AJP - Heart Information on EB 2010
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Heart Circ Physiol 290: H1157-H1164, 2006. First published September 9, 2005; doi:10.1152/ajpheart.00767.2005
0363-6135/06 $8.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
290/3/H1157    most recent
00767.2005v1
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 Web of Science
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 Web of Science (9)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Soukhova-O'Hare, G. K.
Right arrow Articles by Gozal, D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Soukhova-O'Hare, G. K.
Right arrow Articles by Gozal, D.

Postnatal intermittent hypoxia alters baroreflex function in adult rats

Galia K. Soukhova-O'Hare,1 Zixi (Jack) Cheng,1,2 Andrew M. Roberts,2 and David Gozal1,3

1Department of Pediatrics, Kosair Children's Hospital Research Institute, 2Department of Physiology and Biophysics, and 3Department of Pharmacology and Toxicology, University of Louisville, Louisville, Kentucky

Submitted 19 July 2005 ; accepted in final form 25 August 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Chronic perinatal intermittent hypoxia (IH) could have long-term cardiovascular effects by altering baroreflex function. To examine this hypothesis, we exposed rats (n = 6/group) for postnatal days 1–30 or prenatal embryonic days 5–21 to IH (8% ambient O2 for 90 s after 90 s of 21% of O2, 12 h/day) or to normoxia (control). Baroreflex sensitivity (BRS) and cardiac chronotropic responses were examined in anesthetized animals 3.5–5 mo later by infusing phenylephrine or sodium nitroprusside (6–12 µg/min iv, 1–2 min) during normoxia and after 18 min of acute IH (IHA). In controls after IHA, baroreflex gain was 42% (P < 0.05) less than during normoxia. BRS in the postnatal IH group during normoxia was ~50% less than in control rats and similar to controls after IHA. The heart rate response to phenylephrine in the IH group was also less than in controls (P < 0.05) and was not changed by IHA. BRS and heart rate responses in the prenatal IH group were similar to the normoxic control group. Vagal efferent projections to atrial ganglia neurons in rats after postnatal IH (n = 4) were examined by injecting tracer into the left nucleus ambiguus. After 35 days of postnatal IH, basket ending density was reduced by 17% (P < 0.001) and vagal axon varicose contacts by 56% (P < 0.001) compared with controls. We conclude that reduction of vagal efferent projections in cardiac ganglia could be a cause of long-term modifications in baroreflex function.

perinatal intermittent hypoxia; vagus; atrial ganglia; basket endings

RECURRENT APNEAS THAT CAUSE oxygen desaturation and bradycardia are often present in preterm infants (33). However, the long-term effects of neonatal intermittent hypoxia (IH) on cardiovascular regulation and sympathovagal balance are not well known. Adults who experience IH during sleep-disordered breathing such as obstructive sleep apnea have a high risk of developing hypertension, myocardial ischemia, and heart failure (40, 52). Sympathetic overactivity, decreased baroreflex sensitivity (BRS), and decreased heart rate variability have been found in these patients (6, 30, 36, 37, 46). In a rodent model of obstructive sleep apnea, Fletcher et al. (16) demonstrated blood pressure elevation after 30-day exposure to IH. Involvement of the sympathetic efferent arm of the arterial chemoreflex arc was suggested by prevention of the blood pressure response after pharmacological sympathetic blockade or carotid body deafferentiation (16–19, 50). On the other hand, withdrawal of parasympathetic activity could also contribute to the alteration of sympathovagal balance observed in obstructive sleep apnea, as shown by power spectrum analysis of heart rate variability (26, 35, 55).

Perinatal susceptibility to hypoxia is characterized by the developmental window of vulnerability. Thus the presence of normoxia during the first 7–10 days of postnatal life in rats is critical for development of chemoreceptor sensitivity (1, 2, 7, 15, 29). Hypoxia causes injury to the developing brain (22, 34, 57) and could target structures involved in cardiovascular reflexes (4, 13, 32, 49). For example, neonatal rats demonstrate an increased chemoreceptor response to hypoxia after different paradigms of chronic IH (41, 45).

The nucleus tractus solitarius (NTS) is a major projection site for chemo- and baroreceptors (5, 53) and has direct and indirect connections with presympathetic neurons of the rostral medulla (5, 23) as well as with cardiac vagal premotor nuclei, such as the nucleus ambiguus (NA) and the dorsal motor nucleus of the vagus (DmnX). Tonic and reflex activities of the NA primarily determine the resting heart rate, respiratory sinus arrhythmia, and baroreflex function (47). In rats, parasympathetic tone appears toward the second postnatal week and increases throughout development due to maturation of neural centers and peripheral reflexes (8, 13).

Activation of barosensitive neurons in the NTS involves N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors (49). NMDA receptors are primarily involved in the bradycardia rather than in the pressor responses to chemoreceptor activation (24). IH has also been found to increase apoptosis in NMDA receptor-channel complexes containing neurons, including the NTS and DmnX, in the neonatal piglet brain stem (32). Because NMDA receptors are implicated in the development of specific patterns of synaptic connectivity, the susceptibility of the developing brain to IH may cause profound alterations in cardiovascular control functions later in life.

In the present study, we investigated the hypothesis that neonatal IH is associated with long-term impairment of baroreflex control of heart rate. In 3.5- to 5-mo-old Sprague-Dawley rats exposed to IH during the initial 30 neonatal days of life, there were fewer cardiac vagal projections and reduced baroreflex control of heart rate. In contrast, prenatal exposures to IH did not significantly change baroreflex function.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animal groups. Experimental protocols were approved by the Institutional Animal Use and Care Committee at the University of Louisville and were in accordance with National Institutes of Health requirements on the care and use of laboratory animals.

