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Am J Physiol Regul Integr Comp Physiol 278: R831-R837, 2000;
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Vol. 278, Issue 4, R831-R837, April 2000

Thalamic lesions dissociate breathing inhibition by hypoxia and adenosine in fetal sheep

Brian J. Koos, Andrew Chau, Masahiko Matsuura, Oscar Punla, and Lawrence Kruger

Nicholas S. Assali Perinatal Research Laboratory, Departments of Obstetrics and Gynecology and Neurobiology, Brain Research Institute, University of California Los Angeles School of Medicine, Los Angeles, California 90095-1740


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The effects of diencephalic lesions on respiratory responses to intra-arterially infused adenosine (ADO) were determined in chronically catheterized fetal sheep (>0.8 term). These studies were designed to test the hypothesis that the inhibitory effects of ADO on fetal breathing, like those of hypoxia, are mediated by the parafascicular nuclear complex (Pf) of the posteromedial thalamus. ADO inhibited breathing [control (C): 26 ± 2.6, ADO: 4 ± 1 min/h] in normal fetuses and in a fetus with a lesion that virtually destroyed the thalamus but left intact most of Pf. Neuronal lesions in the diencephalon, produced by injecting ibotenic acid, abolished the inhibitory effects of ADO on breathing (C: 31 ± 5.1, ADO: 30 ± 4.5 min/h) when the lesions encompassed Pf or the sector immediately rostral to Pf that retained the capacity to regulate hypoxic inhibition. Smaller lesions created by the insertion of needles also eliminated the depressant effects of ADO when disruptions were within Pf or a rostral component of the thalamic cortical activating system. It is concluded that 1) a medial thalamic sector is critically involved in ADO-induced apnea and 2) ADO-dependent and ADO-independent mechanisms mediate hypoxic inhibition.

brain; rapid eye movement; respiration; sleep; thalamus


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

BREATHING IN FETAL SHEEP (>0.8 term) occurs in episodes associated with rapid eye movements (REM) and low-voltage electrocortical activity (LV ECoG) (7). Although hypercapnia and chemical acidemia increase the amplitude of breathing, these respiratory stimuli do not change the episodic nature of these movements. Fetal breathing also differs from respiration postnatally in that hypoxia almost completely eliminates breathing activity through an effect on the fetal brain (4, 8, 23).

The mechanism by which hypoxia inhibits fetal breathing has not been established, but experimental evidence supports a role for the neuromodulator adenosine (ADO). For example, hypoxia increases fetal plasma (19) and brain (21) concentrations of ADO and peripheral infusions of ADO that increase plasma ADO concentrations to levels measured during hypoxia, virtually abolish fetal breathing (19). Central administration of long-acting ADO analogs also arrests breathing (3). As with hypoxia, the depressant effects of ADO on breathing are abolished by transection of the pons or midbrain (16). The hypoxic arrest of breathing is blunted by intravascular administration of ADO receptor antagonists that cross the blood-brain barrier (2, 5, 22). Peripherally acting ADO receptor antagonists only minimally reduce the inhibitory effects of hypoxia on fetal breathing (18), indicating that the ADO receptors that depress breathing lie within the fetal brain. Although other factors are likely involved, these observations strongly suggest that central ADO receptors participate in hypoxic inhibition of fetal breathing.

We recently observed that a sector of the thalamus encompassing the parafascicular nuclear complex (Pf) is a crucial part of the neuronal apparatus involved in reducing fetal breathing during acute O2 deficiency (17). Because ADO plays a key role in hypoxic inhibition, experiments were conducted to determine whether the Pf region of the fetal thalamus is involved with the inhibitory effects of ADO on fetal breathing. The findings indicate that Pf is one component of a medial thalamic sector that mediates the inhibitory effects of adenosine on breathing and that ADO-dependent and ADO-independent mechanisms are involved in hypoxic inhibition.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Experiments were conducted in 20 pregnant ewes (Rambouillet-Columbia breed), of which 14 animals were used for hypoxia studies reported previously (17). The ewes (~0.8 term) were operated on under halothane anesthesia. Polyvinyl catheters were inserted in the right carotid artery, brachiocephalic trunk, and external jugular vein of the fetus; other catheters were placed in the trachea and amniotic sac (23). Bipolar stainless steel electrodes were implanted on a lateral orbital ridge for recording eye activity and on the cranial dura for recording electrocortical activity.

In our previous work (16), transection of the rostral midbrain abolished the inhibitory effects of adenosine and hypoxia on fetal breathing. In the present study, a diencephalic disruption was achieved by inserting a 3-mm-wide metal spatula through a trephine hole in parietal bone ~1 mm caudal to the coronal suture. The goal was to produce a forebrain lesion that left intact the inhibitory effects of adenosine on fetal breathing.

