AJP - Regu Fuel your research with LabChart
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Regul Integr Comp Physiol 290: R1664-R1671, 2006. First published February 2, 2006; doi:10.1152/ajpregu.00851.2005
0363-6119/06 $8.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
290/6/R1664    most recent
00851.2005v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (6)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Reeves, S. R.
Right arrow Articles by Gozal, D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Reeves, S. R.
Right arrow Articles by Gozal, D.

DEVELOPMENTAL PHYSIOLOGY AND PREGNANCY

Early postnatal chronic intermittent hypoxia modifies hypoxic respiratory responses and long-term phrenic facilitation in adult rats

Stephen R. Reeves,1,2 Gordon S. Mitchell,3 and David Gozal1,2

1Departments of Pediatrics and 2Pharmacology and Toxicology, Kosair Children’s Hospital Research Institute, University of Louisville School of Medicine, Louisville, Kentucky; and 3Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin

Submitted 5 December 2005 ; accepted in final form 27 January 2006


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Acute isocapnic intermittent hypoxia elicits time-dependent, serotonin-dependent enhancement of phrenic motor output in anesthetized rats (phrenic long-term facilitation, pLTF). In adult rats, pLTF is enhanced by chronic intermittent hypoxia (CIH). To test the hypothesis that early postnatal CIH induces persistent modifications of ventilation and pLTF, we exposed male Sprague-Dawley rat pups on their first day of life to a CIH profile consisting of alternating room air and 10% oxygen every 90 s for 30 days during daylight hours (RAIH) or to comparable exposures consisting of room air throughout (RARA). One month after cessation of CIH, respiratory responses were recorded using whole body plethysmography, and integrated phrenic nerve activity was recorded in urethane-anesthetized, vagotomized, paralyzed, and ventilated rats at baseline and after exposures to three 5-min hypoxic episodes [inspired O2 fraction (FIO2) = 0.11] separated by 5 min of hyperoxia (FIO2 = 0.5). RAIH rats displayed greater normoxic ventilation and also increased burst frequency compared with RARA rats (P < 0.01). Ventilatory responses to hypoxia and short-term phrenic responses during acute hypoxic challenges were reduced in RAIH rats (P < 0.01). Although pLTF was present in both RAIH and RARA rats, it was diminished in RAIH rats (minute activity: 74 ± 2% in RARA vs. 55 ± 5% in RAIH at 60 min; P < 0.01). Thus we conclude that early postnatal CIH modifies normoxic and hypoxic ventilatory and phrenic responses that persist at 1 mo after cessation of CIH (i.e., metaplasticity) and markedly differ from previously reported increased neural plasticity changes induced by CIH in adult rats.

respiratory control; development; plasticity; metaplasticity

ANESTHETIZED ADULT RATS exposed to acute intermittent hypoxia (IH) exhibit a prolonged enhancement of phrenic motor output, a phenomenon termed phrenic long-term facilitation (pLTF) (3, 14, 36). pLTF exemplifies the ability of neural pathways underlying respiratory control to exhibit functional plasticity (16). Although intermittent stimulation of the carotid sinus nerve (CSN) as well as multiple IH protocols elicit pLTF (16, 34–36), pLTF is not elicited by equivalent exposures to sustained hypoxia (SH) (2). Diminished pLTF has been demonstrated in CSN-denervated rats, demonstrating that at least a portion of pLTF can be elicited in the absence of functional carotid body chemoreceptors (5). pLTF is a form of serotonin-dependent plasticity (16). According to current models, activation of postsynaptic 5-HT receptors, principally 5-HT2A subtypes, activates downstream signaling cascades involving several protein kinases, facilitating new protein synthesis (3) and increasing glutamatergic synaptic transmission between bulbospinal respiratory premotor neurons and phrenic motoneurons (3, 14, 33, 37).

Genetic, developmental, and gender-related factors modulate pLTF (8, 17). For example, both phrenic and hypoglossal LTF decrease in middle-aged male Sprague-Dawley rats, an effect possibly associated with age-associated changes in serotonin and/or its receptors (52). However, the opposite (i.e., increased pLTF) is observed in middle-aged female rats, suggesting complex interactions between respiratory control and sex hormones in modulating IH-induced respiratory plasticity (53). pLTF can be enhanced by previous experience, such as prior exposure of adult rats to chronic intermittent hypoxia (CIH) (27, 41).

The respiratory control system exhibits developmental plasticity, a unique effect due to experiences during development that persist into adulthood (9, 37, 45). However, it is unclear whether developmental exposures to CIH influence the expression of other forms of plasticity later in life. Thus a fundamental goal of the present study was to determine whether early developmental exposures to IH influence the subsequent response of adult animals to acute IH and the expression of pLTF. This issue is of considerable concern to the human population because large numbers of children exhibit sleep-disordered breathing (20). The potential impact of such respiratory disorders on ventilatory control later in life is unknown.

In a series of studies addressing the ventilatory changes elicited by long-term CIH exposures (1 mo), Reeves and Gozal (44) demonstrated that ventilatory plasticity progressively declines with advancing age. Furthermore, long-term CIH present either prenatally (23) or postnatally (47) elicited sustained increases in normoxic ventilation that persist into adulthood. In the present study, we examined the hypothesis that early long-term postnatal exposures to CIH lead to persistent modifications of the ventilatory plasticity exemplified by pLTF. In addition, we also studied the hypoxic ventilatory responses (HVR) in similarly exposed freely behaving adult rats to compare and contrast the responses observed in both experimental models.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Timed-pregnant Sprague-Dawley rats were purchased from Charles River Laboratories (Portage, MI), and their male offspring were used for all experiments. Litters were routinely culled to eight pups at birth. The experimental protocols were approved by the Institutional Animal Care and Use Committee and are in close agreement with the National Institutes of Health Guide for the Care and Use of Animals. All efforts were made to minimize animal suffering and to reduce the number of animals used.