Male littermates from time-pregnant Sprague-Dawley rats (Charles River, Wilmington, MA) were grouped (n = 6) as follows: RAIH, which were born in normoxia [room air (RA)] and exposed to chronic IH for postnatal days 1–30; RARA, which were born in normoxia and housed in normoxia; and IHRA, which were born from time-pregnant Sprague-Dawley rats exposed to IH during embryonic days 5–21 and then housed in RA.

Chronic IH exposures. Animals were exposed to IH in chambers with a 12:12-h light-dark cycle (Oxycycler model A44XO; Biospherix, Redfield, NY), using an oxygenation profile that mimics those experienced by patients with obstructive sleep apnea syndrome during sleep (21, 22, 45, 51). The IH pattern was controlled by a computerized system that alternated between 21% and 8% oxygen every 90 s for 12 h/day and kept at 21% O2 for 12 h of night time. Ambient CO2 in the chamber was periodically monitored and maintained at 0.03%, and ambient temperature was 22–24°C.

Surgical procedures and cardiovascular recordings. Rats were anesthetized with pentobarbital sodium (50 mg/kg ip). The trachea was cannulated and rats breathed spontaneously. Body temperature was maintained at 37.5°C with a homeostatic blanket (Harvard) and a rectal probe. Polyethylene catheters (PE-50) were placed in a femoral artery to monitor mean arterial pressure (MAP) and in a vein to infuse phenylephrine (PE) or sodium nitroprusside (SNP). The hind paw pinch withdrawal reflexes were tested, and supplemental doses of pentobarbital sodium (10 mg/kg) were given as needed. MAP was measured by pressure transducers and amplifiers (model P23db; Statham Laboratories, Hato Rey, Puerto Rico, or Buxco with preamplifier model MAX II 2270; Buxco Electronics, Troy, NY). Heart rate was measured by a tachograph (model 7P4H; Grass Instrument, Quincy, MA) triggered from the arterial pressure pulse. MAP and heart rate were recorded by a polygraph (model 7, Grass), and beat-to-beat analog signals were digitized and recorded for subsequent offline analyses.

Analysis of baroreflex function. Baroreflex function was assessed when rats were 3.5–5 mo old and included evaluation of baseline heart rate, MAP, BRS, heart rate variability (maximal range of heart rate) during PE and SNP challenges, and the chronotropic response to PE (changes in heart rate). These variables were measured in RA and in RA after acute IH (IHA; six 90-s cycles of 10% O2 alternating with 21% O2).

SNP and PE (Sigma, St. Louis, MO) were freshly prepared, diluted in 0.9% NaCl, and infused by a syringe pump (Sp101i; Stoelting) for 1–2 min (6–12 µg/min) to achieve a heart rate plateau. Infusions were repeated three or four times to obtain an average response for each animal. At least 10–15 min were allowed between infusions for MAP and heart rate to return to baseline. Baseline MAP and heart rate were measured from a 20-s interval before infusion. The heart rate response was measured at the heart rate plateau, and the MAP response was measured at the point corresponding to the beginning of the heart rate plateau. Data were plotted as change in heart rate vs. changes in MAP in response to PE or SNP.

Data analyses. Because the heart rate-MAP relationship can be approximated by a sigmoidal logistic curve (28), we applied regression analysis to the linear part of the responses for each animal to calculate the slope of the regression line, which was used as an index of BRS or baroreflex gain for the group. The responses were studied in RA and in IHA (18 min). Responses of groups to IHA were compared using two-way ANOVA followed by Newman-Keuls post hoc tests, and responses within each group were analyzed with Student's t-tests. Data are presented as means ± SE. Statistical significance was considered at the level of P < 0.05.

Neuroanatomical studies. Vagal axon tracing was done in RAIH (n = 4) and in RARA (n = 4) rats, as previously described (10, 11). Animals (3–5 mo old) from both groups received a series of unilateral injections of the tracer 1,1'-dioleyl-3,3,3',3'-tetramethylindocarbocyanine methanesulfonate (DiI) (rhodamine red, catalog no. 3886; Molecular Probes, Eugene, OR) into the left NA. Fluorogold (FG; 1 ml of 2 mg/ml ip) was injected to counterstain cardiac ganglia. To ensure the accuracy of DiI injection into the NA, in a reference group (n = 3) with similar body weight, FG (3 mg/ml, 5 µl) was injected into the left cervical vagal trunk to label NA motoneurons, followed by DiI injection into the left NA. The reference group was killed 5 days after FG injection, and the experimental group was killed 21 days after DiI injections (time necessary for DiI transportation to axons terminals). Brain stem tissue and atrial specimens were used to analyze NA axons and terminals in cardiac ganglionic cells.

Animals were anesthetized with pentobarbital sodium (50 mg/kg ip), treated with atropine (1 mg/kg sc), and placed in a stereotaxic apparatus. Dorsal incisions were made over the neck muscles to expose the atlanto-occipital membrane and the cisterna magna and dorsal medulla. The occipital bone was trimmed with a dental drill bit until the caudal cerebellum was visible. The caudal end of the area postrema was used as a reference for stereotaxic coordinates. A glass micropipette, filled with DiI and connected to a picospritzer, was advanced to the NA to inject DiI in small aliquots at nine different sites (–1,600 to +1,600 µm; total volume of 22.5–112.5 nl) separated 400 µm longitudinally.