In 15 fetuses, guide cannulas (CMA/Microdialysis, Stockholm, Sweden) were stereotaxically directed toward the Pf of the thalamus. Two cannulas were symmetrically inserted ~3 mm lateral to the sagittal suture and ~4 mm caudal to the coronal suture. The cannula tips were directed rostrally at ~70° angle to the horizontal plain. The cannulas, secured to the calvarium with dental acrylic, were exteriorized through a Silastic rubber window (21), providing access to the fetal brain under chronic experimental conditions.

Fetal pressures were measured with pressure transducers (Cobe Laboratories, Lakewood, CO), with arterial and tracheal pressure referenced to amniotic fluid pressure. Heart rate was determined from the arterial pulse pressure using a cardiotachometer. These fetal measurements, as well as the electrooculogram (EOG) and electrocorticogram (ECoG), were displayed on a chart recorder (7E, Grass Instruments, Quincy, MA). Heart rate and arterial and tracheal pressures were sampled at 100 Hz by a microcomputer with fetal breathing identified online (6). Minute averages of heart rate, mean arterial pressure (MAP), inspiratory time, breath interval, and breath amplitude were stored on disk for subsequent analysis. Fetal arterial blood gases and pH were measured using blood electrodes (Instrumentation Laboratories, Lexington, MA), with the values corrected to fetal temperature (39.5°C).

Experiments

The experiments were conducted under chronic experimental conditions at least 4 days after surgery. ADO (3.6 mg/ml saline) was infused at 0.25 mg · min-1 · kg-1 for 1 h into the right brachiocephalic trunk to determine whether ADO arrested fetal breathing (22).

Localized disruptions of the diencephalon were achieved by injecting ibotenic acid (Ibo), a glutamate analog that destroys neurons with minimal injury to the vasculature and fibers of passage (11, 32). A microinjection pump (CMA/200, CMA/Microdialysis) was used to infuse Ibo [30 µg/µl synthetic cerebrospinal fluid (CSF) (14)] at 0.33 µl/min for a total injection volume of 0.4-2 µl. Repeated bilateral symmetrical injections were usually carried out with up to four injections on a single day, helping to ensure localized destruction of neurons (12). Fetal breathing responses to ADO infusion were again determined at least 3 days after the Ibo injections.

In other studies, bilateral symmetrical microinjections of synthetic CSF were carried out in the brain of six fetuses using the same coordinates as for Ibo. Fetal breathing responses to ADO and hypoxia were tested before and at least 3 days after these microinjections. Fetal isocapnic hypoxia was induced by having the ewe breathe a hypoxic gas mixture (9% O2-3% CO2-88% N2) for 1 h (23). These experiments determined whether needle insertion and/or microinjection alone altered breathing responses apart from the cellular destruction induced by Ibo. The order of ADO and hypoxia experiments was varied.

Brain Analysis

After the studies were completed, the fetal brains were perfused in situ with a buffered 4% formaldehyde solution. Brains disrupted by transection were cut in 50-µm sagittal sections, whereas brains injected with Ibo were cut in the coronal plane at 35 µm. The sections were stained with cresyl violet, which clearly delineated cell destruction and gliosis induced by Ibo.

A neuroanatomist (L. Kruger) blind to the experimental results provided the anatomic description and drawings of brain lesions based on a modified description of the sheep thalamus by Rose (27). A previously published photomicrograph illustrates the Ibo-induced brain lesions that were used for outline drawings (17). The anatomic nomenclature used in this study has been outlined previously (17).

Data Analysis

Because fetal breathing occurs in episodes, the number of minutes per hour of breathing was used as a measure of respiratory activity. In this analysis, breathing was tabulated if it occurred in at least 20 s of each 1-min epoch (22), and a comparable criterion of analysis was used for determining the incidence of REM.

In nine fetuses from previous experimental work (22) and ten fetuses from the present study, the lower 95% prediction limit for breathing incidence under control conditions was 11 min/h (18% of time), with the upper limit during ADO infusion of 12 min/h (20% of time). Thus breathing was determined to be inhibited if it occurred <12 min/h during the hour of ADO infusion.