Experimental Protocol

This study encompasses two series of experiments using identically exposed rats. Male rats were exposed to 30 consecutive days of 12-h IH during daylight starting at 6 h after delivery in room air (RAIH animals). Animals were exposed with their dams and weaned at the age of 21 days while the IH exposure continued. Control animals were exposed in identical chambers to continuous normoxic conditions with room air being flushed in the chambers in a similar periodicity as that used for IH exposures (RARA animals). After 30-day IH exposures were completed, animals were removed from the environmental chambers and returned to normal rearing conditions in room air until experimentation. In the first series of experiments, adult male rats (n = 8 per group from at least four different litters per exposure group) were used in acute ventilatory studies. The second series of experiments were conducted in separate rats (n = 10 per group from at least four different litters per exposure group) and used an anesthetized model to study pLTF.

IH Exposures

Animals were placed in four identical commercially designed chambers (30 x 20 x 20 in.; Oxycycler model A44XO; Biospherix, Redfield, NY), which were operated under a 12:12-h light-dark cycle (6:00 AM–6:00 PM). Gas was circulated around each of the chambers, attached tubing, and other units at 60 l/min (i.e., 1 complete change per 10 s). The O2 concentration was continuously measured using an O2 analyzer and was changed by a computerized system controlling the gas valve outlets. Deviations from the desired concentrations were met by computer-controlled addition of N2 or O2 through solenoid valves. Ambient CO2 in the chamber was periodically monitored and maintained at <0.01% by adjusting flow rates through the chambers. Humidity was measured and maintained at 40–50% by circulating the gas through a freezer and silica gel. Ambient temperature was kept at 22–24°C. The IH profile consisted of alternating room air and 10% O2 every 90 s for the 12-h light period for 30 days.

Acute Ventilatory Experiments

Ventilatory recordings. Respiratory measures were continuously acquired in the freely behaving animals by using the barometric method (Buxco Electronics, Willmington, NC) (4, 40). To minimize the long-term effect of signal drift due to temperature and pressure changes outside the chamber, we used a reference chamber of equal size in which temperature was measured with a T-type thermocouple. In addition, a correction factor was incorporated into the software routine to account for inspiratory and expiratory barometric asymmetries (13). Environmental temperature was maintained slightly below the thermoneutral range (24–26°C). At least 60 min before the start of each protocol, animals were allowed to acclimate to the chamber, through which humidified air (70–90% relative humidity) was passed at a rate of 5 l/min with the use of a precision flow pump-reservoir system. Pressure changes in the chamber due to the inspiratory and expiratory temperature changes were measured using a high-gain differential pressure transducer (model MP45-1; Validyne). Analog signals were continuously digitized and analyzed online by a microcomputer software program (BioSystem XA; Buxco Electronics). A rejection algorithm was included in the breath-by-breath analysis routine and allowed for accurate rejection of motion-induced artifacts. All ventilatory measurements are reported as BTPS values. Tidal volume (VT), respiratory frequency (f), and expired volume over time VE were computed and stored for subsequent off-line analysis.

Acute hypoxic ventilatory challenges. Animals were weighed and allowed at least 60 min to acclimate to the barometric recording chamber. After stable baseline normoxic values were obtained for at least 30 min, rats were switched to 10% O2-balance N2 with the use of a premixed gas mixture. The hypoxic challenge lasted for 20 min, after which room air was reintroduced into the recording chamber and recovery was recorded for 10 min. Ventilatory measures were averaged in 1-min intervals and plotted.

Acute Neurophysiological Experiments

Preparation. When the rats were between 7 and 9 wk of age, anesthesia was induced with isoflurane (Isoflo; Harvard Apparatus) and then maintained [2.5% isoflurane, inspired oxygen fraction (FIO2) = 0.5, balance N2] through a nose cone, followed by a tracheal cannula placed for mechanical ventilation. After implantation of a catheter (0.80 outer diameter x 0.50 mm inner diameter) in the femoral vein, rats were slowly converted to urethane anesthesia (1.6 g/kg iv) over a period of several minutes. A second identical catheter was implanted in the femoral artery for monitoring of arterial blood pressure and also was used to withdraw blood samples for pH and arterial blood gas analysis (ABL-510; Radiometer, Copenhagen, Denmark). Arterial blood gas values were corrected to rectal temperature measured at the time of sampling, with arterial blood pressure also measured via this catheter. Body temperature was maintained at ~37°C with a homeothermic blanket system (model 50-7053; Harvard Apparatus). Adequacy of anesthesia was assessed periodically throughout the experiment by the observation of blood pressure responses to noxious stimuli (i.e., toe pinch); supplemental urethane (0.2 g/kg iv) was given as necessary. Continuous intravenous infusion (2.5 ml/h) of a 1:11 solution of sodium bicarbonate (5%) and lactated Ringer solution was used to maintain acid-base balance beginning shortly after urethane administration. Animals were bilaterally vagotomized in the midcervical region to avoid entrainment by the ventilator and were paralyzed with pancuronium bromide (2.0 mg/kg iv) to prevent spontaneous respiratory muscle movement. End-tidal carbon dioxide partial pressures (PETCO2) were continuously monitored throughout the protocol with the use of a flow-through carbon dioxide analyzer placed in the exhaust line of the ventilator circuit (Capnogard model 1265; Novametrix). The left phrenic nerve was isolated using a dorsal approach, cut distally, desheathed, placed on a bipolar silver wire electrode, and submerged in mineral oil. Nerve activity was amplified (x10,000), band-pass filtered (100 Hz to 10 kHz; model 1800; A-M Systems, Carlsborg, WA). The signal was then digitized (PowerLab 8/SP; ADInstruments, Sydney, Australia), rectified and integrated (time constant = 50 ms), and processed with commercially available computer software (Chart 5.2; ADInstruments).