Tissue preparation. After 21 days, animals were reanesthetized with an overdose of pentobarbital sodium (100 mg/kg), which was perfused through the heart with 0.9% saline (300 ml) and phosphate-buffered (pH 7.4, 600 ml) 10% formalin. The atria were separated and cut open. The left atrium tissue block included the region of the junction with the pulmonary veins; the right atrium had the superior vena cava, inferior vena cava, and the left precava vein attached. The interatrial septum was separated from the atria. Using extreme care to avoid peeling the ganglionic plexuses from the dorsal surface of the atria, we gently removed tissues surrounding the heart. Tissues were then dehydrated through a graded series (70%, 90%, and 2x 100%) of glycerin and mounted on slides with coverslips applied in 100% glycerin and n-propylgallate (5%) to prevent fading.

Data acquisition and analysis of vagal axonal terminals in cardiac ganglia. Cardiac ganglia specimens were screened with a conventional epifluorescent microscope equipped with filter cubes appropriate for DiI (rhodamine) and FG (UV). When DiI nerve fibers and endings were found in the cardiac ganglion at x200 magnification, their locations were recorded for subsequent detailed confocal microscopic analysis at x400 magnification. Vagal efferent terminals around ganglionic neurons were examined with a confocal microscope and digital tracing system (Neurolucida; MicroBright Field, Colchester, VT). For each group, 50 confocal images and 150 basket-like endings representing axon branching around the ganglionic cell were randomly selected. Basket endings with axon varicosities were digitized (see Fig. 5), and 10 montages of a series of confocal projections were made for each group.


Figure 5
View larger version (184K):
[in this window]
[in a new window]
  		Fig. 5. Microphotographs of vagal axons with basket endings in cardiac ganglia. A: stack of confocal optical sectioned images showing nucleus ambiguus (NA) projections to 2 cells (in the center) with basket-like endings around the cells. Additionally, 2 1,1'-dioleyl-3,3,3',3'-tetramethylindocarbocyanine methanesulfonate (DiI)-labeled fiber bundles and 2 other single fibers crossing from the top to the bottom are apparent. Inset: axon and varicosities of basket ending that were traced, marked, and counted 3-dimensionally. The blue contours are drawings of the cell body of the neuron in different optical sections. B–D: 3 single optical sections, selected from the stack of confocal images in A, showing vagal varicose contacts around the 2 selected neurons in A. Several Fluorogold (FG)-labeled neurons that did not receive DiI-labeled axons are also visible. In the insets of B–D, varicosities are marked by red solid dots and the contours and nucleus of the ganglionic cells are indicated as blue and yellow lines, respectively. Scale bar = 50 µm.

 

   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Effect of IH on somatic growth. During the first postnatal days, IHRA rats weighed less than RAIH or RARA rats (Fig. 1). However, at P15, the RAIH rats weighed less than the other groups. Growth recovery, although incomplete, was apparent in IHRA compared with RARA by postnatal day 30. However, RAIH rats exposed to IH during the first month of postnatal life demonstrated growth retardation. When IH exposure was discontinued after 30 days, their weight was ~25% less then the RARA group. After 3.5–5 mo, RAIH and IHRA rats still weighed less than RARA controls (343 ± 8 g in RAIH, 357 ± 7 g in IHRA, and 398 ± 7 g in RARA; P < 0.01).


Figure 1
View larger version (17K):
[in this window]
[in a new window]
  		Fig. 1. Effects of perinatal intermittent hypoxia (IH) on body weight in Sprague-Dawley rats. Bars indicate weight (means ± SE; n = 16 for each group) of control pups exposed to normoxia (RARA), pups exposed to prenatal IH during embryonic days 5–21 and then raised in normoxia (IHRA), and pups exposed to IH during postnatal days 1–30 and then raised in normoxia (RAIH). RA, room air.

 
Effect of perinatal IH on blood pressure and BRS. In RARA, RAIH, and IHRA rats at ages 3.5–5 mo, MAP averaged 117 ± 5, 113 ± 5, and 120 ± 4 mmHg, respectively. Neither MAP nor heart rate differed significantly between the groups (P > 0.05; Table 1). In RARA, BRS was significantly attenuated by IHA as indicated by a mean 42% decrease in the slope of the regression line compared with responses measured in RA (P < 0.05; Fig. 2, RARA; Table 1). In contrast, RAIH rats that were exposed to IH during the neonatal period exhibited attenuated baroreflex gain in normoxia, which was not altered further by IHA (Fig. 2, RAIH). BRS in RAIH rats was ~50% lower (P < 0.05) than in controls when measured in RA but was similar to the BRS of RARA measured after IHA (Figs. 2 and 3). BRS in IHRA rats was not different from controls (Fig. 3, Table 1).


View this table:
[in this window]
[in a new window]
  		Table 1. Cardiovascular variables in RARA, RAIH, and IHRA rats, measured in normoxia and IHA

 

Figure 2
View larger version (16K):
[in this window]
[in a new window]
  		Fig. 2. Baroreflex function of 3.5- to 5-mo-old RARA (A), RAIH (B), and IHRA (C) rats (n = 6/group). Baroreflex sensitivity (BRS) was assessed from response to intravenous administration of phenylephrine (PE) and sodium nitroprusside during normoxia (RA) and after acute intermittent hypoxia (IHA; 18 min of 90-s intervals alternating between 10 and 21% O2). Changes in heart rate ({Delta}HR) and mean arterial pressure ({Delta}MAP) from baseline for each animal were plotted and fitted by linear regression. Solid lines represent responses during RA, and dashed lines represent responses during IHA. Note that, after IHA, the responses (slopes) of RARA (A) and IHRA (C) rats were less than responses during RA. bpm, Beats/minute.

 

Figure 3
View larger version (12K):
[in this window]
[in a new window]
  		Fig. 3. Comparison of BRS in 3.5- to 5-mo-old RARA, IHRA, and RAIH rats measured in normoxia. Bars represent slopes (means ± SE) of regression lines or BRS in the 3 groups. *P < 0.05. Note that in the RAIH group the gain was about one-half of that of the other groups.