Statistical Analysis

The mean of the 4-h control period for EOG, REM, and breathing was used for statistical comparison of measurements during and after ADO infusion. Repeated measures of ANOVA with post hoc comparison of means were performed using Tukey's least-significant difference criterion with the presence or absence of brain lesions as the within-animal factors. Student's t-test was used for single comparisons of means. Differences were significant at the P < 0.05 level. All values are expressed as means ± SE.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Intrathalamic Transection

Rostral diencephalic disruption resulted in necrosis of virtually the entire thalamus but left intact the habenular nuclei and most of the caudal portion of the Pf (Fig. 1). During hypoxia, fetal arterial PO2 (PaO2) decreased by ~13 Torr [control (C): 25 Torr, hypoxia (H): 12 Torr]. Fetal hypoxemia was associated with a progressive decline in arterial pH (C: 7.362, H: 7.325) and a relatively stable arterial PCO2 (PaCO2; C: 52.3 Torr, H: 49.0 Torr).


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  		Fig. 1.   Outline drawing of a Nissl-stained sagittal section of thalamic transection that failed to abolish adenosine (ADO) inhibition of breathing. Heavy gliosis is dark gray or stippled black when necrotic. Lighter gliosis is lightly stippled. Hb, habenula nucleus; IC, inferior colliculus; Pf, parafascicular nucleus (includes centrum medianum nuclei); Pta, anterior pretectal nucleus; SC, superior colliculus; sPf, subparafascicular nucleus. Animal and section number are indicated below these and in subsequent figures.

Intravascular infusion of ADO did not alter fetal PaO2, PaCO2, or pH compared with the respective control values of 23 Torr, 50.9 Torr, and 7.352. ADO inhibited fetal breathing (C: 24 min/h, ADO: 8 min/h) and REM (C: 25 min/h, ADO: 3 min/h). The ECoG was not recorded.

The connections to other thalamic sectors and rostral brain had been destroyed as a result of the transection, thus the posteromedial thalamus containing Pf apparently contained neurons that can contribute to inhibition of breathing during systemic ADO administration. Further localizing studies were conducted by inserting needles and by injecting Ibo into and adjacent to this caudal thalamic sector.

Forebrain Cannulas

ADO. ADO was infused in nine fetuses with cannulas before needle placement to establish breathing responses before injecting Ibo. Arterial blood gases and pH were within the normal range (PO2 = 23.6 ± 1.5 Torr, PCO2 = 49.0 ± 2.0 Torr, and pH = 7.349 ± 0.009) during the control period. ADO significantly reduced arterial pH to 7.302 ± 0.010 and 7.306 ± 0.009 for measurements after 30 and 60 min of infusion, respectively. Mean PaO2 and PaCO2 were not significantly affected.

Fetal heart rate, averaging 187 ± 6 beats/min during the control period, increased within the first 10 min of ADO administration to 211 ± 9.6 and 231 ± 9.9 beats/min for measurements of 20 and 60 min, respectively, after the start of infusion. MAP was 40.3 ± 2.3 mmHg during the control period and was not significantly affected by ADO infusion.

ADO virtually eliminated fetal breathing, as indicated by the reduced incidence (C: 27 ± 2; ADO: 4 ± 0.7 min/h) and the decreased number of breaths per hour (C: 582 ± 130; ADO: 197 ± 74 breaths/h). The breathing exhibited significantly reduced inspiratory duration and breath amplitude (Table 1). ADO also significantly reduced REM (H: 28 ± 3; ADO: 5 ± 0.7 min/h) without affecting the incidence of LV ECoG (C: 31 ± 3.1; ADO 26 ± 3.9 min/h) or high-voltage (HV) ECoG (C:22 ± 0.6; ADO: 24 ± 4.3 min/h).

                              
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  		Table 1.   Ti, Ttot, and Pt during ADO infusion for fetuses before and after Ibo brain lesions

Diencephalic Microinjections

Ibo. Ibo was microinjected into the diencephalon of nine fetal sheep with guide cannulas. Two to four microinjections were carried out bilaterally at one site on 2-3 separate days (fetuses nos. 23, 69, 163, 511, 633) and at two different sites separated by 3 mm along the axis of the needle in two fetuses (nos. 139, 617). Two symmetrical injections were performed in fetus no. 489, and three injections were carried out on the left in no. 308. In the latter, a microdialysis probe was inserted on the right, but no injections were performed.

Fetal responses to ADO were tested at least 3 days after Ibo microinjections. During the control period, fetal arterial blood gases (PaO2 = 25 ± 0.8 Torr, PaCO2 = 52.4 ± 1.0 Torr) and pH (7.352 ± 0.011) were within the normal range. ADO minimally reduced PaO2 by ~3 Torr during the first 30 min of infusion, but fetal O2 tensions subsequently increased toward the control value. Arterial pH fell slightly to 7.325 ± 0.011 by the end of the infusion, but PaCO2 was not significantly affected.