Phrenic LTF protocol. Preparations were allowed to stabilize for ~30 min after surgery (PETCO2 ~40 Torr). Apneic threshold was determined by hyperventilating the rat until phrenic nerve activity ceased and then slowly increasing PETCO2 by decreasing ventilator rate or supplementing inspired carbon dioxide until rhythmic activity returned. To standardize baseline activity, we maintained preparations 3 Torr above the PETCO2 value at which respiratory activity resumed. After baseline phrenic activity was established (30 min), an arterial blood sample (0.2–0.3 ml in a heparinized glass syringe) was drawn; subsequent blood samples were compared with this baseline value. Rats were then exposed to either three 5-min bouts of hypoxia [arterial oxygen partial pressure (PaO2) ~40 Torr] as the pLTF induction protocol or maintained under baseline conditions as a time control (n = 3 for RAIH and RARA exposure groups). Arterial carbon dioxide partial pressure (PaCO2) was strictly maintained at ±2 Torr of baseline during hypoxia. Blood samples were collected during the final 30 s of the first treatment bout, and if blood gases were not within the acceptable range, FIO2 and/or inspired carbon dioxide fraction (FICO2) was adjusted during the next bout. Rats were returned to FIO2 = 0.5 for 5 min between hypoxic episodes. After the hypoxic episode, phrenic nerve activity was monitored for 1 h while baseline conditions were strictly maintained. Blood samples were collected at 15, 30, and 60 min after pLTF induction to ensure that blood gases remained isocapnic (PaO2 within 1 Torr of baseline). After the 60-min blood sample was collected, a hypercapnic challenge (PETCO2 > 70 mmHg for 10 min) was performed to elicit maximal phrenic output.

Data Analysis

Phrenic activity was averaged online in 30-s bins and stored for subsequent off-line analysis. Changes from baseline in burst amplitude and minute phrenic activity (MPA) were normalized as percentages of maximal values due to significant differences between the groups during basal conditions (RARA = 36.1 ± 1.4 vs. RAIH = 42.0 ± 1.3 burst frequency; P < 0.02 ). Acute hypoxic responses and time-dependent changes in phrenic activity, blood gases, and blood pressure were compared among treatment groups by using one-way or two-way repeated-measures ANOVA followed by Fisher’s least significant difference post hoc tests as appropriate. Effects were considered statistically significant at P < 0.05.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Normoxic Ventilation

RAIH animals weighed significantly less than controls (271.3 ± 4.1 vs. 301.4 ± 9.0 g, respectively; P < 0.01); thus all reported ventilation data were normalized for weight as appropriate. Basal normoxic VE as measured using whole body plethysmography was significantly increased in adult male rats exposed to 30 days of postnatal IH (P < 0.05; Fig. 1A). Ventilatory enhancements were related to significantly greater VT (P < 0.05; Fig. 1B) as well as increased f (P < 0.05; Fig. 1C).


Figure 1
View larger version (9K):
[in this window]
[in a new window]
  		Fig. 1. Measurements (means ± SE; n = 8/group) of baseline normoxic expired volume over time (VE) obtained using whole body plethysmography were increased in adult male rats exposed to early postnatal chronic intermittent hypoxia (RAIH; A). Ventilatory enhancements were related to significantly greater tidal volume (VT; B) and increased respiratory frequency (f; C). *P < 0.05 compared with rats exposed to room air conditions (RARA). BW, body weight.

 
Peak Hypoxic Ventilatory Response

Examination of ventilatory measures during peak HVR revealed that weight-normalized VE was not significantly different between the groups (Fig. 2A). Furthermore, no significant differences occur with respect to weight-normalized VT [RARA = 3.6 ± 0.3 µl vs. RAIH = 3.5 ± 0.2 µl; P = not significant (NS)] or f (RARA = 285.8 ± 23.1 min–1 vs. RAIH = 235.6 ± 36.9 min–1; P = NS) during peak HVR. Conversely, when measures were corrected for their corresponding baseline values, RAIH animals displayed substantially reduced responses to acute hypoxia (P < 0.01; Fig. 2B). Relative changes in VE were accounted for primarily through relative changes in f (RARA = 248.6 ± 27.2% vs. RAIH = 56.3 ± 15.3%; P < 0.001) without significant relative changes in VT (RARA = –1.6 ± 8.9% vs. RAIH = –4.4 ± 5.7%; P = NS).


Figure 2
View larger version (10K):
[in this window]
[in a new window]
  		Fig. 2. Measurements (means ± SE; n = 8/group) of absolute weight-normalized VE (A) and ventilation expressed as the percent change from baseline (%CHNG BL) during peak hypoxic ventilatory responses (HVR). Weight-normalized VE did not significantly differ between groups. However, ventilatory measures corrected for corresponding baseline values reveal that RAIH animals demonstrated significantly attenuated ventilatory responses to acute hypoxia (B) *P < 0.01 compared with RARA rats.