 
Effects of perinatal hypoxia on heart rate responses. Changes in heart rate in response to PE were smaller in RAIH than in RARA rats when responses were measured in RA (19 ± 4 vs. 35 ± 8 beats/min; P = 0.04) and were not altered by IHA (Table 1, Fig. 4). The heart rate responses to PE in RAIH rats were also less than in IHRA rats in RA (P = 0.02; Fig. 3). Although the ranges of heart rate responses to PE and to SNP were smaller in the RAIH group than in the IHRA group, the differences did not reach statistical significance (P > 0.05). In the three treatment groups, IHA did not significantly change either MAP or heart rate compared with the baseline (RA). However, after IHA, heart rate responses to PE decreased in RARA and IHRA rats (Table 1).


Figure 4
View larger version (16K):
[in this window]
[in a new window]
  		Fig. 4. HR responses to PE (means ± SE) measured during normoxia (RA) and immediately after IHA (18 min, 90 s of 10% O2 alternating with 21% O2) in 3.5- to 5-mo-old RARA, RAIH, and IHRA Sprague-Dawley rats (n = 6 per each group). *Significant reduction in {Delta}HR after IHA in RARA group (–49%, P < 0.05); {ddagger}significant reduction in {Delta}HR after IHA in IHRA group (–62%, P < 0.05); {dagger}significant reduction (–56% and –57%, P < 0.05) in {Delta}HR in RAIH rats compared respectively with RARA and RAIH rats. The HR effect to PE in RAIH rats was attenuated in RA and did not change after IHA.

 
Density of basket endings and terminal varicosities in atrial ganglia of IH and normoxia rats. In both RAIH and RARA groups, labeled vagal axons of NA projected to all cardiac ganglia and innervated individual ganglionic cells with basket-like endings. The left NA axons innervated 53% of the ganglionic neurons in normoxic control animals. Visual inspection of the montages showed that vagal innervation of cardiac ganglia in the RAIH group was much sparser, with fewer DiI terminals than in the RARA group (Figs. 5 and 6). This observation was confirmed by examining basket endings in randomly selected images and counting the number of ganglionic cells receiving DiI-labeled axons (basket endings) compared with those not receiving such projections. In 50 frames, the number of basket endings in RAIH was 558 compared with 622 for RARA, and the number of neurons not receiving DiI-labeled projections was 701 for RAIH and 546 for RARA. Overall, the percentage of basket endings was significantly less for RAIH than for RARA (44.3 ± 2.0% and 53.3 ± 2.5%, respectively; P < 0.001; Fig. 7A). By randomly sampling 150 baskets from each group, we found that the vagal terminal varicosities in the IH group (Fig. 7B) were also significantly reduced in RAIH compared with RARA (72 ± 4/neuron and 163 ± 11/neuron, respectively; P < 0.001).


Figure 6
View larger version (69K):
[in this window]
[in a new window]
  		Fig. 6. Top: montages of confocal photomicrographs showing NA efferent axon innervation of atrial ganglionic neurons in 2 rats (A and B). DiI-labeled NA fibers and endings appear red. The soma of the cardiac neurons labeled by FG are also visible. A, top: bundles of vagal efferent fibers enter the atrial ganglia and form a basket-like branching around the ganglionic neuron with numerous varicose terminals. B, top: another example from an RA control shows the dense vagal innervation of atrial ganglion cells. Scale bar = 50 µm. Bottom: examples from 2 rats exposed to IH postnatally (A and B) showing vagal innervation of atrial ganglia cells in rat exposed for 30 postnatal days to IH. In contrast to RA, DiI-labeled basket endings are much sparser. Note that intensities of DiI fluorescence in RA- and IH-exposed animals are comparable. Inset in B, top (RA) and A, bottom (IH): neurolucida drawing of a basket-like ending around neurons, which are indicated by arrows. Scale bar = 50 µm.

 

Figure 7
View larger version (14K):
[in this window]
[in a new window]
  		Fig. 7. A: density of basket endings in the right atrium intramural ganglia of RA and IH rats. The density of the vagal axons that form basket ending in the ganglia cells is reduced in IH by 17% (*P < 0.001). B: comparison of the number of vagal axon varicosities in the atrial ganglia of RA and IH rats. The number of varicose terminals on the ganglionic cells in IH animals was reduced by 56% (*P < 0.001). Efferent projections were evaluated in 2-mo-old rats. IH group was exposed for 30 postnatal days to IH.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
IH during the first 30 days of postnatal life induced a persistent and long-lasting reduction in baroreceptor control of heart rate, which was observed in adult Sprague-Dawley rats. In 3.5- to 5-mo-old RAIH rats, the decrease in heart rate in response to PE administration during normoxia was ~50% of that in control animals. The results also showed that the attenuated BRS and heart rate responses in RAIH rats were associated with a reduction of vagal efferent projections to the heart. Furthermore, after postnatal IH, the long-term reduction in BRS and heart rate responses were not further affected by acute exposures to IH.

Although heart rate responses to PE are primarily mediated by the vagus, it is possible that a sympathetic component could be significantly elevated after IH (19, 37, 50). In association with enhanced sympathetic drive, reflex bradycardia increases due to elevation of vagal tone (31). However, in conditions like hypertension and heart failure, vagal activity is diminished and baroreceptor control of heart rate is reduced mainly due to a reduced vagal component rather than by changes in sympathetic activity (25). Although the present investigation reveals a chronic impairment in sympatho-vagal balance, specific effects on components of the baroreflex arc have not been investigated.