Because the lesion site did not alter cardiovascular responses, all fetuses were grouped together for analysis of heart rate and MAP. The average heart rate of 181 ± 9 beats/min preceding ADO administration did not differ significantly from the baseline value of ADO experiments before the Ibo microinjections. ADO induced a rise in heart rate that began within 10 min of the start of infusion and progressively rose to 206 ± 9.5 and 217 ± 8.2 beats/min for values after 20 and 60 min of infusion, respectively.

MAP (44.8 ± 2.7 mmHg) during the control period was significantly greater than that for ADO studies before Ibo administration. MAP was not significantly changed by ADO administration.

On the basis of breathing responses to adenosine (see METHODS), the fetuses could be separated into those that breathed >20% of the time (ADO inhibition eliminated) and those that breathed <20% of the time (ADO inhibition retained).

Adenosine inhibition eliminated. In seven fetuses, ADO did not reduce the incidence of breathing (Fig. 2) or decrease the number of breaths per hour (C: 860 ± 111; ADO: 1,128 ± 268). ADO increased mean breath amplitude but did not significantly reduce inspiratory time or mean breath interval (Table 1).


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  		Fig. 2.   Ibotenic acid (Ibo) lesions that abolished ADO inhibition. Solid line depicts incidence (min/h) of high (HV) and low (LV) electrocorticogram (ECoG; 4 fetuses), rapid eye movements (5 fetuses), and breathing (6 fetuses) before injecting Ibo. Dotted lines represent ECoG (6 fetuses), rapid eye movements (6 fetuses), and breathing (7 fetuses) responses after Ibo administration. ADO infusion shown by stippled vertical bar. * P < 0.05 compared with respective control mean. dagger  P < 0.05 compared with mean value during ADO infusion before Ibo injection.

The lesions blunted the inhibitory effects of ADO on the incidence of REM compared with responses before Ibo injections but did not significantly alter the incidence of LV or HV ECoG (Fig. 2).

Fetus no. 489 had the smallest lesion that abolished the inhibitory effects of ADO on breathing. Bilateral approximately symmetrical gliosis and neuronal loss were present in a large portion of the "postmedial" thalamic group of Rose (27), involving the nucleus reuniens and Pf (including centrum medianum and the caudal ventral subparafascicular component). Most of ventromedial, arcuate, and mediodorsal nuclei were spared. Portions of central medial, paracentral, rhomboid, and reuniens nuclei were destroyed in their caudal extent but were largely intact in rostral sections. This lesion, which provided the critical zone for analysis in hypoxic inhibition (see Fig. 5 in Ref. 17), established a crucial locus involved in ADO inhibition of breathing.

The other lesions that eliminated the inhibitory effects of ADO on fetal breathing are summarized in Fig. 3. In five of six fetuses, the lesions involved Pf, as previously reported for thalamic lesions that abolish hypoxic inhibition of breathing (17). The exception was fetus no. 511, in which Ibo destroyed a sector immediately rostral to Pf, containing anteromedial, anteroventral, and mediodorsal nuclei (Fig. 3). This rostral lesion eliminated the inhibitory effects of ADO but left intact the depressing effects of hypoxia on fetal breathing, as reported earlier (17).


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  		Fig. 3.   Outline drawings of Nissl-stained transverse sections at an optimal level depicting lesions that abolished ADO inhibition of breathing. Light gliosis is lightly stippled; heavy gliosis is dark gray. Needle tracks are black. AD, anterodorsal nuclei; AH, amygdalo-hippocampal area; Arc, arcuate nuclei; AV, anteroventral nuclei; cl, central lateral nuclei; cm, central medial nuclei; cp, cerebral peduncle; fo, fornix; Hbm, medial habenula nuclei; Hbl, lateral habenula nuclei; LGd, dorsal of lateral geniculate nucleus; mm, medial mammillary nucleus; mt, mammillothalamic tract; Po, posterior group; Ptm, medial pretectal area; Pul, pulvinar; R, reticular nucleus; re, reuniens nucleus; rh, rhomboid nucleus; sn, substantia nigra; VB, ventrobasal nuclear complex; VM, ventromedial nucleus; ZI, zona incerta. Bar = 1 mm. These illustrations of lesions, adapted from Ref. 17, have been regrouped according to ADO responses.