 
Apneic Threshold, Baseline Conditions, Arterial Blood Gases, and Arterial Blood Pressure

The CO2 apneic threshold was not significantly different between RARA and RAIH animals (43.0 ± 0.63 and 42.7 ± 0.42 mmHg PETCO2, respectively; n = 7/group). Because the baseline CO2 for all experiments was set at 3 mmHg above the individual apneic threshold and isocapnia was strictly maintained (PaCO2 ±1 mmHg relative to baseline) throughout the duration of the experiment (see Table 1), any changes in phrenic nerve output should not have been influenced by relative differences in PaCO2 regulation. No differences emerged between the groups with regard to PaO2 with the exception of the 15-min post-IH stimulus time point (P < 0.02; Table 1). It is important to note that PaO2 is well above 100 mmHg at all times following the LTF induction protocol; thus the PaO2 is unlikely to significantly contribute to changes in phrenic output. Representative rectified and integrated phrenic neurograms for each group are shown in Fig. 3. RAIH rats displayed significantly elevated burst frequency compared with RARA rats during baseline conditions (P < 0.02; Fig. 4C); however, burst amplitude (Fig. 4B) and MPA (Fig. 4A) were not significantly different. No significant differences emerged between the groups during exposure to hypercapnia to elicit maximal phrenic activity with respect to burst frequency, burst amplitude, or MPA. Thus all measurements are expressed as percentages of the maximum value.


View this table:
[in this window]
[in a new window]
  		Table 1. PaO2 and PaCO2 measurements collected during the time course of a pLTF experiment in RARA and RAIH rats

 

Figure 3
View larger version (8K):
[in this window]
[in a new window]
  		Fig. 3. Phrenic neurograms of representative phrenic long-term facilitation (pLTF) experiments illustrating integrated phrenic motor output in RARA (A) or RAIH (B) animals. RAIH animals demonstrated attenuated phrenic responses to hypoxia, as well as diminished pLTF. BL, baseline; H1–H3, three 5-min hypoxic episodes; 15', 30', and 60', time of recordings following after pLTF induction.

 

Figure 4
View larger version (10K):
[in this window]
[in a new window]
  		Fig. 4. Measurements (means ± SE; n = 7/group) of minute phrenic activity (MPA; A), burst amplitude ({Delta}Amp; B), and burst frequency (C) during baseline conditions. No significant differences occurred in either MPA or AMP; however, phrenic burst frequency was significantly elevated in RAIH animals (*P < 0.02). Data are expressed as %maximal responses to hypercapnic challenge.

 
Examination of mean arterial blood pressure revealed no significant differences between the groups during baseline conditions (RARA = 88.1 ± 5.7 mmHg vs. RAIH = 86.5 ± 2.6 mmHg; P = NS). Decreases in arterial blood pressures during hypoxic challenges did not differ between groups (RARA = 44.7 ± 1.2 mmHg vs. RAIH = 43.2 ± 1.0 mmHg; P = NS). Measurements of mean arterial blood pressure at 60 min after the LTF induction protocol also revealed no significant differences between the groups (RARA = 85.7 ± 3.81 mmHg vs. RAIH = 86.6 ± 3.80 mmHg).

Short-Term Acute Hypoxic Responses

Phrenic short-term hypoxic responses (STHR), as demonstrated by MPA, were significantly attenuated in RAIH rats despite having similar values (see Table 1) for PaO2 and PaCO2 (P < 0.01; Fig. 5A). The diminished MPA was primarily attributable to decreased burst amplitude (P < 0.01; Fig. 5B) without significant differences in burst frequency (Fig. 5).


Figure 5
View larger version (10K):
[in this window]
[in a new window]
  		Fig. 5. Measurements (means ± SE; n = 7/group) of MPA (A), {Delta}Amp (B), and burst frequency (C) during hypoxic conditions (inspired O2 fraction = 0.11). Short-term hypoxic responses (STHR) were significantly attenuated in RAIH animals as demonstrated by reduced MPA and were primarily related to significant {Delta}Amp. *P < 0.01 compared with RARA animals. No significant differences occurred in phrenic burst frequency during STHR.

 
Phrenic Long-Term Facilitation

Both groups demonstrated significantly increased phrenic output (MPA and burst amplitude) 60 min after the pLTF induction protocol, indicating the development of pLTF (P < 0.05 for each). In addition, RAIH rats demonstrated significantly attenuated phrenic LTF compared with RARA rats (P < 0.05; Fig. 6, A and B). Furthermore, phrenic burst frequency 60 min after the pLTF induction protocol was not significantly different between the groups (Fig. 6C) despite significant differences occurring during baseline conditions.


Figure 6
View larger version (10K):
[in this window]
[in a new window]
  		Fig. 6. Measurements (means ± SE; n = 7/group) of MPA (A), {Delta}Amp (B), and burst frequency (C) 60 min after the pLTF induction protocol. Greater pLTF occurred in RARA animals, as demonstrated by elevated MPA and {Delta}Amp. *P < 0.05 compared with RARA. No significant differences occurred in phrenic burst frequency 60 min after the LTF induction protocol.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
The present study demonstrates that early postnatal exposure to CIH of 1 mo in duration leads to persistent alterations of ventilatory control in 2-mo-old rats. Specifically, developmental CIH resulted in enduring enhancements of both normoxic ventilation and basal phrenic burst frequency. Furthermore, early postnatal CIH was associated with considerable attenuations of the STHR in both freely behaving and acutely instrumented adult rats. Finally, developmental CIH also reduced the magnitude of pLTF. These findings suggest that developmental CIH elicits long-lasting functional modifications of ventilatory control-related networks.