Our neuroanatomical studies showed significant impairment of efferent cardiac vagal innervation after 30 days of postnatal IH. The density of the vagal axon projections in cardiac ganglia was only 83%. In addition, the density of axonal varicose terminals in RAIH animals was only 44% of that found in normoxic controls. In neonates, continuous maturation of peripheral reflexes and centers causes vagal tone to gradually develop (8, 13). Therefore, by interfering with the development of efferent terminals to the heart, postnatal IH may have affected tonic and phasic vagal actions on the heart. It was shown that the conditions when cardiac vagal activity is diminished or unresponsive could include hypertension, heart failure, and sudden cardiac death (14, 54, 56).

The number of vagal axonal varicose terminals around cardiac ganglionic neurons is considered an estimate of the real number of synapses. However, ultrastructural examination might be required to determine whether a varicosity is a genuine synapse. Our combination of confocal microscopy and the analysis of the density of vagal efferent terminals with the number of varicose vagal contacts provided a novel technique to quantify the extent of vagal axon innervation. The densities of vagal basket endings and vagal varicose endings were both reduced by postnatal IH. It is possible that IH could interfere with some tracer transport mechanism and thus obscure the vagal projection to the heart. However, examination of cardiac tissues revealed that DiI fluorescence of vagal axons and terminals in RAIH animals were similar to those found in RARA animals (Figs. 5 and 6). Therefore, we believe that the reduced vagal terminal densities and numbers of varicose endings indeed represent a genuine loss of vagal terminals.

Our group (9) previously showed that NA axons form three times more basket endings in the atrial ganglia cells than DmnX axons. In addition, the finding that domoic acid lesions of the NA largely attenuated the baroreflex, whereas similar lesions of the DmnX did not affect the baroreflex (10, 12), suggests that the NA is the major efferent component in the baroreflex loop and that IH-induced functional deficits could be associated with the NA connections. Because the NA and DmnX project to completely different populations of cardiac ganglionic neurons (11), the DmnX may be involved in other cardiac functions. In contrast to the conventional concept that the cardiac ganglion is a simple relay station of the central nervous system, the cardiac ganglion is now considered as an integration center, which mediates local as well as central cardiac reflexes (43, 44). Thus postnatal IH is associated with long-term functional and anatomical plasticity of the vagal network underlying the baroreflex response of the heart. Long-lasting forms of plasticity, also termed metaplasticity, have been previously reported for respiratory control systems following perinatal hyperoxia (3) and sustained hypoxia (1), as well as by gestational IH (21). The present study is the first to demonstrate that metaplastic alterations can also develop in cardiovascular regulatory structures.

The mechanisms linking hypoxic episodes and reduced vagal efferent projections to the heart include increased generation of oxygen free radicals in the hypoxic tissue, causing cell membrane lipid peroxidation, protein oxidation, and nucleic acid oxidation (34, 57). NMDA receptor-channel complexes, which can regulate neuronal cell death in the developing brain, demonstrate susceptibility to hypoxia that is restricted to a specific period of development (27, 32, 38). Tissue hypoxia modifies NMDA receptors and results in increased Ca2+ influx into the cell and nucleus that could alter transcription of specific genes responsible for the programmed cell death (27, 32).

In 3.5- to 5-mo-old Sprague-Dawley rats, there were no long-term alterations in the resting MAP and heart rate. This indicates that, in normotensive rats, postnatal IH does not have a long-term effect on the setup point for the MAP or on the resting sympatho-vagal balance. However, regulatory deficits, such as a blunted heart rate baroreflex, were apparent after baroreflex activation in RAIH rats. This finding indicates that postnatal IH causes more of an impairment on a phasic component of the baroreflex than on a tonic component in normotensive rats. It should be emphasized, however, that heart rate changes reported herein in response to pressor and depressor pharmacological stimulation are not only due to arterial baroreceptors but also represent responses to vagal afferents innervating the heart. The latter were not specifically studied.

Although IHRA rats had BRS and heart rate responses that were similar to those in the control group, recovery of body weight in this group was not complete by 3.5–5 mo (20, 48). These observations suggest that, with our exposure protocol, the oxidative stress experienced by the fetus was less when the mother was exposed to IH vs. if pups were directly exposed postnatally. Long-term consequences for cardiovascular function, however, were reported with regard to sustained prenatal hypoxia (10%) in rats exposed at embryonic days 5–20 (42). Similar to our present results, in 12-wk-old rats, blood pressure and heart rate were not affected under the resting conditions; however, under cold stress, blood pressure and the variability of blood pressure and heart rate were increased (42).

A limitation of the present investigation is that the results were obtained in anesthetized animals; therefore, the responses may be attenuated. Acute IH in the studied groups did not produce changes in baseline MAP or heart rate measured at the end of exposure in normoxia (Table 1). Furthermore, direct vasodilator effects of acute hypoxia on vessel tone, as well as attenuation of sympathetic activitydue to anesthesia, could obscure the responses. However, the increased sympathetic component present in awake animals exposed to IH, which could interfere with the baroreflex response to PE by compensating the negative chronotropic effects, was avoided. We found strong evidence of impairment in vagal cardiac efferent projections associated with reduced BRS of heart rate control as a consequence of postnatal hypoxia. However, there may also be impairment in the central connectivity, as well as in signal processing dependent on afferent input, which have not been addressed by the present study.