ADO inhibition retained. In two fetuses (nos. 308, 163), ADO decreased the incidence of fetal breathing from 26 ± 2 (control) to 10 ± 2 min/h (ADO). ADO also reduced the number of breaths per hour by 60%, compared with the control of 635 ± 95 breaths/h. ADO did not significantly affect mean inspiratory time (C: 0.57 ± 0.03 s, ADO: 0.60 ± 0.12 s), breath interval (C: 1.80 ± 0.20 s, ADO: 1.95 ± 0.31 s), or breath amplitude (C: 2.5 ± 0.1, ADO: 2.9 ± 0.7 mmHg).

The incidence of REM was also reduced during ADO infusion (C: 32 ± 2; ADO: 19 ± 13 min/h). The incidence of LV (C: 36 ± 2; ADO: 27 ± 6.5 min/h) and HV ECoG (C: 21 ± 1.5; ADO: 30 ± 7.5 min/h) was generally unaffected by the purine nucleoside.

The brain lesions in fetus no. 163 were confined to the cingulate gyrus, with no extension into the diencephalon. The disruptions in fetus no. 308 were asymmetric: medial to Pf on the right side and lateral to Pf on the left side (see Fig. 1 of Ref. 17).

Synthetic CSF. The vehicle (synthetic CSF) was injected bilaterally into the fetal diencephalon in 6 of 15 fetuses with guide cannulas to determine whether smaller lesions created by needle insertion altered breathing responses to ADO.

ADO. In four of six fetuses, fetal responses to ADO were tested before CSF microinjections; in these four fetuses, ADO inhibited REM (C: 29 ± 3.8; ADO: 8 ± 2.6 min/h) and breathing (C: 23 ± 4.5; ADO: 5 ± 0.9 min/h) and decreased the incidence of LV ECoG (C: 36 ± 3; ADO: 26 ± 4.7 min/h, in the 3 fetuses with ECoG recordings).

ADO unexpectedly failed to arrest breathing or REM (C: 25 ± 3.5; ADO: 20 ± 7.5 min/h) after needle insertion and CSF injection in five of six fetuses (Fig. 4). The incidence of LV ECoG (C: 31 ± 1.8; ADO: 28 ± 4 min/h) was also unaffected. Histological examination showed that the sites of injection, as determined by the termination of needle tracks, were within Pf (nos. 137, 175), lateral to Pf (no. 182), in the subparafascicular nuclei (no. 237), and the mediodorsal thalamus (no. 768). The lesions extended about two or three neurons beyond the needle tracks, but rarely exceeding 1 mm in width.


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  		Fig. 4.   Fetal breathing incidence (min/h) during ADO infusion and hypoxia. A: responses before vehicle injection. B: responses after vehicle injection.

In the sixth fetus (no. 167), ADO inhibited breathing (C: 19; ADO: 7 min/h) and REM (C: 32; ADO: 4 min/h) before the microinjections of CSF, but the postinjection response differed in that the inhibitory effects of ADO on breathing (C: 28; ADO: 9 min/h) and REM (C: 30; ADO: 2 min/h) persisted after the microinjections. Histological examination showed that the needle tracks passed through the right mediodorsal and ventrobasal nuclei and involved the medial pulvinar on the left.

Needle insertion associated with CSF or Ibo microinjections had a consistent effect on breathing responses to ADO. In both circumstances, bilateral disruptions of the medial thalamus eliminated the depressant effects of ADO, but lateral thalamic or suprathalamic lesions failed to do so. These results are surprising because such small disruptions did not interfere with hypoxic arrest of breathing activity as previously reported (17) and as described below.

Hypoxia. Fetal responses to hypoxia were also determined to establish whether they would be similar to those for ADO. Hypoxia inhibited breathing (Fig. 4) in the five fetuses tested before CSF microinjections. A mixed response was observed in the three fetuses with EOG recordings: hypoxia inhibited REM in fetus no. 768 (C: 36; H: 5 min/h) but not in the fetuses nos. 137 (C:27; H: 57 min/h) and 167 (C: 27; H: 40 min/h). In the latter two fetuses, cannula placement in the fetal brain apparently disrupted hypoxic depression of eye movements without interfering with the inhibition of breathing. The incidence of LV ECoG fell nearly 50% during hypoxia (C: 35 ± 1.2; H: 18 ± 4.3 min/h).