Methodological Considerations

Before we further discuss the findings of the present study, it is important to acknowledge several technical issues pertaining to portions of the experiments described. First, the acutely instrumented animal preparation used in the present study is an anesthetized, vagotomized, paralyzed, and artificially ventilated model. Although a priori, such a model could compromise the integrity of the normal physiology, the preparation has the powerful advantages that it enables rigorous control of blood gases and the elimination of multiple uncontrolled factors that could modify or mask the expression of pLTF. Furthermore, this is a widely studied model, and key elements of the underlying mechanism of pLTF have been revealed (3, 14, 36). Thus, using this same model, we are able to make comparisons between the present data and a wealth of experience using the same experimental procedures.

Experiments were conducted in adult rats treated with 30 days of CIH beginning on the first day life. After CIH exposure, animals were returned to room air for an additional 30 days and allowed to mature in normoxia until the experiments were initiated. Thus any residual effects of early postnatal CIH shown represent the persistence of a CIH-induced effect long after the IH had ended and thus reflect a long-lasting form of plasticity and metaplasticity induced during development. We do not know whether such effects will endure during longer periods of normoxia after the IH exposure has ended. Indeed, several recent studies have suggested that varying durations and severity of IH profiles are important determinants of the magnitude and duration of the physiological responses elicited by IH (22, 32, 42, 49). Thus it is critically important to take into account the type of IH profiles used in any given experimental setting when attempting to compare among different studies. The profile used in this study was chosen for two major reasons, namely, its unique resemblance to the gas exchange alterations occurring in patients with sleep apnea (21) and also by virtue of the profile’s previously demonstrated ability to elicit alterations in ventilatory control in either adult (46) or neonatal rats (23, 47).

One additional point that deserves further comment is that IH may affect arterial blood pressure and that acute hypoxia also leads to pressor and depressor responses. Thus a component of the differences in respiratory responses presented could be secondary to altered cardiovascular responses. Indeed, a recent study from our laboratory (48) revealed that early postnatal CIH leads to long-term altered baroreflex function. However, although significant differences in heart rate responses and baroreflex sensitivity were identified in adults exposed postnatally to CIH, there were no differences in mean arterial blood pressure during normoxia, a finding that is now replicated by this study. Furthermore, we now report that the depressor responses during acute hypoxia and blood pressures responses upon induction of pLTF also are similar in the two experimental groups.

Developmental Plasticity of Ventilatory Control

Recent evidence has accumulated to support the concept that environmental influences, in addition to genetic influences, may play an important role in determining the functional response properties of pathways underlying ventilatory control (9, 37). In addition, numerous studies have documented profound differences in adaptive responses between adults and developing animals. Such findings have led to the formulation of the concept of "critical periods" of susceptibility during development (for further reading, please refer to Refs. 9 and 24). Briefly, the term critical period as it relates to developmental plasticity refers to a discrete temporal window during which structural and/or functional development of neural networks is uniquely susceptible to environmental influences. This period of time is characterized by accelerated brain growth and synaptogenesis (10, 26), developmental regulation of receptors and associated signaling cascades (25), and neuronal pruning that is regulated through tightly controlled apoptotic mechanisms (43). Consequently, the occurrence of external, environmental modifiers such as early postnatal CIH during this critical period of susceptibility could disrupt the normal maturation of the system and thereby alter its final configuration. In this study, we did not attempt to define the temporal boundaries of the previously identified critical period but, rather, conducted exposures throughout the first month of postnatal life to be all inclusive of this previously defined temporal window of plasticity.

The effects of early postnatal exposure to environmental stimuli have been investigated in several models of ventilatory plasticity. Indeed, developmental hypoxia (7, 12, 15, 38, 39) and hyperoxia both impair ventilatory and phrenic responses in adult rats (6, 18, 29, 30). Such effects can last for the lifetime of the animal (18). Because long-term effects are not observed after hyperoxic exposures in adult rats (28, 29), these forms of plasticity are unique to development.

Recent evidence presented by Bavis et al. (7) has demonstrated that hypoxic phrenic responses are unaffected by perinatal SH in adult rats. Therefore, altered respiratory mechanics are most likely responsible for the changes in ventilatory responses vs. nervous system plasticity following developmental SH (7). Notably, similar exposures to SH in adult rats augment ventilation and hypoxic ventilatory responses via distinct central and peripheral neural mechanisms that are readily reversible (1, 11, 46).

In the present study, we found increased basal phrenic nerve burst frequency as well as increased normoxic ventilation after early postnatal IH exposure. These findings are compatible with previous plethysmographic findings in freely behaving RAIH rats (47). Developmental exposures to CIH alter ventilatory output when presented either prenatally (i.e., gestational IH) (23) or postnatally (44). Furthermore, the effects of developmental CIH are not mechanistically similar to those induced by similar exposure to hyperoxia or SH when assessed in awake, freely behaving rats (47). However, much like the effects of hyperoxia and SH, early postnatal exposure to CIH elicits an age-dependent plasticity such that identical exposures at later ages result in relatively smaller changes in normoxic ventilation (44).

