by NTS neurons.

Hypercapnia selectively attenuates the somato-sympathetic reflex

The effects of hyperoxic hypercapnia (5, 10 or 15% CO2 in O2) on splanchnic sympathetic nerve activity (sSNA) and sympathetic reflexes such as the somato-sympathetic reflex or baroreflex were studied in urethane anaesthetised, paralysed, artificially ventilated and vagotomized Sprague-Dawley rats. Hypercapnia caused a small increase in mean arterial blood pressure (MAP) in the 10% CO2 group and a fall in heart rate (HR) in all three groups. sSNA increased in all three groups. Phrenic frequency and amplitude increased during hypercapnia, with frequency adapting back towards baseline during the CO2 exposure. The somato-sympathetic reflex was attenuated in the 5% CO2 group and abolished in the 10 and 15% CO2 groups, whereas there was little effect on the sSNA baroreflex. Hypercapnia significantly affects phrenic nerve activity (PNA), sSNA and selectively inhibits the somato-sympathetic reflex with little effect on the sSNA baroreflex.

Chronic hypoxia abolishes posthypoxia frequency decline in the anesthetized rat

In anesthetized rats, increases in phrenic nerve amplitude and frequency during brief periods of hypoxia are followed by a reduction in phrenic nerve burst frequency [posthypoxia frequency decline (PHFD)]. We investigated the effects of chronic exposure to hypoxia on PHFD and on peripheral and central O2-sensing mechanisms. In Inactin-anesthetized (100 mg/kg) Sprague-Dawley rats, phrenic nerve discharge and arterial pressure responses to 10 s N2 inhalation were recorded after exposure to hypoxia (10 +/- 0.5% O2) for 6-14 days. Compared with rats maintained at normoxia, PHFD was abolished in chronic hypoxic rats. Because of inhibition of PHFD, the increased phrenic burst frequency and amplitude after N2 inhalation persisted for 1.8-2.8 times longer in chronic hypoxic (70 s) compared with normoxic (25-40 s) rats (P < 0.05). After acute bilateral carotid body denervation, N2 inhalation produced a short depression of phrenic nerve discharge in both chronic hypoxic and normoxic rats. However, the degree and duration of depression of phrenic nerve discharge was smaller in chronic hypoxic compared with normoxic rats (P < 0.05). We conclude that after exposure to chronic hypoxia, a reduction in PHFD contributes to an increased duration of the acute hypoxic ventilatory response in anesthetized rats. Furthermore, after exposure to chronic hypoxia, the central network responsible for respiration is more resistant to the depressant effects of acute hypoxia in anesthetized rats.

Elimination of the post-hypoxic frequency decline in conscious rats lesioned in pontine A5 region

A decrease in the frequency of breathing following a hypoxic exposure that is below baseline values is called the post-hypoxic frequency decline (phfd) and is due to an elongation of expiratory time (TE). We hypothesized that lesioning the pontine A5 region would eliminate the phfd in conscious rats. Fourteen conscious male rats that demonstrated a phfd received lesions either within the A5 region (n=9) or outside this region (controls, n=5). Compared with pre-lesion values, body temperature decreased and frequency of breathing was lower during exposure to air, hypoxia, and hypercapnia in A5-lesioned, but not in the control-lesioned rats. No effect of A5 lesions was noted on tidal volume. Rats with A5 lesions no longer exhibited a phfd, and TE values following hypoxia were comparable to baseline TE values. These data suggest that the A5 region of the ventrolateral pons modulates the phfd in conscious rats and affects frequency of breathing in response to both hypoxia and hypercapnia.

Gender comparisons of control of breathing and metabolism in conscious mice exposed to cold

We evaluated the effects of 2 h of warm (24 degrees C) and cold (6 degrees C) exposure on metabolism and ventilation (V(E)) in conscious male and female Harlan ICR Swiss Webster mice exposed to air, and 8% O(2) in N(2) (hypoxia) and to 5% CO(2) in O(2) (hypercapnia) for 2 min each at both temperatures. All cold-exposed mice increased O(2) consumption (V(O2)), and maintained body temperature. Cold-exposed females doubled their tidal volume, increased their V(E) fivefold, and doubled their ventilatory equivalent to V(O2) (V(E)/V(O2)). In contrast, cold-exposed males decreased tidal volume and doubled V(E) relative to warm exposure. The ventilatory equivalent of males was similar during warm and cold exposure. During warm exposure, mice of both genders increased their ventilatory responses to both hypoxia and to hypercapnia by different mechanisms. In contrast, during cold exposure, these responses were blunted relative to air measurements in females and decreased below air values in males. Thus, cold exposure was able to elicit gender-specific ventilatory and metabolic responses.