In contrast to the ADO studies, hypoxic inhibition of breathing was retained in all six fetuses tested after CSF microinjections (Fig. 4). Hypoxia also inhibited REM (C: 27 ± 1.3; H: 7 ± 3.5) in three (nos. 137, 175, 182) of five fetuses with EOG recordings but not in fetuses nos. 237 (C: 21; H: 23 min/h) and 167 (C: 28; H: 60 min/h). Again, disruptions caused by cannula and needle placement apparently dissociated the effects of hypoxia on breathing and eye movements. Hypoxia also reduced the incidence of LV ECoG (C: 32 ± 1.0; H: 18 ± 2.2 min/h).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The results indicate that the forebrain contains a component of the neural substrate that mediates the inhibitory effects of ADO on fetal breathing. This finding differs from previous studies in fetal sheep (3) and newborn lambs (26) and the neonatal rat brain stem in vitro (9), indicating that ADO modulates synaptic transmission in the brain stem and/or spinal cord. The depressant effects of adenosine on the brain stem have been implicated in the hypoxia-induced reduction or "roll off" in respiration observed in newborns and adults (10, 13, 29). This report provides new information on the extent of the neuronal network mediating the inhibitory effects of ADO on fetal breathing and its possible relevance to respiratory regulation in general.

The studies in the fetus with brain transection suggest that the Pf may be involved in triggering the arrest of breathing by ADO as well as hypoxia (17). This possibility was supported by the Ibo studies in which bilateral lesions that encompassed Pf abolished the respiratory depression of both hypoxia and ADO. More rostral intralaminar lesions also eliminated the inhibitory effects of ADO, but not those of hypoxia; thus the thalamic locus mediating ADO inhibition of breathing appears to encompass a larger sector than that for hypoxia.

Smaller medial thalamic lesions produced by needle placement also eliminated the arrest of breathing by ADO, indicating that relatively restricted disruptions of neural tissue within the medial thalamus can prevent ADO-induced apnea. This effect appears to be confined to the medial thalamus because ADO arrested breathing, as in normal fetuses, with needle disruptions in other forebrain sites. These results suggest that Pf receives contributions from more rostral sectors that modulate the respiratory effects of ADO; however, it cannot be determined whether the critical disruptions that eliminated ADO inhibition resulted from injury of neurons and/or fibers of passage. That these needle lesions of the medial thalamus failed to alter hypoxic inhibition indicates that a mechanism independent of ADO is involved in hypoxic depression of breathing.

The "nonspecific" intralaminar nuclei of the thalamus, including Pf, integrates sensorimotor function and participates in tonic cortical activation associated with increased discharge rates in desynchronized states of wakefulness and REM sleep (1). The latter has particular relevance to the arrest of breathing by hypoxia and adenosine because reduced REM drive to breathing may be critical to the respiratory depression (22, 23). Receiving fibers from raphe nuclei and hypothalamic sectors involved in sleep regulation (33), neurons within Pf project to the cerebral cortex, striatum, subthalamus, raphe nuclei, and various midbrain sectors (25). Electrical stimulation of Pf in rats inhibits spontaneous motor activity (25), which is consistent with the hypothesis that Pf is part of the neuronal apparatus that depresses breathing.

In situ hybridization studies have shown a relatively high expression of the ADO A1-receptor gene in the thalamus of fetal rats (36) but only a transient expression of ADO A2a-receptor gene in the Pf of newborn pups (35). However, immunohistochemical studies indicate that ADO A2a receptors in low concentrations are, in fact, present in parafascicular, anteroventral, mediodorsal, and other thalamic nuclei of the adult rat (28). Thus there is reason to suspect that, in fetal sheep, ADO A1 and/or A2a receptors in the nonspecific nuclei may be involved in respiratory depression. Other possible sites include the preoptic area that contains ADO receptors that modulate sleep and from which fibers project to Pf (33, 34).

The respiratory effects of intravascularly administered ADO depend on maturity. For example, ADO stimulates respiration in adult rabbits through excitation of the carotid bodies, but it depresses ventilation in newborn rabbits through central effects (37). The mechanism underlying this developmental difference in respiratory response has not been elucidated, but increased permeability of the blood-brain barrier to ADO in the neonate may be a critical factor (37).

ADO induced a tachycardia that is elicited predominantly by ADO A2-receptor stimulation of the autonomic nervous system (15). This rise in heart rate with slow infusions contrasts with the transient bradycardia observed in adult animals with intravascular injections (24), a response produced by direct negative chronotropic effects of ADO on the atrial sinus node. The lack of a significant effect of ADO on fetal MAP is consistent with our previous studies (22).

In summary, the Pf of the posteromedial thalamus is a critical part of the neuronal network that mediates hypoxic inhibition of fetal breathing, and a larger sector of the medial thalamus appears to be essential for ADO-induced respiratory depression. Small bilateral disruptions of Pf or a rostral component of the thalamic cortical-activating system eliminate the inhibitory effects of ADO but not those of hypoxia. These findings, along with previous studies, suggest that hypoxic inhibition of breathing results from both ADO-dependent and ADO-independent mechanisms.