Effect of Early Postnatal CIH on pLTF

The effects of CIH on pLTF in adult rats have now been investigated in multiple studies (19, 27, 54). In adult male rats, pretreatment with CIH enhances the STHR, as well as pLTF. In fact, CIH restores diminished pLTF in geriatric female rats (54). Ling et al. (21) demonstrated that enhanced pLTF following CIH is serotonin dependent, albeit not necessarily involving the 5-HT2A serotonin receptors most frequently associated with pLTF (36). In the present study, developmental CIH exposures were associated with markedly different metaplastic changes in phrenic nerve motor output responses. Indeed, developmental CIH attenuated both STHR and pLTF. There are some similar reports of diminished HVR following a CIH profile consisting of 30 min a day for 6 days in piglets (50, 51). However, because previous studies of pLTF have demonstrated a correlation between STHR and pLTF, we cannot rule out the possibility that the attenuation of pLTF following developmental CIH may be linked to the reductions in STHR (16). Furthermore, these results seem to contrast with previous findings by McGuire and Ling (31). In that study, a 7-day CIH profile consisting of alternating 11% O2 for 5 min with 5 min of 21% O2 for 12 h during the night (6:00 PM to 6:00 AM) was associated with enhanced ventilatory LTF in poikilocapnic freely behaving rats. These effects lasted for more than 3 wk but not more than 7 wk. It is critically important to note that the preparation used in the present study is considerably different from the one described by McGuire and Ling. Indeed, we used an anesthetized and vagotomized animal preparation in which isocapnia was strictly maintained (please refer to Methodological Considerations). Thus the two models are obviously and intrinsically different, which in our opinion precludes direct comparisons of the two data sets. Furthermore, notwithstanding inherent differences between the models used in these studies, it also is important to note that the dissimilar CIH profiles also may have contributed to the seemingly disparate results.

In conclusion, we have shown that early postnatal 30-day exposures to CIH lead to lasting alterations in STHR and pLTF in 2-mo-old male rats. In addition, this exposure paradigm results in sustained enhancements of basal respiratory motor output, as well as overall increased normoxic ventilation. However, the mechanisms underlying these phenomena remain to be established.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This study was supported by National Institutes of Health (NIH) Grant HL-69932, The Children’s Foundation Endowment for Sleep and Neurobiology Research, and the Commonwealth of Kentucky Challenge for Excellence Trust Fund. S. R. Reeves is the recipient of predoctoral fellowships from the Ohio Valley Chapter of the American Heart Association and NIH (Grant F30 NS-48770). G. S. Mitchell is supported by NIH Grants HL-63319 and HL-69064.


   ACKNOWLEDGMENTS
 
We are grateful to Julia Wilkerson for invaluable instruction and assistance in the initial phases of this project.


   FOOTNOTES
 

Address for reprint requests and other correspondence: D. Gozal, Kosair Children’s Hospital Research Institute, Univ. of Louisville, Suite 204, 570 South Preston St., Louisville, KY 40202 (e-mail: david.gozal{at}louisville.edu)