Decreased BRS and a decreased range of negative chronotropic cardiac responses caused by IH may represent a mechanism for shifting MAP to higher levels. Indeed, the decreased BRS, characteristic of patients with obstructive sleep apnea, is often associated with elevated blood pressure (6, 39, 46) but can also occur at normal blood pressures (36). Additional factors (including genetic predisposition) that are critical for hypertension, or other pathophysiological complications, might be involved in the further progression of cardiovascular disorders in obstructive sleep apnea. Our data demonstrate that postnatal IH exerts long-term cardiovascular consequences, which include reduced BRS, decreased overall range of heart rate responses, and the range of negative chronotropic responses. Remodeling of vagal cardiac nerve efferents to the heart occurs, which could be a morphological basis for baroreflex deficits caused by postnatal chronic IH.


   ACKNOWLEDGMENTS
 
This study was supported by National Institutes of Health Grants HL-69932 and P20 RR-15576 to D. Gozal, AG-201020 to Z. J. Cheng, and F32 HL-70494 to G. K. Soukhova-O'Hare; by American Heart Association Grant AHA 9930173 to Z. J. Cheng; by The Children's Foundation Endowment for Sleep Research; and by the Commonwealth of Kentucky Challenge for Excellence Trust Fund.


   FOOTNOTES
 

Address for reprint requests and other correspondence: D. Gozal, Kosair Children's Hospital Research Institute, 570 S Preston St., Suite 321, Dept. of Pediatrics, Univ. of Louisville, 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
 REFERENCES
 