    ACKNOWLEDGEMENTS

We thank S. Sampogna for the histological preparations.


    FOOTNOTES

This study was supported in part by National Institute of Child Health and Human Development Grant HD-18478 and the National Institute of Neurological Disorders and Stroke Grant NS-5685.

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. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: B. J. Koos, Dept. of Obstetrics and Gynecology, 22-132 CHS, UCLA School of Medicine, Los Angeles, CA 90095-1740 (E-mail: bkoos{at}mednet.ucla.edu).

Received 8 July 1999; accepted in final form 6 October 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Bentivoglio, M, Balercia G, and Kruger L. The specificity of the nonspecific thalamus: the midline nuclei. Prog Brain Res 87: 53-80, 1991[ISI][Medline].

2.   Bissonnette, JM, Hohimer AR, Chao CR, Knopp SJ, and Notoroberto NF. Theophylline stimulates fetal breathing movements during hypoxia. Pediatr Res 28: 83-86, 1990[ISI][Medline].

3.   Bissonnette, JM, Hohimer AR, and Knopp SJ. The effects of centrally administered adenosine on fetal breathing movements. Respir Physiol 84: 273-285, 1991[ISI][Medline].

4.   Boddy, K, Dawes GS, Fisher R, Pinter S, and Robinson JS. Foetal respiratory movements, electrocortical and cardiovascular responses to hypoxaemia and hypercapnia in sheep. J Physiol (Lond) 243: 599-618, 1974[Abstract/Free Full Text].

5.   Chau, A, and Koos BJ. Metabolic and cardiorespiratory responses to hypoxia in fetal sheep: adenosine receptor blockade. Am J Physiol Regulatory Integrative Comp Physiol 276: R1805-R1811, 1999[Abstract/Free Full Text].

6.   Dale, PS, Ducsay CA, Gilbert RD, Koos BJ, and Longo LD. A microcomputer program for real-time data acquisition in the perinatal physiology laboratory. J Dev Physiol (Eynsham) 11: 185-188, 1989[ISI][Medline].

7.   Dawes, GS, Fox JE, Leduc BM, Liggins GC, and Richards RT. Respiratory movements and rapid eye movement sleep in the foetal lamb. J Physiol (Lond) 220: 119-143, 1972[Abstract/Free Full Text].

8.   Dawes, GS, Gardner WM, Johnston BM, and Walker DW. Breathing in fetal lambs: the effects of brain stem section. J Physiol (Lond) 335: 535-553, 1983[Abstract/Free Full Text].

9.   Dong, X-W, and Feldman JL. Modulation of inspiratory drive to phrenic motoneurons by presynaptic adenosine A1 receptors. J Neurosci 15: 3458-3467, 1995[Abstract].

10.   Easton, PA, and Anthonisen NR. Ventilatory response to sustained hypoxia after pretreatment with aminophylline. J Appl Physiol 64: 1445-1450, 1988[Abstract/Free Full Text].

11.   Guldin, WO, and Markowitsch HJ. Epidural kainate, but not ibotenate, produces lesions in local and distant regions of the brain. A comparison of the intracerebral actions of kainic acid and ibotenic acid. J Neurosci Methods 4: 83-93, 1982.

12.   Jarrard, LE, and Meldrum BS. Selective excitotoxic pathology in the rat hippocampus. Neuropathol Appl Neurobiol 19: 381-389, 1993[ISI][Medline].

13.   Kawai, A, Okada Y, Mückenhoff K, and Scheid P. Theophylline and hypoxic ventilatory response in the rat isolated brainstem-spinal cord. Respir Physiol 100: 25-32, 1995[ISI][Medline].

14.   Koos, BJ. Central stimulation of breathing movements in fetal lambs by prostaglandin synthetase inhibitors. J Physiol (Lond) 362: 455-466, 1985[Abstract/Free Full Text].

15.   Koos, BJ, and Chau A. Fetal cardiovascular and breathing responses to an adenosine A2a receptor agonist in sheep. Am J Physiol Regulatory Integrative Comp Physiol 274: R152-R159, 1998[Abstract/Free Full Text].

16.   Koos, BJ, Chao A, and Doany W. Adenosine stimulates breathing in fetal sheep with brain stem section. J Appl Physiol 72: 94-99, 1992[Abstract/Free Full Text].

17.   Koos, BJ, Chau A, Matsuura M, Punla O, and Kruger L. Thalamic locus mediates hypoxic inhibition of breathing in fetal sheep. J Neurophysiol 79: 2383-2393, 1998[Abstract/Free Full Text].