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
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Aaron EA and Powell FL. Effect of chronic hypoxia on hypoxic ventilatory response in awake rats. J Appl Physiol 74: 1635–1640, 1993.[Abstract/Free Full Text]
  2. Baker TL, Fuller DD, Zabka AG, and Mitchell GS. Respiratory plasticity: differential actions of continuous and episodic hypoxia and hypercapnia. Respir Physiol 129: 25–35, 2001.[CrossRef][ISI][Medline]
  3. Baker-Herman TL, Fuller DD, Bavis RW, Zabka AG, Golder FJ, Doperalski NJ, Johnson RA, Watters JJ, and Mitchell GS. BDNF is necessary and sufficient for spinal respiratory plasticity following intermittent hypoxia. Nat Neurosci 7: 48–55, 2004.[CrossRef][ISI][Medline]
  4. Bartlett D Jr and Tenney SM. Control of breathing in experimental anemia. Respir Physiol 10: 384–395, 1970.[CrossRef][ISI][Medline]
  5. Bavis RW and Mitchell GS. Intermittent hypoxia induces phrenic long-term facilitation in carotid-denervated rats. J Appl Physiol 94: 399–409, 2003.[Abstract/Free Full Text]
  6. Bavis RW, Olson EB Jr, and Mitchell GS. Critical developmental period for hyperoxia-induced blunting of hypoxic phrenic responses in rats. J Appl Physiol 92: 1013–1018, 2002.[Abstract/Free Full Text]
  7. Bavis RW, Olson EB Jr, Vidruk EH, Fuller DD, and Mitchell GS. Developmental plasticity of the hypoxic ventilatory response in rats induced by neonatal hypoxia. J Physiol 557: 645–660, 2004.[Abstract/Free Full Text]
  8. Behan M, Zabka AG, and Mitchell GS. Age and gender effects on serotonin-dependent plasticity in respiratory motor control. Respir Physiol Neurobiol 131: 65–77, 2002.[CrossRef][ISI][Medline]
  9. Carroll JL. Developmental plasticity in respiratory control. J Appl Physiol 94: 375–389, 2003.[Abstract/Free Full Text]
  10. Dobbing J and Sands J. Comparative aspects of the brain growth spurt. Early Hum Dev 3: 79–83, 1979.[CrossRef][ISI][Medline]
  11. Dwinell MR and Powell FL. Chronic hypoxia enhances the phrenic nerve response to arterial chemoreceptor stimulation in anesthetized rats. J Appl Physiol 87: 817–823, 1999.[Abstract/Free Full Text]
  12. Eden GJ and Hanson MA. Effects of chronic hypoxia from birth on the ventilatory response to acute hypoxia in the newborn rat. J Physiol 392: 11–19, 1987.[Abstract/Free Full Text]
  13. Epstein RA, Epstein MA, Haddad GG, and Mellins RB. Practical implementation of the barometric method for measurement of tidal volume. J Appl Physiol 49: 1107–1115, 1980.[Abstract/Free Full Text]
  14. Feldman JL, Mitchell GS, and Nattie EE. Breathing: rhythmicity, plasticity, chemosensitivity. Annu Rev Neurosci 2003: 239–266, 2003.[CrossRef]
  15. Frappell PB and Mortola JP. Hamsters vs. rats: metabolic and ventilatory response to development in chronic hypoxia. J Appl Physiol 77: 2748–2752, 1994.[Abstract/Free Full Text]
  16. Fuller DD, Bach KB, Baker TL, Kinkead R, and Mitchell GS. Long term facilitation of phrenic motor output. Respir Physiol 121: 135–146, 2000.[CrossRef][ISI][Medline]
  17. Fuller DD, Baker TL, Behan M, and Mitchell GS. Expression of hypoglossal long-term facilitation differs between substrains of Sprague-Dawley rat. Physiol Genomics 4: 175–181, 2001.[Abstract/Free Full Text]
  18. Fuller DD, Bavis RW, Vidruk EH, Wang ZY, Olson EB Jr, Bisgard GE, and Mitchell GS. Life-long impairment of hypoxic phrenic responses in rats following 1 month of developmental hyperoxia. J Physiol 538: 947–955, 2002.[Abstract/Free Full Text]
  19. Fuller DD, Johnson SM, Olson EB Jr, and Mitchell GS. Synaptic pathways to phrenic motoneurons are enhanced by chronic intermittent hypoxia after cervical spinal cord injury. J Neurosci 23: 2993–3000, 2003.[Abstract/Free Full Text]
  20. Gozal D. Morbidity of obstructive sleep apnea in children: facts and theory. Sleep Breath 5: 35–42, 2001.[CrossRef][Medline]
  21. Gozal D, Daniel JM, and Dohanich GP. Behavioral and anatomical correlates of chronic episodic hypoxia during sleep in the rat. J Neurosci 21: 2442–2450, 2001.[Abstract/Free Full Text]
  22. Gozal D and Gozal E. Episodic hypoxia enhances late hypoxic ventilation in developing rat: putative role of neuronal NO synthase. Am J Physiol Regul Integr Comp Physiol 276: R17–R22, 1999.[Abstract/Free Full Text]
  23. Gozal D, Reeves SR, Row BW, Neville JJ, Guo SZ, and Lipton AJ. Respiratory effects of gestational intermittent hypoxia in the developing rat. Am J Respir Crit Care Med 167: 1540–1547, 2003.[Abstract/Free Full Text]
  24. Gozal E and Gozal D. Respiratory plasticity following intermittent hypoxia: developmental interactions. J Appl Physiol 90: 1995–1999, 2001.[Abstract/Free Full Text]
  25. Herlenius E and Lagercrantz H. Development of neurotransmitter systems during critical periods. Exp Neurol 190, Suppl 1: S8–S21, 2004.[CrossRef]
  26. Huttenlocher PR. Synapse elimination and plasticity in developing human cerebral cortex. Am J Ment Defic 88: 488–496, 1984.[ISI][Medline]
  27. Ling L, Fuller DD, Bach KB, Kinkead R, Olson EB Jr, and Mitchell GS. Chronic intermittent hypoxia elicits serotonin-dependent plasticity in the central neural control of breathing. J Neurosci 21: 5381–5388, 2001.[Abstract/Free Full Text]
  28. Ling L, Olson EB Jr, Vidruk EH, and Mitchell GS. Attenuation of the hypoxic ventilatory response in adult rats following one month of perinatal hyperoxia. J Physiol 495: 561–571, 1996.[ISI]
  29. Ling L, Olson EB Jr, Vidruk EH, and Mitchell GS. Developmental plasticity of the hypoxic ventilatory response. Respir Physiol 110: 261–268, 1997.[CrossRef][ISI][Medline]
  30. Ling L, Olson EB Jr, Vidruk EH, and Mitchell GS. Slow recovery of impaired phrenic responses to hypoxia following perinatal hyperoxia in rats. J Physiol 511: 599–603, 1998.[Abstract/Free Full Text]
  31. McGuire M and Ling L. Ventilatory long-term facilitation is greater in 1-month- versus 2-month-old awake rats. J Appl Physiol 98: 1195–1201, 2005.[Abstract/Free Full Text]
  32. McGuire M, Zhang Y, White DP, and Ling L. Effect of hypoxic episode number and severity on ventilatory long-term facilitation in awake rats. J Appl Physiol 93: 2155–2161, 2002.[Abstract/Free Full Text]
  33. McGuire M, Zhang Y, White DP, and Ling L. Phrenic long-term facilitation requires NMDA receptors in the phrenic motonucleus in rats. J Physiol 567: 599–611, 2005.[Abstract/Free Full Text]
  34. Millhorn DE, Eldridge FL, and Waldrop TG. Prolonged stimulation of respiration by a new central neural mechanism. Respir Physiol 41: 87–103, 1980.[CrossRef][ISI][Medline]
  35. Millhorn DE, Eldridge FL, and Waldrop TG. Prolonged stimulation of respiration by endogenous central serotonin. Respir Physiol 42: 171–188, 1980.[CrossRef][ISI][Medline]
  36. Mitchell GS, Baker TL, Nanda SA, Fuller DD, Zabka AG, Hodgeman BA, Bavis RW, Mack KJ, and Olson EB Jr. Invited review: Intermittent hypoxia and respiratory plasticity. J Appl Physiol 90: 2466–2475, 2001.[Abstract/Free Full Text]
  37. Mitchell GS and Johnson SM. Plasticity in respiratory motor control. Neuroplasticity in respiratory motor control. J Appl Physiol 94: 358–374, 2003.[Abstract/Free Full Text]
  38. Mortola JP, Morgan CA, and Virgona V. Respiratory adaptation to chronic hypoxia in newborn rats. J Appl Physiol 61: 1329–1336, 1986.[Abstract/Free Full Text]
  39. Okubo S and Mortola JP. Control of ventilation in adult rats hypoxic in the neonatal period. Am J Physiol Regul Integr Comp Physiol 259: R836–R841, 1990.[Abstract/Free Full Text]
  40. Pappenheimer JR. Sleep and respiration of rats during hypoxia. J Physiol 266: 191–207, 1977.[Abstract/Free Full Text]
  41. Peng YJ and Prabhakar NR. Reactive oxygen species in the plasticity of respiratory behavior elicited by chronic intermittent hypoxia. J Appl Physiol 94: 2342–2349, 2003.[Abstract/Free Full Text]
  42. Peng YJ and Prabhakar NR. Effect of two paradigms of chronic intermittent hypoxia on carotid body sensory activity. J Appl Physiol 96: 1236–1242, 2004.[Abstract/Free Full Text]
  43. Rabinowicz T, de Court Petetot JM, Xi G, and de los RE. Human cortex development: estimates of neuronal numbers indicate major loss late during gestation. J Neuropathol Exp Neurol 55: 320–328, 1996.[ISI][Medline]
  44. Reeves SR and Gozal D. Changes in ventilatory adaptations associated with long-term intermittent hypoxia across the age spectrum in the rat. Respir Physiol Neurobiol 150: 135–143, 2005.
  45. Reeves SR and Gozal D. Developmental plasticity of respiratory control following intermittent hypoxia. Respir Physiol Neurobiol 149: 301–311, 2005.[CrossRef][ISI][Medline]
  46. Reeves SR, Gozal E, Guo SZ, Sachleben LR Jr, Brittian KR, Lipton AJ, and Gozal D. Effect of long-term intermittent and sustained hypoxia on hypoxic ventilatory and metabolic responses in the adult rat. J Appl Physiol 95: 1767–1774, 2003.[Abstract/Free Full Text]
  47. Reeves SR, Wang M, Guo SZ, Lipton AJ, Gozal E, and Gozal D. Effects of post-natal intermittent hypoxia on respiratory development (Abstract). Am J Respir Crit Care Med 165: A419, 2002.
  48. Soukhova-O’Hare GK, Cheng Z, Roberts AA, and Gozal D. Postnatal intermittent hypoxia alters baroreflex function in adult rats. Am J Physiol Heart Circ Physiol 290: H1157–H1164, 2006.[Abstract/Free Full Text]
  49. Waters KA and Gozal D. Responses to hypoxia during early development. Respir Physiol Neurobiol 136: 115–129, 2003.[CrossRef][ISI][Medline]
  50. Waters KA, Laferriere A, Paquette J, Goodyer C, and Moss IR. Curtailed respiration by repeated vs. isolated hypoxia in maturing piglets is unrelated to NTS ME or SP levels. J Appl Physiol 83: 522–529, 1997.[Abstract/Free Full Text]
  51. Waters KA and Tinworth KD. Depression of ventilatory responses after daily, cyclic hypercapnic hypoxia in piglets. J Appl Physiol 90: 1065–1073, 2001.[Abstract/Free Full Text]
  52. Zabka AG, Behan M, and Mitchell GS. Long term facilitation of respiratory motor output decreases with age in male rats. J Physiol 531: 509–514, 2001.[Abstract/Free Full Text]
  53. Zabka AG, Behan M, and Mitchell GS. Selected contribution: time-dependent hypoxic respiratory responses in female rats are influenced by age and by the estrus cycle. J Appl Physiol 91: 2831–2838, 2001.[Abstract/Free Full Text]
  54. Zabka AG, Mitchell GS, Olson EB Jr, and Behan M. Selected contribution: chronic intermittent hypoxia enhances respiratory long-term facilitation in geriatric female rats. J Appl Physiol 95: 2614–2623, 2003.[Abstract/Free Full Text]