  1. 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]
  2. Bisgard GE, Olson EB Jr, Wang ZY, Bavis RW, Fuller DD, and Mitchell GS. Adult carotid chemoafferent responses to hypoxia after 1, 2, and 4 wk of postnatal hyperoxia. J Appl Physiol 95: 946–952, 2003.[Abstract/Free Full Text]
  3. Bisgard GE, Olson EB Jr, Bavis RW, Wenninger J, Nordheim EV, and Mitchell GS. Carotid chemoafferent plasticity in adult rats following developmental hyperoxia. Respir Physiol Neurobiol 145: 3–11, 2005.[CrossRef][Web of Science][Medline]
  4. Blanco CE, Dawes GS, Hanson MA, and McCooke HB. The response to hypoxia of arterial chemoreceptors in fetal sheep and new-born lambs. J Physiol 351: 25–37, 1984.[Abstract/Free Full Text]
  5. Blessing WW. The Lower Brainstem and Bodily Homeostasis. New York: Oxford Univ. Press, 1997.
  6. Carlson JT, Hedner JA, Sellgren J, Elam M, and Wallin BG. Depressed baroreflex sensitivity in patients with obstructive sleep apnea. Am J Respir Crit Care Med 154: 1490–1496, 1996.[Abstract]
  7. Carroll JL. Developmental plasticity in respiratory control. J Appl Physiol 94: 375–389, 2003.[Abstract/Free Full Text]
  8. Chatow U, Davidson S, Reichman BL, and Akselrod S. Development and maturation of the autonomic nervous system in premature and full-term infants using spectral analysis of heart rate fluctuations. Pediatr Res 37: 294–302, 1995.[Web of Science][Medline]
  9. Cheng Z and Powley TL. Nucleus ambiguus projections to cardiac ganglia of rat atria:anterograde tracing study. J Comp Neurol 424: 588–606, 2000.[CrossRef][Web of Science][Medline]
  10. Cheng Z, Guo SZ, Lipton AJ, and Gozal D. Domoic acid lesions in nucleus of the solitary tract: time-dependent recovery of hypoxic ventilatory response and peripheral afferent axonal plasticity. J Neurosci 22: 3215–3226, 2002.[Abstract/Free Full Text]
  11. Cheng Z, Hong Z, Guo SZ, and Gozal D. Differential control of vagal efferents from the nucleus ambiguus and the dorsal motor nucleus of the vagus to rat cardiac ganglia: anatomical evidence. Am J Physiol Regul Integr Comp Physiol 286: R625–R633, 2004.[Abstract/Free Full Text]
  12. Cheng Z, Zhang H, Yu J, and Gozal D. Attenuation of baroreflex sensitivity following domoic acid lesion of the nucleus ambiguus (NA) of rats: functional implications. J Appl Physiol 96: 1137–1145, 2004.[Abstract/Free Full Text]
  13. Crossley D II and Altimirs J. Ontogeny of cholinergic and adrenergic cardiovascular regulation in the domestic chicken (Gallus gallus). Am J Physiol Regul Integr Comp Physiol 279: R1091–R1098, 2000.[Abstract/Free Full Text]
  14. Eckberg DL, Drabinsky M, and Braunwald E. Defective cardiac parasympathetic control in patients with heart disease. N Engl J Med 285: 877–883, 1971.[Web of Science][Medline]
  15. Erickson JT, Mayer C, Jawa A, Ling L, Olson EB Jr, Vidruk EH, Mitchell GS, and Katz DM. Chemoafferent degeneration and carotid body hypoplasia following chronic hyperoxia in newborn rats. J Physiol 509: 519–526, 1998.[Abstract/Free Full Text]
  16. Fletcher EC, Lesske J, Behm R, Miller CC, and Unger T. Carotid chemoreceptors, systemic blood pressure, and chronic episodic hypoxia mimicking sleep apnea. J Appl Physiol 72: 1978–1984, 1992.[Abstract/Free Full Text]
  17. Fletcher EC, Lesske J, Qian W, Miller CC, and Unger T. Repetitive episodic hypoxia causes diurnal elevation of systemic blood pressure in rats. Hypertension 19: 555–561, 1992.[Abstract/Free Full Text]
  18. Fletcher EC, Lesske J, Culman J, Miller CC, and Unger T. Sympathetic denervation blocks blood pressure elevation in episodic hypoxia. Hypertension 20: 612–619, 1992.[Abstract/Free Full Text]
  19. Fletcher EC. Sympathetic over activity in the etiology of hypertension of obstructive sleep apnea. Sleep 26: 15–19, 2003.[Web of Science][Medline]
  20. Franklin KA, Holmgren PÅ, Jönsson F, Poromaa N, Stenlund H, and Svanborg E. Snoring, pregnancy-induced hypertension, and growth retardation of the fetus. Chest 117: 137–141, 2000.[Abstract/Free Full Text]
  21. 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]
  22. Gozal E, Row BW, Schurr A, and Gozal D. Developmental differences in cortical and hippocampal vulnerability to intermittent hypoxia in the rat. Neurosci Lett 305: 197–201.
  23. Guyenet PG, Koshiya N, Huangfu D, Baraban SC, Stornetta RI, and Li YW. Role of medulla oblongata in generation of sympathetic and vagal outflows. Prog Brain Res 107: 127–144, 1996.[Web of Science][Medline]
  24. Haibara AS, Colombari E, Chianca-Jr DA, Bonagamba LGH, and Machado BH. NMDA receptors in NTS are involved in bradycardic but not in pressor response to chemoreflex. Am J Physiol Heart Circ Physiol 269: H1421–H1427, 1995.[Abstract/Free Full Text]
  25. Head GA. Cardiac baroreflexes and hypertension. Clin Exp Pharmacol Physiol 21: 791–802, 1994.[Web of Science][Medline]
  26. Hilton MF, Chappell MJ, Bartlett WA, Malhotra A, Beattie JM, and Cayton RM. The sleep apnoea/hypopnoea syndrome depresses waking vagal tone independent of sympathetic activation. Eur Respir J 17: 1258–1266, 2001.[Abstract/Free Full Text]
  27. Ikonomidou C, Bosch F, Miksa M, Bittigau P, Vockler J, Dikranian K, Tenkova TI, Stefovska V, Turski L, and Olney JW. Blockade of NMDA receptors and apoptotic neurodegeneration in the developing brain. Science 283: 70–74, 1999.[Abstract/Free Full Text]
  28. Kent BB, Drane JW, Bluminstein B, and Manning JW. A mathematical model to assess changes in the baroreceptor reflex. Cardiology 57: 295–310, 1972.[Web of Science][Medline]
  29. Kholwadwala D and Donnelly DF. Maturation of carotid chemoreceptor sensitivity to hypoxia: in vitro studies in the newborn rat. J Physiol 453: 461–473, 1992.[Abstract/Free Full Text]
  30. Lanfranchi PA and Somers VK. Arterial baroreflex function and cardiovascular variability: interactions and implications. Am J Physiol Regul Integr Comp Physiol 283: R815–R826, 2002.[Abstract/Free Full Text]
  31. Levy MN and Zeiske H. Autonomic control of cardiac pacemaker activity and atrioventricular transmission. J Appl Physiol 27: 465–470, 1969.[Free Full Text]
  32. Machaalani R and Waters KA. Increased neuronal cell death after intermittent hypercapnic hypoxia in the developing piglet brainstem. Brain 985: 127–134, 2003.
  33. McNamara F and Sullivan CE. Obstructive sleep apnea in infants: relation to family history of sudden infant death syndrome, apparent life-threatening events, and obstructive sleep apnea. J Pediatr 36: 318–323, 2000.
  34. Mishra OP and Delivoria-Papadopoulos M. Cellular mechanisms of hypoxic injury in the developing brain. Brain Res Bull 48: 233–238, 1999.[CrossRef][Web of Science][Medline]
  35. Narkiewicz K, Montano N, Logliati C, van de Borne PJ, Dyken ME, and Somers VK. Altered cardiovascular variability in obstructive sleep apnea. Circulation 98: 1071–1077, 1998.[Abstract/Free Full Text]
  36. Narkiewicz K, Pesek CA, Kato M, Phillips BG, Davison DE, and Somers VK. Baroreflex control of sympathetic nerve activity and heart rate in obstructive sleep apnea. Hypertension 32: 1039–1043, 1998.[Abstract/Free Full Text]
  37. Narkiewicz K and Somers VK. Sympathetic nerve activity in obstructive sleep apnoea. Acta Physiol Scand 177: 385–390, 2003.[CrossRef][Web of Science][Medline]
  38. Olney JW, Wozniak DF, Jevtovic-Todorovic V, Faber NB, Bittigau P, and Ikonomidou C. Drug-induced apoptotic neurodegeneration in the developing brain. Brain Pathol 12: 488–498, 2002.[Web of Science][Medline]
  39. Parati G, Di Rienzo M, Bonsignore MR, Insalaco G, Marrone O, Castiglione P, Bonsigniore G, and Mancia G. Autonomic cardiac regulation in obstructive sleep apnea syndrome: evidence from spontaneous baroreflex analysis during sleep. J Hypertens 15: 1621–1626, 1997.[CrossRef][Web of Science][Medline]
  40. Parish JM and Shepard JW. Cardiovascular effects of sleep disorders. Chest 97: 1220–1226, 1990.[Abstract/Free Full Text]
  41. Peng YJ, Rennison J, and Prabhakar NR. Intermittent hypoxia augments carotid body and ventilatory response to hypoxia in neonatal rat pups. J Appl Physiol 97: 2020–2025, 2004.[Abstract/Free Full Text]
  42. Peyronnet J, Dalmaz Y, Ehrström M, Mamet J, Roux JC, Pequignot JM, Thorén HP, and Lagercrantz H. Long-lasting adverse effects of prenatal hypoxia on developing autonomic nervous system and cardiovascular parameters in rats. Pflügers Arch 443: 858–865, 2002.[CrossRef][Web of Science][Medline]
  43. Randall WC, Wurster RD, Randal DC, and Xi-Moy SX. From cardioaccelerator and inhibitory nerves to a "heart brain": an evolution of concepts. In: The Autonomic Nervous System, edited by Shepherd JT and Vatner SF. Reading, UK: Harwood, 1996, p. 173–179.
  44. Randall DC. Towards understanding of the function of the intrinsic cardiac ganglia (Abstract). J Physiol 528: 406, 2000.[Free Full Text]
  45. 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 In press.
  46. Resta O, Guido P, Rana L, Procacci V, Scarpelli F, and Picca V. Depressed baroreceptor reflex in patients with obstructive sleep apnea (OSA). Boll Soc Ital Biol Sper 72: 247–254, 1996.[Medline]
  47. Richter DW and Spyer KM. Cardiorespiratory control. In: Central Regulation of Autonomic Function, edited by Loewy AD and Spyer KM. New York: Oxford Univ. Press, 1990, p. 189–207.
  48. Schwartz JE, Kovach A, Meyer J, McConnell C, and Iwamoto HS. Brief intermittent hypoxia restricts fetal growth in Sprague-Dawley rats. Biol Neonate 73: 313–319, 1998.[CrossRef][Web of Science][Medline]
  49. Seagard JL, Dean C, and Hopp FA. Neurochemical transmission of baroreceptor input in the nucleus tractus solitarius. Brain Res Bull 51: 111–118, 2000.[CrossRef][Web of Science][Medline]
  50. Sica AL, Greenberg HE, Ruggiero DA, and Scharf SM. Chronic-intermittent hypoxia: a model of sympathetic activation in the rat. Respir Physiol 121: 173–184, 2000.[CrossRef][Web of Science][Medline]
  51. Soukhova GK, Roberts AM, Lipton AJ, Gozal E, and Gozal D. Early postnatal exposure to intermittent hypoxia attenuates baroreflex sensitivity in adult rats (Abstract). Sleep 25: 335, 2002.
  52. Stradling JR. Review: sleep apnea and systemic hypertension. Thorax 44: 984–989, 1989.[Free Full Text]
  53. Törk I, McRitchie D, Rikard-Bell GC, and Paxinos G. Autonomic regulatory centers in the medulla oblongata. In: The Human Nervous System, edited by Paxinos G. San Diego, CA: Academic, 1990, p. 221–259.
  54. Vanoli E, De Ferrari GM, Stramba-Badiale M, Hull SS Jr, Foreman RD, and Schwartz PJ. Vagal stimulation and prevention of sudden death in conscious dogs with a healed myocardial infarction. Circ Res 68 1471–1481, 1991.
  55. Wiklund U, Olofsson BO, Franclin K, Blom H, Bjerle P, and Niklasson U. Autonomic cardiovascular regulation in patients with obstructive sleep apnea: a study based on spectral analysis of heart rate variability. Clin Physiol 20: 234–241, 2000.[CrossRef][Web of Science][Medline]
  56. Whitescarver SA, Ott CE, and Kotchen TA. Parasympathetic impairment of baroreflex control of heart rate in Dahl S rats. Am J Physiol Regul Integr Comp Physiol 259: R76–R83, 1990.[Abstract/Free Full Text]
  57. Xu W, Xu W, Chi L, Row BW, Xu R, Ke Y, Xu B, Luo C, Kheirandish L, Gozal D, and Liu R. Increased oxidative stress is associated with chronic intermittent hypoxia-mediated brain cortical neuronal cell apoptosis in a mouse model of sleep apnea. Neuroscience 126: 313–323, 2004.[CrossRef][Web of Science][Medline]