18.   Koos, BJ, Chau A, and Ogunyemi D. Adenosine mediates metabolic and cardiovascular responses to hypoxia in fetal sheep. J Physiol (Lond) 448: 761-766, 1995.

19.   Koos, BJ, and Doany W. Role of plasma adenosine in breathing responses to hypoxia in fetal sheep. J Dev Physiol (Eynsham) 16: 81-85, 1991[Medline].

20.   Koos, BJ, Kruger L, and Murray TF. Source of extracellular brain adenosine during hypoxia in fetal sheep. Brain Res 778: 439-442, 1997[ISI][Medline].

21.   Koos, BJ, Mason BA, Punla O, and Adinolfi AM. Hypoxic inhibition of breathing in fetal sheep: relationship to brain adenosine concentrations. J Appl Physiol 77: 2734-2739, 1994[Abstract/Free Full Text].

22.   Koos, BJ, and Matsuda K. Fetal breathing, sleep state, and cardiovascular responses to adenosine in sheep. J Appl Physiol 68: 489-495, 1990[Abstract/Free Full Text].

23.   Koos, BJ, Sameshima H, and Power GG. Fetal breathing movement, sleep state and cardiovascular responses to graded hypoxia in sheep. J Appl Physiol 62: 1033-1039, 1987[Abstract/Free Full Text].

24.   Mason, BA, Ogunyemi D, Punla O, and Koos BJ. Maternal and fetal cardiovascular responses to adenosine in sheep. Am J Obstet Gynecol 168: 1558-1561, 1993[ISI][Medline].

25.   Mikeikovsky, Y, Verevkina SV, and Norzdrachev AD. Effects of stimulation of the frontoparietal cortex and parafascicular nucleus on locomotion in rats. Physiol Behav 55: 267-271, 1994[Medline].

26.   Moore, PJ, Ackland GC, and Hanson MA. Unilateral cooling in the region of the locus coeruleus blocks the fall in respiratory output during hypoxia. Exp Physiol 81: 983-994, 1996[Abstract].

27.   Rose, JE. The thalamus of the sheep: cellular and fibrous structure and comparison with pig, rabbit and cat. J Comp Neurol 77: 469-523, 1942.

28.   Rosen, DL, Robeva A, Woodard RL, Guyenet PG, and Linden J. Immunohistochemical localization of adenosine A2a receptors in the rat central nervous system. J Comp Neurol 401: 163-186, 1998[ISI][Medline].

29.   Runold, M, Lagerkrantz H, Prabhakar N, and Fredholm BB. Role of adenosine in hypoxic ventilatory depression. J Appl Physiol 67: 541-546, 1989[Abstract/Free Full Text].

30.   Ryan, JW, and Waldrop TG. Hypoxic sensitive neurons in the caudal hypothalamus project to periaqueductal gray. Respir Physiol 100: 185-194, 1995[ISI][Medline].

31.   Satoh, S, Matsumura H, Suzuki F, and Hayaishi O. Promotion of sleep mediated by the A2a-adenosine receptor and possible involvement of this receptor in the sleep induced by prostaglandin D2 in rats. Proc Natl Acad Sci USA 93: 5980-5984, 1996[Abstract/Free Full Text].

32.   Schwarcz, R, Hokfelt T, Fuxe G, Jonsson G, Goldstein M, and Terenius L. Ibotenic acid-induced neuronal degeneration: a morphological and neurochemical study. Exp Brain Res 37: 199-216, 1979[ISI][Medline].

33.   Simerly, RB, and Swanson LW. Projections of the medial preoptic nucleus: a Phaseolus vulgaris leucoagglutinin anterograde tract-tracing study in the rat. J Comp Neurol 270: 209-242, 1988[ISI][Medline].

34.   Ticho, SR, and Radulovacki M. Role of adenosine in sleep and temperature regulation in the preoptic area of rats. Pharmacol Biochem Behav 40: 33-40, 1991[ISI][Medline].

35.   Weaver, DR. A2a adenosine receptor expression in developing rat brain. Mol Brain Res 20: 313-327, 1993[Medline].

36.   Weaver, DR. A1-adenosine receptor gene expression in fetal rat brain. Dev Brain Res 94: 205-223, 1996[Medline].

37.   Watt, AH, Buss DC, and Routledge PA. Comparison of respiratory effects of intravenous adenosine in neonatal and adult rabbits. Life Sci 39: 1617-1622, 1986[ISI][Medline].

Am J Physiol Regul Integr Comp Physiol 278(4):R831-R837
0363-6119/00 $5.00 Copyright © 2000 the American Physiological Society


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