This article has been cited by other articles:


Home page
 J. Appl. Physiol.Home page
R. W. Bavis and G. S. Mitchell
Long-term effects of the perinatal environment on respiratory control
J Appl Physiol, April 1, 2008; 104(4): 1220 - 1229.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
C. Julien, A. Bairam, and V. Joseph
Chronic intermittent hypoxia reduces ventilatory long-term facilitation and enhances apnea frequency in newborn rats
Am J Physiol Regulatory Integrative Comp Physiol, April 1, 2008; 294(4): R1356 - R1366.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Heart Circ. Physiol.Home page
H. Gu, M. Lin, J. Liu, D. Gozal, K. E. Scrogin, R. Wurster, M. W. Chapleau, X. Ma, and Z. Cheng
Selective impairment of central mediation of baroreflex in anesthetized young adult Fischer 344 rats after chronic intermittent hypoxia
Am J Physiol Heart Circ Physiol, November 1, 2007; 293(5): H2809 - H2818.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
K. Szdzuy and J. P. Mortola
Ventilatory chemosensitivity of the 1-day-old chicken hatchling after embryonic hypoxia
Am J Physiol Regulatory Integrative Comp Physiol, October 1, 2007; 293(4): R1640 - R1649.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
290/6/R1664    most recent
00851.2005v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (6)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Reeves, S. R.
Right arrow Articles by Gozal, D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Reeves, S. R.
Right arrow Articles by Gozal, D.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2006 by the American Physiological Society.