This article has been cited by other articles:


Home page
 CirculationHome page
D. Gozal, L. Kheirandish-Gozal, L. D. Serpero, O. Sans Capdevila, and E. Dayyat
Obstructive Sleep Apnea and Endothelial Function in School-Aged Nonobese Children: Effect of Adenotonsillectomy
Circulation, November 13, 2007; 116(20): 2307 - 2314.
[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. Heart Circ. Physiol.Home page
M. Lin, R. Liu, D. Gozal, W. B. Wead, M. W. Chapleau, R. Wurster, and Z. Cheng
Chronic intermittent hypoxia impairs baroreflex control of heart rate but enhances heart rate responses to vagal efferent stimulation in anesthetized mice
Am J Physiol Heart Circ Physiol, August 1, 2007; 293(2): H997 - H1006.
[Abstract] [Full Text] [PDF]

Home page
 J. Neurophysiol.Home page
K. J. S. Griffioen, H. W. Kamendi, C. J. Gorini, E. Bouairi, and D. Mendelowitz
Reactive Oxygen Species Mediate Central Cardiorespiratory Network Responses to Acute Intermittent Hypoxia
J Neurophysiol, March 1, 2007; 97(3): 2059 - 2066.
[Abstract] [Full Text] [PDF]

Home page
 Exp PhysiolHome page
V. A. Braga, R. N. Soriano, and B. H. Machado
Sympathoexcitatory response to peripheral chemoreflex activation is enhanced in juvenile rats exposed to chronic intermittent hypoxia
Exp Physiol, November 1, 2006; 91(6): 1025 - 1031.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
S. R. Reeves, G. S. Mitchell, and D. Gozal
Early postnatal chronic intermittent hypoxia modifies hypoxic respiratory responses and long-term phrenic facilitation in adult rats
Am J Physiol Regulatory Integrative Comp Physiol, June 1, 2006; 290(6): R1664 - R1671.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
290/3/H1157    most recent
00767.2005v1
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 Web of Science
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 Web of Science (9)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Soukhova-O'Hare, G. K.
Right arrow Articles by Gozal, D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Soukhova-O'Hare, G. K.
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.